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

Abstract

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.

Keywords: callus formation; cathepsin K; fracture healing; osteoclast; rabbit.

Figure 1

(A) Cartilage and bony callus retention in fractured calluses from Study 1 at day 41 post‐osteotomy. Representative images from paraffin‐embedded sections from the fractured ulnae of (a) vehicle‐, (b) ODN‐, and (c) ALN‐treated rabbits and stained with Safranin‐O/Fast Green to identify cartilaginous (red) and mineralized callus formation (green) at the osteotomy site. Note the presence of cartilage in the osteotomy gap in the ALN‐treated group. (B–D) Histomorphometric analysis to quantitate the area of cartilaginous region, bony callus, and periosteal callus of fractured ulnae post‐osteotomy. (B: bone, BM: bone marrow, C: cartilage). Double scale bar, 1.5 mm. N = 8–9/group; mean ± SEM.

Figure 2

Osteoclast recruitment to fractured callus from Study 1 at day 41 post‐osteotomy. Representative images from serial paraffin‐embedded sections from the same fractured ulnae showed in Figure 1, including (a) vehicle‐, (b) ODN‐, and (c) ALN‐treated rabbits and immunostained with anti‐cathepsin K monoclonal antibody and then counterstained with toluidine‐blue. (a–c) Double scale bar, 1.5 mm. (d–f) Large multinucleated osteoclasts (Oc, yellow arrows) are shown in the respective inset from a–c, viewed at higher magnification with scale bar, 100 μm. (B: bone, BM: bone marrow, C: cartilage).

Figure 3

Schematic of the Study design 2. Veh: vehicle; ODN: odanacatib; ALN: alendronate. In vivo radiographic (X‐ray) scanning and ex vivo DXA are noted.

Figure 4

Representative radiographic images of the ulnar fracture sites from Study 2 at day 28 and week 15 post‐osteotomy. Arrows indicate the original osteotomy sites.

Figure 5

DXA‐based bone mineral density of intact (A) and fractured ulnae (B) from Study 2 at week 15 post‐osteotomy. (A) aBMD of the intact ulna did not show differences between treatment groups; (B) aBMD of the fractured ulnae showed significant increases with treatment either with ALN or with ODN administered continuously or right after fracture. N = 13–15/group. Data are expressed as mean ± SEM.