Cover Page

Contents

Cover

Half Title page

Title page

Copyright page

Dedication

Preface

Contributing Authors

Chapter 1: History of Drug Discovery

1 Introduction

2 Antibacterials

3 Cancer Drugs

4 Cardiovascular Drugs

5 Cholesterol Drugs

6 CNS Drugs

7 Anti-inflammatory Drugs

8 Antiulcer drugs

9 Antiviral Drugs

References

Chapter 2: Target Identification and Validation

1 Introduction

2 Definition of Drug Targets

3 Classification of Currently Utilized Drug Targets

4 Receptors as Drug Targets

5 Enzymes as Drug Targets

6 Transporter Proteins as Drug Targets

7 Modern Technologies Employed in Target Identification and Validation

8 Impact of Therapeutic Modalities on the Selection Drug Targets

9 The Future

References

Chapter 3: In Vitro and in Vivo Assays

1 Introduction

2 The Testing Funnel

3 In Vitro Assays

4 In Vivo Assays

5 Outlook

References

Chapter 4: Drug Metabolism and Pharmacokinetics In Drug Discovery

1 Introduction

2 Drug Metabolism

3 Pharmacokinetic Fundamentals

4 Pharmacokinetic Studies in Support of Drug Optimization

5 Absorption and Permeability

6 Drug Transporters

7 Protein Binding

8 Pharmacokinetics and Pharmacodynamics

9 Predicting Human Pharmacokinetics

10 Summary

References

Chapter 5: Cardiovascular Drugs

1 Introduction

2 Early History of Coronary Heart Disease (CHD)

3 Lipid-Lowering Agents

4 Antihypertensive Agents

5 Antithrombotic Drugs

6 Thrombolytic Agents

7 Antianginal Agents

8 Heart Failure Drugs

9 The Future

References

Chapter 6: Diabetes Drugs

1 Introduction

2 Current Therapies for Type 2 Diabetes

3 Other Treatments for Type 2 Diabetes

4 Novel Mechanisms of Action: Future Treatments for Type 2 Diabetes

5 Current Therapies for Type 1 Diabetes

6 Future Treatments for Type 1 Diabetes

7 Future Prospects for New Diabetes Drugs

Acknowledgment:

References

Chapter 7: CNS Drugs

1 Introduction

2 Antipsychotic Drugs

3 Antidepressant Drugs

4 Drugs for Epilepsy and Bipolar Disorder

5 Anxiolytic Drugs

6 Centrally Acting Analgesic Drugs

7 Drugs for Treating Substance Abuse and ADHD

8 Drugs for Neurodegenerative Diseases

9 Future Prospects for New CNS Drugs

References

Chapter 8: Cancer Drugs

1 Introduction

2 Historical Perspective of Cancer Drugs

3 Antimetabolites

4 Alkylating Agents

5 Platinum Complexes

6 Plant- and Marine-Based Natural Products

7 Topoisomerase Inhibitors

8 Antitumor Antibiotics

9 Tyrosine Kinase Inhibitors (TKIs)

10 Hormones

11 Histone Deacetylase (HDAC) Inhibitors

12 Miscellaneous Cancer Drugs

13 Conclusion

References

Chapter 9: Anti-Inflammatory and Immunomodulatory Drugs

1 Introduction

2 Arachidonic Acid Cascade

3 Leukotriene Pathway Inhibitors

4 Antihistamines

5 Corticosteroids

6 Rheumatoid Arthritis

7 Osteoarthritis

8 Chronic Inflammatory Arthritis and Gout

9 Multiple Sclerosis

10 Transplantation

11 Biological Agents That Suppress Cytokine Production or Signaling

12 B-Cell Therapy

13 Cytotoxic T-lymphocyte Antigen 4 (CTLA4)

14 Interleukins

15 Safety

16 Summary

References

Chapter 10: Antibacterial Drugs

1 Introduction

2 The Rise and Decline of Antibiotics

3 The Unique Challenges of Antibacterial Drug Discovery

4 Antibiotic Classes

5 Emerging Strategies to Discover New Antibacterial Drugs

6 Conclusions

References

Chapter 11: Antiviral Drug Discovery

1 Introduction

2 Human Immunodeficiency Virus-1 inhibitors

3 Hepatitis B Virus Inhibitors

4 Hepatitis C Virus Inhibitors

5 Inhibitors of Respiratory Viruses—Influenza and Respiratory Syncytial Virus

6 Herpesviridae Inhibitors

7 Epilogue

References

Index

Drug Discovery

Title Page

Dedicated To
Professor Paul Floreancig

Preface

The primary audience of this book is scientists in drug discovery in general and novice medicinal chemists in particular. In academia, most of us are trained as synthetic organic chemists. It takes us many years to learn the trade of drug discovery “on-the-job.” This book is to jump-start our understanding of the landscape of drug discovery. Veteran medicinal chemists and process chemists will benefit from this book as well by learning the perspectives of drug discovery authored by experts in different fields of drug discovery.

We are indebted to contributing authors, all of whom are the world’s leading experts in drug discovery, for their hard work and care in writing definitive summaries of their respective fields. The pharmaceutical industry is going through an interesting time. We hope that this book will help all scientists in drug discovery appreciate the practices, processes, and perspectives of drug discovery.

Jack Li and E. J. Corey
August 1, 2012

Contributing Authors

Dr. Narendra B. Ambhaikar
Dr. Reddy’s Laboratories CPS
Bollaram Road, Miyapur
Hyderabad-500 049
Andhra Pradesh, India

Dr. Makonen Belema
Medicinal Chemistry
Bristol-Myers Squibb Company
5 Research Parkway
Wallingford, CT 06492, United States

Dr. Ana B. Bueno DCR&T
Eli Lilly and Company
Avda de la Industria,
28108-Alcobendas
Madrid, Spain

Dr. Ana M. Castaño DCR&T
Eli Lilly and Company
Avda de la Industria, 30
28108-Alcobendas
Madrid, Spain

Dr. Audrey Chan
Cubist Pharmaceuticals, Inc.
65 Hayden Ave
Lexington, MA 02421, United States

Dr. Christopher W. Cianci Virology
Bristol-Myers Squibb Company
5 Research Parkway
Wallingford, CT 06492, United States

Dr. Jason Cross
Cubist Pharmaceuticals, Inc.
65 Hayden Ave
Lexington, MA 02421, United States

Dr. Stanley V. D’Andrea
Medicinal Chemistry
Bristol-Myers Squibb Company
5 Research Parkway
Wallingford, CT 06492, United State

Dr. Ira B. Dicker Virology
Bristol-Myers Squibb Company
5 Research Parkway
Wallingford, CT 06492, United State

Dr. Jeremy J. Edmunds
Medicinal Chemistry
Abbott Bioresearch Center
381 Plantation St.
Worcester, MA, 01605, United States

Dr. Yong He
Cubist Pharmaceuticals, Inc.
65 Hayden Ave
Lexington, MA 02421, United States

Dr. Adam R. Johnson
Biochemical Pharmacology
Genentech Inc.
1 DNA Way
South San Francisco, CA 94080
United States

Dr. Mark R. Krystal Virology
Bristol-Myers Squibb Company
5 Research Parkway
Wallingford, CT 06492, United States

Dr. Kimberley Lentz
Metabolism and Pharmacokinetics
Bristol-Myers Squibb Company
5 Research Parkway
Wallingford, CT 06492, United States

Dr. Jie Jack Li
Medicinal Chemistry
Bristol-Myers Squibb Company
Route 206 and Province Line Road
Princeton, NJ 08540, United States

Dr. Blaise Lippa
Cubist Pharmaceuticals, Inc.
65 Hayden Ave
Lexington, MA 02421, United States

Dr. John A. Lowe, III
JL3Pharma LLC
28 Coveside Lane
Stonington CT 06378, United States

Dr. Anthony M. Manning
Research & Preclinical Development
Aileron Therapeutics
281 Albany St.
Cambridge, MA 02139, United States

Dr. Nicholas A. Meanwell
Medicinal Chemistry
Bristol-Myers Squibb Company
5 Research Parkway
Wallingford, CT 06492, United States

Dr. Joseph Raybon
Metabolism and Pharmacokinetics
Bristol-Myers Squibb Company
5 Research Parkway
Wallingford, CT 06492, United States

Dr. Angel Rodríguez
Medical Affairs-Diabetes
Eli Lilly and Company
Avda de la Industria, 30
28108-Alcobendas
Madrid, Spain

Dr. M. Dominic Ryan
Cubist Pharmaceuticals, Inc.
65 Hayden Ave
Lexington, MA 02421, United States

Dr. Michael W. Sinz
Metabolism and Pharmacokinetics
Bristol-Myers Squibb Company
5 Research Parkway
Wallingford, CT 06492, United States

Dr. Drago R. Sliskovic
AAPharmasyn, LLC.
3985 Research Park Drive
Ann Arbor, MI 48108, United States

Dr. Kap-Sun Yeung
Medicinal Chemistry
Bristol-Myers Squibb Company
5 Research Parkway
Wallingford, CT 06492, United States

Chapter 1

History of Drug Discovery

Jie Jack Li

1 Introduction

The history of drug discovery is as ancient as our glorious history of humanity. Shen Nung, The Divine Farmer, is fabled to have sampled 365 herbs himself to evaluate their medicinal value as early as 2337 B.C. in China. In 400 B.C. in Greece, Hippocrates, The Father of Medicine, decreed the Hippocratic Oath, in which a physician is to pledge “I will preserve the purity of my life and my arts”. Another Greek Physician, Galen (129–199), influenced 45 generations with his teachings of medicine, transforming medicine from art to science. During the Renaissance, Paracelsus (1493–1541) from Switzerland represented the pinnacle of Western medicine. Since then, a parade of luminaries began to unveil the myth of life. Andreas Vesalius (1514–1564) of Brussels founded the Science of Anatomy. William Harvey (1578–1657) of England made one of the greatest discoveries in medicine—Circulation of Blood. Dutchman Antonie von Leeuwenhoek (1632–1723) opened our eyes to a whole new world of microbes by inventing the microscope.

The intellectual contributions of these great men established the foundation of modern medicine and heralded the golden age of contemporary drug discovery.

2 Antibacterials

2.1 Lister and Carbolic Acid

Although many contributed to the germ theory, French chemist Louis Pasteur (1822-1895) transformed medicine with his well-designed experiments and a treatise entitled Organized Corpuscles Existing in Atmosphere published in 1855.1 Pasteur’s tour de force officially introduced and cemented germ theory within mainstream science. The pasteurization process, heating the food at a specific temperature for a certain time and then cooling it immediately, is still the standard practice. Merely one year later, Joseph Lister (1827–1912) in England successfully applied the germ theory by using carbolic acid (phenol) as an antiseptic during surgery to kill bacteria.

As a surgeon, Lister was appalled at the postsurgical infections, which killed patients with an astonishing 40–60% mortality rate. However, simple and direct application of the pasteurization process would not be acceptable during surgery—after all one could not simply boil the patient in hot water! Inspired by anecdotal success stories of carbolic acid (an ingredient isolated from coal tar, a waste from coal gas production) in deodorizing sewage and in controlling typhoid, Lister introduced carbolic acid as an antiseptic in surgery.2 It works by solubilizing the phospholipids in cell membranes, thus disrupting the cell membranes. Today, asepsis has largely replaced antisepsis in the operating rooms.

2.2 Dr. Ehrlich’s Magic Bullet

In the 1890s, together with Emil von Behring, Paul Ehrlich (1854–1915) developed a horse serum antitoxin to quell diphtheria. The vaccine saved thousands of children’s lives during the 1891 outbreak, for which he won the Nobel Prize in 1908. Since 1910, working with his Japanese associate Sachachio Hata, Ehrlich experimented with numerous chemicals to treat syphilis.3 They had some success with an arsenic compound atoxyl, which was efficacious but was too toxic. His chemist Alfred Bertheim (1879–1914) at first elucidated the chemical constitution of atoxyl and later synthesized innumerable arsenobenzene compounds including arsphenamine (Ehrlich’s 606), which was efficacious with an acceptable safety profile.4 Ehrlich licensed the drug to Hoeschst, who sold it under the trade name Salvarsan. To find less toxic and more water-soluble ant-syphilitics, Bertheim synthesized neoarsphenamine (Neosalvarsan). Both Salvarsan and Neosalvarsan had a tremendous impact on fighting syphilis, wiping out half of the syphilis infections in Europe in a mere five years (although syphilis was not completely eradicated until the introduction of penicillin in the 1940s).

Ehrlich was also the first to propose the side chain theory and the receptor theory to explain how drugs worked. While he is immortalized as the father of chemotherapy and with his concept of magic bullets, Bertheim, probably the first medicinal chemist in history, is largely forgotten. In addition to his important contributions to the discovery of Salvarsan and Neosalvarsan, Bertheim also published a book Ein Handbuch der organischen Arsenverbindungen (A Handbook of Organic Arsenic Compounds).

2.3 Domagk and Sulfa Drugs

In 1932, Gerhard Domagk (1895–1964), the head of the Bacteriology Laboratory of I. G. Farbenindustrie Aktiengesellschaft (I. G. Farben), experimented with different dyes available to him in search of antibacterial drugs.5 Looking for antibacterials from dyes was most likely influenced by Ehrlich’s experiences with staining. By injecting dyes in mice infected with Streptococcus pyogenes bacterium, Domagk discovered that 2’,4’-diaminoazobenzene-4-sulfonamide, later branded as Prontosil, was effective in killing the bacterium without unacceptable toxic effects. The dye was prepared by Josef Klarer (1898–1953), a chemist at the Bayer Company, a branch of I. G. Farben. Later on, another Bayer chemist, Fritz Mietzsch (1896–1958), prepared the salt of Prontosil, enabling liquid formulation that was more amenable for injection. After Domagk’s first disclosure in 1935, Prontosil quickly became widely prescribed for streptococcal infections.

In 1935, a French husband and wife team, Professor Jacques Tréfouël and Madame Therèse Tréfouël, discovered that Prontosil was not active in vitro. The real active ingredient for its antibacterial activity is sulfanilamide (mistakenly being called sulfonamide even today), which is generated from in vivo metabolism of Prontosil. The mechanism of action (MOA) of sulfanilamides (antimetabolites) is through folate antagonism. Since the structure of sulfanilamide is similar to that of para-aminobenzoic acid (PABA), an essential ingredient for cell synthesis, it interrupts bacterial growth.

Domagk was bestowed the Nobel Prize in 1939. He received numerous letters from patients and doctors, expressing their gratitude for his discovery of Prontosil. In contrast, Klarer, the chemist who first synthesized it, received none.

2.4 Fleming, Florey, Chain, and Penicillin

Alexander Fleming (1881–1955)6 actually discovered penicillin in 1928 in England, 4 years before Domagk’s Prontosil. However, more than 15 years elapsed until Howard Florey (1898–1968)7 and Ernst Chain (1907–1979)8 isolated enough penicillin and demonstrated its curative effects in both mice and humans. Penicillin quickly replaced Ehrlich’s 606 and Domagk’s sulfa drugs as the most widely used antibiotic. It works for Gram-positive bacterial infections, including strep and staph infections, pneumonia, gangrene, meningitis, as well as gonorrhea (now, however, a resistant form has emerged) and syphilis. The MOA of penicillin is through inhibition of cell wall synthesis. Because animals, including humans, lack a cell wall, penicillin exerts a bactericidal action selectively on growing or multiplying germs.

2.5 Waksman, Schatz, and Streptomycin

Inspired by Fleming’s success with penicillin, Selman A. Waksman (1888–1973), a professor of soil microbiology at Rutgers College, began to look for antibiotics in soil in 1939.9 At first his group isolated a small molecule antibiotic, actinomycin, and then streptothricin. Although both of them killed Gram-negative bacteria, they were so toxic that they also killed test animals. In October 1943, Waksman’s student Albert Schatz isolated streptomycin, an aminosugar. With assistance from Merck for large-scale production and the Mayo Clinic for animal testing and clinical trials, streptomycin was proven to be both safe and effective in treating tuberculosis. Astonishingly, only 3 years elapsed from its discovery to the first successful treatment of a human patient. Nowadays, it generally takes 12 years and over $1.3 billion to bring a drug to the market.

Streptomycin was the first drug to be effective against Gram-negative bacteria. It was particularly interesting at the time because of its activity against human tubercle bacillus, which made it the first specific agent effective in treating tuberculosis. Streptomycin works by inducing the binding of “wrong” tRNA-amino-acid complexes, resulting in synthesis of false protein.

2.6 Duggar, Conover, and Tetracyclines

In 1945, 73 year old botanist Benjamin M. Duggar was a consultant for Lederle and led their screening efforts in the hunt for antibiotics. Coincidently, a sample from the University of Missouri, where Duggar taught botany 40 years earlier, yielded an antibiotic later named chlortetracycline. Lederle sold chlortetracycline under the brand name of Aureomycin in 1948. Nowadays, Benjamin Duggar is considered the pioneer of tetracycline antibiotics.10

In 1949, a yellow powder with strong antibiotic properties was isolated by Pfizer scientists from a soil sample. The soil organism was Streptomyces rimosus and the compound was generically known as oxytetracycline. Backtracking revealed that the soil sample was collected at the Terre Haute factory in Indiana owned by Pfizer, which later sold oxytetracycline under the brand name Terramycin.

Later on, Lloyd Conover at Pfizer stunned his colleagues by preparing another powerful antibiotic chemically from chlortetracycline. Under carefully controlled catalytic hydrogenation conditions, Conover converted Lederle’s chlortetracycline to tetracycline. That was the first example of a semisynthetic compound with antibiotic activities.

Tetracyclines are inhibitors of protein synthesis by inhibiting the binding of tRNA-amino-acid complexes. They are bacteriostatic.

2.7 Quinolones and Zyvox

In 1946, while pursuing better antimalarial drugs by synthesizing chloroquine, George Y. Lesher (1926–1990) at Sterling Winthrop Research Institute at Rensselaer isolated a by-product, nalidixic acid.11 It was found to be an antibacterial agent during routine screening. But it did not become popular until 1962 when Lesher introduced it into clinical practice for kidney infections. It was also used to treat urinary tract infections because it was excreted via urine in high concentration. Shortly after, the quinolone antibacterial field flourished, rendering thousands of 4-quinolone derivatives as represented by pipemidic acid. Nalidixic acid and pipemidic acid are considered the first-generation quinolone antibacterials. The drawbacks of these drugs were their moderate activity toward susceptible bacteria and poor absorption by the body.

In the early 1980s, fluorinated quinolone (fluoroquinolone) antibacterials were discovered to possess longer half-lives and better oral efficacy than the first-generation quinolones. These so-called second-generation quinolone antibacterials are exemplified by norfloxacin (the first fluoroquinolone discovered in 1980) and ciprofloxacin (Cipro).12

The third generation of quinolone antibacterials is still being actively investigated and is urgently needed due to the rapid development of resistance by bacteria toward existing antibacterial drugs. Examples of the third generation of quinolone antibacterials include fleroxacin and tosufloxacin. They are endowed with sufficiently long half-lives to enable a once-daily regimen, along with enhanced activity toward a variety of bacteria.

The MOA of quinolones is through inhibition of bacterial gyrase (topoisomerase II), thus inhibiting DNA function. Quinolones do not affect human cells because topoisomerase II only exists in bacteria.

Another important category of antibacterial agents is the oxazolidinones as exemplified by linezolid (Zyvox).13 The genesis of Zyvox began in 1978 when a DuPont patent described some novel oxazolidinones for controlling fungal and bacterial plant pathogens. These compounds and two subsequently optimized drug candidates, DuP 721 and DuP 105, did not become marketed drugs due to their unacceptable toxicities. Steven J. Brickner, a medicinal chemist working at Upjohn, was intrigued by some attributes of this class of compounds after learning of Dupont’s exploits in 1987. He immediately began an exploratory oxazolidinone project. Working with two other groups led by Michael R. Barbachyn and Douglas K. Hutchinson, they prepared eperezolid and linezolid and commenced clinical trials for both compounds in 1995. Since linezolid was more advantageous than eperezolid in terms of its pharmacokinetics, linezolid was selected to move forward and won FDA approval in 2000. Pharmacia, which took over Upjohn in 1995, sold it under the trade name Zyvox.

Zyvox works by inhibition of the initial phase of bacterial protein synthesis. It is also a MAO (monoamine oxidase)-B inhibitor but without significant blood pressure liability—bringing an interesting closure to the origin of MAO inhibitors for depression, which came about from the improvement in mood in patients with tuberculosis treated with the original MAO inhibiting agents (see Section 6.2).

3 Cancer Drugs

3.1 The Origin of Cancer

Today, the oncogene theory prevails for explaining the origins of cancer.14,15 The current view is that cancer is a multistep process, characterized by mutations in several oncogenes and the loss of function of tumor-suppressing genes. Oncogenes are activated by either an inherited defect or exposure to an outside agent, a carcinogen. However, oncogenes are normally kept inactive by tumor-suppressing genes. In some cases those controlling genes may be mutated, or removed, allowing oncogenes to run rampant. For a tumor to develop, a subject has to lose one tumor-suppressing gene in addition to having two or more oncogenes.

3.2 Chemotherapy

Chemotherapy, despite its devastating side effects, saves lives. Yet it originated at first from a chemical weapon, mustard gas. During WWII, the USS John Harvey loaded with 100 tons of mustard gas was sunk by Luftwaffe Ju-88 bombers at the Bari Harbor on the Adriatic Sea. Subsequent autopsies of 617 victims revealed that mustard gas destroyed most of their white blood cells,16 which suggested that it attacked bone marrow preferentially. Since fast division of cells is a hallmark of cancer, mustard-gas-based drugs could be applicable in cancer treatment because they slow the rate of cancer cell division. Nitrogen mustard was invented because mustard gas was impossible to administer due to its volatility. Its mechanism of action is through alkylation of DNA. Subsequently, more elaborate alkylating agents, such as cyclophosphamide (Cytoxan), discovered at Asta-Werke AG in Germany in 1956, have become the staple of chemotherapy.

Cisplatin’s chemotherapeutic effect was discovered by Barnett Rosenberg, a physics professor at Michigan State University in 1967.17 While experimenting with the effects of electric fields on cell growing in culture, he observed that an electric current interfered with cell divisions of E. coli bacteria in suspension. Further investigation revealed that the electric current generated by the platinum electrodes resulted in the formation of cationic platinum. Rosenberg tested cisplatin as a drug against intestinal bacteria first and tumors later. Bristol-Myers Squibb (BMS) successfully developed cisplatin and won the FDA’s approval in 1978. Cisplatin (Platinol) has become one of the most widely used chemotherapeutics in treating metastatic testicular cancer, ovarian tumors, and bladder cancer. BMS developed a follow-up to cisplatin in collaboration with Johnson Matthey: carboplatin. Sanofi-Aventis licensed oxaliplatin from Switzerland’s Debiopharm and developed it for treating colorectal cancer. Approved in 2002 in the U.S. under the name Eloxatin, it is usually administered with 5-fluorouracil and leucovorin in a combination called Folfox. During the past 40 years, a few thousand platinum complexes have been evaluated and about 30 have reached the clinic, but none has surpassed the three original ones which possess superior efficacy and safety profiles.

In 1958, during their pursuit of diabetes drugs, R. L. Noble and C. T. Beer at the University of Western Ontario in London, Ontario, tested extracts from Madagascar periwinkle (Vinca rosea) on rabbits. The animals subsequently developed critically low counts of white blood cells, leaving them with damaged bone marrow and defenseless against bacterial infections. Noble and Beer then tried the plant extract on animals with transplanted tumors because again cancer is characterized by abnormal proliferation of white blood cells. After seeing tumor shrinkage, they further investigated and isolated two important cancer drugs from vinca alkaloids: vincristine and vinblastine.18 The vinca alkaloids work by serving as a “spindle poison”. They bind to tubulin, one of the key constituents of microtubules, thus preventing the cell from making the spindles it needs to divide. Thanks to vincristine (Oncovin) and vinblastine (Velban), Hodgkin’s disease patients now have a 90% chance of survival.

Vinca alkaloids are not the only class of anticancer drugs derived from plants. Taxol is another prominent example. In 1962, Arthur Barclay, under the aegis of the NCI-USDA (National Cancer Institute-United States Department of Agriculture) plant-screening program, traveled to the Gifford Pinchot Forest in Washington State. He collected samples of twigs, leaves, and fruits of a then little known pacific yew tree, Taxus brevifolia, and shipped them to the NCI.19 One of the NCI’s contractors was the Wisconsin Alumni Research Foundation, which tested the extracts and found them to be cytotoxic. In 1966, after being rejected by many other laboratories for fear of toxicity, the stem barks found their way to the hands of Monroe E. Wall, chief chemist of the Fractionation and Isolation Laboratory at the Research Triangle Institute in North Carolina. Wall and his colleague, Mansukh C. Wani, isolated the active principle using the “bioactivity-directed fractionation” process. They also elucidated the compound’s intricate structure and christened it taxol. The interest in taxol was greatly piqued when Susan Howitz, a professor at the Albert Einstein College of Medicine, discovered that taxol had a completely novel mechanism of action. It exerts its action by stabilizing microtubules (one of the components of the cytoskeleton), resulting in inhibition of mitosis and induction of apoptosis.

The NCI commenced the Phase I clinical trials of taxol in 1984 and Phase II trials in 1987 with positive results. BMS, the only major U.S. pharmaceutical company to have made a bid, was awarded the molecule. BMS successfully carried out the Phase III trials for taxol. The FDA approved taxol for use in refractory ovarian cancer in 1992, for breast cancer in 1994, and later for non-small-cell lung cancer and Kaposi’s sarcoma. Meanwhile, French company Rhône-Poulenc marketed a competing drug docetaxel (Taxotere), discovered by French chemist Pierre Potier, who invented docetaxel by a minor modification of taxol (replacing the benzyl amide group on taxol with a tert-butoxyl-carbamate group).

3.3 Hormone Treatment

The correlation between sex hormones and cancer has been well established. Breast cancer in particular is linked to estrogen abnormality. Tamoxifen, the most frequently prescribed anticancer drug in the world, was initially made as a contraceptive. In the 1960s, Imperial Chemical Industries Ltd. (ICI) chemist Dora M. Richardson discovered ICI-147,741, the trans-isomer of triphenylethylene, which would later become tamoxifen. Although it was shown to be an effective contraceptive in rats, it induced ovulation in women, exactly the opposite of what it did to rats. Fortunately, Arthur Walpole along with his endocrinologist colleague Michael J. K. Harper included cancer as an indication in tamoxifen’s clinical trials.20 Tamoxifen was found to be efficacious. It was marketed in the UK as a breast cancer treatment (1973) and as an inducer of ovulation (1975). In 1978, the FDA approved tamoxifen for treatment of estrogen receptor-positive metastatic breast cancer in the US.

Tamoxifen is a selective estrogen receptor modulator (SERM). It modulates the estrogen hormone level by mimicking estrogen. Therefore it is especially effective in treating hormone-receptor-positive breast cancer. A newer SERM, raloxifene (Evista), marketed by Eli Lilly as an osteoporosis drug, renders a 58% reduction of breast cancer.

3.4 Small-Molecule Protein Kinase Inhibitors

In contrast to the carpet-bombing approach of old chemotherapy, protein kinase inhibitors, as targeted cancer drugs, are a more effective method for cancer treatment with fewer side effects. Protein kinases are enzymes inside the cell that are capable of donating phosphate groups to target proteins. Protein kinases comprise a family of more than 150 members. They are responsible for signal transduction, turning on and off the switches that control cancer cell growth. Many protein kinases have been implicated in cancer. It has been shown that blocking the functions of protein kinases can stop cancer growth.

Trastuzumab (Herceptin), a bioengineered human monoclonal antibody developed by Genentech, was approved for the treatment of breast cancer in 1998.21 The success of Herceptin inspired the pursuit of small-molecule protein kinase inhibitors. Using a protein kinase C-α inhibitor as a starting point, Ciba-Geigy chemist Jürg Zimmermann carried out methodical structure-activity relationship (SAR) investigations.22 From among over 300 analogs emerged imatinib (Gleevec), a selective Bcr-Abl-tyrasine kinase inhibitor. It was launched in 2001 for the treatment of chronic myeloid leukemia (CML). Later study revealed that Gleevec actually blocks a panel of at least 8 protein kinases, including Bcr-Abl, platelet-derived growth factor receptor (PDGFR), and c-kit.

AstraZeneca’s gefitinib (Iressa) and OSI’s erlotinib (Tarceva), both EGFR inhibitors, entered the market in 2003. Sugen’s sunitinib (Sutent) exhibits potent antiangiogenic activity through the inhibition of multiple receptor tyrosine kinases (RTKs). Specifically sunitinib inhibits vascular endothelial growth factor receptors VEGFR1, VEGFR2, and VEGFR3 and platelet-derived growth factor receptors PDGFR-α and PDGFR-β. In addition, sunitinib also targets receptors implicated in tumerogenesis, including fetal liver tyrosine kinase receptor 3 (Flt3) and stem cell factor receptor (c-KIT). Bayer’s sorafenib (Nexavar), launched in 2005, is a VEGFR inhibitor. Similar to Gleevec, BMS’s dasanitib (Sprycel) and Narvotis’s nilotinib (Tasignal) came to market in 2006 and 2007, respectively. They both block the Bcr-Abl kinase, which is largely responsible for causing chronic myeloid leukemia (CML).

4 Cardiovascular Drugs

British physician William Harvey’s (1578–1657) discovery of the circulation of blood is considered one the greatest discoveries made in physiology.23 His most celebrated monograph, Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus (Anatomical Exercise on the Motion of the Heart and Blood of Animals) was published in 1628. Over a century later, another Englishman, William Withering, discovered the medicinal use of foxglove in treating cardiac diseases.

4.1 Withering and Digitalis

William Withering (1741–1799), a physician in Birmingham, bought from a Gypsy lady the secret formula of her special herbal tea, which showed remarkable results in treating dropsy (similar to today’s congestive heart failure).24 Through careful investigation, Withering correctly concluded that among 20 or so ingredients in the herbal tea, purple foxglove was the active ingredient. Withering spent the next decade exploring the curative effects of digitalis, the major principle. Digitalis, as well as many cardiac glycosides, is toxic when taken in large doses. If the dose is too low, then it is ineffective. Digitalis is only effective when administered at or near toxic dose, so finding the correct dosage is very important. Withering’s finding in choosing the precise dosage propelled digitalis to become one of the most valuable cardiac drugs ever discovered.

Digitalis was originally prepared from the powdered leaves of foxglove. Nowadays, digitalis isolated from foxglove leaves is further crystallized to afford cardiac glycosides such as digoxin and digitoxin, which are more easily quantified.

4.2 Sobrero, Nobel, and Nitroglycerin

Italian chemist Ascanio Sobrero (1812–1888) first synthesized nitroglycerin in 1847,25 by nitration of glycerol using a cold mixture of nitric acid and sulfuric acid. When he tasted it, he found it sweet, pungent, and aromatic; but a very minute quantity put upon the tongue produced a violent headache for several hours. That was the first recorded vasodilating effect attributed to a drug.

Alfred Nobel (1833–1896) from Stockholm created dynamite using a porous silica gel to absorb the unstable nitroglycerin. The patented detonator made a fortune for the Nobel family. In explosive production plants, munitions workers often experienced facial flush and severe headache when they returned to work after being away from the factory over the weekend. Further investigation revealed that nitroglycerin was a powerful vasodilator which led to its use as a circulatory system vasodilator.

The MOA of nitroglycerin’s treatment of angina was not known until the 1980s. It turns out that soluble guanylated cyclase could be activated by free radicals such as nitric oxide (NO) generated from nitroglycerin. Nitric oxide is a signaling molecule in the cardiovascular system.

In addition to nitroglycerin, many organic nitrates are used to treat angina pectoris. These organic nitrates are typically prepared by nitration of polyols (molecules with many alcohol functionalities). Examples include isosorbide dinitrate (ISDN), pentaerythritol tetranitrate, and erythrityl tetranitrate.

4.3 Vogl and Diuretics

Diuretics, also known as water pills, and β-blockers are the most prescribed drugs for heart conditions. Serendipity played an important role in the discovery of the first mercurial diuretic.

In 1919, Alfred Vogl, a third-year medical student in Vienna, gave a new mercurial anti-syphilitic, merbaphen (Novasurol), to treat a patient’s congenital syphilis.26 To his astonishment, the patient’s urine output was 5–6 times the normal amount. By removing fluid, the pressure on the heart was removed. Mercurial diuretics revolutionized the treatment of severe edema from congestive heart failure and were the primary treatment for this disease until the late 1950s and the emergence of thiazide diuretics.

In 1957, in pursuit of diuretic agents, Merck chemist Frederick C. Novello wanted to make some analogs of an older sulfa drug, dichlorophenamide.27 Surprisingly, the reaction gave the ring formation product rather than the linear derivatization product. The bicyclic ring formed was a benzothiadiazine derivative, chlorothiazide. Further testing proved it to be a potent diuretic without elevation of bicarbonate excretion, an undesired side effect. Chlorothiazide was the first ever nonmercurial, orally active diuretic drug whose activity was not dependent on carbonic anhydrase inhibition, such as acetazolamide.

Currently the most frequently prescribed diuretic is hydrochlorothiazide discovered by Ciba scientists led by George deStevens.28 In 1957, George deStevens became aware of the research of Frederick Novello on the synthesis of disulfonamides in general and chlorothiazide in particular. A simple modification from a double bond on chlorothiazide to a single bond gave him a hydrochlorothiazide that was 10-fold more potent than the prototype. Hydrochlorothiazide was introduced into medical practice in 1959 and within a short time became the drug of choice in the treatment of mild hypertension.

4.4 Snake Venom and ACE Inhibitors

Angiotensin converting enzyme (ACE) inhibitors are widely used in treating hypertension, congestive heart failure, and heart attacks. In 1967, John Vane at Oxford University and his colleague Mick Bakhle tested a poisonous venom extract of the Brazilian pit viper Bothrops jararaca.29 It was found to be a potent ACE inhibitor in vitro.

Vane suggested to the Squibb Institute that they study snake venom extract for its effects on the cardiovascular system. Biochemist David Cushman and organic chemist Miguel A. Ondetti at Squibb-isolated a nonapeptide, teprotide. Using teprotide as a starting point, Cushman and Ondetti curtailed the molecule and replaced its carboxylate group with a thiol (-SH) and achieved a 2,000-fold increase in potency in ACE inhibition. The resulting drug became the first oral ACE inhibitor, captopril (Capoten).

To improve on captopril, which suffered some side effects due to the presence of the thiol group, Merck scientists led by Arthur A. Patchett started by replacing the thiol group with the original carboxylate due to thiol’s liabilities.30 The loss in potency of the carboxylate was compensated by modification of the molecule elsewhere. They arrived at enalaprilat, which suffered poor oral bioavailability. They simply converted the acid into its corresponding ethyl ester, creating enalapril, a prodrug of enalaprilat, with excellent oral bioavailability. One advantage of a prodrug is the delay in onset of action, which can be beneficial for a drug to treat blood pressure. The longer duration of action also allows a once-daily dosage. It is also devoid of the side effects associated with the thiol group, including bone marrow growth suppression (due to a decrease in circulating white blood cells), skin rash, and loss of taste. In 1981, Merck sold enalapril using the brand name Vasotec, which became its first billion-dollar drug in 1988. Another popular ACE inhibitor is quinapril hydrochloride (Accupril), discovered by Parke–Davis.

Since angiotensin II is a potent vasoconstrictor, blocking its action would result in vasodilation. Dupont–Merck Pharmaceuticals exploited the angiotensin II receptor in the early 1980s. The fruit of the effort was losartan (Cozaar), which quickly became one of the most important drugs for the treatment of high blood pressure after its launch in 1995. Other angiotensin II receptor antagonists (also known as angiotensin receptor blockers, ARBs) include Novartis’s valsartan (Diovan), Sanofi-Synthélabo’s irbesartan (Avapro), AstraZeneca’s candesartan (Atacand), and Boehringer Ingelheim’s telmisartan (Micardis). They proved to be superior to ACE inhibitors because they did not cause the irritating cough that occurs in a small percentage of patients taking ACE inhibitors.

4.5 Black and Beta-Blockers

As early as 1948, Raymond P. Ahlquist at the Medical College of Georgia speculated that there were two types of adrenergic receptors (adrenoceptors in short), which he termed α-adrenoceptor and β-adrenoceptor. In 1957, Irwin H. Slater and C. E. Powell at Eli Lilly prepared dichloroisoprenaline (DCI, the dichloro analog of isoprenaline) and it was later demonstrated to be the first selective β-adrenoreceptor blocking reagent, also known as a β-blocker. However, DCI was not further pursued as a drug because it had a marked undesirable stimulant effect on the heart, an intrinsic sympathomimetic action (ISA).

Ahlquist’s theory of two adrenergic receptors inspired British pharmacologist James Black to look for drugs with β-receptor blocking properties in a systematic way later known as rational drug design. Before then, drugs were mostly discovered by screening compounds randomly against animal models.

Black famously said, “The most fruitful basis for the discovery of a new drug is to start from an old drug.”31 At Imperial Chemical Industries (ICI) Pharmaceutical Division, together with chemist James Stephenson, Black led a team to look for β-blockers that were devoid of the stimulant effect on the heart since 1958. In 1962, Stephenson and his colleagues succeeded in making a β-blocker pronethalol using DCI as the starting point (pronethalol replaced the dichlorobenzene ring with a naphthaline ring). Unfortunately, pronethalol was withdrawn from further development when it was found to cause thymic tumors in mice. In 1964 ICI eventually produced the drug propranolol (Inderal), which possessed a better efficacy and safety profile. Propranolol is now widely used in the management of angina, hypertension, arrhythmia, and migraine headaches. Two additional β-blockers, atenolol (Tenormin) and practolol (Dalzic), were later discovered and marketed by ICI.

At one point or another, almost all the major pharmaceutical companies had a me-too β-blocker on the market. It brought a windfall to the drug industry.

4.6 Renin Inhibitor

Since renin is extremely specific for angiotensinogen and the first and rate-limiting enzyme of the renin-angiotensin system (RAS), renin inhibition was recognized for decades as an attractive approach for the treatment of hypertension and hypertension-related target organ damage.32

Ciba-Geigy was a pioneer in the rennin field in the race to develop orally available renin inhibitors, but the clinical development of aliskiren was in jeopardy following the merger of Ciba-Geigy and Sandoz to form Novartis in 1996 and the successful launch of the antihypertensive angiotensin receptor blocker (ARB) valsartan (Diovan) in 1997. A group of former Ciba-Geigy employees convinced Novartis to out-license the Phase I/II development of aliskiren and formed the biopharmaceutical company Speedel to accomplish this task.33 Speedel was successful in developing a commercially viable process and demonstrating clinical proof of concept (POC), and Novartis exercised a call-back option in 2002. Following an extensive Phase III development program, aliskiren (Tekturna) has been on the market for treatment of hypertension since 2007.

4.7 Fleckenstein and Calcium Channel Blockers

Calcium channel blockers (CCBs), also known as calcium channel antagonists or calcium entry blockers, are drugs that inhibit the influx of Ca2+ ions into cells without affecting inward Na+ or outward K+ currents to a significant degree. They are widely used in the treatment of high blood pressure, angina, and rapid heartbeat (tachycardia), including arterial fibrillation.

In 1963, Albrecht Fleckenstein at the University of Freiburg in Germany investigated two newly synthesized coronary vasodilators, prenylamine and verapamil, which had unexplained cardiodepressant side effects.34 Fleckenstein and his colleagues observed that both compounds exerted a negative inotropic effect caused by calcium. They concluded that this negative inotropism was due to an ability of these drugs to block excitation-induced calcium influx. In 1966 Fleckenstein then coined the term “calcium antagonists” because both drugs mimicked the cardiac effects of simple calcium withdrawal.

In 1969, Professor Kroneberg, the leading pharmacologist of Bayer Company, handed Fleckenstein Bay-a-1040 and Bay-a-7168. Both compounds were strong coronary vasodilators and exerted significant negative inotropic effects on the myocardium. Fleckenstein also found that the mechanism of action of those two drugs appeared to be similar to that of verapamil. Later on Bay-a-1040 and Bay-a-7168 were given generic names nifepidine and niludipine, respectively. Nifepidine (Adalat) and niludipine heralded the beginning of one of the most important classes of calcium antagonists: 1,4-dihydropyridines.35

Bayer’s nifepidine is a short-acting calcium channel blocker and thus has to be taken several times a day. On the other hand, Pfizer’s amlodipine (Norvasc) has a high bioavailability and a longer half-life in plasma; thus it can be taken once daily.

4.8 Blood Thinners, from Heparin to Plavix

Thrombosis and embolus take place when platelets are overactive, causing promotion of blood coagulation. Blood thinners attenuate the functions of platelets and fibrin to prevent thrombosis or embolus. The first blood thinner in clinical use was heparin, acidic sulfated polysaccharides. Jay McLean (1890–1957), working in the laboratories of Prof. William Henry Howell (1860–1945) at the Johns Hopkins University, discovered the anticoagulation substance from dog’s liver isolate in 1917. Two years later, Howell and his student L. Emmett Holt, Jr. further purified the anticoagulation substance and christened it heparin.36 Charles Best of the insulin fame returned at the University of Toronto and embarked on a journey to produce purified heparin since 1929. Working with organic chemists Arthur Charles and David Scott, they prepared the barium and later the sodium salt of heparin, which rendered uniformly consistent composition and potency. That, in turn, enabled Best and Gordon Murray to carry out clinical trials with purified heparin in 1935. Nowadays, small-molecular-weight heparin fractions are used in clinics because only a fraction of the heparin molecule is active in blocking blood clotting factor Xa (heparin’s MOA).

Heparin, highly sulfated glycosaminoglycan, has to be given intravenously. The first identified oral anticoagulant was warfarin, discovered by Karl Paul Link (1901–1978) at the University of Wisconsin in the 1940s. In 1933, Link began to investigate the cause of cattle death due to internal hemorrhage after ingestion of sweet clovers. By 1939, his group isolated and characterized the active principle as dicumarol. In analogy to hemorrhage of cows, Link sought to look for rodenticide from the dicumarol analogs to kill rats by hemorrhage. One of them, WARF-42, emerged as the best rat poison in 1948.37 In the early 1950s, clinical trials for warfarin as a blood thinner commenced. It now is the most used oral anticoagulant in history. The MOA of warfarin is inhibition of VKORC1 (vitamin K epoxide reductase complex, subunit 1) and vitamin K epoxide reductase.

Aspirin was synthesized in 1897 by Bayer’s Felix Hoffmann as an analgesic. In the 1940s, Lawrence Craven, a family doctor in California, observed aspirin’s anticlotting properties. Harvey J. Weiss at Columbia University was one of the first to discover aspirin’s antiplatelet effect in 1967. Since 1985, an aspirin a day has been a popular prophylaxis to prevent a second heart attack. Aspirin works by inhibiting prostaglandin synthetase, which explains most of its antiplatelet, antipyretic, and anti-inflammatory properties.

In 1972, Jean-Pierre Maffrand of Sanofi made analogs of Yoshitomi’s tinoridine (Nonflamin) to find drugs with improved anti-inflammatory properties. But the compounds they made had no anti-inflammatory properties at all. Further scrutiny revealed that they inhibited blood platelet aggregation. Immediately realizing the important roles that platelets played in myocardial infarction and brain ischemia, the team set out to find a platelet aggregation inhibitor superior to aspirin. The fruit of labor was ticlopidine (Ticlid). Unfortunately, a rare but potentially fatal side effect called thrombotic thrombocytopenic purpura (TTP) and several other side effects severely limited ticlopidine’s use in patients.38

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