REGN2810 in Pediatric Patients With Relapsed, Refractory Solid, or Central Nervous System (CNS) Tumors and Safety and Efficacy of REGN2810 in Combination With Radiotherapy in Pediatric Patients With Newly Diagnosed or Recurrent Glioma

Phase 1: * To confirm the safety and anticipated recommended phase 2 dose (RP2D) of REGN2810 (cemiplimab) for children with recurrent or refractory solid or Central Nervous System (CNS) tumors * To characterize the pharmacokinetics (PK) of REGN2810 given in children with recurrent or refractory solid or CNS tumors Phase 2 (Efficacy Phase): * To confirm the safety and anticipated RP2D of REGN2810 to be given concomitantly with conventionally fractionated or hypofractionated radiation among patients with newly diagnosed diffuse intrinsic pontine glioma (DIPG) * To confirm the safety and anticipated RP2D of REGN2810 given concomitantly with conventionally fractionated or hypofractionated radiation among patients with newly diagnosed high-grade glioma (HGG) * To confirm the safety and anticipated RP2D of REGN2810 given concomitantly with re-irradiation in patients with recurrent HGG * To assess PK of REGN2810 in pediatric patients with newly diagnosed DIPG, newly diagnosed HGG, or recurrent HGG when given in combination with radiation * To assess anti-tumor activity of REGN2810 in combination with radiation in improving overall survival at 12 months (OS12) among patients with newly diagnosed DIPG * To assess anti-tumor activity of REGN2810 in combination with radiation in improving progression-free survival at 12 months (PFS12) among patients with newly diagnosed HGG * To assess anti-tumor activity of REGN2810 in combination with radiation in improving overall survival at OS12 among patients with recurrent HGG