An international research team says it has conclusive proof Researchers Find "Conclusive Proof" That Tamoxifen Prevents Breast Cancer (dateline June 4, 2003) | Breast Health News | Imaginis - The Women's Health & Wellness Resource Network

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Researchers Find "Conclusive Proof" That Tamoxifen Prevents Breast Cancer (dateline June 4, 2003)

An international research team says it has conclusive proof that the drug tamoxifen can reduce the likelihood that women at high risk of breast cancer will develop the disease. The researchers analyzed results of several tamoxifen clinical trials to reach their conclusion. Though the results are positive and tamoxifen is already approved by the Food and Drug Administration (FDA) to prevent breast cancer in high-risk women in the United States, the researchers say the next step is to reduce the potentially harmful side effects of tamoxifen before it should become widely used to prevent breast cancer.

Tamoxifen (brand name, Nolvadex) is an "anti-estrogen" drug that has been used for over 20 years to treat breast cancer. Many breast cancer cells depend on the hormone estrogen to grow and survive. Tamoxifen works by competing with estrogen to bind to estrogen receptors on breast cancer cells. By blocking estrogen from these cells, the cancer cells are essentially "starved." Researchers are also investigating whether tamoxifen can be effective in women whose cancers do not have estrogen-receptors (so-called estrogen receptor-negative, or ER-negative, cancers).

In 1998, the U.S. FDA approved tamoxifen to help prevent breast cancer in women at high-risk for the disease (based on family history, etc.). This decision came after a large clinical trial (the National Adjuvant Breast and Bowel Project’s Breast Cancer Prevention Trial) found that tamoxifen could reduce the risk of breast cancer by approximately 50% compared to an inactive pill (placebo). In fact, the trial was stopped early so women who were being given the placebo could also receive the benefits of tamoxifen.

In the latest research on tamoxifen, Professor Jack Cuzick, PhD, Senior Researcher at Cancer Research UK, a British charity, and colleagues analyzed the results of five breast cancer prevention trials in which tamoxifen and a similar drug, raloxifene (brand name, Evista) were given.

"In our analysis we combined all the available evidence from studies using tamoxifen for breast cancer prevention – collectively involving over 40,000 women – and it is clear to us now that the drug can reduce the chance of high-risk women developing the disease," said Professor Jack Cuzick, in a Cancer Research UK statement. Specifically, Professor Cuzick and colleagues found a 38% reduction in the likelihood of developing breast cancer among women who had taken tamoxifen.

However, the researchers noted that the side effects of tamoxifen limit its accessibility. "The evidence to date clearly shows that tamoxifen can reduce the risk of breast cancers stimulated by the hormone estrogen," said Professor Cuzick. "However, it is crucial that we follow all the trials to their conclusions and find ways to reduce the side-effects of tamoxifen before we can recommend that high-risk women take the drug to prevent breast cancer."

The most common side effect of tamoxifen is a higher occurrence of hot flashes. Other side effects include irregular menstrual cycles, unusual vaginal discharge or bleeding, and irritation of skin around the vagina. Tamoxifen also increases a woman's chances of developing serious health problems including:

  • endometrial cancer (cancer of the lining of the uterus)
  • deep vein thrombosis (blood clots in large veins, particularly in the legs)
  • pulmonary embolism (blood clot in the lung)
  • possibly stroke

The risks of these complications are generally small and are discussed in detail at

Professor Cuzick suggested that possible ways to reduce these side effects include lowering the dose of tamoxifen or adding a small dose of aspirin.

The team’s research also showed that the drug raloxifene appears promising in preventing breast cancer, but the final results of a large clinical trial called STAR, the Study of Tamoxifen and Raloxifene, are needed. STAR involves 22,000 American women at high risk of developing breast cancer. Other drugs, such as anastrozole (brand name, Armidex) may also be effective at preventing breast cancer.

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