Noninvasive insulin delivery: the great potential of cell-penetrating peptides

Insulin, a potent therapeutic peptide used in the treatment of diabetes, is administered to patients via subcutaneous injections because of the poor pharmacokinetics associated with alternative routes of administration such as oral, nasal and pulmonary delivery. Noninvasive nasal and oral formulations are appealing to patients who need consecutive daily treatments of insulin. However, to achieve mucosal absorption of insulin via oral or nasal administration, two barriers must be overcome: the impermeability of insulin through the epithelial membranes and local digestion and enzymatic degradation. Cell-penetrating peptides (CPPs), which efficiently bring exogenous proteins and nucleic acids into cells, have great potential to facilitate insulin permeation from the intestinal lumen or nasal cavity into systemic circulation via efficient uptake by epithelial cells. In fact, the coadministration of insulin with the peptide penetratin, a typical CPP, increased intestinal and nasal insulin bioavailability to 35 and 50%, respectively. In this review, the authors describe recent findings using this novel CPP-based formulation for noninvasive delivery of insulin.