Improved synthesis and in vitro characterization of chitosan–thioethylamidine conjugate

The aim of this study was to establish improved reaction conditions for the synthesis of chitosan–thioethylamidine (Ch-TEA) conjugate and to evaluate the properties of the obtained Ch-TEA conjugate. The influence of different factors on the coupling reaction, such as concentration of chitosan solution, employment of reducing agent and deprotection of S-acetyl groups, was evaluated. The cohesive properties and stability of the obtained conjugate were evaluated by disintegration test and by oxidation experiments, respectively. The adhesive properties of Ch-TEA conjugate were evaluated in vitro on freshly excised porcine mucosa via tensile studies and the rotating cylinder method. The permeation-enhancing effect of Ch-TEA conjugate was evaluated in Ussing-type chambers by using rhodamine 123 as model compound. The resulting conjugate displayed 225 μmol immobilized free thiol groups and 102 μmol disulfide bonds per gram polymer. The degree of modification depends mostly on the chitosan concentration employed and the deprotection of S-acetyl groups with hydroxylamine. During oxidation studies the amount of thiol groups decreased by 61%. Disintegration studies of tablets comprising Ch-TEA demonstrated stability for 48 h. In tensile studies, the total work of adhesion of the conjugate was determined to be 5.1-fold increased in comparison to unmodified chitosan. Results from the rotating cylinder method showed more than a 13-fold increase in the adhesion time of thiolated chitosan versus unmodified chitosan. The apparent permeability coefficient (Papp) of the system 0.5% (w/v) Ch-TEA conjugate with 5% (w/v) glutathione was calculated to be 5.35×10−8 cm/s, while the Papp value of the system 0.5% (w/v) unmodified chitosan was 1.73×10−8 cm/s. These features should render Ch-TEA useful as an excipient for various drug delivery systems.