Background: ONC201 is a first-in-class DRD2 antagonist and ClpP agonist that has demonstrated promising activity in high-grade glioma preclinical models and radiographic regressions with single agent ONC201 in recurrent H3 K27M-mutant glioma patients . The recommended phase 2 dose (RP2D) of 625mg ONC201 orally once a week has been established in adult patients as well tolerated and biologically active. ONC201 efficacy has been shown in high-grade glioma preclinical models and radiographic regressions with single agent ONC201 have been reported in adult recurrent H3 K27M-mutant glioma patients. We report results from the first Phase I pediatric clinical trial of ONC201. Methods: This open-label, multi-center trial for pediatric H3 K27M-mutant glioma or non-biopsied DIPG employed a 3+3 dose-escalation and dose-expansion design with 6 arms. Arms A and E, which have completed accrual, determined the RP2D of ONC201 using oral capsule and liquid formulations in post-radiation pediatric H3 K27M-mutant glioma patients ONC201, respectively. Arm B aims to determine the RP2D for ONC201 in combination with radiotherapy in patients with newly diagnosed DIPG. Arms C and D aim to measure intratumoral ONC201 concentrations in midline glioma patients and the impact of ONC201 on H3 K27M DNA levels in CSF, respectively. Arm F was recently opened to study ONC201 as a single agent in patients with progressive H3 K27M-mutant tumors (excluding DIPG and spinal cord tumors) following radiotherapy. After determining the RP2D, a dose-expansion cohort will evaluate the safety, radiographic response, and activity of ONC201. Results: An RP2D of weekly 625mg ONC201 scaled by body weight as a capsule or in liquid formulation was established in the primary endpoints of arms A, B and E alone or in combination with radiation, without incidence of dose-limiting toxicity (DLT). Pharmacokinetic profiles were similar to those observed in adults (T_1/2: 8.4h; T_max: 2.1h; C_max: 2.3ug/mL; AUC_0-tlast: 16.4ug/mL), with similar exposure across body weights. Conclusions: ONC201 was well tolerated without DLTs at the same adult RP2D scaled by body weight as monotherapy or in combination with radiotherapy in pediatric H3 K27M-mutant glioma patients. Further investigation of ONC201 to treat H3 K27M-mutant glioma and DIPG is warranted. Clinical trial information: NCT03416530.