Background: Survival for high-risk neuroblastoma (NBL) remains poor with over 50% of patients experiencing disease relapse. The addition of dinutuximab (DIN) and granulocyte macrophage colony-stimulating factor (GM-CSF) to irinotecan (IRN) and temozolomide (TMZ) in patients with first relapse achieved an overall response rate of 42% in the study ANBL1221¹ and is now standard of care. However, there remains a significant unmet need to improve outcomes for children with relapsed/refractory NBL. Deregulation of the Cyclin D/cyclin-dependent kinase (CDK) 4 & 6 pathway has been observed in NBL, rendering CDK4 & 6 inhibition logical to explore. Abemaciclib is an orally administered, selective CDK4 & 6 inhibitor approved to treat breast cancer. JPCS (NCT04238819) is designed to assess the pharmacokinetics, safety, and tolerability of abemaciclib added to standard treatment regimens for relapsed/refractory pediatric solid tumors. Part A defined the recommended phase 2 dose (RP2D) of abemaciclib in combination with IRN and TMZ. In Part C, the addition of DIN + GM-CSF to the JPCS Part A regimen will be evaluated. The co-primary objectives of Part C are to determine the RP2D and to estimate the anti-tumor activity of abemaciclib added to the ANBL1221 chemoimmunotherapy backbone in patients with first relapse/refractory NBL. Methods: JPCS Part C is a two-stage, non-randomized, open-label, Phase 1b/2 study for patients < 21 years of age with first relapse/refractory NBL. Stage 1 aims to identify the maximum tolerated dose (MTD) of abemaciclib at a starting dose of 55 mg/m² combined with DIN, GM-CSF, IRN and TMZ using a 3+3 design. One dose de-escalation (30 mg/m²) will be possible if the starting dose is not tolerated. Further tolerability and initial anti-tumor activity will be evaluated in an expansion cohort at the MTD. Up to 11 evaluable patients will enroll in Stage 1 at the MTD; the MTD will be declared the RP2D if adequate safety and tolerability are observed and ≥6 patients achieve minimal response or better per International Neuroblastoma Response Criteria (INRC). Following determination of the RP2D, Stage 2 will enroll 35 patients. If ≥26 of 46 total patients achieve an INRC minimum response or better, further investigation of this regimen could be warranted. As of 31 Jan 2023, Part C is open for enrollment at one site (US) and additional sites are being evaluated. References: Mody et al. J Clin Oncol. 2020;38(19):2160-2169. Clinical trial information: NCT04238819.