LAUSR

233Background: Ra-223, a targeted alpha therapy, extended survival and had a favorable safety profile at 3 years’ follow up in pts with mCRPC in the pivotal phase 3 ALSYMPCA trial. The maturing global, prospective, single-arm, observational REASSURE study, designed to evaluate long-term safety at 7 years’ follow up, enrolled pts with mCRPC with bone metastases planned to receive Ra-223. Methods: We performed a descriptive analysis of safety and BL characteristics of US pts according to prior or concomitant abiraterone/enzalutamide (abi/enza) or prior docetaxel/cabazitaxel (chemo) using data from the first planned interim analysis (pts receiving ≥1 Ra-223 dose; median follow up 8 mo). Results: 244 US pts were included; 80% had no prior chemo. Prior abi/enza and/or chemo pts had higher median BL PSA and were less likely to complete 5-6 doses (Table). Subgroups had similar median ALP, LDH and Hb. Overall, drug-related tx-emergent AEs occurred in 71 pts (29%) and serious AEs in 9 (3.7%). Most common AEs were ...