HDAC3-IN-4 is a selective and orally active inhibitor of HDAC3, boasting an IC50 of 89 nM. It induces the degradation of PD-L1 by modulating cathepsin B (CTSB) in lysosomes, with a DC50 of 5.7 μM. Compared to HDAC1, HDAC6, HDAC7, and HDAC8, HDAC3-IN-4 demonstrates superior selectivity for HDAC3.

HDAC1:730 nM, HDAC6:>10 μM, HDAC8:>10 μM, HDAC7:>10 μM, HDAC3:89 nM

HDAC3-IN-4 (compound HQ-30) exhibits potent antiproliferative effects, with IC50 values of 0.09 μM for Jurkat (T lymphoma), 0.43 μM for HCT-116 (colorectal cancer), 1.20 μM for B16-F10 (melanoma), 2.94 μM for MCF-7 (breast cancer), and 0.24 μM for HepG2 (liver cancer). At concentrations ranging from 0.5 to 8 μM over 48 hours, HDAC3-IN-4 induces apoptosis in B16-F10 cells in a concentration-dependent manner. Furthermore, at doses from 0.5 to 4 μM for 48 hours, it increases the percentage of B16-F10 cells in the G2/M phase of the cell cycle, while decreasing the percentage in the G0/G1 phase. Additionally, at 0.5 to 4 μM for 24 hours, HDAC3-IN-4 significantly upregulates acetylated H3 (Ac-H3). The compound also modulates PD-L1 expression through the lysosomal pathway.

HDAC3-IN-4 (compound HQ-30; 25 mg/kg; oral administration; daily; for 9 days) effectively decreases tumor volume and weight, consequently inhibiting tumor growth.