Temporal changes in systemic and local expression of bone turnover markers during six months of sclerostin antibody administration to ovariectomized rats

- Scl-Ab administration to mature OVX rats increased bone formation and bone volume.
- Stimulation of bone formation was greater after 6 versus 26 weeks of Scl-Ab administration.
- Scl-Ab administration decreased bone resorption parameters through 26 weeks.
- Skeletal expression of numerous osteocyte genes was increased by Scl-Ab administration, with sost showing the greatest induction.
- Increased skeletal sost expression may contribute to the attenuation of anabolic responses to Scl-Ab with longer-term administration.

Sclerostin (Scl) is an osteocyte protein that decreases bone formation, and its inhibition by neutralizing antibodies (Scl-Ab) increases bone formation, mass and strength. We investigated the effects of Scl-Ab in mature ovariectomized (OVX) rats with a mechanistic focus on longer-term responses of osteoclasts, osteoblasts and osteocytes. Four-month-old Sprague-Dawley rats had OVX or sham surgery. Two months later, sham controls received sc vehicle while OVX rats received vehicle (OVX-Veh) or Scl-Ab (25 mg/kg) once weekly for 6 or 26 weeks followed by necropsy (n = 12/group). Terminal blood was collected for biochemistry, non-adherent marrow cells were harvested from femurs for ex vivo osteoclast formation assays, and vertebrae and tibiae were collected for dynamic histomorphometry and mRNA analyses. Scl-Ab treatment led to progressively thicker but fewer trabeculae in the vertebra, leading to increased trabecular bone volume and reduced trabecular surfaces. Scl-Ab also increased cortical bone volume in the tibia, via early periosteal expansion and progressive endocortical contraction. Scl-Ab significantly reduced parameters of bone resorption at week 6 relative to OVX-Veh controls, including reduced serum TRACP-5b, reduced capacity of marrow cells to form osteoclasts ex vivo, and > 80% reductions in vertebral trabecular and tibial endocortical eroded surfaces. At week 26, serum TRACP-5b and ex vivo osteoclast formation were no longer reduced in the Scl-Ab group, but eroded surfaces remained > 80% lower than in OVX-Veh controls without evidence for altered skeletal mRNA expression of opg or rankl. Scl-Ab significantly increased parameters of bone formation at week 6 relative to OVX-Veh controls, including increases in serum P1NP and osteocalcin, and increased trabecular, endocortical and periosteal bone formation rates (BFRs). At week 26, surface-referent trabecular BFR remained significantly increased in the Scl-Ab group versus OVX-Veh controls, but after adjusting for a reduced extent of trabecular surfaces, overall (referent-independent) trabecular BFR was no longer significantly elevated. Similarly, serum P1NP and osteocalcin were no longer significantly increased in the Scl-Ab group at week 26. Tibial endocortical and periosteal BFR were increased at week 6 in the Scl-Ab group versus OVX-Veh controls, while at week 26 only endocortical BFR remained increased. The Scl-Ab group exhibited significant increments in skeletal mRNA expression of several osteocyte genes, with sost showing the greatest induction in both the tibia and vertebra. We propose that Scl-Ab administration, and/or the gains in bone volume that result, may have increased osteocytic expression of Scl as a possible means of regulating gains in bone mass.