A new drug that helped more than 90 percent of patients with a rare form of leukemia won government approval Thursday. Gleevec works by blocking chemical signals sent by cancerous cells and researchers hope it will also prove useful against a form of stomach tumor and perhaps other types of cancer as well. 

Health and Human Services Secretary Tommy Thompson said the drug is based on the principle of molecular targeting, killing leukemia cells while leaving normal white cells alone. 

"We believe such targeting is the wave of the future," he said. 

The results of clinical trials of the drug, formerly known as STI-571, generated excitement among cancer researchers. 

Chronic myeloid leukemia, a relatively rare disease, is caused by a protein produced an abnormal chromosome. It leads to a huge increase in the number of white blood cells in the body, which can interfere with the functioning of other organs. The disease kills about 2,300 Americans annually. 

In clinical trials financed by the manufacturer, Novartis Pharmaceuticals, more than 90 percent of patients in the first phase of CML saw their cancer go into remission within the first six months of taking the pill. The findings were presented in December at a meeting of the American Society of Hematology. 

A study of patients in the second phase of the disease showed more than 90 percent responded positively to the treatment. In 63 percent, the cancer went into remission. The trials involved 530 first-phase and 230 second-phase patients. 

The early success has propelled researchers to test the drug on almost 3,000 patients around the world. 

"This is not a miracle drug," but it is a model for future cancer study because it targets the cause of the disease without damaging other cells, Edward Benz, president of the Dana-Farber Cancer Institute at Harvard Medical School, said at the hematology meeting. 

Gleevec blocks a signal that the abnormal protein sends out, preventing the abnormal growth and production of other cancerous cells. 

Currently the only treatments for CML are bone marrow transplants, which can be dangerous, or interferon, which can extend a leukemia patient's life by up to two years but can have side effects that cause about 20 percent of patients to stop using it. 

Gleevec has been studied on humans for only about two years, so how long it will prolong a patient's life is not yet known. But it has had few side effects.