After the primary TW 12 analysis of the MK-5172 arms in the Vanguard cohort, patients receiving the 400 mg and 800 mg doses in the Second Cohort were down-dosed due to elevated liver transaminases and began to receive 100 mg in an open-label fashion between weeks 3 and 12 of MK-5172 therapy.Current Status of Study All patients in both cohorts of the study (termed the Combined Cohort when analyzed together) have reached the primary endpoint of the study, cEVR, or have discontinued early. cEVR values reflect both those patients with both undetectable (TND) and detectable unquantifiable (TD(u)) HCV RNA. In the Second Cohort, 134 of 156 patients randomized into the MK-5172 arms are receiving PR (n=17) or are in the follow-up phase (n=117) of the study. All patients in the Vanguard Cohort who received MK-5172 are in the follow-up phase of the study or have discontinued early. Of the patients randomized to the MK-5172 arms, 2.3 percent (6/266) met the protocol-defined criteria for virologic failure: one (1) patient was re-infected with genotype 3 infection; and four patients had no detectable (n=3) or low (n=1) levels of MK-5172 at time of failure and for a period of time prior. The primary efficacy analysis included the full analysis set (FAS) of all randomized patients who received at least one dose of study treatment. SVR12 Results in the Vanguard Cohort In the Vanguard Cohort, higher SVR12 rates were achieved across all MK-5172 arms compared to control (FAS) – 96.2 percent (25/26) in the MK-5172 100 mg arm; 86.7 percent (26/30) in the MK-5172 200 mg arm; 87.0 percent (20/23) in the MK-5172 400 mg arm; and 81.5 percent (22/27) in the MK-5172 800 mg arm; versus 54.2 percent (13/24) in the control arm. In these results, all patients had undetectable (TND) HCV RNA.