Shares of Vertex Pharmaceuticals Inc. soared more than 55.5 percent in after-hours trading on the Nasdaq exchange Thursday after the Cambridge biotechnology company released positive clinical data on an experimental treatment for cystic fibrosis.

Vertex said a mid-stage study of its VX-661 drug candidate in combination with its approved cystic fibrosis drug Kalydeco “showed statistically significant improvements in lung function” among adult patients with two copies of a common cystic fibrosis regulator gene mutation.

“These new results provide further validation of our strategy” of combining therapies to develop a portfolio of treatments for patients with different forms of cystic fibrosis, Vertex chief executive Jeffrey Leiden said in a conference call with investors.

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The life-threatening disease, passed down through families, causes mucus to build up in the lungs and digestive tract.

Vertex stock closed down 2.2 percent at $52.87 in Thursday trading. But when Vertex released its clinical data after the market closed, the share price quickly vaulted more than $29.