The US Food and Drug Administration is refusing to extend the target patient group for Vertex Pharmaceuticals’ cystic fibrosis (CF) drug Kalydeco.

The regulator has issued a complete response letter after reviewing the firm’s request to market the therapy for CF patients carrying one of 23 residual function mutations, concluding that it “cannot approve the application in its current form”.

Kalydeco (ivacaftor) is already cleared for use in CF patients aged two and older who have one of 10 mutations in the CFTR gene (G551D, G1244E, G1349D, G178R, G551S, S1251N, S1255P, S549N, S549R or R117H), and Vertex is seeking “to get the drug to more people as quickly as possible," according to chief medical officer Jeffrey Chodakewitz.

"Based on the established safety profile of Kalydeco and our increasing understanding of the biology of these specific residual function mutations and their response to ivacaftor, we believe that people with these mutations would benefit from treatment with this medicine,” he said.

CF is a serious genetic disorder that results in the formation of thick mucus that builds up in the lungs, digestive tract and other parts of the body leading to severe respiratory and digestive problems, as well as other complications such as infections and diabetes.

More than 1,500 CF patients in the US (aged two and above) have one of the 23 residual function mutations included in the application, Vertex noted.