Background: Ra-223, a 1st-in-class alpha-pharmaceutical, targets bone metastases (mets) with high-energy alpha-particles of short range (<100 μm). ALSYMPCA, a phase III, double-blind, randomized, multinational study, compared Ra-223 plus best standard of care (BSC) v placebo (pbo) plus BSC in patients (pts) with bone mets in CRPC. The primary endpoint was OS; secondary endpoints included SREs and ECOG PS. Methods: Eligible pts had progressive, symptomatic CRPC with ≥ 2 bone mets on scintigraphy and no known visceral mets; were receiving BSC; and either previously received docetaxel, were docetaxel ineligible, or refused docetaxel. Pts were randomized 2:1 to 6 injections of Ra‑223 (50 kBq/kg IV) q4 wks or matching pbo and stratified by prior docetaxel use, baseline ALP level, and current bisphosphonate use. Results: 921 pts were randomized from 6/2008-2/2011. In a planned interim analysis (n = 809), Ra-223 significantly improved OS v pbo (median OS 14.0 v 11.2 mo, respectively; two-sided P = .00185; HR = .695; 95% CI, .552-.875).SREs were lower in the Ra-223 v pbo group, and time to 1st SRE was significantly delayed (median time to SRE 13.6 mo v 8.4 mo, respectively; P = .00046; HR = .610; 95% CI, .461-.807). The proportion of pts with ECOG PS deterioration (≥ 2 points) was less in Ra-223 v pbo group at Wk 12 and Wk 24 (4%, 15/389 v 9%, 16/180 and 7%, 16/236 v 12%, 10/83, respectively). Time to ECOG PS deterioration (≥ 2 points) was significantly delayed by Ra-223 v pbo (P = .003; HR = .62; 95% CI, .46-.85). Conclusions: Ra-233 significantly delayed time to 1st SRE and SRE components, notably SCC. Fewer pts in the Ra-223 group had ECOG PS deterioration. Ra-223 improves OS with excellent safety and may provide a new standard of care for CRPC pts with bone mets.

SRE component	No. (%) of patients		Time to 1st event (Ra-223 vs Pbo)
	Ra-223 n = 541	Pbo n = 268	p value*	HR (95%CI)
External beam radiotherapy	122 (23)	72 (27)	.0038	.65 (.48-.87)
Spinal cord compression	17 (3)	16 (6)	.016	.44 (.22-.88)
Pathologic bone fracture	20 (4)	18 (7)	.013	.45 (.24-.86)
Surgical intervention	9 (2)	5 (2)	.69	.80 (.27-2.4)

*Not adjusted for multiplicity.