Abstract 976: BRAF inhibitor GSK2118436 - single agent and combination activity with other targeted agents in BRAF V600 mutant melanoma

Abstract

Abstract Although BRAF inhibitors display promising clinical response in BRAFV600 mutant melanoma patients, not all patients respond to these agents and the durability of the response is limited. To improve BRAF targeted therapy, we characterized a panel of BRAFV600 mutant melanoma cell lines and determined their response to cell growth inhibition, apoptosis induction and cell signaling changes by GSK2118436 alone and in combination with the MEK inhibitor GSK1120212 or PI3K/mTOR inhibitor GSK2126458. Nine of 11 BRAFV600E, 4/5 BRAFV600K and 1/1 BRAFV600D mutant lines were sensitive to GSK2118436 (IC50<0.3µM). These sensitive lines had wild-type MEK1/2 with or without PTEN mutations. Two of the three lines insensitive to GSK2118436 had either MEK1P124S or MEK2K66_K68del mutations. GSK2118436 treatment decreased MEK and ERK phosphorylation and cyclin D1 in both the sensitive and insensitive lines. However, GSK2118436 decreased S6 ribosomal protein phosphorylation (pS6P) only in the sensitive lines. Cell lines sensitive to GSK2118436 were also sensitive to the allosteric MEK inhibitor GSK1120212 (IC50<0.01µM). The combination of GSK2118436 and GSK1120212 had a synergistic to additive effect on cell growth inhibition in the sensitive lines, but had minimal benefit in the three insensitive lines. Although 5/6 PTEN NULL lines were sensitive to GSK2118436 based on their IC50s, growth of these lines was inhibited less effectively by GSK2118436 compared to the PTEN WT lines evaluated by IC70. PTEN NULL cells displayed higher levels of basal AKT phosphorylation (pAKT) than PTEN WT lines. Treatment of PTEN NULL lines with GSK2118436 increased pAKT while similar treatment of PTEN WT lines decreased pAKT and increased PTEN expression. In combination with the PI3K/mTOR inhibitor GSK2126458, GSK2118436 or GSK1120212 effectively inhibited MEK/ERK and PI3K/mTOR signaling, enhanced cell growth inhibition, and induced apoptosis evaluated by caspase 3/7 activation and/or PARP cleavage in both PTEN WT and PTEN NULL lines. In PTEN deficient lines, these combinations were more effective than the combination of GSK2118436 and GSK1120212. These results indicate MEK1/2 mutations associate with lack of sensitivity to single agent BRAF and MEK inhibitors. In addition, PTEN deficiency may reduce the effectiveness of BRAF or MEK inhibitors. The combination of GSK2118436 with GSK1120212 or GSK2126458 may improve the effectiveness of BRAF targeted therapy in BRAFV600 mutant melanoma. Our results support the testing of GSK2118436 with these combinations in the clinic and warrant further investigation of MEK and PTEN mutation status and pS6P as a pharmacodynamic marker to improve treatment outcome in BRAFV600 mutant melanoma. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 976. doi:1538-7445.AM2012-976