Background: NB is the most common extracranial solid tumor in children and half of patients present with high-risk disease. Bone and BM are frequent sites of metastatic disease and can serve as a reservoir for residual disease driving relapse. Naxitamab, a GD2-binding monoclonal antibody, was recently approved in the United States in combination with GM-CSF for the treatment of pediatric patients ≥1 year of age and adult patients with R/R HR-NB in the bone/BM who have demonstrated a partial response (PR), minor response (MR), or stable disease (SD) to prior therapy. Here we describe outcomes from the registrational Trial 201 (NCT03363373) detailed by bone/BM involvement. Methods: HR-NB patients with primary refractory disease or incomplete response to salvage treatment following relapse or progressive disease (PD) (in both cases including SD, MR and PR) with disease limited to bone and/or BM were eligible. Naxitamab was administered over ≥30 min in the outpatient setting on Days 1, 3 and 5 at 3 mg/kg/infusion (9 mg/kg/cycle) in combination with GM-CSF at 250 µg/m2/day on Days -4 to 0 and at 500 µg/m2/day on days 1 to 5. Treatment cycles were repeated every 4 weeks. Response was assessed after Cycle 2 and then every 2-3 months by revised International Neuroblastoma Response Criteria (INRC) using BM biopsies/aspirates and 123I-MIBG scintigraphy or FDG-PET. Effectiveness was concluded if the lower limit of the Clopper-Pearson exact 95% confidence interval (CI) of overall response rate (ORR) was >20%. We report efficacy data on 22 patients and safety data on the first 25 patients enrolled. Results: 13 (59%) patients had NB in bone, 2 (9%) had NB in BM, and 7 (32%) had NB in both bone and BM. Summary of overall response and response by compartment. Conclusions: Naxitamab provided clinically meaningful activity in both bone and BM with ORR of 68% and had a manageable AE profile. Clinical trial information: NCT03363373

^*2/20 (10%) patients had PR, 2/20 (10%) had SD, and 2/20 (10%) had PD. ^**1/9 (11%) patient had minimal disease and 1/9 (11%) was not evaluable. ^a INRC denotes that osseous lesions without soft tissue mass are considered non-measurable by RECIST and do not require confirmatory assessment of response Median time to first CR was 5.6 weeks (range 5.3-29.3). 5/25 (20%) patients reported 6 naxitamab-related serious adverse events (SAEs): 4 anaphylactic reaction, 1 pyrexia, 1 respiratory depression. 3/25 (12%) patients discontinued treatment due to naxitamab-related Grade 4 AE: 2 anaphylactic reaction, 1 respiratory depression. No fatal events were reported.