TMET-02. CLINICAL EFFICACY OF ONC201 IN H3K27M-MUTANT DIFFUSE MIDLINE GLIOMAS IS DRIVEN BY DISRUPTION OF INTEGRATED METABOLIC AND EPIGENETIC PATHWAYS

H3K27M-mutant diffuse midline glioma (DMG) patients have no proven effective therapies beyond radiation. ONC201, a mitochondrial protease ClpP agonist, has recently demonstrated efficacy in these patients, but the mechanism behind this remains unknown. We assessed clinical outcomes, tumor sequencing, and tissue samples from patients treated in two completed multi-site clinical studies (n=71). Patients treated with ONC201 monotherapy following initial radiation but prior to recurrence (n=35) demonstrated a median overall survival of 21.7 months and a median progression-free survival of 12.2 months. Radiographic response was associated with increased expression of key tricarboxylic acid cycle-related genes in baseline tumor sequencing. Similar survival benefits were observed using in utero electroporation and orthotopic mouse models of H3K27M-DMG (median 107 versus 77 days [p=0.02] and median 97 versus 141 days [p=0.004], respectively). Integrated transcriptomic and metabolomic analysis of ONC201-treated H3K27M-DMG cells revealed downregulation of TCA cycle, glycolysis, and pyruvate metabolism genes, most notably oxoglutarate dehydrogenase (OGDH), and concomitant changes in levels of associated metabolites including ɑ-ketoglutarate (ɑ-KG). Interestingly, L-2-hydroxyglutarate (L-2HG), a known inhibitor of the Jumonji C domain family of histone lysine demethylases, was also increased. This corresponded with increases in repressive H3K27me3 in vitro and in human tumors accompanied by epigenetic downregulation of cell cycle regulation and neuro-glial differentiation genes. Genetic knockdown of OGDH recapitulated ONC201-mediated increases in H3K27me3. In contrast, genetic knockdown of ClpP or lactate dehydrogenase A or overexpression of L-2-hydroxyglutarate dehydrogenase (L-2HGDH), but not D-2HGDH, abrogated ONC201-mediated increases in H3K27me3, demonstrating that ONC201 induces the production of L-2HG in H3K27M-DMG cells leading to increased H3K27me3. Overall, our data demonstrates the efficacy of ONC201 in H3K27M-mutant DMG and supports ONC201 as the first monotherapy to improve outcomes in patients with H3K27M-mutant DMG for whom few therapeutic options currently exist. Mechanistically, ONC201 disrupts integrated metabolic and epigenetic pathways and reverses pathognomonic H3K27me3 reduction.