B1486 is a highly potent VprBP inhibitor capable of blocking osteoclast differentiation and bone resorption

Bone is a dynamic tissue that is continuously remodeled by tightly-coordinated actions of bone-resorbing osteoclasts and bone-forming osteoblasts. Uncontrolled osteoclastogenesis has a severe impact on skeletal health and thus represents an important therapeutic target in osteolytic diseases. VprBP is a recently identified kinase and plays an important role in stimulating osteoclast differentiation and bone resorption through histone H2AT120 phosphorylation (H2AT120p). We previously developed B32B3 as the first VprBP inhibitor, but its high IC₅₀ value has been a major challenge in using it for osteoclast-triggered bone diseases. In this study, we designed and characterized a series of small molecule compounds derived from B32B3 to identify a novel second-generation inhibitor of VprBP. Our initial screening of B32B3 derivatives identified three compounds, designated as B1486, B3703, and B6756, that are effective in interfering with VprBP kinase activity toward H2AT120p and RANKL-induced differentiation of osteoclast precursor cells (OCPs). However, when their cellular efficacy was systematically accessed over different concentration ranges, B1486 was more potent than B3703 and B6756 at hindering VprBP function in the process of OCP cell differentiation. Consistent with these in vitro findings, B1486 showed significantly improved inhibitory activity against VprBP-driven bone resorption and low bone mass phenotypes in our preclinical studies using zebrafish and mouse models. Taken together, our results indicate that B1486 is a highly potent inhibitor for blocking VprBP-mediated H2AT120p and osteoclastogenic gene silencing, as well as for overcoming osteoporosis generated by excessive osteoclast differentiation.