We report a direct comparison of RANKL inhibition (RANK-Fc) with bisphosphonate treatment (ALN) from infancy through early adulthood in a mouse model of Osteogenesis Imperfecta. Both ALN and RANK-Fc decreased fracture incidence to the same degree with increases in metaphyseal bone volume via increased number of thinner trabeculae. The potential therapeutic benefit of RANKL inhibitors in OI is under investigation. We report a direct comparison of RANKL inhibition (RANK-Fc) with bisphosphonate treatment (alendronate; ALN) from infancy through early adulthood in a model of OI, the oim/oim mouse. Two week old oim/oim, oim/+ and wildtype (+/+) mice were treated with RANK-Fc 1.5 mg/kg twice per week, ALN 0.21 mg/kg/week or saline (n = 12-20 per group) for 12 weeks. ALN and RANK-Fc both decreased fracture incidence (9.0 ± 3.0 saline 4.4 ± 2.7 ALN, 4.3 ± 3.0 RANK-Fc fractures per mouse). Serum TRACP-5b activity decreased to 65% after 1 month in all treated mice, but increased to 130-200% at sacrifice with RANK-Fc. Metaphyseal density was significantly increased with ALN in +/+ and oim/oim mice (p < 0.05) and tended to increase with RANK-Fc in +/+ mice. No changes in oim/oim femur biomechanical parameters occurred with treatment. Both ALN and RANK-Fc significantly increased trabecular number (ie 3.73±0.77 1/mm for oim/oim saline vs 7.93±0.67ALN and 7.34±1.38 RANK-Fc) and decreased trabecular thickness (ie 0.045 mm ±0.003 for oim/oim saline vs 0.034±0.003 ALN and 0.032±0.002RANK-Fc) and separation in all genotypes (ie 0.28±0.08 mm for oim/oim saline vs 0.12±0.010 ALN and 13±0.03 RANK-Fc)., with significant increase in bone volume fraction (BVF) with ALN, and a trend towards increased BVF in RANK-Fc. Treatment of oim/oim mice with either a bisphosphonate or a RANK-Fc causes similar decreases in fracture incidence with increases in metaphyseal bone volume via increased number of thinner trabeculae.

Background Receptor Activator of Nuclear Factor-κB ligand (RANKL) inhibitors are being considered for use in children with osteogenesis imperfecta (OI). We sought to assess efficacy of two doses of a RANKL inhibitor, OPG-Fc, in a growing animal model of OI, the col1α2-deficient mouse (oim/oim) and its wildtype controls (+/+). Methods Treated mice showed runting and radiographic evidence of osteopetrosis with either high (20 mg/kg twice weekly) or low dose (1 mg/kg/week) OPG-Fc. Because of this adverse event, OPG-Fc treatment was halted and the mice were euthanized or monitored for recovery with monthly radiographs and assessment of serum osteoclast activity (TRACP-5b) until 25 weeks of age. Results Twelve weeks of OPG-Fc treatment resulted in radiographic and histologic osteopetrosis with no evidence of bone modeling and negative Tartrate-resistant acid phosphatase (TRAP) staining, root dentin abnormalities, and TRACP-5b activity suppression. Signs of recovery appeared four to eight weeks post-treatment cessation. Conclusion Both high and low dose OPG-Fc treatment resulted in osteopetrotic changes in infant mice, an outcome not seen in studies with the RANKL inhibitor RANK – Immunoglobulin Fc segment complex (RANK-Fc), or in studies with older animals. Further investigations of RANKL inhibitors prior to their consideration for use in children are necessary.