Localized delivery of β-NGF via injectable microrods accelerates endochondral fracture repair

Currently, there are no biological approaches to accelerate bone fracture repair. Osteobiologics that promote endochondral ossification are an exciting alternative to surgically implanted bone grafts, however, the translation of osteobiologics remains elusive because of the need for localized and sustained delivery that is both safe and effective. In this regard, an injectable system composed of hydrogel-based microparticles designed to release osteobiologics in a controlled and localized manner is ideal in the context of bone fracture repair. Here, we describe poly (ethylene glycol) dimethacrylate (PEGDMA)-based microparticles, in the form of microrods, engineered to be loaded with beta nerve growth factor ({beta}-NGF) for use in a murine tibial fracture model. In-vitro studies demonstrated that protein-loading efficiency is readily altered by varying PEGDMA macromer concentration and that {beta}-NGF loaded onto PEGDMA microrods exhibited sustained release over a period of 7 days. In-vitro bioactivity of {beta}-NGF was confirmed using a tyrosine receptor kinase A (Trk-A) expressing cell line, TF-1. Moreover, TF-1 cell proliferation significantly increased when incubated with {beta}-NGF loaded PEGDMA microrods versus {beta}-NGF in media. In-vivo studies show that PEGDMA microrods injected into the fracture calluses of mice remained in the callus for over 7 days. Importantly, a single injection of {beta}-NGF-loaded PEGDMA microrods resulted in significantly improved fracture healing as indicated by significant increases in bone volume, trabecular connective density, and bone mineral density and a significant decrease in cartilage despite a remarkably lower dose ([~]111 fold) than the {beta}-NGF in media. In conclusion, we demonstrate a novel and translational method of delivering {beta}-NGF via injectable PEGDMA microrods to improve bone fracture repair.