Cathepsin K (CatK), a cysteine protease, has been implicated in the process of bone resorption and inflammation. Selective inhibitors of CatK are promising therapeutic agents for the treatment of diseases associated with excessive bone loss and osseous inflammation, such as osteoarthritis, rheumatoid arthritis, periodontitis, osteoporosis, and multiple myeloma. Multiple reports have emerged over the last several years demonstrating the effect of different CatK inhibitors on osteo-inflammatory conditions. Therefore, the study of CatK inhibition as a target to prevent bone loss and inflammation, and influence bone marrow osseous progenitor cells, in a large animal model, is the subject of this dissertation. The horse was selected as the large animal model because this species suffers from ailments of adaptive bone remodeling in their sport performance and CatK inhibitors may serve as therapeutics in this species as well as serve as a large animal model for human applications.
In the first phase of this work, we determined an optimal dose and dose interval for a CatK inhibitor (CatKI), VEL-0230, in healthy adult horses. Plasma pharmacokinetic (PK) and bone resorption biomarker [carboxy-terminal cross-linking telopeptide of type I collagen (CTX-1)] analyses were performed following single and multiple oral dose protocols of a CatKI (VEL-0230) in horses. Weekly administration of VEL-0230, at a dose of 4 mg/ kg body weight, provided effective inhibition of bone resorption in young exercising horses that returned to baseline within 7 days after drug withdrawal even after multiple doses.
In the second phase of this work, we evaluated bone structure and turnover in healthy young exercising horses receiving repeated oral dosing of a CatKI in a randomized, controlled, double-blinded, prospective, sufficiently powered clinical trial. With the objectives of: 1. To determine whether repeated dosing of a CatKI produced a desired inhibition of the bone resorption biomarker CTX-1, 2. To determine the effect of repeated dosing of this CatKI on bone homeostasis as evaluated by quantitative imaging, bone histomorphometric measurements, immuno-staining of bone CatK and the quantitative analysis of bone turnover related gene expression. We found that repetitive dosing for 4 weeks of this CatKI transiently and repetitively inhibited plasma CTX-1 (reflecting inhibition of bone collagen resorption) and increased bone plasma osteocalcin levels (reflecting increased bone formation), however, over the 4 week administration time period, CatKI did not prevent normal adaptive bone remodeling to exercise as both the control and CatK-treated groups demonstrated radiographic evidence of remodeling. Bone morphology, and density did not differ between groups by all intensive measures in these normal animals over this 4 weeks period. We concluded that this CatKI had minimal negative influence on adaptive remodeling in normal young exercising animals, permitting endogenous mechanical and biologic signals of exercise to control bone density and mineralization.
In the third phase of this work, we compared the effects of in vivo CatKI administration for 4 weeks to control vehicle administration for 4 weeks on equine bone marrow-derived stem and progenitor cells. Bone marrow was harvested from the sternum of six horses (3 assigned to receive CatKI and 3 assigned to receive control vehicle) prior the start of the study and from twelve horses 24 hours after the fourth weekly dose of CatKI (n=6) or vehicle (n=6). Bone marrow was cultured in monolayer to select/deselect bone marrow derived-mesenchymal stromal cells (BMD-MSCs) and cell characterization by flow cytometry and differentiation capacities were studied. We hypothesized that neither exercise nor repeated oral administration of VEL-0230, a potent and selective CatKI, would affect the characterization or differentiation capacity of equine bone marrow derived-stem and progenitor cell, specifically chondrogenic, osteogenic or osteoclastogenic tri-lineage differentiation or expression of stem cell markers. Bone marrow cells from both groups were multi-potent and showed strong capacity to differentiate into chondrogenic, osteogenic and osteoclastic pathways with no difference in staining scores between the CatKI and control groups. There was no significant difference in expression of CD90, CD34, CD11b, MHCI, and MHCII molecules from before exercise to after exercise in either the control or CatKI groups. There was no difference in viability or differentiation capacity between control and CatKI groups as determined by 7AAD, Toluidine blue, von Kossa, Alizarin red, and Tartrate-resistant acid phosphatase (TRAP) staining. Cathepsin K inhibition for 4 weeks did not alter the regenerative ability of bone marrow, specifically for bone formation (osteoblastogenesis) or remodeling (osteoclastogenesis).
In the fourth phase of work, we have studied two inflammatory models in equine bone marrow nucleated cells (BMNCs); the Lipopolysaccharide (LPS) and the unmethylated CpG stimulation with the following objectives: 1. To determine whether CatK inhibition will alter the cytokine secretion by stimulated BMNCs; specifically IL-1ß, IL-6, and TNF-a, and 2. To determine the changes in BMNCs surface markers’ expression and MHC molecules under CatK inhibition. Equine BMNCs were exposed to VEL-0230 at concentrations of 0, 1, and 10 µM in cell culture media with and without LPS (1 µg/ml) and unmethylated CpG (5 µg/ml). Cathepsin K inhibition promoted BMNC viability and reduced cell apoptosis. Moreover, CatK inhibition significantly decreased cytokine secretion of either naive or stimulated BMNCs, and decreased their MHC molecules expression. In conclusion, CatK inhibition in horses did affect BMNCs other than mature osteoclasts rendering them hypo-responsive to both TLR4- and TLR9-induced inflammation, predicting a proteolytic activity for CatK within the MyD88 pathway and/or the following proteolytic events required for the cytokines secretion.