Abstract 2707: An <i>ex vivo</i> culture platform of invasive lobular breast cancer for the screening of novel drugs targeting the tumor microenvironment

The study of invasive lobular breast cancer (ILC) and the development of effective ILC-specific therapies is hampered by the lack of suitable model systems that accurately reflect the human disease. Here, we present a set of four genetically engineered mouse models (GEMMs) of ILC, based on the most common genetic alterations found in human ILC. These GEMMs represent the predominant subtypes of ILC and closely mimic human ILC in growth pattern, invasiveness, tumor microenvironment (TME) composition, and subtype-specific molecular profiling. Using these GEMMs, we developed a novel ex vivo culture platform that maintains tissue architecture, TME composition and gene expression for up to seven days. Using this platform, we investigated whether targeting cancer-associated fibroblasts (CAFs), which are commonly found in invasive lobular carcinoma (ILC) and known to hinder therapy effectiveness, in combination with tumor cells, could enhance anti-cancer drug efficacy. To identify potential drug combinations, we conducted single-cell transcriptomics on our four ILC mouse models to investigate the complex interactions between CAFs and other cells in the TME. Building on previous research showing that immune-regulatory CAFs (iCAFs) recruit myeloid cells and promote tumor cell invasion, as well as our current findings of extensive CXCL- and complement-mediated crosstalk between iCAFs and macrophages, we hypothesized that inhibiting these iCAF functions could synergize with immunotherapy; a treatment typically ineffective in ILC. Indeed, inhibiting iCAF functions improved immunotherapy efficacy in ex vivo cultures, as measured by enhanced T-cell activation, a finding that was subsequently validated in mice. Furthermore, our ex vivo platform enabled the culture and drug screening of human ILC specimens. Overall, our ILC GEMMs effectively model a wide range of human ILC molecular subtypes, and our ex vivo culture platform facilitates the faster and more accurate identification of new therapeutic vulnerabilities compared to traditional 2D or 3D cultures. Taken together, these models have the potential to significantly advance ILC research. Citation Format: Konstantina Strepi, Daisy Batenburg, Martine van Miltenburg, Pim Kloosterman, Eline van der Burg, Anne Paulien Drenth, Frank van Diepen, Guido de Roo, Lex de Vrije, Mariska van Geldrop, Serge Zander, Jos Jonkers, Julia M. Houthuijzen. An ex vivo culture platform of invasive lobular breast cancer for the screening of novel drugs targeting the tumor microenvironment [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 2707.