Combination therapies may help to decrease toxicity by allowing use of lower concentrations of individual agents to get a similar biologic effect and/or overcoming clinical resistance to single agent therapy. Recently, we have shown that by the inhibition of PKCβ, a protein kinase increasing survival and proliferation signals, we can shift the balance toward histone deacetylase inhibitor (HDI) induced stress response and the irreversible engagement of the apoptotic cascade in lymphoma cells. In the present study, we investigated the cytotoxicity and mechanisms of cell death of combination of BKM120, a PI3k inhibitor with panobinostat (LBH589), a histone deacetylase inhibitor in B-cell and T-cell lymphoma cell lines.

Flow cytometry assays and PARP cleavage analysis were applied for the study of the apoptosis induction. Cellular protein levels were measured by Western blot. Cytotoxicity of single agents and their combinations was determined by WST-1 proliferation assay. The interaction between BKM120 and panobinostat was examined according to the Chou and Talalay method by using the Calcusyn software.

Combined BKM120/panobinostat treatment synergistically inhibited proliferation and induced apoptosis in tested cell lines. Panobinostat single treatment (concentrations used in combined treatment) induced a slight proliferation inhibition and apoptosis. Interestingly, this treatment increased the level of phosphorylated AKT that might result in an attenuation of further apoptosis. The elevated AKT phosphorylation was significantly diminished by simultaneous treatment with BKM120, which may explain increased induction of apoptosis and inhibition of proliferation in cells treated by BKM120/panobinostat. Moreover, in comparison to the effects of single agent treatments, combined BKM120/panobinostat treatment resulted in significantly lower phospho-RPS6, phospho-GSK3 and p21 protein levels and an increased level of acetylated histone H3.

Our data suggest that PI3k inhibitors in combination with HDI might have potential therapeutic value in lymphoma treatment. The inhibition of PI3k by BKM120 markedly sensitized lymphoma cells to the induction of apoptosis by panobinostat and synergistically inhibited the proliferation of all tested lymphoma cell lines. However, the molecular events underlying the synergistic effect using PI3k inhibitors in combination with HDI remain to be fully elucidated.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr C181.