Genomic analysis of circulating cell-free DNA (cfDNA) to investigate mechanisms of resistance to enzalutamide (ENZ) in metastatic castration-resistant prostate cancer (mCRPC) patients (pts).

157 Background: Factors driving clinical resistance to ENZ are poorly understood. Genomic analysis of cfDNA is a promising and minimally invasive approach for investigating mechanisms of therapeutic resistance in mCRPC. Methods: Baseline plasma samples were collected from 53 mCRPC pts commencing ENZ. In 31 of these pts, 12-week and treatment cessation samples were also obtained. DNA was extracted and subjected to array Comparative Genomic Hybridization (aCGH) for chromosome copy number (CN) analysis and androgen receptor (AR) gene sequencing (MiSeq) for mutation analysis. Endpoints were i) PSA50 or PSA30 response rates (RR) (PSA decline ≥ 50% or 30% for ≥ 3 weeks); and ii) radiographic/clinical progression-free survival (PFS). Results: CN changes on baseline aCGH included 8p loss (26%), 8q gain (32%), MYC gain (28%), CCND1 gain/amplification (amp) (11%), MET gain (13%), RB1 loss (21%) and AR gain/amp (30%). Correlation of clinical and genomic data showed multiple links between AR CN status and outcomes on...