Inhibitors of hepatitis C virus NS3.4A protease: an overdue line of therapy

Identification of novel small molecules as inhibitors of hepatitis C virus by structure-based virtual screening

Hepatitis C virus (HCV) NS3/NS4A serine protease is essential for viral replication, which is regarded as a promising drug target for developing direct-acting anti-HCV agents. In this study, sixteen novel compounds with cell-based HCV replicon activity ranging from 3.0 to 28.2 μM (IC₅₀) were successfully identified by means of structure-based virtual screening. Compound 5 and compound 11, with an IC₅₀ of 3.0 μM and 5.1 μM, respectively, are the two most potent molecules with low cytotoxicity.

Discovery and development of telaprevir: an NS3-4A protease inhibitor for treating genotype 1 chronic hepatitis C virus

Infection with hepatitis C virus (HCV) is a major medical problem with over 170 million people infected worldwide. Substantial morbidity and mortality are associated with hepatic manifestations (cirrhosis and hepatocellular carcinoma), which develop with increasing frequency in people infected with HCV for more than 20 years. Less well known is the burden of HCV disease associated with extrahepatic manifestations (diabetes, B-cell proliferative disorders, depression, cognitive disorders, arthritis and Sjögren's syndrome). For patients infected with genotype 1 HCV, treatment with polyethylene glycol decorated interferon (peginterferon) α and ribavirin (PR) is associated with a low (40-50%) success rate, substantial treatment-limiting side effects and a long (48-week) duration of treatment. In the past 15 years, major scientific advances have enabled the development of new classes of HCV therapy, the direct-acting antiviral agents, also known as specifically targeted antiviral therapy for hepatitis C (STAT-C). In combination with PR, the HCV NS3-4A protease inhibitor telaprevir has recently been approved for treatment of genotype 1 chronic HCV in the United States, Canada, European Union and Japan. Compared with PR, telaprevir combination therapy offers significantly improved viral cure rates and the possibility of shortened treatment duration for diverse patient populations. Developers of innovative drugs have to blaze a new path with few validated sign posts to guide the way. Indeed, telaprevir's development was once put on hold because of its performance in a standard IC(50) assay. Data from new hypotheses and novel experiments were required to justify further investment and reduce risk that the drug might fail in the clinic. In addition, the poor drug-like properties of telaprevir were a formidable hurdle, which the manufacturing and formulation teams had to overcome to make the drug. Finally, novel clinical trial designs were developed to improve efficacy and shorten treatment in parallel instead of sequentially. Lessons learned from the development of telaprevir suggest that makers of innovative medicines cannot rely solely on traditional drug discovery metrics, but must develop innovative, scientifically guided pathways for success.

The therapeutic potential of NS3 protease inhibitors in HCV infection

Hepatitis C virus (HCV) infection is a serious cause of chronic liver disease worldwide and afflicts > 170 million people. The HCV-encoded NS3 protease is essential for viral replication and has long been recognised as a prime target for antiviral drugs. However, the peculiar active site structure of this enzyme, a shallow dent on the surface of the protein, has rendered the development of small-molecule inhibitors a highly challenging task. Nevertheless, perseverance and creativity has led to significant progress in this field over the last few years resulting in three compounds that are reported to enter the clinic. The impressive reduction of HCV RNA plasma levels observed with two of these inhibitors (ciluprevir and VX-950) in clinical trials has undoubtedly illustrated the potential of this viral enzyme-targeted drug discovery approach.

P4 and P1' optimization of bicycloproline P2 bearing tetrapeptidyl alpha-ketoamides as HCV protease inhibitors

With the aim of improving HCV protease inhibitors reported in our previous manuscripts, we synthesized and evaluated a series of 1a-based tetrapeptidyl alpha-ketoamides with additional P4 modification. The promising analog discovered through this SAR, 5a, was further derivatized at P1' or P1 position. As a result of these efforts, we found that replacement of the P4 valine as seen in 1a with cyclohexylglycine (Chg) resulted in the discovery of 5a, 5c, and 5e endowed with improved cellular activity in comparison to 1a.

Combination of a hepatitis C virus NS3-NS4A protease inhibitor and alpha interferon synergistically inhibits viral RNA replication and facilitates viral RNA clearance in replicon cells

The present standard of care for hepatitis C virus (HCV) infection is pegylated alpha interferon (IFN-alpha) in combination with ribavirin. However, specific antivirals such as HCV NS3-NS4A protease inhibitors are now in clinical development, and these agents can potentially be used in combination with the present treatments. Therefore, it is important to investigate the potential benefits or adverse effects of these new combinations by using available in vitro HCV culture systems first. In the present study we demonstrate that the combination of a specific HCV NS3-NS4A protease inhibitor and IFN-alpha synergistically inhibits HCV RNA replication in replicon cells, with little or no increase in cytotoxicity. Furthermore, the benefit of the combination was sustained over time, such that a greater than 3-log reduction in HCV RNA levels was achieved following 9 days of treatment. The viral RNA appeared to be cleared from the replicon cells after 14 days of treatment, and no viral RNA rebound was observed upon withdrawal of the inhibitors. In each case, the antiviral effects obtained with higher concentrations of either the protease inhibitor alone or IFN-alpha alone can be achieved by a combination of both agents at lower concentrations, which may potentially reduce the risk of possible adverse effects associated with high doses of either agent.

Inhibitors of hepatitis C virus NS3.4A protease. Part 3: P2 proline variants

We recently described the identification of an optimized alpha-ketoamide warhead for our series of HCV NS3.4A inhibitors. We report herein a series of HCV protease inhibitors incorporating 3-alkyl-substituted prolines in P(2). These compounds show exceptional enzymatic and cellular potency given their relatively small size. The marked enhancement of activity of these 3-substituted proline derivatives relative to previously reported 4-hydroxyproline derivatives constitutes additional evidence for the importance of the S(2) binding pocket as the defining pharmacophore for inhibition of the NS3.4A enzyme.

Inhibitors of hepatitis C virus NS3.4A protease 2. Warhead SAR and optimization

The alpha-ketoamide warhead (e.g., 15) was found to be a practical replacement for aliphatic aldehydes in a series of HCV NS3.4A protease inhibitors. Structure-activity relationships and prime side optimization are discussed.

P4 cap modified tetrapeptidyl α-ketoamides as potent HCV NS3 protease inhibitors

We describe herein the design, syntheses, and biological evaluation of new series of P4 tetrazole and adipic acid, ester, amide capped tetrapeptidyl alpha-ketoamide based HCV protease inhibitors.

Bovine viral diarrhea virus as a surrogate model of hepatitis C virus for the evaluation of antiviral agents

The identification and development of new antiviral agents that can be used to combat hepatitis C virus (HCV) infection has been complicated by both technical and logistic issues. There are few, if any, robust methods by which HCV virions can be grown in vitro. The development of HCV RNA replicons has been a great breakthrough that has allowed for the undertaking of significant screening efforts to identify inhibitors of HCV intracellular replication. However, since replicons do not undergo a complete replication cycle, drug screening programs and mechanism of action studies based solely on these assays will not identify compounds targeting either early (virion attachment, entry, uncoating) or late (virion assembly, egress) stages of the viral replication cycle. Drugs that negatively affect the infectivity of new virions will also not be identified using HCV RNA replicons. Bovine viral diarrhea virus (BVDV) shares a similar structural organization with HCV, and both viruses generally cause chronic long-term infections in their respective hosts. The BVDV surrogate model is attractive, since it is a virus-based system. It is easy to culture the virus in vitro, molecular clones are available for genetic studies, and the virus undergoes a complete replication cycle. Like HCV, BVDV utilizes the LDL receptor to enter cells, uses a functionally similar internal ribosome entry site (IRES) for translation, uses an NS4A cofactor with its homologous NS3 protease, has a similar NS3 helicase/NTPase, a mechanistically similar NS5B RNA-dependent RNA polymerase, and a seemingly equivalent mechanism of virion maturation, assembly and egress. While the concordance between drugs active in either BVDV or HCV is largely unknown at this time, BVDV remains a popular model system with which drugs can be evaluated for potential antiviral activity against HCV and in studies of drug mechanism of action.

Inhibitors of hepatitis C virus NS3·4A protease 1. Non-Charged tetrapeptide variants

Tetrapeptide-based peptidomimetic compounds have been shown to effectively inhibit the hepatitis C virus NS3.4A protease without the need of a charged functionality. An aldehyde is used as a prototype reversible electrophilic warhead. The SAR of the P1 and P2 inhibitor positions is discussed.