Indoxyl sulphate-TNFα axis mediates uremic encephalopathy in rodent acute kidney injury

Acute kidney injury (AKI) is often accompanied by uremic encephalopathy resulting from accumulation of uremic toxins in brain possibly due to impaired blood-brain barrier (BBB) function. Anionic uremic toxins are substrates or inhibitors of organic anionic transporters (OATs). In this study we investigated the CNS behaviors and expression/function of BBB OAT3 in AKI rats and mice, which received intraperitoneal injection of cisplatin 8 and 20 mg/kg, respectively. We showed that cisplatin treatment significantly inhibited the expressions of OAT3, synaptophysin and microtubule-associated protein 2 (MAP2), impaired locomotor and exploration activities, and increased accumulation of uremic toxins in the brain of AKI rats and mice. In vitro studies showed that uremic toxins neither alter OAT3 expression in human cerebral microvascular endothelial cells, nor synaptophysin and MAP2 expressions in human neuroblastoma (SH-SY5Y) cells. In contrast, tumour necrosis factor alpha (TNFα) and the conditioned medium (CM) from RAW264.7 cells treated with indoxyl sulfate (IS) significantly impaired OAT3 expression. TNFα and CM from IS-treated BV-2 cells also inhibited synaptophysin and MAP2 expressions in SH-SY5Y cells. The alterations caused by TNFα and CMs in vitro, and by AKI and TNFα in vivo were abolished by infliximab, a monoclonal antibody designed to intercept and neutralize TNFα, suggesting that AKI impaired the expressions of OAT3, synaptophysin and MAP2 in the brain via IS-induced TNFα release from macrophages or microglia (termed as IS-TNFα axis). Treatment of mice with TNFα (0.5 mg·kg-1·d-1, i.p. for 3 days) significantly increased p-p65 expression and reduced the expressions of Nrf2 and HO-1. Inhibiting NF-κB pathway, silencing p65, or activating Nrf2 and HO-1 obviously attenuated TNFα-induced downregulation of OAT3, synaptophysin and MAP2 expressions. Significantly increased p-p65 and decreased Nrf2 and HO-1 protein levels were also detected in brain of AKI mice and rats. We conclude that AKI inhibits the expressions of OAT3, synaptophysin and MAP2 due to IS-induced TNFα release from macrophages or microglia. TNFα impairs the expressions of OAT3, synaptophysin and MAP2 partly via activating NF-κB pathway and inhibiting Nrf2-HO-1 pathway.

Inhibitory interaction of flavonoids with organic anion transporter 3 and their structure-activity relationships for predicting nephroprotective effects

The organic anion transporter 3 (OAT3), an important renal uptake transporter, is associated with drug-induced acute kidney injury (AKI). Screening and identifying potent OAT3 inhibitors with little toxicity in natural products, especially flavonoids, in reducing OAT3-mediated AKI is of great value. The five strongest OAT3 inhibitors from the 97 flavonoids markedly decreased aristolochic acid I-induced cytotoxicity and alleviated methotrexate-induced nephrotoxicity. The pharmacophore model clarified hydrogen bond acceptors and hydrophobic groups are the critical pharmacophores. These findings would provide valuable information in predicting the potential risks of flavonoid-containing food/herb-drug interactions and optimizing flavonoid structure to alleviate OAT3-related AKI.

The Impact of Inflammation on the In Vivo Activity of the Renal Transporters OAT1/3 in Pregnant Women Diagnosed with Acute Pyelonephritis

Inflammation can regulate hepatic drug metabolism enzymes and transporters. The impact of inflammation on renal drug transporters remains to be elucidated. We aimed to quantify the effect of inflammation (caused by acute pyelonephritis) on the in vivo activity of renal OAT1/3, using the probe drug furosemide. Pregnant women (second or third trimester) received a single oral dose of furosemide 40 mg during acute pyelonephritis (Phase 1; n = 7) and after its resolution (Phase 2; n = 7; by treatment with intravenous cefuroxime 750 mg TID for 3-7 days), separated by 10 to 14 days. The IL-6, IFN-γ, TNF-α, MCP-1, and C-reactive protein plasma concentrations were higher in Phase I vs. Phase II. The pregnant women had a lower geometric mean [CV%] furosemide CLsecretion (3.9 [43.4] vs. 6.7 [43.8] L/h) and formation clearance to the glucuronide (1.1 [85.9] vs. 2.3 [64.1] L/h) in Phase 1 vs. Phase 2. Inflammation reduced the in vivo activity of renal OAT1/3 (mediating furosemide CLsecretion) and UGT1A9/1A1 (mediating the formation of furosemide glucuronide) by approximately 40% and 54%, respectively, presumably by elevating the plasma cytokine concentrations. The dosing regimens of narrow therapeutic window OAT drug substrates may need to be adjusted during inflammatory conditions.

Novel (sulfated) thyroid hormone transporters in the solute carrier 22 family

Objective

Thyroid hormone (TH) transport represents a critical first step in governing intracellular TH regulation. It is still unknown whether the full repertoire of TH transporters has been identified. Members of the solute carrier (SLC) 22 family have substrates in common with the known TH transporters of the organic anion-transporting peptide family. Therefore, we screened the SLC22 family for TH transporters.

Methods

Uptake of 1 nM of iodothyronines or sulfated iodothyronines in COS1 cells expressing SLC22 proteins was performed.

Results

We first tested 25 mouse (m) SLC22 proteins for TH uptake and found that the majority of the organic anion transporter (OAT) clade were capable of 3,3',5-triiodothyronine and/or thyroxine (T4) transport. Based on phylogenetic tree analysis of the mouse and human (h) SLC22 family, we selected eight hSLC22s that grouped with the newly identified mouse TH transporters. Of these, four tested positive for uptake of one or more substrates, particularly hSLC22A11 showed robust (3-fold over control) uptake of T4. Uptake of sulfated iodothyronines was strongly (up to 17-fold) induced by some SLC22s, most notably SLC22A8, hSLC22A9, mSLC22A27 and mSLC22A29. Finally, the zebrafish orthologues of SLC22A6/8 drOatx and drSlc22a6l also transported almost all (sulfated) iodothyronines tested. The OAT inhibitors lesinurad and probenecid inhibited most SLC22 proteins.

Conclusions

Our results demonstrated that members of the OAT clade of the SLC22 family constitute a novel, evolutionary conserved group of transporters for (sulfated) iodothyronines. Future studies should reveal the relevance of these transporters in TH homeostasis and physiology.

OAT, OATP, and MRP Drug Transporters and the Remote Sensing and Signaling Theory

The coordinated movement of organic anions (e.g., drugs, metabolites, signaling molecules, nutrients, antioxidants, gut microbiome products) between tissues and body fluids depends, in large part, on organic anion transporters (OATs) [solute carrier 22 (SLC22)], organic anion transporting polypeptides (OATPs) [solute carrier organic (SLCO)], and multidrug resistance proteins (MRPs) [ATP-binding cassette, subfamily C (ABCC)]. Depending on the range of substrates, transporters in these families can be considered multispecific, oligospecific, or (relatively) monospecific. Systems biology analyses of these transporters in the context of expression patterns reveal they are hubs in networks involved in interorgan and interorganismal communication. The remote sensing and signaling theory explains how the coordinated functions of drug transporters, drug-metabolizing enzymes, and regulatory proteins play a role in optimizing systemic and local levels of important endogenous small molecules. We focus on the role of OATs, OATPs, and MRPs in endogenous metabolism and how their substrates (e.g., bile acids, short chain fatty acids, urate, uremic toxins) mediate interorgan and interorganismal communication and help maintain and restore homeostasis in healthy and disease states.

Functional Characterization of Rare Variants in OAT1/SLC22A6 and OAT3/SLC22A8 Urate Transporters Identified in a Gout and Hyperuricemia Cohort

The OAT1 (SLC22A6) and OAT3 (SLC22A8) urate transporters are located on the basolateral membrane of the proximal renal tubules, where they ensure the uptake of uric acid from the urine back into the body. In a cohort of 150 Czech patients with primary hyperuricemia and gout, we examined the coding regions of both genes using PCR amplification and Sanger sequencing. Variants p.P104L (rs11568627) and p.A190T (rs146282438) were identified in the gene for solute carrier family 22 member 6 (SLC22A6) and variants p.R149C (rs45566039), p.V448I (rs11568486) and p.R513Q (rs145474422) in the gene solute carrier family 22 member 8 (SLC22A8). We performed a functional study of these rare non-synonymous variants using the HEK293T cell line. We found that only p.R149C significantly reduced uric acid transport in vitro. Our results could deepen the understanding of uric acid handling in the kidneys and the molecular mechanism of uric acid transport by the OAT family of organic ion transporters.

SLC22 Transporters in the Fly Renal System Regulate Response to Oxidative Stress In Vivo

Several SLC22 transporters in the human kidney and other tissues are thought to regulate endogenous small antioxidant molecules such as uric acid, ergothioneine, carnitine, and carnitine derivatives. These transporters include those from the organic anion transporter (OAT), OCTN/OCTN-related, and organic cation transporter (OCT) subgroups. In mammals, it has been difficult to show a clear in vivo role for these transporters during oxidative stress. Ubiquitous knockdowns of related Drosophila SLC22s-including transporters homologous to those previously identified by us in mammals such as the "Fly-Like Putative Transporters" FLIPT1 (SLC22A15) and FLIPT2 (SLC22A16)-have shown modest protection against oxidative stress. However, these fly transporters tend to be broadly expressed, and it is unclear if there is an organ in which their expression is critical. Using two tissue-selective knockdown strategies, we were able to demonstrate much greater and longer protection from oxidative stress compared to previous whole fly knockdowns as well as both parent and WT strains (CG6126: p < 0.001, CG4630: p < 0.01, CG16727: p < 0.0001 and CG6006: p < 0.01). Expression in the Malpighian tubule and likely other tissues as well (e.g., gut, fat body, nervous system) appear critical for managing oxidative stress. These four Drosophila SLC22 genes are similar to human SLC22 transporters (CG6126: SLC22A16, CG16727: SLC22A7, CG4630: SLC22A3, and CG6006: SLC22A1, SLC22A2, SLC22A3, SLC22A6, SLC22A7, SLC22A8, SLC22A11, SLC22A12 (URAT1), SLC22A13, SLC22A14)-many of which are highly expressed in the kidney. Consistent with the Remote Sensing and Signaling Theory, this indicates an important in vivo role in the oxidative stress response for multiple SLC22 transporters within the fly renal system, perhaps through interaction with SLC22 counterparts in non-renal tissues. We also note that many of the human relatives are well-known drug transporters. Our work not only indicates the importance of SLC22 transporters in the fly renal system but also sets the stage for in vivo studies by examining their role in mammalian oxidative stress and organ crosstalk.

Transporters (OATs and OATPs) contribute to illustrate the mechanism of medicinal compatibility of ingredients with different properties in yuanhuzhitong prescription

Various medicinal ingredients with different tastes are combined according to the theory of compatibility in Chinese materia medica to achieve a better efficacy, while the mechanism was not very clear. Here, the authors studied the interaction between ingredients and human transporters such as the kidney transporters OAT1 and OAT3, the liver transporters OATP1B1 and OATP1B3, and the intestine transporter OATP2B1 to discern the compatibility mechanism of ingredients with different tastes in the Yuanhuzhitong preparation (YHP) comprising Corydalis yanhusuo (CYH) and Angelica dahurica (AD), which could relieve pain by restraining the central system. The results show that tetrahydropalmatine (TDE), the major component of CYH, could be transported by OAT3 into kidney, OATP1B1 and OATP1B3 into liver, while imperatorin (IPT) and isoimperatorin (ISP), the two key components of AD, and AD extract showed strong inhibition to OAT1 and OAT3. What's more, AD extract also exerted strongly inhibition to human transporters OATP1B1 and OATP1B3. It was also detected that IPT, ISP, and AD extract significantly downregulated the expression of Oatp1a1, Oatp1a4, and Oatp1b2 of liver in mice. The in vivo results show that the concentration of TDE in liver and kidney significantly decreased, while the TDE concentration in blood and brain were both significantly enhanced in the presence of IPT, ISP, and AD extract. These results suggest that the ingredients in AD with pungent taste could enhance the exposure of TDE in blood and brain by inhibiting the uptake of TDE in liver and kidney. That is to say, TDE with bitter taste could "flood up" into the central nervous system to play its therapeutic effect by the cut-off of that into liver and kidney in the presence of ingredients within AD. This paper not only proves the meridian distribution of CYH in liver and kidney with the role of OAT3, OATP1B1, and OATP1B3, but also illustrates how to improve the efficacy of CYH by reasonable compatibility with AD. This study may offer a valuable clue to illustrate the mechanism of compatibility theory.

Acamprosate Is a Substrate of the Human Organic Anion Transporter (OAT) 1 without OAT3 Inhibitory Properties: Implications for Renal Acamprosate Secretion and Drug-Drug Interactions

Acamprosate is an anionic drug substance widely used in treating symptoms of alcohol withdrawal. It was recently shown that oral acamprosate absorption is likely due to paracellular transport. In contrast, little is known about the eliminating mechanism clearing acamprosate from the blood in the kidneys, despite the fact that studies have shown renal secretion of acamprosate. The hypothesis of the present study was therefore that renal organic anion transporters (OATs) facilitate the renal excretion of acamprosate in humans. The aim of the present study was to establish and apply OAT1 (gene product of SLC22A6) and OAT3 (gene product of SLC22A8) expressing cell lines to investigate whether acamprosate is a substrate or inhibitor of OAT1 and/or OAT3. The studies were performed in HEK293-Flp-In cells stably transfected with SLC22A6 or SLC22A8. Protein and functional data showed that the established cell lines are useful for studying OAT1- and OAT3-mediated transport in bi-laboratory studies. Acamprosate inhibited OAT1-mediated p-aminohippuric acid (PAH) uptake but did not inhibit substrate uptake via OAT3 expressing cells, neither when applied concomitantly nor after a 3 h preincubation with acamprosate. The uptake of PAH via OAT1 was inhibited in a competitive manner by acamprosate and cellular uptake studies showed that acamprosate is a substrate for OAT1 with a K_m-value of approximately 700 µM. Probenecid inhibited OAT1-mediated acamprosate uptake with a K_i-value of approximately 13 µM, which may translate into an estimated clinically significant DDI index. In conclusion, acamprosate was identified as a substrate of OAT1 but not OAT3.

Differential interactions of the β-lactam cloxacillin with human renal organic anion transporters (OATs)

The β-lactam penicillin antibiotic cloxacillin (CLX) presents wide inter-individual pharmacokinetics variability. To better understand its molecular basis, the precise identification of the detoxifying actors involved in CLX disposition and elimination would be useful, notably with respect to renal secretion known to play a notable role in CLX elimination. The present study was consequently designed to analyze the interactions of CLX with the solute carrier transporters organic anion transporter (OAT) 1 and OAT3, implicated in tubular secretion through mediating drug entry at the basolateral pole of renal proximal cells. CLX was first shown to block OAT1 and OAT3 activity in cultured OAT-overexpressing HEK293 cells. Half maximal inhibitory concentration (IC₅₀ ) value for OAT3 (13 µm) was however much lower than that for OAT1 (560 µm); clinical inhibition of OAT activity and drug-drug interactions may consequently be predicted for OAT3, but not OAT1. OAT3, unlike OAT1, was next shown to mediate CLX uptake in OAT-overexpressing HEK293 cells. Kinetic parameters for this OAT3-mediated transport of CLX (K_m  = 10.7 µm) were consistent with a possible in vivo saturation of this process for high CLX plasma concentrations. OAT3 is consequently likely to play a pivotal role in renal CLX secretion and consequently in total renal CLX elimination, owing to the low plasma unbound fraction of the antibiotic. OAT3 genetic polymorphisms as well as co-administered drugs inhibiting in vivo OAT3 activity may therefore be considered as potential sources of CLX pharmacokinetics variability.

Organic anion transporters 1 and 3 influence cellular energy metabolism in renal proximal tubule cells

Organic anion transporters (OATs) 1 and 3 are, besides being uptake transporters, key in several cellular metabolic pathways. The underlying mechanisms are largely unknown. Hence, we used human conditionally immortalized proximal tubule epithelial cells (ciPTEC) overexpressing OAT1 or OAT3 to gain insight into these mechanisms. In ciPTEC-OAT1 and -OAT3, extracellular lactate levels were decreased (by 77% and 71%, respectively), while intracellular ATP levels remained unchanged, suggesting a shift towards an oxidative phenotype upon OAT1 or OAT3 overexpression. This was confirmed by increased respiration of ciPTEC-OAT1 and -OAT3 (1.4-fold), a decreased sensitivity to respiratory inhibition, and characterized by a higher demand on mitochondrial oxidative capacity. In-depth profiling of tricarboxylic acid (TCA) cycle metabolites revealed reduced levels of intermediates converging into α-ketoglutarate in ciPTEC-OAT1 and -OAT3, which via 2-hydroxyglutarate metabolism explains the increased respiration. These interactions with TCA cycle metabolites were in agreement with metabolomic network modeling studies published earlier. Further studies using OAT or oxidative phosphorylation (OXPHOS) inhibitors confirmed our idea that OATs are responsible for increased use and synthesis of α-ketoglutarate. In conclusion, our results indicate an increased α-ketoglutarate efflux by OAT1 and OAT3, resulting in a metabolic shift towards an oxidative phenotype.

The SLC22 Transporter Family: A Paradigm for the Impact of Drug Transporters on Metabolic Pathways, Signaling, and Disease

The SLC22 transporter family consists of more than two dozen members, which are expressed in the kidney, the liver, and other tissues. Evolutionary analysis indicates that SLC22 transporters fall into at least six subfamilies: OAT (organic anion transporter), OAT-like, OAT-related, OCT (organic cation transporter), OCTN (organic cation/carnitine transporter), and OCT/OCTN-related. Some-including OAT1 [SLC22A6 or NKT (novel kidney transporter)] and OAT3 (SLC22A8), as well as OCT1 (SLC22A1) and OCT2 (SLC22A2)-are widely studied drug transporters. Nevertheless, analyses of knockout mice and other data indicate that SLC22 transporters regulate key metabolic pathways and levels of signaling molecules (e.g., gut microbiome products, bile acids, tricarboxylic acid cycle intermediates, dietary flavonoids and other nutrients, prostaglandins, vitamins, short-chain fatty acids, urate, and ergothioneine), as well as uremic toxins associated with chronic kidney disease. Certain SLC22 transporters-such as URAT1 (SLC22A12) and OCTN2 (SLC22A5)-are mutated in inherited metabolic diseases. A new systems biology view of transporters is emerging. As proposed in the remote sensing and signaling hypothesis, SLC22 transporters, together with other SLC and ABC transporters, have key roles in interorgan and interorganism small-molecule communication and, together with the neuroendocrine, growth factor-cytokine, and other homeostatic systems, regulate local and whole-body homeostasis.

The drug transporter OAT3 (SLC22A8) and endogenous metabolite communication via the gut-liver-kidney axis

The organic anion transporters OAT1 (SLC22A6) and OAT3 (SLC22A8) have similar substrate specificity for drugs, but it is far from clear whether this holds for endogenous substrates. By analysis of more than 600 metabolites in the Oat3KO (Oat3 knockout) by LC/MS, we demonstrate OAT3 involvement in the movement of gut microbiome products, key metabolites, and signaling molecules, including those flowing through the gut-liver-kidney axis. Major pathways affected included those involved in metabolism of bile acids, flavonoids, nutrients, amino acids (including tryptophan-derivatives that are uremic toxins), and lipids. OAT3 is also critical in elimination of liver-derived phase II metabolites, particularly those undergoing glucuronidation. Analysis of physicochemical features revealed nine distinct metabolite groups; at least one member of most clusters has been previously validated in transport assays. In contrast to drugs interacting with the OATs, endogenous metabolites accumulating in the Oat1KO (Oat1 knockout) versus Oat3KO have distinct differences in their physicochemical properties; they are very different in size, number of rings, hydrophobicity, and molecular complexity. Consistent with the Remote Sensing and Signaling Hypothesis, the data support the importance of the OAT transporters in inter-organ and inter-organismal remote communication via transporter-mediated movement of key metabolites and signaling molecules (e.g. gut microbiome-to-intestine-to-blood-to-liver-to-kidney-to-urine). We discuss the possibility of an intimate connection between OATs and metabolite sensing and signaling pathways (e.g. bile acids). Furthermore, the metabolomics and pathway analysis support the view that OAT1 plays a greater role in kidney proximal tubule metabolism and OAT3 appears relatively more important in systemic metabolism, modulating levels of metabolites flowing through intestine, liver, and kidney.

Role of gemfibrozil as an inhibitor of CYP2C8 and membrane transporters

INTRODUCTION

Cytochrome P450 (CYP) 2C8 is a drug metabolizing enzyme of major importance. The lipid-lowering drug gemfibrozil has been identified as a strong inhibitor of CYP2C8 in vivo. This effect is due to mechanism-based inhibition of CYP2C8 by gemfibrozil 1-O-β-glucuronide. In vivo, gemfibrozil is a fairly selective CYP2C8 inhibitor, which lacks significant inhibitory effect on other CYP enzymes. Gemfibrozil can, however, have a smaller but clinically meaningful inhibitory effect on membrane transporters, such as organic anion transporting polypeptide 1B1 and organic anion transporter 3. Areas covered: This review describes the inhibitory effects of gemfibrozil on CYP enzymes and membrane transporters. The clinical drug interactions caused by gemfibrozil and the different mechanisms contributing to the interactions are reviewed in detail. Expert opinion: Gemfibrozil is a useful probe inhibitor of CYP2C8 in vivo, but its effect on membrane transporters has to be taken into account in study design and interpretation. Moreover, gemfibrozil could be used to boost the pharmacokinetics of CYP2C8 substrate drugs. Identification of gemfibrozil 1-O-β-glucuronide as a potent mechanism-based inhibitor of CYP2C8 has led to recognition of glucuronide metabolites as perpetrators of drug-drug interactions. Recently, also acyl glucuronide metabolites of clopidogrel and deleobuvir have been shown to strongly inhibit CYP2C8.

Transmembrane transport of steviol glucuronide and its potential interaction with selected drugs and natural compounds

Steviol glucuronide (SVG) is the major metabolite derived from steviol, the aglycone of stevioside and rebaudioside A. After the ingestion of stevioside and rebaudioside A, SVG is formed and excreted into the urine in humans. In the present study, transporter mediated efflux and uptake of SVG was investigated in order to understand molecular mechanisms underlying its renal clearance. Results showed that SVG was not a substrate of efflux transporters BCRP, MRP2, MATE1 or P-gp. In contrast, OAT3 played a predominant role in the uptake of SVG in comparison to OATP1B1, OATP1B3, or OATP2B1. Quercetin, telmisartan, diclofenac, and mulberrin displayed a relatively strong inhibition against OAT3 mediated uptake of SVG with IC50 values of 1.8, 2.9, 8.0, and 10.0 μM, respectively. Because OAT3 is a major uptake transporter in the kidney, inhibition of OAT3 activity may alter SVG's renal clearance by drugs and natural compounds that are used concomitantly with stevia leaf extracts.

Uric acid, indoxyl sulfate, and methylguanidine activate bulbospinal neurons in the RVLM via their specific transporters and by producing oxidative stress

Patients with chronic renal failure often have hypertension, but the cause of hypertension, other than an excess of body fluid, is not well known. We hypothesized that the bulbospinal neurons in the rostral ventrolateral medulla (RVLM) are stimulated by uremic toxins in patients with chronic renal failure. To investigate whether RVLM neurons are sensitive to uremic toxins, such as uric acid, indoxyl sulfate, or methylguanidine, we examined changes in the membrane potentials (MPs) of bulbospinal RVLM neurons of Wister rats using the whole-cell patch-clamp technique during superfusion with these toxins. A brainstem-spinal cord preparation that preserved the sympathetic nervous system was used for the experiments. During uric acid, indoxyl sulfate, or methylguanidine superfusion, almost all the RVLM neurons were depolarized. To examine the transporters for these toxins on RVLM neurons, histological examinations were performed. The uric acid-, indoxyl sulfate-, and methylguanidine-depolarized RVLM neurons showed the presence of urate transporter 1 (URAT 1), organic anion transporter (OAT)1 or OAT3, and organic cation transporter (OCT)3, respectively. Furthermore, the toxin-induced activities of the RVLM neurons were suppressed by the addition of an anti-oxidation drug (VAS2870, an NAD(P)H oxidase inhibitor), and a histological examination revealed the presence of NAD(P)H oxidase (nox)2 and nox4 in these RVLM neurons. The present results show that uric acid, indoxyl sulfate, and methylguanidine directly stimulate bulbospinal RVLM neurons via specific transporters on these neurons and by producing oxidative stress. These uremic toxins may cause hypertension by activating RVLM neurons.

Organic anion transporter 3- and organic anion transporting polypeptides 1B1- and 1B3-mediated transport of catalposide

We investigated the in vitro transport characteristics of catalposide in HEK293 cells overexpressing organic anion transporter 1 (OAT1), OAT3, organic anion transporting polypeptide 1B1 (OATP1B1), OATP1B3, organic cation transporter 1 (OCT1), OCT2, P-glycoprotein (P-gp), and breast cancer resistance protein (BCRP). The transport mechanism of catalposide was investigated in HEK293 and LLC-PK1 cells overexpressing the relevant transporters. The uptake of catalposide was 319-, 13.6-, and 9.3-fold greater in HEK293 cells overexpressing OAT3, OATP1B1, and OATP1B3 transporters, respectively, than in HEK293 control cells. The increased uptake of catalposide via the OAT3, OATP1B1, and OATP1B3 transporters was decreased to basal levels in the presence of representative inhibitors such as probenecid, furosemide, and cimetidine (for OAT3) and cyclosporin A, gemfibrozil, and rifampin (for OATP1B1 and OATP1B3). The concentration-dependent OAT3-mediated uptake of catalposide revealed the following kinetic parameters: Michaelis constant (K_m) =41.5 μM, maximum uptake rate (V_max) =46.2 pmol/minute, and intrinsic clearance (CL_int) =1.11 μL/minute. OATP1B1- and OATP1B3-mediated catalposide uptake also showed concentration dependency, with low CL_int values of 0.035 and 0.034 μL/minute, respectively. However, the OCT1, OCT2, OAT1, P-gp, and BCRP transporters were apparently not involved in the uptake of catalposide into cells. In addition, catalposide inhibited the transport activities of OAT3, OATP1B1, and OATP1B3 with half-maximal inhibitory concentration values of 83, 200, and 235 μM, respectively. However, catalposide did not significantly inhibit the transport activities of OCT1, OCT2, OAT1, P-gp, or BCRP. In conclusion, OAT3, OATP1B1, and OATP1B3 are major transporters that may regulate the pharmacokinetic properties and may cause herb–drug interactions of catalposide, although their clinical relevance awaits further evaluation.

Evaluation of drug-drug interaction between the novel cPLA2 inhibitor AK106-001616 and methotrexate in rheumatoid arthritis patients

1. Drug interaction potential between AK106-001616, a novel cytosolic phospholipase A2 inhibitor, and methotrexate (MTX) in rheumatoid arthritis patients was investigated. This trial is registered with ClinicalTrials.gov, number NCT00902369. 2. In the clinical study, the 90% confidence intervals (CIs) for the geometric mean ratio (GMR) of AUC0-t of MTX administered after AK106-001616 200 mg compared to the MTX without AK106-001616 were within 80-125%. However, administration of AK106-001616 at doses of 400 and 600 mg exceeded the 125% threshold. As small but statistically significant increases in AUC0-t were observed, we investigated the mechanism for this drug-drug interaction between MTX and AK106-001616. 3. In vitro, AK106-001616 inhibited OAT1 (IC50 = 18.4 μM, Ki = 33.6 μM) in a non-competitive manner and OAT3 (IC50 = 1.80 μM, Ki = 1.49 μM) in a competitive manner. Both transporters are involved in MTX transport in renal proximal tubules. 4. AK106-001616 has a weak drug interaction with MTX. In vitro studies provide a mechanistic understanding of the in vivo inhibition of transporters by AK106-001616.

Reduced renal clearance of cefotaxime in asians with a low-frequency polymorphism of OAT3 (SLC22A8)

Organic anion transporter 3 (OAT3, SLC22A8), a transporter expressed on the basolateral membrane of the proximal tubule, plays a critical role in the renal excretion of organic anions including many therapeutic drugs. The goal of this study was to evaluate the in vivo effects of the OAT3-Ile305Phe variant (rs11568482), present at 3.5% allele frequency in Asians, on drug disposition with a focus on cefotaxime, a cephalosporin antibiotic. In HEK293-Flp-In cells, the OAT3-Ile305Phe variant had a lower maximum cefotaxime transport activity, Vmax , [159 ± 3 nmol*(mg protein)(-1) /min (mean ± SD)] compared with the reference OAT3 [305 ± 28 nmol*(mg protein)(-1) /min, (mean ± SD), p < 0.01], whereas the Michaelis-Menten constant values (Km ) did not differ. In healthy volunteers, we found volunteers that were heterozygous for the Ile305Phe variant and had a significantly lower cefotaxime renal clearance (CLR ; mean ± SD: 84.8 ± 32.1 mL/min, n = 5) compared with volunteers that were homozygous for the reference allele (158 ± 44.1 mL/min, n = 10; p = 0.006). Furthermore, the net secretory component of cefotaxime renal clearance (CLsec ) was reduced in volunteers heterozygous for the variant allele [33.3 ± 31.8 mL/min (mean ± SD)] compared with volunteers homozygous for the OAT3 reference allele [97.0 ± 42.2 mL/min (mean ± SD), p = 0.01]. In summary, our study suggests that a low-frequency reduced-function polymorphism of OAT3 associates with reduced cefotaxime CLR and CL(sec) .

Transport of Kynurenic Acid by Rat Organic Anion Transporters rOAT1 and rOAT3: Species Difference between Human and Rat in OAT1

A tryptophan catabolite, kynurenic acid, is involved in schizophrenia and uremia; there is little information on the mechanism of its disposition. Recently, our laboratory showed that kynurenic acid is a good substrate of human organic anion transporters hOAT1 and hOAT3. In this study, we performed uptake experiment using Xenopus laevis oocytes to characterize the transport of kynurenic acid by rat homologs of the transporters, rOAT1, and rOAT3. These transporters stimulated the uptake of kynurenic acid into oocytes, and transport by rOAT3 was marked. The Km values of the transport were estimated to be 8.46 μM for rOAT1 and 4.81 μM for rOAT3, and these values are comparable to their human homologs. The transport activity of kynurenic acid by rOAT1 was about one quarter of that of p-aminohippurate, although they were at the similar levels in hOAT1. A comparative experiment with hOAT1 was added in this study, showing that uptake amounts of kynurenic acid by hOAT1-expressing oocytes were 4 times greater than rOAT1-expressing oocytes. rOAT3 transported kynurenic acid as efficiently as estrone sulfate; this phenomenon was also observed in hOAT3. In conclusion, transport of kynurenic acid by rOAT1 and rOAT3 was shown. The characteristics of rOAT3 were similar to hOAT3, but low transport activity of kynurenic acid by rOAT1 was exhibited compared with hOAT1.

Assessment of the role of renal organic anion transporters in drug-induced nephrotoxicity

In the present review we have attempted to assess the involvement of the organic anion transporters OAT1, OAT2, OAT3, and OAT4, belonging to the SLC22 family of polyspecific carriers, in drug-induced renal damage in humans. We have focused on drugs with widely recognized nephrotoxic potential, which have previously been reported to interact with OAT family members, and whose underlying pathogenic mechanism suggests the participation of tubular transport. Thus, only compounds generally believed to cause kidney injury either by means of direct tubular toxicity or crystal nephropathy have been considered. For each drug, or class of agents, the evidence for actual transport mediated by individual OATs under in vivo conditions is discussed. We have then examined their role in the context of other carriers present in the renal proximal tubule sharing certain substrates with OATs, as these are critical determinants of the overall contribution of OAT-dependent transport to intracellular accumulation and transepithelial drug secretion, and thus the impact it may have in drug-induced nephrotoxicity.

Renal elimination of organic anions in cholestasis

The disposition of most drugs is highly dependent on specialized transporters. OAT1 and OAT3 are two organic anion transporters expressed in the basolateral membrane of renal proximal tubule cells, identified as contributors to xenobiotic and endogenous organic anion secretion. It is well known that cholestasis may cause renal damage. Impairment of kidney function produces modifications in the renal elimination of drugs. Recent studies have demonstrated that the renal abundance of OAT1 and OAT3 plays an important role in the renal elimination of organic anions in the presence of extrahepatic cholestasis. Time elapsed after obstructive cholestasis has an important impact on the regulation of both types of organic anion transporters. The renal expression of OAT1 and OAT3 should be taken into account in order to improve pharmacotherapeutic efficacy and to prevent drug toxicity during the onset of this hepatic disease.