Daclatasvir-d is intended for use as an internal standard for the quantification of daclatasvir by GC- or LC-MS. Daclatasvir is a first generation direct-acting inhibitor of hepatitis C virus (HCV) non-structural protein 5A (NS5A; Ks = 8 and 210 nM for the NS5A and NS5A residues of HCV genotype 1b, respectively). It potently inhibits HCV replication in multiple HCV replicon genotypes (ECs = 9-146 pM) with the highest potency in genotypes 1b and 4a (ECs = 9 and 12 pM, respectively). Daclatasvir disrupts the subcellular localization of NS5A in Huh7.5 cells and inhibits viral RNA synthesis and virion assembly and secretion when used at a concentration of 1 nM in HCV-infected Huh7 cells. Daclatasvir also inhibits organic anion transport polypeptides 1B1 (OAT1B1) and OAT1B3 (ICs = 1.5 and 3.27 µM, respectively). Formulations containing daclatasvir have been used alone and in combination with NS3/4A and NS5B inhibitors in the treatment of HCV.