Intestinal epithelium membrane permeability

Among these testing methods, the small animal GI model and the Caco-2 cell culture model have shown the best correlation with oral absorption in vivo. The Caco-2 culture system consists of a monolayer of human intestinal epithelial cells grown on semipermeable supports such as polycarbonate membranes. Because the cells are human in origin, they exhibit many characteristics of the human small intestinal epithelium. The permeability coefficients relative to the extents of human drug absorption are listed here ... 

Thus, the apparent membrane permeability characteristics of hydrophilic compounds listed in Table 3.4 indicate that colonic epithelium is different from small intestinal epithelium in selectivity, or size or density distribution of the paracellular pathway. This is further complicated because of the possible involvement of unidentified carriers or channels for some compounds, as suggested for glycerol and D-xylose. However, the colon-to-SI ratios of the apparent membrane permeability are generally comparable with (or lower than) those calculated considering the morphological surface area, suggesting that such factors are not in favor for colonic absorption in most cases. Matching... 

Cell monolayers grown on permeable culture inserts form confluent mono-layers with barrier properties and can be used for drug absorption experiments. The most well-known cell line for the in vitro determination of intestinal drug permeability is the human colon adenocarcinoma Caco-2 [20, 21], The utility of the Caco-2 cell line is due to its spontaneous differentiation to enterocytes under conventional cell culture conditions upon reaching confluency on a porous membrane to resemble the intestinal epithelium. This cell model displays small intestinal carriers, brush borders, villous cell model, tight junctions, and high resistance [22], Caco-2 cells express active transport systems, brush border enzymes, and phase I and II enzymes [22-24], Permeability models... 

Effervescence may produce physiological changes within the body. Carbon dioxide bubbling directly onto the intestinal epithelium induced enhanced drug permeability due to an alteration of the paracellular pathway. This, in addition to fluid flow and membrane hydrophobicity concepts, may account for observed increases in drug flux. " ... 

Drug absorption generally occurs either through passive transcellular or paracellu-lar diffusion, active carrier transport, or active efflux mechanisms. Several methods have been developed to aid in the understanding of the absorption of new lead compotmds. The most common ones use an immortalized cell line (e.g., Caco-2, Madin-Darby canine kidney, and the like) to mimic the intestinal epithelium. These in vitro models provide more predictive permeability information than the artificial membrane systems (i.e., PAMPA and permeability assays, described previously) based on the cells ability to promote (active transport) or resist (efflux) transport. Various in vitro methods are listed in the U.S. FDA guidelines. These are acceptable to evaluate the permeability of a drug substance, and includes a monolayer of suitable epithelial cells, and one such epithelial cell line that has been widely used as a model system of intestinal permeability is the Caco-2 cell line. 

Permeability and Associated Assays Orally administered drugs need to cross the intestinal epithelium cell layer to reach the systemic circulation. Therefore, membrane permeability is a major determining factor of intestinal absorption and oral bioavailability for drug molecules. Several in vitro systems mimicking the epithelium cell layer are routinely used in ADME profiling to assess membrane permeability of NCEs. The parallel artificial membrane permeability assay (PAMPA) uses a dual chamber sandwich plate separated by an artificial lipid membrane to simulate the epithelium layer (Figure 6.6A) [75]. Compounds are... 

A more permeable epithelium obviously facilitates greater absorption. Some epithelia are relatively more permeable than others. For example, the skin is an extremely impermeable barrier, whereas the permeability of the lung membranes towards many compounds is much higher than the skin and is also higher than that of the small intestine and other mucosal routes. The vaginal epithelium is relatively permeable, particularly at certain stages of the menstrual cycle. 

In the filterwell or Transwell , systems, cells are grown on a permeable membrane between two separated liquid phases (McCall et al, 1981). In this condition confluent epithelial cells can achieve their full polarized functional state without the stresses induced by doming of epithelial monolayers on glass or plastic surfaces (Rabito et al, 1980). Filterwell culture has proved valuable for modelling the epithelium of the human intestine (Hidalgo et al., 1989). This technique is rapidly becoming the preferred culture system for many studies using epithelial cultures. 

The rate and extent of intestinal permeation is dependent on the physicochemical properties of the compound (see Sections 16.1.2 and 16.4.3) and the physiological factors. Drugs are mainly absorbed in the small intestine due to its much larger surface area and less tight epithelium in comparison to the colon [17]. The permeation of the intestine may be affected by the presence of an aqueous boundary layer and mucus adjacent to cells, but for a majority of substances the epithelial barrier is the most important barrier to drug absorption. The lipoidal cell membrane restricts the permeability of hydrophilic and charged compounds, whereas large molecules are restricted by the ordered structure of the lipid bilayer. 

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