Expression of the MRP2 gene-encoded conjugate export pump in human kidney proximal tubules and in renal cell carcinoma

Transport mechanism of human bilirubin transporter ABCC2 tuned by the inter-module regulatory domain

Bilirubin is mainly generated from the breakdown of heme when red blood cells reach the end of their lifespan. Accumulation of bilirubin in human body usually leads to various disorders, including jaundice and liver disease. Bilirubin is conjugated in hepatocytes and excreted to bile duct via the ATP-binding cassette transporter ABCC2, dysfunction of which would lead to Dubin-Johnson syndrome. Here we determine the structures of ABCC2 in the apo, substrate-bound and ATP/ADP-bound forms using the cryo-electron microscopy, exhibiting a full transporter with a regulatory (R) domain inserted between the two half modules. Combined with substrate-stimulated ATPase and transport activity assays, structural analysis enables us to figure out transport cycle of ABCC2 with the R domain adopting various conformations. At the rest state, the R domain binding to the translocation cavity functions as an affinity filter that allows the substrates of high affinity to be transported in priority. Upon substrate binding, the R domain is expelled from the cavity and docks to the lateral of transmembrane domain following ATP hydrolysis. Our findings provide structural insights into a transport mechanism of ABC transporters finely tuned by the R domain.

Regulation of ABC transporters by sex steroids may explain differences in drug resistance between sexes

Drug efficacy is dependent on the pharmacokinetics and pharmacodynamics of therapeutic agents. Tight junctions, detoxification enzymes, and drug transporters, due to their localization on epithelial barriers, modulate the absorption, distribution, and the elimination of a drug. The epithelial barriers which control the pharmacokinetic processes are sex steroid hormone targets, and in this way, sex hormones may also control the drug transport across these barriers. Thus, sex steroids contribute to sex differences in drug resistance and have a relevant impact on the sex-related efficacy of many therapeutic drugs. As a consequence, for the further development and optimization of therapeutic strategies, the sex of the individuals must be taken into consideration. Here, we gather and discuss the evidence about the regulation of ATP-binding cassette transporters by sex steroids, and we also describe the signaling pathways by which sex steroids modulate ATP-binding cassette transporters expression, with a focus in the most important ATP-binding cassette transporters involved in multidrug resistance.

Effects of rifampicin on porphyrin metabolism in healthy volunteers

Pregnane X receptor (PXR) is known to stimulate haem synthesis, but detailed knowledge on the effects of PXR activation on porphyrin metabolism in humans is lacking. We utilized a randomized, crossover, open (blinded laboratory) and placebo-controlled trial with 600-mg rifampicin or placebo dosed for a week to investigate the effects of PXR activation on erythrocyte, plasma, faecal and urine porphyrins. Sixteen healthy volunteers participated on the trial, but the number of volunteers for blood and urine porphyrin analyses was 15 while the number of samples for faecal analyses was 14. Rifampicin increased urine pentaporphyrin concentration 3.7-fold (mean 1.80 ± 0.6 vs. 6.73 ± 4.4 nmol/L, p = 0.003) in comparison with placebo. Urine coproporphyrin I increased 23% (p = 0.036). Faecal protoporphyrin IX decreased (mean 31.6 ± 23.5 vs. 19.2 ± 27.8 nmol/g, p = 0.023). The number of blood erythrocytes was slightly elevated, and plasma bilirubin, catabolic metabolite of haem, was decreased. In conclusion, rifampicin dosing elevated the excretion of certain urinary porphyrin metabolites and decreased faecal protoporphyrin IX excretion. As urine pentaporphyrin and coproporphyrin I are not precursors in haem biosynthesis, increased excretion may serve as a hepatoprotective shunt when haem synthesis or porphyrin levels are increased.

Polymorphic renal transporters and cisplatin's toxicity in urinary bladder cancer patients: current perspectives and future directions

Urinary bladder cancer (UBC) holds a potentially profound social burden and affects over 573,278 new cases annually. The disease's primary risk factors include occupational tobacco smoke exposure and inherited genetic susceptibility. Over the past 30 years, a number of treatment modalities have emerged, including cisplatin, a platinum molecule that has demonstrated effectiveness against UBC. Nevertheless, it has severe dose-limiting side effects, such as nephrotoxicity, among others. Since intracellular accumulation of platinum anticancer drugs is necessary for cytotoxicity, decreased uptake or enhanced efflux are the root causes of platinum resistance and response failure. Evidence suggests that genetic variations in any transporter involved in the entry or efflux of platinum drugs alter their kinetics and, to a significant extent, determine patients' responses to them. This review aims to consolidate and describe the major transporters and their polymorphic variants in relation to cisplatin-induced toxicities and resistance in UBC patients. We concluded that the efflux transporters ABCB1, ABCC2, SLC25A21, ATP7A, and the uptake transporter OCT2, as well as the organic anion uptake transporters OAT1 and OAT2, are linked to cisplatin accumulation, toxicity, and resistance in urinary bladder cancer patients. While suppressing the CTR1 gene's expression reduced cisplatin-induced nephrotoxicity and ototoxicity, inhibiting the expression of the MATE1 and MATE2-K genes has been shown to increase cisplatin's nephrotoxicity and resistance. The roles of ABCC5, ABCA8, ABCC10, ABCB10, ABCG1, ATP7B, ABCG2, and mitochondrial SLC25A10 in platinum-receiving urinary bladder cancer patients should be the subject of further investigation.

Impact of ABCB1, ABCC1, ABCC2, and ABCG2 variants in predicting prognosis and clinical outcomes of north Indian lung cancer patients undergoing platinum-based doublet chemotherapy

BACKGROUND

ABC transporters are membrane proteins expressed in the lungs and are crucial for efflux of various chemotherapeutic agents. Polymorphisms of ABC transporters have a certain impact on the transporter activity because their expression levels may influence the extent and longevity of chemotherapeutic drug outflow, affecting patient outcomes. The present study aimed to assess the impact of ABCB1, ABCC1/2, and ABCG2 gene variants in predicting prognosis and clinical outcomes in lung carcinoma patients.

METHODS

In total, 502 lung cancer patients undergoing platinum-based chemotherapy were recruited in this prospective study. Genotyping of ABCB1 (C¹²³⁶ T, C³⁴³⁵ T, and G²⁶⁷⁷ T/A), ABCC1 (G³¹⁷³ A and G²¹⁶⁸ A), ABCC2 (G⁴⁵⁴⁴ A), and ABCG2 (C⁴²¹ A) polymorphisms in Northern Indian lung carcinoma patients were evaluated using polymerase chain reaction-restriction fragment length polymorphism analysis.

RESULTS

Poor survival outcomes were noted in patients carrying a heterozygous genotype (CT) for the ABCB1 C¹²³⁶ T polymorphism compared to the wild-type genotype (CC) (p = 0.04). The mutant genotype (AA) for ABCC1 G³¹⁷³ A exhibited a lower median survival time compared to the reference genotype (GG) (p = 0.009). Lower survival was observed in individuals carrying a heterozygous genotype (GA) for ABCC2 G⁴⁵⁴⁴ A polymorphism compared to the wild-type genotype (GG) (p = 0.017). Small cell lung cancer patients with the ABCB1 G²⁶⁷⁷ A polymorphism having a heterozygous genotype (GA) showed poor survival compared to the wild-type genotype (GG) (p = 0.03). For ABCC1 G³¹⁷³ A, adenocarcinoma patients having a mutant genotype (AA) had reduced survival compared to the wild-type (GG) genotype (p = 0.03). For ABCB1 C³⁴³⁵ T, individuals carrying a heterozygous (CT) (p = 0.018) and mutant (TT) genotype (p = 0.007) had poor survival compared to the wild-type (CC) genotype in patients treated with pemetrexed and cisplatin. The patients administered cisplatin and irinotecan and having mutant alleles (AA) for the ABCB1 G²⁶⁷⁷ A polymorphism showed a lower survival compared to the individuals carrying wild-type alleles (GG) (p = 0.009).

CONCLUSIONS

Our findings suggest that ABCB1 C¹²³⁶ T, ABCB1 C³⁴³⁵ T, ABCB1 G²⁶⁷⁷ A, ABCC1 G³¹⁷³ A, and ABCC2 G⁴⁵⁴⁴ A are involved in predicting prognosis. Genotyping of the ABC polymorphism is essential for predicting prognosis in lung carcinoma patients.

Effect of Rifampicin on the Pharmacokinetics of Evogliptin in Healthy Volunteers

Purpose

Evogliptin (DA-1229) is a novel, potent, and selective dipeptidyl peptidase 4 (DPP-4) inhibitor for treating type 2 diabetes mellitus. This study investigates the effect of rifampicin on evogliptin pharmacokinetics.

Patients and Methods

An open-label, crossover, one-sequence study was conducted on 12 healthy subjects. Reference baseline pharmacokinetic samples were collected on day 1 after the subjects were administered a single dose of 5 mg evogliptin. After a washout period, the subjects were administered 600 mg rifampicin once daily for 10 days, from days 8 to 17, for full induction of hepatic enzyme activity. On day 17, single doses of evogliptin (5 mg) were administered along with rifampicin (600 mg). The test pharmacokinetic samples were collected with a sampling schedule identical to that used for the reference.

Results

Maximum concentration (C_max) and area under the plasma drug concentration-time curve (AUC_0-96h) of evogliptin with and without co-administration of rifampicin were compared. Reference and test C_max and AUC_0-96h values of evogliptin were 4.70 ng/mL vs 4.86 ng/mL and 153.97 ng∙h/mL vs 58.83 ng∙h/mL, respectively. All adverse events were mild in intensity and considered unrelated to evogliptin administration.

Conclusion

Rifampicin decreased the AUC_0-96h of evogliptin by 61.8% without significantly affecting C_max. The mechanism underlying the decrease in AUC_0-96h is thought to be the induction of cytochrome P450 (CYP), especially 3A, by rifampicin. The adverse events, none of which were serious, were not significantly altered by the concomitant administration of evogliptin and rifampicin. Nevertheless, it would be prudent that evogliptin dosing should be carefully considered when co-administered with CYP3A inducers.

Drug resistance: from bacteria to cancer

The phenomenon of drug resistance has been a hindrance to therapeutic medicine since the late 1940s. There is a plethora of factors and mechanisms contributing to progression of drug resistance. From prokaryotes to complex cancers, drug resistance is a prevailing issue in clinical medicine. Although there are numerous factors causing and influencing the phenomenon of drug resistance, cellular transporters contribute to a noticeable majority. Efflux transporters form a huge family of proteins and are found in a vast number of species spanning from prokaryotes to complex organisms such as humans. During the last couple of decades, various approaches in analyses of biochemistry and pharmacology of transporters have led us to understand much more about drug resistance. In this review, we have discussed the structure, function, potential causes, and mechanisms of multidrug resistance in bacteria as well as cancers.

MRP4 is responsible for the efflux transport of mycophenolic acid β-d glucuronide (MPAG) from hepatocytes to blood

Mycophenolic acid (MPA) has become a cornerstone of immunosuppressive therapy, in particular for transplant patients. In the gastrointestinal tract, the liver and the kidney, MPA is mainly metabolized into phenyl-β-d glucuronide (MPAG). Knowledge about the interactions between MPA/MPAG and membrane transporters is still fragmented. The aim of the present study was to explore these interactions with the basolateral hepatic MRP4 transporter. The inhibition of the MRP4-driven transport by various drugs which can be concomitantly prescribed was also evaluated. In vitro experiments using vesicles overexpressing MRP4 showed an ATP-dependent transport of MPAG driven by MRP4 (Michaelis-Menten constant of 233.9 ± 32.8 µM). MPA was not effluxed by MRP4. MRP4-mediated transport of MPAG was inhibited (from -43% to -84%) by ibuprofen, cefazolin, cefotaxime and micafungin. An in silico approach based on molecular docking and molecular dynamics simulations rationalized the mode of binding of MPAG to MRP4. The presence of the glucuronide moiety in MPAG was highlighted as key, being prone to make electrostatic and H-bond interactions with specific residues of the MRP4 protein chamber. This explains why MPAG is a substrate of MRP4 whereas MPA is not.

Vectorial transport of the arginine derivatives asymmetric dimethylarginine (ADMA) and L-homoarginine by OATP4C1 and P-glycoprotein studied in double-transfected MDCK cells

Elevated plasma concentrations of the uremic toxin asymmetric dimethylarginine (ADMA) and low plasma concentrations of l-homoarginine are independently associated with cardiovascular events and mortality. Key enzymes involved in the homeostasis of both arginine derivatives are expressed in proximal tubule cells of the kidney. To get access to these enzymes, transport proteins are important. One of the transporters mediating the transport of ADMA and l-homoarginine is the solute carrier superfamily (SLC) member OATP4C1, located in the basolateral membrane of proximal tubule cells. To gain insights into the role of export pumps in the transport of both substances, we established a double-transfected MDCK cell line expressing OATP4C1 and the export pump P-glycoprotein (P-gp). Using MDCK cell monolayers, we demonstrated in time-dependent and concentration-dependent vectorial transport experiments that ADMA and l-homoarginine are transported from the basolateral to the apical compartment of MDCK-OATP4C1-P-gp cells with significantly higher transport rates compared to single-transfected MDCK-OATP4C1, MDCK-P-gp and MDCK-VC (control) cells (e.g. transport ratio MDCK-OATP4C1-P-gp/MDCK-VC: for 50 µM ADMA = 2.0-fold, for 50 µM l-homoarginine  = 3.4-fold). These results indicate that both OATP4C1 and P-gp transport the arginine derivatives ADMA and l-homoarginine and are, therefore, important for the homoeostasis of both substances.

Protein Abundance of Clinically Relevant Drug Transporters in The Human Kidneys

Renal drug transporters such as the organic cation transporters (OCTs), organic anion transporters (OATs) and multidrug resistance proteins (MRPs) play an important role in the tubular secretion of many drugs influencing their efficacy and safety. However, only little is known about the distinct protein abundance of these transporters in human kidneys, and about the impact of age and gender as potential factors of inter-subject variability in their expression and function. The aim of this study was to determine the protein abundance of MDR1, MRP1-4, BCRP, OAT1-3, OCT2-3, MATE1, PEPT1/2, and ORCTL2 by liquid chromatography-tandem mass spectrometry-based targeted proteomics in a set of 36 human cortex kidney samples (20 males, 16 females; median age 53 and 55 years, respectively). OAT1 and 3, OCT2 and ORCTL2 were found to be most abundant renal SLC transporters while MDR1, MRP1 and MRP4 were the dominating ABC transporters. Only the expression levels of MDR1 and ORCTL2 were significantly higher abundant in older donors. Moreover, we found several significant correlations between different transporters, which may indicate their functional interplay in renal vectorial transport processes. Our data may contribute to a better understanding of the molecular processes determining renal excretion of drugs.

PBPK Modeling of Coproporphyrin I as an Endogenous Biomarker for Drug Interactions Involving Inhibition of Hepatic OATP1B1 and OATP1B3

The aim of the present study was to establish a physiologically based pharmacokinetic (PBPK) model for coproporphyrin I (CP-I), a biomarker supporting the prediction of drug-drug interactions (DDIs) involving hepatic organic anion transporting polypeptide 1B (OATP1B), using clinical DDI data with an OATP1B inhibitor rifampicin (300 and 600 mg, orally). The in vivo inhibition constants of rifampicin used as initial input parameters for OATP1Bs (K_i,u,OATP1Bs ) and multidrug resistance-associated protein two-mediated biliary excretion were estimated as 0.23 and 0.87 μM, respectively, from previous reports. Sensitivity analysis demonstrated that the K_i,u,OATP1Bs and biosynthesis rate of CP-I affected the magnitude of the interaction. K_i,u,OATP1Bs values optimized by nonlinear least-squares fitting were ~0.5-fold of the initial value. It was determined that the blood concentration-time profiles of four statins were well-predicted using corrected individual K_i,u,OATP1B values (ratio of in vitro K_i,u(statin) /in vitro K_i,u(CP-I) ). In conclusion, PBPK modeling of CP-I supports dynamic prediction of OATP1B-mediated DDIs.

Adefovir dipivoxil induced hypophosphatemic osteomalacia in chronic hepatitis B: a comparative study of Chinese and foreign case series

BACKGROUND

Adefovir dipivoxil (ADV)-induced renal tubular dysfunction and hypophosphatemic osteomalacia (HO) have been given great consideration in the past few years. However, no standard guidance is available due to a lack of powerful evidence from appropriate long-term prospective case-control studies and variations in the definition of renal adverse events. The aim of this study is to clarify clinical features of ADV-related HO in Chinese chronic hepatitis B patients with long-term ADV treatment in Chinese and non-Chinese comparative case series.

METHODS

Retrieval of case reports was based on Pubmed, CNKI, Wan Fang and VIP databases using the key words adefovir dipivoxil, hypophosphatemia, osteomalacia and Fanconi syndrome. We divided patients into Chinese (C group) and Foreign (F group) groups according to their nationality. Comparisons involving demographics, clinical manifestations, tests, treatment and prognosis were conducted between the two groups.

RESULTS

Of the patients screened, 120 Chinese patients were identified in the C group, and 32 non-Chinese patients were identified in the F group. The average age of the C group was younger than that of the F group (51.89 years ±10.96 years versus 56.47 years ±11.36 years, t = - 2.084, P = 0.039). No significant difference was found in gender (male to female, 3.29:1 versus 3:1, χ ²  = 0.039, P = 0.844). Although there was no significant difference in the duration of ADV therapy before ostalgia onset, the C group tended to develop adverse events earlier, by 2-3 years, while the F group developed adverse events at 4-5 years (Z = - 1.517, P = 0.129). Prognosis was good after adjustment of the ADV dose and supplemental administration of phosphate and calcitriol. Time to resolution of tubular dysfunction was commenced at the first month, and Chinese patients were more prone to recover in the first 3 months than non-Chinese patients (91.3% of patients in the C group versus 56.3% in the F group, Z = - 3.013, P = 0.003).

CONCLUSIONS

Sufficient attention is required for middle-aged males before and during exposure to long-term ADV therapy, regardless of nationality. The clinical picture, laboratory and radiograph alterations are important clues for those patients and are usually characterized by polyarthralgia, renal tubular dysfunction and mineralization defects. Implementation of an early renal tubular injury index is recommended for patients with higher risk, which would prevent further renal injury.

Usefulness of the MRP2 promoter to overcome the chemoresistance of gastrointestinal and liver tumors by enhancing the expression of the drug transporter OATP1B1

Tumor response to chemotherapy is often limited by drug export through ABC proteins. To overcome this problem, here we have investigated the usefulness of inducing the expression of the multidrug uptake transporter OATP1B1 under the control of the MRP2 promoter (MRP2pr). Human hepatoma cells (Alexander) were transfected with MRP2pr fragments of different length fused to the firefly luciferase ORF in order to select the shortest fragment with the highest response to dexamethasone, which was subsequently used to generate the chimeric construct MRP2pr-OATP1B1-V5. Hepatoma cells transduced with MRP2pr-OATP1B1-V5 resulted in dexamethasone-sensitive inducible OATP1B1 expression and enhanced selective antitumor response to OATP1B1 substrates (paclitaxel, Bamet-R2 and Bamet-UD2). In human colon cancer cells LS174T/R, used as a model of endogenous chemoresistance due to MRP2 overexpression, MRP2pr-OATP1B1 induced OATP1B1 expression together with chemosensitivity to OATP1B1 substrates. In nude mice, xenografted tumors formed by LS174T/R cells transduced with MRP2pr-OATP1B1 plus treatment with dexamethasone were markedly sensitized to Bamet-UD2. In conclusion, the induced expression of anticancer drug uptake transporters, under the control of promoters of ABC proteins involved in chemoresistance, constitutes an interesting approach to overcome the poor response of cancer to chemotherapy due to reduced drug uptake and/or enhanced drug export.

Disposition and clinical implications of protein-bound uremic toxins

In patients with chronic kidney disease (CKD), adequate renal clearance is compromised, resulting in the accumulation of a plethora of uremic solutes. These uremic retention solutes, also named uremic toxins, are a heterogeneous group of organic compounds with intrinsic biological activities, many of which are too large to be filtered and/or are protein bound. The renal excretion of protein-bound toxins depends largely on active tubular secretion, which shifts the binding and allows for active secretion of the free fraction. To facilitate this process, renal proximal tubule cells are equipped with a range of transporters that co-operate in basolateral uptake and luminal excretion. Many of these transporters have been characterized as mediators of drug disposition, but have recently been recognized for their importance in the proximal renal tubular transport of uremic toxins as well. This also indicates that during uremia, drug disposition may be severely affected as a result of drug–uremic toxin interaction. In addition, CKD patients receive various drugs to treat their complications potentially resulting in drug–drug interactions (DDIs), also for drugs that are non-renally excreted. This review discusses the current knowledge on formation, disposition and removal of protein-bound uremic toxins. Furthermore, implications associated with drug treatment in kidney failure, as well as innovative renal replacement therapies targetting the protein-bound uremic toxins are being discussed. It will become clear that the complex problems associated with uremia warrant a transdisciplinary approach that unites research experts in the area of fundamental biomedical research with their colleagues in clinical nephrology.

Effect of Liver Disease on Hepatic Transporter Expression and Function

Liver disease can alter the disposition of xenobiotics and endogenous substances. Regulatory agencies such as the Food and Drug Administration and the European Medicines Evaluation Agency recommend, if possible, studying the effect of liver disease on drugs under development to guide specific dose recommendations in these patients. Although extensive research has been conducted to characterize the effect of liver disease on drug-metabolizing enzymes, emerging data have implicated that the expression and function of hepatobiliary transport proteins also are altered in liver disease. This review summarizes recent developments in the field, which may have implications for understanding altered disposition, safety, and efficacy of new and existing drugs. A brief review of liver physiology and hepatic transporter localization/function is provided. Then, the expression and function of hepatic transporters in cholestasis, hepatitis C infection, hepatocellular carcinoma, human immunodeficiency virus infection, nonalcoholic fatty liver disease and nonalcoholic steatohepatitis, and primary biliary cirrhosis are reviewed. In the absence of clinical data, nonclinical information in animal models is presented. This review aims to advance the understanding of altered expression and function of hepatic transporters in liver disease and the implications of such changes on drug disposition.

MRP3 as a novel resistance factor for sorafenib in hepatocellular carcinoma

The mechanism of resistance of hepatocellular carcinoma (HCC) to sorafenib is unknown and no useful predictive biomarker for sorafenib treatment has been reported. Accordingly, we established sorafenib-resistant HCC cells and investigated the underlying mechanism of resistance to sorafenib. Sorafenib-resistant cell lines were established from the HCC cell line PLC/PRF5 by cultivation under continuous exposure to increasing concentration of sorafenib. The IC50 values of the 2 resistant clones PLC/PRF5-R1 and PLC-PRF5-R2 were 9.2±0.47 μM (1.8-fold) and 25±5.1 μM (4.6-fold) respectively, which were significantly higher than that of parental PLC/PRF5 cells (5.4±0.17 μM) (p < 0.01 respectively), as determined by MTT assay. Western blot analysis of signal transduction-related proteins showed no significant differences in expression of AKT/pAKT, mTOR/pmTOR, or ERK/pERK between the 2 resistant clones versus parent cells, suggesting no activation of an alternative signal transduction pathway. Likewise, when expression of membrane transporter proteins was determined, there were no significant differences in expression levels of BSEP, MDR1, MRP2, BCRP, MRP4 and OCT1 between resistant clones and parent cells. However, the expression levels of MRP3 in the 2 resistant clones were significantly higher than that of parent cells. When MRP3 gene was knocked down by siRNA in PLC-PRF5-R2 cells, the sensitivity of the cells to sorafenib was restored. In the analysis of gene mutation, there was no mutation in the activation segment of Raf1 kinase in the resistant clones. Our data clearly demonstrate that the efflux transporter MRP3 plays an important role in resistance to sorafenib in HCC cells.

Low heritability in pharmacokinetics of talinolol: a pharmacogenetic twin study on the heritability of the pharmacokinetics of talinolol, a putative probe drug of MDR1 and other membrane transporters

BACKGROUND

Efflux transporters like MDR1 and MRP2 may modulate the pharmacokinetics of about 50 % of all drugs. It is currently unknown how much of the variation in the activities of important drug membrane transporters like MDR1 or MRP2 is determined by genetic or by environmental factors. In this study we assessed the heritability of the pharmacokinetics of talinolol as a putative probe drug for MDR1 and possibly other membrane transporters.

METHODS

Talinolol pharmacokinetics were investigated in a repeated dose study in 42 monozygotic and 13 same-sex dizygotic twin pairs. The oral clearance of talinolol was predefined as the primary parameter. Heritability was analyzed by structural equation modeling and by within- and between-subject variance and talinolol clearance was correlated with polymorphisms in MDR1, MRP2, BCRP, MDR5, OATP1B1, and OCT1.

RESULTS

Talinolol clearance varied approximately ninefold in the studied sample of healthy volunteers. The correlation of clearances between siblings was not significantly different for the monozygotic and dizygotic pairs. All data analyses consistently showed that variation of talinolol pharmacokinetics was mainly determined by environmental effects. Structural equation modeling attributed 53.5 % of the variation of oral clearance to common environmental effects influencing both siblings to the same extent and 46.5 % to unique environmental effects randomly affecting individual subjects. Talinolol pharmacokinetics were significantly dependent on sex, body mass index, total protein consumption, and vegetable consumption.

CONCLUSIONS

The twin study revealed that environmental factors explained much more of the variation in pharmacokinetics of talinolol than genetic factors.

TRIAL REGISTRATION

European clinical trials database number: EUDRA-CT 2008-006223-31. Registered 26 September 2008. ClinicalTrials.gov number: NCT01845194 .

The Effect of Albumin on MRP2 and BCRP in the Vesicular Transport Assay

The ABC transporters multidrug resistance associated protein 2 (MRP2) and breast cancer resistance protein (BCRP) are of interest in drug development, since they affect the pharmacokinetics of several drugs. Membrane vesicle transport assays are widely used to study interactions with these proteins. Since albumin has been found to affect the kinetics of metabolic enzymes in similar membrane preparations, we investigated whether albumin affects the kinetic parameters of efflux transport. We found that albumin increased the Vmax of 5(6)-carboxy-2',7'-dichlorofluorescein (CDCF) and estradiol-17-β-D-glucuronide uptake into MRP2 vesicles in the presence of 0.1% bovine serum albumin (BSA) by 2 and 1.5-fold, respectively, while BSA increased Lucifer yellow uptake by 30% in BCRP vesicles. Km values increased slightly, but the change was not statistically significant. The effect of BSA on substrate uptake was dependent on the vesicle amount, while increasing BSA concentration did not significantly improve substrate uptake. These results indicate a minor effect of albumin on MRP2 and BCRP, but it should be considered if albumin is added to transporter assays for example as a solubilizer, since the effect may be substrate or transporter specific.

Renal drug transporters and their significance in drug-drug interactions

The kidney is a vital organ for the elimination of therapeutic drugs and their metabolites. Renal drug transporters, which are primarily located in the renal proximal tubules, play an important role in tubular secretion and reabsorption of drug molecules in the kidney. Tubular secretion is characterized by high clearance capacities, broad substrate specificities, and distinct charge selectivity for organic cations and anions. In the past two decades, substantial progress has been made in understanding the roles of transporters in drug disposition, efficacy, toxicity and drug-drug interactions (DDIs). In the kidney, several transporters are involved in renal handling of organic cation (OC) and organic anion (OA) drugs. These transporters are increasingly recognized as the target for clinically significant DDIs. This review focuses on the functional characteristics of major human renal drug transporters and their involvement in clinically significant DDIs.

Mechanisms involved in the transport of mercuric ions in target tissues

Mercury exists in the environment in various forms, all of which pose a risk to human health. Despite guidelines regulating the industrial release of mercury into the environment, humans continue to be exposed regularly to various forms of this metal via inhalation or ingestion. Following exposure, mercuric ions are taken up by and accumulate in numerous organs, including brain, intestine, kidney, liver, and placenta. In order to understand the toxicological effects of exposure to mercury, a thorough understanding of the mechanisms that facilitate entry of mercuric ions into target cells must first be obtained. A number of mechanisms for the transport of mercuric ions into target cells and organs have been proposed in recent years. However, the ability of these mechanisms to transport mercuric ions and the regulatory features of these carriers have not been characterized completely. The purpose of this review is to summarize the current findings related to the mechanisms that may be involved in the transport of inorganic and organic forms of mercury in target tissues and organs. This review will describe mechanisms known to be involved in the transport of mercury and will also propose additional mechanisms that may potentially be involved in the transport of mercuric ions into target cells.

The role of cGMP and its signaling pathways in kidney disease

Cyclic nucleotide signal transduction pathways are an emerging research field in kidney disease. Activated cell surface receptors transduce their signals via intracellular second messengers such as cAMP and cGMP. There is increasing evidence that regulation of the cGMP-cGMP-dependent protein kinase 1-phosphodiesterase (cGMP-cGK1-PDE) signaling pathway may be renoprotective. Selective PDE5 inhibitors have shown potential in treating kidney fibrosis in patients with chronic kidney disease (CKD), via their downstream signaling, and these inhibitors also have known activity as antithrombotic and anticancer agents. This review gives an outline of the cGMP-cGK1-PDE signaling pathways and details the downstream signaling and regulatory functions that are modulated by cGK1 and PDE inhibitors with regard to antifibrotic, antithrombotic, and antitumor activity. Current evidence that supports the renoprotective effects of regulating cGMP-cGK1-PDE signaling is also summarized. Finally, the effects of icariin, a natural plant extract with PDE5 inhibitory function, are discussed. We conclude that regulation of cGMP-cGK1-PDE signaling might provide novel, therapeutic strategies for the worsening global public health problem of CKD.

ABCC2 polymorphisms and survival in the Princess Margaret cohort study and the NCIC clinical trials group BR.24 trial of platinum-treated advanced stage non-small cell lung cancer patients

BACKGROUND

The drug transporter ABCC2 is upregulated in non-small cell lung cancer (NSCLC) and implicated in platinum resistance. We evaluated the association between germline polymorphisms in the ABCC2 gene and survival outcomes of platinum-treated advanced NSCLC patients.

MATERIAL AND METHODS

Ten candidate and tagging germline polymorphisms in the ABCC2 gene were genotyped in a discovery cohort of 170 platinum-treated stage IV NSCLC patients from the Princess Margaret Cancer Centre. Associations with overall survival were assessed using multivariate Cox proportional hazard models adjusted for prognostic variables. To validate our results, we analyzed the association of the two top polymorphisms in the ABCC2 gene on survival outcomes of 219 stage IIIB-IV NSCLC patients enrolled on the NCIC Clinical Trials Group BR.24 clinical trial.

RESULTS

Only one polymorphism was validated across both cohorts for an association with overall survival: the A allele of the ABCC2 polymorphism, rs8187710 (4544G>A), was associated with adverse overall survival (adjusted hazard ratio [aHR] 2.22; 95% CI: 1.2-4.0; p=0.009) among our stage IV NSCLC patients. A significant association with overall survival (aHR 1.73; 95% CI: 1.0-2.9; p=0.036) was observed for the same ABCC2 polymorphism in the BR.24 validation cohort. No other ABCC2 polymorphisms were associated with outcome.

CONCLUSION

The ABCC2 polymorphism, rs8187710 (4544G>A), is associated with overall survival in platinum-treated advanced NSCLC patients. Additional studies are needed to evaluate the predictive versus prognostic nature of this relationship, and to explore the functional effect of this polymorphism on the pharmacokinetics of platinum drugs.

Fabrication of Collagen Gel Hollow Fibers by Covalent Cross-Linking for Construction of Bioengineering Renal Tubules

Collagen, the most used natural biomacromolecule, has been extensively utilized to make scaffolds for cell cultures in tissue engineering, but has never been fabricated into the configuration of a hollow fiber (HF) for cell culture due to its poor mechanical properties. In this study, renal tubular cell-laden collagen hollow fiber (Col HF) was fabricated by dissolving sacrificial Ca-alginate cores from collagen shells strengthened by carbodiimide cross-linking. The inner/outer diameters of the Col HF were precisely controlled by the flow rates of core alginate/shell collagen solution in the microfluidic device. As found, the renal tubular cells self-assembled into renal tubules with diameters of 50-200 μm post to the culture in Col HF for 10 days. According to the 3D reconstructed confocal images or HE staining, the renal cells appeared as a tight tubular monolayer on the Col HF inner surface, sustaining more 3D cell morphology than the cell layer on the 2D flat collagen gel surface. Moreover, compared with the cultures in either a Transwell or polymer HF membrane, the renal tubules in Col HF exhibited at least 1-fold higher activity on brush border enzymes of alkaline phosphatase and γ-glutamyltransferase, consistent with their gene expressions. The enhancement occurred similarly on multidrug resistance protein 2 and glucose uptake. Such bioengineered renal tubules in Col HF will present great potential as alternatives to synthetic HF in both clinical use and pharmaceutical investigation.

Toxicological significance of renal Bcrp: Another potential transporter in the elimination of mercuric ions from proximal tubular cells

Secretion of inorganic mercury (Hg(2+)) from proximal tubular cells into the tubular lumen has been shown to involve the multidrug resistance-associated protein 2 (Mrp2). Considering similarities in localization and substrate specificity between Mrp2 and the breast cancer resistance protein (Bcrp), we hypothesize that Bcrp may also play a role in the proximal tubular secretion of mercuric species. In order to test this hypothesis, the uptake of Hg(2+) was examined initially using inside-out membrane vesicles containing Bcrp. The results of these studies suggest that Bcrp may be capable of transporting certain conjugates of Hg(2+). To further characterize the role of Bcrp in the handling of mercuric ions and in the induction of Hg(2+)-induced nephropathy, Sprague-Dawley and Bcrp knockout (bcrp(-/-)) rats were exposed intravenously to a non-nephrotoxic (0.5 μmol · kg(-1)), a moderately nephrotoxic (1.5 μmol · kg(-1)) or a significantly nephrotoxic (2.0 μmol · kg(-1)) dose of HgCl2. In general, the accumulation of Hg(2+) was greater in organs of bcrp(-/-) rats than in Sprague-Dawley rats, suggesting that Bcrp may play a role in the export of Hg(2+) from target cells. Within the kidney, cellular injury and necrosis was more severe in bcrp(-/-) rats than in controls. The pattern of necrosis, which was localized in the inner cortex and the outer stripe of the outer medulla, was significantly different from that observed in Mrp2-deficient animals. These findings suggest that Bcrp may be involved in the cellular export of select mercuric species and that its role in this export may differ from that of Mrp2.

Regulation of the expression of renal drug transporters in KEAP1-knockdown human tubular cells

The kidney secretes various xenobiotics through a well-established transport system. The transcription factor NF-E2-related factor 2 (NRF2) up-regulates a subset of genes encoding antioxidant and detoxification proteins. Kelch-like ECH-associated protein 1 (KEAP1) down-regulates NRF2 by facilitating continuous degradation of NRF2 protein. Here, we investigated the role of NRF2 in the expression of renal drug transporters by using a stable KEAP1 knockdown renal tubular HK-2 cell line (shKEAP1). KEAP1 knockdown resulted in a significant increase in the expression of four renal transporters, namely, multidrug resistance protein 1 (MDR1; ABCB1), breast cancer resistance protein (BCRP; ABCG2), multidrug resistance-associated protein 2 (MRP2; ABCC2), and MRP3 (ABCC3). In western blot and immunocytochemical analyses, protein levels of these transporters were also significantly higher in the knockdown group. Consequently, shKEAP1 cells released more Hoechst 33342 fluorescent dye and doxorubicin, and they were more resistant to doxorubicin than the control cells. In addition, cisplatin resistance of shKEAP1 decreased upon co-incubation with a transporter inhibitor. Whereas, a short term incubation (24h) with sulforaphane did not show noticeable changes in the expression of transporter. Collectively, these results indicate that NRF2 regulates the expression of MDR1, BCRP, MRP2, and MRP3 in human tubular epithelial cells. Altered expression of these transporters affects drug secretion in these cells, which may result in the renal cellular damage upon exposure to nephrotoxic xenobiotics.

Mechanism underlying an elevated serum bile acid level in chronic renal failure patients

INTRODUCTION

Bile acids play an important role in the digestion of dietary lipids. Bile acid metabolism is regulated by the digestive system. The kidney is an important organ of the urinary system and is believed to play a minor role in bile acid excretion; however, many recent studies have reported an increased serum bile acid level and alterations in bile acid homeostasis in both clinical and animal model studies on chronic renal failure. The existing research findings on the mechanisms underlying this phenomenon were mostly derived from animal model studies, but clinical investigations have been limited.

MATERIALS AND METHODS

Kidney tissues and serum and urine samples from CRF patients and normal controls were studied.

RESULTS

We found increased serum bile acid levels and decreased urine bile acid output levels in chronic renal failure patients. Mesangial cell and endothelial cell proliferation, glomerular sclerosis, renal interstitial fibrosis, and intrarenal vascular sclerosis were observed based on hematoxylin-eosin and Masson trichrome staining pathology analysis. Scatter diagram and Pearson correlation analysis showed that in chronic renal failure patients, the estimated glomerular filtration rate and serum bile acid level were interrelated. Reverse transcription polymerase chain reaction and Western blotting results indicated that reabsorption and secretion of bile acid at the apical surface of the proximal renal tubular did not contribute to the elevated serum BA level.

CONCLUSION

The increase in plasma bile acid is due to decreased bile acid filtration through the kidneys in CRF patients.

Amelioration of mercury nephrotoxicity after pharmacological manipulation of organic anion transporter 1 (Oat1) and multidrug resistance-associated protein 2 (Mrp2) with furosemide

Furosemide improves HgCl₂-induced tubule injury up-regulating Oat1 and Mrp2, thus increasing renal elimination of mercuric ions.

Novel Hg2+-induced nephropathy in rats and mice lacking Mrp2: evidence of axial heterogeneity in the handling of Hg2+ along the proximal tubule

The role of the multi-resistance protein 2 (Mrp2) in the nephropathy induced by inorganic mercuric mercury (Hg(2+)) was studied in rats (TR(-)) and mice (Mrp2(-/-)), which lack functional Mrp2, and control animals. Animals were exposed to nephrotoxic doses of HgCl2. Forty-eight or 24 hours after exposure, tissues were harvested and analyzed for Hg content and markers of injury. Histological analyses revealed that the proximal tubular segments affected pathologically by Hg(2+) were significantly different between Mrp2-deficient animals and controls. In the absence of Mrp2, cellular injury localized almost exclusively in proximal tubular segments in the subcapsular (S1) to midcortical regions (early S2) of the kidney. In control animals, cellular death occurred mainly in the proximal tubular segments in the inner cortex (late S2) and outer stripe of the outer medulla (S3). These differences in renal pathology indicate that axial heterogeneity exists along the proximal tubule with respect to how mercuric ions are handled. Total renal and hepatic accumulation of mercury was also greater in animals lacking Mrp2 than in controls, indicating that Mrp2 normally plays a significant role in eliminating mercuric ions from within proximal tubular cells and hepatocytes. Analyses of plasma creatinine, BUN, and renal expression of Kim-1 and Ngal tend to support the severity of the nephropathies detected histologically. Collectively, our findings indicate that a fraction of mercuric ions is normally secreted by Mrp2 in early portions of proximal tubules into the lumen and then is absorbed downstream in straight portions, where mercuric species typically induce toxic effects.

A clearer view of the molecular complexity of clear cell renal cell carcinoma

The von Hippel-Lindau (VHL) tumor suppressor gene is mutated as an early event in almost all cases of clear cell renal cell carcinoma (ccRCC), the most frequent form of kidney cancer. In this review we discuss recent advances in understanding how dysregulation of the many hypoxia-inducible factor α-dependent and -independent functions of the VHL tumor suppressor protein (pVHL) can contribute to tumor initiation and progression. Recent evidence showing extensive inter- and intratumoral genetic diversity has given rise to the idea that ccRCC should actually be considered as a series of molecularly related, yet distinct, diseases defined by the pattern of combinatorial genetic alterations present within the cells of the tumor. We highlight the range of genetic and epigenetic alterations that recur in ccRCC and discuss the mechanisms through which these events appear to function cooperatively with a loss of pVHL function in tumorigenesis.

Importance of ABCC1 for cancer therapy and prognosis

Multidrug resistance presents one of the most important causes of cancer treatment failure. Numerous in vitro and in vivo data have made it clear that multidrug resistance is frequently caused by enhanced expression of ATP-binding cassette (ABC) transporters. ABC transporters are membrane-bound proteins involved in cellular defense mechanisms, namely, in outward transport of xenobiotics and physiological substrates. Their function thus prevents toxicity as carcinogenesis on one hand but may contribute to the resistance of tumor cells to a number of drugs including chemotherapeutics on the other. Within 48 members of the human ABC superfamily there are several multidrug resistance-associated transporters. Due to the well documented susceptibility of numerous drugs to efflux via ABC transporters it is highly desirable to assess the status of ABC transporters for individualization of treatment by their substrates. The multidrug resistance associated protein 1 (MRP1) encoded by ABCC1 gene is one of the most studied ABC transporters. Despite the fact that its structure and functions have already been explored in detail, there are significant gaps in knowledge which preclude clinical applications. Tissue-specific patterns of expression and broad genetic variability make ABCC1/MRP1 an optimal candidate for use as a marker or member of multi-marker panel for prediction of chemotherapy resistance. The purpose of this review was to summarize investigations about associations of gene and protein expression and genetic variability with prognosis and therapy outcome of major cancers. Major advances in the knowledge have been identified and future research directions are highlighted.

The role of canalicular ABC transporters in cholestasis

Cholestasis, a hallmark feature of hepatobiliary disease, is characterized by the retention of biliary constituents. Some of these constituents, such as bile acids, inflict damage to hepatocytes and bile duct cells. This damage may lead to inflammation, fibrosis, cirrhosis, and eventually carcinogenesis, sequelae that aggravate the underlying disease and deteriorate clinical outcome. Canalicular ATP-binding cassette (ABC) transporters, which mediate the excretion of individual bile constituents, play a key role in bile formation and cholestasis. The study of these transporters and their regulatory nuclear receptors has revolutionized our understanding of cholestatic disease. This knowledge has served as a template to develop novel treatment strategies, some of which are currently already undergoing phase III clinical trials. In this review we aim to provide an overview of the structure, function, and regulation of canalicular ABC transporters. In addition, we will focus on the role of these transporters in the pathogenesis and treatment of cholestatic bile duct and liver diseases.

Molecular diagnosis of ATP-binding cassette transporter-related diseases

ATP-binding cassette (ABC) transporters are involved in a variety of physiologic processes such as xenobiotic defense, lipid metabolism, ion homeostasis and immune functions. A large number of ABC proteins have been causatively linked to rare and common human genetic diseases including familial high-density lipoprotein deficiency, retinopathies, cystic fibrosis, diabetes and cardiomyopathies. Furthermore, genetic variations in ABC transporter genes and dysregulated expression patterns of these molecules significantly contribute to drug resistance in human cancer cells and alter the pharmacokinetic properties of a variety of drugs. In order to analyze DNA sequence alterations or define disease-associated mRNA expression patterns of the complete ABC transporter superfamily, novel high-throughput molecular methods such as quantitative real-time PCR and DNA microarray analysis are emerging. The aim of this review is to provide an overview and to present some examples of human ABC transporters involved in monogenic diseases, cancer and pharmacogenetics. Methodologic aspects of molecular diagnostics applied to analyze genetic variations, mRNA and protein expression levels and functional characteristics of ABC transporters are discussed.

Renal transport of organic anions and cations

Organic anions and cations (OAs and OCs, respectively) comprise an extraordinarily diverse array of compounds of physiological, pharmacological, and toxicological importance. The kidney, primarily the renal proximal tubule, plays a critical role in regulating the plasma concentrations of these organic electrolytes and in clearing the body of potentially toxic xenobiotics agents, a process that involves active, transepithelial secretion. This transepithelial transport involves separate entry and exit steps at the basolateral and luminal aspects of renal tubular cells. Basolateral and luminal OA and OC transport reflects the concerted activity of a suite of separate proteins 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 OAs and OCs. New information on naturally occurring genetic variation of many of these processes provides insight into the basis of observed variability of drug efficacy and unwanted drug-drug interactions in human populations. The present review examines recent work on these issues.

Glutathione status and the renal elimination of inorganic mercury in the Mrp2(-/-) mouse

Multidrug resistance-associated proteins (MRP) 2 and 4 are localized in proximal tubular epithelial cells and participate in the renal elimination of xenobiotics. MRP2 has also been implicated in the renal and hepatic elimination of mercury. The current study tested the hypothesis that MRP2 and MRP4 are involved in renal and hepatic handling of inorganic mercury (Hg²⁺). We examined the disposition of Hg²⁺ in Mrp2^−/− mice and assessed the transport of mercuric conjugates in inside-out membrane vesicles containing human MRP4. Since MRP2 has been shown to utilize glutathione (GSH) for transport of select substrates, we examined renal concentrations of GSH and cysteine and the expression of glutamate cysteine ligase (GCL) in Mrp2^−/− and FVB mice. The effect of Hg²⁺ exposure on renal GSH levels was also assessed in these mice. Our data suggest that MRP2, but not MRP4, is involved in proximal tubular export of Hg²⁺. In addition, GSH levels are greater in Mrp2^−/− mice and exposure to Hg²⁺ reduced renal levels of GSH. Expression of GCL was also altered in Mrp2^−/− mice under normal conditions and following exposure to HgCl₂. This study provides important novel data regarding the transport of Hg²⁺ and the effect of Hg²⁺ exposure on GSH levels.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Expression of multidrug resistance-associated protein 2 in human gallbladder carcinoma

Gallbladder carcinoma (GBCA) is one of the most aggressive malignancies. It is usually diagnosed at an advanced stage, and prognosis remains poor despite advances in imaging techniques and aggressive surgical treatment. Overexpression of multidrug resistance-associated proteins (MRPs) in tumor cells is a major cause of the intrinsic multidrug resistance phenotype. Despite the documented importance of MRP expression in many carcinomas, the prognostic significance of MRP2 expression in primary GBCA is not known. Immunostaining for MRP2 was performed on tissue samples obtained from 143 patients with GBCA. We examined the association between MRP expression and clinicopathological characteristics and outcome of patients with GBCA. GBCA demonstrated MRP2 immunoreactivity in the apicolateral membranes of epithelial cells. MRP2 expression was positive in 53.1% (76/143) of GBCA samples. Positive MRP2 expression was significantly associated with the presence of local recurrence (P = 0.038), lymphatic invasion (P = 0.038), vascular invasion (P = 0.023), and perineural invasion (P = 0.006). In addition, the median survival time of patients with MRP2-positive GBCA (15 months) was significantly shorter than that of patients with MRP2-negative GBCA (85 months, P = 0.011). We found that the expression of MRP2 in GBCA contributed to aggressive tumor behavior and poor prognosis, suggesting that MRP2 expression can be used as a potential prognostic biomarker of GBCA.

Transporters and drug-drug interactions: important determinants of drug disposition and effects

Uptake and efflux transporters determine plasma and tissue concentrations of a broad variety of drugs. They are localized in organs such as small intestine, liver, and kidney, which are critical for drug absorption and elimination. Moreover, they can be found in important blood-tissue barriers such as the blood-brain barrier. Inhibition or induction of drug transporters by coadministered drugs can alter pharmacokinetics and pharmacodynamics of the victim drugs. This review will summarize in particular clinically observed drug-drug interactions attributable to inhibition or induction of intestinal export transporters [P-glycoprotein (P-gp), breast cancer resistance protein (BCRP)], to inhibition of hepatic uptake transporters [organic anion transporting polypeptides (OATPs)], or to inhibition of transporter-mediated [organic anion transporters (OATs), organic cation transporter 2 (OCT2), multidrug and toxin extrusion proteins (MATEs), P-gp] renal secretion of xenobiotics. Available data on the impact of nutrition on transport processes as well as genotype-dependent, transporter-mediated drug-drug interactions will be discussed. We will also present and discuss data on the variable extent to which information on the impact of transporters on drug disposition is included in summaries of product characteristics of selected countries (SPCs). Further work is required regarding a better understanding of the role of the drug metabolism-drug transport interplay for drug-drug interactions and on the extrapolation of in vitro findings to the in vivo (human) situation.

Polymorphisms in genes involved in the metabolism and transport of soy isoflavones affect the urinary metabolite profile in premenopausal women following consumption of a commercial soy supplement as a single bolus dose

SCOPE

Genetic variation in relevant enzymes and transporters may contribute to discordant observations concerning health outcomes of dietary isoflavone consumption, so we examined the association of the UGT1A1*28 promoter polymorphism and of other SNPs with isoflavone metabolites in urine.

METHODS AND RESULTS

We genotyped prospectively for polymorphisms in UGT1A1 (UGT1A1*28), LPH (666G>A), CBG (1368T>A), ABCG2 (421C>A), and ABCC2 (1249G>A) to select 100 women (18-50 years) to receive a commercial soy supplement as a single dose and collect all urine over 24 h for analysis by RP-HPLC. We observed large differences in isoflavone recovery (mean 39%, eightfold variation) and metabolites. Glucuronides were the major metabolites (72% of total). UGT1A1*28 was associated only with percentage of glycitein as sulphate (positive; p = 0.046), but excluding five participants with both minor alleles of CBG and ABCG2 uncovered additional associations with percentage of glycitein as glucuronide (negative; p = 0.028), combined isoflavones as sulphate (positive; p = 0.035) and sulphate-to-glucuronide ratio for combined isoflavones (positive; p = 0.036). CBG1368T>A, ABCG2 421C>A, and ABCC2 1249G>A were also associated with differences in isoflavone metabolites in urine.

CONCLUSION

Genetic variation in UGT1A1, CBG, ABCG2, and ABCC2 influences isoflavone metabolism so may affect benefits of dietary consumption.

Generation and characterization of a novel multidrug resistance protein 2 humanized mouse line

The multidrug resistance protein (MRP) 2 is predominantly expressed in liver, intestine, and kidney, where it plays an important role in the excretion of a range of drugs and their metabolites or endogenous compounds into bile, feces, and urine. Mrp knockout [Mrp2(-/-)] mice have been used recently to study the role of MRP2 in drug disposition. Here, we describe the first generation and initial characterization of a mouse line humanized for MRP2 (huMRP2), which is nulled for the mouse Mrp2 gene and expresses the human transporter in the organs and cell types where MRP2 is normally expressed. Analysis of the mRNA expression for selected cytochrome P450 and transporter genes revealed no major changes in huMRP2 mice compared with wild-type controls. We show that human MRP2 is able to compensate functionally for the loss of the mouse transporter as demonstrated by comparable bilirubin levels in the humanized mice and wild-type controls, in contrast to the hyperbilirubinemia phenotype that is observed in MRP2(-/-) mice. The huMRP2 mouse provides a model to study the role of the human transporter in drug disposition and in assessing the in vivo consequences of inhibiting this transporter by compounds interacting with human MRP2.

A new herb-drug interaction of Polygonum cuspidatum, a resveratrol-rich nutraceutical, with carbamazepine in rats

Carbamazepine (CBZ), an antiepileptic with narrow therapeutic window, is a substrate of CYP 3A which metabolizes CBZ to carbamazepine-10,11-epoxide (CBZE), an active metabolite. This study investigated the acute and chronic effects of Polygonum cuspidatum (PC), a resveratrol-rich nutraceutical, on the pharmacokinetics of CBZ in rats and the underlying mechanisms. Rats were orally administered CBZ (200 mg/kg) alone and coadministered with a single dose and the 7th dose of PC (2 g/kg) in a crossover design. The concentrations of CBZ and CBZE in serum and various tissues were determined by HPLC method. The results showed that PC significantly increased the AUC(0-t) of CBZ and CBZE, whereas the formation rate of CBZE was decreased. Tissue analysis showed that the concentrations of CBZ and CBZE in brain, liver and kidney were significantly increased by PC. Cell studies indicated that the efflux function of MRP 2 was inhibited by the serum metabolites of PC. In conclusion, PC markedly increased the systemic exposure and brain concentration of CBZ and CBZE through inhibiting the activities of CYP 3A and MRP 2.

Relationships between the renal handling of DMPS and DMSA and the renal handling of mercury

Within the body of this review, we provide updates on the mechanisms involved in the renal handling mercury (Hg) and the vicinal dithiol complexing/chelating agents, 2,3-bis(sulfanyl)propane-1-sulfonate (known formerly as 2,3-dimercaptopropane-1-sulfonate, DMPS) and meso-2,3-bis(sulfanyl)succinate (known formerly as meso-2,3-dimercaptosuccinate, DMSA), with a focus on the therapeutic effects of these dithiols following exposure to different chemical forms of Hg. We begin by reviewing briefly some of the chemical properties of Hg, with an emphasis on the high bonding affinity between mercuric ions and reduced sulfur atoms, principally those contained in protein and nonprotein thiols. A discussion is provided on the current body of knowledge pertaining to the handling of various mercuric species within the kidneys, focusing on the primary cellular targets that take up and are affected adversely by these species of Hg, namely, proximal tubular epithelial cells. Subsequently, we provide a brief update on the current knowledge on the handling of DMPS and DMSA in the kidneys. In particular, parallels are drawn between the mechanisms participating in the uptake of various thiol S-conjugates of Hg in proximal tubular cells and mechanisms by which DMPS and DMSA gain entry into these target epithelial cells. Finally, we discuss factors that permit DMPS and DMSA to bind intracellular mercuric ions and mechanisms transporting DMPS and DMSA S-conjugates of Hg out of proximal tubular epithelial cells into the luminal compartment of the nephron, and promoting urinary excretion.

Hepatobiliary transport in health and disease

Bile salts, cholesterol and phosphatidylcholine are secreted across the canalicular membrane of hepatocytes into bile by ATP-binding cassette (ABC) transporters. Secretion of bile salts by ABCB11 is essential for bile flow and for absorption of lipids and fat-soluble vitamins. ABCG5 and ABCG8 eliminate excess cholesterol and sterols from the body by secreting them into bile. There are two mechanisms to protect the canalicular membrane from solubilization by bile salts; ABCB4 secretes phosphatidylcholine into bile to form mixed micelles with bile salts, and ATP8B1 maintains the canalicular membrane in a liquid-ordered state. Three different forms of progressive familial intrahepatic cholestasis (PFIC) disorders, PFIC1, PFIC2 and PFIC3, are caused by mutations in ATP8B1, ABCB11 and ABCB4, respectively. Sitosterolemia is caused by mutations in ABCG5 and ABCG8. This article reviews the physiological roles of these canalicular transporters, and the pathophysiological processes and clinical features associated with their mutations.

Fluoroquinolone disposition: identification of the contribution of renal secretory and reabsorptive drug transporters

INTRODUCTION

Fluoroquinolones (FQs) exist as charged molecules in blood and urine making their absorption, distribution, and elimination likely to be influenced by active transport mechanisms. Greater understanding of in vivo FQ clearance mechanisms should help improve the predictability of drug-drug interactions, enhance the clinical safety and efficacy, and aid future novel drug design strategies.

AREAS COVERED

The authors present an overview of FQ development and associated drug-drug interactions, followed by systematic quantitative review of the physicochemical and in vivo pharmacokinetic properties for 15 representative FQs using historical clinical literature. These results were correlated with in vitro studies implicating drug transporters in FQ clearance to link clinical and in vitro evidence supporting the contribution of drug transport mechanisms to FQ disposition. Specific transporters likely to handle FQs in human renal proximal tubule cells are also identified.

EXPERT OPINION

Renal handling, that is, tubular secretion and reabsorption, appears to be the main determinant of FQ plasma half-life, clinical duration of action, and drug-drug interactions. Due to their zwitterionic nature, FQs are likely to interact with organic anion and cation transporters within the solute carrier (SLC) superfamily, including OAT1, OAT3, OCT2, OCTN1, OCTN2, MATE1, and MATE2. The ATP-binding cassette (ABC) transporters MDR1, MRP2, MRP4, and BCRP also may interact with FQs.