In Search of the Ideal Bone Morphogenetic Protein Delivery System: In Vitro Studies on Demineralized Bone Matrix, Purified, and Recombinant Bone Morphogenetic Protein

The clinical use of recombinant bone morphogenetic protein (rBMP) is limited by the lack of a suitable delivery system. The bone morphogenetic protein (BMP) delivery system provided by nature is highly effective, and by studying purified BMP (BMP/NCP) and demineralized bone matrix (DBM), it may be possible to learn how to emulate nature's success. The current study used an in vitro muscle cell model to study the activity of BMP/NCP and DBM and the effects of extracellular matrix on BMP activity. C2C12 cells transiently exposed to recombinant human BMP-4 (rhBMP-4) rapidly increased their alkaline phosphatase (AP) activity to day 5, after which it steadily declined. Cells exposed to BMP/NCP or DBM continued to increase their AP activity over the 14-day culture. If BMP/NCP was treated to remove a 22-kd protein, it became water-soluble and exhibited a similar activity pattern to rhBMP-4. Cells cultured on collagen type I, fibronectin, and hyaluronic-coated surfaces demonstrated increased AP activity when exposed to rhBMP-4 or BMP/NCP compared with cells cultured on bovine serum albumin or poly-l-lysine. These results suggest that the natural BMP delivery system operates both by binding to the BMP molecule and slowly releasing it into the extracellular milieu and by interacting with the responding cells through cell-matrix receptors to enhance the cellular response to BMP.