Spatial Profiling of Patient-Matched HER2 Positive Gastric Cancer Reveals Resistance Mechanisms to Trastuzumab and Trastuzumab Deruxtecan

Abstract PURPOSE HER2-positive gastric cancer (HER2+ GC) exhibits significant intra-tumoral heterogeneity and frequent development of resistance to HER2-targeted therapies. This study aimed to characterize the spatial tumor microenvironment (TME) in HER2+ GC and identify mechanisms of resistance to HER2 blockade including trastuzumab and trastuzumab deruxtecan (T-DXd), with the goal of informing novel therapeutic strategies. PATIENTS AND METHODS We performed spatial transcriptomics on pre-and post-treatment samples from patients with HER2+ metastatic GC who received trastuzumab-based therapy. We also established patient-derived organoids (PDOs) to investigate mechanisms of trastuzumab resistance in vitro . RESULTS ERBB2 -high tumor regions were found to be "immune cold", characterized by low PD-L1 expression and reduced lymphocyte infiltration. We identified two distinct mechanisms of acquired trastuzumab resistance: epithelial-mesenchymal transition (EMT) and upregulation of the endoplasmic reticulum-associated protein degradation (ERAD) pathway. EMT-positive tumors showed increased expression of immune checkpoints, including PD-L1 , and the chemokine CCL2 . Non-EMT tumors exhibited upregulation of the ERAD pathway, highlighting it as a potential therapeutic target. Importantly, we observed increased expression of the promising therapeutic target CLDN18.2, in trastuzumab-resistant tumors. Additionally, loss of HLA was identified as a potential mechanism of resistance to trastuzumab deruxtecan (T-DXd). CONCLUSION Our spatial profiling study reveals distinct TME features and resistance mechanisms in HER2+ GC, providing a valuable resource for future research and therapeutic development. The identification of potential therapeutic targets, such as CLDN18.2, may pave the way for novel treatment strategies to overcome resistance and improve outcomes for patients with HER2+ GC.