STEM-15. THERAPY-INDUCED CHANGES BY BRAF AND MEK INHIBITORS IN BRAF V600E-MUTATED GLIOMA MODELS PROVIDE POTENTIAL NOVEL THERAPEUTIC OPPORTUNITIES

Abstract

Abstract Combinations of the MEK inhibitor trametinib, and BRAF inhibitor dabrafenib (BRAFi+MEKi) show rapid and sustained responses in patients with BRAF V600E-mutated low-grade glioma, but tumor rebound after treatment discontinuation is frequent. Moreover, a lack of response is common in patient with high-grade glioma raising the need for further research into BRAFi+MEKi effects on tumors. We showed previously that BRAF V600E-mutated glioma cells positive for CD133 (Prominin-1), a marker of brain tumor stem cells, show decreased sensitivity to BRAFi, indicative of their role in promoting therapy resistance. BRAF V600E-mutated murine and patient-derived glioma cell lines (STN-10049, SU-aGBM5) were generated and together with established BRAF V600E-mutated cell lines (DBTRG, AM38) were analyzed for changes in gene expression in response to 48 hrs treatment with BRAFi dabrafenib and MEKi trametinib. Cells were analyzed by RNA-seq and gene enrichment analyses while cell culture supernatant was analyzed for cytokine production using an ELISA. Syngeneic, orthotopic BRAF V600E-mutated tumor allograft-bearing mice were treated with BRAFi+MEKi, with therapeutic antibodies against immune checkpoint molecules (anti-PD-L1 and anti-CTLA-4) and with combination of all four agents, and tumors were analyzed by mass cytometry and immunofluorescence for stem and T cell markers. BRAFi+MEKi treatment induced an interferon gamma (IFNg) response gene signature in BRAF V600E-mutated glioma cells and increased HLA gene expression. The frequency of tumor-infiltrating CD4+ CD8+ T cells in syngeneic BRAF V600E-mutated tumor allografts increased with BRAFi+MEKi treatment. Combining BRAFi+MEKi with anti-PD-L1 and anti-CTLA-4 treatment decreased CD133+ cells more effectively than either therapy alone, and resulted in a T cell-dependent survival benefit of mice with orthotopic BRAF V600E-mutated high-grade glioma. Combination of BRAFi+MEKi with immune checkpoint inhibition should be further explored as a viable option to prevent tumor rebound and therapy resistance in patients with BRAF V600E-mutated glioma.