Circulating Tumor Cell (Ctc) Enumeration In Men With Metastatic Castration-Resistant Prostate Cancer (Mcrpc) Treated With Enzalutamide Post-Chemotherapy (Phase 3 Affirm Study).

5035 Background: CTC enumeration is a biomarker associated with clinical outcomes in patients (pts) with mCRPC. CTC count at baseline (BL) and post-treatment is prognostic for survival. Enzalutamide (ENZA) was approved for post-chemotherapy CRPC based on the overall survival (OS) benefit (AFFIRM trial). In this trial, CTC enumeration was embedded as a biomarker endpoint. Methods: AFFIRM was a phase 3 study (N = 1199; NCT00974311) randomizing men who received ≤ 2 docetaxel-based chemotherapy regimens to ENZA 160 mg/day or placebo (PBO) (2:1). CTC samples were collected at sites selected for logistics and investigator interest. Conversion rate was defined as a decline from ≥ 5 (unfavorable) to u003c 5 (favorable) CTCs/7.5 mL of blood from BL to best post-BL result. Results: BL CTCs were obtained from 447 of 1199 pts at 89 of 156 study sites; BL demographics and disease history were similar to pts in whom samples were not collected. 382 pts had both BL and post-BL counts; OS, rPFS and select disease characterist...