Cellular Target Engagement and Dissociation Kinetics of Class I-Selective Histone Deacetylase (HDAC) Inhibitors

Histone deacetylases (HDACs) 1-3 are key regulators of gene expression and represent important therapeutic targets in cancer, neurodegenerative, and immune disorders. Many potent class I HDAC inhibitors display slow- and tight-binding kinetics, which profoundly influence their efficacy and pharmacodynamics. In particular, their dissociation rate (off-kinetic) is critical, since prolonged target engagement greatly influences drug efficacy in vivo. However, the off-kinetics of HDAC inhibitors are often overlooked in the early stages of drug development. Here, we investigated the dissociation kinetics of tucidinostat, trapoxin A, and TNG260 in comparison to the pan-HDAC inhibitor vorinostat. Using biochemical 100-fold jump dilution assays, NanoBRET assays, and cellular washout experiments, we characterized the dissociation of these compounds from purified proteins and in a cellular context. Tucidinostat showed moderately slow off-kinetics, while the clinical candidate TNG260 demonstrated pronounced tight-binding properties. Trapoxin A displayed remarkable discrepancies between assays, as it showed fast dissociation kinetics in the biochemical assay, but tight-binding properties in a cellular setting. These findings not only address the previously unexplored dissociation kinetics of two clinically relevant inhibitors, but also underscore the importance of comprehensive kinetic profiling of novel HDAC inhibitors in cellular models.