Estradiol 17 beta-D-glucuronide is a high-affinity substrate for oatp organic anion transporter

Novel testing strategy for prediction of rat biliary excretion of intravenously administered estradiol-17β glucuronide

The aim of the present study was to develop a generic rat physiologically based kinetic (PBK) model that includes a novel testing strategy where active biliary excretion is incorporated using estradiol-17β glucuronide (E217βG) as the model substance. A major challenge was the definition of the scaling factor for the in vitro to in vivo conversion of the PBK-model parameter Vmax. In vitro values for the Vmax and Km for transport of E217βG were found in the literature in four different studies based on experiments with primary rat hepatocytes. The required scaling factor was defined based on fitting the PBK model-based predicted values to reported experimental data on E217βG blood levels and cumulative biliary E217βG excretion. This resulted in a scaling factor of 129 mg protein/g liver. With this scaling factor the PBK model predicted the in vivo data for blood and cumulative biliary E217βG levels with on average of less than 1.8-fold deviation. The study provides a proof of principle on how biliary excretion can be included in a generic PBK model using primary hepatocytes to define the kinetic parameters that describe the biliary excretion.

Ion Mobility Spectrometry and Tandem Mass Spectrometry Analysis of Estradiol Glucuronide Isomers

Estradiol is an estrogenic steroid that can undergo glucuronidation at two different sites, which results in two estradiol glucuronide regioisomers. These isomers can be produced by different enzymes and can have different biological activities before being eliminated from the body. Although there have been previous methods that can distinguish the two isomers, these methods often have long acquisition times or high cost per analysis. In this study, traveling wave ion mobility spectrometry (TWIMS) coupled with mass spectrometry (MS) was employed to separate estradiol glucuronides using alkali metal adduction in positive ion mode, where the sodiated dimer adduct provided adequate separation both in single-component standards and in two-component mixtures. Additionally, in negative ion mode, tandem mass spectrometry (MS/MS) was used to quantitatively determine the relative composition of the two isomers. This was possible due to differences in the energetic requirements for loss of the glucuronic acid, which was characterized by energy-resolved collision-induced dissociation (CID). This work demonstrated that the intensity of the glucuronic acid neutral loss product as compared with the intensity of the intact precursor ion can be used to determine the percentage of each isomer present in a mixture. Overall, TWIMS successfully separated estradiol glucuronide isomers in positive ion mode and MS/MS via CID enables relative quantitation of each isomer in negative ion mode.

Drug transporters in tissues and cells relevant to sexual transmission of HIV: Implications for drug delivery

Efflux and uptake transporters of drugs are key regulators of the pharmacokinetics of many antiretroviral drugs. A growing body of literature has revealed the expression and functionality of multiple transporters in female genital tract (FGT), colorectal tissue, and immune cells. Drug transporters could play a significant role in the efficacy of preventative strategies for HIV-1 acquisition. Pre-exposure prophylaxis (PrEP) is a promising strategy, which utilizes topically (vaginally or rectally), orally or other systemically administered antiretroviral drugs to prevent the sexual transmission of HIV to receptive partners. The drug concentration in the receptive mucosal tissues and target immune cells for HIV is critical for PrEP effectiveness. Hence, there is an emerging interest in utilizing transporter information to explain tissue disposition patterns of PrEP drugs, to interpret inter-individual variability in PrEP drug pharmacokinetics and effectiveness, and to improve tissue drug exposure through modulation of the cervicovaginal, colorectal, or immune cell transporters. In this review, the existing literature on transporter expression, functionality and regulation in the transmission-related tissues and cells is summarized. In addition, the relevance of transporter function for drug delivery and strategies that could exploit transporters for increased drug concentration at target locales is discussed. The overall goal is to facilitate an understanding of drug transporters for PrEP optimization.

Assessment of gadoxetate DCE-MRI as a biomarker of hepatobiliary transporter inhibition

Drug-induced liver injury (DILI) is a clinically important adverse drug reaction, which prevents the development of many otherwise safe and effective new drugs. Currently, there is a lack of sensitive and specific biomarkers that can be used to predict, assess and manage this toxicity. The aim of this work was to evaluate gadoxetate-enhanced MRI as a potential novel biomarker of hepatobiliary transporter inhibition in the rat. Initially, the volume fraction of extracellular space in the liver was determined using gadopentetate to enable an estimation of the gadoxetate concentration in hepatocytes. Using this information, a compartmental model was developed to characterise the pharmacokinetics of hepatic uptake and biliary excretion of gadoxetate. Subsequently, we explored the impact of an investigational hepatobiliary transporter inhibitor on the parameters of the model in vivo in rats. The investigational hepatobiliary transporter inhibitor reduced both the rate of uptake of gadoxetate into the hepatocyte, k1 , and the Michaelis-Menten constant, Vmax , characterising its excretion into bile, whereas KM values for biliary efflux were increased. These effects were dose dependent and correlated with effects on plasma chemistry markers of liver dysfunction, in particular bilirubin and bile acids. These results indicate that gadoxetate-enhanced MRI provides a novel functional biomarker of inhibition of transporter-mediated hepatic uptake and clearance in the rat. Since gadoxetate is used clinically, the technology has the potential to provide a translatable biomarker of drug-induced perturbation of hepatic transporters that may also be useful in humans to explore deleterious functional alterations caused by transporter inhibition.

Molecular and cellular physiology of renal organic cation and anion transport

Organic cations and anions (OCs and OAs, respectively) constitute an extraordinarily diverse array of compounds of physiological, pharmacological, and toxicological importance. Renal secretion of these compounds, which occurs principally along the proximal portion of the nephron, plays a critical role in regulating their plasma concentrations and in clearing the body of potentially toxic xenobiotics agents. The transepithelial transport involves separate entry and exit steps at the basolateral and luminal aspects of renal tubular cells. It is increasingly apparent that basolateral and luminal OC and OA transport reflects the concerted activity of a suite of separate transport processes arranged in parallel in each pole of proximal tubule cells. The cloning of multiple members of several distinct transport families, the subsequent characterization of their activity, and their subcellular localization within distinct regions of the kidney now allows the development of models describing the molecular basis of the renal secretion of OCs and OAs. This review examines recent work on this issue, with particular emphasis on attempts to integrate information concerning the activity of cloned transporters in heterologous expression systems to that observed in studies of physiologically intact renal systems.

Organic cation transporters

Over the last 15 years, a number of transporters that translocate organic cations were characterized functionally and also identified on the molecular level. Organic cations include endogenous compounds such as monoamine neurotransmitters, choline, and coenzymes, but also numerous drugs and xenobiotics. Some of the cloned organic cation transporters accept one main substrate or structurally similar compounds (oligospecific transporters), while others translocate a variety of structurally diverse organic cations (polyspecific transporters). This review provides a survey of cloned organic cation transporters and tentative models that illustrate how different types of organic cation transporters, expressed at specific subcellular sites in hepatocytes and renal proximal tubular cells, are assembled into an integrated functional framework. We briefly describe oligospecific Na(+)- and Cl(-)-dependent monoamine neurotransmitter transporters ( SLC6-family), high-affinity choline transporters ( SLC5-family), and high-affinity thiamine transporters ( SLC19-family), as well as polyspecific transporters that translocate some organic cations next to their preferred, noncationic substrates. The polyspecific cation transporters of the SLC22 family including the subtypes OCT1-3 and OCTN1-2 are presented in detail, covering the current knowledge about distribution, substrate specificity, and recent data on their electrical properties and regulation. Moreover, we discuss artificial and spontaneous mutations of transporters of the SLC22 family that provide novel insight as to the function of specific protein domains. Finally, we discuss the clinical potential of the increasing knowledge about polymorphisms and mutations in polyspecific organic cation transporters.

Organic anion transporter 3 (Slc22a8) is a dicarboxylate exchanger indirectly coupled to the Na+ gradient

Basolateral uptake of organic anions in renal proximal tubule cells is indirectly coupled to the Na(+) gradient through Na(+)-dicarboxylate cotransport and organic anion/dicarboxylate exchange. One member of the organic anion transporter (OAT) family, Oat1, is expressed in the proximal tubule and is an organic anion/dicarboxylate exchanger. However, a second organic anion carrier, Oat3, is also highly expressed in the renal proximal tubule, but its mechanism is unclear. Thus we have assessed Oat3 function in Xenopus laevis oocytes and rat renal cortical slices. Probenecid-sensitive uptake of p-aminohippurate (PAH, an Oat1 and Oat3 substrate) and estrone sulfate (ES, an Oat3 substrate) in rat Oat3-expressing oocytes was significantly trans-stimulated by preloading the oocytes with the dicarboxylate glutarate (GA). GA stimulation of ES transport by oocytes coexpressing rabbit Na(+)-dicarboxylate cotransporter 1 and rat Oat3 was significantly inhibited when the preloading medium contained Li(+) or methylsuccinate (MS) or when Na(+) was absent. All these treatments inhibit the Na(+)-dicarboxylate cotransporter, but not rat Oat3. Li(+), MS, and Na(+) removal had no effect when applied during the ES uptake step, rather than during the GA preloading step. Concentrative ES uptake in rat renal cortical slices was also demonstrated to be probenecid and Na(+) sensitive. Accumulation of ES was stimulated by GA, and this stimulation was completely blocked by probenecid, Li(+), MS, taurocholate, and removal of Na(+). Thus Oat3 functions as an organic anion/dicarboxylate exchanger that couples organic anion uptake indirectly to the Na(+) gradient.

Gender difference in the Oatp1-mediated tubular reabsorption of estradiol 17beta-D-glucuronide in rats

The gender difference in the urinary excretion of estradiol-17beta-glucuronide (E(2)-17betaG) was examined in rats. The urinary clearance of E(2)-17betaG was >250 times lower in male than in female rats. No such major gender difference was observed in its biliary excretion or metabolism in kidney homogenate. Both plasma protein binding and inulin clearance were comparable in male and female rats, suggesting that this gender difference cannot be explained by glomerular filtration. The urinary clearance with respect to the plasma unbound E(2)-17betaG in male rats was <1% of the glomerular filtration rate, indicating its potential reabsorption by the kidney, and this increased to a level comparable with that found in female rats when dibromosulfophthalein was coinfused. A marked increase in E(2)-17betaG urinary excretion was also observed in male rats that had undergone orchidectomy. Testosterone injections given to female rats reduced the urinary excretion to a level comparable with that of control male rats. The concomitant change in the expression of the gene product for organic anion-transporting polypeptide Oatp1, of which E(2)-17betaG is a typical substrate, was found in the kidney membrane fractions after these treatments. These results suggest that urinary E(2)-17betaG excretion is subject to hormonal regulation and that the large gender difference can be explained by regulation in Oatp1-mediated reabsorption.

Molecular aspects of renal anionic drug transport

Multiple organic anion transporters in the proximal tubule of the kidney are involved in the secretion of drugs, toxic compounds, and their metabolites. Many of these compounds are potentially hazardous on accumulation, and it is therefore not surprising that the proximal tubule is also an important target for toxicity. In the past few years, considerable progress has been made in the cloning of these transporters and their functional characterization following heterologous expression. Members of the organic anion transporter (OAT), organic anion transporting polypeptide (OATP), multidrug resistance protein (MRP), sodium-phosphate transporter (NPT), and peptide transporter (PEPT) families have been identified in the kidney. In this review, we summarize our current knowledge on their localization, molecular and functional characteristics, and substrate and inhibitor specificity. A major challenge for the future will be to understand how these transporters work in concert to accomplish the renal secretion of specific anionic substrates.

Comparative inhibitory effects of different compounds on rat oatpl (slc21a1)- and Oatp2 (Slc21a5)-mediated transport

PURPOSE

The purpose of the present study is to examine the selectivity of various inhibitors towards the rat organic anion transporting polypeptides 1 (Oatp1: gene symbol Slc21a1) and 2 (Oatp2: Slc21a5).

METHODS

The inhibitory effects of 20 compounds on the Oatpl-mediated transport of estradiol 17beta-D-glucuronide and on the Oatp2-mediated transport of digoxin were examined in cDNA-transfected LLC-PK1cells.

RESULTS

Among the compounds examined in this study, nonsteroidal anti-inflammatory drugs, deoxycorticosterone. and quinidine preferentially inhibited Oatpl. whereas digoxin, quinine, and rifampicin preferentially inhibited Oatp2 at low concentrations. On the other hand, propionic acid, re-ketoglutarate and p-aminohippurate showed no inhibitory effects on either transporter up to a concentration of 1,000 microM. The Ki values of ibuprofen and quinidine were estimated to be 19 and 13 times lower for Oatpl compared with Oatp2, whereas the values for rifampicin, quinine, and digoxin were 13, 20, and 100< times lower for Oatp2 compared with Oatpl.

CONCLUSIONS

At low concentrations, some of the tested inhibitors exert selective inhibition of either Oatpl- or Oatp2-mediated substrate transport. These selective inhibitors may be used at appropriate concentrations to estimate the maximum contribution of Oatp1 or Oatp2 to the total substrate uptake into rat hepatocytes.

Estrogen metabolism by conjugation

The involvement of estrogens in carcinogenic processes within estrogen-responsive tissues has been recognized for a number of years. Classically, mitogenicity associated with estrogen receptor-mediated cellular events was believed to be the mechanism by which estrogens contributed to carcinogenesis. Recently, the possibility that estrogens might contribute directly to mutagenesis resulting from DNA damage has been investigated. That damage is apparently a result of the formation of catechol estrogens that can be further oxidized to semiquinones and quinones. Those molecules represent reactive oxygen species and electrophilic molecules that can form depurinating DNA adducts, thus having the potential to result in permanent nucleotide mutation. Conjugation of parent estrogens to sulfate and glucuronide moieties; of catechol estrogens to methyl, sulfate, and glucuronide conjugates; and of catechol estrogen quinones to glutathione conjugates all represent potential "detoxification" reactions that may protect the cell from estrogen-mediated mitogenicity and mutagenesis. In this chapter, the biochemistry and molecular genetics of those conjugative reaction pathways are discussed. When applicable, the involvement of specific enzymatic isoforms is presented. Finally, the activity of many of these conjugative biotransformation reactions is subject to large interindividual variation--often due to the presence of common nucleotide polymorphisms within the genes encoding those enzymes. Functionally significant genetic polymorphisms that might contribute to variable conjugation of estrogens and catechol estrogens are also discussed.

Molecular pharmacology of renal organic anion transporters

Renal organic anion transport systems play an important role in the elimination of drugs, toxic compounds, and their metabolites, many of which are potentially harmful to the body. The renal proximal tubule is the primary site of carrier-mediated transport from blood to urine of a wide variety of anionic substrates. Recent studies have shown that organic anion secretion in renal proximal tubule is mediated by distinct sodium-dependent and sodium-independent transport systems. Knowledge of the molecular identity of these transporters and their substrate specificity has increased considerably in the past few years by cloning of various carrier proteins. However, a number of fundamental questions still have to be answered to elucidate the participation of the cloned transporters in the overall tubular secretion of anionic xenobiotics. This review summarizes the latest knowledge on molecular and pharmacological properties of renal organic anion transporters and homologs, with special reference to their nephron and plasma membrane localization, transport characteristics, and substrate and inhibitor specificity. A number of the recently cloned transporters, such as the p-aminohippurate/dicarboxylate exchanger OAT1, the anion/sulfate exchanger SAT1, the peptide transporters PEPT1 and PEPT2, and the nucleoside transporters CNT1 and CNT2, are key proteins in organic anion handling that possess the same characteristics as has been predicted from previous physiological studies. The role of other cloned transporters, such as MRP1, MRP2, OATP1, OAT-K1, and OAT-K2, is still poorly characterized, whereas the only information that is available on the homologs OAT2, OAT3, OATP3, and MRP3-6 is that they are expressed in the kidney, but their localization, not to mention their function, remains to be elucidated.

A novel human organic anion transporting polypeptide localized to the basolateral hepatocyte membrane

We cloned and expressed a new organic anion transporting polypeptide (OATP), termed human OATP2, (OATP-C, LST-1; symbol SLC21A6), involved in the uptake of various lipophilic anions into human liver. The cDNA encoding OATP2 comprised 2073 base pairs, corresponding to a protein of 691 amino acids, which were 44% identical to the known human OATP. An antibody directed against the carboxy terminus localized OATP2 to the basolateral membrane of human hepatocytes. Northern blot analysis indicated a strong expression of OATP2 only in human liver. Transport mediated by recombinant OATP2 and its localization were studied in stably transfected Madin-Darby canine kidney strain II (MDCKII) and HEK293 cells. Confocal microscopy localized recombinant OATP2 protein to the lateral membrane of MDCKII cells. Substrates included 17beta-glucuronosyl estradiol, monoglucuronosyl bilirubin, dehydroepiandrosterone sulfate, and cholyltaurine. 17beta-Glucuronosyl estradiol was a preferred substrate, with a Michaelis-Menten constant value of 8.2 microM; its uptake was Na(+) independent and was inhibited by sulfobromophthalein, with a inhibition constant value of 44 nM. Our results indicate that OATP2 is important for the uptake of organic anions, including bilirubin conjugates and sulfobromophthalein, in human liver.

Isolation of a multispecific organic anion and cardiac glycoside transporter from rat brain

A novel multispecific organic anion transporting polypeptide (oatp2) has been isolated from rat brain. The cloned cDNA contains 3,640 bp. The coding region extends over 1,983 nucleotides, thus encoding a polypeptide of 661 amino acids. Oatp2 is homologous to other members of the oatp gene family of membrane transporters with 12 predicted transmembrane domains, five potential glycosylation, and six potential protein kinase C phosphorylation sites. In functional expression studies in Xenopus laevis oocytes, oatp2 mediated uptake of the bile acids taurocholate (Km approximately 35 microM) and cholate (Km approximately 46 microM), the estrogen conjugates 17beta-estradiol-glucuronide (Km approximately 3 microM) and estrone-3-sulfate (Km approximately 11 microM), and the cardiac gylcosides ouabain (Km approximately 470 microM) and digoxin (Km approximately 0.24 microM). Although most of the tested compounds are common substrates of several oatp-related transporters, high-affinity uptake of digoxin is a unique feature of the newly cloned oatp2. On the basis of Northern blot analysis under high-stringency conditions, oatp2 is highly expressed in brain, liver, and kidney but not in heart, spleen, lung, skeletal muscle, and testes. These results provide further support for the overall significance of oatps as a new family of multispecific organic anion transporters. They indicate that oatp2 may play an especially important role in the brain accumulation and toxicity of digoxin and in the hepatobiliary and renal excretion of cardiac glycosides from the body.

Cloning, in vitro expression, and tissue distribution of a human prostaglandin transporter cDNA(hPGT)

We recently identified a cDNA in the rat that encodes a broadly expressed PG transporter (PGT). Because PGs play diverse and important roles in human health and disease, we cloned human PGT (hPGT) from an adult human kidney cDNA library. A consensus sequence (4.0 kb) derived from several clones, plus 3' polymerase chain reaction amplification, exhibited 74% nucleic acid identity and 82% amino acid identity compared to rat PGT. When transiently expressed in HeLa cells, a full-length clone catalyzed the transport of PGE1, PGE2, PGD2, PGF2alpha, and, to a lesser degree, TXB2. Northern blotting revealed mRNA transcripts of many different sizes in adult human heart, placenta, brain, lung, liver, skeletal muscle, pancreas, kidney, spleen, prostate, ovary, small intestine, and colon. hPGT mRNAs are also strongly expressed in human fetal brain, lung, liver, and kidney. The broad tissue distribution and substrate profile of hPGT suggest a role in the transport and/or metabolic clearance of PGs in diverse human tissues.