Transport transcellular route

Kelder et al. [19] have shown that PSA can be used to model oral absorption and brain penetration of drugs that are transported by the transcellular route. A good correlation was found between brain penetration and PSA (n=45, r=0.917). From analyzing a set of 2366 central nervous system (CNS) and non-CNS oral drugs that have reached at least phase 11 clinical trials it was concluded that orally active drugs that are transported passively by the transcellular route should have PSA< 120 Al In addition, different PSA distributions were found for CNS and non-CNS drugs. 

The perturbation of monolayers with agents (e.g., disodium ethylenediamine tetraacetate, Ca+2-free medium, sodium citrate, cytochalasin D) to open tight junctions and the effect on the transmonolayer flux of permeants are addressed in this section. It has been observed that permeants taking predominantly the trans-cellular route are not affected by perturbants of the paracellular route, compared to extracellular or relatively hydrophilic permeants (Artursson and Magnusson, 1990). Let us put these general observations into a quantitative intepretation in the light of the transmonolayer kinetic studies of steroids in this section and of paracellular permeants in Section III. There are three cases to consider (1) ABL-controlled permeants, (2) monolayer-controlled permeants transported principally by the transcellular route, and (3) monolayer-controlled permeants for which the paracellular route dominates. 

There are two principal routes of drug transport across any epithelium transcellu-lar and paracellular (Fig. 6). In the transcellular route, drugs are transported... 

For the evaluation of a possible relationship between the molecular structure of a potential candidate and its transport abilities to cross the epithelial membrane of the gut, the mechanism or route of transport must be known [1,4]. This is due to the structural requirements for the transcellular route being different from the paracellular route. During the lead optimization phase - when many mechanistically based studies are performed - the cell culture-based models can also be used with great confidence. 

Molecules with a large molecular weight or size are confined to the transcellular route and its requirements related to the hydrophobicity of the molecule. The transcellular pathway has been evaluated for many years and is thought to be the main route of absorption of many drugs, both with respect to carrier-mediated transport and passive diffusion. The most well-known requirement for the passive part of this route is hydrophobicity, and a relationship between permeability coefficients across cell monolayers such as the Caco-2 versus log P and log D 7.4 or 6.5 have been established [102, 117]. However, this relationship appears to be nonlinear and reaches a plateau at around log P of 2, while higher lipophilicities result in reduced permeability [102, 117, 118]. Because of this, much more attention has recently been paid towards molecular descriptors other than lipophilicity [86, 119-125] (see section 5.5.6.). The relative contribution between the para-cellular and transcellular components has also been evaluated using Caco-2 cells, and for a variety of compounds with different charges [110, 112] and sizes [112] (see Section 5.4.5). 

This refers to the transport across the epithelial cells, which can occur by passive diffusion, carrier-mediated transport, and/or endocytic processes (e.g., transcytosis). Traditionally, the transcellular route of nasal mucosa has been simply viewed as primarily crossing the lipoidal barrier, in which the absorption of a drug is determined by the magnitude of its partition coefficient and molecular size. However, several investigators have reported the lack of linear correlation between penetrant lipophilicity and permeability [9], which implies that cell membranes of nasal epithelium cannot be regarded as a simple lipoidal barrier. Recently, compounds whose transport could not be fully explained by passive simple diffusion have been investigated to test if they could be utilized as specific substrates for various transporters which have been identified in the... 

Secretory epithelia control transport of water and solutes from the subluminal compartment (blood) into the lumen or body exterior. At present, there is no single unifying model for transepithelial fluid or water transport. In some epithelia, transcellular routes of fluid transport via water channels may predominate [88a], However, in other types of epithelia, such as the cervical-vaginal epithelia, transport of fluids usually occurs via the paracellular route [1, 14], In the latter, movement of fluid can be driven by three main mechanisms (Figure 15.1C) ... 

Orally active drugs that are transported passively by the transcellular route can be tailored to brain permeation by decreasing the PSA to less than 60-70 A2. 

The use of in vitro models for prediction of compounds that are predominantly absorbed passively by the transcellular route is generally good with these models. Predicting compounds which are absorbed paracellularly or via active uptake or efflux mechanisms is more difficult. There is a lack of understanding of expression levels of transporters in the gut, which makes in vivo predictions difficult. 

Beside membrane transporters such as PepTl and PepT2, which act as absorptive systems, there are transporters like P-gp and the MRP 15, which transport certain drugs actively back into the intestinal lumen. These efflux pumps are located in several tissues including liver, kidney, brain, and intestine [90,91]. In the intestine, efflux systems are predominantly located at the apical side of the epithelial cells. Lipophilic drugs are usually absorbed by the transcellular route so that they are mostly affected by these systems. Interestingly, the intracellular occurring CYP3A metabolizes compounds to substrates that are eliminated by P-gp [92],... 

The most efficient rectal absorption enhancers, which have been studied, include surfactants, bile acids, sodium salicylate (NaSA), medium-chain glycerides (MCG), NaCIO, enamine derivatives, EDTA, and others [45 17]. Transport from the rectal epithelium primarily involves two routes, i.e., the paracellular route and the transcellular route. The paracellular transport mechanism implies that drugs diffuse through a space between epithelial cells. On the other hand, an uptake mechanism which depends on lipophilicity involves a typical transcellular transport route, and active transport for amino acids, carrier-mediated transport for (3-lactam antibiotics and dipeptides, and endocytosis are also involved in the transcellular transport system, but these transporters are unlikely to express in rectum (Figure 8.7). Table 8.3 summarizes the typical absorption enhancers in rectal routes. 

The corneal route is often considered to be the main pathway for ocular absorption. Most drugs cross this membrane into the intraocular tissues by either intercellular or transcellular diffusion. Lipophilic drugs are transported via the transcellular route, and hydrophilic drugs penetrate mainly through the intercellular... 

Prognosis of a compounds permeability should be made stressing limitations of the model. There is no bioavailability prognosis from in vitro data - a cellular assay can provide only permeability potential through a biological membrane. The membrane, in most cases CACO-2 cells, is very similar to what we observe in vivo in the small intestine and resembles many characteristics to in vivo enterocytes. CACO-2 cells can be used for prediction of different pathways across intestinal cells. Best correlation occurs for passive transcellular route of diffusion. Passive paracellular pathway is less permeable in CACO-2 and correlations are rather qualitative than quantitative for that pathway. CACO-2 cells are an accepted model for identification of compounds with permeability problems, for ranking of compounds and selection of best compounds within a series. Carrier-mediated transport can be studied as well using careful characterization of transporters in the cell batch or clone as a prerequisite for transporter studies. 

Studies using TEM and in situ precipitation to follow the pathway of topically applied compounds have focused on distinguishing between the intracellular and intercellular routes of transport of substances across the SC. In 1968, Silberberg [24] first used this technique to provide evidence that mercury, after topical application of 0.1% aqueous mercuric chloride, traverses across the SC in vitro via the intercellular spaces. But difficulties with fixation and processing prevented demonstration that the mercury aggregates were also present in the SC cells. Thus, the possibility that mercury may also have taken a transcellular route through the SC could not be excluded. 

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