Both Kalydeco and VX-661 are designed to treat the underlying cause of CF, a faulty gene and its protein product, known as CFTR.

“Today's announcement is more terrific news for the CF community,” said Robert J. Beall, Ph.D., president and CEO of the Cystic Fibrosis Foundation. “These results show that another potential therapy from our ongoing collaboration with Vertex can be paired with Kalydeco and achieve promising improvements in health. The data further validate our strategy of using small molecule compounds to address the basic genetic defect in cystic fibrosis.”

The results showed a significant improvement in lung function in people with two copies of the most common CF mutation, Delta F508, who took the combination treatment, compared with those who received a placebo.

About 50 percent of people with CF in the United States have two copies of the Delta F508 mutation.

The Phase 2 study enrolled 128 people ages 18 and older with two copies of the Delta F508 mutation of CF. Four different doses of VX-661 were evaluated in combination with Kalydeco. Those who received the two highest doses of VX-661 showed the greatest improvement in lung function.

VX-661 is the second potential therapy that Vertex has studied in combination with Kalydeco in people with two copies of the Delta F508 mutation. Vertex is also studying Kalydeco with VX-809 in two Phase 3 trials that will enroll about 1,000 people at approximately 200 clinical trial sites in North America, Europe and Australia.

Vertex plans to conduct additional studies of VX-661 and Kalydeco in combination, pending discussions with regulatory agencies.

In today's announcement, Vertex also said it plans to begin a Phase 1 study later this year to evaluate Kalydeco in combination with the potential CF drug VX-983 in people with two copies of the Delta F508 mutation.

The CF Foundation played a key role in the development of Kalydeco, VX-809, VX-661 and VX-983, providing significant scientific, clinical and financial support. The U.S. Food and Drug Administration approved Kalydeco in January 2012 for people with the G551D mutation of CF.

"The CF Foundation is deeply grateful to the people with CF, families and medical professionals who have played a role in these studies, and to the Vertex scientists who remain dedicated to advancing CF drug development," said Beall.

Read the press release from Vertex.