Thiolated chitosan as an improved bioadhesive polymer in drug delivery

Abstract Chitosan, a naturally occurring linear polymer obtained from the exoskeleton of crustaceans is considered to possess many properties including mucoadhesive property, biodegradability, biocompatibility, improved solubility, and enhanced bioavailability. Due to these properties, chitosan is used in many pharmaceutical formulations. Multifunctional or modified chitosan is found to be more advantageous than unmodified chitosan. Thiolated chitosan is a type of modified chitosan which can be synthesized from chitosan by attaching thiol group to chitosan backbone. These thiolated chitosan derivatives can be chitosan–cysteine, chitosan–N-acetyl cysteine, chitosan–thioglycolic acid, chitosan–thioethylamidine, chitosan–glutathione, chitosan–4-thiobutylamidine, chitosan–6-mercaptonicotinic acid, and chitosan–4-mercaptobenzoic acid. Thiolated chitosan has a greater amount of amidine moieties which enhance the interaction of thiolated chitosan with mucous membrane and help in greater mucoadhesive behavior. Intermolecular or intramolecular disulfide bond formation in thiolated chitosan is responsible for cohesive property and pH-dependent sol–gel behavior of formulation. This thiolated chitosan can disturb the electric resistivity and can also open the tight junctions in order to improve the permeation behavior of the formulations. Further, this chapter highlighted some of the important applications of chitosan such as its use for controlled release drug delivery, noninvasive peptide delivery, gene or DNA delivery, targeted delivery, ocular delivery, coating of nanoparticles, tissue engineering, and wound dressing. In all these applications thiolated chitosan is found to be more advantageous when compared to unmodified chitosan.