Combined BRAF and MEK blockade for BRAF-mutant gliomas.

BRAF is a common finding in glioma (about 10-60% depending on histopathologic subclassification). BRAF monotherapy shows modest preclinical efficacy against BRAF gliomas and also induces adverse secondary skin malignancies. Here, we examine the molecular mechanism of intrinsic resistance to BRAF inhibition in glioma. Furthermore, we investigate BRAF/MEK combination therapy that overcomes intrinsic resistance to BRAF inhibitor and also prevents BRAF inhibitor induced secondary malignancies. Immunoblotting and Human Phospho-Receptor Tyrosine Kinase Array assays were used to interrogate MAPK pathway activation. The cellular effect of BRAF and MEK inhibition was determined by WST-1 viability assay and cell cycle analysis. Flanked and orthotopic GBM mouse models were used to investigate the in vivo efficacy of BRAF/MEK combination therapy and the effect on secondary malignancies. BRAF inhibition leads to recovery of ERK phosphorylation. Combined BRAF and MEK inhibition prevents reactivation of the MAPK signaling, which correlates with decreased cell viability and augmented cell cycle arrest. Similarly, mice bearing BRAF glioma showed reduced tumor growth when treated with a combination of BRAF and MEK inhibitor compared to BRAF inhibition alone. Additional benefit of BRAF/MEK inhibition was reflected by reduced cutaneous squamous-cell carcinoma (cSCC) growth (a surrogate for RAS-driven secondary maligancies). In glioma, recovery of MAPK signaling upon BRAF inhibition accounts for intrinsic resistance to BRAF inhibitor. Combined BRAF and MEK inhibition prevents rebound of MAPK activation, resulting in enhanced antitumor efficacy and also reduces the risk of secondary malignancy development.