Background: Insulin-like growth factor receptor-1 (IGF-1R) signaling activates the PI3K/AKT pathway and may lead to androgen receptor (AR) transactivation and progression to endocrine treatment resistance. Xe, an IGF-ligand-neutralizing antibody, binds to IGF-1 and IGF-2 and inhibits IGF-1R signaling. This multi-center randomized phase II trial (NCT02204072) evaluated anti-tumor activity of Xe plus En in mCRPC. Methods: Men with histologically/cytologically confirmed mCRPC and progression after DCt+Abi were randomized to receive Xe 1000mg IV QW + En 160mg/day oral, or En alone (28-day cycles until progression or intolerable adverse events [AEs]). Primary endpoint: progression-free survival by investigator assessment (PFS-IA); secondary: PFS by central review (PFS-CR), overall survival (OS), AEs. Results: Overall, 43 patients were randomized per arm; 70% Caucasian and 29% Asian (median age 70 y; range 46–88). At baseline (BL) the two arms were generally well balanced, although 33% v 47% were ECOG PS O, and 72% v 56% had a Gleason total score ≥8. By data cut-off (23 October 2017), 39/43 (Xe+En) and 38/43 patients (En) had discontinued, most due to disease progression. The median PFS-IA was 7.4 m for Xe+En (95% CI: 3.5–8.7) and 6.2 m for En (3.5–11.1) [HR = 0.99 (0.56–1.73); p = 0.96]. The results were similar after adjusting for BL ECOG PS and Gleason score. The median PFS-CR was 3.6 m for Xe+En (3.5–8.1) and 6.2 m for En (3.6–8.3) (HR = 1.22 [0.70–2.13]; p = 0.48). OS data are immature. For the two arms, prostate-specific antigen (PSA) response rates were 21% and 19%; maximum decline in PSA: -20 v -9 μg/L; PSA change at week 12: 19% v 18%; maximum decline in circulating tumor cells (CTC): -52% v -35%; and CTC response: 16% v 11%. The most frequently reported AEs were: fatigue 67% v 49%; decreased appetite 56% v 54%; weight reduction 37% v 12%; anemia 33% v 44%; back pain 30% v 37%. Nine patients discontinued Xe due to AEs. Conclusions: Addition of Xe to En did not prolong PFS in mCRPC compared with En alone. There were no notable differences in PSA-related endpoints and CTC between arms. Clinical trial information: NCT02204072