LONG-TERM RELEASE OF BIOACTIVE BMP2 FROM BCP CERAMICS VIA CELL-MEDIATED GROWTH FACTOR LIBERATION

Abstract

The reconstruction of bone defects with biomaterials represents a potential alternative to the transplantation of autologous and allogenic bone. Ceramic materials can be combined with growth factors (i.e. BMPs) to render them osteoinductive. Coating of biomaterials with growth factors has mostly been attempted by adsorption onto the material’s surface. The superficial deposition usually results in an immediate passive release of the proteins, thus restricting their temporal availability during bone healing. It was hypothesized that a co-precipitation of proteins onto calcium phosphate ceramics may provide the possibility to achieve a prolonged release of proteins from the material without impairing the biologic activity of growth factors.

Tritium labelled bovine serum albumin ([3H]BSA) and recombinant human BMP2 (rhBMP2) were coated onto biphasic calcium phosphate (BCP) ceramics using a coprecipitation technique of proteins together with calcium phosphate (Liu Y et al. 2001). The co-precipitation was compared to conventional adsorption of proteins to ceramic materials. The passive and cell-mediated release of [3H]BSA was investigated during 19 days. To analyze the cell-mediated protein release, murine bone marrow cells were seeded onto ceramics and differentiated to osteoclasts or to monocytes/macrophages. To assess whether rhBMP2 co-precipitated to BCP ceramics retained its biologic activity the growth factor’s ability to induce the differentiation of primary murine osteoblasts was studied.

After 19 days 71.7±5.3% of the adsorbed [3H]BSA was passively released (63.0±6.0% within 4 days). The passive liberation of [3H]BSA was effectively reduced using the coprecipitation technique (12.5±2.0% within 19 days, 10.1±2.3% within 4 days, p< 0.001). Further analysis demonstrated a sustained, osteoclast-mediated release of coprecipitated [3H]BSA from calcium phosphate ceramics which was blocked by the addition of calcitonin. Passive release of adsorbed and co-precipitated BMP2 led to a temporally restricted stimulation of murine osteoblasts. Cell-mediated liberation of co-precipitated BMP2 induced a sustained stimulation of the differentiation of osteoblasts.

The successful application of exogenously added growth factors depends critically on the mode of delivery. It has been shown that a sustained availability of BMP2 is beneficial for bone healing. Application of the co-precipitation technique resulted in a long-term release of proteins from BCP ceramics mediated by active resorbing osteoclasts without impairing the biologic activity of rhBMP2. Co-precipitating growth factors onto BCP ceramics provides a potential to shift the initial extensive liberation to a sustained release of bioactive proteins. This method of protein delivery may represent a possibility to achieve a more physiological availability of growth factors during bone regeneration.