Merimepodib (MMPD) is an orally administered, inosine monophosphate dehydrogenase inhibitor that has shown antiviral activity in nonresponders with chronic hepatitis C (CHC) when combined with pegylated interferon alfa 2a (Peg-IFN-alfa-2a) and ribavirin (RBV). We conducted a randomized, double-blind, multicenter, phase 2b study to evaluate the antiviral activity, safety, and tolerability of MMPD in combination with Peg-IFN-alfa-2a and RBV in patients with genotype 1 CHC who were nonresponders to prior therapy with Peg-IFN and RBV. Patients received 50 mg MMPD, 100 mg MMPD, or placebo every 12 hours, in addition to Peg-IFN-alfa-2a and RBV, for 24 weeks. Patients with a 2-log or more decrease from baseline or undetectable hepatitis C virus (HCV) RNA levels at week 24 were then eligible to continue Peg-IFN-alfa-2a and RBV for a further 24 weeks, followed by 24 weeks of follow-up. The primary efficacy endpoint was sustained virological response (SVR) rate at week 72 in all randomized patients who received at least one dose of study drug and had a history of nonresponse to standard therapy. A total of 354 patients were randomized to treatment (117 to placebo; 119 to 50 mg MMPD; 118 to 100 mg MMPD), and 286 completed the core study. The proportion of patients who achieved SVR was similar among the treatment groups: 6% (6/107) for 50 mg MMPD, 4% (5/112) for 100 mg MMPD, and 5% (5/104) for placebo (P = 0.8431). Adverse-event profiles for the MMPD combination groups were similar to that for Peg-IFN-alfa and RBV alone. Nausea, arthralgia, cough, dyspnea, neutropenia, and anemia were more common in patients taking MMPD.The addition of MMPD to Peg-IFN-alfa-2a and RBV combination therapy did not increase the proportion of nonresponder patients with genotype 1 CHC achieving an SVR.