CAMBRIDGE, Mass. (AP) — Vertex Pharmaceuticals said Thursday that its drug combination to treat cystic fibrosis improved lung function in adults with the inherited disease, sending shares rocketing more than 50 percent in after-hours trading.

THE SPARK: Vertex reported mid-stage results from a study of 128 people taking its experimental drug VX-661 combined with Kalydeco, its approved drug for the deadly disease.

The company said patients taking the combination for 28 days showed a statistically significant improvement in lung function, on average, compared with placebo. The drugs were well tolerated with mild to moderate side effects.

The drug developer said it plans to conduct additional studies of VX-661 and could move the drug into late-stage testing based on those results and discussions with regulators.

THI BIG PICTURE: Cystic fibrosis causes sticky mucus buildup in the lungs and other organs, leading to infections, digestive problems and usually early death.

The typical life expectancy among patients who survive to adulthood is about 37 years, according to the National Institutes of Health. Patients with the disease have various genetic mutations that interfere with the flow of chloride and water across cell walls, causing buildup of internal mucus.

The Food and Drug Administration approved Kalydeco last year for people with a very rare form of cystic fibrosis. VX-661, which is not yet approved, is designed to treat the most common form of the disease. Vertex is testing the drugs in combination to see if they improve results for patients with the more prevalent form of the disease.

SHARE ACTION: Shares of Vertex Pharmaceuticals Inc. rocketed $28.26, or 53 percent, to $81.13 in after-hours trading. Shares fell $1.17, or 2.2 percent, to $52.87 in regular trading.