Ra223 is a life-prolonging alpha-emitter bone targeted therapy for mCRPC patients with bone metastases. However, evidence on biomarkers that may help us in patient selection are lacking. Total ALP (tALP) appeared to be a potential marker of Ra223 effect in early studies (Sartor, Ann Oncol, 2017). Other bone-related markers, as bone-specific ALP (BALP), have demonstrated its prognostic value in mCRPC patients with bone metastases (Fizazi K, Eur Urol, 2015; Lara PN, J Natl Cancer Inst, 2014). PRORADIUM (NCT022925702) is a prospective multicentre cohort study in mCRPC patients treated with Ra223. The primary aim was to assess the impact of baseline serum biomarkers of bone formation (BALP and C-terminal of type 1 collagen propeptide CICP) on overall survival (OS). Secondary aims include the correlation of progression-free survival (PFS), time to PSA progression (TTPP) and skeletal-related events free-survival (SRE-FS) with serum bone markers. 142 out of 168 pts enrolled in the study were included in this preliminary analysis. Median age was 74 yrs, 85.5% pts had ECOG 0-1, 52% pts completed 5-6 cycles of Ra223. Higher baseline levels of BALP and CICP were associated to number of metastases in bone-scan (p=0.007 and p=0.016, respectively) and baseline pain (p=0.014 and p=0.050, respectively). After a median follow-up of 18 months, 91 deaths were observed, with a median OS of 11.5 months (95%CI: 9.1-13.9). Patients with baseline BALP and CICP values above the median showed a trend to shorter TTPP (BALP: 2.8 vs 3.1 m, p=0.016; CICP: 2.8 vs 3.0 m, p=0.091) and PFS (BALP: 4.4 vs 5.1 m, p=0.070; CICP: 4.2 vs 5.5 m, p=0.281), respectively. The elevation of bone markers above the median was significantly associated with worse OS (BALP: 8.8 vs 17.9 m, p<0.001; CICP: 9.4 vs 16.8 m, p=0.001). There were not associations found with SRE-FS. Our results suggest that baseline serum markers of bone formation may serve as biomarkers for prognosis in mCRPC patients treated with Ra233. NCT022925702. IBIMA and CNIO. Bayer, CRIS Cancer Foundation, Grant from Instituto de Salud Carlos III (PI16/01565). N. Romero Laorden: Honoraria (self), Travel / Accommodation / Expenses: Bayer, Astellas Pharma, Janssen-Cilag, Sanofi-Aventis, PharmaMar, MSD, Roche. R. Lozano Mejorada: Honoraria (self): Roche, Janssen-Cilag, Bayer; Research grant / Funding (institution): Bayer, Janssen-Cilag; Travel / Accommodation / Expenses: Roche, Janssen-Cilag, Astellas Pharma. E. Almagro Casado: Honoraria (self): MSD; Travel / Accommodation / Expenses: BMS. E. Gonzalez Billalabeitia: Travel / Accommodation / Expenses: BMS, Pfizer, Janssen-Cilag, Astellas Pharma, Sanofi. A. Montesa: Advisory / Consultancy: Janssen-Cilag, Pfizer, Sanofi, Astellas Pharma; Travel / Accommodation / Expenses: Pfizer. G.A. De Velasco Oria de Rueda: Honoraria (self), Advisory / Consultancy: Pfizer, Novartis, Ipsen, Astellas Pharma, BMS, Bayer, MSD, Roche. R. Morales Barrera: Honoraria (self): MSD, Sanofi, AstraZeneca, Janssen, Roche/Genentech; Advisory / Consultancy: MSD, Sanofi, AstraZeneca, Janssen, Roche/Genentech; Speaker Bureau / Expert testimony: MSD, Sanofi, AstraZeneca, Asofarma, Janssen; Research grant / Funding (institution): AB Science, Aragon Pharmaceuticals, INC, Astellas Pharma, AstraZeneca, Aveo Pharmaceuticals, Bayer, Blueprint Medicines Corporation, BN Immunotherapeutics, Boehringer Ingelheim España, BMS, Clovis Oncology, Cougar Technology, Deciphera Pharmaceuticals LL; Travel / Accommodation / Expenses: MSD, Roche, Lilly, Clovis Oncology, Bayer, Janssen-Cilag, Astellas Pharma, AstraZeneca. D. Lorente: Honoraria (self): Janssen-Cilag, Bayer, Astellas Pharma, Sanofi; Advisory / Consultancy: Janssen-Cilag, Bayer, Sanofi; Travel / Accommodation / Expenses: Sanofi, Astellas, Janssen-Cilag, Celgene. E. Castro: Honoraria (self): Astellas Pharma, Janssen-Cilag, AstraZeneca, Bayer, Pfizer; Advisory / Consultancy: Bayer, Janssen-Cilag; Research grant / Funding (institution): Janssen-Cilag, AstraZeneca, Bayer, Genentech, Roche, Pfizer, Astellas Medivation, Tokai Pharmaceuticals; Travel / Accommodation / Expenses: Bayer, Janssen-Cilag, Roche, Astellas Pharma. D. Olmos Hidalgo: Honoraria (self): Janssen-Cilag, Bayer, Sanofi; Advisory / Consultancy: Bayer, Janssen-Cilag, AstraZeneca, Clovis Oncology; Research grant / Funding (institution): Bayer, Janssen, AstraZeneca, Roche/Genentech, Medivation/Pfizer, Astellas, Tokai, MSD, GSK; Travel / Accommodation / Expenses: Bayer, Janssen-Cilag, Ipsen. All other authors have declared no conflicts of interest.