A Phase 3 Study to Evaluate the Efficacy and Safety of Ivacaftor and VX-661 in Combination With Ivacaftor in Subjects Aged 12 Years and Older With Cystic Fibrosis, Heterozygous for the F508del-cystic Fibrosis Transmembrane Conductance Regulator (CFTR) Mutation

Inclusion Criteria:

- Heterozygous for F508del-CFTR and a second allele with a CFTR mutation predicted to
have residual function

- Forced Expiratory Volume in 1 Second (FEV1) greater than or equal to (≥) 40 percent
(%) and less than or equal to (≤) 90% of predicted normal for age, sex, and height
during screening

- Sweat chloride value ≥60 millimole per liter (mmol/L) during screening OR as
documented in the participant's medical record

- 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
participant

- An acute upper or lower respiratory infection, pulmonary exacerbation

- History of solid organ or hematological transplantation

- Ongoing or prior participation in an investigational drug study (including studies
investigating VX-661, lumacaftor [VX-809], and/or ivacaftor) within 30 days of
screening

- Pregnant and nursing females

- Sexually active participants of reproductive potential who are not willing to follow
the contraception requirements