Background: In ALSYMPCA, the first-in-class α-emitter Ra-223 had a highly favorable safety profile and was well tolerated. Safety monitoring of Ra-223 is essential for a complete safety profile. Here are final safety data including long-term follow-up safety 3 years after last pt’s first injection (inj). Methods: Pts received 6 inj and entered designated follow-up from 4 wks after their last inj to 3 years after first inj. Pts were to be evaluated during tx period and 9 follow-up visits. All adverse events (AEs) were collected until 12 wks after last inj; thereafter, only AEs deemed tx-related were collected. Additional long-term safety data were assessed by specific diseases including acute myelogenous leukemia (AML), myelodysplastic syndrome (MDS), aplastic anemia, and new primary cancer in bone or other organs. Results: Safety population (pts receiving ≥ 1 inj) included 901 pts (Ra-223, n = 600; placebo [pbo], n = 301); 572 pts (Ra-223, n = 405; pbo, n = 167) entered follow-up. 60 pts (Ra-223, n = 49; pbo, n = 11) completed all follow-up visits. Overall, 564 (94%) Ra-223 and 292 (97%) pbo pts had ≥ 1 tx-emergent AE (Table). During long-term follow-up, there were no reports of AML, MDS, or new primary bone cancer. New primary cancers in other organs were reported: 2 Ra-223 (1 bladder, 1 lymph node mets), 3 pbo (2 skin, 1 adenocarcinoma rectum and sigmoideum), and 2 pbo cross-over pts (1 skin, 1 meningioma). Aplastic anemia was reported in 1 Ra-223 pt. Conclusions: Ra-223 remained safe and well tolerated 3 years after the last pt’s first inj. No major safety issues were identified during the ALSYMPCA 3-year long-term follow-up. Clinical trial information: NCT00699751

*AEs that started on or after first inj up to 12 wks after last inj.