Passive transcellular transport

Passive transcellular transport across the intestinal epithelium involves three discrete steps (1) uptake across the apical membrane, (2) diffusion through the cytoplasm, and (3) efflux across the basolateral membrane. Occasionally, drug molecules without favorable physicochemical properties traverse the intestinal epithelium using endogenous membrane transporters.6-8 In addition, the intestinal mucosa, with its numerous drug-metabolizing enzymes and efflux transporters, such as P-glycoprotein (Pgp), functions as a biochemical barrier.9... 

This agrees to internal VolSurf models derived for PAMPA membrane transport [163] to understand passive transcellular transport across membranes. One of our internal models based on 29 compounds characterized by immobilized artificial membrane chromatography by Salminen etal. ]164] shows an of 0.81 and = 0.70 for two PLS components derived using VolSurf descriptors. This is one of the rare examples where ionized starting molecules led to slightly better PLS statistics, while the general chemical interpretation is not affected. 

According to Gasteiger et al. [59], the correlation coefficient r between bioavailability and HIA is 0.498 for 161 compounds. This conclusion inspires us to propose the use of aqueous solubility, descriptors of HIA models, and some rule-based descriptors to predict first-pass metabolism, to model bioavailability. Another research direction for the prediction of oral bioavailability is to develop separate prediction models for different components involved in oral bioavailability, including passive transcellular transport, paracellular transport, carrier-mediated transport, and first-pass metabolism, and then integrate them together. At present, the development of an integrated model is really difficult or even impossible because the predictions for some mechanisms involved in oral bioavailability are really unreliable. 

Action on the membrane components Numerous studies have shown that the passive transcellular transport of hydrophilic compounds, including macromolecules such as peptides, can be enhanced by interaction of the penetration enhancers with both the phospholipid bilayer and the integrated proteins, thereby making the membrane more fluid and thus more permeable to both lipophilic and hydrophilic compounds. 

Al cells paracellular and both active and passive transcellular transport... 

Compoimds that were likely to have a different mechanism (paracellular or active transport) than the others (passive transcellular transport) were omitted. 

FIG. 2 Mechanisms of drug transfer in the cellular layers that line different compartments in the body. These mechanisms regulate drug absorption, distribution, and elimination. The figure illustrates these mechanisms in the intestinal wall. (1) Passive transcellular diffusion across the lipid bilayers, (2) paracellular passive diffusion, (3) efflux by P-glycoprotein, (4) metabolism during drug absorption, (5) active transport, and (6) transcytosis [251]. 

PAMPA is typically used to make a prediction of the passive, transcellular absorption of a compound. Compounds which may be absorbed by a paracellular mechanism or may be substrates for active transport (uptake or efflux) are usually better assessed in a cell based system. A combination of assays can be applied to gain a greater understanding of the permeability and transport properties of a compound. 

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). 

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. 

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... 

X. Liu, and L. Z. Benet. Contributions of saturable active secretion, passive transcellular, and paracellular diffusion to the overall transport of furosemide across adenocarcinoma (Caco-2) cells,/. Pharm. Sci. 2002,... 

Let us conclude this section by proposing that provided that the drug is sufficiently soluble in the gastrointestinal fluids, the complex process of intestinal drug absorption can often be satisfactorily described by focusing on passive transport across the cell membrane, and that the development of models that predict passive transcellular permeability is particularly important. Such models are the focus of the remaining part of this chapter. 

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... 

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. 

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. 

Sugano et al. studied the membrane permeation of 51 benzamidine-based thrombin inhibitors in a rat everted sac permeability model [197]. They reported significant membrane permeabilities in this in vitro model, which they attributed to passive paracellular transport, a different absorption mechanism to transcellular permeability. 

Lipophilic drug molecules are absorbed across the nasal epithelium by passive transcellular diffusion. For small, unionized molecules, this provides a rapid efficient transport mechanism, often resulting in plasma concentration profiles resembling that of intravenous injection and bioavailabilities of up to 100%. 

Transcellular transport Basic mechanisms of transepithelial transport of drugs include passive transport of small molecules, active transport of ionic and polar compounds, and endocytosis and transcytosis of macromolecules. 

It has been shown that antibodies can reach the systemic circulation after oral administration, but only to a very small extent. The antibodies pass the intestinal epithelium not by passive transcellular but by receptor-mediated transcellular or paracellular transport. The Fc part of the antibody is responsible for the saturable receptor-mediated transport, especially IgG in breast-fed neonates. As the receptor is found primarily in the gastrointestinal tract of neonates, it was called Fc-Rn (Fc receptor neonatal). Apart from this location, Fc-Rn has also been discovered in other tissues such as the liver. Its role will be further discussed in Section 3.9.3. 

Permeation of mAbs across the cells or tissues is accomplished by transcellular or paracellular transport, involving the processes of diffusion, convection, and cellular uptake. Due to their physico-chemical properties, the extent of passive diffusion of classical mAbs across cell membranes in transcellular transport is minimal. Convection as the transport of molecules within a fluid movement is the major means of paracellular passage. The driving forces of the moving fluid containing mAbs from (1) the blood to the interstitial space of tissue or (2) the interstitial space to the blood via the lymphatic system, are gradients in hydrostatic pressure and/or osmotic pressure. In addition, the size and nature of the paracellular pores determine the rate and extent of paracellular transport. The pores of the lymphatic system are larger than those in the vascular endothelium. Convection is also affected by tortuosity, which is a measure of hindrance posed to the diffusion process, and defined as the additional distance a molecule must travel in a particular human fluid (i. e., in vivo) compared to an aqueous solution (i. e., in vitro). 

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...

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