Zoledronic acid protects against local and systemic bone loss in tumor necrosis factor-mediated arthritis

Receptor activator of nuclear factor kappaB ligand and osteoprotegerin regulation of bone remodeling in health and disease

Osteoclasts and osteoblasts dictate skeletal mass, structure, and strength via their respective roles in resorbing and forming bone. Bone remodeling is a spatially coordinated lifelong process whereby old bone is removed by osteoclasts and replaced by bone-forming osteoblasts. The refilling of resorption cavities is incomplete in many pathological states, which leads to a net loss of bone mass with each remodeling cycle. Postmenopausal osteoporosis and other conditions are associated with an increased rate of bone remodeling, which leads to accelerated bone loss and increased risk of fracture. Bone resorption is dependent on a cytokine known as RANKL (receptor activator of nuclear factor kappaB ligand), a TNF family member that is essential for osteoclast formation, activity, and survival in normal and pathological states of bone remodeling. The catabolic effects of RANKL are prevented by osteoprotegerin (OPG), a TNF receptor family member that binds RANKL and thereby prevents activation of its single cognate receptor called RANK. Osteoclast activity is likely to depend, at least in part, on the relative balance of RANKL and OPG. Studies in numerous animal models of bone disease show that RANKL inhibition leads to marked suppression of bone resorption and increases in cortical and cancellous bone volume, density, and strength. RANKL inhibitors also prevent focal bone loss that occurs in animal models of rheumatoid arthritis and bone metastasis. Clinical trials are exploring the effects of denosumab, a fully human anti-RANKL antibody, on bone loss in patients with osteoporosis, bone metastasis, myeloma, and rheumatoid arthritis.

Bone degeneration and recovery after early and late bisphosphonate treatment of ovariectomized wistar rats assessed by in vivo micro-computed tomography

Bisphosphonates are antiresorptive drugs commonly used to treat osteoporosis. It is not clear, however, what the influence of the time point of treatment is. Recently developed in vivo micro-computed tomographic (CT) scanners offer the possibility to study such effects on bone microstructure in rats. The aim of this study was to determine the influence of early and late zoledronic acid treatment on bone in ovariectomized rats, using in vivo micro-CT. Twenty-nine female Wistar rats were divided into the following groups: ovariectomy (OVX, n = 5), OVX and zoledronic acid (ZOL) at week 0 (n = 8), OVX and ZOL at week 8 (n = 7), and sham (n = 9). CT scans were made of the proximal tibia at weeks 0, 2, 4, 8, 12, and 16; and bone structural parameters were determined in the metaphysis. Two fluorescent labels were administered to calculate dynamic histomorphometric parameters. At week 16, all groups were significantly different from each other in bone volume fraction (BV/TV), connectivity density, and trabecular number (Tb.N), except for the early ZOL and control groups which were not significantly different for any structural parameter. After ZOL treatment at week 8, BV/TV, structure model index, Tb.N, and trabecular thickness significantly improved in the late ZOL group. The OVX and ZOL groups showed, respectively, higher and lower bone formation rates than the control group. Early ZOL treatment inhibited all bone microstructural changes seen after OVX. Late ZOL treatment significantly improved bone microstructure, although the structure did not recover to original levels. Early ZOL treatment resulted in a significantly better microstructure than late treatment. However, late treatment was still significantly better than no treatment.

Longitudinal assessment of synovial, lymph node, and bone volumes in inflammatory arthritis in mice by in vivo magnetic resonance imaging and microfocal computed tomography

OBJECTIVE

To develop longitudinal 3-dimensional (3-D) measures of outcomes of inflammation and bone erosion in murine arthritis using contrast-enhanced magnetic resonance imaging (CE-MRI) and in vivo microfocal computed tomography (micro-CT) and, in a pilot study, to determine the value of entry criteria based on age versus synovial volume in therapeutic intervention studies.

METHODS

CE-MRI and in vivo micro-CT were performed on tumor necrosis factor-transgenic (TNF-Tg) mice and their wild-type littermates to quantify the synovial and popliteal lymph node volumes and the patella and talus bone volumes, respectively, which were validated histologically. These longitudinal outcome measures were used to assess the natural history of erosive inflammatory arthritis. We also performed anti-TNF versus placebo efficacy studies in TNF-Tg mice in which treatment was initiated according to either age (4-5 months) or synovial volume (3 mm(3) as detected by CE-MRI). Linear regression was performed to analyze the correlation between synovitis and focal erosion.

RESULTS

CE-MRI demonstrated the highly variable nature of TNF-induced joint inflammation. Initiation of treatment by synovial volume produced significantly larger treatment effects on the synovial volume (P = 0.04) and the lymph node volume (P < 0.01) than did initiation by age. By correlating the MRI and micro-CT data, we were able to demonstrate a significant relationship between changes in synovial and patellar volumes (R(2) = 0.75, P < 0.01).

CONCLUSION

In vivo CE-MRI and micro-CT 3-D outcome measures are powerful tools that accurately demonstrate the progression of erosive inflammatory arthritis in mice. These methods can be used to identify mice with arthritis of similar severity before intervention studies are initiated, thus minimizing heterogeneity in outcome studies of chronic arthritis seen between genetically identical littermates.

Erosive arthritis

Inflammation and degradation of bone are two closely linked processes. Chronic inflammatory arthritis not only leads to inflammatory bone loss but it also involves local erosion of articular bone. This osteo-destructive feature of chronic inflammatory arthritis is a major cause of disability in patients with rheumatoid arthritis. Osteoclasts are essential for the resorption of mineralized cartilage and subchondral bone in chronic arthritis. The observed up-regulation of osteoclast differentiation factors (receptor activator of nuclear factor-κB ligand [RANKL]) in the synovial membrane of chronically inflamed joints indicates that osteoclasts are abundant in this setting, leading to rapid degradation of mineralized tissue. Blockade of osteoclast formation is thus a key strategy in preventing structural damage in arthritis. Denosumab, a humanized antibody that neutralizes RANKL, is an attractive candidate agent to inhibit inflammatory bone loss.

Antiinflammatory treatment with bisphosphonates in ankylosing spondylitis

PURPOSE OF REVIEW

Bisphosphonates are antiosteoclastic agents widely used in the treatment of bone diseases. They also have antiinflammatory properties suggested by a clinical amelioration in animal models of arthritis. Bisphosphonates act on cells from the monocyte/macrophage lineage and may modulate the generation of proinflammatory cytokines. Ankylosing spondylitis is characterized by bone marrow subchondral inflammation with the presence of T cells and macrophages, and osteoporosis is a well known complication of the disease. Thus, bisphosphonates may be reasonably used as a therapeutic agent in ankylosing spondylitis.

RECENT FINDINGS

Different open trials have shown that the amino bisphosphonate pamidronate ameliorated the clinical symptoms of ankylosing spondylitis, mainly axial disease, and in one study, peripheral arthritis. Laboratory parameters of inflammation were, in general, not influenced while biochemical markers of bone turnover fell significantly under pamidronate. Imaging modifications of the inflammatory lesions using MRI were also improved with pamidronate. A dose-controlled study demonstrated a higher efficacy for 60 mg pamidronate compared with a 10 mg dose.

SUMMARY

The amino bisphosphonate pamidronate has shown clinical and radiological amelioration in ankylosing spondylitis patients, although this improvement is mild and transient. Additional studies are required to better define the real impact of pamidronate on ankylosing spondylitis and its place among the different treatment options for the disease.

Pathological role of osteoclast costimulation in arthritis-induced bone loss

Abnormal T cell immune responses induce aberrant expression of inflammatory cytokines such as TNF-alpha, leading to osteoclastmediated bone erosion and osteoporosis in autoimmune arthritis. However, the mechanism underlying enhanced osteoclastogenesis in arthritis is not completely understood. Here we show that TNF-alpha contributes to inflammatory bone loss by enhancing the osteoclastogenic potential of osteoclast precursor cells through inducing paired Ig-like receptor-A (PIR-A), a costimulatory receptor for receptor activator of NF-kappaB (RANK). In fact, bone erosion and osteoporosis, but not inflammation, caused by aberrant TNF-alpha expression were ameliorated in mice deficient in Fc receptor common gamma subunit or beta(2)-microglobulin, in which the expression of PIR-As and PIR-A ligands is impaired, respectively. These results establish the pathological role of costimulatory receptors for RANK in bone loss in arthritis and may provide a molecular basis for the future therapy of inflammatory diseases.

Mechanisms of bone loss in rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune inflammatory disease in which destruction of bone in the joints causes major morbidity. Recent research has shed light on the cell and molecular mechanisms that lead to this osteolysis, all due directly or indirectly to the chronic inflammation. The aspects of this research covered in this review include the alteration of cell proliferation and survival that results in growth of the RA synovium. This process depends upon an increase in angiogenesis and local blood flow, which is also a feature of increased bone turnover. In addition, the inflammatory environment increases expression of chemokines, which are involved in the recruitment of monocytic osteoclast precursors. Chronic inflammation also promotes an overall catabolic state, with increased osteoclast differentiation and resorptive activity, driven by disregulation of receptor activator of NF-kappaB ligand (RANKL) and the synergistic activity of inflammatory cytokines such as tumor necrosis factor-alpha and interleukin-1. Osteoclast survival is increased in this environment, but osteoblast differentiation and survival are decreased, with a consequent reduction in bone formation and a net loss of bone. Recognition of these processes and the factors involved will enable more effective and targeted treatments for RA.

Cells of the synovium in rheumatoid arthritis. Osteoclasts

Osteoclasts are multinucleated cells of hematopoietic origin and are the primary bone resorbing cells. Numerous osteoclasts are found within the synovial tissue at sites adjacent to bone, creating resorption pits and local bone destruction. They are equipped with specific enzymes and a proton pump that enable them to degrade bone matrix and solubilize calcium, respectively. The synovial tissue of inflamed joints has a particularly high potential to accumulate osteoclasts because it harbors monocytes/macrophages, which function as osteoclast precursors, as well as cells that provide the specific molecular signals that drive osteoclast formation. Osteoclasts thus represent a link between joint inflammation and structural damage since they resorb mineralized tissue adjacent to the joint and destroy the joint architecture.

Inflammation-induced bone loss: can it be prevented?

Inflammatory synovitis induces profound bone loss and OCLs are the instrument of this destruction. TNF blockers have an established role in the prevention of inflammatory bone loss in RA; however, not all patients respond to anti-TNF therapy and side effects may prevent long-term treatment in others. The B-cell--depleting antibody rituximab and the T-cell costimulation blocker abatacept are emerging as major treatment options for patients who are resistant to anti-TNF [96,97]. Proof-of-concept studies demonstrate that targeting RANK-mediated osteoclastogenesis prevents inflammatory bone loss and clinical application has only just begun. The efficacy of RANKL inhibition has been witnessed in trials of Denosumab, and RANKL-neutralizing antibodies are likely to become the treatment of choice for blocking RANKL in RA [77,78]. A major limitation of RANKL antagonism is that it does not treat synovitis. Therefore, anti-RANKL therapy most likely will be used in the context of MTX therapy. There is uncertainty about the possible extraskeletal adverse effects of long-term effects of long-term RANKL blockade. In particular, anti-RANKL therapy could jeopardize dendritic cell function or survival. The demonstrable role of OCLs in inflammation-induced bone loss also invites a reconsideration of the new BPs for bone protection [98]. Studies of ZA in preclinical models indicate that bone protection is comparable to that afforded by OPG. One possible caveat is that intravenous BPs are linked to jaw osteonecrosis [99], although the incidence is confined mainly to intensive treatment in the oncology setting. Although pulsed PTH stimulated bone formation in arthritic models, it has yet to be proven clinically in the context of powerful OCL inhibition with TNF or RANKL antagonists. With strategies that normalize OCL numbers, clinicians are poised to accomplish effective prevention of inflammation-induced bone loss.

Suppression of inflammation and structural damage in experimental arthritis through molecular targeted therapy with PPI-2458

OBJECTIVE

To determine the disease-modifying activity and mechanism of action of the orally available methionine aminopeptidase type 2 inhibitor, [(1R)-1-carbamoyl-2-methyl-propyl]-carbamic acid-(3R,4S,5S,6R)-5-methoxy-4-[(2R,3R)-2-methyl-3-(3-methyl-but-2-enyl)-oxiranyl]-1-oxa-spiro [2.5] oct-6-yl ester (PPI-2458), in a rat model of peptidoglycan-polysaccharide (PG-PS)-induced arthritis.

METHODS

Arthritis was induced in rats by administration of PG-PS, causing tarsal joint swelling and histopathologic changes characteristic of rheumatoid arthritis (RA). PPI-2458, a potent irreversible methionine aminopeptidase type 2 inhibitor, was administered orally every other day at 1, 5, or 10 mg/kg.

RESULTS

In an in vitro osteoclastogenesis model, PPI-2458 potently inhibited osteoclast differentiation and bone resorption. In the rat PG-PS arthritis model, PPI-2458 afforded significant protection against established disease after therapeutic dosing. This in vivo activity of PPI-2458 was linked to the inhibition of methionine aminopeptidase type 2. Histopathologic assessment of affected joints showed improvement in processes of inflammation, bone resorption, and cartilage erosion, associated with significant improvement in all clinical indices. The protective effects of PPI-2458 against bone destruction in vivo, including the structural preservation of affected hind joints, correlated with improvements in bone histomorphometric markers, as determined by microfocal computed tomography and a significant decrease in systemic C-telopeptide of type I collagen, suggesting decreased osteoclast activity in vivo. Moreover, PPI-2458 prevented cartilage erosion as shown by a significant decrease in systemic cartilage oligomeric matrix protein.

CONCLUSION

The findings of this study suggest that PPI-2458 exerts disease-modifying activity in experimental arthritis through its direct inhibition of several pathophysiologic processes of this disease. These results provide a rationale for assessing the potential of PPI-2458 as a novel RA therapy.

Antitumor Effects of Clinical Dosing Regimens of Bisphosphonates in Experimental Breast Cancer Bone Metastasis

BACKGROUND

Bisphosphonates exhibit direct antitumor activity in animal models, but only at high doses that are incompatible with the clinical dosing regimens approved for the treatment of cancer patients with skeletal metastases. We compared the antitumor effects of clinical dosing regimens of the bisphosphonates zoledronic acid and clodronate in a mouse model of bone metastasis.

METHODS

Mice (n = 6-10 per group) were treated with zoledronic acid, clodronate, or vehicle starting before (preventive protocols) or after (treatment protocols) intravenous injection with human B02/GFP.2 breast cancer cells, which express green fluorescent protein (GFP) and luciferase and metastasize to bone. Zoledronic acid was given as daily, weekly, or single doses at a cumulative dose of 98-100 microg/kg body weight, equivalent to the 4-mg intravenous dose given to patients. Clodronate was given as a daily dose (530 microg/kg/day), equivalent to the daily 1600-mg oral clinical dose given to patients. Bone destruction was measured by radiography, x-ray absorptiometry or tomography, and histomorphometry (as the ratio of bone volume to tissue volume). Skeletal tumor burden was measured by histomorphometry (as the ratio of tumor burden to soft tissue volume [TB/STV]) and luciferase activity. All statistical tests were two-sided.

RESULTS

In treatment protocols, daily clodronate was less effective at decreasing the TB/STV ratio than daily (53% versus 87%, difference = 34%, 95% confidence interval [CI] = 16% to 44%, P < .001) or weekly (53% versus 90%, difference = 37%, 95% CI = 19% to 46%, P < .001) zoledronic acid-dosing regimens. Compared with vehicle, a single dose of zoledronic acid decreased tumor burden by only 16% (95% CI = 9% to 22%, P < .001). In preventive protocols, daily clodronate and daily or weekly zoledronic acid decreased the TB/STV ratio by 49% (95% CI = 40% to 57%, P = .006), 83% (95% CI = 68% to 98%, P < .001), and 66% (95% CI = 47% to 84%, P < .001), respectively, compared with vehicle, whereas a single dose of zoledronic acid decreased tumor burden by only 13% (95% CI = -2% to 28%, P = .84). Mice treated with a daily preventive regimen of clodronate or with a daily or weekly preventive regimen of zoledronic acid showed a decreased B02/GFP.2 cell tumor burden compared with vehicle, whereas a single preventive dose of zoledronic acid had no effect.

CONCLUSION

Daily or repeated intermittent therapy with clinical doses of bisphosphonates inhibits skeletal tumor growth in a mouse model.

Fcgamma receptors directly mediate cartilage, but not bone, destruction in murine antigen-induced arthritis: uncoupling of cartilage damage from bone erosion and joint inflammation

OBJECTIVE

To determine the relationship between synovial inflammation and the concomitant occurrence of cartilage and bone erosion during conditions of variable inflammation using various Fcgamma receptor knockout (FcgammaR(-/-)) mice.

METHODS

Antigen-induced arthritis (AIA) was introduced in the knee joints of various FcgammaR(-/-) mice and wild-type controls. Joint inflammation and cartilage and bone destruction levels were determined by histologic analysis. Cathepsin K, RANKL, and osteoprotegerin (OPG) levels were detected by immunolocalization.

RESULTS

In FcgammaRIIb(-/-) mice, which lack the inhibiting Fcgamma receptor IIb, levels of joint inflammation and cartilage and bone destruction were significantly higher (infiltrate 93%, exudate 200%, cartilage 100%, bone 156%). AIA in mice lacking activating FcgammaR types I, III, and IV, but not FcgammaRIIb (FcR gamma-chain(-/-) mice), prevented cartilage destruction completely. In contrast, levels of bone erosion and joint inflammation were comparable with their wild-type controls. Of great interest, in arthritic mice lacking activating FcgammaR types I, II, and III, but not IV (FcgammaRI/II/III(-/-) mice), levels of joint inflammation were highly elevated (infiltrate and exudate, 100% and 188%, respectively). Cartilage destruction levels were decreased by 92%, whereas bone erosion was increased by 200%. Cathepsin K, a crucial mediator of osteoclasts, showed a strong correlation with the amount of inflammation but not with the amount of activating FcgammaR, which was low in osteoclasts. RANKL, but not OPG, levels were higher in the inflammatory cells of arthritic knee joints of FcgammaRI/II/III(-/-) mice versus wild-type mice.

CONCLUSION

Activating FcgammaR are crucial in mediating cartilage destruction independently of joint inflammation. In contrast, FcgammaR are not directly involved in bone erosion. Indirectly, FcgammaR drive bone destruction by regulating joint inflammation.

Extra-skeletal effects of bisphosphonates

Bisphosphonates are pharmacological agents which are currently used both in osteoporosis than in other pathological conditions characterised by an increased bone resorption, such as Paget's disease of bone, malign hypocalcaemia during myeloma, osteolytic bone metastasis and fibrous dysplasia of bone. The most important biological effect of bisphosphonates is the reduction of bone remodelling through the inhibition of osteoclastic activity, but there are many clinical and experimental evidences of extra-skeletal biological effects of bisphosphonates. It has been shown that bisphosphonates exert their effects not only on bone tissue cells, but also on those of the immune system with an "immuno-modulating" effect, influencing the production of pro- and anti-inflammatory cytokines and changing the molecular expression involved in the immune processes and anti-inflammatory response. Although the available data are conflicting, there are several reports concerning the beneficial effects of bisphosphonates in controlling the progression of chronic joint inflammatory diseases, suggesting a wider use for these therapeutic agents in clinical practice.

Pathogenesis and Prevention of Bone Loss in Patients Who Have Kidney Disease and Receive Long-Term Immunosuppression

The coexistence of kidney disease with a need for immunosuppressive therapy leads to the convergence of several threats to bone. These comprise general effects of the primary disease, e.g., inflammatory state, more specific effects of acute renal failure or chronic kidney disease, and effects of therapies. Multisystem inflammatory disease that requires immunosuppression is associated frequently with kidney damage, and any reduction of kidney function that takes the patient into or beyond chronic kidney disease stage 2 for more than a short time is likely to have a negative impact on bone health. Bone mineral density frequently is low and fracture rates are high, although correlations often are poor. Chronic inflammation leads to local and systemic imbalance between bone formation and resorption. Upregulation of NF-kappabeta ligand (RANKL) and variable downregulation of osteoprotegerin are implicated, and bone health may improve in response to treatment of the inflammatory state. Certain immunosuppressive agents, especially glucocorticoids and calcineurin inhibitors, contribute further to bone loss. Antiresorptive agents such as bisphosphonates are used widely and, although able to prevent loss of bone mineral density, have uncertain effects on fracture rates. Augmentation of anabolic activity is desirable but elusive. Synthetic parathyroid hormone is untested but has potential. Manipulation of the RANKL/osteoprotegerin system now is feasible using antibodies to RANKL or synthetic osteoprotegerin. In the future, manipulation of the calcium-sensing receptor using calcimimetic or calcilytic agents may allow the anabolic effects of parathyroid hormone to be harnessed to good effect. With all of these therapies, it will be important to assess response in relation to important clinical end points such as fracture.

Multimodal imaging analysis of tumor progression and bone resorption in a murine cancer model

OBJECTIVE

This study evaluates the use of multimodal imaging to qualitatively and quantitatively measure tumor progression and bone resorption in a xenotransplanted tumor model of human neuroblastoma.

METHODS

Human neuroblastoma cells expressing a luciferase reporter gene were injected into the femur of nu/nu mice. Tumor progression with and without zoledronic acid treatment was monitored using radiographs, D-luciferin-induced luminescence, micro-computer tomography (CT) and micro-magnetic resonance imaging (MRI).

RESULTS

We observed a gradual increase in D-luciferin-based bioluminescence concomitant with detectable osteolytic lesions. Tumor growth was inhibited (P=0.003-0.07) with zoledronic acid treatment. Micro-CT analysis in vivo provided a method to quantify bone loss, and its prevention by zoledronic acid. High-resolution MRI images allowed the observation of tumor cells within the bone marrow cavity, as well as distant metastasis.

CONCLUSION

Multimodal imaging allows to measure tumor growth and bone resorption simultaneously in vivo and also proved useful in the detection distant metastasis.

Bisphosphonate therapy in rheumatoid arthritis

Focal bone damage and generalized bone loss are features of rheumatoid arthritis (RA). The introduction of TNFalpha antagonists has radically improved the management of RA by providing a means of slowing or preventing the occurrence of focal bone damage. However, some patients with severe RA have contraindications to TNFalpha antagonist therapy and others either fail to respond or fail to tolerate TNFalpha antagonists. In addition, whether TNFalpha antagonists effectively combat generalized bone loss remains unknown. Bisphosphonates can prevent generalized bone loss. Their main target is the osteoclast, which has been identified as the culprit in focal bone damage caused by inflammatory diseases. As a result, the potential effects of bisphosphonates on focal bone damage related to RA are generating strong interest. Although results from the few studies in humans have been disappointing, new insights into the mechanisms of action of amino-bisphosphonates and recent data obtained in animals, most notably with new-generation bisphosphonates, have rekindled the hope that bisphosphonates may be beneficial in RA. We review herein the main studies of the effects of bisphosphonate therapy on focal bone damage and generalized bone loss in patients with RA.

Activation of p38 MAPK is a key step in tumor necrosis factor-mediated inflammatory bone destruction

OBJECTIVE

To investigate whether activation of p38 MAPK is a crucial signaling factor in inflammatory bone destruction mediated by tumor necrosis factor (TNF). Mice overexpressing TNF were treated with 2 different inhibitors of p38 MAPK, and the effect of this treatment on joint inflammation and structural damage was assessed.

METHODS

Human TNF-transgenic mice received systemic treatment with 2 different p38 MAPK inhibitors (RO4399247 and AVE8677). Treatment was started at the time of symptom onset and lasted for 6 weeks. Mice were assessed for clinical signs of arthritis, bone erosion, and cartilage damage. In addition, the effect of these inhibitors on osteoclast generation in vitro and in vivo was assessed.

RESULTS

Both p38 MAPK inhibitors significantly reduced clinical signs of TNF-mediated arthritis. This was attributable to reducing synovial inflammation by 50% without affecting the cellular composition of the infiltrate. Synovial expression of interleukin-1 and RANKL was reduced upon p38 MAPK blockade, and activation of the molecular target MAPK-activated protein kinase 2 (MAPKAP-2) was also inhibited. Proteoglycan loss of articular cartilage was reduced by 50%, although p38 MAPK inhibition did not change matrix molecule synthesis by cultivated chondrocytes. Importantly, bone loss was almost completely prevented by p38 MAPK inhibition. The numbers of synovial osteoclasts and precursors were dramatically reduced, and both p38 MAPK inhibitors also inhibited in vitro osteoclastogenesis at micromolar concentrations and blocked activation of MAPKAP-2 as well as differentiation markers in cultured osteoclast precursors.

CONCLUSION

These results suggest the major importance of p38 MAPK for TNF-mediated inflammatory bone destruction in arthritis and suggest that inhibition of p38 MAPK might be an important tool for reducing structural damage in rheumatoid arthritis.

Bone loss in inflammatory arthritis: mechanisms and therapeutic approaches with bisphosphonates

The inflammatory process in rheumatoid arthritis provokes intense bone resorption, evidenced as bone erosions, juxta-articular osteopenia and generalized osteoporosis. These types of bone loss share a common pathogenesis, and the role of osteoclasts in focal bone erosion was verified in elegant animal studies. The tumour necrosis factor (TNF) family of cytokines and receptors--specifically TNF-alpha, RANKL, RANK and OPG--are dominant regulators of osteoclastic bone resorption in rheumatoid arthritis. The confirmation of the osteoclast mechanism provides new insight into the structural joint protection afforded by disease-modifying drugs and suggests innovative approaches to limit bone destruction. Emerging treatment strategies for bone disease in rheumatoid arthritis are the use of monoclonal antibodies to neutralize RANKL, and powerful bisphosphonates that target pathogenic osteoclasts.

Biologic therapies in clinical development for the treatment of rheumatoid arthritis

The therapeutic objective in patients with rheumatoid arthritis (RA) is reduction of disease activity with an ultimate goal of disease remission. Limitations of currently available disease-modifying antirheumatic drugs and biologic therapies suggest that there remains an unmet need for agents that advance these goals in a greater proportion of patients. Progress in our understanding of the regulatory molecules and pathways that mediate the immune and inflammatory responses necessary for the initiation and perpetuation of RA has led to the identification of new targets for therapy. It is expected that the therapeutic modulation of these targets, which include proinflammatory cytokines, T and B cells, adhesion molecules, chemokines, and intra- and extracellular signaling pathways, can provide new treatment strategies in patients with RA and other autoimmune disorders. Toward this end, a series of novel agents with diverse mechanisms of action are in development. Although many of these agents are still beyond the clinical horizon, several of them have shown promise in recent trials. This article reviews a few of the many treatment strategies currently being evaluated, which are hoped to lead to greater benefits and better disease management in the clinical setting.

Suppression of adjuvant-induced arthritic bone destruction by cyclooxygenase-2 selective agents with and without inhibitory potency against carbonic anhydrase II

In vitro assays revealed that COX-2 inhibitors with CA II inhibitory potency suppressed both differentiation and activity of osteoclasts, whereas that without the potency reduced only osteoclast differentiation. However, all COX-2 inhibitors similarly suppressed bone destruction in adjuvant-induced arthritic rats, indicating that suppression of osteoclast differentiation is more effective than that of osteoclast activity for the treatment.

INTRODUCTION

Cyclooxygenase (COX)-2 and carbonic anhydrase II (CA II) are known to play important roles in the differentiation of osteoclasts and the activity of mature osteoclasts, respectively. Because several COX-2 selective agents were recently found to possess an inhibitory potency against CA II, this study compared the bone sparing effects of COX-2 selective agents with and without the CA II inhibitory potency.

MATERIALS AND METHODS

Osteoclast differentiation was determined by the mouse co-culture system of osteoblasts and bone marrow cells, and mature osteoclast activity was measured by the pit area on a dentine slice resorbed by osteoclasts generated and isolated from bone marrow cells. In vivo effects on arthritic bone destruction were determined by radiological and histological analyses of hind-paws of adjuvant-induced arthritic (AIA) rats.

RESULTS

CA II was expressed predominantly in mature osteoclasts, but not in the precursors. CA II activity was inhibited by sulfonamide-type COX-2 selective agents celecoxib and JTE-522 similarly to a CA II inhibitor acetazolamide, but not by a methylsulfone-type COX-2 inhibitor rofecoxib. In vitro assays clearly revealed that celecoxib and JTE-522 suppressed both differentiation and activity of osteoclasts, whereas rofecoxib and acetazolamide suppressed only osteoclast differentiation and activation, respectively. However, bone destruction in AIA rats was potently and similarly suppressed by all COX-2 selective agents whether with or without CA II inhibitory potency, although only moderately by acetazolamide.

CONCLUSIONS

Suppression of osteoclast differentiation by COX-2 inhibition is more effective than suppression of mature osteoclast activity by CA II inhibition for the treatment of arthritic bone destruction.

Rheumatic diseases: the effects of inflammation on bone

Rheumatoid arthritis, juvenile idiopathic arthritis, the seronegative spondyloarthropathies including psoriatic arthritis, and systemic lupus erythematosus are all examples of rheumatic diseases in which inflammation is associated with skeletal pathology. Although some of the mechanisms of skeletal remodeling are shared among these diseases, each disease has a unique impact on articular bone or on the axial or appendicular skeleton. Studies in human disease and in animal models of arthritis have identified the osteoclast as the predominant cell type mediating bone loss in arthritis. Many of the cytokines and growth factors implicated in the inflammatory processes in rheumatic diseases have also been demonstrated to impact osteoclast differentiation and function either directly, by acting on cells of the osteoclast-lineage, or indirectly, by acting on other cell types to modulate expression of the key osteoclastogenic factor receptor activator of nuclear factor (NF) kappaB ligand (RANKL) and/or its inhibitor osteoprotegerin (OPG). Further elucidation of the mechanisms responsible for inflammation-induced bone loss will potentially lead to the identification of novel therapeutic strategies for the prevention of bone loss in these diseases. In this review, we provide an overview of the cell types, inflammatory mediators, and mechanisms that are implicated in bone loss and new bone formation in inflammatory joint diseases.

Zoledronic acid in the treatment of Paget's disease and other benign bone disorders

Zoledronic acid is a potent bisphosphonate widely used to counteract cancer-related bone loss. Once-yearly or even less frequent doses may be an effective therapy for benign bone disorders and may also result in improved compliance and tolerability. Data from two Phase III clinical trials have demonstrated the efficacy and safety of zoledronic acid in the treatment of Paget's disease of bone. Other studies have provided preliminary evidence for the utility of zoledronic acid in osteoporosis and other benign bone disorders. The major adverse effects associated with zoledronic acid infusions are transient flu-like symptoms. Ongoing clinical trials will provide key data on the ability of this agent to treat osteoporosis and prevent fractures in high-risk patients and on its long-term safety profile.

Synovial fibroblasts promote osteoclast formation by RANKL in a novel model of spontaneous erosive arthritis

OBJECTIVE

Erosion of cartilage and bone is a hallmark of rheumatoid arthritis (RA). This study was undertaken to explore the roles of hyperproliferating synovial fibroblasts and macrophages in abnormal osteoclast formation, using the recently described BXD2 mouse model of RA.

METHODS

Cell distribution in the joints was analyzed by immunohistochemistry, using tartrate-resistant acid phosphatase (TRAP) staining to identify osteoclasts. To identify the defective cells in BXD2 mice, mouse synovial fibroblasts (MSFs) were cultured with bone marrow-derived macrophages. Osteoclast formation was assayed by TRAP staining and bone resorption pit assay, and the cytokine profiles of the MSFs and macrophages were determined by quantitative real-time polymerase chain reaction and enzyme-linked immunosorbent assay.

RESULTS

In BXD2 mice, TRAP-positive osteoclasts were found at sites of active bone erosion, in close proximity to hyperproliferating synovial fibroblasts. On coculture, MSFs from BXD2 mice, but not C57BL/6 mice, produced high levels of RANKL messenger RNA, induced macrophages to form osteoclasts, and actively eroded bone slices, through a mechanism(s) that could be blocked by pretreatment with osteoprotegerin. Although macrophages from BXD2 mice expressed higher basal levels of tumor necrosis factor alpha (TNFalpha), interleukin-1beta (IL-1beta), and IL-6 than those from C57BL/6 mice, abnormal osteoclast formation was not due to enhanced sensitivity of the BXD2 mouse macrophages to RANKL. TNFalpha, produced by both BXD2 MSFs and BXD2 mouse macrophages, had a strong stimulatory effect on RANKL expression.

CONCLUSION

BXD2 MSFs produce RANKL and induce the development of osteoclasts from macrophages. The enhanced production of RANKL is possibly due to autocrine stimulation, together with paracrine stimulation by factors produced by macrophages.

Tumor necrosis factor prevents alendronate-induced osteoclast apoptosis in vivo by stimulating Bcl-xL expression through Ets-2

OBJECTIVE

To investigate why bisphosphonates are less effective at preventing focal bone loss in rheumatoid arthritis (RA) patients than in those with generalized osteoporosis, and the mechanisms involved.

METHODS

The response of osteoclasts to alendronate (ALN) in tumor necrosis factor-transgenic (TNF-Tg) mice that develop erosive arthritis and in wild-type littermates was studied. TNF-Tg and wild-type mice were given ALN, and the osteoclast numbers in the inflamed joints and in the long bones were compared. The expression levels of Bcl-xL in the osteoclasts of TNF-Tg and wild-type mice were examined by immunostaining. The effect of overexpression of Bcl-xL and Ets-2 proteins on ALN-induced osteoclast apoptosis was determined using an in vitro osteoclast survival assay and retrovirus transfer approach.

RESULTS

ALN reduced osteoclast numbers in the metaphyses by 97%, but by only 46% in the adjacent inflamed joints. Bcl-xL expression was markedly higher in osteoclasts in the joints than in those in the metaphyses of TNF-Tg mice. Bcl-xL or Ets-2 overexpression protected osteoclasts from ALN-induced apoptosis, and TNF stimulated Bcl-xL and Ets-2 expression in osteoclasts. Overexpression of Ets-2 increased Bcl-xL messenger RNA in osteoclasts, while a dominant-negative form of the Ets-2 blocked the protective effect of Bcl-xL or TNF on ALN-induced apoptosis.

CONCLUSION

The reduced efficacy of bisphosphonates to stop bone erosion in the inflamed joints of RA patients may result from local high levels of TNF up-regulating Ets-2 expression in osteoclasts, which in turn stimulates Bcl-xL expression in them and reduces their susceptibility to bisphosphonate-induced apoptosis.

Mechanisms of Disease: the link between RANKL and arthritic bone disease

Chronic inflammation and bone loss are closely linked pathophysiologic events. The most typical example of inflammatory bone loss is seen in patients with rheumatoid arthritis who develop systemic osteopenia as well as local breakdown of bone in the direct vicinity of inflamed joints. Understanding the mechanisms of arthritic bone degradation is crucial for designing therapies that can specifically protect joints from structural damage. Since osteoclast differentiation and activity are key events in arthritic bone damage, the signals that trigger osteoclastogenesis are potential therapeutic targets. Receptor activator of nuclear factor-kappaB (RANK) is activated by its ligand, RANKL, an essential molecule for osteoclast development: in the absence of RANKL or RANK, osteoclast differentiation from monocyte precursors does not occur. RANKL is expressed on T cells and fibroblasts within the synovial inflammatory tissue of patients with RA and its expression is regulated by proinflammatory cytokines. In animal models of arthritis, blockade of RANKL-RANK interactions, or a genetic absence of RANKL or RANK, protects against joint damage despite the presence of joint inflammation. Therefore, inhibition of RANKL is regarded as a promising future strategy for inhibiting inflammatory bone loss in patients with chronic inflammatory arthritis.

The use of bisphosphonates in the treatment of osteoporosis

PURPOSE OF REVIEW

The bisphosphonates alendronate and risedronate, given orally once weekly, are the cornerstone of treatment of postmenopausal osteoporosis, as well as of male and secondary osteoporosis. They reduce significantly the risk of vertebral and nonvertebral fractures; their effects appear early, within 6-12 months, and appear to be sustained. Several questions remain unanswered, however. In addition, data on a new bisphosphonate became available in 2004.

RECENT FINDINGS

The optimal duration of treatment has not been clearly established. Long-term data with alendronate are now available, indicating a persistence of alendronate effects on bone mineral density and bone turnover markers several years after stopping treatment given for 5 years. Whether these effects translate into sustained reduction of fractures needs to be further analyzed. Because of their efficacy, bisphosphonate use has been explored in other forms of osteoporosis, such as after androgen deprivation therapy for prostatic cancer. The challenge of long-term compliance with treatment of osteoporosis has triggered the use of intermittent bisphosphonate. The effects of intermittent oral and intravenous ibandronate on bone mineral density, bone turnover, and fractures have been recently reported.

SUMMARY

The mechanism by which bisphosphonates improve bone strength is not yet fully understood but probably involves complex effects on different components of bone strength, such as microarchitecture.

RANKL is a marker and mediator of local and systemic bone loss in two rat models of inflammatory arthritis

RANKL is an essential mediator of bone erosions, but the role of RANKL in systemic bone loss had not been studied in arthritis. RANKL protein was increased in rat joint extracts and serum at the earliest stages of arthritis. Osteoprotegerin (OPG) treatment reversed local and systemic bone loss, suggesting that RANKL is both a marker and mediator of bone loss in arthritis.

INTRODUCTION

RANKL is well established as an essential mediator of bone erosions in inflammatory arthritis, but the role of RANKL in systemic bone loss in arthritis had not been studied. We hypothesized that serum RANKL could serve as both a mediator and as a novel biomarker for local and systemic bone loss in arthritis. We challenged this hypothesis in two established rat models of inflammatory arthritis. We sought to determine whether serum RANKL was elevated early in disease progression and whether RANKL suppression could prevent both local and systemic bone loss in these models.

MATERIALS AND METHODS

Detailed time-course studies were conducted in animals with collagen-induced (CIA) or adjuvant-induced (AIA) arthritis to evaluate the onset and progression of inflammation (paw swelling), bone erosions, osteoclast numbers, and RANKL protein levels in arthritic joints and in serum. Additional CIA and AIA rats (n=8/group) received placebo (PBS) or recombinant OPG (3 mg/kg three times weekly) for 10 days beginning 4 days after disease onset (first macroscopic evidence of hind paw erythema and edema) to assess the role of RANKL in local and systemic bone loss.

RESULTS

RANKL protein was significantly elevated in the joints and serum of CIA and AIA rats within 1-2 days of disease onset. Increased RANKL levels were associated with local (hind paw) and systemic (vertebral) osteopenia in both models. The RANKL inhibitor OPG prevented local and systemic osteopenia in both models of established disease.

CONCLUSIONS

RANKL protein is significantly increased both locally and systemically during the earliest stages of inflammatory arthritis in rats, suggesting that serum RANKL might have prognostic value for bone erosions and systemic osteopenia in this condition. RANKL inhibition through OPG prevented local and systemic bone loss in these arthritis models, suggesting that RANKL inhibition is a promising new approach for treating bone loss in arthritis.

Interaction between Synovial Inflammatory Tissue and Bone Marrow in Rheumatoid Arthritis

Rheumatoid arthritis (RA) leads to destruction of cartilage and bone. Whether rheumatoid arthritis also affects the adjacent bone marrow is less clear. In this study, we investigated subcortical bone marrow changes in joints from patients with RA. We describe penetration of the cortical barrier by synovial inflammatory tissue, invasion into the bone marrow cavity and formation of mononuclear cell aggregates with B cells as the predominant cell phenotype. B cells expressed common B cell markers, such as CD20, CD45RA, and CD79a, and were mature B cells, as indicated by CD27 expression. Plasma cells were also present and were enriched in the regions between aggregates and inflammatory tissue. Moreover, molecules for B cell chemoattraction, such as BCA-1 and CCL-21, homing, mucosal addressin cell adhesion molecule-1 and survival, BAFF, were expressed. Endosteal bone next to subcortical bone marrow aggregates showed an accumulation of osteoblasts and osteoid deposition. In summary, we show that synovial inflammatory tissue can reach the adjacent bone marrow by fully breaking the cortical barrier, which results in formation of B cell-rich aggregates as well as increased formation of new bone. This suggests that bone marrow is an additional compartment in the disease process of RA.

Influence of bisphosphonates on the production of pro-inflammatory cytokines by activated human articular chondrocytes

Bisphosphonates have anti-inflammatory effects in rheumatoid arthritis and chondroprotective effects in animal arthritis models but their influence on chondrocytes is not known. The aim of this study is to investigate whether bisphosphonates could influence the production of pro-inflammatory cytokines by activated chondrocytes. Therefore human articular cartilage explants were incubated at 48 h with clodronate, pamidronate or risedronate (10(-6) and 10(-8)mol/L), and dexamethasone (10(-8)mol/L). Subsequently, cultures were stimulated with IL-1, 10 ng/mL (n=6) or 1 ng/mL (n=10) for 48 h. Co-incubation was performed with or without bisphosphonates or dexamethasone. A flow cytometric microsphere-based immunoassay was used for the detection of the pro-inflammatory cytokines IL-6, IL-8, TNF-alpha, IL-1 and the regulatory cytokines IL-12p70 and IL-10 in the supernatants. Stimulation with IL-1 resulted in a dose dependent induction of IL-6 and IL-8, but no production of the other cytokines could be demonstrated. This production of IL-6 and IL-8 was neither inhibited nor enhanced by bisphosphonates. Only dexamethasone caused an inhibition of IL-6 production. In conclusion, there is no evidence on the level of articular cartilage cells that bisphosphonates would suppress or enhance IL-6 and IL-8 mediated joint destruction.

Incadronate disodium inhibits joint destruction and periarticular bone loss only in the early phase of rat adjuvant-induced arthritis

Destruction of articular cartilage and subchondral bone loss in the affected joints of rat adjuvant arthritis have never been quantified histologically. This study aimed to evaluate the effect of incadronate disodium on joint destruction and periarticular bone loss, using histomorphometric measurements. Seven-week-old female Lewis rats were injected with 0.1 mg of heat-killed Mycobacterium butyricum into the tail base. Immediately after sensitization, vehicle, or incadronate at 10 or 100 microg/kg per day, was administered subcutaneously, three times per week. Hind-paw volume was measured weekly and the animals were killed at 2, 4, 6, and 10 weeks after sensitization. After taking X-rays, decalcified sagittal sections of the ankle joint were prepared and stained with toluidine blue and tartarate-resistant acid phosphatase. Articular cartilage destruction and subchondral bone loss were evaluated histomorphometrically. At 2 weeks after sensitization, no radiographic or histologic changes were observed. However, at 4 weeks, severe articular cartilage destruction and subchondral bone loss were found in the arthritic control group, while these changes were inhibited dose-dependently by incadronate treatment. At 6 and 10 weeks, both the destructive changes and the bone loss had further progressed, and they were not inhibited by incadronate treatment. Incadronate dose-dependently inhibited articular cartilage destruction and subchondral bone loss at 4 weeks after sensitization in this adjuvant arthritis model. However, the suppressive effects of incadronate did not continue until 6 and 10 weeks.

Analysis of the kinetics of osteoclastogenesis in arthritic rats

OBJECTIVE

To analyze the kinetics of osteoclastogenesis in 2 models of chronic immune-mediated arthritis and 1 model of acute arthritis.

METHODS

Adjuvant-induced arthritis (AIA) and collagen-induced arthritis (CIA) in Lewis rats were used as models of chronic arthritis. Acute arthritis was induced in Lewis rats by injecting carrageenan into the hind paw. Osteoclasts were identified by cathepsin K immunohistochemistry at various time points after the onset of arthritis. The location, size, and nucleation of osteoclasts were also analyzed.

RESULTS

In both AIA and CIA, multinucleated and cathepsin K-positive osteoclasts first were observed on the day of disease onset. Initially, osteoclasts were localized at the periosteum next to the synovial membrane and in subchondral bone channels. The number, size, and nucleation of osteoclasts rapidly increased, leading to severe bone loss within days after disease onset. In addition, numerous mononucleated cathepsin K-positive osteoclast precursor cells emerged in the synovial membrane. All osteoclasts (cathepsin K-positive, multinucleated, attached to bone) and osteoclast precursors (cathepsin K-positive, mononucleated or multinucleated, within synovial tissue) were also positive for a macrophage-specific marker. Upon induction of acute arthritis with carrageenan, osteoclasts formed transiently in subchondral bone, but regressed 7 days after disease onset.

CONCLUSION

Functional osteoclasts are generated at the earliest stage of arthritis, and new precursors are continuously formed in the synovial membrane to replenish the osteoclast pool. These data indicate that anti-resorptive therapies may provide the most effective bone protection, when treatment is started soon after the onset of arthritis.