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

Protein-bound uremic retention solutes

Protein-bound uremic retention solutes are molecules with low molecular weight (MW) but should be considered middle or high MW substances. This article describes the best known substances of this group, which include p-cresol, indoxyl sulfate, hippuric acid, 3-carboxy-4-methyl-5-propyl-2-furan-propionic acid (CMPF), and homocysteine. At concentrations encountered during uremia, p-cresol inhibits phagocyte function and decreases leukocyte adhesion to cytokine-stimulated endothelial cells. CMPF has been implicated in anemia and neurologic abnormalities of uremia. CMPF could alter the metabolism of drugs of inhibiting their binding to albumin and their tubular excretion. Indoxyl sulfate administrated to uremic rats increases the rate of progression of renal failure. Hippuric acid inhibits glucose utilization in the muscle, and its serum concentration is correlated with neurologic symptoms of uremia. Homocysteine predisposes uremic patients to cardiovascular disease through impairment of endothelial and smooth muscle cell functions. The removal of protein-bound compounds by conventional hemodialysis is low. Other strategies to decrease their concentrations include increase in dialyze pore size, daily hemodialysis, peritoneal dialysis, reduction of production or acceleration of degradation, and preservation of residual renal function.

Impact of drug transporter studies on drug discovery and development

Drug transporters are expressed in many tissues such as the intestine, liver, kidney, and brain, and play key roles in drug absorption, distribution, and excretion. The information on the functional characteristics of drug transporters provides important information to allow improvements in drug delivery or drug design by targeting specific transporter proteins. In this article we summarize the significant role played by drug transporters in drug disposition, focusing particularly on their potential use during the drug discovery and development process. The use of transporter function offers the possibility of delivering a drug to the target organ, avoiding distribution to other organs (thereby reducing the chance of toxic side effects), controlling the elimination process, and/or improving oral bioavailability. It is useful to select a lead compound that may or may not interact with transporters, depending on whether such an interaction is desirable. The expression system of transporters is an efficient tool for screening the activity of individual transport processes. The changes in pharmacokinetics due to genetic polymorphisms and drug-drug interactions involving transporters can often have a direct and adverse effect on the therapeutic safety and efficacy of many important drugs. To obtain detailed information about these interindividual differences, the contribution made by transporters to drug absorption, distribution, and excretion needs to be taken into account throughout the drug discovery and development process.

[Molecular mechanisms of drug influx and efflux transport at the blood-brain barrier]

The blood-brain barrier (BBB) segregates the circulating blood from interstitial fluid in the brain and restricts drug permeability into the brain. Recent studies have revealed that the BBB exhibits not only blood-to-brain influx transport for the supply of nutrients, but also brain-to-blood efflux transport to excrete drugs and endogenous compounds. The influx transport system allows drugs to enter the brain. (L)-DOPA is transported into the brain by the large neutral amino acid transport system, system L. A cationic mu-opioid peptide analogue enters the brain by adsorptive-mediated endocytosis. In contrast, efflux transport limits the distribution of drugs in the brain. The ATP binding cassette transporter B1 (ABCB1) mediates the efflux transport of lipophilic drugs at the BBB by using ATP energy. Furthermore, organic anion transporter 3 (OAT3) is expressed at the BBB and mediates the efflux transport of homovanillic acid, a dopamine metabolite. This efflux transport is also likely to be involved in the transport of anionic drugs such as 6-mercaptopurine and acyclovir. Clarifying the BBB transport could give us important information allowing the development of better CNS drugs and improving our understanding of the relationship between CNS diseases and BBB functions.

Contribution of organic anion transporter 3 (Slc22a8) to the elimination of p-aminohippuric acid and benzylpenicillin across the blood-brain barrier

The role of rat organic anion transporter 3 (rOat3; Slc22a8) in the efflux transport at the blood-brain barrier (BBB) was characterized. The expression of rOat1, rOat2, and rOat3 in the brain capillary endothelial cells (BCEC) was examined using reverse transcription-polymerase chain reaction analysis, which showed that there was no expression of rOat1 or rOat2, but moderate expression of rOat3. The expression of rOat3 in the BCEC was further confirmed by Western blotting. Immunohistochemical staining showed that rOat3 is located on the abluminal and, possibly, luminal membrane of the BCEC. The contribution of rOat3 to the efflux of para-aminohippuric acid (PAH) and benzylpenicillin (PCG), substrates of rOat3, from the cerebrum into the blood circulation across the BBB was evaluated using the Brain Efflux Index method. PAH and PCG were eliminated from the cerebrum with rate constants of 0.039 and 0.043 min-1, respectively, and the elimination was saturated at high substrate concentrations. Taking account of the dilution in the brain, the Km values for the elimination of PAH and PCG were estimated to be 168 and 29 micro M, respectively. The efflux of PAH and PCG across the BBB was inhibited in a dose-dependent manner by unlabeled PCG and PAH, respectively. The Ki value of PAH for the efflux of PCG was 106 micro M and that of PCG for the efflux of PAH was 58 micro M. These values were comparable with their Km values, suggesting that they share the same efflux mechanism at the BBB. Furthermore, cimetidine and pravastatin, which are also substrates and inhibitors of rOat3, significantly inhibited the efflux of PAH and PCG from the cerebrum. These results suggest that rOat3 is responsible for the elimination of PAH and PCG from the brain across the BBB.

Rat organic anion transporter 3 (rOAT3) is responsible for brain-to-blood efflux of homovanillic acid at the abluminal membrane of brain capillary endothelial cells

The mechanism that removes homovanillic acid (HVA), an end metabolite of dopamine, from the brain is still poorly understood. The purpose of this study is to identify and characterize the brain-to-blood HVA efflux transporter at the rat blood-brain barrier (BBB). Using the Brain Efflux Index method, the apparent in vivo efflux rate constant of [3H]HVA from the brain, k(eff), was determined to be 1.69 x 10(-2) minute(-1). This elimination was significantly inhibited by para-aminohippuric acid (PAH), benzylpenicillin, indoxyl sulfate, and cimetidine, suggesting the involvement of rat organic anion transporter 3 (rOAT3). rOAT3-expressing oocytes exhibited [3H]HVA uptake (K(m) = 274 micromol/L), which was inhibited by several organic anions, such as PAH, indoxyl sulfate, octanoic acid, and metabolites of monoamine neurotransmitters. Neurotransmitters themselves did not affect the uptake. Furthermore, immunohistochemical analysis suggested that rOAT3 is localized at the abluminal membrane of brain capillary endothelial cells. These results provide the first evidence that rOAT3 is expressed at the abluminal membrane of the rat BBB and is involved in the brain-to-blood transport of HVA. This HVA efflux transport system is likely to play an important role in controlling the level of HVA in the CNS.

Recent advances in the brain-to-blood efflux transport across the blood-brain barrier

Elucidating the details of the blood-brain barrier (BBB) transport mechanism is a very important step towards successful drug targeting to the brain and understanding what happens in the brain. Although several brain uptake methods have been developed to characterize transport at the BBB, these are mainly useful for investigating influx transport across the BBB. In 1992, P-glycoprotein was found to act as an efflux pump for anti-cancer drugs at the BBB using primary cultured bovine brain endothelial cells. In order to determine the direct efflux transport from the brain to the circulating blood of exogenous compounds in vivo, the Brain Efflux Index method was developed to characterize several BBB efflux transport systems. Recently, we have established conditionally immortalized rat (TR-BBB) and mouse (TM-BBB) brain capillary endothelial cell lines from transgenic rats and mice harboring temperature-sensitive simian virus 40 large T-antigen gene to characterize the transport mechanisms at the BBB in vitro. TR-BBB and TM-BBB cells possess certain in vivo transport functions and express mRNAs for the BBB. Using a combination of newly developed in vivo and in vitro methods, we have elucidated the efflux transport mechanism at the BBB for neurosteroids, excitatory neurotransmitters, suppressive neurotransmitters, amino acids, and other organic anions to understand the physiological role played by the BBB as a detoxifying organ for the brain.

Application of In Vivo Brain Microdialysis to the Study of Blood-brain Barrier Transport of Drugs

Recent advances in blood-brain barrier (BBB) research have led to a new understanding of drug transport processes at the BBB. The BBB acts as a dynamic regulatory interface at which nutrients necessary for neural activity are actively taken up into the brain from the blood circulation, and actively excludes metabolites that might interfere with the maintenance of brain homeostasis. Such influx and efflux transport functions at the BBB would also control the concentrations of various drugs in the brain interstitial fluid (ISF), which are an important determinant of the central nervous system (CNS) effects. Thus, direct measurement of the brain ISF concentration of drugs can provide significant information for clarifying the influx and efflux transport functions of drugs across the BBB. Although several experimental techniques have been developed to investigate transport functions across the BBB, in vivo brain microdialysis seems to be one of the most suitable techniques for characterizing the influx and efflux transport functions across the BBB under physiological and pathological conditions. This review covers studies during the past decade, in which the influx and efflux transport of drugs across the BBB was kinetically and mechanistically evaluated by means of the brain microdialysis technique. Some applications of brain microdialysis to studies on neuronal function and neurotherapeutics are also included.