Determination of Slow-binding HDAC Inhibitor Potency and Subclass Selectivity

ABSTRACT

Histone deacetylases (HDACs) 1–3 regulate chromatin structure and gene expression. These three enzymes are targets for cancer chemotherapy and are studied for the treatment of immune disorders and neurodegeneration, but there is a lack of selective pharmacological tool compounds to unravel their individual roles. Potent inhibitors of HDACs 1–3 often display slow-binding kinetics, which causes a delay in inhibitor–enzyme equilibration and may affect assay readout. Here, we compare the potency and selectivity of slow-binding inhibitors measured by discontinuous and continuous assays. We find that entinostat, a clinical candidate, inhibits HDACs 1–3 by a two-step, slow-binding mechanism with lower potencies than previously reported. In addition, we show that RGFP966, commercialized as HDAC3-selective probe, is a slow-binding inhibitor with inhibitor constants of 57 nM, 31 nM, and 13 nM against HDACs 1–3, respectively. These data highlight a need for thorough kinetic investigation in the development of selective HDAC probes.