A Study to Evaluate Efficacy and Safety of Ivacaftor in Subjects With Cystic Fibrosis Aged 3 Through 5 Years Who Have a Specified CFTR Gating Mutation

Inclusion Criteria: * Male or female with confirmed diagnosis of CF. * Must have 1 of the following CFTR gating mutations on at least 1 allele: G551D, G178R, S549N, S549R, G551S, G1244E, S1251N, S1255P, or G1349D. * Hematology, serum chemistry, and coagulation at Screening with no clinically significant abnormalities or concomitant diagnosis that would interfere with the LCI and CT scan study assessments, as judged by the investigator. Exclusion Criteria: * An acute upper or lower respiratory infection, pulmonary exacerbation, or changes in therapy (including antibiotics) for pulmonary disease within 4 weeks before Day 1 * Any clinically significant laboratory abnormalities at the Screening Visit that would interfere with the study assessments or pose an undue risk for the subject (in the opinion of the investigator) * Abnormal liver function, at Screening, defined as ≥3 × upper limit of normal (ULN), of any 3 or more of the following: serum aspartate transaminase (AST), serum alanine transaminase (ALT), gamma-glutamyl transpeptidase (GGT), serum alkaline phosphatase (ALP), and total bilirubin * History of solid organ or hematological transplantation * Any clinically significant "non-CF-related" illness within 2 weeks before Day 1 * Use of any moderate or strong inducers or inhibitors of cytochrome P450 (CYP) 3A within 2 weeks before Day 1 * Participation in a clinical study involving administration of either an investigational or a marketed drug within 30 days or 5 terminal half-lives (whichever is longer or as determined by the local requirements) before Screening