Concomitant characterization of androgen receptor (AC) and immune checkpoints (ICs) in cell-free (cf) DNA and RNA from patients with metastatic castration resistant prostate cancer (mCRPC).

286Background: Immunotherapy (IO) appears to benefit a subset of patients (pt) with mCPRC, with recent data showing disease control > 6 mo in 11% of patients (De Bono et al ASCO 2018). We interrogate cfDNA/cfRNA from pt with mCRPC to ascertain relationships between AR and IC markers, and offer preliminary correlation with response to IC inhibitors (ICIs). Methods: Pt in this series were on active therapy for mCRPC and received cfDNA/cfRNA assessment for 26 analytes using the CLIA-certified NantHealth LiquidGPS platform during the course of routine clinical care. For this assay, 10 mL of blood were separately collected in DNA and RNA stabilizing solutions, respectively. DNA was amplified via mutant allele-specific blocker or allele-specific PCR assays. Wild type (WT) DNA was PCR amplified in an area where mutations had not been detected, and allele fractions were determined by the ratio of mutant to WT alleles. RNA was reverse transcribed to cDNA using random hexamer primers and amplified via gene specific...