Abstract GS3-05: Evaluation of the Sensitivity to Endocrine Therapy (SETER/PR) assay to predict benefit from extended endocrine therapy in the NRG/NSABP B-42 trial

Abstract Background: SETER/PR index of sensitivity to endocrine therapy (ET) measures endocrine receptor-related transcription from fixed paraffin-embedded tissue and is highly reproducible within and between laboratories. SETER/PR index is correlated with receptor ligand binding activity and predicts early pharmacodynamic response to ET and survival outcomes following ET in palliative and adjuvant treatment settings. We evaluated whether SETER/PR index predicts benefit from extended letrozole therapy (ELT) in the NSABP B-42 trial, which evaluated ELT vs placebo in HR+ postmenopausal breast cancer patients (pts) who had received 5 yrs of adjuvant ET with aromatase inhibitor (AI), or tamoxifen followed by AI. Methods: SETER/PR index was measured using the QuantiGene Plex bead-based hybridization assay (ThermoFisher) using two unstained sections and an H&E-stained slide in B-42 HER2- samples. Cut points for analysis of SETER/PR index were pre-defined from the inter-quartile range (1.10 ≤SETER/PR index ≤2.10) and the mean SETER/PR index of 1.50 from a reference cohort of samples, representative of B-42 eligibility. Primary endpoint was breast cancer-free interval (BCFI). Stratified Cox proportional hazards models were used to calculate hazard ratio with 95% confidence interval (HR, 95%CI) with significance at p