Differential Effect of the Histone Deacetylase Inhibitors Vorinostat and MGCD0103 in Cell-Free and Cell-Based Assays

Recent clinical trials demonstrated a promising clinical activity of a variety of Histone deacetylase inhibitors (HDACi) in patients with relapsed non-Hodgkin and Hodgkin lymphoma (HL). In this study, we examined the in vitro activity of the pan- DAC inhibitor vorinostat and the isotype selective HDAC inhibitor MGCD0103. Using a cell free fluorogenic peptide-based substrate assay, both MGCD0103 and vorinostat were potent inhibitors of class I HDACs, with vorinostat also effectively inhibiting HDAC6. Subsequently, we examined the effect of these two compounds on three HL-derived cell lines (HD-LM2, L-428, and KM-H2). Although these cell lines had a similar expression level of class-I HDAC proteins, as determined by western blot analysis, they were more sensitive to MGCD0103 in short term culture. After 72 hours of incubation, the IC50 of MGCD0103 was consistently <0.5 μM compared with >1.5 μM for vorinostat. This differential antiproliferative effect was also observed at the molecular level. Submicromolar concentrations (as low as 0.1 μM) of MGCD0103 effectively induced the expression of p21 and the acetylation of H3 histones, whereas higher concentration of vorinostat were required to achieve a similar effect. Furthermore, we examined the effect of equimolar concentrations of MGCD0103 and vorinostat on cytokine and chemokine secretion, using a multiplex cytokine array (34 cytokines/chemokines), on a selected panel of gene expression using pathway PCR array, and on selected protein expression using pathway protein array (phospho kinases and apoptosis pathways) and multicolor FACS experiments. While vorinostat and MGCD0103 shared similar effects on a variety of these elements, MGCD0103 was more effective in upregulating p21 expression, activating the intrinsic caspase pathway, and favoring the secretion of Th1-type cytokines. Given the recent reported results from two independent phase-II studies in patients with relapsed HL that demonstrated higher response rate achieved with MGCD0103 compared with vorinostat, our results suggest that cell-based assays may have a better predictor value of the potential clinical activity of HDACi in HL compared with the cell-free assays.