An orally active cathepsin K inhibitor, furan-2-carboxylic acid, 1-{1-[4-fluoro-2-(2-oxo-pyrrolidin-1-yl)-phenyl]-3-oxo-piperidin-4-ylcarbamoyl}-cyclohexyl)-amide (OST-4077), inhibits osteoclast activity in vitro and bone loss in ovariectomized rats

In-vivo assessment of the osteo-protective effects of eugenol in alveolar bone tissues

Estrogen deficiency following menopausal provokes alveolar bone loss, remodeling and inflammation. Eugenol is a phenolic compound with wide dental applications and anti-inflammatory properties. In the present study, the potential protective role of eugenol against alveolar bone deformities was investigated in an ovariectomized (OVX) rodent model. Two doses of eugenol (2.5 and 5 mg/kg/d) were administered to OVX animals for 12 weeks. In Serum, markers of bone metabolism and pro-inflammatory cytokines were estimated using ELISA. Alveolar bone morphometry was analyzed using high-resolution micro-computed tomography (CT). Bone histological analysis (H&E stain) was also performed. Alveolar bone expression of osteoclastogenesis modulating factors, such as osteoprotegerin (OPG), receptor activator of nuclear factor kappa-b ligand (RANKL) and inflammatory mediators, were measured using immunohistochemistry. Eugenol failed to correct elevated body weights and uterine atrophy in OVX rats. The significant elevation of bone metabolic markers and inflammatory cytokines in OVX animals were markedly improved by eugenol treatment, particularly the higher dose. Eugenol treatment considerably attenuated morphometric trabecular alterations of the alveolar bone and improved alveolar resorption and gingival infiltration. Alveolar bone of OVX animals showed augmented expression of RANKL, OPG and inflammatory cytokines, which were corrected by eugenol treatment. Alveolar bone loss and remodeling associated with estrogen insufficiency was ameliorated by eugenol owing to its anti-inflammatory properties, suggesting an extra dental impact for eugenol.

Effect of SI-591, a new class of cathepsin K inhibitor with peptidomimetic structure, on bone metabolism in vitro and in vivo

SI-591[N-[1-[[[(1S)-3-[[(3S)-hexahydro-2-oxo-1H-azepin-3-yl]amino]-1-(1-methylethyl)-2,3-dioxopropyl]amino]carbonyl]cyclohexyl]-2-furancarboxamide] is an orally bioavailable compound that was synthesized as one of several unique peptidomimetic compounds without a basic group. This compound was found to have the ability to inhibit cathepsin K, a lysosomal cysteine protease. Cathepsin K is known to be expressed in osteoclasts and involved in bone loss processes. In this study, SI-591 was shown to inhibit the activity of various purified cathepsin molecules at nanomolar concentrations but had high selectivity for cathepsin K over other subtypes including B and L. SI-591 also decreased the level of CTX-I, a bone resorption marker, which was released from osteoclasts in vitro in a dose-dependent manner. The mobilization of calcium from the bones to the blood stream is known to increase in rats fed with a low calcium diet; SI-591 inhibited this increase in serum calcium level at an oral dose of 3mg/kg. Furthermore, SI-591 significantly decreased the level of CTX-I and DPD, bone resorption markers, at oral doses of 10mg/kg or less in ovariectomized rats, while it did not affect the level of BGP, a bone formation marker. In addition, SI-591 prevented bone mineral density loss in the lumber vertebrae and femurs in ovariectomized rats. These results suggest that SI-591 inhibits bone resorption without affecting osteoblast maturation. Therefore, SI-591, a novel cathepsin K inhibitor, could be a promising agent for the treatment of postmenopausal osteoporosis.

Azanitrile Cathepsin K Inhibitors: Effects on Cell Toxicity, Osteoblast-Induced Mineralization and Osteoclast-Mediated Bone Resorption

Aim

The cysteine protease cathepsin K (CatK), abundantly expressed in osteoclasts, is responsible for the degradation of bone matrix proteins, including collagen type 1. Thus, CatK is an attractive target for new anti-resorptive osteoporosis therapies, but the wider effects of CatK inhibitors on bone cells also need to be evaluated to assess their effects on bone. Therefore, we selected, among a series of synthetized isothiosemicarbazides, two molecules which are highly selective CatK inhibitors (CKIs) to test their effects on osteoblasts and osteoclasts.

Research Design and Methods

Cell viability upon treatment of CKIs were was assayed on human osteoblast-like Saos-2, mouse monocyte cell line RAW 264.7 and mature mouse osteoclasts differentiated from bone marrow. Osteoblast-induced mineralization in Saos-2 cells and in mouse primary osteoblasts from calvaria, with or without CKIs,; were was monitored by Alizarin Red staining and alkaline phosphatase activity, while osteoclast-induced bone resorption was performed on bovine slices.

Results

Treatments with two CKIs, CKI-8 and CKI-13 in human osteoblast-like Saos-2, murine RAW 264.7 macrophages stimulated with RANKL and mouse osteoclasts differentiated from bone marrow stimulated with RANKL and MCSF were found not to be toxic at doses of up to 100 nM. As probed by Alizarin Red staining, CKI-8 did not inhibit osteoblast-induced mineralization in mouse primary osteoblasts as well as in osteoblast-like Saos-2 cells. However, CKI-13 led to a reduction in mineralization of around 40% at 10–100 nM concentrations in osteoblast-like Saos-2 cells while it did not in primary cells. After a 48-hour incubation, both CKI-8 and CKI-13 decreased bone resorption on bovine bone slices. CKI-13 was more efficient than the commercial inhibitor E-64 in inhibiting bone resorption induced by osteoclasts on bovine bone slices. Both CKI-8 and CKI-13 created smaller bone resorption pits on bovine bone slices, suggesting that the mobility of osteoclasts was slowed down by the addition of CKI-8 and CKI-13.

Conclusion

CKI-8 and CKI-13 screened here show promise as antiresorptive osteoporosis therapeutics but some off target effects on osteoblasts were found with CKI-13.

The antidepressant bupropion exerts alleviating properties in an ovariectomized osteoporotic rat model

AIM

Depression is a risk factor for impaired bone mass and micro-architecture, but several antidepressants were found to increase the incidence of osteoporotic fractures. In the present study we used ovariectomized (OVX) rats as a model of osteoporosis to investigate the effects of the antidepressant bupropion on the femoral bones.

METHODS

OVX animals were treated with bupropion (30, 60 mg·kg(-1)·d(-1)) for six weeks. Bone turnover biomarkers (urinary DPD/Cr ratio, serum BALP, OC, TRAcP 5b, CTX and sRANKL levels) and inflammatory cytokines (TNF-α, IL-1β and IL-6) were determined using ELISA. Inductively coupled plasma mass spectroscopy (ICP-MS) was used to determine the femoral bone mineral concentrations. The cortical and trabecular morphometric parameters of femoral bones were determined using micro-CT scan and histopathology.

RESULTS

In OVX rats, the levels of bone turnover biomarkers and inflammatory cytokines were significantly elevated and femoral bone Ca(2+) and PO4(3-) concentrations were significantly reduced. Moreover, cortical and trabecular morphometric parameters and histopathology of femoral bones were severely altered by ovariectomy. Bupropion dose-dependently inhibited the increases in bone turnover biomarkers and inflammatory cytokines. OVX rats treated with the high dose of bupropion showed normal mineral concentrations in femoral bones. The altered morphometric parameters and histopathology of femoral bones were markedly attenuated by the treatment.

CONCLUSION

Bupropion exerts osteo-protective action in OVX rats through suppressing osteoclastogenesis-inducing factors and inflammation, which stabilize the osteoclasts and decrease bone matrix degradation or resorption.

ONO-5334, a cathepsin K inhibitor, improves bone strength by preferentially increasing cortical bone mass in ovariectomized rats

This study compared the effects of ONO-5334, a cathepsin K inhibitor, with those of alendronate on bone mass and strength in ovariectomized rats. Ovariectomy resulted in significant elevation in urinary deoxypyridinoline and plasma C-terminal cross-linking telopeptide of type I collagen (CTX) 8 weeks after surgery. Peripheral quantitative computed tomography analysis showed that total, trabecular, and cortical bone mineral content (BMC) decreased in the proximal tibia, which was paralleled with a significant decline in bone strength. Treatment with ONO-5334 (0.12, 0.6, 3 or 15 mg/kg) once daily for 8 weeks dose-dependently restored the decrease in total BMC and bone mineral density (BMD) in the proximal tibia and suppressed urinary deoxypyridinoline and plasma CTX levels. Alendronate (1 mg/kg, once daily) also fully restored these bone mass parameters. Separate analysis of trabecular and cortical bones, however, showed that ONO-5334 only partially restored trabecular BMD and BMC at 15 mg/kg, whereas alendronate fully restored these parameters. On the other hand, ONO-5334 increased both cortical BMD and BMC with an effect more potent than that of alendronate. Bone geometric analysis indicated that ONO-5334 at 15 mg/kg decreased endosteal circumference without affecting periosteal circumference, resulting in marked increase in cortical thickness. Interestingly, the effects of ONO-5334 on bone strength parameters were more prominent than those of alendronate, although the two test compounds had a similar effect on total BMC. Taken together, our results indicate that ONO-5334 has pharmacological characteristics different from those of alendronate and may offer a unique therapy for patients with osteoporosis.

Increased fracture callus mineralization and strength in cathepsin K knockout mice

Cathepsin K (CatK) is a cysteine protease, expressed predominantly in osteoclasts (OC) which degrades demineralized bone matrix. Novel selective inhibitors of CatK are currently being developed for the treatment of postmenopausal osteoporosis. Pharmacological inhibition of CatK reduces OC resorption activity while preserving bone formation in preclinical models. Disruption of the CatK gene in mice also results in high bone mass due to impaired bone resorption and elevated formation. Here, we assessed mid-shaft femoral fracture healing in 8-10week old CatK knock-out (KO) versus wild type (WT) mice. Fracture healing and callus formation were determined in vivo weekly via X-ray, and ex vivo at days 14, 18, 28 and 42 post-fracture by radiographic scoring, micro-computed tomography (μCT), histomorphometry and terminal mechanical four point bend strength testing. Radiological evaluation indicated accelerated bone healing and remodeling for CatK KO animals based on increased total radiographic scores that included callus opacity and bridging at days 28 and 42 post-fracture. Micro-CT based total callus volume was similar in CatK KO and WT mice at day 14. Callus size in CatK KO mice was 25% smaller than that in WT mice at day 18, statistically significant by day 28 and exhibited significantly higher mineralized tissue volume and volumetric BMD as compared to WT by day 18 onward. Osteoclast surface and osteoid surface trended higher in CatK KO calluses at all time-points and osteoblast number was also significantly increased at day 28. Increased CatK KO callus mineral density was reflected in significant increases in peak load and stiffness over WT at day 42 post-fracture. Regression analysis indicated a positive correlation (r=0.8671; p<0.001) between callus BMC and peak load indicating normal mineral properties in CatK KO calluses. Taken together, gene deletion of cathepsin K in mice accelerated callus size resolution, significantly increased callus mineralized mass, and improved mechanical strength as compared to wild type mice.

Effects of pharmacological inhibition of cathepsin K on fracture repair in mice

Cathepsin K inhibitors (CatK-I) have been developed and established to restore bone mass in both animal models of bone loss and postmenopausal osteoporotic patients. We investigated the effects of a CatK-I L-006235 on bone repair and compared to alendronate (ALN) for its known effects on fracture healing in preclinical models. Femoral fractures were performed on wild type mice that were given vehicle (CON), CatK-I or ALN from day 0 post-fracture until euthanasia. Radiologic and micro-CT analyses demonstrated that CatK-I enhanced mineralization within the calluses at day 21 post-fracture, but to a lesser degree than ALN. Histological analyses showed residual unmineralized and mineralized cartilage in the calluses of CatK-I and ALN treated groups at day 21 post-fracture compared to that in CON. CatK-I enhanced the number of tartrate-resistant acid phosphatase positive (TRAP+) osteoclasts in the fracture calluses compared to ALN and CON treated groups. However, relative levels of serum C-terminal telopeptides of type I collagen (CTX) normalized to the number of TRAP+ osteoclasts within the calluses were significantly decreased in both CatK-I and ALN groups compared to CON. Additionally, the percentages of osteoblast surface over mineralized calluses and levels of the bone formation marker serum N-terminal propeptide of type I procollagen (P1NP) were comparable between CatK-I versus CON groups, while these bone formation parameters were decreased by ALN. Taken together, these results indicate that unlike ALN, CatK-I inhibits osteoclastic activity without changing bone formation, and the inhibition of CatK delayed but did not abrogate callus remodeling during bone repair.

The role of bone turnover markers in monitoring treatment in postmenopausal osteoporosis

Bone metabolism is assessed using biochemical bone turnover markers (BTM). BTM reflect the metabolic effect of drugs on bone turnover, help to establish the lowest dose inducing the largest change in the BTM, predict treatment-related reduction in fracture risk, and are helpful in bridging studies. Changes in BTM during anti-osteoporotic therapy depend on the cellular mechanism of action of the drug, degree of change in bone turnover rate and route of administration. BTM help to establish the optimal dose of anti-osteoporotic drugs because treatment-related changes in BTM are more rapid compared with change in BMD. A greater decrease in BTM levels during the first year of tantiresorptive treatment is associated with greater antifracture efficacy over 3 years. According to preliminary data, measurement of BTM can improve persistence with anti-resorptive treatment. The use of BTM to monitor anti-osteoporotic therapy in "real life" is limited at this stage.

Effects of ONO-5334, a novel orally-active inhibitor of cathepsin K, on bone metabolism

In the present study, we examined the in vitro and in vivo pharmacological effects of ONO-5334, a novel inhibitor of cathepsin K, on bone metabolism. In vitro experiments indicated that ONO-5334 is a potent inhibitor of cathepsin K with Ki value of 0.1 nM. Although this compound inhibited other cysteine proteases, such as cathepsin S, L and B, its inhibitory activity for these enzymes was 8 to 320 fold lower than that for cathepsin K. ONO-5334 also inhibited human osteoclasts bone resorption in vitro at a concentration more than 100 fold lower than that of alendronate, a bisphosphonate. While alendronate disrupted actin ring and induced pyknotic nuclei in osteoclasts, ONO-5334 did not have such effects, suggesting that this compound does not affect osteoclasts viability. In in vivo experiments, oral administration of ONO-5334 dose-dependently reduced plasma calcium level increased by parathyroid hormone related peptide in thyroparathyroidectomized rats. Furthermore, in vivo experiment using normal monkeys demonstrated that ONO-5334 decreases serum and urine C-telopeptide of type I collagen level, a bone resorption marker, soon after oral dosing. These levels were consistently decreased below pre-dose levels by repeated oral dosing with ONO-5334 for 7 days. ONO-5334 on the other hand did not affect bone formation markers, serum osteocalcin and bone specific alkaline phosphatase. These findings indicate that ONO-5334 is a specific inhibitor for cathepsin K and thus may be a novel therapeutic agent for metabolic bone diseases.

Cysteinyl cathepsins and mast cell proteases in the pathogenesis and therapeutics of cardiovascular diseases

The initiation and progression of cardiovascular diseases involve extensive arterial wall matrix protein degradation. Proteases are essential to these pathological events. Recent discoveries suggest that proteases do more than catabolize matrix proteins. During the pathogenesis of atherosclerosis, abdominal aortic aneuryms, and associated complications, cysteinyl cathepsins and mast cell tryptases and chymases participate importantly in vascular cell apoptosis, foam cell formation, matrix protein gene expression, and pro-enzyme, latent cytokine, chemokine, and growth factor activation. Experimental animal disease models have been invaluable in examining each of these protease functions. Deficiency and pharmacological inhibition of cathepsins or mast cell proteases have allowed their in vivo evaluation in the setting of pathological conditions. Recent discoveries of highly selective and potent inhibitors of cathepsins, chymase, and tryptase, and their applications in vascular diseases in animal models and non-vascular diseases in human trials, have led to the hypothesis that selective inhibition of cathepsins, chymases, and tryptase will benefit patients suffering from cardiovascular diseases. This review highlights recent discoveries from in vitro cell-based studies to experimental animal cardiovascular disease models, from protease knockout mice to treatments with recently developed selective and potent protease inhibitors, and from patients with cathepsin-associated non-vascular diseases to those affected by cardiovascular complications.

Targeting the alphavbeta3 integrin for small-animal PET/CT of osteolytic bone metastases

This article describes the evaluation of the radiopharmaceutical (64)Cu-CB-TE2A-c(RGDyK) ((64)Cu-RGD) as an imaging agent for osteolytic bone metastases and their associated inflammation by targeting of the alpha(v)beta(3) integrin on osteoclasts and the proinflammatory cells involved at the bone metastatic site.

METHODS

The (64)Cu-RGD radiotracer was evaluated in the transgenic mouse expressing Tax (Tax(+)), which spontaneously develops osteolytic tumors throughout the vertebrae and hind limbs, using biodistribution studies and small-animal PET/CT. Histologic analysis was also performed on Tax(+) mouse tails, using hematoxylin and eosin and tartrate-resistant acid phosphatase to confirm the presence of osteolytic bone lesions and the presence of osteoclasts, respectively. Additionally, a proof-of-principle study was conducted with a small group of Tax(+) animals presenting with osteolytic lesions. These animals were treated with the bisphosphonate zoledronic acid and imaged with (64)Cu-RGD to determine whether this radiopharmaceutical was sensitive enough to detect a response to the bisphosphonate therapy.

RESULTS

Biodistribution studies using (64)Cu-RGD demonstrated that Tax(+) mice between the ages of 6 and 12 mo had a greater accumulation of activity in their tail vertebrae than did the wild-type (WT) cohort (P = 0.013). Additionally, Tax(+) mice between the ages of 6 and 12 mo had significantly more tracer activity associated with their tail vertebrae than did Tax(+) mice older than 12 mo (P = 0.003), suggesting that earlier bone metastases cause an increased recruitment of alpha(v)beta(3)-expressing cells. Small-animal PET/CT with (64)Cu-RGD was conducted on Tax(+) and WT mice. On the basis of standardized uptake value analysis, Tax(+) mice had approximately 2-fold more tail-associated activity than did WT animals (P = 0.0157). Additionally, decreases in uptake were observed in the tails of Tax(+) mice after treatment with the osteoclast inhibitor zoledronic acid, and histologic analysis of Tax(+) mouse-tail vertebrae revealed the presence of Tax(+) tumor cells, osteoclasts, and proinflammatory cells within the bone microenvironment.

CONCLUSION

Together, these data suggest that (64)Cu-RGD has the potential to effectively image osteolytic bone metastases and monitor the physiologic changes in the bone metastatic microenvironment after osteoclast-inhibiting bisphosphonate therapy.

The anabolic action of intermittent PTH in combination with cathepsin K inhibitor or alendronate differs depending on the remodeling status in bone in ovariectomized mice

We hypothesized that the anabolic action of parathyroid hormone (PTH) with the anti-catabolic agents cathepsin K inhibitor and alendronate differs depending on the remodeling status in the bone. C57/BL/6J mice, 8 weeks of age, were subjected to ovariectomized (OVX) or sham surgery. At 6 weeks after surgery, the mice were treated with cathepsin K inhibitor, alendronate, or a vehicle (daily, for 8 weeks), with or without PTH (1-34) (5 times/week, for the last 4 weeks). We assessed the bone chemical markers of the serum and urine, bone mineral density (BMD), histomorphomery in the primary and secondary spongiosa of the proximal tibia after fluorescence labeling, primary cell culture, and mRNA expressions in bone marrow cells. Cathepsin K inhibitor and alendronate significantly increased the BMD and the bone volume of the primary and secondary spongiosa, with a reduction of the urinary C-telopeptide of type I collagen that was increased by OVX, respectively. Cathepsin K inhibitor augmented the anabolic action of PTH on the BMD and bone volume at both the primary and secondary spongiosa, while alendronate had the same effect on the BMD and bone volume only at the primary spongiosa. Cathepsin K inhibitor did not decrease serum osteocalcin with or without PTH, while alendronate did decrease it. Cathepsin K inhibitor did not decrease the values of osteoclast number or bone formation rate with or without PTH, while alendronate decreased those values and increased osteoclast apoptosis. The combination of PTH and cathepsin K inhibitor increased alkaline phosphatase-positive CFU-f formation and c-fos, osterix, and osteocalcin mRNA expressions of bone marrow cells as well as PTH alone, while the combination of PTH and alendronate decreased those values. This study demonstrated that alendronate enhances the anabolic action of PTH at the primary spongiosa, but blunts it in the remodeling trabecular bone, while cathepsin K inhibitor enhances the action at both sites in OVX mice. In conclusion, the anabolic action of intermittent PTH in combination with cathepsin K inhibitor or alendronate differs depending on the remodeling status of bone in OVX mice.

The effect of TAK-778 on gene expression of osteoblastic cells is mediated through estrogen receptor

This study evaluated the effect of TAK-778 [(2R, 4S)-(-)-N-(4-diethoxyphosphorylmethylphenyl)-1,2,4,5-tetrahydro-4-methyl-7,8-methylenedioxy-5-oxo-3-benzothiepin-2-carboxamide)] on in vitro osteogenic events and on gene expression of osteoblastic cells derived from human alveolar bone and the participation of estrogen receptors (ERs) on such effect. Osteoblastic cells were subcultured, with or without TAK-778 (10(-5) M), to evaluate cell growth and viability, total protein content, and alkaline phosphatase (ALP) activity at 7, 14, and 21 days; bone-like formation at 21 days; and gene expression, using cDNA microarray, at 7 days. Also, osteoblastic cells were exposed to TAK-778 (10(-5) M) combined to ICI182,780, a nonspecific ER antagonist (10(-6) M), and gene expression was evaluated by real-time polymerase chain reaction (PCR) at 7 days. TAK-778 induced a reduction in culture growth and an increase in cell synthesis, ALP activity, and bone-like formation. The cDNA microarray showed genes associated with cell adhesion and differentiation, skeletal development, ossification, and transforming growth factor-beta receptor signaling pathway, with a tendency to be higher expressed in cells exposed to TAK-778. The gene expression of ALP, osteocalcin, Msh homeobox 2, receptor activator of NF-kappa B ligand, and intercellular adhesion molecule 1 was increased by TAK-778 as demonstrated by real-time PCR, and this effect was antagonized by ICI182,780. The present results demonstrated that TAK-778 acts at a transcriptional level to enhance the in vitro osteogenic process and that its effect on gene expression of osteoblastic cells is mediated, at least partially, through ERs. Based on these findings, TAK-778 could be considered in the treatment of bone metabolic disorders.

Biochemical markers of bone turnover: potential use in the investigation and management of postmenopausal osteoporosis

INTRODUCTION

The aim was to analyse data on the use of biochemical bone turnover markers (BTM) in postmenopausal osteoporosis.

METHODS

We carried out a comparative analysis of the most important papers concerning BTM in postmenopausal osteoporosis that have been published recently.

RESULTS

The BTM levels are influenced by several factors. They are moderately correlated with BMD and subsequent bone loss. Increased levels of bone resorption markers are associated with a higher risk of fracture. Changes in the BTM during the anti-osteoporotic treatment (including combination therapy) reflect the mechanisms of action of the drugs and help to establish their effective doses. Changes in the BTM during the anti-resorptive treatment are correlated with their anti-fracture efficacy.

CONCLUSION

Biological samples should be obtained in a standardised way. BTM cannot be used for prediction of the accelerated bone loss at the level of the individual. BTM help to detect postmenopausal women who are at high risk of fracture; however, adequate practical guidelines are lacking. BTM measurements taken during the anti-resorptive therapy help to identify non-compliers. They may improve adherence to the anti-resorptive therapy and the fall in the BTM levels that exceeds the predefined threshold improves patients' persistence with the treatment. There are no guidelines concerning the use of BTM in monitoring anti-osteoporotic therapy in postmenopausal women.

Cysteine cathepsin proteases as pharmacological targets in cancer

Proteolytic activity is required for several key pro-tumorigenic processes: angiogenesis, invasion and metastasis. Consequently, increases in protease expression and activity are frequently reported in human cancers, and correlate with malignant progression and poor patient prognosis. Cysteine cathepsin proteases have recently emerged as an important class of proteolytic enzymes in cancer development, and cysteine cathepsin inhibitors have been proposed as anticancer agents. In this review, we highlight recent studies that now allow us to evaluate critically whether cysteine cathepsin inhibition represents a viable therapeutic strategy for the treatment of cancer.

Pretreatment with anticatabolic agents blunts but does not eliminate the skeletal anabolic response to parathyroid hormone in oophorectomized mice

Previous studies have indicated that bisphosphonate pretreatment can inhibit the anabolic actions of PTH. We examined the capacity of two anticatabolic agents with different mechanisms of action, alendronate and osteoprotegerin (OPG), to influence the anabolic activity of PTH. Oophorectomized mice were pretreated for 30 d with alendronate or OPG and then treated for 30 d with the respective anticatabolic alone or the respective anticatabolic plus PTH(1-34). Bones were analyzed by bone mineral density (BMD), microcomputed tomography, histology and histomorphometry, and biochemical bone markers. OPG pretreatment produced a greater inhibition of bone turnover and a greater increase in bone than alendronate. Increases in bone were sustained during subsequent treatment with vehicle or continued administration of the anticatabolic. Pretreatment with each anticatabolic blunted the capacity of PTH to increase BMD and bone volume and continued treatment with each anticatabolic agent also reduced the effectiveness of PTH. Although both anticatabolics decreased the maximal PTH effect, BMD and bone volume increased more when PTH was added than when only anticatabolics were used. These results demonstrate that mechanistically distinct anticatabolics may reduce PTH efficacy, that the characteristics of this inhibition may reflect the different modes of action of the anticatabolics, but that the addition of PTH still provides a skeletal benefit even if the anabolic effect is submaximal.