Transforming growth factor- β1 incorporation in an α-tricalcium phosphate/dicalcium phosphate dihydrate/tetracalcium phosphate monoxide cement: release characteristics and physicochemical properties

Abstract

The osteoconductive properties of calcium phosphate cements (CPCs) may be improved by the addition of growth factors, such as recombinant human transforming growth factor- β1 (rhTGF- β1). Previously we have shown that rhTGF- β1 was released from cement enriched with rhTGF- β1 and subsequently stimulated the differentiation of pre-osteoblastic cells from adult rat long bones. It is unknown whether the addition of rhTGF- β1 changes the material properties of this α-tricalcium-phosphate ( α-TCP)/tetracalcium-phosphate-monoxide (TeCP)/dicalcium-phosphate-dihydrate (DCPD) cement, and what the characteristics of the release of rhTGF- β1 from this CPC are. Therefore, in the present study we determined the release of rhTGF- β1 from cement pellets in vitro. The possible intervening effects of the CPC modification for intermixing rhTGF- β1 on physicochemical properties were studied by assessing the compressive strength and setting time, as well as crystallinity, calcium to phosphorus ratio, porosity and microscopic structure. Most of the previously incorporated rhTGF- β1 in the cement pellets was released within the first 48 h. For all concentrations of rhTGF- β1 intermixed (100 ng–2.5 mg/g CPC), approximately 0.5% of the amount of rhTGF- β1 incorporated initially was released in the first 2 h, increasing to 1.0% after 48 h. The release of rhTGF- β1 continued hereafter at a rate of about 0.1% up to 1 week, after which no additional release was found. The initial setting time, nor the final setting time was changed in control cement without rhTGF- β1 (standard CPC) or in cement modified for rhTGF- β1 (modified CPC) at 20°C and 37°C. Setting times were more than six times decreased at 37°C compared to 20°C. The compressive strength was initially low for both standard CPC and modified CPC, after which it increased between 24 h and 8 weeks. The compressive strength for the modified CPC was significantly higher compared with standard at 1, 2, and 8 weeks after mixing. X-ray diffraction revealed that both standard and modified CPC changed similarly from the original components into crystalline apatite. The calcium to phosphorus ratio as determined by an electron microprobe did not differ at all time points measured for standard CPC and modified CPC. In both standard CPC and modified CPC the separated particles became connected by crystals, forming a structure in which the particles could hardly be recognised in a densifying matrix with some small pores. The present study shows that the calcium phosphate cement is not severely changed by modification for the addition of rhTGF- β1. The addition of rhTGF- β1 in CPC enhances the biologic response as shown in our previous study and did not interfere with the aimed physical and chemical properties as shown in this study. We conclude that the addition of rhTGF- β1 enlarges the potential of the CPC in bone replacement therapy.