Models to Study Active Transporters

Studies were undertaken to quantify transporters and examine regional expression in the intestine. mRNA levels in the gut were studied by Englund et al. [35]. Nine transporters were examined and eight were shown to have significant regional differences in expression. In addition, up to a 20-fold difference in expression was observed for certain transporters between intestinal tissue and Caco-2 cells. Expression of transporters in cell models compared to normal tissue can be markedly different, depending on the age of the cells, passage number and culture conditions [35]. Cell models may under- or overexpress transporters. In addition cell lines may express transporters which may not be relevant in vivo. 

The relevance of P-gp at the BBB is considered highly important due to the low drug concentrations in the systemic circulation. The interest in transporters led to development of cell lines that overexpress them, for example, MDCK-MDRl and MDCK-MDR2. MDCK-MDRl, which overexpresses P-gp, is one of the most widely used models in early drug discovery screening [38, 39]. 

In addition it was also suggested that cell lines that have limited expression of transporters (e.g., 2/4/Al) are of use in understanding passive permeability. Saturation of transporters or inhibition can also be used to understand the contribution of passive permeation and active transport. 

Correlation of in Vitro Models to Human P frand Fraction Absorbed Data 

Correlation of in vitro data with human Pefjand Fa% was undertaken with a number of models [25, 27]. Although Fa% data is useful and more broadly available, the introduction of the Loc-I-Gut model by Lennernas allowed direct measurement of PefF. In the Loc-I-Gut model, a segment of the small intestine in humans is isolated using a balloon catheter, disappearance of the drug is measured and the Pgfr can be generated [40]. 

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On the other hand, artificial membranes represent a further simplification of the human GIT than Caco-2 monolayers as a number of features are not present, like the paracellular pathway, active transporters, or efflux systems. Caco-2 is a useful model to study actively transported compounds as well as substrates and inhibitors of efflux systems (PgP, MRP). Caco-2 monolayers also have some potential to model paracellular transport, although the junctions appear tighter than in the upper GIT. The consequence is that some paracellular transported compounds are properly transported while others are underestimated (Figure 15.6R). 

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