Clinical activity of enzalutamide against metastatic castration-resistant prostate cancer (mCRPC) in patients who have progressed on abiraterone acetate: The Princess Margaret experience.
159 Background: In men with mCRPC enzalutamide and abiraterone acetate have been shown individually to prolong survival after progression on chemotherapy with docetaxel. Little is known about the sequential use of enzalutamide and abiraterone. A PSA response rate of 8% and progression free survival (PFS) of 2.7 months has been reported for 38 men who received abiraterone following enzalutamide [Loriot, Ann Onc, 2013]. Here we report our experience with enzalutamide following abiraterone. Methods: We reviewed all patients with mCRPC treated with enzalutamide following abiraterone and docetaxel at our institution. Primary outcomes were PSA response rate (confirmed decline ≥ 50%) and time to treatment failure (TTF, defined as the time from treatment initiation to treatment discontinuation for any reason, including disease progression, treatment toxicity, patient preference, or death.). Data were analyzed using the Kaplan-Meier method. Results: Twenty-six patients were treated between August 2012 and July 2013. Median age was 72 years (range 56-88); 85% had ECOG performance status 0 or 1; metastatic sites included bone (N=25, 96%), lymph nodes (N=19, 73%) and visceral (N=1, 3.4%). Median number of prior cycles of docetaxel was 8 (range 1 – 12), 6 patients (29%) had previous exposure to ketoconazole, and median duration of previous abiraterone was 8.7 (range 1.4-22.7) months. Seven pts (27%) had a PSA response ≥ 50% and an additional 7 patients (27%) had a ≥ 30% PSA response. Median TTF on enzalutamide was 4.9 (95% CI 3.8-6.2) months. Reasons for discontinuation of enzalutamide were clinical and/or biochemical progression in 24 patients (92%), and toxicity (fatigue grade 3-4) in 3 patients (11.5%). An update of patient numbers and analysis will be presented at the meeting. Conclusions: Treatment of patients with mCRPC with enzalutamide after progression on docetaxel and abiraterone has modest clinical activity.