© 2018 Massachussetts Medical Society. All rights reserved. Background: The next-generation cystic fibrosis transmembrane conductance regulator (CFTR) corrector VX-659, in triple combination with tezacaftor and ivacaftor (VX-659- tezacaftor-ivacaftor), was developed to restore the function of Phe508del CFTR protein in patients with cystic fibrosis. Methods: We evaluated the effects of VX-659-tezacaftor-ivacaftor on the processing, trafficking, and function of Phe508del CFTR protein using human bronchial epithelial cells. A range of oral VX-659-tezacaftor-ivacaftor doses in triple combination were then evaluated in randomized, controlled, double-blind, multicenter trials involving patients with cystic fibrosis who were heterozygous for the Phe508del CFTR mutation and a minimal-function CFTR mutation (Phe508del-MF genotypes) or homozygous for the Phe508del CFTR mutation (Phe508del-Phe508del genotype). The primary end points were safety and the absolute change from baseline in the percentage of predicted forced expiratory volume in 1 second (FEV1). Results: VX-659-tezacaftor-ivacaftor significantly improved the processing and trafficking of Phe508del CFTR protein as well as chloride transport in vitro. In patients, VX-659- tezacaftor-ivacaftor had an acceptable safety and side-effect profile. Most adverse events were mild or moderate. VX-659-tezacaftor-ivacaftor resulted in significant mean increases in the percentage of predicted FEV1 through day 29 (P<0.001) of up to 13.3 points in patients with Phe508del-MF genotypes; in patients with the Phe508del-Phe508del genotype already receiving tezacaftor-ivacaftor, adding VX-659 resulted in a further 9.7-point increase in the percentage of predicted FEV1. The sweat chloride concentrations and scores on the respiratory domain of the Cystic Fibrosis Questionnaire-Revised improved in both patient populations. Conclusions: Robust in vitro activity of VX-659-tezacaftor-ivacaftor targeting Phe508del CFTR protein translated into improvements for patients with Phe508del-MF or Phe508del- Phe508del genotypes. VX-659 triple-combination regimens have the potential to treat the underlying cause of disease in approximately 90% of patients with cystic fibrosis.