transporters are expressed in the transfected cell lines when using a transfected
cell line to map transporter substrate. The vector control transfected cells and a
known transporter inhibitor are always recommended to be used together with
the transporter transfected cells to get more definitive results.
The IC 50 andKiof transporter inhibitors can be determined using selective
transporter substrate byin vitroassays. For uptake transporters, hepatocyte,
transporter transfected cells, oocytes, and yeast have been used asin vitro
models; for efflux transporter, Caco-2 cells, and transporter transfected cells
and membranes are commonly used models.
6.6.2 In situ/Ex vivoModels
Compared toin vitroassays, the isolated perfused intestine, liver, kidney, or
brain allows a more accurate determination of the transporter functions in
intestinal absorption, biliary elimination, renal excretion, or brain penetration
and the interplay with drug metabolism enzymes. Liver perfusion studies have
shown that transporter and metabolism enzymes acted in concert to govern
drug metabolism in hepatocytes and elimination into the bile (Benet et al.,
2004; Lau et al., 2004, 2006; Liu and Pang, 2005). In rat liver perfusion studies,
the AUC of digoxin (a substrate of OATP2, P-gp, and CYP3A4) in perfusate
was increased by rifampicin treatment (an OATP2 inhibitor) and decreased by
quinidine treatment (a P-gp inhibitor). It is concluded that rifampicin limits
the hepatic enzyme exposure to digoxin by inhibiting the sinusoidal Oatp2-
mediated transport, whereas quinidine increased digoxin exposure to the
hepatic enzymes by inhibiting the canalicular P-gp efflux. These data
emphasize the importance of uptake and efflux transporters on hepatic drug
metabolism (Lau et al., 2004). Perfused kidney studies proved the P-gp-
mediated digoxin renal elimination and the interaction between digoxin and
CsA (Hori et al., 1993; Okamura et al., 1993). By comparing the perfused
tissue from transporter deficient animals with that from normal animals, the
function of that specific deficient transporter can be evaluated. The
disadvantage of the perfusion techniques is the need for specific surgical skills
and sophisticated equipments.
6.6.3 In vivoModels
6.6.3.1 Transporter Genetic Knockout Mice or Natural Mutant Animals
Genetic knockout mice or natural mutant animal models have become
important tools in understanding the physiological functions of drug
transporters and in evaluating the effect of transporters on the pharmacoki-
netics and pharmacodynamics of drugs (Chen et al., 2003a; Loescher and
Potschka, 2005; Xia et al., 2005b). Genetic knockout mice are generated by
disrupting the endogenous transporter gene in order to investigate the specific
targeted transporter. Naturally, mutant animals are subpopulations that have
a spontaneous mutation in a transporter gene. Naturally occurring P-gp
182 DRUG TRANSPORTERS IN DRUG DISPOSITION, DRUG INTERACTIONS