– HDAC-IN-56 ((S)-17b), an orally active inhibitor of class I histone deacetylase (HDAC), exhibits inhibitory constants (IC 50) of 56.0 ± 6.0 nM for HDAC1, 90.0 ± 5.9 nM for HDAC2, 422.2 ± 105.1 nM for HDAC3, and greater than 10,000 nM for HDAC4-11. This compound displays robust antitumor activity [1], marked by its ability to significantly elevate intracellular acetylhistone H3 and P21 levels, while also inducing G1 cell cycle arrest and apoptosis effectively.