Abstract
Despite years of clinical use and extensive research efforts, the mechanism of action of ribavirin (RBV) is not well understood. Although it has only a mild, transient antiviral effect on HCV replication when administered as monotherapy, when combined with interferon, RBV improves sustained virological response (SVR) rates by approximately 25-30%. Proposed mechanisms of action for RBV against HCV include (1) a direct effect against the HCV RNA dependent RNA polymerase; (2) induction of misincorporation of nucleotides leading to lethal mutagenesis; (3) depletion of intracellular pools via inhibition of inosine monophosphate dehydrogenase; (4) alteration in the cytokine balance between a Th2 profile (anti-inflammatory) to a Th1 profile (pro-inflammatory); and (5) potentiating the effect of interferon via up-regulation of genes involved in interferon signalling. Given the lack of a clear understanding of RBV mechanism of action, it has been challenging to confidently position this drug with new direct antiviral agents (DAA). However, early clinical studies provide strong evidence for a benefit of RBV in combination with DAAs for both IFN containing and sparing regimens. The addition of RBV reduces viral breakthroughs and/or relapses, at least when drugs with low to moderate genetic barriers to resistance are paired together. This is particularly true in patients harbouring HCV subtype 1a. Ongoing studies are now addressing the utility of RBV in nucleoside containing DAA regimens, which offer both potent antiviral activity as well as a high genetic barrier to resistance. It is remarkable that the age-old question of the role of RBV in the future of HCV therapy remains as real today as it was two decades ago.