Thiolated Chitosans: Novel Polymers for Mucoadhesive Drug Delivery - A Review

Chitosan is a natural polycationic copolymer consisting of glucosamine and N-acetylglucosamine units. The polymer has valuable properties as a biomaterial because it is considered to be biocompatible, biodegradable and non-toxic. The purpose of this review article is to provide detailed information about thiolated chitosans which are gaining popularity because of their high mucoadhsiveness and extended drug release properties. The derivatization of the primary amino groups of chitosan with coupling reagents bearing thiol functions leads to the formation of thiolated chitosans. Various properties of chitosan are improved by the immobilization of thiol groups. Due to the formation of disulfide bonds with mucus glycoproteins, mucoadhesiveness is augmented. The permeation of paracellular markers through mucosa can be enhanced by utilizing thiolated instead of unmodified chitosan. Moreover, thiolated chitosans display in situ gelling features due to the pH-dependent formation of inter-as well as intra-molecular disulfide bonds. This latter process provides, strong cohesion and stability of carrier matrices, being based on thiolated chitosans. The in situ gel formation within the pH range of 5 to 6.8 makes the application of thiolated chitosans on vaginal, nasal and ocular mucosa also possible. Thiolated chitosans can guarantee prolonged controlled release of embedded therapeutic ingredients