Background: Ra-223 is a first-in-class alpha-pharmaceutical targeting bone metastases (mets) with high-energy, short-range (< 100 μm) alpha-particles. In the phase 3, double-blind, randomized, multinational ALSYMPCA study, Ra-223 significantly improved OS in CRPC patients (pts) with bone mets by a median increase of 3.6 months compared with placebo (median OS: 14.9 vs 11.3 mo; P < 0.001; HR = 0.69; 95% CI: 0.58-0.83). An updated analysis of the SRE secondary endpoint is presented. Methods: Eligible pts had progressive, symptomatic CRPC with ≥ 2 bone mets on scintigraphy and no known visceral mets; were receiving best standard of care (BSoC); and either previously received docetaxel, or did not because they were docetaxel ineligible, or refused docetaxel. Pts were randomized 2:1 to receive 6 injections of Ra‑223 (50 kBq/kg IV) q4wk or matching placebo and stratified by prior docetaxel use, baseline alkaline phosphatase level, and current bisphosphonate use. Results: 921 pts were randomized (Ra-223, n = 614; placebo, n = 307); 40% had >20 mets. Ra-223 significantly delayed time to first SRE versus placebo by a median increase of 5.8 months (median time to SRE: 15.6 vs 9.8 mo; P < 0.001; HR = 0.66; 95% CI: 0.52-0.83). Ra-223 also reduced the risk of time to first event for all 4 SRE components versus placebo. Conclusions: Ra-223 significantly delayed time to first SRE with a reduction in risk observed for all 4 SRE components. Despite the longer time at risk, Ra-223 patients had an approximately 50% reduction in risk for SCC. Ra-223 is an effective therapy with a highly favorable safety profile and may provide a new standard of care for treatment of CRPC pts with bone mets. Clinical trial information: NCT00699751.

*Not adjusted for multiplicity.