Evaluation of antiviral drug synergy in an infectious HCV system

Background: Direct-acting antivirals (DAAs) have greatly improved the treatment of HCV infection. To improve response and prevent resistance, combination regimens have been the focus of clinical development. Regimens are often first assessed in vitro, with most combination studies to date using subgenomic replicon systems, which do not replicate the complete HCV life cycle and preclude study of entry and assembly inhibitors. Infectious full-length HCV systems have been developed and are being used to test drug efficacy. Methods: Using cell-based HCV Con1b replicon and an infectious full-length HCV (HCVcc-Luc) infection system, we systematically tested the synergy, additivity or antagonism of combinations of protease, NS5A and nucleotide NS5B inhibitor classes as well as the combination of these DAAs with host-targeting agent cyclosporin A or non-antibody entry inhibitor (S)-chlorcyclizine. Two computational software packages, MacSynergyII and CalcuSyn, were used for data analysis. Results: Combinations between different classes showed good consistency across the two viral assay systems and two software platforms. Combinations between NS5A and nucleotide NS5B inhibitors were synergistic, while combinations of protease inhibitors with the other two classes were additive to slightly antagonistic. As expected, combinations of antivirals of the same class were additive. Combination studies between these DAA classes and cyclosporin A or (S)-chlorcyclizine demonstrated additive to synergistic effects and highly synergistic effects, respectively. Combinations of these drugs did not show any added or unexpected cytotoxicity. Conclusions: Our results show that in vitro combination studies of anti-HCV DAAs in the HCVcc system may provide useful guidance for drug combination designs in clinical studies. We also demonstrate that these DAAs in combination with host-targeting agents or entry inhibitors may improve HCV treatment response.