Carrier-mediated transporters

There are numerous abnormalities of cysteine metabolism. Cystine, lysine, arginine, and ornithine are excreted in cystine-lysinuria (cystinuria), a defect in renal reabsorption. Apart from cystine calculi, cystinuria is benign. The mixed disulfide of L-cysteine and L-homocysteine (Figure 30-9) excreted by cystinuric patients is more soluble than cystine and reduces formation of cystine calculi. Several metabolic defects result in vitamin Bg-responsive or -unresponsive ho-mocystinurias. Defective carrier-mediated transport of cystine results in cystinosis (cystine storage disease) with deposition of cystine crystals in tissues and early mortality from acute renal failure. Despite... 

Fig. 9 Schematic representation depicting the movement of molecules from the absorbing (mucosal or apical) surface of the GIT to the basolateral membrane and from there to blood. (A) transcellular movement through the epithelial cell. (B) Paracellular transport via movement between epithelial cells. (Q Specialized carrier-mediated transport into the epithelial cell. (D) Carrier-mediated efflux transport of drug out of the epithelial cell. (Copyright 2000 Saguaro Technical Press, Inc., used with permission.)...

Hilgendorf, C. Spahn-Langguth,H. Regardh,C. G. Lipka, E. Amidon,G. L. Langguth, P., Caco-2 vs Caco-2/HT29-MTX co-cultured cell lines Permeabilities via diffusion, inside- and outside-directed carrier-mediated transport, J. Pharm. Sci. 89, 63-75 (2000). 

Carrier-mediated transport is linear with mucosal solute concentration until this concentration exceeds the number of available carriers. At this point the maximal solute flux (7max) is independent of further increases in mucosal solute concentration. In the linear range of solute flux versus mucosal concentration (C), the proportionality constant is the ratio of / to the solute-carrier affinity constant (Km). This description of Michaelis-Menten kinetics is directly analogous to time changes in mass per unit volume (velocity of concentration change) found in enzyme kinetics, while here the appropriate description is the time change in solute mass per unit surface area of membrane supporting the carrier. 

Carrier-mediated transport of nutrients in small intestinal epithelia is often promoted by the maintenance of transmucosal ion gradients. A mathematical descrip-... 

Figure 8 A peptide carrier mediated transport to improve oral absorption.

Enalaprilat and SQ27,519 are angiotensin-converting enzyme (ACE) inhibitors with poor oral absorption. Enalapril and fosinopril are dipeptide and amino acid derivatives of enalaprilat and SQ27,519, respectively [51] (Fig. 10). Both prodrugs are converted via deesterification to the active drug by hepatic biotransformation. In situ rat perfusion of enalapril indicated a nonpassive absorption mechanism via the small peptide carrier-mediated transport system. In contrast to the active parent, enalapril renders enalaprilat more peptide-like, with higher apparent affinity for the peptide carrier. The absorption of fosinopril was predominantly passive. Carrier-mediated transport was not demonstrated, but neither was its existence ruled out. 

Figure 1 General pathways through which molecules can actively or passively cross a monolayer of cells. (A) Endocytosis of solutes and fusion of the membrane vesicle with the opposite plasma membrane in an active process called transcytosis. (B) Similar to A, but the solute associates with the membrane via specific (e.g., receptor) or nonspecific (e.g., charge) interactions. (C) Passive diffusion between the cells through the paracellular space. (C, C") Passive diffusion (C ) through the cell membranes and cytoplasm or (C") via partitioning into and lateral diffusion within the cell membrane. (D) Active or carrier-mediated transport of an otherwise poorly membrane permeable solute into and/or out of a cellular barrier.

W Wang, VHL Lee. (1991). Carrier-mediated transport of peptides in the rabbit conjunctiva. Pharm Res 8(Suppl) S-129. 

Y Horibe, KJ Kim, VHL Lee. (1998). Carrier-mediated transport of monocarboxy-late drugs in the pigmented rabbit conjunctiva. Invest Ophthalmol Vis Sci 39 1436-1443. 

The rat intestinal cell line IEC-18 has been evaluated as a model to study small intestinal epithelial permeability. This cell line forms very leaky monolayers with TER of 50 n cm2 and permeability to mannitol of 8 x 10-6 cm s 1. The IEC-18 model was proposed to be a better model than the Caco-2 monolayers for evaluating the small intestinal paracellular permeation of hydrophilic molecules. However, the leakier paracellular pathway is related to the poor differentiation level of the cells and an undeveloped paracellular barrier lacking peri-junctional actin-belt. In addition, due to the poor differentiation the cells have minute expression of transporters and are therefore not useful for studies of carrier-mediated transport [82, 84]... 

If screening for carrier-mediated transport in any direction = both a nongradient and a pH gradient system should be used and bidirectional transport data should be reported... 

Several attempts have been made to estimate the dose required in humans in relation to a drug s potency, and to put this into the context of solubility and permeability for an optimal oral drug [2, 3]. A relatively simple example of this is where a 1.0 mg kg-1 dose is required in humans, then 52 pg mL"1 solubility is needed if the permeability is intermediate (20-80%) [3]. This solubility corresponds approximately to 100 pM of a compound with a MW of 400 g mol-1. Most screening activities for permeability determinations in, e.g., Caco-2, are made at a concentration of 10 pM or lower due to solubility restrictions. The first implication of this is that the required potency for these compounds needs to correspond to a dose of <0.1 mg kg-1 in humans if the drug should be considered orally active. Another implication would be the influence of carrier-mediated transport (uptake or efflux), which is more evident at low concentrations. This could result in low permeability coefficients for compounds interacting with efflux transporters at the intestinal membrane and which could either be saturated or of no clinical relevance at higher concentrations or doses. 

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). 

Solvents used to increase solubility for compounds during screening of permeability across the cell monolayers, together with commonly used excipients for formulations, can also affect the barrier as they contain ingredients which enhance drug absorption [100, 151]. There are different mechanisms by which these compounds can modulate the barrier [4, 149, 150] for example, they may increase the tight junctional pathway inhibiting carrier-mediated transport, or cholesterol... 

Hidalgo, I. J., Li, J., Carrier-mediated transport and efflux mechanisms in Caco-2 cells, Adv. Drug Delivery Rev. 1996, 22, 53-66. 

Tamai, I. [Molecular characterization of intestinal absorption of drugs by carrier-mediated transport mechanisms]. Yakugaku Zasshi 1997, 117, 415-434. 

Swaan, P. W. and J. J. Tukker. Carrier-mediated transport mechanism of foscamet (trisodium phosphono-formate hexahydrate) in rat intestinal tissue. J. Pharmacol. Exp. Ther. 1995, 272, 242-247. 

Ishizawa, T., et al. Sodium and pH dependent carrier-mediated transport of antibiotic, fosfomydn, in the rat intestinal brush-border membrane. J. Pharmacobiodyn. 1990, 13, 292—300. 

Carrier-mediated transport, Active Efflux, Passive (trans and para cellular) diffusion... 

In addition to the mechanistic simulation of absorptive and secretive saturable carrier-mediated transport, we have developed a model of saturable metabolism for the gut and liver that simulates nonlinear responses in drug bioavailability and pharmacokinetics [19]. Hepatic extraction is modeled using a modified venous equilibrium model that is applicable under transient and nonlinear conditions. For drugs undergoing gut metabolism by the same enzymes responsible for liver metabolism (e.g., CYPs 3A4 and 2D6), gut metabolism kinetic parameters are scaled from liver metabolism parameters by scaling Vmax by the ratios of the amounts of metabolizing enzymes in each of the intestinal enterocyte compart-... 

In other studies, bisphosphonate-pamidronate or alendronate were linked to the terminal carboxylic acid of the stabilized dipeptide Pro-Phe to improve the bioavailability of bisphosphonates by hPepTl-mediated absorption. In-situ single-pass perfused rat intestine studies revealed competitive inhibition of transport by Pro-Phe, suggesting carrier-mediated transport. Oral administration of the dipeptidyl prodrugs resulted in a 3-fold increase in drug absorption following oral administration to rats. The authors suggested that oral bioavailability of bisphosphonates may be improved by PepTl-mediated absorption when administered as peptidyl prodrugs [53]. Future mechanistic studies may prove if hPepTl is involved in the absorption process. 

Lastly, pharmacogenomics could provide new tools for the design of more specific and active CNS pharmaceuticals. The efficacy of a broad spectrum of neuro-pharmaceutical drugs is often complicated by their inability to reach their site of action because of the BBB. One way to overcome this is to use carrier-mediated transport at the luminal and/or abluminal membranes of the endothelial cells of the BBB. This will provide a physiologically based drug delivery strategy for the brain by designing new chemical entities or fused proteins that can cross the BBB via these transporters. 

Hydrophilic peptides and proteins are frequently large molecules they may enter the brain by carrier-mediated transport, receptor-mediated transcytosis, or by adsorptive-mediated transcytosis. Small peptides, such as di- and tripeptides are transported by the specific transporters, PepTl and PepT2, but neither of them is present at the BBB. Nevertheless, there is saturable brain uptake of the tripeptide glutathione and of several opioid peptides, suggesting that specific transporters, as... 

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