Background: Diffuse midline gliomas, H3 K27M-mutant are associated with a poor prognosis compared to H3 wild-type gliomas and have no effective therapy following first-line radiation. ONC201 is a bitopic DRD2 antagonist and allosteric ClpP agonist that has shown encouraging single agent efficacy in recurrent H3 K27M-mutant gliomas located in various midline structures of the brain. In addition to tumor and immune cells, the pharmacodynamics of ONC201 extend to stromal cells that can mediate a bystander antitumor response in preclinical models. Given this observation and that the thalamus has the highest extrastriatal expression of DRD2, we report the clinical experience of ONC201 in a subgroup of H3 K27M-mutant glioma patients with primary tumors located in the thalamus. Methods: We analyzed 29 thalamic H3 K27M-mutant glioma patients treated with ONC201 in clinical trials enrolled as of 5/22/19. Nineteen enrolled with recurrent disease whereas 10 enrolled following radiation prior to recurrence. Twelve patients enrolled on NCT03295396, 10 NCT03416530, 4 NCT02525692, and 3 expanded access. Median age was 22 years old (range: 5-70) and baseline KPS was 80 (range: 60-90). Median time from radiation to start of ONC201 was 1.8 months (range: 0.2-8.7) for non-recurrent patients and 7.2 months (range: 1.4-102.0) for recurrent patients. Results: As of 12/18/2019, PFS6 and OS12 measured relative to initiation of ONC201 are 26.3% and 36.8%, respectively, in the recurrent group. For patients initiating ONC201 post-radiation prior to recurrence, median PFS or OS have not been reached with a median follow up of 21.9 months (8.6-26.6) from diagnosis, which surpass historical OS of 13.5 months. Best response for evaluable recurrent patients by RANO: 1 CR, 3 PR, 4 SD, 8 PD, 3 not reported; for non-recurrent patients: 2 PR, 4 SD, 1 PD, 3 not reported. Median duration of response for recurrent patients is 14.0 months (2.0-33.1). ONC201 was well tolerated and no dose-limiting toxicities or treatment discontinuations due to toxicity occurred. Furthermore, H3 K27M cell-free tumor DNA in plasma and CSF correlated with MRI response. Conclusions: In summary, single agent ONC201 administered at recurrence or following radiation, demonstrates promising clinical efficacy in thalamic H3 K27M-mutant glioma patients. Investigations are ongoing to assess whether micro-environmental DRD2 expression correlates with responses of thalamic H3 K27M-mutant glioma to ONC201. Clinical trial information: NCT03295396, NCT03416530, NCT02525692.