Background:Large bone defects are challenging to manage clinically and usually require treatment with bone graft or bone graft substitute. This study evaluated the effect of romosozumab, a sclerostin antibody, in combination with demineralized bone matrix (DBM) on bone regeneration in a critical-size ulnar defect model in nonhuman primates. Methods:In cynomolgus monkeys (N = 22, male, 10-12 years old), a full-cortex bone defect (0.5 cm long) was created in the left ulnar shaft and filled with DBM. Animals were randomized to receive vehicle (n = 10) or romosozumab (n = 12; 30 mg/kg) subcutaneously, every 2 weeks for 28 weeks. Radiographs of the left ulna were taken every 2 weeks for 28 weeks to monitor bone regeneration response. Ulnae were excised and analyzed by ex-vivo x-ray and micro-computed tomography (micro-CT) to evaluate bone repair, and lumbar vertebrae were excised for bone histomorphometric analysis to evaluate the systemic anabolic response. Results:In-vivo and ex-vivo x-ray images of surgical ulnae demonstrated that the critical-size ulnar defect fully bridged in 3 romosozumab-treated monkeys at week 28 but not in any vehicle-treated monkey. Micro-CT analysis demonstrated that average new bone volume and new bone area within the defect region were 118 % and 105 % greater, respectively, with romosozumab versus vehicle. Trabecular bone volume per tissue volume and trabecular thickness of lumbar vertebral body were 72 % and 92 % greater, and eroded surface was significantly lower with romosozumab versus vehicle. Conclusion:High-dose romosozumab in combination with DBM improved bone regeneration in a critical-size ulnar defect model and increased bone mass in non-surgical bone in nonhuman primates. The translational potential of this article:Clinical management of large bone defect is complex and challenging. More effective management is needed. This paper reports the first nonhuman primate study that evaluated high-dose romosozumab in combination with demineralized bone matrix in a critical-size defect model and provides perspective for the future research evaluating the combination of romosozumab and bone graft or bone graft substitutes in various relevant clinical conditions.

Sclerostin antibody (Scl-Ab) restored bone mass and strength in the ovariectomized rat model of postmenopausal osteoporosis. Increased bone mineral density (BMD) and decreased skeletal fragility fracture risk have been reported in postmenopausal osteoporotic women receiving Scl-Ab. In males, loss of androgen leads to rapid decreases in BMD and an increased risk of fragility fractures. We hypothesized that Scl-Ab could reverse the loss of bone mass and strength caused by androgen ablation in the orchiectomized (ORX) rat model of male osteoporosis. We treated 9-month-old ORX Sprague Dawley rats (3 months after ORX) subcutaneously twice weekly with vehicle or Scl-Ab (5 or 25 mg/kg) for 6 weeks (n = 10 per group). Both doses of Scl-Ab fully reversed the BMD deficit in the lumbar spine and femur and tibia in ORX rats. Microcomputed tomography showed that the bone mass in the fifth lumbar vertebral body, femur diaphysis, and femoral neck were dose-dependently restored by Scl-Ab. The bone strength at these sites increased significantly with Scl-Ab to levels matching those of sham-operated controls and correlated positively with improvements in bone mineral content, demonstrating bone quality maintenance. Dynamic histomorphometry of the tibial diaphysis and second lumbar vertebral body demonstrated that Scl-Ab significantly increased bone formation on periosteal, endocortical, and trabecular surfaces and significantly decreased bone resorption on endocortical and trabecular surfaces. The effects of Scl-Ab on increasing bone formation and decreasing bone resorption led to restoration of bone mass and strength in androgen-deficient rats. These findings support the ongoing evaluation of Scl-Ab as a potential therapeutic agent for osteoporosis in men.

Alveolar bone is a mechanosensitive tissue that provides structural support for teeth. Alveolar bone loss is common with aging, menopause, tooth loss, and periodontitis and can lead to additional tooth loss, reduced denture fixation, and challenges in placing dental implants. The current studies suggest that sclerostin and DKK1, which are established osteocyte-derived inhibitors of bone formation, contribute to alveolar bone loss associated with estrogen ablation and edentulism in rats. Estrogen-deficient ovariectomized rats showed significant mandibular bone loss that was reversed by systemic administration of sclerostin antibody (SAB) alone and in combination with DKK1 antibody (DAB). Osteocytes in the dentate and edentulous rat maxilla expressed Sost (sclerostin) and Dkk1 (DKK1) mRNA, and molar extraction appeared to acutely increase DKK1 expression. In a chronic rat maxillary molar extraction model, systemic SAB administration augmented the volume and height of atrophic alveolar ridges, effects that were enhanced by coadministering DAB. SAB and SAB+DAB also fully reversed bone loss that developed in the opposing mandible as a result of hypo-occlusion. In both treatment studies, alveolar bone augmentation with SAB or SAB+DAB was accompanied by increased bone mass in the postcranial skeleton. Jaw bone biomechanics showed that intact sclerostin-deficient mice exhibited stronger and denser mandibles as compared with wild-type controls. These studies show that sclerostin inhibition, with and without DKK1 coinhibition, augmented alveolar bone volume and architecture in rats with alveolar bone loss. These noninvasive approaches may have utility for the conservative augmentation of alveolar bone.

Sustained elevation of parathyroid hormone (PTH) is catabolic to cortical bone, as evidenced by deterioration in bone structure (cortical porosity), and is a major factor for increased fracture risk in chronic kidney disease (CKD). Etelcalcetide (AMG 416), a novel peptide agonist of the calcium-sensing receptor, reduces PTH levels in subtotal nephrectomized (Nx) rats and in hemodialysis patients with secondary hyperparathyroidism (SHPT) in clinical studies; however, effects of etelcalcetide on bone have not been determined. In a rat model of established SHPT with renal osteodystrophy, etelcalcetide or vehicle was administered by subcutaneous (s.c.) injection to subtotal Nx rats with elevated PTH (>750pg/mL) once per day for 6weeks. Sham-operated rats receiving vehicle (s.c.) served as non-SHPT controls. Prior to treatment, significant increases in serum creatinine (2-fold), blood urea nitrogen (BUN, 3-fold), PTH (5-fold), fibroblast growth factor-23 (FGF23; 13-fold) and osteocalcin (12-fold) were observed in SHPT rats compared to non-SHPT controls. Elevations in serum creatinine and BUN were unaffected by treatment with vehicle or etelcalcetide. In contrast, etelcalcetide significantly decreased PTH, FGF23 and osteocalcin, whereas vehicle treatment did not. Cortical bone porosity increased and bone strength decreased in vehicle-treated SHPT rats compared to non-SHPT controls. Cortical bone structure improved and energy to failure was significantly greater in SHPT rats treated with etelcalcetide compared to vehicle. Mineralization lag time and marrow fibrosis were significantly reduced by etelcalcetide. In conclusion, etelcalcetide reduced bone turnover, attenuated mineralization defect and marrow fibrosis, and preserved cortical bone structure and bone strength by lowering PTH in subtotal Nx rats with established SHPT.

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.

ABSTRACT Results of prior studies suggest that fibroblast growth factor 21 (FGF21) may be involved in bone turnover and in the actions of peroxisome proliferator-activated receptor (PPAR) α and γ in mice. We have conducted independent studies to examine the effects of FGF21 on bone homeostasis and the role of FGF21 in PPARα and γ actions. High-fat-diet-induced obesity (DIO) mice were administered vehicle or recombinant human FGF21 (rhFGF21) intraperitoneally at 0 (vehicle), 0.1, 1, and 3 mg/kg daily for 2 weeks. Additional groups of DIO mice received water or 10 mg/kg rosiglitazone daily. Mice treated with rhFGF21 or rosiglitazone showed expected metabolic improvements in glucose, insulin, and lipid levels. However, bone loss was not detected in rhFGF21-treated mice by dual-energy X-ray absorptiometry (DXA), micro-CT, and histomorphometric analyses. Mineral apposition rate, a key bone formation parameter, was unchanged by rhFGF21, while significantly decreased by rosiglitazone in DIO mice. Bone resorption markers, OPG/RANKL mRNA expression, and histological bone resorption indices were unchanged by rhFGF21 or rosiglitazone. Bone marrow fat was unchanged by rhFGF21, while increased by rosiglitazone. Furthermore, FGF21 knockout mice did not show high bone mass phenotype. Treatment with PPARα or PPARγ agonists caused similar metabolic effects in FGF21 knockout and wild-type mice. These results contrast with previous findings and suggest that FGF21 is not critical for bone homeostasis or actions of PPARα and PPARγ. © 2016 American Society for Bone and Mineral Research.

ABSTRACT Denosumab (DMAb) administration to postmenopausal women with osteoporosis is associated with continued bone mineral density (BMD) increases and low fracture incidence through 8 years, despite persistently reduced bone turnover markers and limited fluorochrome labeling in iliac crest bone biopsies. BMD increases were hypothesized to result from additional accrual of bone matrix via modeling-based bone formation—a hypothesis that was tested by examining fluorochrome labeling patterns in sections from ovariectomized (OVX) cynomolgus monkeys (cynos) treated with DMAb for 16 months. Mature OVX or Sham cynos were treated monthly with vehicle for 16 months, whereas other OVX cynos received monthly 25 or 50 mg/kg DMAb. DMAb groups exhibited very low serum bone resorption and formation biomarkers and near-absent fluorochrome labeling in proximal femur cancellous bone. Despite these reductions, femoral neck dual-energy X-ray absorptiometry (DXA) BMD continued to rise in DMAb-treated cynos, from a 4.6% increase at month 6 to 9.8% above baseline at month 16. Further examination of cortical bone in the proximal femur demonstrated consistent and prominent labeling on the superior endocortex and the inferior periosteal surface, typically containing multiple superimposed labels from month 6 to 16 over smooth cement lines, consistent with continuous modeling-based bone formation. These findings were evident in all groups. Quantitative analysis at another modeling site, the ninth rib, demonstrated that DMAb did not alter the surface extent of modeling-based labels, or the cortical area bound by them, relative to OVX controls, while significantly reducing remodeling-based bone formation and eroded surface. This conservation of modeling-based formation occurred concomitantly with increased femoral neck strength and, when coupled with a reduction in remodeling-based bone loss, is likely to contribute to increases in bone mass with DMAb treatment. Thus, this study provides preclinical evidence for a potential mechanism that could contribute to the clinical observations of continued BMD increases and low fracture rates with long-term DMAb administration. © 2015 American Society for Bone and Mineral Research.

In 1, the cover image of the May 2015 issue was misattributed. The image comes from the following article, Sustained Modeling-Based Bone Formation During Adulthood in Cynomolgus Monkeys May Contribute to Continuous BMD Gains with Denosumab. doi: 10.1002/jbmr.2480 The publisher regrets the error. This erratum corrects the error.

The effects of up to 26 weeks of sclerostin antibody (Scl-Ab) treatment were investigated in ovariectomized (OVX) rats. Two months after surgery, 6-month-old osteopenic OVX rats were treated with vehicle or Scl-Ab (25 mg/kg, sc, one time per week) for 6, 12, or 26 weeks. In vivo dual-energy x-ray absorptiometry analysis demonstrated that the bone mineral density of lumbar vertebrae and femur-tibia increased progressively through 26 weeks of Scl-Ab treatment along with progressive increases in trabecular and cortical bone mass and bone strength at multiple sites. There was a strong correlation between bone mass and maximum load at lumbar vertebra, femoral neck, and diaphysis at weeks 6 and 26. Dynamic histomorphometric analysis showed that lumbar trabecular and tibial shaft endocortical and periosteal bone formation rates (BFR/BS) increased and peaked at week 6 with Scl-Ab-treatment; thereafter trabecular and endocortical BFR/BS gradually declined but remained significantly greater than OVX controls at week 26, whereas periosteal BFR/BS returned to OVX control levels at week 26. In the tibia metaphysis, trabecular BFR/BS in the Scl-Ab treated group remained elevated from week 6 to week 26. The osteoclast surface and eroded surface were significantly lower in Scl-Ab-treated rats than in OVX controls at all times. In summary, bone mass and strength increased progressively over 26 weeks of Scl-Ab treatment in adult OVX rats. The early gains were accompanied by increased cortical and trabecular bone formation and reduced osteoclast activity, whereas later gains were attributed to residual endocortical and trabecular osteoblast stimulation and persistently low osteoclast activity.

Sclerostin (Scl) is an osteocyte protein that decreases bone formation, and its inhibition by neutralizing antibodies (Scl-Ab) increases bone formation, mass and strength. We investigated the effects of Scl-Ab in mature ovariectomized (OVX) rats with a mechanistic focus on longer-term responses of osteoclasts, osteoblasts and osteocytes. Four-month-old Sprague-Dawley rats had OVX or sham surgery. Two months later, sham controls received sc vehicle while OVX rats received vehicle (OVX-Veh) or Scl-Ab (25mg/kg) once weekly for 6 or 26weeks followed by necropsy (n=12/group). Terminal blood was collected for biochemistry, non-adherent marrow cells were harvested from femurs for ex vivo osteoclast formation assays, and vertebrae and tibiae were collected for dynamic histomorphometry and mRNA analyses. Scl-Ab treatment led to progressively thicker but fewer trabeculae in the vertebra, leading to increased trabecular bone volume and reduced trabecular surfaces. Scl-Ab also increased cortical bone volume in the tibia, via early periosteal expansion and progressive endocortical contraction. Scl-Ab significantly reduced parameters of bone resorption at week 6 relative to OVX-Veh controls, including reduced serum TRACP-5b, reduced capacity of marrow cells to form osteoclasts ex vivo, and >80% reductions in vertebral trabecular and tibial endocortical eroded surfaces. At week 26, serum TRACP-5b and ex vivo osteoclast formation were no longer reduced in the Scl-Ab group, but eroded surfaces remained >80% lower than in OVX-Veh controls without evidence for altered skeletal mRNA expression of opg or rankl. Scl-Ab significantly increased parameters of bone formation at week 6 relative to OVX-Veh controls, including increases in serum P1NP and osteocalcin, and increased trabecular, endocortical and periosteal bone formation rates (BFRs). At week 26, surface-referent trabecular BFR remained significantly increased in the Scl-Ab group versus OVX-Veh controls, but after adjusting for a reduced extent of trabecular surfaces, overall (referent-independent) trabecular BFR was no longer significantly elevated. Similarly, serum P1NP and osteocalcin were no longer significantly increased in the Scl-Ab group at week 26. Tibial endocortical and periosteal BFR were increased at week 6 in the Scl-Ab group versus OVX-Veh controls, while at week 26 only endocortical BFR remained increased. The Scl-Ab group exhibited significant increments in skeletal mRNA expression of several osteocyte genes, with sost showing the greatest induction in both the tibia and vertebra. We propose that Scl-Ab administration, and/or the gains in bone volume that result, may have increased osteocytic expression of Scl as a possible means of regulating gains in bone mass.

Background Some Scl-Ab-induced BMD gains gradually reversed after treatment discontinuation in rats. Objectives To examine if follow-up treatment with a RANKL inhibitor, OPG-Fc, would maintain Scl-Ab-induced bone mass gains in OVX rats. Methods Six-month-old OVX rats (2 months post-OVX) were treated with Scl-Ab (Scl-Ab VI, 25 mg/kg, SC, 1x/wk) for 6 wks and then transitioned to vehicle (Veh) or OPG-Fc (10 mg/kg, SC, 2x/wk) for another 6 or 20 wks (n=12-14/group). Veh-treated Sham and OVX control rats were euthanized at wks 0, 6, 12, and 26, and a group of Scl-Ab-treated OVX rats were euthanized at wk 6 (n=10-12/group). Results In vivo DXA analysis showed that after 6 wks, Scl-Ab-treated OVX rats increased lumbar spine and femur-tibia BMD to levels significantly above OVX and Sham controls. These treatment-related increases were maintained by transition to OPG-Fc, and gradually reversed by transition to Veh (Fig.). Histomorphometry revealed significant increases in vertebral trabecular bone volume (BV/TV) and tibial cortical thickness (Ct.Th) after 6 wks of Scl-Ab treatment. While transition to OPG-Fc maintained BV/TV and Ct.Th at the peak levels achieved with Scl-Ab, transition to Veh led to gradual declines in these parameters to levels that remained above OVX controls at wk 26. Significant increases in trabecular, endocortical, and periosteal bone formation rates and significant decreases in trabecular and endocortical eroded surfaces were also associated with Scl-Ab treatment (6 wks). Transition to OPG-Fc led to reductions in each of these bone resorption and formation parameters, however transition to Veh reversed these changes. At wk 26, maximum load of lumbar vertebral bodies was significantly greater in the OPG-Fc vs Veh transitioned group. Conclusions After discontinuing Scl-Ab treatment, transitioning to the RANKL inhibitor OPG-Fc effectively inhibited bone resorption and maintained Scl-Ab-induced bone mass and bone strength gains. Disclosure of Interest M. Ominsky Shareholder of: Amgen Inc., Employee of: Amgen Inc., X. Li Shareholder of: Amgen Inc., Employee of: Amgen Inc., K. Warmington Shareholder of: Amgen Inc., Employee of: Amgen Inc., Q.-T. Niu Shareholder of: Amgen Inc., Employee of: Amgen Inc., F. Asuncion Shareholder of: Amgen Inc., Employee of: Amgen Inc., D. Dwyer Shareholder of: Amgen Inc., Employee of: Amgen Inc., M. Grisanti Shareholder of: Amgen Inc., Employee of: Amgen Inc., C.-Y. Han Shareholder of: Amgen Inc., Employee of: Amgen Inc., P. Kostenuik Shareholder of: Amgen Inc., Employee of: Amgen Inc., M. Stolina Shareholder of: Amgen Inc., Employee of: Amgen Inc., H. Ke Shareholder of: Amgen Inc., Employee of: Amgen Inc.

Background Bone formation on previously quiescent surfaces occurs through a process known as bone modeling, which differs from the resorption-coupled formation that occurs during bone remodeling. The robust anabolic effects of sclerostin antibody (Scl-Ab) on trabecular surfaces were previously demonstrated to occur almost entirely via modeling in both OVX rats1 and adolescent non-human primates.2 Objectives In the current analysis, we quantified the effects of Scl-Ab on modeling- and remodeling-based bone formation on the femur endocortical surface in male cynomolgus monkeys. Methods Adolescent (4-5-year-old) monkeys were treated biweekly with subcutaneous vehicle (Veh) or 30 mg/kg Scl-Ab for 10 weeks. Tetracycline (25 mg/kg) was injected on days 14 and 24, and calcein (8 mg/kg) was injected on days 56 and 66. Histomorphometry was performed on cross-sections of the left femur diaphysis. Results Calcein-based mineralizing surface (MS/BS) was increased from 14% in Veh to 88% in the Scl-Ab group (p<0.001). Bone forming surfaces were characterized as either modeling (MBF) or remodeling (RBF) surfaces based on the morphology of the underlying cement lines. The Scl-Ab-mediated increases in bone forming surface were almost entirely modeling-based, with an increase in MBF from 7% of the total bone surface in Veh to 77% in the Scl-Ab group (p<0.001). No significant change was observed in RBF between the Veh and Scl-Ab groups (10% vs 15%, p=0.20), and no eroded surface was observed in any Scl-Ab-treated animal (vs 6% in Veh, p<0.01). The period of bone formation on surfaces was extended in the Scl-Ab group, with 62% of calcein-labeled surfaces having both underlying tetracycline labels (active from day 14) as compared to only 4% in Veh (p<0.001). Scl-Ab also increased osteoblast activity as reflected in mineral apposition rate (MAR) by 48% on modeling surfaces (1.18 vs 0.80 μm/day, p<0.05), while remodeling MAR was not significantly changed. Conclusions These results demonstrate that Scl-Ab markedly increased modeling-based bone formation and reduced bone resorption in cortical bone, similar to the findings in trabecular bone. These contrasting effects on bone formation and resorption illustrate the unique tissue-level mechanism of Scl-Ab, which was consistent across both trabecular and cortical sites. Based on these results in non-human primates, the therapeutic profile of Scl-Ab provides a rapid and efficient process by which bone mass can be increased. References Li et al. J Bone Min Res. 2007;22(S1):S36. Ominsky et al. J Bone Min Res. 2010;25(S1):S53. Disclosure of Interest M. Ominsky Shareholder of: Amgen Inc., Employee of: Amgen Inc., Q.-T. Niu Shareholder of: Amgen Inc., Employee of: Amgen Inc., H. Z. Ke Shareholder of: Amgen Inc., Employee of: Amgen Inc.

OBJECTIVESclerostin plays a major role in regulating skeletal bone mass, but its effects in articular cartilage are not known. The purpose of this study was to determine whether genetic loss or pharmacologic inhibition of sclerostin has an impact on knee joint articular cartilage.METHODSExpression of sclerostin was determined in articular cartilage and bone tissue obtained from mice, rats, and human subjects, including patients with knee osteoarthritis (OA). Mice with genetic knockout (KO) of sclerostin and pharmacologic inhibition of sclerostin with a sclerostin-neutralizing monoclonal antibody (Scl-Ab) in aged male rats and ovariectomized (OVX) female rats were used to study the effects of sclerostin on pathologic processes in the knee joint. The rat medial meniscus tear (MMT) model of OA was used to investigate the pharmacologic efficacy of systemic Scl-Ab or intraarticular (IA) delivery of a sclerostin antibody-Fab (Scl-Fab) fragment.RESULTSSclerostin expression was detected in rodent and human articular chondrocytes. No difference was observed in the magnitude or distribution of sclerostin expression between normal and OA cartilage or bone. Sclerostin-KO mice showed no difference in histopathologic features of the knee joint compared to age-matched wild-type mice. Pharmacologic treatment of intact aged male rats or OVX female rats with Scl-Ab had no effect on morphologic characteristics of the articular cartilage. In the rat MMT model, pharmacologic treatment of animals with either systemic Scl-Ab or IA injection of Scl-Fab had no effect on lesion development or severity.CONCLUSIONGenetic absence of sclerostin does not alter the normal development of age-dependent OA in mice, and pharmacologic inhibition of sclerostin with Scl-Ab has no impact on articular cartilage remodeling in rats with posttraumatic OA.

ABSTRACT Bone formation can be remodeling-based (RBF) or modeling-based (MBF), the former coupled to bone resorption and the latter occurring directly on quiescent surfaces. Unlike osteoanabolic therapies such as parathyroid hormone (PTH) 1-34 that increase bone remodeling and thus both formation and resorption, sclerostin antibody (Scl-Ab) increases bone formation while decreasing bone resorption. With this unique profile, we tested our hypothesis that Scl-Ab primarily elicited MBF by examining bones from Scl-Ab–treated ovariectomized (OVX) rats and male cynomolgus monkeys (cynos). Histomorphometry was performed to quantify and characterize bone surfaces in OVX rats administered vehicle or Scl-Ab (25 mg/kg) subcutaneously (sc) twice/week for 5 weeks and in adolescent cynos administered vehicle or Scl-Ab (30 mg/kg) sc every 2 weeks for 10 weeks. Fluorochrome-labeled surfaces in L2 vertebra and femur endocortex (cynos only) were considered to be MBF or RBF based on characteristics of their associated cement lines. In OVX rats, Scl-Ab increased MBF by eightfold (from 7% to 63% of bone surface, compared to vehicle). In cynos, Scl-Ab markedly increased MBF on trabecular (from 0.6% to 34%) and endocortical surfaces (from 7% to 77%) relative to vehicle. Scl-Ab did not significantly affect RBF in rats or cynos despite decreased resorption surface in both species. In cynos, Scl-Ab resulted in a greater proportion of RBF and MBF containing sequential labels from week 2, indicating an increase in the lifespan of the formative site. This extended formation period was associated with robust increases in the percent of new bone volume formed. These results demonstrate that Scl-Ab increased bone volume by increasing MBF and prolonged the formation period at both modeling and remodeling sites while reducing bone resorption. Through these unique effects on bone formation and resorption, Scl-Ab may prove to be an effective therapeutic to rapidly increase bone mass in diseases such as osteoporosis. © 2014 American Society for Bone and Mineral Research.

The physiological role of Dickkopf-1 (Dkk1) during postnatal bone growth in rodents and in adult rodents was examined utilizing an antibody to Dkk1 (Dkk1-Ab) that blocked Dkk1 binding to both low density lipoprotein receptor-related protein 6 (LRP6) and Kremen2, thereby preventing the Wnt inhibitory activity of Dkk1. Treatment of growing mice and rats with Dkk1-Ab resulted in a significant increase in bone mineral density because of increased bone formation. In contrast, treatment of adult ovariectomized rats did not appreciably impact bone, an effect that was associated with decreased Dkk1 expression in the serum and bone of older rats. Finally, we showed that Dkk1 plays a prominent role in adult bone by mediating fracture healing in adult rodents. These data suggest that, whereas Dkk1 significantly regulates bone formation in younger animals, its role in older animals is limited to pathologies that lead to the induction of Dkk1 expression in bone and/or serum, such as traumatic injury. (C) 2011 American Society for Bone and Mineral Research.