Passive transcellular pathway

While both paracellular and passive transcellular pathways are available to a solute, the relative contribution of each to the observed transport will depend on the properties of the solute and the membrane in question. Generally, polar membrane-impermeant molecules diffuse through the paracellular route, which is dominated by tight junctions (Section III.A). Exceptions include molecules that are actively transported across one or both membrane domains of a polarized cell (Fig. 2). The tight junction provides a rate-limiting barrier for many ions, small molecules, and macromolecules depending on the shape, size, and charge of the solute and the selectivity and dimensions of the pathway. 

Passive Transcellular Pathway pH Partition Theory as the Basis of Understanding Membrane Permeability... 

Two distinguishing features of gastrointestinal active and facilitated transport processes are that they are capacity-limited and inhibitable. Passive transcellular solute flux is proportional to mucosal solute concentration (C), where the proportionality constant is the ratio of the product of membrane diffusion coefficient (Dm) and distribution coefficient (Kd) to the length of the transcellular pathway (Lm). 

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). 

There are two pathways by which a drug molecule can cross the epithelial cell the transcellular pathway, which requires the drug to permeate the cell membranes, and the paracellular pathway, in which diffusion occurs through water-filled pores of the tight junctions between the cells. Both the passive and the active transport processes may contribute to the permeability of drugs via the transcellular pathway. These transport pathways are distinctly different, and the molecular properties that influence drug transport by these routes are also different (Fig. 

Keywords Colon Controlled release Sustained release Rat Single-pass perfusion Recirculation Closed loop Carrier-mediated transport Passive transport Membrane permeability P-glycoprotein Paracellular pathway Transcellular pathway... 

Major transport pathways in Caco-2 monolayers. A Passive transcellular B Passive paracellular C Transporter-mediated apical uptake D Transporter-mediated apical efflux E Transporter-mediated basolateral efflux F Transporter-mediated basolateral uptake. 

Fig. 3.1 Schematic presentation of absorption pathways through the intestinal epithelium. A passive transcellular B active, carrier-mediated C passive, paracellular D efflux transporters E transcytosis...

The transcellular pathway involves the movement of the drag across the epithelial cell, by active and/or passive processes (Figure 1.3), which are discussed in detail below. 

The surface area of brush border membranes is 1000 fold larger than paracellular surface area (Pap-penheimer and Reiss 1987). Therefore, the probability for transcellular permeability is much higher than for paracellular permeability. Indeed, lipophilic drugs with rapid and complete absorption have a high probability for passive transcellular route. Hydrophilic drags tend to pass cellular membranes via water filled pores in the paracellular pathway (for review, see Lee et al. 1991). However, there is also a part of hydrophilic molecules passing membranes by transcellular route (Nellans 1991). The paracellular pathway is used by some positively charged compounds whereas transcellular pathway is preferred with unionised compounds. 

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. 

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). 

Biomimetic artifical membrane-paracellular pathways-Renkin function The purpose of this study was to construct and examine the prediction model for total passive permeation through the intestinal membrane. The paracellular pathway prediction model based on Renkin function (PP-RF) was combined with a bio-mimetic artificial membrane permeation assay (BAMPA), which is an in vitro method to predict transcellular pathway permeation, to construct the prediction model (BAMPA-PP-RF model). The parameters of the BAMPA-PP-RF model, for example, apparent pore radius and potential drop of the paracellular pathway, were calculated from BAMPA permeability, the dissociation constant, the molecular radius, and the fraction of a dose absorbed in humans consisting of 80 structurally diverse compounds. The apparent pore radius and the apparent potential drop obtained in this study were 5.61-5.65 A and 75-86 mV, respectively, and these were in accordance with the previously reported values. The mean square root error of the BAMPA-PP-RF model was 13-14%. The BAMPA-PP-RF model was shown to be able to predict the total passive permeability more adequately than BAMPA alone. 

Figure 8,1, Routes and mechanisms of solute transport across epithelial membranes. In general, routes 2-5 are transcellular pathways (i.e., compounds move through the cells), whereas route 1 is considered a paracellular pathway (i.e., a compound moves between the cells). (l)Tight junctional pathway (2) drug efflux pathway (e.g., P-glycoprotein mediated) (3) passive diffiision (4)receptor-mediated endocytosis and/or transc3dosis pathways (5) carrier-mediated route. Note that receptor and carrier proteins in epithelial cells are expressed on both the apical and basolateral surfaces.

With respect to transcellular permeability, the relationship of solute structure with permeability depends on the mechanism. Historically, a passive diffusion pathway is assumed for most solutes. Nevertheless, a great number of solutes are identified as being associated with active absorption and secretary processes in intestinal epithelial cells. Additionally, although active transport involves specific interactions between a solute and transporter, passive diffusion is dependent on solute partitioning into the cellular plasma membrane and the diffusion coefficient within the membrane. 

Most drugs appear to be absorbed in humans by passive diffusion (linear or first-order kinetics). The predominant pathway taken by most drugs is through the epithelial cell, the transcellular route. It is this route that requires the compound to have a reasonable K0/w... 

The permeability of the cell monolayer consists of parallel transcellular and paracellular pathways. In passive diffusional transport, it is generally taken that uncharged molecules are capable of partitioning into the cell membrane and... 

Pade, V., Stavchansky, S., Estimation of the relative contribution of the transcellular and paracellular pathway to the transport of passively absorbed drugs in the Caco-2 cell culture model, Pharm. Res. 1997, 34, 1210-1215. 

Figure 8.2 Possible drug transport pathways across the intestinal mucosa, illustrating transcellular (1) and paracellular (2) modes of passive transport, transcytosis (3), carrier-mediated transport (4), and efflux transport (5). A combination of these routes often defines the overall transepithelial transport rate of nutrients and drugs.

Cellular Cl- replenishment is maintained by a basolateral anion-exchanger (Cl /OH or Cr/HCOJ) or via the Na+/K+/2C1 co-transporter, whose activities are closely tied synergistically through action of Na+/K+-ATPase, K+ channels, and the IIC()7/Na+ co-transporter (with a 3 1 stoichiometry) that extrudes HCOJ [61]. Passive Cl- diffusion through the paracellular pathway can occur because of the greater mobility of Cl- than Na+ in the paracellular space ( <1.3) [13]. Electroneutrality is maintained by transcellular Na+ transport in the luminal to subluminal direction, accomplished by both the apical Na+/H+ exchanger (NHE-3) and a basolateral HCOJ/Na+ co-transporter (with a 3 1 stoichiometry). 

Transport across the cell membrane may occur via different routes. Some of these transport processes are energy dependent and therefore termed active others are independent from energy, thus passive. Passive transport phenomena, for example, transcellular transport, are triggered by external driving forces, such as concentration differences, and do not require metabolic activity. However, generally, they are restricted to small lipophilic compounds. In contrast, active transport phenomena, such as active carrier-mediated transport or vesicular pathways, take course independent from external driving... 

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