An aqueous nanoparticulate delivery system containing oppositely charged polymers polyethylenimine (PEI) and DS with zinc as a stabilizer was developed by Tiyaboonchai et al. (43). The pH of PEI solutions, the weight ratio of the two polymers, and zinc sulfate concentrations affected the particle size of the nanoparticles. Spherical particles of 250 nm mean diameter with a zeta potential of approximately +30 mV were produced under optimal conditions. Insulin could be loaded up to 90% in these nanoparticles. Circular dichroism spectra showed no significant conformational changes compared with free insulin under optimized formulation conditions. In contrast to rapid release of insulin in vitro, the hypoglycemic activity in streptozotocin-induced diabetic rats was prolonged. This system offered a number of advantages, including ease of manufacturing under mild preparation conditions, completely aqueous processing conditions, use of biocompatible polymers, ability to control particle size, a high level of drug entrapment, and an ability to preserve secondary structure and biological activity of protein.

Simon et al. prepared nanosized insulin-complexes based on amine modified comb-like polyesters (44). Protection of insulin in nanocomplexes from enzymatic degradation was investigated. The interaction with enterocyte-like Caco-2 cells in terms of cytotoxicity, transport through and uptake in the cell layers was evaluated by measuring transepithelial electrical resistance (TEER), release of lactate dehydrogenase (LDH), and insulin transport. The protection capacity of the nanocomplexes and their interaction with Caco-2 cells varied strongly as a function of lactide-grafting (hydrophobicity). With increasing lactide-grafting (P(26)P(26)-1(LL)P(26)-2(LL)) Nanocomplexes protected as much as 95% of the insulin against degradation by trypsin.

Colon-targeted delivery systems

Yakugaku Zasshi developed two types of microcapsular devices containing new acrylate-based nanogels with a specific solute-permeability for the delayed- or thermosensitive-release of peptide drugs (46). A nanogel-particle of acrylic terpolymer, ethyl acrylate-methyl methacrylate-2-hydroxy-ethyl methacrylate, was newly synthesized by emulsion polymerization to construct delayed-release microcapsules. The insulin-loaded lactose particles were spray coated with the acrylic terpolymers. These microcapsules showed a pH-in-dependent delayed-release profile. Oral administration of the microcapsules with the lag time of 6 h to beagle dogs resulted in significantly reduced blood glucose concentration, leading to colon-specific insulin delivery with pharmacological availability of 5%. Meanwhile, poly(N-isopropylcarylamide) (p(NIPAAm)] nanogel-particles with a reversible temperature-dependent swelling property were prepared by dispersion polymerization to fabricate microcapsular membranes with thermosensitively changeable permeability. The microcapsules constructed by coating of drug-loaded CaCO₃ particles with a blend mixture of the p(NIPAAm) nanogels and ethylcellulose pseudo-latex exhibited an on-off positively thermosensitive drug-release; the release rate was remarkably enhanced at higher temperatures possibly due to the formation of voids through the shrinkage of p(NIPAAm) nanogels in the membrane. A possible application of this type of microcapsules can be found in externally temperature-activated pulsatile peptide delivery.