Transporters uptake

Some drugs with low intrinsic permeability achieve acceptable oral bioavailability because they are substrates for uptake transporters, which normally function in nutrient uptake. The most prominent example is the peptide transporter, PepTl, which is active toward peptidomimetic antibiotics such as cephalexin, the antiviral agent valacyclovir [24] and other drugs. PepTl is natively expressed in Caco-2 cells, and adenovirus transduction has been used to increase PepTl expression levels [25]. However, the expression of PepTl was not polarized in this system and this expressed system appears to be of limited value as an improved screening model. PepTl has also been expressed in Chinese hamster ovary cells and a variety of other mammalian systems [26, 27]. 

In addition to PepTl, there is a related renal transporter, PepT2. There are also a series of other uptake transporters (organic anion transporting polypeptides, organic anion transporters, organic cation transporters), which are expressed primarily in the liver and other tissues but are not known to affect oral bioavailability. 

1 Nelson, J. A., Dutt, A., Allen, L. H., Weight, D. A., Functional expression of renal organic cation transporter and P-glycoprotein in Xenopus laevis oocytes, Cancer Chemother. Pharmacol. 1995, 37, 187-189. 

2 Kinoshita, S., Suzuki, H., Ito, K., Shimizu, T., Sugiyama, Y., Transfected rat cMOAT is functionally expressed on the apical membrane of Madin—Darby canine kidney (MDCK) cells, Pharm. Res. 1998, 15, 1851-1856. 

Paulusma, C. C., Oude Elferink, R. P., Baas, F., Schinkel, A. H., Borst, P., Drug export activity of the human canalicular multispecific organic anion transporter in polarized kidney MDCK cells expression cMOAT (MRP2) cDNA, J. Clin. Invest. 1988, 101, 1310-1319. 

Carrier-mediated transport of drugs and their metabolites has recently been recognized as an important issue in pharmaceutical research. There is a wealth of information that suggests that transporters are responsible both for the uptake and efflux of drugs and other xenobiotics, and may be key determinants of the disposition of a drug [91,92]. Transporter proteins are divided into two categories (1) the adenosine triphosphate (ATP) binding cassette (ABC) transporter superfamily and (2) the solute carrier (SLC) family of proteins. 

2 ABCC1 (Multidrug Resistance Related Proteinl, MRP1) 

3 ABCC2 (Multidrug Resistance Related Protein2, MRP2) 

4 ABCG2 (Breast Cancer Resistance Protein, BCRP) 

The drug uptake (SLC) family of transporter is the largest superfamily of transporters. This family includes 31 transporters from organic anion transporter polypeptides (OATPs), organic anion transporters (OATs), organic cation transporters (OCTs), peptide cotransporters (PEPTs), and sodium-bile acid cotransporter classes. Only OATP, OAT, OCT, and PEPT are primarily involved with the transport of drugs/xenobiotics. 

The reabsorption of conjugated bile acids is mediated by ASBT, which is localized on the apical membrane of ileal enterocytes in mammals. ASBT is a drug target not only to lower plasma cholesterol level but also to improve intestinal permeability [46]. Although available monolayer cell lines do not express ASBT, it has been expressed in MDCK cells [47]. Human intestine also expresses multiple MCTisoforms [48]. These MCTs are responsible for the absorption of short-chain fatty acid. Expression of MCT in Caco-2 allows it to be an appropriate model to study short-chain fatty acid transport [9, 49, 50]. 

In addition to the above uptake transporters that predominately express in the intestine, uptake transporters that function in other tissues also express in the intestine. For example, brain transporter OATP1A2, hepatic transporter OATP1B1, and OATP1B3 have been detected in human intestine [51]. In the intestine, OATP1A2 is localized on the apical membrane, and is the key uptake transporter for fexofenadine [51] and levofloxacin [12]. OATP1A2 was reported to be natively expressed in 

Caco-2 cells and was thought to be the transporter for levofloxadn uptake in Caco-2 cells [12]. 

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An important factor in determining the course of uptake, transport, and distribution of xenobiotics is their polarity. Compounds of low polarity tend to be lipophilic and of low water solubility. Compounds of high polarity tend to be hydrophilic and of low fat solubility. The balance between the lipophilicity and hydrophilicity of any compound is indicated by its octanol-water partition coefficient (K J, a value determined when equilibrium is reached between the two adjoining phases ... 

Monoamines ean also be found in terminals at both symmetrie and asymmetrie synapses, but this may be partly beeause they eo-exist with the elassieal transmitters glutamate and GABA. The faet that vesieular and neuronal uptake transporters for the monoamines ean be deteeted outside a synapse along with appropriate postsynaptie reeeptors does suggest, however, that some monoamine effeets ean oeeur distant from the synaptie junetion (see Piekel, Nirenberg and Milner 1996, and Chapter 6). 

Weis, J.S. and Weis, P., Metal uptake, transport and release by wetlands plants implication for phytoremediation and restoration, Environment International, 30 (5), 739-753, 2004. 

R.M. Welch, W.A. Norvell, USDA Agricultural Research Studying the uptake, transport, and... 

Cui, Y., J. Konig, and D. Keppler. Vectorial transport by double-transfected cells expressing the human uptake transporter SLC21A8 and the apical export pump ABCC2, Mol. Pharmacol. 2001, 60, 934—943... 

Cell lines, such as the Caco-2 and MDCK cells [27, 35, 47, 49, 57, 67, 128-133], have been used frequently to study different transporters in the GI tract. These cell lines have been evaluated for transport both in absorptive and secretory direction and in addition also been transfected with specified transporter systems of interests to yield new clones [23, 31, 72, 79, 80, 134] or co-cultures [135], Some of the uptake transporters belonging to the organic cation transporter (OCT) family have also been identified in cell lines such as the pig kidney cell line LLC-PK1, and MDCK [67, 136]. In fact, its presence in Caco-2 cells needs to be further elucidated as reports have shown both the absence and presence of transporters from this family of transporters [136-138],... 

In cell lines, the organic anion transporters (OAT and OATP) have been identified and cloned into cells of kidney origin such as LLC-PK1, MDCK, HK-2, and Caco-2 [129]. The most well-known uptake transporters, which transport the substrate over the membrane into the organism are the amino acid- [35, 42, 139] and oligopeptide-carriers (PepTl and PepT2) [139-142]. These two transporter families are abundantly expressed in the small intestine of most animals, and can therefore be involved in the absorption process of pharmaceutical drugs. The PepTl is expressed in the cell lines Caco-2 and HT-29 [140-142]. 

Both active and passive transport occur simultaneously, and their quantitative roles differ at different concentration gradients. At low substrate concentrations, active transport plays a major role, whilst above the concentration of saturation passive diffusion is the major transport process. This very simple rule can be studied in an experimental system using cell culture-based models, and the concentration dependency of the transport of a compound as well as asymmetric transport over the membrane are two factors used to evaluate the presence and influence of transporters. Previous data have indicated that the permeability of actively absorbed compounds may be underestimated in the Caco-2 model due to a lack of (or low) expression of some uptake transporters. However, many data which show a lack of influence of transporters are usually derived from experiments... 

Recently, molecular biology studies have been carried out on hepatic uptake transporters. With regard to the Na+-dependent hepatic uptake of bile acids, Na+-taurocholate cotransporting polypeptide (Ntcp/NTCP) has been cloned from both rodents and humans [14-17]. Ntcp/NTCP accepts bile salts, such as taurocholate and glycocholate, as well as some anionic compounds such as dehydroepian-drosterone sulfate and bromosulfophthalein [16, 18]. However, the presence of unidentified Na+-dependent transporters for anionic drugs (e.g., bumetanide) has also been suggested [19, 20]. 

Fig. 12.1. Phylogenic trees for uptake transporters. hOAT4 reported in Ref. [39]. hOAT4 reported in Ref. [37],...

Michalski, C., Cui, Y., Nies, A. T., Nuessler, A. K., Neuhaus, P., Zanger, U. M., Klein, K., Eichel-baum, M., Keppler, D., Konig, J., A naturally occurring mutation in the SLC21A6 gene causing impaired membrane localization of the hepatocyte uptake transporter, J. Biol. Chem. 2002, 277, 43058-43063. 

Tab. 9.2 Summary of genetic polymorphisms in drug uptake transporters...

The OATP family of uptake transporters consists of 52 members comprising 12 different subfamilies across eight species. In humans, 11 different OATP family... 

Figure 15.2 Transport proteins involved in the intestinal absorption and the renal and hepatic excretion of drugs. In the intestine, drugs are taken up from the luminal side into enterocytes before the subsequent elimination into blood. In hepatocytes, drugs are taken up from the blood over the basolateral membrane and excreted over the canalicular membrane into bile. In the renal epithelium, drugs undergo secretion (drugs are taken up from the blood and excreted into the urine) or reabsorption (drugs are taken up from the urine and are excreted back into blood). Uptake transporters belonging to the SLC transporter superfamily are shown in red and export pumps...

Figure 15.3 Cell systems used for the analysis of transport processes, (a) Single-transfected MDCKII cell stably expressing an uptake transporter. These cell systems are useful for the analysis of single transport processes. A substance (drug) can be added into the basolateral compartment and the uptake can be measured by analyzingthe radioactivity in the cell at respective time points, (b) Double-transfected MDCKII cells stably expressing an uptake...

In the renal epithelium, many uptake transporters are localized in the basolateral membrane and efflux transporters are localized in the apical membrane (Figure 15.2). As a result, vectorial transport of endogenous substances and of drugs from the blood into the urine is achieved. The important uptake transporters are members of the SLC22 family of solute carriers, especially the family members OCT2 and OAT2, which are highly expressed in human kidney. 

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