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 character ized for their ability to transport and metabolize endogenous compound s as well as xenobiotics. All three clones were able to form a homogen eous well-differentiated epithelial monolayer as demonstrated by the p resence of microvilli at the apical pole of the cells and a high trans epithelial electrical resistance. These cell monolayers were further c haracterized for their ability to transport different probes such as m annitol for the paracellular route, testosterone for passive diffusion and taurocholic acid for the presence of active biliary acids transpo rters. Only small differences were observed between the parental cell line Caco-2 and the different clones in terms of transepithelial elect rical resistance, mannitol paracellular transport and testosterone pas sive 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 charac terized for the expression of various enzyme systems involved in the b iotransformation of endogenous compounds and xenobiotics, such as cyto chromes P450 and UDP-glucuronosyltransferases. All cell types expresse d cytochrome P450IA1, as demonstrated by the O-deethylation of 7-ethox yresorufin. However, a 3 day beta-naphthoflavone pretreatment induced 10.1 +/- 3.0- and 10.4 +/- 5.9-fold increases in 7-ethoxyresorufin O-d eethylation 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-glucuronos yltransferases. Since the intestinal epithelium plays an important rol e in the rate of absorption of intact drugs following their oral admin istration, both transport and metabolic characteristics make the Caco- 2/TC-7 clone a suitable in vitro model for studying the intestinal dis position of drugs.