Inhibition of inosine monophosphate dehydrogenase (IMPDH) is one of several proposed mechanisms of action for ribavirin (RBV), a critical component of the current treatment for chronic hepatitis C (CHC). This study was a double-blind, placebo-controlled dose-escalation study of a novel, selective, orally active small molecule inhibitor of IMPDH, merimepodib (VX-497 or MMPD) in combination with standard interferon-alpha (IFN-α). Fifty-four treatment-naive patients with genotype-1 CHC were randomized to receive IFN-α 3 MIU subcutaneously three times a week, alone or in combination with 100 mg or 300 mg (every 8 h) of MMPD for 4 weeks. At the end of 4 weeks, all patients were offered 48 weeks of treatment with IFN-α/RBV. The objectives of the study were to evaluate the tolerability of the IFN-α/MMPD combination and to evaluate whether MMPD had an on-treatment effect on HCV-RNA, similar to RBV when added to IFN-α. The drug combination was generally well tolerated; one patient at the higher dose discontinued because of elevated alanine aminotransferase levels. No pharmacokinetic interactions were evident between the two drugs. Analysis of covariance that adjusted for a baseline imbalance in HCV-RNA in the intent-to-treat population did not show any significant differences between the treatment groups, or between MMPD plus IFN-α compared with IFN-α alone. However, the per-protocol primary efficacy analysis based on treatment-compliant patients demonstrated a greater reduction in mean HCV-RNA in the combination of 100 mg MMPD plus IFN-α compared with IFN-α alone (-1.78 log vs -0.86 log, P=0.037). In conclusion, the addition of a selective IMPDH inhibitor to IFN-α was well tolerated. In a low-dose range, the addition of MMPD may have the potential to add to the antiviral efficacy of IFN-α. Larger, longer duration trials incorporating pegylated IFN would be required to determine whether this combination, alone or with RBV, would increase either early or sustained virological response rates.