The company said on Thursday the favorable results were seen in patients who took the drug, known as VX-661, in combination with its already approved treatment for cystic fibrosis, Kalydeco (ivacaftor) in a short 28-day study.

The Phase II study involved 128 people with cystic fibrosis who had two copies of the most common gene mutation responsible for the debilitating condition in which the lungs are unable to flush out salt and therefore become flooded with mucous and can become easily infected.

Patients treated in the two highest dose groups had a mean relative increase in lung function of between 7.5 percent and 9 percent, compared with patients given a placebo.

The results were "impressive," Wells Fargo analyst Brian Abrahams said in a research note. He added that they "meaningfully increase the likelihood of success" for the company's program to develop combination treatments for cystic fibrosis.

He has estimated the market for a dual cystic fibrosis treatment at over $3 billion.

Vertex said the experimental drug was generally well tolerated when used alone and in combination with Kalydeco.

Vertex shares were up 54 percent at $81.55 in after-hours trading from their closing of $52.87 Thursday on the Nasdaq.