COMPARISON OF CYTOTOXIC EFFECTS OF BISPHOSPHONATES IN-VITRO AND IN-VIVO

We compared the cytotoxic effects of alendronate (ALN) and incadronate (YM175) on isolated rabbit osteoclasts in vitro and on rats in vivo. In the in vitro experiment, each bisphosphonate was added to the cultu re of isolated osteoclasts at the final concentration of 3 x 10(-5), 3 x 10(-4), or 3 x 10(-3) M, and the amount of creatine phosphokinase ( CPK) released into the medium was taken as an index of cytotoxicity at 5, 10, and 24 hours after the treatment. Also viability of osteoclast s, measured in terms of trypan blue exclusion, was assessed at 24 hour s after the treatment. In YM175-treated groups, CPK activity in the me dium increased in a concentration-dependent manner with time, and phas e-contrast microscopic observation revealed damaged cell membranes and nuclear deterioration in YM175-treated osteoclasts. As a result, the viability of the osteoclasts was decreased at the concentrations of 3 x 10(-4) and 3 x 10(-3) M. However, in the ALN-treated groups, neither CPK activity nor viability of isolated osteoclasts changed significan tly compared with control levels even at 3 x 10(-3) M for up to 24 hou rs. In the in vivo experiment, each bisphosphonate was administered se parately to normal rats (7 weeks old, Sprague-Dawley) by intravenous i njection at 1, 5, or 25 mmol/kg. Two days after the injection, the ani mals were euthanized, and the plasma Ca concentration and total CPK ac tivity were measured. In YMI175-injected rats, the CPK activity increa sed at 25 mmol/kg, and a slight decrease in the plasma Ca level was se en at this dose. In contrast, in ALN-injected rats, CPK activity did n ot increase even at 5 or 25 mmol/kg, and the plasma Ca level did decre ase significantly compared with controls. Isozyme analysis revealed th at, not only was CPK activity increased in the BE type in YM175-inject ed rats, it was also increased in the MB and MM types. In conclusion, alendronate, unlike YM175, does not have any cytotoxic effects on oste oclasts either in vitro or in vivo.