Real-time assessment of circulating tumor cells refines the indication for HER2-targeted therapy in metastatic gastric cancer

HER2-targeted therapies have improved outcomes in metastatic gastric cancer (mGC), yet assessment of HER2 status in tumor tissues remains limited by heterogeneity and temporal changes. This study aimed to evaluate real-time HER2 expression on circulating tumor cells (CTCs) using the On-chip Sort system. CTCs were enriched from blood samples of 27 mGC patients, identified by cytokeratin staining, and assessed for HER2 expression via fluorescent labeling. The epithelial-mesenchymal transition (EMT) index was calculated based on co-expression of vimentin and cytokeratin. CTCs also underwent whole-genome amplification and targeted sequencing using a cancer gene panel. Patients were stratified into three groups: Group A (n = 13), HER2-positive in tissue; Group B (n = 8), tissue HER2-negative but CTC HER2-positive; and Group C (n = 6), HER2-negative in both tissue and CTCs. All patients received cytotoxic chemotherapy; only Group A received additional HER2-targeted therapy. Group B showed the poorest progression-free survival (PFS: 7.0 months), compared to Group A (15.7 months) and Group C (not reached). CTC HER2 expression correlated with EMT index; Groups A and B also exhibited higher EMT indices and shared EMT-related mutations. These findings suggest that CTC-based HER2 monitoring reflects tumor aggressiveness and may help identify patients who could benefit from HER2-targeted therapy despite negative tissue HER2 status.