Surface functional modification of self-assembled insulin nanospheres for improving intestinal absorption.

In this work we fabricated therapeutic protein drugs such as insulin as free-carrier delivery system to improve their oral absorption efficiency. The formulation involved self-assembly of insulin into nanospheres (INS) by a novel thermal induced phase separation method. In consideration of harsh environment in gastrointestinal tract, surface functional modification of INS with ɛ-poly-L-lysine (EPL) was employed to form a core-shell structure (INS@EPL) and protect them from too fast dissociation before their arriving at target uptake sites. Both INS and INS@EPL were characterized as uniformly spherical particles with mean diameter size of 150-300 nm. The process of transient thermal treatment did not change their biological potency retention significantly. In vitro dissolution studies showed that shell cross-linked of INS with EPL improved the release profiles of insulin from the self-assembled nanospheres at intestinal pH. Confocal microscopy visualization and transport experiments proved the enhanced paracellular permeability of INS@EPL in Caco-2 cells. Compared to that of INS, enteral administration of INS@EPL at 20 IU/kg resulted in more significant hypoglycemic effects in diabetic rats up to 12 h. Accordingly, the results indicated that surface functional modification of self-assembled insulin nanospheres with shell cross-linked polycationic peptide could be a promising candidate for oral therapeutic protein delivery.