DIPG-80. CLINICAL AND RADIOGRAPHIC RESPONSE TO ONC201 IN A PEDIATRIC PATIENT WITH A THALAMIC H3K27M AND BRAFV600E MUTANT DIFFUSE MIDLINE HIGH GRADE GLIOMA

Recent improved understanding of the molecular markers of high grade glioma has shifted the approach to these aggressive CNS tumors to increasingly use molecularly guided targeted therapies. Treatment of patients with BRAFV600E mutant high grade gliomas with BRAF inhibitors has shown efficacy, however the impact of concomitant H3K27M mutation is unknown. ONC201 targets dopamine receptor D2 (DRD2), which is shown to be broadly overexpressed in the thalamus as well as multiple tumor types; its antagonism has demonstrated anti-tumor efficacy and immunomodulatory properties in preclinical studies. ONC201 has also demonstrated clinical efficacy in patients with H3K27M mutant gliomas. We present the case of a 9-year-old male with a right thalamic H3.3K27M mutant diffuse midline glioma with a concomitant BRAFV600E mutation with an ongoing partial response to ONC201 treatment. The patient was diagnosed in May 2018. He underwent biopsy, followed by standard focal proton radiation therapy (54Gy) and subsequent treatment with dasatinib, bevacizumab and everolimus over the course of five months. After continued radiographic progression on serial imaging, in April 2019 he started ONC201 375mg orally once per week through an expanded access trial. He has tolerated the medication well with grade 1 nausea and fatigue. Over the next nine months, he demonstrated clinical and radiographic improvement with modest increased use of his left side and MRIs showing progressive decrease in size of the thalamic lesion with a 70 % decrease in the target lesion (measuring 53x62mm prior to treatment, decreased to 38x26mm in January 2020).