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

Inclusion Criteria: * 18 years of age or older * Has a confirmed diagnosis of CF with at least one allele of the following CFTR gating mutations: G551D, G1244E, G1349D, G178R, G551S, S1251N, S1255P, S549N, and S549R. * Has been stable on Kalydeco therapy for at least 3 months prior to screening * Has FEV1 ≥ 60% of predicted normal for age, sex, and height at screening and baseline (Day 1) assessments * Weighs at least 40 kg at screening * Patients of either gender and women of childbearing potential must be willing to use a medically highly effective form of birth control during the treatment period and 30 days after the last dose of study treatment. Exclusion Criteria: * Acute upper respiratory infection or lower respiratory infection, pulmonary exacerbation, or changes in therapy within 4 weeks of study treatment * Uncontrolled type 2 diabetes, or uncontrolled CF-related diabetes * History of hepatitis C or chronic active hepatitis B infection * History of pulmonary tuberculosis, non-tuberculosis mycobacterial infections or allergic bronchopulmonary aspergillosis (ABPA) treated during screening or within 2 years prior to screening * Colonization with B. cenocepacia, B. dolosa, B. multivorans, and/or M. abcessus within 2 years prior to Screening * Abnormal liver function * History of abnormal renal function * History of prolonged QTcF \> 450 msec for males or QTcF \> 470 msec for females * History of solid organ or hematological transplantation * Using any inhibitor or inducer of cytochrome P450/3A during the study or within 30 days of screening * Women who are pregnant or lactating, or have plans to become pregnant during the study or within 1 month following the last dose