Profound Effect of Plasma Protein Binding on the Polarized Transport of Furosemide and Verapamil in the Caco-2 Model

Caco-2 cell monolayers have been widely used as a very valuable in vitro model to study intestinal permeability of drugs for correlation with or prediction of human absorption, to rapidly screen a large number of potential drug candidates for intestinal absorbability, and to study kinetics and mechanisms of intestinal absorption, exsorption and metabolism (18). In most studies reported to date, aqueous buffered solutions were used as media in the basolateral compartment for measurement of the apical-to-basolateral (A→B) absorptive permeability or transport clearance (9,10), basolateral-toapical (B→A) efflux (exsorption) permeability or clearance (9,10), and net secretion ratio (B→A/A→B) involving efflux transporters such as P-glycoprotein (P-gp). Since mesenteric blood perfuses enterocytes near the basolateral side and the plasma protein binding may theoretically affect drug absorption and exsorption in vivo, it was decided to study the effect of using human plasma as a basolateral medium on the bidirectional transport of two efflux transporter substrates, furosemide (6) and verapamil (11), both of them being extensively bound (99% and 90%, respectively) to plasma proteins (12).