Antitumour Activity and Safety of Enzalutamide in Patients with Metastatic Castration-resistant Prostate Cancer Previously Treated with Abiraterone Acetate Plus Prednisone for ≥24 weeks in Europe

Background: Enzalutamide and abiraterone acetate plus prednisone, which target the androgen receptor axis, have expanded the treatment of advanced prostate cancer. Retrospective analyses suggest some cross-resistance between these two drugs when used sequentially, but robust, prospective studies have not yet been reported. Objective: To fulfil a regulatory postregistration commitment by evaluating the efficacy and safety of enzalutamide in patients with metastatic castration-resistant prostate cancer (mCRPC) who progressed following abiraterone acetate plus prednisone treatment. Design, setting, and participants: Multicentre, single-arm, open-label study, enrolled patients with progressing mCRPC after ≥24 wk of abiraterone acetate plus prednisone treatment. All patients maintained castration therapy during the trial. Prior chemotherapy was allowed but not required. Intervention: Patients received enzalutamide 160 mg/d orally. Outcome measurements and statistical analysis: The primary endpoint was radiographic progression-free survival. Secondary endpoints were overall survival, prostate-specific antigen (PSA) response, and time-to-PSA progression. Safety data were also assessed. Kaplan-Meier methods were used to descriptively analyse time-to-event endpoints. Results and limitations: Overall, 214 patients received enzalutamide treatment, 145 of whom were chemotherapy-naïve. Median radiographic progression-free survival was 8.1 mo (95% confidence interval: 6.1–8.3); median overall survival had not been reached. Unconfirmed PSA response rate was 27% (48 of 181). Median time-to-PSA progression was 5.7 mo (95% confidence interval: 5.6–5.8). The most common treatment-emergent adverse events were fatigue (32%), decreased appetite (25%), asthenia (18%), back pain (17%), and arthralgia (16%). No seizures were reported. Conclusions: Enzalutamide showed antitumour activity in some patients with mCRPC who had previously progressed following ≥24 wk of abiraterone acetate plus prednisone treatment. Patient summary: Patients with mCRPC who progressed on previous abiraterone acetate plus prednisone treatment, with or without prior chemotherapy, received enzalutamide. Although cross-resistance between the two agents was observed in a majority of patients, some still benefited from enzalutamide treatment. In this prospective, single-arm study, enzalutamide showed antitumour activity in some patients with metastatic castration-resistant prostate cancer who had previously progressed following at least 24 wk of treatment with abiraterone acetate. Adverse events were consistent with the established safety profile of enzalutamide.