Abstract PS10-03: Impact of Baseline Oestradiol and Testosterone on the Preventive Effect of Anastrozole

Abstract It is well known that serum levels of oestradiol and testosterone, esp free hormone levels, influence the risk of developing breast cancer in postmenopausal women (Thomas et al 1997, Hankinson et al 1998, Kaaks et al 2005, Tin Tin et al 2021). However very little is known about how these hormone levels influence the effectiveness of aromatase inhibitors. In the IBIS-II Prevention Trial we compared anastrozole to placebo in 3864 women at high risk of breast cancer (Cuzick et al 2020). Of these women 3644 (94.3%) had a baseline blood sample. In those with a valid blood sample, 72 in the anastrozole arm and 142 in the placebo arm developed breast cancer (including DCIS) after 12.9 years of follow up (OR = 0.49, 95% CI 0.37–0.66 P 0.0001). For each case two controls were selected, matched on age, treatment arm and follow up longer than the matching case. In these women oestradiol (E2), testosterone (Testo) and SHBG were measured by liquid chromatography – tandem mass spectroscopy, and E2/SHBG and Testo/SHBG ratios were computed to approximate free hormone levels, and analysed in quartiles. Hormone replacement therapy was not allowed during the trial, and women with use within 3 months prior to entry or outlier hormone values were excluded from these analyses. In the placebo arm higher levels of both of these ratios were associated with a higher breast cancer rate (OR per quartile 1.25 (1.04-1.59), P=0.018) for E2/SHBG and (OR per quartile 1.22 (1.02-1.47), P = 0.032) for Testo/SHBG), whereas no significant effect was seen in the anastrozole arm (OR = 1.05 (0.81-1.37), and 1.16 (0.90-1.49), resp) (Table). Neither treatment interaction was significant. Absolute numbers of cases were similar between the anastrozole and placebo arms in the lowest quartile of E2/SHBG (18 anastrozole, 22 placebo), but higher numbers were seen in the placebo arm for the other quartiles. Similar results were seen for testo/SHBG, and supportive, but weaker results were seen for these two hormones without an SHBG adjustment, SHBG and BMI at entry. Adjusting for other risk factors had no influence on these findings. Effect sizes were similar during the 5 year treatment period and thereafter. There were too few ER negative cases to compare results by receptor status. Vasomotor side effects were higher with increasing E2/SHBG levels in both arms, but no other hormone showed an effect, and no hormone level was significantly associated with gynaecologic or musculoskeletal side effects. As aromatase inhibitors effectively eliminate the production of oestradiol in postmenopausal women, these results suggest they may be more effective in postmenopausal women with high oestradiol or testosterone levels, and raise questions about their value in women in the lower quartile of serum levels for these hormones for prevention and possibly adjuvant treatment. Table: Risk of breast cancer by quartiles of the oestradiol/SHBG in ratio high risk women treated with anastrozole or placebo. Citation Format: Jack Cuzick, Kim Chu, Brian Keevil, Antony Howell, Bernardo Bonanni, Evans Gareth, Kaija Holli, Sibylle Loibl, Nicholas Zdenkowski, Steven Cummings, Mitch Dowsett. Impact of Baseline Oestradiol and Testosterone on the Preventive Effect of Anastrozole abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS10-03.