Abstract PS3-04-16: Decoding resistance to trastuzumab deruxtecan in metastatic breast cancer: S100P signaling and immune-niche barriers

Abstract Background: Trastuzumab deruxtecan (T-DXd) has shown strong clinical efficacy in HER2-high and HER2-low metastatic breast cancer (MBC). However, many patients develop intrinsic or acquired resistance, particularly within sanctuary sites like the brain and bone. These niche-specific mechanisms remain poorly understood, limiting durable treatment responses and the design of effective combination therapies. Methods: We analyzed 109 MBC patients treated with T-DXd, stratifying response using clinical and radiographic criteria. Pre-treatment biopsies from 31 patients underwent spatial immune profiling using GeoMX Digital Spatial Profiler. Eight brain and bone metastases were profiled using a 570-plex antibody panel with spatial transcriptomics. Single-cell spatial transcriptomics (CosMX) was performed on 6 brain metastases. Functional validation included CRISPR-Cas9–mediated knockout and lentiviral overexpression of S100P, RAGE inhibition with azeliragon, and in vivo xenograft models. Results: T-DXd response was not associated with HER2 (p=0.128), ER (p=0.832), PR (p=0.617), or Ki-67 (p=0.212). Prior T-DM1 exposure predicted improved response (OR=5.27, p=0.048). Spatial profiling identified enhanced immune activation (CD3, CD8α, FABP4) in responders, especially in bone lesions. In contrast, brain metastases displayed conserved immunosuppressive profiles regardless of response. S100P was the most upregulated protein in resistant tumors (6.8-fold, p=0.0018), with marked spatial heterogeneity. S100P knockout sensitized HER2+ cells to T-DXd, while overexpression induced resistance. RAGE inhibition enhanced T-DXd cytotoxicity (∼49-fold reduction in EC50). Single-cell analysis of resistant brain metastases revealed astrocytic upregulation of CAT and FN1, and macrophage expression of DR5 and CD163, consistent with a neuroprotective, immune-silent niche impeding ADC activity. Conclusions: Resistance to T-DXd is driven by tumor-intrinsic S100P-RAGE signaling and reinforced by niche-specific microenvironments—particularly in the brain—characterized by immunosuppressive astrocyte and macrophage programs. These findings nominate S100P, RAGE, and brain niche-derived stromal pathways as actionable targets to overcome resistance and extend the efficacy of T-DXd in MBC. Citation Format: G. Das, S. Wang, C. Xue, M. Vasquez, W. Yang, L. Xiaoxian, H. Zhao, W. Dong, S. T. Wong. Decoding resistance to trastuzumab deruxtecan in metastatic breast cancer: S100P signaling and immune-niche barriers abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-04-16.