Edited by Ja´nos Fischer and C. Robin Ganellin Analogue-based Drug Discovery II
Related Titles Faller, B., Urban, L. (...
165 downloads
1799 Views
7MB Size
Report
This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Report copyright / DMCA form
Edited by Ja´nos Fischer and C. Robin Ganellin Analogue-based Drug Discovery II
Related Titles Faller, B., Urban, L. (eds.)
Hit and Lead Profiling Identification and Optimization of Drug-like Molecules 2009 ISBN: 978-3-527-32331-9
Chorghade, M. S. (ed.)
Drug Discovery and Development 2 Volume Set 2007 ISBN: 978-0-471-39846-2
Johnson, D. S., Li, J. J. (eds.)
The Art of Drug Synthesis 2007 ISBN: 978-0-471-75215-8
IUPAC / Fischer, János / Ganellin, C. Robin (eds.)
Analogue-based Drug Discovery 2006 ISBN-13: 978-3-527-31257-3
Lednicer, Daniel
New Drug Discovery and Development 2006 ISBN-13: 978-0-470-00750-1
Edited by János Fischer and C. Robin Ganellin
Analogue-based Drug Discovery II
The Editors Prof. Dr. János Fischer Richter Plc Gyömröi ut 30 1103 Budapest Hungary Prof. Dr. C. Robin Ganellin University College London Department of Chemistry 20 Gordon Street London WC1H OAJ United Kingdom
Supported by The International Union of Pure and Applied Chemistry (IUPAC) Chemistry and Human Health Devision PO Box 13757 Research Triangle Park, NC 27709-3757 USA
All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.: applied for British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.d-nb.de. # 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Cover Design Adam Design, Weinheim Typesetting Thomson Digital, Noida, India Printing and Binding Strauss GmbH, Mörlenbach Printed in the Federal Republic of Germany Printed on acid-free paper ISBN: 978-3-527-32549-8
V
Contents Preface XV Introduction XVII Abbreviations XXI
1
Part I
General Aspects
1
Optimizing Drug Therapy by Analogues 3 János Fischer, C. Robin Ganellin, John Proudfoot, and Erika M. Alapi Introduction 3 Pharmacodynamic Characteristics 4 Potency 4 Improving the Ratio of Main Activity and Adverse Effects 5 Improving Selectivity Through Receptor Subtypes 6 Improving Selectivity Through Unrelated Receptors 7 Improving Selectivity by Tissue Distribution 7 Improving Selectivity of Nonreceptor-Mediated Effects 10 Improving the Physicochemical Properties with Analogues 10 Analogues to Reduce the Resistance to Anti-Infective Drugs 11 Antibiotics 11 Antifungal Drugs 12 Antiviral Drugs 12 Analogue Research in Resistance to Drug Therapies in Cancer Treatment 15 Pharmacokinetic Characteristics 15 Improving Oral Bioavailability 15 Improving Absorption 16 Improving Metabolic Stability 16 Drugs with a Long Duration of Action 17 Ultrashort-Acting Drugs 18 Decreasing Interindividual Pharmacokinetic Differences 20 Decreasing Systemic Activity 21
1.1 1.2 1.2.1 1.2.2 1.2.2.1 1.2.2.2 1.2.2.3 1.2.2.4 1.2.3 1.2.4 1.2.4.1 1.2.4.2 1.2.4.3 1.2.5 1.3 1.3.1 1.3.1.1 1.3.1.2 1.3.2 1.3.3 1.3.4 1.3.5
Analogue-based Drug Discovery II. Edited by János Fischer and C. Robin Ganellin Copyright Ó 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32549-8
VI
Contents
1.4 1.4.1 1.4.2 1.5
Drug Interactions 22 Decreasing Drug Interactions 22 Increasing Drug Interactions 23 Summary 23 References 24
2
Standalone Drugs 29 János Fischer, C. Robin Ganellin, Arun Ganesan, and John Proudfoot Acetaminophen (Paracetamol) 30 Acetylsalicylic Acid (Aspirin) 33 Aripiprazole 35 First Generation ‘‘Typical’’ Antipsychotic Drugs (Other Names: Neuroleptics, Conventional Antipsychotics) 36 Second-Generation ‘‘Atypical’’ Antipsychotic Drugs 37 A New Approach: Aripiprazole, a Dopamine Partial Agonist 38 Bupropion 39 Ezetimibe 42 Lamotrigine 46 Metformin 47 Topiramate 49 Valproate 51 Summary 52 References 53
2.1 2.2 2.3 2.3.1 2.3.2 2.3.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10
3
3.1 3.2 3.3 3.4 3.5 3.6 3.7
4 4.1 4.2 4.3 4.4 4.5 4.6 4.7
Application of Molecular Modeling in Analogue-Based Drug Discovery 61 György G. Ferenczy Introduction 61 Cilazapril: An ACE Inhibitor 62 Atorvastatin: A HMG-CoA Reductase Inhibitor 66 PDE4 Inhibitors 70 GPIIb/IIIa Antagonists 73 HIV Protease Inhibitors 74 Epilogue 79 References 79 Issues for the Patenting of Analogues 83 Stephen C. Smith Introduction 83 Patents: Some Fundamentals 84 Patentability 85 Important Elements of the International Patent System Priority 87 Novelty 88 Inventive Step: Nonobviousness 90
86
Contents
4.8 4.9 4.10 4.11 4.12 4.12.1 4.12.2 4.12.3 4.12.4 4.12.5 4.12.6 4.12.7 4.12.8 4.12.9 4.12.10 4.13
Utility: Industrial Application 93 Selection Inventions 93 Enantiomers 94 Prodrugs and Active Metabolites 95 The Patenting Process from the Inventors Standpoint 97 Inventorship 98 The Priority Patent Application 98 Prior Art Disclosure 98 Patent Specification Review 99 ‘‘Best Mode’’ of Carrying Out the Invention 99 Foreign Patent Applications 99 Patent Application Publication 100 Patent Examination 100 Opposition to Grant 101 Patent Litigation 102 Pitfalls for the Unwary: Granted Versus Published Patents, Scientific Publications 102 References 105
Part II
Analogue Classes
5
Dipeptidyl Peptidase IV Inhibitors for the Treatment of Type 2 Diabetes 109 Jens-Uwe Peters and Patrizio Mattei Introduction 109 In Vitro Assays and Animal Models for the Assessment of DPP-IV Inhibitors 110 Substrate-Based DPP-IV Inhibitors 110 Sitagliptin and Analogues 119 Xanthines and Analogues 122 Pharmacological Comparison of DPP-IV Inhibitors 125 Concluding Remarks 127 References 128
5.1 5.2 5.3 5.4 5.5 5.6 5.7
6 6.1 6.2 6.2.1 6.2.2 6.2.3 6.2.4 6.3 6.3.1 6.3.2
107
Phosphodiesterase 5 Inhibitors to Treat Erectile Dysfunction 135 Nils Griebenow, Helmut Haning, and Erwin Bischoff Introduction 135 Pharmacology of Phosphodiesterases 136 The Phosphodiesterase Family 136 Pharmacological Effects of cGMP 137 PDE5: Regulation, Activation, and Structure 138 PDE5 Inhibitors and Erectile Dysfunction 143 Pyrimidinone PDE5 Inhibitors 147 Xanthines and cGMP Analogues 147 PDE5 Inhibitors Incorporating the Purinone Nucleus 150
VII
VIII
Contents
6.3.2.1 6.3.2.2 6.3.3 6.3.3.1 6.3.3.2 6.3.4 6.3.5 6.3.6 6.3.7 6.4 6.4.1 6.5
Zaprinast 150 Purinones 150 Pyrazolopyrimidinone PDE5 Inhibitors 151 Pyrazolo[3,4-d]Pyrimidin-4-One PDE5 Inhibitors 151 1,6-Dihydro-7H-Pyrazolo[4,3-d]Pyrimidin-4-One PDE5 Inhibitors 152 Imidazotriazinone PDE5 Inhibitors 154 Imidazoquinazolinones 155 Pyrazolopyridopyrimidines 156 Miscellaneous Heterocylic-Fused Pyrimidinone PDE5 Inhibitors 156 Nonpyrimidone PDE5 Inhibitors 160 Hexahydropyrazino-Pyrido-Indole-1,4-Diones 160 Conclusions 162 References 162
7
Rifamycins, Antibacterial Antibiotics and Their New Applications 173 Enrico Selva and Giancarlo Lancini Discovery of the Pioneer Drug 173 Clinically Used Rifamycins 173 Mode of Action of Rifamycins and Structural Requirements for Activity 174 Modulation of Chemotherapeutic Properties 177 Profiles of Rifamycins Targeted at Tuberculosis Treatment 177 Rifampicin (INN), Rifampin (USAN) 178 Rifapentine 180 Rifabutin 181 Rifamycins Beyond Tuberculosis 181 Rifamycin SV and Rifamide 182 Rifaximin 182 Trials for Other Therapeutic Indications 183 Summary 183 References 184
7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.13
8 8.1 8.2 8.2.1 8.2.1.1 8.2.2 8.2.3 8.2.3.1 8.2.3.2 8.3 8.3.1
Monoterpenoid Indole Alkaloids, CNS and Anticancer Drugs 189 András Nemes Introduction 189 Vincamine and Derivatives: Cerebrovascular and Neuroprotective Agents 190 Medicinal Chemistry of Vincamine Derivatives 190 Structure–Activity Relationships 192 Synthesis of Vincamine Derivatives 193 Pharmacological Properties of Vincamine Derivatives 193 Mechanism of Action 193 Clinical Pharmacology 194 Antitumor Dimeric Vinca Alkaloids 195 Medicinal Chemistry of Dimeric Vinca Alkaloid Derivatives 195
Contents
8.3.1.1 8.3.2 8.3.3 8.3.3.1 8.3.3.2 8.4 8.4.1 8.4.1.1 8.4.2 8.4.3 8.4.3.1 8.4.3.2 8.5
Structure–Activity Relationships 196 Synthesis of Dimeric Vinca Alkaloid Derivatives 198 Pharmacological Properties of Dimeric Vinca Alkaloid Derivatives Mechanism of Action 199 Clinical Pharmacology 199 Antitumor Camptothecin Derivatives 201 Medicinal Chemistry of Camptothecin Derivatives 201 Structure–Activity Relationships 202 Synthesis of Camptothecin Derivatives 203 Pharmacological Properties of Camptothecin Derivatives 204 Mechanism of Action 204 Clinical Pharmacology 205 Summary and Conclusions 207 References 207
9
Anthracyclines, Optimizing Anticancer Analogues 217 Federico-Maria Arcamone Introduction: Biosynthetic Antitumor Anthracyclines 217 Analogues with Modification of the Aminosugar Moiety 219 Analogues with Modifications in the Anthraquinone Moiety 223 Analogues Modified on Ring A of the Aglycone 226 Disaccharide Analogues 229 Other Compounds 232 Summary and Final Remarks 233 References 234
9.1 9.2 9.3 9.4 9.5 9.6 9.7
10 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9
11
11.1 11.2
Paclitaxel and Epothilone Analogues, Anticancer Drugs 243 Paul W. Erhardt and Mohammad El-Dakdouki Introduction 243 Discovery and Development of Paclitaxel 243 Clinical Success and Shortcomings of Paclitaxel 245 ABDD Leading to Docetaxel 247 Additional Structural Analogues 249 The Pursuit of Microtubule-Stabilizing Pharmacological Analogues 250 The Epothilones 252 ABDD and Development Leading to Ixabepilone 258 Conclusions 260 References 263 Selective Serotonin Reuptake Inhibitors for the Treatment of Depression 269 Wayne E. Childers Jr. and David P. Rotella Introduction 269 Neurochemistry and Mechanism of Action 270
199
IX
X
Contents
11.3 11.3.1 11.3.2 11.3.3 11.3.4 11.3.5 11.4 11.4.1 11.4.2 11.4.3 11.4.4 11.4.5 11.5 11.6
Preclinical Pharmacology 271 Sertraline 271 Escitalopram 272 Fluvoxamine 273 Fluoxetine 274 Paroxetine 275 Medicinal Chemistry 276 Sertraline 276 Escitalopram 278 Fluvoxamine 279 Fluoxetine 281 Paroxetine 284 Comparison of SSRIs and Other Uses 285 Summary 288 References 288
12
Muscarinic Receptor Antagonists in the Treatment of COPD 297 Matthias Grauert, Michael P. Pieper, and Paola Casarosa Introduction 297 Muscarinic Receptor Subtypes 298 Structures of Muscarinic Agonists and Antagonists 299 Muscarinic Agonists 299 Antimuscarinics 300 Discovery of Quaternary Antimuscarinics 303 Once-Daily Quaternary Antimuscarinics: Tiotropium Bromide as the Gold Standard 305 Preclinical Pharmacology: Comparison of Ipratropium and Tiotropium 309 Bronchoconstriction in Conscious Guinea Pigs According to the Method of Kallos and Pagel 310 Bronchoconstriction in Anaesthetized Dogs 310 Clinical Pharmacology 311 Antimuscarinics in Clinical Development for the Treatment of COPD 313 Summary 313 References 314
12.1 12.2 12.3 12.3.1 12.3.2 12.3.3 12.3.4 12.4 12.4.1 12.4.2 12.5 12.6 12.7
13 13.1 13.2 13.3 13.4
b-Adrenoceptor Agonists and Asthma 319 Giovanni Gaviraghi Introduction 319 First-Generation b2-Agonists: The Short-Acting Bronchodilators Second-Generation b2-Agonists: Further Derivatives of Salbutamol 321 Third-Generation b2-Agonists: The Long-Acting Bronchodilators 321
319
Contents
13.5 13.6
Combination Therapy with LABA and Corticosteroids 326 Future Directions: Once-a-Day Therapy and Bifunctional Muscarinic Antagonist–b2-Agonist (MABA) 327 References 329
Part III
Case Histories
14
Liraglutide, a GLP-1 Analogue to Treat Diabetes 335 Lotte B. Knudsen Introduction 335 Discussion 338 Physiology of Native GLP-1 338 Development of Liraglutide: A GLP-1 Analogue 339 The Pharmacology of Liraglutide 346 Clinical Evidence with Liraglutide 349 Summary 350 References 351
14.1 14.2 14.2.1 14.2.2 14.2.3 14.2.4 14.3
15 15.1 15.2 15.3 15.4 15.5 15.6 15.7
16 16.1 16.1.1 16.2 16.2.1 16.2.2 16.3 16.4 16.4.1 16.5 16.6 16.7 16.8 16.9 16.9.1
333
Eplerenone: Selective Aldosterone Antagonist 359 Jaroslav Kalvoda and Marc de Gasparo Introduction 359 Development of a Specific and Selective Aldosterone Antagonist 360 Eplerenone: Selectivity and Specificity 367 Preclinical Development of Eplerenone: From Animal to Man 373 Further Development of Eplerenone 375 Conclusions 376 Epilogue 376 References 377 Clevudine, to Treat Hepatitis B Viral Infection 383 Ashoke Sharon, Ashok K. Jha, and Chung K. Chu Current Status of Anti-HBV Agents 383 Nucleoside Reverse Transcriptase Inhibitors 386 Chemical Evolution of Clevudine 387 Development of Synthetic Routes 387 Structure–Activity Relationships 388 Metabolism and Mechanism of Action 390 Pharmacokinetics 392 Woodchuck Studies 393 Clinical Studies 394 Drug Resistance 396 Toxicity and Tolerability 398 Dosage and Administration 399 Combination Therapy 399 Combination of Clevudine with Other Agents 399
XI
XII
Contents
16.9.2 16.10
Combination of Clevudine with Vaccine 400 Summary 400 References 401
17
Rilpivirine, a Non-nucleoside Reverse Transcriptase Inhibitor to Treat HIV-1 409 Jerome Guillemont, Luc Geeraert, Jan Heeres, and Paul J. Lewi Introduction 409 Chemistry 412 Synthesis of TMC278 and Close Analogues 412 Modulation of the Central Heterocycle Core 418 C-5 Substitution of the Pyrimidine Core 419 Structure–Activity Relationships 420 Introduction of G Spacer Between the Aryl Ring and the Cyano Group 421 Modulation of Substituents at C-2 and C-6 on the Left Wing and of the Linker Between Left Wing and Pyrimidine Core 423 Subsitution at C-5 Position of the Pyrimidine Heterocycle 424 Modification of the Central Heterocycle Core 426 TMC278: Physicochemical Properties 429 Modeling of TMC278 and Crystal Structure 430 Pharmacokinetic and Phase II Studies of TMC278 431 Conclusions 434 References 434
17.1 17.2 17.2.1 17.2.2 17.2.3 17.3 17.3.1 17.3.2 17.3.3 17.3.4 17.4 17.5 17.6 17.7
18
18.1 18.2 18.2.1 18.3 18.3.1 18.3.2 18.3.3 18.3.3.1 18.3.3.2 18.3.3.3 18.4 18.4.1 18.4.2 18.4.3 18.5 18.6
Tipranavir, a Non-Peptidic Protease Inhibitor for Multi-drug Resistant HIV 443 Suvit Thaisrivongs, Joseph W. Strohbach, and Steve R. Turner Human Immunodeficiency Virus 443 HIV Protease 443 HIV PIs 444 Approaches to Identifying and Developing PI Leads 446 Focused Screening 446 Broad Screening for Nonpeptidic Leads 447 Structure-Based Drug Design 448 PNU-96988, A First-Generation Clinical Candidate 449 PNU-103017, A Second-Generation Clinical Candidate 450 Tipranavir, The Third Generation 453 Characteristics of Tipranavir 454 In Vitro Activity 454 Pharmacokinetics 455 Highlights of Clinical Data 456 Fragment-Based Lead Development? 457 Summary 458 References 459
Contents
19 19.1 19.2 19.3 19.4 19.4.1 19.4.2 19.4.3 19.5 19.6 19.7
20
20.1 20.2 20.3
21
21.1 21.2 21.3 21.4 21.4.1 21.4.2 21.4.3 21.4.4 21.5 21.6 21.7 21.8 21.8.1 21.8.1.1 21.8.1.2 21.8.1.3 21.8.2
Lapatinib, an Anticancer Kinase Inhibitor 465 Karen Lackey Introduction 465 Aims 467 Chemical Evolution and Proof-of-Mechanistic Approach Using Small Molecules 469 Final Set of Analogues that Led to the Discovery of Lapatinib 6-Furanyl Quinazoline Series 474 6-Thiazolylquinazoline Series 479 Alkynylpyrimidine Series 480 Final Selection Criteria and Data 482 Early Clinical Results 487 Prospects for Kinase Inhibitors 489 References 490 Dasatinib, a Kinase Inhibitor to Treat Chronic Myelogenous Leukemia 493 Jagabandhu Das and Joel C. Barrish 493 Introduction 493 Discussion 494 Clinical Findings and Summary 502 References 503 Venlafaxine and Desvenlafaxine, Selective Norepinephrine and Serotonin Reuptake Inhibitors to Treat Major Depressive Disorder 507 Magid Abou-Gharbia and Wayne E. Childers Jr. Introduction 507 Major Depressive Disorder 510 MDD Pharmacotherapy 511 The Discovery of Venlafaxine 511 Identification of an Early Lead (WY-44362) 511 Structure–Activity Relationship Studies 512 In Vivo Animal Models of Preclinical Efficacy 514 Selection of WY-45030 for Clinical Trials 514 Clinical Efficacy of Effexor1 515 An Extended Release Formulation – Effexor XR1 516 Discovery of a Second-Generation SNRI – O-Desmethylvanlafaxine 516 Effexor and Pristiq – Additional Considerations 518 Effexor 518 Onset of Action 518 Treatment of Some Anxiety Disorders 519 Painful Somatic Symptoms 519 Pristiq 519
474
XIII
XIV
Contents
21.8.2.1 21.8.2.2 21.9
Anxiety and Painful Symptoms 519 Symptoms Associated with Menopause Conclusions 520 References 520 Index
525
519
XV
Preface The positive response to the first volume stimulated the editors to continue beyond the well-received book. Three very important facts supported this feeling. All copies of the book ‘‘Analogue-Based Drug Discovery’’ were sold within 18 months after its publication in February 2006. 2) The Journal of Medicinal Chemistry in its very positive review recommended the book for teaching of medicinal chemistry. 3) Last, but not the least Wiley-VCH, and, personally, Dr. Frank O. Weinreich welcomed the idea of the continuation.
1)
We started to collect new topics at the beginning of 2008. We have continued to study the general aspects of ‘‘Analogue-Based Drug Discovery’’ with the help of the chapters that describe how analogues optimize drug therapy. In a separate chapter on standalone drugs, we demonstrate that in the case of a minor number of drugs, the pioneer drug could not (or not yet) be optimized. These standalone drugs can always challenge the medicinal chemistry researchers because, as existing drugs, they can serve as starting points for researchers. We are grateful again to the IUPAC (International Union of Pure and Applied Chemistry), which supported this activity in projects. The Subcommittee for Medicinal Chemistry and Drug Development and the Division of Chemistry and Human Health provided the opportunity to the editors to discuss this work with other experts of medicinal chemistry. We are grateful for the participation of all the contributors. Many authors of the book played an important role as inventors who discovered valuable drugs, and their chapters carry a high credibility either as an analogue class study or as a case history of a drug. We are very much obliged to the helpful reviewing work done by many colleagues, whose names are as follows: Karl-H. Baringhaus, Jozsef Bódi, Derek Buckle, Mark Bunnage, Duane Burnett, Neal Castagnoli, Jonathan B. Chaires, Mukund Chorghade, Erik De Clercq, Duncan Curley, György Domány, Joelle Dubois, Andrew Fensome, Tom Heightman, Bastian Hengerer, Duy H. Hua, Robert Jones, Dale
Analogue-based Drug Discovery II. Edited by János Fischer and C. Robin Ganellin Copyright Ó 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32549-8
XVI
Preface
Kempf, Karsten Krohn, K.H. Lee, John Lowe III, Frank C. Odds, Eckhard Ottow, Tom Perun, István Polgár, Dominick Quagliato, Waldemar Priebe, Graham Robertson, Romano Silvestri, László Szabó, Károly Tihanyi, Edwin B. Villhauer, Niels Vrang, Richard White, Michael Williams, and Puwen Zhang. All these colleagues contributed to the quality of this second volume. We express special thanks to reviewers Derek Buckle, John Lowe III, Bruce E. Maryanoff, Lester A. Mitscher, and Dominick Quagliato, who each corrected the language, and Eckhard Ottow, who corrected the structures, of a whole chapter. Some authors, besides the editors, also served as reviewers. Our thanks are due to these authors and reviewers as follows: Giovanni Gaviraghi, John Proudfoot, and David Rotella. J.F. thanks the Alexander von Humboldt Foundation (Bonn) for a fellowship in 2008 and 2009. We hope that the second volume will also be well received and that it will contribute in some way to help the experts in drug discovery and students of medicinal chemistry. October 2009 Budapest and London
János Fischer and C. Robin Ganellin
XVII
Introduction Janos Fischer and C. Robin Ganellin Analogy plays a very important role in scientific research and especially in applied research. This is certainly true for the medicinal chemist searching for new drugs to treat diseases. The chemical structure and the similarities and differences in chemical and biological properties between compounds help guide the researcher in deciding where to position a new molecule in comparison to what is already known about other compounds. Medicinal chemistry is a relatively young science that spanned the whole of the twentieth century. In the first half of the century, new drug research was dominated by organic chemistry, and researchers sought improved drugs among structurally similar compounds. Full analogues (see below) dominated this kind of research. The latter half of the century saw a much greater contribution from biochemistry and pharmacology, and many pioneer drugs were discovered. This opened the way for researchers to seek to improve upon these drugs by investigating analogues. The first volume of Analogue-Based Drug Discovery focused on an important segment of medicinal chemistry, where an existing drug was selected as a lead compound and the research had, as a goal, to improve upon the lead by synthesizing and testing analogues. The chemical structure and main biological activity of such an analogue were often similar to the lead drug. Thus, the researchers generally sought a structural and pharmacological analogue (more simply called a full analogue) or if the pharmacophores were the same, a direct analogue. Usually, the aim was to achieve an improved biological activity profile, with a greater potency. The first volume included a description of many well-established analogue classes of drug that are indispensable nowadays for the treatment of peptic ulcer disease, gastroesophageal reflux disease, prevention of cardiovascular diseases (e.g., antihypertensives, cholesterol-lowering agents, calcium antagonists, and beta-adrenergic receptor blocking agents), pain (e.g., opioid analgesics), and many other diseases. The last two decades, however, have witnessed great changes in the chemical and biological methods for generating a lead compound. Combinatorial chemistry affords many more compounds than traditional synthetic methods and these are tested very rapidly by high-throughput screening (HTS) to deliver new hit and lead molecules. This procedure often paves the way for new types of structures for drug research thereby decreasing the importance of having chemical similarity. At the Analogue-based Drug Discovery II. Edited by János Fischer and C. Robin Ganellin Copyright Ó 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32549-8
XVIII
Introduction
same time, this provides a better opportunity for novelty and therefore for patenting. This also gives rise to a greater need to compare the biological properties of these new lead compounds in order to arrive at the best pharmacological analogue. Analogue-based drug discovery (ABDD) is not a simple research method, but it is a way of thinking that, in addition to organic synthesis, uses most of the procedures that are now available to medicinal chemists, such as i. ii. iii. iv. v.
investigation of structure–activity relationships, molecular modeling, structure-based drug discovery, fragment-based drug discovery, early recognition of drug distribution properties and avoidance of potential toxicities.
Analogue-based drug discovery has the merit that the therapeutic target is already validated, but it carries the hazard of potentially losing out to competitors who may start from the same approach at about the same time. This second volume of Analogue-Based Drug Discovery has a broader scope than the first volume. The book not only contains descriptions of full analogues but also includes several pharmacological analogues. The book is divided into three parts: 1) 2) 3)
General Aspects of Analogue-Based Drug Discovery Analogue Classes Case Histories
General Aspects
The opening chapter summarizes various possibilities exemplifying how the properties of a drug may be modified to give a new drug analogue that improves patient drug therapy. There are 12 principles exemplified and within some of these principles there are several methods; hence this chapter gives a broad overview. A small number of the pioneer drugs remain without successful analogues; we describe these by the term standalone drugs. Among the most frequently used 100 drugs, 9 such standalone drugs can be identified. Their history and present situation may be used to initiate a new research activity to make their analogues. In addition to the traditional structure–activity relationship (SAR) studies, molecular modeling is the most important method that can help the medicinal chemist to find a new drug analogue. The chapter discusses several useful examples of molecular modeling in analogue research. Patenting activity is one of the basic tasks of drug research. Patents mostly concern a group of direct analogues; therefore, the first claim of a patent contains a general structure that describes this group of compounds. The chapter gives an overview of some of the issues that can affect the commercial protection of the discoveries made by medicinal chemists.
Case Histories
Analogue Classes
The discovery of dipeptidyl peptidase IV inhibitors has opened a promising chapter for the treatment of type 2 diabetes. The pioneer drug sitagliptin has been followed by several analogues in order to obtain more potent and longer acting derivatives. Serendipitous clinical observation afforded the pioneer drug sildenafil. Several analogues have been found that have optimized its properties (e.g., selectivity, duration of action). Rifamycins are antibacterial antibiotics derived from fermentation. Analoguebased drug research afforded more potent derivatives. One of the derivatives, the poorly absorbed rifaximin, has a promising application for the treatment of irritable bowel syndrome. Three analogue classes of monoterpenoid indole alkaloids are discussed: (i) vincamine derivatives, (ii) dimeric vinca alkaloid analogues, and (iii) camptothecin analogues. The successful natural product direct analogues are applied for the treatment of cerebral insufficiencies and cancer. The natural product doxorubicin is an anthracycline antibiotic used to treat a wide range of cancers, but it has a cardiotoxic adverse effect. The research into direct analogues had a goal to obtain drugs with a better therapeutic index. Paclitaxel and epothilone analogues are also examples of how natural product drugs can be used to initiate analogue-based drug research to afford new drug analogues with better properties as anticancer agents. The selective serotonin reuptake inhibitors (SSRIs) are pharmacological analogues that revolutionized antidepressant therapy. The structurally different SSRIs have different profiles for inhibiting uptake of the neurotransmitters: serotonin, dopamine, and norepinephrine. The modification of naturally occurring tropane alkaloids afforded the quaternary ammonium salts ipratropium and tiotropium, which are important drugs used for treating chronic obstructive pulmonary disease. Tiotropium, as a result of analogue-based drug discovery, has a longer duration of action that enables a once-daily dosing. The natural product adrenaline (epinephrine) was the starting point for drug research into b-adrenoreceptor agonists. From isoprenaline (isoproterenol) through the selectively acting salbutamol, and on to salmeterol, analogue research resulted in selective, more potent, and longer acting analogues with different PK profiles, which are important drugs in asthma therapy.
Case Histories
Eight case histories are described by their inventors. Liraglutide is a new antidiabetic drug, an analogue of the natural product glucagonlike peptide 1. Among the acylated GLP-1 analogues liraglutide has been developed for a once-daily treatment.
XIX
XX
Introduction
Eplerenone is a spironolactone analogue for treating hypertension that has a greater selectivity for the mineralocorticoid receptor and reduced sexual side effects. Clevudine is a new drug for the treatment of the chronic hepatitis B virus (HBV) infection, which belongs to the class of nucleoside reverse transcriptase inhibitors. Tipranavir is a new anti-HIV agent that is a protease inhibitor. The discovery of tipranavir used structure-based and fragment-based drug design and its long discovery process started from warfarin, which is a weak HIV-1 protease inhibitor. Dasatinib can be regarded as a pharmacological analogue of imatinib. Dasatinib is more potent and it can be used in imatinib-resistant cases for the treatment of chronic myelogenous leukemia (CML). Lapatinib can be regarded as a pharmacological analogue of erlotinib. It is a tyrosine kinase inhibitor and was first approved for the treatment of solid tumors such as in breast cancer. Venlafaxine is the first marketed serotonin/norepinephrine reuptake inhibitor (SNRI) and is used for the treatment of deep depression. Its active metabolite is desvenlafaxine, which has some advantageous properties; for example, it has a more favorable metabolic profile compared to venlafaxine. Rilpivirine is a new HIV-1 nonnucleoside reverse transcriptase inhibitor (NNRTI), an analogue of etravirine. Rilpivirine is highly potent also against strains that are resistant to the first-generation NNRTI drugs. The first volume of Analogue-Based Drug Discovery discussed mostly well-established drugs. This second volume also opens the door to new drug discoveries and the editors hope that, like the first volume, all of the drugs discussed in this book will have a bright future.
XXI
Abbreviations ABC ABDD ABPM ACAT ACE ACTH ADMET AFC AIDS ALT ALL AMP cAMP ANDA a-APA APV AR ATP AUC AZT BBB Bcr-Abl BG b.i.d. BOC CBF CC50 b-CCE CGI CHB CK CL CLR
ATP binding cassette analogue-based drug discovery ambulatory blood pressure monitoring acyl-CoA:cholesterol acyltransferase angiotensin-converting enzyme adrenocorticotropic hormone absorption, distribution, metabolism, excretion and toxicity 7-amino-4-trifluoromethylcoumarin acquired immunodeficiency syndrome alanine aminotransferase acute lymphoblastic leukemia amprenavir cyclic 30 ,50 -adenosine monophosphate Abbreviated New Drug Application a-anilinophenylacetamide amprenavir androgen receptor adenosine triphosphate area under the curve azidothymidine blood-brain-barrier Breakpoint cluster region - Abelson blood glucose twice a day (from Latin bis in die) t-butoxycarbonyl cerebral blood flow 50% cytotoxic concentration ethyl b-carboline-3-carboxylate Clinical Global Impressions Scale chronic hepatitis B creatine kinase clearance renal clearance
Analogue-based Drug Discovery II. Edited by János Fischer and C. Robin Ganellin Copyright Ó 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32549-8
XXII
Abbreviations
CLT CLV CLV-TP CML CMRglc CNS COBP COPD COX-1 COX-2 CPI/r CPT CRC CYP DA 10-DAB DAPY DATA dCK DNA cDNA cccDNA mtDNA DOC DOCA DPP-4 DSM-III EBV EC50 ED EFS EGFR EMEA EPA EPO EPS Erk ETC FAAH FBDD FDA L-FEAU FEV L-FMAU
total clearance clevudine clevudine triphosphate chronic myelogenogenous leukemia cerebral metabolic rate of glucose central nervous system chronic obstructive broncho-pneumopathies chronic obstructive pulmonary disease cyclooxygenase-1 cyclooxygenase-2 comparator protease inhibitor boosted with ritonavir camptothecin colorectal cancer cytochrome P450 isoenzyme dopamine 10-deacetyl-baccatin diarylpyrimidine diaryltriazine deoxycytidine kinase desoxyribonucleic acid complementary deoxyribonucleic acid covalently closed circular DNA mitochondrial DNA deoxycorticosterone deoxycorticosterone acetate dipeptidyl peptidase 4 Diagnostic and Statistical Manual of Mental Disorders, third edition Epstein-Barr virus effective concentration 50 erectile dysfunction electric field stimulation epidermal growth factor receptor European Medicines Agency Environmental Protection Agency European Patent Office exprapyramidal side effect extracellularly regulated kinase emtricitabine fatty acid amide hydrolase fragment-based drug design Food and Drug Administration 1-(20 -deoxy-20 -fluoro-b-L-arabinofuranosyl)-5-ethyluridine forced expiratory volume L-20 -fluoro-5-methyl-b-L-arabinofuranosyluracil
Abbreviations
GABAA GAD GI GIP GLP-1 cGMP GPIIb/IIIa HA HAART HA/ACTH HAM-A HAM-D HbA1c HBV HBcAg HBeAg HbsAg HCC HCV HDV hERG HFB HIAA HIV HIV PR HMG-CoA 5-HT 5-HTP HTS IBMX IC50 pIC50 ICS IDR IDV i.m. IND INN IOPY i.p. i.v. Ki LABA Lck hLck
gamma-aminobutyric acid A generalized anxiety disorder growth inhibition glucose-dependent insulinotropic polypeptide glucagon-like peptide-1 cyclic 30 ,50 -guanosine monophosphate glycoprotein IIb/IIIa heavy atom Highly Active Antiretroviral Therapy histamine-induced adrenocorticotropic hormone Hamilton Anxiety Taring Scale Hamilton Depression Rating Scale glycosylated haemoglobin hepatitis B virus hepatitis B core antigen hepatitis B e antigen hepatitis B surface antigen hepatocellular carcinoma hepatitis C virus hepatitis delta virus human ether-a-go-go-related gene human foreskin fibroblast 5-hydroxy-indole acetic acid human immunodeficiency virus HIV protease 3-hydroxy-3-methylglutaryl coenzyme A 5-hydroxytryptamine (serotonin) 5-hydroxytryptophan high-throughput screening isobutylmethylxanthine inhibitory concentration 50 log IC50 inhaled corticosteroids idarubicin indinavir intramuscular Investigational New Drug International Nonproprietary Name iodophenoxypyridone intraperitoneal intravenous inhibitory constant long-acting b2-agonist lymphocyte specific kinase human Lck
XXIII
XXIV
Abbreviations
mLck LDL-C LE LPV LVEF MADRS MAOI M1 MAP rMD MDD MDR MED MES MIC MR MRP MTD NAPQI NCE NCI NDA NE NMR NNRTI NO NPs NPC1L1 NRIs NRTI NSAIDs NSCLC OADs OC OCD OGTT PCA PCF PCT PDEs PDGFR PEP PGE1 PGE2 P-gp
murine Lck low-density lipoprotein-cholesterol ligand efficiency lopinavir left ventricular ejection fraction Montgomery-Asberg Depression Rating Scale monoamine oxidase inhibitor muscarinic receptor M1 subtype mitogen-activated protein restrained molecular dynamics major depressive disorder multidrug resistance minimal effective dose maximal electroshock seizure minimal inhibitory concentration mineralocorticoid receptor multidrug resistance-associated protein maximum tolerated dose N-acetyl-p-benzoquinone imine New Chemical Entity National Cancer Institute New Drug Application norepinephrine nuclear magnetic resonance nonnucleoside reverse transcriptase inhibitor nitric oxide natural products Niemann-Pick C1-Like-1 norepinephrine reuptake inhibitors nucleoside reverse transcriptase inhibitor nonsteroidal anti-inflammatory drugs non-small cell lung cancer oral antidiabetic drugs ovarian cancer obsessive-compulsive disorder oral glucose tolerance test p-chloroamphetamine plant cell fermentation Patent Cooperation Treaty phosphodiesterases platelet derived growth factor receptor prolyl endopeptidase prostaglandin E1 prostaglandin E2 permeability glyocoprotein
Abbreviations
Ph (+) PK PKG POMS PPCE PR QSAR q.d. or QD RBA RGD RNA RNApol mRNA RT RTV SAR SBDD s.c. SCID SCLC SEDDS SEF SI SIV SMC SNRI SQV Src SRI SSRIs TCR TDF TGFa TI TIBO t.i.d. TK TMPK TPV TPV/r TPT TRIPs TTP UDP
Philadelphia chromosome positive pharmakokinetic protein kinase G profile of mood state postproline cleaving enzyme progesterone receptor quantitative structure-activity relationship once a day (from Latin quaque die) relative binding affinity arginine-glycine-aspartic acid ribonucleic acid RNA polymerase messenger RNA reverse transcriptase ritonavir structure-activity relationship structure-based drug design subcutaneous severe combined immunodeficient small-cell lung cancer self-emulsifying drug delivery system sodium excreting factor selectivity index simian immunodeficiency virus smooth muscle cell serotonin/norepinephrine reuptake inhibitor saquinavir sarcoma serotonin reuptake inhibitor selective serotonin reuptake inhibitors T-cell antigen receptor tenofovir disoproxil fumarate tansforming growth factor-a tumor inhibition 4,5,6,7-tetrahydro-5-methylimidazo[4,5,1-jk]benzodiazepin-2 (1H)-one three times daily thymidine kinase thymidylate kinase tipranavir tipranavir/ritonavir combination topotecan Trade-related Aspects of Intellectual Property Rights time to progression uridine diphosphate
XXV
XXVI
Abbreviations
UGT USAN VEGFR VMS VSMC VSS WBC WHcAg WHsAg WHV WTO
uridine diphosphate glucuronyl transferase United States Adopted Names vascular endothelial growth factor receptor vasomotor symptoms vascular smooth muscle cell steady-state volume white blood cell woodchick hepatitis virus core antigen woodchuck hepatitis virus surface antigen woodchuck hepatitis virus World Trade Organization
Part I General Aspects
Analogue-based Drug Discovery II. Edited by János Fischer and C. Robin Ganellin Copyright Ó 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32549-8
j3
1 Optimizing Drug Therapy by Analogues Janos Fischer, C. Robin Ganellin, John Proudfoot, and Erika M. Alapi The medicinal chemist has a complex task to discover a useful new drug molecule. Among the different ways to accomplish this, the use of analogue research has been the most successful over the years, and most of the improvements in drugs have been obtained this way. This chapter will summarize the main aspects and demonstrate how analogues in a class of drugs are used to optimize drug therapy.
1.1 Introduction
The term analogue (from the Greek analogos) means proportionate, but in everyday terms we use it to indicate similarities between things. In medicinal chemistry, an analogue drug [1] has a chemical and/or pharmacological relationship with another drug. Structural analogues are drugs that have a similar chemical structure but quite different pharmacological properties, whereas pharmacological analogues are drugs that have a similar pharmacological activity without any discernible chemical or structural relationship. About half the drugs are analogues in both respects, and we therefore call them full analogues that are structural and pharmacological analogues. One special class of full analogues is considered to be direct analogues if they have identical pharmacophores – that is, they can be described by a general structure that includes most of the chemical skeleton. There are only a few drugs for which no successful analogues have been discovered, these we have termed standalone drugs [2]. Because of the experimental nature of structure–activity relationships (SARs), the term analogue is descriptive and not exact, nevertheless it is a useful tool in classifying the 6000 different drugs used nowadays. An analogue class is a group of drugs that displays similar in vitro and in vivo pharmacological properties; therefore, they are either pharmacological or full analogues. The first drug in the class (often termed first in class) used as a lead for the development of analogues can be considered a pioneer drug. A pioneer drug is therefore the first marketed drug in an analogue class. After the discovery of a pioneer
Analogue-based Drug Discovery II. Edited by János Fischer and C. Robin Ganellin Copyright Ó 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32549-8
j 1 Optimizing Drug Therapy by Analogues
4
drug, there are almost always efforts to improve upon it with analogues in order to obtain new drugs with better therapeutic properties. A drug can be characterized by several chemical and pharmacological properties. Even a minor structural change can sometimes modify all these properties. In order to illustrate key concepts in the optimization of drug therapy by analogues, it is necessary to simplify this question and we will always focus on a dominant property in a given analogue class.
1.2 Pharmacodynamic Characteristics
Pharmacodynamics refers to the study of drug actions in living organisms and includes the dominant activity along with possible adverse effects. 1.2.1 Potency
The potency of a drug refers to the amount of drug required to achieve a defined biological activity. The smaller the dose required, the more potent the drug. The discovery of the histamine H2-receptor antagonist cimetidine in 1971 was a pioneer invention in the treatment and prevention of peptic ulcer and gastroesophagitis. Cimetidine is used in a daily dose of 800 mg. The application of analogue-based drug research provided ranitidine (1976), roxatidine (1979), famotidine (1979), and nizatidine (1980), which are more potent drugs and are effective at a lower dosage. Table 1.1 shows daily doses in comparison to cimetidine. Famotidine is the most potent member of the analogue class (Figure 1.1) [3]. Statins are HMG-CoA reductase inhibitors (Figure 1.2). Based on their in vitro effects, rosuvastatin is the most potent analogue and is followed in rank order by atorvastatin, simvastatin, and pravastatin [4] (Table 1.2). The same rank order of potency has been observed in clinical trials.
Table 1.1 Comparison of approximately equivalent daily doses of H2-receptor antagonists.
Drug cimetidine nizatidine ranitidine roxatidine famotidine a)
Daily dose (mg) 800 300 300 150 40
Administered as acetate hydrochloride.
Molecular weight (Da) 252 331 314 385a) 337
1.2 Pharmacodynamic Characteristics
j5
CH3
N H
NH
NH
N
S NC
CH3
H3C
CH3
N
O
NH
S
NH
N O 2N
cimetidine
ranitidine
N
S O
NH
N
O
N
S
N
H2N
NH2 O
NH2
O CH3
O
roxatidine
famotidine
CH3 H3C
N
N
NH
S
S
NH
CH3
O 2N
nizatidine Figure 1.1 Structures of H2-receptor antagonists.
1.2.2 Improving the Ratio of Main Activity and Adverse Effects
There are no drugs without some adverse effects. One important goal of analoguebased discovery approaches is to design improved drugs with a better ratio of efficacy to adverse effects.
Table 1.2 Inhibitory effects of various statins in vitro.
Drug pravastatin simvastatin atorvastatin rosuvastatin
CH3
IC50 (nM)
Molecular weight (Da)
44.1 11.2 8.2 5.4
424 419 559 482
O S
NH2
j 1 Optimizing Drug Therapy by Analogues
6
HO
O
HO
O
O O H3C
OH
O O
H3C
OH
O
H3C
H
CH3
CH3
CH3
H3C
H
CH3
HO pravastatin
simvastatin
HO
HO
OH
OH O
O
OH
OH F
F CH3
CH3 N CH3
CH3 N
N H3C O
S
N O
NH O
CH3
rosuvastatin
atorvastatin
Figure 1.2 Structures of HMG-CoA reductase inhibitors.
1.2.2.1 Improving Selectivity Through Receptor Subtypes The opportunity for improvement is clear if the mechanism of the adverse effect is known and one such example is found in the case of the adrenergic b-receptor blockers (Figure 1.3). The pioneer drug of b-receptor blockers is propranolol, invented in 1962. Subsequently, it was discovered that b-receptors occur as subtypes, for example, b1 (in the heart) and b2 (mediating smooth muscle relaxation). Propranolol blocks both b1- and b2-receptors. However, blocking b2-receptors in bronchitis and asthma can be harmful, and analogue research successfully focused on producing selective b1-blockers. The first b1-selective blocker practolol was invented in 1964, but it was withdrawn from the market due to an unusual side effect, an oculomucocutaneous reaction that can lead to blindness. This is fortunately not a class effect, and many other selective blockers were developed and used in cardiology, such as atenolol, betaxolol, metoprolol, celiprolol, nebivolol, and bisoprolol [5] as b1-selective blockers.
1.2 Pharmacodynamic Characteristics OH
OH O
NH
CH3 CH3
O H3C
CH3
NH
O
CH3
NH
practolol
propranolol
OH
OH O H3C
j7
NH
CH3 CH3
O
O
CH3
H2N
metoprolol
atenolol
OH
O
OH
CH3
NH
O
CH3
NH
O
NH
O
CH3
CH3 CH3
O O H3C
H3C
H3C
N
O
OH O
O H3C
betaxolol
bisoprolol
CH3
NH
CH3 CH3 CH3
NH
O
celiprolol
H
OH
OH NH
O H
H
F
F
nebivolol
Figure 1.3 Structures of adrenergic b-receptor blockers.
1.2.2.2 Improving Selectivity Through Unrelated Receptors Treatment with cimetidine for antiulcer therapy, the pioneer H2-receptor histamine antagonist drug, resulted in a low incidence of gynecomastia as an unwanted side effect. This was traced to a low level of antiandrogenic activity. Thus, cimetidine was shown to competitively inhibit the binding of 3 ½Hdihydrotestosterone to its cytoplasmic receptor and to decrease its specific nuclear uptake in rat ventral prostate slices [6]. Subsequent H2-receptor antagonist analogues were an improvement since they generally did not show this effect. 1.2.2.3 Improving Selectivity by Tissue Distribution Antihistamines, useful in allaying the symptoms of allergic responses such as rhinitis and itching eyes when the pollen count is high, have been in general use for over 60 years. Their use has, however, been somewhat limited by a high incidence of drowsiness or sedation. Early attempts to separate the sedative effects from the antihistaminic action were not very successful, and it took many years until it was realized that these activities were connected. In particular, the work of Schwartz and
j 1 Optimizing Drug Therapy by Analogues
8
F
HO
N
OH
NH
O
Cl
haloperidol
azacyclonol
Figure 1.4 Structures of haloperidol and azacyclonol.
his colleagues in Paris demonstrated that histamine is a neurotransmitter in histaminergic nerves in the central nervous system (CNS) [7]. Histamine, acting on the histamine H1-receptor in the brain, was shown to be a stimulant of wakefulness; blocking these receptors leads to a loss of alertness resulting in drowsiness. Some attempts were made to synthesize compounds that were selective for the peripheral versus the central H1-receptors. Although such claims were made, they were not substantiated and, indeed, the use of molecular biology has demonstrated that there is no difference between the central and peripheral histamine H1receptors. There are some differences in the H1-receptors between species but not within a species. It follows that if it is not possible to separate the activity between peripheral and central histamine H1-receptors, then it is necessary to limit the access of antihistamine drugs to the CNS. The first compound to establish itself on this basis was terfenadine, but it was not originally developed as an antihistamine. Terfenadine was developed for CNS actions (dopamine antagonism) and calcium ion-channel blocking [8]. It was chemically related (as a combination of analogues) to haloperidol and azacyclonol (Figure 1.4) but found to be restricted to peripheral systems and to act as an antihistamine; indeed, it became the first member of a new class of drugs identified as nonsedative antihistamines and was very successful. This was followed by astemizole and then by other drugs that were synthesized as analogues of previously established active but sedating antihistamines (see Table 1.3 and Figure 1.5 for some examples). Astemizole arose from a research program at Janssen Pharmaceutica reportedly aimed at antihistamines having a long duration of action and a low risk of provoking central and anticholinergic effects [9]. Some of the above-mentioned products were later withdrawn because of an unwelcome side effect on heart caused by the blockade of the hERG (human ether-a-go-go-related gene) potassium ion channel that gives rise to cardiac arrhythmias. This led to a search for new analogues. The reasons why the compounds have low concentrations in the brain are complex and not fully understood. Some important factors that determine low brain con-
1.2 Pharmacodynamic Characteristics
j9
OH Cl
O
N
N
N N OH O
O H3C OH
O H3C
R
H3C H3C
CH3
cetirizine
ebastine
R = Me, terfenadine R = COOH, fexofenadine Cl CH3
N
N O
N HO
N
O
O
loratadine
acrivastine
CH3
F
F
N
N
N
N NH
N N
H3C
N
N HN
O O
mizolastine Figure 1.5 Structures of H1-antihistamines.
CH3
astemizole
j 1 Optimizing Drug Therapy by Analogues
10
Table 1.3 H1-antihistamines that have low incidence of sedative effects.
Drug
Type
Launch
terfenadine astemizole cetirizine [10] acrivastine [11] loratadine [12] ebastine [13] fexofenadine mizolastine
pioneer, chance discovery pharmacological analogue metabolite of hydroxizine analogue of triprolidine analogue of azatidine analogue of diphenylpyraline and terfenadine metabolite of terfenadine analogue of astemizole and temelastine
1981 (withdrawn, 1997) 1983 (withdrawn, 1999) 1987 1988 1988 1990 1996 1998
centrations are (i) high binding to brain tissue proteins, (ii) poor penetration of the blood–brain barrier, and (iii) high binding to the P-glycoprotein (Pgp) efflux pump. 1.2.2.4 Improving Selectivity of Nonreceptor-Mediated Effects In many cases, the mechanisms causing side effects are not known. Platinum compounds (Figure 1.6) play a major role in oncology. Cisplatin may cause regression and control of various tumors, such as testicular, ovarian, head, neck, and colon carcinoma. However, among its side effects, renal damage can be observed. Analogues have overcome this problem; thus, for carboplatin, the nephrotoxicity is much lower [14], and oxaliplatin is devoid of nephrotoxicity [15].
O H3N H3N
Cl Pt Cl
H3N H3N
O
O Pt
Pt O O
cisplatin
O
H2 N
carboplatin
N H2
O O
oxaliplatin
Figure 1.6 Structures of platinum compounds.
1.2.3 Improving the Physicochemical Properties with Analogues
Benzylpenicillin (penicillin G) (Figure 1.7), which is acid-sensitive, is rapidly destroyed by gastric fluid at pH 2. The pioneer penicillin drug was administered by intramuscular injection to avoid this problem, but this was not convenient for the patients. Through analogues, where an electron-withdrawing substituent was introduced in the side chain, the acid sensitivity of the b-lactam ring was reduced. A range
1.2 Pharmacodynamic Characteristics
j11
NH2
NH
S
O
CH3
N
O
CH3
O
NH O R
O
OH
S N
CH3 CH3
O
OH
ampicillin, R = H amoxicillin, R = OH
benzylpenicillin
CH3
O N
H3C
NH
R1 R
O O
S N O
O
CH3
NH
S
CH3 OH
H3C
O
O
N O O
R = H, R1 = H, oxacillin R = H, R1 = Cl, cloxacillin
methicillin
R = F, R1 = Cl, flucloxacillin R = Cl, R1 = Cl, dicloxacillin Figure 1.7 Benzylpenicillin and analogues.
of such analogues proved to be resistant to acid hydrolysis and they could be given orally (e.g., ampicillin [16]). 1.2.4 Analogues to Reduce the Resistance to Anti-Infective Drugs 1.2.4.1 Antibiotics Resistance to antibiotics has become an increasing problem all over the world, and the need to find new agents continues [17]. The widespread use of penicillin G led to an alarming increase of penicillinresistant Staphylococcus aureus infections in 1960. A solution to the problem was the design of penicillinase-resistant penicillins. The first such analogue was methicillin [18]; however, it was acid-sensitive and it was inactive against Gramnegative bacteria. Methicillin is no longer used clinically because better analogues (Figure 1.7) have been discovered, such as oxacillin [19], cloxacillin [20], flucloxacillin [21], and dicloxacillin [21] that were stable to b-lactamase enzyme of S. aureus and had acid stability.
CH3 CH3 OH
j 1 Optimizing Drug Therapy by Analogues
12
OH CH3
OH N N
N N
N
N
N N
F
N
N
F
F fluconazole
F
N
F voriconazole
Figure 1.8 Structures of fluconazole and voriconazole.
1.2.4.2 Antifungal Drugs A very important subclass of antifungal drugs is the azoles. Until the 1980s, while these drugs were used only topically, no resistance was detected. Since the introduction of the systemically active azoles, resistance has emerged. Even in the case of fluconazole, which is the most potent member of this analogue class, a resistance was observed in Candida infections. In this case, voriconazole, an analogue of fluconazole (Figure 1.8), has been used with a good result [22]. 1.2.4.3 Antiviral Drugs Although the first generation of anti-AIDS drugs provided key medicines for the treatment of the disease, the emergence of HIV-1 strains resistant to various drug classes prompted efforts to discover analogues with broader activity profiles. Successful analogue design approaches have led to improved pharmacological profiles that include suppression of viral resistance for two of the three classes where multiple drugs have reached the market, the nonnucleoside reverse transcriptase inhibitors (NNRTIs) and the protease inhibitors. For the nucleoside reverse transcriptase inhibitor class, the marketed drugs were generally discovered while generating analogues of zidovudine (stavudine, didanosine, tenofovir disoproxil, abacavir, and lamivudine) (Figure 1.9) with improved therapeutic index, and benefits with regard to resistance profiles were empirically uncovered [23]. Etravirine [24] (Figure 1.10), the most recent NNRTI to reach the market, is a pharmacological analogue of the first-generation NNRTIs and displays activity against a broad panel of drug-resistant reverse transcriptase (RT) mutants. Structurally, it bears no resemblance to any of the first-generation inhibitors and it emerged by iterative design starting with the earlier alpha-APA class over a period of many years. Optimization was guided throughout by structural information. Molecular conformational flexibility is apparently a key attribute that allows the molecule to effectively bind in multiple modes to the conformationally mobile NNRTI binding site of numerous clinically relevant RTmutants. The close structural analogue, rilpivirine [25], is in phase III trials.
1.2 Pharmacodynamic Characteristics
O
O CH3
HN O
CH3
HN N
O
N
O
O OH
N3
OH stavudine
zidovudine
NH2
O N
HN
N
N
N
N
O
N
O
P
N
O
O
CH3
O O
HO
H3C tenofovir disoproxil
didanosine
NH2 HN N
N
N
O
N
N
N
O S HO HO
abacavir
lamivudine
Figure 1.9 Structures of zidovudine and analogues.
O
O
O
O
H2N
j13
O CH3
CH3 CH3
j 1 Optimizing Drug Therapy by Analogues
14
CH3
CH3 O
N
NH
N
Br CH3
NC
NH
CN
NC
N
NH N
CH3
CN
NH2
rilpivirine
etravirine Figure 1.10 Structures of etravirine and rilpivirine.
Among the HIV-1 protease inhibitors, analogue approaches specifically attempting to improve resistance profiles have also given drugs that have reached the market. Cocrystal structure information of ritonavir (Figure 1.11), a first-generation protease inhibitor, bound to HIV-1 protease, was used to design lopinavir [26]. Removal of the isopropylthiazole group of ritonavir that interacts with valine-82 led to decreased sensitivity to the protease mutants selected by ritonavir. This modification, along with the modification of the other thiazole ring in ritonavir, also substantially improved the CYP inhibition profile of lopinavir. The design of amprenavir [27] and darunavir [28] (Figure 1.12) emphasized the importance of interactions between the inhibitors and the conserved elements of the
O
N
NH
NH
NH
S
N
O
NH
O
CH3
NH
O
CH3
CH3
H3C
OH
CH3
O
O
CH3
S
H3C
O
HO
N
ritonavir
H3C
N
NH
O CH3
lopinavir
Figure 1.11 Structures of ritonavir and lopinavir.
O O
O
O NH
N OH
O
H
O
O
S O
CH3
O H
NH2
N H
N
Figure 1.12 Structures of amprenavir and darunavir.
S
OH NH 2
CH3
amprenavir
O
O
darunavir
1.3 Pharmacokinetic Characteristics
N
N Cl
N H3C
O
NH
S
N
N
NH
NH
N
NH
N
N
N
N
O
CH3
CH3
CH3
dasatinib
imatinib
CF3 N
O N
N
H3C
NH
j15
NH
N
N CH3
nilotinib Figure 1.13 Structures of imatinib, dasatinib, and nilotinib.
protein backbone of the wild-type enzyme as a means to attaining excellent potency against mutant strains resistant to the first-generation inhibitors. 1.2.5 Analogue Research in Resistance to Drug Therapies in Cancer Treatment
Imatinib (Figure 1.13) is the pioneer drug for the treatment of chronic myologenous leukemia (CML). However, a significant number of patients develop resistance to imatinib. New analogues, such as dasatinib [29] and nilotinib [30], have been introduced recently, and it is hoped that these analogues will be effective also in imatinib-resistant cases.
1.3 Pharmacokinetic Characteristics
Pharmacokinetics is the study of the metabolism of drugs with particular emphasis on the time required for absorption, duration of action, distribution in the body, and method of excretion. Through analogue design, the pharmacokinetic parameters (ADME) of a pioneer drug or a drug class can be optimized. 1.3.1 Improving Oral Bioavailability
A good oral bioavailability is necessary in most cases because the oral application of a drug is preferred to an injection therapy.
OH
j 1 Optimizing Drug Therapy by Analogues
16
HO
O
NH2 HO
CH3
O
N NH
N
NH O
O
OH
enalaprilat
O
O
OH
lisinopril
Figure 1.14 Structures of enalaprilat and lisinopril.
1.3.1.1 Improving Absorption The angiotensin-converting enzyme (ACE) inhibitor enalaprilat (Figure 1.14) is not orally absorbed but is available for intravenous administration when oral therapy is not appropriate, for example, in hypertensive emergencies. The ethyl ester prodrug, enalapril has an excellent oral bioavailability but requires hydrolysis by esterases. The analogue-based drug research afforded the lysylproline analogue, lisinopril [31], which has an acceptable bioavailability and it does not require metabolic activation. 1.3.1.2 Improving Metabolic Stability The pioneer antifungal miconazole (Figure 1.15) and its analogue tioconazole [32] are clinically effective drugs, when administered by the topical route, against fungal infections of vagina and skin. Unfortunately, tioconazole and other imidazole derivatives of that time showed only poor efficacy in animal models of fungal infection when given by either the intravenous or the oral routes. Pharmacokinetic studies indicated that these agents were very susceptible to metabolic inactivation, resulting in low oral bioavailability and low plasma levels. They were also very lipophilic and highly bound to plasma proteins, which resulted in very low circulating levels of the unbound, active form. The first orally active antifungal drug was ketoconazole [33], which was discovered at Janssen Pharmaceutica. Ketoconazole was metabolically less susceptible than earlier imidazole derivatives, resulting in good oral bioavailability; however, it was metabolized such that less than 1% of unchanged drug was excreted through urine. In addition, although ketoconazole was less lipophilic than the earlier derivatives, leading to high blood levels, it remained highly protein-bound with less than 1% in unbound form. Introducing a polar hydroxyl group and a more polar 1,2,4-triazole ring led to UK47265. It was 100 times more potent than ketoconazole when dosed via either the oral or the intravenous routes; however, its in vitro activity against fungi was modest. Unfortunately, UK-47265 proved to be hepatotoxic in mice and dogs and teratogenic in rats; therefore, it was not developed further. Researchers at Pfizer continued to
1.3 Pharmacokinetic Characteristics
j17
O CH3 N N
Cl
Cl O
Cl Cl
Cl
O
O
O
Cl
N
Cl O
N
Cl N
N
N
N
ketoconazole
miconazole
tioconazole F
Cl
F
Cl OH N
N
N
OH
N
N
N
N
N
N
N N
UK-47265
N
fluconazole
Figure 1.15 Structures of miconazole and analogues.
study further analogues, and the 2,4-difluoro derivative (fluconazole, UK-49858) showed high efficacy without any safety problem. Fluconazole [34] has a plasma half time of 5.1 h and 75% of the drug is excreted unchanged through urine. 1.3.2 Drugs with a Long Duration of Action
Captopril [35] was the first orally active ACE inhibitor (angiotensin-converting enzyme) to reach the marketplace. It is rapidly absorbed with a bioavailability of about 75%. Peak drug concentrations in plasma occur within 1 h of dosing, and the drug is then cleared rapidly (t1/2 about 2 h); therefore, a dosage regimen of two- to
Cl S
j 1 Optimizing Drug Therapy by Analogues
18
Table 1.4 Elimination half-life values of ACE inhibitors [31].
Drug captopril benazepril cilazapril enalapril fosinopril lisinopril perindopril ramipril trandolapril
Elimination half-life (h) 2 11 10 11 12 12 >24 8–14 16–24
three times daily is necessary. Since food reduces the oral bioavailability of captopril, the drug should be given 1 h before meals. Hypertension usually requires a lifelong treatment; therefore, the long-acting analogues of captopril are advantageous (Table 1.4, Figure 1.16). The long-acting once-daily dosing has practical benefits by improving patient compliance. The clinical advantage of full 24-h control of blood pressure is important to prevent cardiovascular events (e.g., myocardial infarction and stroke). The oral bioavailability of the long-acting ACE inhibitors is only slightly reduced by food. The case of the calcium channel antagonists, dihydropyridines, shows a similar picture. The pioneer drug nifedipine has a short duration of action. The long-acting analogues, such as felodipine, lacidipine, and amlodipine are more convenient for the lifelong treatment of hypertension (Table 1.5, Figure 1.17). Table 1.5 Elimination half-life values of calcium antagonist dihydropyridines [36].
Drug nifedipine felodipine lacidipine amlodipine
Elimination half-life (h) 2–4 10–15 7–18 30–50
1.3.3 Ultrashort-Acting Drugs
Esmolol [37] (Figure 1.18) is a b1-selective blocker with a very short duration of action. It is administered intravenously and used when b-blockade of short duration is desired in patients in whom adverse effects of bradycardia, heart failure, or hypotension may necessitate its rapid withdrawal. It is used in emergency situations during critical care medication.
1.3 Pharmacokinetic Characteristics
HS
H3C
O
O
N
N
H3C O
NH OH
O
O HO
captopril H3C
O
j19
O
benazepril
O
CH3
O
O
H3C
N
N
N
NH
NH
O
O
OH
O
H3C O
NH2
O
P O
OH
O
H3C
H3C
N
NH
O
O
OH
O CH3
O
OH
O
ramipril
CH3 N
NH O
N
NH
perindopril
O
O
CH3
O
H3C
O
fosinopril
lisinopril
O
O N
N
NH
O
CH3
H3C
O
O
H3C
OH
cilazapril
enalapril
HO
O
O
trandolapril Figure 1.16 Structures of ACE inhibitors.
OH
O
OH
j 1 Optimizing Drug Therapy by Analogues
20
Cl NO2 MeOOC
H3C
COOMe
N H
Cl MeOOC
CH3
H3C
nifedipine
COOEt
N H
CH3
felodipine
HO Cl
O EtOOC
H3C
COOEt
N H
MeOOC
CH3
lacidipine
COOEt
H3C
N H
O
NH2
amlodipine
Figure 1.17 Structures of nifedipine and analogues.
This special analogue type is called a soft drug. It is active only as an ester that loses its activity after metabolic hydrolysis. Esmolols distribution half-life is 2 min and its elimination half-life is 9 min. The same principle is used in the case of loteprednol etabonate (Figure 1.19), which is a glucocorticoid soft drug that has a good local activity after it is topically administered to the eye. Since it is rapidly deactivated after reaching the general circulation, it does not display systemic side effects. 1.3.4 Decreasing Interindividual Pharmacokinetic Differences
Interindividual pharmacokinetic differences result when a significant number of patients require a higher or multiple doses of a drug to achieve symptom relief and healing. Omeprazole [38] is the pioneer proton pump inhibitor that shows an interindividual variability in pharmacokinetics. Analogue-based drug discovery afforded pantoprazole (Figure 1.20), which possesses linear, highly predictable pharmacokinetics. It has a lower variability in pharmacokinetics compared to omeprazole,
1.3 Pharmacokinetic Characteristics
j21
OH O
NH
O
H3C
CH3
H3C O
Figure 1.18 Structure of esmolol.
HO
O
O
Cl
CH3
O
O
H
CH3 H
CH3
O H
O Figure 1.19 Structure of loteprednol etabonate.
particularly with respect to bioavailability. The pharmacokinetics of pantoprazolesodium [39] is almost the same in patients with gastrointestinal diseases and those with renal failure, and in the elderly, so that no dose adjustment is required. The drawback of omeprazole could also be overcome by using its (S)-enantiomer (esomeprazole) whose bioavailability is about double that of the racemate.
CH3 H3C
N
O
H3C
S N
O
O
O CH3
N H
esomeprazole
H3C
CH3 N
O
S N
O
N H
pantoprazole
Figure 1.20 Structures of proton pump inhibitors.
1.3.5 Decreasing Systemic Activity
In the intranasal and inhalation application of corticosteroids in the treatment of asthma and rhinitis, it is important to decrease the systemic availability of these drugs to avoid their adverse effects, such as adrenocortical insufficiency and osteoporosis. Analogue research afforded budenoside and fluticasone with low oral (systemic) bioavailability (Table 1.6, Figure 1.21).
F
O F
j 1 Optimizing Drug Therapy by Analogues
22
Table 1.6 Oral bioavailability of inhaled corticosteroids [40].
Drug
Oral (systemic) bioavailability (%)
beclomethasone budenoside fluticasone
15–20 10 <2
OH
O
O CH3
HO CH3 H
F CH3
O
HO
O
H
F
O
O
CH3
O O CH3
H
H3C
H
S
H
O F fluticasone furoate
budesonide
OH O CH3
HO CH3
CH3
H
Cl
OH
H
O beclomethasone Figure 1.21 Structures of corticosteroids.
1.4 Drug Interactions 1.4.1 Decreasing Drug Interactions
Cimetidine inhibits CYPs (e.g., CYP1A2, CYP2C9, and CYP2D6), an important class of drug-metabolizing enzymes. This interaction inhibits the metabolism of certain
1.5 Summary
drugs such as propranolol, warfarin, diazepam, and theophylline, thus producing effects equivalent to an overdose of these medicines. It is therefore advisable to avoid coadministration. These effects are avoided by analogues such as ranitidine and famotidine [41]. 1.4.2 Increasing Drug Interactions
Because of the potency of inhibition of cytochrome P-450 by ritonavir, it was found that combination of other HIV-1 protease inhibitors with ritonavir led to increased plasma levels of these drugs. Analogue research starting from ritonavir afforded the more potent lopinavir (Figure 1.11). However, lopinavirs plasma halflife is low. In a combination of ritonavir and lopinavir, known as Kaletra, ritonavir inhibits the P-450-mediated metabolism of lopinavir and therefore the combination is remarkably effective [42]. This CYP3A4 inhibition by ritonavir has been exploited to increase the exposure of other anti-AIDS drugs that are metabolized by this particular CYP.
1.5 Summary
The chapter gives an overview of the possibilities of analogue research, illustrated with examples. Drug therapy can be optimized with the help of analogue research in the following 12 ways: . .
. . . .
. . . . . .
Increasing potency Improving the ratio of the main activity and the adverse effects: –Improving selectivity through receptor subtypes –Improving selectivity through unrelated receptors –Improving selectivity by tissue distribution –Improving selectivity of nonreceptor-mediated effects Improving the physicochemical properties with the help of analogues Decreasing resistance to anti-infective drugs Decreasing resistance to anticancer agents Improving oral bioavailability –Improving absorption –Improving metabolic stability Long-acting drugs for chronic diseases Ultrashort-acting drugs in emergency cases Decreasing interindividual pharmacokinetic differences Decreasing systemic activities Decreasing drug interactions with the help of analogues Synergistic interactions between analogues
j23
j 1 Optimizing Drug Therapy by Analogues
24
Acknowledgments
We thank Derek Buckle, John Lowe III, and Thomas J. Perun for carefully reading and reviewing the manuscript. J.F. thanks the Alexander von Humboldt-Stiftung (Bonn) for a fellowship in 2008 and 2009.
References 1 Fischer, J. and Ganellin, C.R. (eds) (2006)
2
3
4
5
6
7
8
9
Analogue-based Drug Discovery, Wiley-VCH Verlag GmbH, Weinheim, p. XXIII. Fischer, J. (2006–2008) IUPAC Project Standalone Drugs, www.iupac.org. Yanagisawa, I., Hirata, Y., and Ishii, Y. (1987) Studies on histamine H2 receptor antagonists. 2. Synthesis and pharmacological activities of N-sulfamoyl and N-sulfonyl amidine derivatives. J. Med. Chem., 30, 1787–1793. Stein, E.A. (2001) New statins and new doses of old statins. Curr. Atheroscler. Rep., 3, 14–18. Harting, J., Becker, K.H., Bergmann, R., Bourgois, R., Enenkel, H.J., Fuchs, A., Jonas, R., Lettenbaur, H., Minck, K.O., Schelling, P., and Schulze, E. (1986) Pharmacodynamic profile of the selective b1-adrenoceptor antagonist bisoprolol. Arzneim.-Forsch. Drug Res., 36 (1), 200–208. Winters, S.J., Banks, J.L., and Loriaux, D.L. (1979) Cimetidine is an antiandrogen in the rat. Gastroenterology, 76, 504–508. Schwartz, J.-C., Barbin, G., Duchemin, A.-M., Garbarg, M., Llorens, C., Pollard, H., Quach, T.T., and Rose, C. (1982) Pharmacology of Histamine Receptors (eds C.R. Ganellin and M.E. Parsons), Wright PSG, Bristol. Carr, A.A. and Meyer, D.R. (1982) Synthesis of terfenadine. Arzneim.-Forsch. Drug Res., 32, 1157. Janssens, F., Torremans, J., Janssen, R.A., Stokbroekx, M., Luyckx, M., and Janssen, Paul A.J. (1985) New antihistaminic N-heterocyclic 4-piperidinamines. 1. Synthesis and antihistaminic activity of N-(4-piperidinyl)-1H-benzimidazol-2amines. J. Med. Chem., 28, 1925.
10 Pechadre, J.C., Vernay, D., Trolese, F.F.,
11
12
13
14
15
16
17
18
Bloom, M., Dupont, P., and Rihoux, J.P. (1988) Comparison of the central and peripheral effects of cetirizine and terfenadine. Eur. J. Clin. Pharmacol., 35, 255. Barnett, A., Lorio, L.C., Kreutner, W., Tosszi, S., Ahn, H.S., and Gulbekian, A. (1984) Evaluation of the CNS properties of SCH29851, a potential non-sedating antihistamine. Agents Actions, 14, 590. Leighton, H.J., Batz, R.F., and Findlay, J.W.A. (1986) BW 825C: a potent antihistamine with low sedation potential. Pharmacologist, 25, 163. Ratner, P., Falques, M., Chuecos, F., Esbri, R., Gispert, J., Peris, F., Luria, X., and Rosales, M.J. (2005) Meta-analysis of the efficacy of ebastine 20 mg compared to loratidine 10 mg and placebo in the symptomatic treatment of seasonal allergic rhinitis. Allergy Immunol., 138 (4), 312–318. Thatcher, N. and Lind, M. (1990) Carboplatin in small cell lung cancer. Semin. Oncol., 17 (1 Suppl. 2), 40–48. Haioun, C. and Mondor, C.H. (2005) Oxaliplatin in lymphoma. Haematol. Rep., 1 (8), 106–107. Doyle, F.P., Nayler, J.H.C., Smith, H., and Stove, E.R. (1961) Some novel acid-stable penicillins. Nature, 191, 1091–1092. von Nussbaum, F., Brands, M., Hinzen, B., Weigand, S., and H€abich, D. (2006) Antibacterial products in medicinal chemistry: exodus or revival? Angew. Chem. Int. Ed., 45, 5072–5129. Doyle, F.P., Hardy, K., Nayler, J.H.C., Soulal, M.J., Stove, E.R., and Waddington, H.R.J. (1962) Derivatives of 6aminopenicillanic acid. Part III. 2, 6Dialkoxybenzoyl derivatives. J. Chem. Soc., 1453–1458.
References 19 Doyle, F.P., Long, A.A., Nayler, J.H.C., and
20
21
22
23
24
25
Stove, E.R. (1961) New penicillins stable towards both acid and penicillinase. Nature, 192, 1183. Doyle, F.P., Hanson, J.C., Long, A.A.W., Nayler, J.H.C., and Stove, E.R. (1963) Derivatives of 6-aminopenicillanic acid. Part VI. Penicillins from 3- and 5phenylisoxazole-4-carboxylic acids and their alkyl and halogen derivatives. J. Chem. Soc., 5838. Naylor, J.H.C. (1960) (Beecham) British Patent 905,778; Naylor, J.H.C. (1962) (Beecham) British Patent 978,299. Gothard, P. and Rogers, T.R. (2004) Voriconazole for serious fungal infections. Int. J. Clin. Pract., 58 (1), 74–80. Sluis-Cremer, N. and Ross, T. (2006) HIV-1 reverse transcriptase inhibitors: drug resistance and drug development. Curr. Pharm. Des., 12 (15), 1809–1810. Das, K., Clark, A.D., Jr., Lewi, J.J., Heeres, J., De Jonge, M.R., Koymans, L.M., Vinkers, H.M., Daeyaert, F., Ludovici, D.W., Kukla, M.J., De Corte, B., Kavash, R.W., Ho, C.Y., Lichtenstein, M.A., Andries, K., Pauwels, R., De Bethune, M.P., Boyer, P.L., Clark, P., Hughes, S.H., Janssen, P.A.J., and Arnold, E. (2004) Roles of conformational and positional adaptability in structure-based design of TMC125-R165335 (etravirine) and related non-nucleoside reverse transcriptase inhibitors that are highly potent and effective against wild-type and drug-resistant HIV-1 variants. J. Med. Chem., 47, 2550–2560. Janssen, P.A.J., Lewi, P., Arnold, E., Daeyaert, F., De Jonge, M., Heeres, J., Koymans, L., Vinkers, M., Guillemont, J., Pasquier, E., Kukla, M., Ludovici, D., Koen, A., De Bethune, M.P., Pauwels, R., Das, K., Clark, A.D., Jr., Volovik Frenkel, Y., Hughes, S.H., Medaer, B., De Knaep, F., Bohets, H., De Clerck, F., Lampo, A., Williams, P., and Stoffels, P. (2005) In search of a novel anti-HIV drug: multidisciplinary coordination in the discovery of 4-[[4-[[(1E)-2-cyanoethenyl]2,6-dimethylphenyl]amino]-2pyrimidinyl]-aminobenzonitrile
26
27
28
29
30
31
32
(R278474, rilpivirine). J. Med. Chem., 48, 1901–1909. Sham, H.L., Kempf, D.J., Molla, A., Marsh, K.C., Kumar, G.N., Chen, C.-M., Kati, W., Stewart, K., Lal, R., Hsu, A., Betebenner, D., Korneyeva, M., Vasavanonda, S., McDonald, E., Saldivar, A., Wideburg, N., Chen, X., Niu, P., Park, C., Jayanti, V., Grabowski, B., Granneman, G.R., Sun, E., Japour, A.J., Leonard, J.M., Plattner, J.J., and Norbeck, D.W. (1998) ABT-378, a highly potent inhibitor of the human immunodeficiency virus protease. Antimicrob. Agents Chemother., 42, 3218–3224. Kim, E.E., Baker, C.T., Dwyer, M.D., Murcko, M.A., Rao, B.G., Tung, R.D., and Navia, M.A. (1995) Crystal structure of HIV-1 protease in complex with VX-478, a potent and orally bioavailable inhibitor of the enzyme. J. Am. Chem. Soc., 117, 1181–1182. Ghosh, A.K., Kincaid, J.F., Cho, W., Walters, D.E., Krishan, K., Hussain, K.A., Koo, Y., Cho, H., Rudall, C., Holland, L., and Buthod, J. (1998) Potent HIV protease inhibitors incorporating high-affinity P2-ligands and (R)-[(hydroxyethyl)amino] sulfonamide isostere. Bioorg. Med. Chem. Lett., 8, 687–690. Shah, N.P., Tran, C., Le, F.Y., Chen, P., Norris, D., and Sawyers, C.L. (2005) Overriding imatinib resistance with novel Abl kinase inhibitor. Science, 305, 399–401. Kantarjian, H., Giles, F., Wunderle, L., Bhalla, K., OBrien, S., Wassmann, B., Tanaka, C., Manley, P., Rae, P., Mietlowski, W., Bochinski, K., Hochhaus, A., Griffin, J.D., Hoelzer, D., Albitar, M., Dugan, M., Cortes, J., Alland, L., and Ottmann, O.G. (2006) Nilotinib in imatinib-resistant CML and philadelphia chromosome-positive ALL. N. Engl. J. Med., 354, 2542–2551. Alf€oldi, S. and Fischer, J. (2006) Optimizing antihypertensive therapy by angiotensin-converting enzyme inhibitors, in Analogue-Based Drug Discovery (eds J. Fischer and C.R. Ganellin), Wiley-VCH Verlag GmbH, Weinheim. Jevons, S., Gymer, G.E., Brammer, K.W., Cox, D.A., and Leeming, M.R.G. (1979)
j25
j 1 Optimizing Drug Therapy by Analogues
26
33
34
35
36
37
Antifungal activity of tioconazole (UK20,349), a new imidazole derivative. Antimicrob. Agents Chemother., 15, 597–602. Thienpont, D., Van Cutsem, J., Van Gerven, F., Heeres, J., and Janssen, P.A.J. (1979) Ketoconazole: a new broad spectrum orally active antimycotic. Experientia, 35, 606–607. Richardson, K., Cooper, K., Marriott, M.S., Tarbit, M.H., Troke, P.F., and Whittle, P.J. (1990) Discovery of fluconazole, a novel antifungal agent. Rev. Infect. Dis., 12 (Suppl. 3), S267–S271. Ondetti, M.A., Rubin, B., and Cushman, D.W. (1977) Design of specific inhibitors of angiotensin-converting enzyme: new class of orally active antihypertensive agents. Science, 196, 441–444. Gaviraghi, G. (2006) Case study of lacidipine in the research of new calcium antagonists, in Analogue-Based Drug Discovery (eds J. Fischer and C.R. Ganellin), Wiley-VCH Verlag GmbH, Weinheim. Erhardt, P.W. (1993) Esmolol. Chron. Drug Discov., 3, 191–206.
38 Larsson, H., Carlsson, E., Junggren, U.,
39
40
41
42
Olbe, L., Sj€ostrand, S.E., Ska:7pt :61ptnberg, I., and Sundell, G. (1983) Inhibition of gastric acid secretion by omeprazole in the dog and rat. Gastroenterology, 85, 900–907. Huber, R., Kohl, B., Sachs, G., SennBilfinger, J., Simon, W.A., and Sturm, E. (1995) Review article: the continuing development of proton-pump inhibitors with particular reference to pantoprazole. Aliment. Pharmacol. Ther., 9, 363–378. Miller, D.D., Brueggemeier, R.W., and Dalton, J.T. (2002) Adrenocorticoids, in Foyes Principles of Medicinal Chemistry, 5th edn (eds D.A. Williams and T.L. Lemke), Lippincott Williams & Wilkins, Philadelphia. Hoogerwerf, W.A. and Pasricha, P.J. (2006) Pharmacotherapy of gastric acidity, peptic ulcers, and gastrooesophageal reflux disease, in Goodman & Gilmans The Pharmacological Basis of Therapeutics, 11th edn (eds L.L. Brunton, J.S. Lazo, and K.L. Parker), McGraw-Hill, New York. Silverman, R.B. (2004) The Organic Chemistry of Drug Design and Drug Action, 2nd edn, Elsevier/Academic Press, p. 273.
Erika M. Alapi
Richter Plc., Gyömro˝i út 19/21, 1103 Budapest, Hungary Erika M. Alapi was born in Pasztó, Hungary, in 1974. She received her MSc degree in chemistry at the Faculty of Science of E€otv€os Lorand University in Budapest. She is working as a medicinal chemist in the field of CNS and preparing her PhD thesis Optimizing Therapy by Analogues under the supervision of Professor Janos Fischer at Gedeon Richter Pharmaceutical Plc. She contributed to Analogue-Based Drug Discovery I as a coauthor.
References
Janos Fischer
Professor, Richter Plc., Gyömro˝i út 19/21, 1103 Budapest, Hungary Janos Fischer is a Senior Research Scientist at Richter Plc., Budapest, Hungary. He received his MSc and PhD degrees in organic chemistry from the Eotvos University of Budapest under Professor A. Kucsman. Between 1976 and 1978, he was a Humboldt Fellow at the University of Bonn under Professor W. Steglich. He has worked at Richter Plc. since 1981 where he participated in the research and development of leading cardiovascular drugs in Hungary. His main interest is analoguebased drug discovery. He is the author of some 100 patents and scientific publications. In 2004, he was elected as a Titular member of the Chemistry and Human Health Division of IUPAC. He received an honorary professorship at the Technical University of Budapest.
C. Robin Ganellin
Professor, University College London, Department of Chemistry, 20 Gordon Street, London WC1H 0AJ, UK C. Robin Ganellin studied chemistry at London University, receiving a PhD in 1958 under Professor Michael Dewar, and was a Research Associate at MIT with Arthur Cope in 1960. He then joined Smith Kline & French Laboratories in the UK and was one of the coinventors of the revolutionary drug, cimetidine (also known as Tagamet). In 1986, he was made a Fellow of the Royal Society and appointed to the SK&F Chair of Medicinal Chemistry at University College London, where he is now Professor Emeritus of Medicinal Chemistry. Professor Ganellin is coinventor of over 160 patents and has authored over 260 scientific publications. He was President of the Medicinal Chemistry Section of the IUPAC and is Chairman of the IUPAC Subcommittee on Medicinal Chemistry and Drug Development.
j27
j 1 Optimizing Drug Therapy by Analogues
28
John Proudfoot
Boehringer Ingelheim Pharmaceuticals Inc., 900 Ridgebury Rd., Ridgefield, CT 06877-0368, USA John Proudfoot is a Medicinal Chemist with over 20 years of experience in drug design, ranging from fragment-based lead identification to late-stage lead optimization. He received his doctorate from the University College Dublin in Ireland, working with Professor Dervilla Donnelly. After postdoctoral studies under Professor Carl Djerassi at Stanford University and under Professor John Cashman at the University of California, San Francisco, he joined Boehringer Ingelheim Pharma in 1987. He is a coinventor of 28 issued US patents, including the one for the marketed drug, nevirapine, the first nonnucleoside HIV-1 reverse transcriptase inhibitor to receive regulatory approval.
j29
2 Standalone Drugs Janos Fischer, C. Robin Ganellin, Arun Ganesan, and John Proudfoot
In volume I of Analogue-Based Drug Discovery we focused on analogue drugs with a double similarity, that is, those that are similar to another drug from both chemical and pharmacological viewpoints. These were referred to as structural and pharmacological analogues or, alternatively, full analogues. One special class of these analogues is considered to be direct analogues if they have identical pharmacophores – that is, they can be described by a general structure that includes most of the chemical skeleton. The book also discussed some examples of structural analogues, in which the similarity is limited to their chemical structures since they possess different pharmacological properties. We also proposed the term pharmacological analogues for drugs that have completely different chemical structures but similar pharmacological properties. Analogues are based on following a lead compound, and if this compound were a drug that opened up a new therapeutic treatment then we use the term pioneer drug. Such a drug is often referred to as first in class. One can subdivide pioneer drugs into those that have analogues and those that do not. Thus, the latter group provides a special subclass called standalone drugs,1) which are pioneer drugs with no effective analogues. An analysis of the Top 100 most important drugs based on the sales data of IMS is revealing. In 2006, the global sales of drugs amounted to US$ 638 billion. The sales of the Top 100 drugs were about 51% of the global sales, that is, US$ 326 billion. Table 2.1 shows that 84.4% of sales of the Top 100 drugs derive from small-molecule drugs, whereas the sales of macromolecular drugs and biological entities amount to 9.2% and the sales of vitamins and a hormone are 6.4%. The importance of small-molecule drugs remains very high in spite of the increasing role of macromolecular drugs (proteins) and biological entities (monoclonal antibodies).
1) In 2006, a new project of IUPACs Subcommittee on Medicinal Chemistry and Drug Development (Division VII, Chemistry and Human Health) was started to study the role and the importance of standalone drug. The project was chaired by Janos Fischer with the coauthors of this chapter as project members. Analogue-based Drug Discovery II. Edited by János Fischer and C. Robin Ganellin Copyright Ó 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32549-8
j 2 Standalone Drugs
30
Table 2.1 Drug types of the Top 100 drugs.
Drug types Small-molecule drugs Macromolecules and biological entities Vitamins and hormone
Sales (2006) US$ (billion)
Ratio (%)
Numbers
275 30 21
84.4 9.2 6.4
80 10 10
Table 2.2 Types of small-molecule drugs among the Top 100 drugs.
Small-molecule drug type Pioneer drugs Full analogues Pharmacological analogues Structural analogues Standalone drugs
Number 10 51 8 2 9
Table 2.2 analyzes the small-molecule drugs among the Top 100 drugs according to their analogue status. It is remarkable how many (51) of full analogues are among the Top 100 drugs. The analogue drugs belong to 42 different analogue classes of drug action, as defined by the main pharmacological effect, for example, ACE inhibitors, a1-blockers, AT1 antagonists, b-lactamase inhibitors, and so on, and there are only 10 pioneer drugs (first in class) among the Top 100 drugs from the analogue classes. This situation derived from the fact that the pioneer drugs were improved through making analogues and then continuous optimization has afforded additional analogue drugs that are even better. The nine standalone drugs from among the Top 100 drugs (2006), acetaminophen, acetylsalicylic acid, aripiprazole, bupropion, ezetimibe, lamotrigine, metformin, topiramate, and valproate semisodium, will be discussed one by one.
2.1 Acetaminophen (Paracetamol)
Based on their structures and biological activity acetanilide, phenacetin, and paracetamol (Figure 2.1) are direct analogues; however, they also have a close metabolic relationship since paracetamol is the active metabolite of both acetanilide and phenacetin. Acetanilide (1) was discovered as an antipyretic drug after it was mistakenly used instead of naphthalene in an antiworming regimen in 1886 [1]. It was marketed under the name Antifebrin in 1886 by the company Kalle, then from 1908 by
2.1 Acetaminophen (Paracetamol)
HN
O
HN
O
O
acetanilide
phenacetin
HN
O
OH
paracetamol
Figure 2.1 Paracetamol and its former analogues.
Hoescht Dyeworks. Acetanilide was cheaper and easier to prepare than other antipyretics and it remained in use for many decades. In 1887, the Bayer Company introduced the 4-ethoxy derivative, phenacetin, as a less toxic analogue of acetanilide. It was a highly successful product and established the Bayer Company as a leading pharmaceutical manufacturer. Phenacetin was widely used for about 90 years until mounting concerns over carcinogenicity and kidney-damaging properties in chronic treatment [2] led to restrictions on its use in most countries. It is, however, even nowadays available in some combinations in some countries. Paracetamol (acetaminophen) was identified as an active antipyretic and analgesic agent in a clinical trial in 1893 by Joseph von Mering [3] in cooperation with the Bayer Company. However, he believed mistakenly that it produced methemoglobinemia as an adverse effect and the compound was ignored for a half century. This situation changed when Lester and Greenberg [4], and subsequently Flinn and Brodie [5], confirmed that paracetamol is the active metabolite of phenacetin and Brodie and Axelrod found that paracetamol is the main metabolite of acetanilide [6]. At the same time, Flinn and Brodie found that, even at high oral doses, paracetamol is not attended by the formation of methemoglobin. In 1953, paracetamol was marketed by the Sterling-Winthrop Company and, especially after the withdrawal of phenacetin, assumed an important role as an antipyretic and mild analgesic drug. It is used in monotherapy and in several combination preparations, which are available on the market as OTC products. In 2007, according to the IMS data, the value of paracetamol sales was greater than US$ 7.5 billion and it ranked fourth among drugs. Paracetamol has no anti-inflammatory activity and its analgesic effect is comparable to aspirin. In patients who are sensitive to aspirin, the use of paracetamol is indicated. Despite its extensive use, the mechanism of action of paracetamol remained unknown. It can be supposed that it has a primarily CNS activity. In a recent study, Hoegestaett et al. showed that paracetamol undergoes a primary deacetylation step in the liver, brain, and spinal cord that is followed by N-acylation of the resulting 4-aminophenol with arachidonic acid by FAAH to form AM404 (Figure 2.2) in the CNS [7]. The biological significance of this route was confirmed by Ottani et al.
j31
j 2 Standalone Drugs
32
OH
O N H
Figure 2.2 N-Arachidonoyl-phenolamine (AM404).
who showed that the effect of paracetamol in pain models is prevented by cannabinoid receptor antagonists [8]. According to Hinz et al., paracetamol is a selective cyclooxygenase-2 (COX-2) inhibitor in man [9]; however, paracetamol has no significant anti-inflammatory activity. With normal use, therapeutic levels of paracetamol are considered to be safe for humans [10]; however, an overdose can cause severe liver toxicity. Paracetamol is metabolized primarily by O-glucuronidation and O-sulfation. However, a small amount is metabolized via a third metabolic pathway, in which oxidation by microsomal cytochrome P-450 gives a toxic metabolite NAPQI (N-acetyl-p-benzoquinone imine). This toxic metabolite is normally rapidly neutralized by glutathione by formation of a nontoxic adduct (Figure 2.3). The problem occurs when glutathione stores are diminished; NAPQI is no longer detoxified and covalently binds to lipid bilayer of hepatocytes, resulting in hepatotoxicity. Paracetamol toxicity is the leading cause of hospital admission for acute liver failure in the United States of America accounting for approximately 40% of all liver transplants [11]. Several direct analogues of paracetamol are summarized by Vermeulen and coworkers [12]. For example, N-methyl-paracetamol caused no hepatic necrosis in mice, rats, or hamsters in doses that caused massive hepatic necrosis in the same animals when paracetamol was administered [13]. No development is reported on these analogues. In 2007, new paracetamol analogues were described by Bazan and coworkers, in which a benzothiazolonyl moiety is linked to the p-acylaminophenol fragment [14]. In 2008, Imming and coworkers described N-(1H-indazol-5-yl)acetamide and N-(4-hydroxybenzyl)acetamide, which displayed analgesic activities comparable to paracetamol [15]. All of these analogues (Figure 2.4) try to avoid the formation of NAPQI. More time is needed to evaluate these efforts to optimize paracetamol.
HN
O
cytochrome
N
O
P450
HN
O
G-SH S-G
OH
O NAPQI
Figure 2.3 Formation of N-acetyl-p-benzoquinone imine.
glutathione
OH
2.2 Acetylsalicylic Acid (Aspirin)
O O S N N
O
HN
OH
O
O
N N H
OH Bazan (2006)
Nelson (1978)
O
HN
Imming (2008)
Figure 2.4 Some direct analogues of paracetamol (experimental phase).
2.2 Acetylsalicylic Acid (Aspirin)
Willow bark [16] and leaves have been used as antipyretic and anti-inflammatory analgesics in ancient civilizations. In 1828, Buchner isolated salicin from boiled willow bark (Figure 2.5) and in 1833 pharmacist Merck in Darmstadt used highly purified salicin as an antipyretic agent for half the price of quinine; however, salicin was poorly palatable because of its bitter taste and its structure at that time was not yet known. In 1839, Piria obtained salicylic acid from salicin by an oxidative cleavage [17]. In 1876, Maclagan [18] found that salicin and salicylic acid are equally efficacious in treating acute rheumatism, which is understandable, since salicin is a prodrug of salicylic acid. Salicylic acid became a cheap chemical through the Kolbe synthesis [19] and in 1876 it was introduced as an analgesic antirheumatic drug at the Charite in Berlin [20]. Despite the undoubted benefits of salicylic acid in the treatment of pain, fever, and inflammatory disorders, there were several side effects at high doses of 4–6 g per day, for example, stomach irritation, often associated with nausea and vomiting, and hearing disorders (tinnitus). In 1897, Eichengr€ un and Hoffmann at Bayer discovered acetylsalicylic acid (aspirin), which was not only as effective as sodium salicylate but also better tolerated. Since the beginning of the last century, aspirin has been the most widely used drug in the world.
OH HO HO
O
O
OH O
OH
O
HO
HO
OH
O O
salicin
salicylic acid
Figure 2.5 History of acetylsalicylic acid.
acetylsalicylic acid
j33
j 2 Standalone Drugs
34
More than 60 years passed before the widely used nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, indometacin, flurbiprofen, diclofenac, naproxen, ketoprofen, piroxicam, and sulindac (Figure 2.6), were discovered. These drugs were all discovered between 1961 and 1969 without the knowledge of their biochemical mechanism of action. In 1971, Vane discovered the mechanism by which aspirin and NSAIDs exert their anti-inflammatory, analgesic, and antipyretic actions. He proved that aspirin and NSAIDs inhibit prostaglandin synthesis [21]. It was already known that PGE1 was
Cl
O N
COOH O
COOH ibuprofen (1961)
indometacin (1961)
COOH Cl H N
COOH
Cl
F
diclofenac (1965)
flurbiprofen (1964)
COOH
COOH
O O naproxen (1967)
ketoprofen (1967) O S
O
O S
N
H N
N
OH O
F COOH
piroxicam (1968)
sulindac (1969)
Figure 2.6 Nonsteroidal anti-inflammatory drugs.
2.3 Aripiprazole
O
HO
NH2 O
HO
O
O
O
OH
HO
F
HO F salicylamide
salsalate
diflunisal
Figure 2.7 Salicylic acid analogues.
a potent pyretic agent [22] and prostaglandins had also been detected in inflammatory exudates [23]. The cyclooxygenase-1 (COX-1) enzyme, which catalyzes the prostaglandin formation, was isolated in 1976 [24]. Aspirin selectively acetylates the hydroxy group of one-serine residue (Ser 530) located 70 amino acids from the C-terminus of COX-1 [25]. In 1991, the COX-2 enzyme was discovered, which produces prostaglandins, mostly PGE2, during inflammatory reactions [26]. At low concentrations, aspirin acetylates COX-1 rapidly (within minutes) and selectively. At high concentrations, over longer time periods, aspirin will also nonspecifically acetylate a variety of proteins and also COX-2 at its serine residue (Ser 516). The pharmacology of aspirin is unique because its activity derives from a reactive acetate and salicylate [27]. There is a significant difference between salicylic acid and aspirin concerning the COX-1 inhibition. In contrast to aspirin, salicylic acid prevents binding of arachidonic acid by competitive inhibition. The nonaspirin NSAIDs (Figure 2.6) also inhibit competitively the COX enzymes, which is an important distinction from aspirin effects. Another important activity of aspirin is the inhibition of thromboxane formation by blocking prostaglandin biosynthesis in platelets [28]. Aspirin is the only antiplatelet agent in clinical use with this mechanism of action. There is no successful aspirin analogue on the market. Salicylamide, salsalate, and diflunisal (Figure 2.7) are existing drugs, which are all salicylic acid analogues. Salicylamide is without anti-inflammatory activity and diflunisal has no antipyretic effect. There have been several attempts to improve aspirin by prodrug approaches, which are reviewed in a recent article by Gilmer and coworkers [29].
2.3 Aripiprazole
Schizophrenia, which affects 1–2% of the global population, is a serious brain disorder characterized by abnormal mental functions and disturbed behavior [30]. The positive symptoms include disorganized thoughts, delusions, and hallucinations, whereas the negative symptoms are characterized by social withdrawal, lack of motivation, and disturbances in basic cognitive functions. Depression and bipolar disorder are highly comorbid in schizophrenia and these are the reasons for a higher risk of suicide in this disorder.
j35
j 2 Standalone Drugs
36
H3C
N
CH3 N CH3
CH3 CH3
N
N
S
S
promethazine antihistamine 1949 (launch)
Cl
chlorpromazine antipsychotic 1952 (launch)
Figure 2.8 Promethazine and chlorpromazine.
2.3.1 First Generation Typical Antipsychotic Drugs (Other Names: Neuroleptics, Conventional Antipsychotics)
Chlorpromazine, the first antipsychotic drug, was obtained from the antihistaminic and sedative promethazine. Charpentier et al. [31] synthesized and Courvoisier et al. investigated many analogues in order to enhance the sedative effect of promethazine [32] (Figure 2.8). Chlorpromazine was marketed by Rhône-Poulenc in 1952. Chlorpromazine initiated an analogue-based drug research, and several direct analogues were discovered and introduced as antipsychotic drugs without the knowledge of their mechanism of action. In vivo pharmacological models, such as conditioned avoidance response test, were typically used to drive these discovery efforts. A completely different approach in the Janssen Laboratories in 1958 afforded more potent drugs. The sedative effect of an analgesic pethidine derivative (R1187) was reminiscent of the sedation produced by chlorpromazine and, after exploring hundreds of derivatives, haloperidol was discovered. Haloperidol was 50–100 times potent than chlorpromazine, with a better side-effect profile (Figure 2.9) [33]. Chlorpromazine and haloperidol and their analogues helped many patients, but treatment with these first-generation antipsychotic drugs was associated with the development of extrapyramidal side effects (EPS), for example, dystonias, akathisias,
O
O
OH Cl
O
O
N
N F R1187 pethidine derivative analgesic effect Figure 2.9 Pethidine derivative and haloperidol.
haloperidol antipsychotic 1959 (launch)
2.3 Aripiprazole
and tardive dyskinesia (TD) in about 20% of patients and, as further adverse effects, elevated prolactin levels, and weight gain are observed. About 30% of patients do not respond to the first-generation antipsychotic drugs. 2.3.2 Second-Generation Atypical Antipsychotic Drugs
The pioneer drug clozapine was discovered in 1960 by the Wander company in Switzerland. The SAR of these tricyclic antidepressants afforded amoxapine and its 7-hydroxy metabolite had an antipsychotic activity. In the case of clozapine, the beneficial effect of the chloro substituent in position 8 and not in position 2 as in some classical tricyclic antidepressants, such as in amoxapine (Figure 2.10), was a key step in the discovery process. Clozapine was poised to revolutionize the treatment of schizophrenia because it had no EPS adverse effect in humans [34]. Clozapine was introduced in 1972 in some European countries, but agranulocytosis, a severe adverse effect, nearly terminated the use of this drug. Careful blood monitoring could decrease this severe adverse effect and clozapine has found an important place in the treatment of schizophrenia. More atypical antipsychotic drugs, such as olanzapine and quetiapine, were discovered by structural modification of clozapine, whereas haloperidol is similar to risperidone, ziprasidone, and sertindole in structure. It was proposed in 1954 that schizophrenia resulted from overactive serotonin neurotransmission [35] whereas, according to the dopamine hypothesis of schizophrenia, a functional hyperactivity of dopamine in the neuronal systems of the brain has a major contribution to the disease [36]. Dopamine receptors were first classified into D1 and D2 subtypes [37]. Typical and atypical antipsychotic drugs (Figure 2.11) are antagonists at D2 receptors; however, the atypical antipsychotics display an increased affinity for serotonin 5-HT2A/2C receptors. The atypical antipsychotic drugs have only a limited effect in the treatment of refractory schizophrenia. Metabolic adverse effects (obesity, diabetes) have been found in the case of clozapine and olanzapine. A prolongation of QT-interval has been observed after the use of quetiapine, ziprasidone, and sertindole.
H N
H N N
N N C
A
2 Cl
Cl 8
N A
amoxapine antidepressant
C N H
O
clozapine antipsychotic 1972 (launch)
Figure 2.10 Amoxapine and clozapine.
j37
j 2 Standalone Drugs
38
O OH
CH3 N
N
N
N
N
N
N H
S
olanzapine
CH3
S
F
quetiapine S
O
N
N
O
N
N
N
N
O N
CH3 Cl
N H
Cl
risperidone
ziprasidone
N
O HN
N
F
N sertindole
Figure 2.11 Atypical antipsychotic drugs.
2.3.3 A New Approach: Aripiprazole, a Dopamine Partial Agonist
All the typical and atypical antipsychotic drugs inhibit dopamine neurotransmission by blocking the postsynaptic dopamine receptors. Unfortunately, dopamine antagonism is also responsible for the most serious side effects of these agents, for example, extrapyramidal syndrome, tardive dyskinesia, and hyperprolactinemia. Carlsson proposed that compounds stabilizing the dopaminergic system without inducing hypodopaminergia would be advantageous in the treatment of schizophrenia [38]. Researchers at Otsuka (Japan) serendipitously found among 2(1H)-quinolone derivatives new compounds with neuroleptic-like activity. The best compound was OPC-4392 (Figure 2.12), which was found to be a dopamine autoreceptor agonist and a weak postsynaptic dopamine D2 antagonist [39]. In clinical investigations, OPC-4392 proved to be active in the treatment of the negative symptoms, but it was not sufficiently potent for the treatment of positive symptoms. A more potent postsynaptic dopamine D2 antagonist was needed. Among the direct analogues of OPC-4392 (lead), aripiprazole was found to fulfill these criteria. In the apomorphine-induced stereotypy in mice, OPC-4392 had an ED50 of
2.4 Bupropion
O
N H
O
H3C
N N
CH3
OPC-4392
O
N H
O
Cl
N N
Cl
aripiprazole Figure 2.12 OPC-4392 and aripiprazole.
41 mmol/kg p.o., whereas aripiprazole showed the highest potency with an ED50 of 0.6 mmol/kg p.o. [40]. Aripiprazole proved to be a presynaptic partial agonist of dopamine D2/3 receptors. The introduction [41] of aripiprazole in 2002 was the first breakthrough in antipsychotic treatment since the discovery of clozapine and analogues. However, unmet medical needs still remain to find a more effective drug for the treatment of refractory patients, to improve treatment of negative symptoms and cognitive dysfunction [42].
2.4 Bupropion
Bupropion, also known as Amfebutamone, is an antidepressant that is also used as an anticigarette smoking treatment. Soroko et al. [43], from the Wellcome Research Laboratories (Research Triangle Park, NC, USA), describe how they sought a compound that would be active in antidepressant screening models but differ chemically and pharmacologically from the well-known tricyclic antidepressants (such as amitriptyline and imipramine), and would not be sympathomimetic, cholinolytic, or an inhibitor of monoamine oxidase. Mehta, who synthesized bupropion in the 1960s, describes [44] that they used as a screen the tetrabenazine-induced sedation [45] in mouse and rat models since most clinically useful antidepressants prevent this. As he stated [44] optimally a compound should prevent tetrabenazine-induced sedation without itself being a stimulant at antidepressant doses. Several observations from their earlier studies [46] of compounds related to bupropions chemical structure (Figure 2.13) were used in the overall design of bupropion. They were as follows: 1)
The tert-alkyl group on the nitrogen atom was found to be less susceptible to N-dealkylation than other alkyl groups.
j39
j 2 Standalone Drugs
40
O H N H3C
Cl
CH3 H3C CH 3
Figure 2.13 Bupropion.
2)
The substituent in the meta position on the phenyl ring would mitigate the possibility of para-hydroxylation and consequent elimination of the drug as a glucuronide. 3) Alkoxy and alkyl substituents on the phenyl ring should be avoided since they have been found to confer antiarrhythmic activity and inhibition of free fatty acid mobilization. 4) a-Hydroxy derivatives with two-carbon side chains (e.g., isoproterenol) have usually been associated with sympathomimetic properties.
The first compound in the series of halogen substituted N-tert-butyl-2-phenylpropanolamines to show the CNS activity, using the potentiation of amphetamine as a possible screen for antidepressant activity, was 3,4-dichlorophenyl-N-tert-butylaminopropanol (Figure 2.14) [44]. Mehta continues to observe [44], since, a priori, it was conceivable that the chemically stable aminoketones and aminoalcohols could be interconvertible in vivo, there was uncertainty as to which form was responsible for the observed activity. Thus a series of aminoketones was made and tested; bupropion was the fourth compound in this series to be designed and screened as a potential antidepressant using the modified antitetrabenazine screen developed by Sulser and Soroko [45]. Mehta observes [44] that it was quite apparent that the aminoketones rather than the aminoalcohols were of primary interest and, as the amine moiety was tert-alkyl, bupropion was rather stable. Bupropion possessed the optimal variations in its chemical structure necessary to achieve maximum activity and therapeutic index. The drug was expected to result in vivo in relatively innocuous metabolites and to have a high degree of both water and lipid solubility, resulting in good systemic absorption after oral administration [44]. US patents protecting the compound were published in 1974 and 1975 [47], and there is an earlier German patent (1971) describing its synthesis [48]. Bupropion, like amitriptyline, produced a dose-dependent prevention of tetrabenazine-induced sedation in mice [43]. It also prevented tetrabenazine-induced
OH Cl
H N
Cl
H3C
CH3 H3C CH3
Figure 2.14 3,4-Dichlorophenyl-N-tert-butylaminopropanol.
2.4 Bupropion
blepharospasm and fall in rectal temperature, and it corrected the hunched posture characteristic of tetrabenazine action [43]. In addition, like amitriptyline, it markedly antagonized the 6 C drop in rectal temperature seen after reserpine, and was not a locomotor stimulant in mice [43]. Unlike amitriptyline, bupropion antagonized the intense motor activity produced by dextroamphetamine in mice [43]. Bupropion did not inhibit monoamine oxidase; it was approximately 60-fold less potent than amitriptyline as an inhibitor of norepinephrine uptake in rat hypothalamus synaptosomes (IC50 ¼ 6.5 mM) whereas it was 20-fold more potent than amitriptyline as an inhibitor of dopamine uptake in striatal synaptosomes (IC50 ¼ 3.4 mM) [43]. It had little effect on 5-HT uptake in concentrations as high as 10 mM [43]. Bupropion is chiral and it has been separated into its two enantiomers [49]. No significant differences were found [49] in the activities in the tetrabenazine-induced sedation model in mice or in the uptake of norepinephrine and dopamine into synaptosomes. However, the enantiomers are not very stable and the racemization half-life for the ()enantiomer, under physiological conditions (pH 7.6), was suggested to be approximately 44 min [49]. A phase I clinical study in 12 healthy subjects revealed no differences from lactose and it was concluded [50] that bupropion, at the doses studied (of 50 and 100 mg), was devoid of stimulant or sedative properties, or cardiovascular effects. Bupropion was approved by the US Food and Drug Administration (FDA) as an antidepressant on December 30, 1985 and marketed under the name of WellbutrinÔ. However, a significant incidence of seizures at the originally recommended dosage (400–600 mg) caused the drug to be withdrawn in 1986. Subsequently, the risk of seizures was found to be highly dose dependent, and bupropion was reintroduced to the market in 1989 with a maximum recommended dose of 450 mg/day. A sustained release formulation was approved by the FDA in 1996 and has been used clinically to treat depression. Placebo-controlled double-blind studies have confirmed the efficacy of bupropion for clinical depression [51]. Furthermore, available clinical data suggest that bupropion is an effective and generally welltolerated option in the treatment of major depressive disorder [52]. Perhaps then, bupropion should not be regarded as a standalone drug, although its mechanism of action appears to be different from other antidepressant drugs; like other antidepressants, its actions involve biogenic amine transmission, but it exerts a different combination of effects on biogenic amines. Bupropion is, however, the first non-nicotine medication to gain approval as an aid to smoking cessation and it was placed on the market for this purpose in 1997 under the name ZybanÔ. This use arose from finding that smokers who were taking bupropion (as Wellbutrin) as antidepressant medication found that they had a reduced desire or craving to smoke cigarettes [53]. It is administered in 150 mg doses in a sustained release formulation twice daily and reduces the severity of nicotine cravings and withdrawal symptoms. The treatment course lasts for 7 to 12 weeks with the patient halting the use of tobacco about 10 days into the course. The efficacy of bupropion is reported to be similar to that of nicotine replacement therapy [54].
j41
j 2 Standalone Drugs
42
N HN N Figure 2.15 Varenicline.
Bupropion has, however, been joined by another product on the market since 2006, when Pfizer launched varenicline (Figure 2.15) as ChantixÔ in the United States and ChampixÔ in Europe. Varenicline acts by a different mechanism since it is an a4b2 nicotinic acetylcholine receptor partial agonist [55]. Both drugs, bupropion and varenicline, now have black box warnings from the US FDA (July 2009) [56] to highlight the risk of serious mental health side effects including depression, hostility, and suicidal thoughts. The suicide aspect seems to be much worse for varenicline that, since its approval in 2006, is reported to have been associated with 98 suicides and 188 attempts. Bupropion, since 1997, has been associated with 14 suicides and 17 attempts.
2.5 Ezetimibe
High plasma concentrations of low-density lipoprotein-cholesterol (LDL-C) are known to play an active role in the development of atherosclerotic coronary heart disease. The latter being a major cause of death and cardiovascular morbidity in the Western world. For this reason, much effort has been expended in seeking to reduce the serum cholesterol levels by drug treatment. The two significant sources for cholesterol are endogenously synthesized cholesterol and dietary cholesterol. Early discoveries found ways to reduce the equilibrium level of cholesterol. Cholestyramine and colestipol, for example, are basic anion exchange resins that sequester bile acids in the gastrointestinal tract to prevent reabsorption. The sequestered bile acids are then excreted, which results in plasma cholesterol being converted into bile acids to normalize the bile acid levels. This conversion of cholesterol lowers the plasma concentration of LDL-C. Clofibrate, and the various other fibrates that followed, reduces the plasma triglyceride concentrations. In most patients, plasma cholesterol and LDL-C concentrations also fall. The fibrates were subsequently shown to be agonists of the PPAR-a receptor (peroxisome proliferator-activated receptors that modulate carbohydrate and fat metabolism). The statins are specific inhibitors of hydroxymethylglutaryl coenzyme A (HMGCoA) reductase that were introduced into clinical practice in the 1980s. In low doses, the statins exerted a relatively selective effect on hepatic cholesterol synthesis. As the statins became established, their potencies, or dose levels, were increased in order to have a more profound effect on lowering of the LDL-C levels seen in the blood [57]. However, this gave rise to an increased incidence of such an unwelcome side effect as rhabdomyolysis especially when used in combination with fibrates.
2.5 Ezetimibe
Cholesterol absorption inhibitors have been described [58a] by scientists at Pfizer. These are synthetic saponins, tigogenin cellobiosides, tiqueside, and pharmaqueside. The latter was stated in 2002 to be in phase III clinical trials for the treatment of hypercholesterolemia, but further results do not appear to have been reported. Other efforts to inhibit the absorption of dietary cholesterol focused on the inhibition of Acyl-CoA:cholesterol acyltransferase (ACAT), the major enzyme associated with cholesterol esterification. Inhibition of this enzyme blocks absorption of intestinal cholesterol. As part of a broad chemical program to discover novel ACAT inhibitors [58b], scientists at Schering-Plough Research Institute (in Kenilworth, NJ, USA) investigated the synthesis and in vivo testing of conformationally restricted compounds related to two known chemical classes of ACAT inhibitors, namely, SA 58035 and CI 976 (Figure 2.16). Burnett et al. describe [58b] bridging of the two carbon atoms of the ethyl linkage and the nitrogen atom in SA 58035 by using 2-azetidinone containing two phenyl groups. The desired compound A was not active, but a secondary compound B, arising from acylation, did show some activity, pariculary in vivo. The latter provided an initial lead that was followed up and, by analogy with their previous work in ACAT, methoxy groups were introduced. This work eventually led to the first drug candidate, the chiral trans-azetidinone SCH 48461, for development (Figure 2.17). Surprisingly there was a strong divergence between the in vitro ACAT assay (measuring inhibition of the esterification of cholesterol with oleic acid) and the in vivo test that measured the inhibition of the rise in plasma cholesterol levels in cholesterol-fed hamsters. These results suggested the existence of another mechanism of action and further work suggested that it was novel and upstream from ACAT. Furthermore, SCH 48461 was shown to reduce serum cholesterol in human clinical trials [59]. The above findings led to abandonment of the in vitro assay. Having to rely only on an in vivo assay made it difficult to separate potential effects on intrinsic potency from effects on pharmacokinetics. This was especially difficult since SCH 48461 was extensively metabolized in vivo, making it unclear as to exactly what was the active species. Van Heeck et al. [60a] describe that an experimental protocol was devised to determine whether there were active metabolites and how they might contribute to the in vivo profile of SCH 48461. The studies revealed the rapid appearance of a complex metabolic mixture in the bile, which was more potent as an inhibitor of
O
CH3(CH2)9 Me
SiMe2(C10H21)
HN O
OMe
HN MeO
SA 58035 Figure 2.16 ACAT inhibitors.
CI 976
OMe
j43
j 2 Standalone Drugs
44
PhCH2 CO
N
N
O
O
OMe
OMe
compound B
compound A
OMe
OH OH
F N
N
O
O
OMe
F
SCH 48461
SCH 58235 (ezetimibe)
Figure 2.17 Compounds leading to ezetimibe.
cholesterol absorption. Rosenblum et al. listed [60b] the putative sites of metabolism of SCH 48461, as shown in Figure 2.18, and directed their synthesis effort to making the most probable metabolites for testing. Their results led them inter alia to introduce two hydroxyl groups as metabolic products and two fluorine atoms to block metabolism sites (Figure 2.19). The result was the compound SCH 58235, ezetimibe [61], which was reported to be 50 times more potent than SCH 48461 in the
demethylation benzylic oxidation
OMe
N
demethylation
O pendant phenyl oxidation
OMe ring opening
Figure 2.18 Putative sites of metabolism of SCH 48461.
2.5 Ezetimibe
dealkylated stereoselectively oxidized
OH OH F N O F oxidation blocked
oxidation blocked Figure 2.19 Design of (SCH 58235) ezetimibe.
cholesterol-fed hamster assay. This result provides an impressive story of serendipitous discovery of a drug working by a novel mechanism, together with outstanding integration of synthetic and medicinal chemistry with metabolic and pharmacological studies, to make a potent therapeutic product. The discovery of ezetimibe led to a 10-year search for its mechanism of action. The researchers at Schering-Plough showed [62] that Niemann-Pick C1-Like 1 (NPC1L1) protein plays a critical role in the absorption of intestinal cholesterol across the plasma membrane of the intestinal enterocyte. This protein is believed to be a molecular target [63] of ezetimibe. By inhibiting cholesterol absorption at the level of the brush border of the intestine, ezetimibe reduces the amount of lipoprotein cholesterol circulated to the liver. In response to reduced cholesterol delivery, the liver reacts by upregulating LDL-C receptor that, in turn, leads to increased clearance of cholesterol from the blood [64]. Ezetimibe is marketed as a 10 mg tablet under the name ZetiaÔ and is also available in combination with simvastatin; it has also been approved for a combination with fenofibrate. Ezetimibe is a standalone product as, so far, there are no analogues yet available, probably because the mechanism of action was published only in 2005. Thus, this remarkable example of drug discovery started out as an attempt to make an analogue of leads that, though characterized pharmacologically, had not yet yielded a clinically useful product. A serendipitous discovery of a different mechanism of action led to the characterization of a previously unknown mechanism of cholesterol uptake and the clever design of a very potent candidate drug. This work represents a true breakthrough in the search for drugs that reduce the intake of dietary cholesterol and provides an outstanding example of analogue-based drug research leading to the discovery of a pioneer drug.
j45
j 2 Standalone Drugs
46
2.6 Lamotrigine
Lamotrigine (Figure 2.20) is a widely used anticonvulsive agent. Its predominant indication is in the treatment of epilepsy and bipolar disorder [65, 66]. The discovery process was partly serendipitious, based on the observation that folic acid produces epileptogenic foci and that antiepileptic drugs such as phenytoin, phenobarbital, and carbamazepine have antifolate properties [67, 68]. Thus, a correlation between antiepileptic and antifolate properties was assumed. A series of aminotriazine compounds was prepared by chemists at Wellcome in the United Kingdom as potential antifolate agents. Lamotrigine was the lead from this series and was first disclosed in a 1981 patent. The patent reports significant anticonvulsant activity with an ED50 of 2.4 mg/kg in mice by oral administration [69]. Nevertheless, the compound had little effect on folate levels, and subsequent work confirmed that the original hypothesis linking folates and antiepileptics was misguided. In vitro studies demonstrated that lamotrigine primarily acts as a blocker of voltage-sensitive sodium ion channels [70]. The promising activity and tolerability of lamotrigine led to its progression to clinical studies, with phase I studies confirming its safety and potential superiority to established drugs such as phenytoin and diazepam [71]. Lamotrigine was first approved in 1994 as an antiepileptic drug for the treatment of partial seizures. Lamotrigine is the only medication besides topiramate used in the treatment of seizures associated with Lennox–Gastaut syndrome, a severe form of epilepsy. Following its widespread use as an antiepileptic, it was noticed that patients reported higher levels of happiness and mastery, or perceived internal locus of control, independent of seizure control [72]. This led to investigation of lamotrigine as an antipsychotic and it received approval in 2003 for the treatment of bipolar I disorder, the first drug to do so since lithium. In addition, off-label uses of lamotrigine include the treatment of peripheral neuropathy, migraines, borderline personality disorder, and refractory unipolar depression. Lamotrigine has virtually complete bioavailability with no first-pass effect. It is 55% bound to plasma proteins and reaches a peak plasma concentration in 2.5 h. The halflife in healthy adults is 24–35 h, with clearance primarily as the glucuronide conjugate in urine. Metabolism by uridine diphosphate glucuronyl transferases is competitive and susceptible to drug–drug interactions. However, lamotrigine does not induce cytochrome P-450 isozymes and thus does not affect plasma levels of other Cl Cl N H2N
NH2 N N
Figure 2.20 Lamotrigine.
2.7 Metformin
Cl Cl
Cl
N H2N
NH2 N
Figure 2.21 JZP-4.
medications. It is a well-tolerated drug, with skin rash being the most common side effect with an incidence of 5–10%. Other side effects include headaches, dizziness, and insomnia. The pharmacodynamics of lamotrigine may be more complex than its action on sodium channels, as its side-effect profile is different from other drugs such as oxcarbazepine that act on sodium channels [73]. Overall, lamotrigine acts as a mood stabilizer and inhibits the release of excitatory amino acid neurotransmitters such as aspartate and glutamate. The standalone status of lamotrigine has recently been challenged by a compound, elpetrigine, from GlaxoSmithKline (Figure 2.21). The switch from triazine to pyrazine and from dichloro- to trichlorophenyl results in increased potency for sodium and calcium channels, and potential reduction of the dermatological side effects [74, 75]. Elpetrigine is currently in clinical development, with phase I trials completed.
2.7 Metformin
Metformin is the most widely used oral hypoglycemic agent. It is the first-line drug for the treatment of type 2 diabetes and the only oral antidiabetic besides glibenclamide in the World Health Organization Model List of Essential Medicines [76]. Unusually for a drug molecule, it is a simple biguanide in structure and has more nitrogen than carbon atoms. The discovery can be traced back to medieval times, when extracts of the French lilac Galega officinalis were used to treat the frequent and painful urination accompanying diabetes mellitus [77]. The active constituent was later identified as galegine or isoprenylguanidine (Figure 2.22). Guanidine itself was shown by Watanabe to be elevated in the blood following removal of the parathyroid gland and suggested as the reason for decreased blood sugar levels following
HN
H N NH2 galegine
HN
NH2 NH2
guanidine
Figure 2.22 First antidiabetic guanidines.
j47
j 2 Standalone Drugs
48
NH2
H N
HN
N H
NH2
Synthalin NH2
H N
HN
NH
N H
NH2
NH
Synthalin B
Figure 2.23 First marketed antidiabetic guanidine derivatives.
parathyroidectomy [78]. In supporting animal studies, rabbits injected with guanidine show a fall in blood sugar levels. Guanidine was too toxic for therapeutic use, while galegine was less so, and extracts of G. officinalis were briefly used as an antidiabetic in the early 1920s. These observations stimulated interest in the evaluation of guanidine derivatives, particularly diguanidines and biguanides. Frank at the University of Breslau prepared a series of diguanidines separated by a methylene chain. These compounds were more potent and less toxic than the monoguanidines [79]. From this series, Synthalin (Figure 2.23) progressed to clinical trials in 1926 and was marketed by Schering AG for the treatment of mild diabetes. Reports of adverse effects led to the introduction of Synthalin B with a longer methylene chain as a safer alternative. Nevertheless, both compounds were withdrawn in the 1940s due to potential liver and kidney toxicity. Meanwhile, depression of blood sugar levels was inhibitory to the reproduction of trypanosomes and Synthalin was found to have trypanocidal activity. This eventually led to the discovery of the amidine drugs stilbamidine and pentamidine, still in use for the treatment of trypanosomiasis and other protozoal infections. At the University of Vienna, Slotta and Tschesche reported a series of biguanides, including metformin, with hypoglycemic activity [80]. The compounds were nontoxic in animals, but these studies were not extended to humans and essentially forgotten for decades with the introduction of insulin. In the 1950s, Jean Sterne a physician at the Hospital Laennec and Aron Laboratories in Paris independently investigated biguanides as antidiabetic agents and selected metformin (Figure 2.24) for clinical development, naming it Glucophage (glucose eater) [81]. Simultaneously, the US Vitamin Corporation had examined a series of biguanides, from which phenformin emerged as the clinical candidate [82]. A year later, Mehnert in Germany reported studies with buformin [83].
NH Me
N Me
NH
N H
metformin
NH 2
NH N H
N H
NH NH 2
phenformin
Figure 2.24 Metformin and its withdrawn derivatives.
NH N H
NH
N H
buformin
NH 2
2.8 Topiramate
SAR studies with biguanides indicate that hypoglycemic activity is lost if more than one nitrogen is substituted. Among mono- and disubstituted biguanides, chains longer than five atoms were not tolerated. Despite extensive investigation, the precise mechanism of action of biguanides remains unknown. They do not increase insulin production in either the fasting or postprandial state. The primary effect of metformin is inhibition of gluconeogenesis in the liver, and increased glucose uptake across the cell membrane in peripheral tissues. A number of molecular targets have been proposed, although separating the primary effector from downstream consequences of glucose regulation is complicated. Phenformin and buformin were the first biguanides to receive clinical approval due to their greater potency than metformin [84]. Lactic acidosis is the most serious side effect of biguanides. It is more pronounced with phenformin and buformin than metformin, probably due to decreased metabolism of the alkyl chain in some patients leading to prolonged drug exposure. Both phenformin and buformin were withdrawn from the market in most countries in the 1970s, leading to renewed interest in metformin. Clinical studies demonstrated that metformin is unique in having antihyperglycemic activity without causing overt hypoglycemia or weight gain unlike sulfonylureas or insulin. Furthermore, the drug is still efficacious in cases of insulin resistance. Metformin was first marketed in France in 1979 by Lipha, now Merck, that had acquired Aron Laboratories and later received FDA approval in 1994. Metformin has 50–60% oral bioavailability with negligible binding to plasma proteins. Its plasma half-life is estimated at 1.5–4.9 h, reaching a peak concentration 1–2 h after oral dosing. Metformin does not undergo significant metabolism, and is excreted intact in urine. The most serious potential side effect is lactic acidosis, and for this reason it is contraindicated in patients with impaired liver or kidney function. Lactic acidosis is believed to be an on-target effect as metformin inhibits gluconeogenesis, the pathway by which lactate buildup is normally prevented by conversion to glucose. The most common adverse effect is gastrointestinal upset, which is rare upon prolonged steady use of the drug. In addition to antidiabetic therapy, metformin is under investigation for other diseases that feature insulin resistance such as polycystic ovary syndrome, nonalcoholic fatty liver disease, and premature puberty. The favorable effects of metformin in lowering blood pressure, serum triglycerides, total cholesterol, and low-density lipoprotein-cholesterol suggest that it is beneficial for the treatment of cardiovascular disease. Metformin has a remarkable history in terms of its discovery. The compound was first prepared in 1922 but did not receive approval for more than 50 years. Since then, it has become established as the first-line drug for the treatment of type 2 diabetes and is likely to be useful for additional therapeutic indications.
2.8 Topiramate
Topiramate (Figure 2.25) was identified in the early 1980s as an anticonvulsant agent in a pharmacological model that measures the ability of compounds to prevent
j49
j 2 Standalone Drugs
50
O O S O NH2
O O
O O
O
topiramate
O O S O NH2
O O O S O O
H N
NH2 S O O
O O
RWJ 37947
H N
NH2 S O O
S JNJ-26990990
benzylsulfamide
Figure 2.25 Topiramate and related compounds.
maximal electroshock seizures in mouse [85, 86]. Structurally, it bears no resemblance to previously used anticonvulsant agents and was originally synthesized as a chemical intermediate during a program to identify inhibitors of fructose-1,6bisphosphatase as potential antidiabetic agents [87, 88]. In combination with the sulfamate functionality, which appears to be essential for activity, many variations on the scaffold are tolerated. These range from relatively simple phenylethyl groups to the rather sophisticated sugar-derived core of topiramate itself. The key publication on the discovery process emphasizes the importance of oral bioavailability, duration of action, and minimal neurotoxicity as important features in the selection of the drug for clinical development [85]. Topiramate is generally used in patients who are unresponsive to other anticonvulsant medications or is used in combination with other anticonvulsants to treat intractable seizures. Topiramate has also been approved for the prophylactic treatment of migraine headaches in some countries and this is now the most common use. Topiramate is a structurally unique anticonvulsant agent. There are no former abandoned analogues. Topiramate displays the following actions related to seizure reduction: (a) antagonism of the AMPA/kainate subtype of the glutamate receptor, (b) blockage of voltage-dependent sodium channels in nerve cells, decreasing excessive nerve-cell firing, (c) increase in the activity of the neurotransmitter gamma-aminobutyrate at some subtypes of the GABA-A receptor [89]. Topiramate also inhibits carbonic anhydrase, particularly isozymes II and IV. The relation of this activity to seizure reduction is unclear although recent patent applications covering topiramate analogues indicate that reduction of carbonic anhydrase activity could be beneficial in decreasing side effects seen with topiramate, such as paresthesia. Because there is so far no single clear mechanism of action that links to the pharmacological effects, a target-based screening approach to the identification of new pharmacological analogues is not applicable. A number of publications and patents from the original discovery group have appeared and some more potent analogues of topiramate have been disclosed [90, 91]. These efforts employed the same pharmacological models as in the original discovery. One analogue from these efforts, RWJ 37947, a cyclic sulfate analogue of topiramate, reportedly entered development, but no new information on this compound has appeared since 2002. A recent report on the design of additional topiramate analogues [92] indicates that the high potency of this compound against
2.9 Valproate
j51
carbonic anhydrase enzymes was thought likely to lead to undesirable side effects in certain patients and prevented its further development. Two recent patents and one publication disclose analogues, such as JNJ-26990990 (Figure 2.25), which bear some resemblance to the structural motifs in the original manuscript on topiramate [92–94]. A full report that details the optimization process leading to this clinical candidate has appeared. The switch to a sulfamide series was motivated by a desire to generate topiramate analogues with andiabetic and antiobesity effects but devoid of anticonvulsant activity. However, subsequent screening of these sulfamides with simplified structures revealed promising anticonvulsant activity. JNJ-26990990 in particular shows broad-spectrum activity in a number of seizure models and is only a very weak inhibitor of carbonic anhydrase enzymes. JNJ26990990 entered phase II clinical trials in October 2007. Somewhat analogous anticonvulsants, such as benzylsulfamide (Figure 2.25), have also been discovered from a pharmacophore-based design approach, but they appear not to have progressed to clinical development [95] (Figure 2.25).
2.9 Valproate
The usefulness of valproic acid (Figure 2.26) as an anticonvulsant was made as a serendipitous discovery [96, 97] during the investigation of khelline derivatives as potential anticonvulsants. After finding it difficult to dissolve some derivatives in common organic solvents, a suggestion led to the use of valproic acid as a solubilizer. Anticonvulsant activity was observed in all the samples and valproic acid itself was found to be the anticonvulsant agent. The initial human epilepsy trials were reported in 1964 followed by first approval in France in 1967. Valproate is a widely used anticonvulsant agent because of its activity against different types of seizures. It is also approved for the treatment of acute and chronic migraine and for mania. Valproic acid is a pioneer drug, that is, the first of its class to reach the market. There are no former or previously used analogues. Valproic acid is associated with significant side effects such as reproductive dysfunction and teratogenicity, and there is interest in developing improved analogues [98]. A number of structurally similar compounds have a valproate-like profile in animal seizure models. Valpromide, the amide derivative of valproic acid, is slightly more potent in animal models and is rapidly transformed to valproate in man,
O
O
O OH
NH2
O NH2
NH2 F
valproic acid
valpromide
Figure 2.26 Valproic acid and related compounds.
valnoctamide
j 2 Standalone Drugs
52
essentially acting as a prodrug. It is marketed as an antiepileptic drug and antipsychotic agent in Europe. Valnoctamide, a metabolically stable analogue of valpromide, has been marketed as an anxiolytic and sedative since the mid-1960s. It also exhibits greater anticonvulsant potency than valproate in animal models and is significantly less embryotoxic than valproic acid; however, its usefulness as an antiepileptic in man remains to be determined. Recently, a constrained cyclopropane analogue of valpromide (Figure 2.26) was reported to have superior activity in the MES models used in the discovery of vaproate and topiramate [99]. Since the broad spectrum of utility of valproic acid probably arises from its diverse actions at the molecular level, a target-based screening approach to the identification of pharmacological analogues remains problematic.
2.10 Summary
Nearly every drug has one or more analogues, which are similar to the original drug both in their structures and in their pharmacological mechanism of action (full analogues) or their similarity is restricted to the pharmacological mechanism of action (pharmacological analogues). We analyzed the Top 100 most frequently used drugs based on their sales data in 2006 and, interestingly, their value amounted to 51% of the global sales of all drugs, which indicates their importance. Among these 100 drugs, 9 standalone drugs were identified, that is, compounds that are pioneer drugs for which there are no effective analogues. In this chapter, we have described the serendipitous discovery of these standalone drugs and the past and present efforts to design analogues that would improve upon their properties. Six standalone drugs, namely, acetaminophen, bupropion, lamotrigine, metformin, topiramate, and valproate, have no clear mechanism of action. They display some weak in vitro activities; however, there is no proven correlation between these in vitro activities and their pharmacological properties in animals. Acetylsalicylic acid is a selective irreversible COX-1 inhibitor, but in a higher dosage it loses its selectivity. Aripiprazole is a partial agonist of the D2/3 receptors, but other targets (5-HT2A, 5-HT2B) may also play a role in its in vivo activity. The mechanism of action of ezetimibe was discovered only recently (2005). Elucidation of the mechanism of action and serendipitous observations may lead to new useful analogues of these standalone drugs.
Acknowledgment
We thank Duane Burnett, William Greenlee, and Bruce E. Maryanoff for carefully reading and reviewing the manuscript.
References
References 1 Cahn, A. and Hepp, P. (1886) Das
2
3
4 5
6
7
8
9
10
11
12
13
antifebrin, ein neues fiebermittel. Centralbl. Klin. Med., 7, 561. US Food and Drug Administration ordered the withdrawal of drugs containing phenacetin in 1983. von Mering, J. (1893) Beitr€age zur kenntnis der antipyretica. Ther. Monatsch., 7, 577–587. Lester, D. and Greenberg, L.A. (1947) J. Pharm. Exp. Ther., 90, 68. Flinn, F.B. and Brodie, B.B. (1948) The effect on the pain threshold of N-acetyl p-aminophenol. A product derived in the body from acetanilide. J. Pharm. Exp. Ther., 94, 76. Brodie, B.B. and Axelrod, J. (1948) The fate of acetanilide in man. J. Pharm. Exp. Ther., 94, 29–38. H€ ogest€att, E.D., Jonsson, B.A., Ermund, A., Andersson, D.A., Bjork, H., Alexander, J.P., Cravatt, B.F., Basbaum, A.I., and Zygmunt, P.M. (2005) Conversion of acetaminophen to the bioactive Nacylphenolamine AM404 via fatty acid amide hydrolase-dependent arachidonic conjugation in the nervous system. J. Biol. Chem., 280, 31405–31412. Ottani, A., Leone, S., Sandrini, M., Ferrari, A., and Bertolini, A. (2006) The analgesic activity of paracetamol is prevented by the blockade of cannabinoid CB1 receptors. Eur. J. Pharmacol., 531 (1–3), 280–281. Hinz, B., Cheremina, O., and Brune, K. (2008) Acetaminophen (paracetamol) is a selective cyclooxygenase-2 inhibitor in man. FASEB J., 22, 383–390. Thomas, S.H. (1993) Paracetamol (acetaminophen) poisoning. Pharmacol. Ther., 60, 91–120. Wallace, J.L. (2004) Acetaminophen hepatotoxicity: NO to the rescue. Br. J. Pharmacol., 143, 1–2. Bessems, J.G.M. and Vermeulen, N.P.E. (2001) Paracetamol(acetaminophen)induced toxicity: molecular and biochemical mechanisms, analogues and protective approaches. Crit. Rev. Toxicol., 31, 55–138. Nelson, S.D., Forte, A.J., and McMurtry, R.J. (1978) Decreased toxicity of the
14
15
16
17
18
19
20
21
22
23
24
N-methyl analogues of acetaminophen and phenacetin. Res. Commun. Chem. Pathol. Pharmacol., 22 (1), 61–71. Vaccarino, A.L., Paul, D., Mukherjee, P.K., Rodrigez de Turco, E., Marcheselli, V.L., Xu, L., Trudell, M.L., Minguez, J.M., Matia, M.P., Sunkel, C., Alvarez-Builla, J., and Bazan, N.G. (2007) Bioorg. Med. Chem., 15, 2206–2215. Sinning, C., Watzer, B., Coste, O., N€ using, R.M., Ott, I., Ligresti, A., Di Marzo, V., and Imming, P. (2008) New analgesics synthetically derived from paracetamol metabolite N-(4-hydroxyphenyl)(5Z,8Z,11Z,14Z)-icosatetra5,8,11,14-enamide. J. Med. Chem., 51 (24), 7800–7805. Schmid, B., K€otter, I., and Heide, L. (2001) Pharmacokinetics of salicin after oral administration of a standardised willow bark extract. Eur. J. Clin. Pharmacol., 57, 387–391. Piria, R. (1839) Recherches sur la salicine et les produits qui en derivent. C.R. Acad. Sci. (Paris), 8, 479–485. Maclagan, T. (1876) The treatment of rheumatism by salicin and salicylic acid. Br. Med. J., 1, 627. Kolbe, H. (1860) Über die synthese der salicyls€aure. Ann. Chem. Pharm., 113, 125–127. Stricker, S. (1876) Über die resultate der bahandlung der polyathritis rheumatica mit salicyls€aure. Berl. Klin. Wochenschr., 13, 1–2, 15–16, 99–103. Vane, J.R. (1971) Inhibition of prostaglandin synthesis as a mechanism of action for aspirin-like drugs. Nat. New Biol., 231, 232–235. Milton, A.S. and Wendlandt, S. (1970) A possible role for prostaglandin E1 as a modulator of temperature regulator in the central nervous system of the cat. J. Physiol., 207, 76–77. Di Rosa, M., Giroud, J.P., and Willoughby, D.A. (1971) Studies of the mediators of the acute inflammatory response induced in rats in different sites by carrageenin and turpentine. J. Physiol., 104, 15–29. Hemler, M. and Lands, W.E.M. (1976) Purification of the cyclooxygenase that
j53
j 2 Standalone Drugs
54
25
26
27 28
29
30
31
32
33
34
forms prostaglandins. J. Biol. Chem., 251, 5575–5579. Roth, G.J. and Majerus, P.W. (1975) The mechanism of the effect of aspirin on human platelets: 1. Acetylation of a particulate fraction protein. J. Clin. Invest., 56, 624–632. Xie, W., Chipman, J.G., Robertson, D.L., Erikson, R.L., and Simmons, D.L. (1991) Expression of a mitogen-responsive gene encoding prostaglandin synthase is regulated by mRNA splicing. Proc. Natl. Sci. USA, 88, 2692–2696. Schr€or, K. (2009) Acetylsalicylic Acid, Wiley-Blackwell, p. 35. Smith, J.B. and Willis, A.L. (1971) Aspirin selectively inhibits prostaglandin production in human platelets. Nature, 231, 235–237. Moriarty, L.M., Lally, M.N., Carolan, C.G., Jones, M., Clancy, J.M., and Gilmer, J.F. (2008) Discovery of a True aspirin prodrug. J. Med. Chem., 51, 7991–7999. Lewis, D.A. and Lieberman, J.A. (2000) Catching up on schizophrenia: natural history and neurobiology. Neuron, 28, 325–334. Charpentier, P., Gaillot, P., Jacob, R., Gaudechon, J., and Buisson, P. (1952) Recherches sur les dimethylaminopropylN-phenothiazines substituees. Comptes Rendus de lAcademie des Sciences, 235, 59–60. Courvoisier, S., Fournel, J., Ducrot, R. et al. (1953) Proprietes pharmacodynamiques du chlorhydrate de chloro-(3dimethylamino-30 -propyl)-10phenothiazine (4560 R.P.). Arch. Int. Pharmacodyn., 92, 305. Janssen, P.A.J., Van deWesteringh, C., Jagneau, A.W.M., Demoen, P.J.A., Hermans, B.K.F., Van Daele, G.H.P., Schelleck, K.H.L., Van der Eycken, C.A.M., and Niemegerm, C.J.E. (1959) Chemistry and pharmacology of CNS depressants related to 4-(4-hydroxy-4phenylpiperidino)butyrophenone. Part I. Synthesis and screening data in mice. J. Med. Pharm. Chem., 1, 281–287. Schmutz, J. and Eichenberger, E. (1982) Chronicles of Drug Discoveries, vol. 1 (eds J.S. Bindra and D. Lednicer), John Wiley & Sons, Inc., New York, pp. 39–60.
35 Wooley, D.W. and Shaw, E. (1954) A
36
37
38
39
40
41
42
biochemical and pharmacological suggestion about certain mental disorders. Proc. Natl. Acad. Sci. USA, 40, 228. Carlsson, A. and Lindquist, M. (1963) Effect of chlorpromazine and haloperidol on formation of 3-methoxytyramine and normetanephrine in mouse brain. Acta Pharmacol. Toxicol., 20, 140–144. Kebabian, J.W. and Calne, D.B. (1979) Multiple receptors for dopamine. Nature, 277, 93. Carlsson, A., Waters, N., Holm-Waters, S., Tedroff, J., Nilsson, M., and Carlsson, M.L. (2001) Interaction between monoamines, glutamate, and GABA in schizophrenia: new evidence. Annu. Rev. Pharmacol. Toxicol., 41, 237–260. Banno, K., Fujioka, T., Kikuchi, T., Oshiro, Y., Hiyama, T., and Nakagawa, K. (1988) Studies on 2(1H)-quinolone derivatives as neuroleptic agents. I. Synthesis and biological activities of (4-phenyl-1piperazinyl)-propoxy-2(1H)-quinolone derivatives. Chem. Pharm. Bull., 36 (11), 4377–4388. Oshiro, Y., Sato, S. Kurahashi, N., Tanaka, T., Kikuchi, T., Tottori, K., Uwahodo, Y., and Nishi, T. (1198) Novel antipsychotic agents with dopamine autoreceptor agonist properties; synthesis and pharmacology of 7-[4-(4-phenyl-1piperazinyl)butoxy]-3,4-dihydro-2(1H)quinolone derivatives. J. Med. Chem., 41, 658–667. Williams, M.A. (2004) A return to the fundamentals of drug discovery. Curr. Opin. Invest. Drugs, 5, 29–33. Butini, S., Gemma, S., Campiani, G., Franceschini, S., Trotta, F., Borriello, M., Ceres, N., Ros, S., Coccone, S.S., Bernetti, M., De Angelis, M., Brindisi, M., Nacci, V., Fiorini, I., Novellino, E., Cagnotto, A., Mennini, T., Sandager-Nielsen, K., Andreasen, J.T., Scheel-Kruger, J., Mikkelsen, J.K., and Fattorusso, C. (2009) Discovery of a new class of potential multifunctional atypical antipsychotic agents targeting dopamine D3 and serotonin 5-HT1A and 5-HT2A receptors: design, synthesis, and effects on behavior. J. Med. Chem., 52, 151–169.
References 43 Soroko, F.E., Mehta, N.B., Maxwell, R.A.,
44
45
46
47
48
49
50
51
52
53
Ferris, R.M., and Schroeder, D.H. (1977) Bupropion hydrochloride (() a-tbutylamino-3-chloropropiophenone HCl): a novel antidepressant agent. J. Pharm. Pharmacol., 29, 767–770. Mehta, N.B. (1983) The chemistry of bupropion. J. Clin. Psychiatry, 44 (Sec 2), 56–59. Sulser, F. and Soroko, F.E. (1965) On the role of brain norepinephrine release in the antibenzoquinolizine actions of desipramine. Psychopharmacolgy, 8, 191–200. Baltzly, R. and Mehta, N.B. (1968) N-sec and N-t-alkyl derivatives of methoxamine and related compounds. J. Med. Chem., 11, 833–844. Mehta, N.B. (1974) Meta-chloro substituted a-butylaminopropiophenones. US Patent 3,819,706 to Burroughs Wellcome Co.; US Patent 3,885,046 (1975). Mehta, N.B. and Yeowell, D.A. (1971) German Patent 2,059,618 to Wellcome Foundation. Musso, D.L., Mehta, N.B., Soroko, F.E., Ferris, R.M., Hollingsworth, E.B., and Kenney, B.T. (1993) Synthesis and evaluation of the antidepressant activity of the enantiomers of bupropion. Chirality, 5, 495–500. Peck, A.W., Bye, C.E., Clubley, M., Henson, T., and Riddington, C. (1979) A comparison of bupropion hydrochloride with desamphetamine and amitriptyline in healthy subjects. Br. J. Clin. Pharmacol., 7, 469–478. Fava, M., Rush, A.J., Thase, M.E., Clayton, A., Stahl, S.M., Pradko, J.F., and Johnston, J.A. (2005) 15 years of clinical experience with bupropion HCl: from bupropion to bupropion SR to bupropion XL. Prim. Care Companion J. Clin. Psychiatry, 7, 106–113. Dhillon, S., Yang, L.P., and Curran, M.P. (2008) Bupropion: a review of its use in the management of major depressive disorder. Drugs, 68, 653–689. Tonneson, P., Tonstad, S., Hjalmarson, A., Lebargy, F., Van Spiegel, P.I., Hider, A., Sweet, R., and Townsend, J. (2003) A multicentre randomized double-blind, placebo-controlled 1 year study of
54
55
56
57
58
59
bupropion SR for smoking cessation. J. Intern. Med., 254, 184–192. Wu, P., Wilson, K., Dimoulas, P., and Mills, E.J. (2006) Effectiveness of smoking cessation therapies: a systematic review and meta-analysis. BMC Public Health, 6, 300–315. Jorenby, D.E., Hays, J.T., Rigotti, N.A., Azoulay, S., Watsky, E.J., Williams, K.E., Billing, C.B., Gong, J., and Reeves, K.R. (2006) Efficacy of varenicline, an alpha4beta2 nicotinic acetylcholine receptor partial agonist, vs placebo or sustained-release bupropion for smoking cessation: a randomized controlled trial. JAMA, 296, 56–63. Osterwell, N. (2009) Chantix and Zyban to receive boxed warnings for serious neuropsychiatric symptoms. Medscape Med. News, http://www.medscape.com/ viewarticle/705163. Kerpel-Fronius, S. and Fischer, J. (2006) Optimizing the clinical pharmacologic properties of the HMG-CoA reductase inhibitors, Chapter 4, in Analogue-Based Drug Discovery (eds J. Fischer and C.R. Ganellin), IUPAC, Wiley-VCH Verlag GmbH, Weinheim, pp. 137–156. (a) McCarthy, P.A., DeNinno, M.P., Morehouse, L.A., Chandler, C.E., Bangerter, F.W., Wilson, T.C., Urban, F.J., Walinsky, S.W., Cosgrove, P.G., Duplantier, L., Etienne, J.B., Fowler, M.A., Lambert, J.F., ODonnell, J.P., Pezzullo, S.L., Watson, H.A., Wilkins, R.W., Jr., Zaccaro, L.M., and Zawistoski, M.P. (1996) 11-Ketotigogenin cellobioside (pamaqueside): a potent cholesterol absorption inhibitor in the hamster. J. Med. Chem., 39, 1935–1937; (b) Burnett, D.A., Caplen, M.A., Davis, H.R., Jr., Burrier, R.E., and Clader, J.W. (1994) 2-Azetidinones as inhibitors of cholesterol absorption. J. Med. Chem., 37, 1733–1736. Bergman, M., Morales, H., Mellars, L., Kosoglou, T., Burrier, R., Davis, H.R., Sybertz, E.J., and Pollare, T. (1995) The clinical development of a novel cholesterol absorption inhibitor. Proceedings of the XII International Symposium on Drugs Affecting Lipid Metabolism, Houston, TX, November 7–10, 1995.
j55
j 2 Standalone Drugs
56
60 (a) Van Heeck, M., France, C.F., Compton,
66 Bhagwagar, Z. and Goodwin, G.M. (2005)
D.S., Mcleod, R.L., Yumibe, N.P., Alton, K.B., Sybertz, E.J., and Davis, H.R., Jr. (1997) In vivo metabolism-based discovery of a potent cholesterol absorption inhibitor, SCH58235, in the rat and rhesus monkey through the identification of the active metabolites of SCH48461. J. Pharmacol. Exp. Ther., 283, 157–163; (b) Rosenblum, S.B., Huynh, T., Afonso, A., Davis, H.R., Jr., Yumibe, N., Clader, J.W., and Burnett, D.A. (1998) Discovery of 1-(4-fluorophenyl)-(3R)-[3-(4fluorophenyl)-(3S)-hydroxypropyl]-(4S)(4-hydroxyphenyl)-2-azetidinone (SCH 58235): a designed, potent, orally active inhibitor of cholesterol absorption. J. Med. Chem., 41, 973–980. Clader, J.W. (2004) The discovery of ezetemibe: a view from outside the receptor. J. Med. Chem., 47, 1–9. Altmann, S.W., Davis, H.R., Jr., Zhu, L.J., Yao, X., Hoos, L.M., Tetzloff, G., Iyer, S.P., Maguire, M., Golovko, A., Zeng, M., Wang, L., Murgolo, N., and Graziano, M.P. (2004) Niemann-Pick C1 Like 1 protein is critical for intestinal cholesterol absorption. Science, 303, 1201–1204. Garcia-Calvoa, M., Lisnock, J.M., Bull, H.G., Hawes, B.E., Burnett, D.A., Braun, M.P., Crona, J.H., Davis, H.R., Jr., Dean, D.C., Detmers, P.A., Graziano, M.P., Hughes, M., MacIntyre, D.E., Ogawa, A., ONeill, K.A., Iyer, S.A.N., Shevell, D.E., Smith, M.M., Tang, Y.S., Makarewicz, A.M., Ujjainwalla, F., Altmann, S.W., Chapman, K.T., and Thornberry, N.A. (2005) The target of ezetimibe is Niemann-Pick C1-Like 1 (NPC1L1). Proc. Natl. Acad. Sci. USA, 102, 8132–8137. Lammert, F. and Wang, D.Q.H. (2005) New insights into the genetic regulation of intestinal cholesterol absorption. Gastroenterology, 129, 718–734. Choi, H. and Morrell, M.J. (2003) Review of lamotrigine and its applications in epilepsy. Ex. Opin. Pharmacother., 4, 243–251.
Lamotrigine in the treatment of bipolar disorder. Exp. Opin. Pharmacother., 6, 1401–1408. Reynolds, E.H., Chanarin, I., Milner, G., and Matthews, D.M. (1966) Anticonvulsant therapy, folic acid and vitamin B12 metabolism and mental symptoms. Epilepsia, 7, 261–270. Leach, M.J., Marden, C.M., and Miller, A.A. (1966) Anticonvulsant therapy, megaloblastic hematopoeisis and folic acid metabolism. Q. J. Med., 35, 521–537. Baxter, M.G., Elphick, A.R., Miller, A.A., and Sawyer, D.A. (1981) 1,2,4-Triazine derivatives, pharmaceutical compositions and intermediates utilized for their preparation. European Patent 21121. Leach, M.J., Marden, C.M., and Miller, A.A. (1986) Pharmacological studies on lamotrigine, a novel potential antiepileptic drug: II. Neurochemical studies on the mechanism of action. Epilepsia, 27, 490–497. Cohen, A.F., Ashby, L., Crowley, D., Land, G., Peck, A.W., and Miller, A.A. (1985) Lamotrigine (BW430C), a potential anticonvulsant. Effects on the central nervous system in comparison with phenytoin and diazepam. Br. J. Clin. Pharmacol., 20, 619–629. Smith, D., Baker, G., Davies, G., Dewey, M., and Chadwick, D.W. (1993) Outcomes of add-on treatment with lamotrigine in partial epilepsy. Epilepsia, 34, 312–322. Leach, M.J., Randall, A.D., Stefani, A., and Hainsworth, A.H. (2002) Lamotrigine: mechanisms of action, in Antiepleptic Drugs, 5th edn (eds R.H. Levy, R.H. Mattson, B.S. Meldrum, and E. Perucca), Lippincott Williams & Wilkins, Philadelphia, pp. 363–369. Foreman, M.M., Hanania, T., Stratton, S.C., Wilcox, K.S., White, S., Stable, J.P., and Eller, M. (2008) In vivo pharmacological effects of JZP-4, a novel anticonvulsant, in models for anticonvulsant, antimania and antidepressant activity. Pharmacol. Biochem. Behav., 89, 523–534. Foreman, M.M., Hanania, T., and Eller, M. (2009) Anxiolytic effects of lamotrigine and JZP-4 in the elevated plus maze and in
61
62
63
64
65
67
68
69
70
71
72
73
74
75
References
76
77
78
79
80
81
82
83
84
85
86
87
the four plate conflict test. Eur. J. Pharmacol., 602, 316–320. Goodarzi, M.O. and Bryer-Ash, M. (2005) Metformin revisited: re-evaluation of its properties and role in the pharmacopoeia of modern antidiabetic agents. Diabetes Obesity Metab., 7, 654–665. Bailey, C.J. and Day, C. (2004) Metformin: its botanical background. Pract. Diabetes Int., 21, 115–117. Watanabe, C.K. (1918) Studies in the metabolism changes induced by administration of guanidine bases. I. Influence of injected guanidine hydrochloride upon blood sugar content. J. Biol. Chem., 33, 253–265. Frank, E., Nothmann, M., and Wagner, A. (1926) Über synthetisch dargestellte K€ orper mit insulinartiger Wirkung auf den normalen und den diabetischen organismus. Klin. Woch., 5, 2100–2107. Slotta, K.H. and Tschesche, R. (1929) Die blutzuckersenkende Wirkung der biguanide. Chem. Ber., 62, 1398–1405. Sterne, J. (1957) Du nouveau dans les antidiabetiques. La N,Ndimethylamineguanyl guanide (N.N.D.G.). Maroc Med., 36, 1295–1296. Ungar, G., Freedman, L., and Shapiro, S.L. (1957) Pharmacological studies of a new oral hypoglycaemic drug. Proc. Soc. Exp. Biol. Med., 100, 190–192. Mehnert, H. and Seitz, W. (1958) Weitere ergebnisse der diabetesbehandlung mit blutzuckersenkenden biguaniden. Munch. Med. Woch., 100, 1849–1851. Schafer, G. (1983) Biguanides: a review of history, pharmacodynamics and therapy. Diabetes Metab. Rev., 9, 148–163. Maryanoff, B.E., Nortey, S.O., Gardocki, J.F., Shank, R.P., and Dodgson, S.P. (1987) Anticonvulsant O-alkyl sulfamates. 2,3:4,5-Bis-O-(1-methylethylidene)-betaD-fructopyranose sulfamate and related compounds. J. Med. Chem., 30, 880–887. Maryanoff, B.E. and Gardocki, J.F. (1985) Anticonvulsant sulfamate derivatives, US Patent 4,513,006. Van Passel, L., Arif, H., and Hirsch, L.J. (2006) Topiramate for the treatment of epilepsy and other nervous system disorders. Exp. Rev. Neurother., 6, 19–31.
88 http://www.orangebookblog.com/
Ortho_20motion_20PI.pdf(2006). 89 Rogawski, M.A. (2006) Molecular targets
90
91
92
93
94
95
96
versus models for new antiepileptic drug discovery. Epilepsy Res., 68, 22–28. Maryanoff, B.E., Costanzo, M.J., Shank, R.P., Schupsky, J.J., Ortegon, M.E., and Vaught, J.L. (1993) Anticonvulsant sugar sulfamates. Potent cyclic sulfate and cyclic sulfite analogues of topiramate. Bioorg. Med. Chem. Lett., 3, 2653–2656. Maryanoff, B.E., Costanzo, M.J., Nortey, S.O., Greco, M.N., Shank, R.P., Schupsky, J.J., Ortegon, M.E., and Vaught, J.L. (1998) Structure–activity studies on anticonvulsant sugar sulfamates related to topiramate. Enhanced potency with cyclic sulfate derivatives. J. Med. Chem., 41, 1315–1343. Parker, M.H., Smith-Swintosky, V.L., McComsey, D.F., Huang, Y., Brenneman, D., Klein, B., Malatynska, E., White, H.S., Milewski, M.E., Herb, M., Finley, M.F.A., Liu, Y., Lubin, M.L., Qin, N., Iannucci, R., Leclercq, L., Cuyckens, F., Reitz, A.B., and Maryanoff, B.E. (2009) Novel, broad-spectrum anticonvulsants containing a sulfamide group: advancement of N-((benzo[b]thien-3-yl)methyl)sulfamide (JNJ-26990990) into human clinical studies. J. Med. Chem., 52 (23), 7528–7536. Parker, M.H., Reitz, A.B., and Maryanoff, B.E., Preparation of benzo-fused heteroaryl sulfamide derivatives for the treatment of epilepsy and related disorders, WO2006023861(2006). McComsey, D.F., Parker, M.N., Reitz, A.B., and Maryanoff, B.E.,Preparation of sulfamates and sulfamides with oxygen containing ring systems useful for the treatment of epilepsy and related disorders, WO2006007435(2006). Gavernet, L., Barrios, I.A., Sella Cravero, M., and Bruno-Blanchl, L.E. (2007) Design, synthesis, and anticonvulsant activity of some sulfamides. Bioorg. Med. Chem., 15, 5604–5614. Sneader, W. (1995) Drug Prototypes and their Exploitation, John Wiley & Sons, Ltd., Chichester, p. 752.
j57
j 2 Standalone Drugs
58
97 Henry, T.R. (2003) The history of valproate
in clinical neuroscience. Psychopharmacol. Bull., 37 (Suppl. 2), 5–16. 98 Rogawski, M.A. (2006) Diverse mechanisms of antiepileptic drugs in the development pipeline. Epilepsy Res., 69, 273–294.
99 Pessah, N., Bialer, M., Wlodarczyk, B.,
Finnell, R.H., and Yagen, B. (2009) 1Fluoro-2,2,3,3-tetramethylcyclopropane carboxamide, a novel potent anticonvulsant derivative of a cyclic analogue of valproic acid. J. Med. Chem., 52, 2233–2242.
Janos Fischer
Professor, Richter Plc., Gyömro˝i út 19/21, 1103 Budapest, Hungary Janos Fischer is a Senior Research Scientist at Richter Plc., Budapest, Hungary. He received his MSc and PhD degrees in organic chemistry from the Eotvos University of Budapest under Professor A. Kucsman. Between 1976 and 1978, he was a Humboldt Fellow at the University of Bonn under Professor W. Steglich. He has worked at Richter Plc. since 1981 where he participated in the research and development of leading cardiovascular drugs in Hungary. His main interest is analoguebased drug discovery. He is the author of some 100 patents and scientific publications. In 2004, he was elected as a Titular member of the Chemistry and Human Health Division of IUPAC. He received an honorary professorship at the Technical University of Budapest.
C. Robin Ganellin
Professor, University College London, Department of Chemistry, 20 Gordon Street, London WC1H 0AJ, UK C. Robin Ganellin studied chemistry at London University, receiving a PhD in 1958 under Professor Michael Dewar, and was a Research Associate at MIT with Arthur Cope in 1960. He then joined Smith Kline & French Laboratories in the UK and was one of the coinventors of the revolutionary drug, cimetidine (also known as Tagamet). In 1986, he was made a Fellow of the Royal Society and appointed to the SK&F Chair of Medicinal Chemistry at University College London, where he is now Professor Emeritus of Medicinal Chemistry. Professor Ganellin is coinventor of over 160 patents and has authored over 260 scientific publications. He was President of the Medicinal Chemistry Section of the IUPAC and is Chairman of the IUPAC Subcommittee on Medicinal Chemistry and Drug Development.
References
Arun Ganesan
Professor, University of Southampton, School of Chemistry, Southampton S017 1BJ, UK A. Ganesan was born in Malaysia and graduated from the National University of Singapore, with a degree in chemistry and microbiology (1985) and an Honours degree in chemistry (1986). He obtained his PhD in 1992 from the University of California-Berkeley, working under the supervision of Professor Clayton H. Heathcock in the area of synthetic methodology and natural product total synthesis. Following a postdoctoral position with Professor Gregory L. Verdine at Harvard University in 1992–1993 working on DNA binding proteins, he began his independent career in 1993 as a Senior Research Chemist at the Center for Natural Product Research in Singapore. In 1996, he was promoted to Principal Investigator at the Institute, where he headed the Medicinal and Combinatorial Chemistry Group. He moved to Southampton in 1999 as a Reader to join National Combinatorial Center of Excellence. Ganesan is a Treasurer of the Royal Society of Chemistrys High Throughput Chemistry and New Technologies subject group and a member of the Organic Division Executive. He serves on the IUPAC Subcommitee for Medicinal Chemistry and Drug Development. He is a past Chair of the Gordon Research Conference and was awarded a JSPS fellowship as visiting professor at the Tokyo Institute of Technology, Japan. He is on the advisory board of QSAR and Combinatorial Science and Current Chemical Biology. He is a cofounder and Director of Karus Therapeutics (http://www.karustherapeutics.com), a spin-off of the University of the Southampton, and a Business Fellow of the London Technology Network. John Proudfoot
Boehringer Ingelheim Pharmaceuticals Inc., 900 Ridgebury Rd., Ridgefield, CT 06877-0368, USA John Proudfoot is a Medicinal Chemist with over 20 years of experience in drug design, ranging from fragment-based lead identification to late-stage lead optimization. He received his doctorate from the University College Dublin in Ireland, working with Professor Dervilla Donnelly. After postdoctoral studies under Professor Carl Djerassi at Stanford University and under Professor John Cashman at the University of California, San Francisco, he joined Boehringer Ingelheim Pharma in 1987. He is a coinventor of 28 issued US patents, including the one for the marketed drug, nevirapine, the first nonnucleoside HIV-1 reverse transcriptase inhibitor to receive regulatory approval.
j59
j61
3 Application of Molecular Modeling in Analogue-Based Drug Discovery Gy€orgy G. Ferenczy
3.1 Introduction
Analogue-based drug discovery benefits from molecular modeling and computational chemistry in various ways. The search for new medicaments that are similar to existing ones is an approach where the knowledge accumulated for the parent molecule drives drug discovery efforts. This requires a thorough understanding of all aspects of the effect of the parent molecule and this is supported by computational studies ranging from bioinformatics to pharmacokinetics. When research is not restricted to close structural analogues, the role of molecular modeling includes the extension of the notion of analogy; important features related to, but not apparent from the structural formula, can be identified and exploited in the design of analogues. One can mention, as examples, the better appreciation of three-dimensional structures by conformational analysis, the derivation of certain properties from the electronic structure of molecules and pharmacophore models that help to distinguish between properties that are important and those that are unimportant for binding to the target. The identification of features or structural motifs that are not relevant for interaction with the target is usually also fundamental, as it helps to design molecules that are outside competitor patent space that is a critical point in analogue-based research. Moreover, the unimportant parts of a molecule are suitable points for modifications in order to improve selectivity profile and pharmacokinetic properties that may be crucial aspects in developing an analogue with properties superior to the original molecule. The availability of information for the original molecule is highly advantageous for modeling approaches. In fact, a critical aspect of molecular modeling is to apply it at the appropriate time when, on the one hand, enough information is available to develop predictive models and, on the other hand, the model is able to provide nonobvious information that represents a useful contribution to the research. Analogue-based discovery, by definition, is based on information gathered for the original drug molecule that can be used as input for modeling work. Structure-based approaches that exploit the atomic resolution structure of the target are efficient and increasingly applied in molecular modeling and they may also
Analogue-based Drug Discovery II. Edited by János Fischer and C. Robin Ganellin Copyright Ó 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32549-8
j 3 Application of Molecular Modeling in Analogue-Based Drug Discovery
62
be applied in analogue-based research. Experimental or homology-based enzyme/ receptor structures may make it possible to design structurally loosely related functional analogues. The above discussion already indicates that, although analogue-based drug research has some particular features, the molecular modeling approaches applied are not specific. This is all the more so since molecular modeling – and we can state that to a large extent drug discovery in general – is largely based on the concept of analogy even if research is directed to the discovery of a novel molecule rather than an analogue of an existing drug. One can mention here that lead optimization is basically the search of an improved analogue of the lead molecule, and even lead identification by virtual screening often exploits the information available for known ligands of a target protein to optimize the virtual screening protocol. In this chapter, the discussion is restricted to examples where the lead is a drug, drug candidate, or an endogenous ligand of the target. Perhaps, it is not fair to term some of these drugs as analogues for two reasons. One is that they deviate considerably from their parent structure and this is partly due to the involvement of molecular modeling in the research process. The other reason is that when the parent molecule is not a marketed drug that already passed all stages of the development, then the research directed toward a structural analogue has an increased risk. In the past, molecular modeling was considered to be most useful in optimizing binding to the target. However, with increasing importance of ADMET properties in the early phases of drug research, molecular modeling plays an increasingly important role in optimizing properties such as metabolism, CYP inhibition and induction, and hERG channel activity. Nevertheless, these aspects of molecular modeling are less pronounced in the following discussions mainly because these drug discovery programs started several years ago. The examples presented are based on scientific publications that describe, quote, or at leastindicatetheuseofmodelingtechniquesinthediscoveryofanalogues.Incertaincases, the discovery process has been published in detail, while in some other cases it can only be approximately recovered. Thus, the examples presented may not reflect the complete contribution of modeling techniques in the course of a particular analogue research project. Nevertheless, they illustrate the contribution of molecular modeling in this field. No attempt has been made to describe all aspects of the discovery processes; rather the focus is on molecular modeling contributions. The research that led to the drugs or drug candidates described below began several years ago and since then, structural information on some of the targets has become available. This makes it possible to reevaluate certain hypotheses and observations made in the course of the discovery and it is particularly instructive for future applications.
3.2 Cilazapril: An ACE Inhibitor
Angiotensin-converting enzyme (ACE), part of the renin–angiotensin system, is an appropriate target for antihypertensive drugs [1]. Captopril (1) of Bristol-Meyers
3.2 Cilazapril: An ACE Inhibitor
Squibb was the first marketed nonpeptidic ACE inhibitor to be launched in the early 1980s. N SH
O O
OH
1, captopril
The development of captopril was preceded by the identification of a polypeptide that was first isolated and then synthesized and was shown to inhibit ACE [2]. A schematic model of the ACE active site (vide infra) was built after the recognition of its similarity to pancreatic carboxypeptidase A [3]. The model guided structure–activity studies that led to the discovery of SQ20881, a nonapeptide, and later to captopril. The clinical investigations of these compounds proved that they are effective antihypertensives. Side effects of captopril were attributed to the mercapto function and its replacement by weaker chelating functions led to the discovery of enalapril (2, Merck) [4].
O
O N H
N O O
OH
2, enalapril
The design of another ACE inhibitor by researchers at Roche is a well-documented example where computational methods made a significant contribution. The research used as starting points the two-dimensional model of the active site [3] (Figure 3.1) and the earlier structure–activity studies of captopril and related compounds [3–6]. An analysis of the available conformations of captopril generated limits for the relative positions of the three interaction points defined by the carboxy, amide, and thiol functions [7]. Three related bicyclic systems (3, 4, 5) were chosen so that they were able to hold these functions and their functionalization was expected to predictably influence their conformations. O HS N n
N N
O O 3
OH
j63
j 3 Application of Molecular Modeling in Analogue-Based Drug Discovery
64
O R1
N
R2
N O O
OH
4 O
R1 R2 O
COOH 5
A series of compounds was prepared, conformations were investigated by NMR, and X-ray crystallography and biological activities were measured. The collected data revealed relationships between the relative positions of the functional groups and the ACE inhibition demonstrated by the molecules. Even the best molecules were, however, less active than captopril and enalapril. This was attributed to the finding that, according to NMR and X-ray experiments, the ring systems hold the functional groups in a relative position that corresponds to a high-energy conformation of alanylproline as it was calculated by molecular mechanics [8]. Therefore, new 7,6 and 8,6-bicyclic systems (6, 7) were synthesized in accordance with molecular graphic studies that suggested conformations with appropriate orientation of the carboxyl, carbonyl, and zinc-coordinating groups.
“CARBOXYPEPTIDASE A”
Zn++
NH
R
O
CH
C
“ANGIOTENSIN–CONVERTING ENZYME”
Zn++
+
CH2 O– NH
CH
C O
NH
R2
O
CH
C –O
–O
O C
C O
NH
R1
O
CH
C
R3
O
O C
CH2 CH
C
–S
CH2 CH
CH2 O– CH2 CH
H
+
R NH CH
C O O–
N
CH
N
CH
C O O–
CH3 O C
O–
C O
Figure 3.1 Schematic representation of the binding of substrates and inhibitors at the active site of pancreatic carboxypeptidase A, and the hypothetical active site of angiotensin-converting enzyme. Reprinted with permission from Ondetti et al. [3]. Copyright (1977) AAAS.
3.2 Cilazapril: An ACE Inhibitor
Y N N R2
O
CO2 R1
6
Y N N R2
O
CO2R1
7
Among the synthesized compounds, a dicarboxylic acid derivative of 7 was found to be the most active ACE inhibitor. Its monoethylester, cilazapril (8), is a prodrug with superior biological characteristics [8] and was launched in 1990.
N O
N
O
N H
O
CO2H
8, cilazapril
Cilazapril thus contains interacting functions similar to capropril, but the chemical nature of these functions and the skeleton that holds them are significantly different. The replacement of the thiol group with homophenylalanine and the introduction of the ester group are equivalent to the modifications appearing in enalapril with respect to captopril. On the other hand, a completely new feature of cilazapril is the bicyclic skeleton that is able to present the functional groups in a favorable orientation, and owing to its rigidity offers free energy gain by decreasing the entropy penalty upon binding. The crystal structures of several ACE–ligand complexes were published later. They include the complexes of captopril and enalapril [9]. The experimental structures confirm the presence of the presumed principal interactions, and the conformations of the ligands in the complex are consistent with the results of modeling studies (Figure 3.2).
j65
j 3 Application of Molecular Modeling in Analogue-Based Drug Discovery
66
Figure 3.2 Important interactions in the Captopril–ACE complex (PDB ID: 1UZF [9]). Captopril carbons are shown in green and carbons of interacting residues are white. N-atoms are blue, O-atoms are red, and the
S-atom is yellow. Only polar H-atoms are shown and they are colored blue green. H-bonds are indicated by dashed yellow lines and other interactions are indicated by magenta lines.
3.3 Atorvastatin: A HMG-CoA Reductase Inhibitor
The biosynthesis of cholesterol involves a key step in which 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) is transformed to mevalonate by HMG-CoA reductase (HMGR). The inhibition of this process is an effective way to decrease serum cholesterol levels, and hypocholesterolemic agents decrease the risk of coronary artery disease [10]. The first HMGR inhibitors discovered were either fungal metabolites or synthetic derivatives thereof, such as compactin (9, Sankyo), lovastatin (10, Merck & Co), simvastatin (11, Merck & Co), and pravastatin (12, Sankyo). O
HO O
O O H
9, compactin
3.3 Atorvastatin: A HMG-CoA Reductase Inhibitor
O
HO O
O O
H
10, lovastatin
O
HO O
O O
H
11, simvastatin
O
OH
HO HO
O O
H
HO
12, pravastatin
Another series of HMGR inhibitors (13) with a different lipophilic group was disclosed by Merck Sharpe and Dohme [11–13].
j67
j 3 Application of Molecular Modeling in Analogue-Based Drug Discovery
68
O
HO O
A
B Y X
13
This initiated at Parke-Davis a search for inhibitors containing a lactone and a hydrophobic moiety together with the optimal spacer in between. A series of molecules with the general formula 14 was synthesized and their cholesterol synthesis inhibition was measured [14]. O
HO O X R1
N
R2
14
First, X ¼ ethyl was identified as the optimal spacer. Then, structure–activity studies and the comparative conformational analysis of this series, compactin (9), and a molecule in the Merck series (13) led to the proposal that the X ¼ ethyl bridge in 14 (ethylene 13) adopts a nearly perpendicular position with respect to the pyrrole ring. For certain molecules, for example, Merck (13), this is the only allowed conformation, while in some other cases, the observed lower potency is attributed to the fact that this conformation is energetically disfavored. The authors of Ref. [14] also note that the calculated minimum energy conformation does not agree with the one found experimentally in the crystal of compactin [15]. The latter X-ray conformation corresponds to a secondary minimum with slightly higher energy. On the basis of the observed SAR of the series 14, it was postulated that for an optimal potency the extension of the R1 and R2 substituents cannot exceed 5.9 and 3.3 A, respectively. Further studies aimed at finding correlation between the potency and the charge distribution of the R1 group. This was prompted by the polar nature of the isobutyric ester containing fungal metabolite inhibitors (cf. molecules 9 and 10). However, it was concluded that potency is insensitive to the polarity of the R1 group. The relatively low potency of compounds of type 14 with respect to certain members of series 13 prompted the structural comparison of these molecules. The superposition of the structures revealed that a methyl group (in para position with
3.3 Atorvastatin: A HMG-CoA Reductase Inhibitor
O
HO O F F H3C
N
CH3
CH3 Figure 3.3 Overlay of HMGR inhibitor templates. Drawing from Roth [16], with permission from Elsevier. Copyright (2002).
respect to the ethylene) in 13 occupies a position that is vacant in series 14 [16] (Figure 3.3). Indeed, the 3,4-dichloro- and 3,4-dibromo-pyrrole derivatives of 14 exhibited a significant potency increase. As the dihalogen-substituted compounds were found to be toxic, additional substituents were investigated [16, 17]. Among them, the Ph and CONHPh substitutions were found to be highly potent and appear in atorvastatin calcium (15, Parke-Davis) that has been marketed since 1997. 2+
Ca
O
O HO HO F N O HN
2 15, atorvastatin calcium
The X-ray structure of HMGR-atorvastatin [18] complex makes it now possible to interpret former findings and to assess the predictions made by modeling (Figure 3.4). The binding pocket and its interactions with the ligand are in line with former observations concerning the size restriction of the R1 and R2 substituents of molecule 14, the insensitivity of the affinity to the polarity of R1 and the availability
j69
j 3 Application of Molecular Modeling in Analogue-Based Drug Discovery
70
Figure 3.4 Atorvastatin in the binding pocket of HMGR (PDB ID: 1HWK [18]). The pocket is colored according to the electrostatic potential: blue – negative; red – positive. Annotations highlight features discussed in the text.
of space for substituents at the 3 and 4 positions of pyrrole. Moreover, the predicted nearly perpendicular orientation of the ethyl spacer with respect to the pyrrole ring was indeed observed in the experimental complex structure.
3.4 PDE4 Inhibitors
Phosphodiesterases (PDEs) are involved in the transformation of cyclic nucleotides into their corresponding 50 -monophosphates. Several subtypes of PDEs have been described. PDE4 is a cyclic 30 ,50 -adenosine monophosphate (cAMP) specific isozyme that is dominant in immune and inflammatory cells and also in airway smooth muscle. The inhibition of PDE4 is proposed for the treatment of asthma and chronic obstructive pulmonary disease (COPD) [19]. A pioneering compound in PDE4-specific inhibition is rolipram (16) developed at Schering AG. O NH
O
O 16, rolipram
This compound did not reach the market due to its undesirable side effects, but it served as a prototype of selective PDE4 inhibitors. A large number of rolipram
3.4 PDE4 Inhibitors
congeners have been synthesized and tested for PDE4 inhibition. These studies led to the identification of the features essential for PDE4 binding. This pharmacophore model helped to find inhibitors with an improved side-effect profile compared to rolipram. The structure–activity studies identified a dialkoxyphenyl group and a hydrogen bond acceptor (carbonyl of pyrrolidone in rolipram) as a minimum requirement for efficient PDE4 binding [20, 21]. It was also found that steric hindrance limits the size of the para-alkoxy substituent. The relative orientation of the alkoxy groups and the hydrogen acceptor moiety was investigated by conformational analysis of rolipram analogues and by superposition of some rigid molecules. No unambiguous assignment of a bioactive conformation was possible, but investigations suggested a basically extended conformation in which the carbonyl group points away from the para-alkoxy substituent [20, 21]. The typical distance between the dialkoxy phenyl group and the carbonyl dipole was proposed to have a value of 8–9 A [20]. The replacement of the pyrrolidone in rolipram by a 1,4-substituted cyclohexane ring fulfilled the criteria set by the above-described pharmacophore model and allowed extension of the structure–activity studies [21]. The introduction of the cyclohexane ring offered the advantage of a more rigid structure without a chiral center. The optimization of this series led to the discovery of cilomilast (17, GlaxoSmithKline) that was subject of an approval letter by FDA. Later, however, cilomilast development was discontinued. O
N O
O
OH 17, cilomilast
In other rolipram-based series, the pyrrolidone ring was replaced by benzamide derivatives and analogues. Among them, roflumilast (18, Nycomed) is under registration [22].
O O Cl O
F
N
N H
F
Cl 18, roflumilast
The crystal structures of several PDE4 complexes have been published since 2000 [23] and it is instructive to compare these experimental complex structures with the pharmacophore model that was derived with no atomic resolution structural knowledge of the target protein. The complexes of rolipram [24–27], cilomilast [27],
j71
j 3 Application of Molecular Modeling in Analogue-Based Drug Discovery
72
and some more molecules confirm the crucial role of the dialkoxyphenyl moiety. The two ether oxygen atoms are hydrogen bonded to the side-chain amide N-atom of a glutamine residue, and the phenyl ring is surrounded by hydrophobic residues. This part of the molecule essentially replaces the adenine part of cAMP, the natural ligand of PDE4 [28]. The role of the pyrrolidone ring is not straightforward to interpret. In all except one X-ray structure [25], it apparently does not make any direct interaction with either the metals or the enzyme residues. The pyrrolidone carbonyl group is hydrogen bonded to a water molecule that, in turn, is hydrogen bonded to other water molecules having hydrogen bonds with the enzyme. The lack of more direct interactions is somewhat unexpected as the sensitivity of the binding affinity to this structural motif of the ligand was clearly observed. On the other hand, the comparison of the structures of rolipram and cilomilast (Figure 3.5) reveals that the relative position of the hydrogen bond acceptor with respect to the dialkoxy groups is not unequivocally defined, even though the presence of both seems to be important for binding. A possible explanation is that the ligand protrudes into an ordered set of water molecules contributing to the coordination of the metal ions and residues in the pocket and then for an efficient binding, the ligand has to provide appropriately positioned functions whose interactions are able to compensate for the disturbed hydrogen bonding network. An additional difficulty in interpreting rolipram binding has also to be noted. Rolipram binds to the full-length PDE4 with two different affinities [29], but the high-affinity binding is not observed in the truncated protein containing the catalytic domain appearing in the X-ray structures. As the large PDE4 binding pocket is not fully occupied by either rolipram or cilomilast, larger molecules may also be well accommodated. However, they can bind at the expense of expelling ordered water molecules. The free energy balance of such a binding is difficult to predict, although results in Ref. [30] indicate that it may be advantageous in the case of large-enough enthalpy gain. This would open the possibility to design inhibitors that create more direct interactions with the enzyme.
Figure 3.5 Superposition of rolipram (blue; PDB ID: 1OYN [24]) and cilomilast (brown; PDB ID: 1XOM [27]) in the PDE4 binding pocket. The water molecules are removed.
3.5 GPIIb/IIIa Antagonists
3.5 GPIIb/IIIa Antagonists
GPIIb/IIIa is a membrane-associated glycoprotein belonging to the integrin family of receptors. It plays a critical role in platelet aggregation. Antagonists prevent aggregation and thus thrombosis and are potentially suitable for the treatment of cardiovascular, cerebrovascular, and peripheral vascular diseases [31]. The peptide sequence Arg-Gly-Asp (RGD) has been recognized as the adhesive motif present in macromolecules, such as fibrinogen, that mediate platelet aggregation. Fibrinogen is a covalent dimer, the two halves of which have identical sequences, and it binds to GPIIb/IIIa on adjacent platelets and cross-links them. Small RGD containing peptides inhibit fibrinogen binding to platelets and inhibit platelet aggregation [32]. Thus, small peptides, peptidomimetics, or other molecules capable of binding to the RGD recognizing motif of GPIIb/IIIa may find therapeutic applications [33], and research was focused on developing RGD analogues. SK&F-107260 (19, developed by SmithKline Beecham) is a cyclic RGD analogue that owing to its more rigid structure exhibits higher affinity for GPIIb/IIIa than for RGD itself.
S O H2N
N
H N
S HN O
H N
O OH
N H
O
NH
O
19, SK&F-107260
The synthetic peptide eptifibatide (20, jointly developed by COR Therapeutics and Schering-Plough and launched in 1998) is also an RGD analogue whose improved profile is probably also due to its relative rigidity. NH NH2 O
N H H N
HN O
OH
O
O
N H
O
HN O
S
S H2N
N H O
20, eptifibatide
O N
NH
j73
j 3 Application of Molecular Modeling in Analogue-Based Drug Discovery
74
Structure–activity studies with peptides identified the critical structural characteristics for RGD analogues. They include a basic group (guanidino of arginine) and an acidic group (carboxylate of aspartic acid) with a distance of 12–15 A between them. The spacer exerts an effect on the affinity by its length, rigidity, influence on the acidity and basicity of the end groups, and potentially also through direct interactions with the receptor. A large number of structural studies on the conformation of RGD containing peptides and mimics have been performed to clarify the relative orientation of the basic and acidic groups. The information accumulated suggests that the end groups are optimally linked by a linear spacer whose rigidity is a significant factor for affinity (see, e.g., Ref. [34]). Indeed, it was found that affinity is a linear function of the number of rotatable bonds with a slope of 0.7 kcal/mol (G.G. Ferenczy, unpublished), the value associated with the free energy cost of freezing a rotatable bond upon binding. Several small molecules that bind to the GPIIb/IIIa receptor and inhibit platelet aggregation in the nanomolar range have been described. An early representative, tirofiban hydrochloride [35] (21, Merck & Co), has been marketed since 1998. O S O OH
HN
O O H Cl
21, tirofiban hydrochloride
3.6 HIV Protease Inhibitors
Human immunodeficiency virus (HIV) is the causative agent of acquired immunodeficiency syndrome (AIDS). HIV protease is responsible for the cleavage of precursor polyproteins to structural and functional proteins. This aspartic protease is one of the targets of anti-HIV therapy. First-generation protease inhibitors (PIs), such as saquinavir (22), indinavir (23), ritonavir (24), nelfinavir (25), and amprenavir (26), are all transition-state analogues with a hydroxyethylene core.
O H N
N
O N H
O 22, saquinavir
NH2 O
N OH H
H N
H
3.6 HIV Protease Inhibitors
N
OH
H N
N
N O
NH
N
H N
OH
O
23, indinavir S N
OH
O N H
O
S
H N
O
N
O
24, ritonavir
HO
H N
O
S
O
N
N H
H
OH H
25, nelfinavir
H N
O O
O
OH N O
S
NH2 O
26, amprenavir
The rapid discovery and development of these peptidomimetic agents were supported by atomic resolution X-ray crystallographic structures of the HIV protease and its complexes. These first-generation PIs have several shortcomings. Most important is drug resistance that comes from mutations as the response of the virus to drug treatments. In addition, inadequate pharmacokinetics manifests itself in one or several ways such as short plasma half-life, poor aqueous solubility, high protein binding, and modest oral bioavailability.
j75
j 3 Application of Molecular Modeling in Analogue-Based Drug Discovery
76
The development of second-generation PIs was focused on addressing the abovementioned issues. First-generation PIs were used as starting points to develop improved agents. In order to reduce the effect of mutations on the activity of inhibitors, some interactions with the side chains of HIV protease were replaced by interactions with the protein backbone. The search for improved PIs and the interpretation of their activity were supported by structural biology and molecular modeling. The analysis of the X-ray structure of HIV protease with ritonavir (24) revealed that the isopropyl group on the thiazolyl ring interacts with the isopropyl group of the Val82 side chain of the S3 pocket [36]. This is in line with the reduced Ki value of ritonavir toward proteases mutated at this position. The omission of the P3 group reduced the activity but with the advantage of low serum binding. The activity was partially regained with the introduction of conformational constraint by cyclic urea (27) [36].
OH
O N
HN
N H
O
S
H N
N
O O
27, A-1555564
Twoalternativemodelsforthepositionandinteractionsofthecyclicureamoietywereset up. One of them is hydrogen-bonded with Asp29 and the other is hydrogen-bonded with Asp30 of the HIV protease. The degree of the intramolecular van der Waals interaction between the urea and the P1 benzyl group is also different in the two models [36]. Finally, the P20 (thiazolyl)methoxycarbonyl group was replaced by the dimethylphenoxyacetyl group leading to lopinavir (28).
OH
O N
HN O
N H
H N
O O
28, lopinavir
HIV protease inhibitors with a dimethylphenoxyacetyl group were first described in Refs [37, 38]. The X-ray structures of complexes with this group showed a shift of main-chain and side-chain atoms of residues 29 and 30 revealing a new region of conformational flexibility in the HIV protease [37]. The advantage of the dimethyl with respect to the monomethyl substitution was attributed, in part, to more favorable binding entropy of the former, as it always puts one of the methyl groups in a position with a beneficial interaction. The X-ray structure also showed that the methyl group
3.6 HIV Protease Inhibitors
j77
Table 3.1 Similarity coefficients between parent compounds and optimized drugs.
Parent compound
Optimized compound
Similarity coefficient [41]
N O
N
O
N SH
N H
O O
O
O
0.34
O
OH cilazapril
captopril
O
HO O
O O
H
O
O
0.14
HO compactin
HO O
HO
F O
0.29
N
F
O HN Cl atorvastatin
Ref. [13] compound 100
O O Cl
O NH
O
O
F F
O
rolipram
N
N H
0.27
Cl roflumilast (Continued)
j 3 Application of Molecular Modeling in Analogue-Based Drug Discovery
78
Table 3.1 (Continued)
Parent compound
Optimized compound
Similarity coefficient [41]
NH H2N
H N
O
N H
NH2
O
O
N H
S
HO
HN
O
HO
0.43
O OH O
O
O tirofiban hydrochloride
RGD S N HN N O O
N
NH
O
O HN HN
NH
NH O
0.39
OH
OH
O
O
O
S N ritonavir
lopinavir
has an optimal size and that larger substituents on the phenyl ring probably result in clashes with protein residues [38]. The X-ray crystal structure of lopinavir in complex with the HIV protease was published in 2002 [39]. It confirmed that in line with the design strategy the interaction of lopinavir with Val82 is significantly reduced. The dimethylphenoxyacetyl group occupies a position that was anticipated. The cyclic urea binds both to the backbone and to the side chain of Asp29 and it is accompanied with a slight movement of the protein backbone near Gly48. Lopinavir is an improved analogue of ritonavir with an increased HIV protease inhibitory activity that is moderately affected by mutations of Val82. Its bioavailability is highly enhanced when coadministered with ritonavir, a potent CYP3A4 inhibitor [40]. Lopinavir, coformulated with ritonavir, was launched by Abbott first in the United States in 2000.
References
3.7 Epilogue
Five examples of analogue-based drug discovery were presented. In all these cases, molecular modeling made a substantial contribution to the introduction of significant structural modifications. An indication of the important differences between the parent compounds and the optimized drugs is their low similarity coefficients presented in Table 3.1 [41]. In all these examples, the focus was on the interaction with the target and how the information and models available for the ligand–target interaction could be used to keep or improve affinity while other, typically pharmacokinetic, characteristics were optimized. On the other hand, modeling and computational methods are more and more able to predict properties relevant to pharmacokinetics, thus providing additional support to the optimization in analogue-based drug discovery projects.
References 1 Wong, J., Patel, R.A.,and Kowey, P.R.
2
3
4
5
(2004) The clinical use of angiotensinconverting enzyme inhibitors. Prog. Cardiovasc. Dis., 47, 116–130. Ondetti, M.A., Williams, N., Sabo, E.F., Pluscec, J., Weaver, E.R.,and Kocy, O. (1971) Angiotensin-converting enzyme inhibitors from the venom of Bothrops jararaca. Isolation, elucidation of structure, and synthesis. Biochemistry, 10, 4033–4039. Ondetti, M.A., Rubin, B.,and Cushman, D.W. (1977) Design of specific inhibitors of angiotensin-converting enzyme: new class of orally active antihypertensive agents. Science, 196, 441–444. Patchett, A.A., Harris, E., Tristram, E.W., Wyvratt, M.J., Wu, M.T., Taub, D., Peterson, E.R., Ikeler, T.J., Ten Broeke, J., Payne, L.G., Ondeyka, D.L., Thorsett, E.D., Greenlee, W.J., Lohr, N.S., Hoffsommer, R.D., Joshua, H., Ruyle, W.V., Rothrock, J.W., Aster, S.D., Maycock, A.L., Robinson, F.M., Hirschmann, R., Sweet, C.S., Ulm, E.H., Gross, D.M., Vassil, T.C.,and Stone, C.A. (1980) A new class of angiotensinconverting enzyme inhibitors. Nature, 288, 280–283. McEvoy, F.J., Lai, F.M.,and Albright, J.D. (1983) Antihypertensive agents:
6
7
8
9
angiotensin converting enzyme inhibitors. 1-[3-(Acylthio)-3aroylpropionyl]-L-prolines. J. Med. Chem., 26, 381–393. Kim, D.H., Guinosso, C.J., Buzby, G.C., Herbst, D.R., McCaully, R.J., Wicks, T.C., and Wendt, R.L. (1983) (Mercaptopropanoyl)indoline-2carboxylic acids and related compounds as potent angiotensin converting enzyme inhibitors and antihypertensive agents. J. Med. Chem., 26, 394–403. Hassall, C.H., Kr€ohn, A., Moody, C.J.,and Thomas, W.A. (1984) The design and synthesis of new triazolo, pyrazolo-, and pyridazo-pyridazine derivatives as inhibitors of angiotensin converting enzyme. J. Chem. Soc. Perkin. Trans. I, 155–164. Attwood, M.R., Hassal, C.H., Kr€ohn, A., Lawton, G.,and Redshaw, S. (1986) The design and synthesis of the angiotensin converting enzyme inhibitor cilazapril and related bicyclic compounds. J. Chem. Soc. Perkin. Trans. I, 1011–1019. Natesh, R., Schwager, S.L.U., Evans, H.R., Sturrock, E.D.,and Acharya, K.R. (2004) Structural details on the binding of antihypertensive drugs captopril and enalaprilat to human testicular
j79
j 3 Application of Molecular Modeling in Analogue-Based Drug Discovery
80
10
11
12
13
14
15
16
angiotensin I-converting enzyme. Biochemistry, 43, 8718–8724. Eisenberg, D.A. (1998) Cholesterol lowering in the management of coronary artery disease: the clinical implications of recent trials. Am. J. Med., 104 (Suppl. 1), 2S–5S. Williard, A.K., Novello, F.C., Hoffman, W.F.,and Cragoe, E.J., Jr., Substituted pyranone inhibitors of cholesterol synthesis. US Patent 4,459,422 (1984). Stokker, G., Hoffman, W., Alberts, A., Cragoe, E., Deana, A., Gilfillan, J., Huff, J., Novello, F., Prugh, J., Smith, R.,and Willard, A. (1985) 3-Hydroxy-3methylglutaryl-coenzyme A reductase inhibitors. I. Structural modification of 5-substituted 3,5-dihydroxypentanoic acids and their lactone derivatives. J. Med. Chem., 28, 347–358. Stokker, G.E., Alberts, A.W., Anderson, P.S., Cragoe, E.J., Gilfillan, A.A., Hirshfield, J., Holtz, W.J., Hoffman, W.F., Huff, J.W., Lee, T.J., Novello, F.C., Prugh, J.D., Rooney, C.S., Smith, R.L.,and Willard, A.K. (1986) 3-Hydroxy-3methylglutaryl-coenzyme A reductase inhibitors. 3. 7-(3,5-Disubstituted-[1,10 biphenyl]-2-yl)-3,5-dihydroxy-6-heptenoic acids and their lactone derivatives. J. Med. Chem., 29, 170–181. Roth, B.D., Ortwine, D.F., Hefle, M.L., Stratton, C.D., Sliskovic, D.R., Wilson, M.W.,and Newton, R.S. (1990) Inhibitors of cholesterol biosynthesis. 1. trans-6-(2-Pyrrol-1-ylethyl)-4hydroxypyran-2-ones, a novel series of HMG-CoA reductase inhibitors. 1. Effects of structural modifications at the 2- and 5-positions of the pyrrole nucleus. J. Med. Chem., 33, 21–31. Endo, A., Kuroda, M.,and Tanzawa, K. (1976) Competitive inhibition of 3hydroxy-3-methylglutaryl coenzyme a reductase by ML-236A and ML-236B fungal metabolites, having hypocholesterolemic activity. FEBS Lett., 72, 323–326. Roth, B.D. (2002) The discovery and development of atorvastatin, a potent novel hypolipidemic agent. Progr. Med. Chem., 40, 1–22.
17 Roth, B.D., Blankley, C.J., Chucholowski,
18
19
20
21
22 23
24
A.W., Ferguson, E., Hoefle, M.L., Ortwine, D.F., Newton, R.S., Sekerke, C.S., Sliskovic, D.R., Stratton, C.D.,and Wilson, M.W. (1991) Inhibitors of cholesterol biosynthesis. 3. Tetrahydro-4-hydroxy-6[2-(1H-pyrrol-1-yl)ethyl]-2H-pyran 2-one inhibitors of HMG-CoA reductase. 2. Effects of introducing substituents at positions three and four of the pyrrole nucleus. J. Med. Chem., 34, 357–366. Istvan, E.S.and Deisenhofer, J. (2001) Structural mechanism for statin inhibition of HMG-CoA reductase. Science, 292, 1160–1164. Burnouf, C.and Pruniaux, M.P. (2002) Recent advances in PDE4 inhibitors as immunoregulators and antiinflammatory drugs. Curr. Pharm. Des., 8, 1255–1296. Marivet, M.C., Bourguignon, J.-J., Lugnier, C., Mann, A., Stoclet, J.-C.,and Wermuth, C.-G. (1989) Inhibition of cyclic adenosine-30 ,50 -monophosphate phosphodiesterase from vascular smooth muscle by rolipram analogs. J. Med. Chem., 32, 1450–1457. Christensen, S.B., Guider, A., Forster, C.J., Gleason, J.G., Bender, P.E., Karpinski, J.M., DeWolf, W.E., Jr., Barnette, M.S., Underwood, D.C., Griswold, D.E., Cieslinski, L.B., Burman, M., Bochnowicz, S., Osborn, R.R., Manning, C.D., Grous, M., Hillegas, L.M., Bartus, J.O., Ryan, M.D., Eggleston, D.S., Haltiwanger, R.C.,and Trophy, T.J. (1998) 1,4-Cyclohexanecarboxylates: potent and selective inhibitors of phosophodiesterase 4 for the treatment of asthma. J. Med. Chem., 41, 821–835. http://www.nycomed.com/en/Menu/ RD/Pipeline/ accessed March 2009. Xu, R.X., Hassell, A.M., Vanderwall, D., Lambert, M.H., Holmes, W.D., Luther, M.A., Rocque, W.J., Milburn, M.V., Zhao, Y., Ke, H.,and Nolte, R.T. (2000) Atomic structure of PDE4: insights into phosphodiesterase mechanism and specificity. Science, 288, 1822–1825. Huai, Q., Wang, H., Sun, Y., Kim, H.Y., Liu, Y.,and Ke, H. (2003) Threedimensional structures of PDE4D in
References
25
26
27
28
29
30
31
32
complex with roliprams and implication on inhibitor selectivity. Structure, 11, 865–873. Xu, R.X., Rocque, W.J., Lambert, M.H., Vanderwall, D.E., Luther, M.A.,and Nolte, R.T. (2004) Crystal structures of the catalytic domain of phosphodiesterase 4B complexed with AMP, 8-Br-AMP, and rolipram. J. Mol. Biol., 337, 355–365. Zhang, K.Y.J., Card, G.L., Suzuki, Y., Artis, D.R., Fong, D., Gillette, S., Hsieh, D., Neiman, J., West, B.L., Zhang, C., Milburn, M.V., Kim, S.-H., Schlessinger, J.,and Bollag, G. (2004) A glutamine switch mechanism for nucleotide selectivity by phosphodiesterases. Mol. Cell, 15, 279–286. Card, G.L., England, B.P., Suzuki, Y., Fong, D., Powell, B., Lee, B., Luu, C., Tabrizizad, M., Gillette, S., Ibrahim, P.N., Artis, D.R., Bollag, G., Milburn, M.V., Kim, S.-H., Schlessinger, J.,and Zhang, K.Y.J. (2004) Structural basis for the activity of drugs that inhibit phosphodiesterases. Structure, 12, 2233–2247. Ke, H., Colicelli, J.,and Huai, Q. (2003) The crystal structure of AMP-bound PDE4 suggests a mechanism for phosphodiesterase catalysis. Biochemistry, 42, 13220–13226. Rocque, W.J., Tian, G., Wiseman, J.S., Holmes, W.D., Zajac-Thompson, I., Willard, D.H., Patel, I.R., Wisely, G.B., Clay, W.C., Kadwell, S.H., Hoffman, C.R., and Luther, M.A. (1997) Human recombinant phosphodiesterase 4B2B binds (R)-rolipram at a single site with two affinities. Biochemistry, 36, 14250–14261. Hamelberg, D.,and McCammon, J.A. (2004) Standard free energy of releasing a localized water molecule from the binding pockets of proteins: doubledecoupling method. J. Am. Chem. Soc., 126, 7683–7689. Huang, F.and Hong, E. (2004) Platelet glycoprotein IIb/IIIa inhibition and its clinical use. Curr. Med. Chem. Cardiovasc. Hematol. Agents, 2, 187–196. Plow, E.F., Pierschbacher, M.D., Ruoslahti, E., Marguerie, G.,and
33
34
35
36
37
38
Ginsburg, M.H. (1985) The effect of Arg-Gly-Asp-containing peptides on fibrinogen and von Willebrand factor binding to platelets. Proc. Natl. Acad. Sci. USA, 82, 8057–8061. Shebuski, R.J., Berry, D.E., Bennett, D.B., Romoff, T., Storer, B.L., Ali, F.,and Samanen, J. (1989) Demonstration of Ac-Arg-Gly-Asp-Ser-NH2 as an antiaggregatory agent in the dog by intracoronary administration. Thromb. Haemost., 61, 183–188. Fischer, M.J., Bruce, G., Harms, C.S., Kline, A.D., Mullaney, J.T., Nunes, A., Scarborough, R.M., Arfsten, A.E., Skelton, M.A., Um, S.L., Utterback, B.G.,and Jakubowski, J.A. (1997) Non-peptide RGD surrogates which mimic a Gly-Asp b-turn: potent antagonists of platelet glycoprotein IIbIIIa. J. Med. Chem., 40, 2085–2101. Egbertson, M.S., Chang, C.T.-C., Duggan, M.E., Gould, R.J., Halczenko, W., Hartman, G.D., Laswell, W.L., Lynch, J.J., Jr., Lynch, R.J., Manno, P.D., Naylor, A.M., Prugh, J.D., Ramjit, D.R., Sitko, G.R., Smith, R.J., Turchi, L.M.,and Zhang, G. (1994) Non-peptide fibrinogen receptor antagonists. 2. Optimization of a tyrosine template as a mimic for Arg-Gly-Asp. J. Med. Chem., 37, 2537–2551. Sham, H.L., Kempf, D.J., Molla, A., Marsh, K.C., Kumar, G.N., Chen, C.-M., Kati, W., Stewart, K., Lal, R., Hsu, A., Betebenner, D., Korneyeva, M., Vasavanoda, S., McDonald, E., Saldivar, A., Wideburg, N., Chen, X., Niu, P., Park, C., Jayanti, V., Grabowski, B., Granneman, G.R., Sun, E., Japour, A.J., Leonard, J.M., Plattner, J.J.,and Norbecki, D.W. (1998) ABT-378, a highly potent inhibitor of the human immunodeficiency virus protease. Antimicrob. Agents Chemother., 32, 3218–3224. Tong, L., Pav, S., Mui, S., Lamarre, D., Yoakim, C., Beaulieu, P.,and Anderson, P.C. (1995) Crystal structures of HIV-2 protease in complex with inhibitors containing the hydroxyethylamine dipeptide isostere. Structure, 3, 33–40. Beaulieu, P.L., Anderson, P.C., Cameron, D.R., Croteau, G., Gorys, V., Grand-Matre,
j81
j 3 Application of Molecular Modeling in Analogue-Based Drug Discovery
82
C., Lamarre, D., Liard, F., Paris, W., Plamondon, L., Soucy, F., Thibeault, D., Wernic, D., Yoakim, C., Pav, S.,and Tong, L. (2000) 20 ,60 -Dimethylphenoxyacetyl: a new achiral high affinity P3-P2 ligand for peptidomimetic-based HIV protease inhibitors. J. Med. Chem., 43, 1094–1108. 39 Stoll, V., Qin, W., Stewart, K.D., Jakob, C., Park, C., Walter, K., Simmer, R.L., Helfrich, R., Bussiere, D., Kao, J., Kempf, D., Sham, H.L.,and Norbeck, D.W. (2002) X-ray crystallographic structure of
ABT-378 (lopinavir) bound to HIV-1 protease. Bioorg. Med. Chem., 10, 2803–2806. 40 Chrusciel, R.A., Thalsrivongs, S.,and Nicholas, J.A. (2001) HIV protease inhibitors in early development, in Protease Inhibitors in AIDS Therapy (eds R.C. Ogdenand C.W. Flexner), Marcel Dekker, New York, pp. 119–137. 41 Tanimoto similarity coefficients calculated with FCFP_4 fingerprints by SciTegic Pipeline Pilot 7.0.1.100 from Accelrys Software Inc.
Gy€ orgy G. Ferenczy
Chinoin Zrt., Sanofi-aventis, Tó u. 1-5, 1045 Budapest, Hungary Gy€orgy G. Ferenczy received his PhD from the E€ otv€ os University in Budapest in 1988. His early career involved quantum chemical method development and applications to extended systems. He was a postdoctoral fellow at the University of Oxford, UK, and at the University of Nancy, France. He was in the Drug Design Group of Gedeon Richter Plc., Budapest, as a Research Scientist and Group Leader. In 1999, he joined Chinoin Ltd., a member of the Sanofi-Aventis group. He has authored and coauthored 50 scientific papers and patents.
j83
4 Issues for the Patenting of Analogues Stephen C. Smith
4.1 Introduction
The statement in the first edition of this book that The most fruitful basis to discover a new drug is to start with an old drug, and this has been the most common and reliable route to new products [1] encapsulates the main issue in patenting new drug analogues, that is, they are in fact related to existing drugs and so may attract the charge of being obvious over them. For any invention to be patentable, it has to be new in a strict formal sense over what is already known, inventive, which is not obvious over what is already known, and be useful in a broadly industrial context. Drug products are no different, but because most are in some sense analogues of a known drug, it is the requirement to be inventive over what is known that generally provides the biggest hurdle to patenting new analogues. Given the enormous expense of bringing a new drug to sale, ensuring that patent protection is obtained in most major markets and maintained for a reasonable period to recover the investment and generate profit for shareholders is critical. No company can afford to find that, shortly after launching a new product, a competitor is able to introduce the same product without their permission. Although patent attorneys and other legal professionals will do their best to provide adequate protection and ameliorate any problems that may arise, the role of the inventor and associated research staff is critical to ensuring that drug patents have the best possible foundation. Getting a patent granted by satisfying the requirements of patent offices is of course the initial objective, but having its validity survive prolonged attack from determined competitors is the ultimate aim. Any product with annual US sales of the order of US$ 80 million plus is almost certain to find the validity of its US patent challenged as part of an Abbreviated New Drug Application (ANDA) related challenge to introduce a generic version. Consequently, it is necessary to try to challenge proof the validity of any patent protecting a new chemical entity (NCE) by taking extra care during the application stage. Here, the inventors – who usually include one or more medicinal chemists – have a critical role to play.
Analogue-based Drug Discovery II. Edited by János Fischer and C. Robin Ganellin Copyright Ó 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-32549-8
j 4 Issues for the Patenting of Analogues
84
This chapter aims to give medicinal chemists and others involved in new drug research a basic survival guide to the patent system and how it is applied, in particular to patenting drug analogues. What follows can necessarily be only a broad overview of the situation for drug analogues, but it is hoped it will give medicinal chemists and others involved in new drug research enough understanding and practical information to avoid some of the pitfalls in the patenting process and allow them to have an informed dialogue with their legal advisers.
4.2 Patents: Some Fundamentals
Patents are exclusive rights granted for inventions in all fields of technology that meet the basic criteria for patentability. They are not available for designs, artistic works, or intellectual activities for which other intellectual property (IP) rights may be available such as registered designs and copyright. Like other property rights, patents may be sold, licensed, mortgaged, or otherwise commercialized. Patents are national rights that allow the owner to stop other people from doing what is claimed in the patent without the owners permission during a set time within the state concerned and while renewal fees are being paid. They do not give any positive right to commercialize an invention since specific permission may be required from other rights holders or the state (e.g., marketing authorization for a drug product). A patent is a special legal instrument first developed in Europe1) in the fifteenth century to promote technological development by providing timelimited protection from competition (monopoly) in exchange for public disclosure of how to make and use the technology concerned. They require a formal application accompanied by a patent specification that sets out in detail what is protected by legal claims (typically found at the end of the patent specification) and the nature of the invention and how it can be made and used, including in the case of chemical inventions detailed examples of making and often testing of compounds falling within the legal claims. In all major countries, the specifications of unexamined patent applications are now published or made available on-line 18 months after the earliest of their filing and priority dates.2) The process of obtaining a granted patent varies (typically from 2 to 10 years) from the filing of the application during which time the patentability is examined and argued with the applicant against set criteria. Until relatively recently, many states restricted the availability of patent protection for pharmaceutical inventions, permitting only claims to processes for manufacturing an active ingredient. But now all major countries grant patents for pharmaceutical 1) The earliest known English patent for invention was granted by Henry VI to Flemish-born John of Utynam in 1449 giving him a 20-year monopoly for a method previously unknown in England for making stained glass that was needed for the windows of Eton College. 2) In the United States of America, it is still possible to prevent publication of a patent
application after 18 months by, at the time of filing, so requesting on the basis that the application will not be filed in a foreign country that itself provides for publication after 18 months. However, securing patents in at least the EU and Japan as well as the USA is the norm for analogue patenting and so this possibility is of little practical value.
4.3 Patentability
inventions with claims to chemical products, formulations, and processes for manufacture. These patents are granted for a period of at least 20 years, starting from the date when the application was filed.3) The advantage of patents that claim the chemical products themselves (product patents) is that they have the effect of preventing unauthorized commercial use of a claimed compound for any use (not just that found by the patentee) irrespective of how it has been made. A claim to a process for manufacturing a product by contrast only restricts the use of the process itself. This may be difficult for the patent owner to prove in practice without access to the manufacturing plant and so many patent systems particularly in Europe reverse the normal evidence requirements (the burden of proof) where infringement of a patented process is suspected. The alleged infringer then has to prove that the claimed process is not being used. Both granted patents and patent applications give rise to publications destroying the novelty of any of the subject matter disclosed in the specification should this be claimed in patent applications filed later. Granted patents in addition may restrict the freedom of competitors to commercialize anything within the patent claims without a license from the patent owner (patentee).
4.3 Patentability
The criteria that must be satisfied in deciding the patentability of an invention, that is, whether a patent may be granted or whether it should remain in force, are essentially the same in all major countries. In summary, the subject matter must be suitable material for a patent – it must not be specifically excluded by national law: thus, a method of treatment or diagnosis practiced on the human or animal body is excluded in most countries other than the United States of America; (2) novel in comparison with what is in the prior art – this includes publications of any sort (including abstracts of conference proceedings) and documented uses of the invention anywhere in the world in any language before the priority date of the application; (3) inventive over the relevant closest prior art – this includes publications and uses as in No. 2 above, but they must be relevant to the technical area and normally in relation to a single document rather than a combination of documents unless they are directly related to each other or else establish the level of general knowledge in the technical area, such as textbooks, manuals, and so on; (4) useful or capable of use primarily in an industrial context – in the United States of America useful has been very broadly interpreted to include methods of doing business or carrying out research, but in most other countries some technical character is needed; and (1)
3) In the United States of America, the term of drug analogue patents may be extended by up to 5 years for delays in the regulatory approval process and in Europe similar extended protection in the form of patent-like supplementary protection certificates is also available for up to 5 years.
j85
j 4 Issues for the Patenting of Analogues
86
(5)
adequately described – there must be sufficient information in the patent specification for someone of reasonable skill in the relevant art to make and use the invention; for example, in the pharmaceutical context, the information must be sufficient for a medicinal chemist to make a compound and understand how it would be used in a drug.
For drug analogue patenting, the first criterion does not normally give problems, but satisfying the requirements of the remaining criteria, and especially that of being inventive, presents the most challenges both when patents are being written (drafted) and when they are being defended in litigation.
4.4 Important Elements of the International Patent System
It is important to stress at the outset that there is no such thing as a world patent giving rights everywhere. However, there are a number of unified patent examination and processing systems such as that provided by the European Patent Office (EPO) securing protection in up to 36 European states as designated by the applicant.4) Such systems allow a single patent application to be filed and examined in an official language (English, French, or German in the case of the EPO) leading eventually to a group of separate patents applicable in the individual nation states required. The international patent system is underpinned by three key major treaties: Paris Convention: 5) This allows a person to file a patent application in his own country, establishing a priority date for the invention, and then wait up to a year before deciding to file in other countries and still rely on the priority date of the first patent application, thus allowing opportunity to further evaluate the invention before incurring the substantial cost of foreign patent applications. Patent Cooperation Treaty (PCT): 6) This enables a person to obtain patent protection eventually in a large number of states that are signatories of the treaty 4) See EPO web site for more information: http://www.epo.org. 5) The Paris Convention for the Protection of Industrial Property, governing patents, trade marks, industrial designs, and so on, was originally signed in Paris in 1883; it has been amended several times, most recently in Stockholm in 1967 and is now acceded to by 173 states (May 2009); it requires signatories to grant reciprocal protection to a national of another signatory state and sets out basic rules that all contracting states must follow in relation to patent and other IP rights; see also World IP Organisation web site at http://www.wipo.int/treaties/en/ip/paris/. 6) The Patent Cooperation Treaty is administered by the World Intellectual Property
Organisation (WIPO) based in Geneva, Switzerland, and is open to any state that is a member of the Paris Convention; under the treaty, anyone who is a national or resident of a PCT contracting State may file an International Application either with the national patent office of the contracting State of which the applicant is a national or resident or, at the applicants option, with the International Bureau of WIPO in Geneva; filing a single PCT application allows protection to be obtained in all signatory states (141 with the accession of Peru and Chile in March 2009) on completion in due course of the relevant national patent requirements; see also the PCT web site at http://www.wipo.int/pct/en/.
4.5 Priority
by filing a single International Patent Application (often referred to as a PCT application). Besides reducing overall costs, the use of the PCT has the advantage of increasing by up to 18 months the time before a decision has to be made on foreign filing. This is useful where there is a continuing research activity such as on particular drug analogues where initially promising leads may disappear and the direction of research change, requiring a change in the patenting strategy. As a result, most organizations seeking patent protection internationally for pharmaceutical inventions now use the PCT as their preferred route. Agreement on Trade-related Aspects of Intellectual Property Rights (TRIPs): This was established in 1995 as part of a global open trade deal and sets out the minimum levels of protection that each government has to give to the patents and other intellectual property rights of fellow members of the World Trade Organization. This was particularly important in establishing that it was improper to discriminate against particular technologies such as pharmaceuticals or whether they were imported or locally produced. Adoption of TRIPs led to changes in the patent law of both developed and developing countries including the United States of America and India and established a minimum patent term of 20 years from the date of filing the applications.
4.5 Priority
In competitive and rapidly evolving technology areas such as drug research, it is important to secure early patent protection for inventions as soon as possible after they are made so that priority is established for the invention. This is because in all countries except the United States the patent is accorded to the first person to file a patent application. In the United States, the patent is granted to the first inventor rather than the first person to apply for a patent and an elaborate practice of patent interferences has been developed to determine the true first inventor. The Paris Convention (see above) was developed to allow a single early-filed patent application in one country to be used for up to 1 year as the basis for any number of corresponding foreign applications claiming the priority of the original national application (Figure 4.1). Everything in the second application that is literally within (or properly based on) the first application is regarded as having the same filing date as the first application – the
1st (priority) application filed
0 months
2nd updated application filed – claiming priority for subject matter also in 1st application
12 months
Figure 4.1 Priority under the Paris convention.
j87
j 4 Issues for the Patenting of Analogues
88
priority date – when considering patentability over the prior art. To retain priority, the subject matter in the later application must be directly and unambiguously derivable from that in the first, the so-called priority application. In principle, there is no limit to the number of priority establishing applications that may be filed within the year from the first application and from which the later foreign applications may claim priority. However, they will ultimately need to be translated into the language of the state in which the priority is being asserted and so in practice for drug analogue inventions it is rare for there to be more than three priority applications. The advantage of this arrangement for analogue research is that it allows the first application to include claims amounting to a years research plan based on initial activity in a few lead compounds that have so far been prepared and tested. Then, as the research plan is explored and additional interesting compounds are discovered these can be either added to a new priority establishing application or else included in the foreign patent applications. For example, suppose a first application was filed claiming naphthalene derivatives having a halogen substituent based on the activity of a chloronaphthalene included in the patent specification, then a claim to halogenonaphthalenes in a second application filed up to a year later would be entitled to the priority date of the first application. This would be important if a competitor filed an application also claiming halogenonaphthalenes or if there were to be a publication of the chloronaphthalene itself some time between the first and the second patent applications since, if the priority was not maintained, then a valid patent could not be obtained by the originator.
4.6 Novelty
Deciding whether claimed subject matter is novel in relation to the prior art is critical in any assessment of validity of a patent; if novelty is not present there, then consideration of inventive step (which itself can always be argued or refuted with evidence of unexpected effects) simply is not possible. In the case of chemical inventions, it does not matter at all for what purpose, if any, the compound was used and so disclosure or use in any technical art may be considered. It is enough that the compound was made somewhere else to destroy the novelty of a later patent claim to the compound. However, it may well be that other aspects related to the known compound are still novel and inventive, such as pharmaceutical formulations or new medical uses. Thus, for example, 2,6-diisopropylphenol (Figure 4.2), the active entity of the intravenous anesthetic propofol, was first described as a stabilizer for rubber and so could not at the time when its anesthetic properties were discovered in 1977 be claimed as a compound itself but only as a special injectable formulation.7) 7) The first medical use for a known compound can now be protected under European law, that is, a product claim in the form of known substance X for use as a medicine is now permissible in Europe. Methods of medical treatment involving administration of a known substance have always been patentable in the United States.
4.6 Novelty
OH
Figure 4.2 Propofol (DiprivanÒ ).
O
O S
O H
NH2 X
O O
O O H H O (a)
CH2OSO2NHR1
R5
R2 R4
R3 (b)
Figure 4.3 (a) Topiramate (TopamaxÒ ) and (b) generic structure including topiramate.
The objective of all patent claims is to define the limits of the invention in such a way that all essential features are included and defined as generally as possible without covering embodiments (examples) that are already known or which are obvious from what is known or those that cannot be made or which simply do not work. This may be relatively straightforward for mechanical inventions but can be difficult for chemical inventions such as new drug analogues where the draftsman wants to include in the claims all of those analogues that could readily be conceived by a competitor based on the compounds actually exemplified in the patent specification. This frequently leads to the use of complex definitions in patent claims involving a generalized structural formula with individual substituents defined in lists of alternatives such as that shown above for the McNeil anticonvulsant, topiramate (Figure 4.3). Such claims8) typically include thousands or even millions of alternative compounds. Fortunately, no patent office takes the view that all possible compounds that can theoretically be listed within a general formula are implicitly described once the structure is published. What then is the effect of such a published patent? The novelty is certainly destroyed for all compounds that are specifically mentioned or listed in worked examples and the rest of a patent specification once it is published. Sometimes, subtle linguistic devices are employed by patent draftsman to indicate which compounds have actually been prepared and evaluated in bioassays and which are mere possibilities. For example, the following compounds were obtained implies that actual synthesis has occurred whereas the compounds in the table may be obtained implies that it is simply possible to make the listed compounds. In the latter case, such disclosures will still be novelty destroying unless it can be shown that such compounds cannot be made by any of the processes
8) See Appendix 4.B.
j89
j 4 Issues for the Patenting of Analogues
90
described in the specification or other well-known means – if they can be so made the disclosure is often referred to as enabling, that is, there is sufficient description for a chemist with reasonable basic skills to make the compound(s) concerned.
4.7 Inventive Step: Nonobviousness
The problem with most inventions is that with hindsight they may appear to be obvious or to lack the necessary inventive step over what is already known. Consequently, showing inventive step is almost always the biggest hurdle that drug analogue patents must clear since, by definition, they start from an existing molecule already known to have useful pharmacological properties. The fact that increasing logic and theory has been applied in recent years to research processes to increase the likelihood of candidate drugs emerging quickly from drug discovery programs can make it more difficult to show that arriving at a useful analogue of an existing drug is other than predictable. From the patent perspective, so far at least the discovery process and subsequent development of clinically useful analogues of existing drugs continue to remain uncertain and regularly give rise to unexpected results. Lack of predictability and/or unexpected results in comparison with the relevant close prior art are required if inventive step is to be shown. The US Supreme Court set down in a decision in 1966 [2] (which is still followed today) the key components of obviousness determinations: the determination of nonobviousness is made after establishing the scope and content of prior art, the differences between the prior art and the claims at issue, and the level of ordinary skill in the pertinent art. The Court also considered possible secondary considerations that might be taken into account as evidence of nonobviousness, such as commercial success, long-felt but unmet needs, and the lack of success of others to solve the underlying problem. So, it is necessary to decide what is the claimed invention and consider both chemical structural differences and the underlying properties of the closest compounds in the art in deciding whether an inventive step exists. The size of the inventive step is of no concern – there simply has to be one, the so-called scintilla of invention. For pharmacological analogues, where the new drug molecule has similar pharmacological activities but there are no real chemical or structural similarities with the prior art drug, then an inventive step is clearly present. However, for most drug analogues some sort of structural similarity can be discerned between the new molecule and the starting drug. This means that in virtually all applications for drug analogues patents, examiners will allege that the invention lacks an inventive step simply because of perceived structural similarity with the closest compound in the prior art (the so-called a priori or prima facie obviousness). Analogues of a natural product whose structure has not previously been published are of course a clear exception. The degree of structural or chemical closeness that makes one compound obvious from another is of course subjective and therefore arguable. The person required to
4.7 Inventive Step: Nonobviousness
judge this is the notional person of ordinary skill in the art, that is, for drug analogues, usually a skilled medicinal chemist. This hypothetical person is considered to be aware of all the background literature, have all the skills of a synthetic chemist at the time the application was filed, and have a general understanding of how molecules may be evaluated for potential drug use, but this person may not be in any way inventive. In complex cases, the notional person may be considered as a team, spanning a number of specialist skill sets such as pharmacology and formulation science. It is possible to give some rough guidance on what is likely to attract an obviousness allegation. Thus, if the relationship with the prior art is one of isomerism (positional or spatial), homology (such as propyl or ethyl for methyl), or involves replacing groups within a series (such as bromo for chloro) or generally recognized to be similar in properties (such as cyano or nitro for halogeno), then an a priori obviousness objection is likely to be made. Successfully countering (rebutting) such an objection by argument alone may be difficult and instead it may be necessary to use evidence of unexpected properties or difficulties in moving from the prior art to the claimed molecules. What then is the closest relevant prior art for assessing inventive step? First, this may not necessarily be the same as the known drug that was the starting point for the research. Thus, the structurally closest compounds to the claimed analogues may have a totally unrelated set of industrially useful properties (e.g., as dyestuffs) or they may have been simply published without any apparent utility in an academic paper. In both of these cases, it should be possible to counter an obviousness argument by saying that the useful pharmacological properties of the new analogues were unexpected and could not have been predicted from what was known in the prior art. In general, patent specifications only need to contain an indication of the relevant prior art without disclosing the closest piece of prior art, although applicants will usually have to get close to specifying it in order to properly set out the advantages of the invention. If it is clear from the outset that the new analogues have a very close relationship with a known drug, then the nature of the unexpected properties (such as unexpectedly superior activity, bioavailability, stability, lack of side effects or toxicity, etc.) and details of how they can be demonstrated (e.g., any special testing systems) should be set out in general terms in the patent specification. However, it is generally better to await the nature of any obviousness challenge before presenting actual evidence of the unexpected properties since these are likely to be required in comparison with what is cited as the closest prior art by the relevant patent examiner. While overcoming objections of obviousness by providing suitable evidence of unexpected effects is attractive, it may give rise to problems when a patent is the subject of litigation, especially in the United States of America. The evidence and those who produced it are then subjected to close scrutiny by the parties to the litigation examining the nature and extent of the comparisons made and the conduct of the testing. Consequently, evidence should only be offered to dispose of an obviousness objection after counterarguments have failed. Such counterarguments
j91
j 4 Issues for the Patenting of Analogues
92
might be based on structural dissimilarities with the prior art compounds, contrary teaching in the literature such as structure–activity relationships leading away from the new analogues, a long-felt want for the invention indicated by false starts or failures, or difficulties in synthesizing the new analogues. However, mere evidence of the commercial success of the patented drug may not be sufficient since there may be other reasons for this such as substantial expenditure on marketing. The finding that the new analogues possess additional useful properties, even if unexpected, may not always be sufficient if the analogues exhibit similar underlying properties to those of the original drug. Thus, such properties might be seen as a bonus rather than as demonstrating an inventive step if a series of novel betareceptor antagonists structurally very similar to the known drug propranolol were found to have similar levels of antagonist properties but had greater stability. However, if the extra property was perhaps to be cardioselectivity and it was to be shown that this had previously been difficult to achieve, then this would probably have supported the presence of an inventive step. Although all the major developing countries employ similar principles when approaching assessment of inventive step to those set out by the US Supreme Court above, in practice the approaches may differ. Thus, the European Patent Office in examining an inventive step adopts the so-called problem–solution approach that, after determining the closest relevant prior art, establishes what it considers to be the objective technical problem to be solved by inspecting the patent specification and looking for the features that distinguish the invention over the prior art and then decides whether the solution – the claimed invention – would have been obvious to the notional skilled person starting from the closest prior art. In this context, when setting up the notional technical problem to be solved, it is not appropriate to use compounds from the prior art that although structurally closest to the claimed compounds would have been disregarded as irrelevant by the person skilled in the art.9) US courts have taken a similar view and held that it is not reasonable to expect that a skilled medicinal chemist would remove a structural feature known to convey superior properties to a known drug when starting to search for new improved analogues [3]. They have also rejected an attempt to suggest that one structurally analogous compound would be necessarily chosen from among the 54 in total described in an earlier patent as the starting point for improved analogues without clear teaching that it was preferred [4]. Finally, it must be mentioned that it is not usually acceptable to construct an obviousness argument based on a mosaic of parts from many different unrelated prior art documents. If several documents are used, they must be clearly linked and it must be possible to show how the skilled man would inevitably be led to combine them to reach the claimed invention.
9) European Technical Board of Appeal, decision T334/92, Eisai Benzodiazines: here the EPO was held to have improperly based the notional problem on a 20-year old reference that, although including structurally very similar compounds to the antiangina agents claimed, had been disregarded by other groups working in the area as a starting point for new antiangina drugs; see http://legal.europeanpatent-office.org/dg3/biblio/t920334eu1.htm.
4.9 Selection Inventions
4.8 Utility: Industrial Application
Unless compounds have some useful property that can be used in some industrial context, they cannot be validly patented. Consequently, it is important to ensure that all the molecules included in a drug analogue patent specification possess the underlying biological activity at a level that is useful. It is frequently tempting, during a large synthetic program to produce drug analogues, to include every compound made irrespective of the extent of biological activity shown in the assay systems. Yet including any significant number of such inactive compounds introduces a fatal weakness into any eventual patent since a determined competitor may well be able to discover such an area of inactivity and thus challenge the patent for lack of utility. Showing useful pharmacological properties in the target disease situation in humans or animals is normally not possible since most patents on drug analogues are applied for well before any clinical evaluation has been possible. However, data from in vitro assays or testing in an animal surrogate and which is reasonably correlated with the end disease can be used to satisfy the therapeutic utility requirement for patentability. This information correlating any assay or animal testing with the disease should be provided in the specification together with test data at least in general terms for the analogues described in the examples. Before the 1980s, in the United States of America patent applications asserting use in treating cancer in humans were regularly rejected as lacking utility because such use was considered incredible. However, this is fortunately no longer the case and [Patent] Office personnel must determine if the asserted utility for the invention is credible based on the information disclosed in the application [5].
4.9 Selection Inventions
Selection inventions, as the name implies, involve the selection of a small number of compounds previously included within a known broadly defined group of compounds (usually within the scope of the claims of an earlier patent) and finding that the selected compounds possess some unexpected useful properties such as surprisingly higher activity. However, selection inventions still have to satisfy the novelty and inventive step criteria for patentability but the resultant patents are self-standing and have an independent 20-year term. Consequently, they can be particularly useful where there is a new drug analogue research program continuing over several years and the initial patents may not contain the eventual drug development candidate. Of course, if the selection has been made from within the broad groups of compounds claimed in a patent that is still in force and that has been granted to a competitor, then commercializing one of the selected compounds will require a license under the competitors patent. Equally, the competitor could not commercialize a compound claimed in the selection patent without a license. Normally, such a situation would be dealt with by cross-licensing of both patents.
j93
j 4 Issues for the Patenting of Analogues
94
Establishing novelty for a selection invention can be very difficult. None of the compounds in the selection must have been described before or suggested as possibly obtainable anywhere in the prior art and for any purpose. This includes not being mentioned in patent claims. As mentioned before, a typical main claim to drug analogues may nominally include hundreds of thousands of possibilities, the majority of which have not been made. Making a small selection of compounds from such a large number of possibilities is not usually a problem in terms of novelty, unless the earlier patent specification suggests that certain optional groups of compounds are preferred alternatives. However, the subsidiary claims in the patent will likely contain a general formula with short lists of alternative substituents including perhaps only a few tens or hundreds of compounds. The novelty of compounds selected from such narrow claims may thus be in doubt. It is not possible to give hard and fast rules about what size of group will be considered as novel over the prior disclosed broad group since such assessments are highly fact specific and there is considerable variation among the United States of America, Europe, and Japan. Suffice it to say that selection patents must necessarily have narrowly defined claims in terms of the number of molecules included and ideally contain only a handful. In addition to novelty, an inventive step must be shown over the prior art, that is, the known compounds from which the selection is made. This can either be based on a new and valuable property, such as the one making the compounds useful in a new disease situation, a substantial reduction in adverse properties such as toxicity or side effects, or else a substantial increase in the underlying activity of the prior art compounds, but in all cases where the new property or size of the change in properties could not have been predicted from the prior art. The nature and extent of the properties providing the inventive step and how they can be demonstrated must be properly set out in the patent specification. Also, it is even more important than in ordinary patents on drug analogues to ensure that all claimed compounds within the selection possess the unexpected and useful properties.
4.10 Enantiomers
Enantiomers of racemic drugs are probably the closest structural analogues and a patent protecting an enantiomeric form, the ultimate selection invention of one from two molecules. The first hurdle to patentability is to demonstrate novelty over the known racemic form. Fortunately, most countries take the view that an enantiomer of a known racemic form that has never been known or existed except in the presence of the other enantiomer is formally novel and move the inquiry to inventive step. At one time, it was unusual to consider separating individual enantiomers of clinically useful drugs partly because of the additional cost of manufacture, but today it is the norm and many drugs are now used as single enantiomers. It has also become easier with modern technology to produce such enantiomers on a large-scale commercial basis either by stereoselective synthesis or by physical separation
4.11 Prodrugs and Active Metabolites
F
N
O N Figure 4.4 Escitalopram (LexaproÒ ).
techniques such as chiral HPLC. Consequently, it is difficult now to argue that the skilled person would not routinely attempt to separate a racemic candidate drug molecule to establish how the useful biological property, side effects, and toxicity were distributed between the individual enantiomers. However, things that are obvious to try are not necessarily obvious in fact because there may be all sorts of reasons why success cannot be achieved [6]. As a result, patents claiming a single enantiomer often start with a presumption that they lack an inventive step over the known racemic form of the drug. Such a presumption can be disposed of only by concrete evidence of unexpected properties in the enantiomers. In addition, there can be difficulties arising from whether the information in the patent specification is sufficient to support a product claim to the enantiomer however prepared and without any use limitation. An interesting set of legal judgments in the English courts [7] illustrate many of the issues of patentability referred to in this chapter and involve the enantiomeric antidepressant drug escitalopram (Figure 4.4). These decisions are self-contained and present the various issues in clear terms.
4.11 Prodrugs and Active Metabolites
Both of these may be regarded as analogues of known drugs and may even be interrelated. Prodrugs are normally chemically simple derivatives or precursors of the parent drug that are not active until the drug moiety is presented in vivo and they are usually designed to overcome some deficiency in the original drug, for example, esters, ethers, oxidizable precursors, and so on of the drug molecule. Prodrugs may be difficult to patent and the issued patents susceptible to challenge when the drug itself is already known. They first need to pass the novelty hurdle that may be difficult since many simple derivatives such as alkyl esters will probably have appeared routinely together with phrases such as or a prodrug thereof in drug patent specifications. The prodrug concept is well known and so prodrugs of existing drugs are at face value obvious unless the added element forming the prodrug itself contains some special or unusual structural component or it can be shown that the prodrug possesses surprising or unexpected benefits, such as dramatically improved bioavailability, duration of effect, and so on.
j95
j 4 Issues for the Patenting of Analogues
96
O
S OH N
N N
O
H
O
Figure 4.5 Hetacillin.
NH2 NH
H S N
O O
OH O Figure 4.6 Ampicillin.
One prodrug that was the subject of extensive patent litigation was hetacillin, the acetone condensate of the antibiotic ampicillin (Figures 4.5 and 4.6). This prodrug is inactive as an antibiotic until administered to humans when ampicillin is rapidly regenerated in the blood stream. As a result, its sale was eventually held in the United Kingdom to be an infringement of the existing patent on ampicillin [8]. Active metabolites may be patented insofar as their existence has not been published and it is not obvious that they will be produced from the drug itself. Since such metabolites are generally found only when the drug is administered to humans, any patent application is likely to be filed several years after the application protecting the drug itself and so will lead to later expiring patents. Consequently, there is the opportunity to maintain effective patent protection for a product even after the patent protecting the active ingredient has expired by alleging infringement of the active metabolite patent when the drug is administered to patients. Examples of patented active metabolites that resulted in litigation include the antihistamines fexofenadine, which is a carboxylic acid formed rapidly in vivo following administration of terfenadine (Figures 4.7 and 4.8) and desloratidine, which is the major metabolite of loratidine (Figures 4.9 and 4.10).
OH O
N OH OH Figure 4.7 Fexofenadine (AllegraÒ ).
4.12 The Patenting Process from the Inventors Standpoint
N OH OH
Figure 4.8 Terfenadine (SeldaneÒ ).
H N
N Cl Figure 4.9 Desloratidine (ClarinexÒ ).
O
O
CH3
N
N Cl Figure 4.10 Loratadine (ClaritinÒ ).
Attempts to enforce the relevant patents on these active metabolites against companies commercializing the parent drug have met with varying results [9] in different countries, demonstrating the problems particularly of novelty in patenting this type of drug analogue.
4.12 The Patenting Process from the Inventors Standpoint
Having a patent application filed for a drug invention is a major achievement for most medicinal chemists especially if they are one of the named inventors. However, it sets in train a series of events that can involve the inventor with the patent system for many years especially where a successful drug product results. The whole patenting process to obtain patents in the United States of America, Europe, Japan, and other commercially important countries can easily take up to 10–12 years by which time most medicinal chemists will have moved onto new projects if not new job roles.
j97
j 4 Issues for the Patenting of Analogues
98
However, their involvement with the patents on which they are inventors may nonetheless continue, often in the following respects. 4.12.1 Inventorship
Deciding who are to be the inventors for a patent application – the inventorship – is an important legal matter affecting the ultimate validity of any patent and is resolved by the patent attorney handling the application. Inventorship is not decided like authorship of scientific papers but in the light of all the facts surrounding the invention, how it was arrived at, and who was associated with the critical steps involved. However, in the case of a new set of analogues based on an existing drug, it is probable that there will be at least one medicinal chemist inventor. 4.12.2 The Priority Patent Application
The medicinal chemist inventor is first likely to be involved when filing a priority establishing patent application – the priority application – is being considered. At this stage, a decision must be made on the extent of the compounds to be included in the claims and which synthetic methods are to be included. Then, there will be a request to supply written examples of compounds that have been made and tested. It is important at this stage to inform the patent draftsman of any compounds that are inactive so that they can be excluded from the claims either by careful construction of the substituent definitions in the general formula or by specific disclaimer.10) 4.12.3 Prior Art Disclosure
The patent draftsman must also be told about any relevant publications, seminars, or the like – prior art – that seem at all relevant to the compounds being patented. It is probable that a formal search for relevant prior art will be carried out that will pick up patent specifications and scientific journal articles, but the research team working on the invention is best placed to identify less well-documented prior art such as conference abstracts. Under the requirements of US law, the inventors and others in the research team associated with the patented invention must provide the patent attorney handling any corresponding US patent application with publications and other pertinent information so that this can be submitted to the US patent examiner who is assigned to process the application. This is a continuing obligation under US law until a US 10) Disclaimers are found at the end of the claim and are typically in the form: what we claim is a compound of the formula A wherein the substituents R1, R2 and R3 have any of the following meanings. . .; but excluding the compounds of the formula A wherein R1 is methyl or ethyl, R2 is hydrogen and R3 is fluoro. . . .
4.12 The Patenting Process from the Inventors Standpoint
patent is granted – failure to comply may lead to a charge of fraud on the patent office and result in the patent being unenforceable, should the patent be litigated. 4.12.4 Patent Specification Review
It is tempting for inventors to read only the examples and technical description of patent specifications, particularly because the documents tend to be repetitive and legalistic and contain seemingly standard wording about dosages, formulations, and the like – the so-called boiler-plate text. However, especially at the drafting stage, inventors should make sure they read all of the patent specification carefully. It is easy for cut and paste word-processing errors to creep in and it is very likely that the inventor will be the person on the witness stand asked to explain what he or she meant by the text! Also, it is not possible to correct errors other than those for which a correction is obvious by reading the text once patent applications have been filed. That said, it is still important to complete reviewing the draft quickly so that the application can be filed with the earliest possible date – the inventors are on the critical path to doing this and may sometimes be the rate-limiting step in the patent application process. 4.12.5 Best Mode of Carrying Out the Invention
A US patent application must comply with another special requirement and the specification must set forth the best mode contemplated by the inventor of carrying out his invention. In the case of analogue patents, this means that the best analogue known to the inventor at the time the application is filed must be included somewhere in the specification. It simply has to appear somewhere but does not have to be flagged up as the best mode. The penalty for not disclosing it is invalidity of the eventual US patent. It must also be present in a foreign-filed priority application if priority is to be maintained in the United States of America. 4.12.6 Foreign Patent Applications
Some nine months after the priority application has been filed, a decision has to be made whether to continue with the patenting process by filing further patent applications in foreign countries, updating the material in, and claiming priority from, the earlier application. These new filings may be made via a single PCT application and/or individual national applications. If a PCTapplication is filed, then after some degree of centralized examination of formal matters as well as a search for prior art affecting novelty and inventive step, it will lead to a series of national applications or regional applications (such as through the European Patent Office) that will then be further examined before any patents are granted. Under the Paris Convention, these applications must be filed within 12 months of the earliest priority application which is an inextensible deadline. However, since translations are often
j99
j 4 Issues for the Patenting of Analogues
100
required, the process to prepare the updated specification for the new applications is usually started several months before the deadline. At this stage, the inventor again needs to supply further examples of analogues based on the original invention and confirm their useful biological properties and mention any inactivity of the analogues. The best mode requirement must also be updated by including the latest preferred analogues. The updated specification will also need to be carefully checked since this may be the final point at which it is possible to easily correct errors and omissions – new matter cannot generally be added after filing which is one reason why patent specifications tend to be so verbose–everything needs to be present that may be needed during the lifetime of the patent, which can be up to 25 years for a drug analogue patent. 4.12.7 Patent Application Publication
Assuming the priority application is maintained by filing one or more corresponding applications, the patent specification will be published more or less as originally filed 18 months after the earliest priority application, or shortly afterwards. This date is important for inventors since patent applications from competitors published after this date, apart from in the United States of America,11) must necessarily be based on later-filed priority applications and cannot then validly claim the same invention. Usually, the application is published together with an official search report of the prior art although this may be published separately. This report indicates which prior published material is considered most relevant to the novelty and inventive step of the invention and gives an early indication of how difficult it may be to obtain a granted patent. 4.12.8 Patent Examination
All patent offices have some backlog of applications and so it may be some time (perhaps as long as 2–3 years) after a patent application has been filed and examination requested before the relevant patent office will issue an official letter – sometimes referred to as an office action – with a fixed date for reply. The letter12) will be 11) As mentioned earlier, in the United States of America it is the person who is first to make the invention and not the first patent applicant who is awarded an eventual patent following a complex and lengthy patent interference procedure. For drug analogue patents, this procedure critically depends on being able to establish dates on which an example from the patent was first conceived and made and tested in the laboratory for its therapeutic purpose. It is most important therefore to keep proper dated laboratory documentation of the preparation and test-
ing of compounds with the pages signed and dated by the person who carried out the work and the signature corroborated by someone independent of the research work. 12) The official letters issued to patent applicants may contain some off-putting language like all claims rejected, final rejection, and the like. However, these are formal official positions that can be argued against and usually overcome by the patent attorney with technical assistance from the inventor.
4.12 The Patenting Process from the Inventors Standpoint
received first by the patent attorney handling the application who will usually ask for technical input from the inventors about replying to any objections about patentability over cited references. This input has to be given high priority over other work since, although extensions may be obtainable (often with payment of a fine), failure to reply on time to the patent office may lead to the patent application being considered abandoned and all rights lost. In some patent offices (such as in the United States), it is possible to meet with the examiner in person, in an attempt to resolve difficulties and the inventor may usefully also attend, as may the patent attorney. Several official letters and replies may be required before either a notice of the intention to grant a patent or confirmation of its rejection is received. In the latter situation, it is usual to appeal the rejection to the appropriate Appeal Board within the patent office or to the relevant court. In both cases, the inventor will be required to provide technical inputs to help the legal professionals argue the appeal that is likely to be critical to its eventual success. Considerable care should be exercised before any additional experimental data is generated for the purpose of overcoming objections in official letters, especially comparative test data with prior art molecules. Negative data from such testing cannot be ignored and must also be communicated to the examiner considering a US patent application, even if the testing was carried out in connection with a non-US application. 4.12.9 Opposition to Grant
In Europe, before a European patent becomes effective in the national states designated when the application was filed, it is possible for an interested party to oppose the patent grant. This must be done within 9 months of the publication date of the patent specification in the form in which it is to be granted. Thus, the end of the European opposition period is another important milestone in the patenting process. If an opposition is filed, then the inventors input is central to its legal defense and further time commitment can be expected for several years.13) If no opposition is filed, then the application immediately becomes a bundle of national patents that can then be enforced, albeit separately, in the countries concerned. Afterward, these patents may only be challenged before the individual national courts and so successfully opposing patent grant can be a cost-effective way of disposing of a European patent.
13) Decisions of the Opposition Division of the EPO can be appealed adding still further time until the matter is finally resolved – as an extreme example, European Patent 58481 protecting continuous release depot formulations for polypeptide drug moieties such as the anticancer agent goserelin (Zo
