Vertex Pharmaceuticals said on Tuesday its Kalydeco cystic fibrosis treatment given
with an experimental drug demonstrated significant improvements in lung function in a pair of late-stage trials the company plans to use to seek approval for the combination therapy.

Shares of Vertex surged more than 20 percent in premarket trade on Wednesday.

Patient discontinuations of the Kalydeco/tezacaftor combination due to adverse side effects, including respiratory-related side effects, were low and similar to placebo. That is important as bronchial constriction has led up to 30 percent of those prescribed Vertex’s Orkambi combination treatment to stop taking it.

“If we could get the same efficacy but have a cleaner, safer drug (than Orkambi) that would be a win and the results are actually better than that,” said Dr. Patrick Flume, an investigator on both trials.

“I think they should be very excited about the news,” added Flume, professor of medicine and pediatrics at Medical University of South Carolina, who said he expects the combination to win approval.

Vertex plans to apply for U.S. and European approvals in the third quarter. It also owns tezacaftor.

CF is a rare, life-shortening genetic disease affecting about 75,000 people in North America, Europe and Australia that causes progressive lung damage and loss of lung function. The studies tested CF patients aged 12 and over.

In a study of more than 500 patients with two copies of the F508del genetic mutation, which represents about 50 percent of the CF population, the Kalydeco/tezacaftor combination led to an improvement of 4 percentage points versus placebo in volume of air exhaled in one second, a measure known as FEV1.

Analysts have said an improvement of 4 points or better with few discontinuations would likely be viewed favorably by investors.

In addition to the primary goal of the study, the oral drugs showed significant improvements on secondary goals, including a 35 percent reduction in the annualized rate of lung infections known as pulmonary exacerbations that cause hospitalization and speed loss of lung function.

In the other study, the combination led to a mean absolute FEV1 improvement of 6.8 percentage points compared to placebo in patients with one copy of the F508del mutation and a second genetic mutation, representing a small percentage of CF patients. The improvement with Kalydeco alone was 4.7 percentage points versus placebo in those patients.

Kalydeco, known chemically as ivacaftor, was given every 12 hours, while tezacaftor, previously known as VX-661, was taken once a day. Kalydeco was the first approved drug that addressed the underlying cause of CF rather than symptoms.

Vertex’s approved medicines address about 29,000 CF patients. It is testing drugs for a triple combination with Kalydeco and tezacaftor that could eventually treat 80-90 percent of all CF patients, with data expected later this year.

CNBC contributed to this report.