Study to Evaluate the Safety and Efficacy of CTP-656 in Patients With Cystic Fibrosis With CFTR Gating Mutations

This is a randomized, parallel-group, double-blind, placebo controlled multicenter study to evaluate the safety and efficacy of CTP-656 in CF patients with CFTR gating mutations, compared to Kalydeco, for a total of 28 days. Subjects will be randomized to receive either double-blind CTP-656 or placebo, or open-label Kalydeco.