Drugs that block estrogen may lower women's risk of breast cancer for 10 years, according to a new review of studies.
Postmenopausal women in the studies who took drugs called selective estrogen receptor modulators (SERMs), such as tamoxifen, were 38 percent less likely to develop any type of breast cancer over a 10-year period, compared with women who weren’t taking SERMs. The studies also involved the SERMs raloxifene, arzoxifene and lasofoxifene.
The review shows that "each of these drugs will prevent breast cancer in populations of women," said Dr. V. Craig Jordan, scientific director of oncology at the Lombardi Comprehensive Cancer Center at Georgetown University.
The review "places into perspective the 40-year journey" of SERM research, said Jordan, whose studies in the 1970s showed that tamoxifen prevented mammary cancer in rats. He was not involved in the new study.
Only tamoxifen and raloxifene are approved by the Food and Drug Administration for the prevention of breast cancer, and only tamoxifen is approved for use in premenopausal women who are at high risk for breast cancer. The other drugs in the studies are currently being studied as treatments for osteoporosis, but they also lower the risk of breast cancer, Jordan said.
The drugs come with side effects that have made women reluctant to take them, he said. The common side effects are similar to symptoms of menopause, including hot flashes and vaginal dryness.
"There is the belief that the gains do not exceed the discomfort," Jordan said. "I think it is now time for women to seriously consider taking advantage" of the drugs' effects. Women should talk with their doctors about their options, he noted.
In the new review, researchers looked at nine studies involving more than 83,000 women. Eight of the studies compared taking a SERM with taking a placebo, while one study compared raloxifene to tamoxifen.
Researchers found that 4.2 percent of women taking a SERM developed breast cancer, while 6.3 percent of women in the control groups developed breast cancer.
The reduction in risk was primarily seen with estrogen-receptor positive breast cancers, which are fueled by estrogen, according to the study.
Jordan said that in one "pivotal" trial included in the review, researchers found that both raloxifene and tamoxifen reduced women's risk of breast cancer by 50 percent while they were taking the drugs, but only the women who took tamoxifen continued to see a reduced risk after they stopped taking the drug.
The new study also showed that only tamoxifen reduced the risk of ductal carcinoma in situ, a precancerous condition. "The other ones don't tend to do that. One of the mysteries has been why only tamoxifen really reduces DCS. "We still don’t know why," Jordan said.
The more serious side but less common effects of SERMs include an increased of endometrial cancer, or cancer of the uterine lining. Among women in the studies, endometrial cancer was indeed more common in those taking SERMs, although a small number — 168 women — in the studies developed endometrial cancer.
The drugs also increase the risk deep-vein thrombosis.
The study did not look at whether the drugs might help women live longer. To get data on mortality rates, more time is needed, Jordan said. The drugs have not been used for long enough for an effect on mortality rates to be seen.
The study is published online Tuesday (April 30) in the journal Lancet.
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