Gene vector loaded biphasic calcium phosphate ceramic induces ectopic bone formation in rats

TricOs is a resorbable bone substitute already clinically used. It is a biphasic calcium phosphate ceramic composed of a mixture of hydroxyapatite and beta-tricalcium phosphate combining structural stability with osteogenic capacity. For better healing of large bone defects optimization of the biological properties would be an advantage. This could be achieved by localized gene delivery using vectors coding for e.g. growth factors like BMP-2. Based on our knowledge of localized gene delivery using copolymer protected gene vectors (COPROGs) and organic matrices such as collagen sponges or fibrin glue we examined whether a combination of COPROGs and the anorganic calcium ceramic for matrix mediated gene delivery is possible. For in vitro studies we used a novel reporter gene, Metridia luciferase-a protein which is secreted in the tissue culture supernatant. We could demonstrate that our gene vector formulation immobilized on TricOs revealed Metridia luciferase expression over 48 days with a maximum at day 5. Detection of GFP expression exhibited a bright fluorescence of cells which colonized the TricOs granules. For in vivo studies we implanted TricOs granules loaded with COPROGs coding either for BMP-2 or firefly luciferase as control in the musculus lattisimus dorsi in rats. At different time points the bone substitutes were explanted and the formation of newly formed bone was examined by muCT, conventional CT and PETscan. In contrast to the luciferase implants BMP-2 gene vector loaded granules exhibited a better turnover of the bone substitute into newly formed bone and a higher bone density. The finding that ectopic bone formation in the muscle of rats suggests that matrix mediated gene delivery is useful in bone regeneration applications.