Early assessment of PSA response in patients with mCRPC treated with enzalutamide and abiraterone.

e574Background: Abiraterone (Abi) and enzaleutamide (Enza) are first-line agents for the treatment of metastatic castrate-resistant prostate cancer (mCRPC). Primary resistance is well-documented, but little data exists for rapid treatment responders. This study intended to further characterize patients with early prostate-specific antigen (PSA) decline. Methods: A single-institution retrospective review was performed on 123 mCRPC patients treated with Abi and/or Enza. PSA was recorded every 4 weeks for the duration of treatment. The primary endpoint was to describe PSA response, including sensitivities and specificities, as a predictor of later treatment response (defined as ≥50% decrease in PSA from baseline). Additional clinical covariates were also evaluated as treatment-response predictors. Results: A PSA response to Abi was achieved in 52/123 (42%) of patients. Median time to PSA nadir was 37 days. 30/52 (58%) patients responded to the drug within 4 weeks. Median length of time on drug was 110 days. ...