Combination of predicted sensitivity to endocrine therapy (SET2,3 index) and the Recurrence Score in node-positive breast cancer: Independent validation in the PACS-01 trial.

565 Background: SET2,3 index measures hormone receptor-related transcriptional activity (SET ER/PR ) adjusted for a baseline prognosis index (BPI) derived from tumor size, nodes involved and a 4-gene molecular subtype (RNA4). SET2,3 added prognostic information to the 21-gene Breast Recurrence Score (RS) in the SWOG 8814 trial (1). To confirm this, we independently evaluated both tests performed on node-positive, hormone receptor-positive (HR+) cancers from the PACS-01 trial (Roche et al, JCO 2006, PMID: 17116941) that evaluated the addition of adjuvant docetaxel to anthracycline-based chemotherapy. Adjuvant endocrine therapy became standard for premenopausal patients during PACS-01, justifying a sensitivity analysis in the population who received adjuvant endocrine therapy. Methods: SET2,3 was measured with the QuantiGene Plex bead-based hybridization assay (ThermoFisher) using an aliquot of residual RNA from prior RS testing in 791 HR+ tumor samples. Pre-defined cut points defined higher risk if RS >25 or SET2,3 25, with 5-year DRFI 64.6% 95%CI 56.9;71.3. SET ER/PR index of endocrine transcriptional activity and the BPI, each contributed additional prognostic information to RS status in the sensitivity population (Table). Conclusions: SET2,3 index added prognostic information to the Recurrence Score in this independent blinded validation study of the PACS-01 trial for node-positive breast cancer. 1. Speers et al, JCO 2023, PMID: 36649570. Table: see text