Exploration of the Key Factors for Optimizing the in Vivo Oral Delivery of Insulin by Using a Noncovalent Strategy with Cell-Penetrating Peptides.

This present study aimed to determine the optimal oral insulin delivery conditions that would maximize the utility of cell-penetrating peptides (CPPs) by using a noncovalent strategy. We first compared the effectiveness of two potential CPPs, penetratin and its analog PenetraMax, as absorption enhancers for insulin. The combined effect was evaluated under in vivo oral administration conditions. Both D-forms of CPPs were highly effective for increasing the oral absorption of insulin, and D-PenetraMax showed a more rapid onset of absorption enhancement effects compared with those of D-penetratin. However, synergistic absorption enhancement effects after combination treatment were not observed. Next, we tried a theoretical approach to establish optimized oral insulin delivery conditions. A surface plasmon resonance (SPR)-based analysis demonstrated that binding between insulin and penetratin (2 mM) might be saturated at 100-500 µM penetratin, while the bound concentration of penetratin could increase in accordance with an increased concentration of mixed insulin. To test this hypothesis, we investigated the effectiveness of different insulin doses in the gastric pH-neutralized mice. The results showed that the dissociation of noncovalent complexes of insulin and CPPs at the low gastric pH was prevented in these mice. Our findings clearly suggested that a noncovalent strategy with CPPs represents an effective approach for the L-form of CPP to increase the concentration of CPP-bound insulin to attain greater absorption of insulin, although this approach may not be appropriate for the D-form of CPP. Our findings will contribute to the development of oral dosage forms of insulin for noncovalent strategies involving CPP.