Abstract GS1-06: Evaluation of the Breast Cancer Index in premenopausal women with early-stage HR+ breast cancer in the SOFT trial

Abstract Background: The landmark Suppression of Ovarian Function Trial (SOFT) in premenopausal breast cancer patients revealed that the addition of ovarian function suppression (OFS) to adjuvant endocrine therapy with either tamoxifen (T+OFS) or exemestane (E+OFS) reduces the risk of recurrence compared with adjuvant tamoxifen alone. The benefit from the addition of OFS was most clinically meaningful for patients at higher clinico-pathologic risk of recurrence. There are no biomarkers to aid decision-making about intensification of endocrine therapy with OFS and its resultant toxicities. The Breast Cancer Index (BCI) is a gene expression–based signature that stratifies patients based on the risk of overall (0-10 years) and late (post-5 years) distant recurrence (DR) and predicts the likelihood of benefit from extended endocrine therapy in early stage HR+ breast cancer. The purpose of this study is to assess BCI’s prognostic and predictive ability in premenopausal women randomly assigned to 5-years treatment with E+OFS or T+OFS vs T alone in the SOFT trial. Methods: All available FFPE tumor samples from the SOFT trial (n=1718 of 3047) were included in the study. BCI testing was performed blinded to clinical characteristics, treatment and outcome. Median follow-up was 13 years. Primary endpoint was breast cancer-free interval (BCFI). Secondary endpoints were distant recurrence-free interval (DRFI) and disease-free survival (DFS). Kaplan-Meier analysis and Cox proportional hazards regression models, stratified by prior chemotherapy and lymph node status, were used to evaluate the predictive performance of BCI (H/I) status (High vs Low), and secondarily H/I as a continuous score. Hypothesis testing for interaction was performed by stratified log-rank tests. Results: Tumor samples from 1687 (98%) patients (30.4% < 40 years, 64.1% T1, 50.1% G2, 65.8% N0, 85.5%% HER2-, 53.3% received prior chemotherapy) were successful in BCI testing and included in the final analysis. Patient characteristics in the translational cohort are representative of the parent SOFT trial. 42.4% of patients’ tumors had H/I-High status. Prognostic and predictive analyses are ongoing and will be presented at the meeting. Citation Format: Ruth O’Regan, Yi Zhang, Gini F. Fleming, Prudence Francis, Roswitha Kammler, Giuseppe Viale, István Láng, Meritxell Bellet Ezquerra, Hervé R. Bonnefoi, Sherene Loi, Marco Colleoni, Catherine A. Schnabel, Kai Treuner, Meredith Regan. Evaluation of the Breast Cancer Index in premenopausal women with early-stage HR+ breast cancer in the SOFT trial [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr GS1-06.