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.

HDAC2:90.0 nM, HDAC3:422.2 nM, HDAC1:56.0 nM

HDAC-IN-56 exhibits a strong selective inhibitory effect on class I HDACs, specifically 1, 2, and 3, surpassing that of MS-275 [1]. At a concentration of 0.1 μM over 2 hours, HDAC-IN-56 displays significant species-specific metabolic differences across human, monkey, dog, rat, and mouse hepatocytes, yet remains metabolically stable in all five species [1]. When used at 0.01-1 μM for 72 hours, HDAC-IN-56 effectively induces G1 phase cell cycle arrest and apoptosis [1]. Treatment at these concentrations also elevates intracellular levels of acetyl-histone H3 and p21 more effectively than Tucidinostat or MS-275, indicating potent inhibition of class I histone deacetylases [1]. The IC50 of HDAC-IN-56 for SKM-1 is 139.0 ± 8.0 nM [1]. In cell cycle analysis of SKM-1 cells at 0.01, 0.1, and 1 μM for 72 hours, HDAC-IN-56 downregulated c-Myc and CDK4 expression at 0.1 μM more efficiently than MS-275 or Tucidinostat [1]. Apoptosis analysis in SKM-1 cells showed that HDAC-IN-56 triggered strong apoptosis, as evidenced by Annexin V/PI staining, with greater efficacy than MS-275 or Tucidinostat [1]. Western blot analysis demonstrated that HDAC-IN-56 increases the intracellular levels of acetyl-histone H3 and p21 in SKM-1 cells at these concentrations over 72 hours, outperforming Tucidinostat or MS-275 [1].

HDAC-IN-56, administered orally at 10-80 mg/kg daily for one month, does not cause significant weight changes even at the highest dose of 80 mg/kg [1]. It exhibits a favorable pharmacokinetic profile, showing oral bioavailability of 47.7% in ICR mice and 39.5% in SD rats when administered to SD rats (10, 20 mg/kg) and ICR mice (20, 40 mg/kg) after a fasting period [1]. HDAC-IN-56 at doses of 20-60 mg/kg effectively inhibits tumor growth of MC38 cells in nude mice and demonstrates enhanced tumor inhibition in immunocompetent C57BL/6 mice, suggesting that the immune system may be involved and activated to enhance the antitumor effect [1]. Animal models used include male SD rats or ICR mice for pharmacokinetic studies and SKM-1 or MC-38 cell xenografts for efficacy evaluation [1].