Novel drug transporter substrates identification: An innovative approach based on metabolomic profiling, in silico ligand screening and biological validation

Solute carrier (SLC) transport proteins are fundamental for the translocation of endogenous compounds and drugs across membranes, thus playing a critical role in disease susceptibility and drug response. Because only a limited number of transporter substrates are currently known, the function of a large number of SLC transporters is elusive. Here, we describe the proof-of-concept of a novel strategy to identify SLC transporter substrates exemplarily for the proton-coupled peptide transporter (PEPT) 2 (SLC15A2) and multidrug and toxin extrusion (MATE) 1 transporter (SLC47A1), which are important renal transporters of drug reabsorption and excretion, respectively. By combining metabolomic profiling of mice with genetically-disrupted transporters, in silico ligand screening and in vitro transport studies for experimental validation, we identified nucleobases and nucleoside-derived anticancer and antiviral agents (flucytosine, cytarabine, gemcitabine, capecitabine) as novel drug substrates of the MATE1 transporter. Our data confirms the successful applicability of this new approach for the identification of transporter substrates in general, which may prove particularly relevant in drug research.

Hepatic Expression of the Na+-Taurocholate Cotransporting Polypeptide Is Independent from Genetic Variation

The hepatic Na+-taurocholate cotransporting polypeptide NTCP/SLC10A1 is important for the uptake of bile salts and selected drugs. Its inhibition results in increased systemic bile salt concentrations. NTCP is also the entry receptor for the hepatitis B/D virus. We investigated interindividual hepatic SLC10A1/NTCP expression using various omics technologies. SLC10A1/NTCP mRNA expression/protein abundance was quantified in well-characterized 143 human livers by real-time PCR and LC-MS/MS-based targeted proteomics. Genome-wide SNP arrays and SLC10A1 next-generation sequencing were used for genomic analyses. SLC10A1 DNA methylation was assessed through MALDI-TOF MS. Transcriptomics and untargeted metabolomics (UHPLC-Q-TOF-MS) were correlated to identify NTCP-related metabolic pathways. SLC10A1 mRNA and NTCP protein levels varied 44-fold and 10.4-fold, respectively. Non-genetic factors (e.g., smoking, alcohol consumption) influenced significantly NTCP expression. Genetic variants in SLC10A1 or other genes do not explain expression variability which was validated in livers (n = 50) from The Cancer Genome Atlas. The identified two missense SLC10A1 variants did not impair transport function in transfectants. Specific CpG sites in SLC10A1 as well as single metabolic alterations and pathways (e.g., peroxisomal and bile acid synthesis) were significantly associated with expression. Inter-individual variability of NTCP expression is multifactorial with the contribution of clinical factors, DNA methylation, transcriptional regulation as well as hepatic metabolism, but not genetic variation.

Raman Imaging and Fluorescence Lifetime Imaging Microscopy for Diagnosis of Cancer State and Metabolic Monitoring

Hurdles for effective tumor therapy are delayed detection and limited effectiveness of systemic drug therapies by patient-specific multidrug resistance. Non-invasive bioimaging tools such as fluorescence lifetime imaging microscopy (FLIM) and Raman-microspectroscopy have evolved over the last decade, providing the potential to be translated into clinics for early-stage disease detection, in vitro drug screening, and drug efficacy studies in personalized medicine. Accessing tissue- and cell-specific spectral signatures, Raman microspectroscopy has emerged as a diagnostic tool to identify precancerous lesions, cancer stages, or cell malignancy. In vivo Raman measurements have been enabled by recent technological advances in Raman endoscopy and signal-enhancing setups such as coherent anti-stokes Raman spectroscopy or surface-enhanced Raman spectroscopy. FLIM enables in situ investigations of metabolic processes such as glycolysis, oxidative stress, or mitochondrial activity by using the autofluorescence of co-enzymes NADH and FAD, which are associated with intrinsic proteins as a direct measure of tumor metabolism, cell death stages and drug efficacy. The combination of non-invasive and molecular-sensitive in situ techniques and advanced 3D tumor models such as patient-derived organoids or microtumors allows the recapitulation of tumor physiology and metabolism in vitro and facilitates the screening for patient-individualized drug treatment options.

Two experts and a newbie: [18F]PARPi vs [18F]FTT vs [18F]FPyPARP-a comparison of PARP imaging agents

Purpose

Imaging of PARP expression has emerged as valuable strategy for prediction of tumor malignancy. While [¹⁸F]PARPi and [¹⁸F]FTT are already in clinical translation, both suffer from mainly hepatobiliary clearance hampering their use for detection of abdominal lesions, e.g., liver metastases. Our novel radiotracer [¹⁸F]FPyPARP aims to bridge this gap with a higher renal clearance and an easily translatable synthesis route for potential clinical application.

Methods

We developed a less lipophilic variant of [¹⁸F]PARPi by exchange of the fluorobenzoyl residue with a fluoronicotinoyl group and automated the radiosyntheses of the three radiotracers. We then conducted a comparative side-by-side study of [¹⁸F]PARPi, [¹⁸F]FPyPARP, and [¹⁸F]FTT in NOD.CB17-Prkdc^scid/J mice bearing HCC1937 xenografts to assess xenograft uptake and pharmacokinetics focusing on excretion pathways.

Results

Together with decent uptake of all three radiotracers in the xenografts (tumor-to-blood ratios 3.41 ± 0.83, 3.99 ± 0.99, and 2.46 ± 0.35, respectively, for [¹⁸F]PARPi, [¹⁸F]FPyPARP, and [¹⁸F]FTT), a partial shift from hepatobiliary to renal clearance of [¹⁸F]FPyPARP was observed, whereas [¹⁸F]PARPi and [¹⁸F]FTT show almost exclusive hepatobiliary clearance.

Conclusion

These findings imply that [¹⁸F]FPyPARP is an alternative to [¹⁸F]PARPi and [¹⁸F]FTT for PET imaging of PARP enzymes.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-021-05436-7.

Effects of a Common Eight Base Pairs Duplication at the Exon 7-Intron 7 Junction on Splicing, Expression, and Function of OCT1

Organic cation transporter 1 (OCT1, SLC22A1) is localized in the sinusoidal membrane of human hepatocytes and mediates hepatic uptake of weakly basic or cationic drugs and endogenous compounds. Common amino acid substitutions in OCT1 were associated with altered pharmacokinetics and efficacy of drugs like sumatriptan and fenoterol. Recently, the common splice variant rs35854239 has also been suggested to affect OCT1 function. rs35854239 represents an 8 bp duplication of the donor splice site at the exon 7-intron 7 junction. Here we quantified the extent to which this duplication affects OCT1 splicing and, as a consequence, the expression and the function of OCT1. We used pyrosequencing and deep RNA-sequencing to quantify the effect of rs35854239 on splicing after minigene expression of this variant in HepG2 and Huh7 cells and directly in human liver samples. Further, we analyzed the effects of rs35854239 on OCT1 mRNA expression in total, localization and activity of the resulting OCT1 protein, and on the pharmacokinetics of sumatriptan and fenoterol. The 8 bp duplication caused alternative splicing in 38% (deep RNA-sequencing) to 52% (pyrosequencing) of the minigene transcripts when analyzed in HepG2 and Huh7 cells. The alternatively spliced transcript encodes for a truncated protein that after transient transfection in HEK293 cells was not localized in the plasma membrane and was not able to transport the OCT1 model substrate ASP⁺. In human liver, however, the alternatively spliced OCT1 transcript was detectable only at very low levels (0.3% in heterozygous and 0.6% in homozygous carriers of the 8 bp duplication, deep RNA-sequencing). The 8 bp duplication was associated with a significant reduction of OCT1 expression in the human liver, but explained only 9% of the general variability in OCT1 expression and was not associated with significant changes in the pharmacokinetics of sumatriptan and fenoterol. Therefore, the rs35854239 variant only partially changes splicing, causing moderate changes in OCT1 expression and may be of only limited therapeutic relevance.

Inhibition of organic cation transporter 3 activity by tyrosine kinase inhibitors

Organic cation transporter (OCT) 3 (SLC22A3) is a widely expressed drug transporter, handling notably metformin and platinum derivatives, as well as endogenous compounds like monoamine neurotransmitters. OCT3 has been shown to be inhibited by a few marketed tyrosine kinase inhibitors (TKIs). The present study was designed to determine whether additional TKIs may interact with OCT3. For this purpose, the effects of 25 TKIs toward OCT3 activity were analyzed using OCT3-overexpressing HEK293 cells. 13/25 TKIs, each used at 10 µM, were found to behave as moderate or strong inhibitors of OCT3 activity, that is, they decreased OCT3-mediated uptake of the fluorescent dye 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide by at least 50% or 80%, respectively. This OCT3 inhibition was correlated to some molecular descriptors of TKIs, such as the percentage of H atoms and that of cationic forms at pH = 7.4. It was concentration-dependent, notably for brigatinib, ceritinib, and crizotinib, which exhibited low half maximal inhibitory concentration (IC₅₀ ) values in the 28-106 nM range. Clinical concentrations of these three marketed TKIs, as well as those of pacritinib, were next predicted to inhibit in vivo OCT3 activity according to regulatory criteria. Cellular TKI accumulation experiments as well as trans-stimulation assays, however, demonstrated that OCT3 does not transport brigatinib, ceritinib, crizotinib, and pacritinib, thus discarding any implication of OCT3 in the pharmacokinetics of these TKIs. Taken together, these data suggest that some TKIs may act as potent inhibitors of OCT3 activity, which may have consequences in terms of drug-drug interactions and toxicity.

Characterization of cytochrome P450 (CYP) 2D6 drugs as substrates of human organic cation transporters and multidrug and toxin extrusion proteins

BACKGROUND AND PURPOSE

The metabolic activity of cytochrome P450 (CYP) 2D6 is highly variable and CYP2D6 genotypes insufficiently explain the extensive and intermediate metabolic phenotypes, limiting the prediction of drug response plus adverse drug reactions. Since CYP2D6 prototypic substrates are positively charged, the aim of this study was to evaluate the organic cation transporters (OCTs) and multidrug and toxin extrusion proteins (MATEs) as potential contributors to the variability of CYP2D6 hydroxylation of debrisoquine, dextromethorphan, diphenhydramine, perhexiline and sparteine.

EXPERIMENTAL APPROACH

OCT1/SLC22A1-, OCT2/SLC22A2-, OCT3/SLC22A3-, MATE1/SLC47A1-, and MATE2K/SLC47A2-overexpressing cell lines were used to investigate the transport of the selected drugs. Individuals from a study cohort, well defined with respect to CYP2D6 genotype and sparteine pharmacokinetics, were genotyped for the common OCT1 variants rs12208357 (OCT1-R61C), rs34130495 (OCT1-G401S), rs202220802 (OCT1-Met420del), rs34059508 (OCT1-G465R), OCT2 variant rs316019 (OCT2-A270S) and MATE1 variant rs2289669. Sparteine pharmacokinetics was stratified according to CYP2D6 and OCT1, OCT2 or MATE1 genotype.

KEY RESULTS

OCTs and MATE1 transport sparteine and debrisoquine with high affinity in vitro, but OCT- and MATE1-dependent transport of dextromethorphan, diphenhydramine and perhexiline was not detected. Sparteine and debrisoquine transport depends on OCT1 genotype; however, sparteine pharmacokinetics is independent from OCT1 genotype.

CONCLUSIONS AND IMPLICATIONS

Some drugs that are substrates of CYP2D6 are also substrates of OCTs and MATE1, suggesting overlapping specificities. Variability in sparteine hydroxylation in extensive and intermediate metabolizers cannot be explained by OCT1 genetic variants indicating presence of other factors. Dose-dependent toxicities of dextromethorphan, diphenhydramine and perhexiline appear to be independent from OCTs and MATEs.

Differential in vitro interactions of the Janus kinase inhibitor ruxolitinib with human SLC drug transporters

Interactions of the Janus kinase (JAK) inhibitor ruxolitinib with solute carriers (SLCs) remain incompletely characterised. The present study was therefore designed to investigate this issue.The interactions of ruxolitinib with SLCs were analysed using transporter-overexpressing human embryonic kidney HEK293 cells. Substrate accumulation was detected by spectrofluorimetry, liquid chromatography coupled to tandem mass spectrometry or scintillation counting.Ruxolitinib was found to potently inhibit the activities of organic anion transporter 3 (OAT3), organic cation transporter 2 (OCT2), multidrug and toxin extrusion 1 (MATE1) and MATE2-K (half maximal inhibitory concentration (IC₅₀) < 10 µM). It blocked OAT1, OAT4, OATP1B1, OATP1B3, OATP2B1 and OCT3, but in a weaker manner (IC₅₀ > 10 µM), whereas OCT1 was not impacted. No time-dependent inhibition was highlighted. When applying the US Food and Drug Administration (FDA) criteria for transporters-related drug-drug interaction risk, OCT2 and MATE2-K, unlike MATE1 and OAT3, were predicted to be in vivo inhibited by ruxolitinib. Cellular uptake studies additionally indicated that ruxolitinib is a substrate for MATE1 and MATE2-K, but not for OAT3 and OCT2.Ruxolitinib in vitro blocked activities of most of SLC transporters. Only OCT2 and MATE-2K may be however clinically inhibited by the JAK inhibitor, with the caution for OCT2 that in vitro inhibition data were generated with an FDA-non recommended fluorescent substrate. Ruxolitinib MATEs-mediated transport may additionally deserve attention for its possible pharmacological consequences in MATE-positive cells.

Genetic and Epigenetic Regulation of Organic Cation Transporters

Organic cation transporters (OCTs) of the solute carrier family (SLC) 22 are the subject of intensive research because they mediate the transport of many clinically-relevant drugs such as the antidiabetic agent metformin, the opioid tramadol, and the antimigraine agent sumatriptan. OCT1 (SLC22A1) and OCT2 (SLC22A2) are highly expressed in human liver and kidney, respectively, while OCT3 (SLC22A3) shows a broader tissue distribution. As suggested from studies using knockout mice, particularly OCT2 and OCT3 appear to be of relevance for brain physiological function and drug response. The knowledge of genetic factors and epigenetic modifications affecting function and expression of OCTs is important for a better understanding of disease mechanisms and for personalized treatment of patients. This review briefly summarizes the impact of genetic variants and epigenetic regulation of OCTs in general. A comprehensive overview is given on the consequences of OCT2 and OCT3 knockout in mice and the implications of genetic OCT2 and OCT3 variants on central nervous system function in humans.

Pharmacoresponse in genetic generalized epilepsy: a genome-wide association study

Aim: Pharmacoresistance is a major burden in epilepsy treatment. We aimed to identify genetic biomarkers in response to specific antiepileptic drugs (AEDs) in genetic generalized epilepsies (GGE). Materials & methods: We conducted a genome-wide association study (GWAS) of 3.3 million autosomal SNPs in 893 European subjects with GGE – responsive or nonresponsive to lamotrigine, levetiracetam and valproic acid. Results: Our GWAS of AED response revealed suggestive evidence for association at 29 genomic loci (p <10-5) but no significant association reflecting its limited power. The suggestive associations highlight candidate genes that are implicated in epileptogenesis and neurodevelopment. Conclusion: This first GWAS of AED response in GGE provides a comprehensive reference of SNP associations for hypothesis-driven candidate gene analyses in upcoming pharmacogenetic studies.

Clinically Relevant OATP2B1 Inhibitors in Marketed Drug Space

OATP2B1 is an intestinal and hepatic drug uptake transporter. Intestinal OATP2B1 has been elucidated as the mechanism of unexpected clinical drug-drug interactions (DDIs), where drug exposure was unexpectedly decreased with unchanged half-life. Hepatic OATP2B1 may be an understudied clinical DDI mechanism. The aim of the present work was to understand the prevalence of clinically relevant intestinal and hepatic OATP2B1 inhibitors in marketed drug space. HEK293 cells stably overexpressing human OATP2B1 or vector control were generated and cultured for 72 h in a 96-well format. OATP2B1-mediated uptake of dibromofluorescein (DBF) was found to be optimal at 10 μM concentration and 30 min incubation time. A total of 294 drugs (top 300 marketed drugs, excluding biologics and restricted drugs, supplemented with ∼100 small-molecule drugs) were screened for OATP2B1 inhibition at 10 μM. Drugs demonstrating ≥50% inhibition in this screen were advanced for IC₅₀ determination, which was extrapolated to clinical intestinal and hepatic OATP2B1 inhibition as per 2017 FDA DDI guidance. Of the 294 drugs screened, 67 elicited ≥50% inhibition of OATP2B1-mediated DBF uptake at 10 μM screening concentration. For the 67 drugs flagged in the single-concentration inhibition screen, upon evaluation of a full concentration range, IC₅₀ values could be determined for 58 drugs. OATP2B1 IC₅₀ values established for these 58 drugs were extrapolated as potentially clinically relevant at the intestinal level for 38 orally administered drugs (I_gut/IC₅₀ ≥ 10), and 17 were flagged as potential clinical inhibitors of hepatic OATP2B1 uptake (1 + I_in,max,u/IC₅₀ ≥ 1.1). This analysis of 294 drugs demonstrated prevalence of clinically relevant intestinal and hepatic OATP2B1 inhibitors to be 13 and 6%, respectively. As OATP2B1-inhibitor drugs are not exceedingly rare, these results suggest that clinical OATP2B1 DDIs have been rarely observed because OATP2B1 is uncommonly the predominant determinant of drug disposition.

The Membrane Transporter OAT7 (SLC22A9) Is Not a Susceptibility Factor for Osteoporosis in Europeans

Bone production, maintenance, and modeling are a well-balanced process involving mineralization by osteoblasts and resorption by osteoclasts. Sex steroid hormones, including their conjugated forms, contribute majorly to maintaining this balance. Recently, variants in the SLC22A9 gene have been associated with osteoporosis in Korean females. We had recently shown that SLC22A9, encoding organic anion transporter 7 (OAT7), is an uptake transporter of estrone sulfate and identified several genetic variants in Europeans leading to functional consequences in vitro. We therefore hypothesized that SLC22A9 genetic variants may contribute to the pathophysiology of osteoporosis in Europeans. To test this hypothesis, we examined the associations of SLC22A9 variants with bone quality, fractures, and bone turnover markers. We genotyped SLC22A9 variants in 5,701 (2,930 female) subjects (age range, 20-93 years) extracted from the population-based Study of Health in Pomerania (SHIP and SHIP-TREND) covered by the Illumina Infinium HumanExome BeadChip version v1.0 (Exome Chip). Descriptive data (e.g., history of fractures), ultrasonography of the calcaneus, as well as serum concentrations of carboxy-terminal telopeptide of type I collagen, amino-terminal propeptide of type I procollagen, and vitamin D were determined. Comprehensive statistical analyses revealed no association between low-frequency and rare SLC22A9 variants and bone quality, fractures, and bone turnover markers. Our results indicate that single genetic SLC22A9 variants do not have a major impact on osteoporosis risk prediction in Europeans, yet findings need to be replicated in larger-scale studies.

Sorafenib Activity and Disposition in Liver Cancer Does Not Depend on Organic Cation Transporter 1

Systemic therapy of advanced hepatocellular carcinoma (HCC) with the small-molecule multikinase inhibitor sorafenib is associated with large interindividual pharmacokinetic variability and unpredictable side effects potentially requiring dose reduction or treatment termination. Organic cation transporter (OCT1; gene SLC22A1) has been proposed as a clinical biomarker of HCC response. Because proof is lacking that OCT1 transports sorafenib, we used a combinatorial approach to define how OCT1 contributes to sorafenib transport. Overexpression of functional OCT1 protein in Xenopus laevis oocytes and mammalian cell lines did not facilitate sorafenib transport. Otherwise, sorafenib considerably accumulated in liver cancer cell lines despite negligible OCT1 mRNA and protein levels. Sorafenib pharmacokinetics was independent of OCT1 genotype in mice. Finally, SLC22A1 mRNA expression was significantly reduced by DNA methylation in The Cancer Genome Atlas HCC cohort. These results clearly demonstrate OCT1-independent cellular sorafenib uptake indicating that OCT1 is apparently not a valid biomarker of sorafenib response in HCC.

Characterization of the breast cancer resistance protein (BCRP/ABCG2) in clear cell renal cell carcinoma

The efflux transporter breast cancer resistance protein BCRP/ABCG2 is well-known for its contribution to multi-drug resistance in cancer. Its relevance in cancer biology independent from drug efflux remains largely elusive. Our study aimed at elucidating the biological relevance and regulatory mechanisms of BCRP/ABCG2 in clear cell renal cell carcinoma (ccRCC) and disease progression. Two independent ccRCC-cohorts [Cohort 1 (KIRC/TCGA): n = 453, Cohort 2: n = 64] were investigated to elucidate BCRP/ABCG2 mRNA and protein expression and their association with survival. The impact of BCRP/ABCG2 on response to sunitinib treatment was investigated in two independent sunitinib-treated ccRCC-cohorts based on mRNA levels. Moreover, underlying regulatory mechanisms for interindividual variability of BCRP/ABCG2 expression were systematically assessed. Owing to redundant functional properties, mRNA and protein expression of the multidrug resistance protein MDR1/ABCB1 were additionally evaluated in these cohorts. In independent ccRCC-cohorts, low BCRP/ABCG2 and MDR1/ABCB1 mRNA and protein expression were associated with severity (e.g., tumor stage) of ccRCC and poor cancer-specific survival. BCRP/ABCG2 and MDR1/ABCB1 mRNA expression were linked to decreased progression-free survival after sunitinib treatment. Germline and somatic variants influenced interindividual variability of BCRP/ABCG2 expression only moderately. miR-212-3p and miR-132-3p were identified to regulate BCRP/ABCG2 posttranscriptionally by interaction with the ABCG2 3'UTR as confirmed through reporter gene assays in RCC cell lines. In summary, BCRP/ABCG2 expression in ccRCC underlies considerable interindividual variability with impact on patient survival and response to sunitinib treatment. While germline or somatic genetic variants and DNA methylation cannot explain aberrant BCRP/ABCG2 expression, miR-212-3p and miR-132-3p were identified to contribute to posttranscriptional regulation of BCRP/ABCG2.

Cellular Uptake of the Atypical Antipsychotic Clozapine Is a Carrier-Mediated Process

The weak base antipsychotic clozapine is the most effective medication for treating refractory schizophrenia. The brain-to-plasma concentration of unbound clozapine is greater than unity, indicating transporter-mediated uptake, which has been insufficiently studied. This is important, because it could have a significant impact on clozapine's efficacy, drug-drug interaction, and safety profile. A major limitation of clozapine's use is the risk of clozapine-induced agranulocytosis/granulocytopenia (CIAG), which is a rare but severe hematological adverse drug reaction. We first studied the uptake of clozapine into human brain endothelial cells (hCMEC/D3). Clozapine uptake into cells was consistent with a carrier-mediated process, which was time-dependent and saturable ( V_max = 3299 pmol/million cells/min, K_m = 35.9 μM). The chemical inhibitors lamotrigine, quetiapine, olanzapine, prazosin, verapamil, indatraline, and chlorpromazine reduced the uptake of clozapine by up to 95%. This could in part explain the in vivo interactions observed in rodents or humans for these compounds. An extensive set of studies utilizing transporter-overexpressing cell lines and siRNA-mediated transporter knockdown in hCMEC/D3 cells showed that clozapine was not a substrate of OCT1 (SLC22A1), OCT3 (SLC22A3), OCTN1 (SLC22A4), OCTN2 (SLC22A5), ENT1 (SLC29A1), ENT2 (SLC29A2), and ENT4/PMAT (SLC29A4). In a recent genome-wide analysis, the hepatic uptake transporters SLCO1B1 (OATP1B1) and SLCO1B3 (OATP1B3) were identified as additional candidate transporters. We therefore also investigated clozapine transport into OATP1B-transfected cells and found that clozapine was neither a substrate nor an inhibitor of OATP1B1 and OATP1B3. In summary, we have identified a carrier-mediated process for clozapine uptake into brain, which may be partly responsible for clozapine's high unbound accumulation in the brain and its drug-drug interaction profile. Cellular clozapine uptake is independent from currently known drug transporters, and thus, molecular identification of the clozapine transporter will help to understand clozapine's efficacy and safety profile.

The fruit fly Drosophila melanogaster as an innovative preclinical ADME model for solute carrier membrane transporters, with consequences for pharmacology and drug therapy

Solute carrier membrane transporters (SLCs) control cell exposure to small-molecule drugs, thereby contributing to drug efficacy and failure and/or adverse effects. Moreover, SLCs are genetically linked to various diseases. Hence, in-depth knowledge of SLC function is fundamental for a better understanding of disease pathophysiology and the drug development process. Given that the model organism Drosophila melanogaster (fruit fly) expresses SLCs, such as for the excretion of endogenous and toxic compounds by the hindgut and Malpighian tubules, equivalent to human intestine and kidney, this system appears to be a promising preclinical model to use to study human SLCs. Here, we systematically compare current knowledge of SLCs in Drosophila and humans and describe the Drosophila model as an innovative tool for drug development.

Systemic regulation of bilirubin homeostasis: Potential benefits of hyperbilirubinemia

Neurotoxic bilirubin is the end product of heme catabolism in mammals. Bilirubin is solely conjugated by uridine diphospho-glucuronosyltransferase 1A1, which is a membrane-bound enzyme that catalyzes the transfer of glucuronic acid. Due to low function of hepatic and intestinal uridine diphospho-glucuronosyltransferase 1A1 during the neonatal period, human neonates develop mild to severe physiological hyperbilirubinemia. Accumulation of bilirubin in the brain leads to the onset of irreversible brain damage, termed kernicterus. Breastfeeding is one of the most significant factors that increase the risk of developing kernicterus in infants. Why does this most natural way of feeding increase the risk of brain damage or even death? This question leads to the hypothesis that breast milk-induced hyperbilirubinemia might bring certain benefits that outweigh those risks. While bilirubin is neurotoxic and cytotoxic, this compound is also a potent antioxidant. There are studies showing improved clinical conditions in patients with hyperbilirubinemia. Accumulating evidence also shows that genetic polymorphisms linked to hyperbilirubinemia are beneficial against various diseases. In this review article, we first introduce the production, metabolism, and transport of bilirubin. We then discuss the potential benefits of neonatal and adult hyperbilirubinemia. Finally, epigenetic factors as well as metabolomic information associated with hyperbilirubinemia are described. This review article advances the understanding of the physiological importance of the paradoxical compound bilirubin. (Hepatology 2018;67:1609-1619).

OCTN1 Is a High-Affinity Carrier of Nucleoside Analogues

Resistance to xenobiotic nucleosides used to treat acute myeloid leukemia (AML) and other cancers remains a major obstacle to clinical management. One process suggested to participate in resistance is reduced uptake into tumor cells via nucleoside transporters, although precise mechanisms are not understood. Through transcriptomic profiling, we determined that low expression of the ergothioneine transporter OCTN1 (SLC22A4; ETT) strongly predicts poor event-free survival and overall survival in multiple cohorts of AML patients receiving treatment with the cytidine nucleoside analogue cytarabine. Cell biological studies confirmed OCTN1-mediated transport of cytarabine and various structurally related cytidine analogues, such as 2'deoxycytidine and gemcitabine, occurs through a saturable process that is highly sensitive to inhibition by the classic nucleoside transporter inhibitors dipyridamole and nitrobenzylmercaptopurine ribonucleoside. Our findings have immediate clinical implications given the potential of the identified transport system to help refine strategies that could improve patient survival across multiple cancer types where nucleoside analogues are used in cancer treatment. Cancer Res; 77(8); 2102-11. ©2017 AACR.

Development of Human Membrane Transporters: Drug Disposition and Pharmacogenetics

Membrane transporters play an essential role in the transport of endogenous and exogenous compounds, and consequently they mediate the uptake, distribution, and excretion of many drugs. The clinical relevance of transporters in drug disposition and their effect in adults have been shown in drug–drug interaction and pharmacogenomic studies. Little is known, however, about the ontogeny of human membrane transporters and their roles in pediatric pharmacotherapy. As they are involved in the transport of endogenous substrates, growth and development may be important determinants of their expression and activity. This review presents an overview of our current knowledge on human membrane transporters in pediatric drug disposition and effect. Existing pharmacokinetic and pharmacogenetic data on membrane substrate drugs frequently used in children are presented and related, where possible, to existing ex vivo data, providing a basis for developmental patterns for individual human membrane transporters. As data for individual transporters are currently still scarce, there is a striking information gap regarding the role of human membrane transporters in drug therapy in children.

Impact of Membrane Drug Transporters on Resistance to Small-Molecule Tyrosine Kinase Inhibitors

Small-molecule inhibitors of tyrosine kinases (TKIs) are the mainstay of treatment for many malignancies and represent novel treatment options for other diseases such as idiopathic pulmonary fibrosis. Twenty-five TKIs are currently FDA-approved and >130 are being evaluated in clinical trials. Increasing evidence suggests that drug exposure of TKIs may significantly contribute to drug resistance, independently from somatic variation of TKI target genes. Membrane transport proteins may limit the amount of TKI reaching the target cells. This review highlights current knowledge on the basic and clinical pharmacology of membrane transporters involved in TKI disposition and their contribution to drug efficacy and adverse drug effects. In addition to non-genetic and epigenetic factors, genetic variants, particularly rare ones, in transporter genes are promising novel factors to explain interindividual variability in the response to TKI therapy.

MCT4 surpasses the prognostic relevance of the ancillary protein CD147 in clear cell renal cell carcinoma

Cluster of differentiation 147 (CD147/BSG) is a transmembrane glycoprotein mediating oncogenic processes partly through its role as binding partner for monocarboxylate transporter MCT4/SLC16A3. As demonstrated for MCT4, CD147 is proposed to be associated with progression in clear cell renal cell carcinoma (ccRCC). In this study, we evaluated the prognostic relevance of CD147 in comparison to MCT4/SLC16A3 expression and DNA methylation.

Methylomes of renal cell lines and tumors or metastases differ significantly with impact on pharmacogenes

Current therapies for metastatic clear cell renal cell carcinoma (ccRCC) show limited efficacy. Drug efficacy, typically investigated in preclinical cell line models during drug development, is influenced by pharmacogenes involved in targeting and disposition of drugs. Here we show through genome-wide DNA methylation profiling, that methylation patterns are concordant between primary ccRCC and macro-metastases irrespective of metastatic sites (rs ≥ 0.92). However, 195,038 (41%) of all investigated CpG sites, including sites within pharmacogenes, were differentially methylated (adjusted P < 0.05) in five established RCC cell lines compared to primary tumors, resulting in altered transcriptional expression. Exemplarily, gene-specific analyses of DNA methylation, mRNA and protein expression demonstrate lack of expression of the clinically important drug transporter OCT2 (encoded by SLC22A2) in cell lines due to hypermethylation compared to tumors or metastases. Our findings provide evidence that RCC cell lines are of limited benefit for prediction of drug effects due to epigenetic alterations. Similar epigenetic landscape of ccRCC-metastases and tumors opens new avenue for future therapeutic strategies.

Abstract 257: Evaluation of organic cation transporter 1 (OCT1, SLC22A1) as transporter for sorafenib

The multikinase inhibitor sorafenib is approved for the treatment of advanced clear-cell renal and hepatocellular carcinomas, yet, patients’ survival is prolonged for only several months (Wilhelm et al., Mol Cancer Ther 2008). The understanding of this poor efficacy remains incomplete. One reason for treatment failure may be the high interindividual variability in pharmacokinetics of the orally-given sorafenib. Membrane transporters may contribute to pharmacokinetics of sorafenib and its metabolites as recently reported for the hepatic solute carrier (SLC) organic anion transporter family member 1B1 (OATP1B1) and OATP1B3 (Zimmerman et al, Clin Cancer Res 2013). In contrast, the role of organic cation transporter 1 (OCT1), which is also a major hepatic uptake transporter (Nies et al, Handb Exp Pharmacol 2011), for cellular uptake of sorafenib is controversial (Hu et al, Clin Cancer Res 2009; Swift et al, Drug Metab Dispos 2013; Herraez et al, Hepatology 2013). We therefore sought to comprehensively elucidate the impact of OCT1 on cellular sorafenib uptake by using a combination of different in vitro cell culture models and in vivo approaches involving experiments with Oct1 knockout (-/-) mice. Firstly, we analyzed cellular accumulation of radiolabeled sorafenib by the mammalian cell line HEK293 stably expressing OCT1. Despite overexpression of a functional OCT1 transporter, cellular uptake of neither sorafenib nor its active metabolite sorafenib N-oxide was increased. Next, we studied sorafenib and sorafenib N-oxide pharmacokinetics in mice with a homozygous genetic deletion of the Oct1 transporter. The deficiency of Oct1 had no influence on plasma and hepatic sorafenib and sorafenib N-oxide concentrations. Finally, we investigated sorafenib transport into the human hepatocellular carcinoma cell lines HepG2 and HuH7. Although neither OCT1 mRNA nor protein was detected, a considerable cellular accumulation of sorafenib was observed in both cell lines. Expression profiling of a panel of 55 relevant SLC drug uptake transporters identified several potential novel candidates mediating sorafenib accumulation.

We conclude from these findings that cellular uptake of sorafenib and sorafenib N-oxide is independent of OCT1 and that other, as yet unidentified, transport mechanisms are responsible for cellular sorafenib uptake.

Supported by the Robert Bosch Foundation, Stuttgart, Germany and the ICEPHA Grant Tübingen-Stuttgart, Germany.

Citation Format: Claudia Neul, Sharyn D. Baker, Alex Sparreboom, Elke Schaeffeler, Stefan Laufer, Matthias Schwab, Anne T. Nies. Evaluation of organic cation transporter 1 (OCT1, SLC22A1) as transporter for sorafenib. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 257.

Impact of Genetic Polymorphisms of ABCB1 (MDR1, P-Glycoprotein) on Drug Disposition and Potential Clinical Implications: Update of the Literature

ATP-binding cassette transporter B1 (ABCB1; P-glycoprotein; multidrug resistance protein 1) is an adenosine triphosphate (ATP)-dependent efflux transporter located in the plasma membrane of many different cell types. Numerous structurally unrelated compounds, including drugs and environmental toxins, have been identified as substrates. ABCB1 limits the absorption of xenobiotics from the gut lumen, protects sensitive tissues (e.g. the brain, fetus and testes) from xenobiotics and is involved in biliary and renal secretion of its substrates. In recent years, a large number of polymorphisms of the ABCB1 [ATP-binding cassette, sub-family B (MDR/TAP), member 1] gene have been described. The variants 1236C>T (rs1128503, p.G412G), 2677G>T/A (rs2032582, p.A893S/T) and 3435C>T (rs1045642, p.I1145I) occur at high allele frequencies and create a common haplotype; therefore, they have been most widely studied. This review provides an overview of clinical studies published between 2002 and March 2015. In summary, the effect of ABCB1 variation on P-glycoprotein expression (messenger RNA and protein expression) and/or activity in various tissues (e.g. the liver, gut and heart) appears to be small. Although polymorphisms and haplotypes of ABCB1 have been associated with alterations in drug disposition and drug response, including adverse events with various ABCB1 substrates in different ethnic populations, the results have been majorly conflicting, with limited clinical relevance. Future research activities are warranted, considering a deep-sequencing approach, as well as well-designed clinical studies with appropriate sample sizes to elucidate the impact of rare ABCB1 variants and their potential consequences for effect sizes.

Functional characterization of common protein variants in the efflux transporter ABCC11 and identification of T546M as functionally damaging variant

Multidrug resistance protein 8 (ABCC11) is an efflux transporter for anionic lipophilic compounds, conferring resistance to antiviral and anticancer agents like 5-fluorouracil (5-FU). ABCC11 missense variants may contribute to variability in drug response but functional consequences, except for the 'earwax variant' c.538G>A, are unknown. Using the 'Screen and Insert' technology, we generated human embryonic kidney 293 cells stably expressing ABCC11 missense variants frequently occurring in different ethnic populations: c.57G>A, c.538G>A, c.950C>A, c.1637C>T, c.1942G>A, c.4032A>G. A series of in silico prediction analyses and in vitro plasma membrane vesicle uptake, immunoblotting and immunolocalization experiments were undertaken to investigate functional consequences. We identified c.1637C>T (T546M), previously associated with 5-FU-related toxicity, as a novel functionally damaging ABCC11 variant exhibiting markedly reduced transport function of 5-FdUMP, the active cytotoxic metabolite of 5-FU. Detailed analysis of 14 subpopulations revealed highest allele frequencies of c.1637C>T in Europeans and Americans (up to 11%) compared with Africans and Asians (up to 3%).

Stratified medicine for the use of antidiabetic medication in treatment of type II diabetes and cancer: where do we go from here?

At present, the global diabetes epidemic is affecting 347 million individuals, 90% of whom are diagnosed with type II diabetes mellitus (T2DM). T2DM is commonly treated with more than one type of therapy, including oral antidiabetic drugs (OADs) and agents used in the treatment of diabetic complications. Several pharmacological classes of OADs are currently available for the treatment of T2DM, of which insulin secretagogues (i.e. sulphonylureas and meglitinides), insulin sensitizers [thiazolidinediones (TZDs)] and biguanides are the most commonly prescribed. Although many of these OADs have been used for more than half a century in the treatment of T2DM, the pharmacogenomic characteristics of these compounds have only recently been investigated, primarily in retrospective studies. Recent advances in pharmacogenomics have led to the identification of polymorphisms that affect the expression and function of drug-metabolizing enzymes and drug transporters, as well as drug targets and receptors. These polymorphisms have been shown to affect the therapeutic response to and side effects associated with OADs. The aim of this review was to provide an up-to-date summary of some of the pharmacogenomic data obtained from studies of T2DM treatment, with a focus on polymorphisms in genes affecting pharmacokinetics, pharmacodynamics and treatment outcome of the most commonly prescribed OADs. In addition, the implications of pharmacogenomics in the use of the OAD metformin in cancer will be briefly discussed. Finally, we will focus on recent advances in novel 'omics' technologies and discuss how these might aid in the personalized management of T2DM.

Differential expression of drug uptake and efflux transporters in Japanese patients with hepatocellular carcinoma

Targeted chemotherapy for hepatocellular carcinoma (HCC) is impaired by intrinsic and/or acquired drug resistance. Because drugs used in HCC therapy (e.g., anthracyclines or the tyrosine kinase inhibitor sorafenib) are substrates of uptake and/or efflux transporters, variable expression of these transporters at the plasma membrane of tumor cells may contribute to drug resistance and subsequent clinical response. In this study, the variability of expression of uptake transporters [organic cation transporter (OCT) 1 and OCT3] and efflux transporters [multidrug resistance 1 (MDR1)/P-glycoprotein, multidrug resistance protein (MRP) 1, MRP2, and breast cancer resistance protein (BCRP)], selected for their implication in transporting drugs used in HCC therapy, was investigated. HCC and corresponding nontumor tissue samples were collected from 24 Japanese patients at the time of surgery. Protein expression was determined by immunohistochemistry. Expression data were correlated with clinicopathological characteristics and patients' outcome (median follow-up, 53 months). Generally, expression was highly variable among individual tumor samples. Yet median expression of OCT1, OCT3, and MDR1 in HCC was significantly lower (1.4-, 2.7-, and 2-fold, respectively) than in nontumor tissue, while expression of MRP2 persisted and BCRP showed a trend of increased levels in HCC. Patients with low BCRP expression had significantly shorter overall and recurrence-free survival times. Results suggest different expression patterns of drug transporters in HCC, which are associated only in part with clinicopathological characteristics. Detailed information on expression of drug transporters in HCC may be promising for individualization and optimization of drug therapy for liver cancer.

Genetic biomarkers in epilepsy

The identification of valid biomarkers for outcome prediction of diseases and improvement of drug response, as well as avoidance of side effects is an emerging field of interest in medicine. The concept of individualized therapy is becoming increasingly important in the treatment of patients with epilepsy, as predictive markers for disease prognosis and treatment outcome are still limited. Currently, the clinical decision process for selection of an antiepileptic drug (AED) is predominately based on the patient's epileptic syndrome and side effect profiles of the AEDs, but not on effectiveness data. Although standard dosages of AEDs are used, supplemented, in part, by therapeutic monitoring, the response of an individual patient to a specific AED is generally unpredictable, and the standard care of patients in antiepileptic treatment is more or less based on trial and error. Therefore, there is an urgent need for valid predictive biomarkers to guide patient-tailored individualized treatment strategies in epilepsy, a research area that is still in its infancy. This review focuses on genomic factors as part of an individual concept for AED therapy summarizing examples that influence the prognosis of the disease and the response to AEDs, including side effects.

Solute carrier transporter and drug-related nephrotoxicity: the impact of proximal tubule cell models for preclinical research

INTRODUCTION

The final excretion step of several drugs is facilitated by membrane transporters of the Solute carrier (SLC) family expressed in the proximal tubules of the kidney. Membrane transporters contribute substantially to the pharmacokinetic profile of drugs and play important roles in drug-induced nephrotoxicity. Different cell models have been applied as tools for the assessment of nephrotoxic effects caused by drugs.

AREAS COVERED

This review gives an overview over clinically relevant SLC transporters involved in the renal elimination of drug agents and their specific role in drug-induced nephrotoxicity. Most widely applied cell models are described and their advantages and limitations are outlined.

EXPERT OPINION

In vitro cell culture models (e.g., continuous and primary renal cell lines, polarized cell monolayers) represent valuable tools for early assessment of the nephrotoxic potential of drugs. Since SLC transporters contribute to drug excretion in a large part, in vitro cell culture models might be very helpful to study transport pathways and/or potential drug-drug interactions at an early stage of the drug development process to predict nephrotoxic effects.

Cellular uptake of imatinib into leukemic cells is independent of human organic cation transporter 1 (OCT1)

PURPOSE

In addition to mutated BCR-ABL1 kinase, the organic cation transporter 1 (OCT1, encoded by SLC22A1) has been considered to contribute to imatinib resistance in patients with chronic myeloid leukemia (CML). As data are conflicting as to whether OCT1 transports imatinib and may serve as a clinical biomarker, we used a combination of different approaches including animal experiments to elucidate comprehensively the impact of OCT1 on cellular imatinib uptake.

EXPERIMENTAL DESIGN

Transport of imatinib was studied using OCT1-expressing Xenopus oocytes, mammalian cell lines (HEK293, MDCK, V79) stably expressing OCT1, human leukemic cells, and Oct1-knockout mice. OCT1 mRNA and protein expression were analyzed in leukemic cells from patients with imatinib-naïve CML as well as in cell lines.

RESULTS

Transport and inhibition studies showed that overexpression of functional OCT1 protein in Xenopus oocytes or mammalian cell lines did not lead to an increased cellular accumulation of imatinib. The CML cell lines (K562, Meg-01, LAMA84) and leukemic cells from patients expressed neither OCT1 mRNA nor protein as demonstrated by immunoblotting and immunofluorescence microscopy, yet they showed a considerable imatinib uptake. Oct1 deficiency in mice had no influence on plasma and hepatic imatinib concentrations.

CONCLUSIONS

These data clearly demonstrate that cellular uptake of imatinib is independent of OCT1, and therefore OCT1 is apparently not a valid biomarker for imatinib resistance.

DNA methylation of the SLC16A3 promoter regulates expression of the human lactate transporter MCT4 in renal cancer with consequences for clinical outcome

PURPOSE

The monocarboxylate transporter 4 (MCT4) is a metabolic target in tumor biology because it mediates lactate transport across membranes resulting in antiapoptotic effects. Cell experiments support the importance of MCT4 in clear cell renal cell carcinoma (ccRCC). In this study, we assessed the prognostic potential of MCT4 expression in ccRCC and its epigenetic regulation by DNA methylation as novel predictive marker for patient outcome using independent ccRCC cohorts.

EXPERIMENTAL DESIGN

MCT4 protein expression was quantified in 207 ccRCC and corresponding nontumor tissues. Data of an independent ccRCC cohort from The Cancer Genome Atlas (TCGA) were analyzed on MCT4 mRNA (n = 482) and DNA methylation (n = 283) level. The findings on MCT4 expression and DNA methylation in the SLC16A3 promoter were validated in a third cohort (n = 64). Promoter activity assays were conducted in four RCC cell lines.

RESULTS

MCT4 protein expression was upregulated (P < 0.0001) in ccRCC and showed significant association with cancer-related death. Upregulation of MCT4 mRNA expression (P < 0.00001) was confirmed in the TCGA cohort. Single CpG sites correlated inversely with mRNA expression and were associated with overall survival in Kaplan-Meier analyses [HR = 0.39; 95% confidence interval (CI), 0.24-0.64; P[log-rank] = 1.23e(-04)]. Promoter activity studies confirmed MCT4 regulation by DNA methylation. The significant correlation between MCT4 protein and gene expression or DNA methylation at single CpG sites was validated in a third cohort. Again, higher methylation at individual CpG sites was associated with prolonged survival [HR = 0.05; 95% CI, 0.01-0.40; P[log-rank] = 6.91e(-05)].

CONCLUSION

We identified SLC16A3 promoter DNA methylation as a novel epigenetic mechanism for MCT4 regulation in ccRCC with first evidence of a biological rationale for prognosis and clinical outcome.

Genetics is a major determinant of expression of the human hepatic uptake transporter OATP1B1, but not of OATP1B3 and OATP2B1

Background

Organic anion transporting polypeptide (OATP) 1B1, OATP1B3, and OATP2B1 (encoded by SLCO1B1, SLCO1B3, SLCO2B1) mediate the hepatic uptake of endogenous compounds like bile acids and of drugs, for example, the lipid-lowering atorvastatin, thereby influencing hepatobiliary elimination. Here we systematically elucidated the contribution of SLCO variants on expression of the three hepatic OATPs under consideration of additional important covariates.

Methods

Expression was quantified by RT-PCR and immunoblotting in 143 Caucasian liver samples. A total of 109 rare and common variants in the SLCO1B3-SLCO1B1 genomic region and the SLCO2B1 gene were genotyped by MALDI-TOF mass spectrometry and genome-wide SNP microarray technology. SLCO1B1 haplotypes affecting hepatic OATP1B1 expression were associated with pharmacokinetic data of the OATP1B1 substrate atorvastatin (n = 82).

Results

Expression of OATP1B1, OATP1B3, and OATP2B1 at the mRNA and protein levels showed marked interindividual variability. All three OATPs were expressed in a coordinated fashion. By a multivariate regression analysis adjusted for non-genetic and transcription covariates, increased OATP1B1 expression was associated with the coding SLCO1B1 variant c.388A > G (rs2306283) even after correction for multiple testing (P = 0.00034). This held true for haplotypes harboring c.388A > G but not the functional variant c.521T > C (rs4149056) associated with statin-related myopathy. c.388A > G also significantly affected atorvastatin pharmacokinetics. SLCO variants and non-genetic and regulatory covariates together accounted for 59% of variability of OATP1B1 expression.

Conclusions

Our results show that expression of OATP1B1, but not of OATP1B3 and OATP2B1, is significantly affected by genetic variants. The SLCO1B1 variant c.388A > G is the major determinant with additional consequences on atorvastatin plasma levels.

Metformin and cancer: from the old medicine cabinet to pharmacological pitfalls and prospects

Metformin is a biguanide derivative used in the treatment of type II diabetes (T2D) and one of the world's most widely prescribed drugs. Owing to its safety profile, it has been recently promoted for a range of other indications, particularly for its role in cancer prevention. There is evidence from studies in diabetic cohorts, as well as laboratory studies, that the action of metformin depends on a balance between the concentration and duration of exposure, which depends crucially on cell- and tissue-specific pharmacological factors. Mechanistic studies have revealed the involvement of increasingly complex pathways. Yet, there are several missing links regarding the role of drug transporters and drug-drug interactions, as well as the expression levels of transporters in normal versus tumor tissues, which may affect patient exposure and dosing when metformin is used in cancer prevention. This review highlights the current knowledge on metformin action and pharmacology, including novel insights into genomic factors, with a specific focus on cancer prevention. Furthermore, future challenges that may influence therapeutic outcome will be discussed.

Multidrug and toxin extrusion proteins as transporters of antimicrobial drugs

INTRODUCTION

Antimicrobial drugs are essential in the treatment of infectious diseases. A better understanding of transport processes involved in drug disposition will improve the predictability of drug-drug interactions with consequences for drug response. Multidrug And Toxin Extrusion (MATE; SLC47A) proteins are efflux transporters mediating the excretion of several antimicrobial drugs as well as other organic compounds into bile and urine, thereby contributing to drug disposition.

AREAS COVERED

This review summarizes current knowledge of the structural and molecular features of human MATE transporters including their functional role in drug transport with a specific focus on antimicrobial drugs. The PubMed database was searched using the terms "MATE1," "MATE-2K," "MATE2," "SLC47A1," "SLC47A2," and "toxin extrusion protein" (up to June 2012).

EXPERT OPINION

MATE proteins have been recognized as important transporters mediating the final excretion step of cationic drugs into bile and urine. These include the antiviral drugs acyclovir, amprenavir, and ganciclovir, the antibiotics cephalexin, cephradine and levofloxacin, as well as the antimalarial agents chloroquine and quinine. It is therefore important to enhance our understanding of the role of MATEs in drug extrusion with particular emphasis on the functional consequences of genetic variants on disposition of these antimicrobial drugs.

DNA methylation is associated with downregulation of the organic cation transporter OCT1 (SLC22A1) in human hepatocellular carcinoma

BACKGROUND

Organic cation transporters (OCTs) determine not only physiological processes but are also involved in the cellular uptake of anticancer agents. Based on microarray analyses in hepatocellular carcinoma (HCC), SLC22A1/OCT1 mRNA seems to be downregulated, but systematic protein expression data are currently missing. Moreover, the underlying molecular mechanisms responsible for altered SLC22A1 expression in HCC are not fully understood. Therefore, we investigated the role of DNA methylation in the transcriptional regulation of the family members SLC22A1/OCT1, SLC22A2/OCT2 and SLC22A3/OCT3 in HCC.

METHODS

Semiquantitative immunohistochemistry of SLC22A1 protein expression was performed in paired HCC and histological normal adjacent liver tissues (n = 71) using tissue microarray analyses, and the results were correlated with clinicopathological features. DNA methylation, quantified by MALDI-TOF mass spectrometry and gene expression of SLC22A1, SLC22A2 and SLC22A3 were investigated using fresh-frozen HCC (n = 22) and non-tumor adjacent liver tissues as well as histologically normal liver samples (n = 120) from a large-scale liverbank.

RESULTS

Based on tissue microarray analyses, we observed a significant downregulation of SLC22A1 protein expression in HCC compared to normal adjacent tissue (P < 0.0001). SLC22A1 expression was significantly inverse correlated with expression of the proliferation marker MIB1/Ki-67 (rs = -0.464, P < 0.0001). DNA methylation of SLC22A1 was significantly higher in HCC compared with non-tumor adjacent liver tissue and was lowest in histologically normal liver tissue. Methylation levels for SLC22A1 in combination with RASSF1A resulted in a specificity of > 90% and a sensitivity of 82% for discriminating HCC and tumor-free liver tissue.

CONCLUSIONS

DNA methylation of SLC22A1 is associated with downregulation of SLC22A1 in HCC and might be a new biomarker for HCC diagnosis and prognosis. Moreover, targeting SLC22A1 methylation by demethylating agents may offer a novel strategy for anticancer therapy of HCC.

Mammalian MATE (SLC47A) transport proteins: impact on efflux of endogenous substrates and xenobiotics

Multidrug and toxin extrusion proteins (MATEs; SLC47A) are mammalian transporters being predominately expressed in the brush-border membrane of proximal tubule epithelial cells in the kidney and the canalicular membrane of hepatocytes. Functionally, MATEs act as efflux transporters for organic compounds, thereby mediating the elimination process. Two isoforms, MATE1 and 2, have been identified, and, so far, only a limited number of substrates, including clinically used drugs such as metformin and cimetidine, are known. A knockout mouse model has been established, as well, and is a valuable tool for further systematic pharmacokinetic analyses. In this review, we summarize the progress in MATE research on structural, molecular, functional, and pathophysiological aspects. Consequences of genetic variants for pharmacokinetic alterations and drug therapy are discussed.

Proton pump inhibitors inhibit metformin uptake by organic cation transporters (OCTs)

Metformin, an oral insulin-sensitizing drug, is actively transported into cells by organic cation transporters (OCT) 1, 2, and 3 (encoded by SLC22A1, SLC22A2, or SLC22A3), which are tissue specifically expressed at significant levels in various organs such as liver, muscle, and kidney. Because metformin does not undergo hepatic metabolism, drug-drug interaction by inhibition of OCT transporters may be important. So far, comprehensive data on the interaction of proton pump inhibitors (PPIs) with OCTs are missing although PPIs are frequently used in metformin-treated patients. Using in silico modeling and computational analyses, we derived pharmacophore models indicating that PPIs (i.e. omeprazole, pantoprazole, lansoprazole, rabeprazole, and tenatoprazole) are potent OCT inhibitors. We then established stably transfected cell lines expressing the human uptake transporters OCT1, OCT2, or OCT3 and tested whether these PPIs inhibit OCT-mediated metformin uptake in vitro. All tested PPIs significantly inhibited metformin uptake by OCT1, OCT2, and OCT3 in a concentration-dependent manner. Half-maximal inhibitory concentration values (IC(50)) were in the low micromolar range (3-36 µM) and thereby in the range of IC(50) values of other potent OCT drug inhibitors. Finally, we tested whether the PPIs are also transported by OCTs, but did not identify PPIs as OCT substrates. In conclusion, PPIs are potent inhibitors of the OCT-mediated metformin transport in vitro. Further studies are needed to elucidate the clinical relevance of this drug-drug interaction with potential consequences on metformin disposition and/or efficacy.

Organic cation transporters (OCTs, MATEs), in vitro and in vivo evidence for the importance in drug therapy

Organic cation transporters (OCTs) of the solute carrier family (SLC) 22 and multidrug and toxin extrusion (MATE) transporters of the SLC47 family have been identified as uptake and efflux transporters, respectively, for xenobiotics including several clinically used drugs such as the antidiabetic agent metformin, the antiviral agent lamivudine, and the anticancer drug oxaliplatin. Expression of human OCT1 (SLC22A1) and OCT2 (SLC22A2) is highly restricted to the liver and kidney, respectively. By contrast, OCT3 (SLC22A3) is more widely distributed. MATEs (SLC47A1, SLC47A2) are predominantly expressed in human kidney. Data on in vitro studies reporting a large number of substrates and inhibitors of OCTs and MATEs are systematically summarized. Several genetic variants of human OCTs and in part of MATE1 have been reported, and some of them result in reduced in vitro transport activity corroborating data from studies with knockout mice. A comprehensive overview is given on currently known genotype-phenotype correlations for variants in OCTs and MATE1 related to protein expression, pharmacokinetics/-dynamics of transporter substrates, treatment outcome, and disease susceptibility.

Expression of organic cation transporters OCT1 (SLC22A1) and OCT3 (SLC22A3) is affected by genetic factors and cholestasis in human liver

An important function of hepatocytes is the biotransformation and elimination of various drugs, many of which are organic cations and are taken up by organic cation transporters (OCTs) of the solute carrier family 22 (SLC22). Because interindividual variability of OCT expression may affect response to cationic drugs such as metformin, we systematically investigated genetic and nongenetic factors of OCT1/SLC22A1 and OCT3/SLC22A3 expression in human liver. OCT1 and OCT3 expression (messenger RNA [mRNA], protein) was analyzed in liver tissue samples from 150 Caucasian subjects. Hepatic OCTs were localized by way of immunofluorescence microscopy. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and genome-wide single-nucleotide polymorphism microarray technology served to genotype 92 variants in the SLC22A1-A3/OCT1-3 gene cluster. Transport of metformin by recombinant human OCT1 and OCT3 was compared using transfected cells. OCT1 mRNA and protein expression varied 113- and 83-fold, respectively; OCT3 mRNA expression varied 27-fold. OCT1 transcript levels were on average 15-fold higher compared with OCT3. We localized the OCT3 protein to the basolateral hepatocyte membrane and identified metformin as an OCT3 substrate. OCT1 and OCT3 expression are independent of age and sex but were significantly reduced in liver donors diagnosed as cholestatic (P < or = 0.01). Several haplotypes for OCT1 and OCT3 were identified. Multivariate analysis adjusted for multiple testing showed that only the OCT1-Arg61Cys variant (rs12208357) strongly correlated with decreased OCT1 protein expression (P < 0.0001), and four variants in OCT3 (rs2292334, rs2048327, rs1810126, rs3088442) were associated with reduced OCT3 mRNA levels (P = 0.03).

CONCLUSION

We identified cholestasis and genetic variants as critical determinants for considerable interindividual variability of hepatic OCT1 and OCT3 expression. This indicates consequences for hepatic elimination of and response to OCT substrates such as metformin.

Interplay of conjugating enzymes with OATP uptake transporters and ABCC/MRP efflux pumps in the elimination of drugs

BACKGROUND

Biliary excretion is a major elimination route of many drugs and their metabolites. Hepatobiliary elimination is a vectorial process involving uptake transporters in the basolateral hepatocyte membrane, possibly Phase I and Phase II metabolizing enzymes, and ATP-dependent efflux pumps in the apical hepatocyte membrane.

OBJECTIVES

Because many drugs and their metabolites are anions, this review focuses on transporters involved in their hepatocellular uptake (members of the organic anion transporting polypeptide (OATP) family) and biliary elimination (apical conjugate efflux pump ABCC2/MRP2).

METHODS

The molecular and functional characteristics of the human OATP and ABCC/MRP transporters are presented, including a detailed overview of endogenous and drug substrates. Examples illustrate the interplay of transporters with Phase II conjugating enzymes. Model systems to study the vectorial transport of organic anions are also discussed.

RESULTS/CONCLUSIONS

OATP uptake transporters, conjugating enzymes, and ABCC2/MRP2 work in concert to enable the hepatobiliary elimination of anionic drugs and their metabolites. It is increasingly important to understand how genetic variants of these transporters and enzymes influence the interindividual variability of drug elimination.