Circulating tumor cell (CTC) expression patterns of cell surface targets in metastatic prostate cancer.

232 Background: Resistance to first-line Androgen Receptor Pathway Inhibitors (ARPI) in metastatic prostate cancer (mPC) is universal and can be driven by androgen receptor (AR) alterations driving constitutive AR signaling, or lineage state transitions that bypass AR and culminate in small cell/neuroendocrine PC (NEPC). Recent advances in cell surface targeted therapies, including radioligand therapies and antibody drug conjugates show promise for treatment of metastatic castrate-resistant prostate cancer (mCRPC). Circulating tumor cells (CTCs) are a minimally-invasive source of tumor material to track these known and novel targets for therapeutic development. We report differential expression of cell surface targets using the largest CTC RNA sequencing cohort of patients with mPC across lineage states and clinical outcomes. Methods: We isolated CTCs using automated microfluidic technology on a 273-sample cohort from patients with histologically confirmed mPC. CTCs purified via anti-EpCAM conjugated magnetic beads were analyzed via RNA-seq. We examined CTC gene expression of cell surface targets, including PSMA, TROP2, B7H3, DLL3, stratified by: disease category (metastatic castration sensitive PC [mCSPC], mCRPC or NEPC), metastatic site (bone +/- lymph nodes, liver, other visceral), and prior ARPI treatment. Results: CTCs from patient with mCRPC showed high expression of canonical adenocarcinoma cell surface targets STEAP1 , KLK2 and FOLH1 (PSMA) and epithelial TACSTD2 (TROP2), intermediate tumor immune checkpoint cell surface protein CD276 (B7H3), and low expression of NEPC targets DLL3 and SSTR2 . CTCs from NEPC patients, as expected, demonstrated lower expression of most adenocarcinoma targets and a trend towards higher DLL3 and SSTR2 and associates with worse OS regardless of disease site in multivariate analysis (HR 4.76 [1.09-8.44], p=0.011). CTCs from patients with mCSPC had highest TACSTD2 (p=0.0076) and a trend towards lower DLL3 expression. When comparing metastatic sites, CTCs from non-NEPC mCRPC patients with liver metastases had significantly higher expression of DLL3 (p=0.012) compared to bone or non-liver visceral metastases and can be detected in high-risk adenocarcinomas though expression is heterogeneous. Conversely, CTCs from mCRPC with liver metastases had lower TACSTD2 expression (p=0.039). Prior ARPI therapy associated with lower TACSTD2 (p=0.025) and a trend toward higher DLL3 in patients with early evidence of lineage transition. Conclusions: We report differential cell surface target expression across lineage states and disease sites using novel CTC RNA-seq and single CTC phenotyping. Altered target expression was observed in PC lineage transitions. These findings have implications for treatment response towards surface targeted therapies, and future studies will utilize these biomarkers to explore mechanisms of response and resistance.