No abstract

DEB025/Debio 025 (Alisporivir) is a cyclophilin (Cyp)-binding molecule with potent anti-hepatitis C virus (HCV) activity both in vitro and in vivo. It is currently being evaluated in phase II clinical trials. DEB025 binds to CypA, a peptidyl-prolyl cis-trans isomerase which is a crucial cofactor for HCV replication. Here we report that it was very difficult to select resistant replicons (genotype 1b) to DEB025, requiring an average of 20 weeks (four independent experiments), compared to the typically <2 weeks with protease or polymerase inhibitors. This indicates a high genetic barrier to resistance for DEB025. Mutation D320E in NS5A was the only mutation consistently selected in the replicon genome. This mutation alone conferred a low-level (3.9-fold) resistance. Replacing the NS5A gene (but not the NS5B gene) from the wild type (WT) genome with the corresponding sequence from the DEB025res replicon resulted in transfer of resistance. Cross-resistance with cyclosporine A (CsA) was observed, whereas NS3 protease and NS5B polymerase inhibitors retained WT-activity against DEB025res replicons. Unlike WT, DEB025res replicon replicated efficiently in CypA knock down cells. However, DEB025 disrupted the interaction between CypA and NS5A regardless of whether the NS5A protein was derived from WT or DEB025res replicon. NMR titration experiments with peptides derived from the WT or the DEB025res domain II of NS5A corroborated this observation in a quantitative manner. Interestingly, comparative NMR studies on two 20-mer NS5A peptides that contain D320 or E320 revealed a shift in population between the major and minor conformers. These data suggest that D320E conferred low-level resistance to DEB025 probably by reducing the need for CypA-dependent isomerisation of NS5A. Prolonged DEB025 treatment and multiple genotypic changes may be necessary to generate significant resistance to DEB025, underlying the high barrier to resistance.

Hepatitis C virus (HCV) represents a major health burden. An estimated 170 million to 180 million people worldwide are chronically infected with this virus and are at increased risk of developing liver cirrhosis and/or hepatocellular carcinoma (64). The current standard of care for chronic hepatitis C consists of pegylated alpha interferon (IFN) in combination with ribavirin (RBV) (12). This therapy is, however, associated with serious side effects and results in a sustained virological response in only 50 to 60% of patients (depending on the genotype). There is thus an urgent need for more effective and better-tolerated drugs.Selective inhibitors of HCV replication that target the NS3 protease and the NS5B RNA-dependent RNA polymerase (RdRp) in particular have been pursued as potential new therapies (38). BILN 2061 (culprivir), a peptidomimetic inhibitor of the HCV NS3 protease (the first selective inhibitor of HCV to be administered to patients), resulted in a rapid and pronounced decline in the level of viral replication in patients chronically infected with HCV genotype 1. Its clinical development was, however, halted because of cardiotoxicity (22,31). Currently, four NS3 protease inhibitors are in clinical development, i.e., VX-950 (telaprevir), SCH-503034 (boceprevir), and TMC435350 (2,51,56). Telaprevir and boceprevir are at the most advanced stages of development and are being evaluated in combination with the standard therapy in phase III and phase II clinical trials, respectively (3,39,45,50). Several nucleoside polymerase inhibitors (NIs) and nonnucleoside polymerase inhibitors (NNIs) are or have been in development. Nucleoside analogue inhibitors of HCV replication basically act as chain terminators of the polymerization process once they have been phosphorylated to their 5Ј-triphosphate metabolite (13).