In vitro evaluation of the potential of thiomers for the nasal administration of Leu-enkephalin
Purpose. To improve the mucoadhesive properties of polycarbophil by the introduction of sulfhydryl groups. Methods. Mediated by a carbodiimide, cysteine was covalently bound to polycarbophil (PCP) forming amide bonds between the primary amino group of the amino acid and the carboxylic acid moieties of the polymer. The amount of covalently attached cysteine and the formation of disulfide bonds within the modified polymer were determined by quantifying the share of thiol groups on the polymer conjugates with Ellman's reagent. The adhesive properties of polycarbophil-cysteine conjugates were evaluated in vitro on excised porcine intestinal mucosa by determining the total work of adhesion (TWA). Results. Depending on the weight-ratio of polycarbophil to cysteine at the coupling reaction, e.g., 16:1 and 2:1, 0.6 ± 0.7 μmole and 5.3 ± 2.4 μmole cysteine, respectively, were covalently bound per g polymer. The modified polymer displayed improved internal cohesive properties due to the formation of interchain disulfide bonds within the polymer in aqueous solutions at pH-values above 5. Adhesion studies revealed strongly improved adhesive properties. Whereas the TWA was determined to be 104 ± 21 μJ for the unmodified polymer, it was 191 ± 47 μJ for the polymer-cysteine conjugate 16:1 and 280 ± 67 μJ for the polymer-cysteine conjugate 2:1. Conclusions. Polymers with thiol groups might represent a new generation of mucoadhesive polymers displaying comparatively stronger adhesive properties.