Instability of bovine insulin in poly(lactide-co-glycolide) (PLGA) microspheres

Biodegradable poly(lactide-co-glycolide) (PLGA; 50/50) microspheres containing bovine insulin as a model protein was prepared by an oil-in-oil (o/o) emulsion solvent evaporation process. When aluminum tristearate (0.15% (w/v)) was employed as a dispersing agent, the loading efficiency of insulin was almost 100% and the yield was over 80%. The average diameter of the PLGA microspheres always ranged between 100 and 200 microns. Morphology study using a scanning electron micrograph showed smooth, spherical, fairly monodispersed PLGA microspheres containing insulin. In relation to release profile, the very low release rate of insulin was demonstrated (only 1% of insulin released after 7 d release test in pH 7.4 Tris buffer) for the PLGA microspheres. Nevertheless, the degradation of bovine insulin in PLGA microspheres was confirmed by high performance liquid chromatography. This degradation seemed to be caused by an acidic condition caused by poly(lactide-co-glycolide) polymer.