Byproduct-Free Intact Modification of Insulin by Cholesterol End-Modified Poly(ethylene glycol) for in Vivo Protein Delivery

Insulin is a key peptide hormone used for the treatment of both type I and type II diabetes. To maximize the effect of the treatment of these diseases, the addition of poly­(ethylene glycol) (PEGylation) methods for the insulin are widely developed. Here, to make these PEGylation methods the simplest, we report the byproduct-free intact modification of insulin by cholesterol end-modified poly­(ethylene glycol) with urethane, propyl, and methoxy groups (that is, Chol–U–Pr–mPEG). The noncovalent PEGylation by the Chol–U–Pr–mPEG has been achieved by the simple mixing of insulin with the Chol–U–Pr–mPEG in aqueous solution, followed by freeze-drying. The formation of the Chol–U–Pr–mPEG/insulin complex has proceeded without byproducts, such as N-hydroxysuccinimide, formed by the conventional covalent PEGylation using an active ester group. The byproduct-free PEGylation has preserved insulin conformation as well as primary structure. The intact PEGylation has protected insulin from hydrolysis by protease. The resulting insulin modified by the Chol–U–Pr–mPEG has sustainably suppressed the level of blood glucose, as compared to naked insulin, in mice. Consequently, the Chol–U–Pr–mPEG/insulin complex formation offers the byproduct-free intact PEGylation of insulin for in vivo protein delivery.