Role of P-glycoprotein-mediated secretion in absorptive drug permeability:An approach using passive membrane permeability and affinity to P-glycoprotein

It has been shown in vivo and in vitro that P-glycoprotein (P-gp) may be ab le to influence the permeability of its substrates across biological membra nes. However, the quantitative contribution of the secretion process mediat ed by P-gp on the overall permeability of membranes has not been determined yet. In particular, observations need to be clarified in which substrates showing high affinity to P-glycoprotein, e.g., verapamil, apparently do not seem to be greatly influenced by P-gp in their permeability and consequent ly also with respect to their extent of GI-absorption after oral administra tion, whereas weaker substrates of P-gp, e.g., talinolol, have clearly show n P-gp-related absorption phenomena such as nonlinear intestinal permeabili ty and bioavailability. Experiments with Caco-2 cell monolayers and mathema tical simulations based on a mechanistic permeation model should aid in cla rifying the underlying mechanism for these observations and quantifying the influence of passive membrane permeability and affinity to P-gp to the ove rall transmembrane drug flux. In addition, the concentration range of drug ai which P-glycoprotein-mediated transport across the biological membrane i s relevant should be examined, The permeability of various drugs in Caco-2 monolayers was determined experimentally and modeled using a combination of passive absorptive membrane permeability and a Michaelis-Menten-type trans port process in the secretory direction. The passive permeabilities were ex perimentally obtained for the apical and basolateral membrane by efflux exp eriments using Caco-2 monolayers in the presence of a P-gp inhibitor. The M ichaelis-Menten parameters were determined by a newly developed radioligand -binding assay for the quantification of drug affinity to P-gp. The model w as able to accurately simulate the permeability of P-glycoprotein substrate s, with differing passive membrane permeabilities and P-glycoprotein affini ties. Using the outlined approach, permeability vs donor-concentration prof iles were calculated, and the relative contribution of passive and active t ransport processes to the overall membrane permeability was evaluated. A mo del is presented to quantitatively describe and predict direction-dependent drug fluxes in Caco-2 monolayers by knowing the affinity of a compound to the exsorptive transporter P-gp and its passive membrane permeability. It w as shown that a combination of high P-gp affinity with good passive membran e permeability, e.g., in the case of verapamil, will readily compensate for the P-gp-mediated reduction of intestinal permeability, resulting in a nar row range in which the permeability depends on the apical drug concentratio n. On the other hand, the permeability of compounds with low passive membra ne permeability (e.g., talinolol) might be affected over a wide concentrati on range despite low affinity to P-gp.