Effects of AKT inhibitor therapy in response and resistance to BRAF inhibition in melanoma

BACKGROUND: The clinical use of BRAF inhibitors for treatment of metastatic melanoma is limited by the development of drug resistance. In this study we investigated whether co-targeting the MAPK and the PI3K-AKT pathway can prevent emergence of resistance or provide additional growth inhibitory effects in vitro. METHODS: Anti-tumor effects of the combination of the BRAF inhibitor (BRAFi) dabrafenib and GSK2141795B (AKTi) in a panel of 23 BRAF mutated melanoma cell lines were evaluated on growth inhibition by an ATP-based luminescent assay, on cell cycle and apoptosis by flow cytometry and on cell signaling by western blot. Moreover, we investigated the possibilities of delaying or reversing resistance or achieving further growth inhibition by combining AKTi with dabrafenib and/or the MEK inhibitor (MEKi) trametinib by using long term cultures. RESULTS: More than 40% of the cell lines, including PTEN-/- and AKT mutants showed sensitivity to AKTi (IC(50) < 1.5 μM). The combination of dabrafenib and AKTi synergistically potentiated growth inhibition in the majority of cell lines with IC(50) > 5 nM dabrafenib. Combinatorial treatment induced apoptosis only in cell lines sensitive to AKTi. In long term cultures of a PTEN-/- cell line, combinatorial treatment with the MAPK inhibitors, dabrafenib and trametinib, and AKTi markedly delayed the emergence of drug resistance. Moreover, combining AKTi with the MAPK inhibitors from the beginning provided superior growth inhibitory effects compared to addition of AKTi upon development of resistance to MAPK inhibitors in this particular cell line. CONCLUSIONS: AKTi combined with BRAFi-based therapy may benefit patients with tumors harboring BRAF mutations and particularly PTEN deletions or AKT mutations.