Effect of L-000845704, an alphaVbeta3 integrin antagonist, on markers of bone turnover and bone mineral density in postmenopausal osteoporotic women

Integrins as therapeutic targets: lessons and opportunities

The integrins are a large family of cell adhesion molecules that are essential for the regulation of cell growth and function. The identification of key roles for integrins in a diverse range of diseases, including cancer, infection, thrombosis and autoimmune disorders, has revealed their substantial potential as therapeutic targets. However, so far, pharmacological inhibitors for only three integrins have received marketing approval. This article discusses the structure and function of integrins, their roles in disease and the chequered history of the approved integrin antagonists. Recent advances in the understanding of integrin function, ligand interaction and signalling pathways suggest novel strategies for inhibiting integrin function that could help harness their full potential as therapeutic targets.

Current, new and future treatments of osteoporosis

Osteoporosis as a common chronic disease is challenging human health. Although different therapeutic options are routinely used for prevention/treatment of osteoporosis, their side effects and benefits are under question. Increasing our knowledge about signaling pathways in bone and osteocytes as well as osteoblasts and osteoclasts will help us in designing new therapeutic modalities for osteoporosis. In the present study, all new therapeutic measures of osteoporosis have been reviewed. For this purpose, search engines like Pubmed, Web of Science, Scopus, Google Scholar were searched and all relevant articles were found. The study was limited to the year 1998-2010. Bisphosphonates are the cornerstone of osteoporosis treatment, but there are not enough relevant studies that investigated their equivalencies in comparison with each other or the other medications. Therefore, medication selection is empirical and subjective. Furthermore, no eminent study has compared certain combinations. There are new hopes for treatment of osteoporosis, which are more specific with less harm. Our results show that new and emerging therapies are more potent and target specified which more individualize osteoporosis treatment; however, more investigations on their safety and efficacy in comparison with current medications are highly recommended.

Prevalent and Emerging Therapies for Osteoporosis

Osteoporosis and fractures associated with it constitute a real and serious socio-medical problem, which only recently has come to the forefront of social consciousness. With increasing number of exservicemen and their dependents, osteoporosis management has become very important in our setup. Currently available pharmacological therapies for prevention of fragility fractures are limited in scope, efficacy and acceptability to patients. Oral bisphosphonates are the standard treatment for osteoporosis which are associated with significant gastrointestinal side effects and thus poor patient compliance. Newer regimens, including intravenous (IV) formulations of bisphosphonates, have successfully come in vogue with greater patient compliance and equal or better benefits. The real need in osteoporosis treatment is for additional anabolic drugs. The only currently approved anabolic agent for treating osteoporosis is teriparatide (recombinant human parathyroid hormone 1-34), which stimulates new bone formation. Considerable efforts are being made to develop new, more effective treatment for osteoporosis. These novel drugs under trial include those primarily inhibiting osteoclastic bone resorption (like bisphosphonates) such as inhibitors of receptor activator of nuclear factor-kappa B ligand (RANKL) signalling, cathepsin K inhibitors, c-Src kinase inhibitors, integrin inhibitors, chloride channel inhibitors and the drugs with osteo-anabolic actions such as orally active parathyroid hormone (PTH) analogues, calcium sensing receptor antagonists, PTH-related peptide analogues and agents that induce osteoblast anabolism via pathways involving key, recently identified, molecular targets (wnt low-density lipoprotein receptor-related protein-5 signalling; sclerostin antibodies).

New treatment targets in osteoporosis

Postmenopausal osteoporosis is characterized by bone remodeling alterations with an imbalance between excessive bone resorption and inadequate bone formation. At present, osteoporosis treatment rests on bone resorption inhibitors and, more specifically, on bisphosphonates. However, the introduction of anabolic agents such as parathyroid hormone that stimulate bone formation has expanded the range of treatment options. New treatment targets have been identified via improved knowledge on bone pathophysiology, bone remodeling, bone cells and intracellular signaling pathways. RANKL inhibition by anti-RANKL antibodies is undergoing considerable development as a treatment for osteoporosis. Also under development are anti-catabolic drugs that target the molecular mechanisms involved in bone resorption, including cathepsin K inhibitors and integrin alpha(v)beta(3) antagonists. The identification of new pathways involved in bone formation is directing clinical research efforts toward the development of anabolic agents. The signaling pathways involved in bone formation, most notably the Wnt-pathway, hold considerable promise as treatment targets in conditions characterized by insufficient bone formation. Current focuses of interest include antibodies against naturally occurring Wnt-pathway antagonists (e.g., sclerostin and Dkk1) and modulators of parathyroid hormone production (calcilytic agents). Thus, active research is ongoing to improve the treatment of osteoporosis, a disease whose high prevalence and considerable functional and socioeconomic impact will raise formidable challenges in the near future.

Molecular biology of bone remodeling: implications for new therapeutic targets for osteoporosis

Osteoporosis is a major public health problem for adults over age 55 years costing billions of euros/dollars. Over the last 20 years anti-resorptive drugs were the treatment of choice for osteoporosis and most were derived from the bisphosphonate molecule. In the last 7 years remarkable advances in molecular biology and genetics have led to a detailed understanding of the bone remodeling cycle and as a result new therapeutic targets for treatment emerged. These new compounds have different modes of action depending on their role in the bone remodeling cycle. A major discovery was the important role of RANKL (receptor activator of nuclear factor kappa B ligand) secreted by osteoblasts and responsible for stimulating osteoclastic bone resorption. This led to development of a potent monoclonal antibody that blocks its action. This drug should be available soon as a new treatment for osteoporosis. Other molecular targets in resorption have been identified and several specific antagonists are potential treatments. However, a significant limiting factor for a new anti-resorptive drug is the cost of bringing it to the market because of the huge costs of a fracture trial. Although anti-resorptive agents have been the backbone of osteoporosis treatment they do not rebuild bone architecture and development of anabolic agents is needed. These are likely to evolve from an understanding of the LRP/Wnt signaling pathway. Already an antibody against sclerostin has shown promise in animal studies, and not to forget parathyroid hormone which was the first clinically useful anabolic treatment for osteoporosis.

Connective tissue growth factor promotes articular damage by increased osteoclastogenesis in patients with rheumatoid arthritis

Introduction

A protein analysis using a mass spectrometry indicated that there are serum proteins showing significant quantitative changes after the administration of infliximab. Among them, connective tissue growth factor (CTGF) seems to be related to the pathogenesis of rheumatoid arthritis (RA). Therefore, this study was conducted to investigate how CTGF is associated with the disease progression of RA.

Methods

Serum samples were collected from RA patients in active or inactive disease stages, and before or after treatments with infliximab. CTGF production was evaluated by ELISA, RT-PCR, indirect immunofluorescence microscopy, and immunoblotting. Osteoclastogenesis was evaluated using tartrate-resistant acid phosphatase (TRAP) staining, a bone resorption assay and osteoclasts specific catalytic enzymes productions.

Results

The serum concentrations of CTGF in RA were greater than in normal healthy controls and disease controls. Interestingly, those were significantly higher in active RA patients compared to inactive RA patients. Furthermore, the CTGF levels significantly were decreased by infliximab concomitant with the disease amelioration. In addition, tumour necrosis factor (TNF)α can induce the CTGF production from synovial fibroblasts even though TNFα can oppositely inhibit the production of CTGF from chondrocytes. CTGF promoted the induction of the quantitative and qualitative activities of osteoclasts in combination with M-CSF and receptor activator of NF-κB ligand (RANKL). In addition, we newly found integrin αVβ3 on the osteoclasts as a CTGF receptor.

Conclusions

These results indicate that aberrant CTGF production induced by TNFα plays a central role for the abnormal osteoclastic activation in RA patients. Restoration of aberrant CTGF production may contribute to the inhibition of articular destruction in infliximab treatment.

Osteoclast-targeting small molecules for the treatment of neoplastic bone metastases

Osteoclasts are highly specialized cells that resorb bone, and their abnormal activity is implicated in a variety of human bone diseases. In neoplastic bone metastasis, the bone destruction caused by osteoclasts is not only associated with the formation and progression of metastatic lesions, but also could contribute to frequent complications such as severe pain and pathological fractures, which greatly diminish the quality of life of patients. Bisphosphonates, potent antiresorptive drugs, have been shown to have efficacy for treating bone metastases in many types of cancer, and the development of various molecularly targeted agents is currently proceeding. Thus, inhibition of osteoclast function is now established as an important treatment strategy for bony metastases. This review focuses on promising small molecules that disrupt osteoclast function and introduces our chemical/biological approach for identifying osteoclast-targeting small molecular inhibitors.

Bone signaling pathways and treatment of osteoporosis

Osteoporotic fractures are a major healthcare burden costing over US$50 billion/per year. Bone turnover is a continuous process regulated by the coupled activities of osteocytes, osteoclasts and osteoblasts that maintain bone mass and strength. Osteoclastic bone resorption is regulated by the RANKL/osteoprotegerin/RANK pathway, while osteoblastic bone formation is controlled by canonical Wnt signaling. Antiresorptive bisphosphonates remain the mainstay of treatment but recombinant parathyroid hormone is increasingly being used as an anabolic agent. Nevertheless, these drugs are limited by patient compliance, efficacy and cost. Cathepsin K inhibitors and RANKL antibodies have been developed as new antiresorptive drugs, while short-acting calcilytics and antibodies to Dickkopf-1 and sclerostin are promising anabolics. The recent identification of adipocytes and duodenal enterochromaffin cells as novel regulators of bone mass represent exciting opportunities for future drug development.

[Austrian guidance for the pharmacological treatment of osteoporosis in postmenopausal women--update 2009]

Osteoporosis is a systemic skeletal disease characterized by diminished bone mass and deterioration of bone microarchitecture, leading to increased fragility and subsequent increased fracture risk. Therapeutic measures therefore aim at reducing individual fracture risk. In Austria, the following drugs, all of which have been proven to reduce fracture risk, are currently registered for the treatment of postmenopausal osteoporosis: alendronate, risedronate, etidronate, ibandronate, raloxifene, teriparatide (1-34 PTH), 1-84 PTH, strontium ranelate and salmon calcitonin. Fluorides are still available, but their role in daily practice has become negligible. Currently, there is no evidence that a combination of two or more of these drugs could improve anti-fracture potency. However, treatment with PTH should be followed by the treatment with an anticatabolic drug such as bisphosphonates. Calcium and vitamin D constitute an important adjunct to any osteoporosis treatment.

Emerging drugs for postmenopausal osteoporosis

Postmenopausal osteoporosis (PMO) is a common skeletal disease with serious consequences due to fractures, including increased risk of disability and death. The risk of fractures can be reduced with medications that are currently available; however, these drugs are frequently not prescribed due to failure to recognize that a patient is at high risk for fracture; fear of adverse drug effects; or, sometimes, high cost. When these drugs are prescribed, long-term adherence to therapy is poor. Efforts to improve the clinical effectiveness of pharmacological therapies have included lengthening the interval between doses, simplifying drug administration, and manipulating the molecular structure of drugs in existing therapeutic classes. Recent improvement in understanding the pathophysiology of PMO at the molecular level has fostered the development of new therapeutic agents with novel mechanisms of action. This is a review of the data on the efficacy and safety of emerging drugs for the treatment of PMO, including agents with novel mechanisms of action (denosumab, odanacatib, antibody to sclerostin), new estrogen agonists/antagonists (lasofoxifene, bazedoxifene, arzoxifene), new delivery systems for existing drugs (salmon calcitonin, teriparatide), and drug combinations given concurrently, sequentially, or cyclically. These new therapeutic agents, new delivery systems, and new methods of combining drugs may ultimately reduce the great personal and economic burden of osteoporotic fractures.

A novel role for SED1 (MFG-E8) in maintaining the integrity of the epididymal epithelium

The epididymis is a highly convoluted tubule that connects the testis with the vas deferens, and in which mammalian sperm acquire the ability to fertilize eggs. The most proximal portion of the epididymis, or initial segment, secretes numerous factors that are critical for sperm maturation and storage. One such factor is SED1 (also known as MFG-E8) a bi-motif protein composed of two N-terminal EGF domains, the second of which contains an RGD motif, and two C-terminal discoidin domains (also known as F5/8 type C domains). Previous studies have reported that SED1 is secreted into the epididymal lumen, where it coats sperm and later facilitates sperm-egg binding. Herein, we report that SED1-null males also harbor unexpected epididymal pathologies, including detached epithelia and spermatic granulomas. We therefore examined whether SED1 has a tissue-intrinsic role in the epididymis, in addition to its role in sperm-egg adhesion. Improved fixation protocols revealed that SED1 is found in the basolateral domains of epididymal epithelial cells in vivo, and similarly, SED1 is secreted both apically and basally from polarized epididymal cells in vitro. The basolateral distribution of SED1 suggests that it may play a novel role in epididymal cell adhesion. Consistent with this, in vitro assays showed that SED1 supports epididymal cell adhesion via RGD binding to alphaV integrin receptors on epididymal epithelial cells. Finally, epididymal cells from SED1-null males showed reduced adhesion in vitro, a phenotype that can be rescued with exogenous SED1. These results suggest that SED1 facilitates epididymal cell adhesion, and that its loss leads to breakdown of the epididymal epithelium and consequent development of spermatic granulomas.

Giant cell formation and function

PURPOSE OF REVIEW

To provide insight into the current state of understanding regarding the molecular and cellular mechanisms underlying the formation and function of various types of multinucleated giant cells.

RECENT FINDINGS

Recent studies involving mainly osteoclasts and foreign body giant cells have revealed a number of common factors, for example, vitronectin, an adhesion protein, dendritic cell-specific transmembrane protein, a fusion factor, and macrophage fusion receptor, that contribute to giant cell formation and function. Insight into common molecules, receptors, and mediators of adhesion and fusion mechanisms of giant cell formation have been complicated by the wide diversity of species, models, and cell types utilized in these studies.

SUMMARY

These recently identified factors together with the well known osteoclast receptor, alphavbeta3, may serve as potential therapeutic targets for the modulation and inhibition of multinucleated giant cell formation and function. Further studies on intracellular and intercellular signaling mechanisms modulating multinucleated giant cell formation and function are necessary for the identification of therapeutic targets as well as a better understanding of giant cell biology.

Integrins, insulin like growth factors, and the skeletal response to load

Bone loss during skeletal unloading, whether due to neurotrauma resulting in paralysis or prolonged immobilization due to a variety of medical illnesses, accelerates bone loss. In this review the evidence that skeletal unloading leads to bone loss, at least in part, due to disrupted insulin like growth factor (IGF) signaling, resulting in reduced osteoblast proliferation and differentiation, will be examined. The mechanism underlying this disruption in IGF signaling appears to involve integrins, the expression of which is reduced during skeletal unloading. Integrins play an important, albeit not well defined, role in facilitating signaling not only by IGF but also by other growth factors. However, the interaction between selected integrins such as alphaupsilonbeta3 and beta1 integrins and the IGF receptor are of especial importance with respect to the ability of bone to respond to mechanical load. Disruption of this interaction blocks IGF signaling and results in bone loss.

New strategies for the treatment of metastatic bone disease

The introduction of bisphosphonates represents an important advance in the care of patients with metastatic bone disease. Nonetheless, we remain unable to prevent metastatic bone destruction. This review will discuss several novel therapies, including inhibitors of receptor activator of nuclear factor-kappabeta, c-Src, mammalian target of rapamycin, cathepsin K, and alpha(5)beta(3) integrins, which could improve our control over this devastating complication.

Studies with an orally bioavailable alpha V integrin antagonist in animal models of ocular vasculopathy: retinal neovascularization in mice and retinal vascular permeability in diabetic rats

The alpha(V) integrins are key receptors involved in mediating cell migration and angiogenesis. In age-related macular degeneration (AMD) and diabetic retinopathy, angiogenesis plays a critical role in the loss of vision. These ocular vasculopathies might be treatable with a suitable alpha(V) antagonist, and an oral drug would offer a distinct advantage over current therapies. (3,S,beta,S)-1,2,3,4-Tetrahydro-beta-[[1-[1-oxo-3-(1,5,6,7-tetrahydro-1,8-naphthyridin-2-yl)propyl]-4-piperidinyl]methyl]-3-quinolinepropanoic acid (JNJ-26076713) is a potent, orally bioavailable, nonpeptide alpha(V) antagonist derived from the arginine-glycine-asparagine binding motif in the matrix protein ligands (e.g., vitronectin). This compound inhibits alpha(V)beta(3) and alpha(V)beta(5) binding to vitronectin in the low nanomolar range, it has excellent selectivity over integrins alpha(IIb)beta(3) and alpha(5)beta(1), and it prevents adhesion to human, rat, and mouse endothelial cells. JNJ-26076713 blocks cell migration induced by vascular endothelial growth factor, fibroblast growth factor (FGF), and serum, and angiogenesis induced by FGF in the chick chorioallantoic membrane model. JNJ-26076713 is the first alpha(V) antagonist reported to inhibit retinal neovascularization in an oxygen-induced model of retinopathy of prematurity after oral administration. In diabetic rats, orally administered JNJ-26076713 markedly inhibits retinal vascular permeability, a key early event in diabetic macular edema and AMD. Given this profile, JNJ-26076713 represents a potential therapeutic candidate for the treatment of age-related macular degeneration, macular edema, and proliferative diabetic retinopathy.

Emerging pharmacologic therapies for osteoporosis

Osteoporotic fractures are an important public health problem, contributing substantially to morbidity and mortality in an ageing world population and consuming considerable health resources. Presently available pharmacologic therapies for prevention of fragility fractures are limited in scope, efficacy and acceptability to patients. Considerable efforts are being made to develop new, more effective treatments for osteoporosis, and to refine/optimize existing therapies. These novel treatments include an expanding array of drugs that primarily inhibit osteoclastic bone resorption: estrogenic compounds, bisphosphonates, inhibitors of receptor activator of NF-kappaB ligand signaling, cathepsin K inhibitors, c-src kinase inhibitors, integrin inhibitors and chloride channel inhibitors. The advent of intermittent parathyroid hormone (PTH) therapy has provided proof-of-principle that osteoblast-targeted (anabolic) agents can effectively prevent osteoporotic fractures, and is likely to be followed by the introduction of other therapies based on PTH (orally active PTH analogs, antagonists of the calcium sensing receptor, PTH-related peptide analogs) and/or agents that induce osteoblast anabolism by means of pathways involving key, recently identified, molecular targets (wnt-low-density lipoprotein receptor-related protein 5 signaling, sclerostin and matrix extracellular phosphoglycoprotein).

Integrin beta3 Leu33Pro polymorphism and risk of hip fracture: 25 years follow-up of 9233 adults from the general population

OBJECTIVE

Integrin alphavbeta3 is essential for mature osteoclast function and therefore important for the development of osteoporosis and osteoporotic fractures. Integrin alphavbeta3 antagonists have antiresorptive effects in bone. We tested the hypothesis that the Leu33Pro polymorphism in the integrin beta3-subunit associates with risk of hip fracture.

METHODS

We included 9233 men and women selected at random to represent the Danish general population as participants in the Copenhagen City Heart Study. First-ever hip fractures (n=267) were registered during 25 years follow-up. Log-rank statistics and Cox regression were used to compare fracture incidences and risk estimates between genotypes.

RESULTS

Genotyping rendered 69.9% noncarriers, 27.3% heterozygotes and 2.7% homozygotes. Incidence of hip fracture was 2.8 and 1.5 per 1000 person-years in homozygotes and noncarriers (log-rank: P=0.02), respectively. Multifactorial adjusted Cox regression revealed a hazard ratio of 2.0 (95% confidence interval: 1.1-3.5) for hip fracture in homozygotes versus noncarriers. After stratification by sex, equivalent hazard ratios were 2.0 (1.0-4.1) in women and 2.0 (0.8-5.0) in men. In the 2193 postmenopausal women, hazard ratio for hip fracture in homozygotes versus noncarriers after additional adjustment for age at menopause and use of hormone replacement therapy was 2.6 (1.2-5.3). Hazard ratio for hip fracture in heterozygotes versus noncarriers did not differ from 1.0.

CONCLUSIONS

Individuals homozygous for the integrin beta3 Leu33Pro polymorphism have a two-fold risk of hip fracture, mainly confined to postmenopausal women. Integrin beta3 Leu33Pro homozygosity could prove a useful marker for risk of future hip fracture and may contribute to pharmacogenetic variation in effects of integrin alphavbeta3 antagonists.

Syk, c-Src, the alphavbeta3 integrin, and ITAM immunoreceptors, in concert, regulate osteoclastic bone resorption

In this study, we establish that the tyrosine kinase Syk is essential for osteoclast function in vitro and in vivo. Syk(-/-) osteoclasts fail to organize their cytoskeleton, and, as such, their bone-resorptive capacity is arrested. This defect results in increased skeletal mass in Syk(-/-) embryos and dampened basal and stimulated bone resorption in chimeric mice whose osteoclasts lack the kinase. The skeletal impact of Syk deficiency reflects diminished activity of the mature osteoclast and not impaired differentiation. Syk regulates bone resorption by its inclusion with the alpha v beta3 integrin and c-Src in a signaling complex, which is generated only when alpha v beta3 is activated. Upon integrin occupancy, c-Src phosphorylates Syk. Alpha v beta3-induced phosphorylation of Syk and the latter's capacity to associate with c-Src is mediated by the immunoreceptor tyrosine-based activation motif (ITAM) proteins Dap12 and FcRgamma. Thus, in conjunction with ITAM-bearing proteins, Syk, c-Src, and alpha v beta3 represent an essential signaling complex in the bone-resorbing osteoclast, and, therefore, each is a candidate therapeutic target.

Osteoclasts: what do they do and how do they do it?

As Americans live longer, degenerative skeletal diseases, such as osteoporosis, become increasingly prevalent. Regardless of cause, osteoporosis reflects a relative enhancement of osteoclast activity. Thus, this unique bone resorptive cell is a prominent therapeutic target. A number of key observations provide insights into the mechanisms by which precursors commit to the osteoclast phenotype and how the mature cell degrades bone. The osteoclast is a member of the monocyte/macrophage family that differentiates under the aegis of two critical cytokines, namely RANK ligand and M-CSF. Tumor necrosis factor (TNF)-alpha also promotes osteoclastogenesis, particularly in states of inflammatory osteolysis such as that attending rheumatoid arthritis. Once differentiated, the osteoclast forms an intimate relationship with the bone surface via the alphavbeta3 integrin, which transmits matrix-derived, cytoskeleton-organizing, signals. These integrin-transmitted signals include activation of the associated proteins, c-src, syk, Vav3, and Rho GTPases. The organized cytoskeleton generates an isolated microenvironment between the cell's plasma membrane and the bone surface in which matrix mineral is mobilized by the acidic milieu and organic matrix is degraded by the lysosomal protease, cathepsin K. This review focuses on these and other molecules that mediate osteoclast differentiation or function and thus serve as candidate anti-osteoporosis therapeutic targets.

Identification of a human peripheral blood monocyte subset that differentiates into osteoclasts

Increased bone resorption mediated by osteoclasts causes various diseases such as osteoporosis and bone erosion in rheumatoid arthritis (RA). Osteoclasts are derived from the monocyte/macrophage lineage, but the precise origin remains unclear. In the present study, we show that the purified CD16^- human peripheral blood monocyte subset, but not the CD16⁺ monocyte subset, differentiates into osteoclast by stimulation with receptor activator of NF-κB ligand (RANKL) in combination with macrophage colony-stimulating factor (M-CSF). Integrin-β3 mRNA and the integrin-αvβ3 heterodimer were only expressed on CD16^- monocytes, when they were stimulated with RANKL + M-CSF. Downregulation of β3-subunit expression by small interfering RNA targeting β3 abrogated osteoclastogenesis from the CD16^- monocyte subset. In contrast, the CD16⁺ monocyte subset expressed larger amounts of tumor necrosis factor alpha and IL-6 than the CD16^- subset, which was further enhanced by RANKL stimulation. Examination of RA synovial tissue showed accumulation of both CD16⁺ and CD16^- macrophages. Our results suggest that peripheral blood monocytes consist of two functionally heterogeneous subsets with distinct responses to RANKL. Osteoclasts seem to originate from CD16^- monocytes, and integrin β3 is necessary for osteoclastogenesis. Blockade of accumulation and activation of CD16^- monocytes could therefore be a beneficial approach as an anti-bone resorptive therapy, especially for RA.

Therapy insight: the risks and benefits of bisphosphonates for the treatment of tumor-induced bone disease

Bisphosphonates are a valuable class of drugs with potent anti-resorptive actions that make them ideal for skeletal protection in osteoporosis, cancer bone metastasis, multiple myeloma, and Paget's disease of bone. It has become apparent, however, that these drugs also have the potential to cause a number of adverse effects. While these do not limit bisphosphonate use, the incidence of these adverse events can be minimized if appropriate care is taken with their administration, and by maintaining appropriate surveillance and patient care. We review the range of adverse reactions to bisphosphonate therapy with a particular emphasis on the recently identified association between long-term bisphosphonate treatment and osteonecrosis of the jaw. This is a potentially serious side effect seen mostly in patients with multiple myeloma or breast cancer bone metastases who receive intravenous bisphosphonate treatment. While the etiology is uncertain, a strong association with dental pathology and interventions highlights the need for close attention to dental health in this patient group.

Drug insight: Existing and emerging therapies for osteoporosis

Osteoporosis is a major public health problem that is characterized by microarchitectural deterioration, low bone mass, and increased risk of fractures. Currently, many women and men affected with this disease are not diagnosed or treated. As osteoporosis is often clinically silent, risk-factor assessment and measurement of BMD are needed to identify those who may benefit from osteoporosis therapy. Although adequate daily intake of calcium and vitamin D, and regular weight-bearing exercise are important for skeletal health, they are not adequate treatments for individuals with osteoporosis. Therapies approved for treatment and/or prevention of osteoporosis in the United States include oral bisphosphonates (alendronate, ibandronate and risedronate), calcitonin, estrogens, teriparatide (parathyroid hormone fragment [1-34]), and raloxifene. For most patients, oral bisphosphonates are the treatment of choice, given the large-scale randomized-trial data demonstrating efficacy in fracture reduction, although bisphosphonates that reduce spine and nonspine fractures (e.g. alendronate and risedronate) are preferred. For high-risk patients (those with very low bone density, or with fractures), teriparatide therapy for 2 years should be considered. The treatment paradigm for osteoporosis will evolve further as promising new treatments progress through clinical development.

Involvement of alpha(v)beta3 integrins in osteoclast function

Integrins are heterodimeric adhesion receptors that mediate cell-matrix interaction. Osteoclast exhibits high expression of the alpha(v)beta(3) integrin, which binds to a variety of extracellular matrix proteins including vitronectin, osteopontin, and bone sialoprotein. Arg-Gly-Asp (RGD)-containing peptides, RGD-mimetics, and blocking antibodies to alpha(v)beta(3) integrin were shown to inhibit bone resorption in vitro and in vivo, suggesting that this integrin may play an important role in regulating osteoclast function. Several lines of evidence have demonstrated that a number of signaling molecules are involved in the alpha(v)beta(3) integrin-dependent signaling pathway, including c-Src, Pyk2, c-Cbl, and p130(Cas). In this article, we review the history of "alpha(v)beta(3) integrin and osteoclasts" and discuss the involvement of alpha(v)beta(3) integrins in osteoclast function at tissue, cellular, and molecular levels. A better understanding of the role of alpha(v)beta(3) integrin in osteoclastic bone resorption would provide opportunities for developing new therapeutics to treat human bone diseases, including rheumatoid arthritis, osteoporosis, and periodontal disease.

Discovery of small molecule integrin alphavbeta3 antagonists as novel anticancer agents

Integrin alphavbeta3 has been implicated in multiple aspects of tumor progression and metastasis. Many tumors have high expression of alphavbeta3 that correlates with tumor progression. Therefore, alphavbeta3 receptor is an excellent target for drug design and delivery. We have discovered a series of novel alphavbeta3 antagonists utilizing common feature pharmacophore models. Upon validation using a database of known alphavbeta3 receptor antagonists, a highly discriminative pharmacophore model was used as a 3D query. A search of a database of 600 000 compounds using the pharmacophore Hypo5 yielded 832 compounds. On the basis of structural novelty, 29 compounds were tested in alphavbeta3 receptor specific binding assay and four compounds showed excellent binding affinity. A limited SAR analysis on the active compound 26 resulted in the discovery of two compounds with nanomolar to subnanomolar binding affinity. These small-molecule compounds could be conjugated to paclitaxel for selective delivery to alphavbeta3 positive metastatic cancer cells.

Osteoclasts and integrins

The osteoclast is the unique bone resorptive cell that accomplishes its mission by forming an isolated acidified microenvironment between itself and the bone surface. Creation of this compartment is the first step in bone degradation and establishes that an intimate physical relationship must exist between the osteoclast and bone. Thus, identification of the mechanisms by which the osteoclast attaches to bone is essential to understanding how the cell degrades skeletal tissue. Our studies have investigated whether absence of the alphavbeta3 integrin modifies the ability of c-Fms to induce Rho GTPases, and the implications for formation of the osteoclast cytoskeleton.

Targeting integrin structure and function in disease

Initially linked to the pathogenesis of inflammatory and hematologic diseases, integrins have become validated drug targets with the approval of five drugs. Moreover, there are several promising drug candidates in preclinical and clinical stages of development for multiple clinical indications. Integrins are attractive drug targets as their antagonism can block several steps in disease progression or maintenance. Integrin inhibitors can block the proliferation, migration, or tissue localization of inflammatory, angiogenic, and tumor cells, as well as signaling and gene expression contributing to disease. There has been a rapid increase in the elucidation of integrin structure, their allosteric mechanisms of bidirectional signaling, and the structure of complexes with drugs. This information brings greater focus to how integrins support various cellular functions and how they have been and may be targeted to develop novel drugs. Here we review conformational switches, including an internal ligand, which allosterically regulate the transition from low- to high-affinity ligand binding. We address some of the successes, disappointments, and challenges in targeting competitive or allosteric sites to develop therapeutics. We also discuss new opportunities, including a structure-based approach to discover novel drugs to treat inflammatory and other diseases. This approach targets structural relatives of the von Willebrand factor A-domain present in integrins and many functionally diverse proteins.

alphavbeta3 and macrophage colony-stimulating factor: partners in osteoclast biology

Osteoclasts, the sole bone-resorbing cells, arise by fusion and differentiation of monocyte/macrophage precursors. Matrix degradation requires adhesion of the osteoclast to bone, an integrin alphavbeta3-mediated event that also stimulates signals which polarize the cell and secrete resorptive molecules such as hydrochloric acid and acidic proteases. Two cytokines are necessary and sufficient for osteoclastogenesis, receptor activator of nuclear factor kappaB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF), both produced by mesenchymal cells in the bone marrow environment. M-CSF promotes survival and proliferation of osteoclast precursors. It also contributes to their differentiation and regulates the cytoskeletal changes that accompany bone resorption. Binding of M-CSF to c-Fms, its receptor, recruits adapter proteins and cytosolic kinases, thereby activating a variety of intracellular signals. We herein review how alphavbeta3 and M-CSF, alone and in concert, impact production, survival, and function of the osteoclast, thereby controlling skeletal mass. Signals from alphavbeta3 and/or c-Fms activate Syk and Vav3, originally defined by their function in lymphoid cells. Genetic depletion of either protein generates a strong bone phenotype, underscoring the promise of osteoimmunobiology.

A functional radioreceptor assay of alpha-V-beta-3 (αvβ3) inhibitors in plasma: Application as an ex vivo pharmacodynamic model

Development of alphavbeta3-integrin inhibitors has been hampered by a lack of pharmacodynamic endpoints to identify doses that inhibit alphavbeta3 in vivo. To address this need, we developed an alphavbeta3 radioreceptor assay (RRA) that could be performed in 100% plasma. The RRA was based on 125I-echistatin binding to plate-immobilized alphavbeta3. Small molecule alphavbeta3 inhibitors efficiently competed echistatin binding to alphavbeta3 when the assay was carried out in buffer. However, when carried out in 100% plasma, the RRA revealed a 45 to >3000-fold loss in compound potencies. The losses in potency reflected, in part, the high plasma protein binding by the compounds examined. The RRA was adapted as an ex vivo pharmacodynamic model. Echistatin binding was measured in the presence of plasma harvested at timed intervals from rats dosed with select compounds. Using this pharmacodynamic model, compound and dose selection was optimized for further testing in models of corneal angiogenesis. Moderate anti-angiogenic activity was achieved when rats were dosed sufficient to achieve sustained (>50%) plasma inhibition through the trough interval. Thus, the RRA provided a simple technique to rank order compound potency in plasma, and could find general use as an ex vivo pharmacodynamic assay to select compounds and doses for preclinical and clinical proof-of-principle studies.

Osteoclasts; culprits in inflammatory osteolysis

Periarticular osteolysis, a crippling complication of rheumatoid arthritis, is the product of enhanced osteoclast recruitment and activation. The osteoclast, which is a member of the monocyte/macrophage family, is the exclusive bone resorptive cell, and its differentiation and activation are under the aegis of a variety of cytokines. Receptor activator of NF-kappaB ligand (RANKL) and macrophage colony-stimulating factor are the essential osteoclastogenic cytokines and are increased in inflammatory joint disease. Tumor necrosis factor-alpha, which perpetrates arthritic bone loss, exerts its osteoclastogenic effect in the context of RANKL with which it synergizes. Achieving an understanding of the mechanisms by which the three cytokines affect the osteoclast has resulted in a number of active and candidate therapeutic targets.

[Partnership between academic research and industry to study a new anti-osteoporotic drug]

The activity of the osteoclast, the cell responsible for bone resorption, is subjected to different regulation factors. Amongst these, those issued from the matrix, particularly released minerals such as calcium, are determinants. We have shown that variations in calcium concentration in the medium regulates resorption activity and duration of the osteoclast lifespan. The development of a new therapeutic agent, strontium ranelate, has shown very interesting clinical effects reliant on the stimulation of bone formation activity by osteoblasts and modulation of bone resorption activity. From our knowledge regarding osteoclast physiology, in particular calcium signaling pathways, and the control of different osteoclast cellular models, a consequent collaboration was formed between our laboratory and Servier in order to elaborate on the effects of strontium ranelate on the osteoclast. In several years, this collaboration has been further enriched by other collaborators in order to better understand this mechanism. It has also been shown that strontium likely interacts with the calcium-sensing receptor and that the pathways of intracellular signaling pathways activated by calcium and strontium ranelate via this receptor are different. In fact, within the scope of this collaboration with Servier, exchanges with other academic laboratories were initiated and collaboration on numerous techniques became possible. Then, it has been possible to confirm the presence of the calcium-sensing receptor on the osteoclasts and to demonstrate its role in the molecular events associated with strontium ranelate's effects on the osteoclast.