Long-term evaluation of bone formation by osteogenic protein 1 in the baboon and relative efficacy of bone-derived bone morphogenetic proteins delivered by irradiated xenogeneic collagenous matrices

To investigate the long-term efficacy of irradiated recombinant human osteo genic protein 1 (hOP-1) in bone regeneration and morphogenesis, hOP-1 was c ombined with a bovine collagenous matrix carrier (0, 0.1, 0.5, and 2.5 mg h OP-1/g of matrix), sterilized with 2.5 Mrads of gamma-irradiation, and impl anted in 80 calvarial defects in 20 adult baboons (Papio ursinus). The rela tive efficacy of partially purified bone-derived baboon bone morphogenetic proteins (BMPs), known to contain several osteogenic proteins, was compared with the recombinant hOP-1 device in an additional four baboons. Histology and histomorphometry on serial undecalcified sections prepared from the sp ecimens harvested on day 90 and day 365 showed that gamma-irradiated hOP-1 devices induced regeneration of the calvarial defects by day 90, although w ith reduced bane area compared with a previous published series of calvaria l defects treated with nonirradiated hOP-1 devices. One year after applicat ion of the irradiated hOP-1 devices, bone and osteoid volumes and generated bone tissue areas were comparable with nonirradiated hOP-1 specimens. More over, 365 days after healing regenerates induced by 0.5 mg and 2.5 mg of ir radiated hOP-1 devices showed greater amounts of bone and osteoid volumes w hen compared with those induced by nonirradiated hOP-1 devices. On day 90, defects treated with 0.1 mg and 0.5 mg of bone-derived baboon BMPs, combine d with irradiated matrix, showed significantly less bone compared with defe cts receiving irradiated devices containing 0.1 mg and 0.5 mg hOP-1; 2.5 mg of partially purified BMPs induced bone and osteoid volumes comparable wit h the 0.1-mg and 0.5-mg hOP-1 devices. Control specimens of gamma-irradiate d collagenous matrix without hOP-1 displayed a nearly 2-fold reduction in o steoconductive bone repair when compared with nonirradiated controls. These findings suggest that the reduction in bone volume and bone tissue area on day 90 may be caused by a reduced performance of the irradiated collagenou s matrix substratum rather than to a reduction in the biological activity o f the irradiated recombinant osteogenic protein. This is supported by the r esults of in vitro and in vivo studies performed to determine the structura l integrity of the recovered gamma-irradiated hOP-1 before application in t he baboon. Recoveries by high-performance liquid chromatography (HPLC) and sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS/PAGE)/immuno blot analyses indicated that doses of 2.5-3 Mrads of gamma-irradiation did not significantly affect the structural integrity of the recovered hOP-1. B iological activity of the recovered hOP-1 was confirmed in vitro by showing induction of alkaline phosphatase activity in rat osteosarcoma cells (ROS) and in vivo by de novo endochondral bone formation in the subcutaneous spa ce of the rat. These findings in the adult primate indicate that a single a pplication of gamma-irradiated hOP-1 combined with the irradiated xenogenei c bovine collagenous matrix carrier is effective in regenerating and mainta ining the architecture of the induced bone at doses of 0.5 mg/g and 2.5 mg/ g of carrier matrix.