Abstract
Drug absorption from the gastro-intestinal tract is often restricted by efflux transport by P-glycoprotein (P-gp) and metabolism by cytochrome P450 (CYP) 3A4. Both localize in the epithelial cells and thus their activities are directly affected by the intracellular drug concentration which should be regulated by the ratio of permeability between apical (A) and basal (B) membranes. In this study, using Caco-2 cells with forced expression of CYP3A4, we assessed the transcellular permeation of A-to-B and B-to-A directions and the efflux from the preloaded cells to the both sides of 12 representative P-gp or CYP3A4 substrate drugs, and obtained the parameters for permeabilities, transport, metabolism, and unbound fraction in the enterocytes (fent) using simultaneous and dynamic model analysis. The membrane permeability ratios for B to A (RBA) and fent varied by 8.8-fold and by more than 3,000-fold, respectively, among the drugs. The RBA values for digoxin, repaglinide, fexofenadine, and atorvastatin were greater than 1.0 (3.44, 2.39, 2.27, and 1.90, respectively) in the presence of a P-gp inhibitor, thus suggesting the potential involvement of transporters in the B membrane. The Km for quinidine for P-gp transport was 0.077 µM for the intracellular unbound concentration. These parameters were used to predict overall intestinal availability (FAFG) by applying an intestinal pharmacokinetic model, ATOM, in which permeability of A and B membranes accounted separately. The model predicted changes in the absorption location for P-gp substrates according to its inhibition, and FAFG values of 10/12 drugs, including quinidine at varying doses, were explained appropriately.
Significance Statement Pharmacokinetics has improved predictability by identifying the molecular entities of metabolism and transport, and by using mathematical models to appropriately describe drug concentrations at the locations where they act. However, analyses of intestinal absorption so far have not been able to accurately consider the concentrations in the epithelial cells where P-glycoprotein and CYP3A4 exert effects. In this study, the limitation was removed by measuring the apical and basal membrane permeability separately and then analyzing these values using new appropriate models.
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