The present invention relates to optimal compositions useful in treating HCV infections in humans. These compositions comprise alpha-interferon or pegylated alpha-interferon and an IMPDH inhibitor selected from VX-148 or VX-944, wherein the IMPDH inhibitor is present in an amount such that a ratio of Cavg/Cmin is between 1 to 10, wherein: Cavg is average plasma concentration produced by said IMPDH inhibitor in said human; and Cmin is estimated trough concentration produced by said IMPDH inhibitor in said human. The present invention also relates to methods of producing and using the optimal compositions to treat HCV infections in humans.