Clinical validation of circulating tumor cell synaptophysin and androgen receptor expression in metastatic prostate cancer phenotypes.

5041 Background: Chronic exposure to androgen receptor (AR) targeted therapies can drive lineage plasticity, promoting the emergence of neuroendocrine (NE) prostate cancer (NEPC) and treatment resistance. NEPC is characterized by a phenotypic shift, with loss of AR expression and emergence of markers (e.g., synaptophysin, SYP). To definitively diagnose NEPC, a metastatic biopsy is typically required; however, there are numerous barriers to obtaining a timely tissue diagnosis. As such, there is an unmet need for blood-based assays that can accurately identify treatment-emergent NEPC. In this study, we evaluate concordance between circulating tumor cell (CTC) SYP and AR expression patterns in patients with and without evidence of NE differentiation on tissue histology. Methods: Blood for CTC analysis was prospectively collected from patients with advanced prostate cancer. Patients were retrospectively assigned to either an adenocarcinoma (AR-positive) or NE differentiated (SYP-positive) group based on clinical and patho-genomic factors. Amphicrine PC was included in NE differentiated cohort. The RareCyte system was used to generate slides for CTC-based immunofluorescent (IF) analysis for SYP and AR. Associations between CTC SYP and AR expression, PSA at time of CTC collection, and overall survival (OS) were assessed for patients with ≥ 5 CTCs using Wilcoxon rank-sum test, Fisher’s exact test, and Kaplan-Meier analyses. Results: We enrolled 52 patients with metastatic castration-resistant prostate cancer (mCRPC); 26 were classified as NE differentiated and 26 as AR-positive adenocarcinoma. Median time between biopsy and CTC collection was 10.6 months (IQR 4.1–34.3) and 19.4 months (IQR 3.5–66.4) for the NE differentiated and AR-positive adenocarcinoma cohorts respectively. NE differentiated patients had a significantly higher fraction of SYP-positive CTCs (median 28.6 % vs 0%, p = 0.0084), whereas patients in the adenocarcinoma cohort had a higher fraction of AR-positive CTCs (median 96.7% vs 0.0%, p = 0.0004). Phenotype classification based on high AR or SYP expression ( > 80% of CTCs positive) significantly differentiated cohorts and was strongly associated with tissue histology (p = 0.0048). PSA level at the time of CTC collection was significantly lower in the NE differentiated cohort (median 3.8 vs 486.9 ng/mL, p = 0.0001). There was no difference in OS from time of diagnosis or time of metastasis between groups. Conclusions: SYP and AR CTC expression demonstrated strong concordance with tissue histology and established clinical features of NEPC and AR-positive adenocarcinoma. These early findings support the feasibility of CTC expression profiling to identify NE biomarker expression. However, results are only hypothesis-generating and prospective validation with matched tissue biopsy in a larger cohort with adequate follow up is warranted.