Spatial transcriptomic and proteomic insight into Trop-2, HER2, and AR expression: A pathway to tailored therapies in triple-negative breast cancer.

e12576 Background: Triple-negative breast cancer’s (TNBC) dismal prognosis demands more effective therapy selection. Previously described molecular subtypes reveal its heterogeneity but lack spatial insight and are not currently used in clinical practice. Here, we aim to evaluate by immunohistochemistry (IHC) the expression of key biomarkers in TNBC and its correlation with spatial gene expression obtained by spatial transcriptomics (ST). Methods: We studied a retrospective cohort of 92 patients (pts) treated at the Institut Jules Bordet in Belgium with primary surgery for early TNBC. ST (Visium Spatial Gene Expression, 10X Genomics) was previously performed on fresh frozen surgical samples. Using serial sections of these samples, we performed duplex IHC staining for Trop-2/androgen receptor (AR) and HER2/Ki-67. IHC staining was scored using H-score (Trop-2), Allred score (AR), and current guidelines (Ki67/HER2), adding the ultra-low and null HER2 categories. Statistical analyses included descriptive statistics, Spearman correlation, Mann-Whitney test, and Kaplan-Meier with log-rank tests for estimating outcome distribution. Results: Of 92 pts included, 4 (4.3%), 25 (27.2%), 28 (30.4%), and 35 (38%) pts had HER2 2+, 1+, ultra-low, and null, respectively, tumors. Importantly, we observed a significant difference in tumor cell-derived ERBB2 gene expression of HER2 ultra-low versus HER2 null tumors ( p 5 and Ki67 ≤ 40%, with a PPV of 93% and an NPV of 96%. Conclusions: ST data has the potential to improve easily accessible IHC classifiers, with the aim of refining patient selection for ADC and anti-AR therapies in TNBC.