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 confoundthe results of the study or pose an additional risk in administering study drug to thesubject.

• An acute upper or lower respiratory infection, pulmonary exacerbation, or changes intherapy (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 thecontraception requirements.

• Any contraindication to undergoing chest imaging, as per the site's institutionalguidelines