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

Absolute Change From Baseline in Cystic Fibrosis Questionnaire-Revised (CFQ-R) Respiratory Domain Score Through Week 12

The CFQ-R is a validated participant-reported outcome measuring health-related quality of life for participants with cystic fibrosis. Respiratory domain assessed respiratory symptoms (for example, coughing, congestion, wheezing), score range: 0-100; higher scores indicating fewer symptoms and better health-related quality of life.

Time frame: Baseline, Through Week 12

Number of Pulmonary Exacerbation Events

Pulmonary exacerbation was defined as the treatment with new or changed antibiotic therapy (intravenous, inhaled, or oral) for greater than or equal to 4 sinopulmonary signs/symptoms. The number of events were reported.

Time frame: Baseline through Week 12

Number of Pulmonary Exacerbation Events Per Year

Pulmonary exacerbation was defined as the treatment with new or changed antibiotic therapy (intravenous, inhaled, or oral) for greater than or equal to 4 sinopulmonary signs/symptoms. Total number of days on study is equal to the Week 12 date or the last dose date (whichever occurs last) minus first dose date plus 1. The total number of years (48 weeks) on study is equal to the total number of days on study divided by 336. Pulmonary exacerbation events per year (48 weeks) are reported.

Time frame: Baseline through Week 12

Absolute Change From Baseline in Body Mass Index (BMI) at Week 12

BMI was defined as weight in kilogram (kg) divided by height\*height in square meter (m\^2).

Time frame: Baseline, Week 12

Relative Change From Baseline in Percent Predicted FEV1 Through Week 12

FEV1 is the volume of air that can forcibly be blown out in one second, after full inspiration. Hankinson and Wang standards were used to calculate percent predicted FEV1 (for age, gender, and height). The Hankinson standard was used for male participants 18 years and older and female participants 16 years and older. The Wang standard was used for male participants aged 12 to 17 years and for female participants aged 12 to 15 years.

Time frame: Baseline, Through Week 12

Absolute Change From Baseline in Sweat Chloride Through Week 12

Sweat samples were collected using an approved collection device.

Time frame: Baseline, Through Week 12

Number of Participants With at Least One Pulmonary Exacerbation Through Week 12

Pulmonary exacerbation was defined as the treatment with new or changed antibiotic therapy (intravenous, inhaled, or oral) for greater than or equal to 4 sinopulmonary signs/symptoms. Time-to-first pulmonary exacerbation was planned to be estimated using Kaplan-Meier (KM) estimates. However, due to less than 50% of events, time-to-first event data was not estimated. Instead, number of participants with at least one pulmonary exacerbation event were collected and are reported.

Time frame: Baseline through Week 12

Absolute Change From Baseline in BMI Z-score at Week 12 (in Participants Less Than [<] 20 Years Old at the Time of Screening)

Z-score is a statistical measure to evaluate how a single data point compares to a standard. It describes whether a mean was above or below the standard and how unusual the measurement is, with range from infinity to +infinity; where 0: same mean, \>0: a greater mean, and \<0: a lesser mean than the standard. BMI, adjusted for age and sex, was analyzed as BMI-for-age Z-score (BMI z-score). BMI-for-age z-score was calculated by using centers for disease control and prevention (CDC) growth charts for the paediatric population.

Time frame: Baseline, Week 12

Absolute Change From Baseline in Body Weight at Week 12

Time frame: Baseline, Week 12

Number of Participants With Treatment-Emergent Adverse Events (AEs) and Serious Adverse Events (SAEs)

AE: any untoward medical occurrence in a participant during the study; the event does not necessarily have a causal relationship with the treatment. This includes any newly occurring event or previous condition that has increased in severity or frequency after the informed consent form is signed. AE includes serious as well as non-serious AEs. SAE (subset of AE): medical event or condition, which falls into any of the following categories, regardless of its relationship to the study drug: death, life threatening adverse experience, inpatient hospitalization/prolongation of hospitalization, persistent/significant disability or incapacity, congenital anomaly/birth defect, important medical event. Any AE that increased in severity or newly developed at or after initial dosing of study drug to Week 16 was considered treatment-emergent.

Time frame: Baseline up to Week 16

Trough Plasma Concentrations (Ctrough) of VX-661, VX-661 Metabolite (M1 VX-661), Ivacaftor (IVA) and IVA Metabolite (M1-IVA)

This outcome was not planned to be assessed in Placebo arm.

Time frame: Pre-morning dose on Week 2, Week 4, Week 8 and Week 12