BOSTON (AP) _ An experimental clot-dissolving drug given to heart attack victims also shows promise as a treatment for dangerous blood clots in the lungs, researchers report.

The researchers estimate that more than 50,000 Americans die each year from the lung clots, known as pulmonary embolisms, and another 300,000 are hospitalized annually with the condition.

So far, the medicine has been given to 40 people with lung clots, and it quickly broke up the clots in 37 of them.

The drug is tissue plasminogen activator, or TPA. It is being widely studied as a way to halt heart attacks by dissolving clots in the arteries that feed the heart. The latest research is the first experimental use of the substance in lung clots.

Initial results of the pilot study, directed by Dr. Samuel Z. Goldhaber at Brigham and Women's Hospital in Boston, were released last March at a meeting of the American College of Cardiology in Atlanta. A more complete account of the research is published in Friday's issue of the British journal Lancet.

Pulmonary embolisms often form in an artery in the leg or pelvis. They travel along the bloodstream, pass through the heart and lodge in the arteries that carry blood from the heart to the lungs. Such clots are especially common in people who are bedridden or have undergone surgery.

A potential hazard of the new treatment is internal bleeding that results when blood clots are dissolved. In the latest study, 6 percent of the patients treated suffered serious bleeding.

Lingering clots can damage lung tissue and cause heart failure. By disrupting the flow of blood to the lungs to gather oxygen, they cause shortness of breath.

Currently doctors give victims blood-thinning drugs to keep more clots from forming, but they must wait for the body's own processes to break up the clot.

The researchers recommended that a large-scale study be conducted to compare TPA with conventional therapy. No comparison group was included in the newly published study.