Thiolated Nanoparticles Overcome the Mucus Barrier and Epithelial Barrier for Oral Delivery of Insulin

Oral administration is an ideal alternative for drug delivery due to its convenience and safety. However, oral protein delivery is limited by biological barriers such as the mucus barrier and epithelial barrier, which hamper drugs from entering the blood successfully. Here we presented PC6/CS NPs, a thiolated-polymer-based nanodrug delivery system in the form of poly(acrylic acid)-cysteine-6-mercaptonicotinic acid (PAA-Cys-6MNA, PC6), which is a kind of preactivated thiolated polymer, coated on chitosan (CS) nanoparticles (NPs). Its ability to overcome the mucus barrier and epithelial barrier was investigated. The existence of PC6 made the NPs prone to penetrate the mucus layer as well as strengthened the transcellular transport of insulin on epithelial cells. PC6/CS NPs efficiently enhanced the oral bioavailability of insulin to 16.2%. The improvement resulted from the function of PC6: (1) "diluting" mucus to promote nanoparticle penetration, (2) opening a tight junction to help insulin transport via the paracellular pathway, (3) making the nanoparticle more electrically neutral during the penetration process, and (4) uncoating from PC6/CS NPs so that positive CS NPs were adhered and uptaken by epithelial cells. Our study proves that PC6/CS NPs, which can achieve mucus penetration and epithelial permeation efficiently, are a potential nanocarrier for oral protein delivery.