title: Cathepsin K inhibitors increase distal femoral bone mineral density in rapidly growing rabbits.
abstract: BACKGROUND
  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.
  
  
  METHODS
  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
  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.
  
  
  CONCLUSIONS
  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.
authors: Brenda L
authors: BL
authors: Pennypacker
authors: Renata M
authors: RM
authors: Oballa
authors: Sonia
authors: S
authors: Levesque
authors: Donald B
authors: DB
authors: Kimmel
authors: Merck Research Laboratories, Merck & Co,, Inc,, P,O, Box 100, Whitehouse Station, NJ 08889, USA. le_duong@merck.com.
authors: Le T
authors: LT
authors: Duong
mesh_terms: D000818:Animals; D001549:Benzamides / Q000494:pharmacology; D001713:Biphenyl Compounds / Q000494:pharmacology; D015519:Bone Density / Q000187:drug effects*; D050071:Bone Density Conservation Agents / Q000494:pharmacology*; D056657:Cathepsin K / Q000187:drug effects*; D005260:Female; D005269:Femur / Q000187:drug effects; D006801:Humans; D023421:Models, Animal; D009570:Nitriles / Q000494:pharmacology; D010879:Piperazines / Q000494:pharmacology; D011817:Rabbits; D011897:Random Allocation; D013844:Thiazoles / Q000494:pharmacology
chemical_list: D001549:Benzamides; D001713:Biphenyl Compounds; D050071:Bone Density Conservation Agents; C507712:CRA 013783; C527129:L-873724; C476986:N-(cyanomethyl)-4-methyl-2-(4'-(1-piperazinyl)(1,1'-biphenyl)-3-yl)pentanamide; D009570:Nitriles; D010879:Piperazines; D013844:Thiazoles; D056657:Cathepsin K; C527128:odanacatib