SAN ANTONIO (AP) - A new class of estrogen-blocking drugs appears to be slightly more effective than the standard medicine for treating women with early-stage breast cancer.

The drug tamoxifen has long been used to help prevent cancer from coming back after surgery and chemotherapy. Some cancers are fueled by estrogen, the female sex hormone, and tamoxifen interferes with this.

However, a new study, presented Monday at the San Antonio Breast Cancer Symposium, suggests that the drug anastrozole is possibly even more effective.

``After 20 years, tamoxifen's established benefits in early breast cancer are being challenged,'' said the study's director, Dr. Aman Buzdar of the University of Texas M.D. Anderson Cancer Center in Houston.

The study, described by researchers as the largest ever in breast cancer treatment, was sponsored by AstraZeneca, which makes anastrozole. The drug was approved for advanced breast cancer use last year by the Food and Drug Administration and is sold under the brand name Arimidex.

Anastrozole is one of a class of drugs called aromatase inhibitors that reduces estrogen levels circulating in a woman's body. Tamoxifen works by blocking estrogen from reaching cancer cells.

The study involved 9,366 early-stage breast cancer patients in 21 countries who were randomly assigned to get tamoxifen, anastrozole or a combination of the two. After almost three years of follow-up, 10 percent of those getting anastrozole alone had suffered a return of cancer or died, compared with 12 percent getting either tamoxifen alone or the combination.

``Basically this confirms that this class of drugs is superior to tamoxifen, which has been the gold standard for the last 30 years,'' said Dr. Helen Chew of the University of California-Davis Cancer Center. ``It has looked great in advanced (cancer) cases, and it's looking just as good in early cases.''

However, the researchers cautioned that more follow-up will be needed to learn whether anastrozole actually improves breast cancer survival. Dr. Michael Baum of University College London, who is working on the study, said he does not advocate switching patients now taking tamoxifen to the new drug.

Other side effects and benefits of the drugs may also influence doctors' decisions. Women taking tamoxifen were more likely to experience vaginal bleeding, hot flashes and weight gain, but they were less likely to have broken bones, since tamoxifen slows osteoporosis.

From Healthy.net