Vertex Pharmaceuticals' (VRTX -0.69%) blockbuster cystic fibrosis drug Trikafta might soon have a larger addressable market. The company announced Thursday that a phase 3 trial of the drug, in which it was administered to 6- to 11-year-old patients, met both its primary endpoint (for safety and tolerability) and several secondary endpoints.

The next stage, said Vertex, is for the company to file a supplemental New Drug Application (NDA) for patients in that age group, which it will do in the fourth quarter.

Trikafta was first approved by the Food and Drug Administration in October, but only for use in patients 12 and older -- hence the need for a supplemental NDA.

Doctor with container of pills writing a prescription.

Image source: Getty Images.

It rapidly became a blockbuster product, and today it's far and away Vertex's top seller. Described by some as a "miracle drug," it treats people afflicted with the disease by targeting cystic fibrosis transmembrane conductance regulators that are defective because of certain genetic mutations. An estimated 75,000 people worldwide suffer from cystic fibrosis, according to the company.

In the phase 3 study with children, 66 patients were tested across 24 weeks of treatment. According to Vertex, the benefit-risk profile of Trikafta was similar to its performance in the late-stage trials with older patients that led to the FDA's original approval.

"Our aim is to extend eligibility to all patients who may benefit from this transformative medicine, and the positive results from the study in children ages 6 through 11 years old allows us to take another step forward toward this goal," said CEO Carmen Bozic.

Despite the encouraging news, Vertex was trading 3.3% lower as of 3:25 p.m. EDT on Thursday, a steeper decline than the S&P 500 index, which was off by about 1.9%.