PRCA-03 PATIENT-DERIVED ORGANOIDS FOR MODELING CENTRAL NERVOUS SYSTEM (CNS) METASTATIC BREAST CANCER

Abstract Developing patient-derived organoids (PDOs) from metastatic breast cancers offers opportunities for functional drug screening and biomarker discovery, yet their establishment remains technically demanding. To test this methodology for generating PDOs from CNS breast cancer metastases, we successfully derived a PDO from a solid CNS metastasis. The PDOs continued to express proliferative markers and Estrogen Receptor during passaging, demonstrating the feasibility of this approach. We next sought to generate PDOs from breast cancer leptomeningeal disease (LMD), isolating metastatic breast cancer cells from the cerebrospinal fluid (CSF) of affected patients who consented to donate tissue for research. We compared previously reported Type 1 and Type 2 medium formulations for growing breast cancer organoids, as well as a recently described Ocello medium that incorporates conditioned medium from leptomeningeal cells. In our initial experience, we found that LMD organoids could survive in Ocello medium but exhibited slow or no expansion in culture. For comparison, we also tested the growth of metastatic cancers that had spread to other fluid-filled compartments, leading to malignant effusions in the pleural and peritoneal spaces. In contrast to LMD, we found that PDOs from pleural effusions and ascites exhibited rapid expansion of epithelial components for the majority of cases, expressed expected breast cancer markers, and maintained intratumoral heterogeneity. We also demonstrate the ability to isolate T cells from malignant fluid for co-culturing with PDOs to test immunotherapy agents. These results underscore both the potential and limitations of current ex vivo approaches in modeling metastatic disease within fluid compartments. While PDOs from pleural effusions and ascites show promise, refining culture conditions for LMD remains critical. Our future studies will aim to optimize ex vivo expansion by testing additional modifications to culture conditions and continuing the slow expansion of existing viable cells. We will also incorporate in vivo growth efforts using mice.