Sofosbuvir susceptibility of genotype 1 to 6 HCV from DAA-naïve subjects.

High sequence diversity of HCV may lead to variation in susceptibility to antiviral agents amongst different genotypes and subtypes of the virus. We assessed the susceptibility to sofosbuvir of chimeric replicons carrying the full length NS5B coding region from 479 HCV infected, treatment-naïve patients, including 15 subtypes in 6 genotypes. NS5B replicon vectors with subtype 1b, subtype 4a and subtype 6a backbone were modified to support testing of patient samples. We also evaluated sofosbuvir susceptibility in a panel of 331 replicons containing engineered NS5B inhibitor resistance-associated substitutions. The mean 50% effective sofosbuvir concentration (EC50) amongst different genotypes ranged from 32 (subtype 2a) to 130 nM (genotype 4); while some variation in susceptibility amongst patient isolates was observed, the 95th percentile for any genotype did not exceed 189 nM. Levels of resistance to sofosbuvir in replicons containing S282T were between 2.4 and 18 fold-change in EC50; no other single NS5B resistance-associated substitution demonstrated reduced sofosbuvir susceptibility. These data suggest that S282T is the only known substitution that confers detectable resistance to sofosbuvir in vitro. Sofosbuvir displayed potent antiviral activity across a diverse range of NS5B mutants and HCV clinical isolates in multiple subtypes of genotypes 1 to 6.