Effects of Anti-Sclerostin Antibody Release from Porous Microparticles on Bone Resorption Inhibition of Osteoblasts

Background: Anti-sclerostin antibodies are among the most efficient drugs for the treatment of osteoporosis, and have been also expected for the treatment of local bone disorders. We have previously developed porous microparticles of hydroxyapatite and chondroitin sulfate loading anti-sclerostin antibodies formulated with zinc cations. However, the biological behavior and concentration dependence of anti-sclerostin antibodies in vitro released from the microparticles remain unclear. Objective: Bolus administration and the subsequent release of anti-sclerostin antibodies from the microparticles formulated with or without zinc cations were investigated; bone-resorptive inhibitory effects on mouse MC3T3-E1 osteoblast function were revealed by cell culture using a cell culture insert plate. Methods: Differentiation induction culture of osteoblasts was performed after maintaining the concentrations of anti-sclerostin antibodies and sclerostin at previously reported concentrations of 5.0 and 1.0 µg/mL for the first 3 days. Subsequently, the medium was replaced with fresh medium that did not contain anti-sclerostin antibodies but microparticles loading anti-sclerostin antibodies (20 or 80 µg/mg) with or without zinc cations in the cell culture insert. After 11 days of incubation, the bioactivity of the osteoblasts was evaluated using the polymerase chain reaction method. Results: The formulation using zinc cations showed an increase of anti-sclerostin antibodies released from the microparticles, which increased the expression of receptor activator of the nuclear factor kappa-B ligand in the osteoblasts on day 14. This result indicates the inhibition of sclerostin-mediated bone resorption. However, the increase of loading amounts of anti-sclerostin antibodies extremely enhanced the subsequent release of anti-sclerostin antibodies, which decreased the inhibition of bone resorption contrary to expectations. Conclusion: The moderately sustained release of anti-sclerostin antibodies from the microparticles can promote the inhibition of bone resorption in osteoblasts, supporting the potential of this formulation for the treatment of localized bone disorders.