In vitro and in vivo responses to high and low doses of nitrogen-containing bisphosphonates suggest engagement of different mechanisms for inhibition of osteoclastic bone resorption.

The effects of nitrogen-containing bisphosphonates (N-BPs) on osteoclasts (Ocs) may differ with dose and regimen. N-BPs reduce Oc bone resorption by inhibiting the enzyme farnesyl diphosphate synthase (FPPS), an effect counteracted by geranylgeraniol (GGOH), which restores geranylgeranylation downstream of FPPS. We assessed GGOH effects on inhibition of bone resorption by the N-BPs alendronate (ALN), ibandronate (IBN), and zoledronate (ZOL) in an assay of rabbit Oc resorption of bovine cortical bone. GGOH blocked inhibition of resorption at low, but not high, N-BP concentrations, with a 14- to 20-fold increase in IC50 values for each N-BP. In vivo, growing male rats were administered doses calculated to mimic bioavailable exposures in daily (ALN, IBN), weekly (ALN), monthly (IBN), and yearly (ZOL) clinical regimens. Tibiae were harvested at 48 h, and metaphyses were analyzed. With lower ALN and IBN doses, Oc numbers rose by 26-48 %, morphology was normal, and there was no increase in apoptotic Ocs. In contrast, with higher IBN and ZOL doses, bone-associated Ocs were generally rounded in appearance and numbers of nuclei/Oc versus vehicle increased 42 and 31 %, respectively (P < 0.05). With ZOL, there was no rise in Oc number, but there was a 6.5-fold increase in apoptotic Ocs versus vehicle and a ≥13.5-fold increase versus lower-dose ALN or IBN (P < 0.05). With higher-dose IBN there was no rise in Oc number but 7- and 14-fold increases in Oc apoptosis versus low-dose ALN and IBN (P < 0.02). These results suggest that different mechanisms may come into play across the dosing spectrum of N-BPs.