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.