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According to results recently presented at the 2006 annual San Antonio Breast Cancer Symposium, tamoxifen (Nolvadex®) appears to reduce the risk of developing cancer years following completion of preventive therapy among women who are at a high risk of developing breast cancer.

Breast cancer causes roughly 40,000 deaths annually in the U.S. alone. The majority of women with breast cancer have cancer that is estrogen receptor-positive (ER-positive). This type of breast cancer is stimulated to grow by the female hormones estrogen and/or progesterone.

Tamoxifen, an antiestrogen agent, reduces exposure of breast cells to estrogen, therefore reducing estrogen’s ability to stimulate the development of breast cancer. Some postmenopausal women at a high risk of developing breast cancer can be treated with tamoxifen to significantly reduce their risk of developing breast cancer.

However, serious side effects associated with tamoxifen may prevent physicians from recommending its use or may stop patients from taking the agent for breast cancer prevention. These side effects include an increased risk of endometrial (uterine) cancer and the development of blood clots.

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Researchers recently conducted long-term analysis from a trial referred to as the IBIS-1 trial. This trial included 7,145 women who were at a high-risk for developing breast cancer. Women were treated with either tamoxifen or placebo (inactive substitute) for five years. Women treated with tamoxifen had a significantly reduced risk of developing breast cancer after five years of treatment.

Data from this trial were again evaluated at 10 years’ follow-up. Results are as follows:

  • Breast cancer was reduced by 29% among women treated with tamoxifen compared to those taking placebo.
  • Hormone-positive breast cancer was reduced by 34% among women treated with tamoxifen compared to those taking placebo.
  • The preventive effect on breast cancer, specifically hormone-positive breast caner, was actually improved a 10 years compared to the five-year follow-up.
  • Among women who had never taken hormone-replacement therapy (HRT) or had stopped before beginning the trial, those treated with tamoxifen had over a 50% reduction in the rate of hormone-positive breast cancer compared to those taking placebo. However, no significant differences in breast cancer rates among women taking tamoxifen or placebo were seen among women taking HRT.
  • Side effects from tamoxifen were minor following the completion of five years of therapy.

The researchers concluded that women who are at a high risk of developing breast cancer continue to benefit from tamoxifen, even five years following completion of therapy. However, it is important for every patient to speak with her physician regarding individual risks and benefits of using tamoxifen as preventive therapy for breast cancer.

Reference: Cuzick, J, et al. Long-term efficacy of tamoxifen for chemoprevention-results of the IBIS-I study. Proceedings from the 2006 San Antonio Breast Cancer Symposium. December 2006. Abstract #34.