The drug, called Pulmozyme, is not a cure for cystic fibrosis, a life-threatening genetic disease characterized by thick mucous secretions in the lungs, but it was shown in clinical trials to improve patients' quality of life significantly.
The trials showed that Pulmozyme reduced the risk of respiratory tract infections requiring antibiotic injections by 27 percent, improved lung function, and reduced hospitalizations and related medical costs.
Cystic fibrosis affects about 30,000 Americans, 3,000 Canadians and more than 20,000 Europeans.
"This is an example of biotech delivering on its promise," said Dr. David A. Kessler, the FDA commissioner. "The bottom line is the stuff works."
Pulmozyme is a genetically engineered copy of a naturally occurring enzyme that breaks up the mucus in cystic fibrosis patients' lungs.
The patients lack a gene, first identified earlier this year, which normally prompts the body to produce such an enzyme.
In the long term, gene replacement therapy could provide a cure for cystic fibrosis, but such a drug is many years away, experts say.
Pulmozyme will be used in addition to current cystic fibrosis treatments, which include antibiotics and mucous drainage procedures.
Genentech has not set a price for Pulmozyme.
But Viren Mehta, an analyst with Mehta & Isaly, estimates the cost for cystic fibrosis patients at $10,000 a year, and roughly a third of that for chronic bronchitis, which would require a smaller dose.
Chronic bronchitis, sometimes a euphemism for emphysema, presented a potentially far larger market for the drug, he said. "The potential is that many emphysema patients will benefit," Mr. Mehta said.