Background: Almost half of pts with NB present with high-risk disease, most of whom experience persistence of disease or disease progression. Further, residual disease in the bone/bone marrow (BM) can drive relapse. NAX is a humanized GD2-binding monoclonal antibody indicated under accelerated approval in the US with GM-CSF for the treatment of R/R HR-NB in the bone/BM in pts ≥1 year of age with a partial response (PR), minor response (MR), or stable disease (SD) to prior therapy. Here we characterize the response data of NAX overall and efficacy in pt subgroups. Methods: Trial 201 is an ongoing registrational phase II trial evaluating NAX in pts with R/R HR-NB. Pts with progressive or residual soft tissue disease were excluded. NAX was administered over 30-60 min in the outpatient setting on Days 1/3/5 at 3 mg/kg/day with GM-CSF at 250 µg/m²/day on Days –4 to 0 and 500 µg/m²/day on Days 1 to 5; cycles were every 4 wks. Efficacy was evaluated by independent pathology and radiology review per revised International Neuroblastoma Response Criteria. Results: Results presented here are from an ad hoc interim analysis (Aug 5, 2020) of 48 pts (safety population) and 36 (efficacy population). Pts in the efficacy population received a median of 7 cycles (range 1–11). Overall response rate (ORR; complete response [CR] + PR) was 58% (44% CR; Table). Of the 9 responders who had PR initially, 4 improved to CR. Median number of cycles before onset of response (CR or PR) was 2 (range 2–8) and median time to onset of response (CR or PR) was 7.3 wks (95% CI 6–17). Median duration of response was 24.9 wks (95% CI 25–not estimable). The most common adverse events (CTCAE graded) were infusion-related reactions including hypotension and pain, which occurred in 98% and 96% of pts, respectively. Adverse events were managed with established guidelines. Conclusions: NAX provided a clinically meaningful response in pts with R/R HR-NB limited to bone/BM – frequent sites of residual disease. Response rates were generally consistent across subgroups. These efficacy results combined with a manageable safety profile and use in the outpatient setting demonstrate that NAX addresses a significant unmet medical need. Clinical trial information: NCT03363373.

^aIncomplete response (SD, MR, or PR) in bone/BM to initial therapy. ^bIncomplete response in bone/BM to therapy for actively progressing or relapsed disease. ^cGroups not mutually exclusive, ie, pts with bone disease may also have BM disease and vice versa.