Organic anion-transporting polypeptide B (OATP-B) and its functional comparison with three other OATPs of human liver

The Impact of Organic Anion-Transporting Polypeptide (OATP) Variants on the Side Effects of Direct-Acting Antivirals in Hepatitis C Patients

Background Organic anion-transporting polypeptides (OATPs) are responsible for the cellular uptake of a broad range of endogenous compounds and xenobiotics in multiple tissues. The aim of our study was to determine whether variations in OATP1B1 and OATP1B3 affect the side effects experienced by hepatitis C patients treated with direct-acting antivirals (DAAs). Methods This study included 199 hepatitis C patients treated with DAAs. Ledipasvir/sofosbuvir or ombitasvir/paritaprevir/ritonavir ± dasabuvir and 162 control individuals without hepatitis C. Treatment-related side effects were recorded. The OATP1B1 gene variations c.388A>G and c.521T>C and the OATP1B3 gene variations c.334T>G and c.699G>A were analyzed via the polymerase chain reaction-restriction fragment length polymorphism method. Allele/genotype combinations of OATP1B1 and OATP1B3 haplotypes were evaluated. Results Side effects were observed in 53 (26.6%) of 199 hepatitis C patients. There were skin mucosal lesions in 19 patients (36%), fatigue in 18 patients (34%), pruritus in 11 patients (20.5%), and other in five patients (9.5%). There was a significant relationship between the c.334T>G variant and side effects (p = 0.030). The frequency distribution of the c.334T>G variant was in Hardy-Weinberg equilibrium. The frequencies of the patient group and the control group were 65.3% and 63%, respectively. We found a significant difference between the patient and control groups in terms of the haplotype ratios of c.388A>G and c.521T>C (p = 0.036). Conclusions We found a significant relationship between the c.334T>G variant in OATP1B3 and DAA-related side effects in hepatitis C patients.

Organic anion transporting polypeptides: Pharmacology, toxicology, structure, and transport mechanisms

Organic anion transporting polypeptides (OATPs) are membrane proteins that mediate the uptake of a wide range of substrates across the plasma membrane of various cells and tissues. They are classified into 6 subfamilies, OATP1 through OATP6. Humans contain 12 OATPs encoded by 11 solute carrier of organic anion transporting polypeptide (SLCO) genes: OATP1A2, OATP1B1, OATP1B3, the splice variant OATP1B3-1B7, OATP1C1, OATP2A1, OATP2B1, OATP3A1, OATP4A1, OATP4C1, OATP5A1, and OATP6A1. Most of these proteins are expressed in epithelial cells, where they mediate the uptake of structurally unrelated organic anions, cations, and even neutral compounds into the cytoplasm. The best-characterized members are OATP1B1 and OATP1B3, which have an important role in drug metabolism by mediating drug uptake into the liver and are involved in drug-drug interactions. In this review, we aimed to (1) provide a historical perspective on the identification of OATPs and their nomenclature and discuss their phylogenic relationships and molecular characteristics; (2) review the current knowledge of the broad substrate specificity and their role in drug disposition and drug-drug interactions, with a special emphasis on human hepatic OATPs; (3) summarize the different experimental systems that are used to study the function of OATPs and discuss their advantages and disadvantages; (4) review the available experimental 3-dimensional structures and examine how they can help elucidate the transport mechanisms of OATPs; and (5) finally, summarize the current knowledge of the regulation of OATP expression, discuss clinically important single-nucleotide polymorphisms, and outline challenges of physiologically based pharmacokinetic modeling and in vitro to in vivo extrapolation. SIGNIFICANCE STATEMENT: Organic anion transporting polypeptides (OATPs) are a family of 12 uptake transporters in the solute carrier superfamily. Several members, particularly the liver-expressed OATP1B1 and OATP1B3, are important drug transporters. They mediate the uptake of several endobiotics and xenobiotics, including statins and numerous other drugs, into hepatocytes, and their inhibition by other drugs or reduced expression due to single-nucleotide polymorphisms can lead to adverse drug effects. Their recently solved 3-dimensional structures should help to elucidate their transport mechanisms and broad substrate specificities.

Efflux and uptake of androgen sulfates using transporter-overexpressing HEK293 cells and membrane vesicles

Hydrophilic steroid conjugates require active and facilitated transport mechanisms for their distribution into tissues and excretion from the body. The ATP-binding cassette (ABC) and solute carrier organic anion (SLCO) transporters involved in androgen sulfate (-S) disposition have been poorly characterized. In this study, we investigated the in vitro transport of testosterone-S, epitestosterone-S, dehydroepiandrosterone-S (DHEA-S), androsterone-S, and etiocholanolone-S by the multidrug resistance-associated proteins 2-4 (MRP2-4, ABCC2-4), breast cancer resistance protein (BCRP, ABCG2), and organic anion-transporting polypeptides (OATP) 1B1, 1B3, and 2B1 (SLCO1B1, SLCO1B3, and SLCO2B1) using human transporter-overexpressing HEK293 cells and membrane vesicles. We found testosterone-S, epitestosterone-S, and DHEA-S to be selectively transported by BCRP and/or MRP4, whereas all studied androgen sulfates were substrates of MRP3, OATP1B1, OATP1B3, and OATP2B1. MRP2 did not transport any of the studied compounds. Evaluation of transport kinetics revealed MRP4 to interact with its substrates at high to moderate affinity, whereas the observed affinities towards MRP3, BCRP, and OATPs were mostly moderate. These results help to build a better mechanistic understanding of the disposition of androgen sulfates in the human body. Additionally, this data may be used to assess the feasibility of androgen sulfates as additional biomarkers in doping detection.

Regionality and Temporal Dynamics of Sequestration and Relocation of Cardenolides in the Monarch Butterfly, Danaus plexippus

The adaptation of monarch butterflies (Danaus plexippus) to milkweed plants and their ability to sequester toxic cardenolides is a model system for plant-herbivore coevolution. However, the physiological mechanisms underlying cardenolide sequestration and its temporal dynamics remain largely unknown. Here, we show that the polar cardenolide ouabain passes through the isolated midgut epithelium of D. plexippus in vitro and is also absorbed into the body cavity of monarch caterpillars. Remarkably, the same pattern was observed in caterpillars of the related, but non-sequestering milkweed butterfly Euploea core, and even in the non-adapted Solanaceae specialist Manduca sexta, although uptake across gut epithelia occurred at a lower rate. Furthermore, we demonstrated that cardenolides begin to cross the epithelium in the anterior part of the intestine and can be detected in body tissues as soon as one minute after ingestion. Finally, we show that not all cardenolides are translocated into butterfly tissues during metamorphosis, and that the most apolar cardenolides are removed with the last caterpillar exuviae. As a result, adult butterflies contain no cardenolides less polar than the milkweed cardenolide calactin. We conclude that uptake by the intestinal epithelium is a very rapid process and that quantitative differences in cardenolide sequestration among lepidopteran caterpillars are only partially mediated by the gut epithelium, likely involving additional mechanisms such as metabolism or excretion. In addition, the translocation of cardenolides from the caterpillar is a selective process which may be due to the limited mobility of highly apolar cardenolides.

Impacts of prenatal environmental exposures on fetal-placental-maternal bile acid homeostasis and long-term health in offspring

During pregnancy, the maternal body undergoes a series of adaptative physiological changes, leading to a slight increase in serum bile acid (BA) levels. Although the fetal liver can synthesize BAs since the first trimester through the alternative pathway, the BA metabolic system is immature in the fetus. Compared to adults, the fetus has a distinct composition of BA pool and limited expression of BA synthesis enzymes and transporters. Besides, the "enterohepatic circulation" of BAs is absent in fetus. Thus, fetal BAs need to be transported to the mother through the placenta for further metabolism and excretion, and maternal BAs can also be transported to the fetus. That is what we call the "fetal-placental-maternal BA circulation". Various BA transporters and nuclear receptors are essential for maintaining the balance of this BA circulation to ensure normal fetal development. However, prenatal adverse environments can alter fetal BA metabolism, resulting in intrauterine developmental abnormalities and susceptibility to a variety of adult chronic diseases. This review summarizes the current understanding of the fetal-placental-maternal BA circulation and discusses the effects of prenatal adverse environments on this particular BA circulation, aiming to provide a theoretical basis for exploring early prevention and treatment strategies for BA metabolism-associated adverse pregnancy outcomes and long-term impairments.

The Competitive Counterflow Assay for Identifying Drugs Transported by Solute Carriers: Principle, Applications, Challenges/Limits, and Perspectives

The identification of substrates for solute carriers (SLCs) handling drugs is an important challenge, owing to the major implication of these plasma membrane transporters in pharmacokinetics and drug-drug interactions. In this context, the competitive counterflow (CCF) assay has been proposed as a practical and less expensive approach than the reference functional uptake assays for discriminating SLC substrates and non-substrates. The present article was designed to summarize and discuss key-findings about the CCF assay, including its principle, applications, challenges and limits, and perspectives. The CCF assay is based on the decrease of the steady-state accumulation of a tracer substrate in SLC-positive cells, caused by candidate substrates. Reviewed data highlight the fact that the CCF assay has been used to identify substrates and non-substrates for organic cation transporters (OCTs), organic anion transporters (OATs), and organic anion transporting polypeptides (OATPs). The performance values of the CCF assay, calculated from available CCF study data compared with reference functional uptake assay data, are, however, rather mitigated, indicating that the predictability of the CCF method for assessing SLC-mediated transportability of drugs is currently not optimal. Further studies, notably aimed at standardizing the CCF assay and developing CCF-based high-throughput approaches, are therefore required in order to fully precise the interest and relevance of the CCF assay for identifying substrates and non-substrates of SLCs.

Impact of OATP2B1 on Pharmacokinetics of Atorvastatin Investigated in rSlco2b1-Knockout and SLCO2B1-Knockin Rats

The organic anion transporting polypeptide (OATP) 2B1 is considered an emerging drug transporter that is found expressed in pharmacokinetically relevant organs such as the liver, small intestine, and kidney. Despite its interaction with various substrate drugs, the understanding of its in vivo relevance is still limited. In this study, we first validated the interaction of atorvastatin with rat OATP2B1 using transiently transfected HeLa cells. Moreover, we characterized our rSlco2b1-knockout and SLCO2B1-knockin rats for mRNA, protein expression, and localization of OATP2B1 in the liver, small intestine, and kidney. The transporter showed the highest expression in the liver followed by the small intestine. In humanized rats, human OATP2B1 is localized on the sinusoidal membrane of hepatocytes. In enterocytes of wild-type and humanized rats, the transporter was detected in the luminal membrane with the vast majority being localized subapical. Subsequently, we assessed atorvastatin pharmacokinetics in male wild-type, rSlco2b1-knockout, and SLCO2B1-knockin rats after a single-dose administration (orally and intravenously). Investigating the contribution of rat OATP2B1 or human OATP2B1 to oral atorvastatin pharmacokinetics revealed no differences in concentration-time profiles or pharmacokinetic parameters. However, when comparing the pharmacokinetics of atorvastatin after intravenous administration in SLCO2B1-humanized rats and knockout animals, notable differences were observed. In particular, the systemic exposure (area under the curve) decreased by approximately 40% in humanized animals, whereas the clearance was 57% higher in animals expressing human OATP2B1. These findings indicate that human OATP2B1 influences pharmacokinetics of atorvastatin after intravenous administration, most likely by contributing to the hepatic uptake. SIGNIFICANCE STATEMENT: Wild-type, rSlco2b1-knockout, and SLCO2B1-humanized Wistar rats were characterized for the expression of rat and human SLCO2B1/OATP2B1. Pharmacokinetic studies of atorvastatin over 24 hours were conducted in male wild-type, rSlco2b1-knockout, and SLCO2B1-humanized rats. After a single-dose intravenous administration, a lower systemic exposure and an increase in clearance were observed in SLCO2B1-humanized rats compared with knockout animals indicating a contribution of OATP2B1 to the hepatic clearance.

Impact of genetic variants in the solute carrier (SLC) genes encoding drug uptake transporters on the response to anticancer chemotherapy

Cancer drug resistance constitutes a severe limitation for the satisfactory outcome of these patients. This is a complex problem due to the co-existence in cancer cells of multiple and synergistic mechanisms of chemoresistance (MOC). These mechanisms are accounted for by the expression of a set of genes included in the so-called resistome, whose effectiveness often leads to a lack of response to pharmacological treatment. Additionally, genetic variants affecting these genes further increase the complexity of the question. This review focuses on a set of genes encoding members of the transportome involved in drug uptake, which have been classified into the MOC-1A subgroup of the resistome. These proteins belong to the solute carrier (SLC) superfamily. More precisely, we have considered here several members of families SLC2, SLC7, SLC19, SLC22, SLCO, SLC28, SLC29, SLC31, SLC46, and SLC47 due to the impact of their expression and genetic variants in anticancer drug uptake by tumor cells or, in some cases, general bioavailability. Changes in their expression levels and the appearance of genetic variants can contribute to the Darwinian selection of more resistant clones and, hence, to the development of a more malignant phenotype. Accordingly, to address this issue in future personalized medicine, it is necessary to characterize both changes in resistome genes that can affect their function. It is also essential to consider the time-dependent dimension of these features, as the genetic expression and the appearance of genetic variants can change during tumor progression and in response to treatment.

Quantitative prediction of transporter-mediated drug-drug interactions using the mechanistic static pharmacokinetic (MSPK) model

Guidance/guidelines on drug-drug interactions (DDIs) have been issued in Japan, the United States, and Europe. These guidance/guidelines provide decision trees for conducting metabolizing enzyme-mediated clinical DDI studies; however, the decision trees for transporter-mediated DDIs lack quantitative prediction methods. In this study, the accuracy of a net-effect mechanistic static pharmacokinetics (MSPK) model containing the fraction transported (ft) of transporters was examined to predict transporter-mediated DDIs. This study collected information on 25 oral drugs with new active reagents that were used in clinical DDI studies as perpetrators (42 cases) from drugs approved in Japan between April 2016 and June 2020. The AUCRs (AUC ratios with and without perpetrators) of victim drugs were predicted using the net-effect MSPK model. As a result, 83 and 95% of the predicted AUCRs were within 1.5- and 2-fold error in the observed AUCRs, respectively. In cases where the victims were statins in which pharmacokinetics several transporters are involved, 70 and 91% of the predicted AUCRs were within 1.5- and 2-fold errors, respectively. Therefore, the net-effect MSPK model was applicable for predicting the AUCRs of victims, which are substrates for multiple transporters.

Dissimilar Effect of P-Glycoprotein and Breast Cancer Resistance Protein Inhibition on the Distribution of Erlotinib to the Retina and Brain in Humans and Mice

P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) are two ATP-binding cassette efflux transporters that are coexpressed at the human blood-brain barrier (BBB) and blood-retina barrier (BRB). While pharmacological inhibition of P-gp and/or BCRP results in increased brain distribution of dual P-gp/BCRP substrate drugs, such as the tyrosine kinase inhibitor erlotinib, the effect of P-gp and/or BCRP inhibition on the retinal distribution of such drugs has hardly been investigated. In this study, we used positron emission tomography (PET) imaging to assess the effect of transporter inhibition on the distribution of [11C]erlotinib to the human retina and brain. Twenty two healthy volunteers underwent two PET scans after intravenous (i.v.) injection of a microdose (<5 μg) of [11C]erlotinib, a baseline scan, and a second scan either with concurrent i.v. infusion of tariquidar to inhibit P-gp (n = 5) or after oral intake of single ascending doses of erlotinib (300 mg, 650 mg, or 1000 mg, n = 17) to saturate erlotinib transport. In addition, transport of [3H]erlotinib to the retina and brain was assessed in mice by in situ carotid perfusion under various drug transporter inhibition settings. In comparison to the baseline PET scan, coadministration of tariquidar or erlotinib led to a significant decrease of [11C]erlotinib total volume of distribution (VT) in the human retina by -25 ± 8% (p ≤ 0.05) and -41 ± 16% (p ≤ 0.001), respectively. In contrast, erlotinib intake led to a significant increase in [11C]erlotinib VT in the human brain (+20 ± 16%, p ≤ 0.001), while administration of tariquidar did not result in any significant changes. In situ carotid perfusion experiments showed that both P-gp and BCRP significantly limit the distribution of erlotinib to the mouse retina and brain but revealed a similar discordant effect at the mouse BRB and BBB following co-perfusion with tariquidar and erlotinib as in humans. Co-perfusion with prototypical inhibitors of solute carrier transporters did not reveal a significant contribution of organic cation transporters (e.g., OCTs and OCTNs) and organic anion-transporting polypeptides (e.g., OATP2B1) to the retinal and cerebral distribution of erlotinib. In conclusion, we observed a dissimilar effect after P-gp and/or BCRP inhibition on the retinal and cerebral distribution of [11C]erlotinib. The exact mechanism for this discrepancy remains unclear but may be related to the function of an unidentified erlotinib uptake carrier sensitive to tariquidar inhibition at the BRB. Our study highlights the great potential of PET to study drug distribution to the human retina and to assess the functional impact of membrane transporters on ocular drug distribution.

The fluorescence-based competitive counterflow assay developed for organic anion transporting polypeptides 1A2, 1B1, 1B3 and 2B1 identifies pentamidine as a selective OATP1A2 substrate

Organic anion transporting polypeptides OATP1A2, OATP1B1, OATP1B3 and OATP2B1 are Na+ - and ATP-independent exchangers of large, organic compounds, encompassing structurally diverse xenobiotics, including various drugs. These OATPs influence intestinal absorption (OATP2B1), hepatic clearance (OATP1B1/3) and blood to brain penetration (OATP1A2, OATP2B1) of their drug substrates. Consequently, OATP-mediated drug or food interactions may lead to altered pharmacokinetics and toxicity. During drug development, investigation of hepatic OATP1B1 and OATP1B3 is recommended by international regulatory agencies. Most frequently, OATP-drug interactions are investigated in an indirect assay, i.e., by examining uptake inhibition of a radioactive or fluorescent probe. However, indirect assays do not distinguish between transported substrates and non-transported OATP inhibitors. To fill this hiatus, a novel assay, termed competitive counterflow (CCF) has been developed and has since been applied for several OATPs to differentiate between substrates and non-transported inhibitors. However, previous OATP CCF assays, with the exception of that for OATP1B1, used radioactive probes. In the current study, we demonstrate that sulforhodamine 101 or pyranine can be used as fluorescent probes in a CCF assay to identify transported substrates of OATP1A2, or OATPs 1B1, 1B3 and 2B1, respectively. With the help of the newly developed fluorescence-based CCF method, we identify the FDA-approved anti-protozoal drug, pentamidine as a unique substrate of OATP1A2. Furthermore, we confirm the selective, OATP1A2-mediated uptake of pentamidine in a cytotoxicity assay. Based on our results, OATP1A2 may be an important determinant of pentamidine transport through the blood-brain barrier.

Role of the Sodium-Dependent Organic Anion Transporter (SOAT/SLC10A6) in Physiology and Pathophysiology

The sodium-dependent organic anion transporter (SOAT, gene symbol SLC10A6) specifically transports 3'- and 17'-monosulfated steroid hormones, such as estrone sulfate and dehydroepiandrosterone sulfate, into specific target cells. These biologically inactive sulfo-conjugated steroids occur in high concentrations in the blood circulation and serve as precursors for the intracrine formation of active estrogens and androgens that contribute to the overall regulation of steroids in many peripheral tissues. Although SOAT expression has been detected in several hormone-responsive peripheral tissues, its quantitative contribution to steroid sulfate uptake in different organs is still not completely clear. Given this fact, the present review provides a comprehensive overview of the current knowledge about the SOAT by summarizing all experimental findings obtained since its first cloning in 2004 and by processing SOAT/SLC10A6-related data from genome-wide protein and mRNA expression databases. In conclusion, despite a significantly increased understanding of the function and physiological significance of the SOAT over the past 20 years, further studies are needed to finally establish it as a potential drug target for endocrine-based therapy of steroid-responsive diseases such as hormone-dependent breast cancer.

Development of a Suite of Gadolinium-Free OATP1-Targeted Paramagnetic Probes for Liver MRI

Five amphiphilic, anionic Mn(II) complexes were synthesized as contrast agents targeted to organic anion transporting polypeptide transporters (OATP) for liver magnetic resonance imaging (MRI). The Mn(II) complexes are synthesized in three steps, each from the commercially available trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid (CDTA) chelator, with T1-relaxivity of complexes ranging between 2.3 and 3.0 mM-1 s-1 in phosphate buffered saline at an applied field strength of 3.0 T. Pharmacokinetics were assessed in female BALB/c mice by acquiring T1-weighted images dynamically for 70 min after agent administration and determining contrast enhancement and washout in various organs. Uptake of Mn(II) complexes in human OATPs was investigated through in vitro assays using MDA-MB-231 cells engineered to express either OATP1B1 or OATP1B3 isoforms. Our study introduces a new class of Mn-based OATP-targeted contrast that can be broadly tuned via simple synthetic protocols.

Organic anion-transporting polypeptide 2B1 knockout and humanized mice; insights into the handling of bilirubin and drugs

Organic anion transporting polypeptide 2B1 (OATP2B1/SLCO2B1) facilitates uptake transport of structurally diverse endogenous and exogenous compounds. To investigate the roles of OATP2B1 in physiology and pharmacology, we established and characterized Oatp2b1 knockout (single Slco2b1^-/- and combination Slco1a/1b/2b1^-/-) and humanized hepatic and intestinal OATP2B1 transgenic mouse models. While viable and fertile, these strains exhibited a modestly increased body weight. In males, unconjugated bilirubin levels were markedly reduced in Slco2b1^-/- compared to wild-type mice, whereas bilirubin monoglucuronide levels were modestly increased in Slco1a/1b/2b1^-/- compared to Slco1a/1b^-/- mice. Single Slco2b1^-/- mice showed no significant changes in oral pharmacokinetics of several tested drugs. However, markedly higher or lower plasma exposure of pravastatin and the erlotinib metabolite OSI-420, respectively, were found in Slco1a/1b/2b1^-/- compared to Slco1a/1b^-/- mice, while oral rosuvastatin and fluvastatin behaved similarly between the strains. In males, humanized OATP2B1 strains showed lower conjugated and unconjugated bilirubin levels than control Slco1a/1b/2b1-deficient mice. Moreover, hepatic expression of human OATP2B1 partially or completely rescued the impaired hepatic uptake of OSI-420, rosuvastatin, pravastatin, and fluvastatin in Slco1a/1b/2b1^-/- mice, establishing an important role in hepatic uptake. Expression of human OATP2B1 in the intestine was basolateral and markedly reduced the oral availability of rosuvastatin and pravastatin, but not of OSI-420 and fluvastatin. Neither lack of Oatp2b1, nor overexpression of human OATP2B1 had any effect on fexofenadine oral pharmacokinetics. While these mouse models still have limitations for human translation, with additional work we expect they will provide powerful tools to further understand the physiological and pharmacological roles of OATP2B1.

Fluorescence-based methods for studying activity and drug-drug interactions of hepatic solute carrier and ATP binding cassette proteins involved in ADME-Tox

In humans, approximately 70% of drugs are eliminated through the liver. This process is governed by the concerted action of membrane transporters and metabolic enzymes. Transporters mediating hepatocellular uptake of drugs belong to the SLC (Solute carrier) superfamily of transporters. Drug efflux either toward the portal vein or into the bile is mainly mediated by active transporters of the ABC (ATP Binding Cassette) family. Alteration in the function and/or expression of liver transporters due to mutations, disease conditions, or co-administration of drugs or food components can result in altered pharmacokinetics. On the other hand, drugs or food components interacting with liver transporters may also interfere with liver function (e.g., bile acid homeostasis) and may even cause liver toxicity. Accordingly, certain transporters of the liver should be investigated already at an early stage of drug development. Most frequently radioactive probes are applied in these drug-transporter interaction tests. However, fluorescent probes are cost-effective and sensitive alternatives to radioligands, and are gaining wider application in drug-transporter interaction tests. In our review, we summarize our current understanding about hepatocyte ABC and SLC transporters affected by drug interactions. We provide an update of the available fluorescent and fluorogenic/activable probes applicable in in vitro or in vivo testing of these ABC and SLC transporters, including near-infrared transporter probes especially suitable for in vivo imaging. Furthermore, our review gives a comprehensive overview of the available fluorescence-based methods, not directly relying on the transport of the probe, suitable for the investigation of hepatic ABC or SLC-type drug transporters.

Transcriptional Regulation of Liver-Type OATP1B3 (Lt-OATP1B3) and Cancer-Type OATP1B3 (Ct-OATP1B3) Studied in Hepatocyte-Derived and Colon Cancer-Derived Cell Lines

Due to alternative splicing, the SLCO1B3 gene encodes two protein variants; the hepatic uptake transporter liver-type OATP1B3 (Lt-OATP1B3) and the cancer-type OATP1B3 (Ct-OATP1B3) expressed in several cancerous tissues. There is limited information about the cell type-specific transcriptional regulation of both variants and about transcription factors regulating this differential expression. Therefore, we cloned DNA fragments from the promoter regions of the Lt-SLCO1B3 and the Ct-SLCO1B3 gene and investigated their luciferase activity in hepatocellular and colorectal cancer cell lines. Both promoters showed differences in their luciferase activity depending on the used cell lines. We identified the first 100 bp upstream of the transcriptional start site as the core promoter region of the Ct-SLCO1B3 gene. In silico predicted binding sites for the transcription factors ZKSCAN3, SOX9 and HNF1α localized within these fragments were further analyzed. The mutagenesis of the ZKSCAN3 binding site reduced the luciferase activity of the Ct-SLCO1B3 reporter gene construct in the colorectal cancer cell lines DLD1 and T84 to 29.9% and 14.3%, respectively. In contrast, using the liver-derived Hep3B cells, 71.6% residual activity could be measured. This indicates that the transcription factors ZKSCAN3 and SOX9 are important for the cell type-specific transcriptional regulation of the Ct-SLCO1B3 gene.

Transporter-Mediated Drug Delivery

Transmembrane transport of small organic and inorganic molecules is one of the cornerstones of cellular metabolism. Among transmembrane transporters, solute carrier (SLC) proteins form the largest, albeit very diverse, superfamily with over 400 members. It was recognized early on that xenobiotics can directly interact with SLCs and that this interaction can fundamentally determine their efficacy, including bioavailability and intertissue distribution. Apart from the well-established prodrug strategy, the chemical ligation of transporter substrates to nanoparticles of various chemical compositions has recently been used as a means to enhance their targeting and absorption. In this review, we summarize efforts in drug design exploiting interactions with specific SLC transporters to optimize their therapeutic effects. Furthermore, we describe current and future challenges as well as new directions for the advanced development of therapeutics that target SLC transporters.

Thyroid Hormone Transporters in Pregnancy and Fetal Development

Thyroid hormone is essential for fetal (brain) development. Plasma membrane transporters control the intracellular bioavailability of thyroid hormone. In the past few decades, 15 human thyroid hormone transporters have been identified, and among them, mutations in monocarboxylate transporter (MCT)8 and organic anion transporting peptide (OATP)1C1 are associated with clinical phenotypes. Different animal and human models have been employed to unravel the (patho)-physiological role of thyroid hormone transporters. However, most studies on thyroid hormone transporters focus on postnatal development. This review summarizes the research on the thyroid hormone transporters in pregnancy and fetal development, including their substrate preference, expression and tissue distribution, and physiological and pathophysiological role in thyroid homeostasis and clinical disorders. As the fetus depends on the maternal thyroid hormone supply, especially during the first half of pregnancy, the review also elaborates on thyroid hormone transport across the human placental barrier. Future studies may reveal how the different transporters contribute to thyroid hormone homeostasis in fetal tissues to properly facilitate development. Employing state-of-the-art human models will enable a better understanding of their roles in thyroid hormone homeostasis.

Polygonum multiflorum Thunb. Induces hepatotoxicity in SD rats and hepatocyte spheroids by Disrupting the metabolism of bilirubin and bile acid

ETHNOPHARMACOLOGICAL RELEVANCE

The liver damage associated with Polygonum multiflorum Thunb. (P. multiflorum) and its preparations have aroused widespread concern. Opinions on the toxicity mechanisms and targets of P. multiflorum vary, and the toxic components are even more controversial. However, based on the current research results, we believed that any single component in P. multiflorum could not directly lead to liver injury, but may be the synergistic effect of multiple components. In addition, the toxicity mechanism also involved multiple targets.

AIM OF THE STUDY

This study aimed to elucidate the mechanism and target of the hepatotoxicity of P. multiflorum.

MATERIALS AND METHODS

In this study, the manifestations of liver injury triggered by P. multiflorum and the associated metabolic enzymes/transporters in the metabolic pathways of bilirubin and bile acid were investigated to elucidate the mechanism and target of the hepatotoxicity of P. multiflorum and related components. First, the hepatotoxicity and potential effect of P. multiflorum on both metabolic pathways were studied in rats administered P. multiflorum extracts (in 70% ethanol) for 42 days. Then, in vitro cultured hepatocyte spheroids were used to determine the hepatotoxicity of monomer components.

RESULTS

This revealed that P. multiflorum could simultaneously block bilirubin(BIL) and bile acid(BA) metabolism pathways, subsequently leading to liver damage. The targets and modes of action include reducing the activity of UGT1A1, the only metabolic enzyme of BIL, downregulating BIL and BA uptake transporters NTCP, OATP1B1, OATP1B3, efflux transporters MRP2, and BSEP, and upregulating efflux transporter MRP3. Furthermore, our data indicated that 2,3,5,4'-tetrahydroxystilbene-2-O-β-glucoside (TSG) and emodin-8-O-β-D-glucoside (EG) are the main toxic components in P. multiflorum. TSG accounts for 3.71% of the total content of P. multiflorum. In addition to markedly downregulating UGT1A1, TSG can upregulate OATP1B1/3 and promote the uptakes of bilirubin and bile acid, producing synergistic toxicity. EG accounts for 0.29% of the total content and demonstrates direct hepatotoxicity and extensive substrate overlap with bilirubin and bile acids. It can affect these two metabolic pathways simultaneously, promoting the accumulation of both bilirubin and bile acid for further toxic effects. Emodin is other major component, accounting for 0.01% of the total content, and its hepatotoxicity mechanisms include direct toxicity and inhibitory effects on bilirubin metabolizing enzymes. However, emodin is mainly distributed in the kidneys, so its hepatotoxicity risk is relatively low.

CONCLUSION

The simultaneous blockade of bilirubin and bile acid metabolic pathways as the critical toxic mechanism of P. multiflorum-induced liver injury, and potential toxic components were TSG and EG.

Using a competitive counterflow assay to identify novel cationic substrates of OATP1B1 and OATP1B3

OATP1B1 and OATP1B3 are two drug transporters that mediate the uptake of multiple endo- and xenobiotics, including many drugs, into human hepatocytes. Numerous inhibitors have been identified, and for some of them, it is not clear whether they are also substrates. Historically radiolabeled substrates or LC-MS/MS methods were needed to test for transported substrates, both of which can be limiting in time and money. However, the competitive counterflow (CCF) assay originally described for OCT2 and, more recently, for OCT1, OATP2B1, and OATP1A2 does not require radiolabeled substrates or LC-MS/MS methods and, as a result, is a more cost-effective approach to identifying substrates of multidrug transporters. We used a CCF assay based on the stimulated efflux of the common model substrate estradiol-17β-glucuronide (E17βG) and tested 30 compounds for OATP1B1- and OATP1B3-mediated transport. Chinese Hamster Ovary (CHO) cells stably expressing OATP1B1 or OATP1B3 were preloaded with 10 nM [³H]-estradiol-17β-glucuronide. After the addition of known substrates like unlabeled estradiol-17β-glucuronide, estrone-3-sulfate, bromosulfophthalein, protoporphyrin X, rifampicin, and taurocholate to the outside of the preloaded CHO cells, we observed efflux of [³H]-estradiol-17β-glucuronide due to exchange with the added compounds. Of the tested 30 compounds, some organic cation transporter substrates like diphenhydramine, metformin, and salbutamol did not induce [³H]-estradiol-17β-glucuronide efflux, indicating that the two OATPs do not transport them. However, 22 (for OATP1B1) and 16 (for OATP1B3) of the tested compounds resulted in [³H]-estradiol-17β-glucuronide efflux, suggesting that they are OATP substrates. Among these compounds, we further tested clarithromycin, indomethacin, reserpine, and verapamil and confirmed that they are substrates of the two OATPs. These results demonstrate that the substrate spectrum of the well-characterized organic anion transporting polypeptides includes several organic cations. Furthermore, as for other drug uptake transporters, the CCF assay is an easy-to-use screening tool to identify novel OATP substrates.

Cloning and characterization of a novel functional organic anion transporting polypeptide 3A1 isoform highly expressed in the human brain and testis

Organic anion transporting polypeptide 3A1 (OATP3A1, encoded by the SLCO3A1 gene) is a prostaglandin, oligopeptide, and steroid/thyroid hormone transporter with wide tissue distribution, expressed, e.g., in the human brain and testis. Although the physiological importance of OATP3A1 has not yet been clarified, based on its expression pattern, substrate recognition, and evolutionary conservation, OATP3A1 is a potential pharmacological target. Previously, two isoforms of OATP3A1, termed as V1 and V2, have been characterized. Here, we describe the cloning and functional characterization of a third isoform, OATP3A1_V3. The mRNA of isoform V3 is formed by alternative splicing and results in an OATP3A1 protein with an altered C-terminus compared to isoforms V1 and V2. Based on quantitative PCR, we demonstrate the widespread expression of SLCO3A1_V3 mRNA in human organs, with the highest expression in the brain and testis. By generation of an isoform V3-specific antibody and immunostaining, we show that the encoded protein is expressed in the human choroid plexus, neurons, and both germ and Sertoli cells of the testis. Moreover, we demonstrate that in contrast to isoform V1, OATP3A1_V3 localizes to the apical membrane of polarized MDCKII cells. Using HEK-293 cells engineered to overexpress OATP3A1_V3, we verify the protein’s functionality and identify dehydroepiandrosterone sulfate as a novel OATP3A1 substrate. Based on their distinct expression patterns but overlapping functions, OATP3A1 isoforms may contribute to transcellular (neuro)steroid transport in the central nervous system.

KNIME workflow for retrieving causal drug and protein interactions, building networks, and performing topological enrichment analysis demonstrated by a DILI case study

As an alternative to one drug-one target approaches, systems biology methods can provide a deeper insight into the holistic effects of drugs. Network-based approaches are tools of systems biology, that can represent valuable methods for visualizing and analysing drug-protein and protein-protein interactions. In this study, a KNIME workflow is presented which connects drugs to causal target proteins and target proteins to their causal protein interactors. With the collected data, networks can be constructed for visualizing and interpreting the connections. The last part of the workflow provides a topological enrichment test for identifying relevant pathways and processes connected to the submitted data. The workflow is based on openly available databases and their web services. As a case study, compounds of DILIRank were analysed. DILIRank is the benchmark dataset for Drug-Induced Liver Injury by the FDA, where compounds are categorized by their likeliness of causing DILI. The study includes the drugs that are most likely to cause DILI ("mostDILI") and the ones that are not likely to cause DILI ("noDILI"). After selecting the compounds of interest, down- and upregulated proteins connected to the mostDILI group were identified; furthermore, a liver-specific subset of those was created. The downregulated sub-list had considerably more entries, therefore, network and causal interactome were constructed and topological pathway enrichment analysis was performed with this list. The workflow identified proteins such as Prostaglandin G7H synthase 1 and UDP-glucuronosyltransferase 1A9 as key participants in the potential toxic events disclosing the possible mode of action. The topological network analysis resulted in pathways such as recycling of bile acids and salts and glucuronidation, indicating their involvement in DILI. The KNIME pipeline was built to support target and network-based approaches to analyse any sets of drug data and identify their target proteins, mode of actions and processes they are involved in. The fragments of the pipeline can be used separately or can be combined as required.

The Importance of Val386 in Transmembrane Domain 8 for the Activation of OATP1B3 by Epigallocatechin Gallate

Estrone-3-sulfate (E3S) uptake mediated by organic anion transporting polypeptide 1B3 (OATP1B3) can be activated by epigallocatechin gallate (EGCG). In this study, by using chimeric transporters and site-directed mutagenesis, we found that Val386 in transmembrane domain 8 (TM8) is essential for OATP1B3's activation by EGCG. Kinetic studies showed that the loss of activation of 1B3-TM8 and 1B3-V386F in the presence of EGCG is due to their decreased substrate binding affinity and reduced maximal transport rate. The overall transport efficiencies of OATP1B3, 1B3-TM8, and 1B3-V386F in the absence and presence of EGCG are 8.6 ± 0.7 vs 15.9 ± 1.4 (p < 0.05), 11.2 ± 2.1 vs 2.7 ± 0.3 (p < 0.05), and 10.2 ± 1.0 vs 2.5 ± 0.3 (p < 0.05), respectively. While 1B3-V386F cannot be activated by EGCG, its transport activity for EGCG is also diminished. OATP1B3's activation by EGCG is substrate-dependent as EGCG inhibits OATP1B3-mediated pravastatin uptake. Furthermore, the activation of OATP1B3-mediated E3S uptake by quercetin 3-O-α-l-arabinopyranosyl(1 → 2)-α-l-rhamnopyranoside is not affected by TM8 and V386F. Taken together, the activation of OATP1B3 by small molecules is substrate- and modulator-dependent, and V386 in TM8 plays a critical role in the activation of OATP1B3-mediated E3S uptake by EGCG.

Localization of Xenobiotic Transporters Expressed at the Human Blood-Testis Barrier

The blood-testis barrier (BTB) is formed by basal tight junctions between adjacent Sertoli cells (SCs) of the seminiferous tubules and acts as a physical barrier to protect developing germ cells in the adluminal compartment from reproductive toxicants. Xenobiotics, including antivirals, male contraceptives, and cancer chemotherapeutics, are known to cross the BTB, although the mechanisms that permit barrier circumvention are generally unknown. This study used immunohistological staining of human testicular tissue to determine the site of expression for xenobiotic transporters that facilitate transport across the BTB. Organic anion transporter (OAT) 1, OAT2, and organic cation transporter, novel (OCTN) 1 primarily localized to the basal membrane of SCs, whereas OCTN2, multidrug resistance protein (MRP) 3, MRP6, and MRP7 localized to SC basal membranes and peritubular myoid cells (PMCs) surrounding the seminiferous tubules. Concentrative nucleoside transporter (CNT) 2 localized to Leydig cells (LCs), PMCs, and SC apicolateral membranes. Organic cation transporter (OCT) 1, OCT2, and OCT3 mostly localized to PMCs and LCs, although there was minor staining in developing germ cells for OCT3. Organic anion transporting polypeptide (OATP) 1A2, OATP1B1, OATP1B3, OATP2A1, OATP2B1, and OATP3A1-v2 localized to SC basal membranes with diffuse staining for some transporters. Notably, OATP1C1 and OATP4A1 primarily localized to LCs. Positive staining for multidrug and toxin extrusion protein (MATE) 1 was only observed throughout the adluminal compartment. Definitive staining for CNT1, OAT3, MATE2, and OATP6A1 was not observed. The location of these transporters is consistent with their involvement in the movement of xenobiotics across the BTB. Altogether, the localization of these transporters provides insight into the mechanisms of drug disposition across the BTB and will be useful in developing tools to overcome the pharmacokinetic and pharmacodynamic difficulties presented by the BTB. SIGNIFICANCE STATEMENT: Although the total mRNA and protein expression of drug transporters in the testes has been explored, the localization of many transporters at the blood-testis barrier (BTB) has not been determined. This study applied immunohistological staining in human testicular tissues to identify the cellular localization of drug transporters in the testes. The observations made in this study have implications for the development of drugs that can effectively use transporters expressed at the basal membranes of Sertoli cells to bypass the BTB.

Pharmacogenomics for the efficacy and side effects of antihistamines

Antihistamines, especially H1 antihistamines, are widely used in the treatment of allergic diseases such as urticaria and allergic rhinitis, mainly for reversing elevated histamine and anti-allergic effects. Antihistamines are generally safe, but some patients experience adverse reactions, such as cardiotoxicity, central inhibition, and anticholinergic effects. There are also individual differences in antihistamine efficacy in clinical practice. The concept of individualized medicine has been deeply rooted in people's minds since it was put forward. Pharmacogenomics is the study of the role of inheritance in individual variations in drug response. In recent decades, pharmacogenomics has been developing rapidly, which provides new ideas for individualized medicine. Polymorphisms in the genes encoding metabolic enzymes, transporters, and target receptors have been shown to affect the efficacy of antihistamines. In addition, recent evidence suggests that gene polymorphisms influence urticaria susceptibility and antihistamine therapy. Here, we summarize current reports in this area, aiming to contribute to future research in antihistamines and clinical guidance for antihistamines use in individualized medicine.

Inhibitory Effects of Cranberry Juice and Its Components on Intestinal OATP1A2 and OATP2B1: Identification of Avicularin as a Novel Inhibitor

Organic anion-transporting polypeptide (OATP) 1A2 and OATP2B1 mediate the intestinal absorption of drugs. This study aimed to identify fruit juices or fruit juice components that inhibit OATPs and assess the risk of associated food-drug interactions. Inhibitory potency was assessed by examining the uptake of [³H]estrone 3-sulfate and [³H]fexofenadine into HEK293 cells expressing OATP1A2 or OATP2B1. In vivo experiments were conducted using mice to evaluate the effects of cranberry juice on the pharmacokinetics of orally administered fexofenadine. Of eight examined fruit juices, cranberry juice inhibited the functions of both OATPs most potently. Avicularin, a component of cranberry juice, was identified as a novel OATP inhibitor. It exhibited IC₅₀ values of 9.0 and 37 μM for the inhibition of estrone 3-sulfate uptake mediated by OATP1A2 and OATP2B1, respectively. A pharmacokinetic experiment revealed that fexofenadine exposure was significantly reduced (by 50%) by cranberry juice. Cranberry juice may cause drug interactions with OATP substrates.

Transplacental Pharmacokinetic Model of Digoxin Based on Ex Vivo Human Placental Perfusion Study

Digoxin is used as first-line therapy to treat fetal supraventricular tachycardia; however, because of the narrow therapeutic window, it is essential to estimate digoxin exposure in the fetus. The data from ex vivo human placental perfusion study are used to predict in vivo fetal exposure noninvasively, but the ex vivo fetal-to-maternal concentration (F:M) ratios observed in digoxin perfusion studies were much lower than those in vivo. In the present study, we developed a human transplacental pharmacokinetic model of digoxin using previously reported ex vivo human placental perfusion data. The model consists of maternal intervillous, fetal capillary, non-perfused tissue, and syncytiotrophoblast compartments, with multidrug resistance protein (MDR) 1 and influx transporter at the microvillous membrane (MVM) and influx and efflux transporters at the basal plasma membrane (BM). The model-predicted F:M ratio was 0.66, which is consistent with the mean in vivo value of 0.77 (95% confidence interval: 0.64-0.91). The time to achieve the steady state from the ex vivo perfusion study was estimated as 1,500 minutes, which is considerably longer than the reported ex vivo experimental durations, and this difference is considered to account for the inconsistency between ex vivo and in vivo F:M ratios. Reported digoxin concentrations in a drug-drug interaction study with MDR1 inhibitors quinidine and verapamil were consistent with the profiles simulated by our model incorporating inhibition of efflux transporter at the BM in addition to MVM. Our modeling and simulation approach should be a powerful tool to predict fetal exposure and DDIs in human placenta. SIGNIFICANCE STATEMENT: We developed a human transplacental pharmacokinetic model of digoxin based on ex vivo human placental perfusion studies in order to resolve inconsistencies between reported ex vivo and in vivo fetal-to-maternal concentration ratios. The model successfully predicted the in vivo fetal exposure to digoxin and the drug-drug interactions of digoxin and P-glycoprotein/multidrug resistance protein 1 inhibitors in human placenta.

Constituents of Passiflora incarnata, but Not of Valeriana officinalis, Interact with the Organic Anion Transporting Polypeptides (OATP)2B1 and OATP1A2

Herbal medication used in the treatment of sleep disorders and anxiety often contain extracts of Valeriana officinalis or Passiflora incarnata. Valerenic acid in V. officinalis and apigenin, orientin, and vitexin in P. incarnata are thought to contribute to their therapeutic effect. It was the aim of this study to test whether these constituents of herbal extracts are interacting with the uptake of estrone 3-sulfate, pregnenolone sulfate, and dehydroepiandrosterone sulfate mediated by the uptake transporters organic anion transporting polypeptide 2B1 (OATP2B1) or organic anion transporting polypeptide 1A2 (OATP1A2). Madin-Darby canine kidney cells overexpressing OATP2B1 or OATP1A2 were used to determine the influence of the constituents on the cellular accumulation of the sulfated steroids. Subsequently, competitive counterflow experiments were applied to test whether identified inhibitors are also substrates of the transporters. Valerenic acid only interacted with OATP2B1, whereas apigenin, orientin, and vitexin interacted with OATP2B1 and OATP1A2. Competitive counterflow revealed that orientin is a substrate of both transporters, while apigenin was transported by OATP1A2 and vitexin by OATP2B1. In a next step, commercially available P. incarnata preparations were assessed for their influence on the transporters, revealing inhibition of transporter-mediated estrone 3-sulfate uptake. HPLC-UV-MS analysis confirmed the presence of orientin and vitexin in these preparations, thereby suggesting that these constituents are involved in the interaction. Our data indicate that constituents of P. incarnata may alter the function of OATP2B1 and OATP1A2, which could affect the uptake of other compounds relying on uptake mediated by the transporters.

Zebrafish (Danio rerio) Oatp2b1 as a functional ortholog of the human OATP2B1 transporter

OATP2B1 belongs to a highly conserved organic anion transporting polypeptide (OATP) family of transporters, involved in the cellular uptake of both endogenous and exogenous compounds. The reported substrates of human OATP2B1 include steroid conjugates, bile salts, and thyroid hormones, as well as pharmaceuticals. Human OATP2B1 has orthologous genes in other vertebrate species, including zebrafish (Danio rerio), a widely used model organism in biomedical and environmental research. Our previous studies showed that zebrafish Oatp2b1 was phylogenetically closest to mammalian OATP2B1/Oatp2b1 and that it shares a similar tissue expression pattern. In this study, we aimed at discovering whether zebrafish Oatp2b1 could be a functional ortholog of human and rodent OATP2B1. To test this hypothesis, our primary goal was to obtain the first in vitro and in silico insights related to the structure and potential substrate preferences of zebrafish Oatp2b1. We generated cells transiently and stably transfected with zebrafish Oatp2b1 cloned from zebrafish liver, constructed an Oatp2b1 homology model, developed transport activity assays with model fluorescent substrate Lucifer yellow, and finally utilized this assay to analyze the interaction of zebrafish Oatp2b1 with both physiological and xenobiotic substances. Apart from structure similarities, our data revealed the strongest interaction of zebrafish Oatp2b1 with bile acids, steroid sulfates, thyroid hormones, and bilirubin, as well as xenobiotics bromosulfophthalein and sulfasalazine, which indicates its functional orthology with human OATP2B1.

The utility of endogenous glycochenodeoxycholate-3-sulfate and 4β-hydroxycholesterol to evaluate the hepatic disposition of atorvastatin in rats

The liver is an important organ for drugs disposition, and thus how to accurately evaluate hepatic clearance is essential for proper drug dosing. However, there are many limitations in drug dosage adjustment based on liver function and pharmacogenomic testing. In this study, we evaluated the ability of endogenous glycochenodeoxycholate-3-sulfate (GCDCA-S) and 4β-hydroxycholesterol (4β-HC) plasma levels to evaluate organic anion-transporting polypeptide (Oatps)-mediated hepatic uptake and Cyp3a-meidated metabolism of atorvastatin (ATV) in rats. The concentration of ATV and its metabolites, 2-OH ATV and 4-OH ATV, was markedly increased after a single injection of rifampicin (RIF), an inhibitor of Oatps. Concurrently, plasma GCDCA-S levels were also elevated. After a single injection of the Cyp3a inhibitor ketoconazole (KTZ), plasma ATV concentrations were significantly increased and 2-OH ATV concentrations were decreased, consistent with the metabolism of ATV by Cyp3a. However, plasma 4β-HC was not affected by KTZ treatment despite it being a Cyp3a metabolite of cholesterol. After repeated oral administration of RIF, plasma concentrations of ATV, 2-OH ATV and 4-OH ATV were markedly increased and the hepatic uptake ratio of ATV and GCDCA-S was decreased. KTZ did not affect plasma concentrations of ATV, 2-OH ATV and 4-OH ATV, but significantly decreased the metabolic ratio of total and 4-OH ATV. However, the plasma level and hepatic metabolism of 4β-HC were not changed by KTZ. The inhibition of hepatic uptake of GCDCA-S by RIF was fully reversed after a 7-d washout of RIF. Plasma concentration and hepatic uptake ratio of GCDCA-S were correlated with the plasma level and hepatic uptake of ATV in rats with ANIT-induced liver injury, respectively. These results demonstrate that plasma GCDCA-S is a sensitive probe for the assessment of Oatps-mediated hepatic uptake of ATV. However, Cyp3a-mediated metabolism of ATV was not predicted by plasma 4β-HC levels in rats.

Selective antiproliferative effect of C-2 halogenated 13α-estrones on cells expressing Organic anion-transporting polypeptide 2B1 (OATP2B1)

Organic anion-transporting polypeptide 2B1 (OATP2B1) is a multispecific transporter mediating the cellular uptake of steroids and numerous drugs. OATP2B1 is abundantly expressed in the intestine and is also present in various tumors. Increased steroid hormone uptake by OATP2B1 has been suggested to promote progression of hormone dependent tumors. 13α-estrones are effective inhibitors of endogenous estrogen formation and are potential candidates to inhibit proliferation of hormone dependent cancers. Recently, we have identified a variety of 13α/β-estrone-based inhibitors of OATP2B1. However, the nature of this interaction, whether these inhibitors are potential transported substrates of OATP2B1 and hence may be enriched in OATP2B1-overexpressing cells, has not yet been investigated. In the current study we explored the antiproliferative effect of the most effective OATP2B1 inhibitor 13α/β-estrones in control and OATP2B1-overexpressing A431 carcinoma cells. We found an increased antiproliferative effect of 3-O-benzyl 13α/β-estrones in both mock transfected and OATP2B1-overexpressing cells. However, C-2 halogenated 13α-estrones had a selective OATP2B1-mediated cell growth inhibitory effect. In order to demonstrate that increased sensitization can be attributed to OATP2B1-mediated cellular uptake, tritium labeled 2-bromo-13α-estrone was synthesized for direct transport measurements. These experiments revealed increased accumulation of [³H]2-bromo-13α-estrone due to OATP2B1 function. Our results indicate that C-2 halogenated 13α-estrones are good candidates in the design of anti-cancer drugs targeting OATP2B1.

Differences in transport function of the human and rat orthologue of the Organic Anion Transporting Polypeptide 2B1 (OATP2B1)

The human drug transporter Organic Anion Transporting Polypeptide (hOATP)2B1 facilitates cellular uptake of its substrates. Various studies suggest that hOATP2B1 is involved in intestinal absorption, but preclinical evaluations performed in rodents do not support this. Thus, our study aimed to compare the expression and function of hOATP2B1 with its orthologue in rats (rOatp2b1). Even if the general expression pattern was comparable, the transporters exhibited substantial differences on functional level. While bromosulfophthalein and atorvastatin were substrates of both transporters, the steroid sulfate conjugates estrone 3-sulfate (E₁S), progesterone sulfate and dehydroepiandrosterone sulfate were only transported by hOATP2B1. To further elucidate these functional differences, experiments searching for the E₁S substrate recognition site were conducted generating human-rat chimera as well as partly humanized variants of rOatp2b1. The rOatp2b1-329-hOATP2B1 chimera led to a significant increase in E₁S uptake suggesting the C-terminal part of the human transporter is involved. However, humanization of various regions within this part, namely of the transmembrane domain (TMD)-9, TMD-10 or the extracellular loop-5 did not significantly change E₁S transport function. Replacement of the intracellular loop-3, slightly enhanced cellular accumulation of sulfated steroids. Taken together, we report that OATP2B1 exhibited differences in recognition of steroid sulfate conjugates comparing the rat and human orthologues.

Organic Anion Transporting Polypeptide 2B1 (OATP2B1) Genetic Variants: In Vitro Functional Characterization and Association With Circulating Concentrations of Endogenous Substrates

Organic anion transporting polypeptide 2B1 (OATP2B1, gene SLCO2B1) is an uptake transporter that is thought to determine drug disposition and in particular, the oral absorption of medications. At present, the clinical relevance of SLCO2B1 genetic variation on pharmacokinetics is poorly understood. We sought to determine the functional activity of 5 of the most common missense OATP2B1 variants (c.76_84del, c.601G>A, c.917G>A, c.935G>A, and c.1457C>T) and a predicted dysfunctional variant (c.332G>A) in vitro. Furthermore, we measured the basal plasma concentrations of endogenous OATP2B1 substrates, namely estrone sulfate, dehydroepiandrosterone sulfate (DHEAS), pregnenolone sulfate, coproporphyrin I (CPI), and CPIII, and assessed their relationships with SLCO2B1 genotypes in 93 healthy participants. Compared to reference OATP2B1, the transport activities of the c.332G>A, c.601G>A and c.1457C>T variants were reduced among the substrates examined (estrone sulfate, DHEAS, CPI, CPIII and rosuvastatin), although there were substrate-dependent effects. Lower transport function of OATP2B1 variants could be explained by diminished cell surface expression. Other OATP2B1 variants (c.76-84del, c.917G>A and c.935G>A) had similar activity to the reference transporter. In the clinical cohort, the SLCO2B1 c.935G>A allele was associated with both higher plasma CPI (42%) and CPIII (31%) concentrations, while SLCO2B1 c.917G>A was linked to lower plasma CPIII by 28% after accounting for the effects of age, sex, and SLCO1B1 genotypes. No association was observed between SLCO2B1 variant alleles and estrone sulfate or DHEAS plasma concentrations, however 45% higher plasma pregnenolone sulfate level was associated with SLCO2B1 c.1457C>T. Taken together, we found that the impacts of OATP2B1 variants on transport activities in vitro were not fully aligned with their associations to plasma concentrations of endogenous substrates in vivo. Additional studies are required to determine whether circulating endogenous substrates reflect OATP2B1 activity.

Pre-incubation with OATP1B1 and OATP1B3 inhibitors potentiates inhibitory effects in physiologically relevant sandwich-cultured primary human hepatocytes

Organic anion transporting polypeptides (OATP)1B1 and OATP1B3 are liver-specific transport proteins that express on the basolateral membrane of human hepatocytes and mediate hepatic uptake of many drugs, including statins. They are important determinants of transporter-mediated drug-drug interactions (DDIs). It has been reported that pre-incubation with some OATP1B1 and OATP1B3 inhibitors potentiates the inhibitory effects, yielding reduced IC50 values. The US FDA draft guidance has recently recommended to use the lower IC50 values after inhibitor-preincubation to assess OATP1B1 and OATP1B3-mediated DDIs. However, it remains unknown whether the potentiation effects of inhibitor-preincubation on IC50 values occur in a physiologically relevant cell model. The current study was designed to determine the IC50 values of OATP1B1 and OATP1B3 inhibitors everolimus (EVR), sirolimus (SIR), and dasatinib against OATP1B substrates in physiologically relevant primary human hepatocytes with or without inhibitor-preincubation and to compare the OATP-mediated DDI prediction using data from primary human hepatocytes and that reported previously in transporter-expressing cell lines. Primary human hepatocytes were cultured in a sandwich configuration. Accumulation of [3H]-CCK-8 (1 µM, 1.5 min), [3H]-rosuvastatin (0.5 µM, 2 min) and [3H]-pitavastatin (1 µM, 0.5 min) was determined in human sandwich-cultured hepatocytes (SCH) in the presence of vehicle control or an inhibitor with or without inhibitor-preincubation at designated concentrations, and was utilized to determine the IC50 values for these inhibitors. R-value models were used to predict OATP-mediated DDIs. Pre-incubation with EVR at a clinically relevant concentration of 0.2 µM significantly reduced accumulation of [3H]-CCK-8 and [3H]-rosuvastatin even after washing. Reduced IC50 values following inhibitor pre-incubation were observed for all three inhibitors using [3H]-CCK-8 and [3H]-rosuvastatin as substrates in human SCH. The IC50 values after inhibitor-preincubation were lower or comparable in transporter-expressing cell lines compared with that in human SCH. For dasatinib, R-values from both cell lines and human SCH were greater than the US FDA cut-off value of 1.1. For EVR, R values from cell lines were 1.23 and were lowered to near 1.1 (1.08-1.09) in human SCH. For SIR, R values from either cell type were less than the cut-off values of 1.1. In conclusion, the current study is the first to report that pre-incubation with OATP1B inhibitors potentiates inhibitory effects in physiologically relevant primary human hepatocytes, supporting the rationale of the current US FDA draft guidance of including an inhibitor-preincubation step when assessing OATP-mediated DDIs in vitro. IC50 values after inhibitor-preincubation in transporter-expressing cell lines may be used for DDI prediction for the purpose of mitigating false-negative OATP-mediated DDI prediction.

The Complexity of the Blood-Brain Barrier and the Concept of Age-Related Brain Targeting: Challenges and Potential of Novel Solid Lipid-Based Formulations

Diseases that affect the Central Nervous System (CNS) are one of the most exciting challenges of recent years, as they are ubiquitous and affect all ages. Although these disorders show different etiologies, all treatments share the same difficulty represented by the Blood-Brain Barrier (BBB). This barrier acts as a protective system of the delicate cerebral microenvironment, isolating it and making extremely arduous delivering drugs to the brain. To overtake the obstacles provided by the BBB it is essential to explore the changes that affect it, to understand how to exploit these findings in the study and design of innovative brain targeted formulations. Interestingly, the concept of age-related targeting could prove to be a winning choice, as it allows to consider the type of treatment according to the different needs and peculiarities depending on the disease and the age of onset. In this review was considered the prospective contribution of lipid-based formulations, namely Solid Lipid Nanoparticles (SLNs) and Nanostructured Lipid Carriers (NLCs), which have been highlighted as able to overcome some limitations of other innovative approaches, thus representing a promising strategy for the non-invasive specific treatment of CNS-related diseases.

8-acetoxy-trisulfopyrene as the first activatable fluorogenic probe for add-and-read assessment of Organic anion-transporting polypeptides, OATP1B1, OATP1B3, and OATP2B1

Organic anion-transporting polypeptides, OATP1B1, OATP1B3, and OATP2B1 are multispecific membrane proteins mediating the hepatocellular uptake of structurally diverse endo- and exogenous compounds, including various kinds of drugs. Co-administration of OATP1B/2B1 substrates may lead to altered pharmacokinetics or even toxicity. Therefore, the study of the interaction with these OATPs is essential in drug development and is recommended by international regulatory agencies, the FDA, EMA, and PMDA. In general, radiolabeled indicators are used to measure drug interactions of OATPs, and, lately, fluorescent probes are also gaining wider application in OATP tests. However, all of the currently available methods (either radioactive or fluorescence-based) comprise multiple steps, including the removal of the indicator in the end of the experiment. Hence, they are not ideally suited for high-throughput screening. In the current study, in order to find an indicator allowing real-time assessment of hepatic OATP function, we searched for an activatable fluorogenic OATP substrate. Here, we show that 8-acetoxypyrene-1,3,6-trisulfonate (Ace), a fluorogenic derivative of the hepatic OATP substrate pyranine (8-hydroxypyrene-1,3,6-trisulfonate) enters the cells via OATP1B1/3 or OATP2B1 function. In living cells, Ace is then converted into highly fluorescent pyranine, allowing "no-wash" measurement of OATP function and drug interactions. Furthermore, we demonstrate that Ace can be used in an indirect assay termed as competitive counterflow suitable to distinguish between transported substrates and inhibitors of OATP1B1. The fluorescence-based methods described here are unique and open the way toward high-throughput screening of interactions between new molecular entities and OATPs.

Imaging-Based Characterization of a Slco2b1(-/-) Mouse Model Using [11C]Erlotinib and [99mTc]Mebrofenin as Probe Substrates

Organic anion-transporting polypeptide 2B1 (OATP2B1) is co-localized with OATP1B1 and OATP1B3 in the basolateral hepatocyte membrane, where it is thought to contribute to the hepatic uptake of drugs. We characterized a novel Slco2b1(-/-) mouse model using positron emission tomography (PET) imaging with [11C]erlotinib (a putative OATP2B1-selective substrate) and planar scintigraphic imaging with [99mTc]mebrofenin (an OATP1B1/1B3 substrate, which is not transported by OATP2B1). Dynamic 40-min scans were performed after intravenous injection of either [11C]erlotinib or [99mTc]mebrofenin in wild-type and Slco2b1(-/-) mice. A pharmacokinetic model was used to estimate the hepatic uptake clearance (CL1) and the rate constants for transfer of radioactivity from the liver to the blood (k2) and excreted bile (k3). CL1 was significantly reduced in Slco2b1(-/-) mice for both radiotracers (p < 0.05), and k2 was significantly lower (p < 0.01) in Slco2b1(-/-) mice for [11C]erlotinib, but not for [99mTc]mebrofenin. Our data support previous evidence that OATP transporters may contribute to the hepatic uptake of [11C]erlotinib. However, the decreased hepatic uptake of the OATP1B1/1B3 substrate [99mTc]mebrofenin in Slco2b1(-/-) mice questions the utility of this mouse model to assess the relative contribution of OATP2B1 to the liver uptake of drugs which are substrates of multiple OATPs.

Expression and Function of Organic Anion Transporting Polypeptides in the Human Brain: Physiological and Pharmacological Implications

The central nervous system (CNS) is an important pharmacological target, but it is very effectively protected by the blood–brain barrier (BBB), thereby impairing the efficacy of many potential active compounds as they are unable to cross this barrier. Among others, membranous efflux transporters like P-Glycoprotein are involved in the integrity of this barrier. In addition to these, however, uptake transporters have also been found to selectively uptake certain compounds into the CNS. These transporters are localized in the BBB as well as in neurons or in the choroid plexus. Among them, from a pharmacological point of view, representatives of the organic anion transporting polypeptides (OATPs) are of particular interest, as they mediate the cellular entry of a variety of different pharmaceutical compounds. Thus, OATPs in the BBB potentially offer the possibility of CNS targeting approaches. For these purposes, a profound understanding of the expression and localization of these transporters is crucial. This review therefore summarizes the current state of knowledge of the expression and localization of OATPs in the CNS, gives an overview of their possible physiological role, and outlines their possible pharmacological relevance using selected examples.

Characterization of Novel Fluorescent Bile Salt Derivatives for Studying Human Bile Salt and Organic Anion Transporters

Bile salts, such as cholate, glycocholate, taurocholate, and glycochenodeoxycholate, are taken up from the portal blood into hepatocytes via transporters, such as the Na+-taurocholate-cotransporting polypeptide (NTCP) and organic anion-transporting polypeptides (OATPs). These bile salts are later secreted into bile across the canalicular membrane, which is facilitated by the bile salt export pump (BSEP). Apart from bile salt transport, some of these proteins (e.g., OATPs) are also key transporters for drug uptake into hepatocytes. In vivo studies of transporter function in patients by using tracer compounds have emerged as an important diagnostic tool to complement classic liver parameter measurements by determining dynamic liver function both for diagnosis and monitoring progression or improvement of liver diseases. Such approaches include use of radioactively labeled bile salts (e.g., for positron emission tomography) and fluorescent bile salt derivatives or dyes (e.g., indocyanine green). To expand the list of liver function markers, we synthesized fluorescent derivatives of cholic and chenodeoxycholic acid by conjugating small organic dyes to the bile acid side chain. These novel fluorescent probes were able to block substrate transport in a concentration-dependent manner of NTCP, OATP1B1, OATP1B3, OATP2B1, BSEP, and intestinal apical sodium-dependent bile salt transporter (ASBT). Whereas the fluorescent bile acid derivatives themselves were transported across the membrane by OATP1B1, OATP1B3, and OATP2B1, they were not transport substrates for NTCP, ASBT, BSEP, and multidrug resistance-related protein 2. Accordingly, these novel fluorescent bile acid probes can potentially be used as imaging agents to monitor the function of OATPs. SIGNIFICANCE STATEMENT: Synthetic modification of common bile acids by attachment of small organic fluorescent dyes to the bile acid side chain resulted in bright, fluorescent probes that interact with hepatic and intestinal organic anion [organic anion-transporting polypeptide (OATP) 1B1, OATP1B3, OATP2B1], bile salt uptake (Na+-taurocholate-cotransporting polypeptide, apical sodium-dependent bile salt transporter), and bile salt efflux (bile salt export pump, multidrug resistance-related protein 2) transporters. Although the fluorescent bile salt derivatives are taken up into cells via the OATPs, the efflux transporters do not transport any of them but one.

OATP2B1 - The underrated member of the organic anion transporting polypeptide family of drug transporters?

The organic anion transporting polypeptide 2B1 (OATP2B1) was one of the first cloned members of the SLCO family. However, its physiological and pharmacological role is still poorly understood, and object of a current debate on the transporter's relevance. Within this commentary, we summarize the data currently available on the transporter's expression and its substrates and highlight the strength and difficulties of the methods that have been applied to gather these data. The conclusion drawn from these findings was that OATP2B1 due to its intestinal expression is most likely involved in oral drug absorption of its substrate and therefore prone for interactions. This has been tested in in vivo drug interaction and/or pharmacogenetic studies. While some of these support the notion of OATP2B1 being of relevance in drug absorption, the pharmacogenetic findings are rather inconclusive. We will explain our thoughts why OATP2B1 may not influence the general systemic pharmacokinetic of certain substrates, but possibly local distribution processes, like the transfer across the blood-brain-barrier. Besides the pharmacokinetic aspects, there are data on endogenous molecules like coproporphyrins and sulfated steroids. Therefore, we will also highlight possible physiological roles of OATP2B1, which are driven by its expression pattern in the tubular cells of the kidney as well as its expression in the blood brain barrier. Finally we also deal with the advantages and disadvantages in the use of animal models to decipher the role of OATP2B1 in pharmacokinetics of its substrates and beyond.

Bile Acid Toxicity and Protein Kinases

If the bile acids reach to pathological concentrations due to cholestasis, accumulation of hydrophobic bile acids within the hepatocyte may result in cell death. Thus, hydrophobic bile acids induce apoptosis in hepatocytes, while hydrophilic bile acids increase intracellular adenosine 3',5'-monophosphate (cAMP) levels and activate mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) pathways to protect hepatocytes from apoptosis.Two apoptotic pathways have been described in bile acids-induced death. Both are controlled by multiple protein kinase signaling pathways. In mitochondria-controlled pathway, caspase-8 is activated with death domain-independent manner, whereas, Fas-dependent classical pathway involves ligand-independent oligomerization of Fas.Hydrophobic bile acids dose-dependently upregulate the inflammatory response by further stimulating production of inflammatory cytokines. Death receptor-mediated apoptosis is regulated at the cell surface by the receptor expression, at the death-inducing signaling complex (DISC) by expression of procaspase-8, the death receptors Fas-associated death domain (FADD), and cellular FADD-like interleukin 1-beta (IL-1β)-converting enzyme (FLICE) inhibitory protein (cFLIP). Bile acids prevent cFLIP recruitment to the DISC and thereby enhance initiator caspase activation and lead to cholestatic apoptosis. At mitochondria, the expression of B-cell leukemia/lymphoma-2 (Bcl-2) family proteins contribute to apoptosis by regulating mitochondrial cytochrome c release via Bcl-2, Bcl-2 homology 3 (BH3) interacting domain death agonist (Bid), or Bcl-2 associated protein x (Bax). Fas receptor CD95 activation by hydrophobic bile acids is initiated by reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-dependent reactive oxygen species (ROS) signaling. However, activation of necroptosis by ligands of death receptors requires the kinase activity of receptor interacting protein1 (RIP1), which mediates the activation of RIP3 and mixed lineage kinase domain-like protein (MLKL). In this chapter, mainly the effect of protein kinases signal transduction on the mechanisms of hydrophobic bile acids-induced inflammation, apoptosis, necroptosis and necrosis are discussed.

Endogenous Coproporphyrin I and III are Altered in Multidrug Resistance-Associated Protein 2-Deficient (TR-) Rats

Recent studies have focused on coproporphyrin (CP)-I and CP-III (CPs) as endogenous biomarkers for organic anion transporting polypeptides (OATPs). Previous data showed that CPs are also substrates of multidrug resistance-associated protein (MRP/Mrp) 2 and 3. This study was designed to examine the impact of loss of Mrp2 function on the routes of excretion of endogenous CPs in wild-type (WT) Wistar compared to Mrp2-deficient TR- rats. To exclude possible confounding effects of rat Oatps, the transport of CPs was investigated in Oatp-overexpressing HeLa cells. Results indicated that CPs are substrates of rodent Oatp1b2, and that CP-III is a substrate of Oatp2b1. Quantitative targeted absolute proteomic (QTAP) analysis revealed no differences in Oatps, but an expected significant increase in Mrp3 protein levels in TR- compared to WT rat livers. CP-I and CP-III concentrations measured by LC-MS/MS were elevated in TR- compared to WT rat liver, while CP-I and CP-III estimated biliary clearance was decreased 75- and 840-fold in TR- compared to WT rats, respectively. CP-III concentrations were decreased 14-fold in the feces of TR- compared to WT rats, but differences in CP-I were not significant. In summary, the disposition of CPs was markedly altered by loss of Mrp2 and increased Mrp3 function as measured in TR- rats.

Structure and Function of Hepatobiliary ATP Binding Cassette Transporters

The liver is beyond any doubt the most important metabolic organ of the human body. This function requires an intensive crosstalk within liver cellular structures, but also with other organs. Membrane transport proteins are therefore of upmost importance as they represent the sensors and mediators that shuttle signals from outside to the inside of liver cells and/or vice versa. In this review, we summarize the known literature of liver transport proteins with a clear emphasis on functional and structural information on ATP binding cassette (ABC) transporters, which are expressed in the human liver. These primary active membrane transporters form one of the largest families of membrane proteins. In the liver, they play an essential role in for example bile formation or xenobiotic export. Our review provides a state of the art and comprehensive summary of the current knowledge of hepatobiliary ABC transporters. Clearly, our knowledge has improved with a breath-taking speed over the last few years and will expand further. Thus, this review will provide the status quo and will lay the foundation for new and exciting avenues in liver membrane transporter research.

Inhibitory effect of sixteen pharmaceutical excipients on six major organic cation and anion uptake transporters

To date, relatively little is known about the interactions of pharmaceutical excipients with hepatic and renal drug uptake transporters. The present study was designed to systematically evaluate the effects of 16 commonly consumed excipients on human organic cation transporter 1 and 2 (hOCT1 and hOCT2), human organic anion transporter 1 and 3 (hOAT1 and hOAT3) and human organic anion transporting polypeptide 1B1 and 1B3 (hOATP1B1 and hOATP1B3). The inhibitory effects and mechanisms of excipients on transporters were investigated using in vitro uptake studies, cell viability assays, concentration-dependent studies, and the Lineweaver-Burk plot method. Triton X-100 is a non-competitive inhibitor for all six transporters. Tween 20 inhibits hOCT2, hOAT1, hOAT3, and hOATP1B3 in a mixed way, whereas it competitively inhibits hOATP1B1. The inhibition of Tween 80 is competitive for hOCT2, non-competitive for hOATP1B1 and hOATP1B3, and mixed for hOAT1 and hOAT3. Concentration-dependent studies identify Triton X-100 as a strong inhibitor of hOCT1 and hOCT2 with IC₅₀ values of 20.1 and 4.54 μg/mL, respectively. Additionally, Triton X-100, Tween 20, and Tween 80 strongly inhibit hOAT3 with IC₅₀ values ≤31.0 μg/mL. The present study is significant in understanding the excipient-drug interactions and provides valuable information for excipient selection in drug development.

Bromosulfophthalein suppresses inflammatory effects in lipopolysaccharide-stimulated RAW264.7 macrophages

OBJECTIVE

It has been reported that glutathione (GSH), the most abundant cellular antioxidant, can inhibit production of pro-inflammatory cytokines by activated macrophages. Bromosulfophthalein (BSP) has been recognized as an inhibitor of the efflux of reduced GSH from cells, leading to an increase in the intracellular GSH level. In this study, we evaluated, for the first time, whether BSP possessed anti-inflammatory effects in lipopolysaccharide (LPS)-stimulated macrophages.

MATERIALS AND METHODS

RAW 264.7 cells were treated with BSP and the levels of proinflammatory cytokines, GSH, and nitrite were assessed. Gene expression of inducible nitric oxide synthase (iNOS), tumor necrosis factor alpha (TNF α), interleukin-1beta (IL-1β), and interleukin-6 (IL-6) was analyzed via quantitative RT-PCR. We also examined various inflammatory signaling pathways including Akt/forkhead box protein O1 (FoxO1)/toll-like receptor 4 (TLR4), mitogen-activated protein kinases (MAPKs), and Fas protein by Western blot and flow cytometry analysis.

RESULTS

Our study demonstrated that BSP induced an increase in intracellular GSH level in LPS-stimulated macrophages. BSP inhibited production of nitric oxide and proinflammatory cytokines. BSP increased phosphorylation of Akt and nuclear exclusion of FoxO1 and suppressed TLR4 expression. Additionally, BSP decreased MAPKs activation and Fas expression.

DISCUSSION AND CONCLUSION

Taken together, these data suggest that BSP can attenuate inflammation through multiple signaling pathways. These findings highlight the potential of BSP as a new anti-inflammatory agent.