An experimental drug that quickly dissolves blood clots may become the most promising treatment yet for prolonging the lives of heart attack victims, a government-sponsored study suggests.

The chemical, found naturally in the body in small amounts, can be produced in quantity by genetic engineering.

Intravenous administration quickly dissolved blood clots blocking a coronary artery in about two-thirds of patients who received it within hours of their heart attacks, the preliminary research project found. This was about twice as effective as emergency use of another drug now being used experimentally in many hospitals.

"This offers the potential of being the first practical and direct means of limiting heart muscle damage in heart attack victims," said Dr. Eugene Passamani, an associate director of the National Heart, Lung and Blood Institute who supervised the study conducted at 13 medical centers around the country, including George Washington University.

Passamani said a larger study will try to determine how effective the chemical, a peptide known as tissue-type plasminogen activator, is in reducing illness and preventing death following a heart attack.

Passamani estimated it would be a year or two before the drug could be approved for more general use. But initial results were so striking that the study was stopped earlier than planned and the findings rushed into print in this week's New England Journal of Medicine.

Journal editor Dr. Arnold Relman said in an editorial that he made an exception to normal policy by publishing the preliminary report because the new drug might prove to be of "immense clinical value."

Heart disease, the nation's No. 1 killer, results in more than 1 million heart attacks a year in the United States. Passamani said that about 300,000 people die immediately after heart attacks, but that about 680,000 heart attack victims are hospitalized and remain at great risk of heart damage and death.

Time is of the essence, he said, because death of heart muscle tissue and increased likelihood of heart failure seem greatest within the first six hours after the attack begins. Seven to 15 percent of hospitalized heart attack patients die in the hospital, and 20 percent die within six months of the attack, Passamani said.

In the past, the major treatment for heart attacks was bed rest. In the late 1970s, doctors began to investigate more aggressive approaches, particularly the use of a drug, streptokinase, that could be injected directly into the blocked heart artery through a catheter.

Although this proved 60 to 80 percent effective in opening the arteries, Passamani said, it required complicated equipment and experienced personnel and took time. So researchers began looking at the easier and quicker method of injecting drugs intravenously.

Although use of streptokinase in this way met with "mixed results," he said, scientists found a "new generation of clot-dissolving drugs" chemically attracted to the clot.

The study reported yesterday compared streptokinase and the plasminogen activator in a group of 214 heart attack patients who entered the experiment within seven hours after the onset of chest pain.

"The surprising thing was that the new drug was twice as effective in opening the artery" within 90 minutes of injection into the bloodstream, dissolving the clots blocking coronary arteries in 66 percent of the patients, Passamani said.