Abstract PS3-09-21: Evaluation of Sensitivity to Endocrine Therapy (SET) as a Prognostic Biomarker in the Latin American Breast Cancer Cohort Study

Abstract Background: The SET2,3 index, which combines the SETER/PR index—a measure of transcriptional activity related to estrogen and progesterone receptor (ER, PR) —with the baseline prognostic index (BPI), has been validated as prognostic and predict endocrine treatment sensitivity in hormone receptor-positive (HR+) breast cancers from U.S. and European patients. However, its prognostic utility remains to be determined in Latin American (LA) breast cancer patients, who possess a unique and diverse genetic admixture. Methods: Clinical and pathological retrospective data were collected from a multi-country Latin American cohort of breast cancer patients. Tumor subtypes were previously characterized using PAM50 classification, Ki67%, and immunohistochemical analysis for ER, PR, and Human Epidermal Growth Factor Receptor 2 (HER2) status. The SET2,3 index was measured using the QuantiGene Plex Assay on archival tumor tissue samples from the HR+/HER2 Negative (-) cancers, blinded to clinical outcomes. Disease-free survival (DFS) was compared across patient subgroups using the log-rank test, and hazard ratios (HRs) were estimated through Cox proportional hazards models. Results: A total of 292 patients were included, with 38 disease-free survival (DFS) events observed over a median follow-up of 7.15 years. In a multivariable Cox model, use of adjuvant endocrine therapy (Yes vs. No; HR 0.27; p