Polylactic acid-polyethylene glycol block copolymer. A new biodegradable synthetic carrier for bone morphogenetic protein.

A new biodegradable polymer, a polylactic acid-polyethylene glycol (PLA-PEG) block copolymer, proved to be an effective and suitable carrier for bone morphogenetic protein (BMP). Composites of semipurified BMP and PLA-PEG consisting of a PLA segment with a molecular weight (MW) of 650 d and a PEG segment with a MW of 200 d (PLA-PEG 650-200) were implanted under the fasciae of the dorsal muscles of mice. Three weeks after implantation, the PLA-PEG 650-200/BMP composites were completely absorbed and replaced by newly induced bone with hematopoietic marrow. The composites induced twice as much bone as composites of BMP and a 650-d PLA homopolymer. Results indicate that of all the biodegradable synthetic polymers the authors have tested, PLA-PEG 650-200 is the most suitable and effective BMP carrier. Composites of PLA-PEG 650-200/BMP and hydroxyapatite powder (HAP) also induced ectopic bone formation. Because PLA-PEG 650-200/BMP is viscous and semiliquid and PLA-PEG 650-200/BMP/HAP is doughy and plastic, the former can be used as an injectable osteoinductive material and the latter as a plastic mold.