Abstract
: Trastuzumab deruxtecan (T-DXd), an anti-HER2 antibody–drug conjugate with a topoisomerase I inhibitor connected by a cleavable linker, has been approved for patients with HER2-positive gastric or gastroesophageal junction tumors. This biomarker study assessed HER2 expression and immune cell infiltration in relation to the therapeutic response to T-DXd. This retrospective analysis included samples from patients treated with T-DXd in three clinical trials. We performed RNA sequencing and multiplex IHC on archival tumor samples obtained at baseline, during treatment, and after treatment. Flow cytometry was performed on tumor-infiltrating immune cells freshly isolated from tumor tissues. Samples from 28 patients were included in this study. ERBB2 mRNA levels and CD20+ cell infiltration in tumors were significantly higher at baseline in responders than in nonresponders. Patients were classified into three biological groups based on their baseline tumor/stroma-infiltrating immune cell densities. Two groups reported similar response rates, but a trend was observed toward a shorter progression-free survival in the group with more immunosuppressive regulatory T cells and PD-L1 expression at baseline. T-DXd treatment tended to increase the levels of tumor-infiltrating CD8+ T cells and PD1+CD8+ T cells, particularly in responders. Gene expression signatures of CTL and Th cells increased during treatment, whereas signatures related to hypoxia, MYC targets, collagen formation, and IL-10 were downregulated. Our data suggest that HER2 expression levels and baseline tumor microenvironment characteristics correlate with T-DXd efficacy. Furthermore, this treatment may modulate tumor microenvironment immune profiles. Further validation using a larger sample size is warranted. Significance: This biomarker study explored HER2 expression levels and immune cell characteristics that may affect response to T-DXd using tumor tissue samples collected from clinical trial participants. The results suggest that HER2 expression levels and tumor characteristics before the initiation of T-DXd may correlate with the efficacy of the drug.