Artificial polypeptides based polyelectrolyte complexes overcoming the biological barriers for oral insulin delivery

Abstract

To solve current problems of insulin injection, examples of which are critical side effects and inconvenience, the demand for an alternative insulin administration method has increasing. Among various insulin delivery routes, the oral administration is regarded as a powerful candidate because of convenience and minimal aftereffects. In this study, a novel oral insulin delivery system was prepared by the combination of two different artificial polypeptides with insulin. Negatively charged poly(L-glutamate-co-N-3-L-glutamylsulfanilic acid) (PLGS), cationic alpha helical peptide poly-L-lysine (PLL) and insulin formed polyelectrolyte complexes, and were characterized. The size (214.9 $\pm$ 1.4 nm), zeta potential (45.8 $\pm$ 1.0 mV) and the spherical morphology of the polyelectrolyte complexes were confirmed through diverse analyzing techniques including dynamic light scattering method and transmission electron microscope. The property of the polyelectrolyte complexes was examined by in vitro study. The FITC labeled insulin loaded polyelectrolyte complexes were incubated in two different pH conditions simulating the small intestine (pH 6.8) and the stomach (pH 1.2). Significantly higher amount of the loaded FITC insulin was released in the intestinal condition suggesting the controlled release of the polyelectrolyte complexes to protect insulin in the acidic stomach condition while release it in the small intestine. In vitro cellular uptake study with Caco-2 cells also revealed the improved penetration of the loaded FITC labeled insulin. By virtue of the cell penetration enhancing ability of PLL, the permeation of insulin in the small intestine was notably augmented. Furthermore, in vitro cytotoxicity of the polyelectrolyte complexes on Caco-2 cells indicated biocompatible characteristic of the complexes, so that unexpected health problems can be avoided even at high concentration. To confirm the feasibility of the polyelectrolyte complexes, in vivo study was carried out with streptozotocin induced diabetic mice. The polyelectrolyte complexes showed improved hypoglycemic effect suggesting the success of delivery and penetration of the loaded insulin. The blood glucose level was lowered to 80% of the initial value after oral administration of the polyelectrolyte complexes, and the hypoglycemia lasted for more than 14 h. The long lasting hypoglycemic effect of the polyelectrolyte complexes can reduce the number of administration, and it will contribute to the improvement of the quality of patients’ lives. The polyelectrolyte complexes provided reasonable results as a competitive candidate for oral insulin delivery. Introduction of the polyelectrolyte complexes will promote the oral delivery of charged proteins or drugs.