MP87-01 DO PRIMARY HORMONAL THERAPY OUTCOMES PREDICT SUBSEQUENT RESPONSE TO ABIRATERONE OR ENZALUTAMIDE IN METASTATIC CASTRATION-RESISTANT PROSTATE CANCER?

Abstract

INTRODUCTION AND OBJECTIVES: Abiraterone and enzalutamide are two novel androgen-receptor (AR)-targeting therapies that improve survival in men with metastatic castration-resistant prostate cancer (mCRPC). Factors that predict abiraterone and enzalutamide response are lacking. The objective of this study was to determine if prostate-specific antigen (PSA) outcomes on primary androgen deprivation therapy (ADT) could predict outcomes on subsequent novel ARtargeting therapies. METHODS: A multicenter retrospective review was performed to identify mCRPC patients who had received abiraterone or enzalutamide. Eligible patients must have received novel AR-targeting therapy for at least 8 weeks and must have been evaluable for response by PSA criteria. The clinical outcome measures evaluated included prostatespecific antigen (PSA) decline, biochemical progression-free survival (bPFS), and overall survival (OS) after initiating novel hormone therapy. Cox regression models were used to analyze the effects of primary therapy response and other clinicopathologic factors on novel hormone therapy outcomes. Survival probabilities were plotted using the KaplanMeier method and differences assessed with the log-rank test. RESULTS: The population consisted of 80 males who received abiraterone (N1⁄451) or enzalutamide (N1⁄429). The median age at diagnosis was 64.6 years, and the median age at start of novel ADT was 73.2 years. A subset of patients also received docetaxel treatment prior to start of novel ADT (N1⁄427). The median bPFS on primary ADT was 1.9 years (range 0.1-15.9 years) with a median PSA decline of 99%. bPFS on primary ADT showed a significant univariate effect on bPFS and OS after initiating novel ADT (p1⁄40.032 and p1⁄40.028, respectively). Patients who progressed on primary ADT prior to 1 year had a median bPFS on novel AR-targeting therapy of 3.4 months, compared to a median bPFS of 7.6 months and 8.1 months for patients whose time to PSA progression on primary ADT was 1 and < 5 years (p1⁄40.008), and 5 years (p1⁄40.026), respectively. CONCLUSIONS: In patients with mCRPC treated with novel ADT, time to PSA progression on primary ADT had a significant effect on time to progression on novel ADT in the univariate analysis. Patients who progressed on primary ADT within the first year of treatment had shorter bPFS on subsequent AR-targeting therapy. These potential predictive factors couldhelp guideclinical decisionswhen treatingmCRPCpatients.