Characterization of recombinant human bone morphogenetic protein-2 delivery from injectable hyaluronan-based hydrogels by means of 125I-radiolabelling.

This study presents a thorough in vitro and in vivo characterization of the delivery of bone morphogenetic protein 2 (BMP-2) from a hyaluronan-based hydrogel system. The in vitro release of BMP-2 from similar hydrogels has previously been studied by enzyme-linked immunosorbent assay (ELISA), by which only a fraction of the loaded protein is detected. In the current study, (125) I radiolabelling was used instead to monitor BMP-2 in vitro and in vivo. To minimize protein loss during handling, (125) I-BMP-2 adsorption to different tubes was studied at different times and temperatures. The data showed that Protein LoBind tubes exhibited the lowest protein affinity. Furthermore, a biphasic release profile of biologically active BMP-2 was observed both in vitro and in vivo, with the initial fast phase during the first week, followed by a slower release during the remaining 3 weeks. The initial fast-release phase corresponded to the early bone formation observed after 8 days in an ectopic model in rats. Bone volume and mineral content increased until day 14, after which a decrease in bone volume was observed, possibly due to resorption in response to decreased amounts of released BMP-2. Overall, the results suggested that cautious protein handling and a reliable quantification technique are essential factors for successful design of a BMP-2 delivery system.