The Food and Drug Administration will approve today the use of TPA, a heart drug that has proved remarkably effective at dissolving the blood clots that cause at least 80 percent of the 1.5 million heart attacks suffered by Americans each year, sources said.

Studies have shown that TPA, or tissue plasminogen activator, dissolves clots in the heart faster and better than any other drug, and medical experts predict that it will save thousands of lives each year.

"This is a landmark in the history of American medicine," said Dr. Eric Topol, a cardiologist at the University of Michigan Medical School, who in 1984 became the first investigator to use TPA on patients. "It will change our business as much as penicillin did. There is no counting how many lives the drug can save."

Administered soon enough after the beginning of a heart attack, the drug -- a genetically engineered replica of a protein the body produces naturally -- melts the fatty clots that stop the flow of blood to the heart.

Heart attacks are the nation's leading killer. More than 400,000 Americans die from them each year. Chances of survival largely depend on the damage to the patient's heart. And the damage often is determined by how much and how long a clot interrupts normal blood flow to the organ.

Certain to be the first blockbuster product of genetic engineering, the decision to license tissue plasminogen activator, manufactured by Genentech Inc., will bring to an end the most contentious battle over drug approval in recent FDA history.

Last May 29, an FDA advisory committee, citing contradictory information about the drug's safety, recommended against TPA's early approval. Since then, critics have said the FDA has needlessly withheld the drug from the market, risking lives in the process.

Although the committee acknowledged the drug's great promise, it assailed reports presented by Genentech as incomplete and confusing. Questions emerged at the meeting about the drug's safety and effectiveness, its proper dosage, its manufacturing process and the completeness of the company's test results. Genentech was told to get more data.

"We are all glad that its going to get on the market and off our backs," said one official of the FDA last night. Neither he nor any representative of the company would be quoted by name in advance of a news conference scheduled for this morning.

According to agency and industry officials, however, the drug, which will be marketed by Genentech under the brand name Activase, could be on hospital shelves as early as Dec. 1.

FDA's primary concern with the drug had been that at a high dose it appeared to cause bleeding in the brain of up to 2 percent of those who received it. The percentage is now known to be well below 1 percent.

"I don't blame the FDA at all for holding back," said Dr. Topol. "This drug could be given to a million people every year. We all have to be very sure of its safety."

Critics, however, attacked the agency for months after the decision.

The Wall Street Journal has written at least a half dozen editorials critical of the FDA's decision, and one charged the panel with having "sacrificed thousands of American lives on an altar of pedantry."

The drug will be administered in thousands of hospital emergency rooms and cardiac care units by common intravenous bags to patients as soon as heart attacks are suspected.

During clinical studies on 3,700 patients at more than 100 medical centers, TPA was shown to dissolve clots in more than 70 percent of the patients who received it.

Because it dissolves blood clots it can also cause bleeding, which for the most part can be easily controlled. In some patients however -- about 0.4 percent -- there was intracranial bleeding that cased severe brain damage.

Although doctors familiar

with the treatment consider this

to be an acceptable risk for heart

attack victims, the drug would

not be recommended for use in patients at high risk of hemorrhage or with a history of stroke, severe high blood pressure or those who recently have had major surgery.