Abstract 3919: Liquid biopsy derived organoids as a potential platform for personalized cancer therapy in metastatic prostate cancer

Introduction Circulating tumor cells (CTCs) can serve as a source of metastatic tumor material, however their low numbers often limit downstream applications. Diagnostic leukapheresis (DLA) has been shown to substantially increase CTC yield. In this study we isolated CTCs from metastatic prostate cancer (mPCa) patients by DLA to propagate them in vitro as organoid cultures. Furthermore, tumor-derived organoids were used as a model for drug discovery and sensitivity-screening, thereby exploring potential treatment selection. Methods We included 44 mPCa patients into the study and 18 were selected for DLA, based on the presence of ≥5 CTCs/ 7.5 mL blood. We optimized the DLA procedure by comparing low versus high density settings and their impact on CTC isolation efficacy. As the DLA product contains a median of 8.6*10^9 white blood cells (WBC), stringent enrichment methods are needed. CTC enrichment from DLA product was performed by antibody-based WBC depletion alone, or combined with subsequent EpCAM based enrichment. Enriched CTC fractions were cultured in vitro under optimized conditions, to initiate organoid expansion. Results We show that DLA is a safe and efficient method to collect large amounts of CTCs from mPCa patients. With optimized DLA settings we were able to improve CTC enrichment and observed a non-significant increase in CTC yield from DLA (median CTC recovery 15339 vs 5796, P=0.125). WBC depletion alone was found to reduce WBCs by ~2000-fold while retaining >50% of the CTCs, resulting in a WBC to CTC ratio of 545:1. We were able to culture and confirm CTC-derived organoids in 9/18 samples, including one organoid cell line, EMC-PCa-41. Whole Genome Sequencing (WGS) of EMC-PCa-41 revealed a triploid genome characterized by focal amplification of AR, a TMPRSS2-ERG fusion, a PTEN deletion and multiple inter-chromosomal rearrangements. Next we determined copy number profiles in single CTCs and matched organoids from two patients using shallow WGS. These data confirm prior data that CTCs represent the inherent intra-patient heterogeneity and organoids resemble CTCs from the original DLA product. Moreover, we performed an in vitro drug screen with the organoid cell line EMC-PCa-41, and found that it has a limited response to Enzalutamide, which correlated with the relatively short response to Enzalutamide that was observed in the patient. Conclusion Overall our study demonstrates that DLA provides a high CTC yield which enables short-term organoid