628 Acta Orthopaedica 2011; 82 (5): 628–632 Anti-sclerostin antibody and mechanical loading appear to influence metaphyseal bone independently in rats Fredrik Agholme1, Hanna Isaksson2, Xiaodong Li3, Hua Zhu Ke3, and Per Aspenberg1 1Orthopaedics Division, Department of Clinical and Experimental Medicine, Faculty of Medicine, Linköping University, Linköping, Sweden; 2Department of Applied physics, University of Eastern Finland, Kuopio, Finland; 3Metabolic Disorders, Amgen Inc., Thousand Oaks, CA, USA. Correspondence: fredrik.agholme@liu.se Submitted 11-03-18. Accepted 11-06-12 Background and purpose Sclerostin is produced by osteocytes and is an inhibitor of bone formation. Thus, inhibition of scleros- tin by a monoclonal antibody increases bone formation and improves fracture repair. Sclerostin expression is upregulated in unloaded bone and is downregulated by loading. We wanted to determine whether an anti-sclerostin antibody would stimulate metaphyseal healing in unloaded bone in a rat model. Methods 10-week-old male rats (n = 48) were divided into 4 groups, with 12 in each. In 24 rats, the right hind limb was unloaded by paralyzing the calf and thigh muscles with an injec- tion of botulinum toxin A (Botox). 3 days later, all the animals had a steel screw inserted into the right proximal tibia. Starting 3 days after screw insertion, either anti-sclerostin antibody (Scl-Ab) or saline was given twice weekly. The other 24 rats did not receive Botox injections and they were treated with Scl-Ab or saline to serve as normal-loaded controls. Screw pull-out force was mea- sured 4 weeks after insertion, as an indicator of the regenerative response of bone to trauma. Results Unloading reduced the pull-out force. Scl-Ab treat- ment increased the pull-out force, with or without unloading. The response to the antibody was similar in both groups, and no statis- tically significant relationship was found between unloading and antibody treatment. The cancellous bone at a distance from the screw showed changes in bone volume fraction that followed the same pattern as the pull-out force. Interpretation Scl-Ab increases bone formation and screw fixation to a similar degree in loaded and unloaded bone.  The secreted glycoprotein sclerostin is the product of the SOST gene. Sclerostin is an important negative regulator of bone, and naturally occurring mutations of the SOST gene in humans lead to the high bone mass condition sclerostosis (Balemans et al. 2001, Brunkow et al. 2001). This high bone mass pheno- type is also present in animal models of SOST deficiency (Li et al. 2008). Sclerostin asserts its function, in part, by inhibiting canonical Wnt signaling (Li et al. 2005). This is important for osteoblast differentiation (Galli et al. 2010) and also for bone healing and regeneration (Chen et al. 2007, Kim et al. 2007). The SOST gene is expressed almost exclusively in osteocytes (Poole et al. 2005), and sclerostin expression is thought to be a means for osteocytes to locally regulate bone formation (Galli et al. 2010). Sclerostin appears to be vital for the bone to be able to respond to mechanical loading (Robling et al. 2008), and lack of sclerostin prevents osteopenia due to unloading (Lin et al. 2009). One therapeutic option has been to block sclerostin with an antibody. Such treatment has increased bone mass in animal models of postmenopausal osteoporosis (Li et al. 2009) or disuse-induced bone loss (Tian et al. 2011), and in gonad-intact aged male rats and non-human primates (Li et al. 2010, Ominsky et al. 2010). Furthermore, fracture healing has been found to be improved in rodents and non-human primates treated with an anti-sclerostin antibody (Agholme et al. 2010, Ominsky et al. 2011). We have previously shown that inhibition of sclerostin improves bone regeneration and implant fixation during normal loading conditions (Agholme et al. 2010). However, in contrast to laboratory animals, many patients do not bear weight on fractured limbs for a long time. It is therefore important to determine the effect of sclerostin inhibition on bone healing under unloaded conditions. Paralysis of hind limb muscles using botulinum toxin A (Botox) causes rapid bone loss due to reduced weight bearing (Chappard et al. 2001, Warner et al. 2006). We examined the effect of sclerostin inhibition on metaphyseal bone healing in a rat model with Botox injections. Fully weight-bearing ani- mals were included as controls. Materials and methods Forty-eight 10-week-old male Sprague-Dawley rats (Taconic, Lille Skensved, Denmark) with a mean weight of 330 (SD 18) Open Access - This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the source is credited. DOI 10.3109/17453674.2011.625539