Estrogen levels in premenopausal (prem) patients (pts) with hormone-receptor positive (HR+) early breast cancer (BC) receiving adjuvant triptorelin (Trip) plus exemestane (E) or tamoxifen (T) in the SOFT trial: SOFT-EST substudy.

585 Background: Optimal endocrine therapy for prem pts with early HR+ BC may depend on complete estrogen suppression with GnRH-A, and is crucial for those receiving concurrent aromatase inhibitors (AI). SOFT-EST is a prospective substudy of the phase III SOFT trial. Aims of SOFT-EST are to describe estradiol (E2), estrone (E1) and estrone sulphate (E1S) during monthly Trip + E or T and to assess if there is a group in E+Trip with suboptimal estrogen suppression (SES). Methods: All pts enrolled in SOFT from selected centers who consented, selected Trip as ovarian function suppression method and were randomized to E+Trip or T+Trip were enrolled in SOFT-EST until reaching the accrual goal (E=90;T=30 pts). Prem status for SOFT was based on local E2. Blood sampling timepoints (tp) were: 0, 3, 6, 12, 18, 24, 36 & 48 months (m). Serum estrogens were measured by GC/MSMS due to high specificity/sensitivity and no E cross-reactivity. SES for E+Trip for this first pre-planned (12 m) analysis was defined as E2 levels...