Abstract 6754: Spatial-immune multiomics refines prognostication in early-stage, estrogen receptor-positive breast cancer

Abstract Introduction: Despite a strong 5-10 year prognosis, in ER+HER2− breast cancer relapses are common beyond 10 years. Genomic assays employed to determine recurrence risk at diagnosis, and subsequently the required adjuvant treatment, are still ambiguous for a significant proportion of patients with Intermediate risk (i.e. Oncotype Dx Recurrence Score (RS) 16-25). Contemporary management relies on menopausal status as a risk stratifier, which is a source of inaccuracy for the assessment and prescription of personalised treatment regimens. Methods: We sought to refine risk stratification by incorporating features of the tumour-immune microenvironment. We employed a spatial multiomics workflow to a tissue microarray of Irish patients previously enrolled on the TAILORx trial (whole-transcriptome, ROI-based spatial transcriptomics n=410, 7-plex spatial proteomics n=442). We subsequently validated CD8 IHC in whole-resection specimens of the same patient cohort (n=453). Our primary endpoint was invasive disease-free survival. Results: Modelling immune infiltration agnostic of underlying genomic risk produced data congruous with the consensus view of ER+HER2- breast cancer immunogenicity. No tumour-infiltrating lymphocyte (TIL) was independently prognostic across the entire cohort, and the distribution of TILs mirrored the categorical 'hot or not' definition of an immunologically "cold" disease ( 10% TILs). However, modelling by underlying genomic risk revealed monotonic trends of increasing TILs and increased risk. We further demonstrated that macrophage and T-helper cells facilitate and impede cytotoxic T-cells, are associated with extracellular matrix remodelling, M2-like macrophage (SPP1) genes, and cytotoxicity (GZMA, GZMB, PRF1), checkpoints (LAG3, PD-1, PD-L2), and exhaustion (TOX) genes respectively, consistent with restrained T-cell activation. Survival analyses underscored how high cytotoxic T-cell infiltrates were associated with poorer 15-year iDFS in patients receiving additional chemoendocrine therapy (CD8 density: High RS p = 0.017, FDR 0.1. CD8%: Intermediate RS p=0.027, FDR0.1). Critically, in randomised treatment arms of the Intermediate RS, a treatment-biomarker interaction indicated chemotherapy inferiority at higher stromal CD8 density (ΔLR-χ2: 7.36, p = 0.007), which was validated orthogonally on whole-resection specimens from the same cohort (ΔLR-χ2: 7.48, p = 0.006). Conclusions: To our knowledge for the first time, we have outlined a predictive biomarker of chemotherapy inferiority in the Intermediate RS, suggesting a potential treatment change for up to 50% of patients. This discovery requires validation in the larger TAILORx trial before clinical adoption. Citation Format: Zak Kinsella, Arif Chowdhury Jahangir, Hannah Nyarkoah Nyarko, Daria Kalinska-Lysiak, Claudia A. Gonzalez, Verena Murphy, Anthony O'Grady, Joanna Fay, Katherine Sheehan, Arman Rahman, John Crown, Catherine Kelly, Simon Sam McDade, Jochen Prehn, William M. Gallagher, Darran P. O’Connor. Spatial-immune multiomics refines prognostication in early-stage, estrogen receptor-positive breast cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 6754.