Department of Virology, Bristol-Myers Squibb Research & Development, 5 Research Parkway, Wallingford, Connecticut, USA, 06492.
Three Hepatitis C virus (HCV) inhibitors, asunaprevir (ASV, BMS-650032), daclatasvir (DCV, BMS-790052), and BMS-791325, each targeting a different non-structural protein of the virus (NS3, NS5A, and NS5B, respectively), have independently demonstrated encouraging pre-clinical profiles and are currently undergoing clinical evaluation. Since drug-resistant variants have rapidly developed to monotherapy with almost all direct-acting antiviral agents (DAAs) for HCV, the need for combination therapies to effectively eradicate the virus from infected patients is clear. These studies demonstrate the additive/synergistic effects on replicon inhibition and clearance of combining NS3 protease or NS5B RNA polymerase inhibitors with the first-in-class, NS5A replication complex inhibitor daclatasvir (DCV), and reveal new resistance pathways for combinations of two small molecule inhibitors that differ from those that develop during monotherapy. The results suggest that under a specific selective pressure, a balance must be reached in the fitness costs of substitutions in one target gene when substitutions are also present in another target gene. Further synergies and additional novel resistance substitutions were observed during triple combination treatment relative to dual-drug therapy indicating that in combination, HCV inhibitors can exert cross-target influences on resistance development. Enhanced synergies in replicon inhibition and a reduced frequency of resistance together lend strong support to the utility of combinations of DAAs for the treatment of HCV, and the identification of altered resistance profiles during combination treatment provides useful information for monitoring resistance in the clinic.