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

Unraveling the molecular and immunological landscape: Exploring signaling pathways in osteoporosis

Osteoporosis, characterized by compromised bone density and microarchitecture, represents a significant global health challenge, particularly in aging populations. This comprehensive review delves into the intricate signaling pathways implicated in the pathogenesis of osteoporosis, providing valuable insights into the pivotal role of signal transduction in maintaining bone homeostasis. The exploration encompasses cellular signaling pathways such as Wnt, Notch, JAK/STAT, NF-κB, and TGF-β, all of which play crucial roles in bone remodeling. The dysregulation of these pathways is a contributing factor to osteoporosis, necessitating a profound understanding of their complexities to unveil the molecular mechanisms underlying bone loss. The review highlights the pathological significance of disrupted signaling in osteoporosis, emphasizing how these deviations impact the functionality of osteoblasts and osteoclasts, ultimately resulting in heightened bone resorption and compromised bone formation. A nuanced analysis of the intricate crosstalk between these pathways is provided to underscore their relevance in the pathophysiology of osteoporosis. Furthermore, the study addresses some of the most crucial long non-coding RNAs (lncRNAs) associated with osteoporosis, adding an additional layer of academic depth to the exploration of immune system involvement in various types of osteoporosis. Finally, we propose that SKP1 can serve as a potential biomarker in osteoporosis.

Association between serum uric acid and bone mineral density in males from NHANES 2011-2020

Currently, the relationship between serum uric acid (SUA) and bone mineral density (BMD) in men remains controversial. This study aims to investigate the relationship between SUA and lumbar spine BMD in American men using data from the National Health and Nutrition Examination Survey (NHANES). A total of 6254 male subjects aged 12-80 years (mean age 35.52 ± 14.84 years) in the NHANES from 2011 to 2020 were analyzed. SUA was measured by DxC using the timed endpoint method, and lumbar spine BMD was measured by dual-energy X-ray absorptiometry (DXA). Multivariate linear regression models were used to explore the relationship between SUA and BMD by adjusting for age, race/Hispanic origin, drinking behavior, smoking behavior, physical activity, body mass index (BMI), poverty-to-income ratio (PIR), total protein, serum calcium, cholesterol, serum phosphorus, and blood urea nitrogen. After correcting for the above confounders, it was found that SUA was positively associated with lumbar spine BMD in the range of SUA < 5 mg/dL (β = 0.006 95% CI 0.003-0.009, P < 0.001), and BMD of individuals in the highest quartile of SUA was 0.020 g/cm2 higher than those in the lowest quartile of SUA (β = 0.020 95% CI 0.008-0.032, P = 0.003). This study showed that SUA was positively correlated with lumbar spine BMD in American men within a certain range. This gives clinicians some insight into how to monitor SUA levels to predict BMD levels during adolescence when bone is urgently needed for growth and development and during old age when bone loss is rapid.

Prediction of subsequent vertebral compression fractures after thoracolumbar kyphoplasty: a multicenter retrospective analysis

OBJECTIVE

Second fractures at the cemented vertebrae (SFCV) are often seen after percutaneous kyphoplasty, especially at the thoracolumbar junction. Our study aimed to develop and validate a preoperative clinical prediction model for predicting SFCV.

METHODS

A cohort of 224 patients with single-level thoracolumbar osteoporotic vertebral fractures (T11-L2) from 3 medical centers was analyzed between January 2017 and June 2020 to derive a preoperative clinical prediction model for SFCV. Backward-stepwise selection was used to select preoperative predictors. We assigned a score to each selected variable and developed the SFCV scoring system. Internal validation and calibration were conducted for the SFCV score.

RESULTS

Among the 224 patients included, 58 had postoperative SFCV (25.9%). The following preoperative measures on multivariable analysis were summarized in the 5-point SFCV score: bone mineral density (≤-3.05), serum 25-hydroxy vitamin D3 (≤17.55 ng/mL), standardized signal intensity of fractured vertebra on T1-weighted images (≤59.52%), C7-S1 sagittal vertical axis (≥3.25 cm), and intravertebral cleft. Internal validation showed a corrected area under the curve of 0.794. A cutoff of ≤1 point was chosen to classify a low risk of SFCV, for which only 6 of 100 patients (6%) had SFCV. A cutoff of ≥4 points was chosen to classify a high risk of SFCV, for which 28 of 41 (68.3%) had SFCV.

CONCLUSION

The SFCV score was found to be a simple preoperative method for identification of patients at low and high risk of postoperative SFCV. This model could be applied to individual patients and aid in the decision-making before percutaneous kyphoplasty.

Targeting strategies for bone diseases: signaling pathways and clinical studies

Since the proposal of Paul Ehrlich's magic bullet concept over 100 years ago, tremendous advances have occurred in targeted therapy. From the initial selective antibody, antitoxin to targeted drug delivery that emerged in the past decades, more precise therapeutic efficacy is realized in specific pathological sites of clinical diseases. As a highly pyknotic mineralized tissue with lessened blood flow, bone is characterized by a complex remodeling and homeostatic regulation mechanism, which makes drug therapy for skeletal diseases more challenging than other tissues. Bone-targeted therapy has been considered a promising therapeutic approach for handling such drawbacks. With the deepening understanding of bone biology, improvements in some established bone-targeted drugs and novel therapeutic targets for drugs and deliveries have emerged on the horizon. In this review, we provide a panoramic summary of recent advances in therapeutic strategies based on bone targeting. We highlight targeting strategies based on bone structure and remodeling biology. For bone-targeted therapeutic agents, in addition to improvements of the classic denosumab, romosozumab, and PTH1R ligands, potential regulation of the remodeling process targeting other key membrane expressions, cellular crosstalk, and gene expression, of all bone cells has been exploited. For bone-targeted drug delivery, different delivery strategies targeting bone matrix, bone marrow, and specific bone cells are summarized with a comparison between different targeting ligands. Ultimately, this review will summarize recent advances in the clinical translation of bone-targeted therapies and provide a perspective on the challenges for the application of bone-targeted therapy in the clinic and future trends in this area.

Endogenous ligand recognition and structural transition of a human PTH receptor

Endogenous parathyroid hormone (PTH) and PTH-related peptide (PTHrP) bind to the parathyroid hormone receptor 1 (PTH1R) and activate the stimulatory G-protein (Gs) signaling pathway. Intriguingly, the two ligands have distinct signaling and physiological properties: PTH evokes prolonged Gs activation, whereas PTHrP evokes transient Gs activation with reduced bone-resorption effects. The distinct molecular actions are ascribed to the differences in ligand recognition and dissociation kinetics. Here, we report cryoelectron microscopic structures of six forms of the human PTH1R-Gs complex in the presence of PTH or PTHrP at resolutions of 2.8 -4.1 Å. A comparison of the PTH-bound and PTHrP-bound structures reveals distinct ligand-receptor interactions underlying the ligand affinity and selectivity. Furthermore, five distinct PTH-bound structures, combined with computational analyses, provide insights into the unique and complex process of ligand dissociation from the receptor and shed light on the distinct durations of signaling induced by PTH and PTHrP.

Association between bone turnover markers, BMD and height loss of cemented vertebrae after percutaneous vertebroplasty in patients with osteoporotic vertebral compression fractures

Introduction

Percutaneous vertebroplasty (PVP) was recently performed for treating patients with osteoporotic vertebral compression fractures (OVCF). However, recompression of cemented vertebra with significant vertebral height loss occurred in the patients after PVP was observed during the follow-up period. The purpose is to explore the risk factors among several potential predictors for the height loss of treated vertebral bodies after PVP in patients with OVCF.

Methods

A study of 93 patients who had undergone PVP between May 1, 2016, and March 1, 2019, at the Spine Center of Huadong Hospital Affiliated to Fudan University was conducted. The fractured vertebral height loss ratio ≥ 15% at final follow-up were defined as cemented vertebra recompression. The following variables were measured and collected: age, gender, body mass index (BMI), bone mineral density (BMD), volume of bone cement injected, bone cement leakage, fractured vertebra segment, contact between bone cement and endplates, serum of calcium and phosphorus, and six kinds of bone turnover markers.

Results

Mann–Whitney U test and Univariate Logistic regression analysis showed that the cemented vertebra recompression was correlated with BMD, contact between bone cement and endplates, parathyroid hormone (PTH), and 25-hydroxy vitamin D3 (25-OH-D3). Following multivariate modeling, multiple factors logistic regression elucidated that high BMD (P < 0.001, OR = 0.089) and high level of serum 25-OH-D3 (P = 0.012, OR = 0.877) were negatively correlated with the cemented vertebra recompression after PVP.

Conclusion

Decreased BMD and lower level of serum 25-OH-D3 might be two critical and significant risk factors for the height loss of cemented vertebrae after PVP.

Early Addition of Parathyroid Hormone-Related Peptide Regulates the Hypertrophic Differentiation of Mesenchymal Stem Cells

OBJECTIVE

Chondrogenic differentiation of mesenchymal stem cells (MSCs) into hyaline cartilage is complicated by terminal hypertrophic differentiation. In growth plate, parathyroid hormone-related peptide (1-34) (PTHrP) plays a crucial role in maintaining chondrocytes in their proliferation state by counteracting the hypertrophic differentiation. This study aims to test the effect of PTHrP supplementation at different time points on chondrogenic differentiation of MSCs and assess the final quality of differentiated chondrocytes.

METHODS

Human periosteum and bone marrow MSCs isolated from 3 patient samples (donor unmatched) were characterized by flow cytometry and multilineage differentiation. The cells were differentiated into chondrocytes in the presence of transforming growth factor-β (TGF-β) and the PTHrP (1-34) was added from 4th or 14th day of culture. The outcome was analyzed by histology, immunohistochemistry, and gene expression.

RESULTS

Flow cytometry and multilineage differentiation confirmed that the cells isolated from periosteum and bone marrow exhibited the phenotype of MSCs. During chondrogenic differentiation, pellets that received PTHrP from the 4th day of culture showed a significant reduction in hypertrophic markers (COL10A1 and RUNX) than the addition of PTHrP from the 14th day and TGF-β alone treated samples. Furthermore, 4th day supplementation of PTHrP significantly improved the expression of cartilage-specific markers (COL2A1, SOX9, ACAN) in both periosteum and bone marrow-derived MSCs. Histology and immunostaining with collagen type X data corroborated the gene expression outcomes.

CONCLUSION

The outcome showed that supplementing PTHrP from the 4th day of chondrogenic differentiation produced better chondrocytes with less hypertrophic markers in both bone marrow and periosteal-derived MSCs.

Physiological and Pharmacological Roles of PTH and PTHrP in Bone Using Their Shared Receptor, PTH1R

Parathyroid hormone (PTH) and the paracrine factor, PTH-related protein (PTHrP), have preserved in evolution sufficient identities in their amino-terminal domains to share equivalent actions upon a common G protein-coupled receptor, PTH1R, that predominantly uses the cyclic adenosine monophosphate-protein kinase A signaling pathway. Such a relationship between a hormone and local factor poses questions about how their common receptor mediates pharmacological and physiological actions of the two. Mouse genetic studies show that PTHrP is essential for endochondral bone lengthening in the fetus and is essential for bone remodeling. In contrast, the main postnatal function of PTH is hormonal control of calcium homeostasis, with no evidence that PTHrP contributes. Pharmacologically, amino-terminal PTH and PTHrP peptides (teriparatide and abaloparatide) promote bone formation when administered by intermittent (daily) injection. This anabolic effect is remodeling-based with a lesser contribution from modeling. The apparent lesser potency of PTHrP than PTH peptides as skeletal anabolic agents could be explained by lesser bioavailability to PTH1R. By contrast, prolongation of PTH1R stimulation by excessive dosing or infusion, converts the response to a predominantly resorptive one by stimulating osteoclast formation. Physiologically, locally generated PTHrP is better equipped than the circulating hormone to regulate bone remodeling, which occurs asynchronously at widely distributed sites throughout the skeleton where it is needed to replace old or damaged bone. While it remains possible that PTH, circulating within a narrow concentration range, could contribute in some way to remodeling and modeling, its main physiological role is in regulating calcium homeostasis.

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.

New Targets and Emergent Therapies for Osteoporosis

The 11 existing FDA-approved osteoporosis drug treatments include hormone replacement therapy, 2 SERMs (raloxifene and bazedoxifene), 5 inhibitors of bone-resorbing osteoclasts (4 bisphosphonates and anti-RANKL denosumab), 2 parathyroid hormone analogues (teriparatide and abaloparatide), and 1 WNT signaling enhancer (romosozumab). These therapies are effective and provide multiple options for patients and physicians. As the genomic revolution continues, potential novel targets for future drug development are identified. This review takes a wide perspective to describe potentially rewarding topics to explore, including knowledge of genes and pathways involved in bone cell metabolism, the utility of animal models, targeting drugs to bone, and ongoing advances in drug design and delivery.

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.

Association between lumbar bone mineral density and serum uric acid in postmenopausal women: a cross-sectional study of healthy Chinese population

Partial correlation and regression analyses were used in this study. We showed that there is a linear relationship between bone mineral density and serum uric acid within the normal physiologic range, and higher serum uric acid levels had a protective effect on bone loss in postmenopausal osteoporosis.

PURPOSE

The significance of the relationship between lumbar bone mineral density ⁽BMD) and serum uric acid (SUA) levels is unclear. The aims of this study were to investigate on a population-level the association between lumbar BMD and SUA within the normal physiologic range and to determine whether SUA plays a protective role in bone loss in healthy postmenopausal Chinese women.

METHODS

This was a community-based cross-sectional study involving 390 healthy postmenopausal women, 47-89 years of age, conducted in Shenyang, China. The BMD was measured at the lumbar spine using dual-energy X-ray absorptiometry (DXA). The SUA levels were obtained at each DXA visit. Partial correlation and regression analyses were applied to determine the associations.

RESULTS

The SUA levels were significantly different between the normal BMD, osteopenia, and osteoporosis groups. The lumbar BMD was positively correlated with SUA in postmenopausal women after adjustment for age (r = 0.212). After adjustment for age, body mass index, systolic blood pressure, diastolic blood pressure, hip circumference, cigarette smoking, alcohol consumption, milk intake, physical exercise, fracture history, total protein, total bilirubin, triglycerides, total cholesterol, high-density lipoprotein cholesterol, fasting blood glucose, serum calcium, and estimated glomerular filtration rate, the lumbar BMD was associated with SUA and the odds ratio of the third SUA quartile was 0.408 (95%CI, 0.198-0.841, P = 0.015), compared to the first quartile of SUA levels.

CONCLUSION

The lumbar BMD was linearly associated with SUA levels within the normal physiologic range of postmenopausal women. Higher SUA levels had a protective effect on bone loss in postmenopausal osteoporosis.

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.

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 (1-34) reduces alveolar bone loss in type 1 diabetic rats

OBJECTIVE

To investigate the role of parathyroid hormone related protein (PTHrP) in diabetic periodontitis.

METHODS

After injected with 55mg/kg streptozotocin, diabetic rats were treated subcutaneously with low-dose (40μg/kg, once daily for 5days per week), middle-dose (80μg/kg) or high-dose (160μg/kg) PTHrP(1-34) peptide. Treatment continued for 12 weeks. Changes in periodontal tissues were confirmed by micro-computerized tomography assay and H&E analysis. We used tartrate resistant acid phosphatase (TRAP) staining to identify osteoclast cells. The expression of TNF-α, IL-1β and IL-6 was assessed by immunohistochemistry and Western blot.

RESULTS

Tooth-supporting structure loss was observed in periodontal tissues of diabetic rats. PTHrP (1-34) attenuated alveolar bone loss, especially in the middle-dose and high-dose group. Whereas TNF-α, IL-1β and IL-6 protein levels were increased in the diabetic gingival tissues, PTHrP (1-34) treatment inhibited the increase of IL-1β and IL-6, but had no effect on TNF-α.

CONCLUSION

Type 1 diabetes increased the susceptibility to periodontal disease. Intermittent administration of PTHrP (1-34) exhibited an inhibitory effect on alveolar bone resorption and the gingival inflammation in periodontal tissues of diabetic rats.

Influence of Dosing Interval and Administration on the Bone Metabolism, Skeletal Effects, and Clinical Efficacy of Parathyroid Hormone in Treating Osteoporosis: A Narrative Review

Recombinant human parathyroid hormone (PTH) is the key anabolic agent used for preventing fracture in postmenopausal women with osteoporosis. In bone metabolism, PTH signaling is mediated through a G protein–coupled receptor that affects various post‐receptor signaling pathways. Results of preclinical and clinical studies have shown that PTH improves both the structure and strength of bone tissue. Once daily subcutaneous injection of the PTH fragment, teriparatide (PTH [1‐34]), is the most commonly recommended formulation and dosing strategy in clinical practice. However, other dosing intervals, formulations, and routes have been investigated in preclinical and clinical studies. In particular, once‐weekly and cyclical administration have been investigated mainly as a means of reducing the high direct costs of treatment. In preclinical studies, bone formation/resorption markers, bone mineral density measurements, and histomorphometric parameters improved with both once‐daily and once‐weekly administration. However, the magnitude and duration of such improvements were generally greater with once‐daily PTH administration. In clinical studies, reductions in fracture incidence were also noted with both once‐daily and once‐weekly PTH administration, although improvements in nonvertebral fractures are less evident with once‐weekly administration. This narrative review details the differences between PTH formulation and dosing strategies in relation to preclinical and clinical efficacy/safety parameters, although it should be stressed that no head‐to‐head studies allow direct comparisons. This review also seeks to outline practical considerations involved with PTH prescribing and new directions in research regarding routes of administration. © 2017 The Authors. JBMR Plus is published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research.

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.

Abaloparatide

Abaloparatide is an investigational analog of human PTHrP (1-34) being developed for the treatment of osteoporosis. The amino-acid sequence of abaloparatide is identical to that of PTHrP in the first 20 amino-acids, while over half of the remaining amino-acids are different. Some studies in animals and in humans reported that abaloparatide presented a potent anabolic activity with reduced effects on bone resorption as compared to that observed with teriparatide. This may be due to a more transient signaling response of abaloparatide related to differing affinities of the two drugs to the specific conformations of the PTH1 receptor. In the ACTIVE study, a phase 3 fracture prevention trial, 2460 postmenopausal osteoporotic women at high risk for fracture were randomized to receive 18-months of either daily abaloparatide 80 μg s.c., placebo or teriparatide 20 μg s.c. The reduction in vertebral fracture rate with respect to placebo was 86% in the abaloparatide group and 80% in the teriparatide group. Abaloparatide also produced a significant 43% reduction in the rate of nonvertebral fractures (2.7 vs 4.0% with placebo, p=0.04) whereas teriparatide determined a 28% reduction (2.9 vs 4.0% with placebo, p=NS). Abaloparatide or teriparatide showed similar increases in BMD at lumbar spine, while the patients of the abaloparatide group showed significantly greater increases in BMD at both total hip (4.18 vs 3.26%) and femoral neck (3.60 vs 2.66%). Therefore, if the preliminary data of the ACTIVE study is confirmed, abaloparatide may become an important option for the anabolic treatment of postmenopausal osteoporosis.

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.

[Novel anti-osteoporotic drugs on the horizon]

In recent years the great progress in knowledge on bone cell biology has allowed identification of molecular structures that can be targeted with pinpoint precision (druggable targets). Osteoclasts are regulated via the RANK-RANK-ligand (RANKL) signaling pathway and osteoblasts via the Wnt signaling pathway, both of which can be influenced for therapeutic measures. As a result the number of (functional) osteoclasts can be decreased or the genesis of osteoblasts can be increased and bone resorption is inhibited or bone formation is enhanced, respectively. Osteoclasts degrade collagen through cathepsin K and inactivation of this enzyme stabilizes the bone matrix; however, as osteoclasts are still able to maintain a stimulatory cross-talk with osteoblasts, formation of new bone will not be reduced. Parathyroid hormone-related protein plays a role in endochondral ossification and a synthetic analogue of this protein may have potent bone anabolic activity; however, the use of such new and highly efficient therapeutic principles comes with new questions and uncertainties on the sequence of therapies, duration of therapy, long-term side effects, undesired activation of metabolic pathways and effectiveness in comparison to other strategies of fracture prevention.

Osteoporosis and Bone Mass Disorders: From Gene Pathways to Treatments

Genomic technologies have evolved rapidly contributing to the understanding of diseases. Genome-wide association studies (GWAS) and whole-exome sequencing have aided the identification of the genetic determinants of monogenic and complex conditions including osteoporosis and bone mass disorders. Overlap exists between the genes implicated in monogenic and complex forms of bone mass disorders, largely explained by the clustering of genes encoding factors in signaling pathways crucial for mesenchymal cell differentiation, skeletal development, and bone remodeling and metabolism. Numerous of the remaining discovered genes merit functional follow-up studies to elucidate their role in bone biology. The insight provided by genetic studies is serving the identification of biomarkers predictive of disease, redefining disease, response to treatment, and discovery of novel drug targets for skeletal disorders.

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.

Enhancement of Lumbar Fusion and Alleviation of Adjacent Segment Disc Degeneration by Intermittent PTH(1-34) in Ovariectomized Rats

Osteoporosis, which is prevalent in postmenopausal or aged populations, is thought to be a contributing factor to adjacent segment disc degeneration (ASDD), and the incidence and extent of ASDD may be augmented by osteopenia. Parathyroid hormone (PTH) (1-34) has already been shown to be beneficial in osteoporosis, lumbar fusion and matrix homeostasis of intervertebral discs. However, whether PTH(1-34) has a reversing or retarding effect on ASDD in osteopenia has not been confirmed. In the present study, we evaluated the effects of intermittent PTH(1-34) on ASDD in an ovariectomized (OVX) rat model. One hundred 3-month-old female Sprague-Dawley rats underwent L4 -L5 posterolateral lumbar fusion (PLF) with spinous-process wire fixation 4 weeks after OVX surgery. Control groups were established accordingly. PTH(1-34) was intermittently administered immediately after PLF surgery and lasted for 8 weeks using the following groups (n = 20) (V = vehicle): Sham+V, OVX+V, Sham+PLF+V, OVX+PLF+V, OVX+PLF+PTH. The fused segments showed clear evidence of eliminated motion on the fusion-segment based on manual palpation. Greater new bone formation in histology was observed in PTH-treated animals compared to the control group. The extent of ASDD was significantly increased by ovariotomy. Intermittent PTH(1-34) significantly alleviated ASDD by preserving disc height, microvessel density, relative area of vascular buds, endplate thickness and the relative area of endplate calcification. Moreover, protein expression results showed that PTH(1-34) not only inhibited matrix degradation by decreasing MMP-13, ADAMTS-4 and Col-I, but also promote matrix synthesis by increasing Col-II and Aggrecan. In conclusion, PTH(1-34), which effectively improves lumbar fusion and alleviates ASDD in ovariectomized rats, may be a potential candidate to ameliorate the prognosis of lumbar fusion in osteopenia.

Emerging Therapies for Osteoporosis

Although several effective therapies are available for the treatment of osteoporosis in postmenopausal women and older men, there remains a need for the development of even more effective and acceptable drugs. Several new drugs that are in late-stage clinical development will be discussed. Abaloparatide (recombinant parathyroid hormone related peptide [PTHrP] analogue) has anabolic activity like teriparatide. Recent data from the phase 3 fracture prevention trial demonstrate that this agent is effective in reducing fracture risk. Inhibiting cathepsin K reduces bone resorption without decreasing the numbers or activity of osteoclasts, thereby preserving or promoting osteoblast function. Progressive increases in bone mineral density (BMD) have been observed over 5 years. Early data suggest that odanacatib effectively reduces fracture risk. Lastly, inhibiting sclerostin with humanized antibodies promotes rapid, substantial but transient increases in bone formation while inhibiting bone resorption. Marked increases in BMD have been observed in phase 2 studies. Fracture prevention studies are underway. The new therapies with novel and unique mechanisms of action may, alone or in combination, provide more effective treatment options for our patients.

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.

Novel therapies for osteoporosis

Since the identification of osteoporosis as a major health issue in aging populations and the subsequent development of the first treatment modalities for its management, considerable progress has been made in our understanding of the mechanisms controlling bone turnover and disease pathophysiology, thus enabling the pinpointing of new targets for intervention. This progress, along with advances in biotechnology, has rendered possible the development of ever more sophisticated treatments employing novel mechanisms of action. Denosumab, a monoclonal antibody against RANKL, approved for the treatment of postmenopausal and male osteoporosis, significantly and continuously increases bone mineral density (BMD) and maintains a low risk of vertebral, non-vertebral, and hip fractures for up to 8 years. Currently available combinations of estrogens with selective estrogen receptor modulators moderately increase BMD without causing the extra-skeletal adverse effects of each compound alone. The cathepsin K inhibitor odanacatib has recently been shown to decrease vertebral, non-vertebral, and hip fracture rates and is nearing approval. Romosozumab, an anti-sclerosin antibody, and abaloparatide, a PTH-related peptide analog, are at present in advanced stages of clinical evaluation, so far demonstrating efficaciousness together with a favorable safety profile. Several other agents are currently in earlier clinical and preclinical phases of development, including dickkopf-1 antagonists, activin A antagonists, β-arrestin analogs, calcilytics, and Src tyrosine kinase inhibitors.

PDGFB-based stem cell gene therapy increases bone strength in the mouse

Substantial advances have been made in the past two decades in the management of osteoporosis. However, none of the current medications can eliminate the risk of fracture and rejuvenate the skeleton. To this end, we recently reported that transplantation of hematopoietic stem/progenitor cells (HSCs) or Sca1(+) cells engineered to overexpress FGF2 results in a significant increase in lamellar bone matrix formation at the endosteum; but this increase was attended by the development of secondary hyperparathyroidism and severe osteomalacia. Here we switch the therapeutic gene to PDGFB, another potent mitogen for mesenchymal stem cells (MSCs) but potentially safer than FGF2. We found that modest overexpression of PDGFB using a relatively weak phosphoglycerate kinase (PGK) promoter completely avoided osteomalacia and secondary hyperparathyroidism, and simultaneously increased trabecular bone formation and trabecular connectivity, and decreased cortical porosity. These effects led to a 45% increase in the bone strength. Transplantation of PGK-PDGFB-transduced Sca1(+) cells increased MSC proliferation, raising the possibility that PDGF-BB enhances expansion of MSC in the vicinity of the hematopoietic niche where the osteogenic milieu propels the differentiation of MSCs toward an osteogenic destination. Our therapy should have potential clinical applications for patients undergoing HSC transplantation, who are at high risk for osteoporosis and bone fractures after total body irradiation preconditioning. It could eventually have wider application once the therapy can be applied without the preconditioning.

Modulating Bone Resorption and Bone Formation in Opposite Directions in the Treatment of Postmenopausal Osteoporosis

Bone remodeling, the fundamental process for bone renewal, is targeted by treatments of osteoporosis to correct the imbalance between bone resorption and bone formation and reduce the risk of fractures and associated clinical consequences. Currently available therapeutics affect bone resorption and bone formation in the same direction and either decrease (inhibitors of bone resorption) or increase (parathyroid hormone [PTH] peptides) bone remodeling. Studies of patients with rare bone diseases and genetically modified animal models demonstrated that bone resorption and bone formation may not necessarily be coupled, leading to identification of molecular targets in bone cells for the development of novel agents for the treatment of osteoporosis. Application of such agents to the treatment of women with low bone mass confirmed that bone resorption and bone formation can be modulated in different directions and so far two new classes of therapeutics for osteoporosis have been defined with distinct mechanisms of action. Such treatments, if combined with a favorable safety profile, will offer new therapeutic options and will improve the management of patients with osteoporosis.

Osteoblast dysfunctions in bone diseases: from cellular and molecular mechanisms to therapeutic strategies

Several metabolic, genetic and oncogenic bone diseases are characterized by defective or excessive bone formation. These abnormalities are caused by dysfunctions in the commitment, differentiation or survival of cells of the osteoblast lineage. During the recent years, significant advances have been made in our understanding of the cellular and molecular mechanisms underlying the osteoblast dysfunctions in osteoporosis, skeletal dysplasias and primary bone tumors. This led to suggest novel therapeutic approaches to correct these abnormalities such as the modulation of WNT signaling, the pharmacological modulation of proteasome-mediated protein degradation, the induction of osteoprogenitor cell differentiation, the repression of cancer cell proliferation and the manipulation of epigenetic mechanisms. This article reviews our current understanding of the major cellular and molecular mechanisms inducing osteoblastic cell abnormalities in age-related bone loss, genetic skeletal dysplasias and primary bone tumors, and discusses emerging therapeutic strategies to counteract the osteoblast abnormalities in these disorders of bone formation.

Novel approaches to the treatment of osteoporosis

Despite the availability of efficacious treatments for fracture reduction in patients with osteoporosis, there are still unmet needs requiring a broader range of therapeutics. In particular, agents that are capable of replacing already lost bone and that also drastically reduce the risk of non-vertebral fractures are needed. Studies of rare bone diseases in humans and animal genetics have identified targets in bone cells for the development of therapies for osteoporosis with novel mechanisms of action. Here, we review these new developments, with emphasis on inhibitors of cathepsin K in osteoclasts and sclerostin in osteocytes, which are currently studied in phase 3 clinical trials.

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.

Intermittent PTHrP(1-34) exposure augments chondrogenesis and reduces hypertrophy of mesenchymal stromal cells

Phenotype instability and premature hypertrophy prevent the use of human mesenchymal stromal cells (MSCs) for cartilage regeneration. Aim of this study was to investigate whether intermittent supplementation of parathyroid hormone-related protein (PTHrP), as opposed to constant treatment, can beneficially influence MSC chondrogenesis and to explore molecular mechanisms below catabolic and anabolic responses. Human MSCs subjected to chondrogenic induction in high-density culture received PTHrP(1-34), forskolin, dbcAMP, or PTHrP(7-34) either constantly or via 6-h pulses (three times weekly), before proteoglycan, collagen type II, and X deposition; gene expression; and alkaline phosphatase (ALP) activity were assessed. While constant application of PTHrP(1-34) suppressed chondrogenesis of MSCs, pulsed application significantly increased collagen type 2 (COL2A1) gene expression and the collagen type II, proteoglycan, and DNA content of pellets after 6 weeks. Collagen type 10 (COL10A1) gene expression was little affected but Indian hedgehog (IHH) expression and ALP activity were significantly downregulated by pulsed PTHrP. A faster response to PTHrP exposure was recorded for ALP activity over COL2A1 regulation, suggesting that signal duration is critical for catabolic versus anabolic reactions. Stimulation of cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) signaling by forskolin reproduced major effects of both treatment modes, whereas application of PTHrP(7-34) capable of protein kinase C (PKC) signaling was ineffective. Pulsed PTHrP exposure of MSCs stimulated chondrogenesis and reduced endochondral differentiation apparently uncoupling chondrogenic matrix deposition from hypertrophic marker expression. cAMP/PKA was the major signaling pathway triggering the opposing effects of both treatment modes. Intermittent application of PTHrP represents an important novel means to improve chondrogenesis of MSCs and may be considered as a supporting clinical-treatment mode for MSC-based cartilage defect regeneration.

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.

Treatment with N- and C-terminal peptides of parathyroid hormone-related protein partly compensate the skeletal abnormalities in IGF-I deficient mice

Insulin-like growth factor-I (IGF-I) deficiency causes growth delay, and IGF-I has been shown to partially mediate bone anabolism by parathyroid hormone (PTH). PTH-related protein (PTHrP) is abundant in bone, and has osteogenic features by poorly defined mechanisms. We here examined the capacity of PTHrP (1-36) and PTHrP (107-111) (osteostatin) to reverse the skeletal alterations associated with IGF-I deficiency. Igf1-null mice and their wild type littermates were treated with each PTHrP peptide (80 µg/Kg/every other day/2 weeks; 2 males and 4 females for each genotype) or saline vehicle (3 males and 3 females for each genotype). We found that treatment with either PTHrP peptide ameliorated trabecular structure in the femur in both genotypes. However, these peptides were ineffective in normalizing the altered cortical structure at this bone site in Igf1-null mice. An aberrant gene expression of factors associated with osteoblast differentiation and function, namely runx2, osteoprotegerin/receptor activator of NF-κB ligand ratio, Wnt3a , cyclin D1, connexin 43, catalase and Gadd45, as well as in osteocyte sclerostin, was found in the long bones of Igf1-null mice. These mice also displayed a lower amount of trabecular osteoblasts and osteoclasts in the tibial metaphysis than those in wild type mice. These alterations in Igf1-null mice were only partially corrected by each PTHrP peptide treatment. The skeletal expression of Igf2, Igf1 receptor and Irs2 was increased in Igf1-null mice, and this compensatory profile was further improved by treatment with each PTHrP peptide related to ERK1/2 and FoxM1 activation. In vitro, PTHrP (1-36) and osteostatin were effective in promoting bone marrow stromal cell mineralization in normal mice but not in IGF-I-deficient mice. Collectively, these findings indicate that PTHrP (1-36) and osteostatin can exert several osteogenic actions even in the absence of IGF-I in the mouse bone.

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.