Alendronate distributed on bone surfaces inhibits osteoclastic bone resorption in vitro and in experimental hypercalcemia models.

Alendronate is an aminobisphosphonate that acts as a potent inhibitor of osteoclastic bone resorption. To understand the mechanism of action of alendronate in vivo, in this study we investigated the relationship between distribution of [14C]-alendronate in rat bone and its effects on bone resorption in vitro or in rat hypercalcemic models. A single IV dose of 0.05 approximately 1.25 mg/kg inhibited the increase in plasma calcium level induced by bovine PTH or 1 alpha(OH)D3. The minimal effective dose of pamidronate (1.25 mg/kg) and etidronate (over 31.25 mg/kg) were at least 5 times and 25 times, respectively, higher than the dose of alendronate in the rat hypercalcemic model prepared by 1 alpha(OH)D3. The relative potencies of compounds in the hypercalcemic rat models reflected those of inhibitory effects on bone resorption in vitro. We conducted the ivory-slice assay under two conditions: (a) addition of a given bisphosphonate after adherence of the osteoclasts; and (b) preincubation of the ivory slices with a given bisphosphonate. The inhibitory IC50 values of alendronate under condition (b) were similar to those under condition (a). To evaluate the interaction between osteoclasts and alendronate in bone, we investigated the localization of [14C]-alendronate in the tibia of growing rats (4-day-old rats). Alendronate did not distribute uniformly in the tibia. At 1 day after injection (0.05 mg SC), dense labeling was seen primarily under osteoclasts. We injected 0.05 mg/kg of [14C]-alendronate (single i.v.) into rats [14C]-alendronate was rapidly eliminated from plasma, and mainly distributed to the bone in rats. These data suggest that alendronate which distributed on bone surface mainly contributed to the antihypercalcemic action in vivo.

Alendronate modulates osteogenesis of human osteoblastic cells in vitro.

The bisphosphonates, which are carbon-substituted pyrophosphates, have been studied extensively both in vivo and in vitro to elucidate their effects on bone tissues and cells. However, because these agents were shown to have a potent inhibitory effect on bone resorption, the majority of studies have focused on only this aspect of bone metabolism. There appears to be less information regarding the direct effect of bisphosphonates on bone formation, so thus we undertook experiments to investigate the effects of bisphosphonates, especially alendronate, on the mineralization and matrix protein synthesis of human osteoblastic cells in vitro. The data show that the bisphosphonates, alendronate, etidronate and pamidronate, suppressed 1,25-dihydroxycholecalciferol (1,25(OH)2D3)-stimulated mineralization of human osteoblastic cells at high concentrations, while relatively lower concentrations of alendronate and etidronate potentiated mineralization of the cells in the presence of 1,25(OH)2D3. The potentiation of mineralization with alendronate was accompanied by increased synthesis of bone matrix proteins, osteocalcin and collagen, and the mRNA of pro alpha(I) collagen. These findings show that in addition to their well-known effects on bone resorption, bisphosphonates have significant and direct effects on osteogenesis in osteoblasts in vitro. The actual mechanism remains to be further investigated.