Tissue-level mechanisms responsible for the increase in bone formation and bone volume by sclerostin antibody

Nanomaterials-Based Drug Delivery Systems for Therapeutic Applications in Osteoporosis

The etiology of osteoporosis is rooted in the disruption of the intricate equilibrium between bone formation and bone resorption processes. Nevertheless, the conventional anti-osteoporotic medications and hormonal therapeutic regimens currently employed in clinical practice are associated with a multitude of adverse effects, thereby constraining their overall therapeutic efficacy and potential. Recently, nanomaterials have emerged as a promising alternative due to their minimal side effects, efficient drug delivery, and ability to enhance bone formation, aiding in restoring bone balance. This review delves into the fundamental principles of bone remodeling and the bone microenvironment, as well as current clinical treatment approaches for osteoporosis. It subsequently explores the research status of nanomaterial-based drug delivery systems for osteoporosis treatment, encompassing inorganic nanomaterials, organic nanomaterials, cell-mimicking carriers and exosomes mimics and emerging therapies targeting the osteoporosis microenvironment. Finally, the review discusses the potential of nanomedicine in treating osteoporosis and outlines the future trajectory of this burgeoning field. The aim is to provide a comprehensive reference for the application of nanomaterial-based drug delivery strategies in osteoporosis therapy, thereby fostering further advancements and innovations in this critical area of medical research.

Non-invasive quantification of bone (re)modeling dynamics in adults with osteogenesis imperfecta treated with setrusumab using timelapse high-resolution peripheral-quantitative computed tomography

Timelapse imaging using high-resolution peripheral quantitative computed tomography has emerged as a non-invasive method to quantify bone (re)modeling. However, there is no consensus on how to perform the procedure. As part of the ASTEROID phase-2b multicenter trial, we used 29 same-day repeated scans from adults with OI to identify a method that minimized measurement error. We evaluated input image type, registration method, segmentation mask, and for grayscale images various values for the voxel density difference considered formed or resorbed, minimum formation/resorption cluster size, and Gaussian smoothing sigma. We verified the accuracy of our method and then used it on longitudinal scans (baseline, 6, 12, 18, and 24 mo) from 78 participants to assess bone formation and resorption induced by an anabolic (setrusumab) and anti-catabolic (zoledronic acid) treatments as part of the ASTEROID trial. Regardless of image registration method, binary input images resulted in large errors ~13% and ~8% for first- and second-generation scanners, respectively. For the grayscale input images, errors were smaller for 3D compared to matched angle registration. For both scanner generations, a density threshold of 200 mgHA/cm3 combined with Gaussian noise reduction resulted in errors <1%. We verified the method was accurate by showing that similar regions of bone formation and resorption were identified when comparing each scan from the same-day repeated scans with a scan from another timepoint. Timelapse analysis revealed a dose-dependent increase in bone formation and resorption with setrusumab treatment. Zoledronic acid altered bone changes in favor of formation, although no changes reached statistical significance. This study identifies a timelapse method that minimizes measurement error, which can be used in future studies to improve the uniformity of results. This non-invasive imaging biomarker revealed dose dependent bone (re)modeling outcomes from 1 year of setrusumab treatment in adults with OI.

Osteogenesis imperfecta: shifting paradigms in pathophysiology and care in children

The formation of functional bone requires a delicate interplay between osteogenesis and osteolysis. Disturbances in this subtle balance result in an increased risk for fractures. Besides its mechanical function, bone tissue represents a key player in the regulation of calcium homeostasis. Impaired bone formation results in bone fragility, which is especially pronounced in osteogenesis imperfecta (OI). This rare genetic disorder is characterized by frequent fractures as well as extraskeletal manifestations. The current classification of OI includes 23 distinct types. In recent years, several new mutations in different genes have been identified, although the exact pathomechanisms leading to the clinical presentation of OI often remain unclear. While bisphosphonates are still the standard of care, novel therapeutic approaches are emerging. Especially, targeted antibody therapies, originally developed for osteoporosis, are increasingly being investigated in children with OI and represent a promising approach to alleviate the consequences of impaired osteogenesis and improve quality of life in OI patients. This review aims to provide insight into the pathophysiology of OI and the consequences of distinct disease-causing mutations affecting the regulation of bone homeostasis. In this context, we describe the four most recently identified OI-causing genes and provide an update on current approaches for diagnosis and treatment.

Evaluation of romosozumab's effects on bone marrow adiposity in postmenopausal osteoporotic women: results from the FRAME bone biopsy sub-study

Romosozumab, a humanized monoclonal antibody that binds and inhibits sclerostin, produces a marked increase in bone formation with a concomitant decreased bone resorption. This transient rise in bone formation in the first 2 months of treatment is mainly due to an increased modeling-based bone formation. This requires the recruitment and differentiation of osteoblasts, one possibility being a preferential switch in commitment of precursors to osteoblasts over adipocytes. The purpose of this study was to analyze the marrow adiposity in transiliac bone biopsies at months 2 or 12 from the FRAME biopsy sub-study in patients receiving romosozumab or placebo. The total adipocyte area, number, and density were measured on the total cancellous bone area. The size and shape at the individual adipocyte level were assessed including the mean adipocyte area, perimeter, min and max diameters, and aspect ratio. No significant difference in total adipocyte area, number, or density between placebo and romosozumab groups was observed at months 2 and 12, and no difference was observed between 2 and 12 months. After 2 or 12 months, romosozumab did not modify the size or shape of the adipocytes. No relationship between the adipocyte parameters and the dynamic parameters of bone formation could be evidenced. In conclusion, based on the analysis of a small number of biopsies, no effect of romosozumab on bone marrow adiposity of iliac crest was identified after 2 and 12 months suggesting that the modeling-based formation observed at month 2 was not due to a preferential commitment of the precursor to osteoblast over adipocyte cell lines but may result from a reactivation of bone lining cells and from a progenitor pool independent of the marrow adipocyte population.

Association between P1NP value and increases in bone mineral density in patients with postmenopausal osteoporosis treated with romosozumab

OBJECTIVES

The present study aimed to investigate the effectiveness of treatment with romosozumab for 1 year and association between bone turnover markers and changes in bone mineral density (BMD) in patients with postmenopausal osteoporosis.

METHODS

Participants were 53 treatment-naïve postmenopausal osteoporosis patients. Correlations of per cent changes (Δ) in lumbar (L) and total hip (TH) BMD 12 months after initiating romosozumab with baseline demographic factors and parameters of N-terminal propeptide of Type 1 collagen (P1NP) and tartrate-resistant acid phosphatase-5b at baseline and Months 1, 3, and 6 were assessed. Multiple regression analysis was performed on factors significantly correlated with ΔL-BMD and ΔTH-BMD at Month 12.

RESULTS

ΔL-BMD and ΔTH-BMD at Month 12 were 17.5% and 8.1%, respectively. Multiple regression analysis revealed that a high P1NP value at Month 3 predicted large increases in L-BMD and TH-BMD at Month 12. High total amount of P1NP values from baseline to Month 6 was associated with large increases in L-BMD and TH-BMD at Month 12 and was most strongly correlated with the P1NP value at Month 3.

CONCLUSIONS

A high P1NP value at Month 3 predicted large increases in both L-BMD and TH-BMD at Month 12 in postmenopausal osteoporosis patients treated with romosozumab.

Sclerostin blockade inhibits bone resorption through PDGF receptor signaling in osteoblast lineage cells

While sclerostin-neutralizing antibodies (Scl-Abs) transiently stimulate bone formation by activating Wnt signaling in osteoblast lineage cells, they exert sustained inhibition of bone resorption, suggesting an alternate signaling pathway by which Scl-Abs control osteoclast activity. Since sclerostin can activate platelet-derived growth factor receptors (PDGFRs) in osteoblast lineage cells in vitro and PDGFR signaling in these cells induces bone resorption through M-CSF secretion, we hypothesized that the prolonged anticatabolic effect of Scl-Abs could result from PDGFR inhibition. We show here that inhibition of PDGFR signaling in osteoblast lineage cells is sufficient and necessary to mediate prolonged Scl-Ab effects on M-CSF secretion and osteoclast activity in mice. Indeed, sclerostin coactivates PDGFRs independently of Wnt/β-catenin signaling inhibition, by forming a ternary complex with LRP6 and PDGFRs in preosteoblasts. In turn, Scl-Ab prevents sclerostin-mediated coactivation of PDGFR signaling and consequent M-CSF upregulation in preosteoblast cultures, thereby inhibiting osteoclast activity in preosteoblast/osteoclast coculture assays. These results provide a potential mechanism explaining the dissociation between anabolic and antiresorptive effects of long-term Scl-Ab.

Abaloparatide promotes bone repair of vertebral defects in ovariectomized rats by increasing bone formation

Osteoporotic vertebral fracture (OVF) is the most common type of osteoporotic fracture and is associated with immobility and mortality. Bone anabolic agents, such as abaloparatide (ABL), are usually administered to patients with OVF to prevent subsequent fractures. Although several studies have shown that bone anabolic agents promote healing of long bone fractures, there is little evidence of their healing effect on vertebral bone fractures. In the present study, we investigated the effect of ABL on vertebral bone defects using ovariectomized (OVX) rats with vertebral body drill-hole defects, an animal model of OVF. Eight-week-old female Sprague-Dawley rats were subjected to OVX, followed by the 32-36 days of bone depletion period, once-daily subcutaneous ABL was administered to OVX rats at a dose of 30 μg/kg for a maximum of 6 weeks from the day of the vertebral defect surgery. We found that ABL significantly increased bone mineral content and improved trabecular structural parameters at the vertebral defect site. Moreover, ABL significantly increased bone strength of the defected vertebrae. Bone histochemical analysis revealed formation of thick trabecular bone networks at the defect site after ABL administration, consistent with an improvement in trabecular structural parameters by ABL. ABL increased ALPase- and PHOSPHO1-positive osteoblastic cells and ALPase/PCNA double-positive cells, indicating enhanced preosteoblast proliferation as well as bone formation at the defect site. On the other hand, ABL did not affect the number of cathepsin K-positive osteoclasts per bone surface, suggesting that ABL did not promote excessive bone resorption. Our findings suggest that ABL is useful not only for preventing secondary vertebral fractures but also for promoting bone healing in OVF.

Bone fragility and osteoporosis in children and young adults

Osteoporosis is a metabolic bone disorder which increases fragility fracture risk. Elderly individuals, especially postmenopausal women, are particularly susceptible to osteoporosis. Although rare, osteoporosis in children and young adults is becoming increasingly evident, highlighting the need for timely diagnosis, management and follow-up. Early-onset osteoporosis is defined as the presence of a low BMD (Z-score of ≤ -2.0 in individuals aged < 20 years; T-score of ≤ -2.5 in those aged between 20 to 50 years) accompanied by a clinically significant fracture history, or the presence of low-energy vertebral compression fractures even in the absence of osteoporosis. Affected children and young adults should undergo a thorough diagnostic workup, including collection of clinical history, radiography, biochemical investigation and possibly bone biopsy. Once secondary factors and comorbidities are excluded, genetic testing should be considered to determine the possibility of an underlying monogenic cause. Defects in genes related to type I collagen biosynthesis are the commonest contributors of primary osteoporosis, followed by loss-of-function variants in genes encoding key regulatory proteins of canonical WNT signalling (specifically LRP5 and WNT1), the actin-binding plastin-3 protein (encoded by PLS3) resulting in X-linked osteoporosis, and the more recent sphingomyelin synthase 2 (encoded by SGMS2) which is critical for signal transduction affecting sphingomyelin metabolism. Despite these discoveries, genetic causes and underlying mechanisms in early-onset osteoporosis remain largely unknown, and if no causal gene is identified, early-onset osteoporosis is deemed idiopathic. This calls for further research to unravel the molecular mechanisms driving early-onset osteoporosis that consequently will aid in patient management and individualised targeted therapy.

Loss of Nmp4 enhances bone gain from sclerostin antibody administration

Severe osteoporosis is often treated with one of three Food and Drug Administration (FDA)-approved osteoanabolics. These drugs act by (1) parathyroid hormone (PTH) receptor stimulation using analogues to PTH (teriparatide) or PTH-related peptide (abaloparatide) or by (2) monoclonal antibody neutralization of sclerostin, an innate Wnt inhibitor (Scl-mAb, romosozumab-aqqg). The efficacies of both strategies wane over time. The transcription factor Nmp4 (Nuclear Matrix Protein 4) is expressed in all tissues yet mice lacking this gene are healthy and exhibit enhanced PTH-induced bone formation. Conditional deletion of Nmp4 in mesenchymal stem progenitor cells (MSPCs) phenocopies the elevated response to PTH in global Nmp4-/- mice. However, targeted deletion in later osteoblast stages does not replicate this response. In this study we queried whether loss of Nmp4 improves Scl-mAb potency. Experimental cohorts included global Nmp4-/- and Nmp4+/+ littermates and three conditional knockout models. Nmp4-floxed (Nmp4fl/fl) mice were crossed with mice harboring one of three Cre-drivers (i) Prx1Cre+ targeting MSPCs, (ii) BglapCre+ (mature osteocalcin-expressing osteoblasts), and (iii) Dmp1Cre+ (osteocytes). Female mice were treated with Scl-mAb or 0.9 % saline vehicle for 4 or 7 weeks from 10 weeks of age. Skeletal response was assessed using micro-computed tomography, dual-energy X-ray absorptiometry, bone histomorphometry, and serum analysis. Global Nmp4-/- mice exhibited enhanced Scl-mAb-induced increases in trabecular bone in the femur and spine and a heightened increase in whole body areal bone mineral density compared to global Nmp4+/+ controls. This improved Scl-mAb potency was primarily driven by enhanced increases in bone formation. Nmp4fl/fl;PrxCre+ mice showed an exaggerated Scl-mAb-induced increase in femoral bone but not in the spine since Prrx1 is not expressed in vertebra. The Nmp4fl/fl;BglapCre+ and Nmp4fl/fl;Dmp1Cre+ mice did not exhibit an improved Scl-mAb response. We conclude that Nmp4 expression in MSPCs interferes with the bone anabolic response to anti-sclerostin therapy.

Buds of new bone formation within the Femoral Head of Hip Fracture Patients Coincide with Zones of Low Osteocyte Sclerostin

Romosozumab treatment reduces the rate of hip fractures and increases hip bone density, increasing bone formation by inhibiting sclerostin protein. We studied the normal pattern of bone formation and osteocyte expression in the human proximal femur because it is relevant to both antisclerostin treatment effects and fracture. Having visualized and quantified buds of new bone formation in trabeculae, we hypothesized that they would coincide with areas of (a) higher mechanical stress and (b) low sclerostin expression by osteocytes. In patients with hip fracture, we visualized each bud of active modeling-based formation (forming minimodeling structure [FMiS]) in trabecular cores taken from different parts of the femoral head. Trabecular bone structure was also measured with high-resolution imaging. More buds of new bone formation (by volume) were present in the higher stress superomedial zone (FMiS density, N.FMiS/T.Ar) than lower stress superolateral (p < 0.05), and inferomedial (p < 0.001) regions. There were fewer sclerostin expressing osteocytes close to or within FMiS. FMiS density correlated with greater amount, thickness, number, and connectivity of trabeculae (bone volume BV/TV, r = 0.65, p < 0.0001; bone surface BS/TV, r = 0.47, p < 0.01; trabecular thickness Tb.Th, r = 0.55, p < 0.001; trabecular number Tb.N, r = 0.47, p < 0.01; and connectivity density Conn.D, r = 0.40, p < 0.05) and lower trabecular separation (Tb.Sp, r = -0.56, p < 0.001). These results demonstrate modeling-based bone formation in femoral trabeculae from patients with hip fracture as a potential therapeutic target to enhance bone structure. © 2023 American Society for Bone and Mineral Research (ASBMR).

Osteoporosis: Molecular Pathology, Diagnostics, and Therapeutics

Osteoporosis is a major public health concern affecting millions of people worldwide and resulting in significant economic costs. The condition is characterized by changes in bone homeostasis, which lead to reduced bone mass, impaired bone quality, and an increased risk of fractures. The pathophysiology of osteoporosis is complex and multifactorial, involving imbalances in hormones, cytokines, and growth factors. Understanding the cellular and molecular mechanisms underlying osteoporosis is essential for appropriate diagnosis and management of the condition. This paper provides a comprehensive review of the normal cellular and molecular mechanisms of bone homeostasis, followed by an in-depth discussion of the proposed pathophysiology of osteoporosis through the osteoimmunological, gut microbiome, and cellular senescence models. Furthermore, the diagnostic tools used to assess osteoporosis, including bone mineral density measurements, biochemical markers of bone turnover, and diagnostic imaging modalities, are also discussed. Finally, both the current pharmacological and non-pharmacological treatment algorithms and management options for osteoporosis, including an exploration of the management of osteoporotic fragility fractures, are highlighted. This review reveals the need for further research to fully elucidate the molecular mechanisms underlying the condition and to develop more effective therapeutic strategies.

A Novel Long Noncoding RNA in Osteocytes Regulates Bone Formation through the Wnt/β-Catenin Signaling Pathway

The vast majority of transcribed RNAs are noncoding RNAs. Among noncoding RNAs, long noncoding RNAs (lncRNAs), which contain hundreds to thousands of bases, have received attention in many fields. The vast majority of the constituent cells in bone tissue are osteocytes, but their regulatory mechanisms are incompletely understood. Considering the wide range of potential contributions of lncRNAs to physiological processes and pathological conditions, we hypothesized that lncRNAs in osteocytes, which have not been reported, could be involved in bone metabolism. Here, we first isolated osteocytes from femurs of mice with osteocyte-specific GFP expression. Then, through RNA-sequencing, we identified osteocyte-specific lncRNAs and focused on a novel lncRNA, 9530026P05Rik (lncRNA953Rik), which strongly suppressed osteogenic differentiation. In the IDG-SW3 osteocyte line with lncRNA953Rik overexpression, the expression of Osterix and its downstream genes was reduced. RNA pull-down and subsequent LC-MS/MS analysis revealed that lncRNA953Rik bound the nuclear protein CCAR2. We demonstrated that CCAR2 promoted Wnt/β-catenin signaling and that lncRNA953Rik inhibited this pathway. lncRNA953Rik sequestered CCAR2 from HDAC1, leading to deacetylation of H3K27 in the Osterix promoter and consequent transcriptional downregulation of Osterix. This research is the first to clarify the role of a lncRNA in osteocytes. Our findings can pave the way for novel therapeutic options targeting lncRNAs in osteocytes to treat bone metabolic diseases such as osteoporosis.

Nonresponder Considerations for Romosozumab Treatment

Romosozumab can increase bone mineral density (BMD) in patients with osteoporosis, but some patients do not respond to it. This study aimed to identify risk factors for being a nonresponder to romosozumab treatment. This retrospective observational study included 92 patients. Romosozumab (210 mg) was subcutaneously administered to the participants every 4 weeks over 12 months. We excluded patients who previously underwent treatment for osteoporosis to assess the impact of romosozumab alone. We evaluated the proportion of patients who did not respond to romosozumab treatment to the lumbar spine and hip with increased BMD. Nonresponders were defined as those with a bone density change of < 3% after 12 months of treatment. We compared demographics and biochemical markers between responders and nonresponders. We found that 11.5% of patients were nonresponders at the lumbar spine, and 56.8% were nonresponders at the hip. A risk factor for nonresponse at the spine was low type I procollagen N-terminal propeptide (P1NP) values at 1 month. The cutoff value for P1NP at month 1 was 50 ng/ml. We found that 11.5% and 56.8% of patients experienced no significant improvement in the lumbar spine and hip BMD, respectively. Clinicians should use nonresponse risk factors to inform decisions about romosozumab treatment for patients with osteoporosis.

Aging and Bone Metabolism

Changes in bone architecture and metabolism with aging increase the likelihood of osteoporosis and fracture. Age-onset osteoporosis is multifactorial, with contributory extrinsic and intrinsic factors including certain medical problems, specific prescription drugs, estrogen loss, secondary hyperparathyroidism, microenvironmental and cellular alterations in bone tissue, and mechanical unloading or immobilization. At the histological level, there are changes in trabecular and cortical bone as well as marrow cellularity, lineage switching of mesenchymal stem cells to an adipogenic fate, inadequate transduction of signals during skeletal loading, and predisposition toward senescent cell accumulation with production of a senescence-associated secretory phenotype. Cumulatively, these changes result in bone remodeling abnormalities that over time cause net bone loss typically seen in older adults. Age-related osteoporosis is a geriatric syndrome due to the multiple etiologies that converge upon the skeleton to produce the ultimate phenotypic changes that manifest as bone fragility. Bone tissue is dynamic but with tendencies toward poor osteoblastic bone formation and relative osteoclastic bone resorption with aging. Interactions with other aging physiologic systems, such as muscle, may also confer detrimental effects on the aging skeleton. Conversely, individuals who maintain their BMD experience a lower risk of fractures, disability, and mortality, suggesting that this phenotype may be a marker of successful aging. © 2023 American Physiological Society. Compr Physiol 13:4355-4386, 2023.

Bone Cells Metabolic Changes Induced by Ageing

Bone is a living organ that exhibits active metabolic processes, presenting constant bone formation and resorption. The bone cells that maintain local homeostasis are osteoblasts, osteoclasts, osteocytes and bone marrow stem cells, their progenitor cells. Osteoblasts are the main cells that govern bone formation, osteoclasts are involved in bone resorption, and osteocytes, the most abundant bone cells, also participate in bone remodeling. All these cells have active metabolic activities, are interconnected and influence each other, having both autocrine and paracrine effects. Ageing is associated with multiple and complex bone metabolic changes, some of which are currently incompletely elucidated. Ageing causes important functional changes in bone metabolism, influencing all resident cells, including the mineralization process of the extracellular matrix. With advancing age, a decrease in bone mass, the appearance of specific changes in the local microarchitecture, a reduction in mineralized components and in load-bearing capacity, as well as the appearance of an abnormal response to different humoral molecules have been observed. The present review points out the most important data regarding the formation, activation, functioning, and interconnection of these bone cells, as well as data on the metabolic changes that occur due to ageing.

Distinct Responses of Modeling- and Remodeling-Based Bone Formation to the Discontinuation of Intermittent Parathyroid Hormone Treatment in Ovariectomized Rats

Anabolic agents, such as intermittent parathyroid hormone (PTH), exert their treatment efficacy through activation of two distinct bone formation processes, namely, remodeling-based bone formation (RBF, bone formation coupled with prior bone resorption) and modeling-based bone formation (MBF, bone formation without prior activation of bone resorption). However, if not followed by an antiresorptive agent, treatment benefit was quickly lost upon withdrawal from anabolic agents. By using in vivo micro-computed tomography imaging and multiplex cryohistology with sequential immunofluorescence staining, we investigated the temporal response of newly formed bone tissue from MBF and RBF and the preexisting bone tissue to withdrawal from PTH treatment and the associated cellular activity in an ovariectomized (OVX) rat model. We first demonstrated continued mineral apposition at both RBF and MBF sites following PTH discontinuation, resulting in an extended anabolic effect after 1-week withdrawal from PTH. It was further discovered that MBF sites had a greater contribution than RBF sites to the extended anabolic effect upon early withdrawal from PTH, evidenced by a higher percentage of alkaline phosphatase-positive (ALP+) surfaces and far greater bone formation activity at MBF versus RBF sites. Furthermore, significant bone loss occurred after 3 weeks of discontinuation from PTH, resulting from marked loss of newly formed bone tissue from RBF and preexisting bone tissue prior to treatment. In contrast, MBF surfaces had a delayed increase of tartrate-resistant acid phosphatase activity following PTH discontinuation. As a result, newly formed bone tissue from MBF had greater resistance to PTH discontinuation-induced bone loss than those from RBF and preexisting bone. Understanding various responses of two distinct bone formation types and preexisting bone to anabolic treatment discontinuation is critical to inform the design of follow-up treatment or cyclic treatment strategies to maximize treatment benefit of anabolic agents. © 2022 American Society for Bone and Mineral Research (ASBMR).

Reactivation of Bone Lining Cells are Attenuated Over Repeated Anti-sclerostin Antibody Administration

Reactivation of bone lining cells (BLCs) is a crucial mechanism governing the anabolic action of anti-sclerostin antibody (Scl-Ab) via modeling-based bone formation; however, it remains unclear whether this reactivation can be attenuated after persistent administration of Scl-Ab. Here, we aimed to investigate the reproducibility of persistent Scl-Ab administration for the reactivation of BLCs, and to elucidate the relationship between the activity of BLCs and serum levels of N-terminal procollagen type I (P1NP) during chronic Scl-Ab administration. We conducted an osteoblast lineage tracing study. Briefly, Dmp1-CreERt2(+):Rosa26R mice were injected with 1 mg of 4-hydroxy-tamoxifen weekly from postnatal weeks four to eight. Mice were treated twice with either vehicle or Scl-Ab (25 mg/kg) at weeks 12, 16, and 20, and were euthanized at weeks 8, 12, 13, 16, 17, 20, and 21 (4-6 mice in each group). After euthanization, the number and thickness of X-gal (+) cells on the periosteum of the femoral bones and the serum levels of P1NP were quantified at each time point. Scl-Ab induced a significant increase in the thickness of X-gal (+) cells on periosteal bone surfaces at postnatal weeks 13 (after 1st dose), 17 (after 2nd dose), and 21 (after 3rd dose) compared to that in vehicle-treated mice (all P < 0.001). In the Scl-Ab group, significant increases in the thickness of labeled cells were observed between weeks 16 and 17 and weeks 20 and 21 (both P < 0.001). The percentage increase in X-gal (+) cell thickness was 108.9% from week 12 to week 13, 54.6% from week 16 to week 17, and 49.2% from week 20 to week 21 in the Scl-Ab group. Although Scl-Ab treatment increased the serum levels of P1NP at postnatal weeks 13 and 17 compared with those at week 12 (P = 0.017 and P = 0.038, respectively), the same was not observed at week 21 (P = 0.296). A significant increase in P1NP levels was observed between weeks 16 and 17 and weeks 20 and 21 in the Scl-Ab group (P = 0.005 and P = 0.007, respectively). The percentage increase in P1NP levels was 141.7% from weeks 12 to 13, 114.8% from weeks 16 to 17, and 99.4% from weeks 20 to 21. Serum P1NP levels were positively correlated with X-gal (+) cell thickness (R² = 0.732, P < 0.001). Reactivation of BLCs is modestly attenuated, but reproducible, during persistent Scl-Ab administration. Serum P1NP levels appear to be an indicator of the impact of Scl-Ab on the conversion of BLCs into mature osteoblasts on periosteal bone surfaces, thus contributing to modeling-based bone formation.

Where is bone science taking us?

Bone science has over the last decades unraveled many important pathways in bone and mineral metabolism and the interplay between genetic factors and the environment. Some of these discoveries have led to the development of pharmacological treatments of osteoporosis and rare bone diseases. Other scientific avenues have uncovered a role for the gut microbiome in regulating bone mass, which have led to investigations on the possible therapeutic role of probiotics in the prevention of osteoporosis. Huge advances have been made in identifying the genes that cause rare bone diseases, which in some cases have led to therapeutic interventions. Advances have also been made in understanding the genetic basis of the more common polygenic bone diseases, including osteoporosis and Paget's disease of bone (PDB). Polygenic profiles are used for establishing genetic risk scores aiming at early diagnosis and intervention, but also in Mendelian randomization (MR) studies to investigate both desired and undesired effects of targets for drug design.

The effects of romosozumab combined with active vitamin D3 on fracture healing in ovariectomized rats

Background

In this study, we investigated the potential acceleration of fracture healing and bone mineral density-increasing effects of romosozumab and active vitamin D₃ combination therapy for fractures in ovariectomized rats.

Methods

Ovariectomy was performed on 40 24-week-old female Sprague–Dawley rats. After 8 weeks, the rats were subjected to periosteum removal and osteotomy of the femoral shaft followed by osteosynthesis with intramedullary nailing to create fracture models. The rats were then divided into four groups: C group (control), R group (receiving romosozumab at 25 mg/kg once a month via subcutaneous injection), VD group (receiving active vitamin D₃ at 0.2 µg/kg twice a week via subcutaneous injection), and R + VD group. Further, 10 rats were included in a sham group. At 10 weeks after the intervention, both femurs were removed and blood samples were collected from all rats. Soft X-ray imaging was used to evaluate bone union, and microcomputed tomography (micro-CT) was used for bone morphometric evaluation. Toluidine blue staining was used for the histopathological evaluation of the undecalcified specimens, and bone turnover marker levels were measured using enzyme-linked immunosorbent assay.

Results

Bone morphometry analysis via micro-CT revealed increased mineral density of the trabecular bone in the R + VD group femurs, demonstrating the effectiveness of romosozumab plus active vitamin D₃ combination therapy. However, there were no differences in bone union evaluated using soft X-ray imaging, indicating no acceleration of fracture healing.

Conclusions

Although romosozumab and active vitamin D₃ combination therapy increased trabecular bone volume, there was no evidence on its ability to accelerate fracture healing.

Graphical abstract

Interest of Bone Histomorphometry in Bone Pathophysiology Investigation: Foundation, Present, and Future

Despite the development of non-invasive methods, bone histomorphometry remains the only method to analyze bone at the tissue and cell levels. Quantitative analysis of transiliac bone sections requires strict methodologic conditions but since its foundation more 60 years ago, this methodology has progressed. Our purpose was to review the evolution of bone histomorphometry over the years and its contribution to the knowledge of bone tissue metabolism under normal and pathological conditions and the understanding of the action mechanisms of therapeutic drugs in humans. The two main applications of bone histomorphometry are the diagnosis of bone diseases and research. It is warranted for the diagnosis of mineralization defects as in osteomalacia, of other causes of osteoporosis as bone mastocytosis, or the classification of renal osteodystrophy. Bone biopsies are required in clinical trials to evaluate the safety and mechanism of action of new therapeutic agents and were applied to anti-osteoporotic agents such as bisphosphonates and denosumab, an anti-RANKL, which induces a marked reduction of the bone turnover with a consequent elongation of the mineralization period. In contrast, an increased bone turnover with an extension of the formation site is observed with teriparatide. Romosozumab, an anti-sclerostin, has a dual effect with an early increased formation and reduced resorption. Bone histomorphometric studies allow us to understand the mechanism of coupling between formation and resorption and to evaluate the respective role of bone modeling and remodeling. The adaptation of new image analysis techniques will help bone biopsy analysis in the future.

Evidence for the major contribution of remodeling-based bone formation in sclerostin-deficient mice

Bone formation by osteoblasts is achieved through remodeling-based bone formation (RBBF) and modeling-based bone formation (MBBF). The former is when bone formation occurs after osteoclastic bone resorption to maintain bone mass and calcium homeostasis. The latter is when new bone matrices are added on the quiescent bone surfaces. Administration of anti-sclerostin neutralizing antibody promotes MBBF in ovariectomized rats and postmenopausal women. However, it remains to be elucidated which mode of bone formation mainly occurs in Sost-deficient mice under physiological conditions. Here, we show that two-thirds of bone formation involves RBBF in 12-week-old Sost-deficient mice (C57BL/6 background). Micro-computed tomography and histomorphometric analyses showed that the trabecular bone mass in Sost-KO mice was higher than that in Sost^+/- mice. In contrast, the osteoclast number remained unchanged in Sost-KO mice, but the bone resorption marker TRAP5b in serum was slightly higher in those mice. Treatment with anti-RANKL antibody increased the trabecular bone mass of Sost^+/- or Sost-KO mice. Bone formation markers such as osteoid surfaces, the mineral apposition rate, and bone formation rate were almost completely suppressed in Sost^+/- mice treated with anti-RANKL antibody compared with vehicle-treated Sost^+/- mice. In Sost-KO mice, treatment with anti-RANKL antibody suppressed those parameters by more than half. These findings indicate that RBBF accounts for most of the bone formation in Sost^+/- mice, whereas approximately two-thirds of bone formation is estimated to be remodeling-based in 12-week-old Sost-deficient mice. Furthermore, anti-RANKL antibody may be useful for detecting MBBF on trabecular bone.

Nanoplatforms for Promoting Osteogenesis in Ovariectomy-Induced Osteoporosis in the Experimental Model

Background:

Osteoporosis is a debilitating bone ailment characterized by the obvious loss of bone mass and bone microarchitecture impairment.

Objective:

This study aimed to illuminate the in vivo usefulness of nanotechnology as a treatment for osteoporosis via analyzing the effectiveness of nano-hydroxyapatite (nHa), nano-hydroxy- apatite/chitosan (nHa/C), and nano-hydroxyapatite/silver (nHa/S) in mitigation of osteoporosis in ovariectomized rats.

Method:

The characterization of the nHa, nHa/C, and nHa/S was carried out using TEM, SEM, FTIR, and Zeta potential measurements. This in vivo study included 48 adult female rats that were randomized into six groups (8 rats/group): (1) Sham-operated control, (2) osteoporotic, (3) nHa, (4) nHa/C, (5) nHa/S, and (6) Fosamax®. Serum osterix level was quantified using ELISA. Femur bone morphogenetic protein 2 and SMAD1 mRNA levels were evaluated by qPCR. The femur bones were scanned by DEXA for measurement of bone mineral density and bone mineral content. In ad-dition, a histopathological examination of femur bones was performed.

Results:

The present approach denoted that the treatment with nHa, nHa/C, or nHa/S yields a signif-icant rise in serum level of osterix and mRNA levels of bone morphogenetic protein 2 and SMAD1 as well as significant enhancements of bone tissue minerals.

Conclusion:

The findings affirmed the potency of nHa, nHa/C, and nHa/S as auspicious nanoplat-forms for repairing bone defects in the osteoporotic rat model. The positive effect of the inspected nanoformulations arose from bone formation indicators in serum and tissue, and additionally, the reinforcement of bone density and content, which were verified by the histopathological description of bone tissue sections.

Combining sclerostin neutralization with tissue engineering: An improved strategy for craniofacial bone repair

Scaffolds associated with different types of mesenchymal stromal stem cells (MSC) are extensively studied for the development of novel therapies for large bone defects. Moreover, monoclonal antibodies have been recently introduced for the treatment of cancer-associated bone loss and other skeletal pathologies. In particular, antibodies against sclerostin, a key player in bone remodeling regulation, have demonstrated a real benefit for treating osteoporosis but their contribution to bone tissue-engineering remains uncharted. Here, we show that combining implantation of dense collagen hydrogels hosting wild-type (WT) murine dental pulp stem cells (mDPSC) with weekly systemic injections of a sclerostin antibody (Scl-Ab) leads to increased bone regeneration within critical size calvarial defects performed in WT mice. Furthermore, we show that bone formation is equivalent in calvarial defects in WT mice implanted with Sost knock-out (KO) mDPSC and in Sost KO mice, suggesting that the implantation of sclerostin-deficient MSC similarly promotes new bone formation than complete sclerostin deficiency. Altogether, our data demonstrate that an antibody-based therapy can potentialize tissue-engineering strategies for large craniofacial bone defects and urges the need to conduct research for antibody-enabled local inhibition of sclerostin. STATEMENT OF SIGNIFICANCE: The use of monoclonal antibodies is nowadays broadly spread for the treatment of several conditions including skeletal bone diseases. However, their use to potentialize tissue engineering constructs for bone repair remains unmet. Here, we demonstrate that the neutralization of sclerostin, through either a systemic inhibition by a monoclonal antibody or the implantation of sclerostin-deficient mesenchymal stromal stem cells (MSC) directly within the defect, improves the outcome of a tissue engineering approach, combining dense collagen hydrogels and MSC derived from the dental pulp, for the treatment of large craniofacial bone defects.

Applications of Calcium-Based Nanomaterials in Osteoporosis Treatment

With rapidly aging populations worldwide, osteoporosis has become a serious global public health problem. Caused by disordered systemic bone remodeling, osteoporosis manifests as progressive loss of bone mass and microarchitectural deterioration of bone tissue, increasing the risk of fractures and eventually leading to osteoporotic fragility fractures. As fracture risk increases, antiosteoporosis treatments transition from nonpharmacological management to pharmacological intervention, and finally to the treatment of fragility fractures. Calcium-based nanomaterials (CBNMs) have unique advantages in osteoporosis treatment because of several characteristics including similarity to natural bone, excellent biocompatibility, easy preparation and functionalization, low pH-responsive disaggregation, and inherent pro-osteogenic properties. By combining additional ingredients, CBNMs can play multiple roles to construct antiosteoporotic biomaterials with different forms. This review covers recent advances in CBNMs for osteoporosis treatment. For ease of understanding, CBNMs for antiosteoporosis treatment can be classified as locally applied CBNMs, such as implant coatings and filling materials for osteoporotic bone regeneration, and systemically administered CBNMs for antiosteoporosis treatment. Locally applied CBNMs for osteoporotic bone regeneration develop faster than the systemically administered CBNMs, an important consideration given the serious outcomes of fragility fractures. Nevertheless, many innovations in construction strategies and preparation methods have been applied to build systemically administered CBNMs. Furthermore, with increasing interest in delaying osteoporosis progression and avoiding fragility fracture occurrence, research into systemic administration of CBNMs for antiosteoporosis treatment will have more development prospects. Deep understanding of the CBNM preparation process and optimizing CBNM properties will allow for increased application of CBNMs in osteoporosis treatments in the future.

Modeling-Based Bone Formation After 2 Months of Romosozumab Treatment: Results From the FRAME Clinical Trial

The bone-forming agent romosozumab is a monoclonal antibody that inhibits sclerostin, leading to increased bone formation and decreased resorption. The highest levels of bone formation markers in human patients are observed in the first 2 months of treatment. Histomorphometric analysis of bone biopsies from the phase 3 FRAME trial (NCT01575834) showed an early significant increase in bone formation with concomitant decreased resorption. Preclinical studies demonstrated that most new bone formation after romosozumab treatment was modeling-based bone formation (MBBF). Here we analyzed bone biopsies from FRAME to assess the effect of 2 months of romosozumab versus placebo on the surface extent of MBBF and remodeling-based bone formation (RBBF). In FRAME, postmenopausal women aged ≥55 years with osteoporosis were randomized 1:1 to 210 mg romosozumab or placebo sc every month for 12 months, followed by 60 mg denosumab sc every 6 months for 12 months. Participants in the bone biopsy substudy received quadruple tetracycline labeling and underwent transiliac biopsies at month 2. A total of 29 biopsies were suitable for histomorphometry. Using fluorescence microscopy, bone formation at cancellous, endocortical, and periosteal envelopes was classified based on the appearance of underlying cement lines as modeling (smooth) or remodeling (scalloped). Data were compared using the Wilcoxon rank-sum test, without multiplicity adjustment. After 2 months, the median percentage of MBBF referent to the total bone surface was significantly increased with romosozumab versus placebo on cancellous (18.0% versus 3.8%; p = 0.005) and endocortical (36.7% versus 3.0%; p = 0.001), but not on periosteal (5.0% versus 2.0%; p = 0.37) surfaces, with no significant difference in the surface extent of RBBF on all three bone surfaces. These data show that stimulation of bone formation in the first 2 months of romosozumab treatment in postmenopausal women with osteoporosis is predominately due to increased MBBF on endocortical and cancellous surfaces. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Histochemical characteristics on minimodeling-based bone formation induced by anabolic drugs for osteoporotic treatment

Modeling, the changes of bone size and shape, often takes place at the developmental stages, whereas bone remodeling-replacing old bone with new bone-predominantly occurs in adults. Unlike bone remodeling, bone formation induced by modeling i.e., minimodeling (microscopic modeling in cancellous bone) is independent of osteoclastic bone resorption. Although recently-developed drugs for osteoporotic treatment could induce minimodeling-based bone formation in addition to remodeling-based bone formation, few reports have demonstrated the histological aspects of minimodeling-based bone formation. After administration of eldecalcitol or romosozumab, unlike teriparatide treatment, mature osteoblasts formed new bone by minimodeling, without developing thick preosteoblastic layers. The histological characteristics of minimodeling-based bone formation is quite different from remodeling, as it is not related to osteoclastic bone resorption, resulting in convex-shaped new bone and smooth cement lines called arrest lines. In this review, we will show histological properties of minimodeling-based bone formation by osteoporotic drugs.

Activation, development, and attenuation of modeling- and remodeling-based bone formation in adult rats

Activation of modeling-based bone formation (MBF - bone formation without prior activation of bone resorption), has been identified as an important mechanism by which anabolic agents, such as intermittent parathyroid hormone (PTH), rapidly elicit new bone formation. Using a novel cryohistology imaging platform, coupled with sequential multicolor fluorochrome injections, we demonstrated that MBF and remodeling-based bone formation (RBF) in the adult rat tibia model have similar contributions to trabecular bone homeostasis. PTH treatment resulted in a 2.4-4.9 fold greater bone formation rate over bone surface (BFR/BS) by RBF and a 4.3-8.5 fold greater BFR/BS by MBF in male, intact female, and ovariectomized female rats. Moreover, regardless of bone formation type, once a formation site is activated by PTH, mineral deposition continues throughout the entire treatment duration. Furthermore, by tracking the sequence of multicolor fluorochrome labels, we discovered that MBF, a highly efficient but often overlooked regenerative mechanism, is activated more rapidly but attenuated faster than RBF in response to PTH. This suggests that MBF and RBF contribute differently to PTH's anabolic effect in rats: MBF has a greater contribution to the acute elevation in bone mass at the early stage of treatment while RBF contributes to the sustained treatment effect.

Skeletal responses to romosozumab after 12 months of denosumab

Romosozumab, a monoclonal anti‐sclerostin antibody that has the dual effect of increasing bone formation and decreasing bone resorption, reduces fracture risk within 12 months. In a post hoc, exploratory analysis, we evaluated the effects of romosozumab after 12 months of denosumab in postmenopausal women with low bone mass who had not received previous osteoporosis therapy. This phase 2 trial (NCT00896532) enrolled postmenopausal women with a lumbar spine, total hip, or femoral neck T‐score ≤ −2.0 and ≥ −3.5. Individuals were randomized to placebo or various romosozumab dosing regimens from baseline to month 24, were re‐randomized to 12 months of denosumab or placebo (months 24–36), and then all received romosozumab 210 mg monthly for 12 months (months 36–48). Results for the overall population have been previously published. Here, we present results for changes in bone mineral density (BMD) and levels of procollagen type I N‐terminal propeptide (P1NP) and β‐isomer of the C‐terminal telopeptide of type I collagen (β‐CTX) from a subset of women who were randomized to placebo for 24 months, were re‐randomized to receive denosumab (n = 16) or placebo (n = 12) for 12 months, and then received romosozumab for 12 months. In women who were randomized to placebo followed by denosumab, romosozumab treatment for 12 months maintained BMD gained during denosumab treatment at the total hip (mean change from end of denosumab treatment of 0.9%) and further increased BMD gains at the lumbar spine (mean change from end of denosumab treatment of 5.3%). Upon transition to romosozumab (months 36–48), P1NP and β‐CTX levels gradually returned to baseline from their reduced values during denosumab administration. Transitioning to romosozumab after 12 months of denosumab appears to improve lumbar spine BMD and maintain total hip BMD while possibly preventing the rapid increase in levels of bone turnover markers above baseline expected upon denosumab discontinuation. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Sclerostin: from bench to bedside

Skeletal integrity is maintained by a meticulous balance between bone resorption and bone formation, and recent studies have revealed the essential role of canonical Wnt signaling pathways in maintaining skeletal homeostasis. The SOST gene, which encodes sclerostin, a member of Dan family glycoproteins, was originally identified as the gene responsible for two sclerosing bone dysplasias, sclerosteosis and van Buchem disease. Sclerostin is highly expressed by osteocytes, negatively regulates canonical Wnt signaling pathways by binding to low-density lipoprotein receptor-related protein (LRP) 5/6, and suppresses osteoblast differentiation and/or function. Romosozumab, a specific anti-sclerostin antibody, inhibits sclerostin-LRP5/6 interactions and indirectly activates canonical Wnt signaling pathways and bone formation. This review focuses on the mechanism of action of sclerostin and summarizes clinical studies that demonstrated the efficacy of romosozumab to increase bone mineral density and reduce osteoporotic fractures, as well as its cardiovascular safety.

Role of bone-forming agents in the management of osteoporosis

Recent evidence confirms the superiority of osteoanabolic therapy compared to anti-remodeling drugs for rapid improvement in bone density and fracture risk reduction, providing strong justification for the use of these anabolic agents as the initial therapy in high-risk patients, to be followed by anti-remodeling therapy. This review will highlight the results of recent studies and define the current status of osteoanabolic therapy for osteoporosis.

Osteoblastic Wnt1 regulates periosteal bone formation in adult mice

Compelling clinical data together with genetically modified mouse models have demonstrated that Wnt1 is a key Wnt ligand in bone metabolism, regulating both osteoblast activity and osteoclast differentiation. We have previously shown that deletion of Wnt1 in limb mesenchymal cells leads to severe ostepenic bone phenotype and spontaneous fractures very early after birth. However, the function of Wnt1 in mature skeleton remained unknown. To investigate the role of Wnt1 specifically in adult bone metabolism, we generated an osteoblast lineage-targeted inducible Wnt1 knockout mouse model using tetracycline-controlled Osterix-Cre mouse line (Osx-Cre). In this model, the Cre recombinase expression is suppressed by administering doxycycline (Dox) in drinking water. As expected, Wnt1_Osx^-/- mice without Dox developed spontaneous fractures early by 3 weeks of age due to severe trabecular and cortical osteopenia. Administration of Dox to Wnt1_Osx-Dox^-/- and control mice until 4 weeks of age suppressed Wnt1 deletion and completely prevented the fractures. Withdrawal of Dox led to deletion in Wnt1 allele but the fracture incidence progressively decreased in Wnt1_Osx-Dox^-/- mice at 8 or 12 weeks of age (4 and 8 weeks after Dox withdrawal). Interestingly, deletion of Wnt1 at 4 weeks of age resulted only in a modest and transient trabecular osteopenia that was more pronounced in females and was normalized by 12 weeks of age. However, diaphyseal cortical bone mass and cortical thickness in the femurs were significantly decreased in Wnt1_Osx-Dox^-/- mice of both genders. Mechanisticly, this was due to impaired periosteal bone formation. Based on our data, in addition to its essential role in early skeletal growth, Wnt1 is an important regulator of modeling-based bone formation and cortical thickness in adult mice.

Romosozumab in Postmenopausal Korean Women with Osteoporosis: A Randomized, Double-Blind, Placebo-Controlled Efficacy and Safety Study

BACKGROUND

This phase 3 study evaluated the efficacy and safety of 6-month treatment with romosozumab in Korean postmenopausal women with osteoporosis.

METHODS

Sixty-seven postmenopausal women with osteoporosis (bone mineral density [BMD] T-scores ≤-2.5 at the lumbar spine, total hip, or femoral neck) were randomized (1:1) to receive monthly subcutaneous injections of romosozumab (210 mg; n=34) or placebo (n=33) for 6 months.

RESULTS

At month 6, the difference in the least square (LS) mean percent change from baseline in lumbar spine BMD (primary efficacy endpoint) between the romosozumab (9.5%) and placebo (-0.1%) groups was significant (9.6%; 95% confidence interval, 7.6 to 11.5; P&lt;0.001). The difference in the LS mean percent change from baseline was also significant for total hip and femoral neck BMD (secondary efficacy endpoints). After treatment with romosozumab, the percent change from baseline in procollagen type 1 N-terminal propeptide transiently increased at months 1 and 3, while that in C-terminal telopeptide of type 1 collagen showed a sustained decrease. No events of cancer, hypocalcemia, injection site reaction, positively adjudicated atypical femoral fracture or osteonecrosis of the jaw, or positively adjudicated serious cardiovascular adverse events were observed. At month 9, 17.6% and 2.9% of patients in the romosozumab group developed binding and neutralizing antibodies, respectively.

CONCLUSION

Treatment with romosozumab for 6 months was well tolerated and significantly increased lumbar spine, total hip, and femoral neck BMD compared with placebo in Korean postmenopausal women with osteoporosis (ClinicalTrials.gov identifier NCT02791516).

Osteoporosis: Mechanism, Molecular Target and Current Status on Drug Development

CDATA[Osteoporosis is a pathological loss of bone mass due to an imbalance in bone remodeling where osteoclast-mediated bone resorption exceeds osteoblast-mediated bone formation resulting in skeletal fragility and fractures. Anti-resorptive agents, such as bisphosphonates and SERMs, and anabolic drugs that stimulate bone formation, including PTH analogues and sclerostin inhibitors, are current treatments for osteoporosis. Despite their efficacy, severe side effects and loss of potency may limit the long term usage of a single drug. Sequential and combinational use of current drugs, such as switching from an anabolic to an anti-resorptive agent, may provide an alternative approach. Moreover, there are novel drugs being developed against emerging new targets such as Cathepsin K and 17β-HSD2 that may have less side effects. This review will summarize the molecular mechanisms of osteoporosis, current drugs for osteoporosis treatment, and new drug development strategies.

Dual targeting of salt inducible kinases and CSF1R uncouples bone formation and bone resorption

Bone formation and resorption are typically coupled, such that the efficacy of anabolic osteoporosis treatments may be limited by bone destruction. The multi-kinase inhibitor YKL-05-099 potently inhibits salt inducible kinases (SIKs) and may represent a promising new class of bone anabolic agents. Here, we report that YKL-05-099 increases bone formation in hypogonadal female mice without increasing bone resorption. Postnatal mice with inducible, global deletion of SIK2 and SIK3 show increased bone mass, increased bone formation, and, distinct from the effects of YKL-05-099, increased bone resorption. No cell-intrinsic role of SIKs in osteoclasts was noted. In addition to blocking SIKs, YKL-05-099 also binds and inhibits CSF1R, the receptor for the osteoclastogenic cytokine M-CSF. Modeling reveals that YKL-05-099 binds to SIK2 and CSF1R in a similar manner. Dual targeting of SIK2/3 and CSF1R induces bone formation without concomitantly increasing bone resorption and thereby may overcome limitations of most current anabolic osteoporosis therapies.

Frequent administration of abaloparatide shows greater gains in bone anabolic window and bone mineral density in mice: A comparison with teriparatide

Abaloparatide (ABL) is a novel 34-amino acid peptide analog of parathyroid hormone-related protein. In clinical studies, although ABL showed a greater bone mineral density (BMD) increase than teriparatide (TPTD, human parathyroid hormone 1-34), the responses of ABL to bone formation and resorption markers were weaker, making it difficult to understand the relationship between the bone anabolic window (increase in bone formation versus resorption) and bone mass. In the present study, the effects of ABL and TPTD were compared in mice. Given that the rate of bone turnover is higher in rodents than in humans, the comparison was made with several administration regimens providing equivalent daily dosages: once daily (QD, 30 μg/kg every 24 h), twice daily (BID, 15 μg/kg every 12 h), or three times a day (TID, 10 μg/kg every 8 h). Frequent administration of ABL showed higher BMD with enhancement of trabecular and cortical bone mass and structures than that of TPTD, consistent with the clinical results seen with once daily administration. ABL increased bone formation marker levels more than TPTD with more frequent regimens, while bone resorption marker levels were not different between ABL and TPTD in all regimens. Analysis of bone histomorphometry and gene expression also suggested that ABL increased bone formation more than TPTD, while the effect on bone resorption was almost comparable between ABL and TPTD. The bone anabolic windows calculated from bone turnover markers indicated that ABL enhanced the anabolic windows more than TPTD, leading to a robust increase in BMD. The mechanism by which ABL showed a better balance of bone turnover was suggested to be partly due to the enhanced remodeling-based bone formation involved in Ephb4. Taken together, our findings would help elucidate the mechanism by which ABL shows excellent BMD gain and reduction of fractures in patients with osteoporosis.

Bone Lining Cells Could Be Sources of Bone Marrow Adipocytes

BACKGROUND

Recently, lineage-tracing studies demonstrated that parathyroid hormone and anti-sclerostin antibody (Scl-Ab) can convert bone lining cells (BLCs) into active osteoblasts. However, BLCs might also be differentiated into other lineages. Here we investigated whether BLCs could differentiate into bone marrow adipocytes (BMAds) and whether Scl-Ab could suppress this process.

METHODS

Dmp1-CreERt2:mTmG mice were injected with 0.5 mg of 4-hydroxytamoxifen once weekly from postnatal week 4 to week 8. The mice were treated with either vehicle or rosiglitazone for 8 weeks (weeks 12-20). Moreover, they were administered either vehicle or Scl-Ab (50 mg/kg) twice weekly for 4 weeks (weeks 16-20, N = 4-6/group). We chased the GFP+ cells from the endosteal surface to the bone marrow (BM) of the femur. Using immunohistochemical staining, the numbers of perilipin+ or GFP+/perilipin double+ cells in the BM were quantified. In addition, serum N-terminal propeptide of type I procollagen (P1NP) levels were measured at each time point, and bone mass was analyzed at 20 weeks using micro-computed tomography.

RESULTS

Scl-Ab administration significantly reversed the decreases in bone parameters induced by rosiglitazone. Plump GFP+ cells, presumably active osteoblasts, and extremely flat GFP+ cells, presumably BLCs, were present on the endosteal surface of the femur at 8 and 12 weeks, respectively, in line with prior findings. When we chased the GFP+ cells, rosiglitazone significantly increased the number of GFP/perilipin double+ BMAds compared to the effects of the vehicle (P < 0.001), and overlapping Scl-Ab administration decreased the number of GFP/perilipin double + BMAd compared to rosiglitazone alone (P < 0.001). In addition, we found that osteoblast lineage cells such as BLCs might express PPARγ on immunohistochemical staining. When rosiglitazone was administered to Rip-Cre:mTmG mice, GFP+ cells were not present on the endosteal surface or in the BM of the femur; however, they were present in the pancreas.

CONCLUSION

BLCs could be sources of BMAds, and rosiglitazone could stimulate the differentiation of osteoblast lineage cells into BMAds. Suppression of the differentiation of osteoblast lineage cells into BMAds might contribute to anabolic effects resulting from the pharmacologic inhibition of sclerostin.

Anti-Sclerostin Antibodies in Osteoporosis and Other Bone Diseases

The Wnt pathway is a key element of bone remodeling; its activation stimulates bone formation and inhibits bone resorption. The discovery of sclerostin, a natural antagonist of the Wnt pathway, promoted the development of romosozumab, a human monoclonal antibody directed against sclerostin, as well as other anti-sclerostin antibodies. Phase 3 studies have shown the efficacy of romosozumab in the prevention of fractures in postmenopausal women, against placebo but also against alendronate or teriparatide and this treatment also allows bone mineral density (BMD) increase in men. Romosozumab induces the uncoupling of bone remodeling, leading to both an increase in bone formation and a decrease in bone resorption during the first months of treatment. The effect is attenuated over time and reversible when stopped but transition with anti-resorbing agents allows the maintenance or reinforcement of BMD improvements. Some concerns were raised about cardiovascular events. Therefore, romosozumab was recently approved in several countries for the treatment of severe osteoporosis in postmenopausal women with high fracture risk and without a history of heart attack, myocardial infarction or stroke. This review aims to outline the role of sclerostin, the efficacy and safety of anti-sclerostin therapies and in particular romosozumab and their place in therapeutic strategies against osteoporosis or other bone diseases.

Sclerostin antibody stimulates periodontal regeneration in large alveolar bone defects

Destruction of the alveolar bone in the jaws can occur due to periodontitis, trauma or following tumor resection. Common reconstructive therapy can include the use of bone grafts with limited predictability and efficacy. Romosozumab, approved by the FDA in 2019, is a humanized sclerostin-neutralizing antibody (Scl-Ab) indicated in postmenopausal women with osteoporosis at high risk for fracture. Preclinical models show that Scl-Ab administration preserves bone volume during periodontal disease, repairs bone defects surrounding dental implants, and reverses alveolar bone loss following extraction socket remodeling. To date, there are no studies evaluating Scl-Ab to repair osseous defects around teeth or to identify the efficacy of locally-delivered Scl-Ab for targeted drug delivery. In this investigation, the use of systemically-delivered versus low dose locally-delivered Scl-Ab via poly(lactic-co-glycolic) acid (PLGA) microspheres (MSs) was compared at experimentally-created alveolar bone defects in rats. Systemic Scl-Ab administration improved bone regeneration and tended to increase cementogenesis measured by histology and microcomputed tomography, while Scl-Ab delivered by MSs did not result in enhancements in bone or cemental repair compared to MSs alone or control. In conclusion, systemic administration of Scl-Ab promotes bone and cemental regeneration while local, low dose delivery did not heal periodontal osseous defects in this study.

Bone Morphogenetic Protein-2 in Development and Bone Homeostasis

Bone morphogenetic proteins (BMPs) are multi-functional growth factors belonging to the Transforming Growth Factor-Beta (TGF-β) superfamily. These proteins are essential to many developmental processes, including cardiogenesis, neurogenesis, and osteogenesis. Specifically, within the BMP family, Bone Morphogenetic Protein-2 (BMP-2) was the first BMP to be characterized and has been well-studied. BMP-2 has important roles during embryonic development, as well as bone remodeling and homeostasis in adulthood. Some of its specific functions include digit formation and activating osteogenic genes, such as Runt-Related Transcription Factor 2 (RUNX2). Because of its diverse functions and osteogenic potential, the Food and Drug Administration (FDA) approved usage of recombinant human BMP-2 (rhBMP-2) during spinal fusion surgery, tibial shaft repair, and maxillary sinus reconstructive surgery. However, shortly after initial injections of rhBMP-2, several adverse complications were reported, and alternative therapeutics have been developed to limit these side-effects. As the clinical application of BMP-2 is largely implicated in bone, we focus primarily on its role in bone. However, we also describe briefly the role of BMP-2 in development. We then focus on the structure of BMP-2, its activation and regulation signaling pathways, BMP-2 clinical applications, and limitations of using BMP-2 as a therapeutic. Further, this review explores other potential treatments that may be useful in treating bone disorders.

Romosozumab in the treatment of osteoporosis

Osteoporosis is a disease characterized by weakening of the bone architecture, which leads to an increased risk of fracture. There has been interest in the development of osteoanabolic agents that can increase bone mass and reverse the deteriorating architecture of osteoporotic bone. Romosozumab is a new agent for osteoporosis that both promotes bone formation and inhibits bone resorption. It is a monoclonal antibody that inhibits the activity of sclerostin, which allows the Wnt pathway to promote osteoblastogenesis and inhibit the activity of bone-resorbing osteoclasts. In clinical trials, it has proven to be superior to other agents in terms of increasing bone mineral density and reducing the incidence of fractures. This review will highlight the pharmacology, clinical efficacy and safety profile of romosozumab and suggest where this medication may fit within our current management of osteoporosis.

Gene Expression Profile and Acute Gene Expression Response to Sclerostin Inhibition in Osteogenesis Imperfecta Bone

Sclerostin antibody (SclAb) therapy has been suggested as a novel therapeutic approach toward addressing the fragility phenotypic of osteogenesis imperfecta (OI). Observations of cellular and transcriptional responses to SclAb in OI have been limited to mouse models of the disorder, leaving a paucity of data on the human OI osteoblastic cellular response to the treatment. Here, we explore factors associated with response to SclAb therapy in vitro and in a novel xenograft model using OI bone tissue derived from pediatric patients. Bone isolates (approximately 2 mm3) from OI patients (OI type III, type III/IV, and type IV, n = 7; non-OI control, n = 5) were collected to media, randomly assigned to an untreated (UN), low-dose SclAb (TRL, 2.5 μg/mL), or high-dose SclAb (TRH, 25 μg/mL) group, and maintained in vitro at 37°C. Treatment occurred on days 2 and 4 and was removed on day 5 for TaqMan qPCR analysis of genes related to the Wnt pathway. A subset of bone was implanted s.c. into an athymic mouse, representing our xenograft model, and treated (25 mg/kg s.c. 2×/week for 2/4 weeks). Implanted OI bone was evaluated using μCT and histomorphometry. Expression of Wnt/Wnt-related targets varied among untreated OI bone isolates. When treated with SclAb, OI bone showed an upregulation in osteoblast and osteoblast progenitor markers, which was heterogeneous across tissue. Interestingly, the greatest magnitude of response generally corresponded to samples with low untreated expression of progenitor markers. Conversely, samples with high untreated expression of these markers showed a lower response to treatment. in vivo implanted OI bone showed a bone-forming response to SclAb via μCT, which was corroborated by histomorphometry. SclAb induced downstream Wnt targets WISP1 and TWIST1, and elicited a compensatory response in Wnt inhibitors SOST and DKK1 in OI bone with the greatest magnitude from OI cortical bone. Understanding patients' genetic, cellular, and morphological bone phenotypes may play an important role in predicting treatment response. This information may aid in clinical decision-making for pharmacological interventions designed to address fragility in OI. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

[Pharmacological characteristics and clinical study results of romosozumab (EVENITY®; genetical recombination), a drug with novel mechanism of action to treat osteoporosis at high risk of fracture]

Romosozumab (EVENITY^®) is a humanized monoclonal antibody designed to target sclerostin. Sclerostin is a glycoprotein that is secreted by osteocytes and that inhibits Wnt signaling in osteoblast lineage cells, leading to decreased bone formation by osteoblasts and increased bone resorption by osteoclasts. Romosozumab, by binding and inhibiting sclerostin, increases bone formation and decreases bone resorption. Romosozumab is known to mainly enable increase in modeling-based bone formation. In studies using ovariectomized (OVX) models of rats and cynomolgus monkeys, those administered romosozumab showed dose-dependently increased bone mass and strength. In addition, the bone-forming effect of romosozumab decreased after continued administration. In rats, romosozumab caused almost no focal osteoblast hyperplasia or benign or malignant bone tumors, presumably owing to the time-dependent decrease in the bone-forming effect. Clinical studies demonstrated inhibition of new vertebral fractures at 12 months of treatment, and increased bone mineral density at 6 months of treatment. With a dual effect on bone, increasing bone formation and decreasing bone resorption, romosozumab is expected to become a new treatment option, and was approved in January 2019 for the indication of "patients with osteoporosis at high risk for fracture".

Modeling-Based Bone Formation in the Human Femoral Neck in Subjects Treated With Denosumab

Denosumab is associated with continued gains in hip and spine BMD with up to 10 years of treatment in postmenopausal women with osteoporosis. Despite potent inhibition of bone remodeling, findings in nonhuman primates suggest modeling-based bone formation (MBBF) may persist during denosumab treatment. This study assessed whether MBBF in the femoral neck (FN) is preserved in the context of inhibited remodeling in subjects receiving denosumab. This open-label study enrolled postmenopausal women with osteoporosis who had received two or more doses of denosumab (60 mg subcutaneously every 6 months [Q6M]) per standard of care and were planning elective total hip replacement (THR) owing to osteoarthritis of the hip. Transverse sections of the FN were obtained after THR and analyzed histomorphometrically. MBBF, based on fluorochrome labeling and presence of smooth cement lines, was evaluated in cancellous, endocortical, and periosteal envelopes of the FN. Histomorphometric parameters were used to assess MBBF and remodeling-based bone formation (RBBF) in denosumab-treated subjects (n = 4; mean age = 73.5 years; range, 70 to 78 years) and historical female controls (n = 11; mean age = 67.8 years; range, 62 to 80 years) obtained from the placebo group of a prior study and not treated with denosumab. All analyses were descriptive. All subjects in both groups exhibited MBBF in the periosteal envelope; in cancellous and endocortical envelopes, all denosumab-treated subjects and 81.8% of controls showed evidence of MBBF. Compared with controls, denosumab-treated subjects showed 9.4-fold and 2.0-fold higher mean values of MBBF in cancellous and endocortical envelopes, respectively, whereas RBBF mean values were 5.0-fold and 5.3-fold lower. In the periosteal envelope, MBBF and RBBF rates were similar between subjects and controls. These results demonstrate the occurrence of MBBF in the human FN and suggest that denosumab preserves MBBF while inhibiting remodeling, which may contribute to the observed continued gains in BMD over time after remodeling is maximally inhibited. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

Anabolic Therapy and Optimal Treatment Sequences for Patients With Osteoporosis at High Risk for Fracture

OBJECTIVE

Provide an update regarding anabolic medications for osteoporosis, which are often considered to be the last resort for patients with osteoporosis, after multiple fractures have already occurred and other medications have already been administered.

METHODS

Literature review and discussion.

RESULTS

Recent pivotal trial data for anabolic agents and randomized trials comparing anabolic and antiresorptive medications suggest that three anabolic agents (teriparatide, abaloparatide, and romosozumab) reduce nonvertebral and vertebral fractures faster and to a greater extent than potent antiresorptive treatments. Furthermore, bone density accrual is maximized when patients are given anabolic agents first, followed by potent antiresorptive therapy. Since total hip bone density during or after osteoporosis treatment has emerged as an excellent surrogate for future fracture risk, attaining a greater hip bone mineral density is a treatment goal for high-risk osteoporosis patients.

CONCLUSION

This review defines the highest-risk patients and summarizes the rationale for the evolving role of anabolic therapy in the management of postmenopausal women at high risk for fracture.

ABBREVIATIONS

ACTIVE = Abaloparatide Comparator Trial in Vertebral Endpoints; ARCH = Active Controlled Fracture Study in Postmenopausal Women with Osteoporosis at High Risk; BMD = bone mineral density; FRAME = Fracture Study in Postmenopausal Women with Osteoporosis; FRAX = Fracture Risk Assessment Tool; PTH = parathyroid hormone; TBS = trabecular bone score.

Sclerostin-Antibody Treatment Decreases Fracture Rates in Axial Skeleton and Improves the Skeletal Phenotype in Growing oim/oim Mice

In osteogenesis imperfecta (OI), vertebrae brittleness causes thorax deformations and leads to cardiopulmonary failure. As sclerostin-neutralizing antibodies increase bone mass and strength in animal models of osteoporosis, their administration in two murine models of severe OI enhanced the strength of vertebrae in growing female Crtap^-/- mice but not in growing male Col1a1^Jrt/+ mice. However, these two studies ignored the impact of antibodies on spine growth, fracture rates, and compressive mechanical properties. Here, we conducted a randomized controlled trial in oim/oim mice, an established model of human severe OI type III due to a mutation in Col1a2. Five-week-old female WT and oim/oim mice received either PBS or sclerostin antibody (Scl-Ab) for 9 weeks. Analyses included radiography, histomorphometry, pQCT, microcomputed tomography, and biomechanical testing. Though it did not modify vertebral axial growth, Scl-Ab treatment markedly reduced the fracture prevalence in the pelvis and caudal vertebrae, enhanced osteoblast activity (L4), increased cervico-sacral spine BMD, and improved the lumbosacral spine bone cross-sectional area. Scl-Ab did not impact vertebral height and body size but enhanced the cortical thickness and trabecular bone volume significantly in the two Scl-Ab groups. At lumbar vertebrae and tibial metaphysis, the absolute increase in cortical and trabecular bone mass was higher in Scl-Ab WT than in Scl-Ab oim/oim. The effects on trabecular bone mass were mainly due to changes in trabecular number at vertebrae and in trabecular thickness at metaphyses. Additionally, Scl-Ab did not restore a standard trabecular network, but improved bone compressive ultimate load with more robust effects at vertebrae than at metaphysis. Overall, Scl-Ab treatment may be beneficial for reducing vertebral fractures and spine deformities in patients with severe OI.

The Role of Sclerostin in Bone and Ectopic Calcification

Sclerostin, a 22-kDa glycoprotein that is mainly secreted by the osteocytes, is a soluble inhibitor of canonical Wnt signaling. Therefore, when present at increased concentrations, it leads to an increased bone resorption and decreased bone formation. Serum sclerostin levels are known to be increased in the elderly and in patients with chronic kidney disease. In these patient populations, there is a high incidence of ectopic cardiovascular calcification. These calcifications are strongly associated with cardiovascular morbidity and mortality. Although data are still controversial, it is likely that there is a link between ectopic calcification and serum sclerostin levels. The main question, however, remains whether sclerostin exerts either a protective or deleterious role in the ectopic calcification process.

Overview of treatment approaches to osteoporosis

Efficient therapies are available for the treatment of osteoporosis. Anti-resorptive therapies, including bisphosphonates and denosumab, increase bone mineral density (BMD) and reduce the risk of fractures by 20-70%. Bone-forming or dual-action treatments stimulate bone formation and increase BMD more than the anti-resorptive therapies. Two studies have demonstrated that these treatments are superior to anti-resorptives in preventing fractures in patients with severe osteoporosis. Bone-forming or dual-action treatments should be followed by anti-resorptive treatment to maintain the fracture risk reduction. The BMD gains seen with bone-forming and dual-action treatments are greater in treatment-naïve patients compared to patients pretreated with anti-resorptive treatments. However, the antifracture efficacy seems to be preserved. Treatment failure will often lead to switch of treatment from orally to parentally administrated anti-resorptives treatment or from anti-resorptive to bone-forming or dual-action treatment. Osteoporosis is a chronic condition and therefore needs a long-term management plan with a personalized approach to treatment. LINKED ARTICLES: This article is part of a themed issue on The molecular pharmacology of bone and cancer-related bone diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.9/issuetoc.

New perspectives on the treatment of skeletal dysplasia

The last few decades have been marked by the identification of numerous genes implicated in genetic disorders, helping in the elucidation of the underlying pathophysiology of these conditions. This has allowed new therapeutic approaches to emerge such as cellular therapy, gene therapy, or pharmacological therapy for various conditions. Skeletal dysplasias are good models to illustrate these scientific advances. Indeed, several therapeutic strategies are currently being investigated in osteogenesis imperfecta; there are ongoing clinical trials based on pharmacological approaches, targeting signaling pathways in achondroplasia and fibrodysplasia ossificans progressiva or the endoplasmic reticulum stress in metaphyseal dysplasia type Schmid or pseudoachondroplasia. Moreover, the treatment of hypophosphatasia or Morquio A disease illustrates the efficacy of enzyme drug replacement. To provide a highly specialized multidisciplinary approach, these treatments are managed by reference centers. The emergence of treatments in skeletal dysplasia provides new perspectives on the prognosis of these severe conditions and may change prenatal counseling in these diseases over the coming years.

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.