Functions of OATP1A and 1B transporters in vivo: insights from mouse models

Proteome profile differences among human, monkey, and mouse brain microvessels and cultured brain microvascular endothelial cells

BACKGROUND

The blood-brain barrier (BBB) expresses transporters, receptors, and tight junction proteins that regulate the exchange of substances between the blood and brain. The differences in the expression of these proteins in the BBB among different species and cultured BBB model cells should be clarified to interpret the BBB function in model animals and cells. This study aimed to elucidate species differences among humans, monkeys, and mice and in vitro-in vivo differences in the BBB proteome using deep proteomic analysis.

METHODS

Brain microvessels (BMVs) were isolated from frozen cerebral cortices of human and monkey and frozen mouse cerebrums. BMVs and cultured brain microvascular endothelial cells (BMECs), such as hCMEC/D3, HBMEC/ciβ, and primary BMECs, were analyzed via data-independent acquisition using liquid chromatography-mass spectrometry.

RESULTS

Proteomics identified 7,149-8,274 proteins in the BMV fractions and 6,657-7,534 proteins in the brain lysates. Comparative analysis revealed distinct proteomic profiles among the three species, with the human profile being more similar to that of monkeys than that of mice. The expression profile of the solute carrier organic anion transporter family was found to vary among mouse, monkey, and human BMVs. The expression levels of SLC22A6/Slc22a6 and SLC22A8/Slc22a8 were higher in mice than in monkeys and humans, whereas SLC43A3/Slc43a3 expression levels were lower in mice. The expression of amino acid transporters, such as SLC7A5 and SLC3A2/Slc3a2, was higher in BMVs, whereas that of SLC1A5/Slc1a5 and SLC38A9/Slc38a9 was higher in cultured BMECs. MFSD2A/Mfsd2a and SLC27A1/Slc27a1 were highly expressed in BMVs. The expression of tight junction proteins, particularly the claudin family, varied between BMVs and cultured BMECs and among cell lines. Specifically, the expression of claudin-5 was higher in BMVs, and claudin-11 expression was higher in cultured BMECs.

CONCLUSIONS

Deep proteomic analysis revealed species-specific differences in transport-related proteins in the BBB. Furthermore, in vitro and in vivo differences were observed in the transporter and claudin protein expression. This study provides a BBB proteome profile dataset and offers insights for a comprehensive understanding of BBB protein expression across species and between in vivo and in vitro conditions.

Erlotinib-A substrate and inhibitor of OATP2B1: pharmacokinetics and CYP3A-mediated metabolism in rSlco2b1-/- and SLCO2B1+/+ rats

The tyrosine kinase inhibitor erlotinib is recognized as a substrate of cytochrome P450 enzymes and drug transporters. Indeed, erlotinib's extensive metabolism to the active metabolite OSI-420 (desmethyl erlotinib) mainly involves CYP3A enzymes. Additionally, erlotinib is assumed to interact with the organic anion transporting polypeptide (OATP)2B1. In this study, we aimed to investigate the role of human OATP2B1 in erlotinib's metabolism through in vitro and in vivo experiments. Using Madin-Darby canine kidney cells expressing human OATP2B1 for competitive counterflow experiments, we confirmed erlotinib as inhibitor and substrate of the transporter. Moreover, in vitro transport experiments revealed higher cellular accumulation of erlotinib at pH 5.5 than that at pH 7.4. Pharmacokinetic evaluation of orally administered erlotinib in male SLCO2B1+/+ and rSlco2b1-/- rats revealed that the human OATP2B1 does not significantly alter serum levels of erlotinib or its main metabolite OSI-420, although we observed a longer mean residence time of the metabolite in humanized rats. Although there was no difference in the OSI-420:erlotinib ratio over time in SLCO2B1+/+ and rSlco2b1-/- rats, we assessed the role of CYP3A1 and CYP3A2 in the metabolism of erlotinib. In vitro experiments showed a contribution of both enzymes to the formation of OSI-420. For CYP3A1, we found significantly higher expression in liver microsomes of male SLCO2B1+/+ rats, while the knockout genotype showed significantly higher levels of CYP3A2. However, these differences did not affect the systemic exposure of erlotinib or OSI-420 in the rats. Our findings provide further insight into the role of OATP2B1 in the disposition of orally administered erlotinib. SIGNIFICANCE STATEMENT: This study confirms that erlotinib is a substrate of the human organic anion transporting polypeptide 2B1 transporter in vitro. In vivo experiments in rat models, however, showed no significant impact of organic anion transporting polypeptide 2B1 on the systemic exposure of erlotinib or its metabolite, OSI-420. Despite variations in CYP3A enzyme expression in SLCO2B1+/+ rats, the OSI-420:erlotinib ratio remained unchanged. Although SLCO2B1+/+ rats exhibited a longer mean residence time for OSI-420, this did not significantly alter overall exposure in orally treated animals.

OATP1B-type Transport Function Is a Determinant of Aromatase Inhibitor-Associated Arthralgia Susceptibility

SIGNIFICANCE

AIs are effective but often discontinued because of arthralgia. This study explores the role of OATP1B transporters in AI-related side effects and the potential usage of transporter biomarkers to predict and reduce the risk of arthralgia associated with AI treatment.

Characterization of Human Organic Anion Transporter 4 (hOAT4) and Mouse Oat5 (mOat5) As Functional Orthologs for Renal Anion Uptake and Efflux Transport

Organic anions (OAs) are compounds including drugs or toxicants that are negatively charged at physiologic pH and are typically transported by organic anion transporters (OATs). Human OAT4 (SLC22A11) is expressed in the apical membrane of renal proximal tubules. Although there is no rodent ortholog of hOAT4, rodents express Oat5 (Slc22a19), an anion exchanger that is also localized to the apical membrane of renal proximal tubule cells. The purpose of this study was to determine the functional similarity between mouse Oat5 and human OAT4. Chinese hamster ovary (CHO) cells expressing SLC22A11 or Slc22a19 were used to assess the transport characteristics of radiolabeled ochratoxin (OTA). We determined the kinetics of OTA transport; the resulting Michaelis constant (Kt) and maximal rate of mediated substrate transport (Jmax) values were very similar for both hOAT4 and mOat5: Kt 3.9 and 7.2 μM, respectively, and Jmax 4.4 and 3.9 pmol/cm2, respectively. For the profile of OTA inhibition by OAs, IC50 values were determined for several clinically important drugs and toxicants. The resulting IC50 values ranged from 9 μM for indomethacin to ∼600 μM for the diuretic hydrochlorothiazide. We measured the efflux of OTA from preloaded cells; both hOAT4 and mOat5 supported the efflux of OTA. These data support the hypothesis that OAT4 and Oat5 are functional orthologs and share selectivity for OTA both for reabsorption and secretion. SIGNIFICANCE STATEMENT: This study compares the selectivity profile between human organic anion transporter (OAT4) and mouse Oat5. Our data revealed a similar selectivity profile for ochratoxin A (OTA) reabsorption and secretion by these two transporters, thereby supporting the hypothesis that hOAT4 and mOat5, although not genetic orthologs, behave as functional orthologs for both uptake and efflux. These data will be instrumental in selecting an appropriate animal model when studying the renal disposition of anionic drugs and toxicants.

Expression of hepatic genes involved in bile acid metabolism in dairy cows with fatty liver

Bile acids are cholesterol-derived molecules that are primarily produced in the liver. In nonruminants with fatty liver, overproduction of bile acids is associated with liver injury. During the transition period, fatty liver is a metabolic disorder that can affect up to 50% of high-producing dairy cows. The purpose of this study was to provide a comprehensive evaluation on hepatic bile acid metabolism in dairy cows with fatty liver by assessing expression changes of genes involved in bile acid synthesis, export and uptake. The serum activities of aspartate aminotransferase, alanine aminotransferase and glutamate dehydrogenase and concentration of total bile acids were all greater, whereas serum concentration of total cholesterol was lower in cows with fatty liver than in healthy cows. Content of total bile acids was higher but total cholesterol was slightly lower in liver tissues from fatty liver cows than from healthy cows. The hepatic mRNA abundance of cholesterol 7a-hydroxylase (CYP7A1), hydroxy-delta-5-steroid dehydrogenase, 3 β- and steroid delta-isomerase 7 (HSD3B7) and sterol 12α-hydroxylase (CYP8B1), enzymes involved in the classic pathway of bile acid synthesis, was higher in fatty liver cows than in healthy cows. Compared with healthy cows, the hepatic mRNA abundance of alternative bile acid synthesis pathway-related genes sterol 27-hydroxylase (CYP27A1) and oxysterol 7α-hydroxylase (CYP7B1) did not differ in cows with fatty liver. The protein and mRNA abundance of bile acid transporter bile salt efflux pump (BSEP) were lower in the liver of dairy cow with fatty liver. Compared with healthy cows, the hepatic mRNA abundance of bile acid transporters solute carrier family 51 subunit α (SLC51A), ATP binding cassette subfamily C member 1 (ABCC1) and 3 (ABCC3) was greater in cows with fatty liver, whereas the solute carrier family 51 subunit β (SLC51B) did not differ. The expression of genes involved in bile acid uptake, including solute carrier family 10 member 1 (NTCP), solute carrier organic anion transporter family member 1A2 (SLCO1A2) and 2B1 (SLCO2B1) was upregulated in dairy cows with fatty liver. Furthermore, the hepatic protein and mRNA abundance of bile acid metabolism regulators farnesoid X receptor (FXR) and small heterodimer partner (SHP) were lower in cows with fatty liver than in healthy cows. Overall, these data suggest that inhibition of FXR signaling pathway may lead to the increased bile acid synthesis and uptake and decreased secretion of bile acids from hepatocytes to the bile, which elevates hepatic bile acids content in dairy cows with fatty liver. As the hepatotoxicity of bile acids has been demonstrated on nonruminant hepatocytes, it is likely that the liver injury is induced by increased hepatic bile acids content in dairy cows with fatty liver.

Interplay of Ritonavir-Boosted Oral Cabazitaxel with the Organic Anion-Transporting Polypeptide (OATP) Uptake Transporters and Carboxylesterase 1 in Mice

Intravenously administered chemotherapeutic cabazitaxel is used for palliative treatment of prostate cancer. An oral formulation would be more patient-friendly and reduce the need for hospitalization. We therefore study determinants of the oral pharmacokinetics of cabazitaxel in a ritonavir-boosted setting, which reduces the CYP3A-mediated first-pass metabolism of cabazitaxel. We here assessed the role of organic anion-transporting polypeptides (OATPs) in the disposition of orally boosted cabazitaxel and its active metabolites, using the Oatp1a/b-knockout and the OATP1B1/1B3-transgenic mice. These transporters may substantially affect plasma clearance and hepatic and intestinal drug disposition. The pharmacokinetics of cabazitaxel and DM2 were not significantly affected by Oatp1a/b and OATP1B1/1B3 activity. In contrast, the plasma AUC0-120 min of DM1 in Oatp1a/b-/- was 1.9-fold (p < 0.05) higher than that in wild-type mice, and that of docetaxel was 2.4-fold (p < 0.05) higher. We further observed impaired hepatic uptake and intestinal disposition for DM1 and docetaxel in the Oatp-ablated strains. None of these parameters showed rescue by the OATP1B1 or -1B3 transporters in the humanized mouse strains, suggesting a minimal role of OATP1B1/1B3. Ritonavir itself was also a potent substrate for mOatp1a/b, showing a 2.9-fold (p < 0.0001) increased plasma AUC0-120 min and 3.5-fold (p < 0.0001) decreased liver-to-plasma ratio in Oatp1a/b-/- compared to those in wild-type mice. Furthermore, we observed the tight binding of cabazitaxel and its active metabolites, including docetaxel, to plasma carboxylesterase (Ces1c) in mice, which may complicate the interpretation of pharmacokinetic and pharmacodynamic mouse studies. Collectively, these results will help to further optimize (pre)clinical research into the safety and efficacy of orally applied cabazitaxel.

Development of a Pharmacokinetic Model That Accounts for the Plasma Concentrations of Conjugated and Unconjugated Bilirubin Observed in a Variety of Disease States

Introduction

For a large variety of liver pathologies, the plasma unconjugated (UB) and conjugated (CB) bilirubin concentrations appear to be coupled. For example, in alcoholic cirrhosis, UB and CB are roughly the same over a large range of total bilirubin, requiring an initial massive increase (about 40-fold) in plasma CB to reach the level of UB and then similar increases in UB and CB as the disease progresses. This coupling has been either unrecognized or ignored and this paper is the first attempt to try to explain it quantitatively in terms of known hepatic cell metabolic and membrane transport properties.

Methods

A simplified pharmacokinetic model is developed and applied to a variety of hyperbilirubinemic pathologies. A central feature of the model is based on the recent observation that double knockout of the rat OATP1A and OATP1B hepatic transporters produces a roughly 400-fold increase in plasma CB, indicating that there is a normal rapid recycling of CB from the cell to the plasma with reuptake via OATP. We use the experimental rat Km of OATP CB transport to show that OATP uptake becomes saturated at relatively low plasma CB concentrations, decreasing uptake, and producing massive (up to 1000-fold) increases in CB in some pathologies. It is assumed that UB and CB are competing for the OATP transporter, producing the increased plasma UB that is observed in "pure" CB pathologies.

Results

The model accurately describes the clinically observed UB and CB for pure UB (Gilbert's, hemolytic anemia) and CB (Dubin-Johnson, Rotor syndrome, biliary atresia) pathologies as well as in cirrhosis.

Conclusion

This model is a preliminary, first attempt to quantitatively describe UB and CB pharmacokinetics. It is hoped that it will stimulate more detailed measurements and analysis.

Quantitative Prediction of OATP-Mediated Disposition and Biliary Clearance Using Human Liver Chimeric Mice

Drug biliary clearance (CLbile) in vivo is among the most difficult pharmacokinetic parameters to predict accurately and quantitatively because biliary excretion is influenced by metabolic enzymes, transporters, and passive diffusion across hepatocyte membranes. The purpose of this study is to demonstrate the use of Hu-FRG mice [Fah-/-/Rag2-/-/Il2rg-/- (FRG) mice transplanted with human-derived hepatocytes] to quantitatively predict human organic anion transporting polypeptide (OATP)-mediated drug disposition and CLbile To predict OATP-mediated disposition, six OATP substrates (atorvastatin, fexofenadine, glibenclamide, pitavastatin, pravastatin, and rosuvastatin) were administered intravenously to Hu-FRG and Mu-FRG mice (FRG mice transplanted with mouse hepatocytes) with or without rifampicin as an OATP inhibitor. We calculated the hepatic intrinsic clearance (CLh,int) and the change of hepatic clearance (CLh) caused by rifampicin (CLh ratio). We compared the CLh,int of humans with that of Hu-FRG mice and the CLh ratio of humans with that of Hu-FRG and Mu-FRG mice. For predicting CLbile, 20 compounds (two cassette doses of 10 compounds) were administered intravenously to gallbladder-cannulated Hu-FRG and Mu-FRG mice. We evaluated the CLbile and investigated the correlation of human CLbile with that of Hu-FRG and Mu-FRG mice. We found good correlations between humans and Hu-FRG mice in CLh,int (100% within threefold) and CLh ratio (R2 = 0.94). Moreover, we observed a much better relationship between humans and Hu-FRG mice in CLbile (75% within threefold). Our results suggest that OATP-mediated disposition and CLbile can be predicted using Hu-FRG mice, making them a useful in vivo drug discovery tool for quantitatively predicting human liver disposition. SIGNIFICANCE STATEMENT: OATP-mediated disposition and biliary clearance of drugs are likely quantitatively predictable using Hu-FRG mice. The findings can enable the selection of better drug candidates and the development of more effective strategies for managing OATP-mediated DDIs in clinical studies.

A Metabolomics Approach for Predicting OATP1B-Type Transporter-Mediated Drug–Drug Interaction Liabilities

In recent years, various endogenous compounds have been proposed as putative biomarkers for the hepatic uptake transporters OATP1B1 and OATP1B3 that have the potential to predict transporter-mediated drug–drug interactions (DDIs). However, these compounds have often been identified from top–down strategies and have not been fully utilized as a substitute for traditional DDI studies. In an attempt to eliminate observer bias in biomarker selection, we applied a bottom–up, untargeted metabolomics screening approach in mice and found that plasma levels of the conjugated bile acid chenodeoxycholate-24-glucuronide (CDCA-24G) are particularly sensitive to deletion of the orthologous murine transporter Oatp1b2 (31-fold increase vs. wild type) or the entire Oatp1a/1b(−/−)cluster (83-fold increased), whereas the humanized transgenic overexpression of hepatic OATP1B1 or OATP1B3 resulted in the partial restoration of transport function. Validation studies with the OATP1B1/OATP1B3 inhibitors rifampin and paclitaxel in vitro as well as in mice and human subjects confirmed that CDCA-24G is a sensitive and rapid response biomarker to dose-dependent transporter inhibition. Collectively, our study confirmed the ability of CDCA-24G to serve as a sensitive and selective endogenous biomarker of OATP1B-type transport function and suggests a template for the future development of biomarkers for other clinically important xenobiotic transporters.

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.

Drug Transporters ABCB1 (P-gp) and OATP, but not Drug-Metabolizing Enzyme CYP3A4, Affect the Pharmacokinetics of the Psychoactive Alkaloid Ibogaine and its Metabolites

The psychedelic alkaloid ibogaine is increasingly used as an oral treatment for substance use disorders, despite being unlicensed in most countries and having reported adverse events. Using wild-type and genetically modified mice, we investigated the impact of mouse (m)Abcb1a/1b and Abcg2 drug efflux transporters, human and mouse OATP drug uptake transporters, and the CYP3A drug-metabolizing complex on the pharmacokinetics of ibogaine and its main metabolites. Following oral ibogaine administration (10 mg/kg) to mice, we observed a rapid and extensive conversion of ibogaine to noribogaine (active metabolite) and noribogaine glucuronide. Mouse Abcb1a/1b, in combination with mAbcg2, modestly restricted the systemic exposure (plasma AUC) and peak plasma concentration (Cmax) of ibogaine. Accordingly, we found a ∼2-fold decrease in the relative recovery of ibogaine in the small intestine with fecal content in the absence of both transporters compared to the wild-type situation. Ibogaine presented good intrinsic brain penetration even in wild-type mice (brain-to-plasma ratio of 3.4). However, this was further increased by 1.5-fold in Abcb1a/1b;Abcg2−/− mice, but not in Abcg2−/− mice, revealing a stronger effect of mAbcb1a/1b in restricting ibogaine brain penetration. The studied human OATP transporters showed no major impact on ibogaine plasma and tissue disposition, but the mOatp1a/1b proteins modestly affected the plasma exposure of ibogaine metabolites and the tissue disposition of noribogaine glucuronide. No considerable role of mouse Cyp3a knockout or transgenic human CYP3A4 overexpression was observed in the pharmacokinetics of ibogaine and its metabolites. In summary, ABCB1, in combination with ABCG2, limits the oral availability of ibogaine, possibly by mediating its hepatobiliary and/or direct intestinal excretion. Moreover, ABCB1 restricts ibogaine brain penetration. Variation in ABCB1/ABCG2 activity due to genetic variation and/or pharmacologic inhibition might therefore affect ibogaine exposure in patients, but only to a limited extent. The insignificant impact of human CYP3A4 and OATP1B1/1B3 transporters may be clinically advantageous for ibogaine and noribogaine use, as it decreases the risks of undesirable drug interactions or interindividual variation related to CYP3A4 and/or OATP activity.

The role of drug efflux and uptake transporters ABCB1 (P-gp), ABCG2 (BCRP) and OATP1A/1B and of CYP3A4 in the pharmacokinetics of the CDK inhibitor milciclib

The promising anticancer drug milciclib potently inhibits cyclin-dependent kinase (CDK) 2 and tropomyosin receptor kinase (TRK) A, and is currently in phase II clinical studies. To characterize factors affecting milciclib pharmacokinetics, we investigated whether milciclib is a substrate of the multidrug efflux and uptake transporters ABCB1 (P-gp), ABCG2 (BCRP), and OATP1A/1B, and the drug-metabolizing enzyme CYP3A, using genetically-modified mouse models and Madin-Darby Canine Kidney (MDCK-II) cells. In vitro, milciclib was transported by mAbcg2, and this was inhibited by the ABCG2 inhibitor Ko143. Upon oral administration of milciclib, its plasma exposure in Abcb1a/1b^-/-, Abcg2^-/-, and Abcb1a/1b;Abcg2^-/- mice was similar to that found in wild-type mice. Milciclib showed good brain penetration even in wild-type mice (brain-to-plasma ratio of 1.2), but this was further increased by 5.2-fold when both Abcb1 and Abcg2 were ablated, and to a lesser extent in single Abcb1- or Abcg2-deficient mice. Oatp1a/1b deficiency had only a minor impact on the milciclib plasma AUC_0-24h and C_max. The milciclib AUC_0-8h increased 1.9-fold in Cyp3a^-/- mice but decreased only 1.3-fold upon overexpression of human CYP3A4. Thus, ABCB1 and ABCG2 cooperatively limit milciclib brain penetration. The low impact of OATP1 and CYP3A could be clinically favorable for milciclib, reducing the risks of unintended drug-drug interactions or interindividual variation in CYP3A4 activity.

Precipitating factors causing hyperbilirubinemia during chronic hepatitis C treatment with paritaprevir/ritonavir/ombitasvir and dasabuvir

BACKGROUND

Hepatic decompensation is a fatal on-treatment side effect during chronic hepatitis C treatment with paritaprevir/ritonavir/ombitasvir and dasabuvir (PrOD). Prompt bilirubin testing can reveal hepatic failure in susceptible patients, and clinical parameters precipitating early elevation of bilirubin can warn clinicians to avoid PrOD prescription.

METHODS

This retrospective study included 169 Hepatitis C virus (HCV)-genotype 1b patients who underwent a 12-week course of PrOD with or without ribavirin. Laboratory data underwent χ analysis with Fisher's exact test to determine the precipitating factors causing hyperbilirubinemia in patients who had received 1 week of treatment.

RESULTS

Sustained viral response was achieved in 164 patients (97.0%). Total bilirubin was ≥2 mg/dL (21.3%) in 36 patients after 1 week of treatment. Pretreatment white blood cell (WBC) <4500/µL and platelet <100,000/µL correlated with total bilirubin ≥2 mg/dL (relative risk [RR]: 21.64, 95% CI: 5.23-89.64, p < 0.001) after 1 week of treatment. Pretreatment platelet ≥100 000/µL and WBC <4500/µL correlated with direct bilirubin ≥0.45 mg/dL (RR: 6.56, 95% CI: 1.42-30.38, p = 0.016) and indirect bilirubin ≥0.6 mg/dL (RR: 4.77, 95% CI: 1.03-22.15, p = 0.046). Pretreatment platelet <100,000/µL with F3/F4 fibrosis correlated with first week total bilirubin ≥2 mg/dL (RR: 3.57, 95% CI: 1.35-9.09, p = 0.010).

CONCLUSION

PrOD is an effective antiviral regimen for HCV genotype 1b patients. Total bilirubin ≥2 mg/dL after 1 week of treatment serves as an early warning of irreversible progression toward hepatic decompensation, and the current study provides a guide by which to monitor chronic hepatitis C patients undergoing PrOD treatment.

Bile Acids: A Communication Channel in the Gut-Brain Axis

Bile acids are signalling hormones involved in the regulation of several metabolic pathways. The ability of bile acids to bind and signal through their receptors is modulated by the gut microbiome, since the microbiome contributes to the regulation and synthesis of bile acids as well to their physiochemical properties. From the gut, bacteria have been shown to send signals to the central nervous system via their metabolites, thus affecting the behaviour and brain function of the host organism. In the last years it has become increasingly evident that bile acids affect brain function, during normal physiological and pathological conditions. Although bile acids may be synthesized locally in the brain, the majority of brain bile acids are taken up from the systemic circulation. Since the composition of the brain bile acid pool may be regulated by the action of intestinal bacteria, it is possible that bile acids function as a communication bridge between the gut microbiome and the brain. However, little is known about the molecular mechanisms and the physiological roles of bile acids in the central nervous system. The possibility that bile acids may be a direct link between the intestinal microbiome and the brain is also an understudied subject. Here we review the influence of gut bacteria on the bile acid pool composition and properties, as well as striking evidence showing the role of bile acids as neuroactive molecules.

Chronopharmacology of simvastatin on hyperlipidaemia in high-fat diet-fed obese mice

The chronopharmacology refers to the utilization of physiological circadian rhythms to optimize the administration time of drugs, thus increasing their efficacy and safety, or reducing adverse effects. Simvastatin is one of the most widely prescribed drugs for the treatment of hypercholesterolaemia, hyperlipidemia and coronary artery disease. There are conflicting statements regarding the timing of simvastatin administration, and convincing experimental evidence remains unavailable. Thus, we aimed to examine whether different administration times would influence the efficacy of simvastatin. High‐fat diet‐fed mice were treated with simvastatin at zeitgeber time 1 (ZT1) or ZT13, respectively, for nine weeks. Simvastatin showed robust anti‐hypercholesterolaemia and anti‐hyperlipidemia effects on these obese mice, regardless of administration time. However, simvastatin administrated at ZT13, compared to ZT1, was more functional for decreasing serum levels of total cholesterol, triglycerides, non‐esterified free fatty acids and LDL cholesterol, as well as improving liver pathological characteristics. In terms of possible mechanisms, we found that simvastatin did not alter the expression of hepatic circadian clock gene in vivo, although it failed to change the period, phase and amplitude of oscillation patterns in Per2::Luc U2OS and Bmal1::Luc U2OS cells in vitro. In contrast, simvastatin regulated the expression of Hmgcr, Mdr1 and Slco2b1 in a circadian manner, which potentially contributed to the chronopharmacological function of the drug. Taken together, we provide solid evidence to suggest that different administration times affect the lipid‐lowering effects of simvastatin.

OATP1A/1B, CYP3A, ABCB1, and ABCG2 limit oral availability of the NTRK inhibitor larotrectinib, while ABCB1 and ABCG2 also restrict its brain accumulation

BACKGROUND AND PURPOSE

Larotrectinib is a FDA-approved oral small-molecule inhibitor for treatment of neurotrophic tropomyosin receptor kinase fusion-positive cancer. We here investigated the functions of the multidrug efflux transporters ABCB1 and ABCG2, the SLCO1A/1B (OATP1A/1B) uptake transporters, and the multispecific drug-metabolizing enzyme CYP3A in larotrectinib pharmacokinetic behaviour.

EXPERIMENTAL APPROACH

In vitro, transepithelial drug transport and uptake assays were performed. In vivo, larotrectinib (10 mg·kg^-1 ) was administered orally to relevant genetically modified mouse models. Cell medium, plasma samples, and organ homogenates were measured by a sensitive and specific LC-MS/MS larotrectinib assay.

KEY RESULTS

In vitro, larotrectinib was avidly transported by human (h) ABCB1 and mouse (m) Abcg2 efficiently by hABCG2 and modestly by hOATP1A2. In vivo, both mAbcb1a/1b and mAbcg2 markedly limited larotrectinib oral availability and brain and testis accumulation (by 2.1-fold, 10.4-fold, and 2.7-fold, respectively), with mAbcb1a/1b playing a more prominent role. mOatp1a/1b also restricted larotrectinib oral availability (by 3.8-fold) and overall tissue exposure, apparently by mediating substantial uptake into the liver, thus likely facilitating hepatobiliary excretion. Additionally, larotrectinib is an excellent substrate of CYP3A, which restricts the oral availability of larotrectinib and hence its tissue exposure.

CONCLUSIONS AND IMPLICATIONS

ABCG2 and especially ABCB1 limit the oral availability and brain and testis penetration of larotrectinib, while OATP1A/1B transporters restrict its systemic exposure by mediating hepatic uptake, thus allowing hepatobiliary excretion. CYP3A-mediated metabolism can strongly limit larotrectinib oral availability and hence its tissue concentrations. These insights may be useful in the further clinical development of larotrectinib.

Clinical Pharmacokinetics and Pharmacodynamics of Transarterial Chemoembolization and Targeted Therapies in Hepatocellular Carcinoma

The management of hepatocellular carcinoma (HCC) is based on a multidisciplinary decision tree. Treatment includes loco-regional therapy, mainly transarterial chemoembolization, for intermediate-stage HCC and systemic therapy with oral tyrosine kinase inhibitors (TKIs) for advanced HCC. Transarterial chemoembolization involves hepatic intra-arterial infusion with either conventional procedure or drug-eluting-beads. The aim of the loco-regional procedure is to deliver treatment as close as possible to the tumor both to embolize the tumor area and to enhance efficacy and minimize systemic toxicity of the anticancer drug. Pharmacokinetic studies applied to transarterial chemoembolization are rare and pharmacodynamic studies even rarer. However, all available studies lead to the same conclusions: use of the transarterial route lowers systemic exposure to the cytotoxic drug and leads to much higher tumor drug concentrations than does a similar dose via the intravenous route. However, reproducibility of the procedure remains a major problem, and no consensus exists regarding the choice of anticancer drug and its dosage. Systemic therapy with TKIs is based on sorafenib and lenvatinib as first-line treatment and regorafenib and cabozantinib as second-line treatment. Clinical use of TKIs is challenging because of their complex pharmacokinetics, with high liver metabolism yielding both active metabolites and their common toxicities. Changes in liver function over time with the progression of HCC adds further complexity to the use of TKIs. The challenges posed by TKIs and the HCC disease process means monitoring of TKIs is required to improve clinical management. To date, only partial data supporting sorafenib monitoring is available. Results from further pharmacokinetic/pharmacodynamic studies of these four TKIs are eagerly awaited and are expected to permit such monitoring and the development of consensus guidelines.

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.

Interaction Between Sex and Organic Anion-Transporting Polypeptide 1b2 on the Pharmacokinetics of Regorafenib and Its Metabolites Regorafenib-N-Oxide and Regorafenib-Glucuronide in Mice

Regorafenib, a multikinase inhibitor used in the treatment of various solid tumors, undergoes extensive uridine 5′‐diphosphate glucuronosyltransferase (Ugt)1a9‐mediated glucuronidation to form regorafenib‐N‐β‐glucuronide (M7; RG), but the contribution of hepatic uptake transporters, such as organic anion‐transporting polypeptide (Oatp)1b2, to the pharmacokinetics of regorafenib remains poorly understood. Using NONMEM‐based, population‐based, parent‐metabolite modeling, we found that Oatp1b2 and sex strongly impact the systemic exposure to RG in mice receiving oral regorafenib. Metabolic studies revealed that the liver microsomal expression of cytochrome P450 (Cyp)3a11 is twofold lower in female mice, whereas Ugt1a9 levels and function are not sex dependent. This finding is consistent with the metabolism of regorafenib occurring via two competing pathways, and the lack of Oatp1b2 results in decreased clearance of RG. The described model provides mechanistic insights into the in vivo disposition of regorafenib.

High hepatic exposure of furanocoumarins in Radix Angelica dahuricae is associated with transporter mediated active uptake

ETHNOPHARMACOLOGICAL RELEVANCE

Radix Angelica dahuricae (RAD), the roots of Angelica dahurica (Hoffm.) Benth. & Hook.f. ex Franch. & Sav, is a well-known traditional Chinese medicine (TCM) and has been used for centuries to treat headaches, toothaches, nose congestion, abscesses, furunculoses, and acne. This herb is also one of frequently reported TCMs showing the herb-drug interaction potential. Furanocoumarins are main bioactive components of RAD.

AIM OF THE STUDY

This study is designed to characterize the tissue distribution profiles of furanocoumarins after oral administration of RAD extract in rats and to explore the mechanism underlying the high hepatic exposure of the major furanocoumarins.

MATERIALS AND METHODS

The tissue distribution of nine furanocoumarins was determined in rats after an oral dose of 0.46g/kg RAD extract using high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). Unbound fractions (ƒ_u) of major furanocoumarins, including imperatorin (IM), isoimperatorin (IIM), bergapten (BER) and oxypeucedanin hydrate (OXYH), were measured in rat plasma and selected tissue homogenates (liver, kidney, lung and brain) with Rapid Equilibrium Dialysis (RED) method. The temperature dependent hepatic uptake of IM, IIM, BER and OXYH were evaluated in suspended rat primary hepatocytes at 4°C or 37°C by the oil-spin method. The uptake kinetics was conducted in the cells over a wide concentration range. The furanocoumarins were co-incubated with a panel of transporter inhibitors to investigate the involvement of uptake transporters in the hepatic uptake. The transcellular transport characteristics of IM, IIM, BER and OXYH were further assessed using Caco-2 cell monolayer model.

RESULTS

IM, IIM, BER and OXYH were found to be the major bioactive furanocoumarins in rat plasma and tissues, representing more than 90% exposure for all the detected furanocoumarins. The most concentrative organ of major furanocoumarins was the liver, with liver-to-plasma exposure ratio (K_p,_AUC) of 5.1, 6.5 and 4.7 for IM, IIM and BER, and 2.3 for OXYH, respectively. IM, IIM and BER also showed higher concentrations in the kidney with K_p above 2.2. The higher protein binding of the furanocoumarins partially contributed to their higher tissue exposure. In suspended rat primary hepatocyte, the hepatic uptake of IM, IIM, BER and OXYH was temperature-dependent, with considerably higher uptake at 37°C than at 4°C. Uptake kinetics indicated that the hepatic uptake of IM, IIM, BER and OXYH involved both active transport and passive diffusion processes. For IM, IIM and BER, the contribution of the active transport was greater than the passive process, with the CL_active/CL_uptake > 72%. Ritonavir (RTN) and cyclosporine A (CsA), the known inhibitors of organic anion transporting polypeptide (Oatp) significantly inhibited the hepatic uptake of IM and BER, while the inhibitor of the organic anion transporters (Oat) probenecid (PBC) remarkably reduced IIM uptake. In the Caco-2 cell model, the furanocoumarins were highly permeable in the apical to basolateral direction without notable active efflux.

CONCLUSION

The furanocoumarins rapidly and widely distributed into various tissues after oral dose of the RAD extract. IM, IIM, BER and OXYH were the major components detected in both plasma and tissues. Liver was the most distributed tissue of the total and free furanocoumarins. Non-specific protein binding contributed partially to the higher tissue exposures of these bioactive components. The Oatp and Oat mediated active uptake played the primal role in the high hepatic exposure of the furanocoumarins.

Functional shift with maintained regenerative potential following portal vein ligation

Selective portal vein ligation (PVL) allows the two-stage surgical resection of primarily unresectable liver tumours by generating the atrophy and hypertrophy of portally ligated (LL) and non-ligated lobes (NLL), respectively. To evaluate critically important underlying functional alterations, present study characterised in vitro and vivo liver function in male Wistar rats (n = 106; 210-250 g) before, and 24/48/72/168/336 h after PVL. Lobe weights and volumes by magnetic resonance imaging confirmed the atrophy-hypertrophy complex. Proper expression and localization of key liver transporters (Ntcp, Bsep) and tight junction protein ZO-1 in isolated hepatocytes demonstrated constantly present viable and well-polarised cells in both lobes. In vitro taurocholate and bilirubin transport, as well as in vivo immunohistochemical Ntcp and Mrp2 expressions were bilaterally temporarily diminished, whereas LL and NLL structural acinar changes were divergent. In vivo bile and bilirubin-glucuronide excretion mirrored macroscopic changes, whereas serum bilirubin levels remained unaffected. In vivo functional imaging (indocyanine-green clearance test; ^99mTc-mebrofenin hepatobiliary scintigraphy; confocal laser endomicroscopy) indicated transitionally reduced global liver uptake and -excretion. While LL functional involution was permanent, NLL uptake and excretory functions recovered excessively. Following PVL, functioning cells remain even in LL. Despite extensive bilateral morpho-functional changes, NLL functional increment restores temporary declined transport functions, emphasising liver functional assessment.

Paritaprevir and Ritonavir Liver Concentrations in Rats as Assessed by Different Liver Sampling Techniques

The liver is crucial to pharmacology, yet substantial knowledge gaps exist in the understanding of its basic pharmacologic processes. An improved understanding for humans requires reliable and reproducible liver sampling methods. We compared liver concentrations of paritaprevir and ritonavir in rats by using samples collected by fine-needle aspiration (FNA), core needle biopsy (CNB), and surgical resection. Thirteen Sprague-Dawley rats were evaluated, nine of which received paritaprevir/ritonavir at 30/20 mg/kg of body weight by oral gavage daily for 4 or 5 days. Drug concentrations were measured using liquid chromatography-tandem mass spectrometry on samples collected via FNA (21G needle) with 1, 3, or 5 passes (FNA₁, FNA₃, and FNA₅); via CNB (16G needle); and via surgical resection. Drug concentrations in plasma were also assessed. Analyses included noncompartmental pharmacokinetic analysis and use of Bland-Altman techniques. All liver tissue samples had higher paritaprevir and ritonavir concentrations than those in plasma. Resected samples, considered the benchmark measure, resulted in estimations of the highest values for the pharmacokinetic parameters of exposure (maximum concentration of drug in serum [C_max] and area under the concentration-time curve from 0 to 24 h [AUC_0-24]) for paritaprevir and ritonavir. Bland-Altman analyses showed that the best agreement occurred between tissue resection and CNB, with 15% bias, followed by FNA₃ and FNA₅, with 18% bias, and FNA₁ and FNA₃, with a 22% bias for paritaprevir. Paritaprevir and ritonavir are highly concentrated in rat liver. Further research is needed to validate FNA sampling for humans, with the possible derivation and application of correction factors for drug concentration measurements.

Constitutive androstane receptor activation promotes bilirubin clearance in a murine model of alcoholic liver disease

Increased plasma levels of bilirubin have been reported in rat models and patients with alcoholic liver disease (ALD). The constitutive androstane receptor (CAR) is a known xenobiotic receptor, which induces the detoxification and transport of bilirubin. In the present study, the bilirubin transport regulatory mechanisms, and the role of CAR activation in hepatic and extrahepatic bilirubin clearance were investigated in a murine model of ALD. The mice were fed a Lieber-DeCarli ethanol diet or an isocaloric control diet for 4 weeks, followed by the administration of CAR agonists, 1,4-bis-[2-(3,5-dichlorpyridyloxy)]benzene (TCPOBOP) and phenobarbital (PB), and their vehicles to examine the effect of the pharmacological activation of CAR on serum levels of bilirubin and on the bilirubin clearance pathway in ALD by serological survey, western blotting and reverse transcription-quantitative polymerase chain reaction. The results showed that chronic ethanol ingestion impaired the nuclear translocation of CAR, which was accompanied by elevated serum levels of bilirubin, suppression of the expression of hepatic and renal organic anion transporting polypeptide (OATP) 1A1 and hepatic multidrug resistance-associated protein 2 (MRP2), and induction of the expression of UDP-glucuronosyltransferase (UGT) 1A1. The activation of CAR by TCPOBOP and PB resulted in downregulation of the serum levels of bilirubin followed by selective upregulation of the expression levels of OATP1A1, OATP1A4, UGT1A1 and MRP2 in ALD. These results revealed the bilirubin transport regulatory mechanisms and highlighted the importance of CAR in modulating the bilirubin clearance pathway in the ALD mouse model.

The impact of Organic Anion-Transporting Polypeptides (OATPs) on disposition and toxicity of antitumor drugs: Insights from knockout and humanized mice

It is now widely accepted that organic anion-transporting polypeptides (OATPs), especially members of the OATP1A/1B family, can have a major impact on the disposition and elimination of a variety of endogenous molecules and drugs. Owing to their prominent expression in the sinusoidal plasma membrane of hepatocytes, OATP1B1 and OATP1B3 play key roles in the hepatic uptake and plasma clearance of a multitude of structurally diverse anti-cancer and other drugs. Here, we present a thorough assessment of the currently available OATP1A and OATP1B knockout and transgenic mouse models as key tools to study OATP functions in vivo. We discuss recent studies using these models demonstrating the importance of OATPs, primarily in the plasma and hepatic clearance of anticancer drugs such as taxanes, irinotecan/SN-38, methotrexate, doxorubicin, and platinum compounds. We further discuss recent work on OATP-mediated drug-drug interactions in these mouse models, as well as on the role of OATP1A/1B proteins in the phenomenon of hepatocyte hopping, an efficient and flexible way of liver detoxification for both endogenous and exogenous substrates. Interestingly, glucuronide conjugates of both the heme breakdown product bilirubin and the protein tyrosine kinase-targeted anticancer drug sorafenib are strongly affected by this process. The clinical relevance of variation in OATP1A/1B activity in patients has been previously revealed by the effects of polymorphic variants and drug-drug interactions on drug toxicity. The development of in vivo tools to study OATP1A/1B functions has greatly advanced our mechanistic understanding of their functional role in drug pharmacokinetics, and their implications for therapeutic efficacy and toxic side effects of anticancer and other drug treatments.

Influence of 24-Nor-Ursodeoxycholic Acid on Hepatic Disposition of [(18)F]Ciprofloxacin, a Positron Emission Tomography Study in Mice

24-nor-ursodeoxycholic acid (norUDCA) is a novel therapeutic approach to cholestatic liver diseases. In mouse models of cholestasis, norUDCA induces basolateral multidrug resistance-associated proteins 4 (Mrp4) and 3 in hepatocytes, which provide alternative escape routes for bile acids accumulating during cholestasis but could also result in altered hepatic disposition of concomitantly administered substrate drugs. We used positron emission tomography imaging to study the influence of norUDCA on hepatic disposition of the model Mrp4 substrate [(18)F]ciprofloxacin in wild-type and Mdr2((-/-)) mice, a model of cholestasis. Animals underwent [(18)F]ciprofloxacin positron emission tomography at baseline and after norUDCA treatment. After norUDCA treatment, liver-to-blood area under the curve ratio of [(18)F]ciprofloxacin was significantly decreased compared to baseline, both in wild-type (-34.0 ± 2.1%) and Mdr2((-/-)) mice (-20.5 ± 6.0%). [(18)F]Ciprofloxacin uptake clearance from blood into liver was reduced by -17.1 ± 9.0% in wild-type and by -20.1 ± 7.3% in Mdr2((-/-)) mice. Real-time PCR analysis showed significant increases in hepatic Mrp4 and multidrug resistance-associated protein 3 mRNA after norUDCA. Transport experiments in organic anion transporting polypeptide (OATP)1B1-, OATP1B3-, and OATP2B1-transfected cells revealed weak transport of [(14)C]ciprofloxacin by OATP1B3 and OATP2B1 and no inhibition by norUDCA. In conclusion, our data suggest that changes in hepatic [(18)F]ciprofloxacin disposition in mice after norUDCA treatment were caused by induction of basolateral Mrp4 in hepatocytes.

Organic Anion Transporting Polypeptide (OATP)2B1 Contributes to Gastrointestinal Toxicity of Anticancer Drug SN-38, Active Metabolite of Irinotecan Hydrochloride

Gastrointestinal toxicity, such as late-onset diarrhea, is a significant concern in irinotecan hydrochloride (CPT-11)-containing regimens. Prophylaxis of late-onset diarrhea has been reported with use of Japanese traditional (Kampo) medicine containing baicalin and with the antibiotic cefixime, and this has been explained in terms of inhibition of bacterial deconjugation of SN-38-glucuronide since unconjugated SN-38 (active metabolite of CPT-11) is responsible for the gastrointestinal toxicity. It is also prerequisite for SN-38 to be accumulated in intestinal tissues to exert toxicity. Based on the fact that liver-specific organic anion transporting polypeptide (OATP)1B1, a member of the same family as OATP2B1, is known to be involved in hepatic transport of SN-38, we hypothesized that intestinal transporter OATP2B1 contributes to the accumulation of SN-38 in gastrointestinal tissues, and its inhibition would help prevent associated toxicity. We found that uptake of SN-38 by OATP2B1-expressing Xenopus oocytes was significantly higher than that by control oocytes. OATP2B1-mediated uptake of SN-38 was saturable, pH dependent, and decreased in the presence of baicalin, cefixime, or fruit juices such as apple juice. In vivo gastrointestinal toxicity of SN-38 in mice caused by oral administration for consecutive 5 days was prevented by coingestion of apple juice. Thus, OATP2B1 contributes to the uptake of SN-38 by intestinal tissues, triggering gastrointestinal toxicity. So, in addition to the reported inhibition of bacterial β-glucuronidase by cefixime or baicalin, inhibition of OATP2B1 may also contribute to prevention of gastrointestinal toxicity. Apple juice may be helpful for prophylaxis of late-onset diarrhea observed in CPT-11 therapy without disturbance of the intestinal microflora.

Identification of Novel Inhibitors of Organic Anion Transporting Polypeptides 1B1 and 1B3 (OATP1B1 and OATP1B3) Using a Consensus Vote of Six Classification Models

Organic anion transporting polypeptides 1B1 and 1B3 are transporters selectively expressed on the basolateral membrane of the hepatocyte. Several studies reveal that they are involved in drug–drug interactions, cancer, and hyperbilirubinemia. In this study, we developed a set of classification models for OATP1B1 and 1B3 inhibition based on more than 1700 carefully curated compounds from literature, which were validated via cross-validation and by use of an external test set. After combining several sets of descriptors and classifiers, the 6 best models were selected according to their statistical performance and were used for virtual screening of DrugBank. Consensus scoring of the screened compounds resulted in the selection and purchase of nine compounds as potential dual inhibitors and of one compound as potential selective OATP1B3 inhibitor. Biological testing of the compounds confirmed the validity of the models, yielding an accuracy of 90% for OATP1B1 and 80% for OATP1B3, respectively. Moreover, at least half of the new identified inhibitors are associated with hyperbilirubinemia or hepatotoxicity, implying a relationship between OATP inhibition and these severe side effects.

Organic anion uptake by hepatocytes

Many of the compounds taken up by the liver are organic anions that circulate tightly bound to protein carriers such as albumin. The fenestrated sinusoidal endothelium of the liver permits these compounds to have access to hepatocytes. Studies to characterize hepatic uptake of organic anions through kinetic analyses, suggested that it was carrier-mediated. Attempts to identify specific transporters by biochemical approaches were largely unsuccessful and were replaced by studies that utilized expression cloning. These studies led to identification of the organic anion transport proteins (oatps), a family of 12 transmembrane domain glycoproteins that have broad and often overlapping substrate specificities. The oatps mediate Na(+)-independent organic anion uptake. Other studies identified a seven transmembrane domain glycoprotein, Na(+)/taurocholate transporting protein (ntcp) as mediating Na(+)-dependent uptake of bile acids as well as other organic anions. Although mutations or deficiencies of specific members of the oatp family have been associated with transport abnormalities, there have been no such reports for ntcp, and its physiologic role remains to be determined, although expression of ntcp in vitro recapitulates the characteristics of Na(+)-dependent bile acid transport that is seen in vivo. Both ntcp and oatps traffic between the cell surface and intracellular vesicular pools. These vesicles move through the cell on microtubules, using the microtubule based motors dynein and kinesins. Factors that regulate this motility are under study and may provide a unique mechanism that can alter the plasma membrane content of these transporters and consequently their accessibility to circulating ligands.

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

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

Schisandra chinensis regulates drug metabolizing enzymes and drug transporters via activation of Nrf2-mediated signaling pathway

Drug metabolizing enzymes (DMEs) and drug transporters are regulated via epigenetic, transcriptional, posttranscriptional, and translational and posttranslational modifications. Phase I and II DMEs and drug transporters play an important role in the disposition and detoxification of a large number of endogenous and exogenous compounds. The nuclear factor (erythroid-derived 2)-like 2 (Nrf2) is a critical regulator of a variety of important cytoprotective genes that are involved in disposition and detoxification of xenobiotics. Schisandra chinensis (SC) is a commonly used traditional Chinese herbal medicine that has been primarily used to protect the liver because of its potent antioxidative and anti-inflammatory activities. SC can modulate some DMEs and drug transporters, but the underlying mechanisms are unclear. In this study, we aimed to explore the role of Nrf2 in the regulatory effect of SC extract (SCE) on selected DMEs and drug transporters in human hepatocellular liver carcinoma cell line (HepG2) cells. The results showed that SCE, schisandrin A, and schisandrin B significantly increased the expression of NAD(P)H: Nicotinamide Adenine Dinucleotide Phosphate-oxidase or:quinone oxidoreductase 1, heme oxygenase-1, glutamate–cysteine ligase, and glutathione S-transferase A4 at both transcriptional and posttranscriptional levels. Incubation of HepG2 cells with SCE resulted in a significant increase in the intracellular level of glutathione and total glutathione S-transferase content. SCE significantly elevated the messenger ribonucleic acid and protein levels of P-glycoprotein and multidrug resistance-associated protein 2 and 4, whereas the expression of organic anion transporting peptide 1A2 and 1B1 was significantly downregulated by SCE. Knockdown of Nrf2 by small interfering ribonucleic acid attenuated the regulatory effect of SCE on these DMEs and drug transporters. SCE significantly upregulated Nrf2 and promoted the translocation of Nrf2 from cytoplasm to the nuclei. Additionally, SCE significantly suppressed the expression of cytosolic Kelch-like ECH-associated protein 1 (the repressor of Nrf2) and remarkably increased Nrf2 stability in HepG2 cells. Taken together, our findings suggest that the hepatoprotective effects of SCE may be partially ascribed to the modulation of DMEs and drug transporters via Nrf2-mediated signaling pathway. SCE may alter the pharmacokinetics of other coadministered drugs that are substrates of these DMEs and transporters and thus cause unfavorable herb–drug interactions.

What do drug transporters really do?

Potential drug-drug interactions mediated by the ATP-binding cassette (ABC) transporter and solute carrier (SLC) transporter families are of clinical and regulatory concern. However, the endogenous functions of these drug transporters are not well understood. Discussed here is evidence for the roles of ABC and SLC transporters in the handling of diverse substrates, including metabolites, antioxidants, signalling molecules, hormones, nutrients and neurotransmitters. It is suggested that these transporters may be part of a larger system of remote communication ('remote sensing and signalling') between cells, organs, body fluid compartments and perhaps even separate organisms. This broader view may help to clarify disease mechanisms, drug-metabolite interactions and drug effects relevant to diabetes, chronic kidney disease, metabolic syndrome, hypertension, gout, liver disease, neuropsychiatric disorders, inflammatory syndromes and organ injury, as well as prenatal and postnatal development.

Intestinal transport and metabolism of bile acids

In addition to their classical roles as detergents to aid in the process of digestion, bile acids have been identified as important signaling molecules that function through various nuclear and G protein-coupled receptors to regulate a myriad of cellular and molecular functions across both metabolic and nonmetabolic pathways. Signaling via these pathways will vary depending on the tissue and the concentration and chemical structure of the bile acid species. Important determinants of the size and composition of the bile acid pool are their efficient enterohepatic recirculation, their host and microbial metabolism, and the homeostatic feedback mechanisms connecting hepatocytes, enterocytes, and the luminal microbiota. This review focuses on the mammalian intestine, discussing the physiology of bile acid transport, the metabolism of bile acids in the gut, and new developments in our understanding of how intestinal metabolism, particularly by the gut microbiota, affects bile acid signaling.

Quantitative assessment of the multiple processes responsible for bilirubin homeostasis in health and disease

Serum bilirubin measurements are commonly obtained for the evaluation of ill patients and to screen for liver disease in routine physical exams. An enormous research effort has identified the multiple mechanisms involved in the production and metabolism of conjugated (CB) and unconjugated bilirubin (UB). While the qualitative effects of these mechanisms are well understood, their expected quantitative influence on serum bilirubin homeostasis has received less attention. In this review, each of the steps involved in bilirubin production, metabolism, hepatic cell uptake, and excretion is quantitatively examined. We then attempt to predict the expected effect of normal and defective function on serum UB and CB levels in health and disease states including hemolysis, extra- and intrahepatic cholestasis, hepatocellular diseases (eg, cirrhosis, hepatitis), and various congenital defects in bilirubin conjugation and secretion (eg, Gilbert's, Dubin-Johnson, Crigler-Najjar, Rotor syndromes). Novel aspects of this review include: 1) quantitative estimates of the free and total UB and CB in the plasma, hepatocyte, and bile; 2) detailed discussion of the important implications of the recently recognized role of the hepatic OATP transporters in the maintenance of CB homeostasis; 3) discussion of the differences between the standard diazo assay versus chromatographic measurement of CB and UB; 4) pharmacokinetic implications of the extremely high-affinity albumin binding of UB; 5) role of the enterohepatic circulation in physiologic jaundice of newborn and fasting hyperbilirubinemia; and 6) insights concerning the clinical interpretation of bilirubin measurements.

Influence of drug formulation on OATP1B-mediated transport of paclitaxel

Taxane antineoplastic agents are extensively taken up into hepatocytes by OATP1B-type transporters before metabolism and excretion. Because the biodistributional properties imposed upon these agents by different solubilizers drive clinically important pharmacodynamic endpoints, we tested the hypothesis that the in vitro and in vivo interaction of taxanes with OATP1B transporters is affected by the choice of drug delivery system. Transport of paclitaxel, docetaxel, and cabazitaxel was studied in vitro using various cell lines transfected with OATP1B1, OATP1B3, or the rodent equivalent OATP1B2. Pharmacokinetic studies were done in wild-type and OATP1B2-knockout mice in the presence or absence of polysorbate 80 (PS80) or Kolliphor EL (formerly Cremophor EL; CrEL). Paclitaxel and docetaxel, but not cabazitaxel, were transported substrates of OATP1B1, OATP1B3, and OATP1B2, and these in vitro transport processes were strongly reduced in the presence of clinically relevant concentrations of PS80 and CrEL. When paclitaxel was administered without any solubilizers, deficiency of OATP1B2 in mice was associated with a significantly decreased systemic clearance because of a liver distribution defect (P=0.000484). However, this genotype dependence of paclitaxel clearance was masked in the presence of PS80 or CrEL because of significant inhibition of OATP1B2-mediated hepatocellular uptake of the drug (P<0.05). Our findings confirm the importance of OATP1B-type transporters in the hepatic elimination of taxanes and indicate that this process can be inhibited by PS80 and CrEL. These results suggest that the likelihood of drug-drug interactions mediated by these transporters is strongly dependent on the selected taxane solubilizer.

Inhibition of OATP1B1 by tyrosine kinase inhibitors: in vitro-in vivo correlations

BACKGROUND

Several tyrosine kinase inhibitors (TKIs) can decrease docetaxel clearance in patients by an unknown mechanism. We hypothesised that these interactions are mediated by the hepatic uptake transporter OATP1B1.

METHODS

The influence of 16 approved TKIs on transport was studied in vitro using HEK293 cells expressing OATP1B1 or its mouse equivalent Oatp1b2. Pharmacokinetic studies were performed with Oatp1b2-knockout and OATP1B1-transgenic mice.

RESULTS

All docetaxel-interacting TKIs, including sorafenib, were identified as potent inhibitors of OATP1B1 in vitro. Although Oatp1b2 deficiency in vivo was associated with increased docetaxel exposure, single- or multiple-dose sorafenib did not influence docetaxel pharmacokinetics.

CONCLUSION

These findings highlight the importance of identifying proper preclinical models for verifying and predicting TKI-chemotherapy interactions involving transporters.

Utility of Oatp1a/1b-knockout and OATP1B1/3-humanized mice in the study of OATP-mediated pharmacokinetics and tissue distribution: case studies with pravastatin, atorvastatin, simvastatin, and carboxydichlorofluorescein

Although organic anion transporting polypeptide (OATP)-mediated hepatic uptake is generally conserved between rodents and humans at a gross pharmacokinetic level, the presence of three major hepatic OATPs with broad overlap in substrate and inhibitor affinity, and absence of rodent-human orthologs preclude clinical translation of single-gene knockout/knockin findings. At present, changes in pharmacokinetics and tissue distribution of pravastatin, atorvastatin, simvastatin, and carboxydichlorofluorescein were studied in oatp1a/1b-knockout mice lacking the three major hepatic oatp isoforms, and in knockout mice with liver-specific knockin of human OATP1B1 or OATP1B3. Relative to wild-type controls, oatp1a/1b-knockout mice exhibited 1.6- to 19-fold increased intravenous and 2.1- to 115-fold increased oral drug exposure, due to 33%-75% decreased clearance, 14%-60% decreased volume of distribution, and ≤74-fold increased oral bioavailability, with the magnitude of change depending on the contribution of oatp1a/1b to pharmacokinetics. Hepatic drug distribution was 4.2- to 196-fold lower in oatp1a/1b-knockout mice; distributional attenuation was less notable in kidney, brain, cardiac, and skeletal muscle. Knockin of OATP1B1 or OATP1B3 partially restored control clearance, volume, and bioavailability values (24%-142% increase, ≤47% increase, and ≤77% decrease vs. knockout, respectively), such that knockin pharmacokinetic profiles were positioned between knockout and wild-type mice. Consistent with liver-specific humanization, only hepatic drug distribution was partially restored (1.3- to 6.5-fold increase vs. knockout). Exposure and liver distribution changes in OATP1B1-humanized versus knockout mice predicted the clinical impact of OATP1B1 on oral exposure and contribution to human hepatic uptake of statins within 1.7-fold, but only after correcting for human/humanized mouse liver relative protein expression factor (OATP1B1 = 2.2, OATP1B3 = 0.30).

Oatp1a1 requires PDZK1 to traffic to the plasma membrane by selective recruitment of microtubule-based motor proteins

Previous studies identified a family of organic anion transport proteins (OATPs), many of which have C-terminal PDZ binding consensus sequences. In particular, the C-terminal four amino acids of Oatp1a1, a transporter on rat and mouse hepatocytes, comprise a consensus binding site for PDZK1. In PDZK1 knockout mice and in transfected cells where PDZK1 expression was knocked down, Oatp1a1 accumulates in intracellular vesicles. The present study tests the hypothesis that Oatp1a1 traffics to and from the cell surface in vesicles along microtubules, and that PDZK1 guides recruitment of specific motors to these vesicles. Oatp1a1-containing vesicles were prepared from wild-type and PDZK1 knockout mice. As seen by immunofluorescence, kinesin-1, a microtubule plus-end directed motor, was largely associated with vesicles from wild-type mouse liver, whereas dynein, a minus-end directed motor, was largely associated with vesicles from PDZK1 knockout mouse liver. Quantification of motility on directionally marked microtubules following addition of 50 µM ATP showed that wild-type vesicles moved equally toward the plus and minus ends whereas PDZK1 knockout vesicles moved predominantly toward the minus end, consistent with net movement toward the cell interior. These studies provide a novel mechanism by which PDZK1 regulates intracellular trafficking of Oatp1a1 by recruiting specific motors to Oatp1a1-containing vesicles. In the absence of PDZK1, Oatp1a1-containing vesicles cannot recruit kinesin-1 and associate with dynein as a predominant minus-end directed motor. Whether this is a result of direct interaction of the Oatp1a1 cytoplasmic domain with dynein or with a dynein-containing protein complex remains to be established.

Organic anion transporting polypeptide 1B transporters modulate hydroxyurea pharmacokinetics

Hydroxyurea is currently the only FDA-approved drug that ameliorates the pathophysiology of sickle cell anemia. Unfortunately, substantial interpatient variability in the pharmacokinetics (PK) of hydroxyurea may result in variation of the drug's efficacy. However, little is known about mechanisms that modulate hydroxyurea PK. Recent in vitro studies identifying hydroxyurea as a substrate for organic anion transporting polypeptide (OATP1B) transporters prompted the current investigation assessing the role of OATP1B transporters in modulating hydroxyurea PK. Using wild-type and Oatp1b knockout (Oatp1b(-/-)) mice, hydroxyurea PK was analyzed in vivo by measuring [(14)C]hydroxyurea distribution in plasma, kidney, liver, urine, or the exhaled (14)CO2 metabolite. Plasma levels were significantly reduced by 20% in Oatp1b(-/-) mice compared with wild-type (area under the curve of 38.64 or 48.45 μg·h(-1)·ml(-1), respectively) after oral administration, whereas no difference was observed between groups following intravenous administration. Accumulation in the kidney was significantly decreased by twofold in Oatp1b(-/-) mice (356.9 vs. 748.1 pmol/g), which correlated with a significant decrease in urinary excretion. Hydroxyurea accumulation in the liver was also decreased (136.6 vs. 107.3 pmol/g in wild-type or Oatp1b(-/-) mice, respectively) correlating with a decrease in exhaled (14)CO2. These findings illustrate that deficiency of Oatp1b transporters alters the absorption, distribution, and elimination of hydroxyurea thus providing the first in vivo evidence that cell membrane transporters may play a significant role in modulating hydroxyurea PK. Future studies to investigate other transporters and their role in hydroxyurea disposition are warranted for understanding the sources of variation in hydroxyurea's PK.

OATP1A/1B transporters affect irinotecan and SN-38 pharmacokinetics and carboxylesterase expression in knockout and humanized transgenic mice

Organic anion-transporting polypeptides (OATP) mediate the hepatic uptake of many drugs, thus codetermining their clearance. Impaired hepatic clearance due to low-activity polymorphisms in human OATP1B1 may increase systemic exposure to SN-38, the active and toxic metabolite of the anticancer prodrug irinotecan. We investigated the pharmacokinetics and toxicity of irinotecan and SN-38 in Oatp1a/1b-null mice: Plasma exposure of irinotecan and SN-38 was increased 2 to 3-fold after irinotecan dosing (10 mg/kg, i.v.) compared with wild-type mice. Also, liver-to-plasma ratios were significantly reduced, suggesting impaired hepatic uptake of both compounds. After 6 daily doses of irinotecan, Oatp1a/1b-null mice suffered from increased toxicity. However, Oatp1a/1b-null mice had increased levels of carboxylesterase (Ces) enzymes, which caused higher conversion of irinotecan to SN-38 in plasma, potentially complicating pharmacokinetic analyses. Ces inhibitors blocked this increased conversion. Interestingly, liver-specific humanized OATP1B1 and OATP1B3 transgenic mice had normalized hepatic expression of Ces1 genes. While irinotecan liver-to-plasma ratios in these humanized mice were similar to those in Oatp1a/1b-null mice, SN-38 liver-to-plasma ratios returned to wild-type levels, suggesting that human OATP1B proteins mediate SN-38, but not irinotecan uptake in vivo. Upon direct administration of SN-38 (1 mg/kg, i.v.), Oatp1a/1b-null mice had increased SN-38 plasma levels, lower liver concentrations, and decreased cumulative biliary excretion of SN-38. Mouse Oatp1a/1b transporters have a role in the plasma clearance of irinotecan and SN-38, whereas human OATP1B transporters may only affect SN-38 disposition. Oatp1a/1b-null mice have increased expression and activity of Ces1 enzymes, whereas humanized mice provide a rescue of this phenotype.

New insights in bilirubin metabolism and their clinical implications

Bilirubin, a major end product of heme breakdown, is an important constituent of bile, responsible for its characteristic colour. Over recent decades, our understanding of bilirubin metabolism has expanded along with the processes of elimination of other endogenous and exogenous anionic substrates, mediated by the action of multiple transport systems at the sinusoidal and canalicular membrane of hepatocytes. Several inherited disorders characterised by impaired bilirubin conjugation (Crigler-Najjar syndrome type I and type II, Gilbert syndrome) or transport (Dubin-Johnson and Rotor syndrome) result in various degrees of hyperbilirubinemia of either the predominantly unconjugated or predominantly conjugated type. Moreover, disrupted regulation of hepatobiliary transport systems can explain jaundice in many acquired liver disorders. In this review, we discuss the recent data on liver bilirubin handling based on the discovery of the molecular basis of Rotor syndrome. The data show that a substantial fraction of bilirubin conjugates is primarily secreted by MRP3 at the sinusoidal membrane into the blood, from where they are subsequently reuptaken by sinusoidal membrane-bound organic anion transporting polypeptides OATP1B1 and OATP1B3. OATP1B proteins are also responsible for liver clearance of bilirubin conjugated in splanchnic organs, such as the intestine and kidney, and for a number of endogenous compounds, xenobiotics and drugs. Absence of one or both OATP1B proteins thus may have serious impact on toxicity of commonly used drugs cleared by this system such as statins, sartans, methotrexate or rifampicin. The liver-blood cycling of conjugated bilirubin is impaired in cholestatic and parenchymal liver diseases and this impairment most likely contributes to jaundice accompanying these disorders.

Genetically modified mouse models for oral drug absorption and disposition

Intestinal absorption is an essential step in the therapeutic use of most orally administered drugs and often mediated by enterocyte transmembrane transporters. Here we discuss several of these drug transport systems and knockout mouse models to study them. These studies showed that Multidrug resistance-associated protein 2 (Mrp2) can limit intestinal drug absorption. Organic cation transporter n1 (Octn1) and Octn2 might also facilitate intestinal drug absorption, although direct in vivo evidence is lacking. On the other hand, intestinal uptake of drugs is facilitated by the Equilibrative nucleoside transporter 1 (Ent1), Mrp3 and possibly Mrp4. No significant role in intestinal absorption for Oct1 and Oct2 or for Organic anion-transporting polypeptides (Oatp) 1a and 1b was found so far.

Modulation of OATP1B-type transporter function alters cellular uptake and disposition of platinum chemotherapeutics

Expression of the human organic anion transporting polypeptides OATP1B1 and OATP1B3 has been previously believed to be restricted to hepatocytes. Here we show that the gene encoding OATP1B3, but not OATP1B1, is abundantly expressed in multiple human solid tumors that include hepatocellular, lung, and ovarian carcinomas. Surprisingly, OATP1B3 gene expression in a panel of 60 human tumor cell lines was linked with sensitivity to multiple cytotoxic agents, including the platinum anticancer drugs cisplatin, carboplatin, and oxaliplatin. In addition, overexpression of OATP1B3 in mammalian cells increased cellular accumulation of platinum agents and decreased cell survival. In mice with a targeted disruption of the ortholog transporter Oatp1b2, the liver-to-plasma ratio of cisplatin was significantly reduced compared with wild-type mice, without concurrent changes in expression profiles of other transporter genes. Our findings indicate an unexpected role for tumoral and host OATP1B-type carriers in the toxicity and disposition of platinum anticancer drugs, and may provide a foundation for understanding the extensive interindividual pharmacodynamic variability seen with these drugs in patients.

Murine Oatp1a/1b uptake transporters control rosuvastatin systemic exposure without affecting its apparent liver exposure

Organic anion-transporting polypeptides (OATPs) mediate the liver uptake and hence plasma clearance of a broad range of drugs. For rosuvastatin, a cholesterol-lowering drug and OATP1A/1B substrate, the liver represents both its main therapeutic target and its primary clearance organ. Here we studied the impact of Oatp1a/1b uptake transporters on the pharmacokinetics of rosuvastatin using wild-type and Oatp1a/1b-null mice. After oral administration (15 mg/kg), intestinal absorption of rosuvastatin was not impaired in Oatp1a/1b-null mice, but systemic exposure (area under the curve) was 8-fold higher in these mice compared with wild-type. Although liver exposure was comparable between the two mouse strains (despite the increased blood exposure), the liver-to-blood ratios were markedly decreased (>10-fold) in the absence of Oatp1a/1b transporters. After intravenous administration (5 mg/kg), systemic exposure was 3-fold higher in Oatp1a/1b-null mice than in the wild-type mice. Liver, small intestinal, and kidney exposure were slightly, but not significantly, increased in Oatp1a/1b-null mice. The biliary excretion of rosuvastatin was very fast, with 60% of the dose eliminated within 15 minutes after intravenous administration, and also not significantly altered in Oatp1a/1b-null mice. Rosuvastatin renal clearance, although still minor, was increased ∼15-fold in Oatp1a/1b-null males, suggesting a role of Oatp1a1 in the renal reabsorption of rosuvastatin. Absence of Oatp1a/1b uptake transporters increases the systemic exposure of rosuvastatin by reducing its hepatic extraction ratio. However, liver concentrations are not significantly affected, most likely due to the compensatory activity of high-capacity, low-affinity alternative uptake transporters at higher systemic rosuvastatin levels and the absence of efficient alternative rosuvastatin clearance mechanisms.