A slow, tight-binding inhibitor of class I HDACs, progressively binding HDACs and remaining bound after wash-out; inhibits class I HDACs (IC50 = 150, 760, 370, and 5,000 nM for HDAC1, 2, 3, and 8, respectively) but not class II HDACs (IC50 > 180 μM for HDAC4, 5, and 7)