Abstract PS3-11-02: Exploring heterogeneity of HER2 immunohistochemistry staining in patients undergoing neoadjuvant breast cancer treatment in two cancer centers of South America

Abstract Background Intra-tumoral heterogeneity (ITH) is a biological characteristic associated with various key tumor biomarkers in different cancer models. Heterogeneous expression of HER2 has been observed in around 10-40% of patients with early breast cancer (eBC), and its determination has been associated with a worse response to HER2-directed therapies. While ITH characterization has commonly been addressed using gene amplification tests, international guidelines recommend identifying it in immunohistochemistry (IHC) evaluations of HER2 staining. Materials and Methods We included patients with HER2-3+ eBC diagnosis that underwent neoadjuvant treatment in two cancer centers: Instituto Alexander Fleming (Buenos Aires, Argentina) and Instituto Nacional del Cáncer (Asunción, Paraguay). ITH was retrospectively evaluated in the initial tumor biopsies by two different pathologists in each center, and defined as the presence of at least 10% of tumor cells without complete or intense membrane staining. Univariate and multivariate models were used to understand the role of ITH to predict pathological complete response (pCR). Results 116 patients were incorporated in our cohort (female 100%; median age 49IQR 43-56.3; clinically axillary involvement 51.7%). ITH was observed in 15 cases (12.9%), and associated with numerically higher median estrogen receptor (ER) expression (90 IQR 0-98 vs. 5 IQR 0-90; p=0.07). The pCR in our sample was 68.9%, with rates of 26.7% and 68.3% in patients with and without ITH diagnosis, respectively (p=0.004). pCR remained negatively associated with ITH (OR 0.15; CI95% 0.03-0.54; p=0.006) and ER expression (OR 0.24; CI95% 0.09-0.6; p=0.003) in multivariate models after other key variables such as axillary involvement, age and tumor size were concomitantly analyzed. Conclusions ITH defined per IHC analysis was one of the main factors that explained residual disease in our cohort. Our results support the routinary determination of ITH for HER2-staining in initial biopsy results, and the evaluation of this biomarker in novel prognostic assays, as well as in modern clinical trials exploring the role of conjugated antibodies in the eBC setting. Citation Format: M. Ancao, M. Costanzo, M. Gonzalez Donna, M. Pombo, M. Nally, M. Amat, A. Ostinelli, S. Rivero, L. Lapuchesky, A. Nervo, J. Nadal, C. Loza, F. Colo, V. Fabiano, A. Perazzollo, L. Sabatini, T. Vargas, A. Pavetti, M. Cabañas Leon, F. Waisberg. Exploring heterogeneity of HER2 immunohistochemistry staining in patients undergoing neoadjuvant breast cancer treatment in two cancer centers of South America abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-11-02.