PubMed1993
A method to provide near-constant sustained release of high molecular weight, water-soluble proteins from polyanhydride microspheres is described. The polyanhydrides used were poly(fatty acid dimer) (PFAD), poly(sebacic acid) (PSA), and their copolymers [P(FAD-SA)]. P(FAD-SA) microspheres containing proteins of different molecular sizes--lysozyme, trypsin, heparinase, ovalbumin, albumin, and immunoglobulin--were prepared by a solvent evaporation method using a double emulsion. The microspheres containing proteins were spherical, with diameters of 50-125 microns, and encapsulated more than 80% of the protein, irrespective of the protein used. Enzymatic activity studies showed that encapsulation of enzymes inside polyanhydride microspheres can protect them from activity loss. When not placed inside polyanhydride microspheres, trypsin lost 80% of its activity in solution at 37 degrees C at pH 7.4 in 12 hr, whereas inside the polyanhydride microspheres the activity loss was less than 10% under these conditions. About 47% of the enzymatic activity of heparinase encapsulated in the microspheres was lost at 37 degrees C in 24 hr, while in solution it lost over 90% of its activity. The protein-loaded microspheres displayed near-zero-order erosion kinetics over 5 days as judged by the release of sebacic acid (SA) from the microspheres. The microspheres degraded to form SA and FAD monomers. All proteins were released at a near-constant rate without any large initial burst, irrespective of polymer molecular weight and protein loading. The period of protein release was longer than that of SA and continued protein release was observed even after the microsphere matrix had completely degraded. Differential scanning calorimetric studies demonstrated an interaction between protein and the FAD monomers produced with microsphere degradation. It is likely that the protein interaction with FAD monomers permits formation of water-insoluble protein aggregates which slowly dissolve and diffuse out of the matrix, leading to delayed protein release. For trypsin-loaded microspheres, trypsin lost 40% of its activity during microsphere preparation. Activity studies demonstrated that the sonication process was primarily responsible for activity loss. A reduction in the period of ultrasound exposure decreased the loss of protein activity to around 20%.