Optimization of preparative conditions for polylactide (PLA) microspheres containing ovalbumin.

Polylactide (PLA) microspheres containing ovalbumin (OVA) as a model protein were prepared by a water-in-oil-in-water (w/o/w) emulsion solvent evaporation method. The optimization of preparative parameters for the PLA microspheres containing OVA were performed, and the in vitro characteristics of the obtained microspheres were examined. Firstly, a smaller internal aqueous phase volume was found to be advantageous in obtaining high loading efficiency. Secondly, the addition of 2-10% (w/v) NaCl into the external aqueous phase (0.5% (w/v) polyvinyl alcohol solution) also improved OVA loading efficiency. Prepared products showed a sharp release of OVA at the initial phase, but the following phase was characterized by a slow release rate of OVA that continued at least 28 d. The release rate of OVA from microspheres made of PLA with a molecular weight of 15400 was faster than that from microspheres made of PLA with a molecular weight of 58300. However, the LA/GA (lactide/glycolide) ratio was not likely to have much effect on the release profile of OVA. Finally, the effect of PLA microsphere particle size on the release profiles of OVA was examined. The extent of burst release at the initial phase increased as the mean diameter of prepared PLA microspheres decreased. For example, the PLA microspheres with a small mean diameter (5.0 microns) showed a 40% burst release, but almost 30% of OVA remained in the PLA microspheres (confirmed by HPLC method) after the 28 d release test, suggesting the possibility of using this carrier as a long-acting protein delivery system.