Lung clearance index (LCI) is a measure of ventilation inhomogeneity that is derived from a multiple breath washout test using Nitrogen (N2). LCI2.5 represents the number of lung turnovers required to reduce the end tidal inert gas concentration to 1/40th of its starting value.

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. Serious adverse event (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, in-patient hospitalization/prolongation of hospitalization, persistent/significant disability or incapacity, congenital anomaly/birth defect, important medical event. Number of participants with AEs and SAEs are reported. An AE that started at or after initial dosing of study drug, or increased in severity after initial dosing of study drug visit is considered treatment-emergent.

Blood samples drawn during the study will be analyzed to measure the PK parameters, such as concentration, exposure and half-life of VX-809 and its metabolite.

Blood samples drawn during the study will be analyzed to measure the PK parameters, such as concentration, exposure and half-life of VX-770 and its metabolites.