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A drug prescribed to prevent fractures in breast cancer patients whose tumors have spread may actually help slow the cancer itself, US researchers reported.

A drug prescribed to prevent fractures in breast cancer patients whose tumors have spread may actually help slow the cancer itself, US researchers reported on Thursday.

They said Zometa, sold by Swiss drug giant Novartis, appears to have prevented the spread of tumors to the bone.

The study, released in preliminary form ahead of a meeting of the American Society of Clinical Oncology, suggests the drug may be useful for more breast cancer patients.

"Tumor cells are continually being released from the primary tumor," Dr. Rebecca Aft, of Barnes Jewish Hospital and Washington University in St. Louis, said in a statement.

"It is thought that the bone marrow harbors these cells and that these cells are likely to evolve into metastatic disease. We think that zoledronic acid changes the bone marrow so that cancer cells are unable to lodge there."

Full details of the study will be released June 3 at the ASCO meeting in Chicago.

Zometa, known generically as zoledronic acid, is in a class of drugs known as bisphosphonates. They are usually prescribed to treat or prevent osteoporosis, but Zometa is mainly used to strengthen the bones of cancer patients whose tumors have spread to the bone.

Breast and other cancers commonly spread to the bone and patients can be crippled by the pain and fractures that result.

Aft's team studied 120 women with stage 2 or stage 3 breast cancer, which has spread into lymph nodes or other areas near the breast. Some got intravenous infusions of Zometa every three weeks for a year, while others did not.

At the time of diagnosis, none of the patients had evidence of any spread when checked using computed tomography (CT) or positron emission tomography (PET) scans. But bone marrow samples showed about 40 per cent of the patients had detectable breast tumor cells in their bone marrow.

After treatment, the researchers took more bone marrow samples.

They said 23 per cent of women who got Zometa had tumor cells in the bone marrow after three months, compared to 36 per cent of those who did not get the drug.

"If longer follow-up shows that women without tumor cells in their bones do not go on to develop metastatic disease, then it would be reasonable to say that bisphosphonates will likely benefit women with locally advanced breast cancer," Aft said.