LAUSR

292 Background: Recent studies have shown that an early PSA response to AR-targeting agents in mCRPC is associated with a better prognosis. We analyzed the early PSA response to enzalutamide (ENZ) by measuring the PSA doubling time (PSADT) and PSA Velocity while monitoring oncologic outcomes and survival in Japanese patients. Methods: A total of 241 patients with mCRPC treated with ENZ were analyzed. Patients’ median age is 75±7.9 (range 53-93). The patients pre-docetaxel settings were 171 cases (71 %), post-docetaxel settings were 70 cases (29 %). The PSA-PFS and OS were assessed according to PCWG2 criteria. This study was approved by the institutional review board of Gunma University Hospital (No.1595). Results: A case where PSA did not decline at all was defined as Primary Resistance (PR). A case in which PSA once declined after treatment but then progressed was defined as Acquired Resistance (AR). Those in which PSA remained low after treatment were defined as Good Response (GR). We observed 77 PR cases (31.9 %), 125 AR cases (51.9 %) and 39 GR cases (16.2 %).PSA-PFS and OS pre-docetaxel were significantly increased as compared to patients’ post-docetaxcel (PSA-PFS; 47.0 wks vs. 13.4 wks p < 0.001, OS; Not Yet Reached vs. 80.7 wks p < 0.001). Multivariate analysis of prognostic factors, including the PSA response at 4 weeks, was performed using a Cox regression analysis. The PS (0 or 1-2), Hb (≧11.4 or < 11.4), time to CRPC(≧12 m or < 12 m), docetaxel treatment history (none or done) and a PSA decrease of 50% at 4 weeks were all significant factors for the prediction of OS (all variables, p < 0.05). In cases of acquired resistance (n = 125), a multivariate analysis using PSA kinetics factors such as PSADT and PSA Velocity (ng/mL/month) at PSA progression, Hb, time to CRPC(≧12 m or < 12 m), PSADT (≧2 months or < 2 months) and PSA Velocity ( < 20 ng/mL/month or≧20 ng/mL/month), were all factors predicting OS following PSA progression (p < 0.05). Conclusions: Our study has demonstrated that PSA dynamics after ENZ administration may be a useful prognostication factor for mCRPC patients.