Background: ONC201 is a DRD2 antagonist in Phase II trials for cancers that exhibit dysregulation of the dopamine pathway. We previously reported an objective response in the first recurrent H3 K27M midline glioma patient who received ONC201 and that H3 K27M gliomas exhibit enhanced sensitivity to the compound in vitro. Here, we report the clinical experience with ONC201 to date in adult and pediatric H3 K27M glioma. Methods: As of February 1, 2018, 14 patients with H3 K27M glioma have received single agent ONC201 and had at least one post-treatment MRI. This includes 9 adult patients (8 glioblastoma, 1 diffuse midline glioma) and 5 pediatric patients (3 DIPG, 2 other diffuse midline gliomas). All patients had recurrent disease, except for 2 DIPG patients who had completed radiotherapy. Seven patients were enrolled on a clinical trial and seven were enrolled on compassionate use protocols. ONC201 was orally administered at 625 mg to adult patients and scaled based on body weight for pediatric patients. All but one patient were dosed once a week. Results: Six out of the 14 patients remain on therapy with a median follow up of 5.4 months (range: 2.9-22.6) with durable radiographic and/or clinical stability or improvement. This includes the first responder who has now been on treatment for 22 months with a 96% overall regression. All 6 patients who experienced benefit had 1-2 prior lines of therapy, whereas 5 patients who had at least 3 prior lines of therapy did not experience benefit. Among the five patients with thalamic glioma, two experienced complete regressions of their thalamic lesions while another underwent a 30% regression. One patient with previously-irradiated DIPG that exhibited a 300nM IC50 ex vivo has experienced improvements in hemiparesis and other disease-related symptoms. Another previously-irradiated DIPG patient has experienced improvements in her facial palsy and other disease-related symptoms, along with a 40% regression. Conclusions: Preliminary clinical data indicates ONC201 induces durable radiographic regressions and clinical benefit in a subset of patients with H3 K27M glioma. The clinical activity of ONC201 in pediatric and adult H3 K27M gliomas is being evaluated in ongoing clinical trials. Clinical trial information: NCT03295396, NCT02525692, NCT03416530