Sites Of Bone Remodeling Research Articles

Enhancing osteoporosis treatment using a targeted, sustained-release drug delivery system based on macrocyclic amphiphile

Osteoporosis, a prevalent systemic bone metabolic disorder, primarily affects postmenopausal women and is characterized by increased bone fragility and a heightened risk of fractures. The efficacy of current osteoporosis treatments is often limited by non-specific drug targeting and undesirable off-target skeletal side effects. To address this challenge, we have developed a novel hydroxyapatite-responsive drug delivery system. This system utilizes a self-assembled p-phosphonatocalix[4]arene tetradodecyl ether (PC4A12C), engineered to specifically target and sustain the release of osteoporosis medication at sites of bone remodeling. Our focus centers on icariin (ICA), a drug known for its potent osteogenic properties and minimal adverse effects. In vitro, ICA-loaded PC4A12C (ICA@PC4A12C) demonstrated enhanced proliferation, differentiation, and mineralization in bone marrow mesenchymal stem cells (BMSCs). In vivo, ICA@PC4A12C exhibited superior efficacy in specifically targeting bone tissue, ensuring a controlled and slow release of icariin directly within the bone environment. In an osteoporosis mouse model, treatment with ICA@PC4A12C showed notable enhancement in osteogenic activity and a significant increase in bone density compared to ICA alone. These results demonstrate the potential of PC4A12C as an effective drug carrier in the development of advanced antiosteoporotic drug delivery systems.

3D printed O2-generating scaffolds enhance osteoprogenitor- and type H vessel recruitment during bone healing

Oxygen (O2)-delivering tissue substitutes have shown tremendous potential for enhancing tissue regeneration, maturation, and healing. As O2 is both a metabolite and powerful signaling molecule, providing controlled delivery is crucial for optimizing its beneficial effects in the treatment of critical-sized injuries. Here, we report the design and fabrication of 3D-printed, biodegradable, O2-generating bone scaffold comprising calcium peroxide (CPO) that once hydrolytically activated, provides long-term generation of oxygen at a controlled, concentration-dependent manner, and polycaprolactone (PCL), a hydrophobic polymer that regulate the interaction of CPO with water, preventing burst release of O2 at early time points. When anoxic conditions were simulated in vitro, CPO-PCL scaffolds maintained the survival and proliferation of human adipose-derived stem/stromal cells (hASCs) relative to PCL-only controls. We assessed the in vivo osteogenic efficacy of hASC-seeded CPO-PCL scaffolds implanted in a non-healing critical-sized 4-mm calvarial defects in nude mice for 8 weeks. Even without exogenous osteoinductive factors, CPO-PCL scaffolds demonstrated increased new bone volume compared to PCL-only scaffolds as verified by both microcomputed tomography analysis and histological assessments. Lastly, we employed a quantitative 3D lightsheet microscopy platform to determine that O2-generating scaffolds had similar vascular volumes with slightly higher presence of CD31hiEmcnhi pro-osteogenic, type H vessels and increased number of Osterix+ skeletal progenitor cells relative to PCL-only scaffolds. In summary, 3D-printed O2 generating CPO-PCL scaffolds with tunable O2 release rates provide a facile, customizable strategy for effectively treating, craniofacial bone defects. Statement of significanceOxygen(O2)-delivering bone substitutes show promise in defect repair applications by supplying O2 to the cells within or around the graft, improving cell survivability and enhancing bone matrix mineralization. A novel O2-generating bone scaffold has been 3D printed for the first-time which ensures patient and defect specificity. 3D printed calcium peroxide-polycaprolactone (CPO-PCL) bone scaffold provides uninterrupted O2 supply for 22 days allowing cell survival in deprived O2 and nutrient conditions. For the first time, O2-driven bone regenerative environment in mice calvaria has been captured by light-sheet imaging which illuminates abundance of Osterix+ cells, angiogenesis at a single cell resolution indicating active site of bone remodeling and growth in the presence of O2.

Breaking Osteoclast-Acid Vicious Cycle to Rescue Osteoporosis via an Acid Responsive Organic Framework-Based Neutralizing and Gene Editing Platform.

In the osteoporotic microenvironment, the acidic microenvironment generated by excessive osteoclasts not only causes irreversible bone mineral dissolution, but also promotes reactive oxygen species (ROS) production to induce osteoblast senescence and excessive receptor activator of nuclear factor kappa-B ligand (RANKL) production, which help to generate more osteoclasts. Hence, targeting the acidic microenvironment and RANKL production may break this vicious cycle to rescue osteoporosis. To achieve this, an acid-responsive and neutralizing system with high in vivo gene editing capacity is developed by loading sodium bicarbonate (NaHCO3) and RANKL-CRISPR/Cas9(RC) plasmid in a metal-organic framework. This results showed ZIF8-NaHCO3@Cas9 (ZNC) effective neutralized acidic microenvironment and inhibited ROS production . Surprisingly, nanoparticles loaded with NaHCO3 and plasmids show higher transfection efficiency in the acidic environments as compared to the ones loaded with plasmid only. Finally, micro-CT proves complete reversal of bone volume in ovariectomized mice after ZNC injection into the bone remodeling site. Overall, the newly developed nanoparticles show strong effect in neutralizing the acidic microenvironment to achieve bone protection through promoting osteogenesis and inhibiting osteolysis in a bidirectional manner. This study provides new insights into the treatment of osteoporosis for biomedical and clinical therapies.

Targeting ROS-induced osteoblast senescence and RANKL production by Prussian blue nanozyme based gene editing platform to reverse osteoporosis

Blocking the reactive oxygen species (ROS) induced osteoblast senescence and massive RANKL release is the key to alleviate the vicious osteoporotic micro-environment in aged skeleton. In this study, we constructed a composite nanoparticle by confining the growth of Prussian blue nanozyme (PBzyme) on the surface of a sub-50 nm hollow mesoporous silica nanoparticles (HPB), which presented strong ROS scavenging ability. The abundant mesoporous structure made it available to combine receptor activator of nuclear factor kappa-B ligand (RANKL) CRISPR/Cas9 plasmid and skeletal targeting agent alendronate (HPB@RC-ALN) together to modulate the osteoporotic micro-environment. Our results indicate HPB@RC-ALN confined ROS generation and alleviated osteoblast senescence as revealed by dihydroethidium and senescence-associated β-galactosidase staining. The rejuvenated osteoblasts presented robust osteogenic ability as shown by alkaline phosphatase and alizarin red staining in vitro, as well as the micro-computed tomography (μCT) in vivo. Meanwhile, efficient transfection of CRISPR/Cas9 plasmid by HPB@RC-ALN precisely achieved RANKL gene editing target and knocked the RANKL gene out in osteoblasts as confirmed by T7E1 digestion and western blot assays. Hence, the osteoclast formation was significantly suppressed as shutting down the RANKL production by HPB@RC-ALN in the senescent osteoblast. At last, the μCT revealed fully reversed bone volume in ovariectomized mice after HPB@RC-ALN injection into the bone remodeling site. Overall, the constructed nanozyme based gene editing platform achieved bidirectional regulation of osteoblast-osteoclast through rescuing cellular senescence and blocking RANKL production. This strategy provides a new theoretical base for osteoporosis management.

Spatial Organization of Osteoclastic Coupling Factors and Their Receptors at Human Bone Remodeling Sites.

The strictly regulated bone remodeling process ensures that osteoblastic bone formation is coupled to osteoclastic bone resorption. This coupling is regulated by a panel of coupling factors, including clastokines promoting the recruitment, expansion, and differentiation of osteoprogenitor cells within the eroded cavity. The osteoprogenitor cells on eroded surfaces are called reversal cells. They are intermixed with osteoclasts and become bone-forming osteoblast when reaching a critical density and maturity. Several coupling factors have been proposed in the literature, but their effects and expression pattern vary between studies depending on species and experimental setup. In this study, we investigated the mRNA levels of proposed secreted and membrane-bound coupling factors and their receptors in cortical bone remodeling events within the femur of healthy adolescent human controls using high-sensitivity RNA in situ hybridization. Of the proposed coupling factors, human osteoclasts showed mRNA-presence of LIF, PDGFB, SEMA4D, but no presence of EFNB2, and OSM. On the other hand, the osteoblastic reversal cells proximate to osteoclasts presented with LIFR, PDGFRA and PLXNB1, but not PDGFRB, which are all known receptors of the proposed coupling factors. Although EFNB2 was not present in mature osteoclasts, the mRNA of the ligand-receptor pair EFNB2:EPHB4 were abundant near the central blood vessels within intracortical pores with active remodeling. EPHB4 and SEMA4D were also abundant in mature bone-forming osteoblasts. This study highlights that especially LIF:LIFR, PDGFB:PDGFRA, SEMA4D:PLXNB1 may play a critical role in the osteoclast-osteoblast coupling in human remodeling events, as they are expressed within the critical cells.

IL-27 Modulates the Cytokine Secretion in the T Cell-Osteoclast Crosstalk During HIV Infection.

In People with HIV (PWH), chronic immune activation and systemic inflammation are associated with increased risk to develop comorbidities including bone loss. Numerous cells of the immune system, namely, T cells are involved in the regulation of the bone homeostasis and osteoclasts (OCs) activity. IL-27, a cytokine that belongs to the IL-12 family can regulate the secretion of pro- and anti-inflammatory cytokines by T cells, however its role in the setting of HIV is largely unknown. In the present study, we determined the impact of OCs in T cell secretion of cytokines and whether IL-27 can regulate this function. We found that the presence of OCs in the T cell cultures significantly enhanced secretion of IFNγ, TNFα, IL-17, RANKL, and IL-10 in both PWH and healthy controls. In PWH, IL-27 inhibited IL-17 secretion and downregulated surface expression of RANKL in CD4 T cells. All together these results suggest that in the context of HIV infection IL-27 may favor IFNγ and TNFα secretion at the sites of bone remodeling.

Optimal control for a bone metastasis with radiotherapy model using a linear objective functional

Radiation is known to cause genetic damage to highly proliferative cells such as cancer cells. However, the radiotherapy effects to bone cells is not completely known. In this work we present a mathematical modeling framework to test hypotheses related to the radiation-induced effects on bone metastasis. Thus, we pose an optimal control problem based on a Komarova model describing the interactions between cancer cells and bone cells at a single site of bone remodeling. The radiotherapy treatment is included in the form of a functional which minimizes the use of radiation using a penalty function. Moreover, we are interested to model the ‘on’ and the ‘off’ time states of the radiation schedules; so we propose an optimal control problem with a L 1-type objective functional. Bang-bang or singular arc solutions are the obtained optimal control solutions. We characterize both solutions types and explicitly give necessary optimality conditions for them. We present numerical simulations to analyze the different possible radiation effects on the bone and cancer cells. We also evaluate the more significant parameters to shift from a bang-bang solution to a singular arc solution and vice versa. Additionally, we study a fractionated radiotherapy model that yields an output solution that resembles intermittent radiotherapy scheduling.

Influence of Bisphosphonate Drugs on Oral Rehabilitation with Implants

<p style="text-align: justify;">Bisphosphonate (BF) is a drug used to treat some bone pathologies, such as osteoporosis, multiple myeloma, Paget&rsquo;s disease, and to treat bone metastasis. These drugs increase the risk of osteonecrosis, where several factors are considered to be risk factors for this, including surgery for placing dental implants and tooth extraction. The objective of this present study was to conduct a literature review on the emergence of osteonecrosis in patients who are or have used BFand have undergone dental implant surgery. A literature review was carried out using the PUBMED, LILACS, Bireme and Scielo databases. To carry out this research, 24 articles were selected between the years 2008 and 2020, which were related to the topic of the use of BF in dental implantology. There is no clear relationship between BF and osseointegration of dental implants. Studies show that BFs increase the mechanical stability of implants if used locally, but they can also increase the risk of osteonecrosis of the jaw if used systemically. BFs have different effects at bone repair sites and bone remodeling sites. It is known that bone can be formed at the site of repair, but this drug will decrease bone formation to a similar level of bone resorption at the site of remodeling. Despite the risk, implant surgery is not contraindicated in patients undergoing oral treatment. The rate of osteonecrosis related to the use of BF is low, even so the professional must consider the risk and be cautious to prevent this from happening. <p style="text-align: justify;"><strong>Key words:</strong> Osseointegration, Dental implants, Bisphosphonates, Osteonecrosis.

Improved Radium-223 Therapy with Combination Epithelial Sodium Channel Blockade.

Background: Radium-223 dichloride ([223Ra]RaCl2) is the first approved alpha particle-emitting therapy and is indicated for treatment of bone metastatic castrate resistant prostate cancer. Approximately half of the dose is absorbed into the gastrointestinal (GI) tract within minutes of administration, limiting disease-site uptake and contributing to toxicity. Here, we investigate the role of enteric ion channels and their modulation for improved therapeutic efficacy and reduced side effects. Methods: Utilizing primary human duodenal organoids (enteroids) as in vitro models of the functional GI epithelium, we found that Amiloride (ENaC blocker) and NS-1619 (K+ channel activator) presented significant effects in 223Ra membranal transport. The radioactive drug distribution was evaluated for lead combinations in vivo, and in osteosarcoma and prostate cancer models. Results: Amiloride shifted 223Ra uptake in vivo from the gut, to nearly double the uptake at sites of bone remodeling. Bone tumor growth inhibition with the combination as measured by bioluminescent and X-ray imaging was significantly greater than single agents alone, and the combination resulted in no weight loss. Conclusion: This combination of approved agents may be readily implemented as a clinical approach to improve outcomes of bone metastatic cancer patients with the benefit of ameliorated tolerability.

A Critical Role of the Bone Marrow Envelope in Human Bone Remodeling.

Proper bone remodeling depends not only on a team of bone-resorbing osteoclasts and bone-forming osteoblasts. It also depends on the site-specific delivery of a large amount of osteoblast lineage cells to the bone remodeling site. How this delivery occurs is poorly known. Here, we gained insight into this mechanism by analyzing the distribution of markers of osteoblastogenesis on bone surfaces and in their bone marrow neighborhood in human cancellous bone. We found a CD271-positive/PDGFβ-R-positive cell layer surrounding the bone marrow that provides osteoblastogenic potential along all bone surfaces, whether quiescent or remodeling. This bone marrow envelope cell layer takes the appearance of a canopy above remodeling sites, where it then also shows an upregulation of the proliferation marker Ki67, smooth muscle actin (SMA), tenascin C, fibronectin, and MMP13. This indicates that the canopy is a region of the bone marrow envelope where early markers of osteoblastogenesis are activated concurrently with initiation of bone remodeling. Importantly, the high proliferation index in the canopy is not associated with increasing cell densities at the canopy level, but it is at the bone surface level, thereby supporting delivery of cells from the canopy to the bone surface. This delivery route explains why lack of canopies was previously found to coincide with lack of bone formation, and fits current knowledge on the canopies as a target for regulators of bone remodeling. We conclude that the coordination of bone marrow envelope activities and bone surface activities allows integrating osteoblastogenesis and bone remodeling into the same functional unit, and propose that the bone marrow envelope is critical for preserving bone health. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Immunocytochemical assessment of cell differentiation of podoplanin-positive osteoblasts into osteocytes in murine bone

In this study, we examined the immunolocalization of podoplanin/E11, CD44, actin filaments, and phosphorylated ezrin in the osteoblasts on the verge of differentiating into osteocytes in murine femora and tibiae. When observing under stimulated emission depletion microscopy, unlike podoplanin-negative osteoblasts, podoplanin-positive osteoblasts showed a rearranged assembly of actin filaments along the cell membranes which resembled that of embedded osteocytes. In the metaphysis, i.e., the bone remodeling site, CD44-bearing osteoclasts were either proximal to or in contact with podoplanin-positive osteoblasts, but the podoplanin-positive osteoblasts also localized CD44 on their own cell surface. These podoplanin-positive osteoblasts, which either possessed CD44 on their cell surface or were close to CD44-bearing osteoclasts, showed phosphorylated ezrin-positivity on the cell membranes. Therefore, the CD44/podoplanin interaction on the cell surface may be involved in the osteoblastic differentiation into osteocytes in the metaphyses, via the mediation of podoplanin-driven ezrin phosphorylation and the subsequent reorganized assembly of actin filaments. Consistently, the protein expression of phosphorylated ezrin was increased after CD44 administration in calvarial culture. Conversely, in modeling sites such as the cortical bones, podoplanin-positive osteoblasts were uniformly localized at certain intervals even without contact with CD44-positive bone marrow cells; furthermore, they also exhibited phosphorylated ezrin immunoreactivity along their cell membranes. Taken together, it seems likely that the CD44/podoplanin interaction is involved in osteoblastic differentiation into osteocytes in the bone remodeling area but not in modeling sites.

The primary cilium directs osteopontin-induced migration of mesenchymal stem cells by regulating CD44 signaling and Cdc42 activation.

The primary cilium acts as a sensory organelle with diverse receptors and ion channels to detect extracellular cues and regulate cellular functions, including cell migration. The migration of mesenchymal stem cells (MSCs) to bone remodeling sites is important for bone homeostasis. Recently, we have suggested that osteopontin (OPN) is a significant chemoattractant in MSC migration to bone remodeling sites. The objective of this study was to determine whether the primary cilium acts as a chemoattractant sensory unit to detect OPN cues and control MSC migration. We found that the loss of primary cilium induced by silencing of IFT88 reduced OPN-induced migration of MSCs. The effect of IFT88 silencing on cellular attachment, spreading, and proliferation was negligible. The loss of primary cilium did not affect the level of integrinβ1 or CD44, two known receptors for OPN. Interestingly, CD44 was localized to the primary cilium by OPN stimulus. Knockdown of IFT88 or CD44 dysregulated OPN-induced signaling activation and abolished OPN-induced Cdc42 activation. Our findings suggest that the primary cilium acts as a chemoattractant sensor for OPN to regulate MSC migration by controlling not only CD44-mediated OPN signaling, but also Cdc42-mediated actin cytoskeleton rearrangement.

Preclinical Single Photon Emission Computed Tomography of Alpha Particle-Emitting Radium-223.

Objective: Dose optimization and pharmacokinetic evaluation of α-particle emitting radium-223 dichloride (223RaCl2) by planar γ-camera or single photon emission computed tomography (SPECT) imaging are hampered by the low photon abundance and injected activities. In this study, we demonstrate SPECT of 223Ra using phantoms and small animal in vivo models. Methods: Line phantoms and mice bearing 223Ra were imaged using a dedicated small animal SPECT by detecting the low-energy photon emissions from 223Ra. Localization of the therapeutic agent was verified by whole-body and whole-limb autoradiography and its radiobiological effect confirmed by immunofluorescence. Results: A state-of-the-art commercial small animal SPECT system equipped with a highly sensitive collimator enables collection of sufficient counts for three-dimensional reconstruction at reasonable administered activities and acquisition times. Line sources of 223Ra in both air and in a water scattering phantom gave a line spread function with a full-width-at-half-maximum of 1.45 mm. Early and late-phase imaging of the pharmacokinetics of the radiopharmaceutical were captured. Uptake at sites of active bone remodeling was correlated with DNA damage from the α particle emissions. Conclusions: This work demonstrates the capability to noninvasively define the distribution of 223RaCl2, a recently approved α-particle-emitting radionuclide. This approach allows quantitative assessment of 223Ra distribution and may assist radiation-dose optimization strategies to improve therapeutic response and ultimately to enable personalized treatment planning.

Cell Condensation Triggers the Differentiation of Osteoblast Precursor Cells to Osteocyte-Like Cells.

Though the three-dimensional (3D) in vitro culture system has received attention as a powerful tool for conducting biological research, in vitro bone formation and osteocyte differentiation studies have mostly been based on results obtained using two-dimensional (2D) culture systems. Here, we introduced a rotatory culture system to fabricate 3D spheroids, using mouse osteoblast precursor cells. These spheroids, incubated for 2 days without chemical induction by osteogenic supplements, exhibited notably up-regulated osteocyte marker levels; osteoblast marker levels were down-regulated, as compared to those of the conventional 2D monolayer model. The cell condensation achieved with the 3D spheroid structure triggered a greater level of differentiation of osteoblast precursor cells into osteocyte-like cells than that observed during chemical induction. Our study might imply that osteoblasts proliferate and become condensed at the targeted bone remodeling site, because of which osteoblasts achieved the capability to differentiate into osteocytes in vivo.

The Established Nuclear Medicine Modalities for Imaging of Bone Metastases

The skeleton is one of the frequent site of metastases in advanced cancer. Prostate, breast and renal cancers mostly metastasize to bone. Malignant tumors lead to significant morbidity and mortality. Identification of bone lesions is a crucial step in diagnosis of disease at early stage, monitoring of disease progression and evaluation of therapy. Diagnosis of cancer metastases is based on uptake of bone-targeted radioactive tracer at different bone remodeling sites. This manuscript summarizes already established and evolving nuclear medicine modalities (e.g. bone scan, SPECT, SPECT/CT, PET, PET/CT) for imaging of bone metastases.

Osteoclast-derived SLIT3 is a coupling factor linking bone resorption to bone formation.

We identified osteoclast-derived SLIT3 as a new coupling factor using fractionated secretomics. Coupling links bone resorption to bone formation. SLIT3 stimulated the recruitment and proliferation of osteoblasts into bone remodeling sites via activation of β-catenin. Autocrine signaling by SLIT3 also inhibited bone resorption by suppressing the fusion and differentiation of pre-osteoclasts. All mice lacking Slit3 or its receptor Robo1 showed an osteopenic phenotype with low bone formation and high bone resorption. A small truncated recombinant SLIT3 protein increased bone mass in an osteopenic mouse model. These results suggest that SLIT3 is a novel therapeutic target in metabolic bone diseases.

Least-detectable and age-related local in vivo bone remodelling assessed by time-lapse HR-pQCT.

We previously developed an image analysis approach for the determination of local sites of bone remodelling using time-lapse in vivo HR-pQCT. The involved image filtering for removing noise was chosen rather aggressively, and also removed some effects of the bone remodelling. In this paper, we quantify these filtering settings using ex vivo reproducibility HR-pQCT images, and determine the least-detectable bone remodelling using in vivo reproducibility HR-pQCT images, as well as testing whether the approach is capable of capturing age-related bone remodelling by use of in vivo long-term HR-pQCT images. We found that a threshold value of 225 mg HA/cm3 for the filtering led to acceptable results with falsely determined bone remodelling of less than 0.5%, and that the least-detectable bone formation and bone resorption are 2.0 ± 1.0% and 2.2 ± 0.7% respectively. We also found that age-related local bone remodelling can be captured satisfactorily in postmenopausal women. The latter revealed new insights into the effect of ageing on bone remodelling, and showed that bone remodelling seems to take place through a few small formation packets and many large resorption volumes leading to a net bone loss. We conclude that local in vivo bone remodelling can be successfully assessed with time-lapse in vivo HR-pQCT capable of assessing age-related changes in bone remodelling.

Multivalent Presentation of Peptide Targeting Groups Alters Polymer Biodistribution to Target Tissues.

Drug delivery to bone is challenging, whereby drug distribution is commonly <1% of injected dose, despite development of several bone-targeted drug delivery systems specific to hydroxyapatite. These bone-targeted drug delivery systems still suffer from poor target cell localization within bone, as at any given time overall bone volume is far greater than acutely remodeling bone volume, which harbors relevant cell targets (osteoclasts or osteoblasts). Thus, there exists a need to target bone-acting drugs specifically to sites of bone remodeling. To address this need, this study synthesized oligo(ethylene glycol) copolymers based on a peptide with high affinity to tartrate-resistant acid phosphatase (TRAP), an enzyme deposited by osteoclasts during the bone resorption phase of bone remodeling, which provides greater specificity relevant for bone cell drugging. Gradient and random peptide orientations, as well as polymer molecular weights, were investigated. TRAP-targeted, high molecular weight (Mn) random copolymers exhibited superior accumulation in remodeling bone, where fracture accumulation was observed for at least 1 week and accounted for 14% of tissue distribution. Intermediate and low Mn random copolymer accumulation was lower, indicating residence time depends on Mn. High Mn gradient polymers were cleared, with only 2% persisting at fractures after 1 week, suggesting TRAP binding depends on peptide density. Peptide density and Mn are easily modified in this versatile targeting platform, which can be applied to a range of bone drug delivery applications.

Kinetic reconstruction reveals time-dependent effects of romosozumab on bone formation and osteoblast function in vertebral cancellous and cortical bone in cynomolgus monkeys

Romosozumab, a humanized monoclonal sclerostin antibody under development for the treatment of osteoporosis, has a unique mechanism of action on bone—increasing bone formation and decreasing bone resorption. The effects on bone formation are transient, eliciting a rapid increase in bone formation that attenuates with continued treatment. Although bone formation attenuates, bone mineral density (BMD) continues to increase. To explore potential tissue-level mechanisms that could contribute to a progressive increase in spine BMD, we used kinetic reconstruction techniques to examine the effects of romosozumab on modeling and remodeling units in vertebral cancellous bone from adult cynomolgus monkeys administered romosozumab for 10 and 28weeks.The 10-week study duration captured a period of high modeling-based bone formation, and the 28-week study duration followed the self-regulation or attenuation of bone formation in cancellous bone that occurs with long-term treatment. Sequential fluorochrome labels applied for the kinetic reconstruction were also used to evaluate treatment effects on osteoblast function as early as 3weeks, and on bone formation and bone accrual in the vertebral cortex over 28weeks.Kinetic reconstruction of remodeling and modeling formation sites in vertebral cancellous bone revealed that romosozumab effected significant transient increases in mineral apposition rate in remodeling sites at week 3 that was not sustained with continued treatment. However, romosozumab treatment caused sustained improvement in fractional labeling of osteoid, an index of osteoblast efficiency, at remodeling formative sites at both weeks 10 and 28 that was the major contributor to significant increases in final wall thickness (W.Th) of remodeling packets. Remodeling W.Th matched the final W.Th of modeling packets at week 10. At both weeks 10 and 28, romosozumab significantly decreased eroded surface (ES/BS). At week 28, romosozumab also significantly reduced resorption period (Rs.P) and final resorption depth (Rs.De). The reduced final Rs.De combined with the increased W.Th resulted in a significant increase in bone balance (BB) at the level of the remodeling unit. Assessment of bone formation on the vertebral periosteal and endocortical surfaces following 28weeks of treatment revealed that romosozumab significantly increased bone formation on these surfaces, which had attenuated by week 28, resulting in significant increases in new periosteal and endocortical bone by week 28.These data suggest that multiple factors potentially contribute to the increase in spine BMD with romosozumab treatment. In the early period of treatment, increased modeling-based bone formation, increased W.Th at remodeling sites, a decrease in remodeling space secondary to decreased ES/BS in vertebral cancellous bone, and increased periosteal and endocortical bone formation in the vertebral cortex contribute to the early increase in spine BMD. Following the self-regulation of bone formation when modeling-based bone formation has attenuated, a decrease in remodeling space secondary to reduced ES/BS and a positive BB secondary to decreased final Rs.De and increased W.Th contribute to the progressive increase in spine BMD with long-term treatment.

Therapeutic implications of suppressing osteoclast formation versus function.

Anti-resorptive therapy is the principal means of treating osteoporotic disorders. The two families of presently available anti-resorptive drugs, namely bisphosphonates and denosumab, dampen activity of osteoclasts by reducing their number. In consequence, these agents also arrest bone remodelling eventuating suppressed formation as well as resorption. Evidence exists that osteoclasts recruit osteoblasts to sites of bone remodelling by mobilizing chemotactic proteins from matrix and direct secretion of such proteins that attract osteoblast precursors. Thus, anti-resorptive agents, such as the cathepsin K inhibitor odanacatib, that dampen osteoclast function but not number may also preserve osteoblast recruitment by preserving the bone resorptive cell.