Doctors typically prescribe tamoxifen to post-menopausal women who have suffered from early-stage breast cancer to protect them against the spread or recurrence of the disease.

But the benefits of the drug appear to last only five years, because tumors become resistant to it, according to the latest scientific thinking.

In a just-halted clinical trial, women who had exhausted the benefits of tamoxifen and who were prescribed a second drug, letrozole, saw their risk of remaining cancer-free boosted by almost half compared to women given a dummy or placebo.

Specifically, the women on letrozole cut their risk of a recurrence of the cancer by 43 percent after just two-and-a-half years, compared to the placebo group.

In addition, fewer deaths from breast cancer were seen in the letrozole group of women than in the placebo group: nine versus 17, according to the study published on the New England Journal of Medicine's website.

"This very important advance in breast cancer treatment will improve the outlook for many thousands of women," said Andrew von Eschenbach, director of the National Cancer Institute which led the study in the United States.

"This is one more example of the ability to interrupt the progression of a cancer using a drug that blocks a crucial metabolic pathway in the tumor cell."

The findings from the trial were so compelling that the Canadian researchers overseeing it halted it prematurely and offered the drug to the women who were taking the placebo.

Previous studies have shown that half of recurring breast cancer cases occur five years after a woman is first diagnosed with the disease.

"For years, we have thought that we had reached the limit of what we could do to reduce the risk of recurrence with five years of tamoxifen," said Paul Goss, a cancer specialist with Princess Margaret Hospital in Toronto, Canada.

"Our study ushers in a new era of hope."

The drug, trade name Femara, suppresses the production of estrogen, a major growth stimulant in many breast cancers.

More than 5000 women from Europe and North America were enrolled in the trial, which was co-coordinated by the National Cancer Institute of Canada, with agencies in the United States and Europe.

The Swiss pharmaceutical giant, Novartis AG, supplied the drug.

AFP