Transport efflux barrier

Keywords Inner blood-retinal barrier Transporter Influx transport Efflux transport Microdialysis Cell line Drug delivery... 

Carrier-mediated membrane transport proteins on the RPE selectively transport nutrients, metabolites, and xenobiotics between the choriocapillaris and the cells of the distal retina, and include amino acid [33 35], peptide [36], dicarboxylate, glucose [37], monocarboxylic acid [38,39], nucleoside[40], and organic anion and organic cation [41] transporters. Membrane barriers such as the efflux pumps, including multidrug resistance protein (P-gp), and multidrug resistance-associated protein (MRP) pumps have also been identified on the RPE. Exploitation of these transport systems may be the key to circumventing the outer BRB. 

The Efflux Barrier Transport Proteins at the Blood-Brain Barrier... 

The study of active transport mechanisms has grown substantially in recent years, with transport proteins such as P-gp, BCRP, and MRP-2 among the most studied [59]. Several types of in vitro assays to assess substrates of transporters have been established these include assays directed toward intestinal and biliary efflux [60]. Assays that measure passive and active transport are also used to assess penetration of the blood-brain barrier. In addition to the assays described above, transfected cell lines that overexpress transporters present in the blood-brain barrier are also employed [61]. 

In microbes without a permeability barrier, or when the barrier fails, a mechanism must be in place to export metals from the cytoplasm. These active transport systems involve energy-dependent, membrane-bound efflux pumps that can be encoded by either chromosomal- or plasmid-borne genes. Active transport is the most well-studied metal resistance mechanism. Some of these include the ars operon for exporting arsenic from E. coli, the cad system for exporting cadmium from Staphylococcus aureus, and the cop operon for removing excess copper from Enterococcus hiraeP i9A0... 

Culture protocols have been published which describes an accelerated differentiation process where monolayers are ready to be used after 3-7 days of culture [90-92]. One of these systems, the so-called BD BioCoat Intestinal Epithelium Differentiation Environment, is commercially available through BD Bioscience. This system is described to produce monolayers of a quality that are comparable with the typical Caco-2 cells with respect to permeability for drugs transported transcellularly. The paracellular barrier function is however low, as indicated by high mannitol permeability and low TER. The functional capacity for active uptake and efflux is not as thoroughly characterized as for the standard Caco-2 mono-layers. 

From the above, it is clear that the gut wall represents more than just a physical barrier to oral drug absorption. In addition to the requirement to permeate the membrane of the enterocyte, the drug must avoid metabolism by the enzymes present in the gut wall cell as well as counter-absorptive efflux by transport proteins in the gut wall cell membrane. Metabolic enzymes expressed by the enterocyte include the cytochrome P450, glucuronyltransferases, sulfotransferases and esterases. The levels of expression of these enzymes in the small intestine can approach that of the liver. The most well-studied efflux transporter expressed by the enterocyte is P-gp. 

Cholesterol transport and regulation in the central nervous system is distinct from that of peripheral tissues. Blood-borne cholesterol is excluded from the CNS by the blood-brain barrier. Neurons express a form of cytochrome P-450, 46A, that oxidizes cholesterol to 24(S)-hydroxycholesterol [11] and may oxidize it further to 24,25 and 24,27-dihydroxy products [12]. In other tissues hydroxylation of the alkyl side chain of cholesterol at C22 or C27 is known to produce products that diffuse out of cells into the plasma circulation. Although the rate of cholesterol turnover in mature brain is relatively low, 24-hydroxylation may be a principal efflux path to the liver because it is not further oxidized in the CNS [10]. 

Bronger, H., Konig, J., Kopplow, K., Steiner, H.H., Ahmadi, R., Herold-Mende, C., Keppler, D. and Nies, A.T. (2005) ABCC drug efflux pumps and organic anion uptake transporters in human gliomas and the blood-tumor barrier. Cancer Research, 65, 11419-11428. 

K. Katayama, Y. Ohshima, M. Tomi, and K. Hosoya. Application of microdialysis to evaluate the efflux transport of estradiol 17-/7 glucuronide across the rat blood-retinal barrier. J. Neurosci. Methods 156 249-256 (2006). 

C.J. Bachmeier, T.J. Spitzenberger, W.F. Elmquist, and D.W. Miller. Quantitative assessment of HIV-1 protease inhibitor interactions with drug efflux transporters in the blood-brain barrier. Pharm Res. 22 1259-1268 (2005). 

Ohtsuki S, Asaba H, Takanaga H, Deguchi T, Hosoya K, Otagiri M, Terasaki T (2002) Role of blood-brain barrier organic anion transporter 3 (OAT3) in the efflux of indoxyl sulfate, a uremic toxin Its involvement in neurotransmitter metabolite clearance from the brain. J Neurochem 83 57-66... 

Tsuji A (1998) P-glycoprotein-mediated efflux transport of anticancer drugs at the blood- Van Bree rain barrier. Therap Drug Monitor 20 588-590... 

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