Background: Radium-223 (Ra-223), an α emitter, and docetaxel (D), a chemotherapy, prolong overall survival (OS) in metastatic castration-resistant prostate cancer (CRPC) patients (pts). This study examined safety and efficacy of Ra-223 + D v D alone. Preliminary phase IIa prostate-specific antigen (PSA) and alkaline phosphatase (ALP) data were previously reported (Morris et al. ASCO 2015). All pts have been randomized and have completed treatment. Methods: D-eligible pts with progressing CRPC and ≥ 2 bone metastases were randomized 2:1 to Ra-223 (50 kBq/kg q 6 wk × 5) + D (60 mg/m² q 3 wk × 10) v D (75 mg/m² q 3 wk × 10; step-down option to 60 mg/m²). Primary end point was safety; exploratory efficacy end points were PSA and ALP changes, progression-free survival (PFS), and OS. Results: 53 pts (36 Ra-223 + D; 17 D) were randomized; 33 Ra-223 + D and 13 D pts were treated. For pts in the Ra-223 + D arm, 25 (76%) had all 6 Ra-223 injections, and 20 (61%) had all 10 D cycles. For the D-alone pts, 5 (38%) received all 10 cycles (3 received 75 mg/m², and 2 stepped down to 60 mg/m²). Pts in Ra-223 + D arm had a lower percentage of grade 3/4 treatment-emergent adverse events during treatment (Ra-223 + D, 49%; D, 62%); no Ra-223 + D and 2 (15%) D pts had febrile neutropenia; no pts had clinically significant thrombocytopenia in either arm. Ra-223 + D had longer time to all measures of progression and longer PFS (Table). Waterfall plots of PSA and ALP will be presented. Two Ra-223 + D pts and one D pt died of disease progression during 9-mo follow-up. Conclusions: Ra-223 + D was well tolerated. Ra-223 + D treated pts had longer times to progression and fewer adverse events v pts in D arm. Further studies of Ra-223 + D are planned. Clinical trial information: NCT01106352

*Per PCWG2 ^†Per RECIST and PCWG2