BONE INDUCTION BY ADBMP-2/COLLAGEN IMPLANTS

Abstract

Background: Demineralized bone matrix and recombinant human bone morphogenetic protein-2 or 7 (BMP-2 or BMP-7)-containing collagenous matrix have been shown to induce new bone formation in orthotopic and heterotopic sites. We examined the ability of subcutaneous implants of collagen combined with adenoviral vector containing the BMP-2 gene (AdBMP-2) to induce bone formation in rats. We also evaluated whether targeting the AdBMP-2 vector through an alternative receptor pathway, fibroblast growth factor (FGF), would increase the vector's potency. Methods: In a time-course study, rat subcutaneous sites were implanted with (1) AdBMP-2 in rat-bone-derived collagen or (2) rat-bone-derived collagen alone. Samples were collected three, seven, fourteen, or thirty-five days after treatment. In a dose-response study, bone induction by AdBMP-2 in collagen (AdBMP-2/collagen) or by AdBMP-2 and FGF2 Fab' anti-adenovirus knob protein antibody in collagen (FGF2-AdBMP-2/collagen) was tested at fourteen days. Viral vector doses of 1 × 109 PN (viral particle number), 3 × 109 PN, 1 × 1010 PN, 3 × 1010 PN, or 1 × 1011 PN per implant were used. Equal amounts of collagen (25 mg) were used to formulate all implants. Explanted tissues were evaluated histologically to determine bone formation, specific activity of alkaline phosphatase, and calcium content. Results: AdBMP-2/collagen implants induced robust bone formation. New bone was formed by the fourteenth day after implantation. In contrast, little or no bone was induced by the implant containing collagen alone. FGF2-AdBMP-2/collagen implants stimulated significantly more bone formation (p < 0.05) than did AdBMP-2/collagen implants, regardless of the dose of viral particles. Conclusions: Local delivery of AdBMP-2 in a collagen matrix rapidly induces bone formation, and targeting the virus through FGF receptors enhances the osteogenic potential of AdBMP-2. Clinical Relevance: Local delivery of BMP genes in matrices offers an attractive therapeutic approach to bone repair. The potential of sustaining BMP production at the implant site may be desirable to provide a long-lasting osteogenic signal to delayed unions and fracture nonunions. Moreover, gene delivery within a matrix retains the vector at the site and also provides a scaffold for the influx of osteoprogenitor cells. Lower effective vector doses achieved by the FGF2-AdBMP-2 conjugate may be desirable in order to minimize the adenovirus dose and the adenovirus-elicited inflammation in patients while maximizing gene expression.