5049 Background: Disease progression following novel hormonal therapy in CRPC remains bone-dominant and D-responsive. D + Ra223 would be a logical combination but myelosuppression is dose-limiting. Prior combinations have required a reduction in dose intensity (DI) in both agents. Fractionated schedules of D Q2weekly (DQ2) have comparable activity to D 75mg/m 2 Q3weekly with mitigated myelosuppression. We hypothesized that a fractionated schedule of DQ2 with standard Ra223 dosing would be feasible while preserving DI. Methods: Subjects had progressive bone-metastatic CRPC, ECOG PS 0-2, and no bulky visceral disease. Design was phase I, 6+6, dose escalation plus expansion with 28-day cycles. DQ2 was given in a 4-week lead-in, then with Ra223 every 4 weeks. Dose-level (DL) 1: D 40mg/m 2 ; if neutropenia, then 1a: D 40mg/m 2 with G-CSF on Day 16, 2a: D 50mg/m 2 with G-CSF on Day 16. Up to 6 cycles of the combination were given. Primary objective was the feasibility and maximum tolerated DL explored (MTD). Secondary objectives included PSA response, ORR, PFS, OS, and quality of life. Results: 43 patients (pts) enrolled including 34.9% non-white pts and 76.7% with ≥ 4 bony mets. 8 dropped out during the D lead in (1 each neutropenia, stroke, failure to thrive, anorexia/fatigue, thrombocytopenia, infusion reaction and 2 hospitalized for other reasons). At DL1, 2 of 3 had DLT (both neutropenia). No DLT occurred at DL1a (n=5) or DL2a (n=5). MTD was DL2a. 22 subjects enrolled to expansion cohort at DL2a. Of the 35 pts treated with D + Ra223, adverse events of interest listed in the table. No febrile neutropenia, fractures, or G5 toxicity were seen. 19/35 completed all 6 cycles of combination therapy. PSA50 response was seen in 18 (51.4%) and PSA90 in 9 (25.7%) pts. Of 31 pts with evaluable data, best response (RECIST 1.1) was 1 CR, 5 PR, 23 SD, 2 PD. Median PFS was 50.0 weeks (95% CI: 37.3-86.1) and OS was 86.1 weeks (95% CI: 60.0-130.9). 10 pts progressed exclusively in nodal/visceral metastases compared to 6 in bone only. Quality of life measures remained stable on study. Conclusions: Utilizing a D lead-in strategy, combination DQ2 and Ra223 was feasible and well tolerated, with a favorable PFS and evidence of preferential control of osseous metastases. Dose-intensity for both docetaxel and Ra223 was preserved and comparable to the FDA-approved dose-schedules for each of the single agents. The lead-in design and use of G-CSF contributes significantly to the feasibility. Clinical trial information: NCT03737370 . Table: see text