Background: Ra-223 is a targeted alpha therapy that showed a survival advantage and favorable safety profile in the phase 3 ALSYMPCA trial in pts with mCRPC. REASSURE (NCT02141438) is evaluating the long-term safety of Ra-223 in routine clinical practice in pts with mCRPC over a 7-year follow-up period. Methods: In this global, prospective, single-arm, observational study, the second prespecified interim analysis (data cut-off March 2019) evaluated safety and clinical outcomes of Ra-223 in pts with mCRPC. Primary outcome measures were incidence of second primary malignancies (SPM), bone marrow suppression and short- and long-term safety in pts who had ≥1 Ra-223 dose. Secondary outcomes included overall survival (OS). Results: For 1465 pts in the safety analysis, median follow up was 11.5 months. Median PSA (n=1053), ALP (n=1048), and LDH (n=555) levels at baseline were 59 ng/mL, 135 U/L, and 269 U/L, respectively. 81% of pts had bone metastases only at baseline; 19% of pts had other metastatic sites, mostly in the lymph nodes. 19% of pts had <6 metastatic sites, 47% had 6–20 sites, 20% had >20 lesions but not a superscan, and 6% had a superscan. 45%, 38%, 37%, 9%, and 8% of pts received prior abiraterone, docetaxel, enzalutamide, cabazitaxel, or sipuleucel-T as prior therapies, respectively. Median number of Ra-223 doses received was 6; 67% of pts had ≥5 doses. SPM occurred in 1% of pts. The most common treatment-emergent drug-related adverse event (AE) of any grade was diarrhea (11%). 10% of pts had a bone-associated event, 5% had fractures, and 15% had a hematological AE. Median OS was 15.6 months (95% CI 14.6–16.5). Conclusions: In REASSURE, there was a low incidence of SPM, bone fractures, and bone marrow suppression after Ra-223 treatment, with no new AEs identified. This study confirms that in routine clinical practice, Ra-223 AE rates were low, and pts generally received ≥5 doses. Clinical trial information: NCT02141438