GS-9851 (formerly PSI 7851) is an orally available, second generation uridine nucleoside analog polymerase inhibitor of hepatitis C treatment. GS-9851 is a nucleotide prodrug of the nucleoside analog GS-331007 (formerly PSI-6206). GS-9851 potently inhibits HCV NS5B polymerase and has demonstrated pan-genotypic activity in vitro. Preclinical studies of GS-9851 demonstrated a favorable profile in terms of antiviral potency, distribution, and metabolism. GS-9851 is first hydrolyzed to the intermediate GS-566500 (formerly PSI-352707), which is then metabolized to either the inactive metabolite, GS-331007, or the monophosphate GS-606965 (formerly PSI-7411). Inside the hepatocyte, GS-606965 is further phosphorylated by a series of enzymatic steps to an active triphosphate metabolite, GS-461203 (formerly PSI-7409), that selectively inhibits recombinant NS5B. A first-time-in-human study demonstrated that GS-9851 (25 to 800 mg) is generally safe and well tolerated in patients chronically infected with HCV. GS-9851 has a pharmacokinetic profile consistent with once-daily dosing. Administration of GS-9851 led to significant reductions in plasma HCV RNA levels without the evolution of known resistance mutations.