Odanacatib increases mineralized callus during fracture healing in a rabbit ulnar osteotomy model

The effects of the cathepsin K inhibitor odanacatib (ODN) on fracture healing were monitored for ~6 and 15 weeks post-fracture in two separate studies using the unilateral transverse mid-ulnar osteotomy model in skeletally mature female rabbits. Rabbits were pre-treated for 3-4 weeks with vehicle (Veh), ODN (2 mg/kg, po, daily), or alendronate (ALN) (0.3 mg/kg, sc, twice-weekly) prior to osteotomy. In Study 1, the animals were maintained on the same respective treatment for ~6 weeks. In Study 2, the animals were also continued on the same therapy or switched from Veh to ODN or ODN to Veh for 15 weeks. No treatment-related impairment of fracture union was seen by qualitative histological assessments in the first study. Cartilage retention was detected in the calluses of ALN-treated rabbits at week-6, while calluses in the ODN and Veh groups contained bony tissue with significantly less residual cartilage. ODN treatment also markedly increased the number of cathepsin K-(+) osteoclasts in the callus, indicating enhanced callus remodeling. From the second study, ex vivo DXA and pQCT confirmed that ODN treatment pre- and post-osteotomy increased callus bone mineral content and bone mineral density (BMD) versus Veh (p < 0.001) and discontinuation of ODN post-surgery returned callus BMD to Veh. Peak load of ODN- or ALN-treated calluses were comparable to Veh. ODN increased callus yield load (20%, p = 0.056) and stiffness (26%, p < 0.05) versus Veh. These studies demonstrated that ODN increased mineralized callus during the early phase of fracture repair without impairing callus formation or biomechanical integrity at the fracture site.