Thonzonium bromide inhibits RANKL-induced OC formation, the appearance of OC-specific marker genes and bone-resorbing activity in vitro. Thonzonium bromide blocks the RANKL-induced activation of NF-κB, ERK and c-Fos as well as the induction of NFATc1 which is essential for OC formation. Thonzonium bromide disrupts F-actin ring formation resulting in disturbances in cytoskeletal structure in mature OCs during bone resorption. Thonzonium bromide exhibits protective effects in an in vivo murine model of LPS-induced calvarial osteolysis.