Drug transport by Organic Anion Transporters (OATs)

Prominent accumulation in hemodialysis patients of solutes normally cleared by tubular secretion

Dialytic clearance of urea is efficient, but other small solutes normally secreted by the kidney may be cleared less efficiently. This study tested whether the high concentrations of these solutes in hemodialysis patients reflect a failure of passive diffusion methods to duplicate the efficacy of clearance by tubular secretion. We compared the plasma concentrations and clearance rates of four solutes normally cleared by tubular secretion with the plasma concentrations and clearance rates of urea and creatinine in patients receiving maintenance hemodialysis and normal subjects. The predialysis concentrations (relative to normal subjects) of unbound phenylacetylglutamine (122-fold), hippurate (108-fold), indoxyl sulfate (116-fold), and p-cresol sulfate (41-fold) were much greater than the concentrations of urea (5-fold) and creatinine (13-fold). The dialytic clearance rates (relative to normal subjects) of unbound phenylacetylglutamine (0.37-fold), hippurate (0.16-fold), indoxyl sulfate (0.21-fold), and p-cresol sulfate (0.39-fold) were much lower than the rates of urea (4.2-fold) and creatinine (1.3-fold). Mathematical modeling showed that prominent accumulation of the normally secreted solutes in hemodialysis patients could be accounted for by lower dialytic clearance relative to physiologic clearance combined with the intermittency of treatment. Whether or not more efficient removal of normally secreted solutes improves outcomes in dialysis patients remains to be tested.

Glutathione is a low-affinity substrate of the human sodium-dependent dicarboxylate transporter

BACKGROUND/AIMS

During a single pass through the kidneys, more than 80% of glutathione (GSH) is excreted, indicating not only glomerular filtration, but also tubular secretion. The first step in tubular secretion is the uptake of a substance across the basolateral membrane of proximal tubule cells by sodium-dependent and -independent transporters. Due to the dicarboxylate-like structure, we postulated that GSH uptake across the basolateral membrane is mediated by the sodium-dependent dicarboxylate transporter 3 (NaDC3).

METHODS

Tracer uptake and electrophysiologic measurements using a two-electrode voltage clamp device were performed in Xenopus laevis oocytes expressing the human (h)NaDC3.

RESULTS

Uptake of succinate, the reference substrate of hNaDC3, was inhibited by GSH in a dose-dependent manner with an IC50 of 1.88 mM. GSH evoked potential-dependent inward currents, which were abolished under sodium-free conditions. At -60 mV, GSH currents showed saturation kinetics with a KM of 1.65 mM.

CONCLUSION

hNaDC3 present at the basolateral membrane of proximal tubule cells mediates sodium-dependent GSH uptake. The kinetic data show that NaDC3 is a low-affinity GSH transporter.

Organic anion transporter 5 (Oat5) renal expression and urinary excretion in rats treated with cisplatin: a potential biomarker of cisplatin-induced nephrotoxicity

Cisplatin is one of the most potent chemotherapeutic antitumor drugs used in the treatment of a wide range of solid tumors. Its primary dose-limiting side effect is nephrotoxicity. The organic anion transporter 5 (Oat5) is exclusively localized in the kidney. Oat5 urinary excretion was recently proposed as a potential early biomarker of acute kidney injury (AKI). The aim of this study was to evaluate Oat5 renal expression and its urinary excretion in rats exposed to different doses of cisplatin, in comparison with traditional markers of renal injury, like renal histology, creatinine and urea plasma levels, creatinine clearance, protein and glucose urinary levels and urinary alkaline phosphatase (AP) activity. Male Wistar rats were treated with a single injection of cisplatin at different doses of 1, 2, 5 and 10 mg/kg b.w., i.p. (Cis1, Cis2, Cis5 and Cis10, n = 4, respectively) and experiments were carried out 48 h after cisplatin administration. The renal expression of Oat5 was evaluated by immunohistochemistry and Western blotting. Oat5 abundance, AP activity, creatinine, glucose and proteins were assayed in urine. Creatinine clearance and creatinine and urea plasma levels were also evaluated. In this experimental model, plasma urea and creatinine levels, creatinine clearance, AP urinary activity and protein and glucose urinary levels were significantly modified only at the highest cisplatin dose of 10 mg/kg b.w., i.p., as compared to control rats. In contrast, Oat5 urinary abundance was increased in a dose-related manner after the administration of cisplatin. Oat5 urinary abundance was elevated at a dose as low as 1 mg/kg b.w., i.p., implying renal perturbation, when no modifications of traditional markers of renal injury are yet observed. Oat5 renal expression was decreased in a dose-related manner, both in homogenates and apical membranes from cisplatin-treated kidneys. The increase in urinary Oat5 excretion might explain the decrease in the amount of Oat5 molecules in the renal tubule cells. Hence, the preclinical animal results showed in this work propose that Oat5 urinary excretion might potentially serve as a non-invasive early biomarker of cisplatin-induced AKI.

Clinical impact of genetic variants of drug transporters in different ethnic groups within and across regions

Drug transporters, together with drug metabolic enzymes, are major determinants of drug disposition and are known to alter the response to many commonly used drugs. Substantial frequency differences for known variants exist across geographic regions for certain drug transporters. To deliver efficacious medicine with the right dose for each patient, it is important to understand the contribution of genetic variants for drug transporters. Recently, mutual pharmacokinetic data usage among Asian regions, which are thought to be relatively similar in their own genetic background, is expected to accelerate new drug applications and reduce developmental costs. Polymorphisms of drug transporters could be key factors to be considered in implementing multiethnic global clinical trials. This review addresses the current knowledge on genetic variations of major drug transporters affecting drug disposition, efficacy and toxicity, focusing on the east Asian populations, and provides insights into future directions for precision medicine and drug development in east Asia.

Developmental changes in the transcriptome of the rat choroid plexus in relation to neuroprotection

Background

The choroid plexuses are the interface between the blood and the cerebrospinal fluid (CSF) contained within the ventricular spaces of the central nervous system. The tight junctions linking adjacent cells of the choroidal epithelium create a physical barrier to paracellular movement of molecules. Multispecific efflux transporters as well as drug-metabolizing and antioxidant enzymes functioning in these cells contribute to a metabolic barrier. These barrier properties reflect a neuroprotective function of the choroid plexus. The choroid plexuses develop early during embryogenesis and provide pivotal control of the internal environment throughout development when the brain is especially vulnerable to toxic insults. Perinatal injuries like hypoxia and trauma, and exposure to drugs or toxic xenobiotics can have serious consequences on neurogenesis and long-term development. The present study describes the developmental expression pattern of genes involved in the neuroprotective functions of the blood–CSF barrier.

Methods

The transcriptome of rat lateral ventricular choroid plexuses isolated from fifteen-day-old embryos, nineteen-day old fetuses, two-day old pups, and adults was analyzed by a combination of Affymetrix microarrays, Illumina RNA-Sequencing, and quantitative RT-PCR.

Results

Genes coding for proteins involved in junction formation are expressed early during development. Overall perinatal expression levels of genes involved in drug metabolism and antioxidant mechanisms are similar to, or higher than levels measured in adults. A similar developmental pattern was observed for multispecific efflux transporter genes of the Abc and Slc superfamilies. Expression of all these genes was more variable in choroid plexus from fifteen-day-old embryos. A large panel of transcription factors involved in the xenobiotic- or cell stress-mediated induction of detoxifying enzymes and transporters is also expressed throughout development.

Conclusions

This transcriptomic analysis suggests relatively well–established neuroprotective mechanisms at the blood-CSF barrier throughout development of the rat. The expression of many transcription factors early in development raises the possibility of additional protection for the vulnerable developing brain, should the fetus or newborn be exposed to drugs or other xenobiotics.

Fructus Gardenia Extract ameliorates oxonate-induced hyperuricemia with renal dysfunction in mice by regulating organic ion transporters and mOIT3

The potent anti-hyperuricemia activities of Fructus Gardenia Extract (FGE) have been well reported. The aim of this study was to evaluate the uricosuric and nephro-protective effects of FGE and explore its possible mechanisms of action in oxonate-induced hyperuricemic mice. FGE was orally administered to hyperuricemic and normal mice for 1 week. Serum and urinary levels of uric acid, creatinine and blood urea nitrogen (BUN), and fractional excretion of uric acid (FEUA) were measured. The mRNA and protein levels of mouse urate transporter 1 (mURAT1), glucose transporter 9 (mGLUT9), ATP-binding cassette, subfamily G, 2 (mABCG2), organic anion transporter 1 (mOAT1), mOAT3, oncoprotein induced transcript 3 (mOIT3), organic cation/carnitine transporters in the kidney were analyzed. Simultaneously, Tamm-Horsfall glycoprotein (THP) levels in urine and kidney were detected. FGE significantly reduced serum urate levels and increased urinary urate levels and FEUA in hyperuricemic mice. It could also effectively reverse oxonate-induced alterations in renal mURAT1, mGLUT9, mOAT1 and mOIT3 expressions, as well as THP levels, resulting in the enhancement of renal uric acid excretion. Moreover, FGE decreased serum creatinine and BUN levels, and up-regulated expression of organic cation/carnitine transporters, improving renal dysfunction in this model. Furthermore, FGE decreased renal mABCG2 expressions in hyperuricemic mice, contributing to its beneficial actions. However, further investigation is needed in clinical trials of FGE and its bioactive components.

Drugs interacting with organic anion transporter-1 affect uptake of Tc-99m-mercaptoacetyl-triglycine (MAG3) in the human kidney: therapeutic drug interaction in Tc-99m-MAG3 diagnosis of renal function and possible application of Tc-99m-MAG3 for drug development

INTRODUCTION

Renal uptake of Tc-99m-MG3 involves organic anion transporter (OAT). Treatment with drugs showing OAT affinity might interfere with renal uptake of Tc-99m-MAG3, leading to misinterpretation in Tc-99m-MAG3. This study was conducted to discuss a possible drug interference with Tc-99m-MAG3 diagnosis on OAT sites.

METHODS

Renal uptake and plasma clearance of Tc-99m-MAG3 were analyzed in healthy volunteers under control and OAT1 and OAT3 related drug treatment conditions. An in vitro uptake study using OAT1 or OAT3 expressing cells was also conducted.

RESULTS

Both PAH and probenecid treatment induced delays in Tc-99m-MAG3 clearance from blood, and reductions in the renal uptake clearance. As a result, the normalized effective renal plasma flow estimated from Tc-99m-MAG3 clearance was significantly underestimated, whereas the glomerular filtration rate estimated from plasma creatinine levels was unchanged. The transport activity of Tc-99m-MAG3 was higher in OAT1-expressing cells than in OAT3-expressing cells.

CONCLUSION

Drugs with OAT1 affinity affect the renal uptake of Tc-99m-MAG3 and blood clearance. This might cause misinterpretation of functional diagnosis of the kidney using Tc-99m-MAG3.

Plasma membrane transporters in modern liver pharmacology

The liver plays a crucial role in the detoxification of drugs used in the treatment of many diseases. The liver itself is the target for drugs aimed to modify its function or to treat infections and tumours affecting this organ. Both detoxification and pharmacological processes occurring in the liver require the uptake of the drug by hepatic cells and, in some cases, the elimination into bile. These steps have been classified as detoxification phase 0 and phase III, respectively. Since most drugs cannot cross the plasma membrane by simple diffusion, the involvement of transporters is mandatory. Several members of the superfamilies of solute carriers (SLC) and ATP-binding cassette (ABC) proteins, with a minor participation of other families of transporters, account for the uptake and efflux, respectively, of endobiotic and xenobiotic compounds across the basolateral and apical membranes of hepatocytes and cholangiocytes. These transporters are also involved in the sensitivity and refractoriness to the pharmacological treatment of liver tumours. An additional interesting aspect of the role of plasma membrane transporters in liver pharmacology regards the promiscuity of many of these carriers, which accounts for a variety of drug-drug, endogenous substances-drug and food components-drug interactions with clinical relevance.