A novel approach using functional peptides for efficient intestinal absorption of insulin

The aim of this study was to evaluate whether oligoarginine, a cell-penetrating peptide (CPP), can improve intestinal absorption of insulin in rats. Peptides composed of six (R 6), eight (R 8) and 10 (R 10) residues of arginine were used as the CPP. No insulin absorption was observed following administration of insulin solution alone; however, insulin absorption increased dramatically after coadministration of the d-form of R 6 ( d-R 6) and the l-form of R 6 ( l-R 6) in a dose-dependent manner. The effects on insulin absorption were more pronounced for d-R 6 than for l-R 6. Among oligoarginines composed of six, eight, or 10 arginine residues, d-R 8 showed the strongest enhancing effects on insulin intestinal absorption. In contrast, intestinal absorption of other model hydrophilic macromolecules, interferon-β and fluorescein isothiocyanate-labeled dextran 4400, was not affected by coadministration with oligoarginine. Pretreatment by the effective dose of l-R 6 did not induce lactate dehydrogenase leakage or histological damage, suggesting that oligoarginine has no untoward effect on the intestinal mucosa. Our data demonstrate that coadministration of oligoarginine increases intestinal insulin absorption markedly without causing detectable damage in cellular integrity and that the covalent binding between insulin and oligoarginine is not necessary for this effect. We conclude that oligoarginines are likely to become powerful tools for overcoming the low permeability of insulin through the epithelial cell membrane, the major barrier to oral insulin delivery.