Cell Based Systems

Since the early nineties, the use of cell cultures to predict drug permeation across the intestinal barrier has gained popularity (Artursson et al., 2001; Ungell and Karlsson, 2003; Balimane and Chong, 2005). From the different adenocarcinoma cell lines [e.g. Caco-2, HT-29 goblet cells, T84 crypt cells, LS180, and 2/4/A1 (rat fetal duodenal cells)], the Caco-2 cell culture model has become a widely used tool for the determination of the intestinal transport characteristics of drug candidates (Hidalgo et al., 1989; Hilgers et al., 1990; Gan and Thakker, 1997; Balimane et al., 2000; Ungell and Karlsson, 2003). Caco-2 cells differentiate spontaneously under standard culturing conditions to form confluent monolayers; they acquire many features of absorptive intestinal cells during culture (Artursson and Karlsson, 1991; Ungell and Karlsson, 2003). Another cell-based model that is frequently used for permeability assessment is the MDCK (Madin-Darby Canine Kidney) cell culture model. MDCK cells differentiate into columnar epithelial cells and form tight junctions when cultured on microporous membranes. A good correlation between permeation of passively absorbed drugs across MDCK cells and Caco-2 cells has been shown (Irvine et al., 1999). However, the potential difference in expression level of transporters between cells of different origin (canine versus human, kidney versus intestine)

Advantages

Disadvantages

Membrane-based systems easy high- or medium throughput low cost

only passive diffusion variability/stability of columns (IAM)

Cell-based systems good screening tool high- or medium throughput widely recognized cells may have receptors good correlation with human fraction absorbed evaluation of transport mechanism evaluation of absorption enhancing strategies evaluation of toxicity of compounds

no mucus no cellular heterogeneity no natural tissue environment (nerves, blood vessels, regulation by hormones) culture conditions can influence presence of receptors

Tissue-based systems evaluation of transport mechanism segmental absorption evaluation evaluation of absorption enhancing strategies other tissues (nasal, buccal) comparison between species possible presence of nerves (e.g., local control, crypt-villus axis) transporters and enzymes as in vivo

limited database viability of tissue low throughput limited access to (human) tissue

Table 1. Advantages and disadvantages of various absorption models.

Table 1. Advantages and disadvantages of various absorption models.

and the lack of knowledge regarding the correlation between the transport of carrier-mediated compounds in the MDCKmodel and human absorption values, makes this model somewhat less attractive as an early screening tool compared to the Caco-2 model. On the other hand, MDCK cells can easily be transfected to express particular transporter proteins (e.g., P-gp, MDR1 gene product), making the model attractive for mechanistic studies.

In vitro permeation using a monolayer of cultured epithelial cells is currently recognized as a valuable screening tool and has become indispensable in drug discovery and development to estimate intestinal permeability and the absorption potential of drug candidates. Cell culture models can easily be automated and may offer a good compromise between biological relevance and rapidity of screening. In addition, these models can allow evaluation of the 'mechanism' of drug permeation by performing bi-directional transport studies in the presence

In vitro

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