Synergistic activity of mTORC1/2 kinase and MEK inhibitors suppresses pediatric low-grade glioma tumorigenicity and vascularity

Background. Pediatric low-grade glioma (pLGG) is the most common childhood brain tumor. Many patients with unresectable or recurrent/refractory tumors have significant lifelong disability. The majority of pLGG have mutations increasing the activity of the Ras/mitogen-activated protein kinase (MAPK) pathway. Activation of mammalian target of rapamycin (mTOR) is also a hallmark of pLGG. We therefore hypothesized that the dual target of rapamycin complexes 1 and 2 (TORC1/2) kinase inhibitor TAK228 would synergize with the mitogen-activated extracellular signal-regulated kinase (MEK) inhibitor trametinib in pLGG. Methods. We tested TAK228 and trametinib in patient-derived pLGG cell lines harboring drivers of pLGG including BRAF(V600E) and neurofibromatosis type 1 loss. We measured cell proliferation, pathway inhibition, cell death, and senescence. Synergy was analyzed via MTS assay using the Chou-Talalay method. In vivo, we tested for overall survival and pathway inhibition and performed immunohistochemistry for proliferation and vascularization. We performed a scratch assay and measured angiogenesis protein activation in human umbilical vein endothelial cells (HUVECs). Results. TAK228 synergized with trametinib in pLGG at clinically relevant doses in all tested cell lines, suppressing proliferation, inducing apoptosis, and causing senescence in a cell line-dependent manner. Combination treatment increased median survival by 70% and reduced tumor volume compared with monotreatment and control cohorts. Vascularization of tumors decreased as measured by CD31 and CD34. Combination treatment blocked activation of focal adhesion kinase (FAK) and sarcoma proto-oncogene non-receptor tyrosine kinase (SRC) in HUVEC cells and reduced HUVEC migration compared with each drug alone. Conclusions. The combination of TAK228 and trametinib synergized to suppress the growth of pLGG. These agents synergized to reduce tumor vascularity and endothelial cell growth and migration by blocking activation of FAK and SRC.