Absolute measurement of species differences in sodium taurocholate cotransporting polypeptide (NTCP/Ntcp) and its modulation in cultured hepatocytes

Cellular pharmacokinetic of methotrexate and its modulation by folylpolyglutamate synthetase and γ-glutamyl hydrolase in tumor cells

BACKGROUND AND PURPOSE

Clinical studies showed that prolonged infusion of methotrexate (MTX) leads to more severe adverse reactions than short infusion of MTX at the same dose. We hypothesized that it is the saturation of folate polyglutamate synthetase (FPGS) at high MTX concentration that limits the intracellular synthesis rate of methotrexate polyglutamate (MTX-PG). Due to a similar accumulation rate, a longer infusion duration may increase the concentration of MTX-PG and, result in more serious adverse reactions. In this study, we validated this hypothesis.

EXPERIMENTAL APPROACH

A549, BEL-7402 and MHCC97H cell lines were treated with MTX at gradient concentrations. Liquid chromatograph-mass spectrometer (UPLC-MS/MS) was used to quantify the intracellular concentration of MTX-PG and the abundance of FPGS and γ-glutamyl hydrolase (GGH). High quality data were used to fit the cell pharmacokinetic model.

KEY RESULTS

Both cell growth inhibition rate and intracellular MTX-PG concentration showed a nonlinear relationship with MTX concentration. The parameter Vmax in the model, which represents the synthesis rate of MTX-PG, showed a strong correlation with the abundance of intracellular FPGS.

CONCLUSION AND IMPLICATIONS

According to the model fitting results, it was confirmed that the abundance of FPGS is a decisive factor limiting the synthesis rate of MTX-PG. The proposed hypothesis was verified in this study. In addition, based on the intracellular metabolism, a reasonable explanation was provided for the correlation between the severity of adverse reactions of MTX and infusion time. This study provides a new strategy for the individualized treatment and prediction of efficacy/side effects of MTX.

Detection of serum large and middle hepatitis B virus surface proteins: A novel potential diagnostic and prognostic biomarker for chronic hepatitis B

BACKGROUND

The significance of large (LHB) and middle (MHB) HBV surface proteins in chronic hepatitis B (CHB) remains uncertain. This study investigates the role of LHB and MHB in different infection phases and liver diseases.

METHODS

Serum samples from 217 patients with HBV chronic infection, CHB, liver cirrhosis (LC), and hepatocellular carcinoma (HCC) were subjected to quantification of LHB and MHB using ELISA.

RESULTS

Positive correlations were observed among LHB, MHB, and LHB/HBsAg, with HBV serum markers including HBsAg, HBeAg, and HBV DNA. (P < 0.0001). In HBeAg-positive chronic infection, LHB and MHB were higher than in HBeAg-positive CHB (P < 0.01). In HBeAg-negative chronic infection, LHB and MHB were lower than in HBeAg-negative CHB (P < 0.01). ROC analysis identified LHB and MHB as potential discriminators of CHB and chronic infection. LC and HCC exhibited lower LHB, MHB, and MHB/HBsAg than CHB (P < 0.05). Multivariate analysis found that age and the MHB/HBsAg serve as independent factors for the progression of CHB to end stage of liver disease.

CONCLUSIONS

LHB and MHB emerge as novel biomarkers distinguishing chronic infection and CHB. MHB/HBsAg shows promise as a predictor for CHB progression.

Role of sodium taurocholate cotransporting polypeptide (NTCP) in HBV-induced hepatitis: Opportunities for developing novel therapeutics

Hepatitis B is an infectious disease caused by the HBV virus. It presents a significant challenge for treatment due to its chronic nature and the potential for developing severe complications, including hepatocirrhosis and hepatocellular carcinoma. These complications not only cause physical and psychological distress to patients but also impose substantial economic and social burdens on both individuals and society as a whole. The internalization of HBV relies on endocytosis and necessitates the involvement of various proteins, including heparin sulfate proteoglycans, epidermal growth factor receptors, and NTCP. Among these proteins, NTCP is pivotal in HBV internalization and is primarily located in the liver's basement membrane. As a transporter of bile acids, NTCP also serves as a receptor facilitating HBV entry into cells. Numerous molecules have been identified to thwart HBV infection by stifling NTCP activity, although only a handful exhibit low IC50 values. In this systematic review, our primary focus dwells on the structure and regulation of NTCP, as well as the mechanism involved in HBV internalization. We underscore recent drug breakthroughs that specifically target NTCP to combat HBV infection. By shedding light on these advances, this review contributes novel insights into developing effective anti-HBV medications.

Regulation of the HBV Entry Receptor NTCP and its Potential in Hepatitis B Treatment

Hepatitis B virus (HBV) is a globally prevalent human DNA virus responsible for more than 250 million cases of chronic liver infection, a condition that can lead to liver inflammation, cirrhosis, and hepatocellular carcinoma. Sodium taurocholate co-transporting polypeptide (NTCP), a transmembrane protein highly expressed in human hepatocytes and a mediator of bile acid transport, has been identified as the receptor responsible for the cellular entry of both HBV and its satellite, hepatitis delta virus (HDV). This has led to significant advances in our understanding of the HBV life cycle, especially the early steps of infection. HepG2-NTCP cells and human NTCP-expressing transgenic mice have been employed as the primary cell culture and animal models, respectively, for the study of HBV, and represent valuable approaches for investigating its basic biology and developing treatments for infection. However, the mechanisms involved in the regulation of NTCP transcription, translation, post-translational modification, and transport are still largely elusive. Improvements in our understanding of NTCP biology would likely facilitate the design of new therapeutic drugs for the prevention of the de novo infection of naïve hepatocytes. In this review, we provide critical findings regarding NTCP biology and discuss important questions that remain unanswered.

Organic Cation Transporter I and Na+ /taurocholate Co-Transporting Polypeptide are Involved in Retrorsine- and Senecionine-Induced Hepatotoxicity in HepaRG cells

SCOPE

1,2-unsaturated pyrrolizidine alkaloids (PAs) are secondary plant metabolites that are found in many plant species throughout the world. They are of concern for risk assessment as consumption of contaminated foodstuff can cause severe liver damage. Of late, transporter-mediated uptake and transport has advanced as a vital determinant of PA toxicity. In this study, the authors investigate a transporter-mediated uptake of PAs and its implications in PA toxicity.

METHODS AND RESULTS

We show that transporter expression levels are significantly affected by treatment with the PAs senecionine (Sc) and retrorsine (Re) in the human hepatoma cell line HepaRG. Furthermore, the specific contribution to PA uptake of the two transporters Na⁺ /taurocholate co-transporting polypeptide (SLC10A1) and organic cation transporter I (SLC22A1), both belonging to the heterogeneous solute carrier super family, is investigated by means of a siRNA-mediated knockdown approach. Knockdown of both uptake transporters result in reduced uptake of Re and Sc in a time-dependent manner and attenuated PA-mediated cytotoxic effects in HepaRG cells.

CONCLUSION

Our results confirm previous findings of active transport mechanisms of PAs into hepatocytes and highlight the importance of toxicokinetic studies for the risk assessment of PAs.

Lysosomal P-gp-MDR1 Confers Drug Resistance of Brentuximab Vedotin and Its Cytotoxic Payload Monomethyl Auristatin E in Tumor Cells

Antibody-drug conjugates (ADCs) are composed of an antibody linked to cytotoxic anticancer payloads. ADCs recognize tumor-specific cell surface antigens and are internalized into lysosomes where proteolytic enzymes release the cytotoxic payloads. Efflux transporters on plasma membrane that play a significant role on multi-drug resistance in chemotherapy can be internalized on lysosomal membrane and sequester the cytotoxic payloads. In the present study, ATP binding cassette (ABC) efflux transporters including breast cancer resistance protein (BCRP), P-glycoprotein (P-gp-MDR1), multidrug resistance protein (MRP) 2, MRP3 and MRP4 in lysosomal, and plasma membrane of tumor cells were quantified by targeted quantitative proteomics. The cytotoxicity of brentuximab vedotin and its cytotoxic payload monomethyl auristatin E (MMAE) to the tumor cell lines in the presence and absence of elacridar (P-gp-MDR1 inhibitor) or chloroquine (lysosomotropic agent) were evaluated. MMAE is a substrate for P-gp-MDR1, as the apparent efflux ratio in MDR1 transfected MDCK cell monolayers was 44.5, and elacridar abolished the MMAE efflux. Cell lines that highly express P-gp-MDR1 show higher EC₅₀s toward the cell killing effects of MMAE. Co-incubation with chloroquine or elacridar resulted in left shift of MMAE EC₅₀ by 2.9-16-fold and 4.2-22-fold, respectively. Similarly co-incubation with chloroquine or elacridar or in combination of chloroquine and elacridar increased cytotoxic effects of brentuximab vedotin by 2.8- to 21.4-fold on KM-H2 cells that express a specific tumor antigen CD30 and P-gp-MDR1. These findings demonstrate important roles of P-gp-MDR1 on cytotoxic effects of brentuximab vedotin and its payload MMAE. Collectively, ABC transporter-mediated drug extrusion and/or sequestration needs to be early assessed for selection of optimal payloads and linkers when developing ADCs.

Evaluation of Drug Biliary Excretion Using Sandwich-Cultured Human Hepatocytes

Evaluation of hepatobiliary transport of drugs is an important challenge, notably during the development of new molecular identities. In this context, sandwich-cultured human hepatocytes (SCHH) have been proposed as an interesting and integrated tool for predicting in vitro biliary excretion of drugs. The present review was therefore designed to summarize key findings about SCHH, including their establishment, their main functional features and their use for the determination of canalicular transport and the prediction of in vivo biliary clearance and hepatobiliary excretion-related drug-drug interactions. Reviewed data highlight the fact that SCHH represent an original and probably unique holistic in vitro approach to predict biliary clearance in humans, through taking into account sinusoidal drug uptake, passive drug diffusion, drug metabolism and sinusoidal and canalicular drug efflux. Limits and proposed refinements for SCHH-based analysis of drug biliary excretion, as well as putative human alternative in vitro models to SCHH are also discussed.

Quantitative Contribution of Six Major Transporters to the Hepatic Uptake of Drugs: "SLC-Phenotyping" Using Primary Human Hepatocytes

Hepatic uptake transporters [solute carriers (SLCs)], including organic anion transporting polypeptide (OATP) 1B1, OATP1B3, OATP2B1, sodium-dependent taurocholate cotransporting polypeptide (NTCP), and organic anion (OAT2) and organic cation (OCT1) transporters, play a key role in determining the systemic and liver exposure of chemically diverse drugs. Here, we established a phenotyping approach to quantify the contribution of the six SLCs, and passive diffusion, to the overall uptake using plated human hepatocytes (PHHs). First, selective inhibitor conditions were identified by screening about 20 inhibitors across the six SLCs using single-transfected human embryonic kidney 293 cells. Data implied rifamycin SV (20 µM) inhibits three OATPs, while rifampicin (5 µM) inhibits OATP1B1/1B3 only. Further, hepatitis B virus myristoylated-preS1 peptide (0.1 µM), quinidine (100 µM), and ketoprofen (100-300 µM) are relatively selective against NTCP, OCT1, and OAT2, respectively. Second, using these inhibitory conditions, the fraction transported (f_t ) by the individual SLCs was characterized for 20 substrates with PHH. Generally, extended clearance classification system class 1A/3A (e.g., warfarin) and 1B/3B compounds (e.g., statins) showed predominant OAT2 and OATP1B1/1B3 contribution, respectively. OCT1-mediated uptake was prominent for class 2/4 compounds (e.g., metformin). Third, in vitro f_t values were corrected using quantitative proteomics data to obtain "scaled f_t " Fourth, in vitro-in vivo extrapolation of the scaled OATP1B1/1B3 f_t was assessed, leveraging statin clinical drug-drug interaction data with rifampicin as the perpetrator. Finally, we outlined a novel stepwise strategy to implement phenotypic characterization of SLC-mediated hepatic uptake for new molecular entities and drugs in a drug discovery and development setting.

Quantification of Hepatic Organic Anion Transport Proteins OAT2 and OAT7 in Human Liver Tissue and Primary Hepatocytes

Organic anion transporter (OAT) 2 and OAT7 were recently shown to be involved in the hepatic uptake of drugs; however, there is limited understanding of the population variability in the expression of these transporters in liver. There is also a need to derive relative expression-based scaling factors (REFs) that can be used to bridge in vitro functional data to the in vivo drug disposition. To this end, we quantified OAT2 and OAT7 surrogate peptide abundance in a large number of human liver tissue samples ( n = 52), as well as several single-donor cryopreserved human hepatocyte lots ( n = 30) by a novel, validated liquid chromatography tandem mass spectrometry (LC-MS/MS) method. The average surrogate peptide expression of OAT2 and OAT7 in the liver samples was 1.52 ± 0.57 and 4.63 ± 1.58 fmol/μg membrane protein, respectively. While we noted statistically significant differences ( p < 0.05) in hepatocyte and liver tissue abundances for both OAT2 and OAT7, the differences were relatively small (1.8- and 1.5-fold difference in median values, respectively). Large interindividual variability was noted in the hepatic expression of OAT2 (16-fold in liver tissue and 23-fold in hepatocytes). OAT7, on the other hand, showed less interindividual variability (4-fold) in the livers, but high variability for the hepatocyte lots (27-fold). A significant positive correlation in OAT2 and OAT7 expression was observed, but expression levels were neither associated with age nor sex. In conclusion, our data suggest marked interindividual variability in the hepatic expression of OAT2/7, which may contribute to the pharmacokinetic variability of their substrates. Because both transporters were less abundant in hepatocytes than livers, a REF-based approach is recommended when scaling in vitro hepatocyte transport data to predict hepatic drug clearance and liver exposure of OAT2/7 substrates.

Review article: therapeutic bile acids and the risks for hepatotoxicity

BACKGROUND

Bile acids play important roles in cholesterol metabolism and signal through farnesoid X receptor and G protein-coupled receptors. Given their importance in liver biology, bile acid therapy enables therapeutic applications beyond the treatment of cholestatic liver disease. However, predicting hepatotoxicity of bile acids in humans is obscured due to inconsistent extrapolations of animal data to humans.

AIM

To review the evidence that could explain discordant bile acids hepatotoxicity observed in humans and animals.

METHOD

Literature search was conducted in PubMed using keywords "bile acid," "transporter," "hepatotoxicity," "clinical study," "animal study," "species difference," "mechanism," "genetic disorder." Relevant articles were selected for review.

RESULTS

Clinically significant hepatotoxicity was reported in response to certain bile acids, namely chenodeoxycholic acid, which was given a boxed warning for potential hepatotoxicity. The chemical structure, specifically the number and orientation of hydroxyl groups, significantly affects their hydrophobicity, an important factor in bile acid toxicity. Experimental studies show that hydrophobic bile acids can lead to liver injury through various mechanisms, such as death receptor signalling, mitochondrial dysfunction and inflammation. Although animal studies play a central role in investigating bile acid safety, there are considerable differences in bile acid composition, metabolism and hepatobiliary disposition across species. This does not allow appropriate safety inference, especially for predicting hepatotoxicity in humans. Exploring evidences stemming from inborn errors, genetic models of disease and toxicology studies further improves an understanding of bile acid hepatotoxicity.

CONCLUSION

Species differences should be considered in the development of bile acid related therapeutics. Although the mechanism of bile acid hepatotoxicity is still not fully understood, continued mechanistic studies will deepen our understanding.

Comparative untargeted proteomic analysis of ADME proteins and tumor antigens for tumor cell lines

In the present study, total membrane proteins from tumor cell lines including HepG2, Hep3B2, H226, Ovcar3 and N87 were extracted and digested with γLysC and trypsin. The resulting peptide lysate were pre-fractionated and subjected to untargeted quantitative proteomics analysis using a high resolution mass spectrometer. The mass spectra were processed by the MaxQuant and the protein abundances were estimated using total peak area (TPA) method. A total of 6037 proteins were identified, and the analysis resulted in the identification of 2647 membrane proteins. Of those, tumor antigens and absorption, metabolism, disposition and elimination (ADME) proteins including UDP-glucuronosyltransferase, cytochrome P450, solute carriers and ATP-binding cassette transporters were detected and disclosed significant variations among the cell lines. The principal component analysis was performed for the cluster of cell lines. The results demonstrated that H226 is closely related with N87, while Hep3B2 aligned with HepG2. The protein cluster of Ovcar3 was apart from that of other cell lines investigated. By providing for the first time quantitative untargeted proteomics analysis, the results delineated the expression profiles of membrane proteins. These findings provided a useful resource for selecting targets of choice for anticancer therapy through advancing data obtained from preclinical tumor cell line models to clinical outcomes.

Quantitative Characterization of Major Hepatic UDP-Glucuronosyltransferase Enzymes in Human Liver Microsomes: Comparison of Two Proteomic Methods and Correlation with Catalytic Activity

Quantitative characterization of UDP-glucuronosyltransferase (UGT) enzymes is valuable in glucuronidation reaction phenotyping, predicting metabolic clearance and drug-drug interactions using extrapolation exercises based on pharmacokinetic modeling. Different quantitative proteomic workflows have been employed to quantify UGT enzymes in various systems, with reports indicating large variability in expression, which cannot be explained by interindividual variability alone. To evaluate the effect of methodological differences on end-point UGT abundance quantification, eight UGT enzymes were quantified in 24 matched liver microsomal samples by two laboratories using stable isotope-labeled (SIL) peptides or quantitative concatemer (QconCAT) standard, and measurements were assessed against catalytic activity in seven enzymes (n = 59). There was little agreement between individual abundance levels reported by the two methods; only UGT1A1 showed strong correlation [Spearman rank order correlation (Rs) = 0.73, P < 0.0001; R2 = 0.30; n = 24]. SIL-based abundance measurements correlated well with enzyme activities, with correlations ranging from moderate for UGTs 1A6, 1A9, and 2B15 (Rs = 0.52-0.59, P < 0.0001; R2 = 0.34-0.58; n = 59) to strong correlations for UGTs 1A1, 1A3, 1A4, and 2B7 (Rs = 0.79-0.90, P < 0.0001; R2 = 0.69-0.79). QconCAT-based data revealed generally poor correlation with activity, whereas moderate correlations were shown for UGTs 1A1, 1A3, and 2B7. Spurious abundance-activity correlations were identified in the cases of UGT1A4/2B4 and UGT2B7/2B15, which could be explained by correlations of protein expression between these enzymes. Consistent correlation of UGT abundance with catalytic activity, demonstrated by the SIL-based dataset, suggests that quantitative proteomic data should be validated against catalytic activity whenever possible. In addition, metabolic reaction phenotyping exercises should consider spurious abundance-activity correlations to avoid misleading conclusions.

Critical Issues and Optimized Practices in Quantification of Protein Abundance Level to Determine Interindividual Variability in DMET Proteins by LC-MS/MS Proteomics

Protein quantification data on drug metabolizing enzymes and transporters (collectively referred as DMET proteins) in human tissues are useful in predicting interindividual variability in drug disposition. While targeted proteomics is an emerging technique for quantification of DMET proteins, the methodology involves significant technical challenges especially when multiple samples are analyzed in a single study over a long period of time. Therefore, it is important to thoroughly address the critical variables that could affect DMET protein quantification.

Understanding the Interplay Between Uptake and Efflux Transporters Within In Vitro Systems in Defining Hepatocellular Drug Concentrations

One of the most holistic in vitro systems for prediction of intracellular drug concentrations is sandwich-cultured hepatocytes (SCH); however, a comprehensive evaluation of the utility of SCH to estimate uptake and biliary clearances and the need for additional kinetic parameters has yet to be carried out. Toward this end, we have selected 9 compounds (rosuvastatin, valsartan, fexofenadine, pravastatin, repaglinide, telmisartan, atorvastatin, saquinavir, and quinidine) to provide a range of physicochemical and hepatic disposition properties. Uptake clearances were determined in SCH and compared with conventional monolayer and suspension hepatocyte systems, previously reported by our laboratory. CL_uptake ranged from 1 to 41 μL/min/10⁶ cells in SCH which were significantly lower (1%-10%) compared with the other hepatocyte models. The hepatocyte-to-media unbound concentration ratio (Kp_u) has been assessed and ranged 0.7-59, lower compared with other hepatocyte systems (8-280). Estimates of in vitro biliary clearance (CL_bile) for 4 drugs were determined and were scaled to predict in vivo values using both intracellular concentration and media drug concentrations. These studies demonstrate that reduced uptake in rat SCH may limit drug access to canalicular efflux transport proteins and highlight the importance of elucidating the interplay between these proteins for accurate prediction of hepatic clearance.

Hepatobiliary Clearance Prediction: Species Scaling From Monkey, Dog, and Rat, and In Vitro-In Vivo Extrapolation of Sandwich-Cultured Human Hepatocytes Using 17 Drugs

Hepatobiliary elimination can be a major clearance pathway dictating the pharmacokinetics of drugs. Here, we first compared the dose eliminated in bile in preclinical species (monkey, dog, and rat) with that in human and further evaluated single-species scaling (SSS) to predict human hepatobiliary clearance. Six compounds dosed in bile duct-cannulated (BDC) monkeys showed biliary excretion comparable to human; and the SSS of hepatobiliary clearance with plasma fraction unbound correction yielded reasonable predictions (within 3-fold). Although dog SSS also showed reasonable predictions, rat overpredicted hepatobiliary clearance for 13 of 24 compounds. Second, we evaluated the translatability of in vitro sandwich-cultured human hepatocytes (SCHHs) to predict human hepatobiliary clearance for 17 drugs. For drugs with no significant active uptake in SCHH studies (i.e., with or without rifamycin SV), measured intrinsic biliary clearance was directly scalable with good predictability (absolute average fold error [AAFE] = 1.6). Drugs showing significant active uptake in SCHH, however, showed improved predictability when scaled based on extended clearance term (AAFE = 2.0), which incorporated sinusoidal uptake along with a global scaling factor for active uptake and the canalicular efflux clearance. In conclusion, SCHH is a useful tool to predict human hepatobiliary clearance, whereas BDC monkey model may provide further confidence in the prospective predictions.

Database of Optimized Proteomic Quantitative Methods for Human Drug Disposition-Related Proteins for Applications in Physiologically Based Pharmacokinetic Modeling

The purpose of this study was to create an open access repository of validated liquid chromatography tandem mass spectrometry (LC‐MS/MS) multiple reaction monitoring (MRM) methods for quantifying 284 important proteins associated with drug absorption, distribution, metabolism, and excretion (ADME). Various in silico and experimental approaches were used to select surrogate peptides and optimize instrument parameters for LC‐MS/MS quantification of the selected proteins. The final methods were uploaded to an online public database (QPrOmics; www.qpromics.uw.edu/qpromics/assay/), which provides essential information for facile method development in triple quadrupole mass spectrometry (MS) instruments. To validate the utility of the methods, the differential tissue expression of 107 key ADME proteins was characterized in the tryptic digests of the pooled subcellular fractions of human liver, kidneys, intestines, and lungs. These methods and the data are critical for development of physiologically based pharmacokinetic (PBPK) models to predict xenobiotic disposition.

Global Proteomic Analysis of Human Liver Microsomes: Rapid Characterization and Quantification of Hepatic Drug-Metabolizing Enzymes

Many genetic and environmental factors lead to interindividual variations in the metabolism and transport of drugs, profoundly affecting efficacy and toxicity. Precision dosing, that is, targeting drug dose to a well characterized subpopulation, is dependent on quantitative models of the profiles of drug-metabolizing enzymes (DMEs) and transporters within that subpopulation, informed by quantitative proteomics. We report the first use of ion mobility-mass spectrometry for this purpose, allowing rapid, robust, label-free quantification of human liver microsomal (HLM) proteins from distinct individuals. Approximately 1000 proteins were identified and quantified in four samples, including an average of 70 DMEs. Technical and biological variabilities were distinguishable, with technical variability accounting for about 10% of total variability. The biological variation between patients was clearly identified, with samples showing a range of expression profiles for cytochrome P450 and uridine 5'-diphosphoglucuronosyltransferase enzymes. Our results showed excellent agreement with previous data from targeted methods. The label-free method, however, allowed a fuller characterization of the in vitro system, showing, for the first time, that HLMs are significantly heterogeneous. Further, the traditional units of measurement of DMEs (pmol mg^-1 HLM protein) are shown to introduce error arising from variability in unrelated, highly abundant proteins. Simulations of this variability suggest that up to 1.7-fold variation in apparent CYP3A4 abundance is artifactual, as are background positive correlations of up to 0.2 (Spearman correlation coefficient) between the abundances of DMEs. We suggest that protein concentrations used in pharmacokinetic predictions and scaling to in vivo clinical situations (physiologically based pharmacokinetics and in vitro-in vivo extrapolation) should be referenced instead to tissue mass.

Abundance of Hepatic Transporters in Caucasians: A Meta-Analysis

This study aimed to derive quantitative abundance values for key hepatic transporters suitable for in vitro–in vivo extrapolation within a physiologically based pharmacokinetic modeling framework. A meta-analysis was performed whereby data on abundance measurements, sample preparation methods, and donor demography were collated from the literature. To define values for a healthy Caucasian population, a subdatabase was created whereby exclusion criteria were applied to remove samples from non-Caucasian individuals, those with underlying disease, or those with subcellular fractions other than crude membrane. Where a clinically relevant active genotype was known, only samples from individuals with an extensive transporter phenotype were included. Authors were contacted directly when additional information was required. After removing duplicated samples, the weighted mean, geometric mean, standard deviation, coefficient of variation, and between-study homogeneity of transporter abundances were determined. From the complete database containing 24 transporters, suitable abundance data were available for 11 hepatic transporters from nine studies after exclusion criteria were applied. Organic anion transporting polypeptides OATP1B1 and OATP1B3 showed the highest population abundance in healthy adult Caucasians. For several transporters, the variability in abundance was reduced significantly once the exclusion criteria were applied. The highest variability was observed for OATP1B3 > OATP1B1 > multidrug resistance protein 2 > multidrug resistance gene 1. No relationship was found between transporter expression and donor age. To our knowledge, this study provides the first in-depth analysis of current quantitative abundance data for a wide range of hepatic transporters, with the aim of using these data for in vitro–in vivo extrapolation, and highlights the significance of investigating the background of tissue(s) used in quantitative transporter proteomic studies. Similar studies are now warranted for other ethnicities.

In Vitro-In Vivo Extrapolation Scaling Factors for Intestinal P-Glycoprotein and Breast Cancer Resistance Protein: Part I: A Cross-Laboratory Comparison of Transporter-Protein Abundances and Relative Expression Factors in Human Intestine and Caco-2 Cells

Over the last 5 years the quantification of transporter-protein absolute abundances has dramatically increased in parallel to the expanded use of in vitro-in vivo extrapolation (IVIVE) and physiologically based pharmacokinetics (PBPK)-linked models, for decision-making in pharmaceutical company drug development pipelines and regulatory submissions. Although several research groups have developed laboratory-specific proteomic workflows, it is unclear if the large range of reported variability is founded on true interindividual variability or experimental variability resulting from sample preparation or the proteomic methodology used. To assess the potential for methodological bias on end-point abundance quantification, two independent laboratories, the University of Manchester (UoM) and Bertin Pharma (BPh), employing different proteomic workflows, quantified the absolute abundances of Na/K-ATPase, P-gp, and breast cancer resistance protein (BCRP) in the same set of biologic samples from human intestinal and Caco-2 cell membranes. Across all samples, P-gp abundances were significantly correlated (P = 0.04, Rs = 0.72) with a 2.4-fold higher abundance (P = 0.001) generated at UoM compared with BPh. There was a systematically higher BCRP abundance in Caco-2 cell samples quantified by BPh compared with UoM, but not in human intestinal samples. Consequently, a similar intestinal relative expression factor (REF), derived from distal jejunum and Caco-2 monolayer samples, between laboratories was found for P-gp. However, a 2-fold higher intestinal REF was generated by UoM (2.22) versus BPh (1.11). We demonstrate that differences in absolute protein abundance are evident between laboratories and they probably result from laboratory-specific methodologies relating to peptide choice.

Applications of human hepatitis B virus preS domain in bio- and nanotechnology

Human hepatitis B virus (HBV) is a member of the family Hepadnaviridae, and causes acute and chronic infections of the liver. The hepatitis B surface antigen (HBsAg) contains the large (L), middle (M), and small (S) surface proteins. The L protein consists of the S protein, preS1, and preS2. In HBsAg, the preS domain (preS1 + preS2) plays a key role in the infection of hepatocytic cells by HBV and has several immunogenic epitopes. Based on these characteristics of preS, several preS-based diagnostic and therapeutic materials and systems have been developed. PreS1-specific monoclonal antibodies (e.g., MA18/7 and KR127) can be used to inhibit HBV infection. A myristoylated preS1 peptide (amino acids 2-48) also inhibits the attachment of HBV to HepaRG cells, primary human hepatocytes, and primary tupaia hepatocytes. Antibodies and antigens related to the components of HBsAg, preS (preS1 + preS2), or preS1 can be available as diagnostic markers of acute and chronic HBV infections. Hepatocyte-targeting delivery systems for therapeutic molecules (drugs, genes, or proteins) are very important for increasing the clinical efficacy of these molecules and in reducing their adverse effects on other organs. The selective delivery of diagnostic molecules to target hepatocytic cells can also improve the efficiency of diagnosis. In addition to the full-length HBV vector, preS (preS1 + preS2), preS1, and preS1-derived fragments can be useful in hepatocyte-specific targeting. In this review, we discuss the literature concerning the applications of the HBV preS domain in bio- and nanotechnology.

Characterization of Organic Anion Transporter 2 (SLC22A7): A Highly Efficient Transporter for Creatinine and Species-Dependent Renal Tubular Expression

The contribution of organic anion transporter OAT2 (SLC22A7) to the renal tubular secretion of creatinine and its exact localization in the kidney are reportedly controversial. In the present investigation, the transport of creatinine was assessed in human embryonic kidney (HEK) cells that stably expressed human OAT2 (OAT2-HEK) and isolated human renal proximal tubule cells (HRPTCs). The tubular localization of OAT2 in human, monkey, and rat kidney was characterized. The overexpression of OAT2 significantly enhanced the uptake of creatinine in OAT2-HEK cells. Under physiologic conditions (creatinine concentrations of 41.2 and 123.5 µM), the initial rate of OAT2-mediated creatinine transport was approximately 11-, 80-, and 80-fold higher than OCT2, multidrug and toxin extrusion protein (MATE)1, and MATE2K, respectively, resulting in approximately 37-, 1850-, and 80-fold increase of the intrinsic transport clearance when normalized to the transporter protein concentrations. Creatinine intracellular uptake and transcellular transport in HRPTCs were decreased in the presence of 50 µM bromosulfophthalein and 100 µM indomethacin, which inhibited OAT2 more potently than other known creatinine transporters, OCT2 and multidrug and toxin extrusion proteins MATE1 and MATE2K (IC50: 1.3 µM vs. > 100 µM and 2.1 µM vs. > 200 µM for bromosulfophthalein and indomethacin, respectively) Immunohistochemistry analysis showed that OAT2 protein was localized to both basolateral and apical membranes of human and cynomolgus monkey renal proximal tubules, but appeared only on the apical membrane of rat proximal tubules. Collectively, the findings revealed the important role of OAT2 in renal secretion and possible reabsorption of creatinine and suggested a molecular basis for potential species difference in the transporter handling of creatinine.

Polarized location of SLC and ABC drug transporters in monolayer-cultured human hepatocytes

Human hepatocytes cultured in a monolayer configuration represent a well-established in vitro model in liver toxicology, notably used in drug transporter studies. Polarized status of drug transporters, i.e., their coordinated location at sinusoidal or canalicular membranes, remains however incompletely documented in these cultured hepatocytes. The present study was therefore designed to analyze transporter expression and location in such cells. Most of drug transporters were first shown to be present at notable mRNA levels in monolayer-cultured human hepatocytes. Cultured human hepatocytes, which morphologically exhibited bile canaliculi-like structures, were next demonstrated, through immunofluorescence staining, to express the influx transporters organic anion transporting polypeptide (OATP) 1B1, OATP2B1 and organic cation transporter (OCT) 1 and the efflux transporter multidrug resistance-associated protein (MRP) 3 at their sinusoidal pole. In addition, the efflux transporters P-glycoprotein and MRP2 were detected at the canalicular pole of monolayer-cultured human hepatocytes. Moreover, canalicular secretion of reference substrates for the efflux transporters bile salt export pump, MRP2 and P-glycoprotein as well as sinusoidal drug transporter activities were observed. This polarized and functional expression of drug transporters in monolayer-cultured human hepatocytes highlights the interest of using this human in vitro cell model in xenobiotic transport studies.

Endogenous antiviral microRNAs determine permissiveness for hepatitis B virus replication in cultured human fetal and adult hepatocytes

Superior cell culture models for hepatitis B virus (HBV) will help advance insights into host-virus interactions. To identify mechanisms regulating HBV replication, this study used cultured human HepG2 cells and adult or fetal hepatocytes transduced with adenoviral vector to express HBV upstream of green fluorescent protein. The vector efficiently transduced all cell types. In HepG2 cells, replicative viral intermediates, nucleocapsid-associated HBcAg, and HBsAg were expressed. However, in fetal or adult hepatocytes, pregenomic HBV RNA and viral RNAs were expressed, but nucleocapsid-associated HBcAg in cells or HBsAg in culture medium were absent, indicating interruptions in viral replication due to possible microRNA-related interference. MicroRNA profiling demonstrated that a large number of microRNAs with antiviral potential were differentially expressed in hepatocytes after culture. In transfection assays using HepG2 cells, candidate antiviral microRNAs, e.g., hsa-miR-24 or hsa-miR-638 decreased the levels of HBV transcripts or HBV gene products. Since candidate microRNAs could have targeted interferon response genes as an alternative explanation interferon signaling was examined. However, HBV replication in cultured hepatocytes was not restored despite successful inhibition of JAK1/2-STAT signaling by the inhibitor, ruxolitinib. Therefore, HBV was unable to complete replication in cultured hepatocytes due to expression of multiple antiviral microRNAs. This mechanism should help understand restrictions in HBV replication for developing HBV models in cultured cells while providing frameworks for pathophysiological studies of HBV replication in subsets of hepatocytes or stem/progenitor cells during hepatitis.

Interspecies variability in expression of hepatobiliary transporters across human, dog, monkey, and rat as determined by quantitative proteomics

We quantified, by liquid chromatography tandem mass spectrometry, transporter protein expression of BSEP, MATE1, MRP3, MRP4, NTCP, and OCT1 in our human liver bank (n = 55) and determined the relationship between protein expression and sex, age and genotype. These data complement our previous work in the same liver bank where we quantified the protein expression of OATPs, BCRP, MDR1, and MRP2. In addition, we quantified and compared the interspecies differences in expression of the hepatobiliary transporters, corresponding to the above human transporters, in liver tissue and hepatocytes of male beagle dogs, cynomolgus monkeys, Sprague-Dawley rats, and Wistar rats. In all the species, the sinusoidal OATPs/Oatps were the most abundant hepatic transporters. However, there were notable interspecies differences in the relative abundance of the remaining transporters. For example, the next most abundant transporter in humans and monkeys was OCT1/Oct1, whereas it was Mrp2 and Ntcp in dogs/Wistar rats and Sprague-Dawley rats, respectively. In contrast, the protein expression of the efflux transporters BCRP/Bcrp, MDR1/Mdr1, MRP3/Mrp3, MRP4/Mrp4, and MATE1/Mate1 was much lower across all the species. For most transporters, the expression in the liver tissues was comparable to that in the unplated cryopreserved hepatocytes. These data on human liver transporter protein expression complete the picture of the expression of major human hepatobiliary transporters important in drug disposition and toxicity. In addition, the data on expression of the corresponding hepatobiliary transporters in preclinical species will be helpful in interpreting and extrapolating pharmacokinetic, pharmacological, and toxicological results from preclinical studies to humans.

Optimized approaches for quantification of drug transporters in tissues and cells by MRM proteomics

Drug transporter expression in tissues (in vivo) usually differs from that in cell lines used to measure transporter activity (in vitro). Therefore, quantification of transporter expression in tissues and cell lines is important to develop scaling factor for in vitro to in vivo extrapolation (IVIVE) of transporter-mediated drug disposition. Since traditional immunoquantification methods are semiquantitative, targeted proteomics is now emerging as a superior method to quantify proteins, including membrane transporters. This superiority is derived from the selectivity, precision, accuracy, and speed of analysis by liquid chromatography tandem mass spectrometry (LC-MS/MS) in multiple reaction monitoring (MRM) mode. Moreover, LC-MS/MS proteomics has broader applicability because it does not require selective antibodies for individual proteins. There are a number of recent research and review papers that discuss the use of LC-MS/MS for transporter quantification. Here, we have compiled from the literature various elements of MRM proteomics to provide a comprehensive systematic strategy to quantify drug transporters. This review emphasizes practical aspects and challenges in surrogate peptide selection, peptide qualification, peptide synthesis and characterization, membrane protein isolation, protein digestion, sample preparation, LC-MS/MS parameter optimization, method validation, and sample analysis. In particular, bioinformatic tools used in method development and sample analysis are discussed in detail. Various pre-analytical and analytical sources of variability that should be considered during transporter quantification are highlighted. All these steps are illustrated using P-glycoprotein (P-gp) as a case example. Greater use of quantitative transporter proteomics will lead to a better understanding of the role of drug transporters in drug disposition.

Quantitative targeted proteomics for membrane transporter proteins: method and application

Although global proteomics has shown promise for discovery of many new proteins, biomarkers, protein modifications, and polymorphisms, targeted proteomics is emerging in the proteomics research field as a complement to untargeted shotgun proteomics, particularly when a determined set of low-abundance functional proteins need to be measured. The function and expression of proteins related to drug absorption, distribution, metabolism, and excretion (ADME) such as cytochrome P450 enzymes and membrane transporters are of great interest in biopharmaceutical research. Since the variation in ADME-related protein expression is known to be a major complicating factor encountered during in vitro-in vivo and in vivo-in vivo extrapolations (IVIVE), the accurate quantification of the ADME proteins in complex biological systems becomes a fundamental element in establishing IVIVE for pharmacokinetic predictions. In this review, we provide an overview of relevant methodologies followed by a summary of recent applications encompassing mass spectrometry-based targeted quantifications of membrane transporters.

Hepatic uptake of atorvastatin: influence of variability in transporter expression on uptake clearance and drug-drug interactions

Differences in the expression and function of the organic anion transporting polypeptide (OATP) transporters contribute to interindividual variability in atorvastatin clearance. However, the importance of the bile acid transporter sodium taurocholate cotransporting polypeptide (NTCP, SLC10A1) in atorvastatin uptake clearance (CLupt) is not yet clarified. To elucidate this issue, we investigated the relative contribution of NTCP, OATP1B1, OATP1B3, and OATP2B1 to atorvastatin CLupt in 12 human liver samples. The impact of inhibition on atorvastatin CLupt was also studied, using inhibitors of different isoform specificities. Expression levels of the four transport proteins were quantified by liquid chromatography tandem mass spectrometry. These data, together with atorvastatin in vitro kinetics, were used to predict the maximal transport activity (MTA) and interindividual differences in CLupt of each transporter in vivo. Subsequently, hepatic uptake impairment on coadministration of five clinically interacting drugs was predicted using in vitro inhibitory potencies. NTCP and OATP protein expression varied 3.7- to 32-fold among the 12 sample donors. The rank order in expression was OATP1B1 > OATP1B3 ≈ NTCP ≈ OATP2B1. NTCP was found to be of minor importance in atorvastatin disposition. Instead, OATP1B1 and OATP1B3 were confirmed as the major atorvastatin uptake transporters. The average contribution to atorvastatin uptake was OATP1B1 > OATP1B3 >> OATP2B1 > NTCP, although this rank order varied among individuals. The interindividual differences in transporter expression and CLupt resulted in marked differences in drug-drug interactions due to isoform-specific inhibition. We conclude that this variation should be considered in in vitro to in vivo extrapolations.

The impact of solute carrier (SLC) drug uptake transporter loss in human and rat cryopreserved hepatocytes on clearance predictions

Cryopreserved hepatocytes are often used as a convenient tool in studies of hepatic drug metabolism and disposition. In this study, the expression and activity of drug transporters in human and rat fresh and cryopreserved hepatocytes was investigated. In human cryopreserved hepatocytes, Western blot analysis indicated that protein expression of the drug uptake transporters [human Na(+)-taurocholate cotransporting polypeptide (NTCP), human organic anion transporting polypeptides (OATPs), human organic anion transporters, and human organic cation transporters (OCTs)] was considerably reduced compared with liver tissue. In rat cryopreserved cells, the same trend was observed but to a lesser extent. Several rat transporters were reduced as a result of both isolation and cryopreservation procedures. Immunofluorescence showed that a large portion of remaining human OATP1B1 and OATP1B3 transporters were internalized in human cryopreserved hepatocytes. Measuring uptake activity using known substrates of OATPs, OCTs, and NTCP showed decreased activity in cryopreserved as compared with fresh hepatocytes in both species. The reduced uptake in cryopreserved hepatocytes limited the in vitro metabolism of several AstraZeneca compounds. A retrospective analysis of clearance predictions of AstraZeneca compounds suggested systematic lower clearance predicted using metabolic stability data from human cryopreserved hepatocytes compared with human liver microsomes. This observation is consistent with a loss of drug uptake transporters in cryopreserved hepatocytes. In contrast, the predicted metabolic clearance from fresh rat hepatocytes was consistently higher than those predicted from liver microsomes, consistent with retention of uptake transporters. The uptake transporters, which are decreased in cryopreserved hepatocytes, may be rate-limiting for the metabolism of the compounds and thus be one explanation for underpredictions of in vivo metabolic clearance from cryopreserved hepatocytes.

Mass spectrometry-based targeted proteomics as a tool to elucidate the expression and function of intestinal drug transporters

Intestinal transporter proteins affect the oral bioavailability of many drugs in a significant manner. In order to estimate or predict their impact on oral drug absorption, data on their intestinal expression levels are needed. So far, predominantly mRNA expression data are available which are not necessarily correlated with the respective protein content. All available protein data were assessed by immunoblotting techniques such as Western blotting which both possess a number of limitations for reliable protein quantification. In contrast to this, mass spectrometry-based targeted proteomics may represent a promising alternative method to provide comprehensive protein expression data. In this review, we will summarize so far available intestinal mRNA and protein expression data for relevant human multidrug transporters. Moreover, recently observed mass spectrometry-based targeted proteomic data will be presented and discussed with respect to potential functional consequences. Associated to this, we will provide a short tutorial how to set up these methods and emphasize critical aspects in method development. Finally, potential limitations and pitfalls of this emerging technique will be discussed. From our perspective, LC-MS/MS-based targeted proteomics represents a valuable new method to comprehensively analyse the intestinal expression of transporter proteins. The resulting expression data are expected to improve our understanding about the intestinal processing of drugs.

Mass spectrometry-based targeted proteomics as a tool to elucidate the expression and function of intestinal drug transporters

Intestinal transporter proteins affect the oral bioavailability of many drugs in a significant manner. In order to estimate or predict their impact on oral drug absorption, data on their intestinal expression levels are needed. So far, predominantly mRNA expression data are available which are not necessarily correlated with the respective protein content. All available protein data were assessed by immunoblotting techniques such as Western blotting which both possess a number of limitations for reliable protein quantification. In contrast to this, mass spectrometry-based targeted proteomics may represent a promising alternative method to provide comprehensive protein expression data. In this review, we will summarize so far available intestinal mRNA and protein expression data for relevant human multidrug transporters. Moreover, recently observed mass spectrometry-based targeted proteomic data will be presented and discussed with respect to potential functional consequences. Associated to this, we will provide a short tutorial how to set up these methods and emphasize critical aspects in method development. Finally, potential limitations and pitfalls of this emerging technique will be discussed. From our perspective, LC-MS/MS-based targeted proteomics represents a valuable new method to comprehensively analyse the intestinal expression of transporter proteins. The resulting expression data are expected to improve our understanding about the intestinal processing of drugs.