Purpose. To verify or refute the mechanism of permeation enhancement with thiolated polymers via GSH by the use of NaFlu as marker for the paracellular permeation.
Methods. The capability of 0.5% polycarbophil cysteine conjugate (PCP-Cys) to reduce 0.02% oxidized glutathione (GSSG) was evaluated via iodometric titration in aqueous solution. Glutathione in its reduced form (GSH; 0.1%-0.4%) and in combination with 0.5% PCP-Cys were tested for their permeation enhancement of sodium fluorescein (NaFlu) and fluorescence labeled bacitracin (bac-FITC) used as paracellular markers. Permeation studies across guinea pig duodenum were carried out in Ussing-type chambers. Opening of the tight junctions was additionally monitored by transepithelial electrical resistance (TEER) measurements.
Results. PCP-Cys (0.5%) was shown to reduce 22.0% ± 8.2% of GSSG (0.02%) to GSH in aqueous solution at pH 7.0 and 37°C within 3 h. Permeation of NaFlu was shown to depend on the concentration of GSH. The apparent permeability coefficient (Papp) of NaFlu in buffer only was 4.98 ± 0.5*10-6, while in the presence of 0.4% GSH a Papp of 9.31 ± 0.92*10-6 was achieved, representing an enhancement ratio (R = Papp enhancer system/Papp control) of 1.86. The combination of GSH (0.4%) with PCP-Cys (0.5%) led to a significant (p < 0.001) improvement of R for NaFlu up to 2.93 accompanied by a decrease in TEER of 20.3% ± 1.4%. Incubation of bac-FITC with the same GSH / PCP-Cys combination led to an enhancement ratio of 2.06 within 3 h.
Conclusion. GSH plays an important role in the opening of tight junctions of intestinal epithelia. It would appear that PCP-Cys is able to reduce GSSG, prolonging the concentration of GSH at the apical membrane, resulting in significantly enhanced paracellular transport.