To investigate the permeation of two ionisable drug molecules, warfarin and verapamil, across artificial membranes. For the first time since the introduction of the parallel artificial membrane permeation assay (PAMPA) in 1998, in situ permeation-time profiles of drug molecules are studied. The method employs a rotating-diffusion cell where the donor and acceptor compartments are separated by a lipid-impregnated artificial membrane. The permeation of the solute is investigated under well-defined hydrodynamic conditions with control over the unstirred water layer. The flux of the permeating molecule is analysed in situ using UV spectrophotometry. In situ permeation-time profiles are obtained under hydrodynamic control and used to determine permeability coefficients. An advanced analytical transport model is derived to account for the membrane retention, two-way flux and pH gradient between the two compartments. Moreover, a numerical permeation model was developed to rationalise the time-dependent permeation profiles. The membrane permeability, intrinsic permeability and unstirred water permeability coefficients of two drug molecules are obtained from two independent methods, hydrodynamic extrapolation and pH profiling, and the results are compared. Both warfarin and verapamil exhibit high permeability values, which is consistent with the high fraction absorbed in human. Our results demonstrate that a considerable lag-time, varying with the solute lipophilicity and stirring rate, exists in membrane permeation and leads to incorrect compound ranking if it is not treated properly. Comparison of the permeability data as a function of pH and stirring rate suggests that some transport of the ionized molecules occurs, most likely via ion-pairing.
