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Anti-resorptive therapies for osteoporosis
Seminars in Cell & Developmental Biology 19 (2008) 473–478
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Review
Anti-resorptive therapies for osteoporosis Ian R. Reid ∗ Faculty of Medical and Health Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand
a r t i c l e
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Article history: Available online 7 August 2008 Keywords: Bisphosphonate Estrogen Calcium Raloxifene Denosumab
a b s t r a c t The treatment of osteoporosis is largely based around the use of agents that inhibit bone resorption by osteoclasts. The main classes of anti-resorptives currently in use are calcium, bisphosphonates, estrogen, selective estrogen receptor modulators (SERMs) and calcitonin. Novel agents in development are: inhibitors of the osteoclast enzyme, cathepsin K; and a monoclonal antibody against receptor activator of NFB-ligand (RANKL), a factor made by osteoblasts which stimulates osteoclast development. Potent anti-resorptive agents decrease numbers of vertebral fractures by about 50%, and non-vertebral fractures by only 25%. Whether the newer agents can improve on this remains to be seen, though it is possible that anabolic agents which increase bone mass more substantially will be needed to achieve greater reductions in all fracture numbers. © 2008 Elsevier Ltd. All rights reserved.
Contents 1. 2. 3. 4. 5.
6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Calcium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Estrogen and selective estrogen receptor modulators (SERMs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bisphosphonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anti-resorptive agents in development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Cathepsin-K inhibitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Denosumab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Bone is a dynamic tissue which undergoes continuous renewal. This process takes place at foci dotted across the bone surface, two to three million existing in an adult human at any one time. A remodeling site is initiated by loss of the bone lining cells which cover the resting bone surface. The underlying bone surface is then colonized by osteoclasts, which are recruited from the adjacent marrow space. Over a period of about 1 week, a resorption pit is excavated by these osteoclasts. The osteoclasts seal themselves to the bone surface, and secrete protons and proteolytic enzymes (principally cathepsin K) onto the bone surface. The combination of these factors dissolves the bone mineral and hydrolyzes the
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matrix proteins, of which type I collagen is the largest component. This results in a resorption pit, or lacuna. At the end of this time, osteoclast apoptosis occurs, and the vacant resorption pit is then re-colonized by osteoblasts, recruited from stromal cells in the overlying marrow. Over the following 2–3 months, these cells secrete protein matrix and refill the lacuna. Subsequently, calcium and phosphate are deposited as hydroxyapatite crystals between the collagen fibers. By this process, the bone is continually replaced, and on average the entire human skeleton is rebuilt over a period of about 10 years. As humans age, the balance between bone formation and bone resorption is shifted in a negative direction. The number of remodeling sites across the skeleton increases in states of sex hormone deficiency, and some studies suggest that the depth of individual resorption pits increases. As a result of these changes, more and deeper pits are excavated, and sometimes these are large enough to lead to perforation of trabecular plates, which means
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Fig. 1. Sites of action of the major classes of anti-resorptive drugs, each of which is shown in a box. Osteoblasts (OB) express RANKL on their surface, which binds to RANK on pre-osteoclasts (pre-OC) to stimulate osteoclast (OC) development. Denosumab is a monoclonal antibody to RANKL which blocks osteoclastogenesis. Calcium supplementation both stimulates secretion of calcitonin (which acts directly on osteoclasts to reduce bone resorption) and reduces parathyroid hormone (PTH) levels, resulting in diminished RANKL expression by osteoblasts. Estrogen inhibits osteoclastogenesis and the activity of mature osteoclasts, both directly through estrogen receptors, and indirectly through modulation of cytokine production in a variety of cell types. Cathepsin K inhibitors directly block the action of that proteolytic enzyme from osteoclasts, and chloride channel blockers impair the acidification of the space between the ruffled border of the osteoclast and the bone surface. Bisphosphonates are initially laid down on the bone surface and are then taken up by osteoclasts during bone resorption. Once inside the osteoclasts, they inhibit a variety of cellular activities by blocking farnesyl pyrophosphate synthase in the mevalonate pathway. Calcitonin is a weak bone resorption inhibitor still used in some countries for treating osteoporosis. Copyright IR Reid 2008, used with permission.
that the second phase of bone remodeling, bone formation, cannot take place since there is no substratum onto which the bone forming osteoblasts can settle. Advancing age also results in less adequate filling of resorption lacunae by osteoblasts. Thus, even when trabecular perforation does not occur, there is still a negative bone balance at each bone remodeling site. Thus, bone loss is an inevitable component of the ageing process, particularly in postmenopausal women and in men with declining sex hormone levels. Age-related bone loss can be arrested by the use of antiresorptive agents, to restore balance between resorption and formation. The agents available or in development for this purpose, and their mechanisms of action, are shown in Fig. 1. Treatment of established osteoporosis, however, would ideally also use bone anabolic compounds, but parathyroid hormone is the only such agent currently available, and its use is limited by cost and concerns regarding its long-term safety. Thus, anti-resorptives remain the mainstay of all aspects of osteoporosis management at present. The present review will consider the history, efficacy and safety of the available anti-resorptive compounds, as well as discussing novel agents currently in the research pipeline. 2. Calcium Calcium supplementation is the anti-resorptive therapy which has been in use for the longest period of time. While it is often loosely regarded as a general tonic for bone, the doses used as supplements clearly result in suppression of circulating parathyroid hormone levels, with a resultant decrease in bone turnover [1]. After administration of calcium, suppression of bone resorption is apparent within hours [2], and is sustained with long-term use (to
at least 5 years) [3]. These changes in bone turnover are associated with increases in hip bone density of the order of 1.5% over 2 years in men, and 3% over 5 years in women (comparison with placebo) [1,3]. While the beneficial effects of calcium supplementation on bone density are now beyond dispute, its effects on fracture are less clearcut. No study of calcium supplementation as a monotherapy has demonstrated reduced fracture numbers on an intention-to-treat analysis, though this has been achieved when calcium and vitamin D have been given together to vitamin D-deficient populations [4]. One of the difficulties with the long-term use of calcium supplementation is maintaining acceptable compliance levels, since the tablets are bulky, are usually taken in divided doses, and frequently cause gastrointestinal side-effects, principally constipation. As a result, analyses of compliant subjects generally produce more positive results than intention-to treat analyses, and some evidence of anti-fracture efficacy has been apparent when such analyses are carried out on individual trials of calcium alone [3,5]. Metaanalysis of all calcium studies suggests that there is a 10% reduction in the risk of all fractures [6]. However, when this meta-analysis is restricted to the occurrence of hip fractures, there is a statistically significant, 50% increase in those allocated to calcium therapy [7]. This paradoxical result is seen in each of the three studies contributing to this meta-analysis, and has also been reported in a large observational study [8]. The significance of this finding remains uncertain, as does the possible mechanism for such an adverse effect. The femoral neck, like other tubular bones, compensates for age-related bone loss, through periosteal expansion. Calcium supplementation, through suppressing parathyroid hormone, appears to reduce periosteal expansion [9] so this may result in increased fragility at this skeletal site, in spite of the increases in bone density. Calcium supplementation has traditionally been regarded as a safe intervention, with the only common adverse effect being constipation [3]. However, we have recently reported a 50% increase in the occurrence of cardiovascular events in the calcium arm of a randomized, controlled trial [10]. Similar upward trends have been observed in a number of other studies, although they were not statistically significant. In patients with renal failure, both at the pre-dialysis and dialysis stages, increases in vascular calcification, event rate and mortality have been observed in subjects randomized to take calcium supplements [11,12]. There is also observational evidence from a healthy population indicating that carotid artery plaque thickness is related to serum calcium levels in normocalcemic men [13]. One interpretation of these disparate data would be that high serum calcium levels, whether resulting from the ingestion of calcium supplements or occurring spontaneously, results in increased rates of vascular calcification and acceleration of the atherosclerotic process. Further data are awaited to address this possibility more fully. 3. Estrogen and selective estrogen receptor modulators (SERMs) Estrogen is a potent anti-resorptive agent which is key to the normal development of the skeleton, and to the maintenance of skeletal integrity in adults of both sexes. It acts on bone cells both directly and indirectly [14]. Estrogen receptors are present in osteoblasts and osteoclasts. Estrogen promotes the development of osteoblasts in preference to adipocytes from their common precursor cell [15], it increases osteoblast proliferation [16], and it increases production of a number of osteoblast proteins (e.g. insulin-like growth factor-1, type I procollagen, transforming growth factor- (TGF), and bone morphogenetic protein-6). Thus, estrogen tends to have an anabolic effect on the isolated
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osteoblast, which is complemented by its inhibition of apoptosis in these cells [17] and in osteocytes [18]. These effects are reflected in vivo, in increased histomorphometric indices of osteoblast activity (e.g. mean wall thickness) in postmenopausal women treated with estrogen [19]. Estrogen also suppresses osteoclast activity via increased osteoclast apoptosis [20], reduced osteoblast/stromal cell production of RANKL, and by increased production of osteoprotegerin [21]. These direct effects are buttressed by estrogen action on bone marrow stromal and mononuclear cells, to reduce production of cytokines such as interleukin-1 (IL-1), interleukin-6 (IL-6) and tumor necrosis factor-␣ (TNF␣), all of which are potent stimulators of osteoclast recruitment and/or activity [22]. Estrogen also reduces T cell production of TNF␣, via changes in IL-7 [23] and interferon-␥ [14,24]. The use of estrogen in postmenopausal women returns bone turnover markers to premenopausal levels. If instituted at the time of menopause, it completely prevents postmenopausal bone loss and the development of fractures in postmenopausal women [25]. When started in women who are several years postmenopausal, it results in increases of bone density above placebo of 8% at the spine over 3 years [26] and a reduction in hip and spine fracture numbers by one-third [27]. Thus, the widespread use of estrogen replacement in the postmenopause could effectively eliminate the problem of postmenopausal osteoporosis. However, safety concerns have substantially limited uptake of this intervention. There has been longstanding concern regarding an increased incidence of breast cancer. The Women’s Health Initiative confirmed this as a problem when estrogen and progesterone are used in combination [27], though surprisingly there was a substantially downward trend in breast cancer incidence in women treated with conjugated equine estrogens alone [28]. Therapy with estrogen alone is only an option for hysterectomized women, since in the presence of a uterus this treatment results in an increased incidence of endometrial carcinoma. The Women’s Health Initiative also demonstrated that stroke and myocardial infarction are more common with combined estrogen and progesterone use. Estrogen also increases the risk of thromboembolic events [27]. The clear-cut beneficial effects of estrogen on bone together with its adverse event profile, have lead to attempts to develop estrogenlike agents that are free of these problems. These compounds are now known as SERMs, and have a mixture of estrogen agonist and antagonist properties which are tissue-specific [29]. The prototypic SERM was tamoxifen, which was originally designed as an estrogen receptor antagonist for the management of breast cancer. However, it was found that tamoxifen had positive effects on bone density [30] implying that it acted as an estrogen agonist on bone. These findings led to a reappraisal of other estrogen antagonists and to the eventual introduction of raloxifene into clinical use. Raloxifene has much less potent effects on bone resorption and on bone density than does a conventional dose of estrogen [31]. It does reduce the incidence of vertebral fractures [32], however, it has no effect on non-vertebral fractures, so its value in the management of postmenopausal osteoporosis is limited. Its non-bone profile is substantially different from that of estrogen. It reduces the incidence of breast cancers by three-quarters [32] and is neutral with respect to the total number of heart attacks and strokes, though fatal strokes are more common in raloxifene users [33]. Like estrogen, it increases the frequency of thromboembolic events [33]. There have been ongoing efforts to find other SERMs that have more potent bone effects. A number of development programs have been discontinued because the agents have been found to cause uterine prolapse. Phase 3 trials with basedoxifene have been completed. As a monotherapy it appears to be comparable to raloxifene, but it is being further explored for use in combination with estrogen. The hope is that the bone effects of these agents will be
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additive but that the proliferative effects of estrogen on the breast and endometrium will be neutralized by basedoxifene. Phase 2 data with lasofoxifene suggest that its bone effects may be greater than that of the other SERMs [34] but the results of the long-completed phase 3 trial of this agent are still to enter the public domain. Thus, the SERMs are a class of agents that has offered much promise, but has yet to take a major place in the therapy of osteoporosis.
4. Bisphosphonates The bisphosphonates have been the principal anti-resorptive agents used in the therapy of osteoporosis over the last decade. The anti-resorptive effects of these drugs were originally described by Fleisch et al. in the 1960s [35], in studies originally targeted at identifying potential inhibitors of renal stone formation. Subsequently, hundreds of bisphosphonates have been synthesized with a view to optimizing their anti-resorptive potency and minimizing their propensity to inhibit mineralization. The bisphosphonate nucleus consists of two phosphate groups joined through a central carbon atom. This P–C–P bond is not metabolized in humans, so bisphosphonates are extremely stable and able to retain biological activity in vivo over a period of years. The negative charge associated with the phosphate groups gives these compounds a very high affinity for the positively charged surface of the hydroxyapatite crystals of bone. Following intravenous injection of a bisphosphonate, approximately half the dose is sequestered in bone within several hours of administration, the balance being excreted by the kidneys over a similar time-course. Bisphosphonate on the bone surface is ingested by osteoclasts as part of the bone resorption process. These drugs have the potential to interfere with metabolic pathways involving diphosphate moieties. The principal pathway affected by the amino-bisphosphonates is the mevalonate pathway, which leads to the synthesis of cholesterol and to the prenylation of a number of GTPase proteins. These proteins, Rab, Rho and Ras, are critical to the maintenance of the osteoclast cytoskeleton. When their prenylation is blocked by bisphosphonate inhibition of farnesyl pyrophosphate synthase, osteoclast attachment to the bone surface is impaired, and in higher doses, osteoclast apoptosis results. The mevalonate pathway is important in many tissues, but the bisphosphonates are very efficiently targeted to the bone surface by their charge characteristics, and the only cell that ingests bone (and thus the bisphosphonate) is the osteoclast. Thus, in clinical use, these agents are very specific and therefore have a good safety profile. Oral formulations cause upper gastrointestinal inflammation, and this appears to be mediated by inhibition of the mevalonate pathway within gastrointestinal epithelial cells [36]. Recently, use of very high doses of bisphosphonates for treating cancers metastatic to bone, has been associated with mucosal necrosis at dental extraction sites [37] (osteonecrosis of the jaw), and this may have a similar etiology [38]. Following intravenous injection, about 20% of patients experience a transient flu-like illness. This appears to result from accumulation of isopentenyl diphosphate, a compound upstream from the enzyme targeted by the amino bisphosphonates. This compound stimulates release of cytokines (interleukin-6, interferon-␥, tumor necrosis factor-␣) from ␥␦T cells, resulting in the typical symptoms of fever and musculoskeletal pain [39]. Following oral administration, only a few percent of the administered bisphosphonate dose is absorbed. To optimize this, it is important that the dose is taken fasting, with water alone, at least 30 min before other fluid or foods. Co-administration with positively charged ions, such as calcium, iron or other mineral
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supplements, will reduce bioavailability even further. To avert reflux of the tablets into the esophagus with the associated risk of esophagitis and ulceration, patients must remain upright following dosing. These practical difficulties have reduced the acceptability to patients of these drugs, particularly when dosed daily. However, the very long residence time of bisphosphonates in bone has lead to progressively less frequent administration. The most widely used agents, alendronate and risedronate, are now typically administered weekly rather than daily, and ibandronate is used either monthly or three-monthly. The most potent bisphosphonate to enter clinical use is zoledronate, and this has recently been approved as an annual infusion for the treatment for osteoporosis [40]. When used in the treatment of Paget’s disease a single infusion produces remissions that extend well beyond 3 years [41]. Among the amino-bisphosphonates, there is a spectrum of drug affinities for hydroxyapatite, and a different spectrum of drug potencies for inhibition of farnesyl pyrophosphate synthase. It appears that high affinity for bone and high potency of enzyme inhibition together determine in vivo potency of inhibition of bone resorption. Affinity for hydroxyapatite may, in addition, influence duration of action, determining the maximal inter-dose interval which maintains adequate therapeutic efficacy. Bisphosphonates rapidly reduce bone resorption, reductions of up to 90% being possible, depending on the agent and dose used. Bone formation indices decline over the following 3 months to levels roughly comparable to those of the resorption markers. Subsequent increases in bone density are very similar to those seen with estrogen treatment [42], and are broadly comparable across the amino-bisphosphonates currently in clinical use. Bisphosphonates as a class have the best data establishing their anti-fracture efficacy. Even relatively weak anti-resorptives, such as etidronate, appear to reduce vertebral fracture incidence, and alendronate, risedronate and zoledronate have been clearly shown to decrease vertebral, non-vertebral and hip fracture incidence. These agents, like estrogen, decrease fracture risk within the first 6–12 months of treatment, at which time the increases in bone density are only modest. This suggests that the dramatic reduction in bone resorption is more critical for fracture prevention than the increase in bone mass. This is explicable by the fact that in untreated osteoporosis, there is ongoing resorption of critically thinned bone trabeculae. Stabilization of these remaining bone elements produces a dividend in terms of bone strength which is much greater than the changes in bone mass would suggest.
5. Anti-resorptive agents in development Ongoing research into the mechanisms of bone resorption has thrown up a number of novel drug targets, both within the osteoclast and targeted at osteoclast regulating factors. Osteoclastic bone resorption requires attachment of osteoclasts to the bone surface by way of integrin receptors. Integrin antagonists have shown efficacy in animal studies [43] and in early clinical trials [44]. Acid secretion by the osteoclast involves the proton pump and chloride channel on the ruffled border of the osteoclast. Inhibitors of the chloride channel inhibit acidification of the sealed zone beneath the osteoclast, thus inhibiting bone resorption. The efficacy of such factors has been demonstrated in animal studies. The principal proteolytic enzyme to resorb bone matrix is cathepsin-K. This has now entered phase 3 clinical trials and will be discussed in more detail. Similarly, denosumab, which blocks a regulator of osteoclastogenesis, is nearing the end of its phase 3 program, and will receive more detailed discussion.
5.1. Cathepsin-K inhibitors Cathepsin-K is a cysteine protease highly expressed in osteoclasts. There are numerous other cathepsins with wide tissue distributions, so any inhibitors for therapeutic use require high specificity if they are to have acceptable safety profiles. Balicatib advanced into phase 2 clinical trials, but further development was discontinued because of skin and respiratory side-effects [45]. Odanacatib has successfully completed phase 2 trials without obvious safety issues arising. When given as a weekly oral dose, it reduces bone resorption markers by about 70% and produces changes in spine bone densities which are comparable to those seen with potent bisphosphonates [46]. Thus, it appears likely to have comparable efficacy to the bisphosphonates, though confirmation of this awaits the undertaking of phase 3 trials. The need for the development of new drugs which are only comparable in efficacy to those currently available is driven by concerns at the shortcomings of the bisphosphonates. The development of osteonecrosis of the jaw, though overwhelmingly an issue in oncology patients, has raised some anxieties regarding the long-term safety of these drugs, even at osteoporotic doses. The slow offset time of the bisphosphonates causes interference with the use of anabolic agents, such as parathyroid hormone. Therefore, a rapidly reversible anti-resorptive is considered attractive by some, though such a drug will create the need for stricter patient compliance and will not have the benefit of a persisting therapeutic effect for some years following treatment discontinuation, which the bisphosphonates confer. Gastrointestinal side-effects have also proved to be a significant limitation in the use of oral bisphosphonates, so agents such as cathepsin-K inhibitors may well address this shortcoming also. 5.2. Denosumab In the late 1990s an entirely new pathway by which osteoblasts regulate the development of osteoclasts was discovered. It is now apparent that the osteoblasts produce RANKL, a member of the tumor necrosis factor family, which binds to specific receptors (RANK) on the surface of pre-osteoclasts, leading to osteoclastogenesis. The osteoblasts also counter-regulate this mechanism through the production of osteoprotegerin, a member of the tumor necrosis factor receptor family, which binds RANKL, thus preventing its stimulation of osteoclastogenesis [47]. A number of strategies for manipulating this regulatory system have been assessed. The approach which has proceeded the furthest is the development of denosumab, a monoclonal antibody to RANKL, which is administered by subcutaneous injection. This approach has the potential to substantially reduce the number of osteoclasts, and thus precisely regulate bone resorption. Phase 3 trials of denosumab are approaching their conclusion. To date, the phase 1 and 2 programs have demonstrated that this antibody has a half-life of approximately 1 month, and can be dosed at 6-month intervals in women with postmenopausal osteoporosis [48]. The doses used produce a 90% reduction in biochemical markers of bone resorption, slightly greater than that achieved with alendronate. Increases in spine and hip bone density are comparable to or slightly better than those seen with alendronate, though at the cortical bone site of the mid forearm, denosumab appears to have significantly greater efficacy than the bisphosphonates [49,50]. This may be a chance finding, but it could be explained by the fact that bisphosphonates are targeted to sites of high bone turnover, and therefore have a low uptake into cortical bone. In contrast, denosumab influences osteoclastogenesis uniformly throughout the skeleton, so might be expected to have a greater effect on low turnover cortical bone than a bisphosphonate.
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To date, denosumab has shown an acceptable safety profile, but the involvement of the tumor necrosis factor system in the development of the immune system and as a defense against infection, suggests that close surveillance of adverse events is important for some time yet. The presentation of both the efficacy and safety data from the phase 3 program, which will take place late in 2008, is awaited with great interest. 6. Conclusions Anti-resorptive drugs remain the mainstay of therapy for osteoporosis. The bisphosphonates are effective and safe agents for use in this role, and there have been major developments with respect to route and timing of administration which have made them more convenient for patients. The development of agents with different modes of action is likely to overcome the side-effect issues that trouble some patients taking bisphosphonates, and may provide drugs that can be more effectively combined with anabolics, such as parathyroid hormone. With the advent of denosumab, a new route of administration will become available. There is enormous research effort taking place at the present time to develop more effective anabolic agents. This offers the promise of substantially restoring bone mass, and producing greater decreases in fracture rates than simple anti-resorptive therapies can. However, current indications are that such agents will be used in combination or sequentially with anti-resorptive agents, so the central role of this class of compounds is likely to remain for the foreseeable future. All of these advances in the therapeutics of this important bone disease have been made possible through huge expansions in our understanding of the biology which underlies bone formation and resorption. Acknowledgement Funded by the Health Research Council of New Zealand. References [1] Reid IR, Ames R, Mason B, Reid HE, Bacon CJ, Bolland MJ, et al. Randomized controlled trial of calcium supplementation in healthy, non-osteoporotic, older men. Arch Int Med; in press. [2] Reid IR, Schooler BA, Hannon S, Ibbertson HK. The acute biochemical effects of four proprietary calcium supplements. Aust N Z J Med 1986;16:193–7. [3] Reid IR, Mason B, Horne A, Ames R, Reid HE, Bava U, et al. Randomized controlled trial of calcium in healthy older women. Am J Med 2006;119:777–85. [4] Chapuy MC, Arlot ME, Delmas PD, Meunier PJ. Effect of calcium and cholecalciferol treatment for three years on hip fractures in elderly women. BMJ 1994;308:1081–2. [5] Prince RL, Devine A, Dhaliwal SS. Dick IM Effects of calcium supplementation on clinical fracture and bone structure—results of a 5-year, double-blind, placebocontrolled trial in elderly women. Arch Int Med 2006;166:869–75. [6] Tang BMP, Eslick GD, Nowson C, Smith C, Bensoussan A. Use of calcium or calcium in combination with vitamin D supplementation to prevent fractures and bone loss in people aged 50 years and older: a meta-analysis. Lancet 2007;370:657–66. [7] Reid IR, Bolland M, Grey A. Effect of calcium supplementation on hip fractures. Osteoporos Int 2008. [8] Cumming RG, Cummings SR, Nevitt MC, Scott J, Ensrud KE, Vogt TM, et al. Calcium intake and fracture risk: results from the study of osteoporotic fractures. Am J Epidemiol 1997;145:926–34. [9] Chen Z, Beck TJ, Wright NC, LaCroix AZ, Cauley JA, Lewis CE, et al. The effect of calcium plus vitamin D supplement on hip geometric structures: results from the Women’s Health Initiative CaD trial. J Bone Mineral Res 2007;22(Suppl 1):s59. [10] Bolland MJ, Barber PA, Doughty RN, Mason B, Horne A, Ames R, et al. Vascular events in healthy older women receiving calcium supplementation: randomised controlled trial. BMJ 2008;336:262–6. [11] Russo D, Miranda I, Ruocco C, Battaglia Y, Buonanno E, Manzi S, et al. The progression of coronary artery calcification in predialysis patients on calcium carbonate or sevelamer. Kidney Int 2007;72:1255–61. [12] Block GA, Raggi P, Bellasi A, Kooienga L, Spiegel DM. Mortality effect of coronary calcification and phosphate binder choice in incident hemodialysis patients. Kidney Int 2007;71:438–41.
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[13] Rubin MR, Rundek T, McMahon DJ, Lee H-S, Sacco RL, Silverberg SJ. Carotid artery plaque thickness is associated with increased serum calcium levels: the Northern Manhattan study. Atherosclerosis 2007;194:426–32. [14] Reid IR. Menopause. In: Rosen C, editor. Primer on the metabolic bone diseases and disorders of calcium metabolism. 7th ed. American Society for Bone and Mineral Research; 2008. [15] Okazaki R, Inoue D, Shibata M, Saika M, Kido S, Ooka H, et al. Estrogen promotes early osteoblast differentiation and inhibits adipocyte differentiation in mouse bone marrow stromal cell lines that express estrogen receptor (ER) alpha or beta. Endocrinology 2002;143:2349–56. [16] Fujita M, Urano T, Horie K, Ikeda K, Tsukui T, Fukuoka H, et al. Estrogen activates cyclin-dependent kinases 4 and 6 through induction of cyclin D in rat primary osteoblasts. Biochem Biophys Res Commun 2002;299:222–8. [17] Gohel A, McCarthy MB, Gronowicz G. Estrogen prevents glucocorticoidinduced apoptosis in osteoblasts in vivo and in vitro. Endocrinology 1999;140: 5339–47. [18] Tomkinson A, Reeve J, Shaw RW, Noble BS. The death of osteocytes via apoptosis accompanies estrogen withdrawal in human bone. J Clin Endocrinol Metabol 1997;82:3128–35. [19] Bord S, Beavan S, Ireland D, Horner A, Compston JE. Mechanisms by which highdose estrogen therapy produces anabolic skeletal effects in postmenopausal women: role of locally produced growth factors. Bone 2001;29:216– 22. [20] Kameda T, Mano H, Yuasa T, Mori Y, Miyazawa K, Shiokawa M, et al. Estrogen inhibits bone resorption by directly inducing apoptosis of the bone-resorbing osteoclasts. J Exp Med 1997;186:489–95. [21] Syed F, Khosla S. Mechanisms of sex steroid effects on bone. Biochem Biophys Res Commun 2005;328:688–96. [22] Manolagas SC, Jilka RL. Mechanisms of disease: bone marrow, cytokines, and bone remodelling—emerging insights into the pathophysiology of osteoporosis. N Engl J Med 1995;332:305–11. [23] Ryan MR, Shepherd R, Leavey JK, Gao YH, Grassi F, Schnell FJ, et al. An IL-7dependent rebound in thymic T cell output contributes to the bone loss induced by estrogen deficiency. Proc Natl Acad Sci USA 2005;102:16735–40. [24] Cenci S, Toraldo G, Weitzmann MN, Roggia C, Gao YH, Qian WP, et al. Estrogen deficiency induces bone loss by increasing T cell proliferation and lifespan through IFN-gamma-induced class II transactivator. Proc Natl Acad Sci USA 2003;100:10405–10. [25] Lindsay R, Hart DM, Forrest C, Baird C. Prevention of spinal osteoporosis in oophorectomised women. Lancet 1980;ii:1151–3. [26] Bush TL, Wells HB, James MK, Barrett-Connor E, Marcus R, Greendale G, et al. Effects of hormone therapy on bone mineral density—results from the postmenopausal estrogen/progestin interventions (PEPI) trial. JAMA 1996;276:1389–96. [27] Rossouw JE, Anderson GL, Prentice RL, LaCroix AZ, Kooperberg C, Stefanick ML, et al. Risks and benefits of estrogen plus progestin in healthy postmenopausal women - Principal results from the Women’s Health Initiative randomized controlled trial. JAMA 2002;288:321–33. [28] Anderson GL, Limacher M, Assaf AR, Bassford T, Beresford SAA, Black H, et al. Effects of conjugated, equine estrogen in postmenopausal women with hysterectomy—the women’s health initiative randomized controlled trial. JAMA 2004;291:1701–12. [29] Katzenellenbogen BS, Katzenellenbogen JA. Biomedicine—defining the “S” in SERMs. Science 2002;295:2380–1. [30] Grey AB, Stapleton JP, Evans MC, Tatnell MA, Ames RW, Reid IR. The effect of the antiestrogen tamoxifen on bone mineral density in normal late postmenopausal women. Am J Med 1995;99:636–41. [31] Reid IR, Eastell R, Fogelman I, Adachi JD, Rosen A, Netelenbos C, et al. A comparison of the effects of raloxifene and conjugated equine estrogen on bone and lipids in healthy postmenopausal women. Arch Int Med 2004;164: 871–9. [32] Ettinger B, Black DM, Mitlak BH, Knickerbocker RK, Nickelsen T, Genant HK, et al. Reduction of vertebral fracture risk in postmenopausal women with osteoporosis treated with raloxifene—results from a 3-year randomized clinical trial. JAMA 1999;282:637–45. [33] Barrett-Connor E, Mosca L, Collins P, Geiger MJ, Grady D, Kornitzer M, et al. Effects of raloxifene on cardiovascular events and breast cancer in postmenopausal women. N Engl J Med 2006;355:125–37. [34] McClung MR, Siris E, Cummings S, Bolognese M, Ettinger M, Moffett A, et al. Prevention of bone loss in postmenopausal women treated with lasofoxifene compared with raloxifene. Menopause-J N Am Menopause Soc 2006;13:377–86. [35] Fleisch H, Russell RG, Francis MD. Diphosphonates inhibit hydroxyapatite dissolution in vitro and bone resorption in tissue culture and in vivo. Science 1969;165:1262–4. [36] Reszka AA, Halasy-Nagy J, Rodan GA. Nitrogen-bisphosphonates block retinoblastoma phosphorylation and cell growth by inhibiting the cholesterol biosynthetic pathway in a keratinocyte model for esophageal irritation. Mol Pharmacol 2001;59:193–202. [37] Khosla S, Burr D, Cauley J, Dempster DW, Ebeling PR, Felsenberg D, et al. Bisphosphonate-associated osteonecrosis of the jaw: report of a task force of the American Society for Bone and Mineral Research. J Bone Mineral Res 2007;22:1479–91. [38] Reid IR, Bolland MJ, Grey AB. Is bisphosphonate-associated osteonecrosis of the jaw caused by soft tissue toxicity? Bone 2007;41:318–20.
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[39] Thompson K, Rogers MJ. Bisphosphonates and T-cells: new insights into old drugs. BoneKEy-Osteovision; 2006. doi:10.1138/20060224. [40] Black DM, Delmas PD, Eastell R, Reid IR, Boonen S, Cauley JA, et al. Once-yearly zoledronic acid for treatment of postmenopausal osteoporosis. N Engl J Med 2007;356:1809–22. [41] Hosking D, Lyles K, Brown JP, Fraser WD, Miller P, Curiel MD, et al. Long-term control of bone turnover in Paget’s disease with zoledronic acid and risedronate. J Bone Mineral Res 2007;22:142–8. [42] Hosking D, Chilvers CED, Christiansen C, Ravn P, Wasnich R, Ross P, et al. Prevention of bone loss with alendronate in postmenopausal women under 60 years of age. N Engl J Med 1998;338:485–92. [43] Hutchinson JH, Halczenko W, Brashear KM, Breslin MJ, Coleman PJ, Duong LT, et al. Nonpeptide ␣v3 antagonists. 8. In vitro and in vivo evaluation of a potent ␣v3 antagonist for the prevention and treatment of osteoporosis. J Med Chem 2003;46:4790–8. [44] Murphy M, Cerchio K, Stoch S, Gottesdiener K, Wu M, Recker R. Effect of MRL123, an orally administered ␣v3 integrin antagonist, on markers of bone turnover and bone mineral density in postmenopausal osteoporotic women. J Bone Mineral Res 2004;19:s99.
[45] Peroni A, Zini A, Braga V, Colato C, Adami S, Girolomoni G. Drug-induced morphea: report of a case induced by balicatib and review of the literature. J Am Acad Dermatol 2008;59:125–9. [46] Bone HG, McClung M, Verbruggen N, Rybak-Feiglin A, DaSilva C, Santora AC, et al. A randomized double-blind, placebo-controlled study of a cathepsin K inhibitor in the treatment of postmenopausal women with low BMD: one year results. J Bone Mineral Res 2007;22:s37. [47] Boyle WJ, Simonet WS, Lacey DL. Osteoclast differentiation and activation. Nature 2003;423:337–42. [48] Bekker PJ, Holloway DL, Rasmussen AS, Murphy R, Martin SW, Leese PT, et al. A single-dose placebo-controlled study of AMG 162, a fully human monoclonal antibody to RANKL, in postmenopausal women. J Bone Mineral Res 2004;19:1059–66. [49] McClung MR, Lewiecki EM, Cohen SB, Bolognese MA, Woodson GC, Moffett AH, et al. Denosumab in postmenopausal women with low bone mineral density. N Engl J Med 2006;354:821–31. [50] Lewiecki EM, Miller PD, McClung MR, Cohen SB, Bolognese MA, Liu Y, et al. Twoyear treatment with denosumab (AMG 162) in a randomized phase 2 study of postmenopausal women with low BMD. J Bone Mineral Res 2007;22:1832–41.
Contents lists available at ScienceDirect
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Review
Anti-resorptive therapies for osteoporosis Ian R. Reid ∗ Faculty of Medical and Health Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand
a r t i c l e
i n f o
Article history: Available online 7 August 2008 Keywords: Bisphosphonate Estrogen Calcium Raloxifene Denosumab
a b s t r a c t The treatment of osteoporosis is largely based around the use of agents that inhibit bone resorption by osteoclasts. The main classes of anti-resorptives currently in use are calcium, bisphosphonates, estrogen, selective estrogen receptor modulators (SERMs) and calcitonin. Novel agents in development are: inhibitors of the osteoclast enzyme, cathepsin K; and a monoclonal antibody against receptor activator of NFB-ligand (RANKL), a factor made by osteoblasts which stimulates osteoclast development. Potent anti-resorptive agents decrease numbers of vertebral fractures by about 50%, and non-vertebral fractures by only 25%. Whether the newer agents can improve on this remains to be seen, though it is possible that anabolic agents which increase bone mass more substantially will be needed to achieve greater reductions in all fracture numbers. © 2008 Elsevier Ltd. All rights reserved.
Contents 1. 2. 3. 4. 5.
6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Calcium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Estrogen and selective estrogen receptor modulators (SERMs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bisphosphonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anti-resorptive agents in development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Cathepsin-K inhibitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Denosumab . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Bone is a dynamic tissue which undergoes continuous renewal. This process takes place at foci dotted across the bone surface, two to three million existing in an adult human at any one time. A remodeling site is initiated by loss of the bone lining cells which cover the resting bone surface. The underlying bone surface is then colonized by osteoclasts, which are recruited from the adjacent marrow space. Over a period of about 1 week, a resorption pit is excavated by these osteoclasts. The osteoclasts seal themselves to the bone surface, and secrete protons and proteolytic enzymes (principally cathepsin K) onto the bone surface. The combination of these factors dissolves the bone mineral and hydrolyzes the
∗ Tel.: +64 9 3737 599x86259; fax: +64 9 3082 308. E-mail address: [email protected]. 1084-9521/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.semcdb.2008.08.002
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matrix proteins, of which type I collagen is the largest component. This results in a resorption pit, or lacuna. At the end of this time, osteoclast apoptosis occurs, and the vacant resorption pit is then re-colonized by osteoblasts, recruited from stromal cells in the overlying marrow. Over the following 2–3 months, these cells secrete protein matrix and refill the lacuna. Subsequently, calcium and phosphate are deposited as hydroxyapatite crystals between the collagen fibers. By this process, the bone is continually replaced, and on average the entire human skeleton is rebuilt over a period of about 10 years. As humans age, the balance between bone formation and bone resorption is shifted in a negative direction. The number of remodeling sites across the skeleton increases in states of sex hormone deficiency, and some studies suggest that the depth of individual resorption pits increases. As a result of these changes, more and deeper pits are excavated, and sometimes these are large enough to lead to perforation of trabecular plates, which means
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Fig. 1. Sites of action of the major classes of anti-resorptive drugs, each of which is shown in a box. Osteoblasts (OB) express RANKL on their surface, which binds to RANK on pre-osteoclasts (pre-OC) to stimulate osteoclast (OC) development. Denosumab is a monoclonal antibody to RANKL which blocks osteoclastogenesis. Calcium supplementation both stimulates secretion of calcitonin (which acts directly on osteoclasts to reduce bone resorption) and reduces parathyroid hormone (PTH) levels, resulting in diminished RANKL expression by osteoblasts. Estrogen inhibits osteoclastogenesis and the activity of mature osteoclasts, both directly through estrogen receptors, and indirectly through modulation of cytokine production in a variety of cell types. Cathepsin K inhibitors directly block the action of that proteolytic enzyme from osteoclasts, and chloride channel blockers impair the acidification of the space between the ruffled border of the osteoclast and the bone surface. Bisphosphonates are initially laid down on the bone surface and are then taken up by osteoclasts during bone resorption. Once inside the osteoclasts, they inhibit a variety of cellular activities by blocking farnesyl pyrophosphate synthase in the mevalonate pathway. Calcitonin is a weak bone resorption inhibitor still used in some countries for treating osteoporosis. Copyright IR Reid 2008, used with permission.
that the second phase of bone remodeling, bone formation, cannot take place since there is no substratum onto which the bone forming osteoblasts can settle. Advancing age also results in less adequate filling of resorption lacunae by osteoblasts. Thus, even when trabecular perforation does not occur, there is still a negative bone balance at each bone remodeling site. Thus, bone loss is an inevitable component of the ageing process, particularly in postmenopausal women and in men with declining sex hormone levels. Age-related bone loss can be arrested by the use of antiresorptive agents, to restore balance between resorption and formation. The agents available or in development for this purpose, and their mechanisms of action, are shown in Fig. 1. Treatment of established osteoporosis, however, would ideally also use bone anabolic compounds, but parathyroid hormone is the only such agent currently available, and its use is limited by cost and concerns regarding its long-term safety. Thus, anti-resorptives remain the mainstay of all aspects of osteoporosis management at present. The present review will consider the history, efficacy and safety of the available anti-resorptive compounds, as well as discussing novel agents currently in the research pipeline. 2. Calcium Calcium supplementation is the anti-resorptive therapy which has been in use for the longest period of time. While it is often loosely regarded as a general tonic for bone, the doses used as supplements clearly result in suppression of circulating parathyroid hormone levels, with a resultant decrease in bone turnover [1]. After administration of calcium, suppression of bone resorption is apparent within hours [2], and is sustained with long-term use (to
at least 5 years) [3]. These changes in bone turnover are associated with increases in hip bone density of the order of 1.5% over 2 years in men, and 3% over 5 years in women (comparison with placebo) [1,3]. While the beneficial effects of calcium supplementation on bone density are now beyond dispute, its effects on fracture are less clearcut. No study of calcium supplementation as a monotherapy has demonstrated reduced fracture numbers on an intention-to-treat analysis, though this has been achieved when calcium and vitamin D have been given together to vitamin D-deficient populations [4]. One of the difficulties with the long-term use of calcium supplementation is maintaining acceptable compliance levels, since the tablets are bulky, are usually taken in divided doses, and frequently cause gastrointestinal side-effects, principally constipation. As a result, analyses of compliant subjects generally produce more positive results than intention-to treat analyses, and some evidence of anti-fracture efficacy has been apparent when such analyses are carried out on individual trials of calcium alone [3,5]. Metaanalysis of all calcium studies suggests that there is a 10% reduction in the risk of all fractures [6]. However, when this meta-analysis is restricted to the occurrence of hip fractures, there is a statistically significant, 50% increase in those allocated to calcium therapy [7]. This paradoxical result is seen in each of the three studies contributing to this meta-analysis, and has also been reported in a large observational study [8]. The significance of this finding remains uncertain, as does the possible mechanism for such an adverse effect. The femoral neck, like other tubular bones, compensates for age-related bone loss, through periosteal expansion. Calcium supplementation, through suppressing parathyroid hormone, appears to reduce periosteal expansion [9] so this may result in increased fragility at this skeletal site, in spite of the increases in bone density. Calcium supplementation has traditionally been regarded as a safe intervention, with the only common adverse effect being constipation [3]. However, we have recently reported a 50% increase in the occurrence of cardiovascular events in the calcium arm of a randomized, controlled trial [10]. Similar upward trends have been observed in a number of other studies, although they were not statistically significant. In patients with renal failure, both at the pre-dialysis and dialysis stages, increases in vascular calcification, event rate and mortality have been observed in subjects randomized to take calcium supplements [11,12]. There is also observational evidence from a healthy population indicating that carotid artery plaque thickness is related to serum calcium levels in normocalcemic men [13]. One interpretation of these disparate data would be that high serum calcium levels, whether resulting from the ingestion of calcium supplements or occurring spontaneously, results in increased rates of vascular calcification and acceleration of the atherosclerotic process. Further data are awaited to address this possibility more fully. 3. Estrogen and selective estrogen receptor modulators (SERMs) Estrogen is a potent anti-resorptive agent which is key to the normal development of the skeleton, and to the maintenance of skeletal integrity in adults of both sexes. It acts on bone cells both directly and indirectly [14]. Estrogen receptors are present in osteoblasts and osteoclasts. Estrogen promotes the development of osteoblasts in preference to adipocytes from their common precursor cell [15], it increases osteoblast proliferation [16], and it increases production of a number of osteoblast proteins (e.g. insulin-like growth factor-1, type I procollagen, transforming growth factor- (TGF), and bone morphogenetic protein-6). Thus, estrogen tends to have an anabolic effect on the isolated
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osteoblast, which is complemented by its inhibition of apoptosis in these cells [17] and in osteocytes [18]. These effects are reflected in vivo, in increased histomorphometric indices of osteoblast activity (e.g. mean wall thickness) in postmenopausal women treated with estrogen [19]. Estrogen also suppresses osteoclast activity via increased osteoclast apoptosis [20], reduced osteoblast/stromal cell production of RANKL, and by increased production of osteoprotegerin [21]. These direct effects are buttressed by estrogen action on bone marrow stromal and mononuclear cells, to reduce production of cytokines such as interleukin-1 (IL-1), interleukin-6 (IL-6) and tumor necrosis factor-␣ (TNF␣), all of which are potent stimulators of osteoclast recruitment and/or activity [22]. Estrogen also reduces T cell production of TNF␣, via changes in IL-7 [23] and interferon-␥ [14,24]. The use of estrogen in postmenopausal women returns bone turnover markers to premenopausal levels. If instituted at the time of menopause, it completely prevents postmenopausal bone loss and the development of fractures in postmenopausal women [25]. When started in women who are several years postmenopausal, it results in increases of bone density above placebo of 8% at the spine over 3 years [26] and a reduction in hip and spine fracture numbers by one-third [27]. Thus, the widespread use of estrogen replacement in the postmenopause could effectively eliminate the problem of postmenopausal osteoporosis. However, safety concerns have substantially limited uptake of this intervention. There has been longstanding concern regarding an increased incidence of breast cancer. The Women’s Health Initiative confirmed this as a problem when estrogen and progesterone are used in combination [27], though surprisingly there was a substantially downward trend in breast cancer incidence in women treated with conjugated equine estrogens alone [28]. Therapy with estrogen alone is only an option for hysterectomized women, since in the presence of a uterus this treatment results in an increased incidence of endometrial carcinoma. The Women’s Health Initiative also demonstrated that stroke and myocardial infarction are more common with combined estrogen and progesterone use. Estrogen also increases the risk of thromboembolic events [27]. The clear-cut beneficial effects of estrogen on bone together with its adverse event profile, have lead to attempts to develop estrogenlike agents that are free of these problems. These compounds are now known as SERMs, and have a mixture of estrogen agonist and antagonist properties which are tissue-specific [29]. The prototypic SERM was tamoxifen, which was originally designed as an estrogen receptor antagonist for the management of breast cancer. However, it was found that tamoxifen had positive effects on bone density [30] implying that it acted as an estrogen agonist on bone. These findings led to a reappraisal of other estrogen antagonists and to the eventual introduction of raloxifene into clinical use. Raloxifene has much less potent effects on bone resorption and on bone density than does a conventional dose of estrogen [31]. It does reduce the incidence of vertebral fractures [32], however, it has no effect on non-vertebral fractures, so its value in the management of postmenopausal osteoporosis is limited. Its non-bone profile is substantially different from that of estrogen. It reduces the incidence of breast cancers by three-quarters [32] and is neutral with respect to the total number of heart attacks and strokes, though fatal strokes are more common in raloxifene users [33]. Like estrogen, it increases the frequency of thromboembolic events [33]. There have been ongoing efforts to find other SERMs that have more potent bone effects. A number of development programs have been discontinued because the agents have been found to cause uterine prolapse. Phase 3 trials with basedoxifene have been completed. As a monotherapy it appears to be comparable to raloxifene, but it is being further explored for use in combination with estrogen. The hope is that the bone effects of these agents will be
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additive but that the proliferative effects of estrogen on the breast and endometrium will be neutralized by basedoxifene. Phase 2 data with lasofoxifene suggest that its bone effects may be greater than that of the other SERMs [34] but the results of the long-completed phase 3 trial of this agent are still to enter the public domain. Thus, the SERMs are a class of agents that has offered much promise, but has yet to take a major place in the therapy of osteoporosis.
4. Bisphosphonates The bisphosphonates have been the principal anti-resorptive agents used in the therapy of osteoporosis over the last decade. The anti-resorptive effects of these drugs were originally described by Fleisch et al. in the 1960s [35], in studies originally targeted at identifying potential inhibitors of renal stone formation. Subsequently, hundreds of bisphosphonates have been synthesized with a view to optimizing their anti-resorptive potency and minimizing their propensity to inhibit mineralization. The bisphosphonate nucleus consists of two phosphate groups joined through a central carbon atom. This P–C–P bond is not metabolized in humans, so bisphosphonates are extremely stable and able to retain biological activity in vivo over a period of years. The negative charge associated with the phosphate groups gives these compounds a very high affinity for the positively charged surface of the hydroxyapatite crystals of bone. Following intravenous injection of a bisphosphonate, approximately half the dose is sequestered in bone within several hours of administration, the balance being excreted by the kidneys over a similar time-course. Bisphosphonate on the bone surface is ingested by osteoclasts as part of the bone resorption process. These drugs have the potential to interfere with metabolic pathways involving diphosphate moieties. The principal pathway affected by the amino-bisphosphonates is the mevalonate pathway, which leads to the synthesis of cholesterol and to the prenylation of a number of GTPase proteins. These proteins, Rab, Rho and Ras, are critical to the maintenance of the osteoclast cytoskeleton. When their prenylation is blocked by bisphosphonate inhibition of farnesyl pyrophosphate synthase, osteoclast attachment to the bone surface is impaired, and in higher doses, osteoclast apoptosis results. The mevalonate pathway is important in many tissues, but the bisphosphonates are very efficiently targeted to the bone surface by their charge characteristics, and the only cell that ingests bone (and thus the bisphosphonate) is the osteoclast. Thus, in clinical use, these agents are very specific and therefore have a good safety profile. Oral formulations cause upper gastrointestinal inflammation, and this appears to be mediated by inhibition of the mevalonate pathway within gastrointestinal epithelial cells [36]. Recently, use of very high doses of bisphosphonates for treating cancers metastatic to bone, has been associated with mucosal necrosis at dental extraction sites [37] (osteonecrosis of the jaw), and this may have a similar etiology [38]. Following intravenous injection, about 20% of patients experience a transient flu-like illness. This appears to result from accumulation of isopentenyl diphosphate, a compound upstream from the enzyme targeted by the amino bisphosphonates. This compound stimulates release of cytokines (interleukin-6, interferon-␥, tumor necrosis factor-␣) from ␥␦T cells, resulting in the typical symptoms of fever and musculoskeletal pain [39]. Following oral administration, only a few percent of the administered bisphosphonate dose is absorbed. To optimize this, it is important that the dose is taken fasting, with water alone, at least 30 min before other fluid or foods. Co-administration with positively charged ions, such as calcium, iron or other mineral
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supplements, will reduce bioavailability even further. To avert reflux of the tablets into the esophagus with the associated risk of esophagitis and ulceration, patients must remain upright following dosing. These practical difficulties have reduced the acceptability to patients of these drugs, particularly when dosed daily. However, the very long residence time of bisphosphonates in bone has lead to progressively less frequent administration. The most widely used agents, alendronate and risedronate, are now typically administered weekly rather than daily, and ibandronate is used either monthly or three-monthly. The most potent bisphosphonate to enter clinical use is zoledronate, and this has recently been approved as an annual infusion for the treatment for osteoporosis [40]. When used in the treatment of Paget’s disease a single infusion produces remissions that extend well beyond 3 years [41]. Among the amino-bisphosphonates, there is a spectrum of drug affinities for hydroxyapatite, and a different spectrum of drug potencies for inhibition of farnesyl pyrophosphate synthase. It appears that high affinity for bone and high potency of enzyme inhibition together determine in vivo potency of inhibition of bone resorption. Affinity for hydroxyapatite may, in addition, influence duration of action, determining the maximal inter-dose interval which maintains adequate therapeutic efficacy. Bisphosphonates rapidly reduce bone resorption, reductions of up to 90% being possible, depending on the agent and dose used. Bone formation indices decline over the following 3 months to levels roughly comparable to those of the resorption markers. Subsequent increases in bone density are very similar to those seen with estrogen treatment [42], and are broadly comparable across the amino-bisphosphonates currently in clinical use. Bisphosphonates as a class have the best data establishing their anti-fracture efficacy. Even relatively weak anti-resorptives, such as etidronate, appear to reduce vertebral fracture incidence, and alendronate, risedronate and zoledronate have been clearly shown to decrease vertebral, non-vertebral and hip fracture incidence. These agents, like estrogen, decrease fracture risk within the first 6–12 months of treatment, at which time the increases in bone density are only modest. This suggests that the dramatic reduction in bone resorption is more critical for fracture prevention than the increase in bone mass. This is explicable by the fact that in untreated osteoporosis, there is ongoing resorption of critically thinned bone trabeculae. Stabilization of these remaining bone elements produces a dividend in terms of bone strength which is much greater than the changes in bone mass would suggest.
5. Anti-resorptive agents in development Ongoing research into the mechanisms of bone resorption has thrown up a number of novel drug targets, both within the osteoclast and targeted at osteoclast regulating factors. Osteoclastic bone resorption requires attachment of osteoclasts to the bone surface by way of integrin receptors. Integrin antagonists have shown efficacy in animal studies [43] and in early clinical trials [44]. Acid secretion by the osteoclast involves the proton pump and chloride channel on the ruffled border of the osteoclast. Inhibitors of the chloride channel inhibit acidification of the sealed zone beneath the osteoclast, thus inhibiting bone resorption. The efficacy of such factors has been demonstrated in animal studies. The principal proteolytic enzyme to resorb bone matrix is cathepsin-K. This has now entered phase 3 clinical trials and will be discussed in more detail. Similarly, denosumab, which blocks a regulator of osteoclastogenesis, is nearing the end of its phase 3 program, and will receive more detailed discussion.
5.1. Cathepsin-K inhibitors Cathepsin-K is a cysteine protease highly expressed in osteoclasts. There are numerous other cathepsins with wide tissue distributions, so any inhibitors for therapeutic use require high specificity if they are to have acceptable safety profiles. Balicatib advanced into phase 2 clinical trials, but further development was discontinued because of skin and respiratory side-effects [45]. Odanacatib has successfully completed phase 2 trials without obvious safety issues arising. When given as a weekly oral dose, it reduces bone resorption markers by about 70% and produces changes in spine bone densities which are comparable to those seen with potent bisphosphonates [46]. Thus, it appears likely to have comparable efficacy to the bisphosphonates, though confirmation of this awaits the undertaking of phase 3 trials. The need for the development of new drugs which are only comparable in efficacy to those currently available is driven by concerns at the shortcomings of the bisphosphonates. The development of osteonecrosis of the jaw, though overwhelmingly an issue in oncology patients, has raised some anxieties regarding the long-term safety of these drugs, even at osteoporotic doses. The slow offset time of the bisphosphonates causes interference with the use of anabolic agents, such as parathyroid hormone. Therefore, a rapidly reversible anti-resorptive is considered attractive by some, though such a drug will create the need for stricter patient compliance and will not have the benefit of a persisting therapeutic effect for some years following treatment discontinuation, which the bisphosphonates confer. Gastrointestinal side-effects have also proved to be a significant limitation in the use of oral bisphosphonates, so agents such as cathepsin-K inhibitors may well address this shortcoming also. 5.2. Denosumab In the late 1990s an entirely new pathway by which osteoblasts regulate the development of osteoclasts was discovered. It is now apparent that the osteoblasts produce RANKL, a member of the tumor necrosis factor family, which binds to specific receptors (RANK) on the surface of pre-osteoclasts, leading to osteoclastogenesis. The osteoblasts also counter-regulate this mechanism through the production of osteoprotegerin, a member of the tumor necrosis factor receptor family, which binds RANKL, thus preventing its stimulation of osteoclastogenesis [47]. A number of strategies for manipulating this regulatory system have been assessed. The approach which has proceeded the furthest is the development of denosumab, a monoclonal antibody to RANKL, which is administered by subcutaneous injection. This approach has the potential to substantially reduce the number of osteoclasts, and thus precisely regulate bone resorption. Phase 3 trials of denosumab are approaching their conclusion. To date, the phase 1 and 2 programs have demonstrated that this antibody has a half-life of approximately 1 month, and can be dosed at 6-month intervals in women with postmenopausal osteoporosis [48]. The doses used produce a 90% reduction in biochemical markers of bone resorption, slightly greater than that achieved with alendronate. Increases in spine and hip bone density are comparable to or slightly better than those seen with alendronate, though at the cortical bone site of the mid forearm, denosumab appears to have significantly greater efficacy than the bisphosphonates [49,50]. This may be a chance finding, but it could be explained by the fact that bisphosphonates are targeted to sites of high bone turnover, and therefore have a low uptake into cortical bone. In contrast, denosumab influences osteoclastogenesis uniformly throughout the skeleton, so might be expected to have a greater effect on low turnover cortical bone than a bisphosphonate.
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To date, denosumab has shown an acceptable safety profile, but the involvement of the tumor necrosis factor system in the development of the immune system and as a defense against infection, suggests that close surveillance of adverse events is important for some time yet. The presentation of both the efficacy and safety data from the phase 3 program, which will take place late in 2008, is awaited with great interest. 6. Conclusions Anti-resorptive drugs remain the mainstay of therapy for osteoporosis. The bisphosphonates are effective and safe agents for use in this role, and there have been major developments with respect to route and timing of administration which have made them more convenient for patients. The development of agents with different modes of action is likely to overcome the side-effect issues that trouble some patients taking bisphosphonates, and may provide drugs that can be more effectively combined with anabolics, such as parathyroid hormone. With the advent of denosumab, a new route of administration will become available. There is enormous research effort taking place at the present time to develop more effective anabolic agents. This offers the promise of substantially restoring bone mass, and producing greater decreases in fracture rates than simple anti-resorptive therapies can. However, current indications are that such agents will be used in combination or sequentially with anti-resorptive agents, so the central role of this class of compounds is likely to remain for the foreseeable future. All of these advances in the therapeutics of this important bone disease have been made possible through huge expansions in our understanding of the biology which underlies bone formation and resorption. Acknowledgement Funded by the Health Research Council of New Zealand. References [1] Reid IR, Ames R, Mason B, Reid HE, Bacon CJ, Bolland MJ, et al. Randomized controlled trial of calcium supplementation in healthy, non-osteoporotic, older men. Arch Int Med; in press. [2] Reid IR, Schooler BA, Hannon S, Ibbertson HK. The acute biochemical effects of four proprietary calcium supplements. Aust N Z J Med 1986;16:193–7. [3] Reid IR, Mason B, Horne A, Ames R, Reid HE, Bava U, et al. Randomized controlled trial of calcium in healthy older women. Am J Med 2006;119:777–85. [4] Chapuy MC, Arlot ME, Delmas PD, Meunier PJ. Effect of calcium and cholecalciferol treatment for three years on hip fractures in elderly women. BMJ 1994;308:1081–2. [5] Prince RL, Devine A, Dhaliwal SS. Dick IM Effects of calcium supplementation on clinical fracture and bone structure—results of a 5-year, double-blind, placebocontrolled trial in elderly women. Arch Int Med 2006;166:869–75. [6] Tang BMP, Eslick GD, Nowson C, Smith C, Bensoussan A. Use of calcium or calcium in combination with vitamin D supplementation to prevent fractures and bone loss in people aged 50 years and older: a meta-analysis. Lancet 2007;370:657–66. [7] Reid IR, Bolland M, Grey A. Effect of calcium supplementation on hip fractures. Osteoporos Int 2008. [8] Cumming RG, Cummings SR, Nevitt MC, Scott J, Ensrud KE, Vogt TM, et al. Calcium intake and fracture risk: results from the study of osteoporotic fractures. Am J Epidemiol 1997;145:926–34. [9] Chen Z, Beck TJ, Wright NC, LaCroix AZ, Cauley JA, Lewis CE, et al. The effect of calcium plus vitamin D supplement on hip geometric structures: results from the Women’s Health Initiative CaD trial. J Bone Mineral Res 2007;22(Suppl 1):s59. [10] Bolland MJ, Barber PA, Doughty RN, Mason B, Horne A, Ames R, et al. Vascular events in healthy older women receiving calcium supplementation: randomised controlled trial. BMJ 2008;336:262–6. [11] Russo D, Miranda I, Ruocco C, Battaglia Y, Buonanno E, Manzi S, et al. The progression of coronary artery calcification in predialysis patients on calcium carbonate or sevelamer. Kidney Int 2007;72:1255–61. [12] Block GA, Raggi P, Bellasi A, Kooienga L, Spiegel DM. Mortality effect of coronary calcification and phosphate binder choice in incident hemodialysis patients. Kidney Int 2007;71:438–41.
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