The Histone Deacetylase Inhibitor Sodium Valproate Has In Vitro Anti-Neoplastic Activity Against Primary Multiple Myeloma Cells and Cell Lines Both Alone and in Combination with Velcade.

There is growing evidence that histone deacetylase inhibitors (HDAC-I) may have benefit in the treatment of multiple myeloma (MM). We assessed the in vitro responses of the MM cell lines H929, JJN3, KMS11 and U266 to the HDAC-Is trichostatin A (TSA), suberoylanilide hydroxamic acid (SAHA) and sodium valproate (SV). All three HDAC-Is induced dose dependent cell killing and the ranked sensitivity of the lines was remarkably constant with U266 cells being most resistant to all three inhibitors. Real time RT-PCR based analyses of the HDAC expression profiles, identified that the greater resistence of U266 cells was associated with 5–10 fold increased expression of a single HDAC from the sirtuin family of deacetylases (SIRT5). Each HDAC-I displayed different potencies against the cell lines; TSA killed in the range 0.1–1μM, SAHA in the range 1–10μM and SV at the higher doses of 1–10mM. Despite being less potent, the doses of SV are commensurate with those that can be achieved in vivo. Furthermore, we have identified synergy/cooperation between suboptimal SV concentrations and the proteasome inhibitor Velcade (Bortezomib). MTT assays using JJN3 cells, demonstrated that 1mM SV had little effect reducing survival to 89% whereas 0.6 nM velcade reduced cell survival to 42±5.5%. Killing by velcade was not significantly enhanced by the additional presence of 1μM dexamethasone (Dex)(36±3.4%). However, the combination of SV, Dex and Velcade resulted in near total cell killing (4.7±2.6% survival). Collectively, these observations suggested that SV may have value in MM therapy. However, we wished to also investigate the potency of SV against primary MM cells. We have developed an in vitro culture system in which primary MM plasma cells survive for up to 30 days. In this culture system we have observed efficient and selective killing of MM cells by 1mM SV that has been comparable or indeed greater than that observed in response to 1μM SAHA. Thus it appears that primary MM cells are more SV sensitive than MM cell lines. Together our findings indicate that SV may have value in MM therapy and in particular when combined with proteasome inhibitors. Our data also indicate that SIRT5 overexpression may be a resistance factor against such therapies.