Overweight women with breast cancer were found to have more estrogen remaining in their blood after treatment with hormone-suppressing drugs than did women of healthy weight, supporting previous suggestions that aromatase inhibitors might be slightly less effective in women with a high body mass index (BMI).

Obese women have higher levels of estrogen than do women of normal weight, and the new findings show that although aromatase inhibitors do suppress estrogen levels in obese women, estrogen levels remained more than double those of similarly treated women of normal weight.

Despite the fact that estrogen fuels many breast cancers, women with higher BMIs should not be alarmed by the new estrogen-level finding or stop taking their medications, assured the study’s senior author, Professor Mitch Dowsett of the Breakthrough Breast Cancer Research Centre at The Institute of Cancer Research (ICR) in London, United Kingdom. “Our findings are based on laboratory studies, so we would need to carry out clinical trials to tell us whether women with a higher BMI would benefit from changes to their treatment,” explained Dowsett in a statement issued by the ICR to announce the study results, which were published in Journal of Clinical Oncology.

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With recent studies reporting that the aromatase inhibitor anastrozole has lower effectiveness than does tamoxifen in women with high BMI, Dowsett and colleagues evaluated 44 postmenopausal women with early estrogen receptor (ER)-positive breast cancer. The women received anastrozole 1 mg/day for 3 months, followed by another aromatase inhibitor, letrozole, at 2.5 mg/day for the next 3 months, or received those treatments in the opposite sequence.

The investigators learned that baseline values of two types of estrogen, estradiol and estrone sulfate, were significantly correlated with BMI: During both anastrozole therapy and letrozole therapy, estrogen levels were greater among women with higher BMIs than among women of normal weight. However, this difference was only significant in the case of letrozole, not anastrozole. Dowsett’s group also noted that letrozole suppressed both estrogen types to a greater degree across all BMIs than did anastrozole.