The government announced Monday the approval of a new drug to treat the most common form of adult leukemia in the United States.

The Food and Drug Administration said the drug, fludarabine, will be labeled for use in patients with chronic lymphocytic leukemia who are not responding to other therapies.The disease is characterized by a buildup of abnormal white blood cells in the bone marrow, blood, liver, spleen, lymph nodes and other organs. The abnormal cells slowly crowd out normal ones.

The leukemia, most common in people who are over age 50, progresses slowly but is usually fatal. It affects about 8,000 Americans.

The current treatment regimen does not cure the disease and may result in significant side effects, the FDA said. In addition, patients may stop responding to the treatment.

In trials of fludarabine among patients who were not responding to standard therapy, 32 percent to 48 percent showed a positive response, and 13 percent showed complete remission of the leukemia. The trials were sponsored by the National Cancer Institute.

The drug is thought to work by inhibiting reproduction of abnormal cells, and may give patients a better quality of life and allow them to survive longer, the FDA said.

Fludarabine phosphate is administered intravenously. The most common side effects are bone marrow suppression, fever, chills, infections, nausea and fatigue.

The new drug has been available to many patients for a year under government programs that permit early use of potentially effective drugs for people with life-threatening or serious diseases.

The drug is marketed by Berlex Laboratories, of Alameda, Calif., and will be sold under the trade name Fludara.