Serum HER2 extracellular domain as a predictive biomarker for trastuzumab deruxtecan treatment response in HER2-positive gastric cancer: a real-world study

Background: No established predictive biomarker beyond tissue HER2 status currently exists for trastuzumab deruxtecan (T-DXd) treatment response in human epidermal growth factor receptor 2 (HER2)-positive advanced gastric cancer (AGC). Serum HER2 extracellular domain (HER2 ECD) has shown potential in breast cancer; however, its clinical significance in gastric cancer remains unclear. Objectives: To evaluate the efficacy and safety of T-DXd in HER2-positive AGC and investigate the predictive role of serum HER2 ECD for T-DXd response. Design: Retrospective single-institution study. Methods: Data of HER2-positive AGC patients treated with T-DXd were analyzed. According to their baseline serum HER2 ECD levels, the patients were classified into high (⩾12.1 ng/mL) and low (