Parathyroid hormone-related protein for the treatment of postmenopausal osteoporosis: defining the maximal tolerable dose

Biologics: Teriparatide and Newer Anabolics

The landscape of osteoporosis management has evolved significantly over the years, witnessing a paradigm shift from conventional therapies to the emergence of biologic agents. This chapter delves into the intricate mechanisms, potential applications, and future directions of biologic interventions in osteoporosis care. Biologic agents, with their targeted approach to bone health, have revolutionized the field by offering precision-driven strategies that address the underlying mechanisms of bone fragility. This chapter explores the mechanisms of action of various biologics, including Receptor Activator of Nuclear Factor Kappa-B Ligand (RANKL) inhibitors, monoclonal antibodies targeting sclerostin, parathyroid hormone (PTH) analogues, and cathepsin K inhibitors. It discusses their potential benefits, limitations, and safety considerations, while shedding light on the promise of combination therapies that merge biologic agents with traditional approaches. Furthermore, the chapter delves into the potential applications of biologic agents in specific patient populations, the role of biomarkers in predicting treatment responses, and the influence of emerging biological targets. It also explores the advancements in novel targets and drug delivery systems that aim to enhance treatment convenience and effectiveness. By tailoring treatments based on patient characteristics and exploring novel therapeutic targets, the chapter envisions a future of precision medicine in osteoporosis care. As research continues to evolve, the chapter anticipates a transformative impact on bone health outcomes, fracture prevention, and overall quality of life for individuals at risk of osteoporosis-related fractures. Through comprehensive insights into the mechanisms, applications, and future directions of biologic agents, this chapter offers a holistic perspective on the evolving landscape of osteoporosis management.

Bone Remodeling and Modeling: Cellular Targets for Antiresorptive and Anabolic Treatments, Including Approaches Through the Parathyroid Hormone (PTH)/PTH-Related Protein Pathway

Bone is continuously in a state of building and renewal, though the process of remodeling that takes place at many sites asynchronously throughout the skeleton, with bone formation and resorption equal at these sites (bone multicellular units). Remodeling takes place on bone surfaces, both on trabeculae and in the cortex, and serves the purposes of replacing old bone or that damaged by microfractures throughout the skeleton. The bone loss and consequent osteoporotic fractures that result from excess resorption over formation have mainly been prevented or treated by antiresorptive drugs that inhibit osteoclast formation and/or activity. Virtually all of the evidence leading to acceptance of antiresorptive drugs as treatment has depended upon their prevention of vertebral fractures. In recent decades, new prospects came of anabolic treatments that partly restore bone volume and microstructure restore bone that has been lost. The first of these was parathyroid hormone (PTH), shown by daily injection to increase markers of bone formation and prevent fractures. This field of interest enlarged with the discovery of PTH-related protein (PTHrP), so closely related in structure and action to PTH. The structural relationship between PTH and PTHrP is important in assessing their physiological and pharmacological roles, with the N-terminal domains of the 2 having virtually equal actions on target cells. Abaloparatide, a peptide analogue based on the structures of PTHrP and PTH, has been approved in some countries as a therapy for osteoporosis. Treatment through the PTH receptor activation pathway, and probably with any anabolic therapy, needs to be followed by antiresorptive treatment in order to maintain bone that has been restored. No matter how effective anabolic therapies for the skeleton become, it seems highly likely that there will be a continuing need for antiresorptive drugs.

Abaloparatide Has the Same Catabolic Effects on Bones of Mice When Infused as PTH (1-34)

Abaloparatide is a peptide analog of parathyroid hormone-related protein (PTHrP 1-34) and was approved in 2017 as the second osteoanabolic peptide for treating osteoporosis. We previously showed that intermittent abaloparatide is equally as effective as PTH (1-34). This study was designed to compare the catabolic effects of PTH (1-34) and abaloparatide on bone in young female wild-type mice. Two-month-old C57Bl/6J female mice were continuously infused with human PTH (1-34) or abaloparatide at 80 μg/kg BW/day or vehicle for 2 weeks. At euthanasia, DEXA-PIXImus was performed to assess bone mineral density (BMD) in the whole body, femurs, tibiae, and vertebrae. Bone turnover marker levels were measured in sera, femurs were harvested for micro-computer tomography (μCT) analyses and histomorphometry, and tibiae were separated into cortical and trabecular fractions for gene expression analyses. Our results demonstrated that the infusion of abaloparatide resulted in a similar decrease in BMD as infused PTH (1-34) at all sites. μCT and histomorphometry analyses showed similar decreases in cortical bone thickness and BMD associated with an increase in bone turnover from the increased bone formation rate found by in vivo double labeling and serum P1NP and increased bone resorption as shown by osteoclast numbers and serum cross-linked C-telopeptide. Trabecular bone did not show major changes with either treatment. Osteoblastic gene expression analyses of trabecular and cortical bone revealed that infusion of PTH (1-34) or abaloparatide led to similar and different actions in genes of osteoblast differentiation and activity. As with intermittent and in vitro treatment, both infused PTH (1-34) and abaloparatide similarly regulated downstream genes of the PTHR1/SIK/HDAC4 pathway such as Sost and Mmp13 but differed for those of the PTHR1/SIK/CRTC pathway. Taken together, at the same dose, infused abaloparatide causes the same high bone turnover as infused PTH (1-34) with a net resorption in female wild-type mice. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

The Role of BMP Signaling in Osteoclast Regulation

The osteogenic effects of Bone Morphogenetic Proteins (BMPs) were delineated in 1965 when Urist et al. showed that BMPs could induce ectopic bone formation. In subsequent decades, the effects of BMPs on bone formation and maintenance were established. BMPs induce proliferation in osteoprogenitor cells and increase mineralization activity in osteoblasts. The role of BMPs in bone homeostasis and repair led to the approval of BMP2 by the Federal Drug Administration (FDA) for anterior lumbar interbody fusion (ALIF) to increase the bone formation in the treated area. However, the use of BMP2 for treatment of degenerative bone diseases such as osteoporosis is still uncertain as patients treated with BMP2 results in the stimulation of not only osteoblast mineralization, but also osteoclast absorption, leading to early bone graft subsidence. The increase in absorption activity is the result of direct stimulation of osteoclasts by BMP2 working synergistically with the RANK signaling pathway. The dual effect of BMPs on bone resorption and mineralization highlights the essential role of BMP-signaling in bone homeostasis, making it a putative therapeutic target for diseases like osteoporosis. Before the BMP pathway can be utilized in the treatment of osteoporosis a better understanding of how BMP-signaling regulates osteoclasts must be established.

Endocrine role of bone in the regulation of energy metabolism

Bone mainly functions as a supportive framework for the whole body and is the major regulator of calcium homeostasis and hematopoietic function. Recently, an increasing number of studies have characterized the significance of bone as an endocrine organ, suggesting that bone-derived factors regulate local bone metabolism and metabolic functions. In addition, these factors can regulate global energy homeostasis by altering insulin sensitivity, feeding behavior, and adipocyte commitment. These findings may provide a new pathological mechanism for related metabolic diseases or be used in the diagnosis, treatment, and prevention of metabolic diseases such as osteoporosis, obesity, and diabetes mellitus. In this review, we summarize the regulatory effect of bone and bone-derived factors on energy metabolism and discuss directions for future research.

Molecular mechanisms and clinical management of cancer bone metastasis

As one of the most common metastatic sites of malignancies, bone has a unique microenvironment that allows metastatic tumor cells to grow and flourish. The fenestrated capillaries in the bone, bone matrix, and bone cells, including osteoblasts and osteoclasts, together maintain the homeostasis of the bone microenvironment. In contrast, tumor-derived factors act on bone components, leading to subsequent bone resorption or excessive bone formation. The various pathways involved also provide multiple targets for therapeutic strategies against bone metastases. In this review, we summarize the current understanding of the mechanism of bone metastases. Based on the general process of bone metastases, we specifically highlight the complex crosstalk between tumor cells and the bone microenvironment and the current management of cancer bone metastases.

Abaloparatide at the Same Dose Has the Same Effects on Bone as PTH (1-34) in Mice

Abaloparatide, a novel analog of parathyroid hormone-related protein (PTHrP 1-34), became in 2017 the second osteoanabolic therapy for the treatment of osteoporosis. This study aims to compare the effects of PTH (1-34), PTHrP (1-36), and abaloparatide on bone remodeling in male mice. Intermittent daily subcutaneous injections of 80 μg/kg/d were administered to 4-month-old C57Bl/6J male mice for 6 weeks. During treatment, mice were followed by DXA-Piximus to assess changes in bone mineral density (BMD) in the whole body, femur, and tibia. At either 4 or 18 hours after the final injection, femurs were harvested for μCT analyses and histomorphometry, sera were assayed for bone turnover marker levels, and tibias were separated into cortical, trabecular, and bone marrow fractions for gene expression analyses. Our results showed that, compared with PTH (1-34), abaloparatide resulted in a similar increase in BMD at all sites, whereas no changes were found with PTHrP (1-36). With both PTH (1-34) and abaloparatide, μCT and histomorphometry analyses revealed similar increases in bone volume associated with an increased trabecular thickness, in bone formation rate as shown by P1NP serum level and in vivo double labeling, and in bone resorption as shown by CTX levels and osteoclast number. Gene expression analyses of trabecular and cortical bone showed that PTH (1-34) and abaloparatide led to different actions in osteoblast differentiation and activity, with increased Runx2, Col1A1, Alpl, Bsp, Ocn, Sost, Rankl/Opg, and c-fos at different time points. Abaloparatide seems to generate a faster response on osteoblastic gene expression than PTH (1-34). Taken together, abaloparatide at the same dose is as effective as PTH (1-34) as an osteoanabolic, with an increase in bone formation but also an increase in bone resorption in male mice. © 2019 American Society for Bone and Mineral Research.

Antiresorptive and anabolic agents in the prevention and reversal of bone fragility

Bone volume, microstructure and its material composition are maintained by bone remodelling, a cellular activity carried out by bone multicellular units (BMUs). BMUs are focally transient teams of osteoclasts and osteoblasts that respectively resorb a volume of old bone and then deposit an equal volume of new bone at the same location. Around the time of menopause, bone remodelling becomes unbalanced and rapid, and an increased number of BMUs deposit less bone than they resorb, resulting in bone loss, a reduction in bone volume and microstructural deterioration. Cortices become porous and thin, and trabeculae become thin, perforated and disconnected, causing bone fragility. Antiresorptive agents reduce fracture risk by reducing the rate of bone remodelling so that fewer BMUs are available to remodel bone. Bone fragility is not abolished by these drugs because existing microstructural deterioration is not reversed, unsuppressed remodelling continues producing microstructural deterioration and unremodelled bone that becomes more mineralized can become brittle. Anabolic agents reduce fracture risk by stimulating new bone formation, which partly restores bone volume and microstructure. To guide fracture prevention, this Review provides an overview of the structural basis of bone fragility, the mechanisms of remodelling and how anabolic and antiresorptive agents target remodelling defects.

PTH and PTHrP Actions on Bone

Parathyroid hormone (PTH), PTH-related peptide (PTHrP), PTHR, and their cognate G protein-coupled receptor play defining roles in the regulation of extracellular calcium and phosphate metabolism and in controlling skeletal growth and repair. Acting through complex signaling mechanisms that in many instances proceed in a tissue-specific manner, precise control of these processes is achieved. A variety of direct and indirect disease processes, along with genetic anomalies, can cause these schemes to become dysfunctional. Here, we review the basic components of this regulatory network and present both the well-established elements and emerging findings and concepts with the overall objective to provide a framework for understanding the elementary aspects of how PTH and PTHrP behave and as a call to encourage further investigation that will yield more comprehensive understanding of the physiological and pathological steps at play, with a goal toward novel therapeutic interventions.

Abaloparatide exhibits greater osteoanabolic response and higher cAMP stimulation and β-arrestin recruitment than teriparatide

Teriparatide and abaloparatide are parathyroid hormone receptor 1 (PTHR1) analogs with unexplained differential efficacy for the treatment of osteoporosis. Therefore, we compared the effects of abaloparatide and teriparatide on bone structure, turnover, and levels of receptor activator of nuclear factor-kappa B ligand (RANKL) and osteoprotegerin (OPG). Wild-type (WT) female mice were injected daily with vehicle or 20-80 µg/kg/day of teriparatide or abaloparatide for 30 days. Femurs and spines were examined by microcomputed tomography scanning and serum levels of bone turnover markers, RANKL, and OPG, were measured by ELISA. Both analogs similarly increased the distal femoral fractional trabecular bone volume, connectivity, and number, and reduced the structure model index (SMI) at 20-80 µg/kg/day doses. However, only abaloparatide exhibited a significant increase (13%) in trabecular thickness at 20 µg/kg/day dose. Femoral cortical evaluation showed that abaloparatide caused a greater dose-dependent increase in cortical thickness than teriparatide. Both teriparatide and abaloparatide increased lumbar 5 vertebral trabecular connectivity but had no or modest effect on other indices. Biochemical analysis demonstrated that abaloparatide promoted greater elevation of procollagen type 1 intact N-terminal propeptide, a bone formation marker, and tartrate-resistant acid phosphatase 5b levels, a bone resorption marker, and lowered the RANKL/OPG ratio. Furthermore, PTHR1 signaling was compared in cells treated with 0-100 nmol/L analog. Interestingly, abaloparatide had a markedly lower EC50 for cAMP formation (2.3-fold) and β-arrestin recruitment (1.6-fold) than teriparatide. Therefore, abaloparatide-improved efficacy can be attributed to enhanced bone formation and cortical structure, reduced RANKL/OPG ratio, and amplified Gs-cAMP and β-arrestin signaling.

Proteolytic Regulation of Parathyroid Hormone-Related Protein: Functional Implications for Skeletal Malignancy

Parathyroid hormone-related protein (PTHrP), with isoforms ranging from 139 to 173 amino acids, has long been implicated in the development and regulation of multiple tissues, including that of the skeleton, via paracrine and autocrine signaling. PTHrP is also known as a potent mediator of cancer-induced bone disease, contributing to a vicious cycle between tumor cells and the bone microenvironment that drives the formation and progression of metastatic lesions. The abundance of roles ascribed to PTHrP have largely been attributed to the N-terminal 1-36 amino acid region, however, activities for mid-region and C-terminal products as well as additional shorter N-terminal species have also been described. Studies of the protein sequence have indicated that PTHrP is susceptible to post-translational proteolytic cleavage by multiple classes of proteases with emerging evidence pointing to novel functional roles for these PTHrP products in regulating cell behavior in homeostatic and pathological contexts. As a consequence, PTHrP products are also being explored as potential biomarkers of disease. Taken together, our enhanced understanding of the post-translational regulation of PTHrP bioactivity could assist in developing new therapeutic approaches that can effectively treat skeletal malignancies.

Novel therapies in osteoporosis: PTH-related peptide analogs and inhibitors of sclerostin

Bone-forming approaches to treat patients with severe osteoporosis are effective, but treatment options are limited, and there is an unmet clinical need for additional drugs. This review discusses two novel and advanced anabolic therapeutic concepts that have successfully completed phase 3 trials. Romosozumab is a monoclonal antibody that targets the Wnt inhibitor sclerostin. Two phase 3 trials (FRAME and ARCH) of romosozumab for the treatment of postmenopausal osteoporosis have been completed. Both trials successfully reached their primary endpoint by reducing vertebral fractures by 75% compared to placebo (FRAME trial) and 48% compared to alendronate (ARCH trial), respectively. Abaloparatide is a PTH-related protein (PTHrP) analog that has displayed bone anabolic activity. In the phase 3 ACTIVE trial, abaloparatide was compared to placebo and teriparatide for 18 months in postmenopausal women who had already experienced an osteoporotic fracture. Abaloparatide successfully reduced the rate of new vertebral fractures by 86% compared to placebo. Furthermore, abaloparatide achieved greater BMD increases at all measured sites compared to both placebo and teriparatide. Based on these results, abaloparatide was FDA approved in April 2017. This review discusses available data of both agents with regard to efficacy and safety as well as their possible future application.

MANAGEMENT OF ENDOCRINE DISEASE: Novel anabolic treatments for osteoporosis

Skeletal anabolic agents enhance bone formation, which is determined by the number and function of osteoblasts. Signals that influence the differentiation and function of cells of the osteoblast lineage play a role in the mechanism of action of anabolic agents in the skeleton. Wnts induce the differentiation of mesenchymal stem cells toward osteoblasts, and insulin-like growth factor I (IGF-I) enhances the function of mature osteoblasts. The activity of Wnt and IGF-I is controlled by proteins that bind to the growth factor or to its receptors. Sclerostin is a Wnt antagonist that binds to Wnt co-receptors and prevents Wnt signal activation. Teriparatide, a 1-34 amino terminal fragment of parathyroid hormone (PTH), and abaloparatide, a modified 1-34 amino terminal fragment of PTH-related peptide (PTHrp), induce IGF-I, increase bone mineral density (BMD), reduce the incidence of vertebral and non-vertebral fractures and are approved for the treatment of postmenopausal osteoporosis. Romosozumab, a humanized anti-sclerostin antibody, increases bone formation, decreases bone resorption, increases BMD and reduces the incidence of vertebral fractures. An increased incidence of cardiovascular events has been associated with romosozumab, which is yet to be approved for the treatment of osteoporosis. In conclusion, cell and molecular studies have formed the foundation for the development of new anabolic therapies for osteoporosis with proven efficacy on the incidence of new fractures.

Vitamin D and calcium supplementation, skeletal muscle strength and serum testosterone in young healthy adult males: Randomized control trial

BACKGROUND

Cholecalciferol and/or calcium supplementation might increase skeletal muscle strength and serum testosterone in young adult males.

OBJECTIVE

We performed a randomized control trial assessing the effect of cholecalciferol/calcium on skeletal muscle strength and serum testosterone in vitamin D deficient young males.

DESIGN

Two-by-two factorial RCT.

SUBJECT AND INTERVENTION

Two-hundred and twenty-eight young males were block-randomized to (i) double-placebo, (ii) calcium/placebo, (iii) cholecalciferol/placebo and (iv) cholecalciferol/calcium. Doses for cholecalciferol were 60 000 IU/wk for 8 weeks followed by 60 000 IU/fortnightly, and doses for elemental calcium were 500 mg/twice daily for 6 months. A total of 180 subjects completed the study protocol. Their  ean age, body mass index and baseline 25(OH)D were 20.2 ± 2.2 years, 23.0 ± 3.6 kg/m² and 21.5 ± 9.5 nmol/L, respectively.

MEASUREMENTS

Handgrip (primary outcome), pinch-grip strength, distance walked in 6 minutes, dyspnoea-score, quality of life by Short Form 36, serum 25(OH)D, 1,25(OH)₂ D, iPTH, total testosterone and free androgen index (FAI).

RESULTS

After intervention, mean serum 25(OH)D was >75.0 nmol/L in cholecalciferol groups. However, the handgrip strength (29.7 ± 4.4, 29.3 ± 4.6, 30.6 ± 5.0 and 28.8 ± 4.3 kg, P = .28) was comparable in the 4 groups. Subgroups analysis among subjects with baseline serum 25OH)D < 25.0 and <12.0 nmol/L showed similar results. The mean serum testosterone decreased significantly at 6 months; however, delta change was similar in 4 groups. Change in handgrip strength and other outcomes was similar in 4 groups with and without adjustment for delta testosterone and FAI.

CONCLUSIONS

Six months of cholecalciferol/calcium supplementation had no significant effect on skeletal muscle strength and serum testosterone in young adult males.

Autocrine and Paracrine Regulation of the Murine Skeleton by Osteocyte-Derived Parathyroid Hormone-Related Protein

Parathyroid hormone-related protein (PTHrP) and parathyroid hormone (PTH) have N-terminal domains that bind a common receptor, PTHR1. N-terminal PTH (teriparatide) and now a modified N-terminal PTHrP (abaloparatide) are US Food and Drug Administration (FDA)-approved therapies for osteoporosis. In physiology, PTHrP does not normally circulate at significant levels, but acts locally, and osteocytes, cells residing within the bone matrix, express both PTHrP and the PTHR1. Because PTHR1 in osteocytes is required for normal bone resorption, we determined how osteocyte-derived PTHrP influences the skeleton. We observed that adult mice with low PTHrP in osteocytes (targeted with the Dmp1(10kb)-Cre) have low trabecular bone volume and osteoblast numbers, but osteoclast numbers were unaffected. In addition, bone size was normal, but cortical bone strength was impaired. Osteocyte-derived PTHrP therefore stimulates bone formation and bone matrix strength, but is not required for normal osteoclastogenesis. PTHrP knockdown and overexpression studies in cultured osteocytes indicate that osteocyte-secreted PTHrP regulates their expression of genes involved in matrix mineralization. We determined that osteocytes secrete full-length PTHrP with no evidence for secretion of lower molecular weight forms containing the N-terminus. We conclude that osteocyte-derived full-length PTHrP acts through both PTHR1 receptor-mediated and receptor-independent actions in a paracrine/autocrine manner to stimulate bone formation and to modify adult cortical bone strength. © 2017 American Society for Bone and Mineral Research.

Teriparatide treatment of femoral fracture nonunion that autogenous bone grafting failed to heal: a case report

PURPOSE

Management of fracture nonunion is challenging as another surgical intervention for the patient is often a necessity, which has a huge impact on both quality of life and economic burden of the patient. Thus, a less aggressive and better accepted treatment for nonunion is required.

METHODS

We gave teriparatide to a 45-year-old man with femoral fracture nonunion 1 year after he underwent surgery with autogenous bone grafting that failed to heal his initial nonunion. Successful union was obtained after once-daily administration of teriparatide for 9 months.

RESULTS

Our case showed teriparatide could successfully treat a femoral fracture nonunion that autogenous bone grafting failed to heal.

CONCLUSIONS

Teriparatide may provide an alternative treatment for fracture nonunion.

G protein-coupled receptors as anabolic drug targets in osteoporosis

Osteoporosis is a progressive bone disorder characterised by imbalance between bone building (anabolism) and resorption (catabolism). Most therapeutics target inhibition of osteoclast-mediated bone resorption, but more recent attention in early drug discovery has focussed on anabolic targets in osteoblasts or their precursors. Two marketed agents that display anabolic properties, strontium ranelate and teriparatide, mediate their actions via the G protein-coupled calcium-sensing and parathyroid hormone-1 receptors, respectively. This review explores their activity, the potential for improved therapeutics targeting these receptors and other putative anabolic GPCR targets, including Smoothened, Wnt/Frizzled, relaxin family peptide, adenosine, cannabinoid, prostaglandin and sphingosine-1-phosphate receptors.

Dual roles of parathyroid hormone related protein in TGF-β1 signaling and fibronectin up-regulation in mesangial cells

Little is known about the cross-talk between parathyroid hormone (PTH) related protein (PTHrP) and TGF-β1 in mesangial cells (MCs). Our results showed that PTHrP treatment (≤3 h) induced internalization of PTH1R (PTH/PTHrP receptor)–TβRII (TGF-β type 2 receptor) complex and suppressed TGF-β1-mediated Smad2/3 activation and fibronectin (FN) up-regulation. However, prolonged PTHrP treatment (12–48 h) failed to induce PTH1R–TβRII association and internalization. Total protein levels of PTH1R and TβRII were unaffected by PTHrP treatment. These results suggest that internalization of PTH1R and TβRII after short PTHrP treatment might not lead to their proteolytic destruction, allowing the receptors to be recycled back to the plasma membrane during prolonged PTHrP exposure. Receptor re-expression at the cell surface allows PTHrP to switch from its initial inhibitory effect to promote induction of FN. Our study thus demonstrates the dual roles of PTHrP on TGF-β1 signaling and FN up-regulation for the first time in glomerular MCs. These data also provided new insights to guide development of therapy for diabetic kidney disease (DKD).

Development of Potent, Protease-Resistant Agonists of the Parathyroid Hormone Receptor with Broad β Residue Distribution

The parathyroid hormone receptor 1 (PTHR1) is a member of the B-family of GPCRs; these receptors are activated by long polypeptide hormones and constitute targets of drug development efforts. Parathyroid hormone (PTH, 84 residues) and PTH-related protein (PTHrP, 141 residues) are natural agonists of PTHR1, and an N-terminal fragment of PTH, PTH(1-34), is used clinically to treat osteoporosis. Conventional peptides in the 20-40-mer length range are rapidly degraded by proteases, which may limit their biomedical utility. We have used the PTHR1-ligand system to explore the impact of broadly distributed replacement of α-amino acid residues with β-amino acid residues on susceptibility to proteolysis and agonist activity. This effort led us to identify new PTHR1 agonists that contain α → β replacements throughout their sequences, manifest potent agonist activity in cellular assays, and display remarkable resistance to proteolysis, in cases remaining active after extended exposure to simulated gastric fluid. The strategy we have employed suggests a path toward identifying protease-resistant agonists of other B-family GPCRs.

Parathyroid Hormone-Related Peptide: A Novel Endocrine Cardioprotective "Conditioning Mimetic"

An as-yet limited body of evidence suggests that calcium-regulating endocrine hormones-in particular, parathyroid hormone-related peptide (PTHrP)-may have unappreciated cardioprotective effects. The current review focuses on the concept that PTHrP may, via modulation of classic cardioprotective signaling pathways, provide a novel strategy to attenuate myocardial ischemia-reperfusion injury.

Exogenous PTHrP Repairs the Damaged Fracture Healing of PTHrP+/- Mice and Accelerates Fracture Healing of Wild Mice

Bone fracture healing is a complicated physiological regenerative process initiated in response to injury and is similar to bone development. To demonstrate whether an exogenous supply of parathyroid hormone–related protein (PTHrP) helps in bone fracture healing, closed mid-diaphyseal femur fractures were created and stabilized with intramedullary pins in eight-week-old wild-type (WT) PTHrP+/+ and PTHrP+/− mice. After administering PTHrP for two weeks, callus tissue properties were analyzed at one, two, and four weeks post-fracture (PF) by various methods. Bone formation–related genes and protein expression levels were evaluated by real-time reverse transcriptase–polymerase chain reaction and Western blots. At two weeks PF, mineral density of callus, bony callus areas, mRNA levels of alkaline phosphatase (ALP), type I collagen, Runt-related transcription factor 2 (Runx-2), and protein levels of Runx-2 and insulin-like growth factor-1 decreased in PTHrP+/− mice compared with WT mice. At four weeks PF, total collagen-positive bony callus areas, osteoblast number, ALP-positive areas, and type I collagen-positive areas all decreased in PTHrP+/− mice. At both two and four weeks PF, tartrate-resistant acid phosphatase–positive osteoclast number and surface decreased a little in PTHrP+/− mice. The study indicates that exogenous PTHrP provided by subcutaneous injection could redress impaired bone fracture healing, leading to mutation of activated PTHrP by influencing callus areas, endochondral bone formation, osteoblastic bone formation, and bone turnover.

Six Weeks of Daily Abaloparatide Treatment Increased Vertebral and Femoral Bone Mineral Density, Microarchitecture and Strength in Ovariectomized Osteopenic Rats

Abaloparatide is a novel, potent and selective activator of parathyroid hormone receptor 1 (PTHR1) under clinical development for the treatment of osteoporosis. We assessed the effect of 6 weeks of abaloparatide on bone mass, microarchitecture, quality and strength in ovariectomized (OVX) rats. After 8 weeks of post-surgical bone depletion (baseline), OVX rats (n = 20-21/group) received daily subcutaneous vehicle (OVX-Veh) or abaloparatide at 5 or 20 µg/kg. Sham-operated control rats (n = 24) received vehicle. Areal bone mineral density (aBMD) of the lumbar spine (L4), total femur and femur diaphysis was measured at baseline and after 6 weeks of treatment. Femur and vertebral bone architecture and mechanical properties were assessed at the end of the treatment phase. At baseline, OVX-Veh rats exhibited significantly lower aBMD relative to Sham controls. Treatment of OVX rats with abaloparatide at 5 or 20 µg/kg/day increased aBMD dose-dependently in the lumbar spine, total femur and femur diaphysis to levels exceeding OVX-Veh or Sham controls. The abaloparatide 5 and 20 µg/kg groups had improved trabecular microarchitecture relative to OVX vehicle, with trabecular BV/TV exceeding OVX-Veh control values by 57 and 78 % (respectively) at the lumbar spine, and by 145 and 270 % at the distal femur. Femur diaphyseal cortical volume and thickness were significantly greater in the abaloparatide 20 µg/kg group relative to OVX vehicle or Sham controls. Bone strength parameters of the femur diaphysis, femur neck and L4 vertebra were significantly improved in the OVX-ABL groups relative to OVX-Veh controls. Bone mass-strength relationships and estimated intrinsic strength properties suggested maintained or improved bone quality with abaloparatide. These data demonstrate skeletal restoration via abaloparatide treatment of osteopenic OVX rats, in association with improved trabecular microarchitecture, cortical geometry and bone strength at sites that have clinical relevance in patients with osteoporosis.

Hedgehog signaling in bone regulates whole-body energy metabolism through a bone-adipose endocrine relay mediated by PTHrP and adiponectin

Bone plays a role in energy metabolism, but the interplay between bone and other organs in this process is not completely understood. Here, we show that upregulated Hh signaling in bones results in increased whole-body energy expenditure, white adipose tissue (WAT) browning, hypoglycemia and skeletal muscle atrophy. We found that Hh signaling induces PTHrP secretion from bones and causes WAT browning. Injection of PTHrP-neutralizing antibody attenuates WAT browning and improves the circulating blood glucose level while high-fat diet treatment only rescues hypoglycemia. Furthermore, bone-derived PTHrP stimulates adiponectin secretion in WAT and results in systemic increase of fatty acid oxidation and glucose uptake. Mechanistically, PTHrP activates both PKA/cAMP and Akt/Foxo pathways for Ucp1 expression in WAT. PTHrP couples adiponectin actions to activate the AMPK pathway in the skeletal muscles and liver, respectively, for fatty acid oxidation. Our findings establish a new bone-adipose hormonal relay that regulates whole-body energy metabolism.

The Role of Parathyroid Hormone-Related Protein (PTHrP) in Osteoblast Response to Microgravity: Mechanistic Implications for Osteoporosis Development

Prolonged skeletal unloading through bedrest results in bone loss similar to that observed in elderly osteoporotic patients, but with an accelerated timeframe. This rapid effect on weight-bearing bones is also observed in astronauts who can lose up to 2% of their bone mass per month spent in Space. Despite the important implications for Spaceflight travelers and bedridden patients, the exact mechanisms involved in disuse osteoporosis have not been elucidated. Parathyroid hormone-related protein (PTHrP) regulates many physiological processes including skeletal development, and has been proposed as a mechanosensor. To investigate the role of PTHrP in microgravity-induced bone loss, trabecular and calvarial osteoblasts (TOs and COs) from Pthrp^+/+ and ^-/- mice were subjected to actual Spaceflight for 6 days (Foton M3 satellite). Pthrp^+/+, ^+/- and ^-/- osteoblasts were also exposed to simulated microgravity for periods varying from 6 days to 6 weeks. While COs displayed little change in viability in 0g, viability of all TOs rapidly decreased in inverse proportion to PTHrP expression levels. Furthermore, Pthrp^+/+ TOs displayed a sharp viability decline after 2 weeks at 0g. Microarray analysis of Pthrp^+/+ TOs after 6 days in simulated 0g revealed expression changes in genes encoding prolactins, apoptosis/survival molecules, bone metabolism and extra-cellular matrix composition proteins, chemokines, insulin-like growth factor family members and Wnt-related signalling molecules. 88% of 0g-induced expression changes in Pthrp^+/+ cells overlapped those caused by Pthrp ablation in normal gravity, and pulsatile treatment with PTHrP_1-36 not only reversed a large proportion of 0g-induced effects in Pthrp^+/+ TOs but maintained viability over 6-week exposure to microgravity. Our results confirm PTHrP efficacy as an anabolic agent to prevent microgravity-induced cell death in TOs.

Parathyroid Hormone-Related Protein, Its Regulation of Cartilage and Bone Development, and Role in Treating Bone Diseases

Although parathyroid hormone-related protein (PTHrP) was discovered as a cancer-derived hormone, it has been revealed as an important paracrine/autocrine regulator in many tissues, where its effects are context dependent. Thus its location and action in the vasculature explained decades-long observations that injection of PTH into animals rapidly lowered blood pressure by producing vasodilatation. Its roles have been specified in development and maturity in cartilage and bone as a crucial regulator of endochondral bone formation and bone remodeling, respectively. Although it shares actions with parathyroid hormone (PTH) through the use of their common receptor, PTHR1, PTHrP has other actions mediated by regions within the molecule beyond the amino-terminal sequence that resembles PTH, including the ability to promote placental transfer of calcium from mother to fetus. A striking feature of the physiology of PTHrP is that it possesses structural features that equip it to be transported in and out of the nucleus, and makes use of a specific nuclear import mechanism to do so. Evidence from mouse genetic experiments shows that PTHrP generated locally in bone is essential for normal bone remodeling. Whereas the main physiological function of PTH is the hormonal regulation of calcium metabolism, locally generated PTHrP is the important physiological mediator of bone remodeling postnatally. Thus the use of intermittent injection of PTH as an anabolic therapy for bone appears to be a pharmacological application of the physiological function of PTHrP. There is much current interest in the possibility of developing PTHrP analogs that might enhance the therapeutic anabolic effects.

Parathyroid Hormone-Related Protein Analogs as Osteoporosis Therapies

The only bone anabolic agent currently available for osteoporosis treatment is parathyroid hormone (PTH)-either its N-terminal 1-34 fragment or the whole molecule of 1-84 aminoacids-whose intermittent administration stimulates new bone formation by targeting osteoblastogenesis and osteoblast survival. PTH-related protein (PTHrP) is an abundant factor in bone which shows N-terminal homology with PTH and thus exhibits high affinity for the same PTH type 1 receptor in osteoblasts. Therefore, it is not surprising that intermittently administered N-terminal PTHrP peptides induce bone anabolism in animals and humans. Furthermore, the C-terminal region of PTHrP also elicits osteogenic features in vitro in osteoblastic cells and in various animal models of osteoporosis. In this review, we discuss the current concepts about the cellular and molecular mechanisms whereby PTHrP may induce anabolic actions in bone. Pre-clinical studies and clinical data using N-terminal PTHrP analogs are also summarized, pointing to PTHrP as a promising alternative to current bone anabolic therapies.

Exogenous Parathyroid Hormone-Related Peptide Promotes Fracture Healing in Lepr(-/-) Mice

Diabetic osteoporosis continues to surge worldwide, increasing the risk of fracture. We have previously demonstrated that haploinsufficiency of endogenous parathyroid hormone-related peptide (PTHrP) impairs fracture healing. However, whether an exogenous supply of PTHrP can repair bone damage and accelerate fracture healing remains unclear. This study aimed to assess the efficacy and safety of PTHrP in healing fractures. Standardized mid-diaphyseal femur fractures were generated in 12-week-old wild-type and leptin receptor null Lepr(-/-) mice. After administration of PTHrP for 2 weeks, callus tissue properties were analyzed by radiography, micro-computed tomography, histology, histochemistry, immunohistochemistry, and molecular biology techniques. At 2 weeks post-fracture, cartilaginous callus areas were reduced, while total callus and bony callus areas were increased in PTHrP-treated Lepr(-/-) animals and control wild-type mice, compared with vehicle-treated Lepr(-/-) mice. The following parameters were enhanced both in Lepr(-/-) mice after treatment with PTHrP and vehicle-treated wild-type animals, compared with vehicle-treated Lepr(-/-) mice: osteoblast numbers; tissue alkaline phosphatase (ALP) and Type I collagen immunopositive areas; mRNA levels of ALP, Type I collagen, osteoprotegerin, and receptor activator for nuclear factor-κ B ligand; protein levels of Runt-related transcription factor 2 and insulin-like growth factor-1; and the number and surface of osteoclasts. In conclusion, exogenous PTHrP by subcutaneous injection promotes fracture repair in Lepr(-/-) mice by increasing callus formation and accelerating cell transformation: upregulated osteoblastic gene and protein expression, increased endochondral bone formation, osteoblastic bone formation, and osteoclastic bone resorption. However, complete repair was not obtained in PTHrP-treated Lepr(-/-) mice as in control wild-type animals.

PTH receptor-1 signalling-mechanistic insights and therapeutic prospects

Parathyroid hormone/parathyroid hormone-related protein receptor (PTH/PTHrP type 1 receptor; commonly known as PTHR1) is a family B G-protein-coupled receptor (GPCR) that regulates skeletal development, bone turnover and mineral ion homeostasis. PTHR1 transduces stimuli from PTH and PTHrP into the interior of target cells to promote diverse biochemical responses. Evaluation of the signalling properties of structurally modified PTHR1 ligands has helped to elucidate determinants of receptor function and mechanisms of downstream cellular and physiological responses. Analysis of PTHR1 responses induced by structurally modified ligands suggests that PTHR1 can continue to signal through a G-protein-mediated pathway within endosomes. Such findings challenge the longstanding paradigm in GPCR biology that the receptor is transiently activated at the cell membrane, followed by rapid deactivation and receptor internalization. Evaluation of structurally modified PTHR1 ligands has further led to the identification of ligand analogues that differ from PTH or PTHrP in the type, strength and duration of responses induced at the receptor, cellular and organism levels. These modified ligands, and the biochemical principles revealed through their use, might facilitate an improved understanding of PTHR1 function in vivo and enable the treatment of disorders resulting from defects in PTHR1 signalling. This Review discusses current understanding of PTHR1 modes of action and how these findings might be applied in future therapeutic agents.

Effects of aspirin on fracture healing in OPF rats

OBJECTIVES

To study the effect of aspirin on healing process of osteoporotic fracture (OPF) in rats.

METHODS

A total of 50 female Wistar rats aged 3 months were randomly divided into observation group and control group, castration method was adopted to establish the osteoporosis (OP) model. After artificial preparing fractures on the midpoint of left femur, fixing gram needle intramedullary, OPF modeling was complete. Aspirin lavage of 33 mg once a day was adopted in observation group after modeling, same amount of normal saline was used in the control as placebo. From each group, selected 5 rats at the 2nd, 4th, 8th and 12th week after modeling to measure the bone mineral density (BMD) and histological examination of the fracture callus, radiology observation was conducted at the 8th and 12th week. Left femur biomechanical measurement was taken at the 12th week.

RESULTS

BMD values of observation group at each time point were significantly higher than that of the control group after modeling (P<0.05); Histological observation showed that at the 8th week, the endochondral ossification process of observation group was faster than that of observation group, with fuzzy fracture line in observation group and clear fracture line in observation group; at the 12th week, fracture line disappeared in observation group, fracture line of the control group was fuzzy at the same time; three-point bending load of the left femur in observation group rats was significantly higher than that of control group after 12 weeks (P<0.05).

CONCLUSIONS

Aspirin can accelerate the healing of new callus in OPF rats, increase bone density and biomechanics strength, and promote fracture healing of osteoporotic rats.

Novel targets for the prevention of osteoporosis - lessons learned from studies of metabolic bone disorders

INTRODUCTION

Osteoporosis is a major health care problem, and whereas efficacious treatments for vertebral fracture reduction are available for osteoporosis patients, these therapies are still limited with respect to capacity for restoration of bone loss, as well as efficacy on non-vertebral fractures, such as hip fractures, which are the source of morbidity and mortality.

AREAS COVERED

Studies of rare bone diseases in humans, such as osteopetrosis, sclerosteosis, pycnodysostosis and more, have shed light on a series of drug targets in bone that have the potential to result in therapies for osteoporosis with novel mechanisms of action, and the potential to improve the standard of care substantially. We focus on how they are separated from classic treatments for osteoporosis, in terms of novel modes of action, additional beneficial effects on bone turnover and importantly also safety. We focus on the status of anti-sclerostin antibodies, novel parathyroid hormone-related protein analogs, inhibitors of cathepsin K and ClC-7 in osteoclasts, all of which are currently in development.

EXPERT OPINION

There is a good possibility that the treatment of osteoporosis will be greatly improved within the coming years; however, with numerous effective and safe drugs already available careful attention to the safety of these novel candidates is crucial.

Investigational parathyroid hormone receptor analogs for the treatment of osteoporosis

INTRODUCTION

Intermittent parathyroid hormone (PTH) administration, acting through multiple signaling pathways, exerts an osteoanabolic effect on the skeleton that surpasses the effect of other antiosteoporotic agents. However, its efficacy is limited by the coupling effect and relatively common adverse events. Thus, the development of more sophisticated PTH receptor analogs seems imperative.

AREAS COVERED

In this review, the authors summarize the role of PTH signaling pathway in bone remodeling. The authors also summarize investigational analogs targeting this pathway, which may be potential treatments for osteoporosis.

EXPERT OPINION

β-arrestins are multifunctional cytoplasmic molecules that are decisive for regulating intracellular PTH signaling. Recently, in preclinical studies, arrestin analogs have achieved the anabolic bone effect of PTH without an accompanying increase in bone resorption. However, it is not yet known whether these analogs have adverse effects and there are no clinical data for their efficacy to date. On the other hand, several molecules derived either from PTH and PTH-related protein (PTHrP) molecules have been developed. Alternative routes of PTH 1 - 34 delivery (oral, transdermal), the PTH analog ostabolin and the N-terminal PTHrP analogs PTHrP 1 - 36 and abaloparatide, have recently been or are currently being tested in clinical trials and are more likely to become available for use in the near future.

Bone anabolics in osteoporosis: Actuality and perspectives

Vertebral and nonvertebral fractures prevention is the main goal for osteoporosis therapy by inhibiting bone resorption and/or stimulating bone formation. Antiresorptive drugs decrease the activation frequency, thereby determining a secondary decrease in bone formation rate and a low bone turnover. Bisphosphonates are today's mainstay among antiresorptive treatment of osteoporosis. Also, oral selective estrogen receptor modulators and recently denosumab have a negative effect on bone turnover. Agents active on bone formation are considered a better perspective in the treatment of severe osteoporosis. Recombinant-human parathyroid hormone (PTH) has showed to increase bone formation and significantly decrease vertebral fractures in severe patients, but with a modest effect on nonvertebral fractures. The study of Wnt signaling pathway, that induces prevalently an osteoblastic activity, opens large possibilities to antagonists of Wnt-inhibitors, such as sclerostin antibodies and dickkopf-1 antagonists, with potential effects not only on trabecular bone but also on cortical bone.

Emerging drugs for osteoporosis

INTRODUCTION

Osteoporotic fracture is a cause of pain, loss of autonomy and excess mortality. Current drugs however, do not allow for a satisfactory non vertebral fracture risk reduction and the compliance is suboptimal.

AREAS COVERED

Current treatments consist of mainly bisphosphonates, denosumabs, selective estrogen receptor modulators and teriparatides. All drugs currently in development will target some aspect of bone remodeling by using the recent advances in our knowledge of bone biology: cathepsin-K inhibitors (odanacatib) are antiresorptive, antisclerostin monoclonal antibodies (romosozumab and blosozumab) are anabolic agents and PTHrp 1-34 (abaloparatide) is an anabolic agent.

EXPERT OPINION

New drugs with better tolerance and ideally with intermittent administration may improve their compliance. New drugs will have to provide higher efficiency levels with regards to reducing the risk of fractures. They may be second-line options, targeted at patients who are poor responders, or those who display contraindications to the older drugs, as a result of cost issues. In addition, some of these new drugs with potent anabolic effect may be confined to niches, for those patients at high risk of refracture after an initial severe fracture such as a hip fracture or a clinical vertebral fracture.

Bone biology and anabolic therapies for bone: current status and future prospects

Bone is continuously remodelled at many sites asynchronously throughout the skeleton, with bone formation and resorption balanced at these sites to retain bone structure. Negative balance resulting in bone loss and osteoporosis, with consequent fractures, has mainly been prevented or treated by anti-resorptive drugs that inhibit osteoclast formation and/or activity, with new prospects now of anabolic treatments that restore bone that has been lost. The anabolic effectiveness of parathyroid hormone has been established, and an exciting new prospect is presented of neutralising antibody against the osteocyte protein, sclerostin. The cellular actions of these two anabolic treatments differ, and the mechanisms will need to be kept in mind in devising their best use. On present evidence it seems likely that treatment with either of these anabolic agents will need to be followed by anti-resorptive treatment in order to maintain bone that has been restored. No matter how effective anabolic therapies for the skeleton become, it seems highly likely that there will be a continuing need for safe, effective anti-resorptive drugs.

Parathyroid hormone and parathyroid hormone-related protein analogs as therapies for osteoporosis

Osteoporotic fractures result in significant morbidity and mortality. Anabolic agents reverse the negative skeletal balance that characterizes osteoporosis by stimulating osteoblast-dependent bone formation to a greater degree than osteoclast-dependent bone resorption. Parathyroid hormone (PTH) and parathyroid hormone- related protein (PTHrP) are peptide hormones, which have anabolic actions when administered intermittently. The only FDA-approved anabolic bone agent for the treatment of osteoporosis in the United States is PTH 1-34, or teriparatide, administered by daily subcutaneous injections. However, PTH 1-84 is also available in Europe. Synthetic human PTHrP 1-36 and a PTHrP 1-34 analog, BA058, have also been shown to increase lumbar spine bone density. These agents and several other PTH and PTHrP analogs, including some which are not administered as injections, continue to be investigated as potential anabolic therapies for osteoporosis.

A comparison of parathyroid hormone-related protein (1-36) and parathyroid hormone (1-34) on markers of bone turnover and bone density in postmenopausal women: the PrOP study

Parathyroid hormone-related protein (PTHrP)(1-36) increases lumbar spine (LS) bone mineral density (BMD), acting as an anabolic agent when injected intermittently, but it has not been directly compared with parathyroid hormone (PTH)(1-34). We performed a 3-month randomized, prospective study in 105 postmenopausal women with low bone density or osteoporosis, comparing daily subcutaneous injections of PTHrP(1-36) to PTH(1-34). Thirty-five women were randomized to each of three groups: PTHrP(1-36) 400 µg/day; PTHrP(1-36) 600 µg/day; and PTH(1-34) 20 µg/day. The primary outcome measures were changes in amino-terminal telopeptides of procollagen 1 (PINP) and carboxy-terminal telopeptides of collagen 1 (CTX). Secondary measures included safety parameters, 1,25(OH)2 vitamin D, and BMD. The increase in bone resorption (CTX) by PTH(1-34) (92%) (p < 0.005) was greater than for PTHrP(1-36) (30%) (p < 0.05). PTH(1-34) also increased bone formation (PINP) (171%) (p < 0.0005) more than either dose of PTHrP(1-36) (46% and 87%). The increase in PINP was earlier (day 15) and greater than the increase in CTX for all three groups. LS BMD increased equivalently in each group (p < 0.05 for all). Total hip (TH) and femoral neck (FN) BMD increased equivalently in each group but were only significant for the two doses of PTHrP(1-36) (p < 0.05) at the TH and for PTHrP(1-36) 400 (p < 0.05) at the FN. PTHrP(1-36) 400 induced mild, transient (day 15) hypercalcemia. PTHrP(1-36) 600 required a dose reduction for hypercalcemia in three subjects. PTH(1-34) was not associated with hypercalcemia. Each peptide induced a marked biphasic increase in 1,25(OH)2 D. Adverse events (AE) were similar among the three groups. This study demonstrates that PTHrP(1-36) and PTH(1-34) cause similar increases in LS BMD. PTHrP(1-36) also increased hip BMD. PTH(1-34) induced greater changes in bone turnover than PTHrP(1-36). PTHrP(1-36) was associated with mild transient hypercalcemia. Longer-term studies using lower doses of PTHrP(1-36) are needed to define both the optimal dose and full clinical benefits of PTHrP. © 2013 American Society for Bone and Mineral Research.

Activated effects of parathyroid hormone-related protein on human hepatic stellate cells

Background & Aims

After years of experiments and clinical studies, parathyroid hormone-related protein(PTHrP) has been shown to be a bone formation promoter that elicits rapid effects with limited adverse reaction. Recently, PTHrP was reported to promote fibrosis in rat kidney in conjunction with transforming growth factor-beta1 (TGF-β1), which is also a fibrosis promoter in liver. However, the effect of PTHrP in liver has not been determined. In this study, the promoting actions of PTHrP were first investigated in human normal hepatic stellate cells (HSC) and LX-2 cell lines.

Methods

TGF-β1, alpha-smooth muscle actin (α-SMA), matrix metalloproteinase 2 (MMP-2), and collagen I mRNA were quantified by real-time polymerase chain reaction (PCR) after HSCs or LX-2 cells were treated with PTHrP(1–36) or TGF-β1. Protein levels were also assessed by western-blot analysis. Alpha-SMA were also detected by immunofluorescence, and TGF-β1 secretion was measured with enzyme-linked immunosorbent assay (ELISA) of HSC cell culture media.

Results

In cultured human HSCs, mRNA and protein levels of α-SMA, collagen I, MMP-2, and TGF-β1 were increased by PTHrP treatment. A similar increasing pattern was also observed in LX-2 cells. Moreover, PTHrP significantly increased TGF-β1 secretion in cultured media from HSCs.

Conclusions

PTHrP activated HSCs and promoted the fibrosis process in LX-2 cells. These procedures were probably mediated via TGF-β1, highlighting the potential effects of PTHrP in the liver.

Effects of different dosages of parathyroid hormone-related protein 1-34 on the bone metabolism of the ovariectomized rat model of osteoporosis

Intermittent and low-dose parathyroid hormone (PTH) injection to stimulate bone formation has been used in the treatment of osteoporosis. The N-terminal fragment 1-34 of PTH is quite similar in structure and function to N-terminal PTH-related protein (PTHrP). PTH(1-34) and PTHrP also share a coreceptor, the PTH/PTHrP receptor. Therefore, some studies have suggested that PTHrP could effectively stimulate bone formation, similar to PTH. We used an ovariectomized (OVX) rat model of osteoporosis to study the effects of PTHrP(1-34) on bone metabolism by measuring bone mineral density (BMD), bone histomorphometrics, and biomechanical parameters. We found that subcutaneous injection of PTHrP(1-34) (40 or 80 μg/kg body weight every day) in OVX rats increased lumbar and femoral BMD, improved bone biomechanical properties, enhanced bone strength, and promoted bone formation. We selected 40 μg/kg as the preferred therapeutic dose of PTHrP(1-34) and investigated the effects of frequency of treatment (per 1, 2, 3, or 7 days) on bone metabolism in OVX rats. We found that injection of PTHrP(1-34) once per day or every other day significantly improved the BMD and strength of OVX rats. Serum calcium and phosphate levels in all treated rats did not vary significantly from control rats. Based on our results, intermittent low-dose PTHrP(1-34) injection promoted bone formation in OVX rats, suggesting a high potential for therapeutic use in osteoporosis patients.

Skeletal muscle strength in young Asian Indian females after vitamin D and calcium supplementation: a double-blind randomized controlled clinical trial

CONTEXT

Randomized control trials (RCT) of the effect of vitamin D/calcium supplementation on skeletal muscle strength have not shown promising effect in the elderly.

OBJECTIVE

Our objective was to assess the effect of vitamin D and/or calcium on muscle strength in young adults with vitamin D deficiency.

DESIGN AND SETTING

We conducted a RCT using a factorial design at a tertiary-care center from September 2010 to April 2011.

SUBJECTS

A total of 173 healthy females with mean age, body mass index, and 25-hydroxyvitamin D [25(OH)D] of 21.7 ± 4.4 yr, 20.8 ± 2.96 kg/m(2), and 9.3 ± 3.37 ng/ml, respectively, were block randomized to 1) double placebo, 2) calcium/placebo, 3) cholecalciferol/placebo, and 4) cholecalciferol/calcium for 6 months. Cholecalciferol was given at 60,000 IU/wk for 8 wk followed by 60,000 IU/fortnight. Elemental calcium was given in doses of 500 mg twice per day for 6 months.

METHODS

Assessment included hand grip (primary outcome) and pinch grip strength, distance walked in 6 min, dyspnea score, quality of life by Short Form (36) Health Survey (SP-36), serum 25(OH)D, 1,25-dihydroxyvitamin D, and intact PTH.

RESULTS

The serum 25(OH)D increased significantly to 29.9 ± 8.35 and 27.0 ± 9.54 ng/ml in two groups on cholecalciferol. The mean hand grip strength (19.4 ± 3.92, 21.1 ± 3.31, 20.6 ± 3.92, and 20.1 ± 4.00 kg) and its increase from baseline (0.3 ± 2.25, 0.3 ± 2.64, -0.3 ± 2.41, and 0.6 ± 2.30 kg) were comparable in four groups at 6 months. Quality of life, urinary calcium/creatinine ratio, and adverse effects were also comparable in groups.

CONCLUSION

Oral cholecalciferol/calcium supplementation in the dose/schedule used is effective and safe in increasing and maintaining serum 25(OH)D. However, this does not lead to improved skeletal muscle strength in young females.

The osteoclast, bone remodelling and treatment of metabolic bone disease

BACKGROUND

Bone remodelling maintains skeletal integrity by osteoclasts removing foci of damaged bone and osteoblasts replacing them with new bone. Diseases associated with increased bone resorption have increased remodelling often with inadequate bone formation and increased risk of fracture. New therapies are needed for these diseases to reduce resorption and increase formation.

DESIGN

The molecular mechanisms regulating osteoclast and osteoblast functions have become better understood in the past 20 years and have led to questioning of the long-held notion that osteoblastic cells have the dominant regulatory role over osteoclastic cells in bone remodelling. Here, we review current knowledge of how osteoclast formation and functions are regulated and describe how enhanced understanding of these has led to development of new drugs for the management of common bone diseases characterized by increased bone resorption.

RESULTS

Osteoclast formation and functions are regulated by cytokines, especially receptor activator of NF-κB ligand (RANKL) and macrophage-colony-stimulating factor (M-CSF). The differentiation, activity and lifecycle of osteoclasts are regulated in part by other cells that reside within the bone. These include osteoblasts, osteocytes and immune cells, which express these cytokines in response to most factors that promote bone resorption. RANKL and M-CSF activate numerous signalling pathways, which are potential targets for therapeutic intervention. Importantly, osteoclastic cells also function as positive and negative regulators of osteoblastic bone formation.

CONCLUSIONS

There are multiple targets within osteoclasts for pharmacologic intervention to prevent bone loss in osteoporosis and other resorptive bone diseases. However, novel therapies could also affect osteoblastic cell functions.

Recommendations on the management of fragility fracture risk in women younger than 70 years

The risk for fragility fracture represents a problem of enormous magnitude. It is estimated that only a small fraction of women with this risk take the benefit of preventive measures. The relationship between estrogen and bone mass is well known as they are the other factors related to the risk for fracture. There are precise diagnostic methods, including a tool to diagnose the risk for fracture. Yet there continues to be an under-diagnosis, with the unrecoverable delay in instituting preventive measures. Women under the age of 70 years, being much more numerous than those older, and having risk factors, are a group in which it is essential to avoid that first fragility fracture. Today it is usual not to differentiate between the treatment and the prevention of osteoporosis since the common aim is to prevent fragility fractures. Included in this are women with osteoporosis or with low bone mass and increased risk for fracture, for whom risk factors play a primary role. There is clearly controversy over the type of treatment and its duration, especially given the possible adverse effects of long-term use. This justifies the concept of sequential treatment, even more so in women under the age of 70, since they presumably will need treatment for many years. Bone metabolism is age-dependent. In postmenopausal women under 70 years of age, the increase in bone resorption is clearly predominant, related to a sharp drop in estrogens. Thus a logical treatment is the prevention of fragility fractures by hormone replacement therapy (HRT) and, in asymptomatic women, selective estradiol receptor modulators (SERMs). Afterwards, there is a period of greater resorption, albeit less intense but continuous, when one could utilise anti-resorptive treatments such as bisphosphonates or denosumab or a dual agent like strontium ranelate. Bone formation treatment, such as parathyroid hormone (PTH), in women under 70 years will be uncommon. That is because it should be used in cases where the formation is greatly diminished and there is a high risk for fracture, something found in much older women.

Parathyroid hormone-related protein: an update

PTHrP was identified as a cause of hypercalcemia in cancer patients 25 yr ago. In the intervening years, we have learned that PTHrP and PTH are encoded by related genes that are part of a larger "PTH gene family." This evolutionary relationship permits them to bind to the same type 1 PTH/PTHrP receptor, which explains why humoral hypercalcemia of malignancy resembles hyperparathyroidism. This review will outline basic facts about PTHrP biology and its normal physiological functions, with an emphasis on new findings of the past 5-10 yr. The medical and research communities first became aware of PTHrP because of its involvement in a common paraneoplastic syndrome. Now, research into the basic biology of PTHrP has suggested previously unrecognized connections to a variety of disease states such as osteoporosis, osteoarthritis, and breast cancer and has highlighted how PTHrP itself might be used in therapy for osteoporosis and diabetes. Therefore, the story of this remarkable protein is a paradigm for translational research, having gone from bedside to bench and now back to bedside.

Emerging therapies for osteoporosis

This article reviews the conceptual framework for agents that are antiresorptive or anabolic, including pathways that affect bone formation and resorption, and the steps in those pathways that are targets for new therapeutic agents. This article discusses novel antiresorptive and anabolic agents in development. Recent developments that link bone remodeling with serotonin in the gastrointestinal system and the central nervous system via the sympathetic nervous system may change the paradigm for skeletal remodeling. Novel anabolic agents in development include antibodies that target molecules involved in Wnt signaling.