Structure-activity relationships for inhibition of farnesyl diphosphate synthase in vitro nd inhibition of bone resorption in vivo by nitrogen-containing bisphosphonates

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 r eason for these differences in antiresorptive potency have not been explain ed at the level of a pharmacological target. Recently, several nitrogen-con taining bisphosphonates were found to inhibit osteoclast-mediated bone reso rption in vitro by inhibiting farnesyl diphosphate synthase, thereby preven ting protein prenylation in osteoclasts. In this study, we examined the pot ency of a wider range of nitrogen- containing bisphosphonates, including th e 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 resorpt ion in vivo. The activity of recombinant human farnesyl diphosphate synthas e was inhibited at concentrations greater than or equal to1 nM zoledronic a cid or minodronate, the order of potency (zoledronic acid approximate to mi nodronate > risedronate > ibandronate > incadronate > alendronate > pamidro nate) closely matching the order of antiresorptive potency. Furthermore, mi nor changes to the structure of the R-2 side chain of heterocycle-containin g bisphosphonates, giving rise to less potent inhibitors of bone resorption in vivo, also caused a reduction in potency up to similar to 300-fold for inhibition of farnesyl diphosphate synthase in vitro. These data indicate t hat farnesyl diphosphate synthase is the major pharmacological target of th ese 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 farn esyl diphosphate synthase.