Harvard Medical School researchers are cautiously optimistic about a new drug that shrinks and blocks the growth of cancer tumors in mice.

While the promising new drug, endostatin, has yet to be tested in humans, it has successfully starved tumors into submission by preventing their access to the blood vessels in mice. The vessels carry oxygen-rich blood, essential for all body tissue to survive.

The drug is a protein naturally present in the bodies of both mice and humans, giving scientists hope that the drug may one day be used to treat cancer patients.

However, Dr. Michael S. O'Reilly, research fellow in surgery and the drug's discoverer, emphasized that the mouse tests are in their preliminary phases and that "succeeding in mice is tentative at best."

But so far, there is definite cause for encouragement. "We haven't found any [type of] cancer that doesn't respond," O'Reilly added.

The results of the mice experiments were published two weeks ago in the prestigious scientific journal Cell.

The discovery of endostatin is significant because the drug can potentially be targeted at all forms of cancer tumors.

Tumors behave differently in different parts of the body. Traditionally, many other cancer drugs have been effective in treating only particular kinds of tumors.

Since every tumor depends on a blood supply to live, they are all potentially vulnerable to drugs like endostatin.

"They all should be susceptible," O'Reilly said.

O'Reilly's studies have shown that colon, breast and prostate cancers have all been successfully treated with endostatin.

The search for the drug began in the 1970s, when Dr. Judah Folkman, now Andrus professor of pediatric surgery at the Medical School, postulated that tumors, like other body tissue, were dependent upon blood supply to grow.

At that time, his thinking was criticized quite stridently by colleagues.

But his later studies, and those of his assistants like O'Reilly, have made a mockery of the criticism, while expanding treatment possibilities for patients.

O'Reilly discovered two proteins--endostatin and a similar, previously identified protein called angiostatin--in his efforts to test Folkman's hypothesis.