The Food and Drug Administration approved a new cancer drug yesterday that has produced dramatic results in patients with a type of leukemia by targeting an abnormal molecule in the cancer cells that causes their runaway growth.

The FDA took only 2 1/2 months to review the drug, called Gleevec -- the fastest agency action on record for a cancer drug. Health and Human Services Secretary Tommy G. Thompson hailed the drug's approval, calling it the first of what medical experts hope will be a new generation of more precise, less toxic cancer therapies.

"This new drug, we believe, is the picture of the future of cancer treatment," said Richard D. Klausner, director of the National Cancer Institute.

In people with chronic myeloid leukemia (CML), Gleevec -- which can be taken orally -- has produced striking reductions in the number of cancerous white blood cells, along with relatively mild side effects compared with other treatments. Although the majority of treated patients still show some evidence of leukemia on laboratory tests, many feel well and are able to return to work.

"I'm headed for a full remission," said Suzanne Dreger, a CML patient from Falls Church who has been taking the drug for the past year.

But it is too soon to say whether CML patients' positive responses to the drug will translate into improved long-term survival, cautioned Bernard A. Schwetz, acting principal deputy commissioner of the FDA. Additional studies are underway, he said, adding, "We hope that they will confirm that Gleevec's benefits far outweigh its risks."

The FDA approved Gleevec for CML, which is diagnosed in about 4,500 Americans each year. The disease, which most often strikes people during middle age or later, often causes no symptoms in its early stages but eventually progresses to a more aggressive, fatal form of leukemia. A previously approved drug, interferon alfa, produces remissions in many patients but does not cure the disease, and it has severe side effects. Survival with current treatments averages about five years, although some CML patients are cured by bone marrow transplantation.

About 8,000 patients around the world are receiving Gleevec, previously known as STI571 or Glivec, said Daniel Vasella, president of Novartis AG, the drug's manufacturer. Treatment is likely to cost between $2,000 and $2,400 per month, about the same as interferon alfa, Vasella said. Researchers expect CML patients to have to stay on the drug indefinitely. Novartis plans to supply the drug free or at a reduced price to uninsured patients with incomes under $100,000 per year, he added.

Gleevec works in a novel way by turning off an enzyme called a tyrosine kinase that is abnormally active in CML and some other cancers. When the enzyme is turned on, it activates chemical signals that lead to uncontrolled cell division.

Besides CML, Gleevec may be effective against other cancers that develop or grow because of a related molecular abnormality, Klausner said. Results of trials in patients with gastrointestinal stromal tumors, a rare abdominal cancer, are to be announced Sunday at the annual meeting of the American Society of Clinical Oncology. Testing has begun in children and adults with gliomas, a type of brain tumor. The drug may also prove useful in sarcomas (cancers of connective tissue) and in some tumors of the lung, breast and prostate as well as other leukemias and lymphomas, Klausner said.

Gleevec's most common side effects are nausea, fluid retention, muscle cramps, diarrhea and vomiting. Some recipients have developed bleeding, rashes, headache, fatigue or elevated liver enzyme levels. In studies for leukemia, between 1 percent and 5 percent of patients stopped taking the drug because of side effects.

Although some other cancer drugs block the action of substances -- such as hormones -- that promote tumor growth, Gleevec is the first medicine that was designed specifically to target the molecule that causes a type of cancer, Klausner said. As researchers identify molecular abnormalities underlying other malignancies, he predicted, they will be able to develop additional precisely tailored treatments.

"We will be able to cure cancer, but one cancer -- one disease -- at a time," he said.

National Cancer Institute Director Richard D. Klausner, left, with Health Secretary Tommy G. Thompson, hailed Gleevec's approval. He called the drug's precise targeting of molecules the "future of cancer treatment."