Biol. Pharm. Bull. 30(11) 2167—2172 (2007)

confluent monolayers of well-differentiated enterocyte-like cells with functional properties of transporting epithelia, and retains various influx and efflux transporters expressed in the intestine, such as H -coupled oligopeptide transporter 1 (PEPT1), P-glycoprotein (P-gp), multi-drug resistance proteins (MRPs), and breast cancer resistance protein (BCRP). Caco-2 cells have been widely used as a model to investigate the intestinal absorption or secretion of various drugs and other xenobiotics. Caco-2 cell monolayers grown on plastic dishes are useful to mainly investigate the uptake of drugs at the apical membrane. On the other hand, Caco-2 cell monolayers grown on porous membrane filters allow us to investigate transcellular absorption and the secretion of drugs across intestinal epithelial cells. For example, transcellular transport of digoxin was investigated using Caco-2 cells grown on porous membrane filters, and the results indicated that P-gp is involved in the intestinal secretion of digoxin. Chiu et al. investigated the transcellular transport of another P-gp substrate, cyclosporin A, across Caco-2 cell monolayers to examine the jejunal permeability of the drug. Watanabe et al. investigated the effects of progesterone and norethisterone on the apical-to-basolateral and basolateral-to-apical transport of cephalexin, a typical PEPT1 substrate, using Caco-2 cell monolayers cultured on permeable membranes. Irie et al. compared the recognition characteristics of the basolateral peptide transporter for several nonpeptidic compounds, such as valacyclovir, with those of apical PEPT1 using Caco-2 cells grown on porous membrane filters. We previously performed pharmacokinetic analysis of transcellular transport of quinidine across Caco-2 cell monolayers grown on porous membrane filters in order to investigate the mechanism responsible for the intestinal absorption of lipophilic organic cation. The transcellular transport of quinidine was greater in the apical-to-basolateral direction than in the opposite direction. The calculated efflux clearance of quinidine at the apical membrane was 6.1-fold greater than that at the basolateral membrane, suggesting that P-gp is at least partly responsible for the apical efflux of the drug. The influx clearance value at the apical membrane was much greater than that at the basolateral membrane. Therefore, we further investigated the uptake of quinidine at the apical membrane using Caco-2 cells grown on plastic dishes. The apical uptake of quinidine was markedly increased by alkalization of the apical medium, and was diminished by the decrease in temperature (4 °C) of the medium. In addition, the uptake of quinidine in Caco-2 cells grown on plastic dishes was significantly inhibited by the presence of levofloxacin and cationic drugs, such as imipramine. These findings indicated that influx at the apical membrane was the direction-determining step in the transcellular transport of quinidine across Caco-2 cell monolayers, and that some specific transport system was involved in the apical uptake of the drug. In the present study, we performed pharmacokinetic analysis of the transcellular transport of levofloxacin in Caco-2 cells grown on porous membrane filters in order to elucidate whether the specific transport system responsible for the apical uptake of cationic drugs is also involved in the intestinal absorption of levofloxacin. That is, when transcellular drug transport is examined under the condition where the unlabeled drug concentration in the monolayer is equilibrated with that of the incubation medium in the apical and basolateral chambers, the transport data for a small amount of a radio-labeled drug can be analyzed using a linear pharmacokinetic model. In addition, we compared the transport characteristics of levofloxacin in Caco-2 cells with those in the renal epithelial LLC-PK1 cell line, which is known to express the transporters for organic cations. November 2007 2167