Insulin loaded PLGA microspheres: effect of zinc salts on encapsulation, release, and stability

The purpose of this study was to investigate the effect of three zinc salts (i.e., zinc oxide, zinc carbonate, and zinc acetate) on insulin encapsulation efficiency (EE), stability, and in vitro release kinetics from poly(lactic-co-glycolic acid) (PLGA) microspheres. Microspheres were prepared by water-in-oil-in-water (w/o/w) double emulsion solvent evaporation technique and characterized. Integrity of the encapsulated insulin and stability of the released insulin was assessed using a wide range of comprehensive analytical techniques. The EE of the formulation prepared without the addition of a zinc salt was 69%, the secondary structure of the encapsulated insulin in this formulation was found to be altered. Further, desamido insulin and aggregates were observed during in vitro release. When insulin was encapsulated with a zinc salt, EE increased significantly, secondary structure was unaltered, and no degradation or aggregation products were found. Initial burst release and release kinetics were markedly changed with the addition of zinc salts. More than 87% of the encapsulated insulin was released over a 2-week period with the addition of a zinc salt. In conclusion, zinc salts can be useful to increase the EE and stability of insulin in PLGA microspheres prepared by w/o/w technique.