In the present study, we aimed to evaluate the association of circulating AR copy number (CN) and outcome in a cohort of patients with advanced castration-resistant prostate cancer (CRPC) treated with enzalutamide after docetaxel. Fifty-nine CRPC patients were evaluated. AR CN was analyzed with real-time and digital PCR in the serum collected at starting of treatment. Progressive disease was defined on the basis of Prostate Cancer Working Group 2 criteria. AR CN gain was found in 21 of 59 (36%) patients. Median baseline PSA, alkaline phosphatase and lactate dehydrogenase levels were higher in the AR CN gained group (p = 0.007, p = 0.003, p = 0.0009, respectively). Median PFS of patients with AR CN gain was 2.4 (95%CI: 1.9-3.2) vs. 4.0 months (95%CI: 3.0-6.5) of those with no gain (p = 0.0004). Median OS of patients with AR CN gain was 6.1 (95%CI: 3.4-8.6) vs. 14.1 months (95%CI: 8.2-20.5) of those with no gain (p = 0.0003). At multivariate analysis, PSA decline ≥ 50% and AR CN showed a significant association with PFS (p = 0.008 and p = 0.002, respectively) and OS (p = 0.009 and p = 0.001, respectively). These findings indicate that the detection of circulating AR CN gain is a promising non-invasive biomarker for outcome prediction to enzalutamide treatment in CRPC patients
