Determinants of Response to Sequential Pembrolizumab with Trastuzumab Plus Platinum/5FU in HER2-positive Gastric Cancer: A Phase II Chemoimmunotherapy Trial

Abstract Purpose: Adding pembrolizumab to first-line 5FU/platinum chemotherapy plus trastuzumab improves outcomes in advanced HER2+ gastroesophageal adenocarcinomas, but the benefit is largely confined to dual HER2+ and PD-L1+ patients. To assess the contributions of components, we conducted a phase II trial evaluating 5FU/platinum/trastuzumab and added pembrolizumab in cycle 2 in patients with metastatic HER2+ disease. Patients and Methods: Treatment naive advanced HER2+ gastroesophageal cancer patients underwent a baseline biopsy, and received a single dose of 5FU/platinum with trastuzumab followed by repeat biopsy. Pembrolizumab was added, and a third biopsy was performed after 6 cycles. The primary endpoint was the objective response rate (ORR). Secondary endpoints included progression free (PFS), and overall survival. Exploratory biomarker analysis and dynamic changes in HER2 and PD-L1 were prespecified. Results: Sixteen patients were enrolled. The ORR was 69%, and the median PFS was 11.9 months. Serial whole-exome, single-cell RNA, T-cell receptor sequencing and spatial transcriptomics from pre-treatment and on-treatment samples revealed early trastuzumab-induced natural killer cell infiltration in HER2+ tumor beds and an increase in Fc receptor (FcR) gamma III expression in macrophages, suggesting that trastuzumab directs FcR-mediated antibody-dependent cytotoxicity. This favorable remodeling was enhanced by the addition of pembrolizumab, primarily in PD-L1+ samples. We observed TGF-β signaling in HER2-negative tumor regions, which was associated with non-responder status. Conclusions: These data highlight the biology of intratumoral heterogeneity, and the impact of tumor and immune cell features on clinical outcomes and may partly explain the lesser magnitude of pembrolizumab benefit in HER2+ and PD-L1 negative subgroups.