キトサンおよびリポソームのインスリン酵素分解抑制作用と消化管粘膜吸収促進作用

Abstract

We have previously demonstrated that oral administration of chitosan-coated liposomes containing insulin (CS-Lip Ins) lead to a long-term reduction of blood glucose level in rats. In this study, we investigated protective effects of chitosan and liposomal formulation on enzymatic degradation of insulin and promoting effect of chitosan in absorption of insulin in the gastrointestinal tract. A homogenized mucus layer including epithelial cells of rat intestine was used to evaluate the stability of insulin in the gastrointestinal tract. In assessing absorption promoting effects of chitosan, pulse-chaser test, in which the polymer solution was preadministered before insulin administration, was carried out. As the total protein concentration in mucous layer homogenate was increased, the degradation rate of insulin in the solution was accelerated. The enzymatic degradation of insulin was protected in the presence of chitosan or liposomal encapsulation of insulin. Free amino groups of chitosan were supposed to be important in protecting enzymatic degradation of insulin, because carboxymethyl chitin, which possesses a similar structure with chitosan but has no free amino groups, showed little protective effects. Absorption promoting effects of chitosan were not observed in simultaneous administration of chitosan solution with an insulin solution. This result suggested the protective effect of chitosan against enzymatic degradation of insulin is not feasible in vivo. However, co-administration of chitosan solution with liposomal insulin lead to more reduction of blood glucose levels. As the absorption promoting effect was decreased in the pulse-chaser test, the effect was considered to be reversible. In comparing the decreased blood glucose level after simultaneous administration of chitosan solution and liposomal insulin with that of CS-Lip Ins, more prolonged effect was observed for the administration of CS-Lip Ins. These results suggested that both mucoadhesive property of CS-Lip Ins and protective effect of liposomal encapsulation for enzymatic degradation of insulin are important factors in insulin absorption with oral administration of CS-Lip Ins.