Purpose: Bone morphogenetic proteins (BMP) are fragile products that must be protected from degradation and released progressively to achieve maximal efficacy. Release of quantities to the order of 10μg are required at ectopic sites in the rat; in humans 50 mg is required to induce new bone formation. Use of high-dose BMP is costly and the risk of overestimulating mesenchymatous cells remains to be determined. Functional dextranes, or DMCBSU, are inert biological derivatives with random substitution of carboxymethyl, benzylamide and sulfonate units. The affinity of these products for other growth factors led us to propose their use as specific carriers of BMP extracted from bovin bone.
Material and methods: Three different gels (CMDB2, OM27, LS8) and the native dextranes from which they are derived (T40, T500) were tested by to determine their capacity to adsorb and release BMP. Uptake and releasing kinetics were studied by fluorimetry using fluoresceine-labelled BMP. CMDB2 and its native dextrane T500, OM27 and LS8 and their native dextrane TT40, and collagen sponge (control) were implanted in the paravertebral grooves of the rat after impregnating the products with different concentrations of BMP (5 μg, 500 ng, 50 ng, 5 ng). The animals were sacrificed at six weeks. The presence of bone tissue was determined by microradiography and histomorphometry.
Results: The more porous gels (OM27 and LS8) adsorbed the greatest quantities of BMP (96.6 and 95.7 ng/ml respectively). Implantation of BMP associated with certain DMBCSU enabled elaboration of bony tissue in an ectopic site for quantities of BMP starting from 50 ng. This bony tissue formation was obtained for collagen sponge controls with doses 100-fold higher (5 μg). Bony tissue obtained with the BMP:DMCBSU combination was endochondral bone presenting cartilaginous lines, followed by mature bony tissue.
Conclusion: This preliminary study demonstrates that by choosing the right specific carrier for bone growth factors, it is possible to considerably reduce the minimal dose required to induce formation of new bone at an ectopic site. Implantations in bone defects of a critical size are under investigation to validate these results in a model closer to the clinical situation.
