Drug transport and metabolism in in vitro models of the human intestine : By Shamsi D. Raeissi

Abstract: The objective of this thesis was to study the mechanisms of drug transport and metabolism in two different in vitro models of the human intestine: monolayers of human intestinal epithelial cell lines and intestinal microsomes. The first part of the thesis shows that Caco-2 cell monolayers are a good model for mechanistic studies of drug transport. More specifically, the effects of an absorption enhancer, palmitoyl-DL-carnitine (PC), on the permeability of the various drug transport routes in Caco-2 monolayers were examined. PC increased the permeability of the paracellular but not the transcellular route. The Caco-2 cells were also used to study structure-transport relationships for a series of cephalosporins. The results indicated that the presence of an α-amino group will favour cephalosporin transport by the intestinal peptide transporter PEPT1 in Caco-2 cells. Finally, the bidirectional flux of a new anticancer drug, docetaxel, was investigated in the cell monolayers. The results showed that docetaxel was transported by P-glycoprotein (Pgp) in the basolateral to apical direction. In the second part of this thesis, a subclone of Caco-2 cells, TC7, and rabbit intestinal microsomes were used as in vitro models to study intestinal drug metabolism. More specifically, the activity of the CYP3A enzyme expressed in TC7 was compared with the activity of human intestinal CYP3A4, using terfenadine as substrate. The results showed that CYP3A in TC7 cells is not CYP3A4, but probably CYP3A5. In addition, CYP3A in TC7 has a 30-fold lower expression than CYP3A4 in the human intestine, making this cell line useful for qualitative rather than quantitative studies of intestinal CYP3A mediated-metabolism. The interplay between CYP3A-mediated metabolism and Pgp-efflux of terfenadine was also investigated in TC7 cells. CYP3A and Pgp were found to have complementary roles inreducing the absorptive flux of terfenadine and its metabolites across the Caco-2 cell monolayers.Finally, testosterone metabolism in the rabbit intestinal microsomes was investigated. Five oxidation products and a new metabolite were formed which suggests that CYP2B4, CYP2C3, CYP3A6 and a novel glucosyl transfer mechanism are expressed in the rabbit intestine. In conclusion, the results of this thesis indicate that in vitro models, such as cell monolayers of intestinal epithelial cells and intestinal microsomes, are very useful for studies of intestinal drug transport and metabolism.