Different inhibitory actions of immunomodulating agents and immunosuppressive agents on bone resorption of mouse calvaria

In this study, we have investigated the in vitro effects of the immunomodulators lobenzarit and traxanox and a newly synthesized immunosuppressant, mizoribine, as well as cyclosporin A, on bone resorption using neonatal mouse calvariae labelled with 45Ca. As stimulators of bone resorption, bovine parathyroid hormone (PTH), lipopolysaccharide (LPS), interleukin 1β (IL-1β) and tumor necrosis factor-α (TNF-α) were used. Lobenzarit, traxanox, mizoribine and cyclosporin A inhibited or tended to inhibit bone resorption stimulated by PTH, LPS, IL-1β or TNF-α in a dose-dependent manner. Basal bone resorption was inhibited by immunosuppressant cyclosporin A or mizoribine, while immunomodulators lobenzarit and traxanox failed to inhibit basal bone resorption. Removal of lobenzarit from the culture medium resulted in the recovery of bone resorptive activity. These results suggest that the inhibitory effect of immunomodulators on bone resorption is reversible and nonselective. Also, it raises the possibility that immunomodulators and immunosuppressants may affect bone resorption by different mechanisms.