GSK2336805 is an orally bioavailable hepatitis C virus (HCV) inhibitor working through an NS5A-mediated mechanism. This first-time-in-human study was conducted to assess the safety, tolerability, pharmacokinetics, metabolism, and efficacy of GSK2336805 in healthy subjects and subjects infected with HCV genotype 1. We performed a three-part, randomized, doubleblind, placebo-controlled study in 46 healthy subjects and 23 HCV-infected subjects. After an overnight fast, healthy subjects received GSK2336805 as 10 mg, 30 mg, 30 mg plus food, and 60 mg in a single dose and 10 mg (7 days), 30 mg (7 days), and 75 mg (14 days) in a once-daily multiple dose. Subjects with HCV received GSK2336805 as a 1-to 120-mg single dose. In subjects with HCV, reductions in HCV RNA were observed within 4 h and a single dose of GSK2336805 of >10 mg resulted in a statistically significant >2-log reduction in HCV RNA compared with placebo at 24 h postdose. GSK2336805 was readily absorbed in all subjects, and the half-life (t 1/2 ) was suitable for once-daily dosing. Administration of GSK2336805 with food had no effect on plasma GSK2336805 exposure; however, absorption was delayed, with a median t max (time to maximum concentration of drug in serum) of 4.5 versus 2.0 h. Twenty subjects who received GSK2336805 experienced mild to moderate adverse events; none were serious. GSK2336805 was well tolerated and exhibited rapid, significant antiviral activity after a single dose in HCV-infected subjects. These results support the conduct of further studies evaluating GSK2336805 administered once daily for longer durations in combination with peginterferon, ribavirin, and other direct-acting antivirals. (This study has been registered at ClinicalTrials.gov under registration no. NCT01277692.) H epatitis C virus (HCV) is the leading cause of cirrhosis, liver failure, and primary hepatocellular carcinoma and the primary indication for liver transplantation (1). Of the six major HCV genotypes, genotype 1 (GT-1) is the most prevalent and is associated with the highest rate of treatment failure (2-4).Administration of two recently approved HCV nonstructural protein 3 (NS3) serine protease inhibitors, telaprevir and boceprevir, with the standard treatment of peginterferon and ribavirin led to sustained viral response rates of 75% and 68%, respectively (5-8). However, increased side effects, especially skin rash and anemia, are still problematic (9, 10). Therefore, antiviral agents with novel modes of action are necessary for an all-oral combination therapy to further improve sustained viral response rates and reduce side effects (11).NS5A is essential for HCV replication, and NS5A inhibitors have shown potent anti-HCV activity relative to interferon-ribavirin regimens in clinical trials (12)(13)(14)(15). GSK2336805 is an orally bioavailable NS5A inhibitor with selective activity against GT-1a and GT-1b subtypes in HCV replicon systems (50% effective concentration [EC 50 ] for GT-1a, 58.5 pM; for GT-1b, 7.4 pM) in vitro (J. Walker, submitted for publicat...