Abstract 4709: Combined mitogen-activated protein kinase pathway inhibition with short-term romidepsin treatment induces proapoptotic Bim and cell death in BRAF mutant cancers

Abstract

Abstract Solid tumor trials with histone deacetylase inhibitors (HDIs) have been largely disappointing. We recently characterized a romidepsin-resistant T-cell lymphoma cell line that was found to have activation of the mitogen-activated protein kinase (MAPK) pathway, leading to subsequent phosphorylation and degradation of the proapoptotic protein Bim, suggesting that activation of this pathway may also confer resistance to romidepsin and other HDIs. The V600E BRAF mutation has been found in approximately 60% of melanomas and approximately 15% of colon cancers and leads to constitutive activation of the MAPK pathway. We thus hypothesized that combined treatment with romidepsin and BRAF or mitogen-activated protein kinase kinase (MEK) inhibitors may be effective in cancers that harbor the V600E BRAF mutation. To more closely simulate clinical administration of romidepsin, 11 V600E BRAF mutant cell lines (8 melanomas and 3 colorectal cancers) were exposed to 25 ng/ml romidepsin for 6 h after which romidepsin was removed, cells were incubated in fresh medium for an additional 42 h and subsequently apoptosis was measured by Annexin V and propidium iodide staining. Of the 11 cell lines, 10 exhibited significantly higher annexin staining after short-term romidepsin treatment (control 7.1% ± 3.8% vs. treated 34.7% ± 17.8%, p<0.001). Cells were subsequently treated with medium containing 25 ng/ml romidepsin with 250 nM of the MEK inhibitors AZD6244 (selumetinib) and PD0325901 or 1 µM of the mutant BRAF inhibitor PLX4032 (vemurafenib) for 6 h, after which the cells were incubated an additional 42 h in medium containing the inhibitors alone. Combined treatment with romidepsin and AZD6244 resulted in a significantly higher percent of annexin positive cells (p<0.05) in 9 of the 11 BRAF mutant cell lines examined. Immunoblot analysis demonstrated inhibition of the MAPK pathway at the concentrations used and that apoptosis was associated with increased expression of Bim, as well as increased poly (ADP-ribose) polymerase cleavage. Our results suggest that combined treatment with romidpesin and BRAF or MEK inhibitors may be useful in the treatment of cancers that express mutant BRAF. 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 4709. doi:1538-7445.AM2012-4709