Targeting Class I Histone Deacetylases Triggers Antitumor Responses in Colorectal Cancer In Vitro and In Vivo

Class I histone deacetylases (HDACs) are frequently overexpressed
in colorectal cancer (CRC). Combining computational, synthetic, and
biological efforts, we developed novel o-aminobenzamide-based
HDAC inhibitors (HDACis) optimized for class I enzyme-specific targeting.
Compounds 5d and 5i emerged as lead candidates,
showing strong antiproliferative effects in CRC cells with low toxicity
in healthy colon epithelium. Both compounds disrupted the G2/M checkpoint
through distinct mechanisms. 5i, although less potent
(HDAC1 IC50 = 1508 nM), retained selectivity, upregulated
p21, and triggered pronounced apoptosis. 5d (Colrestat), one of the most selective class I HDACis to
date (HDAC1 IC50 = 41.2 nM, HDAC2 IC50 = 52.5
nM, and HDAC3 IC50 = 74.3 nM), induced H3K9 acetylation,
p21 upregulation, and G2/M arrest. The short-term in vitro effects of 5d were modulated by a compensatory upregulation
of autophagy. However, in long-term, this protective mechanism becomes
insufficient to sustain tumor survival, resulting in strong antitumor
efficacy in vivo in the CAM assay for both compounds
even outperforming entinostat.