Tucidinostat (Chidamide/CS055/HBI-8000) is a potent and orally bioavailable HDAC enzymes class I (HDAC1, 2, 3) and class IIb (HDAC10) inhibitor, with IC50s of 95, 160, 67 and 78 nM, less active on HDAC8 and HDAC11 (IC50s, 733 nM, 432 nM, respectively), and shows no effect on HDAC4/5/6/7/9 (IC50s, >30 μM). Tucidinostat shows potent antitumor activity, and inhibits several human derived tumor cell lines, such as HL-60, U2OS, LNCaP with GI50s of 0.4 ± 0.1, 2.0 ± 0.6, and 4.0 ± 1.2 μM, respectively. In addition, Tucidinostat shows less toxic to normal cells from human fetal kidney (CCC-HEK) and liver (CCCHEL).