In Vitro: Pibrentasvir inhibits HCV genotype 1a-H77, 1b-Con1, and 2a-JFH-1 subgenomic replicons with 50% effective concentrations (EC50s) of 1.8, 4.3, and 5.0 pM, respectively. The antiviral activity of Pibrentasvir is attenuated 35- to 47-fold in the presence of 40% human plasma through sequestration of compound due to plasma protein binding. Pibrentasvir retains full activity against all of the genotype 1a and 1b single-position NS5A substitutions tested, except Y93H and Y93N in genotype 1a, which confers a ≤7-fold increase in EC50 to Pibrentasvir.