Abstract / Summary
Background Antibody-drug conjugate (ADC) activity and target-dependent on-target/off-tumor toxicity depends in part on which cells express the target antigen, a feature that bulk assays cannot resolve. Methods We analyzed a publicly available whole-transcriptome CosMx breast tumor dataset including 154,816 cells and 18,934 genes. Eleven clinically relevant ADC targets were profiled across seven annotated lineages. Findings were contextualized using TCGA-BRCA bulk transcriptomes (1,097 tumors and 114 normal tissues) and assessed in two public Xenium breast cancer specimens. Results HER2 ( ERBB2 ), TROP2 ( TACSTD2 ), HER3 ( ERBB3 ), NECTIN4 and MUC1 were enriched in tumor epithelium. B7-H3 ( CD276 ) was highest in endothelial and myoepithelial cells, with similar mean expression in stromal and tumor cells. PSMA ( FOLH1 ) was enriched in endothelium. TCGA-BRCA tumor-normal differences did not identify these cellular sources. No overall-survival association remained after multiple-testing correction or covariate adjustment. HER2 and TROP2 tumor enrichment recurred in both Xenium specimens, whereas EGFR was not tumor restricted. Conclusions Cell-resolved spatial profiling separated ADC targets enriched in tumor epithelium from those expressed in vascular and stromal compartments. B7-H3 and PSMA localization was consistent with prior immunohistochemical reports and is confirmatory. These RNA-level results do not establish surface-protein abundance, ADC response, or clinical toxicity. Low-abundance microenvironmental findings remain based on a single discovery specimen and require protein-level validation. Protein-level confirmation, and correlation with ADC efficacy and toxicity, are required before these maps can inform clinical decisions.