Discovery of Small-Molecule Inhibitors of Receptor Activator of Nuclear Factor-κB Ligand with a Superior Therapeutic Index

Receptor
activator of nuclear factor-κB ligand (RANKL) constitutes
the master mediator of osteoclastogenesis, while its pharmaceutical
inhibition by a monoclonal antibody has been approved for the treatment
of postmenopausal osteoporosis. To date, the pursuit of pharmacologically
more favorable approaches using low-molecular-weight inhibitors has
been hampered by low specificity and high toxicity issues. This study
aimed to discover small-molecule inhibitors targeting RANKL trimer
formation. Through a systematic screening of 39 analogues of SPD-304,
a dual inhibitor of tumor necrosis factor (TNF) and RANKL trimerization,
we identified four compounds (1b, 3b, 4a, and 4c) that selectively inhibited RANKL-induced
osteoclastogenesis in a dose-dependent manner, without affecting TNF
activity or osteoblast differentiation. Based on structure–activity
observations extracted from the most potent and less toxic inhibitors
of RANKL-induced osteoclastogenesis, we synthesized a focused set
of compounds that revealed three potent inhibitors (19a, 19b, and 20a) with remarkably low cell-toxicity
and improved therapeutic indexes as shown by the LC50 to
IC50 ratio. These RANKL-selective inhibitors are an excellent
starting point for the development of small-molecule therapeutics
against osteolytic diseases.