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For Women With a High Risk of Breast Cancer, Tamoxifen Can Provide Long-Lasting Protection

Long-term follow up results confirm the drug can reduce incidence of breast cancer in high-risk women.

“Breast cancer is the epidemic of our century.”

So began Dr. Jack Cuzick’s session at this year’s San Antonio Breast Cancer Symposium, where he presented long-term follow up results from the International Breast Cancer Intervention Study-I (IBIS-I). The trial, supported in part by BCRF, began in 1992 and involved more than 7,000 women between ages 35-70 who had a high risk of breast cancer, mainly because of their family history. The goal was to see if Tamoxifen, a drug used to treat breast cancer could also prevent the disease in high risk women. The FDA approved tamoxifen for prevention of breast cancer in 1999, but this is the first prevention study to date with 20 years of follow up.

The outcome? Women with a high risk of breast cancer who take Tamoxifen, the anti-estrogen drug long used to treat early and advanced stages of the disease, can reduce their incidence of breast cancer by about 30 percent. What’s more, the drug continues providing protection for up to 20 years after a woman stops taking it.

After two decades of follow-up, there were 251 breast cancers among the women who took Tamoxifen for five years, compared to breast cancers in 350 of the women in the placebo group, a reduction of 29 percent. ER+ breast cancer, which accounts for two-thirds of all incidences, was reduced by 35 percent.

“Tamoxifen is a well-established and effective treatment for certain breast cancers, but we now have evidence of its very long-term preventive benefits,” said Dr. Cuzick. “The preventive effect of Tamoxifen is highly significant with a reduction in breast cancer rates of around a third, and this impact has remained strong and unabated for 20 years.”

There was no difference between breast cancer deaths between the Tamoxifen and placebo groups, and Dr. Cuzick advised that further studies would be needed to uncover whether Tamoxifen will help reduce deaths.

Additionally, among women who took hormone replacement therapy while undergoing Tamoxifen treatment, the benefits were significantly lower when compared to those who did not, making it more suitable for pre-menopausal women. The side effects of Tamoxifen, particularly the increased risk of endometrial cancer, create some concern. Endometrial cancer was 3.8 times more common in the women who took Tamoxifen during their 5-year treatment period than in those in the control group, though there was no increased risk in the years following.

Dr. Cuzick hopes that these findings will provide women and their doctors with another option to tackle breast cancer prevention among high-risk, pre-menopausal women.

"Breast cancer prevention has come a long way, with many more options now available to women at high risk of the disease,” Dr. Cuzick said. “For most post-menopausal women, we believe aromatase inhibitors, such as Arimidex (anastrozole) should be the drug of choice, as they are more effective than Tamoxifen and have fewer side effects.”

“However, for most high-risk premenopausal women, Tamoxifen remains the only drug choice for breast cancer prevention and it is a good one, as shown by this new evidence. We hope these results will stimulate more women, particularly younger women, to consider treatment options for breast cancer prevention if they have a family history of the disease or other major risk factors."

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