Retroviral-Based BMP-4 In Vivo Gene Transfer Strategy with Intramedullary Viral Delivery Optimizes Transgene Expression in Rat Femur Fractures

We have developed an intramedullary delivery strategy to administer retroviral vectors expressing a therapeutic gene to promote healing of a closed rat femur fracture. This strategy involves implantation of an indwelling catheter with the stabilizing Kirschner (K)-wire during the surgery prior to fracture of the femur by the three-point bending technique. It uses the openings in the bone that were already created for the stabilizing K-wire and the catheter insertion. In this study, transgene expression and callus bone formation induced by intramedullary delivery of MLV-based vectors expressing the bone morphogenetic protein-2/4 (BMP-2/4) hybrid gene or β-galactosidase (β-gal) gene were compared with those produced by percutaneous injections of the same vectors at the periosteum of the fracture site. The percutaneous injections of MLV-BMP-2/4 vector led to massive but asymmetric transgene expression in surrounding tissues within the fracture callus and large amounts of supraperiosteal as well as asymmetric callus bone formation. In contrast, the intramedullary administration produced a robust and symmetric pattern of transgene expression at the fracture site with very minimal transduction at cells of surrounding tissues, resulting in normal subperiosteal bone development around the entire fracture callus without supraperiosteal bone formation. In summary, we have developed an intramedullary retroviral vector delivery strategy with a rat femur fracture model that led to uniform transgene expression around the entire fracture site, which optimizes the gene therapy-enhanced fracture repair. This strategy should readily be adapted to administer large dosages of any therapeutic vehicle (therapeutic molecules, peptides, or proteins, as well as viral or non-viral vectors) throughout much of early fracture repair, and thus it would be an ideal rat model for in vivo testing of various therapeutic agents to promote fracture repair.