Filibuvir and VX-222 are nonnucleoside inhibitors (NNIs) that bind to the thumb II allosteric pocket of the hepatitis C virus (HCV) RNA-dependent RNA polymerase. Both compounds have shown significant promise in clinical trials and, therefore, it is relevant to better understand their mechanisms of inhibition. In our study, filibuvir and VX-222 inhibited the 1b/Con1 HCV subgenomic replicon, with 50% effective concentrations (EC 50 s) of 70 nM and 5 nM, respectively. Using several RNA templates in biochemical assays, we found that both compounds preferentially inhibited primer-dependent RNA synthesis but had either no or only modest effects on de novo-initiated RNA synthesis. Filibuvir and VX-222 bind to the HCV polymerase with dissociation constants of 29 and 17 nM, respectively. Three potential resistance mutations in the thumb II pocket were analyzed for effects on inhibition by the two compounds. The M423T substitution in the RNA polymerase was at least 100-fold more resistant to filibuvir in the subgenomic replicon and in the enzymatic assays. This resistance was the result of a 250-fold loss in the binding affinity (K d ) of the mutated enzyme to filibuvir. In contrast, the inhibitory activity of VX-222 was only modestly affected by the M423T substitution but more significantly affected by an I482L substitution.
Hepatitis C virus (HCV), the major causative agent of non-A, non-B viral hepatitis, has been estimated to infect over 170 million people worldwide (12,24). Approximately 80% of the infected individuals will develop chronic infection, leading to liver cirrhosis and hepatocellular carcinoma (24). Two protease inhibitors have been recently approved by the FDA and can result in up to a 70% cure rate for genotype 1a-or 1b-infected patients when used in combination with pegylated alpha interferon and ribavirin (1, 17). However, resistance mutations to both ribavirin and the protease inhibitors have already been observed in patients, and there is a need to develop drugs to additional targets in HCV (3). The HCV-encoded NS5B, the RNA-dependent RNA polymerase (RdRp) (5), is a validated drug target, and intense efforts are focused on development of antiviral agents that inhibit its activities.The HCV RdRp can catalyze RNA synthesis in vitro by either a de novo-initiated mechanism or by extension from a primed template (19,40). These two modes of RNA synthesis have been used to classify the effects of HCV polymerase inhibitors (13). De novo initiation usually takes place by the base pairing of the initiating nucleoside triphosphate (NTP; usually a purine triphosphate) to the 3=-most nucleotide of the template RNA (usually a U or a C) (5,19,21,30). This mode of synthesis ensures that no genetic information from the viral genome is lost (18). It is the ratelimiting step in RNA synthesis and requires a higher K m of the initiating NTP (NTPi) than for the other nucleotides incorporated during elongation. Primer extension (PE) takes place when the 3= region of the template RNA loops back on itself to form a hairpin ...