SVR12 rate in the first cohort (n=17) which received sofosbuvir 400mg QD + weight based ribavirin for 12 weeks was surprisingly low with only 59 % (1). In order to better understand which factors were potentially associated with higher risk of relapse the authors performed a multivariate analysis looking at traditional factors associated with relapse (baseline HCV RNA, HCV genotype and IL28B genetics) but also measured ribavirin levels in order to evaluate whether ribavirin levels had an impact on achieving SVR (4).
RBV plasma samples were analyzed using a validated LC-MS/MS assay. Most interestingly, traditional factors did not differ in those that achieved SVR vs. relapse; nor did CD4 count or duration of HCV infection. However, ribavirin concentrations were 52% higher in those that achieved SVR (p=0.01). The ribavirin concentrations over time for patients achieving SVR versus developing relapse are shown in figure 1. No statistical differences in those that achieved SVR versus relapsed were observed with regard to factors such as race, weight or inosine triphosphatase which are all known to potentially affect ribavirin pharmacokinetics. In addition, no statistical difference was found between those that achieved SVR versus relapsed in self-reported adherence.