Evaluation of the effects of systemic treatment with a sclerostin neutralizing antibody on bone repair in a rat femoral defect model

Abstract

Systemic administration of a sclerostin neutralizing antibody (Scl-Ab) has been shown to enhance fracture callus density and strength in several animal models. In order to further evaluate the potential of Scl-Ab to improve healing in a bone defect model, we evaluated Scl-Ab in a 3 mm femoral defect in young male outbred rats. Scl-Ab was given either continuously for 6 or 12 weeks after surgery or with 2 weeks of delay for 10 weeks. Bone formation was assessed by radiographs, m-CT, and histology. Complete bony union was achieved in only a few defects after 12 weeks of healing (Scl-Ab treated 5/30, vehicle treated 1/15). m-CT evaluation demonstrated a significant increase in the BV/TV in the defect in the delayed treatment group (65%, p < 0.05), but a non-significant increase in the continuous group (35%, p ¼ 0.11) compared to control. However, both regimens induced an anabolic response in the bone proximal and distal to the defect and in the un-operated femurs. We demonstrate that treatment with Scl-Ab can enhance bone repair in a bone defect and in the surrounding host bone, but lacks the osteoinductive activity to heal it. This agent seems to be most effective in bone repair scenarios where there is cortical integrity. The Wnt/b-catenin signaling pathway is known to be a key regulator of bone formation and its role in fracture healing has been established in several studies. 1-3This pathway is a target for developing strategies to promote bone fracture healing. 4 In pre-clinical studies activation of the Wnt pathway either by inhibiting Wnt inhibitors such as sclerostin [5][6][7] or DKK1 8 or more recently by liposomal delivery of Wnt3a 9 has been shown to enhance bone repair. Fracture non-union may occur in as many as 10% of all fractures. 10Treatment of non-unions may require multiple surgeries and prolonged hospital stays which leads to increased cost of treatment.11 Autologous bone graft remains the gold standard for management of nonunions, but there are disadvantages including donor site morbidity and limited availability. 12,13 BMPs are osteoinductive proteins that are FDA approved to treat open tibial shaft fractures 14,15 and to facilitate lumbar spine fusions. 16 The success of BMPs in humans has been variable and it is an expensive treatment associated with side effects such as local soft tissue edema 17 and ectopic bone formation.18 Therefore there is interest in identifying systemic agents that can enhance the healing of fractures.Sclerostin, a product of SOST gene, is primarily secreted by osteocytes and inhibits the Wnt pathway by interacting with the LRP5/6. 19 Mutations in SOST gene causing sclerostin loss of function are associated with a clinical syndrome of progressive bone thickening known as sclerosteosis. 20,21 Efficacy of a monoclonal sclerostin neutralizing antibody (Scl-Ab) to increase bone mass and strength and in promoting fracture healing has been demonstrated in several animal models including female osteoporotic rats, aged male rats, and gonad-intact non-human primates....

Endochondral fracture healing with external fixation in the Sost knockout mouse results in earlier fibrocartilage callus removal and increased bone volume fraction and strength

“…Sclerostin negatively regulates bone formation, and the development of Sclerostin inhibitory strategies provides a potential opportunity to enhance bone repair. Deficiency or antagonism of Sclerostin improves intramembranous ossification [22,[24][25][26][27][28][29]38], though the effect on endochondral ossification during fracture repair is not as well defined. We hypothesized that endochondral repair would be improved within the Sost −/− mouse in comparison to C57Bl/ 6J wild type controls, with earlier bony union, and resultant denser and stronger mineralized calluses.…”

“…In particular, the development and subsequent animal and human trials of anti-Sclerostin antibodies have provided evidence of stimulated bone formation and enhanced bone mass [17][18][19][20][21][22][23]. The anabolic effects of Sclerostin deficiency, via genetic knockout mice or anti-Sclerostin antibody treatment, have also been demonstrated in intramembranous bone healing studies [24][25][26][27][28][29]. These anabolic effects include increases in bone mineral density, bone volume and mechanical strength.…”

Combination therapy with BMP-2 and a systemic RANKL inhibitor enhances bone healing in a mouse critical-sized femoral defect

“…Bone formation in the operated femora was quantified in longitudinal H&E sections (6 slides per group, 56-day time point) at 2× magnification according to a previously established protocol [6,21]. The ROI was selected to include any newly formed bone, by marking a rectangular area comprising the defect area, superior and inferior bony bridges.…”

3D printed hyperelastic “bone” scaffolds and regional gene therapy: A novel approach to bone healing

“…Recombinant human Bone Morphogenetic Protein‐2 (rhBMP‐2) is one of the most potent growth factors for bone formation and its combination with a scaffold was initially thought to be the optimal clinical solution for bone regeneration; however, the clinical results have been inconsistent. RhBMP‐2 is traditionally loaded on to a collagen sponge and it has been hypothesized that the rapid release of the Bone Morphogenetic Protein (BMP) from the sponge limits its osteoinductive activity . In order to overcome this limitation, high doses of rhBMP‐2 are used clinically which has been associated with reports of postoperative complications including heterotopic bone formation, soft tissue swelling and osteolysis …”