FDA expanded the approval of Vertex Pharmaceuticals’ Kalydeco (ivacaftor), tripling the number of gene mutations that the drug can now treat for cystic fibrosis.

The agency expanded the indication from 10 mutations to 33, basing its decision on laboratory testing in conjunction with evidence from earlier clinical trials. According to the FDA, the laboratory approach provides a pathway for adding additional, rare mutations of the disease — where small patient populations can make full clinical trials unfeasible.

Results from an in vitro cell-based model system have been shown to reasonably predict clinical response to Kalydeco. When additional mutations responded to Kalydeco in the laboratory test, researchers were thus able to extrapolate clinical benefit demonstrated in earlier clinical trials of other mutations, the agency said.