Background: Neuroblastoma (NB) is the commonest extracranial solid tumor in children. At diagnosis, > 50% have high-risk (HR) disease with poor prognosis despite intensive therapy; thus, there is a high unmet medical need. Naxitamab is an anti-GD2 monoclonal antibody indicated under accelerated approval in the US with GM-CSF for the treatment of relapsed/refractory HR NB in the bone and/or bone marrow in patients (pts) ≥1 year of age with a partial response, minor response, or stable disease to prior therapy. Irinotecan and temozolomide (IT) are commonly used salvage chemotherapies following failure of frontline therapy. The objective of this study is to investigate the efficacy and safety of naxitamab plus IT in pts with HR NB with primary refractory disease or first relapse. Methods: Trial 203 (NCT04560166) is an international, single-arm, multicenter phase 2 trial for HR NB patients ≥12 mo of age, with either evaluable primary refractory disease or first relapse following induction and consolidation therapy. The primary endpoint of the study is the centrally-assessed overall response rate (ORR) (complete [CR] or partial response) at or before completion of 4 cycles of treatment. Key secondary endpoints include ORR after 2 cycles of treatment, duration of response (DoR), CR rate, time to first subsequent therapy or death, progression-free survival (PFS), and overall survival (OS). 52 pts will be enrolled to provide 82% power to demonstrate that response rate is significantly higher than 20%. Key inclusion criteria are documented HR NB at time of diagnosis; received frontline induction/consolidation therapy; active disease despite previous aggressive chemotherapy; and measurable tumor (CT/MRI) that is either MIBG-avid/PET-positive or have MIBG scan with uptake at ≥1 site. Key exclusion criteria include prior treatment with duration ≤3 wks (chemotherapy), ≤6wks (autologous stem cell transplant or therapeutic ¹³¹I-MIBG), ≤4wks (radiation therapy), or progressed while on anti-GD2; residual NB in the bone marrow only; and CNS/leptomeningeal disease in the prior 6 mo. In addition, there should not be < 50% performance status per Lansky or Karnosky scale and life expectancy < 6 mo. Pts will receive irinotecan 50 mg/m²/day IV and temozolomide 100 mg/m²/day orally (both on Days 1-5) in combination with naxitamab 2.25 mg/kg/day IV (Days 2, 4, 8 and 10) and GM-CSF 250 ug/m2/day sc (Days 6-10) in 21 days cycle. Response rates will be assessed using the 2-sided binomial test at the 5% significance level; DoR, PFS, and OS will be estimated using Kaplan-Meier analysis. Adverse events will be graded per CTCAE. Clinical trial information: NCT04560166.