Combinations of BRAF, MEK, and PI3K/mTOR inhibitors overcome acquired resistance to the BRAF inhibitor GSK2118436 dabrafenib, mediated by NRAS or MEK mutations.

Recent results from clinical trials with the BRAF inhibitors GSK2118436 (dabrafenib) and PLX4032 (vemurafenib) have shown encouraging response rates; however, the duration of response has been limited. To identify determinants of acquired resistance to GSK2118436 and strategies to overcome the resistance, we isolated GSK2118436 drug-resistant clones from the A375 BRAF(V600E) and the YUSIT1 BRAF(V600K) melanoma cell lines. These clones also showed reduced sensitivity to the allosteric mitogen-activated protein/extracellular signal-regulated kinase (MEK) inhibitor GSK1120212 (trametinib). Genetic characterization of these clones identified an in-frame deletion in MEK1 (MEK1(K59del)) or NRAS mutation (NRAS(Q61K) and/or NRAS(A146T)) with and without MEK1(P387S) in the BRAF(V600E) background and NRAS(Q61K) in the BRAF(V600K) background. Stable knockdown of NRAS with short hairpin RNA partially restored GSK2118436 sensitivity in mutant NRAS clones, whereas expression of NRAS(Q61K) or NRAS(A146T) in the A375 parental cells decreased sensitivity to GSK2118436. Similarly, expression of MEK1(K59del), but not MEK1(P387S), decreased sensitivity of A375 cells to GSK2118436. The combination of GSK2118436 and GSK1120212 effectively inhibited cell growth, decreased ERK phosphorylation, decreased cyclin D1 protein, and increased p27(kip1) protein in the resistant clones. Moreover, the combination of GSK2118436 or GSK1120212 with the phosphoinositide 3-kinase/mTOR inhibitor GSK2126458 enhanced cell growth inhibition and decreased S6 ribosomal protein phosphorylation in these clones. Our results show that NRAS and/or MEK mutations contribute to BRAF inhibitor resistance in vitro, and the combination of GSK2118436 and GSK1120212 overcomes this resistance. In addition, these resistant clones respond to the combination of GSK2126458 with GSK2118436 or GSK1120212. Clinical trials are ongoing or planned to test these combinations.