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Tamoxifen is an anti-hormone drug that belongs to a class of pharmacological agents known as selective estrogen receptor modulators. Tamoxifen is used for the treatment of certain types of breast cancers that are hormone sensitive. In some women with ductal carcinoma in situ (DCIS), the cancer cells contain (express) estrogen receptors on their surface. Doctors refer to this type of DCIS as "estrogen receptor positive DCIS". This means that when these cancer cells are exposed to estrogen, a naturally occurring female sex hormone, the estrogen binds to the estrogen receptor on the surface of the cells and causes them to actively grow and divide. Tomoxifen counteracts the effects of estrogen by binding to the estrogen receptors on the surface of the estrogen receptor positive breast cancer cells and, thereby, blocking estrogen from binding to the cells and preventing them from growing and dividing.

Tamoxifen is sometimes used as adjuvant therapy (a treatment that improves the outcome of a primary therapy) in women with DCIS who have undergone breast-conserving surgery and radiation therapy. The results of the National Surgical Adjuvant Breast and Bowel Project (NSABP) demonstrated that tomoxifen reduced the likelihood of recurrence of DCIS in the same breast as well the risk of a tumor developing in the contralateral (opposite) breast.

Since its initial approval by the U.S. Food and Drug Administration in 1977, tamoxifen has been found to:

• Lower the risk of recurrence and death in women with early-stage, hormone receptor positive breast cancer

• Reduce the risk of invasive breast cancer following breast-conserving surgery in women with DCIS

• Reduce the risk of breast cancer in high-risk women

In June, 2002 the U.S. Food and Drug Administration and Astra Zeneca added a boxed warning and strengthened the "WARNINGS" section of the label for Nolvadex (tomoxifen citrate) to inform healthcare professionals about new risk information of particular relevance to women with DCIS and women at high risk for developing breast cancer and are receiving or considering Nolvadex therapy to reduce their risk of developing invasive breast cancer. Serious, life-threatening or fatal events associated with Nolvadex in the risk reduction setting (women at high risk for cancer and women with DCIS) include endometrial cancer, uterine sarcoma, stroke, and pulmonary embolism. Healthcare providers should discuss the potential benefits versus the potential risks of these serious events with women considering Nolvadex to reduce their risk of developing breast cancer.