The influence of viscosity-enhancing agents on oral absorption of drugs.

The objective of this study was to evaluate the influence of viscosity-enhancing agents on oral absorption of metoprolol (MPL) and bisoprolol (BPL). Although the viscosity values were similar for MPL and BPL in hydroxypropyl methylcellulose (HPMC, 1.2 % (w/w)) and polyvinyl alcohol (PVA, 8.8 % (w/w)) solutions, the order of diffusion rate constants of the drugs in media were phosphate buffer solution (reference) > HPMC solution > PVA solution. In in vivo rat intestinal absorption experiments showed that the C-max and AUC values of the drugs were lowest when they were administered into the rat jejunum in a PVA solution. In vitro binding studies showed that this may have been due to adsorption of the drugs to PVA molecules, resulting in decreased free fractions of the drugs. Our results indicated that intestinal absorption of the drugs in PVA solution was influenced both by decreased diffusion of the drugs and by interaction with PVA. Since various viscosity-enhancing agents are widely used as pharmaceutical and food additives, these findings may be of significance for understanding therapeutic efficacy and safety of oral drug products.