Tumor and immune microenvironment remodeling with neoadjuvant trastuzumab deruxtecan in HER2-positive gastric cancer: Exploratory analyses from the phase 2 EPOC2003 study.

3129 Background: We previously reported the efficacy and safety of neoadjuvant trastuzumab deruxtecan (T-DXd) for HER2-positive gastric and gastroesophageal junction adenocarcinoma (EPOC2003) (Takahari et al., ASCO GI 2024). This exploratory analysis aimed to investigate the associations of baseline molecular features of the tumor, the immune landscape, and their longitudinal changes with the therapeutic efficacy of T-DXd. Methods: Tumor samples were collected at baseline, pre-cycle 2, post-cycle 3 (before surgery), and at surgery. We performed bulk/single-cell RNA sequencing (scRNA-seq) and multiplex immunohistochemistry to identify correlates of pathological response. Responders were defined as patients achieving a major pathological response according to the Becker criteria. Blood samples were collected at baseline and post-cycle 3 for circulating tumor DNA (ctDNA) analysis using a clinically validated, tumor-informed, personalized assay (Signatera, Natera, Inc.). Results: Baseline HER2 expression did not differ between responders (n= 4) and non-responders (n= 22). In the post-treatment sample analysis, a decreasing trend in proliferation index was observed in responders, whereas this trend was not observed in non-responders, suggesting preferential elimination of proliferative tumor cells by T-DXd. Early on-treatment samples (pre-cycle 2 or post cycle 3) tended to demonstrate upregulation of cytokine signaling, inflammatory, immune response signatures, and CD8+ T cell density in both tumor and stroma regardless of response. Notably, scRNA-seq revealed an expansion of neutrophils, which was more pronounced in responders with a stronger baseline interferon signature. These post-treatment neutrophils displayed upregulated migration, phagocytosis, and degranulation signatures, indicative of enhanced neutrophil mediated anti-tumor activity following T-DXd. ctDNA testing was performed in 18 patients, with a baseline positivity rate of 94.4% (17/18). All responders (3/3) achieved ctDNA clearance after cycle 3, compared with only 35.7% (5/14) of non-responders. Conclusions: Neoadjuvant T-DXd therapy induced biological remodeling of the tumor and immune microenvironment. Together with on-treatment ctDNA kinetics, these findings suggest distinct response-associated biological patterns and may help guide future treatment optimization for HER2-positive gastric and gastroesophageal junction adenocarcinoma. Clinical trial information: NCT05034887 .