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SABCS 2014: IBIS-I Trial Finds Tamoxifen Lowered Breast Cancer Rates Among High-Risk Women

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Key Points

  • Tamoxifen decreased the incidence of breast cancer by 29%.
  • The rates of estrogen receptor–positive breast cancer were reduced by 35%, but no effect was seen for estrogen receptor–negative breast cancers.
  • Larger benefits were seen for women who did not take hormone replacement therapy during the trial.

After a median of 16 years of following women at high risk for breast cancer, the International Breast Cancer Intervention Study-I (IBIS-I) trial found that tamoxifen significantly decreased the incidence of all breast cancers, according to data presented at the 2014 San Antonio Breast Cancer Symposium (Abstract S3-07).

“The International Breast Cancer Intervention Study-I was designed to investigate the long-term risks and benefits of taking tamoxifen to prevent breast cancer in women at high risk for developing the disease,” said Jack Cuzick, PhD, John Snow Professor of Epidemiology at Wolfson Institute of Preventive Medicine at Queen Mary University of London.

“We found that the reduction in breast cancer incidence remains strong and unabated for 20 years,” added Dr. Cuzick. “There are concerns related to endometrial cancer mortality and development of estrogen receptor–negative cancers, but they are statistically nonsignificant and could be chance observations.

“For postmenopausal women, the aromatase inhibitors anastrozole or exemestane are probably better alternatives, both in terms of greater effectiveness and better side-effect profiles, but for premenopausal women, tamoxifen remains the only choice and it is a good one,” said Dr. Cuzick.

IBIS-I Trial

Of the 7,154 pre- and postmenopausal women recruited to the IBIS-I trial, 3,579 were randomly assigned to receive daily 20-mg tamoxifen for 5 years, and 3,575 were randomly assigned to receive a matching placebo for 5 years. Women were aged 35 to 70 years with an increased risk for breast cancer, primarily due to a family history of the disease.

The primary endpoint of this study was the occurrence of noninvasive or invasive breast cancer, and the secondary endpoints included overall mortality, other cancers, and breast cancer–specific mortality.

Study Findings

During a median of 16 years of follow-up, 246 women from the tamoxifen group and 343 from the placebo group developed breast cancers. There was a 29% reduction in breast cancer rates among women from the tamoxifen group. The rates of estrogen receptor–positive breast cancer were reduced by 35%, but no effect was seen for estrogen receptor–negative breast cancers.

Larger benefits were seen for women who did not take hormone replacement therapy during the trial: 45% reduction for estrogen receptor–positive breast cancer and 38% reduction for all breast cancers in noncurrent hormone replacement therapy users.

No differences were seen in breast cancer–related mortality rates between the two groups. However, among women who took tamoxifen, there was a nonsignificant increase in all-cause mortality, but this was smaller than seen in the 96-month follow-up report, according to Dr. Cuzick. There was a nonsignificant increase in other cancers overall, with increases in endometrial cancer and nonmelanoma skin cancer, and decreases in colorectal cancer.

“While the durability of the breast cancer incidence reduction is an important finding, some uncertainties still remain about mortality effects,” said Dr. Cuzick.

This study was funded by Cancer Research UK and AstraZeneca. Dr. Cuzick is an ad hoc speaker for AstraZeneca.

The content in this post has not been reviewed by the American Society of Clinical Oncology, Inc. (ASCO®) and does not necessarily reflect the ideas and opinions of ASCO®.


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