A comparative "in vitro" study of permeability with different synthetic and biological membranes

Permeability coefficients of seven compounds belonging to a true homologous series (4-alkylanilines) through several different synthetic and biological membranes were assayed in a two-chamber diffusion cell. Permeability-lipophilicity relationships for the experimental data were established and compared in order to ascertain whether the behaviour of these membranes was similar to that of the human skin. In all cases, the best fit for the permeation-lipophilicity correlation was provided by the bilinear model. It was demonstrated that this type of correlation, when a dimethylpolysiloxane membrane is used, is due to the existence of a supplementary stagnant aqueous layer adjacent to the membrane in the receptor compartment. This is clear from the fact that when Polysorbate 80 is added to the receptor solution, the effect of this layer is abolished. In these conditions, hyperbolic equation gives, consequently, the best fit for penetration-lipophilicity correlation. On the basis of the data obtained with rat skin and Polysorbate 80 in the receptor solution, it can be concluded that for biological membranes the bilinear model obtained is due to their heterogeneous nature. The optimum lipophilicity value for penetration according to the bilinear model was not the same for all the membranes assayed. Human and rat skin were qualitatively similar in behaviour.