A Study to Evaluate the Efficacy and Safety of VX-661 in Combination With Ivacaftor in Subjects Aged 12 Years and Older With Cystic Fibrosis, Heterozygous for the F508del-CFTR Mutation

Inclusion Criteria:

- Confirmed diagnosis of CF defined as a sweat chloride value greater than or equal to
(>=)60 millimole per liter (mmol/L) by quantitative pilocarpine iontophoresis.

- Heterozygous for the F508del-CFTR mutation and with a second CFTR mutation that is not
likely to respond to VX-661 and/or ivacaftor therapy, genotype to be confirmed via
assessment at the Screening Visit.

- Forced Expiratory Volume in 1 Second (FEV1) >=40 percent (%) and less than or equal to
(<=)90% of predicted normal for age, sex, and height at Screening Visit.

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
participant.

- 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).

- History of solid organ or hematological transplantation.

- Ongoing or prior participation in an investigational drug study or use of commercially
available CFTR modulator within 30 days of screening.

- Pregnant or nursing females.