The inherent low oral bioavailability of therapeutic peptides can be enhanced by the cell-penetrating peptide penetratin and its analogues shuffle and penetramax applied as carriers for delivery of insulin. In this study, the objective was to gain mechanistic insights on the effect of the carrier peptide stereochemistry on their interactions with insulin and on insulin delivery. Insulin-carrier peptide interactions were investigated using small-angle X-ray scattering and cryogenic transmission electron microscopy, while the insulin and peptide stability and transepithelial insulin permeation were evaluated in the Caco-2 cell culture model along with the carrier peptide-induced effects on epithelial integrity and cellular metabolic activity. Interestingly, the insulin transepithelial permeation was influenced by the degree of insulin-carrier peptide complexation and depended on the stereochemistry of penetramax but not of penetratin and shuffle. The l-form of the peptides initially decreased the epithelial integrity comparable to that induced by the d-peptides, suggesting a comparable mechanism of action. The immediate decrease was reversible during exposure of the Caco-2 epithelium to the l-peptides but not during exposure to the d-peptides, likely a result of their higher stability. Overall, exploration of the stereochemistry showed to be an interesting strategy for carrier peptide-mediated insulin delivery.
Keywords: Caco-2 cell culture model; cell-penetrating peptide; mechanism of action; real-time epithelial integrity; stability; transepithelial permeation.