A Study to Evaluate the Effect of VX-661 in Combination With Ivacaftor on Chest Imaging Endpoints in Subjects With Cystic Fibrosis, Homozygous for the F508del CFTR Mutation

Inclusion Criteria: * Homozygous for the F508del CFTR mutation * Confirmed diagnosis of CF * Percent predicted forced expiratory volume (ppFEV1) ≥70% of predicted normal for age, sex, and height during screening. * Stable CF disease as judged by the investigator Exclusion Criteria: * History of any comorbidity 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. * An acute upper or lower respiratory infection, pulmonary exacerbation, or changes in therapy (including antibiotics) for pulmonary disease within 28 days before Day 1 (first dose of study drug) * Pregnant or nursing females. * Sexually active subjects of reproductive potential who are not willing to follow the contraception requirements. * Any contraindication to undergoing chest imaging, as per the site's institutional guidelines