Updated results from this landmark trial also show that the increased risk of fractures associated with Arimidex therapy disappears after treatment stops.
In the study, more than 5,000 women with hormone-receptor-positive tumors were followed for more than three years after treatment was stopped. The researchers show that an additional 25% of recurrences were prevented by Arimidex, compared with tamoxifen, says John F. Forbes, MD, professor of surgery at the University of Newcastle in Australia.
During treatment, nearly 3% of women taking Arimidex had bone fractures vs. only 2% on tamoxifen. More than three years after treatment ended, the percentage was about 1.5% in both groups.
Forbes reported the findings here at the annual San Antonio Breast Cancer Symposium. Results of the study, which was funded by AstraZeneca, maker of Arimidex, were simultaneously published online in the journal Lancet Oncology.
Studies have shown that Arimidex is better at preventing relapses than tamoxifen during the five years that women are being treated with these drugs.
What we didn't know, Forbes says, is what would happen after women stop taking them.
"The news here is that the hoped-for carryover effect seems to be true," William Gradishar, MD, a breast cancer specialist at Northwestern University in Chicago, tells WebMD.
"The safety issue, particularly with regard to bone disease, is reassuring as well," says Gradishar, who was not involved with the work.
Arimidex and other aromatase inhibitors shut down the body's ability to make estrogen.
Arimidex Cuts Breast Cancer Relapses
The landmark study involved postmenopausal women with early-stage breast cancer. They were given either tamoxifen or Arimidex for five years, following breast cancer surgery.
The new analysis followed only about 5,000 of these women who had hormone-receptor-positive tumors. These are the tumors that are targeted by the aromatase inhibitors such as Arimidex.
More than eight years after treatment started -- and more than three years after it stopped -- Arimidex scored better than tamoxifen on almost every measure:
- It lowered the risk of breast cancer relapse by 15% compared to tamoxifen
- It reduced the spread of cancer to other parts of the body, such as the lungs or liver by 16% compared to tamoxifen
- It slashed the chances of a tumor in the other breast by 40% compared to tamoxifen.
At five years, 2.8% fewer women on Arimidex had their cancer recur compared with those on tamoxifen; at nine years, the figure was 4.8%.
But doctors still can't say that Arimidex saves lives. Similar numbers of women in both groups died.
Forbes tells WebMD that's probably because the women are aging -- their average age is now 72 -- so many of them are dying of causes other than breast cancer.
"We should be focused on preventing recurrences. If a woman doesn't have breast cancer, she won't die from it," he says.
In developing countries, about 75% of all breast cancers occur in postmenopausal women; of those, about 80% are fueled by hormones.