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

Uptake Transporters of the SLC21, SLC22A, and SLC15A Families in Anticancer Therapy-Modulators of Cellular Entry or Pharmacokinetics?

Solute carrier transporters comprise a large family of uptake transporters involved in the transmembrane transport of a wide array of endogenous substrates such as hormones, nutrients, and metabolites as well as of clinically important drugs. Several cancer therapeutics, ranging from chemotherapeutics such as topoisomerase inhibitors, DNA-intercalating drugs, and microtubule binders to targeted therapeutics such as tyrosine kinase inhibitors are substrates of solute carrier (SLC) transporters. Given that SLC transporters are expressed both in organs pivotal to drug absorption, distribution, metabolism, and elimination and in tumors, these transporters constitute determinants of cellular drug accumulation influencing intracellular drug concentration required for efficacy of the cancer treatment in tumor cells. In this review, we explore the current understanding of members of three SLC families, namely SLC21 (organic anion transporting polypeptides, OATPs), SLC22A (organic cation transporters, OCTs; organic cation/carnitine transporters, OCTNs; and organic anion transporters OATs), and SLC15A (peptide transporters, PEPTs) in the etiology of cancer, in transport of chemotherapeutic drugs, and their influence on efficacy or toxicity of pharmacotherapy. We further explore the idea to exploit the function of SLC transporters to enhance cancer cell accumulation of chemotherapeutics, which would be expected to reduce toxic side effects in healthy tissue and to improve efficacy.

Probe Cocktail to Assess Transporter Function in Sandwich-Cultured Human Hepatocytes

PURPOSE

Probe substrates are used routinely to assess transporter function in vitro. Administration of multiple probe substrates together as a "cocktail" in sandwich-cultured human hepatocytes (SCHH) could increase the throughput of transporter function assessment in a physiologically-relevant in vitro system. This study was designed to compare transporter function between cocktail and single agent administration in SCHH.

METHODS

Rosuvastatin, digoxin, and metformin were selected as probe substrates of hepatic transporters OATP1B1, OATP1B3, BCRP, P-gp, and OCT1. Total accumulation (Cells+Bile) and biliary excretion index (BEI) values derived from administration of the cocktail were compared to values obtained after administration of single agents in the absence and presence of a model inhibitor, erythromycin estolate.

RESULTS

For rosuvastatin and metformin accumulation, the ratio of means [90% confidence interval (CI)] for cocktail to single agent administration was 100% [94%, 106%] and 90% [82%, 99%], respectively. Therefore, the cocktail and single-agent mode of administration were deemed equivalent per standard equivalence criterion of 80-120% for rosuvastatin and metformin accumulation, but not for digoxin accumulation (77% [62%, 92%]). The ratio of means [90% CI] for rosuvastatin BEI values between the two administration modes (105% [97%, 114%]) also was deemed equivalent. The ratio for digoxin BEI values between the two administration modes was 99% [78%, 120%]. In the presence of erythromycin estolate, the two administration modes were deemed equivalent for evaluation of rosuvastatin, digoxin, and metformin accumulation; the ratio of means [90% CI] was 104% [94%, 115%], 94% [82%, 105%], and 100% [88%, 111%], respectively. However, rosuvastatin and digoxin BEI values were low and quite variable in the presence of the inhibitor, so the BEI results were inconclusive.

CONCLUSIONS

These data suggest that rosuvastatin and metformin can be administered as a cocktail to evaluate the function of OATP1B1, OATP1B3, BCRP, and OCT1 in SCHH, and that digoxin may not be an ideal component of such a cocktail.

Regulation of Organic Anion Transporting Polypeptide 2B1 Expression by MicroRNA in the Human Liver

Organic anion transporting polypeptide 2B1 (OATP2B1, SLCO2B1) is an uptake transporter expressed in several tissues, including the liver, intestine, brain, kidney, and skeletal muscle. Hepatocyte nuclear factor 4 alpha (HNF4α) is known as an important transcriptional factor of OATP2B1 in the liver. It has been reported that there are large interindividual differences in OATP2B1 mRNA and protein expressions in human livers. The mechanism causing the interindividual differences in OATP2B1 expression is still unclear. MicroRNAs (miRNAs) control gene expression by leading translational repression and/or degradation of the target mRNA. There is no significant correlation between OATP2B1 mRNA and protein expression, suggesting that post-transcriptional regulating mechanisms, such as miRNAs, play an important role in the interindividual differences in OATP2B1 expression. In this study, we hypothesized that certain miRNAs cause the interindividual differences in OATP2B1 expression in the human liver. In silico analysis showed that miR-24 was a candidate miRNA regulating OATP2B1 expression. It has been reported that miR-24 degrades HNF4α mRNA expression. We revealed that the miR-24 expression level was negatively correlated with OATP2B1 mRNA, protein, and HNF4α mRNA expression levels in human livers. Transfection by the miR-24 precursor decreased the luciferase activity in the transfected cells with the vector containing the OATP2B1 3' untranslated region (3'UTR) or SLCO2B1 promoter region. In HepaRG cells, miR-24 decreased the OATP2B1 and HNF4α expression levels. These results suggest that miR-24 represses not only the translation of OATP2B1 but also the transcription of OATP2B1 by HNF4α mRNA degradation.

The involvement of human organic anion transporting polypeptides (OATPs) in drug-herb/food interactions

Organic anion transporting polypeptides (OATPs) are important transporter proteins that are expressed at the plasma membrane of cells, where they mediate the influx of endogenous and exogenous substances including hormones, natural compounds and many clinically important drugs. OATP1A2, OATP2B1, OATP1B1 and OATP1B3 are the most important OATP isoforms and influence the pharmacokinetic performance of drugs. These OATPs are highly expressed in the kidney, intestine and liver, where they determine the distribution of drugs to these tissues. Herbal medicines are increasingly popular for their potential health benefits. Humans are also exposed to many natural compounds in fruits, vegetables and other food sources. In consequence, the consumption of herbal medicines or food sources together with a range of important drugs can result in drug-herb/food interactions via competing specific OATPs. Such interactions may lead to adverse clinical outcomes and unexpected toxicities of drug therapies. This review summarises the drug-herb/food interactions of drugs and chemicals that are present in herbal medicines and/or food in relation to human OATPs. This information can contribute to improving clinical outcomes and avoiding unexpected toxicities of drug therapies in patients.

Novel insights into the organic solute transporter alpha/beta, OSTα/β: From the bench to the bedside

Organic solute transporter alpha/beta (OSTα/β) is a heteromeric solute carrier protein that transports bile acids, steroid metabolites and drugs into and out of cells. OSTα/β protein is expressed in various tissues, but its expression is highest in the gastrointestinal tract where it facilitates the recirculation of bile acids from the gut to the liver. Previous studies established that OSTα/β is upregulated in liver tissue of patients with extrahepatic cholestasis, obstructive cholestasis, and primary biliary cholangitis (PBC), conditions that are characterized by elevated bile acid concentrations in the liver and/or systemic circulation. The discovery that OSTα/β is highly upregulated in the liver of patients with nonalcoholic steatohepatitis (NASH) further highlights the clinical relevance of this transporter because the incidence of NASH is increasing at an alarming rate with the obesity epidemic. Since OSTα/β is closely linked to the homeostasis of bile acids, and tightly regulated by the nuclear receptor farnesoid X receptor, OSTα/β is a potential drug target for treatment of cholestatic liver disease, and other bile acid-related metabolic disorders such as obesity and diabetes. Obeticholic acid, a semi-synthetic bile acid used to treat PBC, under review for the treatment of NASH, and in development for the treatment of other metabolic disorders, induces OSTα/β. Some drugs associated with hepatotoxicity inhibit OSTα/β, suggesting a possible role for OSTα/β in drug-induced liver injury (DILI). Furthermore, clinical cases of homozygous genetic defects in both OSTα/β subunits resulting in diarrhea and features of cholestasis have been reported. This review article has been compiled to comprehensively summarize the recent data emerging on OSTα/β, recapitulating the available literature on the structure-function and expression-function relationships of OSTα/β, the regulation of this important transporter, the interaction of drugs and other compounds with OSTα/β, and the comparison of OSTα/β with other solute carrier transporters as well as adenosine triphosphate-binding cassette transporters. Findings from basic to more clinically focused research efforts are described and discussed.

Effect of P-glycoprotein (P-gp) Inducers on Exposure of P-gp Substrates: Review of Clinical Drug-Drug Interaction Studies

Understanding transporter-mediated drug-drug interactions (DDIs) for investigational agents is important during drug development to assess DDI liability, its clinical relevance, and to determine appropriate DDI management strategies. P-glycoprotein (P-gp) is an efflux transporter that influences the pharmacokinetics (PK) of various compounds. Assessing transporter induction in vitro is challenging and is not always predictive of in vivo effects, and hence there is a need to consider clinical DDI studies; however, there is no clear guidance on when clinical evaluation of transporter induction is required. Furthermore, there is no proposed list of index transporter inducers to be used in clinical studies. This review evaluated DDI studies with known P-gp inducers to better understand the mechanism and site of P-gp induction, as well as the magnitude of induction effect on the exposure of P-gp substrates. Our review indicates that P-gp and cytochrome P450 (CYP450) enzymes are co-regulated via the pregnane xenobiotic receptor (PXR) and the constitutive androstane receptor (CAR). The magnitude of the decrease in substrate drug exposure by P-gp induction is generally less than that of CYP3A. Most P-gp inducers reduced total bioavailability with a minor impact on renal clearance, despite known expression of P-gp at the apical membrane of the kidney proximal tubules. Rifampin is the most potent P-gp inducer, resulting in an average reduction in substrate exposure ranging between 20 and 67%. For other inducers, the reduction in P-gp substrate exposure ranged from 12 to 42%. A lower reduction in exposure of the P-gp substrate was observed with a lower dose of the inducer and/or if the administration of the inducer and substrate was simultaneous, i.e. not staggered. These findings suggest that clinical evaluation of the impact of P-gp inducers on the PK of investigational agents that are substrates for P-gp might be warranted only for compounds with a relatively steep exposure-efficacy relationship.

Structural dissection of 13-epiestrones based on the interaction with human Organic anion-transporting polypeptide, OATP2B1

Human OATP2B1 encoded by the SLCO2B1 gene is a multispecific transporter mediating the cellular uptake of large, organic molecules, including hormones, prostaglandins and bile acids. OATP2B1 is ubiquitously expressed in the human body, with highest expression levels in pharmacologically relevant barriers, like enterocytes, hepatocytes and endothelial cells of the blood-brain-barrier. In addition to its endogenous substrates, OATP2B1 also recognizes clinically applied drugs, such as statins, antivirals, antihistamines and chemotherapeutic agents and influences their pharmacokinetics. On the other hand, OATP2B1 is also overexpressed in various tumors. Considering that elevated hormone uptake by OATP2B1 results in increased cell proliferation of hormone dependent tumors (e.g. breast or prostate), inhibition of OATP2B1 can be a good strategy to inhibit the growth of these tumors. 13-epiestrones represent a potential novel strategy in the treatment of hormone dependent cancers by the suppression of local estrogen production due to the inhibition of the key enzyme of estrone metabolism, 17ß-hydroxysteroid-dehydrogenase type 1 (HSD17ß1). Recently, we have demonstrated that various phosphonated 13-epiestrones are dual inhibitors also suppressing OATP2B1 function. In order to gain better insights into the molecular determinants of OATP2B1 13-epiestrone interaction we investigated the effect of C-2 and C-4 halogen or phenylalkynyl modified epiestrones on OATP2B1 transport function. Potent inhibitors (with EC₅₀ values in the low micromolar range) as well as non-inhibitors of OATP2B1 function were identified. Based on the structure-activity relationship (SAR) of the various 13-epiestrone derivatives we could define structural elements important for OATP2B1 inhibition. Our results may help to understand the drug/inhibitor interaction profile of OATP2B1, and also may be a useful strategy to block steroid hormone entry into tumors.

Investigation of Fluorescent Substrates and Substrate-Dependent Interactions of a Drug Transporter Organic Anion Transporting Polypeptide 2B1 (OATP2B1)

PURPOSE

In this study, we investigated organic anion transporting polypeptide 2B1 (OATP2B1)-mediated uptake of fluorescent anions to better identify fluorescent substrates for in vitro OATP2B1 assays. The OATP2B1 is involved in the intestinal absorption and one of the pharmacokinetic determinants of orally administered drugs.

METHODS

A microplate reader was used to determine the cellular accumulation of the fluorescent compounds into the OATP2B1 or the empty vector-transfected HEK293 cells.

RESULTS

Two types of derivatives were found to be OATP2B1 substrates: heavy halogenated derivatives, such as 4',5'-dibromofluorescein (DBF), and carboxylated derivatives, such as 5-carboxyfluorescein (5-CF). The DBF and 5-CF were transported in a time and concentration-dependent manner. The DBF was transported at a broad pH (pH 6.5-8.0) while 5-CF was transported at an acidic pH (pH 5.5-6.5). The K_m values were 0.818 ± 0.067 μM at pH 7.4 for DBF and 8.56 ± 0.41 μM at pH 5.5 for 5-CF. The OATP2B1 inhibitors, including atorvastatin, bromosulfophthalein, glibenclamide, sulfasalazine, talinolol, and estrone 3-sulfate, inhibited the DBF and the 5-CF transport. Contrastively, testosterone, dehydroepiandrosterone sulfate, and progesterone inhibited the DBF transport but stimulated the 5-CF transport. Natural flavonoid aglycones, such as naringenin and baicalein, also exhibited substrate-dependent effects in this manner.

CONCLUSION

We found two fluorescein analogs, DBF and 5-CF as the OATP2B1 substrates that exhibited substrate-dependent interactions.

Fluorescent probes for the dual investigation of MRP2 and OATP1B1 function and drug interactions

Detoxification in hepatocytes is a strictly controlled process, in which the governed action of membrane transporters involved in the uptake and efflux of potentially dangerous molecules has a crucial role. Major transporters of hepatic clearance belong to the ABC (ATP Binding Cassette) and Solute Carrier (SLC) protein families. Organic anion-transporting polypeptide OATP1B1 (encoded by the SLCO1B1 gene) is exclusively expressed in the sinusoidal membrane of hepatocytes, where it mediates the cellular uptake of bile acids, bilirubin, and also that of various drugs. The removal of toxic molecules from hepatocytes to the bile is accomplished by several ABC transporters, including P-glycoprotein (ABCB1), MRP2 (ABCC2) and BCRP (ABCG2). Owing to their pharmacological relevance, monitoring drug interaction with OATP1B1/3 and ABC proteins is recommended. Our aim was to assess the interaction of recently identified fluorescent OATP substrates (various dyes used in cell viability assays, pyranine, Cascade Blue hydrazide (CB) and sulforhodamine 101 (SR101)) (Bakos et al., 2019; Patik et al., 2018) with MRP2 and ABCG2 in order to find fluorescent probes for the simultaneous characterization of both uptake and efflux processes. Transport by MRP2 and ABCG2 was investigated in inside-out membrane vesicles (IOVs) allowing a fast screen of the transport of membrane impermeable substrates by efflux transporters. Next, transcellular transport of shared OATP and ABC transporter substrate dyes was evaluated in MDCKII cells co-expressing OATP1B1 and MRP2 or ABCG2. Our results indicate that pyranine is a general substrate of OATP1B1, OATP1B3 and OATP2B1, and we find that the dye Live/Dead Violet and CB are good tools to investigate ABCG2 function in IOVs. Besides their suitability for MRP2 functional tests in the IOV setup, pyranine, CB and SR101 are the first dual probes that can be used to simultaneously measure OATP1B1 and MRP2 function in polarized cells by a fluorescent method.

Thyroid Hormone Transporters

Thyroid hormone transporters at the plasma membrane govern intracellular bioavailability of thyroid hormone. Monocarboxylate transporter (MCT) 8 and MCT10, organic anion transporting polypeptide (OATP) 1C1, and SLC17A4 are currently known as transporters displaying the highest specificity toward thyroid hormones. Structure-function studies using homology modeling and mutational screens have led to better understanding of the molecular basis of thyroid hormone transport. Mutations in MCT8 and in OATP1C1 have been associated with clinical disorders. Different animal models have provided insight into the functional role of thyroid hormone transporters, in particular MCT8. Different treatment strategies for MCT8 deficiency have been explored, of which thyroid hormone analogue therapy is currently applied in patients. Future studies may reveal the identity of as-yet-undiscovered thyroid hormone transporters. Complementary studies employing animal and human models will provide further insight into the role of transporters in health and disease. (Endocrine Reviews 41: 1 - 55, 2020).

The Emerging Role of the SLCO1B3 Protein in Cancer Resistance

Currently, chemotherapy is one of the mainstays of oncologic therapies. But the efficacy of chemotherapy is often limited by drug resistance and severe side effects. Consequently, it is becoming increasingly important to investigate the underlying mechanism and overcome the problem of anticancer chemotherapy resistance. The solute carrier organic anion transporter family member 1B3 (SLCO1B3), a functional transporter normally expressed in the liver, transports a variety of endogenous and exogenous compounds, including hormones and their conjugates as well as some anticancer drugs. The extrahepatic expression of SLCO1B3 has been detected in different cancer cell lines and cancer tissues. Recently, accumulating data indicates that the abnormal expression and function of SLCO1B3 are involved in resistance to anticancer drugs, such as taxanes, camptothecin and its analogs, SN-38, and Androgen Deprivation Therapy (ADT) in breast, prostate, lung, hepatic, and colorectal cancer, respectively. Thus, more investigations have been implemented to identify the potential SLCO1B3-related mechanisms of cancer drug resistance. In this review, we focus on the emerging roles of SLCO1B3 protein in the development of cancer chemotherapy resistance and briefly discuss the mechanisms of resistance. Elucidating the function of SLCO1B3 in chemoresistance may bring out novel therapeutic strategies for cancer treatment.

Influence of SLCO1B1 polymorphisms on lopinavir Ctrough in Serbian HIV/AIDS patients

AIMS

Lopinavir (LPV) is not a first-line regimen. According to recent WHO data, LPV usage in low- and middle-income countries accounted for approximately 52% of the adult and 23% of the paediatric protease inhibitor market in 2017. Since LPV is a substrate for the SLCO1B1 (OATP1B1) transporter, the aim of this study was to assess the impact of SLCO1B1 polymorphisms (rs11045819, rs4149032 and rs4149056) on LPV trough plasma concentrations (C_trough ) in Serbian patients.

METHODS

Plasma samples from 104 HIV/AIDS Caucasians were collected. LPV C_trough was quantified using liquid-chromatography-mass spectrometry. Genotyping was carried out using real-time-PCR-based allelic discrimination. One-way analysis of variance, t test and linear regression were used for data analysis.

RESULTS

The overall mean (SD) LPV C_trough was 5885 ± 2755 ng/mL. Significant differences were between patients with different rs11045819 genotypes: CC (LPV median C_trough = 6072 ng/mL, interquartile range (IQR) = 4318-7617 ng/mL), CA (LPV median C_trough = 4987 ng/mL, IQR = 4300-6295 ng/mL) and AA (LPV median C_trough = 3648 ng/mL, IQR = 1949-4072 ng/mL) (P = .005). Significant differences were also observed according to rs4149032 genotype: CC (LPV median C_trough = 6027 ng/mL, IQR =4548-8250 ng/mL), CT (LPV median C_trough = 5553 ng/mL, IQR = 4300-6888 ng/mL) and TT (LPV median C_trough = 4408 ng/mL, IQR = 3361-5233 ng/mL) (P = .007). For rs4149056 a statistically significant difference between T-homozygotes (LPV median C_trough = 5434 ng/mL, IQR = 3855-6830 ng/mL), heterozygotes (LPV median C_trough = 6707 ng/mL, IQR = 5088-8063 ng/mL) and C-homozygotes (LPV median C_trough = 13906 ng/mL, IQR = 12946-14866 ng/mL) was observed (P = .002). In multivariate regression analysis, only the SLCO1B1 rs4149056 polymorphism was independently associated with higher LPV C_trough (β = 2834.5 [1442-4226.9] ng/mL [P = .001]).

CONCLUSIONS

Our results demonstrate a statistically significant influence of the SLCO1B1 rs4149056 polymorphism on higher LPV C_trough in Caucasian HIV/AIDS patients.

Transport of Bupropion and its Metabolites by the Model CHO and HEK293 Cell Lines

BACKGROUND

Bupropion (BUP) is widely used as an antidepressant and smoking cessation aid. There are three major pharmacologically active metabolites of BUP, Erythrohydrobupropion (EB), Hydroxybupropion (OHB) and Threohydrobupropion (TB). At present, the mechanisms underlying the overall disposition and systemic clearance of BUP and its metabolites have not been well understood, and the role of transporters has not been studied.

OBJECTIVE

The goal of this study was to investigate whether BUP and its active metabolites are substrates of the major hepatic uptake and efflux transporters.

METHOD

CHO or HEK293 cell lines or plasma membrane vesicles that overexpress OATP1B1, OATP1B3, OATP2B1, OATP4A1, OCT1, BCRP, MRP2 or P-gp were used in cellular or vesicle uptake and inhibition assays. Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) was used to quantify transport activity.

RESULTS

BUP and its major active metabolites were actively transported into the CHO or HEK293 cells overexpressing OATP1B1, OATP1B3 or OATP2B1; however, such cellular active uptake could not be inhibited at all by prototypical inhibitors of any of the OATP transporters. These compounds were not transported by OCT1, BCRP, MRP2 or P-gp either. These results suggest that the major known hepatic transporters likely play a minor role in the overall disposition and systemic clearance of BUP and its active metabolites in humans. We also demonstrated that BUP and its metabolites were not transported by OATP4A1, an uptake transporter on the apical membrane of placental syncytiotrophoblasts, suggesting that OATP4A1 is not responsible for the transfer of BUP and its metabolites from the maternal blood to the fetal compartment across the placental barrier in pregnant women.

CONCLUSION

BUP and metabolites are not substrates of the major hepatic transporters tested and thus these hepatic transporters likely do not play a role in the overall disposition of the drug. Our results also suggest that caution should be taken when using the model CHO and HEK293 cell lines to evaluate potential roles of transporters in drug disposition.

Effects of Maribavir on P-Glycoprotein and CYP2D6 in Healthy Volunteers

Maribavir is an investigational drug being evaluated in transplant recipients with cytomegalovirus infection. To understand potential drug-drug interactions, we examined the effects of multiple doses of maribavir on cytochrome P450 (CYP) 2D6 and P-glycoprotein (P-gp) activity using probe substrates in healthy volunteers. During this phase 1 open-label study (NCT02775240), participants received the probe substrates digoxin (0.5 mg) and dextromethorphan (30 mg) before and after maribavir (400 mg twice daily for 8 days). Serial plasma samples were analyzed for digoxin, dextromethorpha, dextrorphan, and maribavir concentrations. Pharmacokinetic parameters were calculated (noncompartmental analysis) and analyzed with a linear mixed-effects model for treatment comparison to estimate geometric mean ratios (GMRs) and 90% confidence intervals (CIs). CYP2D6 polymorphisms were genotyped using polymerase chain reaction. Overall, 17 of 18 participants (94.4%) completed the study. All participants were genotyped as CYP2D6 intermediate/extensive metabolizers. GMR (90%CI) of digoxin Cmax , AUClast , and AUC0-∞ with and without maribavir was 1.257 (1.139-1.387), 1.187 (1.088-1.296), and 1.217 (1.110-1.335), respectively, outside the "no-effect" window (0.8-1.25). GMR (90%CI) of dextromethorphan AUClast and AUClast ratio of dextromethorphan/dextrorphan were 0.877 (0.692-1.112) and 0.901 (0.717-1.133), respectively, marginally outside the no-effect window, although large variability was observed in these pharmacokinetic parameters. Pharmacokinetic parameters of dextrorphan were unaffected. Maribavir inhibited P-gp activity but did not affect CYP2D6 activity. Maribavir's effect on the pharmacokinetics of P-gp substrates should be evaluated individually, and caution should be exercised with P-gp substrates with narrow therapeutic windows.

Bimodal Molecule as NIR-CT Contrast Agent for Hepatoma Specific Imaging

With currently available molecular imaging techniques, hepatocellular carcinoma (HCC), a liver cancer with high mortality rates and poor treatment responses, is mostly diagnosed at its late stage. This is largely due to the lack of highly sensitive contrast agents with high liver specificity. Herein, we report a novel bimodal contrast agent molecule CNCI-1 for the effective detection of HCC at its early stage both in vitro and in vivo. The agent has high liver specificity with effective X-ray computed tomography (CT)/near-infrared (NIR) imaging functions. It has been successfully applied to in vivo NIR imaging with high sensitivity and high selectivity to the HCC region of the HepG2 tumor-xenografted mice model and LM3 orthotopic hepatoma mice model. Moreover, the agent was found to be noninvasive and hepatocarcinoma cells preferential. Furthermore, it also enhanced the tumor imaging by revealing the blood vessels nearby for the CT image acquisition in the VX2 orthotopic hepatoma rabbit model. Our design strategy provides a new avenue to develop the medical relevant bimodal contrast agents for diagnosis of HCC at its early stage.

Overview of organic anion transporters and organic anion transporter polypeptides and their roles in the liver

Organic anion transporters (OATs) and organic anion transporter polypeptides (OATPs) are classified within two SLC superfamilies, namely, the SLC22A superfamily and the SLCO superfamily (formerly the SLC21A family), respectively. They are expressed in many tissues, such as the liver and kidney, and mediate the absorption and excretion of many endogenous and exogenous substances, including various drugs. Most are composed of 12 transmembrane polypeptide chains with the C-terminus and the N-terminus located in the cell cytoplasm. OATs and OATPs are abundantly expressed in the liver, where they mainly promote the uptake of various endogenous substrates such as bile acids and various exogenous drugs such as antifibrotic and anticancer drugs. However, differences in the locations of glycosylation sites, phosphorylation sites, and amino acids in the OAT and OATP structures lead to different substrates being transported to the liver, which ultimately results in their different roles in the liver. To date, few articles have addressed these aspects of OAT and OATP structures, and we study further the similarities and differences in their structures, tissue distribution, substrates, and roles in liver diseases.

microRNA-206 modulates the hepatic expression of the organic anion-transporting polypeptide 1B1

BACKGROUND & AIMS

The organic anion-transporting polypeptide 1B1 (OATP1B1) is an anion exchanger expressed at the hepatocyte sinusoidal membrane, which mediates the uptake of several endogenous metabolites and drugs. OATP1B1 expression level and activity are major sources of inter-patient variability of pharmacokinetics and pharmacodynamics of several drugs. Besides the genotype, factors that contribute to the inter-individual variability in OATP1B1 expression level are practically unknown. The aim of this work was to uncover novel epigenetic mechanisms of OATP1B1 regulation.

METHODS

A functional screening strategy to assess the effect of microRNAs on the uptake of estrone-3-sulphate, an OATP1B1 substrate, into human hepatocellular carcinoma (Huh-7) cells was used. microRNA-206 (miR-206) expression in human liver tissues was measured by real-time RT-PCR. OATP1B1 expression in Huh-7 and in human liver tissues was assessed by real-time RT-PCR, Western blotting and immunostaining. The mRNA-miRNA interaction was assessed by reporter assay.

RESULTS

miR-206 mimic repressed mRNA and protein expression of OATP1B1 in Huh-7 cells. The intracellular accumulation of estrone-3-sulphate was reduced by 30% in cells overexpressing miR-206. The repressive effect of miR-206 on the activity of the firefly luciferase gene 2 under the control of the OATP1B1 3' untranslated region was lost upon deletion of the predicted miR-206 binding site. Hepatic miR-206 level negatively correlated with OATP1B1 mRNA and protein levels extracted from normal human liver tissues.

CONCLUSIONS

miR-206 exerts a suppressive effect on OATP1B1 expression by an epigenetic mechanism. Individuals with high hepatic levels of miR-206 appear to display lower level of OATP1B1.

Uremic serum residue decreases SN-38 sensitivity through suppression of organic anion transporter polypeptide 2B1 in LS-180 colon cancer cells

Pharmacokinetics of SN-38 in patients with end-stage kidney disease (ESKD) is partially varied because of fluctuations in transporters expression and/or function by high protein bound-uremic toxins concentration. The fluctuations may induce variations in anticancer drugs sensitivity to cancer cells. We aimed to clarify the variations in sensitivity of SN-38 to cancer patients with ESKD and investigate this mechanism, by human colon cancer cells exposed to uremic serum residue. LS180 cells were exposed to normal or uremic serum residue (LS/NSR or LS/USR cells) for a month. IC₅₀ values of SN-38 in LS/NSR or LS/USR cells were calculated from viability of each cells treated SN-38. mRNA expression and intracellular SN-38 accumulation was evaluated by RT-PCR and HPLC-fluorescence methods, respectively. The IC₅₀ value in LS/USR cells was higher than that in LS/NSR cells. Organic anion transporter polypeptide (OATP) 2B1 mRNA expression was lower in LS/USR cells than in LS/NSR cells, and SN-38 accumulation in LS/USR cells was lower than that in LS/NSR cells. Only co-treatment baicalin, which is OATP2B1 inhibitor, almost negated the difference in SN-38 accumulation between LS/NSR and LS/USR. Anticancer effects of substrates of OATP2B1, such as SN-38, were reduced in ESKD patients at the same plasma substrate concentration.

Transporters in the Mammary Gland-Contribution to Presence of Nutrients and Drugs into Milk

A large number of nutrients and bioactive ingredients found in milk play an important role in the nourishment of breast-fed infants and dairy consumers. Some of these ingredients include physiologically relevant compounds such as vitamins, peptides, neuroactive compounds and hormones. Conversely, milk may contain substances-drugs, pesticides, carcinogens, environmental pollutants-which have undesirable effects on health. The transfer of these compounds into milk is unavoidably linked to the function of transport proteins. Expression of transporters belonging to the ATP-binding cassette (ABC-) and Solute Carrier (SLC-) superfamilies varies with the lactation stages of the mammary gland. In particular, Organic Anion Transporting Polypeptides 1A2 (OATP1A2) and 2B1 (OATP2B1), Organic Cation Transporter 1 (OCT1), Novel Organic Cation Transporter 1 (OCTN1), Concentrative Nucleoside Transporters 1, 2 and 3 (CNT1, CNT2 and CNT3), Peptide Transporter 2 (PEPT2), Sodium-dependent Vitamin C Transporter 2 (SVCT2), Multidrug Resistance-associated Protein 5 (ABCC5) and Breast Cancer Resistance Protein (ABCG2) are highly induced during lactation. This review will focus on these transporters overexpressed during lactation and their role in the transfer of products into the milk, including both beneficial and harmful compounds. Furthermore, additional factors, such as regulation, polymorphisms or drug-drug interactions will be described.

Drug-induced liver injury

Drug-induced liver injury (DILI) is an adverse reaction to drugs or other xenobiotics that occurs either as a predictable event when an individual is exposed to toxic doses of some compounds or as an unpredictable event with many drugs in common use. Drugs can be harmful to the liver in susceptible individuals owing to genetic and environmental risk factors. These risk factors modify hepatic metabolism and excretion of the DILI-causative agent leading to cellular stress, cell death, activation of an adaptive immune response and a failure to adapt, with progression to overt liver injury. Idiosyncratic DILI is a relative rare hepatic disorder but can be severe and, in some cases, fatal, presenting with a variety of phenotypes, which mimic other hepatic diseases. The diagnosis of DILI relies on the exclusion of other aetiologies of liver disease as specific biomarkers are still lacking. Clinical scales such as CIOMS/RUCAM can support the diagnostic process but need refinement. A number of clinical variables, validated in prospective cohorts, can be used to predict a more severe DILI outcome. Although no pharmacological therapy has been adequately tested in randomized clinical trials, corticosteroids can be useful, particularly in the emergent form of DILI related to immune-checkpoint inhibitors in patients with cancer.

Fexofenadine and Rosuvastatin Pharmacokinetics in Mice with Targeted Disruption of Organic Anion Transporting Polypeptide 2B1

Organic anion transporting polypeptide 2B1 (OATP2B1) is a widely expressed membrane transporter with diverse substrate specificity. In vitro and clinical studies suggest a role for intestinal OATP2B1 in the oral absorption of medications. Moreover, OATP2B1 is highly expressed in hepatocytes where it is thought to promote liver drug clearance. However, until now, a shortcoming of studies implicating OATP2B1 in drug disposition has been a lack of in vivo models. Here, we report the development of a knockout (KO) mouse model with targeted, global disruption of the Slco2b1 gene to examine the disposition of two confirmed mOATP2B1 substrates, namely, fexofenadine and rosuvastatin. The plasma pharmacokinetics of intravenously administered fexofenadine was not different between KO and wild-type (WT) mice. However, after oral fexofenadine administration, KO mice had 70% and 41% lower maximal plasma concentration (C max) and area under the plasma concentration-time curve (AUC0-last) than WT mice, respectively. In WT mice, coadministration of fexofenadine with grapefruit juice (GFJ) or apple juice (AJ) was associated with reduced C max by 80% and 88%, respectively, while the AUC0-last values were lower by 35% and 70%, respectively. In KO mice, AJ coadministration reduced oral fexofenadine C max and AUC0-last values by 67% and 59%, respectively, while GFJ had no effects. Intravenous and oral rosuvastatin pharmacokinetics were similar among WT and KO mice. We conclude that intestinal OATP2B1 is a determinant of oral fexofenadine absorption, as well as a target for fruit juice interactions. OATP2B1 does not significantly influence rosuvastatin disposition in mice. SIGNIFICANCE STATEMENT: A novel mouse model with targeted disruption of the Slco2b1 gene revealed that OATP2B1 is a determinant of oral absorption but not systemic disposition of fexofenadine, as well as a target of fruit juice interactions. Rosuvastatin oral and intravenous pharmacokinetics were not dependent on OATP2B1. These findings support the utility of the Slco2b1 KO mouse model for defining mechanisms of drug disposition at the intersection of in vitro and clinical pharmacology.

Organic Anion Transporting Polypeptides: Emerging Roles in Cancer Pharmacology

The organic anion transporting polypeptides (OATPs) are a superfamily of drug transporters involved in the uptake and disposition of a wide array of structurally divergent endogenous and exogenous substrates, including steroid hormones, bile acids, and commonly used drugs, such as anti-infectives, antihypertensives, and cholesterol lowering agents. In the past decade, OATPs, primarily OATP1A2, OATP1B1, and OATP1B3, have emerged as potential mediators of chemotherapy disposition, including drugs such as methotrexate, doxorubicin, paclitaxel, docetaxel, irinotecan and its important metabolite 7-ethyl-10-hydroxycamptothecin, and certain tyrosine kinase inhibitors. Furthermore, OATP family members are polymorphic and numerous studies have shown OATP variants to have differential uptake, disposition, and/or pharmacokinetics of numerous drug substrates with important implications for interindividual differences in efficacy and toxicity. Additionally, certain OATPs have been found to be overexpressed in a variety of human solid tumors, including breast, liver, colon, pancreatic, and ovarian cancers, suggesting potential roles for OATPs in tumor development and progression and as novel targets for cancer therapy. This review focuses on the emerging roles for selected OATPs in cancer pharmacology, including preclinical and clinical studies suggesting roles in chemotherapy disposition, the pharmacogenetics of OATPs in cancer therapy, and OATP overexpression in various tumor tissues with implications for OATPs as therapeutic targets.

Influence of OATP1B1 and OATP1B3 mutations and glomerular filtration rate on trough serum digoxin concentration in the Chinese population: A prospective cohort study

Polymorphisms of organic anion transporting polypeptides (OATPs) have been reported to affect trough serum digoxin concentration (SDC). However, the association of these polymorphisms with trough SDC in Chinese heart failure patients has not been studied. We aim to explore whether OATP1B1 388A>G, OATP1B1 521T>C, and OATP1B3 699G>A influence trough SDC in Chinese heart failure patients and to make clinical recommendations.Chinese patients (n = 104) diagnosed with heart failure under long-term digoxin therapy (0.125 mg daily) were enrolled in this study. Blood samples were collected for the analysis of trough SDC (immunofluorescence) and the polymorphisms of OATP1B1 388A>G, OATP1B1 521T>C, and OATP1B3 699G>A (PCR-RFLP and Sanger sequencing).Patients with glomerular filtration rate (GFR) under 30 mL/min had significantly higher trough SDC (1.20 ± 0.50 ng/mL) than recommended trough SDC for heart failure patients. Trough SDC was not significantly influenced by mutations of OATP1B1 388A>G (P = .890), 521T>C (P = .054), and OATP1B3 699G>A (P = .854). Patients with OATP1B1 521T>C mutant-type carrier had slightly higher trough SDC (0.98 ± 0.53 ng/mL) than those with wild-type carrier (0.74 ± 0.40 ng/mL) when they have repaired renal function.Heart failure patients with severe renal dysfunction (GFR<60 mL/min) and/or OATP1B1 521T>C mutant-type carriers are recommended a smaller dosage of digoxin and strict therapeutic drug monitoring.

Identification and Evaluation of Clinical Substrates of Organic Anion Transporting Polypeptides 1B1 and 1B3

Organic anion transporting polypeptides (OATPs) 1B1 and 1B3 facilitate the uptake of drugs and endogenous compounds into the liver. In recent years, the impact of these transporters on drug–drug interactions (DDIs) has become a focus of research, and the evaluation of their role in drug disposition is recommended by regulatory agencies worldwide.^1–3 Although sensitive substrates of OATP1B1/1B3 have been identified in the literature and probe drugs have been proposed by regulatory agencies, there is no general consensus on the ideal in vivo substrate for clinical DDI studies as analysis may be confounded by contribution from other metabolic and/or transport pathways.^1–3 A thorough analysis of the available in vitro and in vivo data regarding OATP1B1/1B3 substrates was performed using the in vitro, clinical, and pharmacogenetic modules in the University of Washington Drug Interaction Database. A total of 34 compounds were identified and further investigated as possible clinical substrates using a novel indexing system. By analyzing the compounds for in vivo characteristics, including sensitivity to inhibition by known OATP1B1/1B3 inhibitors, selectivity for OATP1B1/1B3 compared with other transport and metabolic pathways, and safety profiles, a total of six compounds were identified as potential clinical markers of OATP1B1/1B3 activity.

Genome-wide association meta-analysis for total thyroid hormone levels in Croatian population

Thyroid hormones (THs) are key regulators of cellular growth, development, and metabolism. The thyroid gland secretes two THs, thyroxine (T4) and triiodothyronine (T3), into the plasma where they are almost all bound reversibly to plasma proteins. Free forms of THs are metabolically active, however, they represent a very small fraction of total TH levels. No genome-wide studies have been performed to date on total TH levels, comprising of protein-bound and free forms of THs. To detect genetic variants associated with total TH levels, we carried out the first GWAS meta-analysis of total T4 levels in 1121 individuals from two Croatian cohorts (Split and Korcula). We also performed GWAS analyses of total T3 levels in 577 individuals and T3/T4 ratio in 571 individuals from the Split cohort. The top association in GWAS meta-analysis of total T4 was detected for an intronic variant within SLC22A9 gene (rs12282281, P = 4.00 × 10^-7). Within the same region, a genome-wide significant variant (rs11822642, P = 2.50 × 10^-8) for the T3/T4 ratio was identified. SLC22A9 encodes for an organic anion transporter protein expressed predominantly in the liver and belongs to the superfamily of solute carriers (SLC), a large group of transport membrane proteins. The transport of THs across the plasma membrane in peripheral tissues is facilitated by the membrane proteins, and all TH transport proteins known to date belong to the same SLC superfamily as SLC22A9. These results suggest a potential role for SLC22A9 as a novel transporter protein of THs.

Hyperforin-Induced Activation of the Pregnane X Receptor Is Influenced by the Organic Anion-Transporting Polypeptide 2B1

The herbal remedy St. John's wort (SJW) is used in the treatment of mild depressive symptoms and is known for its drug-drug interaction potential when enhanced expression of CYP3A4 modifies clearance of concomitantly applied substrate drugs. Hyperforin is one constituent of SJW that alters CYP3A4 expression by activation of the nuclear receptor pregnane X receptor (PXR). However, little is known about the transmembrane transport of hyperforin. One membrane protein that modulates cellular entry of drugs is the organic anion-transporting polypeptide (OATP) 2B1. It was the aim of this study to test whether hyperforin interacts with this transport protein. Transport inhibition studies and competitive counterflow experiments suggested that hyperforin is a substrate of OATP2B1. This notion was validated by showing that the presence of OATP2B1 enhanced the hyperforin-induced PXR activation in cell-based luciferase assays. Moreover, in Caco-2 cells transcellular transport of the known OATP2B1 substrate atorvastatin was changed in the presence of hyperforin, resulting in an increased efflux ratio. Eleven commercially available SJW formulations were assessed for their influence on OATP2B1-mediated transport of estrone 3-sulfate and for their impact on CYP3A4 promoter transactivation. The correlation between effect size and the hyperforin content as determined by high-performance liquid chromatography with ultraviolet detection suggested that hyperforin is the major determinant. Our results indicate an interaction between hyperforin and OATP2B1, which is not only known to contribute to hepatocellular uptake but also to intestinal absorption of its substrates. These findings extend the complexity of mechanisms that should be considered when evaluating the interaction potential of SJW preparations.

Macrocyclic Peptides as Drug Candidates: Recent Progress and Remaining Challenges

Peptides as a therapeutic modality attract much attention due to their synthetic accessibility, high degree of specific binding, and the ability to target protein surfaces traditionally considered "undruggable". Unfortunately, at the same time, other pharmacological properties of a generic peptide, such as metabolic stability and cell permeability, are quite poor, which limits the success of de novo discovered biologically active peptides as drug candidates. Here, we review how macrocyclization as well as the incorporation of nonproteogenic amino acids and various conjugation strategies may be utilized to improve on these characteristics to create better drug candidates. We analyze recent progress and remaining challenges in improving individual pharmacological properties of bioactive peptides, and offer our opinion on interfacing these, often conflicting, considerations, to create balanced drug candidates as a potential way to make further progress in this area.

Characterization of Liver- and Cancer-type-Organic Anion Transporting Polypeptide (OATP) 1B3 Messenger RNA Expression in Normal and Cancerous Human Tissues

BACKGROUND

Membrane transport protein organic anion transporting polypeptide (OATP) 1B3 mediates the cellular uptake of many clinically important drugs including anti-cancer drugs (e.g., paclitaxel). In addition to the well-recognized hepatic expression and function of OATP1B3 [herein named liver-type (Lt) OATP1B3], OATP1B3 also expresses in cancers and has been postulated to play a role in cancer therapy, presumably by facilitating the influx of anti-cancer drugs. Recently, a cancer type (Ct)-OATP1B3 mRNA variant was identified in colon and lung cancer tissues, which encodes truncated Ct-OATP1B3 with negligible transport activity. Other than in colon and lung cancers, reports on mRNA expression of OATP1B3 in other cancers cannot distinguish between the Ltand Ct-OATP1B3.

OBJECTIVE

The current studies were designed to characterize the expression of Lt- and Ct-OATP1B3 mRNA in ovarian, prostate, bladder, breast, and lung tissues.

METHODS

Lt- and Ct-OATP1B3 isoform-specific PCR primers were utilized to determine the mRNA levels of Lt- and Ct-OATP1B3, respectively. An expression vector expressing green fluorescent protein (GFP)-tagged Lt-OATP1B3 was transiently transfected into the ovarian cancer cell line SKOV3. Confocal live-cell microscopy was utilized to determine the localization of GFP-Lt-OATP1B3 in SKOV3 cells.

RESULTS

For the first time, Lt-OATP1B3 mRNA was detected in ovarian, prostate, bladder and breast cancers. The localization of GFP-Lt-OATP1B3 on the plasma membrane of SKOV3 cells after transient transfection was readily detected by confocal microscopy.

CONCLUSION

Our findings are supportive of the potential role of Lt-OATP1B3 in cancer cells.

Preventive effect of artemisinin extract against cholestasis induced via lithocholic acid exposure

Obstructive cholestasis characterized by biliary pressure increase leading to leakage of bile back that causes liver injury. The present study aims to evaluate the effects of artemisinin in obstructive cholestasis in mice. The present study was carried out on 40 adult healthy mice that were divided into 4 groups, 10 mice each; the negative control group didn’t receive any medication. The normal group was fed normally with 100 mg/kg of artemisinin extract orally. The cholestatic group fed on 1% lithocholic acid (LCA) mixed into control diet and cholestatic group co-treated with 100 mg/kg of artemisinin extract orally. Mice were treated for 1 month then killed at end of the experiment. A significant increase in alanine aminotransferase, aspartate aminotransferase, and total and direct bilirubin was detected in mice exposed to LCA toxicity. That increase was significantly reduced to normal values in mice co-treated with artemisinin. LCA toxicity causes multiple areas of necrosis of irregular distribution. However, artemisinin co-treatment showed normal hepatic architecture. Moreover, LCA causes down-regulation of hepatic mRNA expressions of a set of genes that are responsible for ATP binding cassette and anions permeability as ATP-binding cassette sub-family G member 8, organic anion-transporting polypeptide, and multidrug resistance-associated protein 2 genes that were ameliorated by artemisinin administration. Similarly, LCA toxicity significantly down-regulated hepatic mRNA expression of constitutive androstane receptor, OATP4, and farnesoid x receptor genes. However, artemisinin treatment showed a reasonable prevention. In conclusion, the current study strikingly revealed that artemisinin treatment can prevent severe hepatotoxicity and cholestasis that led via LCA exposure.

Interaction of soy isoflavones and their main metabolites with hOATP2B1 transporter

Membrane organic anion-transporting polypeptides (OATPs) are responsible for the drug transmembrane transport within the human body. The function of OATP2B1 transporter can be inhibited by various natural compounds. Despite increased research interest in soya as a part of human diet, the effect of its active components to interact with hOATP2B1 has not been elucidated in a complex extent. This in vitro study examined the inhibitory effect of main soy isoflavones (daidzin, daidzein, genistin, genistein, glycitin, glycitein, biochanin A, formononetin) and their metabolites formed in vivo (S-equol, O-desmethylangolensin) towards human OATP2B1 transporter. MDCKII cells overexpressing hOATP2B1 were employed to determine quantitative inhibitory parameters of the tested compounds and to analyze mechanism/s of the inhibitory interaction. The study showed that aglycones of soy isoflavones and the main biologically active metabolite S-equol were able to significantly inhibit hOATP2B1-mediated transport. The Ki values for most of aglycones range from 1 to 20 μM. In contrast, glucosides did not exhibit significant inhibitory effect. The kinetic analysis did not indicate a uniform type of inhibition towards the hOATP2B1 although predominant mechanism of inhibition seemed to be competitive. These findings may suggest that tested soy isoflavones and their metabolites might affect transport of xenobiotics including drugs across tissue barriers via hOATP2B1.

Transmembrane Domain 1 of Human Organic Anion Transporting Polypeptide 2B1 Is Essential for Transporter Function and Stability

Organic anion transporting polypeptides (OATPs, gene symbol SLCO) are important membrane transporter proteins that mediate the uptake of wide ranges of endogenous and exogenous compounds. OATP2B1 has been found in multiple organs and tissues, including the liver, small intestine, kidney, brain, placenta, heart, skin, as well as skeletal muscle, and is proposed to be involved in the uptake of orally administered drugs. Quite a few reports have demonstrated that transmembrane domains (TMs) are crucial for proper functions of OATP family members. Comparative modeling proposed that TM1, along with TM2, 4, and 5 of the N-terminal half of OATP2B1, may be localized within the substrate interaction pocket and are important for uptake function of the transporter. Alanine scanning of the putative transmembrane domain 1 of OATP2B1 revealed that substitution of L58 with alanine dramatically altered the Km value, and mutation of V52, H55, Q59, and L69 resulted in significantly reduced substrate turnover number, whereas A61V, Q62A, and S66A exhibited significant change in both Km and Vmax values. In addition, phenylalanine at position 51 seems to play an important role in maintaining proper folding of OATP2B1 because alanine replacement of F51 caused accelerated degradation of the transporter protein. Although proteasome and lysosome inhibitors could partially recover protein level, the mutant transporter remained nonfunctional. Taken together, the identification of nine essential amino acid residues within TM1 of OATP2B1 suggested that the transmembrane domain is important for maintaining proper function of the transporter.

Thyroid Hormones Are Transport Substrates and Transcriptional Regulators of Organic Anion Transporting Polypeptide 2B1

Levothyroxine replacement therapy forms the cornerstone of hypothyroidism management. Variability in levothyroxine oral absorption may contribute to the well-recognized large interpatient differences in required dose. Moreover, levothyroxine-drug pharmacokinetic interactions are thought to be caused by altered oral bioavailability. Interestingly, little is known regarding the mechanisms contributing to levothyroxine absorption in the gastrointestinal tract. Here, we aimed to determine whether the intestinal drug uptake transporter organic anion transporting polypeptide 2B1 (OATP2B1) may be involved in facilitating intestinal absorption of thyroid hormones. We also explored whether thyroid hormones regulate OATP2B1 gene expression. In cultured Madin-Darby Canine Kidney II/OATP2B1 cells and in OATP2B1-transfected Caco-2 cells, thyroid hormones were found to inhibit OATP2B1-mediated uptake of estrone-3-sulfate. Competitive counter-flow experiments evaluating the influence on the cellular accumulation of estrone-3-sulfate in the steady state indicated that thyroid hormones were substrates of OATP2B1. Additional evidence that thyroid hormones were OATP2B1 substrates was provided by OATP2B1-dependent stimulation of thyroid hormone receptor activation in cell-based reporter assays. Bidirectional transport studies in intestinal Caco-2 cells showed net absorptive flux of thyroid hormones, which was attenuated by the presence of the OATP2B1 inhibitor, atorvastatin. In intestinal Caco-2 and LS180 cells, but not in liver Huh-7 or HepG2 cells, OATP2B1 expression was induced by treatment with thyroid hormones. Reporter gene assays revealed thyroid hormone receptor α-mediated transactivation of the SLCO2B1 1b and the SLCO2B1 1e promoters. We conclude that thyroid hormones are substrates and transcriptional regulators of OATP2B1. These insights provide a potential mechanistic basis for oral levothyroxine dose variability and drug interactions.

Organic Anion Transporting Polypeptide 1a4 is Responsible for the Hepatic Uptake of Cardiac Glycosides in Mice

Among organic anion transporting polypeptide (Oatp) family transporters expressed in the rodent liver, such as Oatp1a1, Oatp1a4, Oatp1b2, and Oatp2b1, Oatp1a4 has a unique character to recognize neutral cardiac glycosides as a substrate in addition to organic anions. The relative contribution of Oatp1a4 to the substrate uptake into hepatocytes has not been clarified. In this study, we investigated the importance of Oatp1a4 in the hepatic uptake of its substrate drugs using Slco1a4-/- mice. The hepatic mRNA expression of Slco1a4 was decreased significantly in Slco1a4-/- mice, whereas no differences were seen in other hepatic transporters between wild-type and Slco1a4-/- mice. We determined the plasma concentrations and liver-to-plasma concentration ratios (Kp,liver) of Oatp1a4 substrates, including ouabain, digoxin, BQ-123, fexofenadine, rosuvastatin, pravastatin, nafcillin, and telmisartan, after continuous intravenous infusion. The plasma concentrations of ouabain and rosuvastatin were 2.1-fold and 1.7-fold higher in Slco1a4-/- mice, and Kp,liver of ouabain and digoxin were 13.4-fold and 4.3-fold lower in Slco1a4-/- mice, respectively. Furthermore, the biliary clearance of ouabain and digoxin with regard to plasma concentration were 21.9-fold and 4.1-fold lower in Slco1a4-/- mice, respectively, accompanied with a marked reduction in their Kp,liver, whereas the systemic clearance of ouabain, but not digoxin, was reduced significantly in Slco1a4-/- mice. These results suggest that Oatp1a4 plays a major role in the hepatic accumulation of cardiac glycosides in mice.

Transporter for sulfated steroid hormones in the testis - expression pattern, biological significance and implications for fertility in men and rodents

In various tissues, steroid hormones may be sulfated, glucuronidated or otherwise modified. For a long time, these hydrophilic molecules have been considered to be merely inactive metabolites for excretion via bile or urine. Nevertheless, different organs such as the placenta and breast tissue produce large amounts of sulfated steroids. After the discovery of the enzyme steroid sulfatase, which is able to re-activate sulfated steroids, these precursor molecules entered the focus of interest again as a local supply for steroid hormone synthesis with a prolonged half-life compared to their unconjugated counterparts. The first descriptions of this so-called sulfatase pathway in the placenta and breast tissue (with special regards to hormone-dependent breast cancer) were quickly followed by studies of steroid sulfate production and function in the testis. These hydrophilic molecules may not permeate the cell membrane by diffusion in the way that unbound steroids can, but need to be transported through the plasma membrane by transport systems. In the testis, a functional sulfatase pathway requires the expression of specific uptake carrier and efflux transporters in testicular cells, i.e. Sertoli, Leydig and germ cells. Main focus has to be placed on Sertoli cells, as these cells build up the blood-testis barrier. In this review, an overview of carrier expression pattern in the human as well as rodent testis is provided with special interest towards implications on fertility.

Differences between carcinoma of the cecum and ascending colon: Evidence based on clinical and embryological data

Developing rapidly from the cecal diverticulum in a 5-week-old embryo, the cecum, which is developed from the caudal limb of the midgut loop, is different from the ascending colon. The aim of this study was to analyze the different clinicopathological and biological characteristics of patients with carcinoma of the cecum and ascending colon. We accessed data for 59,035 patients with adenocarcinomas of the cecum and ascending colon from the Surveillance, Epidemiology and End Results database to explore the potential associations between the clinicopathological characteristics and overall survival. Furthermore, we analyzed the differences in gene expression between the two segments in the Gene Expression Omnibus database. The results were validated in The Cancer Genome Atlas database, as well as with another independent dataset from the First Affiliated Hospital of Xi'an Jiaotong University. The results of this study revealed the potential prognostic differences between adenocarcinoma of the cecum and ascending colon, which may be caused by the differential expression levels of the SLCO1B3 gene. When including the expression levels of SLCO1B3 in intraoperatively examined lymph nodes, 8 factors were found able to predict the prognosis of patients with carcinomas of the cecum and ascending colon. As regards the surgical therapeutic strategies, the resection of >15 local lymph nodes is appropriate for improving the prognosis of patients.

Interaction of Sulfonylureas with Liver Uptake Transporters OATP1B1 and OATP1B3

Sulfonylureas (SUs) such as glibenclamide, gliclazide, glimepiride, glipizide and gliquidone are one of the first oral medicines available for the treatment of type 2 diabetes and are widely used for the treatment of hyperglycaemia. The hepatic transporters, organic anion transporting polypeptide 1B1 (OATP1B1) and organic anion transporting polypeptide 1B3 (OATP1B3), play an important role in the disposition of a variety of drugs by mediating their uptake from blood into hepatocytes. Drug-drug interactions mediated by OATP1B1/1B3 may result in the hepatic transporting change for drug substrates. The inhibitory effects of glibenclamide and glimepiride on sulfobromophthalein (BSP) uptake have been previously studied, and glibenclamide has been reported as the substrate of OATP1B3, but it remains unclear whether other SUs such as gliclazide, glipizide and gliquidone are substrates of OATP1B1 and OATP1B3. Here, we investigated the relationship between the five most commonly applied SUs (glibenclamide, gliclazide, glimepiride, glipizide, gliquidone) and OATP1B1 and OATP1B3. We performed uptake and inhibition assays in HEK293T cells stably expressing OATP1B1 or OATP1B3, respectively, and established a liquid chromatography-mass spectrometry (LC-MS) method for the simultaneous measurement of five SUs. We demonstrated that gliclazide and glimepiride are substrates of OATP1B1 and glibenclamide and glipizide are substrates of OATP1B3. We also confirmed the interaction between these SUs and rosuvastatin. No transporting was observed for gliquidone, suggesting that it is not a substrate of either transporter.

Biomarkers for In Vivo Assessment of Transporter Function

Drug-drug interactions are a major concern not only during clinical practice, but also in drug development. Due to limitations of in vitro-in vivo predictions of transporter-mediated drug-drug interactions, multiple clinical Phase I drug-drug interaction studies may become necessary for a new molecular entity to assess potential drug interaction liabilities. This is a resource-intensive process and exposes study participants, who frequently are healthy volunteers without benefit from study treatment, to the potential risks of a new drug in development. Therefore, there is currently a major interest in new approaches for better prediction of transporter-mediated drug-drug interactions. In particular, researchers in the field attempt to identify endogenous compounds as biomarkers for transporter function, such as hexadecanedioate, tetradecanedioate, coproporphyrins I and III, or glycochenodeoxycholate sulfate for hepatic uptake via organic anion transporting polypeptide 1B or N1-methylnicotinamide for multidrug and toxin extrusion protein-mediated renal secretion. We summarize in this review the currently proposed biomarkers and potential limitations of the substances identified to date. Moreover, we suggest criteria based on current experiences, which may be used to assess the suitability of a biomarker for transporter function. Finally, further alternatives and supplemental approaches to classic drug-drug interaction studies are discussed.

Regulation of Organic Anion Transporting Polypeptides (OATP) 1B1- and OATP1B3-Mediated Transport: An Updated Review in the Context of OATP-Mediated Drug-Drug Interactions

Organic anion transporting polypeptides (OATP) 1B1 and OATP1B3 are important hepatic transporters that mediate the uptake of many clinically important drugs, including statins from the blood into the liver. Reduced transport function of OATP1B1 and OATP1B3 can lead to clinically relevant drug-drug interactions (DDIs). Considering the importance of OATP1B1 and OATP1B3 in hepatic drug disposition, substantial efforts have been given on evaluating OATP1B1/1B3-mediated DDIs in order to avoid unwanted adverse effects of drugs that are OATP substrates due to their altered pharmacokinetics. Growing evidences suggest that the transport function of OATP1B1 and OATP1B3 can be regulated at various levels such as genetic variation, transcriptional and post-translational regulation. The present review summarizes the up to date information on the regulation of OATP1B1 and OATP1B3 transport function at different levels with a focus on potential impact on OATP-mediated DDIs.