Tezacaftor (VX-661) is an investigational compound developed by Vertex Pharmaceuticals to treat cystic fibrosis (CF). It is an oral corrector of the CF transmembrane regulator (CFTR) and is similar to lumacaftor, another N-aryl-1-(2,2-difluoro-1,3-benzodioxol-5-yl)cyclopropanecarboxamide derivative developed by Vertex. Cystic fibrosis is caused by defects in CFTR gene, which encodes an epithelial chloride channel. The most common mutant Δ508CFTR is a misfolded protein that does not reach the cell membrane. VX-661 corrects trafficking of Δ508CFTR and partially restores chloride channel activity. In vitro, a combination of VX-661 and ivacaftor, an FDA approved in 2012 CFTR potentiator which increases the time the CFTR channel is open, allowing chloride ions to flow through the CFTR proteins on the surface of epithelial cells, resulted in greater CFTR activity compared with VX-661 alone. In February 2012, a phase 2, double-blind, placebo-controlled study of VX-661 was initiated in CF patients who were homozygous or heterozygous for the F508del mutation. There is an ongoing Vertex Phase 3 development program of VX-661 in combination with ivacaftor which includes four studies on CF patients 1) with two copies of the F508del mutation, 2) one copy of the F508del mutation and a second mutation that results in residual CFTR function, 3) one copy of the F508del mutation and a second mutation that results in residual CFTR function gating defect in the CFTR protein and 4) one copy of the F508del mutation and a second mutation that results in minimal CFTR function.

Ivacaftor (trade names KALYDECO® (ivacaftor) and ORKAMBI® (lumacaftor/ivacaftor)) is a cystic fibrosis transmembrane conductance regulator potentiator indicated for the treatment of cystic fibrosis in patients age 6 years and older who have one of the following mutations in the CFTR gene: G551D, G1244E, G1349D, G178R, G551S, S1251N, S1255P, S549N, or S549R. One such defect G551D is characterized by a dysfunctional CFTR protein on the cell surface. Although the defective protein is trafficked to the correct area, the epithelial cell surface, while there it cannot transport chloride through the channel. Ivacaftor, a CFTR potentiator, improves the transport of chloride through the ion channel by binding to the channels directly to induce a non-conventional mode of gating which in turn increases the probability that the channel is open. Ivacaftor regulates fluid flow within cells and affects the components of sweat, digestive fluids, and mucus.