- Since the mid-1970s, tamoxifen has been an important weapon in the battle against breast cancer. Tamoxifen is taken as a pill and works by interfering with the activity of the female hormone estrogen. Estrogen is important to the body because of its role in preparing the breast and uterus for reproduction, controlling the production of cholesterol and preserving bone strength. To prepare for reproduction, estrogen causes cells in the breast to grow and divide. This cell proliferation is needed by a woman if she becomes pregnant in order to produce milk.
- Cancer is caused by mutations in a cell's DNA. Although estrogen does not cause mutations to the DNA, its ability to cause cell growth and division means that many more copies of mutated cell are created. The larger number of mutated cells increases the risk that a person will develop cancer. In addition, each time that a cell divides, there is a risk that a mutation will occur.
- Breast cells have receptors where estrogen binds. This forms a new shape that can then bind with coactivators, supporting cell proliferation. Tamoxifen works by binding with the breast cell receptors which prevents the estrogen from binding and blocks cell proliferation. Approximately 75 percent of breast cancers have estrogen receptors (ER-positive). Tamoxifen is only effective against ER-positive breast cancers. Patients who have been treated for early stage breast cancer by surgery or radiation and whose tumors are found to be ER-positive can be put on tamoxifen for five years. According to the National Cancer Institute, the National Surgical Adjuvant Breast and Bowel Project (NSABP) study from 1981 to 1988 showed that 68 percent of the patients taking tamoxifen for five years remained cancer free for over ten years in comparison to 57 percent for those taking a placebo. A continuation of the study from 1987 to 1993 showed that increasing the time taking tamoxifen to ten years did not increase survival rates.
- NSABP ran another study, the Study of Tamoxifen and Raloxifene (STAR), which looked at the benefit of tamoxifen in preventing cancer in high risk patients. High risk factors include a family history of breast cancer, not having children, higher age at first delivery of a child, beginning menstruation early and menopause occurring late in life. The results of this study showed that both tamoxifen and raloxifene reduced the risk of breast cancer by half.
- Tamoxifen has some serious side effects including increased risk of uterine cancer, strokes, blood clots and cataracts. In general, the benefits of taking tamoxifen greatly outweigh the risks. However, the creation of new treatment options such as aromatase inhibitors means that patients and doctors have to consider carefully what is best for each individual.
