A Study to Evaluate the Safety, Pharmacokinetics, and Pharmacodynamics of Ivacaftor in Subjects With Cystic Fibrosis Who Are Less Than 24 Months of Age and Have an Ivacaftor-Responsive CFTR Mutation

Inclusion Criteria: * Confirmed diagnosis of CF by sweat chloride value or CF mutation criteria. * Have 1 of the following 10 CFTR mutations on at least 1 allele: G551D, G178R, S549N, S549R, G551S, G1244E, S1251N, S1255P, G1349D or R117H (eligible in regions where ivacaftor is approved for use). Part A/B group may also have other ivacaftor-responsive mutations. * Hematology, serum chemistry, and vital signs results at screening with no clinically significant abnormalities that would interfere with the study assessments, as judged by the investigator. Exclusion Criteria: * History of any illness or condition that, in the opinion of the investigator, might confound the results of the study or pose an additional risk in administering study drug to the participant * Colonization with organisms associated with a more rapid decline in pulmonary status at screening (Only for Parts A and B) * History of abnormal liver function or abnormal liver function at screening * History of solid organ or hematological transplantation * 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 before screening * Hemoglobin (Hgb) \<9.5 g/dL at screening * Chronic kidney disease of Stage 3 or above * Presence of a non-congenital or progressive lens opacity or cataract at Screening Other protocol defined Inclusion/Exclusion Criteria may apply.