The US Food and Drug Administration has denied the expanded use of Kalydeco in cystic fibrosis patients ages 2 or older who have one of 23 residual function mutations.

Kalydeco, which brought in revenue of $632 million in 2015, is already approved to treat CF patients ages 2 and older with 10 different mutations in the CFTR gene.

The company was exploring the use of the drug in the group of 23 residual function mutations, also known to carry the CFTR protein.

Vertex said it had filed for an application based on studies of these mutations in preclinical trials and an exploratory mid-stage study.

The FDA informed Vertex that it cannot approve the application in its present form. The company said it would meet with the regulator to determine the path forward.

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