Cathepsin K Inhibitors for Osteoporosis: Biology, Potential Clinical Utility, and Lessons Learned

Cigarette Smoke Extract Activates Tartrate-Resistant Acid Phosphatase-Positive Macrophage

BACKGROUND

It has been reported that smoking is one of the strongest positive risk factors for abdominal aortic aneurysms (AAAs). Although many studies have been directed to decipher the effect of smoking on AAA, its effect on macrophage activation has not yet been explored.

OBJECTIVES

We have reported the importance of osteoclastogenesis (OCG) in aneurysm formation. Therefore, we examined the effect of cigarette smoking on OCG and arterial aneurysmal formation by using cigarette smoke extract (CSE) in this study.

METHODS

Macrophage cell lines were stimulated with CSE, and their activation and differentiation were examined in vitro. Since macrophages activated through the OCG pathway are identified by tartrate-resistant acid phosphatase (TRAP) expression, these cells are referred to as TRAP-positive macrophages (TPMs) in this study. We also applied CSE-contained PBS in the calcium chloride-induced mouse carotid aneurysm model in vivo.

RESULTS

Macrophages stimulated with CSE expressed significantly higher levels of nuclear factor of activated T-cells cytoplasmic 1 (NFATc1), TRAP, cathepsin K, matrix metalloproteinase-9 and membrane-type metalloproteinase (MT1-MMP). CSE-treated mouse aneurysms showed increased aneurysm size with increased TPM infiltration and protease expression compared to non-CSE-treated mouse aneurysms.

CONCLUSIONS

These results suggest that CSE intensifies OCG in macrophages and promotes arterial aneurysmal progression.

Disturbed remodeling and delayed fracture healing in pediatric pycnodysostosis patients

Pycnodysostosis is an autosomal recessive disease caused by a gene mutation leading cathepsin K deficiency. Pathological fractures of the long bones are common, but guidelines on fracture treatment in these patients are still lacking. We have treated 5 fractures in 2 pediatric pycnodysostosis patients. We hypothesize that pycnodysostosis patients have an incomplete remodeling process in fracture healing because of cathepsin K deficiency. Therefore, to minimize the role of endochondral bone formation (indirect) after a fracture, it seems prudent to strive for direct bone healing (intramembranous) instead of indirect bone healing. Open reduction with internal fixation should be the goal.

Application of nanotechnology to target and exploit tumour associated proteases

Proteases are hydrolytic enzymes fundamental for a variety of physiological processes, but the loss of their regulation leads to aberrant functions that promote onset and progression of many diseases including cancer. Proteases have been implicated in almost every hallmark of cancer and whilst widely investigated for tumour therapy, clinical adoption of protease inhibitors as drugs remains a challenge due to issues such as off-target toxicity and inability to achieve therapeutic doses at the disease site. Now, nanotechnology-based solutions and strategies are emerging to circumvent these issues. In this review, preclinical advances in approaches to enhance the delivery of protease drugs and the exploitation of tumour-derived protease activities to promote targeting of nanomedicine formulations is examined. Whilst this field is still in its infancy, innovations to date suggest that nanomedicine approaches to protease targeting or inhibition may hold much therapeutic and diagnostic potential.

Coupling factors involved in preserving bone balance

Coupling during bone remodeling refers to the spatial and temporal coordination of bone resorption with bone formation. Studies have assessed the subtle interactions between osteoclasts and osteoblasts to preserve bone balance. Traditionally, coupling research related to osteoclast function has focused on bone resorption activity causing the release of growth factors embedded in the bone matrix. However, considerable evidence from in vitro, animal, and human studies indicates the importance of the osteoclasts themselves in coupling phenomena, and many osteoclast-derived coupling factors have been identified. These include sphingosine-1-phosphate, vesicular-receptor activator of nuclear factor-κB, collagen triple helix repeat containing 1, and cardiotrophin-1. Interestingly, neuronal guidance molecules, such as slit guidance ligand 3, semaphorin (SEMA) 3A, SEMA4D, and netrin-1, originally identified as instructive cues allowing the navigation of growing axons to their targets, have been shown to be involved in the intercellular cross-talk among bone cells. This review discusses osteoclast-osteoblast coupling signals, including recent advances and the potential roles of these signals as therapeutic targets for osteoporosis and as biomarkers predicting human bone health.

Novel cathepsin K inhibitors block osteoclasts in vitro and increase spinal bone density in zebrafish

Cathepsin K (Cat K) is a predominant cysteine protease and highly potent collagenase expressed in osteoclasts. Cat K inhibitors are anti-resorptive agents to treat osteoporosis. A novel scaffold of cathepsin K inhibitors, exemplified by lead compound 1x, was used as the template for designing and synthesizing a total of 61 derivatives that have not been reported before. An exploratory structure–activity relationship analysis identified the potent Cat K inhibitor A22, which displayed an IC₅₀ value of 0.44 μM against Cat K. A22 was very specific for Cat K and caused a significantly higher in vitro inhibition of the enzyme as compared to that of lead compound 1x. A surface plasmon resonance analysis confirmed in vitro binding of A22 to Cat K. Molecular docking studies indicated several favourable interaction sites for A22 within the active pocket of Cat K. Furthermore, A22 also blocked active osteoclasts in vitro and increased spinal bone density in zebrafish, in which it showed an activity that was higher than that of the marketed therapeutic bone metabolizer etidronate disodium. A22 represents a very promising lead compound for the development of novel antiresorptive agents functioning as orthosteric inhibitors of Cat K.

Cathepsin K (Cat K) is a predominant cysteine protease and highly potent collagenase expressed in osteoclasts.

The Alkyne Moiety as a Latent Electrophile in Irreversible Covalent Small Molecule Inhibitors of Cathepsin K

Irreversible covalent inhibitors can have a beneficial pharmacokinetic/pharmacodynamics profile but are still often avoided due to the risk of indiscriminate covalent reactivity and the resulting adverse effects. To overcome this potential liability, we introduced an alkyne moiety as a latent electrophile into small molecule inhibitors of cathepsin K (CatK). Alkyne-based inhibitors do not show indiscriminate thiol reactivity but potently inhibit CatK protease activity by formation of an irreversible covalent bond with the catalytic cysteine residue, confirmed by crystal structure analysis. The rate of covalent bond formation ( kinact) does not correlate with electrophilicity of the alkyne moiety, indicative of a proximity-driven reactivity. Inhibition of CatK-mediated bone resorption is validated in human osteoclasts. Together, this work illustrates the potential of alkynes as latent electrophiles in small molecule inhibitors, enabling the development of irreversible covalent inhibitors with an improved safety profile.

[Research progress in the mechanism of protein factors in regulating bone remodeling]

Objective

To review the role and mechanism of protein factors in bone remodeling, and provides theoretical basis for further elucidating the pathogenesis and clinical treatment of bone-related diseases.

Methods

The relevant research results at home and abroad in recent years were extensively consulted, analyzed, and summarized.

Results

Bone remodeling is an important physiological process to maintain bone homeostasis. Protein, as an important stimulator in bone remodeling, regulates the balance between bone resorption and bone formation.

Conclusion

At present, the research on the mechanism of protein in bone remodeling is insufficient. Therefore, it is necessary to further study the specific time, process, and interaction network of protein in bone remodeling, and to confirm its mechanism in bone remodeling, so as to reveal and treat the pathogenesis of bone-related diseases.

A Mild Inhibition of Cathepsin K Paradoxically Stimulates the Resorptive Activity of Osteoclasts in Culture

Cathepsin K (CatK) inhibition allows reducing bone resorption with specific advantages compared to the existing anti-osteoporosis drugs. Its clinical use appears even more promising with the recent development of ectosteric inhibitors. A confusing observation, however, is that a low dose of the active site CatK inhibitor odanacatib (ODN) was reported to decrease bone mineral density and increase serum levels of the bone resorption marker carboxy-terminal collagen crosslinks (CTX). The present study provides a possible explanation for this paradox. The resorptive activity of human osteoclasts seeded on bone slices was inhibited when subjected to ODN at doses of 20 nM, but about 100-fold lower doses induced a significant increase in CTX levels and in eroded surface (12 repeats). This low-dose-induced stimulation was prevented by inhibition of non-CatK cysteine proteinases, thereby indicating that the stimulation results from an interplay between CatK and other cysteine proteinases. Effective interplay between these proteinases was also shown in enzymatic assays where the CatK-mediated degradation of collagen was enhanced upon addition of cathepsins B or L. Furthermore, extracts of osteoclasts subjected to a low dose of ODN showed higher levels of cathepsin B compared with extracts of control osteoclasts. In conclusion, the low-dose-induced stimulation of resorption observed in the clinical study can be reproduced in osteoclasts cultured in the absence of any other cell. Our data support an osteoclast-intrinsic mechanism where a mild inhibition of CatK results in increased levels of other proteinases contributing to the collagen degradation process.

Proteases: Pivot Points in Functional Proteomics

Proteases drive the life cycle of all proteins, ensuring the transportation and activation of newly minted, would-be proteins into their functional form while recycling spent or unneeded proteins. Far from their image as engines of protein digestion, proteases play fundamental roles in basic physiology and regulation at multiple levels of systems biology. Proteases are intimately associated with disease and modulation of proteolytic activity is the presumed target for successful therapeutics. "Proteases: Pivot Points in Functional Proteomics" examines the crucial roles of proteolysis across a wide range of physiological processes and diseases. The existing and potential impacts of proteolysis-related activity on drug and biomarker development are presented in detail. All told the decisive roles of proteases in four major categories comprising 23 separate subcategories are addressed. Within this construct, 15 sets of subject-specific, tabulated data are presented that include identification of proteases, protease inhibitors, substrates, and their actions. Said data are derived from and confirmed by over 300 references. Cross comparison of datasets indicates that proteases, their inhibitors/promoters and substrates intersect over a range of physiological processes and diseases, both chronic and pathogenic. Indeed, "Proteases: Pivot Points …" closes by dramatizing this very point through association of (pro)Thrombin and Fibrin(ogen) with: hemostasis, innate immunity, cardiovascular and metabolic disease, cancer, neurodegeneration, and bacterial self-defense.

Regulatory aspects of the development of drugs for metabolic bone diseases - FDA and EMA perspective

Regulation of medicines involves complex scientific and public health policies which are reflected in the regulatory approaches used by the European Medicines Agency and the United States Food and Drug Administration for the approval of products developed for metabolic bone diseases. For osteoporosis therapies, utilized by many patients, the approaches and existing guidance for product development of both agencies are similar; confirmatory studies for the approval of osteoporosis products can rely on well-defined efficacy outcome parameters. Therapeutics for rare bone diseases, a rapidly expanding area, often require an individualized regulatory approach. This review outlines key aspects of these regulatory approaches applied by the two agencies for products for metabolic bone diseases.

Preservation of type H vessels and osteoblasts by enhanced preosteoclast platelet-derived growth factor type BB attenuates glucocorticoid-induced osteoporosis in growing mice

Survival of chronic diseases in childhood is often achieved utilizing glucocorticoids, but comes with significant side effects, including glucocorticoid-induced osteoporosis (GIO). Knowledge of the mechanism of GIO is limited to the adult skeleton. We explored the effect of genetic loss and inhibition of cathepsin K (Ctsk) as a potential treatment target in a young GIO mouse model as genetic loss of cathepsin K results in a mild form of osteopetrosis secondary to impaired osteoclast bone resorption with maintenance of bone formation. We first characterized the temporal osteoclast and osteoblast progenitor populations in Ctsk^-/- and wild type (WT) mice in the primary and secondary spongiosa, as sites representative of trabecular bone modeling and remodeling, respectively. In the primary spongiosa, Ctsk^-/- mice had decreased numbers of osteoclasts at young ages (2 and 4 weeks) and increased osteoblast lineage cells at later age (8 weeks) relative to WT littermates. In the secondary spongiosa, Ctsk^-/- mice had greater numbers of osteoclasts and osteoblast lineage cells relative to WT littermates. We next developed a young GIO mouse model with prednisolone 10 mg/m²/day injected intraperitoneally daily from 2 through 6 weeks of age. Overall, WT-prednisolone mice had lower bone volume per tissue volume, whereas Ctsk^-/--prednisolone mice maintained a similar bone volume relative to Ctsk^-/--vehicle controls. WT-prednisolone mice exhibited a decreased number of osteoclasts, tartrate-resistant acid phosphatase and platelet-derived growth factor type BB (PDGF-BB) co-positive cells, type H endothelial cells, and osteoblasts relative to WT-vehicle mice in both the primary and secondary spongiosa. Interestingly, Ctsk^-/--prednisolone mice demonstrated a paradoxical response with increased numbers of all parameters in primary spongiosa and no change in secondary spongiosa. Finally, treatment with a cathepsin K inhibitor prevented WT-prednisolone decline in osteoclasts, osteoblasts, type H vessels, and bone volume. These data demonstrate that cells in the primary and secondary spongiosa respond differently to glucocorticoids and genetic manipulation. Inhibition of osteoclast resorption that preserves osteoclast coupling factors, such as through inhibition of cathepsin K, may be a potential preventive treatment strategy against GIO in the growing skeleton.

Sclerostin vaccination mitigates estrogen deficiency induction of bone mass loss and microstructure deterioration

Sclerostin (SOST) is a Wnt signaling inhibitor detrimental to osteogenic differentiation and bone mineral acquisition. While control of SOST action delays the pathogenesis of skeletal disorders, the effects of SOST vaccination on the estrogen deficiency-induced bone deterioration remain elusive. In this study, we generated a SOST-Fc fusion protein which was composed of a SOST peptide Pro-Asn-Ala-Ile-Gly along with an IgG Fc fragment. SOST-Fc vaccination increased serum anti-SOST antibody levels and reduced serum SOST concentrations in mice. In vitro, anti-SOST serum attenuated the SOST-induced inhibition of osteogenic gene expression in osteoblast cultures. Administration with SOST-Fc increased serum levels of bone formation marker osteocalcin and alleviated the ovariectomy escalation of serum resorption markers CTX-1 and TRAP5b concentrations. It remarkably lessened the estrogen deficiency-mediated deterioration of bone mineral density, morphometric characteristics of trabecular bone, and mechanical strength of femurs and lumbar spines. The SOST-Fc-treated skeletal tissue exhibited moderate responses to the adverse actions of ovariectomy to bone mineral accretion, osteoclast surface, trabecular separation, and fatty marrow histopathology. SOST-Fc treatment increased serum osteoclast-inhibitory factor osteoprotegrin levels in conjunction with strong Wnt3a, β-catenin, and TCF4 immunostaining in osteoblasts, whereas it weakened the estrogen deficiency enhancement of osteoclast-promoting factor receptor activator of nuclear factor-κB ligand. Taken together, blockade of SOST action by SOST-Fc vaccination sustains Wnt signaling, which harmonizes bone mineral accretion and resorption reactions and thereby ameliorates ovariectomy-induced bone loss. This study highlights SOST-Fc fusion protein as a new molecular therapeutic potential for preventing from osteoporotic disorders.

Recent advances in models for screening potential osteoporosis drugs

INTRODUCTION

Osteoporosis is a growing health and health-economic problem due to the increased proportion of elderly people in the population. Basic and clinical advances in research over the past two decades have led to the development of different compounds with antiresorptive or anabolic activity on bone that improved substantially the management of patients with osteoporosis over calcitonin or estrogen replacement. New compounds are in preclinical and clinical development. Areas covered: In this review, the authors review the approaches for the preclinical and clinical development of antiresorptive and anabolic agents for osteoporosis, particularly focusing on the recent advances in technology and in the understanding of skeletal biology, together with their implications on novel osteoporosis drug discovery. Expert opinion: Based on the available evidence from the approved drugs for the treatment osteoporosis as well as from the different compounds under clinical development, it has become clear that long term nonclinical pharmacological studies with either bone quality and off-target effects as the main outcomes should be required for new drugs intended to treat osteoporosis. At the same time, basic and clinical advances in research have underlined the necessity to develop new technologies and new models for a thorough screening of the effects of new drugs on the different components of skeletal aging and bone fragility that cannot be assessed by bone mass measurement.

Translational studies provide insights for the etiology and treatment of cortical bone osteoporosis

Increasing attention is being focused on the important contributions of cortical bone to bone strength, fractures and osteoporosis therapies. Recent progress in human genome wide association studies in combination with high-throughput mouse gene knockout phenotyping efforts of multiple genes and advanced conditional gene inactivation in mouse models have successfully identified genes with crucial roles in cortical bone homeostasis. Particular attention in this review is given to genes, such as WNT16, POSTN and SFRP4, that differentially affect cortical and trabecular bone architecture. We propose that animal models of cortical bone metabolism will substantially contribute to developing anabolic osteoporosis therapies that improve cortical bone mass and reduce non-vertebral fracture risk.

Tanshinones that selectively block the collagenase activity of cathepsin K provide a novel class of ectosteric antiresorptive agents for bone

BACKGROUND AND PURPOSE

Attempts to generate active site-directed cathepsin K (CatK) inhibitors for the treatment of osteoporosis have failed because of side effects. We have previously shown that an ectosteric tanshinone CatK inhibitor isolated from Salvia miltiorrhiza blocked, selectively, the collagenase activity of CatK, without affecting the active site and demonstrated its bone-preserving activity in vivo. Here, we have characterize the antiresorptive potential of other tanshinones, which may provide a scaffold for side effect-free CatK inhibitors.

EXPERIMENTAL APPROACH

Thirty-one tanshinones were tested for their activity against CatK in enzymic and cell-based assays. The inhibitory potency against triple helical and fibrillar collagen degradation was determined in enzymic assays, by scanning electron microscopy and mechanical strength measurements. Human osteoclast assays were used to determine the effects of the inhibitors on bone resorption, its reversibility and osteoclastogenesis. Binding sites were characterized by molecular docking.

KEY RESULTS

Twelve compounds showed highly effective anti-collagenase activity and protected collagen against destruction and mechanical instability without inhibiting the hydrolysis of non-collagenous substrates. Six compounds were highly effective in osteoclast bone resorption assays with IC₅₀ values of <500 nM. None of these tanshinones had effects on cell viability, reversibility of bone resorption inhibition and osteoclastogenesis. The core pharmacophore of the tanshinones appears to be the three-ring system with either a para- or ortho-quinone entity.

CONCLUSIONS AND IMPLICATIONS

Our study identified several potent ectosteric antiresorptive CatK inhibitors from the medicinal plant, S. miltiorrhiza, which may avoid side effects seen with active site-directed inhibitors in clinical trials.

Pycnodysostosis with novel gene mutation and sporadic medullary thyroid carcinoma: A case report

RATIONALE

Pycnodysostosis is a rare autosomal recessive skeletal dysplasia caused by a mutation in the cathepsin K encoded by cathepsin K gene (CTSK). Medullary thyroid carcinoma (MTC) is also a relatively rare type of primary thyroid carcinoma.

PATIENT CONCERNS

A 31-year-old woman presenting a short stature and a palpable nodule in the front of her neck that had gradually increased in size during the last 2 years was referred to our department. She has experienced multiple fractures at lower limbs in the last 2 decades.

DIAGNOSES

The patient's clinical examination revealed short stature, underweight, a prominent forehead, stubby fingers, and a fixed nodule in the right thyroid lobe. Intraoral examination revealed multiple clinically malposed and missing teeth, as well as chronic periodontitis with a narrow and grooved palate. Radiographic examination revealed typical widely separated cranial sutures and an open anterior/posterior fontanel with an obtuse gonial angle, acroosteolysis, and osteosclerosis with narrowed medullary cavities. Ultrasonography of the thyroid gland showed a marked hypoechoic solid nodule in the right lobe in which tumor cell clusters were confirmed by ultrasound-guided fine needle aspiration biopsy and was suspected to be MTC. Laboratory tests revealed dramatically elevated serum calcitonin >2000 pg/L (reference range: 0-5 pg/L) and carcinoembryonic antigen (CEA) 134.37 ng/mL (reference range: 0-5 ng/mL). Genotypic screening revealed compound heterozygous mutations in the CTSK gene (c.158delA, P.Asn53Thr/c.C830T, P.Ala277Val) but no mutation associated with the familial forms of MTC.

INTERVENTIONS

The patient underwent a total thyroidectomy with right-sided functional neck dissection.

OUTCOMES

CEA and serum calcitonin decreased significantly postthyroidectomy, and no further fracture has been reported by the patient so far.

LESSONS

The present study is the first to report a rare case of the coexistence of pycnodysostosis with a compound CTSK gene mutation and sporadic MTC. Radiological techniques and gene analysis play key roles in the definitive diagnosis.

A composite docking approach for the identification and characterization of ectosteric inhibitors of cathepsin K

Cathepsin K (CatK) is a cysteine protease that plays an important role in mammalian intra- and extracellular protein turnover and is known for its unique and potent collagenase activity. Through studies on the mechanism of its collagenase activity, selective ectosteric sites were identified that are remote from the active site. Inhibitors targeting these ectosteric sites are collagenase selective and do not interfere with other proteolytic activities of the enzyme. Potential ectosteric inhibitors were identified using a computational approach to screen the druggable subset of and the entire 281,987 compounds comprising Chemical Repository library of the National Cancer Institute-Developmental Therapeutics Program (NCI-DTP). Compounds were scored based on their affinity for the ectosteric site. Here we compared the scores of three individual molecular docking methods with that of a composite score of all three methods together. The composite docking method was up to five-fold more effective at identifying potent collagenase inhibitors (IC50 < 20 μM) than the individual methods. Of 160 top compounds tested in enzymatic assays, 28 compounds revealed blocking of the collagenase activity of CatK at 100 μM. Two compounds exhibited IC50 values below 5 μM corresponding to a molar protease:inhibitor concentration of <1:12. Both compounds were subsequently tested in osteoclast bone resorption assays where the most potent inhibitor, 10-[2-[bis(2-hydroxyethyl)amino]ethyl]-7,8-diethylbenzo[g]pteridine-2,4-dione, (NSC-374902), displayed an inhibition of bone resorption with an IC50-value of approximately 300 nM and no cell toxicity effects.