Infectious Disorders - Drug Targets (Formerly Current Drug Targets - Infectious Disorders) - Volume 6, Issue 1, 2006

Infection with hepatitis C virus (HCV) affects 170 million people worldwide. Most infections become chronic and slowly evolve to cirrhosis, liver failure, and hepatocellular carcinoma in a considerable fraction of patients. Due the chronic nature of the infection, its high prevalence, and the significant morbidity of the resulting disease, HCV will continue to represent a serious global health threat for many years to come. Our current therapeutic approaches, based on the use of interferon-a and the broad spectrum antiviral agent ribavirin, are effective in only approximately 50% of the treated patients, are associated with severe side-effects and are not suited for all patient groups. Thus, a pressing medical need dictates that we discover and develop new anti-HCV agents that are at the same time more efficacious and better tolerated by patients. After more than 15 years since the molecular identification of HCV, a number of novel agents have been identified that demonstrate anti-HCV activity in preclinical models and some of them have progressed to clinical trials. Aside from the continued improvements to interferon and ribavirin regimens, drugs that target specific steps of the HCV life-cycle are the most promising new agents. For many years, the lack of adequate laboratory animal models and the inability to efficiently propagate the virus in cultured cells have been major obstacles in identifying and evaluating all possible HCV drug targets. Therefore, efforts to develop new therapeutic agents have mostly focused on the development of drugs that inhibit the steps of HCV replication that could be readily studied in the laboratory. Thus, a number of small molecule agents have been developed that target crucial viral enzymes. The HCV NS3-4A protease and the NS5B RNA-dependent RNA polymerase play an essential role in viral replication. Following the successful paradigm established for HIV protease and reverse transcriptase, these enzymes have become preferred targets for the development of small molecule inhibitors. By combining the power of high-throughput screening with rational, knowledge-based drug design, competitive inhibitors of the NS3-4A protease as well as nucleoside and non-nucleoside inhibitors of the NS5B polymerase are being developed. Importantly, inhibitors representative of each of these classes have now demonstrated antiviral activity in early clinical trials. In addition to targeting the crucial viral enzymes, conserved sequences of the viral RNA genome are being targeted with nucleic-acid based drugs such ribozymes, antisense oligonucleotides and, more recently, small-interfering RNA (siRNA). Lin, Kwong and Perni (Discovery and Development of VX-950, a Novel, Covalent, and Reversible Inhibitor of Hepatitis C Virus NS3-4A Serine Protease) review the progress in the development of VX-950, a peptidomimetic inhibitor of the NS3-4 protease that is stabilized into the enzyme's active site by the ability to form a covalent, reversible bond with the catalytic serine residue owing to the inclusion an α-ketoamide moiety. VX-950 displays very potent anti-HCV activity when administered orally in patients, and is now rapidly progressing through clinical trials. Carroll and Olsen (Nucleoside Analog Inhibitors of Hepatitis C Virus Replication) provide a knowledgeable insight into the arena of anti-HCV nucleoside analogs. Ribonucleoside analogs incorporating 2'-C-methyl modification are selectively incorporated by the viral enzyme into the viral genome, therefore acting as potent inhibitors of HCV replication. A special difficulty in developing these agents is due to the fact that nucleoside analogs need to be converted to their respective nucleotide by the host cell metabolic machinery in order to exert their antiviral activity as "chain terminators". The mechanism of action, structure-activity relationship, and metabolic properties of 2'-C-methyl ribonucleosides, and the demonstration of the antiviral activity associated with NM283, a prodrug of 2'-C-methylcytidine, are discussed in this paper. Koch and Narjes (Allosteric Inhibition of the Hepatitis C Virus NS5B RNA-Dependent RNA Polymerase) present an up-to-date and comprehensive summary of the recent developments in the field of non-nucleoside inhibitors (NNIs) of the HCV polymerase. NNIs are almost invariably allosteric inhibitors that are believed to prevent one or more conformational changes that are needed for the enzyme to productively synthesize viral RNA. Strikingly, at least three different binding sites are present on the NS5B polymerase and, consistent with this, compounds binding to the different sites elicit different patterns of mutations that conferred resistance to different agents. This observation raises the hope that multiple NNIs, targeting different sites, may be used in combination regimens thereby minimizing the emergence of resistant viruses. Wilson and Richardson (Future Promise of siRNA and Other Nucleic Acid Based ..........

The hepatitis C virus (HCV) NS3.4A protease, which is essential for viral replication, is considered one of the most attractive targets for developing novel anti-HCV therapies. However, discovery of potent and selective smallmolecule inhibitors of HCV NS3.4A protease as oral drug candidates has been hampered by the shallow substrate-binding groove of the protease. Serine trap warheads have been used to covalently anchor inhibitor scaffolds and to increase their affinity to the protease. This review will examine the evolution of covalent inhibitors of the HCV NS3.4A protease from early aldehyde molecules to α-ketoamide inhibitors. Kinetic and structural studies of α-ketoacid and α-ketoamide inhibitors revealed an unusual mechanism of binding in the catalytic site. Optimization of α-ketoamide scaffolds by scientists at Vertex and Eli Lilly led to the discovery of VX-950, a novel, potent, selective inhibitor of HCV NS3.4A protease. VX-950 possesses excellent antiviral activity in both HCV replicon cells and human fetal hepatocytes infected with HCV-positive patient sera. In addition, VX-950 exhibits a favorable pharmacokinetic profile in several animal species and demonstrates potent inhibition of the HCV NS3.4A protease activity in a mouse model. In a recent phase 1b clinical trial, VX-950 was able to rapidly reduce the plasma viral load of patients chronically infected with genotype 1 HCV by a mean ∼3 log10 in 2 days. The median viral load reduction was 4.4 log10 for the best dose group after 14 days of dosing. The pre-clinical profile and early clinical data of VX-950 will be discussed in this review.

Of the 30 compounds currently marketed in the United States for treatment of viral infections, 15 are nucleoside analogs, demonstrating the utility of this class of compound as a source of antiviral drugs. The success of nucleoside analogs in treating other viral infections provides a compelling rationale for the significant effort that is currently being devoted to the discovery and development of nucleoside analogs to treat infection by hepatitis C virus (HCV) that may lead to improvements in response rates compared to currently available therapies. Several different approaches have been adopted to identify promising analogs, including the use of surrogate viruses in cell culture assays, screening in the cell-based bicistronic HCV replicon assay, and screening nucleoside triphosphates for the ability to inhibit the activity of the HCV RNA-dependent RNA polymerase in vitro. Several classes of ribonucleoside analogs with modifications of the ribose inhibit HCV replication. Nucleoside analogs incorporating a 2'-C-methyl modification are potent inhibitors in the replicon assay in the absence of cytotoxicity, and appear to exert their inhibition by acting as functional chain terminators of RNA synthesis. NM283, a prodrug of 2'-C-methylcytidine, has entered clinical trials and demonstrated viral load reductions in subjects infected with genotype 1 HCV, a genotype known to be difficult to treat effectively with currently approved therapies. Overall, results to date offer encouragement that improved therapies to treat HCV infection including newly developed nucleoside analogs may become available within the next few years.

The human and monetary costs of chronic hepatitis C and the complications arising from this disease emphasize the urgency to find a treatment for Hepatitis C Virus (HCV) infected patients. The current standard of treatment for patients chronically infected with HCV is combination therapy with pegylated interferon plus ribavirin. Recently, viral enzymes have become the target of efforts to develop small molecule inhibitors interfering with the essential steps in the life cycle of the virus. Amongst these enzymes the HCV-encoded NS5B RNA-dependent RNA polymerase (NS5B RdRp) is essential for viral replication and has been recognized as a prime target for therapeutic intervention. Several distinct classes of inhibitors of NS5B RdRp have been disclosed in the literature, including active site inhibitors such as nucleosides and pyrophosphate mimetics, as well as non-nucleoside inhibitors. The latter, based on the success of allosteric inhibitors in the treatment of HIV infection, have been developed into compounds which show activity in the subgenomic cell-culture assay of HCV replication. This review provides an account of the recent developments in this field.

RNA interference (RNAi) is gaining favor as a potential therapeutic option for the treatment of Hepatitis C virus infections. RNAi, first discovered in plants, induces sequence specific degradation of messenger RNA following the introduction of short interference RNA (siRNA). RNAi is a natural defense mechanism used by plants to combat viral infections, and the discovery of RNAi activity in mammalian cells has prompted several drug companies to investigate and exploit RNAi based drugs as a potential therapy against HCV infections. A number of research groups have demonstrated that strong RNAi activity can be induced against HCV using synthetic siRNA duplexes as triggers, or by expressing short hairpin RNAs from plasmid or viral vectors. However, much work remains to improve delivery, maintain specificity and limit the development of virus resistance. HCV is capable of evading RNAi activity through the incorporation escape mutations within the siRNA target sequence, highlighting the importance of implementing strategies to limit the development of resistance. Other nucleic acid based therapies such as antisense oligonucleotides, RNA aptamers and ribozymes have also been considered for use as HCV therapeutics, and we will outline the potential opportunities and obstacles to their use as well as RNAi.

One third of HIV-infected individuals worldwide suffer from chronic hepatitis C virus (HCV) infection. Two main reasons justify considering HCV therapy as a priority in HIV-coinfected patients. First, these patients have more rapid liver disease progression, and second, they have a higher risk of developing hepatotoxicity following the initiation of antiretroviral therapy. Unfortunately, HCV therapy is associated with lower response rates and higher rate of side effects in HIV-coinfected patients. However, recent evidence suggests that when HCV therapy is administered adequately (to optimal candidates; using full doses of ribavirin; at least for 12 months irrespective of the HCV genotype; and with satisfactory drug adherence), treatment responses may not differ much from those seen in HCV-monoinfected individuals. Treatment should be considered up front in antiretroviral-naïve subjects with stable HIV infection. In patients already on antiretroviral therapy, HCV therapy should not be administered before replacing didanosine by another antiretroviral, given the increased risk of mitochondrial toxicities. If possible, zidovudine should be avoided as well, given the high risk of anemia. The histological information provided by either non-invasive procedures (FibroScan, Fibro-test, etc.) or liver biopsy is useful but should not be considered as mandatory before prescribing HCV therapy. In summary, liver disease associated to HCV is a growing problem among HIV-positive individuals. The relatively low efficacy of current anti- HCV medications and their low tolerability clearly indicated the need for new drugs with more potent and direct antiviral activity against HCV.