TMOD-03. GAINING A MECHANISTIC UNDERSTANDING OF THERAPY EVASION FROM DUAL MAPK PATHWAY INHIBITION IN A SYNGENEIC BRAFV600E MUTANT CDKN2A DELETED MOUSE MODEL TO PREEMPT RESISTANCE IN PATIENTS WITH BRAFV600E MUTANT PEDIATRIC GLIOMA

Abstract Pediatric patients with gliomas expressing the constitutively active BRAFV600E-mutant kinase are candidates for clinical trials for small molecule inhibitors specific for BRAFV600E (BRAFi) and MEK (MEKi) (Grossauer et al; Oncotarget 2016, 15:75839). Clinical results from combined BRAF and MEK inhibition (BRAFi+ MEKi) showed promising results with overall response rate of 43% in patients with recurrent high-grade gliomas and younger than 40 years. We previously observed that BRAFi increases intratumoral heterogeneity brought on by glioma stem cells in BRAFV600E-mutant glioblastoma xenografts (Lerner et al; Cancer Res 2015, 75:5355). These data point at glioma stem cells as a culprit in therapy resistance to molecular targeted therapy. Here, we aimed to understand the mechanisms of therapy evasion by studying changes in glioma stem cells and the tumor immune microenvironment in response to dual BRAFi + MEKi our syngeneic, BRAFV600E-mutant CDKN2A-deleted orthotopic mouse model (Grossauer et al; Oncotarget 2016, 15:75839). We found that while BRAFi increases the frequency of glioma stem cells, dual BRAFi+MEKi reduced glioma stem cells frequency, and failed to completely eliminate these malignant cells. BRAFi altered the tumor microenvironment, by increasing the frequency of proliferative cells that express a checkpoint inhibitor molecule. Analyses are ongoing of mechanisms of adaptation by cancer stem cells and of potential immunomodulatory effects of BRAFi + MEKi by single-cell mass spectrometry (CyTOF). Results from these analyses are expected to reveal changes in the tumor stem cell compartment and its immune microenvironment that will inform about mechanisms of therapy evasion to dual MAPK pathway inhibition. Our preclinical studies indicate a survival benefit of combined immune checkpoint inhibition and dual BRAFi + MEKi for animal subjects carrying orthotopic BRAFV600E mutant gliomas. Completion of these studies will bring us one step closer to overcome therapy resistance in patients with BRAFV600E-mutant CDKN2A-deleted glioma.