Ricolinostat restrains osteoclastogenesis through HDAC6-associated regulation of p65 stability

Excessive osteoclast activity is a central driver of pathological bone loss in osteoporosis; however, the mechanisms governing osteoclast differentiation remain incompletely understood, and current therapeutic options remain limited in efficacy. Therefore, we performed high-throughput drug screening to identify candidate agents and further explored the molecular mechanisms underlying targeted osteoclastogenesis inhibition. Here, we identified ricolinostat (RICO), a selective histone deacetylase 6 (HDAC6) inhibitor, as a potent inhibitor of the receptor activator of nuclear factor-kappa B ligand (RANKL)-induced osteoclastogenesis. RICO suppresses osteoclast differentiation and reduces osteoclast-associated gene and protein expression without compromising cell viability. RICO attenuates estrogen deficiency-induced bone loss and reduces osteoclast abundance in vivo. Mechanistically, transcriptomic profiling revealed that RICO broadly suppressed osteoclastogenic, inflammatory, and metabolic programs. Specifically, by inhibiting HDAC6, RICO promoted proteasomal degradation of p65 by increasing its acetylation and ubiquitination, thereby reducing p65 protein abundance in both cytoplasmic and nuclear fractions and suppressing canonical NF-κB signaling. Collectively, these findings identify RICO-as a phase II clinical stage investigational compound with repurposing potential as a promising anti-resorptive candidate for the treatment of osteoporosis and uncover a novel mechanism whereby HDAC6 governs p65 protein stability to inhibit osteoclastogenesis.