Histone deacetylase (HDAC) inhibitor (IC50 = 0.045 μM; EC50 = 0.025 μM in HeLa screening and HeLa cell-based assays, respectively). Exhibits broad spectrum inhibition of HDAC classes I and II in assays using purified recombinant human isoforms. Displays less potent inhibition of HDAC2 (IC50 values are 0.014, 0.020, 0.026, 0.075 and 2.0 μM for HDAC6, HDAC1, HDAC4, HDAC3 and HDAC2 respectively). Displays cytotoxicity against numerous cell lines derived from human tumors. Orally available.