Abstract BACKGROUND Despite the availability of targeted therapies, gliomas carrying the BRAFV600E mutation remain challenging to treat. Combined pharmacologic BRAFV600E and MEK inhibition is clinically used in patients with BRAFV600E mutant gliomas, but primary and acquired resistance is reported in ~70% of high-grade glioma patients. OBJECTIVES To gain mechanistic insights into therapy adaptation and escape for developing combination therapies that enhance clinical effects of combined BRAFV600E inhibition with Dabrafenib and MEK inhibition with Trametinib (BRAFi+MEKi) in high-grade gliomas. APPROACH Patients’ pre- and post-treatment glioma tissues, patient-derived and mouse cell lines, and two novel immunocompetent mouse models for BRAFV600E mutant high-grade gliomas were analyzed for the effect of BRAFi+MEKi using a range of techniques including bulk and single-cell RNA sequencing, immunofluorescence staining, flow cytometry, and mass cytometry. RESULTS We present new data showing that the BRAFi+MEKi combination significantly reduced glioma cell viability while inducing glial differentiation programs, potentially associated with programmed death receptor (PD-1) signaling. This suggests a profound shift in the glioma cell differentiation state that is linked to an inclination toward T cell inhibition. This prompted us to combine BRAFi+MEKi with immune checkpoint inhibitors targeting PD-L1 and CTLA-4, which significantly decreased T cell inhibition. Furthermore, from mouse studies, this combination therapy conferred a survival benefit over single BRAFi+MEKi combination therapy alone. Additionally, BRAFi+MEKi treatment triggered an interferon-gamma response, indicative of cytotoxicity, and enhanced the expression of MHC molecules associated with antigen presentation, only in drug-sensitive but not drug-resistant gliomas. The lack of improved therapeutic efficacy of immune checkpoint blockade in athymic mice, which lack T cells, emphasized the critical role of T cell activity in driving the success of the combination treatment. CLINICAL IMPACT Our preclinical findings highlight the potential of concurrent BRAFi+MEKi treatment and immune checkpoint blockade for overcoming therapy resistance in BRAF mutant high-grade glioma patients.

CNSC-25. CONCURRENT IMMUNE CHECKPOINT BLOCKADE ENHANCES THE SURVIVAL BENEFIT OF CLINICALLY RELEVANT MAPK PATHWAY INHIBITORS IN A T CELL-DEPENDENT MANNER