, according to findings from a case-control study using data from a large breast cancer prevention trial.
In the randomized, placebo-controlled IBIS-II prevention trial of 3864 women aged 40-70 years at increased risk for developing breast cancer, treatment with the aromatase inhibitor anastrozole was associated with a 49% reduction in breast cancer incidence. At median follow-up of 131 months, breast cancer occurred in 85 (4.4%) versus 165 (8.5%) of patients in the anastrozole and placebo arms, respectively.
A preplanned case-control study involving 212 participants from the anastrozole group (72 cases and 140 controls) and 416 from the placebo group (142 cases and 274 controls), showed a significant trend toward increasing breast cancer risk with increasing estradiol-to-sex hormone binding globulin (SHBG) ratio in the placebo group, but not in the anastrozole group (trend per quartile, 1.25 vs 1.06), reported Jack Cuzick, PhD, of the Wolfson Institute of Population Health, Queen Mary University of London, UK, and colleagues.
A weaker but still significant effect was observed for the testosterone-SHBG ratio in the placebo group (trend, 1.21), but again, no such effect was seen in the anastrozole group (trend, 1.18).
A relative benefit was seen for anastrozole in estradiol concentration quartiles 2, 3, and 4 (relative risk [RR], 0.55, 0.54, and 0.56, respectively), but not in quartile 1.
The findings were published online December 6 in The Lancet Oncology.
Study participants were recruited from 153 breast cancer treatment centers across 18 countries and randomized in a 1:1 ratio to receive 1 mg of oral anastrozole daily or placebo. For the case-control analysis, the investigators looked at the effects of baseline estradiol to SHBG ratio on the development of all breast cancers, including ductal carcinoma in situ. They also assessed the relative benefit of anastrozole versus placebo.
Case patients were those diagnosed with breast cancer after trial entry through data cutoff on October 22, 2019, and who had not used hormone replacement therapy within 3 months of trial entry or during the trial. Controls were participants without breast cancer who were randomly selected and matched according to treatment group, age, and follow-up time.
“Although the association between estradiol and breast cancer risk is well established, less is known about whether the concentrations of these hormones have an effect on the efficacy of preventive therapy with selective estrogen receptor modulators or aromatase inhibitors in women at increased risk of developing breast cancer,” the investigators noted, explaining that in the current analysis, they “tested the hypothesis that, for women with a low estradiol–SHBG ratio, anastrozole would provide little or no reduction in the risk of breast cancer.”
The results from the placebo group “confirm the increasing risk of breast cancer associated with higher estradiol and testosterone concentrations, and a decreasing risk associated with increasing SHBG concentrations in women who were not randomly allocated to receive anastrozole,” they said.
“However, to our knowledge, this is the first report of the effect of low concentrations of estradiol or testosterone on a lack of response to aromatase inhibitor treatment, either as a preventive measure or in the adjuvant setting,” they added. “These data provide support for the hypothesis that preventive therapy with an aromatase inhibitor is likely to be most effective for women with higher estradiol-to-SHBG ratios and, conversely, of little or no benefit for those with low estradiol-to-SHBG ratios.”
Thus, measurement of estradiol and SHBG concentrations might be helpful in making decisions about using inhibitors both for treatment and prevention, they continued, underscoring the importance of using assays sensitive enough to measure low estradiol concentrations in the plasma in postmenopausal women.
“We used a very sensitive liquid chromatography–tandem mass spectroscopy assay (lower limit of sensitivity of 3 pmol/L), which allowed us to accurately measure the low concentrations of estradiol and SHBG in the serum samples from our population of postmenopausal women. Wider use of this type of assay or a similar assay will be necessary to implement any of the actions suggested by this study,” they explained.
The findings “suggest a potential role for measuring estradiol, testosterone, and SHBG more widely, both in determining which individuals are at high risk and the likely response to endocrine treatment,” they concluded, noting that measuring serum hormones is inexpensive and, if used more routinely in high-risk clinics and for treatment of early breast cancer, could “substantially improve disease management.”
This study was funded by Cancer Research UK, National Health and Medical Research Council (Australia), Breast Cancer Research Foundation, and DaCosta Fund. Dr. Cuzick reported receiving royalties from Cancer Research UK for commercial use of the IBIS (Tyrer-Cuzick) breast cancer risk evaluation software.