Reiji Nishio

Usefulness of cell-penetrating peptides to improve intestinal insulin absorption

Cell-penetrating peptides (CPPs) are a useful tool for delivering therapeutic macromolecules across cell membranes. We previously devised an approach using CPPs without intermolecular cross-linking and showed the efficient delivery of insulin from the intestine to the systemic circulation using a typical CPP, oligoarginine. However, this approach required relatively high doses of the CPP. Therefore, this study aimed to identify CPPs that are more effective for the delivery of insulin and do not induce toxic effects on the intestine. In this study, we examined the effects of various types of CPPs including arginine-rich peptides and amphipathic peptides that aid insulin absorption from rat ileal segments. Among these peptides, coadministration of insulin with R8, penetratin, pVEC, and RRL helix significantly increased ileal insulin absorption compared with insulin administration alone. In the case of R8, the D-form of the peptide had stronger absorption enhancing ability than the L-form. In contrast, the other three peptides exerted a more significant effect when the L-forms were applied, and L-penetratin had the strongest ability to enhance intestinal insulin absorption. Meanwhile, in a physical mixture of CPP and insulin, aggregates formed in the solution when high concentrations of CPPs were present. L-penetratin enhanced insulin absorption even when administered in an aggregated solution. We then showed that aggregates of L-penetratin and insulin were broken down in the presence of intestinal degradation enzymes. Thus, among CPPs used in this study, L-penetratin had the strongest ability to improve insulin intestinal absorption.

Controlled release of protein drugs from newly developed amphiphilic polymer-based microparticles composed of nanoparticles

A novel formulation of biodegradable microparticles was developed for the sustained release of peptide and protein drugs. The microparticles were formed by the aggregation of protein nanoparticles through water-in-oil (W/O) emulsion-lyophilization and subsequent solid-in-oil-in-water (S/O/W) emulsion-solvent evaporation. Amphiphilic copolymers were used as an emulsifier in the W/O emulsion and matrix of the microparticles. Among the various copolymers investigated, poly(lactide-co-glycolide)-grafted dextran (Dex-g-PLGA) was chosen as the best candidate on the basis of the encapsulation efficiency and in vitro release profile, the near zero-order release without a significant initial burst, of human growth hormone (hGH). The release rate of hGH was controllable by changing the composition of Dex-g-PLGA. The in vivo release studies using normal mice revealed that the plasma concentration of hGH was maintained for 1week without a significant initial burst. The enhancement of biological activity of hGH by sustained release was confirmed by measuring the IGF-1 concentration and body weight of hypophysectomized mice. These results suggest the high potential of the newly developed microparticles for the sustained release of biopharmaceuticals.

Structural requirements of penetratin absorption enhancement efficiency for insulin delivery

Penetratin, a 16-residue peptide, is used widely as a highly efficient delivery carrier for a wide range of poorly permeable therapeutic cargoes. The crucial structural features of penetratin remain unclear, as demonstrated by the difficulties encountered in designing new molecules. The efficiency in enhancing nasal insulin absorption was compared between l-penetratin and 20 of its analogues in rats. We also measured lactate dehydrogenase (LDH) leakage as an indicator of cytotoxicity and scored the histopathological irritation. Substitution of a cationic residue (Arg or Lys) with Leu or addition of tetra-arginine to the C- or N-terminus of penetratin caused considerable reduction in the enhancing efficiency properties of the modified analogues. Mutual exchanging of Arg and Lys in corresponding analogues produced nearly inactive analogues, although changing Arg to Lys in the same analogue produced similar penetratin activity. In addition, activity was impaired markedly upon modification of penetratin within amphiphilic (Trp) or hydrophobic (Ile and Phe) residues. Chain size-modified analogues lacked the ability to induce nasal insulin absorption. In contrast, rearrangement of the modified analogues by C,N-half-exchange and reverse analogues produced activity similar to that of the original penetratin. The enhancing activity was inhibited almost completely upon sequence arrangement of the resulting analogues. Surprisingly, a shuffle (Arg, Lys fix) 2 analogue increased insulin absorption significantly, reaching a relative bioavailability value 1.85-times that of original penetratin. This analogue caused negligible release of LDH in nasal lavage fluid and maintained the integrity of the nasal respiratory epithelium. In conclusion, modulation of amino acid sequences by fixing the cationic residue positions can augment penetratin-enhanced nasal absorption and may lead to improvements in nasal insulin absorption.

Determination of the Optimal Cell-Penetrating Peptide Sequence for Intestinal Insulin Delivery Based on Molecular Orbital Analysis with Self-Organizing Maps

Our recent work has shown that the intestinal absorption of insulin can be improved significantly by coadministration of cell-penetrating peptides (CPPs), especially penetratin. However, a relatively high dose of penetratin is required to adequately stimulate the intestinal absorption of insulin. Therefore, in this study, we sought to determine the CPP that most effectively enhanced intestinal insulin absorption. An in situ loop absorption study using 26 penetratin analogues suggested that the chain length, hydrophobicity, and amphipathicity of the CPPs, as well as their basicity, contribute to their absorption-enhancing efficiency. Moreover, a molecular orbital method with self-organizing maps (SOMs) classification suggested that multiple factors, including the molecular weight, basicity, the lowest unoccupied molecular orbital energy, absolute hardness, and chemical potential of CPPs, are associated with their effects on intestinal insulin absorption. Furthermore, the new CPPs proposed by SOM clustering had a marked capacity to interact with insulin, and their ability to enhance insulin absorption was much stronger than that of the original penetratin. Therefore, the peptide sequence that optimally enhances intestinal insulin absorption could be defined by SOM with the molecular orbital method, and our present work emphasizes the utility of such methodologies in the development of effective drug delivery systems.