■20 CHAPTER I A History of Drug Discovery (Boston,USA).completed the first total synthesis of pen- effect of certain fungi.especially actinomycetes.on bac- metho swerenoteicietforSsprod nomycin acillus but too toxie for use in humans.Few months later Waksman.with Alber Schatz and Elizabeth Buisated ons.The first major developm griseu sever of th 510 was produced in a batch p suffered from the disease worldwide.Sulfonamidesand 6-aminopenicillanicacid(6-APA)with theaid ofaphenylgly- penicillins being ineffective against Mycobacterim muber ulosis,Waksman studie the value of ing that dis Feldman and H.Corwin Hinshaw at the Mayo Clinic (Rochester.USA)confirmed streptomycin's efficacy and S.aureus were di although elatively low toxicity against tuberculosis in guinea pigs the he 20 19% ued after she was treated for Staphylococcus neumonige during the shooting of the ease was visibly arrested,the bacteria disappeared from his ilm "Cleopatra development with great com p only problem the Nobel Prize in Physiol b)streptomycin and antituberculosis drugs discovery of streptomycin (and 17 other antibiotics discov Since 14.Selman A.Waksman (Figure 1.9) screened ppeare ing wing yea ustin Bradfor 1 mgritb p or nom 客 toxicity of streptomy Co.and Hoffmann-LaRoche.Purportedly.after an experi rthan 000 mice and the e ed ultimately as isoniazid and proved especially effective in mixed dosage with strep omycin or PAS.In two decades,after PAS acid (1949)an 1963 erine (195 as capreomvcin.viomvcin.kanamvcin.and amikacin.and ro(ofcin andcipofl acquire bercu (c)Chloramphenicol FIGURE 119 Selman A.Waksman
20 CHAPTER 1 A History of Drug Discovery (Boston, USA), completed the fi rst total synthesis of penicillin and some of its analogs in the early 1950s, but his methods were not effi cient for mass production. The narrow spectrum of activity of the penicillins, along with the selective activity of the orally active phenoxymethylpenicillin, led to the search for derivatives of penicillin which could treat a wider range of infections. The fi rst major development was ampicillin, which offered a broader spectrum of activity than either of the original penicillins. 116 Ampicillin (1961) was produced in a batch process by enzymatic acylation of 6-aminopenicillanic acid (6-APA) with the aid of a phenylglycine derivative such as d -phenylglycine amide. Before even ampicillin, however, in 1960, two companies: Beecham and Bristol brought out methicillin more resistant to the betalactamase enzyme produced by Staphylococcus aureus . But almost immediately, strains of methicillin-resistant S. aureus were discovered, although for many years their number was low. Meanwhile, the new drug seemed to bring a solution to the threat of S. aureus . Famously, the life of the actress Elizabeth Taylor was rescued after she was treated for Staphylococcus pneumoniae during the shooting of the fi lm “ Cleopatra ” . Further development with great commercial impact yielded beta-lactamase-resistant penicillins including fl ucloxacillin, dicloxacillin, and oxacicillin. 117 (b) Streptomycin and antituberculosis drugs Since 1914, Selman A. Waksman ( Figure 1.19 ) screened systematically soil bacteria and fungi and, at the University of California, in 1939 he discovered the marked inhibitory effect of certain fungi, especially actinomycetes, on bacterial growth. In 1940, he and his team were able to isolate actinomycin from Actinomyces griseus (later named Streptomyces griseus ), an antibiotic effective against Koch’s bacillus, but too toxic for use in humans. Few months later, Waksman, with Albert Schatz and Elizabeth Bugie, isolated the fi rst aminoglycoside, streptomycin, from S. griseus . 118 In 1942, several hundred thousand deaths resulted from tuberculosis in Europe, and another 5–10 million people suffered from the disease worldwide. Sulfonamides and penicillins being ineffective against Mycobacterium tuberculosis, Waksman studied the value of streptomycin in treating that disease. Merck immediately started manufacturing streptomycin. Simultaneously, studies by William H. Feldman and H. Corwin Hinshaw at the Mayo Clinic (Rochester, USA) confi rmed streptomycin’s effi cacy and relatively low toxicity against tuberculosis in guinea pigs. On November 20, 1944, doctors administered streptomycin for the fi rst time to a seriously ill tuberculosis patient and observed a rapid, impressive recovery. His advanced disease was visibly arrested, the bacteria disappeared from his sputum, and he made a rapid recovery. The only problem was that the new drug made the patient deaf: streptomycin was particularly toxic on the inner ear. In 1952, Waksman was awarded the Nobel Prize in Physiology or Medicine for his discovery of streptomycin (and 17 other antibiotics discovered under his guidance). A succession of tuberculicid drugs appeared during following years. These were important because with streptomycin monotherapy, resistant mutants began to appear. In 1950, British physician Austin Bradford Hill demonstrated that a combination of streptomycin and p- aminosalicylic acid (PAS) could better cure the disease, although the toxicity of streptomycin was still a problem. By 1951, an even more potent antituberculosis drug was developed simultaneously and independently by the Squibb Co. and Hoffmann-LaRoche. Purportedly, after an experiment on more than 50,000 mice and the examination of more than 5,000 compounds, this drug, isonicotinic acid hydrazide was proved to be able to protect against a lethal inoculum of tubercle bacteria. It was marketed ultimately as isoniazid and proved especially effective in mixed dosage with streptomycin or PAS. In two decades, after PAS acid (1949) and isoniazid (1952), pyrazinamide (1954), cycloserine (1955), ethambutol (1962) and rifampin (1963) were introduced as other weapons against tuberculosis. Aminoglycosides such as capreomycin, viomycin, kanamycin, and amikacin, and recently, the newer quinolones (ofl oxacin and ciprofl oxacin) are only used in drug resistance situations. With acquired immune defi ciency syndrome (AIDS) pandemic, tuberculosis in particular, experienced a dreadful come-back. (c) Chloramphenicol Originally isolated by David Gottlieb (University of FIGURE 1.19 Selman A. Waksman . Illinois, USA) from the soil organism Streptomyces Ch01-P374194.indd 20 h01-P374194.indd 20 5/29/2008 5:42:09 PM /29/2008 5:42:09 PM
Il.Two Hundred Years of Drug Discoveries 21■ e in 1947,has been in the fir Tetracyclines derivatives ohial inst Gra was a bacteriostatic,but at higher concentrations or against negative bacteria and even some protozoa infections. of the (e)Erythromycin red verv In 1949.Abelardo Aguilar.a Filipino scientist.sent soil samples to his er nplover Eli Lilly.Eli Lilly's research (d)Tetracyclines team,led by J.M.McGuire,managed to isolate erythromy Closely congeneric derivative vclic from th metabolic productsof strain of Streptomyce found in tha by Benjamin Minge Duggar.consultant in mycological 948 to Lederle La ncan Cyanamid the Philippines region of lloilo,where it was originally col after formerly being also alled 1n0 time more than one hundred of various molecules of the tetracycline family (Figure 1.20).active against a wide range of doll. of bacteria,were disc The first of these compour a large team of researchers reported the first stereo-con hich exploring the molecular architecture of the broad asymme synthesis of Erythromycn antibiotics oxytetracycline (Terramycin@)and chlortetra- B.eomycin steam and orking with astnucturecosetomaColideschnihaycngo in Har (f)Vancomycir chemically alter an antibiotic to produce other antibiotics that could be very effective In 1952 a missionary in Bo 、Indonesia)sent a ole of dirt to his friend E C Kornfield an organic chem n1952,S over developed tetra (Figure 1.20) ist at Eli Lilly.An organism isolated from that sample orientalis)produced a subst ance ("com oted an industry-wide rch for against mos pos 2 of the important antibiotic discoveries made since then. clostridia,and Neissr were initiated to determine whether the activity of com- pound 05865 would be preserved despite attempts to induce stanc 05865 125 Isolates from other laboratories were also tested and the results were similar.Sub quent animal experi ments sugg t be safe an duhhed "Mississinni mud"because of its hrown color was purified and the resulting drug.which was named mycin"(from the word"vanguish").was made available (g)Cephalosporins (related to the e the FIGURE 1.20 The vcline molecular model cultures of Cephalosporium acremonium from a sewer
II. Two Hundred Years of Drug Discoveries 21 venezuelae in 1947, has been introduced into clinical practice in 1949. It was the fi rst antibiotic to be manufactured synthetically on a large scale. 119 Usually chloramphenicol was a bacteriostatic, but at higher concentrations or against some very susceptible organisms it could be bactericidal. Manufacture of oral chloramphenicol in the United States stopped in 1991, because the hematological toxicity of the drug, which appeared very early in the 1950s. 120 (d) Tetracyclines Closely congeneric derivatives of the polycyclic napthacene-carboxamide were discovered as natural products by Benjamin Minge Duggar, consultant in mycological research to Lederle Laboratories (American Cyanamid), in 1948. The discovery of the tetracycline ring system also enabled further development in antibiotics. 121 Since that time more than one hundred of various molecules of the tetracycline family ( Figure 1.20 ), active against a wide range of bacteria, were discovered. The fi rst of these compounds, chlortetracycline was isolated from Streptomyces aureofaciens . At Pfi zer, Lloyd Conover joined a team which was exploring the molecular architecture of the broad-spectrum antibiotics oxytetracycline (Terramycin®) and chlortetracycline (Aureomycin®). With his team and working with Robert B. Woodward (1965 Nobel laureate in chemistry), in Harvard University, Conover realized it was possible to chemically alter an antibiotic to produce other antibiotics that could be very effective. In 1952, Conover developed tetracycline ( Figure 1.20 ) from chlortetracycline by removal of its chlorine atom by catalytic hydrogenation, and then oxytetracycline. 122 The discovery prompted an industry-wide search for superior structurally modifi ed antibiotics, which has provided most of the important antibiotic discoveries made since then. Tetracyclines including semi-synthetic derivatives like doxycycline and minocycline are offering a wide range of antimicrobial activity against Gram-positive, Gramnegative bacteria and even some protozoa infections. (e) Erythromycin In 1949, Abelardo Aguilar, a Filipino scientist, sent some soil samples to his employer Eli Lilly. Eli Lilly’s research team, led by J. M. McGuire, managed to isolate erythromycin from the metabolic products of a strain of Streptomyces erythreus (designation changed to “ Saccharopolyspora erythraea ” ) found in the samples. The product was launched commercially in 1952 under the brand name Ilosone® (after the Philippines region of Iloilo, where it was originally collected from) after formerly being also called Ilotycin®. Even if the drug has earned American drug giant Eli Lilly billions of dollars, neither Aguilar nor the Philippine government ever received royalties. In 1981, Robert B. Woodward and a large team of researchers reported the fi rst stereo-controlled asymmetric chemical synthesis of Erythromycin. To overcome the acid instability of erythromycin, Taisho Pharmaceutical (Tokyo, Japan) found a new antibiotic, with a structure close to macrolides: clarithromycin. 123 (f) Vancomycin In 1952, a missionary in Borneo (Indonesia) sent a sample of dirt to his friend, E. C. Kornfi eld, an organic chemist at Eli Lilly. An organism isolated from that sample (Streptomyces orientalis) produced a substance ( “ compound 05865 ” ) that was active against most Gram-positive organisms, including penicillin-resistant staphylococci , 124 clostridia , and Neisseria gonorrhea. In vitro experiments were initiated to determine whether the activity of compound 05865 would be preserved despite attempts to induce resistance. After 20 serial passages of staphylococci , resistance to penicillin increased 100,000-fold, compared with only a 4- to 8-fold increase in resistance to compound 05865. 125 Isolates from other laboratories were also tested, and the results were similar. Subsequent animal experiments suggested that compound 05865 might be safe and effective in humans. Before clinical trials, the compound, dubbed “ Mississippi mud ” because of its brown color, was purifi ed and the resulting drug, which was named “ vancomycin ” (from the word “ vanquish ” ), was made available. 126 Vancomycin kept its major interest for Gram-positive infections in which bacteria had proved resistant to methicillingroup antibiotics. (g) Cephalosporins In the early 1960s, came the emergence of the cephalosporins (related to the penicillins) fi rst isolated from FIGURE 1.20 The tetracycline molecular model. cultures of Cephalosporium acremonium from a sewer Ch01-P374194.indd 21 h01-P374194.indd 21 5/29/2008 5:42:10 PM /29/2008 5:42:10 PM
■22 CHAPTER I A History of Drug Discovery in Sardinia in 1948 by Italian scientist Guiseppe Brotzu the ofECetfec ，against at the William Dunn School of Pathology (Oxford.UK)iso iently potent for clinical us The cept ous to the penicillin nucleus 6-aminopenicillan acid tACside-chaims the developmen 19 keted.First-generation (cephalexin.cefazolin.cefadroxil) are moderate spectrum agents. FIGURE 121 George Y.Lesher includes cillinase-produ ing arug or choic BLE I.Quinolones:Structure-Activity hut have no activity Relationship giad1erineatomat cocci They are also more resi generation cephalosporins (cefotaxime. ceftriaxone ceftazidime)have a broad spectrum of activ ity and y aga tho ith a domonas activity)have decreased activity against gram Alkylation of the mpr positive organisms. They may be particularly useful in C-7 ring p ons. cephalosporins (cefepime)are extended-spectrum agents against Gram-positive organisms as osporins.They also have a greater merasesⅡand was used transiently until ocular toxicity was (h)Quinolones Sho afterwards, second-generation agents ope epitomized by ciprofloxa uccdafc f in antibacterial activit 111 asaby-product-naphthyridine.duringn the particular against Gram-negative bacteria.and is effective synthesis of the antima n the treatment of many types of infection.Despite excellen rial compound chloroquine Since covery,the utility of nalic Its in many r repor xic acid was failure in the treatment of bacteril infection.The in the 2001 anthrax scare molecular structures of the quinolones have adapted 4.The problem of resistance over time i non with need (Tabl at key qu it po
22 CHAPTER 1 A History of Drug Discovery in Sardinia in 1948 by Italian scientist Guiseppe Brotzu (University of Cagliari). He noticed that these cultures produced substances that were effective against Salmonella typhi , the cause of typhoid fever. Researchers at the William Dunn School of Pathology (Oxford, UK) isolated cephalosporin C, which had stability to β-lactamases but was not suffi ciently potent for clinical use. The cephalosporin nucleus, 7-aminocephalosporanic acid (7-ACA), was derived from cephalosporin C and proved to be analogous to the penicillin nucleus 6-aminopenicillanic acid. Modifi cation of the 7-ACA side-chains resulted in the development of useful antibiotic agents. The fi rst agent cephalothin was launched by Eli Lilly in 1964. Since 40 years, four “ generations ” of cephalosporins had been marketed. First-generation (cephalexin, cefazolin, cefadroxil) are moderate spectrum agents, with a spectrum that includes penicillinase-producing methicillin-susceptible cocci , though they are not the drugs of choice for such infections. They also have activity against some E. coli, K. Pneumoniae , but have no activity against B. fragilis, enterocci , methicillin-resistant staphylococci, Ps. aeruginosa , etc. The second-generation cephalosporins (cefuroxime cefoxitin) have a larger Gram-negative spectrum while retaining some activity against streptococci or staphylococci . They are also more resistant to β -lactamase. Third generation cephalosporins (cefotaxime, cefoperazone, ceftriaxone, ceftazidime) have a broad spectrum of activity and further increased activity against Gram-negative organisms. Some members of this group (particularly those available in an oral formulation, and those with antipseudomonas activity) have decreased activity against Grampositive organisms. They may be particularly useful in treating hospital-acquired infections, although increasing levels of extended-spectrum β -lactamases are reducing the clinical utility of this class of antibiotics. Fourth generation cephalosporins (cefepime) are extended-spectrum agents with similar activity against Gram-positive organisms as fi rst-generation cephalosporins. They also have a greater resistance to β-lactamases. (h) Quinolones The prolifi c development of the quinolones began in 1962, when George Y. Lesher (Sterling Research, Albany, USA) ( Figure 1.21 ) made the accidental discovery of nalidixic acid as a by-product, 1-8-naphthyridine, during an attempt of the synthesis of the antimalarial compound chloroquine. 127 Since that discovery, the utility of nalidixic acid was largely limited to the treatment of Gram-negative urinary tract infections. Thus, the quinolones have evolved to become important and effective agents in the treatment of bacterial infection. The molecular structures of the quinolones have been adapted over time in association with clinical need ( Table 1.5 ). The addition of specifi cally selected substituents at key positions on the quinolone nucleus made it possible to target specifi c groups of bacteria (topoisomerases II and IV being the lethal targets) and to improve the pharmacokinetics of the earlier quinolone compounds. The fi rst fl uoroquinolone, fl umequine, was used transiently until ocular toxicity was reported. Shortly afterwards, second-generation agents were developed, epitomized by ciprofl oxacin produced after addition of a cyclopropyl group at position N-1. This agent has a wider spectrum of in vitro antibacterial activity, in particular against Gram-negative bacteria, and is effective in the treatment of many types of infection. Despite excellent results in many respiratory infections, reports of failure in pneumococcal infection have limited its use in this area. 128 This drug got celebrity when it later famously resorted to in the 2001 anthrax scare. 4 . The problem of resistance Over time, some bacteria have developed ways to circumvent the effects of antibiotics. René Dubos had the foresight TABLE 1.5 Quinolones: Structure–Activity Relationship One fl uorine atom at position C-6 Increased DNA gyrase Inhibitory activity Second fl uorine atom at position C-8 Increased absorption Longer elimination half-life Increased phototoxicity Piperazine group at position C-7 Greatest activity : aerobic Gram-negative bacteria Increased activity: both staphylococci and Pseudomonas species Alkylation of the C-7 ring Improved activity: aerobic gram-positive bacteria Methyl to the distal nitrogen of the C-7 piperazine ring Increased elimination half-life and improved bioavailability FIGURE 1.21 George Y. Lesher . Ch01-P374194.indd 22 h01-P374194.indd 22 5/29/2008 5:42:11 PM /29/2008 5:42:11 PM
Il.Two Hundred Years of Drug Discoveries 23■ TABLE 1.6 Agent for Which Resistance was Observed Drug agents Introduction First resistance described Penicillin G 1943 1943 Streptomycin 1947 1947 Tetracycline 1952 1952 Methicillin 1960 1961 1964 16d Gentamicin 1967 1969 Cefotaxime 1981 1981 2000 199g to understand the unfortunate potential of antibiotic-resist- a resul stopped FIGURE 1.22 Samuel Broder tations that enable bacteria,viruses,fungi,and parasites to those symptoms were recognized as harbingers of a new and survive powerful drugs.Antimicrobial res adly dis nt to to elm nies cas vere est nd poh difficulty in fighting off microbes leads to an incr sed risk Gallo of the National Cancer Institute (NCD proved that of acauiring infections within hospitals.There is a grea AIDS was caused by a retrovirus (whose replication is linked worry that while many variants of older drugs had beer to a key enzyme.reverse tra crptase).Since the beginning new f molec were not being fo th dideoxynucleosides were disco vered to he potent inhihitor to attack their weak points had not yet borne fruit by the of human immunodeficiency virus (HIV)replicationn early 21st(Table 1.6). The first drug introduced to treat the disease was AZT (3 ceis a growing thr d in able to fight off infect me Horo witz of the Michigan Cancer of antibiotics Resistance mechanisms have been found Foundation (Detroit USa)But AzT beins for every class of antibiotic agent and the search for nev against cancer.Horowitz didnot is a paten n198 e against Ive resea th of AZT for the t de and the r of Samuel Broder (Figure 1.22).aphysician and researcher at the NCI. A6- vity against HI 5.Anti-HIV drugs tase after On June 5,1981,the Centers for Disease Control and osphorylated within the cell to5-tiphosphates.AZ was Prevention(CDC.Atlanta,USA)published an unusual notice only a first step in dev eloping new therapy for AIDS.Its use kly Report:the occu has bee ted with toxicit time in New York s gay me At the of HIV.Other dide of a rare cancer.Kaposi's sarcoma.By the end of 1981. osides with toxicity profiles different from that of AZT had
II. Two Hundred Years of Drug Discoveries 23 to understand the unfortunate potential of antibiotic-resistant bacteria and encouraged prudent use of antibiotics. As a result of this fear, Dubos stopped searching for naturally occurring compounds with antibacterial properties. The widespread use of antibiotics enhanced evolutionarily adaptations that enable bacteria, viruses, fungi, and parasites to survive powerful drugs. Antimicrobial resistance provides a survival benefi t to microbes and makes it harder to eliminate infections from the body. Ultimately, the increasing diffi culty in fi ghting off microbes leads to an increased risk of acquiring infections within hospitals. There is a great worry that while many variants of older drugs had been produced, new families of antibiotics were not being found. The promise of molecular biology through the sequencing of bacterial genomes and the design of chemicals designed to attack their weak points had not yet borne fruit by the early 21st century 129 ( Table 1.6 ). Antimicrobial resistance is a growing threat worldwide, especially within hospitals harboring critically ill patients who are less able to fi ght off infections without the help of antibiotics. Resistance mechanisms have been found for every class of antibiotic agent and the search for new antibiotics effective against various multiresistant germs is probably one of the most diffi cult challenges of medicinal chemistry for the next decade. Development of new classes of antibiotics or more robust versions of old classes will be essential for the future. 130 5 . Anti-HIV drugs On June 5, 1981, the Centers for Disease Control and Prevention (CDC, Atlanta, USA) published an unusual notice in its Morbidity and Mortality Weekly Report : the occurrence of Pneumocystis carinii pneumonia among gay men. At the same time, in New York, a dermatologist encountered cases of a rare cancer, Kaposi’s sarcoma. By the end of 1981, those symptoms were recognized as harbingers of a new and deadly disease later named AIDS. Twenty fi ve years after, more than 40 millions cases were estimated worldwide. In 1984, Luc Montagnier of the Pasteur Institute 131 and Robert Gallo of the National Cancer Institute (NCI) proved that AIDS was caused by a retrovirus (whose replication is linked to a key enzyme, reverse transcriptase). Since the beginning of the epidemics, many of the therapeutic strategies have yielded positive results. 132 In 1985, nucleoside analogs called dideoxynucleosides were discovered to be potent inhibitors of human immunodefi ciency virus (HIV) replication in vitro . The fi rst drug introduced to treat the disease was AZT (3 - azido-3 -deoxythymidine, azidothymidine, zidovudine) a thymidine analog previously developed in 1964 as an anticancer drug by Jerome Horowitz of the Michigan Cancer Foundation (Detroit, USA). But AZT, being ineffective against cancer, Horowitz did not register a patent. 133 In 1987, after 3 years of intensive research, 134 the ultimate approval of AZT as an antiviral treatment for AIDS was the result of pharmacological technology and the personal determination of Samuel Broder ( Figure 1.22 ), a physician and researcher at the NCI. 135 A 6-week clinical trial of AZT was suffi cient to prove its potent antiviral activity against HTLV-III in patients with AIDS or AIDS-related complex. 136 Dideoxynucleosides selectively inhibit HIV reverse transcriptase after they are phosphorylated within the cell to 5 -triphosphates. AZT was only a fi rst step in developing new therapy for AIDS. Its use has been associated with toxicities, particularly bone marrow suppression and several groups have reported the development of AZT-resistant strains of HIV. Other dideoxynucleosides with toxicity profi les different from that of AZT had TABLE 1.6 Agent for Which Resistance was Observed Drug agents Introduction First resistance described Penicillin G 1943 1943 Streptomycin 1947 1947 Tetracycline 1952 1952 Methicillin 1960 1961 Nalidixic acid 1964 1966 Gentamicin 1967 1969 Cefotaxime 1981 1981 Linezolid 2000 1999 FIGURE 1.22 Samuel Broder . Ch01-P374194.indd 23 h01-P374194.indd 23 5/29/2008 5:42:11 PM /29/2008 5:42:11 PM
■24 CHAPTER I A History of Drug Discovery also shown activity against HIV in early clinical studies.In tha TABLE 1.7 Milestones in the Fight Against HIV e)or dd 981 Centers for Disease Control and Prevention (CDC wihAZroni&onpoemem lam 982 The ter AIDS is used for the first time 27th tenofovir,began to be used 1983 CDC (USA)wams blood banks of a possible o the market in 1996 studies have shown that the hinding tral Africa of HIV to lymphocytic CD4 receptor may be blocked by andeRobertcaibhheUnitednnc It has beer a by sub D. area for drug design,develop ent.and production.Thev 985 FDA approves the first constitute the third sub-class of antiretroviral(ARV)drugs screen for antibodies to HIV The discove y of PIs has been hampered by a number remos n the is the every 4h around the clock anti-Hy activity and high oral hioavailability ace ied by a long elimintion half-life to yield sustained virus-sup 991 DDI didanosine):NRT ssive drug level in the blood and infected tissues. 992 soed hard gel y the FDA 994 D4T (stavudine):NRTI which quickly led to viral resistance.It was appr oved again 995 1997 p.In 96 sted with t PL The an H Already.in 2000.the co-formulation of lopinavi and verse transcriptas dose ritonavir (Kaletra)had benehted from the ntra navir is administered alone.This pharmacokinetio intera tion is associated with a high lopinavir trough gene compare hat Neverthe. it i the prepa vir he context with other PIs.Even if the relationship bet plasma and intracellular drug levels has not bee RVtrough plasm D.Drugs for immunosuppression 0 ctive ary theranies usually consisting of two nrtis pl a PI(saquinavir,ritonavir,indinavir,nelfinavir,ampre effects of folic acid on cancer.Some o his results,which or lopin widely use hey prod 100 ubious that L1 e oveand since 1995.AIDS morbidity and mortality that interfere with any hiosynthetic reaction involving the fall by more than 80%. Besides these suc sses,howe ntermediates resembling folie acid to block its action ARV therapies alsc produce side effects h to the 1948 development of methotrexate :140 an apy
24 CHAPTER 1 A History of Drug Discovery also shown activity against HIV in early clinical studies. In September 1995, the results of the “ Delta trial ” showed that combining AZT with ddI (didanosine) or ddC (zalcitabine) did provide a major improvement in treatment compared with AZT on its own. 137 Other nucleoside analogs including stavudine, lamivudine, abacavir, and tenofovir, began to be used in 1994 while non-nucleoside reverse transcriptase inhibitors (non-NRTI), nevirapine, delavirdine, and efavirenz came to the market in 1996. Studies have shown that the binding of HIV to lymphocytic CD4 receptor may be blocked by genetically engineered forms of CD4 protein. It has been proved that HIV protease could be inhibited by substrate analogs. Protease inhibitors (PIs) were thus an excellent area for drug design, development, and production. They constitute the third sub-class of antiretroviral (ARV) drugs. The discovery of PIs has been hampered by a number of signifi cant obstacles. Foremost among these, is the identi- fi cation of inhibitors which simultaneously embody potent anti-HIV activity and high oral bioavailability accompanied by a long elimination half-life to yield sustained virus-suppressive drug levels in the blood and infected tissues. HIV PIs represent a critical milestone on the path to therapeutic effi cacy. 138 Saquinavir was the fi rst PI approved by the FDA in 1995 as Invirase®, a poorly absorbed hard gel capsule which quickly led to viral resistance. It was approved again on November 1997 as Fortovase®, a soft gel capsule reformulated with improved bioavailability. Last step, in 2006, owing to reduction in demand, Fortovase® ceased being in favor of Invirase® boosted with another potent PI ritonavir. Already, in 2000, the co-formulation of lopinavir and lowdose ritonavir (Kaletra®) had benefi ted from the fact that a sub-therapeutic dose of ritonavir, a potent cytochrome P 450 3A4 inhibitor, inhibited the metabolism of lopinavir, resulting in higher lopinavir concentrations than when lopinavir is administered alone. This pharmacokinetic interaction is associated with a high lopinavir trough level and good general tolerability when compared with other PIs. The concept of pharmacokinetic enhancement – boosting – was not new as ritonavir has previously been used in this context with other PIs. Even if the relationship between plasma and intracellular drug levels has not been clarifi ed, ARVtrough plasma concentrations are correlated with virological outcome. On the other hand, Kaletra® has reduced pill-burden and aids compliance. 139 Since 1996, highly active ARV therapies usually consisting of two NRTIs plus a PI (saquinavir, ritonavir, indinavir, nelfi navir, amprenavir or lopinavir) have been widely used. They produce durable suppression of viral replication with undetectable plasma levels of HIV-RNA in more than half of patients. Immunity recovers, and since 1995, AIDS morbidity and mortality fall by more than 80%. Besides these successes, however, ARV therapies also produce numerous side effects. These challenges prompt the search for new drugs and new therapeutic strategies to control chronic viral replication; 140 among them, antisense oligonucleotide therapy could target the regulatory genes of HIV. 141 Nevertheless, it is admitted that the preparation of an effective vaccine is probably the only way to eradicate the disease 142 ( Table 1.7 ). D . Drugs for immunosuppression In the years following World War II, Sidney Farber, a cancer scientist at Boston’s Children’s Hospital, was testing the effects of folic acid on cancer. Some of his results, which now look dubious, suggested that folic acid worsened cancer conditions, inspiring chemists at Lederle to make antimetabolites – structural mimics of essential metabolites that interfere with any biosynthetic reaction involving the intermediates – resembling folic acid to block its action. These events led to the 1948 development of methotrexate, one of the earliest anticancer agents and the mainstay of leukemia chemotherapy. 143 At the time, George Hitchings TABLE 1.7 Milestones in the Fight Against HIV 1981 Centers for Disease Control and Prevention (CDC Atlanta) report an alarming recrudescence of Kaposi’s sarcoma in healthy gay men 1982 The term AIDS is used for the fi rst time on July 27th 1983 CDC (USA) warns blood banks of a possible problem with the blood supply Major outbreak of AIDS in central Africa Luc Montagnier in Pasteur Institute, Paris, France (and later, Robert Gallo in the United States) isolates a retrovirus, later known as human immunodefi ciency virus or HIV 1985 FDA approves the fi rst enzyme linked immunosorbent assay (ELISA) test kit to screen for antibodies to HIV 1987 AZT (zidovudine) is the fi rst anti-HIV drug approved. It has to be taken by one 100 mg capsule every 4 h around the clock 1991 DDI (didanosine): NRTI 1992 DDC (zalcitabine): NRTI The fi rst clinical trial of multiple drugs is held 1994 D4T (stavudine): NRTI 1995 Saquinavir, the fi rst anti-HIV drug as aPI 3TC (lamivudine): NRTI The FDA approves Saquinavir in a record 97 days 1996 • The FDA approves an HIV viral load test • Nevirapine, the fi rst anti-HIV drug of the non-nucleoside reverse transcriptase inhibitors (NNRTI) • Ritonavir: PI • Indinavir: PIs Ch01-P374194.indd 24 h01-P374194.indd 24 5/29/2008 5:42:14 PM /29/2008 5:42:14 PM