Is More of Breast Cancer Drug Better?

Dec. 17, 2007 (San Antonio) -- Women with breast cancer may fare better if they take tamoxifen for longer than the five years for which it is typically prescribed, researchers report.

Early results of a large international study show that women with breast tumors are less likely to get new breast cancers if they take tamoxifen for 10 years than if they take it for five years.

But they were also more likely to get endometrial cancer if they took the drug for the longer period of time, and doctors aren't ready to conclude that 10 years are better.

Doctors already know that women with estrogen-fueled breast tumors are about 50% less likely to get new breast cancers if they take the hormone drug tamoxifen than if they take a placebo.

In an attempt to find out if taking it longer is even better, researchers are studying 11,500 women with early-stage breast cancer who took a tamoxifen pill for five years. Half have been randomly assigned to take tamoxifen for another five years and half are taking a placebo. It's the largest trial ever of a breast cancer treatment.

The preliminary findings of the Adjuvant Tamoxifen -- Longer Against Shorter (ATLAS) trial were presented at the San Antonio Breast Cancer Symposium.

Fewer Recurrences Among Women Who Take Tamoxifen Longer

Five years into the study, cancer has come back in 825 women taking placebo, compared with 739 who are continuing to take tamoxifen.

This translates to a significant 15% further reduction in their risk of recurrence, says Richard Peto, PhD, of the University of Oxford.

But women on tamoxifen were about twice as likely to develop endometrial (uterine) cancer, a known risk of the drug, he says.

"There was an excess of non-fatal endometrial cancers in the tamoxifen group, on the order of 40 vs. 20 in the placebo group," Peto says.

The early results suggest that 10 years of therapy may be doing more good than harm, he tells WebMD.

"But we can't give a woman advice based on that. The key question now is what a total of 10 years of tamoxifen therapy will do to 20-year outcomes," he says.

More Than 5 Years of Tamoxifen

The National Cancer Institute's (NCI) current fact sheet on tamoxifen states that more than five years of therapy is of no value. Peto says the new evidence shows this is now wrong.

"We have to get rid of the wrong idea that going to more than five years will increase the recurrence rate. It lowers the rate, but whether to a degree worth doing is what is still uncertain," Peto says.

Jo Anne Zujewski, MD, investigator in the Division of Cancer Treatment and Diagnosis at the NCI, says NCI fact sheets are periodically updated as new data become available.

"We do anticipate future revisions of NCI fact sheets once additional peer-reviewed data become available from the ATLAS trial as well as ongoing trials of hormonal therapy duration in postmenopausal women," she tells WebMD.

Zujewski notes that in the U.S., aromatase inhibitors (such as Femara, Arimidex, or Aromasin) -- not tamoxifen -- are now a mainstay of hormonal treatment for postmenopausal women with breast cancer.

But the symposium's co-director, C. Kent Osborne, a breast cancer specialist at Baylor University in Houston, tells WebMD that tamoxifen still plays a major role in treating premenopausal women as well as those in developing countries where the aromatase inhibitors are prohibitively expensive.