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.

Synthesis and biological evaluation of alpha-halogenated bisphosphonate and phosphonocarboxylate analogues of risedronate

Alpha-halogenated analogues of the anti-resorptive bisphosphonate risedronate (5, Ris) and its phosphonocarboxylate cognate (7, 3-PEHPC) were synthesized and compared with 5, 7, and the corresponding desoxy analogues in bone mineral affinity and mevalonate pathway inhibition assays. The Ris (5e-h) and 3-PEHPC (7e-h) analogues had decreased bone mineral affinity, confirming that the alpha-OH group in 5 and 7 enhances bone affinity. The 5 alpha-halo-analogues potently inhibited farnesyl pyrophosphate synthase (FPPS) with IC50 values from 16 (alpha-F) to 340 (alpha-Br) nM (5, 6 nM). In contrast, 7 alpha-halo-analogues were ineffective versus FPPS (IC50 > 600 microM), but inhibited Rab geranylgeranyl transferase (RGGT) (IC50 = 16-35 microM) similarly to 7 itself (IC50 = 24 microM). The alpha-F analogue 7e was 1-2 times as active as 7 in J774 cell viability and Rab11 prenylation inhibition assays.

Design and synthesis of potent and selective 1,3,4-trisubstituted-2-oxopiperazine based melanocortin-4 receptor agonists

The design and synthesis of a series of potent 1,3,4-trisubstituted-2-oxopiperazine based MC4 agonists are described. The tripeptidomimetic analogs (12a,b and 23) and the dipeptidomimetic 27 displayed single-nanomolar binding affinity and agonist potency for MC4R and excellent selectivity for MC4R relative to MC1R.

Design, synthesis, and evaluation of proline and pyrrolidine based melanocortin receptor agonists. A conformationally restricted dipeptide mimic approach

The design, synthesis, and structure-activity relationships (SAR) of a series of novel proline and pyrrolidine based melanocortin receptor (MCR) agonists are described. To validate a conformationally constrained Arg-Nal dipeptide analogue strategy, we first synthesized and evaluated a test set of cis-(2R,4R)-proline analogues (21a-g). All of these compounds showed significant binding and agonist potency at the hMC1R, hMC3R, and hMC4R. Potent cis-(2S,4R)-pyrrolidine based MCR agonists (35a-g) were subsequently developed by means of this design approach. A SAR study directed toward probing the effect of the two chiral centers in the pyrrolidine ring on biological activity revealed the importance of the (S) absolute configuration at the 2-position for binding affinity, agonist potency, and receptor selectivity. Among the four sets of the pyrrolidine diastereomers investigated, analogues with the (2S,4R) configuration were the most potent agonists across the three receptors, followed by those possessing the (2S,4S) configuration.

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.

Novel insights into actions of bisphosphonates on bone: differences in interactions with hydroxyapatite

Bisphosphonates are now the most widely used drugs for diseases associated with increased bone resorption, such as osteoporosis. Although bisphosphonates act directly on osteoclasts, and interfere with specific biochemical processes such as protein prenylation, their ability to adsorb to bone mineral also contributes to their potency and duration of action. The aim of the present study was to compare the binding affinities for hydroxyapatite (HAP) of 6 bisphosphonates currently used clinically and to determine the effects of these bisphosphonates on other mineral surface properties including zeta potential and interfacial tension. Affinity constants (K(L)) for the adsorption of bisphosphonates were calculated from kinetic studies on HAP crystal growth using a constant composition method at 37 degrees C and at physiological ionic strength (0.15 M). Under conditions likely to simulate bisphosphonate binding onto bone, there were significant differences in K(L) among the bisphosphonates for HAP growth (pH 7.4) with a rank order of zoledronate > alendronate > ibandronate > risedronate > etidronate > clodronate. The measurements of zeta potential show that the crystal surface is modified by the adsorption of bisphosphonates in a manner best explained by molecular charges related to the protonation of their side-chain moieties, with risedronate showing substantial differences from alendronate, ibandronate, and zoledronate. The studies of the solid/liquid interfacial properties show additional differences among the bisphosphonates that may influence their mechanisms for binding and inhibiting crystal growth and dissolution. The observed differences in kinetic binding affinities, HAP zeta potentials, and interfacial tension are likely to contribute to the biological properties of the various bisphosphonates. In particular, these binding properties may contribute to differences in uptake and persistence in bone and the reversibility of effects. These properties, therefore, have potential clinical implications that may be important in understanding differences among potent bisphosphonates, such as the apparently more prolonged duration of action of alendronate and zoledronate compared with the more readily reversible effects of etidronate and risedronate.

Inhibition of protein prenylation by bisphosphonates causes sustained activation of Rac, Cdc42, and Rho GTPases

N-BPs, which inhibit bone resorption by preventing prenylation of small GTPases, unexpectedly cause the accumulation of GTP-bound, unprenylated Rho family GTPases in macrophages and osteoclasts. In macrophages, this also leads to sustained, Rac-mediated activation of p38. The antiresorptive activity of N-BPs may therefore be caused at least in part, by the accumulation of unprenylated small GTPases, causing inappropriate activation of downstream signaling pathways.

INTRODUCTION

Nitrogen-containing bisphosphonates (N-BPs) are potent inhibitors of bone resorption that act by inhibiting farnesyl diphosphate synthase, thereby indirectly preventing the prenylation of Rho family GTPases that are required for the function and survival of bone-resorbing osteoclasts. However, the effect that these drugs have on the activity of Rho family GTPases has not been determined.

MATERIALS AND METHODS

The effect of N-BPs on the activity of Rho family GTPases in J774 macrophages and osteoclasts was measured using a pull-down assay to isolate the GTP-bound forms. The effect of N-BPs, or decreasing Rac expression using siRNA, on downstream p38 activity was evaluated by Western blotting and apoptosis assessed by measurement of caspase 3/7 activity.

RESULTS

Rather than inhibiting GTPase function, loss of prenylation after treatment with N-BPs caused an increase in the GTP-bound form of Rac, Cdc42, and Rho in J774 cells and osteoclast-like cells, which paralleled the rate of accumulation of unprenylated small GTPases. Activation of Rac also occurred with other inhibitors of prenylation of Rho-family proteins, such as mevastatin and the geranylgeranyl transferase I inhibitor GGTI-298. The Rac-GTP that increased after N-BP treatment was newly translated, cytoplasmic unprenylated protein, because it was not labeled with [(14)C] mevalonate, and the increase in Rac-GTP was prevented by cycloheximide. Furthermore, this unprenylated Rac-GTP retained at least part of its functional activity in J774 cells, because it mediated N-BP-induced activation of p38. Paradoxically, although risedronate induces apoptosis of J774 macrophages by inhibiting protein prenylation, the p38 inhibitor SB203580 enhanced N-BP-induced apoptosis, suggesting that Rac-induced p38 activation partially suppresses the pro-apoptotic effect of N-BPs in these cells.

CONCLUSIONS

N-BP drugs may disrupt the function of osteoclasts in vivo and affect other cell types in vitro by inhibiting protein prenylation, thereby causing inappropriate and sustained activation, rather than inhibition, of some small GTPases and their downstream signaling pathways.

Selective inhibition of Rab prenylation by a phosphonocarboxylate analogue of risedronate induces apoptosis, but not S-phase arrest, in human myeloma cells

Bisphosphonates (BPs) are widely used in the treatment of osteolytic bone disease associated with multiple myeloma, and have been demonstrated to exert antitumor effects both in vitro and in vivo. However, the precise molecular mechanisms involved in the direct antitumor effects of BPs in vitro are not known. Nitrogen-containing BPs, such as risedronate (RIS), act by inhibiting protein prenylation. A phosphonocarboxylate analogue of RIS, 3-PEHPC, has previously been shown in osteoclasts and macrophages to specifically inhibit prenylation of Rab GTPases. The aim of this study was to identify the molecular targets of RIS and 3-PEHPC in human myeloma cells and to determine the cellular effects of selective inhibition of Rab prenylation by 3-PEHPC as compared to nonspecific inhibition of protein prenylation by RIS in human myeloma cells. RIS dose-dependently inhibited prenylation of both Rap1A and Rab6, whereas 3-PEHPC only inhibited Rab6 prenylation. Both RIS and 3-PEHPC dose-dependently increased apoptosis in human myeloma cells. RIS induced an accumulation of cells in the S-phase of the cell cycle, associated with inhibition of DNA replication. In contrast, 3-PEHPC did not cause cell-cycle arrest. Furthermore, geranylgeraniol could prevent inhibition of prenylation, induction of apoptosis, and cell-cycle arrest in response to RIS, but not inhibition of Rab prenylation and apoptosis induced by 3-PEHPC, consistent with specific inhibition of Rab geranylgeranyl transferase by 3-PEHPC. In conclusion, our studies demonstrate that selective inhibition of Rab prenylation induces apoptosis, but not S-phase arrest, thus identifying distinct molecular pathways that mediate the antimyeloma effect of nitrogen-containing BPs.

Synthesis of Tic-D-Phe Psi[CH2-CH2] isostere and its use in the development of melanocortin receptor agonists

The first synthesis of Tic-D-Phe Psi[CH(2)-CH(2)] isostere is described, which features diastereoselective alkylation of the tricyclic lactam 14. The use of this novel dipeptide isostere in the development of melanocortin agonists has been demonstrated by the synthesis of peptidomimetic 7 and non-peptidic ligand 27. Both compounds displayed significant binding and agonist potency at the MC4R.

Design, synthesis, and evaluation of proline based melanocortin receptor ligands

A series of proline based melanocortin ligands has been developed on the basis of initial piperazine leads by using a more conformationally rigid scaffold. A number of these novel ligands showed significant binding affinity for MC3 and MC4 receptors.

Phosphonocarboxylate inhibitors of Rab geranylgeranyl transferase disrupt the prenylation and membrane localization of Rab proteins in osteoclasts in vitro and in vivo

Nitrogen-containing bisphosphonate drugs such as risedronate act by inhibiting farnesyl diphosphate synthase, thereby disrupting protein prenylation in osteoclasts. We recently found that an anti-resorptive phosphonocarboxylate analogue of risedronate, 3-PEHPC (previously referred to as NE10790), selectively prevents prenylation of Rab GTPases in vitro by specifically inhibiting Rab geranylgeranyl transferase. In this study, we demonstrate that unprenylated Rab6 could be detected in J774 cells after treatment with 3-PEHPC or risedronate for as little as 4 h, and reached 50% after 24 h. Furthermore, treatment of J774 cells or osteoclasts with either 3-PEHPC or risedronate disrupted membrane association of several Rab family proteins. Like risedronate, the effects of 3-PEHPC are likely to be restricted to osteoclasts in vivo, since both risedronate and 3-PEHPC inhibited Rab prenylation in osteoclasts, but not in general bone marrow cells, when administered to rabbits in vivo. Analysis of two new phosphonocarboxylate analogues of 3-PEHPC (3-PEPC and 2-PEPC) revealed that, first, the geminal hydroxyl group is not essential for inhibition of Rab prenylation by phosphonocarboxylates, but does contribute to their anti-resorptive potency, most likely by enhancing their affinity for bone mineral. Second, the position of the nitrogen in the side chain of phosphonocarboxylates is crucial for their ability to inhibit Rab prenylation and hence to inhibit bone resorption. In addition, there is a good correlation between the ability of the phosphonocarboxylates to inhibit Rab prenylation and to inhibit bone resorption in vitro, indicating that these compounds are a new class of pharmacological agents that inhibit bone resorption by specifically preventing prenylation of Rab proteins. Furthermore, although phosphonocarboxylates are analogues of bisphosphonates, the structure-activity relationships of phosphonocarboxylates for inhibiting Rab geranylgeranyltransferase appear to differ from the structure-activity relationships of bisphosphonates for inhibiting farnesyl diphosphate synthase.

Bisphosphonates and Cancer-Induced Bone Disease: Beyond Their Antiresorptive Activity: Figure 1

Bisphosphonates are primarily known for their ability to inhibit osteoclast-mediated bone resorption. They are an indispensable part of therapy for patients with cancers that cause osteolysis. However, there is now a growing body of evidence from preclinical research showing that bisphosphonates also exhibit antitumor activity, both in vitro and in vivo. They can affect molecular mechanisms of tumor cell adhesion, invasion, and proliferation; reinforce the effects of cytotoxic agents in a synergistic manner; and exhibit antiangiogenic and immunomodulatory effects. These preclinical findings reveal exciting ways of optimizing bisphosphonate therapy in oncology to fully exploit their antitumor potential.

End-capping of the modified melanocortin tetrapeptide (p-Cl)Phe-D-Phe-Arg-Trp-NH2 as a route to hMC4R agonists

Of the 42 R'-X-(p-Cl)Phe-D-Phe-Arg-Trp-NH(2) (X=CO, SO(2), PO, PS) tested at the human (h)MC1, hMC3, and hMC4 receptors (R), the most potent MC4R agonists (EC(50) of 8-20 nM) were obtained by end-capping with R'=CH(2)CHCH(2) (9), NCCH(2) (16), NH(2)COCH(2) (17), HCONHCH(2) (18), CH(3)NH (19), CH(2)CHCH(2)NH (21), 2-Th (23), PhCH(2) (30) and X=CO. These compounds possess 35-60-fold hMC4 versus hMC1Rs selectivity with urea LK-71 (19) being the most potent at hMC4R and MC4/1R selective (EC(50)=8.5 nM, MC4/1R=100). LK-75 (16) combines high potency at hMC4R and MC4/3R selectivity (EC(50)=10.5 nM, MC4/3R=290). SAR is discussed.

Analogs of sub-nanomolar hMC1R agonist LK-184 [Ph(CH2)3CO-His-d-Phe-Arg-Trp-NH2]. An additional binding site within the human melanocortin receptor 1?

Twenty nine analogs of a superpotent MC1R agonist LK-184 (1) were tested at human melanocortin receptors (hMC1, hMC3, and hMC4Rs). All derivatives with the spacer between the N-terminus and the aromatic ring longer or shorter than C(3) were much less potent at hMC1R than 1. Only LK-312 PhCO(CH(2))(3)CO-His-d-Phe-Arg-Trp-NH(2) (3), partially mimicking the pi-system of 1, had an EC(50) of 0.05 nM at hMC1R, which confirms the localization of the pi-binding zone of the receptor. Truncation of 1 to Ph(CH(2))(3)CO-His-d-Phe-Arg-NH(2) gave a full MC1 agonist, LK-394 (30), with an EC(50) of 5 nM and a weak partial agonism at MC3/4Rs. This suggests the existence of an additional binding site within hMC1R next to that for the core sequence His-d-Phe-Arg-Trp-NH(2).

Differentiating the mechanisms of antiresorptive action of nitrogen containing bisphosphonates

Bisphosphonates (BPS) inhibit bone resorption and are divided into two classes according to their chemical structure and mechanism of action: nonnitrogen containing BPS such as etidronate and clodronate that are of low potency and inhibit osteoclast function via metabolism into toxic ATP-metabolites and nitrogen-containing BPS (NBPS), such as alendronate and risedronate that inhibit the enzyme of the mevalonate biosynthetic pathway farnesyl pyrophosphate synthase (FPPS), resulting in inhibition of the prenylation of small GTP-binding proteins in osteoclasts and disruption of their cytoskeleton. Previously, studies in various cell types suggested, however, that pamidronate functions by mechanism(s) additional or independent of the mevalonate pathway. To examine if such mechanism(s) are also involved in the action of NBPS on osteoclastic bone resorption, we examined the action of alkyl and heterocyclic NBPS with close structural homology on FPPS/isopentenyl pyrophosphate isomerase (IPPI) activity, on osteoclastic resorption, and on reversibility of this effect with GGOH. As expected, both pamidronate and alendronate suppressed bone resorption and FPPS/IPPI activity, the latter with greater potency than the first. Surprisingly, however, unlike alendronate, the antiresorptive effect of pamidronate was only partially reversible with GGOH, indicating the involvement of mechanism(s) of action additional to that of suppression of FPPS. Comparable results were obtained with the heterocyclic NBP NE-21650, a structural analog of risedronate. Thus, despite an effect on FPPS, the actions on bone resorption of some NBPS may involve mechanisms additional to suppression of FPPS. These findings may lead to identification of additional pathways that are important for bone resorption and may help to differentiate among members of the NBP class which are currently distinguished only according to their potency to inhibit bone resorption.

Sub-nanomolar hMC1R agonists by end-capping of the melanocortin tetrapeptide His-D-Phe-Arg-Trp-NH(2)

Twenty three derivatives of the core fragment His(6)-D-Phe(7)-Arg(8)-Trp(9)-NH(2) end-capped with carboxylic and sulfonic acids were synthesized and evaluated at human melanocortin receptors (hMC1, hMC3, and hMC4Rs). The SAR within this series allowed us to map the hMCRs near the His(6) binding site and design a superpotent MC1R agonist, LK-184, Ph(CH(2))(3)CO-His-D-Phe-Arg-Trp-NH(2) (19) with EC(50) 0.01 nM (5 nM at MC3 and MC4Rs).

Novel tetrahydropyran-based peptidomimetics from a bioisosteric transformation of a tripeptide. Evidence of their activity at melanocortin receptors

We have prepared novel peptidomimetics based on a 2,4,6-trisubstituted tetrahydropyran. This scaffold was constructed in an isosteric transformation using conceptual constraints imposed on a tripeptide moiety involving O(i)'-C(i+1)(gamma) and O(i)'-N(i+2) formal cyclization modes. A series of regioselective transformations commencing with a substituted dihydropyran-4-one readily provided the required analogues. Specific tetrahydropyrane analogues modeled on PheArgTrp as a truncated version of the melanocortin receptor message sequence, showed activity at the melanocortin receptors MC4R and MC1R. Thus, the 2,4,6-trisubstituted tetrahydropyran scaffold has provided a potentially useful peptidomimetic lead, and conceptual cyclization of peptide moieties can offer a valuable design strategy in peptidomimetic research.

Synthesis of 2,4,5-trisubstituted tetrahydropyrans as peptidomimetic scaffolds for melanocortin receptor ligands

[reaction: see text] We have synthesized a series of 2,4,5-trisubstituted tetrahydropyran derivatives to determine the utility of this scaffold as a peptidomimetic platform. The key synthetic steps involved a palladium-mediated cross-coupling reaction of a dihydropyran-4-one moiety to introduce R(2) followed by a sequential regio- and diastereoselective reduction of sp(2) carbon centers. Selected compounds have shown biological activity at melanocortin receptors, indicating that this scaffold may be useful in the design of peptidomimetics relating to a tripeptide structure.