UCSF

BRAF^V600E is a common finding in glioma (about 10-60% depending on histopathologic subclassification). BRAF^V600E monotherapy shows modest preclinical efficacy against BRAF^V600E gliomas and also induces adverse secondary skin malignancies. Here, we examine the molecular mechanism of intrinsic resistance to BRAF^V600E inhibition in glioma. Furthermore, we investigate BRAF^V600E/MEK combination therapy that overcomes intrinsic resistance to BRAF^V600E inhibitor and also prevents BRAF^V600E 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^V600E 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^V600E/MEK combination therapy and the effect on secondary malignancies. BRAF^V600E inhibition leads to recovery of ERK phosphorylation. Combined BRAF^V600E and MEK inhibition prevents reactivation of the MAPK signaling, which correlates with decreased cell viability and augmented cell cycle arrest. Similarly, mice bearing BRAF^V600E glioma showed reduced tumor growth when treated with a combination of BRAF^V600E and MEK inhibitor compared to BRAF^V600E inhibition alone. Additional benefit of BRAF^V600E/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^V600E inhibitor. Combined BRAF^V600E and MEK inhibition prevents rebound of MAPK activation, resulting in enhanced antitumor efficacy and also reduces the risk of secondary malignancy development.