A new drug cuts the rate of breast cancer deaths by two-thirds among women at high risk of getting new tumors, a study has found.

The drug, clodronate, is one of a class of compounds known as bisphosphonates that prevent bone loss from osteoporosis and cancer.Another bisphosphonate, pami-dronate, already is given intravenously to women whose breast cancer has spread to their bone marrow. It helps prevent the painful breakdown of bone caused by bone marrow tumors.

But the study in Wednesday's New England Journal of Medicine found that clodronate capsules - coupled with standard surgery, radiation and chemotherapy - also prevented the spread of breast cancer to bone marrow and other organs.

"This is in many ways a very exciting finding," said Dr. Harmon Eyre, chief medical officer of the American Cancer Society.

The study suggests clodronate may interrupt a vicious cycle in which a hormone produced by cancerous bone-marrow cells increases bone breakdown, and a byproduct of bone breakdown stimulates tumor growth, Eyre said. Or it may act in some other way to kill cancerous cells, he added.

"In general, these bisphosphonates are very nontoxic drugs and if they have this significant positive benefit to cancer patients and can be added to the usual chemotherapy or hormonal therapy, that could change the way we manage breast cancer," he said.

Clodronate is sold in Germany but is not currently available in the United States.

German researchers followed 284 patients at the University Hospital Heidelberg who underwent surgery for breast cancer from 1990 to 1995 and had at least one cancerous cell show up in a very sensitive bone marrow test.

The presence of tumor cells in bone marrow signals a high risk that the cancer will spread. However, the women had no detectable secondary tumors, or metastases, when they entered the study.

Half the women took 1,600 milligrams of clodronate daily for two years following breast cancer surgery, in addition to standard treatment. The other half got standard treatment only.

After an average follow-up of three years, six women or 4 percent of the women in the clodronate group had died, compared to 22 women or 15 percent in the control group.