Phase 2 Study of the Safety and Antitumor Activity of Apalutamide (ARN-509), a Potent Androgen Receptor Antagonist, in the High-risk Nonmetastatic Castration-resistant Prostate Cancer Cohort

Background Apalutamide is a potent androgen receptor (AR) antagonist that targets the AR ligand-binding domain and prevents AR nuclear translocation, DNA binding, and transcription of AR gene targets. Objective To evaluate the activity and safety of apalutamide in patients with high-risk nonmetastatic castration-resistant prostate cancer (nmCRPC). Design, setting, and participants We conducted a multicenter phase 2 study of nmCRPC patients with a high risk for progression (prostate-specific antigen [PSA] ≥8 ng/ml or PSA doubling time [PSA DT] ≤10 mo). Intervention Patients received 240 mg/d apalutamide while continuing on androgen-deprivation therapy. Outcome measurements and statistical analysis Primary end point was 12-wk PSA response (Prostate Cancer Working Group 2 criteria). Secondary end points included safety, time to PSA progression (TTPP), and metastasis-free survival (MFS). Results and limitations A total of 51 patients were enrolled; four patients with metastatic disease were excluded from the efficacy analysis. Patient characteristics included median age, 71 yr; Eastern Cooperative Oncology Group performance status 0 (76%); Gleason score ≤7 (57%); median PSA 10.7 ng/ml; and PSA DT ≤10 mo (45%). At median follow-up of 28.0 mo, 18 patients (35%) remained in the study. Overall, 89% of patients had ≥50% PSA decline at 12 wk. Median TTPP was 24.0 mo (95% confidence interval [CI], 16.3 mo–not reached [NR]); median MFS was NR (95% CI, 33.4 mo–NR). Most of the patients discontinued study treatment (n = 33) due to disease progression (n = 11 [22%]) or adverse events (AEs) (n = 9 [18%]). The most common AE was fatigue (any grade, n = 31 [61%]) although grade ≥3 fatigue was uncommon (n = 2 [4%]). These represent the first apalutamide nmCRPC patient clinical data. Conclusions In high-risk nmCRPC patients, apalutamide was safe with robust activity based on durable PSA responses and disease control. Patient summary Antitumor activity and the safety of apalutamide in patients with nonmetastatic castration-resistant prostate cancer support continued development in this setting. Trial registration ClinicalTrials.gov