Decision by Sponsor.

This study will evaluate the efficacy and safety of CTP-656 in patients with cystic fibrosis (CF) who have a cystic fibrosis transmembrane conductance regulator (CFTR) gating mutation.

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.

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