Identification and Characterization of Trimethylamine-N-oxide Uptake and Efflux Transporters.

Trimethylamine-N-oxide (TMAO) is a recently identified predictor of cardiovascular and chronic kidney disease. TMAO is primarily generated through gut-microbiome mediated conversion of dietary choline and carnitine to TMA, which is converted to TMAO by hepatic flavin monooxygenase 3 (FMO3) and subsequently undergoes renal elimination. We investigated the role of uptake and efflux drug transporters in TMAO disposition in vitro and in vivo. After screening a large array of uptake transporters, we show organic cation transporter 2 (OCT2) is the key transporter for TMAO cellular uptake. In Oct1/2 knockout mice, we observed increased plasma TMAO levels with reduced renal retention, suggesting the importance of Oct2 in facilitating the uptake of TMAO into renal tubular cells in vivo. Multiple transporters of the ATP-binding cassette (ABC) family, including ABCG2 (BCRP) and ABCB1 (MDR1), were capable of TMAO efflux. In human subjects, clinical, dietary, and pharmacogenetic covariates were evaluated for contribution to TMAO levels in a cohort of dyslipidemic patients (n = 405). Interestingly, genetic variation in ABCG2, but not other transporters, appeared to play a role in modulating TMAO exposure.

Contribution of Organic Anion-Transporting Polypeptides 1A/1B to Doxorubicin Uptake and Clearance.

The organic anion-transporting polypeptides represent an important family of drug uptake transporters that mediate the cellular uptake of a broad range of substrates including numerous drugs. Doxorubicin is a highly efficacious and well-established anthracycline chemotherapeutic agent commonly used in the treatment of a wide range of cancers. Although doxorubicin is a known substrate for efflux transporters such as P-glycoprotein (P-gp; MDR1, ABCB1), significantly less is known regarding its interactions with drug uptake transporters. Here, we investigated the role of organic anion transporting polypeptide (OATP) transporters to the disposition of doxorubicin. A recombinant vaccinia-based method for expressing uptake transporters in HeLa cells revealed that OATP1A2, but not OATP1B1 or OATP1B3, and the rat ortholog Oatp1a4 were capable of significant doxorubicin uptake. Interestingly, transwell assays using Madin-Darby canine kidney II cell line cells stably expressing specific uptake and/or efflux transporters revealed that OATP1B1, OATP1B3, and OATP1A2, either alone or in combination with MDR1, significantly transported doxorubicin. An assessment of polymorphisms in SLCO1A2 revealed that four variants were associated with significantly impaired doxorubicin transport in vitro. In vivo doxorubicin disposition studies revealed that doxorubicin plasma area under the curve was significantly higher (1.7-fold) in Slco1a/1b-/- versus wild-type mice. The liver-to-plasma ratio of doxorubicin was significantly decreased (2.3-fold) in Slco1a/1b2-/- mice and clearance was reduced by 40% compared with wild-type mice, suggesting Oatp1b transporters are important for doxorubicin hepatic uptake. In conclusion, we demonstrate important roles for OATP1A/1B in transporter-mediated uptake and disposition of doxorubicin.

Contribution of hepatic organic anion-transporting polypeptides to docetaxel uptake and clearance.

The antimicrotubular agent docetaxel is a widely used chemotherapeutic drug for the treatment of multiple solid tumors and is predominantly dependent on hepatic disposition. In this study, we evaluated drug uptake transporters capable of transporting radiolabeled docetaxel. By screening an array of drug uptake transporters in HeLa cells using a recombinant vaccinia-based method, five organic anion-transporting polypeptides (OATP) capable of docetaxel uptake were identified: OATP1A2, OATP1B1, OATP1B3, OATP1C1, and Oatp1b2. Kinetic analysis of docetaxel transport revealed similar kinetic parameters among hepatic OATP1B/1b transporters. An assessment of polymorphisms (SNPs) in SLCO1B1 and SLCO1B3 revealed that a number of OATP1B1 and OATP1B3 variants were associated with impaired docetaxel transport. A Transwell-based vectorial transport assay using MDCKII stable cells showed that docetaxel was transported significantly into the apical compartment of double-transfected (MDCKII-OATP1B1/MDR1 and MDCKII-OATP1B3/MDR1) cells compared with single-transfected (MDCKII-OATP1B1 and MDCKII-OATP1B3) cells (P < 0.05) or control (MDCKII-Co) cells (P < 0.001). In vivo docetaxel transport studies in Slco1b2(-/-) mice showed approximately >5.5-fold higher plasma concentrations (P < 0.01) and approximately 3-fold decreased liver-to-plasma ratio (P < 0.05) of docetaxel compared with wild-type (WT) mice. The plasma clearance of docetaxel in Slco1b2(-/-) mice was 83% lower than WT mice (P < 0.05). In conclusion, this study demonstrates the important roles of OATP1B transporters to the hepatic disposition and clearance of docetaxel, and supporting roles of these transporters for docetaxel pharmacokinetics.

An allosteric mechanism for drug block of the human cardiac potassium channel KCNQ1.

The intracellular aspect of the sixth transmembrane segment within the ion-permeating pore is a common binding site for many voltage-gated ion channel blockers. However, the exact site(s) at which drugs bind remain controversial. We used extensive site-directed mutagenesis coupled with molecular modeling to examine mechanisms in drug block of the human cardiac potassium channel KCNQ1. A total of 48 amino acid residues in the S6 segment, S4-S5 linker, and the proximal C-terminus of the KCNQ1 channel were mutated individually to alanine; alanines were mutated to cysteines. Residues modulating drug block were identified when mutant channels displayed <50% block on exposure to drug concentrations that inhibited wild-type current by ≥90%. Homology modeling of the KCNQ1 channel based on the Kv1.2 structure unexpectedly predicted that the key residue modulating drug block (F351) faces away from the permeating pore. In the open-state channel model, F351 lines a pocket that also includes residues L251 and V254 in S4-S5 linker. Docking calculations indicated that this pocket is large enough to accommodate quinidine. To test this hypothesis, L251A and V254A mutants were generated that display a reduced sensitivity to blockage with quinidine. Thus, our data support a model in which open state block of this channel occurs not via binding to a site directly in the pore but rather by a novel allosteric mechanism: drug access to a side pocket generated in the open-state channel configuration and lined by S6 and S4-S5 residues.

Interaction of three regiospecific amino acid residues is required for OATP1B1 gain of OATP1B3 substrate specificity.

The human organic anion-transporting polypeptides OATP1B1 (SLCO1B1) and OATP1B3 (SLCO1B3) are liver-enriched membrane transporters of major importance to hepatic uptake of numerous endogenous compounds, including bile acids, steroid conjugates, hormones, and drugs, including the 3-hydroxy-3-methylglutaryl Co-A reductase inhibitor (statin) family of cholesterol-lowering compounds. Despite their remarkable substrate overlap, there are notable exceptions: in particular, the gastrointestinal peptide hormone cholecystokinin-8 (CCK-8) is a high affinity substrate for OATP1B3 but not OATP1B1. We utilized homologous recombination of linear DNA by E. coli to generate a library of cDNA containing monomer size chimeric OATP1B1-1B3 and OATP1B3-1B1 transporters with randomly distributed chimeric junctions to identify three discrete regions of the transporter involved in conferring CCK-8 transport activity. Site-directed mutagenesis of three key residues in OATP1B1 transmembrane helices 1 and 10, and extracellular loop 6, to the corresponding residues in OATP1B3, resulted in a gain of CCK-8 transport by OATP1B1. The residues appear specific to CCK-8, as the mutations did not affect transport of the shared OATP1B substrate atorvastatin or the OATP1B1-specific substrate estrone sulfate. Regions involved in gain of CCK-8 transport by OATP1B1, when mapped to the crystal structures of bacterial transporters from the major facilitator superfamily, are positioned to suggest these regions could readily interact with drug substrates. Accordingly, our data provide new insight into the molecular determinants of the substrate specificity of these hepatic uptake transporters with relevance to targeted drug design and prediction of drug-drug interactions.

Functional characterization of genetic variants in the apical sodium-dependent bile acid transporter (ASBT; SLC10A2).

BACKGROUND AND AIM: The major transporter responsible for bile acid uptake from the intestinal lumen is the apical sodium-dependent bile acid transporter (ASBT, SLC10A2). Analysis of the SLC10A2 gene has identified a variety of sequence variants including coding region single nucleotide polymorphisms (SNPs) that may influence bile acid homeostasis/intestinal function. In this study, we systematically characterized the effect of coding SNPs on SLC10A2 protein expression and bile acid transport activity. METHODS: Single nucleotide polymorphisms in SLC10A2 from genomic DNA of ethnically-defined healthy individuals were identified using a polymerase chain reaction (PCR)-based temperature gradient capillary electrophoresis (TGCE) system. A heterologous gene expression system was used to assess transport activity of SLC10A2 nonsynonymous variants and missense mutations. Total and cell surface protein expression of wild-type and variant ASBT was assessed by Western blot analysis and immunofluorescence confocal microscopy. Expression of ASBT mRNA and protein was also measured in human intestinal samples. RESULTS: The studies revealed two nonsynonymous SNPs, 292G>A and 431G>A, with partially impaired in vitro taurocholate transport. A novel variant, 790A>G, was also shown to exhibit near complete loss of taurocholate transport, similar to the previously identified ASBT missense mutations. Examination of ASBT protein expression revealed no significant differences in expression or trafficking to the cell surface among variants versus wild-type ASBT. Analysis of ASBT mRNA and protein expression in human intestinal samples revealed modest intersubject variability. CONCLUSIONS: Genome sequencing and in vitro studies reveal the presence of multiple functionally relevant variants in SLC10A2 that may influence bile acid homeostasis and physiology.

Identification of novel functional organic anion-transporting polypeptide 1B3 polymorphisms and assessment of substrate specificity.

OBJECTIVE: The uptake carrier organic anion-transporting polypeptide 1B3 (OATP1B3, gene SLCO1B3) is involved in the hepatic clearance of xenobiotics including statins, taxanes, and mycophenolic acid. We thought to assess the SLCO1B3 coding region for yet unidentified polymorphisms and to analyze their functional relevance. METHODS: We used DNA of ethnically diverse individuals for polymerase chain reaction, and determined polymorphisms by sequencing or temperature-dependent capillary electrophoresis. We then created variant OATP1B3 expression plasmids by site-directed mutagenesis, which were transiently expressed and functionally characterized in human cervical carcinoma (HeLa) cells using radiolabeled substrates. RESULTS: We identified six nonsynonymous polymorphisms including novel variants such as 439A>G (Thr147Ala), 767G>C (Gly256Ala), 1559A>C (His520Pro), and 1679T>C (Val560Ala). Allelic frequencies occurred to be ethnicity-dependent, with the latter observed only in African-Americans (3.6%). After expression in HeLa cells, His520Pro, Val560Ala, and Met233Ile or Met233Ile_Ser112Ala haplotype variants showed decreased uptake activity compared with wild type for cholecystokinin-8 and rosuvastatin, but not for atorvastatin. Kinetic cholecystokinin-8 analysis showed reduced Vmax without altering Km. His520Pro and Val560Ala exhibited decreased total and plasma membrane protein expressions. Val560 mapped onto a structural model of OATP1B3 showed that this is a key region for substrate-transporter interaction. His520 resides in a predicted extracellular region thought to be critical to the pH-dependent component of OATP1B3 activity. Loss of activity at pH 7.4 and 8.0 relative to pH 6.5 was significantly greater for the Pro520 variant. CONCLUSION: OATP1B3 polymorphisms that result in altered expression, substrate specificity, and pH-dependent activity may be of potential relevance to hepatic clearance of substrate drugs in vivo.

Modulation of drug block of the cardiac potassium channel KCNA5 by the drug transporters OCTN1 and MDR1.

BACKGROUND AND PURPOSE: A common site for drug binding on voltage-gated ion channels is at the interior face of the channel pore. In this study, we tested the hypothesis that the extent of drug block of the human cardiac KCNA5 (K(v) 1.5) channel underlying the atrial-specific, ultra-rapidly activating, delayed K(+) current (I(Kur) ) is modulated by the drug uptake and efflux transporters encoded by organic cation transporter 1 (OCTN1) and multiple drug-resistant gene 1 (MDR1) and expressed in human heart. EXPERIMENTAL APPROACH: Drug block of KCNA5 was assessed in Chinese hamster ovary cells transiently transfected with KCNA5 alone or in combination with the OCTN1 or MDR1 transporter construct, as well as in an MDR1 stably expressed cell line. KEY RESULTS: Co-expression of OCTN1 significantly facilitated block by quinidine (10 µM), verapamil (20 µM), propafenone (5 µM) and clofilium (30 µM). Further evidence of drug transport modulating drug block was the finding that with OCTN1, block developed faster and only partially washed-out, and that block potentiation was prevented by cimetidine, an inhibitor of OCTN1. MDR1 expression attenuated KCNA5 block by erythromycin (an MDR1 substrate). Block was restored by reversin-205 (10 µM, an MDR1 inhibitor). MDR1 did not affect KCNA5 inhibition by KN-93 (1 µM), a blocker acting on the outer mouth of the channel pore. CONCLUSIONS AND IMPLICATIONS: The extent of drug block of KCNA5 can be modulated by drug uptake and efflux transporters. These data provide further support for the idea that modifying intracellular drug concentrations could modulate the effects of blocking ion channels in patients.

Polymorphic variants in the human bile salt export pump (BSEP; ABCB11): functional characterization and interindividual variability.

OBJECTIVES: Our aims were to identify and functionally characterize coding region nonsynonymous single nucleotide polymorphisms in the hepatic efflux transporter, bile salt export pump (BSEP; ABCB11), and to assess interindividual variability in BSEP expression. METHODS: We identified 24 single nucleotide polymorphisms, including nine nonsynonymous variants, in ABCB11 from genomic DNA of approximately 250 ethnically diverse healthy individuals using denaturing high-performance liquid chromatography analysis and DNA sequencing. Wild type and variant BSEP were generated and functionally characterized for taurocholate transport activity in vitro in HeLa cells using a recombinant vaccinia-based method. BSEP expression was assessed by real-time mRNA analysis, western blot analysis, and immunofluorescence confocal microscopy. RESULTS: For the most part, polymorphisms were rare and ethnic-dependent. In vitro functional studies revealed several rare variants, including 616A>G, 1674G>C, 1772A>G, and 3556G>A, to be associated with significantly impaired taurocholate transport activity while the 890A>G variant trended towards impaired function but was not statistically significant. The 3556G>A variant was associated with reduced cell surface to total protein expression compared with wild-type BSEP. Expression of BSEP by mRNA and protein analysis was determined from a bank of human liver samples. Wide interindividual variability was noted in both mRNA (19-fold) and protein (31-fold) expression levels. The common variant 1331T>C was associated with significantly reduced hepatic BSEP mRNA levels. CONCLUSION: Accordingly, our study indicates there are functionally relevant polymorphisms in ABCB11 which may be of potential relevance in the predisposition to acquired liver disorders such as drug-induced cholestasis.

Human skeletal muscle drug transporters determine local exposure and toxicity of statins.

RATIONALE: The 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, or statins, are important drugs used in the treatment and prevention of cardiovascular disease. Although statins are well tolerated, many patients develop myopathy manifesting as muscle aches and pain. Rhabdomyolysis is a rare but severe toxicity of statins. Interindividual differences in the activities of hepatic membrane drug transporters and metabolic enzymes are known to influence statin plasma pharmacokinetics and risk for myopathy. Interestingly, little is known regarding the molecular determinants of statin distribution into skeletal muscle and its relevance to toxicity. OBJECTIVE: We sought to identify statin transporters in human skeletal muscle and determine their impact on statin toxicity in vitro. METHODS AND RESULTS: We demonstrate that the uptake transporter OATP2B1 (human organic anion transporting polypeptide 2B1) and the efflux transporters, multidrug resistance-associated protein (MRP)1, MRP4, and MRP5 are expressed on the sarcolemmal membrane of human skeletal muscle fibers and that atorvastatin and rosuvastatin are substrates of these transporters when assessed using a heterologous expression system. In an in vitro model of differentiated, primary human skeletal muscle myoblast cells, we demonstrate basal membrane expression and drug efflux activity of MRP1, which contributes to reducing intracellular statin accumulation. Furthermore, we show that expression of human OATP2B1 in human skeletal muscle myoblast cells by adenoviral vectors increases intracellular accumulation and toxicity of statins and such effects were abrogated when cells overexpressed MRP1. CONCLUSIONS: These results identify key membrane transporters as modulators of skeletal muscle statin exposure and toxicity.

Breast cancer resistance protein (ABCG2) and drug disposition: intestinal expression, polymorphisms and sulfasalazine as an in vivo probe.

Breast cancer resistance protein (BCRP) is an efflux transporter expressed in tissues that act as barriers to drug entry. Given that single nucleotide polymorphisms (SNPs) in the ABCG2 gene encoding BCRP are common, the possibility exists that these genetic variants may be a determinant of interindividual variability in drug response. The objective of this study is to confirm the human BCRP-mediated transport of sulfasalazine in vitro, evaluate interindividual variation in BCRP expression in human intestine and to determine the role of ABCG2 SNPs to drug disposition in healthy patients using sulfasalazine as a novel in vivo probe. To evaluate these objectives, pinch biopsies were obtained from 18 patients undergoing esophagogastro-duodenoscopy or colonoscopy for determination of BCRP expression in relation to genotype. Wild-type and variant BCRP were expressed in a heterologous expression system to evaluate the effect of SNPs on cell-surface trafficking. A total of 17 healthy individuals participated in a clinical investigation to determine the effect of BCRP SNPs on sulfasalazine pharmacokinetics. In vitro, the cell surface protein expression of the common BCRP 421 C>A variant was reduced in comparison with the wild-type control. Intestinal biopsy samples revealed that BCRP protein and mRNA expression did not significantly differ between patients with 34GG/421CC versus patients with 34GG/421CA genotypes. Remarkably, in subjects with 34GG/421CA genotype, sulfasalazine area under the concentration-time curve was 2.4-fold greater compared with 34GG/421CC subjects (P<0.05). This study links commonly occurring SNPs in BCRP with significantly increased oral sulfasalazine plasma exposure in humans. Accordingly, sulfasalazine may prove to have utility as in vivo probe for assessing the clinical impact of BCRP for the disposition and efficacy of drugs.

A common polymorphism in the bile acid receptor farnesoid X receptor is associated with decreased hepatic target gene expression.

The farnesoid X receptor (FXR or NR1H4) is an important bile-acid-activated, transcriptional regulator of genes involved in bile acid, lipid, and glucose homeostasis. Accordingly, interindividual variations in FXR expression and function could manifest as variable susceptibility to conditions such as cholesterol gallstone disease, atherosclerosis, and diabetes. We performed an FXR polymorphism discovery analysis of European-, African-, Chinese-, and Hispanic-Americans and identified two rare gain-of-function variants and a common single nucleotide polymorphism resulting in a G-1T substitution in the nucleotide adjacent to the translation initiation site (FXR*1B) with population allelic frequencies ranging from 2.5 to 12%. In cell-based transactivation assays, FXR*1B (-1T) activity was reduced compared with FXR*1A (-1G). This reduced activity for FXR*1B resulted from neither decreased translational efficiency nor the potential formation of a truncated translational variant. To further define the relevance of this polymorphism, gene expression was examined in a human liver bank to reveal that levels of the FXR target genes small heterodimer partner and organic anion transporting polypeptide 1B3 were significantly reduced in livers harboring an FXR*1B allele. These findings are the first to identify the presence of a common genetic variant in FXR with functional consequences that could contribute to disease risk or therapeutic outcomes.

Ritonavir, saquinavir, and efavirenz, but not nevirapine, inhibit bile acid transport in human and rat hepatocytes.

Human immunodeficiency virus-infected patients on antiretroviral drug therapy frequently experience hepatotoxicity, the underlying mechanism of which is poorly understood. Hepatotoxicity from other compounds such as bosentan and troglitazone has been attributed, in part, to inhibition of hepatocyte bile acid excretion. This work tested the hypothesis that antiretroviral drugs modulate hepatic bile acid transport. Ritonavir (28 microM), saquinavir (15 microM), and efavirenz (32 microM) inhibited [(3)H]taurocholate transport in bile salt export pump expressing Sf9-derived membrane vesicles by 90, 71, and 33%, respectively. In sandwich-cultured human hepatocytes, the biliary excretion index (BEI) of [(3)H]taurocholate was maximally decreased 59% by ritonavir, 39% by saquinavir, and 20% by efavirenz. Likewise, in sandwich-cultured rat hepatocytes, the BEI of [(3)H]taurocholate was decreased 100% by ritonavir and 94% by saquinavir. Sodium-dependent and -independent initial uptake rates of [(3)H]taurocholate in suspended rat hepatocytes were significantly decreased by ritonavir, saquinavir, and efavirenz. [(3)H]Taurocholate transport by recombinant NTCP and Ntcp was inhibited by ritonavir (IC(50) = 2.1 and 6.4 microM in human and rat, respectively), saquinavir (IC(50) = 6.7 and 20 microM, respectively), and efavirenz (IC(50) = 43 and 97 microM, respectively). Nevirapine (75 microM) had no effect on bile acid transport in any model system. In conclusion, ritonavir, saquinavir, and efavirenz, but not nevirapine, inhibited both the hepatic uptake and biliary excretion of taurocholate.

Drug and bile acid transporters in rosuvastatin hepatic uptake: function, expression, and pharmacogenetics.

BACKGROUND & AIMS: The 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors, or statins, target liver HMG-CoA and are of proven benefit in the prevention of coronary heart disease. Rosuvastatin is an effective statin notable for liver selectivity and lack of significant metabolism. We assessed the extent and relevance of hepatic transporters to rosuvastatin uptake. METHODS: Transporters involved in rosuvastatin uptake were determined through heterologous expression of multiple human and rat uptake transporters. Human organic anion transporting polypeptide (OATP) 1B1 and sodium-dependent taurocholate cotransporting polypeptide (NTCP) allelic variants were also assessed. Expression of OATP and NTCP messenger RNA and protein was determined from a bank of human liver samples. RESULTS: Multiple OATP family members, including 1B1, 1B3, 2B1, and 1A2, were capable of rosuvastatin transport. Naturally occurring polymorphisms in OATP1B1, including *5, *9, *15, and *18, were associated with profound loss of activity toward rosuvastatin. Interestingly, the major human hepatic bile acid uptake transporter NTCP, but not rat Ntcp, also transported rosuvastatin. Human hepatocyte studies suggested that NTCP alone accounted for approximately 35% of rosuvastatin uptake. Remarkably, NTCP*2, a variant known to have a near complete loss of function for bile acids, exhibited a profound gain of function for rosuvastatin. Quantitative messenger RNA analysis revealed marked intersubject variability in expression of OATPs and NTCP. CONCLUSIONS: Multiple transporters mediate the overall hepatic uptake of rosuvastatin, and NTCP may be a heretofore unrecognized transporter important to the disposition of rosuvastatin and possibly other drugs/statins in clinical use. Accordingly, transporter expression and polymorphisms may be key determinants of intersubject variability in response to statin therapy in general.

Development and characterization of LLC-PK1 cells containing Sprague-Dawley rat Abcb1a (Mdr1a): comparison of rat P-glycoprotein transport to human and mouse.

INTRODUCTION: P-glycoprotein is localized in numerous tissues throughout the body and plays an important role in the disposition of many xenobiotics. The contribution of P-glycoprotein-mediated drug transport is being evaluated in early drug discovery stages, particularly for compounds targeted to the central nervous system, using in vitro tools including cell lines expressing P-glycoprotein. Previous work in our laboratory suggests there are species differences in P-glycoprotein transport activity between humans and animals. The rat Abcb1a form of P-glycoprotein (formerly known as Mdr1a), the predominate isoform in the brain, has not been described in a functional cell system. Here, we describe the development and characterization of LLC-PK1 cells expressing rat Abcb1. METHODS: We cloned rat Abcb1a and generated a stable LLC-PK1 cell line. Expression and function of the cells were evaluated by immunoblot analysis, cytotoxicity analysis, cellular accumulation assays, and transcellular transport of probe substrates. The transport ratios of structurally diverse compounds obtained from parental cells or cells stably transfected with human ABCB1, mouse Abcb1a or rat Abcb1a were compared. RESULTS: Two forms of rat Abcb1a were cloned from Sprague-Dawley cDNA that differ by six amino acids and a base pair deletion. The intact form was stably transfected in LLC-PK1 cells. Immunoblot analysis demonstrated expression of the protein. The cells demonstrated P-glycoprotein-mediated function by directional transport of dexamethasone, ritonavir, and vinblastine in a transwell assay that was inhibited in the presence of cyclosporin A, verapamil, or quinidine. Likewise, the cells showed reduced cellular accumulation of Rh123 by FACS analysis that was reversed in the presence of cyclosporin A. These cells showed >or=350-fold resistance to colchicine, doxorubicin, vinblastine, and taxol and were sensitized in the presence of verapamil or cyclosporin A. Of 179 chemically diverse compounds evaluated, approximately 20% of the compounds evaluated were predicted to be substrates in one species but not in other species. DISCUSSION: Taken together, these data suggest these cells will be useful for evaluation of rat Abcb1a-mediated transport and for evaluation of species-specific P-glycoprotein-mediated transport.

Hepatic uptake of the novel antifungal agent caspofungin.

Caspofungin (CANCIDAS, a registered trademark of Merck & Co., Inc.) is a novel echinocandin antifungal agent used in the treatment of esophageal and invasive candidiases, invasive aspergillosis, and neutropenia. Available data suggest that the liver is a key organ responsible for caspofungin elimination in rodents and humans. Caspofungin is primarily eliminated by metabolic transformation; however, the rate of metabolism is slow. Accordingly, it was hypothesized that drug uptake transporters expressed on the basolateral domain of hepatocytes could significantly influence the extent of caspofungin uptake and subsequent elimination. In this study, experiments ranging from perfused rat livers to heterologous expression of individual hepatic uptake transporters were utilized to identify the transporter(s) responsible for the observed liver-specific uptake of this compound. Data from perfused rat liver studies were consistent with the presence of carrier-mediated caspofungin hepatic uptake, although this process appeared to be slow. To identify a relevant hepatic uptake transporter, we developed novel Tet-on HeLa cells expressing OATP1B1 (OATP-C, SLC21A6) and OATP1B3 (OATP8, SLC21A8), whose target gene can be overexpressed by the addition of doxycycline. A modest but statistically significant uptake of caspofungin was observed in cells overexpressing OATP1B1, but not OATP1B3. Taken together, these findings suggest that OATP1B1-mediated hepatic uptake may contribute to the overall elimination of this drug from the body.

Polymorphisms in human organic anion-transporting polypeptide 1A2 (OATP1A2): implications for altered drug disposition and central nervous system drug entry.

Organic anion-transporting polypeptide 1A2 (OATP1A2) is a drug uptake transporter known for broad substrate specificity, including many drugs in clinical use. Therefore, genetic variation in SLCO1A2 may have important implications to the disposition and tissue penetration of substrate drugs. In the present study, we demonstrate OATP1A2 protein expression in human brain capillary and renal distal nephron using immunohistochemistry. We also determined the extent of single nucleotide polymorphisms in SLCO1A2 upon analyses of ethnically defined genomic DNA samples (n = 95 each for African-, Chinese-, European-, and Hispanic-Americans). We identified six nonsynonymous polymorphisms within the coding region of SLCO1A2 (T38C (I13T), A516C (E172D), G559A (A187T), A382T (N128Y), A404T (N135I), and C2003G (T668S)), the allelic frequencies of which appeared to be ethnicity-dependent. In vitro functional assessment revealed that the A516C and A404T variants had markedly reduced capacity for mediating the cellular uptake of OATP1A2 substrates, estrone 3-sulfate and two delta-opioid receptor agonists, deltorphin II, and [D-penicillamine(2,5)]-enkephalin. On the other hand, the G559A and C2003G variants appeared to have substrate-dependent changes in transport activity. Cell surface biotinylation and immunofluorescence confocal microscopy suggested that altered plasma membrane expression of the transporter may contribute to reduced transport activity associated with the A516C, A404T, and C2003G variants. The A404T (N135I) variant also showed a shift in the apparent molecular size, indicative of alterations in glycosylation status. Taken together, these data suggest that SLCO1A2 polymorphisms may be an important yet unrecognized contributor to inter-individual variability in drug disposition and central nervous system entry of substrate drugs.

Identification of amino acids in rat pregnane X receptor that determine species-specific activation.

The pregnane X receptor (PXR) is a nuclear receptor significantly involved in the transcriptional regulation of drug-metabolizing enzymes and transporters. Interestingly, certain PXR ligands such as rifampin have been shown to readily induce human and rabbit but not rodent members of the cytochrome P450 3A. Because drugs of divergent chemical structures seem to be similarly affected, we hypothesized that specific amino acid residue(s) or domains in rat PXR affect receptor activation by certain human PXR ligands. To identify such a domain(s), an array of human-rat and rat-human chimeric PXR cDNAs in a tandem head-to-tail configuration were created using a random chimeragenesis method. Pharmacological characterization of these chimeras revealed a discreet segment within the ligand-binding domain of rat and human PXR to be essential for the rifampin effect. Within this region, the corresponding residues Leu308 and Phe305 of human and rat PXR, respectively, were found to be important for rifampin activation. Homology modeling derived from the recently determined crystal structure of human PXR indicates that these amino acids are located within or neighboring the flexible loop that forms part of the pore to the ligand-binding cavity. Rifampin, paclitaxel, and hyperforin sensitivity was conferred to rat PXR when Phe305 was converted to leucine, whereas attenuation of sensitivity was observed when Leu308 of human PXR was replaced with phenylalanine. Accordingly, our data provide compelling new insight into the importance of the amino acids comprising the pore to the ligand-binding cavity as a critical modulator of PXR response.

Ethnicity-dependent polymorphism in Na+-taurocholate cotransporting polypeptide (SLC10A1) reveals a domain critical for bile acid substrate recognition.

The key transporter responsible for hepatic uptake of bile acids from portal circulation is Na+-taurocholate cotransporting polypeptide (NTCP, SLC10A1). This transporter is thought to be critical for the maintenance of enterohepatic recirculation of bile acids and hepatocyte function. Therefore, functionally relevant polymorphisms in this transporter would be predicted to have an important impact on bile acid homeostasis/liver function. However, little is known regarding genetic heterogeneity in NTCP. In this study, we demonstrate the presence of multiple single nucleotide polymorphisms in NTCP in populations of European, African, Chinese, and Hispanic Americans. Specifically four nonsynonymous single nucleotide polymorphisms associated with a significant loss of transport function were identified. Cell surface biotinylation experiments indicated that the altered transport activity of T668C (Ile223-->Thr), a variant seen only in African Americans, was due at least in part to decreased plasma membrane expression. Similar expression patterns were observed when the variant alleles were expressed in HepG2 cells, and plasma membrane expression was assessed using immunofluorescence confocal microscopy. Interestingly the C800T (Ser267-->Phe) variant, seen only in Chinese Americans, exhibited a near complete loss of function for bile acid uptake yet fully normal transport function for the non-bile acid substrate estrone sulfate, suggesting this position may be part of a region in the transporter critical and specific for bile acid substrate recognition. Accordingly, our study indicates functionally important polymorphisms in NTCP exist and that the likelihood of being carriers of such polymorphisms is dependent on ethnicity.