Open Access Research article

Cathepsin K inhibitors increase distal femoral bone mineral density in rapidly growing rabbits

Brenda L Pennypacker4, Renata M Oballa1, Sonia Levesque2, Donald B Kimmel3 and Le T Duong4*

Author Affiliations

1 Inception Sciences Canada Inc., Suite 210, 887 Great Northern Way, V5T 4T5, Vancouver, BC Canada

2 AniDis, 4850 chemin Bois Franc, Suite 200, St-Laurent, H4S 1A7, Province of Quebec, Canada

3 Osteoporosis Research Center, Creighton University, 601 North 30th Street, Omaha, NE 68131, USA

4 Merck Research Laboratories, Merck & Co., Inc., P.O. Box 100, Whitehouse Station, NJ 08889, USA

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BMC Musculoskeletal Disorders 2013, 14:344  doi:10.1186/1471-2474-14-344

Published: 9 December 2013



Selective and reversible inhibitors of human Cathepsin K (CatK), including odanacatib (ODN), have been developed as potential therapeutics for the treatment of osteoporosis. Inhibitors of human CatK show significantly less potency for the rodent enzymes compared with that for the human or rabbit enzymes; thus the Schenk model in growing rabbit was developed as a screening assay for the in vivo activity of CatK inhibitors in blocking bone resorption.


In this study, the efficacy of the selective inhibitors L-833905, L-006235, L-873724, and L-1037536 (ODN) of human CatK in the rapidly growing rabbit ‘Schenk’ model (age seven weeks) was compared to vehicle, using the bisphosphonate, alendronate (ALN), as a positive control, to assess inhibition of bone resorption. An enzyme inhibition assay (EIA) and an in vitro bone resorption assay using rabbit osteoclasts on bovine cortical bone slices were performed to evaluate the potency of these CatK inhibitors. Bone mineral density of the distal femur (DFBMD) was measured after ten days of treatment using ex vivo DXA densitometry.


Results of the EIA using rabbit CatK and the rabbit bone resorption assay showed that three of the four compounds (L-006235, L-873724, and ODN) had similar potencies in the reduction of collagen degradation. L-833905 appeared to be a weaker inhibitor of CatK. Taking into account the respective in vitro potencies and pharmacokinetic profiles via oral administration, the efficacy of these four CatK inhibitors was demonstrated in a dose-related manner in the growing rabbit. Significant increases in DFBMD in animals dosed with the CatK inhibitors compared to vehicle were seen.


Efficacy of the CatK inhibitors in the Schenk rabbit correlated well with that in the in vitro rabbit bone resorption assay and in the ovariectomized rabbit model as previously published. Hence, these studies validated the rabbit Schenk assay as a rapid and reliable in vivo model for prioritizing human CatK inhibitors as potential therapeutic agents.

Odanacatib; Cathepsin K inhibitor; Osteoclast; Antiresorptives; DXA; Rabbit Schenk assay; Postmenopausal osteoporosis