Zoledronic acid induces formation of a pro-apoptotic ATP analogue and isopentenyl pyrophosphate in osteoclasts in vivo and in MCF-7 cells in vitro

BACKGROUND AND PURPOSE

Bisphosphonates (BPs) are highly effective inhibitors of bone resorption. Nitrogen-containing bisphosphonates (N-BPs), such as zoledronic acid, induce the formation of a novel ATP analogue (1-adenosin-5'-yl ester 3-(3-methylbut-3-enyl) ester triphosphoric acid; ApppI), as a consequence of the inhibition of farnesyl pyrophosphate synthase and the accumulation of isopentenyl pyrophosphate (IPP). ApppI induces apoptosis, as do comparable metabolites of non-nitrogen-containing bisphosphonates (non-N-BPs). In order to further evaluate a pharmacological role for ApppI, we obtained more detailed data on IPP/ApppI formation in vivo and in vitro. Additionally, zoledronic acid-induced ApppI formation from IPP was compared with the metabolism of clodronate (a non-N-BP) to adenosine 5'(beta,gamma-dichloromethylene) triphosphate (AppCCl2p).

EXPERIMENTAL APPROACH

After giving zoledronic acid in vivo to rabbits, IPP/ApppI formation and accumulation was assessed in isolated osteoclasts. The formation of ApppI from IPP was compared with the metabolism of clodronate in MCF-7 cells in vitro. IPP/ApppI and AppCCl2p levels in cell extracts were analysed by mass spectrometry.

KEY RESULTS

Isopentenyl pyrophosphate/ApppI were formed in osteoclasts in vivo, after a single, clinically relevant dose of zoledronic acid. Furthermore, exposure of MCF-7 cells in vitro to zoledronic acid at varying times and concentrations induced time- and dose-dependent accumulation of IPP/ApppI. One hour pulse treatment was sufficient to cause IPP accumulation and subsequent ApppI formation, or the metabolism of clodronate into AppCCl2p.

CONCLUSIONS AND IMPLICATIONS

This study provided the first conclusive evidence that pro-apoptotic ApppI is a biologically significant molecule, and demonstrated that IPP/ApppI analysis is a sensitive tool for investigating pathways involved in BP action.

Fluorescently labeled risedronate and related analogues: "magic linker" synthesis

We report synthesis of the first fluorescently labeled conjugates of risedronate (1), using an epoxide linker strategy enabling conjugation of 1 via its pyridyl nitrogen with the label (carboxyfluorescein). Unlike prior approaches to create fluorescent bisphosphonate probes, the new linking chemistry did not abolish the ability to inhibit protein prenylation in vitro, while significantly retaining hydroxyapatite affinity. The utility of a fluorescent 1 conjugate in visualizing osteoclast resorption in vitro was demonstrated.

Phosphonocarboxylates inhibit the second geranylgeranyl addition by Rab geranylgeranyl transferase

Rab geranylgeranyl transferase (RGGT) catalyzes the post-translational geranylgeranyl (GG) modification of (usually) two C-terminal cysteines in Rab GTPases. Here we studied the mechanism of the Rab geranylgeranylation reaction by bisphosphonate analogs in which one phosphonate group is replaced by a carboxylate (phosphonocarboxylate, PC). The phosphonocarboxylates used were 3-PEHPC, which was previously reported, and 2-hydroxy-3-imidazo[1,2-a]pyridin-3-yl-2-phosphonopropionic acid ((+)-3-IPEHPC), a >25-fold more potent related compound as measured by both IC₅₀ and K_i.(+)-3-IPEHPC behaves as a mixed-type inhibitor with respect to GG pyrophosphate (GGPP) and an uncompetitive inhibitor with respect to Rab substrates. We propose that phosphonocarboxylates prevent only the second GG transfer onto Rabs based on the following evidence. First, geranylgeranylation of Rab proteins ending with a single cysteine motif such as CAAX, is not affected by the inhibitors, either in vitro or in vivo. Second, the addition of an -AAX sequence onto Rab-CC proteins protects the substrate from inhibition by the inhibitors. Third, we demonstrate directly that in the presence of (+)-3-IPEHPC, Rab-CC and Rab-CXC proteins are modified by only a single GG addition. The presence of (+)-3-IPEHPC resulted in a preference for the Rab N-terminal cysteine to be modified first, suggesting an order of cysteine geranylgeranylation in RGGT catalysis. Our results further suggest that the inhibitor binds to a site distinct from the GGPP-binding site on RGGT. We suggest that phosphonocarboxylate inhibitors bind to a GG-cysteine binding site adjacent to the active site, which is necessary to align the mono-GG-Rab for the second GG addition. These inhibitors may represent a novel therapeutic approach in Rab-mediated diseases.

Peripheral blood monocytes are responsible for gammadelta T cell activation induced by zoledronic acid through accumulation of IPP/DMAPP

Nitrogen-containing bisphosphonates indirectly activate Vγ9Vδ2 T cells through inhibition of farnesyl pyrophosphate synthase and intracellular accumulation of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), but the cells responsible for Vγ9Vδ2 T cell activation through IPP/DMAPP accumulation are unknown. Treatment of human peripheral blood mononuclear cells (PBMCs) with a pharmacologically relevant concentration of zoledronic acid induced accumulation of IPP/DMAPP selectively in monocytes, which correlated with efficient drug uptake by these cells. Furthermore, zoledronic acid-pulsed monocytes triggered activation of γδ T cells in a cell contact-dependent manner. These observations identify monocytes as the cell type directly affected by bisphosphonates responsible for Vγ9Vδ2 T cell activation.

New evidence for hominin carcass processing strategies at 1.5 Ma, Koobi Fora, Kenya

Reconstruction of early Pleistocene hominin carcass acquisition and processing behaviors are necessarily based at least in part on butchered fossil bones. This paper provides zooarchaeological and taphonomic analyses and behavioral interpretations of three approximately 1.5 million-year-old archaeofaunas from areas 1A and 103 in the Okote Member of the Koobi Fora Formation, northern Kenya: FwJj14A, FwJj14B, and GaJi14. These sites are all located in similar paleoenvironmental contexts, near shallow water with swampy, seasonally flooded areas, and some evidence for more wooded or gallery forest settings. Both individual specimen--and assemblage-level analyses of butchery-marked bones indicate that the hominins appear to have practiced similar butchery strategies at all of these sites, with butchery (defleshing, disarticulation, and marrow extraction) of both high- and low-ranked skeletal elements with no apparent preference for prey size, skeletal region, limb class, or limb portion. Only four tooth-marked specimens, including one likely crocodile-tooth-marked bone, are preserved in all three archaeofaunas. A paucity of limb epiphyses suggests that bone-crunching hyenids may have deleted these portions subsequent to hominin butchery. Strangely, there are no stone tools preserved with the 292 cut-marked and 27 percussion-marked faunal specimens (out of a total of 6,039 specimens), suggesting that raw material availability may have conditioned hominin lithic discard patterns at these locales. These assemblages increase our knowledge of the dietary behavior and ecology of Homo erectus, and provide support for variability in early Pleistocene hominin carcass foraging patterns.

Mechanisms of action of bisphosphonates: similarities and differences and their potential influence on clinical efficacy

Bisphosphonates (BPs) are well established as the leading drugs for the treatment of osteoporosis. There is new knowledge about how they work. The differences that exist among individual BPs in terms of mineral binding and biochemical actions may explain differences in their clinical behavior and effectiveness.

INTRODUCTION

The classical pharmacological effects of bisphosphonates (BPs) appear to be the result of two key properties: their affinity for bone mineral and their inhibitory effects on osteoclasts.

DISCUSSION

There is new information about both properties. Mineral binding affinities differ among the clinically used BPs and may influence their differential distribution within bone, their biological potency, and their duration of action. The antiresorptive effects of the nitrogen-containing BPs (including alendronate, risedronate, ibandronate, and zoledronate) appear to result from their inhibition of the enzyme farnesyl pyrophosphate synthase (FPPS) in osteoclasts. FPPS is a key enzyme in the mevalonate pathway, which generates isoprenoid lipids utilized for the post-translational modification of small GTP-binding proteins that are essential for osteoclast function. Effects on other cellular targets, such as osteocytes, may also be important. BPs share several common properties as a drug class. However, as with other families of drugs, there are obvious chemical, biochemical, and pharmacological differences among the individual BPs. Each BP has a unique profile that may help to explain potential clinical differences among them, in terms of their speed and duration of action, and effects on fracture reduction.

Visualizing mineral binding and uptake of bisphosphonate by osteoclasts and non-resorbing cells

Bisphosphonates (BPs) target bone due to their high affinity for calcium ions. During osteoclastic resorption, these drugs are released from the acidified bone surface and taken up by osteoclasts, where they act by inhibiting the prenylation of small GTPases essential for osteoclast function. However, it remains unclear exactly how osteoclasts internalise BPs from bone and whether other cells in the bone microenvironment can also take up BPs from the bone surface. We have investigated this using a novel fluorescently-labelled alendronate analogue (FL-ALN), and by examining changes in protein prenylation following treatment of cells with risedronate (RIS). Confocal microscopic analysis showed that FL-ALN was efficiently internalised from solution or from the surface of dentine by resorbing osteoclasts into intracellular vesicles. Accordingly, unprenylated Rap1A accumulated to the same extent whether osteoclasts were cultured on RIS-coated dentine or with RIS in solution. By contrast, J774 macrophages internalised FL-ALN and RIS from solution, but took up comparatively little from dentine, due to their inability to resorb the mineral. Calvarial osteoblasts and MCF-7 tumour cells internalised even less FL-ALN and RIS, both from solution and from the surface of dentine. Accordingly, the viability of J774 and MCF-7 cells was drastically reduced when cultured with RIS in solution, but not when cultured on dentine pre-coated with RIS. However, when J774 macrophages were co-cultured with rabbit osteoclasts, J774 cells that were adjacent to resorbing osteoclasts frequently internalised more FL-ALN than J774 cells more distant from osteoclasts. This was possibly a result of increased availability of BP to these J774 cells due to transcytosis through osteoclasts, since FL-ALN partially co-localised with trancytosed, resorbed matrix protein within osteoclasts. In addition, J774 cells occupying resorption pits internalised more FL-ALN than those on unresorbed surfaces. These data demonstrate that osteoclasts are able to take up large amounts of BP, due to their ability to release the BP from the dentine surface during resorption. By contrast, non-resorbing cells take up only small amounts of BP that becomes available due to natural desorption from the dentine surface. However, BP uptake by non-resorbing cells can be increased when cultured in the presence of resorbing osteoclasts.

Bisphosphonates: an update on mechanisms of action and how these relate to clinical efficacy

The bisphosphonates (BPs) are well established as the treatments of choice for disorders of excessive bone resorption, including Paget's disease of bone, myeloma and bone metastases, and osteoporosis. There is considerable new knowledge about how BPs work. Their classical pharmacological effects appear to result from two key properties: their affinity for bone mineral and their inhibitory effects on osteoclasts. Mineral binding affinities differ among the clinically used BPs and may influence their differential distribution within bone, their biological potency, and their duration of action. The inhibitory effects of the nitrogen-containing BPs (including alendronate, risedronate, ibandronate, and zoledronate) on osteoclasts appear to result from their inhibition of farnesyl pyrophosphate synthase (FPPS), a key branch-point enzyme in the mevalonate pathway. FPPS generates isoprenoid lipids used for the posttranslational modification of small GTP-binding proteins essential for osteoclast function. Effects on other cellular pathways, such as preventing apoptosis in osteocytes, are emerging as other potentially important mechanisms of action. As a class, BPs share several common properties. However, as with other classes of drugs, there are obvious chemical, biochemical, and pharmacological differences among the various individual BPs. Each BP has a unique profile that may help to explain potential important clinical differences among the BPs, in terms of speed of onset of fracture reduction, antifracture efficacy at different skeletal sites, and the degree and duration of suppression of bone turnover. As we approach the 40th anniversary of the discovery of their biological effects, there remain further opportunities for using their properties for medical purposes.

The matricellular protein CYR61 inhibits osteoclastogenesis by a mechanism independent of alphavbeta3 and alphavbeta5

Cysteine-rich protein 61 (CYR61/CCN1) belongs to the family of CCN matricellular proteins. Most of the known effects of CCN proteins appear to be due to binding to extracellular growth factors or integrins, including alpha(v)beta(3) and alpha(v)beta(5). Although CYR61 can stimulate osteoblast differentiation, until now the effect of CYR61 on osteoclasts was unknown. We demonstrate that recombinant human CYR61 inhibits the formation of multinucleated, alpha(v)beta(3)-positive, or tartrate-resistant acid phosphatase-positive human, mouse, and rabbit osteoclasts in vitro. CYR61 markedly reduced the expression of the osteoclast phenotypic markers tartrate-resistant acid phosphatase, matrix metalloproteinase-9, calcitonin receptor, and cathepsin K. However, CYR61 did not affect the formation of multinucleated osteoclasts when added to osteoclast precursors prior to fusion or affect the number or resorptive activity of osteoclasts cultured on dentine discs, indicating that CYR61 affects early osteoclast precursors but not mature osteoclasts. CYR61 did not affect receptor activator of nuclear factor-kappaB (RANK) ligand-induced phosphorylation of p38 or ERK1/2 in human macrophages and did not affect RANK ligand-induced activation of nuclear factor-kappaB, indicating that CYR61 does not appear to inhibit osteoclastogenesis by affecting RANK signaling. Furthermore, a mutant form of CYR61 defective in binding to alpha(v)beta(3) also inhibited osteoclastogenesis, and CYR61 inhibited osteoclastogenesis similarly in cultures of mouse wild-type or beta(5)(-/-) macrophages. Thus, CYR61 does not appear to inhibit osteoclast formation by interacting with alpha(v)beta(3) or alpha(v)beta(5). These observations demonstrate that CYR61 is a hitherto unrecognized inhibitor of osteoclast formation, although the exact mechanism of inhibition remains to be determined. Given that CYR61 also stimulates osteoblasts, CYR61 could represent an important bifunctional local regulator of bone remodeling.

A comparison between the effects of hydrophobic and hydrophilic statins on osteoclast function in vitro and ovariectomy-induced bone loss in vivo

Statins potently inhibit 3-hydroxy-3-methylglutaryl-coenzyme A reductase, blocking downstream biosynthesis of isoprenoid lipids and causing inhibition of protein prenylation. Prenylated signaling molecules are essential for osteoclast function, consistent with our previous observation that mevastatin can inhibit osteoclast activity in vitro. Several reports suggest that statins may also have an anabolic effect on bone and stimulate osteoblast differentiation. This study sought to determine the effects of both hydrophobic and hydrophilic statins, particularly rosuvastatin (RSV), on osteoclast function in vitro and in vivo. All statins tested (RSV, pravastatin [PRA], cerivastatin [CER], and simvastatin [SIM]) caused accumulation of unprenylated Rap-1A in rabbit osteoclast-like cells and J774 macrophages in vitro and inhibited osteoclast-mediated resorption. The order of potency for inhibiting prenylation in vitro (at concentrations of 0.01-50 muM) was CER>SIM>RSV>PRA. The most potent hydrophilic statin (CER, 0.05 and 0.3 mg/kg) inhibited prenylation in rabbit osteoclasts 24 hours after a single subcutaneous (s.c.) injection more effectively than the most potent hydrophobic statin (RSV, 20 mg/kg). However, in a mouse model of osteoporosis, s.c. 0.05 mg/kg/day CER and 2 or 20 mg/kg/day RSV for 3 weeks only mildly prevented loss of cortical and trabecular bone induced by ovariectomy. No increase in bone formation rate was observed with statin treatment, suggesting that this effect was due to inhibition of osteoclast-mediated resorption rather than increased bone formation.

Osteoclast-poor human osteopetrosis due to mutations in the gene encoding RANKL

Autosomal recessive osteopetrosis is usually associated with normal or elevated numbers of nonfunctional osteoclasts. Here we report mutations in the gene encoding RANKL (receptor activator of nuclear factor-KB ligand) in six individuals with autosomal recessive osteopetrosis whose bone biopsy specimens lacked osteoclasts. These individuals did not show any obvious defects in immunological parameters and could not be cured by hematopoietic stem cell transplantation; however, exogenous RANKL induced formation of functional osteoclasts from their monocytes, suggesting that they could, theoretically, benefit from exogenous RANKL administration.

Involvement of PLEKHM1 in osteoclastic vesicular transport and osteopetrosis in incisors absent rats and humans

This study illustrates that Plekhm1 is an essential protein for bone resorption, as loss-of-function mutations were found to underlie the osteopetrotic phenotype of the incisors absent rat as well as an intermediate type of human osteopetrosis. Electron and confocal microscopic analysis demonstrated that monocytes from a patient homozygous for the mutation differentiated into osteoclasts normally, but when cultured on dentine discs, the osteoclasts failed to form ruffled borders and showed little evidence of bone resorption. The presence of both RUN and pleckstrin homology domains suggests that Plekhm1 may be linked to small GTPase signaling. We found that Plekhm1 colocalized with Rab7 to late endosomal/lysosomal vesicles in HEK293 and osteoclast-like cells, an effect that was dependent on the prenylation of Rab7. In conclusion, we believe PLEKHM1 to be a novel gene implicated in the development of osteopetrosis, with a putative critical function in vesicular transport in the osteoclast.

Apomine enhances the antitumor effects of lovastatin on myeloma cells by down-regulating 3-hydroxy-3-methylglutaryl-coenzyme A reductase

Apomine, a 1,1-bisphosphonate-ester with antitumor activity, has previously been reported to strongly down-regulate 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase), the rate-limiting enzyme in the mevalonate pathway responsible for the prenylation of proteins. Here, we show that although apomine down-regulated HMG-CoA reductase protein levels in myeloma cells, it did not inhibit protein prenylation, and apomine-induced apoptosis could not be prevented by mevalonate, indicating that apomine cytotoxicity is independent from its effects on HMG-CoA reductase. Instead, apomine cytotoxicity was prevented by the addition of phosphatidylcholine, which is similar to the previously reported ability of phosphatidylcholine to overcome the cytotoxicity of farnesol, whereas phosphatidylcholine had no effect on down-regulation of HMG-CoA reductase by apomine. These findings raised the possibility that apomine, independent from its own cytotoxic effects, could enhance the antitumor effects of the competitive HMG-CoA reductase inhibitor lovastatin via down-regulating HMG-CoA reductase. Indeed, treatment with apomine in combination with lovastatin resulted in synergistic decreases in viable cell number and induction of apoptosis. At the concentrations used, apomine down-regulated HMG-CoA reductase protein levels without being cytotoxic. Accumulation of unprenylated Rap1A by lovastatin was enhanced in the presence of apomine. Furthermore, synergy was completely prevented by mevalonate, and apomine did not synergize with desoxolovastatin, which does not inhibit HMG-CoA reductase. We conclude that the synergistic drug interaction results from an enhancement by apomine of the effects of lovastatin, mediated by down-regulation of HMG-CoA reductase by apomine. Thus, these findings demonstrate a novel strategy for enhancing the antitumor effects of lovastatin.

Mechanisms of osteopontin and CD44 as metastatic principles in prostate cancer cells

BACKGROUND

The expression level of osteopontin correlates with the metastatic potential of several tumors. Osteopontin is a well-characterized ligand for the alphavbeta3 integrin. The present study was undertaken to elucidate the possible role of osteopontin/alphavbeta3 signaling in prostate cancer cell migration.

RESULTS

We generated stable prostate cancer cell (PC3) lines that over-express osteopontin (PC3/OPN), mutant OPN in the integrin binding-site (PC3/RGDDeltaRGA), and null for OPN (PC3/SiRNA). The following observations were made in PC3/OPN cells as compared with PC3 cells: 1) an increase in multinucleated giant cells and RANKL expression; 2) an increase in CD44 surface expression, interaction of CD44/MMP-9 on the cell surface, MMP-9 activity in the conditioned medium, and cell migration; 3) western blot analysis of concentrated conditioned medium exhibited equal levels of MMP-9 protein in all PC3 cells. However, zymography analysis demonstrated that the levels of MMP-9 activity in the conditioned media reflect the CD44 surface expression pattern of the PC3 cell lines; 4) although MMP-9 and MMP-2 are secreted by PC3 cells, only the secretion of MMP-9 is regulated by OPN expression. A strong down regulation of the above-mentioned processes was observed in PC3/OPN (RGA) and PC3/SiRNA cells. PC3/OPN cells treated with bisphosphonate (BP) reproduce the down-regulation observed in PC3/OPN (RGA) and PC3/SiRNA cells.

CONCLUSION

Rho signaling plays a crucial role in CD44 surface expression. BPs inhibits the mevalonate pathway, which in turn, prevents the prenylation of a number of small GTPases. Attenuation of Rho GTPase activation by BPs may have contributed to the down regulation of cell surface CD44/MMP-9 interaction, MMP-9 activation/secretion, and cell migration. Taken together, these observations suggest that CD44 surface expression is an important event in the activation of MMP-9 and migration of prostate cancer cells. The various steps involved in the above mentioned signaling pathway and/or the molecules regulating the activation of MMP-9 are potential therapeutic target.

A novel method for efficient generation of transfected human osteoclasts

Mature osteoclasts and their precursors are notoriously difficult to transfect using nonviral approaches, a limitation that represents a major technical obstacle in the study of osteoclast biology. Here, we describe a simple electroporation method using Amaxa Nucleofector technology that results in efficient transfection of human blood-derived osteoclast precursors, which can be differentiated in subsequent culture to generate mature osteoclasts that retain expression of the transgene. Moreover, since these osteoclasts maintain the ability to resorb dentine, this technique could prove useful for assessing the role of specific genes/proteins in osteoclast function.

Advances in treating metastatic bone cancer: summary statement for the First Cambridge Conference

The First Cambridge Conference on Advances in Treating Metastatic Bone Cancer, a symposium held in Cambridge, Massachusetts, October 28 to 29, 2005, was convened to discuss recent advances and research related to the natural history of bone metastases and skeletal complications, bone cancer biology, treatment of myeloma and other solid tumors, and treatment-induced bone loss. The conference format combined brief presentations with extended periods of discussion. The conclusions reached during the 2-day meeting are summarized in this article and presented in more detail in the individual articles and accompanying discussion sessions that comprise the conference proceedings.

Molecular mechanisms of action of bisphosphonates: current status

PURPOSE

Bisphosphonates are currently the most important class of antiresorptive agents used in the treatment of metabolic bone diseases, including tumor-associated osteolysis and hypercalcemia. These compounds have high affinity for calcium ions and therefore target bone mineral, where they are internalized by bone-resorbing osteoclasts and inhibit osteoclast function.

EXPERIMENTAL DESIGN

This article reviews the pharmacology of bisphosphonates and the relationship between chemical structure and antiresorptive potency. We also describe new insights into their intracellular molecular mechanisms of action, methods for assessing the effects of bisphosphonates on protein prenylation, and their potential as direct antitumor agents.

RESULTS

Nitrogen-containing bisphosphonates act intracellularly by inhibiting farnesyl diphosphate synthase, an enzyme of the mevalonate pathway, thereby preventing prenylation of small GTPase signaling proteins required for normal cellular function. Inhibition of farnesyl diphosphate synthase also seems to account for their antitumor effects observed in vitro and for the activation of gamma,delta T cells, a feature of the acute-phase response to bisphosphonate treatment in humans. Bisphosphonates that lack a nitrogen in the chemical structure do not inhibit protein prenylation and have a different mode of action that seems to involve primarily the formation of cytotoxic metabolites in osteoclasts.

CONCLUSIONS

Bisphosphonates are highly effective inhibitors of bone resorption that selectively affect osteoclasts in vivo but could also have direct effects on other cell types, such as tumor cells. After >30 years of clinical use, their molecular mechanisms of action on osteoclasts are finally becoming clear but their exact antitumor properties remain to be clarified.

Recent advances in understanding the mechanism of action of bisphosphonates

Bisphosphonates (BPs) are widely used in the treatment of diseases associated with excessive osteoclast-mediated bone resorption, such as osteoporosis. Although several years ago the molecular target of the potent nitrogen-containing BPs (N-BPs) was identified as farnesyl diphosphate synthase, an enzyme in the mevalonate pathway, recent data have shed new light on the precise mechanism of inhibition and demonstrated that the acute-phase reaction, an adverse effect of N-BPs, is also caused by inhibition of this enzyme. In addition, the identification of BP analogues that inhibit different enzymes in the mevalonate pathway could lead to the development of novel inhibitors of bone resorption with potential applications in the treatment of bone disease.

The molecular mechanism of nitrogen-containing bisphosphonates as antiosteoporosis drugs

Osteoporosis and low bone mass are currently estimated to be a major public health risk affecting >50% of the female population over the age of 50. Because of their bone-selective pharmacokinetics, nitrogen-containing bisphosphonates (N-BPs), currently used as clinical inhibitors of bone-resorption diseases, target osteoclast farnesyl pyrophosphate synthase (FPPS) and inhibit protein prenylation. FPPS, a key branchpoint of the mevalonate pathway, catalyzes the successive condensation of isopentenyl pyrophosphate with dimethylallyl pyrophosphate and geranyl pyrophosphate. To understand the molecular events involved in inhibition of FPPS by N-BPs, we used protein crystallography, enzyme kinetics, and isothermal titration calorimetry. We report here high-resolution x-ray structures of the human enzyme in complexes with risedronate and zoledronate, two of the leading N-BPs in clinical use. These agents bind to the dimethylallyl/geranyl pyrophosphate ligand pocket and induce a conformational change. The interactions of the N-BP cyclic nitrogen with Thr-201 and Lys-200 suggest that these inhibitors achieve potency by positioning their nitrogen in the proposed carbocation-binding site. Kinetic analyses reveal that inhibition is competitive with geranyl pyrophosphate and is of a slow, tight binding character, indicating that isomerization of an initial enzyme-inhibitor complex occurs with inhibitor binding. Isothermal titration calorimetry indicates that binding of N-BPs to the apoenzyme is entropy-driven, presumably through desolvation entropy effects. These experiments reveal the molecular binding characteristics of an important pharmacological target and provide a route for further optimization of these important drugs.