rhBMP-2/calcium phosphate matrix accelerates osteotomy-site healing in a nonhuman primate model at multiple treatment times and concentrations.

BACKGROUND: While recombinant human bone morphogenetic protein-2 (rhBMP-2) administered in a calcium phosphate cement accelerates osteotomy-site healing in animal models when administered three hours after surgery, definitive fracture treatment is often delayed. The present study evaluated the ability of rhBMP-2, administered in a new particulating calcium phosphate matrix, to accelerate nonhuman primate fibular osteotomy-site healing following treatment at multiple treatment times and concentrations. METHODS: The ability of 1.5-mg/mL rhBMP-2/calcium phosphate matrix to accelerate osteotomy-site healing when administered three hours, one day, one week, or two weeks after surgery was first evaluated with use of bilateral proximal and distal fibular osteotomy sites in adult male monkeys. In a second study, the healing of osteotomy sites that had been treated with the administration of calcium phosphate matrix alone and with different concentrations of rhBMP-2/calcium phosphate matrix (0.5 mg/mL, 1.5 mg/mL, or 4.5 mg/mL) seven days after surgery was compared with that of contralateral, untreated osteotomy sites. In a third study, the histologic progression of osteotomy-site healing following treatment with 1.5-mg/mL rhBMP-2/calcium phosphate matrix or calcium phosphate matrix alone, administered three hours or one week after surgery to the osteotomy site, was assessed at multiple time points for as long as twenty-four months after surgery. RESULTS: Radiographs demonstrated increased callus area and more rapid healing in response to 1.5-mg/mL rhBMP-2/calcium phosphate matrix administered over the range of treatment times after surgery as compared with the findings of previous reports on untreated osteotomy sites. Bone formation appeared at the osteotomy sites sooner following treatment at one and two weeks as compared with the findings at the earlier time-points. Scintigraphic imaging at one day and one week after surgery showed prolonged retention of rhBMP-2 at the osteotomy site following an initial burst release. In the second study, radiographic, peripheral quantitative computed tomographic, biomechanical, and microscopic evaluation demonstrated that administration of 1.5 and 4.5-mg/mL rhBMP-2/calcium phosphate matrix one week after surgery accelerated osteotomy-site healing by 40% to 50% compared with the findings in untreated controls. The magnitude of acceleration was less in response to 0.5-mg/mL rhBMP-2/calcium phosphate matrix, and calcium phosphate matrix alone did not accelerate osteotomy-site healing. Histological evaluation indicated that an increased cellular infiltrate and increased direct bone formation contributed to the accelerated osteotomy-site healing following administration of rhBMP-2/calcium phosphate matrix at one week compared with three hours after surgery. CONCLUSIONS: A single percutaneous injection of rhBMP-2/calcium phosphate matrix accelerated healing in nonhuman primate fibular osteotomy sites over a wide range of treatment times. Efficacy was optimized in association with the administration of 1.5-mg/mL rhBMP-2/calcium phosphate matrix. Delaying treatment for one week further accelerated healing because of an increase in the number of responding cells and an increase in direct bone formation.

Recombinant human bone morphogenetic protein-2 delivered in an injectable calcium phosphate paste accelerates osteotomy-site healing in a nonhuman primate model.

BACKGROUND: In recent clinical trials demonstrating the efficacy of recombinant human bone morphogenetic protein-2 (rhBMP-2) for the acceleration of bone-healing, investigators used carriers requiring open surgery for administration. In this study, we used a nonhuman primate fibular osteotomy model to evaluate injectable rhBMP-2/carrier formulations that can be administered in closed fractures. METHODS: The fibular osteotomy model was first characterized by evaluating surgically harvested fibular segments containing untreated osteotomy sites (controls) from seventy adult male Cynomolgus monkeys at eight weeks (twenty-four monkeys), ten weeks (thirty-four), twelve weeks (six), and fourteen weeks (six). Fibular segments, from twenty-four animals, in which an osteotomy had not been performed served as normal controls (intact). The contralateral limb of twenty-four of the animals was then used to evaluate the effect of rhBMP-2 administered, three hours after the osteotomy, in eight carrier formulations (buffer, calcium phosphate paste, and hyaluronan gel, hyaluronan paste, and gelatin foam formulated with and without tricalcium phosphate granules). Each carrier was used in three monkeys. At ten weeks, the fibulae with the treated osteotomy sites were harvested and were compared with the contralateral, untreated osteotomized fibulae (paired control). The most promising carrier, calcium phosphate paste (alpha bone substitute material, or alpha-BSM), was then evaluated in eleven additional animals. The outcomes included the findings on radiographs made weekly until the time of fibular harvest, the callus area, the biomechanical properties, and the histologic findings. RESULTS: Radiographic and histologic studies confirmed complete bridging of the control osteotomy sites in most animals by fourteen weeks. The mean torsional stiffness and maximum torque of the control osteotomy sites were 42.7% and 53.7%, 55.2% and 60.4%, 66.7% and 66.4% of the mean torsional stiffness and maximum torque of the intact fibulae at eight, ten, and twelve weeks, respectively, but they were not substantially different from the mean torsional stiffness and maximum torque of the intact fibulae at fourteen weeks (82.3% and 79.8%). In the carrier screening study, outcome measures of healing were more consistently enhanced in the rhBMP-2/alpha-BSM-treated osteotomy sites. In the confirmatory study, the mean callus area, torsional stiffness, and maximum torque were 86%, 72%, and 68% greater in the rhBMP-2/alpha-BSM-treated osteotomy sites than in the paired-control osteotomy sites at ten weeks (p < 0.001). The torsional stiffness and maximum torque in the rhBMP-2/alpha-BSM-treated osteotomy sites were equal to those in the intact fibulae, whereas those parameters in the paired-control osteotomy sites were only 55% and 58%, respectively, of the torsional stiffness and maximum torque of the intact fibulae. Histologic analysis confirmed complete osseous bridging of the rhBMP-2/alpha-BSM-treated osteotomy sites but incomplete bridging of the paired-control osteotomy sites at ten weeks. CONCLUSIONS: A single percutaneous injection of rhBMP-2/alpha-BSM accelerates the healing of fibular osteotomy sites in nonhuman primates by approximately 40% compared with the healing of untreated osteotomy sites.