Case-control analysis of the effect of baseline estradiol serum concentration on the efficacy of anastrozole for preventing breast cancer

The IBIS-II prevention trial randomized postmenopausal women with high breast cancer risk to oral anastrozole (1 mg once daily) or placebo for five years, and found significant prevention of breast cancer by anastrozole. A case-control analysis within the trial now found that this protecting effect was limited to participants with a medium to high (quartiles 2-4) ratios of estradiol to sex hormone binding globulin, while there was no significant effect for participants in the lowest quartile. The authors conclude that measuring serum estradiol might help differentiate which women will benefit most from an aromatase inhibitor for the prevention of breast cancer.