Resensitizing daclatasvir-resistant hepatitis C variants by allosteric modulation of NS5A

The drug daclatasvir (DCV), which inhibits the hepatitis C virus (HCV) non-structural protein 5A (NS5A), can successfully reduce viral load in patients; here, a combination of DCV and an NS5A analogue is shown to enhance DCV potency on multiple genotypes and overcome resistance in vitro and in a mouse model. Mechanism of action of NS5A inhibitors in hepatitis C The recently approved drug daclatasivir can successfully reduce viral load in patients infected with hepatitis C virus (HCV) by inhibiting the virus's non-structural protein 5A (NS5A) replication complex, although HCV NS5A inhibitor resistance mutations arise in some cases. Min Gao and colleagues now show that a combination of daclatasivir and an NS5A analogue can enhance daclatasivir potency, and the combination also overcomes resistance in vitro and in a mouse model. The authors develop a model to explain this effect, whereby inhibitor binding to one resistant NS5A causes a conformational change that is transmitted to adjacent NS5A molecules, restoring drug sensitivity to the resistant molecules. It is estimated that more than 170 million people are infected with hepatitis C virus (HCV) worldwide 1 , 2 . Clinical trials have demonstrated that, for the first time in human history, the potential exists to eradicate a chronic viral disease using combination therapies that contain only direct-acting antiviral agents 3 . HCV non-structural protein 5A (NS5A) is a multifunctional protein required for several stages of the virus replication cycle 4 . NS5A replication complex inhibitors, exemplified by daclatasvir (DCV; also known as BMS-790052 and Daklinza), belong to the most potent class of direct-acting anti-HCV agents described so far, with in vitro activity in the picomolar (pM) to low nanomolar (nM) range. The potency observed in vitro has translated into clinical efficacy, with HCV RNA declining by ~3–4 log 10 in infected patients after administration of single oral doses of DCV. Understanding the exceptional potency of DCV was a key objective of this study. Here we show that although DCV and an NS5A inhibitor analogue (Syn-395) are inactive against certain NS5A resistance variants, combinations of the pair enhance DCV potency by >1,000-fold, restoring activity to the pM range. This synergistic effect was validated in vivo using an HCV-infected chimaeric mouse model. The cooperative interaction of a pair of compounds suggests that NS5A protein molecules communicate with each other: one inhibitor bi