Mechanistic Study of the Uptake/Permeation of Cell-Penetrating Peptides Across a Caco-2 Monolayer and Their Stimulatory Effect on Epithelial Insulin Transport

Region-Dependent Role of Cell-Penetrating Peptides in Insulin Absorption Across the Rat Small Intestinal Membrane

We have reported that the cell-penetrating peptide (CPP) penetratin acts as a potential absorption enhancer in oral insulin delivery systems and that this action occurs through noncovalent intermolecular interactions. However, the region-dependent role of CPPs in intestinal insulin absorption has not been clarified. To identify the intestinal region where CPPs have the most effect in increasing insulin absorption, the region-dependent action of penetratin was investigated using in situ closed intestinal loops in rats. The order of the insulin area under the insulin concentration-time curve (AUC) increase effect by L-penetratin was ileum > jejunum > duodenum > colon. By contrast, the AUC order after coadministration of insulin with D-penetratin was colon > duodenum ≥ jejunum and ileum. We also compared the effects of the L- and D-forms of penetratin, R8, and PenetraMax on ileal insulin absorption. Along with the CPPs used in this study, L- and D-PenetraMax produced the largest insulin AUCs. An absorption study using ilea pretreated with CPPs showed that PenetraMax had no irreversible effect on the intestinal epithelial membrane. The degradation of insulin in the presence of CPPs was assessed in rat intestinal enzymatic fluid. The half-life (t 1/2) of insulin increased from 14.5 to 23.7 and 184.7 min in the presence of L- and D-PenetraMax, respectively. These enzymatic degradation-resistant effects might contribute partly to the increased ileal absorption of insulin induced by D-PenetraMax. In conclusion, this study demonstrated that the ability of the L- and D-forms of penetratin to increase intestinal insulin absorption was maximal in the ileum and the colon, respectively, and that D-PenetraMax is a powerful but transient enhancer of oral insulin absorption.

Effect of different intestinal conditions on the intermolecular interaction between insulin and cell-penetrating peptide penetratin and on its contribution to stimulation of permeation through intestinal epithelium

Our recent studies have shown that the coadministration of cell-penetrating peptides (CPPs) is a potential strategy for oral delivery of peptide- and protein-based biopharmaceuticals. The intermolecular interaction between drug and CPP is an essential factor in the effective delivery of these drugs, but the characteristics of the interaction under the conditions of the intestinal lumen remain unknown. In this study, therefore, we examined the characteristics of binding of the amphipathic CPP penetratin to insulin and the efficiency of its enhancement of epithelial insulin transport at different pH and in simulated intestinal fluids (SIFs). The binding between insulin and penetratin was pH dependent and particularly decreased at pH 5.0. In addition, we clarified that the sodium taurocholate (NaTC) present in two types of SIF (fasted-state SIF [FaSSIF] and fed-state SIF [FeSSIF]) affected binding efficiency. However, the permeation of insulin through a Caco-2 cell monolayer was significantly facilitated by coincubation with l- or d-penetratin at various pH values. Moreover, the permeation-stimulating effect of l-penetratin was observed in FaSSIF containing NaTC and lecithin, but not in 3mM NaTC solution, suggesting that the presence of lecithin was the key factor in maintaining the ability of penetratin to enhance the intestinal absorption of biopharmaceuticals. This report describes the essential considerations for in vivo use and clinical application of a CPP-based oral delivery strategy.

Brain delivery of insulin boosted by intranasal coadministration with cell-penetrating peptides

Intranasal administration is considered as an alternative route to enable effective drug delivery to the central nervous system (CNS) by bypassing the blood-brain barrier. Several reports have proved that macromolecules can be transferred directly from the nasal cavity to the brain. However, strategies to enhance the delivery of macromolecules from the nasal cavity to CNS are needed because of their low delivery efficiencies via this route in general. We hypothesized that the delivery of biopharmaceuticals to the brain parenchyma can be facilitated by increasing the uptake of drugs by the nasal epithelium including supporting and neuronal cells to maximize the potentiality of the intranasal pathway. To test this hypothesis, the CNS-related model peptide insulin was intranasally coadministered with the cell-penetrating peptide (CPP) penetratin to mice. As a result, insulin coadministered with l- or d-penetratin reached the distal regions of the brain from the nasal cavity, including the cerebral cortex, cerebellum, and brain stem. In particular, d-penetratin could intranasally deliver insulin to the brain with a reduced risk of systemic insulin exposure. Thus, the results obtained in this study suggested that CPPs are potential tools for the brain delivery of peptide- and protein-based pharmaceuticals via intranasal administration.