Next-generation CFTR corrector elexacaftor aims to be first cystic fibrosis product with minimal function mutations claim after winning out over VX-659 thanks to tolerability advantage in dueling Phase III programs testing combination with the components of Vertex’ Symdeko.

Already dominant in cystic fibrosis, Vertex hopes its triple regimen will address roughly 90% of the patient population. It also hopes the benefits of the triple will smooth EU reimbursement talks.

With data showing a lung function and sweat chloride benefit for its proprietary CFTR-modulating triplet, Proteostasis says it’s moving ahead. But analysts and investors are concerned the effects are less than seen with Vertex drugs.

Two three-drug regimens for cystic fibrosis hit their primary endpoints in both heterozygous and homozygous patients, with good safety and tolerability, which analysts say increases the likelihood one of them will obtain approval.