Synthesis and in vitro evaluation of thiolated hyaluronic acid for mucoadhesive drug delivery.

It was the aim of this study to synthesize and characterize a novel hyaluronic acid-cysteine ethyl ester (HA-Cys) conjugate providing improved mucoadhesive properties and a significantly lowered biodegradation rate. Mediated by carbodiimide and N-hydroxysuccinimide, L-cysteine ethyl ester hydrochloride was covalently attached to hyaluronic acid (HA, hyaluronan) via the formation of an amide bond. The adhesive properties of HA-Cys conjugates were evaluated in vitro on a freshly excised porcine mucosa via the rotating cylinder method. The cohesive properties of the resulting conjugates were evaluated by oxidation experiments. Biodegradability studies were carried out by viscosity measurements and spectrophotometric assays. Release studies were performed with fluorescein isothiocyanate-dextrans (FD) as model compounds. The obtained conjugate displayed 201.3+/-18.7 micromol immobilized free thiol groups and 85.7+/-22.3 micromol disulfide bonds per gram polymer. Results from the rotating cylinder method showed more than 6.5-fold increase in the adhesion time of HA-Cys versus unmodified HA. In aqueous solutions, the obtained conjugate demonstrated improved cohesive properties. The hydrolysis degree of HA-Cys was lower compared with the corresponding unmodified HA in the framework of viscosity experiments. In addition, the cross-linking process via disulfide bonds additionally reduced the rate of degradation of the new derivative. Cumulative release studies out of matrix tablets comprising HA-Cys and the model compound FD demonstrated a sustained drug release for more than 12h due to in situ formation of inter- and intramolecular disulfide bonds in the thiomer matrix. According to the results of the present study, this novel thiolated polymer seems to represent a promising multifunctional excipient for the development of various drug delivery systems.