Cell-resolved spatial mapping of antibody-drug conjugate targets in breast cancer

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.

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.

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.

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.