Spatially Resolved Transcriptomics Identifies Tumor-Stroma-Immune Networks and Therapeutic Targets in Endocrine-Resistant Advanced Breast Cancer Treated with Everolimus+Letrozole: Insights from the MIRACLE Trial.

Breast cancer is the most commonly diagnosed cancer in women globally. Our previous MIRACLE trial (NCT02313051) demonstrated that everolimus plus letrozole (E+L) significantly improves progression-free survival compared with letrozole (L) monotherapy in premenopausal patients with endocrine therapy-resistant, hormone receptor-positive, HER2-non-amplified advanced breast cancer. This study aims to investigate spatially resolved biomarkers linked to survival benefits from E+L to guide precision therapies. Patients from MIRACLE were stratified by overall survival (OS ≤3 vs. >3 years). Spatial Whole Transcriptome Atlas analysis was used to evaluate tumor-, immune-, and stroma-specific gene expression, co-expression network patterns, and survival correlations. Among patients with shorter survival (OS ≤3 years), we identified a distinctive gene interaction network characterized by tumor-derived S100A9 and CALML5, which is associated with mTORC1 activation. This finding suggests that tasquinimod, an S100A9 inhibitor, could be a viable therapeutic option. Additionally, an interaction between CALML5 and SLPI across tumor and immune areas indicated a potential role in maintaining tumor integrity and mitigating immune-mediated damage. Conversely, patients with longer survival (OS >3 years) exhibited SERPINA1 as a hub gene linked to estrogen receptor activation, and an interaction between FKBP5 and SESN3 associated with AKT/mTORC1 inhibition within tumor-rich regions. Furthermore, the interaction between MMP11 and COL16A1 in stroma-rich regions suggests that cancer-associated fibroblasts may contribute to improved outcomes. Our study underscores the critical role of spatial gene expression analysis in elucidating the tumor microenvironment and its impact on prognosis in patients undergoing E+L treatment, thereby opening new avenues for targeted interventions.