A Study to Evaluate the Pharmacokinetics, Safety, and Tolerability of VX-661/Ivacaftor in Pediatric Subjects With Cystic Fibrosis (CF)

Inclusion Criteria: * Subjects who weigh ≥15 kg without shoes at the Screening Visit. * All genotypes as specified by the study protocol are eligible in Part A. * The following genotypes are eligible in Part B: * homozygous for the F508del CFTR mutation * heterozygous for the F508del CFTR mutation and with a second allele with a CFTR mutation predicted to have residual function. * heterozygous for the F508del CFTR mutation and with a second CFTR allele with a gating defect that is clinically demonstrated to be ivacaftor responsive * Subjects with a confirmed diagnosis of CF defined as a sweat chloride value ≥60 mmol/L or chronic sinopulmonary and/or gastrointestinal disease consistent with a diagnosis of CF. Subjects who are homozygous for the F508del-CFTR mutation must have a sweat chloride value ≥60 mmol/L. * Subjects with ppFEV1 of ≥40 percentage points at the Screening Visit * Subjects with stable CF disease as deemed by the investigator at the Screening Visit. * Subjects who are willing to remain on their stable CF medication regimen through Day 14 (Part A) or through Week 24 (Part B) or, if applicable, through the Safety Follow up Visit. * Subjects who are able to swallow tablets. * Female subjects of childbearing potential must have a negative serum pregnancy test at the Screening Visit and a negative urine pregnancy test at the Day 1 Visit before receiving the first dose of study drug. * Subjects of childbearing potential who are sexually active must meet the contraception requirements Exclusion Criteria: * History of any comorbidity reviewed at the Screening Visit 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 subject. * Any clinically significant laboratory abnormalities at the Screening Visit that would interfere with the study assessments or pose an undue risk for the subject. * An acute upper or lower respiratory infection, pulmonary exacerbation, or changes in therapy for pulmonary disease within 28 days before Day 1 * Colonization with organisms associated with a more rapid decline in pulmonary status. * A standard 12 lead ECG demonstrating QTc \>450 msec at the Screening Visit. * History of solid organ or hematological transplantation at the Screening Visit. * Ongoing or prior participation in an investigational drug study or use of commercially available CFTR modulator (except physician-prescribed Kalydeco for approved indications) within 30 days of screening. * Use of restricted medication or food within a specified duration before the Screening Visit or first dose of study drug and/or unwillingness to maintain the restrictions. * History or evidence of cataract, lens opacity, Y-suture, or lamellar rings determined to be clinically significant by the ophthalmologist during the ophthalmologic examination at the Screening Visit. * Pregnant and nursing females.