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The drug tamoxifen is used to help treat early and late stage breast cancer and to help prevent the disease in women at high risk. The benefits of tamoxifen outweigh the risks for most breast cancer patients. However, some researchers have found that tamoxifen can increase the risk of abnormal uterine bleeding, which can indicate uterine abnormalities or even endometrial cancer, cancer of the lining of the uterus. Now British researchers are investigating whether an intrauterine device can help reduce the chances of uterine abnormalities in women who take tamoxifen. Tamoxifen helps fight breast cancer by blocking estrogen from estrogen-receptors in breast cancer cells. Because many breast cancers depend on estrogen for survival, a lack of estrogen "starves" these cancer cells. While tamoxifen fights estrogen in breast cancer cells, it acts as a weak estrogen in the uterus. Tamoxifen can stimulate the growth of endometrial cells and increase the risk of endometrial cancer in some women. Progesterone is a hormone that can counteract the effects of estrogen on the uterus. Because the intrauterine device, called the levonorgestrel-releasing intrauterine system, releases progestin (a synthetic form of progesterone), it may help decrease uterine abnormalities. The T-shaped intrauterine device is currently used as a form of birth control in Asia and Europe. Mirena, the pharmaceutical company that makes the intrauterine device, is applying for FDA approval of the device in the United States for use as contraception. To conduct their study, the researchers studied 122 post-menopausal women who had taken tamoxifen for at least one year in addition to other breast cancer treatments such as surgery. Half of the women on tamoxifen were fitted with the intrauterine device while the other half of the women continued taking tamoxifen without the device. After examining endometrial samples from both groups one year later, researchers found fewer uterine abnormalities (such as polyps or fibroids) among the women who wore the intrauterine device than among the women that did not use the device. "The levonrgestrel-releasing intrauterine system had a protective action against the uterine effects of tamoxifen," wrote Francis Gardner, MD and his colleagues from the University of Leicester and the Leicester Royal Infirmary in Leicester, England. The study appears in the November 18, 2000 issue of The Lancet. However, the researchers warn that their research is preliminary and needs to be confirmed in a large clinical trial where the effectiveness of the intrauterine device can be evaluated for the duration of tamoxifen treatment. Dr. Gardner said that injections of progestin may have a similar effect as the intrauterine device, although the injections would have to contain relatively high doses of progestin. Also, because the intrauterine device only releases progestin in the uterus, there are few side effects. However, the intrauterine device can cause temporary vaginal bleeding among women who use it. Researchers have suggested that tamoxifen can increase a woman’s risk of developing uterine tumors and other uterine abnormalities. Tamoxifen, which can stimulate thickening and growth of the endometrial (uterine) lining, can also stimulate this tissue when it is found outside the uterus, and may increase symptoms associated with endometriosis (such as pain before and during menstruation, pain during or after sexual intercourse, and heavy or irregular bleeding). Endometriosis is a non-cancerous condition in which endometrial tissue is found outside the uterus in other areas of the body, such as ovaries or the fallopian tubes. Post-menopausal women who take tamoxifen and who have not had a hysterectomy (uterus removal) are also at a slightly higher risk of developing endometrial cancer than women who are not on tamoxifen. In the National Adjuvant Breast and Bowel’s Project’s first Breast Cancer Prevention Trial, endometrial cancer occurred 2.5 times more frequently in women who took tamoxifen compared to women who took a placebo (inactive pill). However, the risk of endometrial cancer is still low among women on tamoxifen: only approximately 2 out of 1000 women on tamoxifen will develop endometrial cancer. Tamoxifen is well-tolerated in most women and is currently the most common drug used to help treat breast cancer. Tamoxifen is approved by the U.S. Food and Drug Administration (FDA) to help treat both early and advanced stages of breast cancer. In late 1998, tamoxifen became the first drug to be approved by the FDA to prevent breast cancer after research showed it reduced the chances of developing breast cancer by nearly 50% in women at high risk of the disease.
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