updated 12/9/2005 3:54:49 PM ET 2005-12-09T20:54:49

Women have better odds of surviving early breast cancer if they are switched to a newer drug after two or three years of tamoxifen, doctors are reporting.

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It is the first evidence that drugs called aromatase inhibitors can save lives, not just prevent cancer from coming back.

Other new research suggests that the longer women take these drugs, the more they may benefit.

“This is a first attempt to get a grip on duration” of treatment, said the leader of one of the studies, Dr. James Ingle of the Mayo Clinic in Rochester, Minn. “Longer is better.”

The findings were reported Friday at a breast cancer conference in Texas.

Tamoxifen has been a mainstay of breast cancer treatment for decades. Taking it for five years after cancer surgery cuts the risk of recurrence in half and improves survival. The drug blunts the effects of estrogen, a hormone that fuels the growth of most tumors that occur in women after menopause.

Aromatase inhibitors keep estrogen from being made in the first place, and do not raise the risk of blood clots and endometrial cancer as tamoxifen does. Three brands are available: AstraZeneca PLC’s Arimidex, Pfizer Inc.’s Aromasin and Novartis Pharmaceuticals’ Femara.

Doctors already know that these drugs can cut the risk of recurrence, as tamoxifen does, but keeping women alive is the ultimate test any cancer treatment must pass.

Researchers led by Dr. Walter Jonat of the University of Kiel in Germany combined information from three large European studies on Arimidex and found that women who were switched to it after several years of tamoxifen were 29 percent more likely to be alive 2½ years later.

About 4.5 percent of women who continued on tamoxifen died, compared with only 3.3 percent of those who switched to Arimidex. More than 3,800 women were included in the analysis, which was paid for by AstraZeneca, as were the individual studies.

“This is the first time survival advantage has been shown,” said Jonat, who has no financial ties to the drug maker.

Women who switched drugs also had a 41 percent lower risk of having cancer again.

Other research reported by Ingle and Dr. Paul Goss of Massachusetts General Hospital involved women taking Femara after they had completed the initially recommended five years of tamoxifen.

This study, involving nearly 5,200 women in North America and Europe and led by Canadian doctors, was stopped after just 2½ years, in October 2003, after researchers saw that women taking Femara had significantly lower rates of cancer recurrence than those who received just tamoxifen.

For this reason, doctors do not know the optimal length of time Femara or any other aromatase inhibitor should be taken.

Goss tried to get at this information by continuing to study women from the original study even though this is not as scientifically rigorous once everybody knows which women got which treatment.

He found that four years after they started the study, those on Femara still had a lower risk of cancer recurrence than women who did not get the drug. For women on Femara, the risk of relapse peaked two years after starting on the drug and continued to fall after that.

As for how long to take Femara, “this remains an unanswered question,” he said.

The biggest unanswered question is whether women are better off taking an aromatase inhibitor up front, or switching to one after a couple years of tamoxifen, said Dr. Eric Winer, an expert on these drugs from the Dana-Farber Cancer Institute in Boston.

A study under way now in Europe should answer that question in the next year or two.

Tamoxifen remains the best treatment for women who get breast cancer before menopause because the newer drugs are not thought to be effective then.

© 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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