Preactivated thiolated pullulan as a versatile excipient for mucosal drug targeting

Aim: The purpose of the present study was to generate a novel mucoadhesive thiolated pullulan with protected thiol moieties and to evaluate its suitability as mucosal drug delivery system. Methods: Two different synthetic pathways: bromination-nucleophilic substitution and reductive amination including periodate cleavage were utilized to synthesize such thiolated pullulans. The thiomer (pullulan-cysteamine) with the highest amount of free thiol groups was further enrolled in a reaction with 6-mercaptonicotinamide and its presence in pullulan structure was confirmed via NMR analysis. Furthermore, unmodified, thiolated and preactivated thiolated pullulan were investigated in terms of mucoadhesion via rotating cylinder studies and rheological synergism method as well as their toxicity potential over Caco-2 cells. Results: Comparing both methods the reductive amination seems to be the method of choice resulting in comparatively higher coupling rates. Using this procedure pullulan-cysteamine conjugate displayed 1522 + 158 mol immobilized thiol groups and 280 + 70 mol free thiol groups per gram polymer. Furthermore, 82% of free thiol groups on this conjugate were linked with 6-mercaptonicotinamide (6-MNA). The adhesion time on the rotating cylinder was up to 46-fold prolonged in case of the thiolated polymer and up to 75-fold in case of the preactivated polymer. Rheological measurements of modified pullulan samples showed 98-fold and 160-fold increase in dynamic viscosity upon the addition of mucus within 60 min, whereas unmodified pullulan did not show an increase in viscosity at all. Both conjugates had a minor effect on Caco-2 cell viability. Conclusion: Because of these features preactivated thiolated pullulan seems to represent a promising type of mucoadhesive polymers for the development of various mucosal drug delivery systems. (C) 2016 Elsevier Ltd. All rights reserved.