Animal studies MDCK cells

Some of the advantages of cell monolayer models include the ability to use human instead of animal cell types as well as the ability to perform cellular uptake and bidirectional cell transport studies for evaluation of absorptive and secretory processes. The potential for automation to achieve higher throughput in the early drug discovery setting is an added attraction. Regardless of the cell type used, the utility of these models in transport studies is based on the correlation between permeability properties determined in these models and those obtained in vivo, such as fraction of dose absorbed (Fig. 3). To date, numerous laboratories have established a correlation between apparent permeability coefficients (P pf) from Caco-2 or MDCK cells and in vivo fraction absorbed of drugs in solution [58—62]. Construction of correlation plots for known compounds or reference markers then provides an opportunity for interpolation of fraction absorbed for NMEs for which in vivo fraction absorbed is unknown. 

Since Intestinal absorption and Intestinal and hepatic first-pass metabolism are the major processes limiting bloavallablllty, the majority of the methods described below will focus on appropriate models for these processes. Nevertheless, It should be emphasized that other barriers, notably the blood-brain barrier (BBB) and virtually all organ-related membrane barriers, also need to be considered when Investigating the distribution of a compound throughout the body and ultimately Its tissue bloavallablllty. A recent study by the European Centre for the Validation of Alternative Methods (ECVAM) compared In vitro BBB methods and characterized several models In relation to In vivo studies Immortalized BBB-derlved endothelial cell lines (SV-ARBEC, MBEC4), non-BBB-derlved cell lines (MDCK, Caco-2, ECV-C6), and primary cells derived from BBB. In all cases, the correlations between In vitro and In vivo assays were low [36], and, moreover, most In vitro methods lacked at least some of the features of the In vivo barrier [37, 38]. Therefore, animal studies will continue to play a major role In studies of the BBB. 

With the difficulties associated with accurate estimation of permeability based only on physicochemical properties, a variety of methods of measuring permeability have been developed and used, among which are (l)cul-tured monolayer cell systems, such as Caco-2 or MDCK ( 2 diffusion cell systems that use small sections of intestinal mucosa between two chambers (3) in situ intestinal perfusion experiments performed in anesthetized animals such as rats and (4)intestinal perfusion studies performed in humans (40,54-62). All of these methods offer opportunities to study transport of drug across biological membranes under well-controlledconditions. Caco-2 mono-layer systems in particular have become increasingly commonly used in recent years and human intestinal perfusion methods are also becoming more commonly available. Correlations between Caco-2 permeability and absorption in humans have been developed in several laboratories (63-72). As shown in Fig. 

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