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Tamoxifen Benefits Hold Up Over Time: Presented at SABCS

By Ed Susman

SAN ANTONIO, TX -- December 18, 2006 -- Updated results of the International Breast Cancer Intervention Study (IBIS-I) confirm that tamoxifen reduces the risk of estrogen-receptor-positive breast cancer in high-risk women by about one third, and this benefit persists for at least 5 years after treatment is stopped, researchers reported at the 29^th Annual San Antonio Breast Cancer Symposium (SABCS).

"Most side effects that were apparent during active treatment disappeared over time, suggesting that the risk-to-benefit ratio will continue to improve over time," said Jack Cuzick, PhD, head, Centre for Epidemiology, Mathematics and Statistics at Cancer Research, and John Snow professor of epidemiology, Wolfson Institute of Preventive Medicine at Queen Mary, University of London, London, United Kingdom.

In the IBIS-1 trial, 7,145 high-risk women were randomized to receive either 20 mg/day of tamoxifen or matching placebo for 5 years. The primary outcome measure was the incidence of breast cancer, including ductal carcinoma in situ.

The results after a median of 96 months show that breast cancer was diagnosed in 4.1% of the 3,579 women in the tamoxifen group and in 5.5% of the 3,575 women in the placebo group. This translates to a significant 27% risk reduction in the treated group (P = .005), Dr. Cuzick said.

A total of 35 women in the treated arm and 35 women in the placebo arm developed estrogen-receptor-negative invasive tumors, a nonsignificant difference.

However, 129 women (3.6%) in the placebo arm and 87 (2.4%) women in the tamoxifen arm developed estrogen-receptor-positive breast cancers, translating to a 34% risk reduction.

"At 10 years, the hazard ratio, 0.66 (95% Confidence Interval [CI] 0.49-0.88), is quite consistent with what we saw at 5 years," Dr. Cuzick said.

He noted that tamoxifen did not appear to benefit women who took hormone replacement therapy during the active study period. A total of 69 of the 941 women in the placebo arm and 66 of the 910 women in the tamoxifen arm who took concurrent HRT developed breast cancer (P = NS).

The risk of thromboembolic events was 30% higher in the treated group than in the placebo arm at 10 years. That compares to a 2.98 fold increase in such events at 5 years.

"Common side effects such as headaches and gynecological and vasomotor adverse events virtually disappeared after stopping therapy," Dr. Cuzick added.


[Presentation title: Long Term Efficacy of Tamoxifen for Chemoprevention Results of the IBIS-I Study. Abstract 34]

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