Viral Kinetics of the Y448H HCV NS5B Polymerase Mutant in Patients Treated with the HCV Inhibitor Tegobuvir (GS-9190)

Abstract

Tegobuvir (GS-9190) is a novel hepatitis C virus (HCV) non-nucleoside NS5B polymerase inhibitor that effectively inhibits HCV replication in HCV-infected genotype (GT) 1 patients. The NS5B Y448H mutation was the most frequent mutation selected in patients receiving tegobuvir. In this study, we employed allele-specific PCR (AS-PCR) to monitor Y448H kinetics and estimate the pre-existing Y448H levels in HCV-infected patients and replicon cells. Y448H AS-PCR was developed with a 0.5% assay cutoff to test replicon cells treated with tegobuvir and samples from HCVinfected GT 1 patients who received 8-days of tegobuvir monotherapy. By population sequencing, Y448H was not detected at baseline in serum samples from any of the 65 patients enrolled in the study. Using the more sensitive AS-PCR, Y448H was assessed in 62/65 patients at baseline and detected at >0.5% in 5/62 patients. Longitudinal patient samples were tested to monitor the Y448H replication kinetics during the 8-day tegobuvir monotherapy. The replication kinetics of the mutant virus were used to extrapolate a median baseline Y448H frequency of 0.025% (-3.6 log 10). For the in vitro selection of GT 1b tegobuvir-treated replicon cells, pre-existing Y448H levels were similarly estimated at 0.015% (-3.8 log 10). Pre-existing levels of the drug-resistant variant Y448H suggest a maximal 3.6 log 10 HCV RNA reduction during monotherapy with optimal doses of NS5B non-nucleoside polymerase inhibitors that select for Y448H. These results aid in the prediction of the patient viral response and study designs to best achieve maximal antiviral response.