HDAC-IN-84 (compound 4d) is a potent HDAC inhibitor, with IC 50 values of 0.0045, 0.015, 0.013, 0.038, 5.8 and 26 μM for HDAC1 , HDAC2 , HDAC3 , HDAC6 , HDAC8 and HDAC11 , respectively. HDAC-IN-84 effectively inhibits the proliferation of leukemia cells without causing toxicity.