The most commonly used breast cancer drug may cause tumors to spread in a small number of women with low levels of a protein which makes cells stick together, British researchers said on Thursday.

The findings could lead to new tests to determine which women are not likely to benefit from tamoxifen and who should be given other drugs, said Stephen Hiscox, a cancer researcher at Cardiff University, who led the study.

"We found something that could be potentially used as a biomarker to determine which kind of drugs they should get because they have low levels of this protein," he said in a telephone interview.

Breast cancer is the leading cause of cancer deaths among women worldwide, according to the American Cancer Society. The group estimates about 465,000 women died of breast cancer globally in 2007 and 1.3 million new cases were diagnosed.

Declining death rates from breast cancer in developed countries have been attributed to early detection through mammography screening and to improved treatment.

Tamoxifen is given to most women for five years after they are diagnosed with breast cancer to prevent the disease from returning, but some develop resistance, which means their tumor is more likely to recur.

Last month, a British team identified another protein — called Pax2 — which helped explain why some women developed resistance to tamoxifen. The generic drug works by blocking oestrogen from causing wild cell growth in breast cancer.

In their study published in the BioMed Central journal Breast Cancer Research, Hiscox and colleagues analyzed breast cancer cells in a lab and a protein which makes cells stick together called E-cadherin.

They found when the protein was at reduced levels tamoxifen caused cancer cells to move around much more aggressively, an initial step in the spread of a tumor, he said.

The researchers did not determine an actual level of the protein that would cause tamoxifen to fail but Hiscox said this would be the next step in their work.

The lab results could also lead to a clinical test to pinpoint women who should be given so-called aromatase inhibitors such as AstraZeneca Plc's Arimidex which deplete oestrogen in a different way, he said.

"People with low levels of E-cadherin may be more suited to these other drugs," Hiscox said.