Sclerostin antibody increases bone mass by stimulating bone formation and inhibiting bone resorption in a hindlimb-immobilization rat model

Sclerostin monoclonal antibody (Scl-Ab) has been shown to increase bone mass and bone strength by stimulating bone formation in an ovariectomy-induced bone loss rat model. The purpose of this study was to determine the effects of Scl-Ab in a rat immobilization/disuse model in which there was both a decrease in bone formation and an increase in bone resorption. Ten-month-old female Sprague Dawley rats were divided into normal weight-bearing (normal-loaded, NL) and right hindlimb-immobilization (under-loaded, UL) groups. Both NL and UL rats were treated with vehicle or Scl-Ab at 5 or 25 mg/kg, twice per week for 4 weeks. Trabecular and cortical bone histomorphometric analyses were performed on the proximal tibial metaphysis (PTM) and tibial shaft (TS). Compared to NL controls, UL rats had reduced body and muscle weights, increased bone marrow fat cells in the PTM, increased trabecular bone resorption and periosteal mineral apposition rate (MAR) as well as decreased trabecular MAR and bone formation rate (BFR/BS). In NL bones, treatment with Scl-Ab significantly increased bone formation and decreased bone resorption, resulting in increased trabecular and cortical bone mass. In UL trabecular bone, treatment with Scl-Ab at 5 or 25 mg/kg induced significant and dose-dependent increases in trabecular bone volume and thickness, mineralized surfaces (MS/BS), MAR and BFR/BS, and a significant decrease in eroded surface (Er.S/BS) compared with UL controls. In UL cortical bone, Scl-Ab treatment induced significant increases in cortical width, periosteal and endocortical MS/BS, MAR and BFR/BS, and significant decreases in endocortical Er.S/BS compared with UL controls. Taken together, these findings suggest that antibody-mediated blockade of sclerostin represents a promising new therapeutic approach for the anabolic treatment of immobilization-induced osteopenia.

Research Highlights
► Immobilization decreased bone formation and increased bone resorption in rats.
► Sclerostin inhibition increased trabecular, endocortical and periosteal bone formation in immobilized tibia.
► The effect of Scl-Ab on trabecular bone in immobilized tibia appears to be less dramatic than that in normal-loaded tibia.