Spatial profiling of patient-matched HER2 positive gastric cancer reveals resistance mechanisms to targeted therapy.

[BACKGROUND] Human epidermal growth factor receptor 2 (HER2; ) is overexpressed or amplified in 15-20% of gastric cancers (HER2+ GC). Within individual HER2+ GCs, HER2/ expression is often variable. Although HER2 therapeutic targeting improves outcomes for HER2+ GC patients, acquired resistance is frequent.

[OBJECTIVE] To spatially interrogate HER2+ GC interpatient and intrapatient heterogeneity and resistance mechanisms associated with HER2-targeting agents (trastuzumab, trastuzumab deruxtecan (T-DXd)).

[DESIGN] Spatial transcriptomic analysis (GeoMx Digital Spatial Profiler) was applied to >1500 regions of interest in 30 GCs-these contained 15 HER2+ GCs treated with trastuzumab and T-DXd subsequently. Analysis of patient-matched samples with acquired trastuzumab or T-DXd resistance revealed escape mechanisms. Results were validated by immunohistochemistry, independent cohorts and patient-derived xenografts and organoids.

[RESULTS] HER2+ tumours exhibited expression within the spatial tumour microenvironment. We observed increased expression of CLDN18.2, a promising therapeutic target, in trastuzumab-resistant tumours. One-third of HER2+ GC patients developed epithelial-mesenchymal transition (EMT) on trastuzumab resistance, associated with and upregulation. Another third of trastuzumab-resistant HER2+ GC patients activated the endoplasmic reticulum-associated degradation (ERAD) pathway including genes such as . HLA loss and increases in oxidative phosphorylation pathways were observed in T-DXd-resistant GCs.

[CONCLUSION] Our results delineate multiple acquired resistance mechanisms to trastuzumab and T-DXd in HER2+ GC . This information may guide trials combining trastuzumab or T-DXd with new agents to enhance the efficacy and durability of HER2 blockade.