Inhibition of bone resorption by alendronate and risedronate does not require osteoclast apoptosis

Alendronate inhibits bone resorption at the bone-screw interface

In the current study, we investigated whether the systemic administration of alendronate, a third-generation bisphosphonate, suppressed the loosening of screws at the bone-screw interface. We systemically administered alendronate to rats fitted with external fixators. External fixators with two half pins were applied to the right femurs of rats, and alendronate was administrated once a week during a 5-week postoperative period. Radiographic, histologic, and immunohistochemical findings subsequently were analyzed. Treatment with alendronate reduced the width of the fibrous loosening membrane and the number of osteoclasts at the bone-screw interface. These findings indicate that systemic treatment with alendronate exerts an inhibitory effect on local bone resorption at the bone-screw interface.

Osteoblast proliferation and maturation by bisphosphonates

Aseptic loosening and osteolysis are currently the most common causes of failure of total joint replacements. Osteolysis is initiated by a macrophage response to wear debris, resulting in localized, osteoclastic peri-implant bone loss. We have previously inhibited osteoclast-mediated bone resorption in a canine total hip arthroplasty model using oral bisphosphonate therapy. Based on serendipitous observations from our canine study, we hypothesized that bisphosphonates have an anabolic effect on osteoblasts, in a manner distinct from their inhibitory effect on osteoclastic bone resorption. We studied the anabolic effects of two FDA-approved bisphosphonates (alendronate and risedronate) on two in vitro models: a primary human trabecular bone cell culture and the MG-63 osteoblast-like cell line. Following treatment with bisphosphonates at varying concentrations and time periods, cells were assayed for proliferation effects and results were quantified using the methods of direct cell count, and the colorimetric MTT (3-dimethylthiazol-2,5-diphenyltetrazolium bromide) assay at 24, 48, and 72 h. The effect of bisphosphonates on the maturation of osteoblasts were tested with alkaline phosphatase bioassay and reverse transcription-polymerase chain reaction for markers of osteoblast differentiation. Results from both the primary human trabecular bone cell culture and the MG-63 osteoblast-like cell line showed that both bisphosphonates significantly increased the cell number over controls, attaining peak levels at a concentration of 10(-8)M. Alkaline phosphatase activity was also increased, representing earlier commitment of osteoprogenitor cells towards the osteoblastic phenotype. Bisphosphonates also enhanced gene expression of BMP-2, Type I collagen and osteocalcin. In summary, bisphosphonates, aside from their role as inhibitors of osteoclastic bone resorption, are promoters of osteoblast proliferation and maturation.

From molds and macrophages to mevalonate: a decade of progress in understanding the molecular mode of action of bisphosphonates

Although bisphosphonates were first used as therapeutic agents to inhibit bone resorption in the early 1970s, their mode of action at the molecular level has only become fully clear within the last few years. One of the reasons for this lack of understanding was the difficulty in isolating large numbers of pure osteoclasts for biochemical studies. In the last decade, the identification of appropriate surrogate models that reflected the antiresorptive potencies of bisphosphonates, such as Dictyostelium slime molds and macrophages, helped overcome this problem and proved to be instrumental in elucidating the molecular pathways by which these compounds inhibit osteoclast-mediated bone resorption. This brief review summarizes our current understanding of these pathways.

Long-term effects of neridronate on human osteoblastic cell cultures

Bisphosphonates (BPs) are widely used in the treatment of a variety of bone-related diseases, particularly where the bone turnover is skewed in favor of osteolysis. The mechanisms by which BPs reduce bone resorption directly acting on osteoclasts are now largely clarified even at molecular level. Researches concerning the BP's effects on osteoblast have instead shown variable results. Many in vitro studies have reported positive effects on osteoblasts proliferation and mineralization for several BPs; however, the observed effects differ, depending on the variety of different model system that has been used.

OBJECTIVES

We have investigated if neridronate, an aminobisphosphonate suitable for pulsatory parenteral administration, could have an effect on human osteoblastic proliferation and differentiation in vitro.

METHODS

We have investigated whether prolonged addition of neridronate (from 10(-3) to 10(-11) M) to different human osteoblasts cultures, obtained from 14 different bone specimens, could affect the cells number, the endogenous cellular alkaline phosphatase (ALKP) activity, and the formation of mineralized nodules.

RESULTS

Our results show that neridronate does not negatively affect in vitro the viability, proliferation, and cellular activity of normal human osteoblasts even after a long period addition of the drug (20 days) at concentrations equal or lower than 10(-5) mol/l (therapeutic dose). In addition, neridronate seems to enhance the differentiation of cultured osteoblasts in mature bone-forming cells. A maximum increase of alkaline phosphatase activity (+50% after 10 days; P < 0.01) and mineralized nodules (+48% after 20 days; P < 0.05) was observed in cultures treated with neridronate 10(-8) M.

CONCLUSIONS

These results encourage the use of neridronate in long-term therapy of demineralizing metabolic bone disorders.

M-CSF, TNFalpha and RANK ligand promote osteoclast survival by signaling through mTOR/S6 kinase

Multinucleated bone-resorbing osteoclasts (Ocl) are cells of hematopoietic origin that play a major role in osteoporosis pathophysiology. Ocl survival and activity require M-CSF and RANK ligand (RANKL). M-CSF signals to Akt, while RANKL, like TNFalpha, activates NF-kappaB. We show here that although these are separate pathways in the Ocl, signaling of all three cytokines converges on mammalian target of rapamycin (mTOR) as part of their antiapoptotic action. Accordingly, rapamycin blocks M-CSF- and RANKL-dependent Ocl survival inducing apoptosis, and suppresses in vitro bone resorption proportional to the reduction in Ocl number. The cytokine signaling intermediates for mTOR/ribosomal protein S6 kinase (S6K) activation include phosphatidylinositol-3 kinase, Akt, Erks and geranylgeranylated proteins. Inhibitors of these intermediates suppress cytokine activation of S6K and induce Ocl apoptosis. mTOR regulates protein translation acting via S6K, 4E-BP1 and S6. We find that inhibition of translation by other mechanisms also induces Ocl apoptosis, demonstrating that Ocl survival is highly sensitive to continuous de novo protein synthesis. This study thus identifies mTOR/S6K as an essential signaling pathway engaged in the stimulation of cell survival in osteoclasts.

Strategic approaches to osteoporosis in transplantation

Recipients of stem cell transplantation are at risk for osteopenia and osteoporosis. Longitudinal studies performed in adults have shown that significant bone demineralization occurs following myeloablative therapy and subsequent immune suppression. Among children and adolescents, cross-sectional analyses indicate that younger patients are also at risk for long-term bone toxicity. Strategies to detect and manage this disorder in pediatric SCT recipients are presented.

Osteogenesis imperfecta

Osteogenesis imperfecta is a genetic disorder of increased bone fragility, low bone mass, and other connective-tissue manifestations. The most frequently used classification outlines four clinical types, which we have expanded to seven distinct types. In most patients the disorder is caused by mutations in one of the two genes encoding collagen type 1, but in some individuals no such mutations are detectable. The most important therapeutic advance is the introduction of bisphosphonate treatment for moderate to severe forms of osteogenesis imperfecta. However, at present, the best treatment regimen and the long-term outcomes of bisphosphonate therapy are unknown. Although this treatment does not constitute a cure, it is an adjunct to physiotherapy, rehabilitation, and orthopaedic care. Gene-based therapy presently remains in the early stages of preclinical research.

The effect of alendronate sodium on spinal fusion: a rabbit model

BACKGROUND CONTEXT

Bisphosphonates affect bone remodeling and increase bone mass through the inhibition of osteoclasts. Their effect on osteoblasts, and the balance between osteoblastic and osteoclastic activity on bone turnover and healing, is not completely understood. Specifically, the effect of bisphosphonates on spinal fusion has yet to be determined. With the increasing use of bisphosphonates in the elderly population, this effect needs to be delineated.

PURPOSE

To evaluate the effect of alendronate sodium after an intertransverse process spinal fusion in a rabbit model.

STUDY DESIGN/SETTING

Randomized double-blinded in vivo study of the effect of alendronate sodium in a spinal fusion model.

METHODS

Fifty New Zealand white rabbits underwent a posterolateral L5-L6 intertransverse process arthrodesis with autogenous iliac crest bone graft. The rabbits were then randomly divided into two groups. Group I received 3 cc of saline placebo per oral gavage, and Group II received 200 micrograms (approximately 0.05 mg/kg/day) of alendronate sodium dissolved in 3 cc of saline per day for 8 weeks. Upon completion, the rabbits were sacrificed and the lumbar spines harvested, radiographed and graded for motion across the fusion site with manual palpation. Two independent pathologists then prepared and sectioned each left and right fusion mass. Three random x10 fields were examined and graded for both the cephalad and caudad ends of each section (516 fields). Fusion quality was graded using an established histological scoring scale (score 0 to 7 based on fibrous and bone content of the fusion mass).

RESULTS

Two rabbits died on the day of operation, and 48 rabbits survived the operation. Five additional rabbits died within the first 2 postoperative weeks. Thus, 43 rabbits (21 in Group I, 22 in Group II) completed the 8-week course of treatment. Grading each side separately, 26 of 42 fusion masses (62%) in Group I and 24 of 44 fusion masses (55%) in Group II had radiographic evidence of fusion (p=.76). With gross palpation, 11 of 21 motion segments (52%) in Group I versus 13 of 22 motion segments (59%) in Group II were determined to have a solid fusion (p=.76). Histologically, Group I had a higher median score (6.0; range, 0 to 7 vs. 1.0; range, 0 to 7; p<.0001) and a higher fusion rate (76% vs. 45%; p=.004) than Group II.

CONCLUSIONS

Alendronate sodium appears to inhibit or delay bone fusion in a rabbit model. Presumably, this occurs as a result of uncoupling the balanced osteoclastic and osteoblastic activity inherent to bone healing. These findings suggest that a discontinuance of alendronate sodium postoperatively during the acute fusion period may be warranted.

Bone as an effect compartment : models for uptake and release of drugs

"Bone-seeking agents" are drugs characterised by high affinity for bone, and are disposed in bone for prolonged periods of time while maintaining remarkably low systemic concentrations. As a consequence, the bone becomes a reservoir for bone-seeking agents, and a site of both desirable and adverse effects, depending on the pharmacological activities of the specific agent. For some agents, significant systemic effects may also be produced following their prolonged release from bone, a process that is governed mostly by the rate of bone remodelling. This review covers the pharmacokinetic and pharmacodynamic features of bone-seeking agents with different pharmacological properties, including drugs (bisphosphonates, drug-bisphosphonate conjugates, radiopharmaceuticals and fluoride), bone markers (tetracycline, bone imaging agents) and toxins (lead, chromium, aluminium). In addition, drugs that do not possess bone-seeking properties but are used for therapy of bone diseases (such as antibacterials for treatment of osteomyelitis) are discussed, along with targeting of these drugs to the bone by conjugation to bone-seeking agents, local delivery systems, and other approaches. The pharmacokinetic and pharmacodynamic behaviour of bone-seeking agents is extremely complex due to heterogeneity in bone morphology and physiology. This complexity, accompanied by difficulties in human bone research caused by ethical and other limitations, gave rise to modelling approaches to study bone drug disposition. This review describes the pharmacokinetic models that have been proposed to describe the pharmacokinetic behaviour of bone-seeking agents and predict bone concentrations of these agents for different doses and patient populations. Models of different types (compartmental and physiologically based) and of different complexity have been applied, but their relevance to drug effects in the bone tissue is limited since they describe the behaviour of the "average" drug molecule. Understanding of the cellular and molecular processes responsible for the heterogeneity of bone tissue will provide better comprehension of the influence of microenvironment on drug bone disposition and the resulting pharmacological response.

Zoledronic acid treatment results in retention of femoral head structure after traumatic osteonecrosis in young Wistar rats

Osteonecrosis (ON) of the femoral head in childhood can lead to loss of femoral head architecture and subsequent deformity. When femoral head ON was surgically induced in 24 rats, zoledronic acid treatment and prophylaxis improved sphericity and maintenance of architecture at 6 weeks. This preliminary experiment supports the use of bisphosphonates in childhood ON.

INTRODUCTION

We hypothesized that the bisphosphonate zoledronic acid could preserve femoral head structure while allowing bone repair.

MATERIALS AND METHODS

Osteonecrosis (ON) was surgically induced in the right femoral head of 24 female Wistar rats. The rats were randomized into three treatment groups and dosed subcutaneously with saline, zoledronic acid (0.1 mg/kg) at 1 and 4 weeks postoperation (ZA post), or zoledronic acid (0.1 mg/kg) given 2 weeks preoperation and at 1 and 4 weeks postoperation (ZA pre-post). After death at 6 weeks postoperation, undecalcified specimens were analyzed by DXA and standardized histomorphometric analysis.

RESULTS

Seventy-one percent of saline-operated femoral heads were aspherical (Mose score > 1), whereas only 13% and 0% of operated heads in the ZA-treated groups were aspherical (p < 0.05). DXA-measured bone mineral density in saline-treated femoral heads was reduced by 34% and 43% compared with the ZA-treated groups (p < 0.01). Histomorphometry showed decreases of 12% and 17% in bone volume (BV/TV) in saline groups compared with ZA post and ZA pre-post (p < 0.05), and a decrease in trabecular number (Tb.N) of 18% and 14% (p < 0.05), respectively. Bone formation rate (BFR) was increased by 56% in saline-treated operated heads over ZA post and was 4.8 times increased over the ZA pre-post group (p < 0.05). The differences in BV/TV and Tb.N in treated groups must therefore be caused by a reduction in bone turnover. Observational histology confirmed the retention of necrotic architecture in treated groups.

CONCLUSIONS

Zoledronic acid treatment and prophylaxis preserved femoral head architecture after traumatic ON in this rat model at 6 weeks. These data indicate that, by conserving femoral head architecture, bone repair may occur in conjunction with improved femoral head shape.

The antineoplastic role of bisphosphonates: from basic research to clinical evidence

Bisphosphonates are now well established as successful agents for the prevention and treatment of postmenopausal osteoporosis, corticosteroid-induced bone loss and Paget's disease. Bisphosphonates have also recently become important in the management of cancer-induced bone disease, and they now have a widely recognized role for patients with multiple myeloma and bone metastases secondary to breast cancer and prostate cancer. Recent studies suggest that, besides the strong antiosteoclastic activity, the efficacy of such compounds in the oncological setting could also be due also to direct antitumor effect, exerted at different levels. Here, after a brief analysis of the chemical structure, we will review the antineoplastic and biological properties of bisphosphonates. We will start from well estabilished mechanisms of action and go on to discuss the latest evidence and hypotheses. In particular, we will review the antiresorptive properties in malignant osteolysis and the recent evidence of a direct antitumor effect. Furthermore, this review will analyze the influence of bisphosphonates on cancer growth factor release, their effect on cancer cell adhesion, invasion and viability, the proapoptotic potential on cancer cells, the antiangiogenic effect, and, finally, the immunomodulating properties of bisphosphonates on the gammadelta T cell population.

Mechanism of action of bisphosphonates

In recent years, substantial progress has been made in understanding the mechanism for bisphosphonate suppression of bone turnover. Bisphosphonates can now be distinguished based on their molecular and cellular mechanisms of action. Simple bisphosphonates such as clodronate and etidronate inhibit bone resorption through induction of osteoclast apoptosis. Clodronate, and perhaps etidronate, triggers apoptosis by generating a toxic analog of adenosine triphosphate, which then targets the mitochondria, the energy center within the cell. For nitrogen-containing bisphosphonates, the direct intracellular target is the enzyme farnesyl diphosphate synthase in the cholesterol biosynthetic pathway. Its inhibition suppresses a process called protein geranylgeranylation, which is essential for the basic cellular processes required for osteoclastic bone resorption. Although nitrogen-containing bisphosphonates can induce osteoclast apoptosis, this is not necessary for their inhibition of bone resorption.

Cellular mechanism of inhibition of osteoclastic resorption of bone and calcified cartilage by long-term pamidronate administration in ovariectomized mature rats

We examined the effects of long-term bisphosphonate (BP, pamidronate) administration at a therapeutic dose (1.5 mg/kg/day) on the distribution, structure, and vacuolar-type H(+)-ATPase expression of osteoclasts, and the resulting trabecular bone volume and structure in ovariectomized (OVX) mature rats. Six-month-old female rats were allocated to sham-operated control, untreated-OVX, and BP-administered OVX groups. Postoperatively, BP was administered intraperitoneally once a day to OVX rats for up to 30 days. On postoperative days 14, 30, and 60, all of the rats were killed and the distal metaphyseal area of the dissected humeri was examined. Quantitative backscattered-electron image analysis revealed that the trabecular bone volume/unit medullary area in untreated OVX rats was significantly (P < 0.05) lower than that in sham-operated controls at 30 and 60 days postoperation. BP administration significantly (P < 0.05) increased trabecular bone volume at 14, 30, and 60 days postoperation in BP-administered OVX rats compared to both sham-operated and untreated OVX rats. Compared to untreated OVX rats, the number of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts along the bone trabeculae in BP-administered OVX rats was not significantly decreased on days 14 and 30, but was significantly decreased on day 60. Ultrastructurally, BP administration caused the disappearance of both the ruffled border (RB) and the clear zone (CZ) structures, and decreased the expression of vacuolar-type H(+)-ATPase in most osteoclasts, but did not significantly induce apoptosis of osteoclasts detected by the terminal dUTP nick end-labeling (TUNEL) method. Our results suggest that long-term BP administration significantly reduces bone and calcified cartilage resorption through impairment of the structure and bone-resorbing function of osteoclasts, and thereby effectively maintains trabecular bone volume and structure in ovariectomy-induced acute estrogen deficiency in mature rats.

Involvement of vacuolar H+ -ATPase in incorporation of risedronate into osteoclasts

Although osteoclasts incorporate bisphosphonates during bone resorption, the mechanism of this incorporation by osteoclasts is not known. We previously reported that bisphosphonates disrupt the actin rings (clear zones) formed in normal osteoclasts, but did not disrupt actin rings in osteoclasts derived from osteosclerotic oc/oc mice, which have a defect in the gene encoding vacuolar H(+)-ATPase (V-ATPase). The present study showed that V-ATPase is directly involved in the incorporation of risedronate, a nitrogen containing bisphosphonate, into osteoclasts. Treatment of osteoclasts with risedronate disrupted actin rings and inhibited pit formation by osteoclasts on dentine slices. Bafilomycin A(1), a V-ATPase inhibitor, inhibited the pit-forming activity of osteoclasts but did not disrupt actin rings. Risedronate failed to disrupt actin rings in the presence of bafilomycin A(1). E-64, a lysosomal cysteine proteinase inhibitor, showed no inhibitory effect on the demineralization of dentine by osteoclasts but inhibited the digestion of dentine matrix proteins without disrupting actin rings. Risedronate disrupted actin rings even in the presence of E-64. Treatment of osteoclasts placed on plastic plates with risedronate also disrupted actin rings. Bafilomycin A(1) but not E64 prevented the disruption of actin rings in osteoclasts treated with risedronate on plastic plates. Inhibition of V-ATPase with bafilomycin A(1) also prevented disruption of actin rings by etidronate, a non-nitrogen-containing bisphosphonate. These results suggest that V-ATPase induced acidification beneath the ruffled borders of osteoclasts and subsequent bone demineralization triggers the incorporation of both nitrogen-containing and non-nitrogen-containing bisphosphonates into osteoclasts.

Beyond the gap: functions of unpaired connexon channels

Gap junctions consist of intercellular channels that connect the cytoplasm of adjacent cells directly and allow the exchange of small molecules. These channels are unique in that they span two plasma membranes--the more orthodox ion or ligand-gated channels span only one. Each cell contributes half of the intercellular channel, and each half is known as a connexon or hemichannel. Recent studies indicate that connexons are also active in single plasma membranes and that they might be essential in intercellular signalling beyond their incorporation into gap junctions.

Osteoclastogenesis, bone resorption, and osteoclast-based therapeutics

Over the past decade, advances in molecular tools, stem cell differentiation, osteoclast and osteoblast signaling mechanisms, and genetically manipulated mice models have resulted in major breakthroughs in understanding osteoclast biology. This review focuses on key advances in our understanding of molecular mechanisms underlying the formation, function, and survival of osteoclasts. These include key signals mediating osteoclast differentiation, including PU.1, RANK, CSF-1/c-fms, and src, and key specializations of the osteoclast including HCl secretion driven by H+-ATPase and the secretion of collagenolytic enzymes including cathepsin K and matrix metalloproteinases (MMPs). These pathways and highly expressed proteins provide targets for specific therapies to modify bone degradation. The main outstanding issues, basic and translational, will be considered in relation to the osteoclast as a target for antiresorptive therapies.

Bisphosphonate mechanism of action

The nitrogen-containing bisphosphonates (N-BPs), alendronate and risedronate, are the only pharmacologic agents shown to prevent spine and nonvertebral fractures associated with postmenopausal and glucocorticoid-induced osteoporosis. At the tissue level, this is achieved through osteoclast inhibition, which leads to reduced bone turnover, increased bone mass, and improved mineralization. The molecular targets of bisphosphonates (BPs) have recently been identified. This review will discuss the mechanism of action of BPs, focusing on alendronate and risedronate, which are the two agents most widely studied. They act on the cholesterol biosynthesis pathway enzyme, farnesyl diphosphate synthase. By inhibiting this enzyme in the osteoclast, they interfere with geranylgeranylation (attachment of the lipid to regulatory proteins), which causes osteoclast inactivation. This mechanism is responsible for N-BP suppression of osteoclastic bone resorption and reduction of bone turnover, which leads to fracture prevention.

The effects of intravenous pamidronate on the bone tissue of children and adolescents with osteogenesis imperfecta

Cyclical pamidronate infusions increase bone mass in children suffering from osteogenesis imperfecta. The histological basis for these effects remains unknown. Therefore, we compared parameters of iliac bone histomorphometry from 45 patients before and after 2.4 +/- 0.6 years of pamidronate treatment (age at the time of the first biopsy, 1.4-17.5 years; 23 girls). Although biopsy size did not change significantly (P = 0.30), cortical width increased by 88%. Cancellous bone volume increased by 46%. This was due to a higher trabecular number, whereas trabecular thickness remained stable. Bone surface-based indicators of cancellous bone remodeling decreased by 26-75%. There was no evidence for a mineralization defect in any of the patients. These results suggest that, in the growing skeleton, pamidronate has a twofold effect. In remodeling, bone resorption and formation are coupled and consequently both processes are inhibited. However, osteoclasts and osteoblasts are active on different surfaces (and are thus uncoupled) during modeling of cortical bone. Therefore resorption is selectively targeted, and continuing bone formation can increase cortical width.