Combinatorial Targeting of Avapritinib-driven Mapk Activation in Pediatric High-grade Glioma

PDGFRA is a frequently altered gene in pHGG, driving aggressive behavior and worse prognoses. Avapritinib, a CNS-penetrant inhibitor of mutated PDGFRα and KIT kinases, has shown promise in vitro, in vivo, and in pHGG patients. Single-agent trials in pHGG often fail due to disease heterogeneity and resistance development; for greater chances of success, rational combinatorial therapy will be needed. We performed a high-throughput kinase-activity mapping (HT-KAM) screen to detect the catalytic activity of >900 kinase-substrate nodes in our pHGG models. Supraphysiological doses of 1uM avapritinib treatment of PDGFRA-altered pHGG cells in vitro resulted in sustained activation of the MAPK pathway. Dose-dependent pERK upregulation in response to avapritinib was confirmed in multiple pHGG in vitro and in vivo models from 100 nM-10 µM. Single-cell RNAseq analysis of avapritinib-treated pHGG tumors in vivo demonstrated that cycling OPC-like cells were primarily responsible for increased expression of MAPK pathway genes. Furthermore, upregulation of the ERK-driven anti-apoptotic protein MCL-1 was found in avapritinib-treated pHGG cells in vitro, and MCL-1 knockout increased sensitivity to avapritinib. Combinatorial treatment of pHGG models with MEK (selumetinib, trametinib), ERK (ulixertinib) and integrated stress/ERK inhibitors (ONC201, ONC206) in vitro eradicated pERK activity. MEK inhibitors demonstrated the strongest knockdown of MAPK activity via HT-KAM assessment in vitro with combinatorial survival benefit in vivo. A tumor organoid model from a patient with PDGFRα D842V-mutant CNS tumor with progression on avapritinib showed synergistic benefit with trametinib treatment. Based on these data, we treated a cohort of five brain tumor patients with avapritinib and a MEK inhibitor (selumetinib or trametinib). A patient with an un-resected PDGFRα-mutant pHGG treated with avapritinib and selumetinib prior to progression demonstrated complete and ongoing tumor regression (12 months+). Considering sustained MAPK activation identified in our study, dual avapritinib-MAPK targeted treatment may be an effective approach for PDGFRα-driven pHGG.