RONC-21. IDENTIFICATION OF EPIGENETIC DRUGS AS RADIOSENSITIZERS IN PEDIATRIC HIGH-GRADE GLIOMAS

Pediatric high-grade gliomas (pHGG) are malignant brain tumors with a high mortality rate. Radiotherapy (RT) is one of the cornerstones of current pHGG treatment, while the efficacy of chemotherapeutics remains inferior. The use of chemotherapeutics that specifically sensitize tumor cells to irradiation are poorly understood, but may help to increase the effect of RT in pHGG treatment. Since recent studies revealed pHGG to be epigenetically dysregulated, we tested 148 epigenetic drugs on eight primary pHGG models in the presence and absence of RT, to assess their radiosensitizing potential. Based on synergy scores, we found 22 compounds that resulted in enhanced cytotoxicity in the presence of RT. The effect of these compounds on pHGG was further investigated by tracking spheroid growth microscopically for 30 days, identifying four molecules that stopped spheroid-expansion solely in combination with RT (p=<0.001, multilevel regression). Parallel cell-viability assays reported identical results. Furthermore, tumor migration in 3D matrigel growth assays, using non-toxic doses of the four identified compounds, revealed that two compounds (the selective HDAC-inhibitors; chidamide and entinostat) stop the infiltrative growth characteristics of pHGG cells, exclusively in combination with RT. RNA-Seq data showed that entinostat and chidamide inhibit DNA-repair pathways like the Fanconi anemia cascade and homologous recombination. Since we anticipate that entinostat- or chidamide-induced radiosensitization can be enhanced by blocking kinase-driven escape mechanisms, we are currently conducting a kinome-wide CRISPR/Cas9 knockout screen in three primary pHGG models to develop combinational therapies. These results highlight entinostat and chidamide as potential radiosensitizers in pHGG treatment.