AR-42 is a potent histone deacetylase inhibitor (HDACi). It induces growth inhibition, cell- cycle arrest, apoptosis, and activation of caspases-3/7. AR-42 also promotes hyperacetylation of H3, H4, and alpha-tubulin, and up-regulation of p21 and  exhibits in vitro and ex vivo biologic activity against malignant mast cells.