The cathepsin K inhibitor AAE581 induces morphological changes in osteoclasts of treated patients

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

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

Clinical and translational pharmacology of the cathepsin K inhibitor odanacatib studied for osteoporosis

Cathepsin K (CatK) is a cysteine protease abundantly expressed by osteoclasts and localized in the lysosomes and resorption lacunae of these cells. CatK is the principal enzyme responsible for the degradation of bone collagen. Odanacatib is a selective, reversible inhibitor of CatK at subnanomolar potency. The pharmacokinetics of odanacatib have been extensively studied and are similar in young healthy men, postmenopausal women and elderly men, and were qualitatively similar throughout Phase 1 development and in-patient studies. Following 3 weeks of 50 mg once weekly dosing the geometric mean area under the curve from 0 to 168 hours was 41.1 μM h, the concentration at 168 hours was 126 nM and the harmonic mean apparent terminal half-life was 84.8 hr. Odanacatib exposure increased in a less than dose proportional manner due to solubility limited absorption. It is estimated that approximately 70% of the absorbed dose of odanacatib is eliminated via metabolism, 20% is excreted as unchanged drug in the bile or faeces, and 10% is excreted as unchanged drug in the urine. The systemic clearance was low (approximately 13 mL/min). Odanacatib decreases the degradation of bone matrix proteins and reduces the efficiency of bone resorption with target engagement confirmed by a robust decrease in serum C-telopeptides of type 1 collagen (approximately 60%), urinary aminoterminal crosslinked telopeptides of type 1 collagen to creatinine ratio (approximately 50%) and total urine deoxypyridinoline/Cr (approximately 30%), with an increase in serum cross-linked carboxy-terminal telopeptide of type 1 collagen (approximately 55%). The 50-mg weekly dosing regimen evaluated in Phase 3 achieved near maximal reduction in bone resorption throughout the treatment period. The extensive clinical programme for odanacatib, together with more limited clinical experience with other CatK inhibitors (balicatib and ONO-5334), provides important insights into the clinical pharmacology of CatK inhibition and the potential role of CatK in bone turnover and mineral homeostasis. Key findings include the ability of this mechanism to: (i) provide sustained reductions in resorption markers, increases in bone mineral density, and demonstrated fracture risk reduction; (ii) be associated with relative formation-sparing effects such that sustained resorption reduction is achieved without accompanying meaningful reductions in bone formation; and (iii) lead to increases in osteoclast number as well as other osteoclast activity (including build-up of CatK enzyme), which may yield transient increases in resorption following treatment discontinuation and the potential for nonmonotonic responses at subtherapeutic doses.

Type I saikosaponins a and d inhibit osteoclastogenesis in bone marrow-derived macrophages and osteolytic activity of metastatic breast cancer cells

Many osteopenic disorders, including a postmenopausal osteoporosis and lytic bone metastasis in breast and prostate cancers, are linked with a hyperosteoclast activity due to increased receptor activator of nuclear factor kappa-B ligand (RANKL) expression in osteoblastic/stromal cells. Therefore, inhibition of RANKL-induced osteoclastogenesis and osteoclast-induced bone resorption is an important approach in controlling pathophysiology of these skeletal diseases. We found that, of seven type I, II, and III saikosaponins isolated from Bupleurum falcatum, saikosaponins A and D, type I saikosaponins with an allyl oxide linkage between position 13 and 28 and two carbohydrate chains that are directly attached to the hydroxyl groups in position 3, exhibited the most potent inhibition on RANKL-induced osteoclast formation at noncytotoxic concentrations. The stereochemistry of the hydroxyl group at C16 did not affect their activity. Saikosaponins A and D inhibited the formation of resorptive pits by reducing the secreted levels of matrix metalloproteinase- (MMP-) 2, MMP-9, and cathepsin K in RANKL-induced osteoclasts. Additionally, saikosaponins A and D inhibited mRNA expression of parathyroid hormone-related protein as well as cell viability and invasion in metastatic human breast cancer cells. Thus, saikosaponins A and D can serve as a beneficial agent for the prevention and treatment of osteoporosis and cancer-induced bone loss.

Genetically Engineered Mouse Models Reveal the Importance of Proteases as Osteoarthritis Drug Targets

More than two decades of research has revealed a combination of proteases that determine cartilage degradation in osteoarthritis. These include metalloproteinases, which degrade the major macromolecules in cartilage, aggrecan and type II collagen, serine proteases, and cysteine proteases, for example cathepsin K. This review summarizes the function of proteases in osteoarthritis progression, as revealed by studies of genetically engineered mouse models. A brief overview of the biochemical characteristics and features of several important proteases is provided, with the objective of increasing understanding of their function. Published data reveal at least three enzymes to be major targets for osteoarthritis drug development: ADAMTS-5, MMP-13, and cathepsin K. In surgical models of osteoarthritis, mice lacking these enzymes are protected from cartilage damage and, to varying degrees, from bone changes. In-vivo studies targeting these proteases with selective small-molecule inhibitors have been performed for a variety of animal models. Mouse models will provide opportunities for future tests of the therapeutic effect of protease inhibitors, both on progression of structural damage to the joint and on associated pain.

The bone resorption inhibitors odanacatib and alendronate affect post-osteoclastic events differently in ovariectomized rabbits

Odanacatib (ODN) is a bone resorption inhibitor which differs from standard antiresorptives by its ability to reduce bone resorption without decreasing bone formation. What is the reason for this difference? In contrast with other antiresorptives, such as alendronate (ALN), ODN targets only the very last step of the resorption process. We hypothesize that ODN may therefore modify the remodeling events immediately following osteoclastic resorption. These events belong to the reversal phase and include recruitment of osteoblasts, which is critical for connecting bone resorption to formation. We performed a histomorphometric study of trabecular remodeling in vertebrae of estrogen-deficient rabbits treated or not with ODN or ALN, a model where ODN, but not ALN, was previously shown to preserve bone formation. In line with our hypothesis, we found that ODN treatment compared to ALN results in a shorter reversal phase, faster initiation of osteoid deposition on the eroded surfaces, and higher osteoblast recruitment. The latter is reflected by higher densities of mature bone forming osteoblasts and an increased subpopulation of cuboidal osteoblasts. Furthermore, we found an increase in the interface between osteoclasts and surrounding osteoblast-lineage cells. This increase is expected to favor the osteoclast-osteoblast interactions required for bone formation. Regarding bone resorption itself, we show that ODN, but not ALN, treatment results in shallower resorption lacunae, a geometry favoring bone stiffness. We conclude that, compared to standard antiresorptives, ODN shows distinctive effects on resorption geometry and on reversal phase activities which positively affect osteoblast recruitment and may therefore favor bone formation.

Therapeutic inhibition of cathepsin K-reducing bone resorption while maintaining bone formation

Osteoporosis is a disease of high bone remodeling with an imbalance of bone resorption over bone formation, resulting in decreased bone mineral density and deterioration of bone microarchitecture. From the emerging understandings of the molecular and cellular regulators of bone remodeling, potential new targets for therapeutic intervention for this disease have been identified. Cathepsin K (CatK), a cysteine protease produced by osteoclasts, is the primary enzyme mediating the degradation of the demineralized bone matrix. Current genetic and pharmacological evidence from studies in multiple preclinical species have consistently demonstrated that inhibition of CatK results in the reduction of bone resorption while allowing bone formation to continue. Early results from clinical studies with several investigational CatK inhibitors indicate that the impact of CatK inhibition on bone formation is distinct from that of either the bisphosphonates or the anti-receptor activator of nuclear factor-κB ligand antibody, denosumab. Odanacatib, a highly selective, reversible and potent inhibitor of CatK, is currently in phase III clinical trials for the treatment of postmenopausal osteoporosis.