Viscosity of pharmacopeial multimolecular ointment vehicles and pharmaceutical availability of a model therapeutic agent.

Viscosity was tested of basic ointment vehicles such as: white petrolatum, yellow petrolatum, anhydrous lanolin and eucerin produced by different manufacturers. Ointment vehicles of definite type differ significantly in rheological parameters. In the same group of products, the experimentally determined viscosity value of some vehicles is two-fold (petrolatum, anhydrous lanolin) or even three-fold (eucerin) higher than that of others. On the basis of rheological tests, using Einstein-Smoluchowski equation (D = kT/ 6pireta), theoretical coefficient was calculated of a model therapeutic agent--salicylic acid diffusion (-log chi2(i) = 1,22) from the tested vehicles to the external compartment. The obtained results were related to the performed in vitro measurements of the rate of salicylic acid release from the above mentioned ointment vehicles to model acceptor fluid. High correlation was observed between theoretical values of diffusion coefficients calculated on the basis of viscosity measurements and tested experimentally pharmaceutical availability of salicylic acid. It was confirmed by describing this dependence with regression equations of high correlation coefficients (r > or = 0,9667). Marked disproportions between rheological parameters of the vehicles of definite type produced by individual manufacturers are the cause of differences in pharmaceutical availability of therapeutic agents contained in these vehicles.