Background: In study COU-AA-302 of men with mCRPC and no prior chemotherapy, AA plus prednisone (hereafter AA) significantly increased radiographic progression-free survival. There is limited information about response to subsequent AS-directed therapies following AA. In this post hoc analysis of pts who received AA during study COU-AA-302, we evaluated clinical response to subsequent AA or enzalutamide (ENZ). Methods: In COU-AA-302, 546 pts were randomized and received AA. Subsequent response and discontinuation data from 88 pts receiving AS-directed therapy after study were collected retrospectively, source verified, and entered into the database. Median time to prostate-specific antigen (PSA) progression with 95% confidence intervals was estimated using the Kaplan-Meier method. Results: As of May 2013, following AA on study, 55 pts received subsequent AA and 33 received subsequent ENZ. 69% (38/55) of pts in the AA then AA group and 67% (22/33) pts in the AA then ENZ group received intervening chemotherapy. Baseline patient characteristics were similar across both groups and to the overall COU-AA-302 population. Median (range) exposure to subsequent therapy was 4 (1-20) months for AA and 5 (1-12) months for ENZ. Response to subsequent AA or ENZ is summarized in the Table. Conclusions: In this post hoc analysis, pts previously treated with AA experienced modest clinical response on subsequent treatment with either AA or ENZ. These data support further studies of subsequent AS-targeted drugs following treatment with AA for mCRPC. Clinical trial information: NCT00887198.

^a More than one reason allowed.