Abstract 1635: Combination of the HDAC inhibitor romidepsin with the MEK inhibitor U0126 results in synergistic plasma cell apoptosis

Abstract Histone deacetylase inhibitors such as vorinostat and panobinostat have been tested as single agents in patients with relapsed myeloma with nominal activity. Yet, when combined with the proteasome inhibitor bortezomib, clinical and laboratory evidence of synergy has been observed. We sought to evaluate the individual signaling effects of the HDACi romidepsin, with the intent of developing additional combinations that may lead to clues of mechanism of action on human myeloma cells. Single agent studies demonstrated IC50 values for romidepsin alone of between 10 and 30nm, further experiments demonstrate that growth inhibition resulting from romidepsin exposure includes apoptosis and cell cycle arrest. Cell death is associated with increased caspase 3, 8, 9 and PARP cleavage. Cell cycle analysis indicates that romidepsin induces G0/G1 cell cycle arrest which is correlated with p21 upregulation. Cell signaling assessment demonstrates that romidepsin inhibits AKT phosphorylation. Surprisingly, romidepsin increases Erk phosphorylation with its resultant effects on cell growth, development and differentiation. Combining romidepsin with the MEK inhibitor U0126 suppresses MEK/ERK signaling, and synergistically enhances the antitumor effect of romidepsin (combination index value <1). Annexin V staining demonstrates significant apoptosis following 18 hr of treatment with U0126 (5µM) and 10nM romidepsin. Western blot confirms the development of significantly increased procasepase and PARP cleavage, as well as substantial downregulation of Mcl-1, Bcl-xL. The combination can overcome the antiapoptotic effect of exogenous cytokines, such as IL-6 and IGF1 in vitro. Further studies evaluating the effect of bone marrow stromal cells will be presented in addition of data evaluating other potential targets for oncogene addiction that can be used for further novel combinations to gain more understanding of the biological context of this combination both in vitro and in vivo. This study will improve the design of effective clinical combination strategies. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1635.