Abstract 1701: Investigating trastuzumab-deruxtecan (DS-8201) resistance mechanisms using clinically relevant PDX models and combination strategies with anti-PD-1 antibody

Abstract Background: Trastuzumab-Deruxtecan (T-Dxd, DS-8201), an antibody-drug conjugate (ADC) targeting HER2, is clinically approved for cancer patients with high and low HER2 expression levels; however, the response rate to DS-8201 are modest, especially in gastric cancer and the mechanisms of resistance remain elusive. In this study, we aimed to address this gap by developing clinically DS-8201 resistant patient derived xenograft (PDX) models in gastric and breast cancer. Our objectives were to elucidate possible mechanisms of resistance with this topoisomerase inhibitor-linked HER2 ADC and explore whether combining DS-8201 with an anti-PD-1 antibody could overcome resistance by using a humanized mouse model. Method: Humanized mice were established by injecting human CD34 positive hematopoietic stem cells into NCG mice, followed by inoculation with PDX tissues from DS-8201-resisatnt gastric or breast cancer patients. Once tumors reached approx.150-200mm3, the mice were administered a single dose of DS-8201, pembrolizumab (twice weekly for 4 weeks) and a combination. Tumor samples were harvested 24h after the final dose, fixed with 10% NFB overnight, and subjected to IHC staining with antibodies targeting CD8, CD4 and HER2 to assess changes in the tumor microenvironment. Furthermore, DNA and RNA were extracted from tumor samples using the K-Cell® kit for subsequent whole-exome sequencing (WES) and RNA sequencing (RNAseq) analysis, respectively. Results: the data revealed limited tumor inhibition in DS-8201 treated group, consistent with clinical findings. Elevated expression of ATP-binding cassette (ABC) transporters, known as drug efflux pumps, was observed in this group, suggesting a potential strategy for overcoming resistance by targeting these transporters. Concurrent administration of pembrolizumab enhanced the efficacy of DS-8201, leading a significant inhibition in tumor growth compared to both the vehicle and monotherapy arms. An increase in CD3+ T cells in blood and CD8+/CD4+ T cell ratio was observed in the combination therapy group, suggesting modulation of the tumor microenvironment. In conclusion, the establishment of novel gastric and breast PDX cancer models, derived from patients exhibiting resistance to DS-8201 in clinical settings, provided a valuable approach for investigating mechanisms of resistance in HER2 targeted ADCs. Our findings suggest that combining pembrolizumab with ADC may represent a promising strategy for overcoming resistance by modulation of tumor-infiltrating lymphocytes. Citation Format: Diandong Jiang, Bin Xie, Yulan Tang, Hongkui Chen, Xin Hou, Cui Xu, Dan Shao, Xinlei Chen, Xiaorong Gu, Loc Van, Josh Caggiula, Danyi Wen. Investigating trastuzumab-deruxtecan (DS-8201) resistance mechanisms using clinically relevant PDX models and combination strategies with anti-PD-1 antibody [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1701.