CHARACTERIZATION OF A NEWLY ISOLATED CACO-2 CLONE (TC-7), AS A MODEL OF TRANSPORT PROCESSES AND BIOTRANSFORMATION OF DRUGS

Three clones isolated from early (clone PD-7 and PF-11) and late (TC-7) passages of the parental Caco-2 epithelial cell line, were characterized for their ability to transport and metabolize endogenous compounds as well as xenobiotics. All three clones were able to form a homogeneous well-differentiated epithelial monolayer as demonstrated by the presence of microvilli at the apical pole of the cells and a high transepithelial electrical resistance. These cell monolayers were further characterized for their ability to transport different probes such as mannitol for the paracellular route, testosterone for passive diffusion and taurocholic acid for the presence of active biliary acids transporters. Only small differences were observed between the parental cell line Caco-2 and the different clones in terms of transepithelial electrical resistance, mannitol paracellular transport and testosterone passive diffusion. However, large differences were observed in the active transport of taurocholic acid with V-max/K-m values of 0.037, 0.048, 0.060 and 0.178 for Caco-2 parental cell line, clones PD-7, PF-11 and TC-7, respectively. Among transport processes, clones were also characterized for the expression of various enzyme systems involved in the biotransformation of endogenous compounds and xenobiotics, such as cytochromes P450 and UDP-glucuronosyltransferases. All cell types expressed cytochrome P450IA1, as demonstrated by the O-deethylation of 7-ethoxyresorufin. However, a 3 day beta-naphthoflavone pretreatment induced 10.1 +/- 3.0- and 10.4 +/- 5.9-fold increases in 7-ethoxyresorufin O-deethylation in Caco-2 cells and PF-11 clone, respectively, while 24.7 +/- 9.6- and 22.7 +/- 8.1-fold increases were observed for PD-7 and TC-7 clones, respectively. These two clones also exhibited a much higher catalytic activity towards 1-naphthol, a substrate for UDP-glucuronosyltransferases. Since the intestinal epithelium plays an important role in the rate of absorption of intact drugs following their oral administration, both transport and metabolic characteristics make the Caco-2/TC-7 clone a suitable in vitro model for studying the intestinal disposition of drugs.