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Breast Cancer Preventive Effects of Anastrozole Continue Long After Treatment Ends

By Megan Brooks

December 13, 2019

NEW YORK (Reuters Health) - The benefits of the aromatase inhibitor anastrozole for breast cancer prevention in high-risk postmenopausal women extend well beyond the five-year treatment period, according to long-term data from the International Breast Cancer Intervention Study II (IBIS-II) Prevention trial.

The new data were featured at a press briefing today at the San Antonio Breast Cancer Symposium and simultaneously published in The Lancet.

"Five years of treatment carries on producing continuing benefits right out to 12 years," study cochair Dr. Jack Cuzick of the Centre for Cancer Prevention, Wolfson Institute of Preventive Medicine, Queen Mary University London, said during the briefing.

From 2003 to 2012, the study enrolled 3,864 postmenopausal women at high risk for breast cancer; 1,920 were randomly assigned to anastrozole (1 mg/day) and 1,944 to placebo for five years, with similar adherence rates (75% for anastrozole and 77% for placebo).

Results reported in 2013, after a median follow-up of five years, showed a 61% reduction in new breast cancers (from 4.6% with placebo to 1.8% with anastrozole), and a number needed to treat to prevent one breast cancer in the first five years of 36, Dr. Cuzick reported.

The new long-term data show there continues to be a "significant" 36% reduction in new cancers in years five to 12 (4.4% in women who took placebo five years vs 3.5% in those who took anastrozole), he said. "This is statistically significant in its own right, and although numerically less than in the active treatment period, not significantly so," he added.

Over the entire 12-year period, there is a 49% risk reduction with five years of anastrozole (from 8.8% with placebo to 5.3% with anastrozole), with a number needed to treat over that 12-year period of 29, "which is substantially bigger than seen with tamoxifen at this stage, where the number needed to treat was 49 to prevent one breast cancer," Dr. Cuzick noted.

No excess of fractures, other cancers, cardiovascular disease or major adverse events was seen in the extended follow-up.

These data further support anastrozole as the preferred therapy for breast cancer prevention in high-risk postmenopausal women, Dr. Cuzick concluded.

This study was supported by AstraZeneca, Cancer Research UK, and the National Health and Medical Research Council Australia. Dr. Cuzick is a consultant for Myriad Genetics.

SOURCE: The Lancet, online December 12, 2019.


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