Objective
To clarify risk factors for PSA progression in patients who received enzalutamide for the treatment of castration resistant prostate cancer (CRPC).

Materials and Methods
We retrospectively reviewed clinical records of CRPC patients who initiated enzalutamide treatment between Nov/2011 and Dec/2017 in our institution. PSA progression was defined as the date that a 25% or greater increase and an absolute increase of 2 ng/ml or more from the nadir. Risk factors for PSA progression were analyzed by a Cox regression model.

Results
Seventy CRPC patients received enzalutamide for the treatment of CRPC. Their median age was 73 (56-88) and the median PSA level was 14.7 ng/ml (1.0-3087.2 ng/ml). The metastatic site was bone (61.4%), lymph node (10.0%), and viscera (11.4%). Sixteen patients had no metastasic foci on imaging at initiation of enzalutamide. Twenty-nine patients had prior docetaxel treatment and 11 patients had prior abiraterone treatment.
PSA decline was observed in 53 patients (77.9%) and mean PSA reduction rate was 54.8±9.8% (95% confidence interval [CI]).The median PSA progression free period was 7months.Response duration of primary androgen deprivation therapy (ADT) less than 20 months (hazard ratio [HR] 2.70, 95% CI 1.47-5.23), prior docetaxel use (HR 3.05, 95% CI 1.62-5.74), prior abiraterone use (HR 2.88, 95% CI 1.20-6.21), presence of metastasis at the initiation of enzalutamide (HR 3.00, 95% CI 1.40-7.43), Neutrophil-lymphocyte ratio ≧3 (HR 1.98, 95% CI 1.49-6.67), and simultaneous steroid use (HR 4.69, 95% CI 2.46-9.90) revealed to be risk factors for PSA progression by univariate analysis. By multivariate analysis, response duration of primary ADT less than 20 months (HR 3.13, 95% CI 1.49-6.67) and simultaneous steroid use (HR 7.73, 95% CI 10.30-30.41) were independent risk factors for PSA progression.

Summary
Duration of primary ADT and simultaneous steroid use were risk factors for PSA progression in the treatment of enzalutamide for CRPC.