HGG-06. Combinatorial targeting of avapritinib-driven MAPK activation in pediatric high-grade glioma

Abstract PDGFRA is a frequently altered gene in pHGG, driving aggressive behavior and worse prognoses. Avapritinib, a potent CNS-penetrant PDGFRA inhibitor, has shown promise in vitro, in vivo, and in pHGG patients. Given the failure of single-agent trials in targeting PDGFRA-altered HGG, combinatorial therapy is likely needed with other targetable pathways for treatment. 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. These results demonstrated that supraphysiological doses of =1uM avapritinib treatment of PDGFRA-altered pHGG cells in vitro results in sustained activation of the MAPK pathway. Specifically, short-term avapritinib treatment with =1uM doses resulted in MEK/ERK (MEK2) and MEK/JNK (MKK4/7) activation, and long-term treatment resulted in sustained MEK/ERK (MEK2) activation across all models. Dose-dependent pERK upregulation in response to avapritinib was confirmed in multiple pHGG in vitro and in vivo models. Single-cell RNA-seq 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 short-term avapritinib-treated pHGG cells in vitro. Combinatorial treatment of pHGG models with MEK (trametinib), ERK (ulixertinib) and integrated stress/ERK inhibitors (ONC201, ONC206) in vitro eradicated pERK activity. Trametinib demonstrated the strongest combinatorial survival benefit among preliminary results in PDGFRA-driven pHGG models in vivo. We subsequently showed in vitro synergy between avapritinib and trametinib in a tumor-derived organoid from a pediatric patient with PDGFRA D842V-mutant metastatic CNS sarcoma that grew on avapritinib. This patient was later treated with this combination and demonstrated stability for five months. Combinatorial therapy with avapritinib and a MEK inhibitor was performed in four additional PDGFRA-driven pHGG patients. In light of sustained MAPK activation identified in our study, dual avapritinib-MAPK targeted treatment may be an effective approach for PDGFRA-driven pHGG.