Porous bone morphogenetic protein-2 microspheres: polymer binding and in vitro release

This research compared the binding and release of recombinant human bone morphogenetic protein 2 (rhBMP-2) with a series of hydrophobic and hydrophilic poly-lactide-co-glycolide (PLGA) copolymers. Porous microspheres were produced via a double emulsion process. Binding and incorporation of protein were achieved by soaking microspheres in buffered protein solutions, filtering, and comparing protein concentration remaining to nonmicrosphere-containing samples. Protein release was determined by soaking bound microspheres in a physiological buffer and measuring protein concentration (by reversed-phase high-performance liquid chromatography) in solution over time. Normalized for specific surface area and paired by polymer molecular weight, microspheres made from hydrophilic 50:50 or 75:25 PLGA bound significantly more protein than microspheres made from the corresponding hydrophobic PLGA. Increased binding capacity correlated with higher polymer acid values. With certain polymers, rhBMP-2 adsorption was decreased or inhibited at high protein concentration, but protein loading could be enhanced by increasing the protein solution:PLGA (volume:mass) ratio or by repetitive soaking. Microspheres of various PLGAs released unbound protein in 3 days, whereas the subsequent bound protein release corresponded to mass loss. RhBMP-2 binding to PLGA was controlled by the acid value, protein concentration, and adsorption technique. The protein released in 2 phases; the first occurred over 3 days regardless of PLGA used and emanated from unbound, incorporated protein, while the second was controlled by mass loss and therefore was dependent on the polymer molecular weight. Overall, control of rhBMP-2 delivery is achievable by selection of PLGA microsphere carriers.