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 -galactosidase (-gal) gene were compared with those pro- duced 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 cal- lus and large amounts of supraperiosteal as well as asymmetric callus bone formation. In contrast, the intramedullary ad- ministration produced a robust and symmetric pattern of transgene expression at the fracture site with very minimal trans- duction at cells of surrounding tissues, resulting in normal subperiosteal bone development around the entire fracture cal- lus 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 op- timizes 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.