Discovered in the Children’s Hospital Boston laboratory of Judah Folkman, Endostatin is a natural substance that blocks the formation of new blood vessels around and in tumors, thereby disrupting their ability to survive and grow. The formation of new blood vessels, a process called angiogenesis, can be helpful — in wound healing, for example — or harmful: it is involved not only in cancer but in diseases such as macular degeneration of the eye, arthritis, and heart disease. One attraction of anti-angiogenic drugs is that they are gentler than conventional chemotherapy drugs, which kill cancer cells but also damage normal cells, often resulting in punishing side effects such as hair loss, nausea and blood cell abnormalities. EntreMed, Inc., of Rockville, Md., is supporting clinical trials of Endostatin. It is among some 80 anti-angiogenic drugs that are being tested in humans as potential treatments to fight or control cancer. All are in early stages of clinical trials and none has moved close to approval for general use. An updated report on Phase 1 trials of Endostatin says it exhibits virtually no toxicity even at high doses, while shrinking tumors in two of 28 advanced cancer patients and slowing disease progression in four others.