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.