Abstract Background Ra223 is a bone-seeking a-emitter targeted therapy that induces double-strand DNA breaks (DSBs). The HR pathway is used to accurately repair DSBs and a significant proportion of mCRPC patients harbor pathogenic germline HR mutations (gHR+). We hypothesize that gHR+ patients, with an impaired ability to repair DSBs, are more likely to respond to Ra223 than gHR- patients. Methods This is an exploratory preplanned analysis of the PRORADIUM study (NCT02925702). PRORADIUM is a prospective observational biomarkers study of mCRPC patients treated with standard-of-care Ra223. Participants were screened for germline mutation in DNA damage and response genes using a multigene panel (Castro et al. J Clin Oncol 2019). Alkaline Phosphatase (ALP) response at 12 weeks was the primary endpoint. PSA response at 12 weeks, overall survival (OS) from Ra223 and clinical and/or radiographic progression free survival (PFS) were also analyzed. Results 161 out of 168 patients enrolled in PRORADIUM with available germline testing results were included. A pathogenic gHR mutation was identified in 14 (8.7%) patients (5 BRCA2, 4 ATM, 1 BRCA1, 1 BRCA1+CHEK2, 1 BRIP1, 1 NBN, 1 BLM). Median age was 74 years (range 46-90). Performance status ECOG 1 was recorded in 91% pts. Median number of prior therapy lines for mCRPC was 2 (range 1-4) including at least one taxane in 63% patients. 54% of patients received 5 or 6 cycles of Ra223. After a median follow-up of 20 months, a > 30% decline in ALP at 12 weeks was observed in 75% of gHR+ patients compared to 43% of gHR- (p = 0.036). No differences in PSA >50% decline at 12 weeks (14% vs 8%, p = 0.437) or in PFS (4.5 vs 5.0 months, p = 0.670) were observed by gHR status. A trend towards more prolonged OS from Ra223 was observed in gHR + (median 14.4 vs 10.6 months, p = 0.066). Conclusions In our study, germline HR mutations were associated with improved ALP responses, suggesting a greater benefit from Ra223 in gHR+ patients. Clinical trial identification NCT02925702. Legal entity responsible for the study The authors. Funding Bayer. Disclosure E. Castro: Honoraria (self), Travel / Accommodation / Expenses: Astellas; Honoraria (self), Advisory / Consultancy, Research grant / Funding (institution), Travel / Accommodation / Expenses: Bayer; Honoraria (self), Advisory / Consultancy, Research grant / Funding (institution): AstraZeneca; Honoraria (self), Advisory / Consultancy, Research grant / Funding (institution), Travel / Accommodation / Expenses: Janssen; Travel / Accommodation / Expenses: Roche; Honoraria (self): Pfizer. R. Lozano Mejorada: Honoraria (self), Travel / Accommodation / Expenses: Roche; Honoraria (self), Travel / Accommodation / Expenses: Janssen. M. Saez: Honoraria (self), Advisory / Consultancy, Travel / Accommodation / Expenses: Bristol-Myers Squibb; Honoraria (self), Advisory / Consultancy, Travel / Accommodation / Expenses: Pfizer; Honoraria (self), Advisory / Consultancy, Travel / Accommodation / Expenses: Novartis; Honoraria (self), Advisory / Consultancy, Travel / Accommodation / Expenses: Bayer; Honoraria (self), Advisory / Consultancy, Travel / Accommodation / Expenses: Janssen; Honoraria (self), Advisory / Consultancy, Travel / Accommodation / Expenses: Sanofi; Honoraria (self), Advisory / Consultancy, Travel / Accommodation / Expenses: Astellas; Honoraria (self), Advisory / Consultancy, Travel / Accommodation / Expenses: Ipsen; Honoraria (self), Advisory / Consultancy, Travel / Accommodation / Expenses: EUSA pharma. U.F.F. De Giorgi: Research grant / Funding (institution): AstraZeneca; Research grant / Funding (institution): Roche; Advisory / Consultancy, Research grant / Funding (institution): Sanofi; Advisory / Consultancy: Astellas; Advisory / Consultancy: Bayer; Advisory / Consultancy: Bristol-Myers-Squibb; Advisory / Consultancy: Ipsen; Advisory / Consultancy: Janssen; Advisory / Consultancy: Merck; Advisory / Consultancy: Pfizer. N. Romero Laorden: Advisory / Consultancy: Bayer; Honoraria (self), Travel / Accommodation / Expenses: Pfizer; Honoraria (self): Astellas; Honoraria (self): Sanofi; Honoraria (self), Travel / Accommodation / Expenses: Janssen; Honoraria (self), Travel / Accommodation / Expenses: Roche; Honoraria (self): PharmaMar. G.A. De Velasco Oria de Rueda: Honoraria (self): Pfizer; Honoraria (self): Novartis; Honoraria (self): Ipsen; Honoraria (self): Astellas; Honoraria (self): Bristol-Myers-Squibb; Honoraria (self): Bayer; Honoraria (self): Roche. J. Puente: Advisory / Consultancy, Research grant / Funding (self), Travel / Accommodation / Expenses: Pfizer; Advisory / Consultancy, Research grant / Funding (self): Astellas; Advisory / Consultancy, Travel / Accommodation / Expenses: Janssen; Advisory / Consultancy: Merck; Advisory / Consultancy: Bayer; Advisory / Consultancy, Travel / Accommodation / Expenses: Roche; Advisory / Consultancy, Travel / Accommodation / Expenses: Bristol-Myers-Squibb; Advisory / Consultancy: AstraZeneca; Advisory / Consultancy: Boehringer Ingelheim; Advisory / Consultancy: Clovis; Advisory / Consultancy: Ipsen; Advisory / Consultancy: Eisai; Advisory / Consultancy: EUSA Pharma; Advisory / Consultancy: Sanofi; Speaker Bureau / Expert testimony: Pierre Fabre; Speaker Bureau / Expert testimony: Celgene; Speaker Bureau / Expert testimony: Kiowa; Speaker Bureau / Expert testimony: Novartis; Speaker Bureau / Expert testimony: Lilly; Advisory / Consultancy, Research grant / Funding (self), Travel / Accommodation / Expenses: Pfizer. A. González del Alba: Advisory / Consultancy, Speaker Bureau / Expert testimony: Astellas; Advisory / Consultancy: Janssen; Advisory / Consultancy: Sanofi; Advisory / Consultancy: Bayer; Advisory / Consultancy, Travel / Accommodation / Expenses: Pfizer; Advisory / Consultancy, Speaker Bureau / Expert testimony, Travel / Accommodation / Expenses: Ipsen; Advisory / Consultancy, Speaker Bureau / Expert testimony, Travel / Accommodation / Expenses: Roche; Advisory / Consultancy: Pierre Fabre; Advisory / Consultancy: Novartis; Advisory / Consultancy: Eusa Pharma; Advisory / Consultancy: Eisai; Advisory / Consultancy, Travel / Accommodation / Expenses: Bristol-Myers-Squibb; Speaker Bureau / Expert testimony, Travel / Accommodation / Expenses: MSD. P. Borrega Garcia: Advisory / Consultancy: Janssen; Advisory / Consultancy: Pfizer; Advisory / Consultancy: Bayer; Advisory / Consultancy: Sanofi. V. Conteduca: Honoraria (self), Travel / Accommodation / Expenses: Astellas; Honoraria (self), Travel / Accommodation / Expenses: Janssen; Honoraria (self), Travel / Accommodation / Expenses: Sanofi; Advisory / Consultancy: Bayer. A. Rodriguez-Vida: Advisory / Consultancy, Speaker Bureau / Expert testimony, Research grant / Funding (self): MSD; Advisory / Consultancy, Speaker Bureau / Expert testimony, Research grant / Funding (self): Pfizer; Advisory / Consultancy, Speaker Bureau / Expert testimony: Bayer; Advisory / Consultancy: Bristol-Myers-Squibb; Advisory / Consultancy, Speaker Bureau / Expert testimony: Astellas; Advisory / Consultancy, Speaker Bureau / Expert testimony: Janssen; Advisory / Consultancy, Speaker Bureau / Expert testimony: Roche; Speaker Bureau / Expert testimony: MSD; Speaker Bureau / Expert testimony: AstraZeneca; Speaker Bureau / Expert testimony: Sanofi; Research grant / Funding (self): Takeda. A. Medina Colmenero: Honoraria (self), Advisory / Consultancy: Astellas Pharma; Honoraria (self), Advisory / Consultancy: Bristol-Myers Squibb; Honoraria (self): Janssen; Honoraria (self), Advisory / Consultancy: Bayer; Honoraria (self): Roche; Honoraria (self): Pfizer; Honoraria (self): Novartis; Advisory / Consultancy: Sanofi. R. Morales Barrera: Honoraria (self), Advisory / Consultancy, Speaker Bureau / Expert testimony, Travel / Accommodation / Expenses: MSD; Honoraria (self), Advisory / Consultancy, Speaker Bureau / Expert testimony: Sanofi; Honoraria (self), Advisory / Consultancy, Speaker Bureau / Expert testimony, Travel / Accommodation / Expenses: AstraZeneca; Honoraria (self), Advisory / Consultancy, Speaker Bureau / Expert testimony: Asofarma; Honoraria (self), Advisory / Consultancy, Speaker Bureau / Expert testimony, Travel / Accommodation / Expenses: Johnson and Johnson; Honoraria (self), Advisory / Consultancy, Travel / Accommodation / Expenses: Roche; Travel / Accommodation / Expenses: Lilly; Travel / Accommodation / Expenses: Clovis; Travel / Accommodation / Expenses: Bayer; Travel / Accommodation / Expenses: Astellas. D. Lorente: Advisory / Consultancy, Speaker Bureau / Expert testimony, Travel / Accommodation / Expenses: Janssen; Advisory / Consultancy, Speaker Bureau / Expert testimony, Travel / Accommodation / Expenses: Sanofi; Advisory / Consultancy, Speaker Bureau / Expert testimony: Bayer; Travel / Accommodation / Expenses: Celgene. D. Olmos Hidalgo: Advisory / Consultancy, Research grant / Funding (institution), Travel / Accommodation / Expenses: Bayer; Advisory / Consultancy, Speaker Bureau / Expert testimony, Research grant / Funding (institution), Travel / Accommodation / Expenses: Janssen; Speaker Bureau / Expert testimony, Research grant / Funding (institution): Sanofi; Advisory / Consultancy, Research grant / Funding (institution): AstraZeneca; Advisory / Consultancy: Clovis; Research grant / Funding (institution): Roche; Research grant / Funding (institution): Astellas; Research grant / Funding (institution): Tokai; Research grant / Funding (institution): Pfizer. All other authors have declared no conflicts of interest.