Lawton Progress in Medicinal Chemistry

1. Auflage 2010
ISBN: 978-0-12-381293-3
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, Band Volume 49, 192 Seiten

Reihe: Progress in Medicinal Chemistry

Progress in Medicinal Chemistry provides a review of eclectic developments in medicinal chemistry. This volume continues in the serial's tradition of providing an insight into the skills required of the modern medicinal chemist; in particular, the use of an appropriate selection of the wide range of tools now available to solve key scientific problems. - Presents the latest research in the field of drug discovery - Publishes on a twice yearly basis to bring you the most innovative updates in medicinal chemistry - Available as an online resource via ScienceDirect

Autoren/Hrsg.

Lawton, G.

Leseproben

1 The Journey to the Discovery of Boceprevir: An NS3–NS4 HCV Protease Inhibitor for the Treatment of Chronic Hepatitis C
Kevin X. Chen; F. George Njoroge    Merck Research Laboratories, 2015 Galloping Hill Road, K-15-A3545, Kenilworth, NJ 07033, USA Publisher Summary
The chapter discusses the discovery of Boceprevir: an NS3–NS4 HCV Protease Inhibitor used for the treatment of chronic hepatitis C. Hepatitis C virus (HCV) infection is a major health problem affecting humans. It is estimated that 3% of the world population, are infected with HCV. Without therapeutic intervention, it can lead to morbidity or mortality in 10–20 years, through either cirrhosis and hepatic failure or hepatocellular carcinoma. HCV infection is the most common cause of liver transplantation. The critical role played by NS3 protease in HCV viral replication makes it an excellent target for the creation of new HCV therapy. The virally encoded protease responsible for processing the non-structural (NS) portion of the polyprotein is located in the N-terminal third of the NS3 protein. The journey of Boceprevir discovery involved initially a structure-based drug-design approach guided by X-ray crystal structures of the enzyme. Then stepwise truncations and systematic depeptidizations on both prime and non-prime sites gave rise to smaller pentapeptides that were potent inhibitors, but that did not possess desirable PK properties. Modifications on the prime side resulted in the discovery of the primary a ketoamide moiety which gave excellent potency. Further structure–activity relationship (SAR) optimization identified P1 cyclobutylalanine, P2 dimethylcyclopropylproline, P3 tert-butylglycine and a tert-butyl urea capping group as the best combination, which led to the discovery of boceprevir. Only with the recent development of the HCV autonomous subgenomic replicon system, has the pre-clinical evaluation of potential anti-HCV agents become possible. Boceprevir and telaprevir have advanced into clinical studies in humans and demonstrated to be safe and efficacious. INTRODUCTION
Hepatitis C virus (HCV) infection is a major health problem affecting humans. It is estimated that 3% of the world population, or 170 million people, including more than 4 million Americans (1.3%) are infected with HCV [1–3]. In roughly 80% of cases, the virus leads to a chronic form of hepatitis, a condition that is incurable in many patients. Without therapeutic intervention, it can lead to morbidity or mortality in 10–20 years, through either cirrhosis and hepatic failure or hepatocellular carcinoma [4–6]. It is anticipated that a significant percentage of those currently infected will develop cirrhosis and other associated hepatic sequelae. HCV infection is the most common cause of liver transplantation [4–6]. Despite significant advances in hepatitis C research with more than 60 antiviral compounds in clinical development, pegylated interferon-a (IFN-a) in combination with oral ribavirin remains the approved standard of care [7–9]. IFN-a is a protein that stimulates the immune system, while ribavirin is a nucleoside analogue that works in concert with IFN-a to control the infection. The efficacy of this combination therapy against the predominant viral genotype (Type 1) affecting North America, Europe and Japan is moderate, with only about 40% of the patients meeting the primary goal of treatment, a sustained virological response (SVR). This is defined clinically as an undetectable serum HCV-RNA level 24 weeks after cessation of therapy. Some patients also experience significant side effects related to the treatment. With few alternatives available, more effective agents with fewer side effects are clearly needed [7–9]. An aetiologic agent of non-A, non-B hepatitis, HCV was identified in 1989 as a member of the Flaviviridae family [10] and is an enveloped, positive-strand RNA virus of approximately 9.6 kilobases. Upon entering a suitable host cell, the HCV genome serves as a template for cap-independent translation through an internal ribosome entry site (IRES) [11, 12]. The resulting single polyprotein contains the structural and non-structural (NS) proteins: Core-E1-E2-P7-NS2-NS3-NS4A-NS4B-NS5A-NS5B. Through host and virally encoded proteases, this polypeptide undergoes both co- and post-translational proteolytic maturation. The virally encoded protease responsible for processing the NS portion of the polyprotein is located in the N-terminal third of the NS3 protein [11, 12]. (Figure 1.1). Besides autoproteolysis of the NS3–NS4A junction, the protease also cleaves the polyprotein at the NS4A–NS4B, NS4B–NS5A and NS5A–NS5B junctions to release the downstream NS functional proteins [11, 12]. The replicative complex, or replisome, is subsequently generated with the mature proteins through self-assembly on the endoplasmic reticulum. Using the viral genome as a template, the replisome generates negative-strand viral RNA intermediates, which are then used as templates to synthesize new positive-strand (genomic) RNAs. The new RNAs are either translated to yield more polyprotein or, later in the infection cycle, encapsulated to generate progeny virions. Inhibition of the proteolytic activities of the NS3 protease would therefore suppress replisome formation, RNA replication and ultimately generation of new virions. Fig. 1.1 Schematic representation of the HCV genome and NS3/NS4A protease-mediated cleavage (shown as arrows). NS3 is a bifunctional protein with protease and helicase activities. On the other hand, the RNA-dependent RNA polymerase (RdRp) contained within the NS5B protein is the catalytic component of the HCV-RNA replication machinery [13, 14]. This enzyme synthesizes RNA using the RNA template. This biochemical activity is not present in mammalian cells, offering the opportunity to identify very selective inhibitors of the viral enzyme. The lack of a robust in vitro cell culture system capable of supporting its replication hampered traditional approaches to develop or evaluate antiviral compounds for many years after HCV was discovered and characterized. In addition, there is no conventional small animal model to conveniently assess the in vivo efficacy. Most of our knowledge of HCV has been derived from surrogate experimental systems that approximate infection and often preclude definitive interpretation. Only with the recent development of the HCV autonomous subgenomic replicon system [15], has the pre-clinical evaluation of potential anti-HCV agents become possible. The chronically infected chimpanzee model [16] and the severe combined immunodeficiency disease (SCID) mouse with chimeric human liver model [17] also proved effective in limited pre-clinical evaluation of anti-HCV therapies, although both animal models suffer from limitations that make them less than ideal for expanded studies. HCV infects only humans and chimpanzees. The chronically infected chimpanzee model, the ‘gold standard’ for HCV studies, is challenging and expensive because one out of three chimpanzees spontaneously resolves the HCV infection. An immune-deficient SCID mouse–human liver xenograft system was developed by researchers at the University of Alberta [17]. In this model, the livers of neonate SCID beige mice are colonized with infused human hepatocytes which rescue them from a fatal transgene. Infection of these human liver grafts by several genotypes of HCV, and the therapeutic effects of INF-a, has been reported. Unfortunately, the animals are fragile and scale up of the colony has been slower than expected, thus limiting access to the system. Extensive research efforts have been directed towards developing drugs that halt HCV replication through the inhibition of NS3 protease and other enzymes. A number of promising small-molecule inhibitors of the NS3/4A protease, the NS5B polymerase and the NS5A are in clinical development [18–21]. Early testing has demonstrated strong antiviral activity both in vitro and in patients. Inhibitors of other potential targets such as Internal Ribosome Entry Site (IRES) and replicase are in earlier stages of pre-clinical investigation [22–24]. Although efforts are ongoing to develop a vaccine, the unusually rapid genetic drift of HCV makes this a daunting task [25]. A major challenge for any successful directly acting anti-HCV therapy is the rapid emergence of drug-resistant viruses under selective pressure [22–24]. The fast turnover rate and the intrinsic low fidelity of the HCV replication machinery endow the virus with the ability to fully explore its genome space and quickly come up with mutations that render it resistant to antiviral drugs. HCV NS3 PROTEASE AS A TARGET
X-ray structures of NS3 protease crystals have been resolved either as an isolated domain or as part of the full-length NS3 protein [26, 27]. The data provided detailed structural insights to facilitate rational inhibitor design. The NS3 protease is in many ways a typical ß-barrel serine protease, with a canonical Asp-His-Ser catalytic triad similar to the well-studied digestive enzymes, trypsin and chymotrypsin. Histidine-57 and aspartic acid-81 of the catalytic triad are located in the N-terminal region, whereas...