Vertex Pharmaceuticals said on Tuesday that a combination of two of its drugs had successfully treated cystic fibrosis in closely watched clinical trials, potentially clearing the way for approval of a new option for nearly half the patients with the genetic disease.

The drug combination, when compared with a placebo, improved the lung function of patients and also reduced pulmonary flare-ups, which can lead to hospitalization.

“These data, the totality and the consistency, were as much or more than we were hoping for,” Dr. Jeffrey Leiden, the chief executive of Vertex, said in an interview. He said the company would submit applications for approval of the combination in the United States and Europe in the fourth quarter.

Still, the improvement in lung function of between 2.6 and 4 percentage points after six months, compared with the lung function of patients who took the placebo, was at the low end of what some doctors would consider meaningful and what some investors were anticipating.

“Would I have rather seen 6, 7 percent? Of course I would have,” said Dr. Bonnie W. Ramsey, a professor of pediatrics at the University of Washington and one of the lead investigators for the studies.

Dr. Ramsey, who was briefed on the results of the test, which were analyzed by Vertex, said that an improvement of 3 percent would be imperceptible to patients. But she said that if the effect lasted longer than six months, it could make a meaningful difference over the long run.

About 30,000 Americans and about 75,000 people globally have cystic fibrosis, an inherited disease caused by mutations in a gene that controls the transport of water and salt into and out of cells. The disease causes mucus to build up in the lungs, which leads to infections and lung damage. Many people with the disease die before reaching 40.

Vertex won approval for one cystic fibrosis drug, Kalydeco, in 2012. But Kalydeco, also known as ivacaftor, works for a mutation found in only about 4 percent of patients.

The combination, consisting of ivacaftor and an experimental drug called lumacaftor, would be applicable to nearly half of cystic fibrosis patients: those with two copies of the most common mutation, known as F508del.

Although there are other drugs available, such as antibiotics and an enzyme to loosen mucus, Kalydeco was the first drug to correct the underlying genetic defect, and lumacaftor could be the second. The gene responsible for the disease was discovered in 1989 by, among others, Dr. Francis S. Collins, now the director of the National Institutes of Health.

The results of Vertex’s trials were being closely watched on Wall Street as a so-called binary event, one that could cause the company’s shares to rise or fall sharply and that could also perhaps influence investor sentiment about the entire biotechnology industry.

“It’s not an exaggeration to say that billions of dollars in value, for Vertex and the overall biotech sector, hinge on” the results, Geoffrey C. Porges, an analyst at Sanford C. Bernstein & Company, wrote in a note last week.

That is because the future of Vertex, which is based in Boston, depends on its being the leader in treating cystic fibrosis. Incivek, its drug for hepatitis C, was approved only three years ago and was a big seller but has already faded in the face of competition from Gilead Science’s Sovaldi.

Although the number of patients with cystic fibrosis is small, analysts say Vertex could reap billions of dollars in annual sales because drugs for rare diseases are expensive. Kalydeco costs more than $300,000 a year and had sales last year of $371 million.

Some doctors have protested the price, including authors of a commentary published in the Journal of the American Medical Association in October.

Dr. Leiden said it was “way too early” to talk about the price of lumacaftor, which previously was called VX-809.

The nonprofit Cystic Fibrosis Foundation financially supported the development of both Kalydeco and lumacaftor and is entitled to royalties on sales of both drugs.

The two trials involved a total of about 1,100 patients, age 12 and over, in North America, Europe and Australia. Each trial had three arms. Patients in two of the arms took ivacaftor plus either a low or high dose of lumacaftor. Patients in the third arm received a placebo.

Lung function was assessed by the maximum a person could exhale in one second, a standard test. At the start of the trial, the patients had average lung function, based on this measure, that was 61 percent of what would be predicted for people their age.

After 24 weeks, lung function had improved in all four groups that got the drug combination, by between 2.2 and 3.6 percentage points, meaning their average lung function was somewhere around 64 percent of what would be normal for their age. The lung function of those who got the placebo fell slightly.

Dr. Leiden emphasized that pulmonary exacerbations, or flare-ups, had been reduced about 30 percent to 40 percent. He also noted that the drug had helped patients, who also tend to have digestive problems, to gain weight.

“Every day you don’t spend in the hospital and every pound you gain is meaningful to that patient and their family,” he said.

About 4.2 percent of those getting the drugs dropped out of the trial because of side effects like liver problems, compared with only 1.6 percent of those receiving the placebo. Shortness of breath was another side effect. But more than 1,000 patients elected to continue on the drug combination after the trial ended.

The results were announced in a news release by the company and have not been peer reviewed by outside experts.

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