Intestinal membrane diffusion across

This permeability barrier shows selectivity in that small hydrophobic molecules can partition into and diffuse across the lipid bilayer of the cell membrane, whereas small hydrophilic molecules can only diffuse between cells (i.e., through the intercellular junctions). In addition, the presence of uptake and efflux transporters complicates our ability to predict intestinal permeability based on physicochemical properties alone because transporters may increase or decrease absorptive flux. The complexity of the permeability process makes it difficult to elucidate permeability pathways in complex biological model systems such as animals and tissues. For this reason, cultured cells in general, and Caco-2 cells in particular, have been used extensively to investigate the role of specific permeability pathways in drug absorption. 

The successful application of in vitro models of intestinal drug absorption depends on the ability of the in vitro model to mimic the relevant characteristics of the in vivo biological barrier. Most compounds are absorbed by passive transcellular diffusion. To undergo tran-scellular transport a molecule must cross the lipid bilayer of the apical and basolateral cell membranes. In recent years, there has been a widespread acceptance of a technique, artificial membrane permeation assay (PAMPA), to estimate intestinal permeability.117118 The principle of the PAMPA is that, diffusion across a lipid layer, mimics transepithelial permeation. Experiments are conducted by applying a drug solution on top of a lipid layer covering a filter that separates top (donor) and bottom (receiver) chambers. The rate of drug appearance in the bottom wells should reflect the diffusion across the lipid layer, and by extrapolation, across the epithelial cell layer. 

Orally administered drugs partition into the lipid bilayer in the process of diffusing across the apical and basolateral membranes of the intestinal brush border cells into the blood, as illustrated in Figure 2. About 1800 such drugs are commercially available. A small surface area of the polar parts of the drug molecule generally favors entry into the hydrophobic interior of... 

The proton pump inhibitors are lipophilic weak bases (pKa 4-5) and after intestinal absorption diffuse readily across lipid membranes into acidified compartments (eg, the parietal cell canaliculus). The prodrug rapidly becomes protonated within the canaliculus and is concentrated more than 1000-fold by Henderson-Hasselbalch trapping (see Chapter 1). There, it rapidly undergoes a molecular conversion to the active form, a reactive thiophilic sulfenamide cation, which forms a covalent disulfide bond with the H +, K+ ATPase, irreversibly inactivating the enzyme. 

Aniline is therefore not absorbed under these conditions (Fig. 3.14). Furthermore, the ionization in the plasma does not facilitate diffusion across the membrane, and with some bases, secretion from the plasma back into the stomach may take place. The situation in the small intestine, where the pH is around 6, is the reverse, as shown in Figure 3.15. 

The analysis of transfer mechanisms of drugs across the intestinal epithelial layer has passed a long way since the theory of lipid pore membrane [118] in which the total pore area of the intestinal membranes was calculated (and found to be low compared with the total surface of the mucosal aspect of the gut), through the Fickian diffusion calculations of the transport of unionized moieties of drug molecules (the Henderson-Hasselbach equation), which led to the conclusion that acidic drugs are absorbed in the stomach [119,120]. 

Memory aid In general, weak organic acids readily diffuse across a biological membrane in an acidic environment, and organic bases can similarly diffuse in a basic environment. This is illustrated quite well in Table 6.2 for the chemical in rat intestine. There are the usual exceptions to the generalizations concerning ionization and membrane transport, and some compounds, such as pralidoxime (2-PAM), paraquat, and diquat, are absorbed to an appreciable extent even in the ionized forms. The mechanisms allowing these exceptions are not well understood. 

Once mobilized in the hepatocyte, chemicals can contact and interact with biotransformation enzymes (Chapter 7). These enzymes generally increase the polarity of the chemical, thus reducing its ability to passively diffuse across the sinusoidal membrane back into the blood. Bio transformation reactions also typically render the xenobiotics susceptible to active transport across the canalicular membrane into the bile canaliculus and, ultimately, the bile duct (Figure 10.3). The bile duct delivers the chemicals, along with other constituents of bile, to the gall bladder that excretes the bile into the intestines for fecal elimination. 

Preliminary pharmacokinetic behavior can be tested through a number of whole cell assays. Most commercially successful drugs are administered orally, meaning the drug must be able to enter the bloodstream by crossing membranes in the intestines. The most common membrane permeability assay is performed by monitoring the absorption and secretion of a compound by colon carcinoma cells (Caco-2). Diffusion across Caco-2 cell membranes is considered to be a valid model for molecular transport in the small intestines.16... 

Passively absorbed compounds diffuse either through the cell itself (transcellular pathway) or in between cells (paracellular pathway). The lipid bilayers of which the mucosal and basolateral epithelial cell membranes are composed of, define the primary transcellular diffusion resistance to solute transport across the intestinal barrier. Transcellular permeabihty, particularly of lipophilic solutes, depends on their partitioning between intestinal membrane and aqueous compartments (Fig. 1). 

The process of intestinal absorption has three components (1) passive diffusion across the membrane, (2) active transport into the membrane, and... 

Bacterial urease. A major source of ammonia in liver (approximately 25%) is produced by the action of certain bacteria in the intestine that possess the enzyme urease. Urea present in the blood circulating through the lower digestive tract diffuses across cell membranes and into the intestinal lumen. Once urea is hydrolyzed by bacterial urease to form ammonia, the latter substance diffuses back into the blood, which transports it to the liver. 

Certain intestinal bacteria can release ammonia from urea molecules that diffuse across the membrane into the intestinal lumen. Treatment with antibiotics kills these organisms, thereby reducing blood ammonia concentration. 

Hpophilicity, but eventually reaches a maxi-mim as diffusion across the aqueous boundary layer becomes a rate-limiting step (see below). Whereas many nutrients and drugs are readily transported across the intestinal mem-brmie, there are a large number of highly water soluble compounds whose transfer across the intestinal membrane is limited as a result of extremely low P values. 

Selection of the octanol-water system is often justified in part beeause, like biological membrane components, oetanol is flexible and contains a polar head and a nonpolar tail. Hence, the tendency of a drug molecule to leave the aqueous phase and partition into oc-tanol is viewed as a measure of how efficiently a drug will partition into and diffuse across biological barriers such as the intestinal membrane. Although the octanol-water partition coefficient is, by far, most commonly used, other solvent systems such as cyclohexane-water and chloroform-water systems offer additional insight into partitioning phenomena. 

Fig. 3. Diagrammatical representation of the effect of bile acid micelles (or vesicles) in overcoming diffusion barrier resistance. In the absence of bile acids, individual lipid molecules must diffuse across the barriers overlying the microvillus border of the intestinal epithelial cell (arrow 1). Hence, uptake of these molecules is largely diffusion limited. In the presence of bile acids (arrow 2) large amounts of these lipid molecules are delivered directly to the aqueous-membrane interface so that the rate of uptake is facilitated [11].

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