It has long been known that small changes to the structure of the R(2) side chain of
nitrogen-containing
bisphosphonates can dramatically affect their potency for inhibiting
bone resorption in vitro and in vivo, although the reason for these differences in antiresorptive potency have not been explained at the level of a pharmacological target. Recently, several
nitrogen-containing
bisphosphonates were found to inhibit osteoclast-mediated
bone resorption in vitro by inhibiting
farnesyl diphosphate synthase, thereby preventing protein prenylation in osteoclasts. In this study, we examined the potency of a wider range of
nitrogen-containing
bisphosphonates, including the highly potent, heterocycle-containing
zoledronic acid and
minodronate (YM-529). We found a clear correlation between the ability to inhibit
farnesyl diphosphate synthase in vitro, to inhibit protein prenylation in cell-free extracts and in purified osteoclasts in vitro, and to inhibit
bone resorption in vivo. The activity of recombinant human
farnesyl diphosphate synthase was inhibited at concentrations > or = 1 nM
zoledronic acid or
minodronate, the order of potency (
zoledronic acid approximately equal to
minodronate >
risedronate >
ibandronate >
incadronate >
alendronate >
pamidronate) closely matching the order of antiresorptive potency. Furthermore, minor changes to the structure of the R(2) side chain of heterocycle-containing
bisphosphonates, giving rise to less potent inhibitors of
bone resorption in vivo, also caused a reduction in potency up to approximately 300-fold for inhibition of
farnesyl diphosphate synthase in vitro. These data indicate that
farnesyl diphosphate synthase is the major pharmacological target of these drugs in vivo, and that small changes to the structure of the R(2) side chain alter antiresorptive potency by affecting the ability to inhibit
farnesyl diphosphate synthase.