Abstract 1007: Histone acetylation mediated by inhibition of Class I histone deacetylases is critical for induction of cell death in multiple myeloma cells.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC Histone deacetylase inhibitors (HDACi) are being evaluated as novel chemotherapeutics in combination with approved and investigational drugs for multiple myeloma (MM) therapy. Despite the rapid advancement in the use of HDACi in MM therapy, it is still not clear which HDAC needs to be inhibited to attain maximal MM cell death. HDAC6, a mediator of the proteasome pathway through its ability to deacetylate tubulin, is considered to be a critical target to promote MM cell apoptosis. However the contribution of Class I HDACs is still unclear. To determine the HDAC that needs to be inhibited in MM, cell death induced by pan-HDACi (LBH589) was compared to Class I HDACi (FK228), selective Class I and HDAC6 inhibitors (ACY-1215, ACY-738) and a specific HDAC6 inhibitor (ACY-738) utilising human myeloma cell lines (HMCL). The specificity of these HDACi was determined by characterizing the acetylation induced in histones (H2A, H2B, H3 and H4) and tubulin through flow cytometry. The histone acetylation pattern of LBH589 and FK228 were similar, ACY-1215 and ACY-738 have comparatively lesser but discernible histone acetylation, while ACY-775 did not acetylate histones. ACY-1215, ACY-738 and ACY-775 acetylate tubulin at amounts significantly higher than LBH589, while FK228 did not acetylate tubulin. To determine if there was a correlation between the pattern of acetylation and MM cell death, eight HMCLs were treated with each HDACi and proportion of cell death measured through flow cytometric enumeration of propidium iodide staining. LBH589 (50 nM) or FK228 (50 nM) induced the same amount of cell death irrespective of the HMCL used whereas ACY-1215 and ACY-738 were able to induce cell death comparable to the LBH589 and FK228 only at the highest concentration used (10 μM). Conversely, ACY-775, which induces negligible histone acetylation, was not able to induce cell death comparable to the other inhibitors except against RPMI-8226 where ACY-775 (10 μM) alone was able to cause a significant amount of cell death. Assessment of apoptosis following exposure of primary MM cells (n=8) to HDACi recapitulated the findings with the HMCL wherein ACY-775 caused the least amount of cell death. ACY-1215 and ACY-738 at higher concentrations (10 μM) were comparable to LBH589 and FK228 in all samples tested and as in the case with the HMCLs, a minority of patients (2/8) showed some cell death with ACY-775 treatment alone. These data suggest that activity against Class I HDACs is more effective than HDAC6 alone in inducing MM cell death. Specific HDAC6 inhibition does induce comparable cell death of some MM cells suggesting that in a subset of patients, inhibiting HDAC6 alone may be as efficient as Class I HDAC inhibition and identifying these patients is important. However, it is clear that in several instances histone acetylation mediated by inhibition of Class I HDAC is sufficient to induce significant MM cell death. Citation Format: Sridurga Mithraprabhu, Tiffany Khong, Simon S. Jones, Andrew Spencer. Histone acetylation mediated by inhibition of Class I histone deacetylases is critical for induction of cell death in multiple myeloma cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1007. doi:10.1158/1538-7445.AM2013-1007