Brush-like branched biodegradable polyesters, part III. Protein release from microspheres of poly(vinyl alcohol)-graft-poly(D,L-lactic-co-glycolic acid).

Brush-like branched polyesters, obtained by grafting poly(lactic-co-glycolic acid), PLGA, onto water-soluble poly(vinyl alcohol) (PVAL) backbones, were investigated regarding their utility for the microencapsulation of proteins. Poly(vinyl alcohol)-graft-poly(lactic-co-glycolic acid), PVAL-g-PLGA, offers additional degrees of freedom to manipulate properties such as e.g. molecular weight, glass transition temperature and hydrophilicity. PLGA chain length was varied at a constant molecular weight (M(w)) of the PVAL backbone and secondly M(w) of the PVAL backbone was varied keeping the PLGA chain lengths constant. The most striking feature of these polymers is their high M(w). Microencapsulation of hydrophilic macromolecules, such as bovine serum albumin, ovalbumin, cytochrome c and FITC-dextran using a w/o/w double emulsion technique was investigated. Surface morphology, particle size, encapsulation efficiencies and protein release profiles were characterized as well. Microencapsulation of model compounds was feasible at temperatures of 0-4 degrees C with yields typically in the range of 60-85% and encapsulation efficiencies of 70-90%. Both, encapsulation efficiency and initial protein release (drug burst) were strongly affected by the glass transition temperature, T(g), of the polymer in contact with water, whereas the in vitro protein release profile depended on the PVAL-g-PLGA structure and composition. In contrast to PLGA, protein release patterns were mostly continuous with lower initial drug bursts. Shorter PLGA chains increased drug release in the erosion phase, whereas initial pore diffusion was affected by the M(w) of PVAL backbone. Release profiles from 2 to 12 weeks could be attained by modification of composition and molecular weight of PVAL-g-PLGA and merit further investigations under in vivo conditions. The in vitro cytotoxicity of PVAL-g-PLGA is comparable to PLGA and therefore, this new class of biodegradable polyesters has considerable potential for parenteral drug delivery systems.