Localization of P-gp (Abcb1) and Mrp2 (Abcc2) in freshly isolated rat hepatocytes

Association between ABCB1 C3435 T polymorphism- and methotrexate-related toxicity in pediatric acute lymphoblastic leukemia: a meta-analysis

BACKGROUND

The effect of methotrexate (MTX)-related toxicity on childhood acute lymphoblastic leukemia (ALL) is controversial. Hence, this meta-analysis aimed to investigate the association between ABCB1 C3435 T polymorphism- and methotrexate-related toxicity in pediatric acute lymphoblastic leukemia.

METHODS

Relevant studies were systematically searched and extracted from multiple databases including PubMed, EMBASE, Web of Science, Cochrane Library, China National Knowledge Infrastructure (CNKI), and Wanfang databases up to 1 June, 2024.

RESULTS

A total of 13 articles met the criteria, including 1506 patients. The results showed that ABCB1 C3435 T polymorphism was significantly associated with MTX-induced hepatotoxicity (CT/TT vs. CC: OR: 2.15, 95% CI: 1.40-3.32). There was no significant difference between ABCB1 C3435 T polymorphism and mucositis, myelosuppression, nephrotoxicity, gastrointestinal toxicity in pediatric ALL treated with MTX.

CONCLUSIONS

Our meta-analysis suggests that the ABCB1 C3435 T mutation may enhance the MTX-induced hepatotoxicity in childhood acute lymphoblastic leukemia.

Proof of Concept of an All-in-One System for Measuring Hepatic Influx, Egress, and Metabolic Clearance Based on the Extended Clearance Concept

Hepatic clearance (CLH ) prediction is a critical parameter to estimate human dose. However, CLH underpredictions are common, especially for slowly metabolized drugs, and may be attributable to drug properties that pose challenges for conventional in vitro absorption, distribution, metabolism, and elimination (ADME) assays, resulting in nonvalid data, which prevents in vitro to in vivo extrapolation and CLH predictions. Other processes, including hepatocyte and biliary distribution via transporters, can also play significant roles in CLH Recent advances in understanding the interplay of metabolism and drug transport for clearance processes have aided in developing the extended clearance model. In this study, we demonstrate proof of concept of a novel two-step assay enabling the measurement of multiple kinetic parameters from a single experiment in plated human primary hepatocytes with and without transporter and cytochrome P450 inhibitors-the hepatocyte uptake and loss assay (HUpLA). HUpLA accurately predicted the CLH of eight of the nine drugs (within twofold of the observed CLH ). Distribution clearances were within threefold of observed literature values in standard uptake and efflux assays. In comparison, the conventional suspension hepatocyte stability assay poorly predicted the CLH The CLH of only two drugs was predicted within twofold of the observed CLH Therefore, HUpLA is advantageous by enabling the measurement of enzymatic and transport processes concurrently within the same system, alleviating the need for applying scaling factors independently. The use of primary human hepatocytes enables physiologically relevant exploration of transporter-enzyme interplay. Most importantly, HUpLA shows promise as a sensitive measure for low-turnover drugs. Further evaluation across different drug characteristics is needed to demonstrate method robustness. SIGNIFICANCE STATEMENT: The hepatocyte uptake and loss assay involves measuring four commonly derived in vitro hepatic clearance endpoints. Since endpoints are generated within a single test system, it blunts experimental error originating from assays otherwise conducted independently. A key advantage is the concept of removing drug-containing media following intracellular drug loading, enabling the measurement of drug reappearance rate in media as well as the measurement of loss of total drug in the test system unencumbered by background quantities of drug in media otherwise present in a conventional assay.

How predictive are isolated perfused liver data of in vivo hepatic clearance? A meta-analysis of isolated perfused rat liver data

Isolated perfused rat liver (IPRL) experiments have been used to answer clearance-related questions, including evaluating the impact of pathological and physiological processes on hepatic clearance (CLH). However, to date, IPRL data has not been evaluated for in vivo CLH prediction accuracy.In addition to a detailed overview of available IPRL literature, we present an in-depth analysis of the performance of IPRL in CLH prediction.While the entire dataset poorly predicted CLH (GAFE = 3.2; 64% within 3-fold), IPRL conducted under optimal experimental conditions, such as in the presence of plasma proteins and with a perfusion rate within 2-fold of physiological liver blood flow and corrected for unbound fraction in the presence of red blood cells, can accurately predict rat CLH (GAFE = 2.0; 78% within 3-fold). Careful consideration of experimental conditions is needed to allow proper data analysis.Further, isolated perfused liver experiments in other species, including human livers, may allow us to address the current in vitro-in vivo disconnects of hepatic metabolic clearance and improve our methodology for CLH predictions.

After 50 Years of Hepatic Clearance Models, Where Should We Go from Here? Improvements and Implications for Physiologically Based Pharmacokinetic Modeling

There is overwhelming preference for application of the unphysiologic, well-stirred model (WSM) over the parallel tube model (PTM) and dispersion model (DM) to predict hepatic drug clearance, CLH , despite that liver blood flow is dispersive and closer to the DM in nature. The reasoning is the ease in computation relating the hepatic intrinsic clearance ( CLint ), hepatic blood flow ( QH ), unbound fraction in blood ( fub ) and the transmembrane clearances ( CLin and CLef ) to CLH for the WSM. However, the WSM, being the least efficient liver model, predicts a lower EH that is associated with the in vitro CLint ( Vmax / Km ), therefore requiring scale-up to predict CLH in vivo. By contrast, the miniPTM, a three-subcompartment tank-in-series model of uniform enzymes, closely mimics the DM and yielded similar patterns for CLint versus EH , substrate concentration [S] , and KL / B , the tissue to outflow blood concentration ratio. We placed these liver models nested within physiologically based pharmacokinetic models to describe the kinetics of the flow-limited, phenolic substrate, harmol, using the WSM (single compartment) and the miniPTM and zonal liver models (ZLMs) of evenly and unevenly distributed glucuronidation and sulfation activities, respectively, to predict CLH For the same, given CLint ( Vmax and Km ), the WSM again furnished the lowest extraction ratio ( EH,WSM = 0.5) compared with the miniPTM and ZLM (>0.68). Values of EH,WSM were elevated to those for EH, PTM and EH, ZLM when the Vmax s for sulfation and glucuronidation were raised 5.7- to 1.15-fold. The miniPTM is easily manageable mathematically and should be the new normal for liver/physiologic modeling. SIGNIFICANCE STATEMENT: Selection of the proper liver clearance model impacts strongly on CLH predictions. The authors recommend use of the tank-in-series miniPTM (3 compartments mini-parallel tube model), which displays similar properties as the dispersion model (DM) in relating CLint and [ S ] to CLH as a stand-in for the DM, which best describes the liver microcirculation. The miniPTM is readily modified to accommodate enzyme and transporter zonation.

Transporter-mediated drug-drug interactions: regulatory guidelines, in vitro and in vivo methodologies and translation, special populations, and the blood-brain barrier

This review, part of a special issue on drug-drug interactions (DDIs) spearheaded by the International Society for the Study of Xenobiotics (ISSX) New Investigators, explores the critical role of drug transporters in absorption, disposition, and clearance in the context of DDIs. Over the past two decades, significant advances have been made in understanding the clinical relevance of these transporters. Current knowledge on key uptake and efflux transporters that affect drug disposition and development is summarized. Regulatory guidelines from the FDA, EMA, and PMDA that inform the evaluation of potential transporter-mediated DDIs are discussed in detail. Methodologies for preclinical and clinical testing to assess potential DDIs are reviewed, with an emphasis on the utility of physiologically based pharmacokinetic (PBPK) modeling. This includes the application of relative abundance and expression factors to predict human pharmacokinetics (PK) using preclinical data, integrating the latest regulatory guidelines. Considerations for assessing transporter-mediated DDIs in special populations, including pediatric, hepatic, and renal impairment groups, are provided. Additionally, the impact of transporters at the blood-brain barrier (BBB) on the disposition of CNS-related drugs is explored. Enhancing the understanding of drug transporters and their role in drug disposition and toxicity can improve efficacy and reduce adverse effects. Continued research is essential to bridge remaining gaps in knowledge, particularly in comparison with cytochrome P450 (CYP) enzymes.

Unravelling the Hepatic Elimination Mechanisms of Colistin

PURPOSE

Colistin is an antibiotic which is increasingly used as a last-resort therapy in critically-ill patients with multidrug resistant Gram-negative infections. The purpose of this study was to evaluate the mechanisms underlying colistin's pharmacokinetic (PK) behavior and to characterize its hepatic metabolism.

METHODS

In vitro incubations were performed using colistin sulfate with rat liver microsomes (RLM) and with rat and human hepatocytes (RH and HH) in suspension. The uptake of colistin in RH/HH and thefraction of unbound colistin in HH (fu,hep) was determined. In vitro to in vivo extrapolation (IVIVE) was employed to predict the hepatic clearance (CLh) of colistin.

RESULTS

Slow metabolism was detected in RH/HH, with intrinsic clearance (CLint) values of 9.34± 0.50 and 3.25 ± 0.27 mL/min/kg, respectively. Assuming the well-stirred model for hepatic drug elimination, the predicted rat CLh was 3.64± 0.22 mL/min/kg which could explain almost 70% of the reported non-renal in vivo clearance. The predicted human CLh was 91.5 ± 8.83 mL/min, which was within two-fold of the reported plasma clearance in healthy volunteers. When colistin was incubated together with the multidrug resistance-associated protein (MRP/Mrp) inhibitor benzbromarone, the intracellular accumulation of colistin in RH/HH increased significantly.

CONCLUSION

These findings indicate the major role of hepatic metabolism in the non-renal clearance of colistin, while MRP/Mrp-mediated efflux is involved in the hepatic disposition of colistin. Our data provide detailed quantitative insights into the hereto unknown mechanisms responsible for non-renal elimination of colistin.

Quantitation of Plasma Membrane Drug Transporters in Kidney Tissue and Cell Lines Using a Novel Proteomic Approach Enabled a Prospective Prediction of Metformin Disposition

The successful prospective incorporation of in vitro transporter kinetics in physiologically based pharmacokinetic (PBPK) models to describe drug disposition remains challenging. Although determination of scaling factors to extrapolate in vitro to in vivo transporter kinetics has been facilitated by quantitative proteomics, no robust assessment comparing membrane recoveries between different cells/tissues has been made. HEK293 cells overexpressing OCT2, MATE1, and MATE2K or human kidney cortex were homogenized and centrifuged to obtain the total membrane fractions, which were subsequently subjected to liquid-liquid extraction followed by centrifugation and precipitation to isolate plasma membrane fractions. Plasma membrane recoveries determined by quantitation of the marker Na+/K+-ATPase in lysate and plasma membrane fractions were ≤20% but within 3-fold across different cells and tissues. A separate study demonstrated that recoveries are comparable between basolateral and apical membranes of renal proximal tubules, as measured by Na+/K+-ATPase and γ-glutamyl transpeptidase 1, respectively. The plasma membrane expression of OCT2, MATE1, and MATE2K was quantified and relative expression factors (REFs) were determined as the ratio between the tissue and cell concentrations. Corrections using plasma membrane recovery had minimal impact on REF values (<2-fold). In vitro transporter kinetics of metformin were extrapolated to in vivo using the corresponding REFs in a PBPK model. The simulated metformin exposures were within 2-fold of clinical exposure. These results demonstrate that transporter REFs based on plasma membrane expression enable a prediction of transporter-mediated drug disposition. Such REFs may be estimated without the correction of plasma membrane recovery when the same procedure is applied between different matrices. SIGNIFICANCE STATEMENT: Transporter REFs based on plasma membrane expression enable in vitro-in vivo extrapolation of transporter kinetics. Plasma membrane recoveries as determined by the quantification of sodium-potassium adenosine triphosphatase were comparable between the in vitro and in vivo systems used in the present study, and therefore had minimal impact on the transporter REF values.

Case Study 4: Application of Basic Enzyme Kinetics to Metabolism Studies-Real-Life Examples

An appreciation of enzyme kinetic principles can be applied in a number of drug metabolism applications. The concept for this chapter arose from a simple discussion on selecting appropriate time points to most efficiently assess metabolite profiles in a human Phase 1a clinical study (Subheading 4). By considering enzyme kinetics, a logical approach to the issue was derived. The dialog was an important learning opportunity for the participants in the discussion, and we have endeavored to capture this experience with other questions related to determination of K_m and V_max parameters, a consideration of the value of hepatocytes vs. liver microsomes, and enzyme inhibition parameters.

Function and Expression of Bile Salt Export Pump in Suspension Human Hepatocytes

The mechanistic understanding of bile salt disposition is not well established in suspension human hepatocytes (SHH) because of the limited information on the expression and function of bile salt export protein (BSEP) in this system. We investigated the transport function of BSEP in SHH using a method involving in situ biosynthesis of bile salts from their precursor bile acids, cholic acid (CA) and chenodeoxycholic acid (CDCA). Our data indicated that glycine- and taurine-conjugated CA and CDCA were generated efficiently and transported out of hepatocytes in a concentration- and time-dependent manner. We also observed that the membrane protein abundance of BSEP was similar between SHH and sandwich-cultured human hepatocytes. Furthermore, known cholestatic agents significantly inhibited G-CA and G-CDCA efflux in SHH. Interestingly, cyclosporine A, troglitazone, itraconazole, loratadine, and lovastatin inhibited G-CA efflux more potently than G-CDCA efflux (3- to 5-fold). Because of these significant differential effects on G-CA and G-CDCA efflux inhibition, we determined the IC50 values of troglitazone for G-CA (9.9 µM) and for G-CDCA (43.1 µM) efflux. The observed discrepancy in the IC50 was attributed to the fact that troglitazone also inhibits organic anion transporting polypeptides and Na+/taurocholate cotransporting polypeptide in addition to BSEP. The hepatocyte uptake study suggested that both active uptake and passive diffusion contribute to the liver uptake of CA, whereas CDCA primarily undergoes passive diffusion into the liver. In summary, these data demonstrated the expression and function of BSEP and its major role in transport of bile salts in cryopreserved SHH. SIGNIFICANCE STATEMENT: BSEP transport function and protein abundance was evident in SHH in the present study. The membrane abundance of BSEP protein was similar between SHH and sandwich-cultured human hepatocytes. The study also illustrated the major role of BSEP relative to basolateral MRP3 and MRP4 in transport of bile salts in SHH. Understanding of BSEP function in SHH may bolster the utility of this platform in mechanistic understanding of bile salt disposition and potentially in the assessment of drugs for BSEP inhibition.

Identification of novel inhibitors of rat Mrp3

Multidrug resistance-associated protein (MRP; ABCC gene family) mediated efflux transport plays an important role in the systemic and tissue exposure profiles of many drugs and their metabolites, and also of endogenous compounds like bile acids and bilirubin conjugates. However, potent and isoform-selective inhibitors of the MRP subfamily are currently lacking. Therefore, the purpose of the present work was to identify novel rat Mrp3 inhibitors. Using 5(6)-carboxy-2',7'-dichlorofluorescein diacetate (CDFDA) as a model-(pro)substrate for Mrp3 in an oil-spin assay with primary rat hepatocytes, the extent of inhibition of CDF efflux was determined for 1584 compounds, yielding 59 hits (excluding the reference inhibitor) that were identified as new Mrp3 inhibitors. A naive Bayesian prediction model was constructed in Pipeline Pilot to elucidate physicochemical and structural features of compounds causing Mrp3 inhibition. The final Bayesian model generated common physicochemical properties of Mrp3 inhibitors. For instance, more than half of the hits contain a phenolic structure. The identified compounds have an AlogP between 2 and 4.5, between 5 to 8 hydrogen bond acceptor atoms, a molecular weight between 260 and 400, and 2 or more aromatic rings. Compared to the depleted dataset (i.e. 90% remaining compounds), the Mrp3 hit rate in the enriched set was 7.5-fold higher (i.e. 17.2% versus 2.3%). Several hits from this first screening approach were confirmed in an additional study using Mrp3 transfected inside-out membrane vesicles. In conclusion, several new and potent inhibitors of Mrp3 mediated efflux were identified in an optimized in vitro rat hepatocyte assay and confirmed using Mrp3 transfected inside-out membrane vesicles. A final naive Bayesian model was developed in an iterative way to reveal common physicochemical and structural features for Mrp3 inhibitors. The final Bayesian model will enable in silico screening of larger libraries and in vitro identification of more potent Mrp3 inhibitors.

Evaluation of the Utility of PXB Chimeric Mice for Predicting Human Liver Partitioning of Hepatic Organic Anion-Transporting Polypeptide Transporter Substrates

The ability to predict human liver-to-plasma unbound partition coefficient (Kpuu) is important to estimate unbound liver concentration for drugs that are substrates of hepatic organic anion-transporting peptide (OATP) transporters with asymmetric distribution into the liver relative to plasma. Herein, we explored the utility of PXB chimeric mice with humanized liver that are highly repopulated with human hepatocytes to predict human hepatic disposition of OATP substrates, including rosuvastatin, pravastatin, pitavastatin, valsartan, and repaglinide. In vitro total uptake clearance and transporter-mediated active uptake clearance in C57 mouse hepatocytes were greater than in PXB chimeric mouse hepatocytes for rosuvastatin, pravastatin, pitavastatin, and valsartan. Consistent with in vitro uptake data, enhanced hepatic uptake and resulting total systemic clearance were observed with the above four compounds in severely compromised immune-deficient (SCID) control mice compared with the PXB chimeric mice, which suggest that mouse has a stronger transporter-mediated hepatic uptake than human. In vivo liver-to-plasma Kpuu from PXB chimeric and SCID control mice were also compared, and rosuvastatin and pravastatin Kpuu in SCID mice were more than 10-fold higher than that in PXB chimeric mice, whereas pitavastatin, valsartan, and repaglinide Kpuu in SCID mice were comparable with Kpuu in PXB chimeric mice. Finally, PXB chimeric mouse liver-to-plasma Kpuu values were compared with the reported human Kpuu, and a good correlation was observed as the PXB Kpuu vales were within 3-fold of human Kpuu Our results indicate that PXB mice could be a useful tool to delineate hepatic uptake and enable prediction of human liver-to-plasma Kpuu of hepatic uptake transporter substrates. SIGNIFICANCE STATEMENT: We evaluated PXB mouse with humanized liver for its ability to predict human liver disposition of five organic anion-transporting polypeptide transporter substrates. Both in vitro and in vivo data suggest that mouse liver has a stronger transporter-mediated hepatic uptake than the humanized liver in PXB mouse. More importantly, PXB liver-to-plasma unbound partition coefficient (Kpuu) values were compared with the reported human Kpuu, and a good correlation was observed. PXB mice could be a useful tool to project human liver-to-plasma Kpuu of hepatic uptake transporter substrates.

Development of a Rat Sandwich-Cultured Hepatocytes Model Expressing Functional Human Organic Anion Transporting Polypeptide (OATP) 1B3: A Potential Screening Tool for Liver-Targeting Compounds

PURPOSE

Organic anion transporting polypeptide (OATP) 1B3 transports many clinically important drugs, including statins, from blood into the liver. It exclusively expresses in human liver under normal physiological conditions. There is no rodent ortholog of human OATP1B3. Tissue targeting of therapeutic molecules mediated by transporters, including liver-targeting via liver-specific OATPs, is an emerging area in drug development. Sandwich-cultured primary hepatocytes (SCH) are a well characterized in vitro model for assessment of hepatic drug uptake and biliary excretion. The current study was designed to develop a novel rat SCH model expressing human OATP1B3 to study the hepatic disposition of OATP1B3 substrates.

METHODS

Primary rat hepatocytes transduced with adenoviral vectors expressing FLAG-tagged OATP1B3 (Ad-OATP1B3), a control vector Ad-LacZ, or that were non-transduced were cultured in a sandwich configuration. FLAG immunoblot and immunofluorescence-staining determined expression and localization of OATP1B3. Uptake of [3H]-cholecystokinin octapeptide (CCK-8), a specific OATP1B3 substrate, was determined. Taurocholate (TC) is a substrate routinely used in SCH to assess biliary excretion via bile canaliculi (BC) and is also a substrate of OATP1B3. [3H]-TC accumulation in cells+BC, cells, biliary excretion index (BEI) and in vitro Clbiliary were determined using B-CLEAR® technology.

RESULTS

OATP1B3 protein was extensively expressed and primarily localized on the plasma membrane in day 4 Ad-OATP1B3-transduced rat SCH. [3H]-CCK-8 accumulation in cells+BC was significantly greater (~5-13 folds, p<0.001) in day 4 SCH with vs. without Ad-OATP1B3-transduction. Expressing OATP1B3 in rat SCH significantly increased [3H]-TC accumulation in cells+BC and cells, without affecting BEI and in vitro Clbiliary.

CONCLUSIONS

Rat SCH expressing human OATP1B3-is a novel in vitro model allowing simultaneous assessment of hepatic uptake, hepatocellular accumulation and biliary excretion process of a human OATP1B3 substrate. This model could be a potential tool for screening for liver-targeting compounds mediated by OATP1B3.

Prediction of Drug Clearance from Enzyme and Transporter Kinetics

Accurate estimation of in vivo clearance in human is pivotal to determine the dose and dosing regimen for drug development. In vitro-in vivo extrapolation (IVIVE) has been performed to predict drug clearance using empirical and physiological scalars. Multiple in vitro systems and mathematical modeling techniques have been employed to estimate in vivo clearance. The models for predicting clearance have significantly improved and have evolved to become more complex by integrating multiple processes such as drug metabolism and transport as well as passive diffusion. This chapter covers the use of conventional as well as recently developed methods to predict metabolic and transporter-mediated clearance along with the advantages and disadvantages of using these methods and the associated experimental considerations. The general approaches to improve IVIVE by use of appropriate scalars, incorporation of extrahepatic metabolism and transport and application of physiologically based pharmacokinetic (PBPK) models with proteomics data are also discussed. The chapter also provides an overview of the advantages of using such dynamic mechanistic models over static models for clearance predictions to improve IVIVE.

Current Advances on 3D-Bioprinted Liver Tissue Models

The liver, the largest gland in the human body, plays a key role in metabolism, bile production, detoxification, and water and electrolyte regulation. The toxins or drugs that the gastrointestinal system absorbs reach the liver first before entering the bloodstream. Liver disease is one of the leading causes of death worldwide. Therefore, an in vitro liver tissue model that reproduces the main functions of the liver can be a reliable platform for investigating liver diseases and developing new drugs. In addition, the limitations in traditional, planar monolayer cell cultures and animal tests for evaluating the toxicity and efficacy of drug candidates can be overcome. Currently, the newly emerging 3D bioprinting technologies have the ability to construct in vitro liver tissue models both in static scaffolds and dynamic liver-on-chip manners. This review mainly focuses on the construction and applications of liver tissue models based on 3D bioprinting. Special attention is given to 3D bioprinting strategies and bioinks for constructing liver tissue models including the cell sources and hydrogel selection. In addition, the main advantages and limitations and the major challenges and future perspectives are discussed, paving the way for the next generation of in vitro liver tissue models.

In Vitro-In Vivo Inaccuracy: The CYP3A4 Anomaly

When predicting hepatic clearance using in vitro to in vivo extrapolation (IVIVE), microsomes or hepatocytes are commonly used. Here, we examine intrinsic clearance values and IVIVE results in human hepatocytes and microsomes for compounds metabolized by a variety of enzymes. The great majority of CYP3A4 substrates examined had higher intrinsic clearance values in microsomes compared with hepatocytes, whereas the values were more similar between the two incubations for substrates of other enzymes. We hypothesize that this may be due to interplay between CYP3A4 and the efflux transporter P-glycoprotein, as they have been shown to exhibit coordinated regulation. When examining the prediction accuracy for substrates of other enzymes between microsomes and hepatocytes, average fold errors as well as overall error were similar, demonstrating once again that IVIVE methods are not adequately defined and understood. SIGNIFICANCE STATEMENT: For CYP3A4 substrates, microsomes give markedly higher predictive in vitro to in vivo extrapolation than for other metabolic enzymes, which is not found for hepatocytes. We hypothesize that this could be a result of CYP3A4-P-glycoprotein interplay or coordinated regulation in hepatocytes that would not be observed in microsomes.

Hepatic clearance concepts and misconceptions: Why the well-stirred model is still used even though it is not physiologic reality?

The liver is the most important drug metabolizing organ, endowed with a plethora of metabolizing enzymes and transporters to facilitate drug entry and removal via metabolism and/or biliary excretion. For this reason, much focus surrounds the development of clearance concepts, which are based on normalizing the rate of removal to the input or arterial concentration. By so doing, some authors have recently claimed that it implies one specific model of hepatic elimination, namely, the widely used well-stirred or venous equilibration model (WSM). This commentary challenges this claim and aims to provide a comprehensive discussion of not only the WSM but other currently applied hepatic clearance models - the parallel tube model (PTM), the dispersion model (DM), the zonal liver model (ZLM), and the heterogeneous capillary transit time model of Goresky and co-workers (GM). The WSM, PTM, and DM differ in the patterns of internal blood flow, assuming bulk, plug, and dispersive flows, respectively, which render different degrees of mixing within the liver that are characterized by the magnitudes of the dispersion number (D_N), resulting in different implications concerning the (unbound) substrate concentration in liver (Cu_H). Early models assumed perfusion rate-limited distribution, which have since been modified to include membrane-limited transport. The recent developments associated with the misconceptions and the sensitivity of the models are hereby addressed. Since the WSM has been and will likely remain widely used, the pros and cons of this model relative to physiological reality are further discussed.

A Comparison of Total and Plasma Membrane Abundance of Transporters in Suspended, Plated, Sandwich-Cultured Human Hepatocytes Versus Human Liver Tissue Using Quantitative Targeted Proteomics and Cell Surface Biotinylation

Suspended (SH), plated (PH), and sandwich-cultured hepatocytes (SCH) are commonly used models to predict in vivo transporter-mediated hepatic uptake (SH or PH) or biliary (SCH) clearance of drugs. When doing so, the total and the plasma membrane abundance (PMA) of transporter are assumed not to differ between hepatocytes and liver tissue (LT). This assumption has never been tested. In this study, we tested this assumption by measuring the total and PMA of the transporters in human hepatocyte models versus LT (total only) from which they were isolated. Total abundance of OATP1B1/2B1/1B3, OCT1, and OAT2 was not significantly different between the hepatocytes and LT. The same was true for the PMA of these transporters across the hepatocyte models. In contrast, total abundance of the sinusoidal efflux transporter, MRP3, and the canalicular efflux transporters, MRP2 and P-gp, was significantly greater (P < 0.05) in SCH versus LT. Of the transporters tested, only the percentage of PMA of OATP1B1, P-gp, and MRP3, in SCH (82.8% ± 7.3%, 57.5% ± 10.9%, 69.3% ± 5.7%) was significantly greater (P < 0.05) than in SH (73.3% ± 6.4%, 27.4% ± 6.4%, 53.6% ± 4.1%). If the transporters measured in the plasma membrane are functional and the PMA in SH is representative of that in LT, these data suggest that SH, PH, and SCH will result in equal prediction of hepatic uptake clearance of drugs mediated by the transporters tested above. However, SCH will predict higher sinusoidal efflux and biliary clearance of drugs if the change in PMA of these transporters is not taken into consideration.

Hepatic Organic Anion Transporting Polypeptide-Mediated Clearance in the Beagle Dog: Assessing In Vitro-In Vivo Relationships and Applying Cross-Species Empirical Scaling Factors to Improve Prediction of Human Clearance

In the present study, the beagle dog was evaluated as a preclinical model to investigate organic anion transporting polypeptide (OATP)-mediated hepatic clearance. In vitro studies were performed with nine OATP substrates in three lots of plated male dog hepatocytes ± OATP inhibitor cocktail to determine total uptake clearance (CLuptake) and total and unbound cell-to-medium concentration ratio (Kpuu). In vivo intrinsic hepatic clearances (CLint,H) were determined following intravenous drug administration (0.1 mg/kg) in male beagle dogs. The in vitro parameters were compared with those previously reported in plated human, monkey, and rat hepatocytes; the ability of cross-species scaling factors to improve prediction of human in vivo clearance was assessed. CLuptake in dog hepatocytes ranged from 9.4 to 135 µl/min/106 cells for fexofenadine and telmisartan, respectively. Active process contributed >75% to CLuptake for 5/9 drugs. Rosuvastatin and valsartan showed Kpuu > 10, whereas cerivastatin, pitavastatin, repaglinide, and telmisartan had Kpuu < 5. The extent of hepatocellular binding in dog was consistent with other preclinical species and humans. The bias (2.73-fold) obtained from comparison of predicted versus in vivo dog CLint,H was applied as an average empirical scaling factor (ESFav) for in vitro-in vivo extrapolation of human CLint,H The ESFav based on dog reduced underprediction of human CLint,H for the same data set (geometric mean fold error = 2.1), highlighting its utility as a preclinical model to investigate OATP-mediated uptake. The ESFav from all preclinical species resulted in comparable improvement of human clearance prediction, in contrast to drug-specific empirical scalars, rationalized by species differences in expression and/or relative contribution of particular transporters to drug hepatic uptake.

Current Research Method in Transporter Study

Transporters play an important role in the absorption, distribution, metabolism, and excretion (ADME) of drugs. In recent years, various in vitro, in situ/ex vivo, and in vivo methods have been established for studying transporter function and drug-transporter interaction. In this chapter, the major types of in vitro models for drug transport studies comprise membrane-based assays, cell-based assays (such as primary cell cultures, immortalized cell lines), and transporter-transfected cell lines with single transporters or multiple transporters. In situ/ex vivo models comprise isolated and perfused organs or tissues. In vivo models comprise transporter gene knockout models, natural mutant animal models, and humanized animal models. This chapter would be focused on the methods for the study of drug transporters in vitro, in situ/ex vivo, and in vivo. The applications, advantages, or limitations of each model and emerging technologies are also mentioned in this chapter.

Meeting the Challenge of Predicting Hepatic Clearance of Compounds Slowly Metabolized by Cytochrome P450 Using a Novel Hepatocyte Model, HepatoPac

Generating accurate in vitro intrinsic clearance data is an important aspect of predicting in vivo human clearance. Primary hepatocytes in suspension are routinely used to predict in vivo clearance; however, incubation times have typically been limited to 4-6 hours, which is not long enough to accurately evaluate the metabolic stability of slowly metabolized compounds. HepatoPac is a micropatterened hepatocyte-fibroblast coculture system that can be used for continuous incubations of up to 7 days. This study evaluated the ability of human HepatoPac to predict the in vivo clearance (CL) of 17 commercially available compounds with low to intermediate clearance (<12 ml/min/kg). In vitro half-life for disappearance of each compound was converted to hepatic clearance using the well stirred model, with and without correction for plasma protein binding. Hepatic CL, using three individual donors, was accurately predicted for 11 of 17 compounds (59%; predicted clearance within 2-fold of observed human in vivo clearance values). The accuracy of prediction increased to 82% (14 of 17 compounds) with an acceptance criterion defined as within 3-fold. When considering only low clearance compounds (<5 ml/min per kg), which represented 10 of the 17 compounds, the accuracy of prediction was 70% within 2-fold and 100% within 3-fold. In addition, the turnover of three slowly metabolized compounds (alprazolam, meloxicam, and tolbutamide) in HepatoPac was directly compared with turnover in suspended hepatocytes. The turnover of alprazolam and tolbutamide was approximately 2-fold greater using HepatoPac compared with suspended hepatocytes, which was roughly in line with the extrapolated values (correcting for the longer incubation time and lower cell number with HepatoPac). HepatoPac, but not suspended hepatocytes, demonstrated significant turnover of meloxicam. These results demonstrate the utility of HepatoPac for prediction of in vivo hepatic clearance, particularly with low clearance compounds.

MRP2 Inhibition by HIV Protease Inhibitors in Rat and Human Hepatocytes: A Quantitative Confocal Microscopy Study

Hepatic drug transporters play a pivotal role in the excretion of drugs from the body, in drug-drug interactions, as well as in drug-induced liver toxicity. Hepatocytes cultured in sandwich configuration are an advantageous model to investigate the interactions of drug candidates with apical efflux transporters in a biorelevant manner. However, the commonly used "offline" assays (i.e., that rely on measuring intracellular accumulated amounts after cell lysis) are time- and resource-consuming, and the data output is often highly variable. In the present study, we used confocal microscopy to investigate the inhibitory effect of all marketed HIV protease inhibitors (10 μM) on the apical efflux transporter multidrug resistance-associated protein 2 (MRP2; ABCC2) by visualizing the biliary accumulation of the fluorescent substrate 5(6)-carboxy-2',7'-dichlorofluorescein (CDF). This method was applied with sandwich-cultured human and rat hepatocytes. Alterations in the biliary excretion index of CDF were calculated on the basis of quantitative analysis of fluorescence intensities in the confocal images. In human hepatocytes, lopinavir followed by tipranavir, saquinavir, atazanavir, and darunavir were the most potent inhibitors of MRP2-mediated efflux of CDF. In rat hepatocytes, tipranavir inhibited Mrp2-mediated CDF efflux most potently, followed by lopinavir and nelfinavir. In conclusion, a comparison of these findings with previously published data generated in offline transporter inhibition assays indicates that this microscopy-based approach enables investigation of the inhibitory effect of drugs on efflux transporters in a very sensitive but nondestructive manner.

Sandwich-Cultured Hepatocytes for Mechanistic Understanding of Hepatic Disposition of Parent Drugs and Metabolites by Transporter-Enzyme Interplay

Functional interplay between transporters and drug-metabolizing enzymes is currently one of the hottest topics in the field of drug metabolism and pharmacokinetics. Uptake transporter-enzyme interplay is important to determine intrinsic hepatic clearance based on the extended clearance concept. Enzyme and efflux transporter interplay, which includes both sinusoidal (basolateral) and canalicular efflux transporters, determines the fate of metabolites formed in the liver. As sandwich-cultured hepatocytes (SCHs) maintain metabolic activities and form a canalicular network, the whole interplay between uptake and efflux transporters and drug-metabolizing enzymes can be investigated simultaneously. In this article, we review the utility and applicability of SCHs for mechanistic understanding of hepatic disposition of both parent drugs and metabolites. In addition, the utility of SCHs for mimicking species-specific disposition of parent drugs and metabolites in vivo is described. We also review application of SCHs for clinically relevant prediction of drug-drug interactions caused by drugs and metabolites. The usefulness of mathematical modeling of hepatic disposition of parent drugs and metabolites in SCHs is described to allow a quantitative understanding of an event in vitro and to develop a more advanced model to predict in vivo disposition.

Simultaneous Assessment In Vitro of Transporter and Metabolic Processes in Hepatic Drug Clearance: Use of a Media Loss Approach

Hepatocyte drug depletion-time assays are well established for determination of metabolic clearance in vitro. The present study focuses on the refinement and evaluation of a "media loss" assay, an adaptation of the conventional depletion assay involving centrifugation of hepatocytes prior to sampling, allowing estimation of uptake in addition to metabolism. Using experimental procedures consistent with a high throughput, a selection of 12 compounds with a range of uptake and metabolism characteristics (atorvastatin, cerivastatin, clarithromycin, erythromycin, indinavir, pitavastatin, repaglinide, rosuvastatin, saquinavir, and valsartan, with two control compounds-midazolam and tolbutamide) were investigated in the presence and absence of the cytochrome P450 inhibitor 1-aminobenzotriazole and organic anion transporter protein inhibitor rifamycin SV in rat hepatocytes. Data were generated simultaneously for a given drug, and provided, through the use of a mechanistic cell model, clearance terms characterizing metabolism, active and passive uptake, together with intracellular binding and partitioning parameters. Results were largely consistent with the particular drug characteristics, with active uptake, passive diffusion, and metabolic clearances ranging between 0.4 and 777, 3 and 383, and 2 and 236 μl/min per milligram protein, respectively. The same experiments provided total and unbound drug cellular partition coefficients ranging between 3.8 and 254 and 2.3 and 8.3, respectively, and intracellular unbound fractions between 0.014 and 0.263. Following in vitro-in vivo extrapolation, the lowest prediction bias was noted using uptake clearance, compared with metabolic clearance or apparent clearance from the media loss assay alone. This approach allows rapid and comprehensive characterization of hepatocyte drug disposition valuable for prediction of hepatic processes in vivo.

In vitro to in vivo extrapolation for high throughput prioritization and decision making

In vitro chemical safety testing methods offer the potential for efficient and economical tools to provide relevant assessments of human health risk. To realize this potential, methods are needed to relate in vitro effects to in vivo responses, i.e., in vitro to in vivo extrapolation (IVIVE). Currently available IVIVE approaches need to be refined before they can be utilized for regulatory decision-making. To explore the capabilities and limitations of IVIVE within this context, the U.S. Environmental Protection Agency Office of Research and Development and the National Toxicology Program Interagency Center for the Evaluation of Alternative Toxicological Methods co-organized a workshop and webinar series. Here, we integrate content from the webinars and workshop to discuss activities and resources that would promote inclusion of IVIVE in regulatory decision-making. We discuss properties of models that successfully generate predictions of in vivo doses from effective in vitro concentration, including the experimental systems that provide input parameters for these models, areas of success, and areas for improvement to reduce model uncertainty. Finally, we provide case studies on the uses of IVIVE in safety assessments, which highlight the respective differences, information requirements, and outcomes across various approaches when applied for decision-making.

Cell cultures in drug discovery and development: The need of reliable in vitro-in vivo extrapolation for pharmacodynamics and pharmacokinetics assessment

For ethical and cost-related reasons, use of animals for the assessment of mode of action, metabolism and/or toxicity of new drug candidates has been increasingly scrutinized in research and industrial applications. Implementation of the 3 "Rs"¹; rule (Reduction, Replacement, Refinement) through development of in silico or in vitro assays has become an essential element of risk assessment. Physiologically based pharmacokinetic (PBPK²) modeling is the most potent in silico tool used for extrapolation of pharmacokinetic parameters to animal or human models from results obtained in vitro. Although, many types of in vitro assays are conducted during drug development, use of cell cultures is the most reliable one. Two-dimensional (2D) cell cultures have been a part of drug development for many years. Nowadays, their role is decreasing in favor of three-dimensional (3D) cell cultures and co-cultures. 3D cultures exhibit protein expression patterns and intercellular junctions that are closer to in vivo states in comparison to classical monolayer cultures. Co-cultures allow for examinations of the mutual influence of different cell lines. However, the complexity and high costs of co-cultures and 3D equipment exclude such methods from high-throughput screening (HTS).³In vitro absorption, distribution, metabolism, and excretion assessment, as well as drug-drug interaction (DDI), are usually performed with the use of various cell culture based assays. Progress in in silico and in vitro methods can lead to better in vitro-in vivo extrapolation (IVIVE⁴) outcomes and have a potential to contribute towards a significant reduction in the number of laboratory animals needed for drug research. As such, concentrated efforts need to be spent towards the development of an HTS in vitro platform with satisfactory IVIVE features.

Actomyosin contractility drives bile regurgitation as an early response during obstructive cholestasis

BACKGROUND & AIMS

A wide range of liver diseases manifest as biliary obstruction, or cholestasis. However, the sequence of molecular events triggered as part of the early hepatocellular homeostatic response in obstructive cholestasis is poorly elucidated. Pericanalicular actin is known to accumulate during obstructive cholestasis. Therefore, we hypothesized that the pericanalicular actin cortex undergoes significant remodeling as a regulatory response to obstructive cholestasis.

METHODS

In vivo investigations were performed in a bile duct-ligated mouse model. Actomyosin contractility was assessed using sandwich-cultured rat hepatocytes transfected with various fluorescently labeled proteins and pharmacological inhibitors of actomyosin contractility.

RESULTS

Actomyosin contractility induces transient deformations along the canalicular membrane, a process we have termed inward blebbing. We show that these membrane intrusions are initiated by local ruptures in the pericanalicular actin cortex; and they typically retract following repair by actin polymerization and actomyosin contraction. However, above a certain osmotic pressure threshold, these inward blebs pinch away from the canalicular membrane into the hepatocyte cytoplasm as large vesicles (2-8μm). Importantly, we show that these vesicles aid in the regurgitation of bile from the bile canaliculi.

CONCLUSION

Actomyosin contractility induces the formation of bile-regurgitative vesicles, thus serving as an early homeostatic mechanism against increased biliary pressure during cholestasis.

LAY SUMMARY

Bile canaliculi expand and contract in response to the amount of secreted bile, and resistance from the surrounding actin bundles. Further expansion due to bile duct blockade leads to the formation of inward blebs, which carry away excess bile to prevent bile build up in the canaliculi.

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.

Novel Method to Predict In Vivo Liver-to-Plasma Kpuu for OATP Substrates Using Suspension Hepatocytes

The ability to predict human liver-to-plasma unbound partition coefficient (Kpuu) is of great importance to estimate unbound liver concentration, develop PK/PD relationships, predict efficacy and toxicity in the liver, and model the drug-drug interaction potential for drugs that are asymmetrically distributed into the liver. A novel in vitro method has been developed to predict in vivo Kpuu with good accuracy using cryopreserved suspension hepatocytes in InVitroGRO HI media with 4% BSA. Validation was performed using six OATP substrates with rat in vivo Kpuu data from i.v. infusion studies where a steady state was achieved. Good in vitro-in vivo correlation (IVIVE) was observed as the in vitro Kpuu values were mostly within 2-fold of in vivo Kpuu Good Kpuu IVIVE in human was also observed with in vivo Kpuu data of dehydropravastatin from positron emission tomography and in vivo Kpuu data from PK/PD modeling for pravastatin and rosuvastatin. Under the specific Kpuu assay conditions, the drug-metabolizing enzymes and influx/efflux transporters appear to function at physiologic levels. No scaling factors are necessary to predict in vivo Kpuu from in vitro data. The novel in vitro Kpuu method provides a useful tool in drug discovery to project in vivo Kpuu.

Sandwich-Cultured Hepatocytes as a Tool to Study Drug Disposition and Drug-Induced Liver Injury

Sandwich-cultured hepatocytes (SCH) are metabolically competent and have proper localization of basolateral and canalicular transporters with functional bile networks. Therefore, this cellular model is a unique tool that can be used to estimate biliary excretion of compounds. SCH have been used widely to assess hepatobiliary disposition of endogenous and exogenous compounds and metabolites. Mechanistic modeling based on SCH data enables estimation of metabolic and transporter-mediated clearances, which can be used to construct physiologically based pharmacokinetic models for prediction of drug disposition and drug-drug interactions in humans. In addition to pharmacokinetic studies, SCH also have been used to study cytotoxicity and perturbation of biological processes by drugs and hepatically generated metabolites. Human SCH can provide mechanistic insights underlying clinical drug-induced liver injury (DILI). In addition, data generated in SCH can be integrated into systems pharmacology models to predict potential DILI in humans. In this review, applications of SCH in studying hepatobiliary drug disposition and bile acid-mediated DILI are discussed. An example is presented to show how data generated in the SCH model were used to establish a quantitative relationship between intracellular bile acids and cytotoxicity, and how this information was incorporated into a systems pharmacology model for DILI prediction.

Determination of Unbound Partition Coefficient and in Vitro-in Vivo Extrapolation for SLC13A Transporter-Mediated Uptake

Unbound partition coefficient (Kpuu) is important to an understanding of the asymmetric free drug distribution of a compound between cells and medium in vitro, as well as between tissue and plasma in vivo, especially for transporter-mediated processes. Kpuu was determined for a set of compounds from the SLC13A family that are inhibitors and substrates of transporters in hepatocytes and transporter-transfected cell lines. Enantioselectivity was observed, with (R)-enantiomers achieving much higher Kpuu (>4) than the (S)-enantiomers (<1) in human hepatocytes and SLC13A5-transfected human embryonic 293 cells. The intracellular free drug concentration correlated directly with in vitro pharmacological activity rather than the nominal concentration in the assay because of the high Kpuu mediated by SLC13A5 transporter uptake. Delivery of the diacid PF-06649298 directly or via hydrolysis of the ethyl ester prodrug PF-06757303 resulted in quite different Kpuu values in human hepatocytes (Kpuu of 3 for diacid versus 59 for prodrug), which was successfully modeled on the basis of passive diffusion, active uptake, and conversion rate from ester to diacid using a compartmental model. Kpuu values changed with drug concentrations; lower values were observed at higher concentrations possibly owing to a saturation of transporters. Michaelis-Menten constant (Km) of SLC13A5 was estimated to be 24 μM for PF-06649298 in human hepatocytes. In vitro Kpuu obtained from rat suspension hepatocytes supplemented with 4% fatty acid free bovine serum albumin showed good correlation with in vivo Kpuu of liver-to-plasma, illustrating the potential of this approach to predict in vivo Kpuu from in vitro systems.

Inhibition of bile salt transport by drugs associated with liver injury in primary hepatocytes from human, monkey, dog, rat, and mouse

Interference of bile salt transport is one of the underlying mechanisms for drug-induced liver injury (DILI). We developed a novel bile salt transport activity assay involving in situ biosynthesis of bile salts from their precursors in primary human, monkey, dog, rat, and mouse hepatocytes in suspension as well as LC-MS/MS determination of extracellular bile salts transported out of hepatocytes. Glycine- and taurine-conjugated bile acids were rapidly formed in hepatocytes and effectively transported into the extracellular medium. The bile salt formation and transport activities were time‒ and bile-acid-concentration‒dependent in primary human hepatocytes. The transport activity was inhibited by the bile salt export pump (BSEP) inhibitors ketoconazole, saquinavir, cyclosporine, and troglitazone. The assay was used to test 86 drugs for their potential to inhibit bile salt transport activity in human hepatocytes, which included 35 drugs associated with severe DILI (sDILI) and 51 with non-severe DILI (non-sDILI). Approximately 60% of the sDILI drugs showed potent inhibition (with IC50 values <50 μM), but only about 20% of the non-sDILI drugs showed this strength of inhibition in primary human hepatocytes and these drugs are associated only with cholestatic and mixed hepatocellular cholestatic (mixed) injuries. The sDILI drugs, which did not show substantial inhibition of bile salt transport activity, are likely to be associated with immune-mediated liver injury. Twenty-four drugs were also tested in monkey, dog, rat and mouse hepatocytes. Species differences in potency were observed with mouse being less sensitive than other species to inhibition of bile salt transport. In summary, a novel assay has been developed using hepatocytes in suspension from human and animal species that can be used to assess the potential for drugs and/or drug-derived metabolites to inhibit bile salt transport and/or formation activity. Drugs causing sDILI, except those by immune-mediated mechanism, are highly associated with potent inhibition of bile salt transport.

In vitro model systems to investigate bile salt export pump (BSEP) activity and drug interactions: A review

The bile salt export pump protein (BSEP), expressed on the canalicular membranes of hepatocytes, is primarily responsible for the biliary excretion of bile salts. The inhibition of BSEP transport activity can lead to an increase in intracellular bile salt levels and liver injury. This review discusses the various in vitro assays currently available for assessing the effect of drugs or other chemical entities to modulate BSEP transport activity. BSEP transporter assays use one of the following platforms: Xenopus laevis oocytes; canalicular membrane vesicles (CMV); BSEP-expressed membrane vesicles; cell lines expressing BSEP; sandwich cultured hepatocytes (SCH); and hepatocytes in suspension. Two of these, BSEP-expressed insect membrane vesicles and sandwich cultured hepatocytes, are the most commonly used assays. BSEP membrane vesicles prepared from transfected insect cells are useful for assessing BSEP inhibition or substrate specificity and exploring mechanisms of BSEP-associated genetic diseases. This model can be applied in a high-throughput format for discovery-drug screening. However, experimental results from use of membrane vesicles may lack physiological relevance and the model does not allow for investigation of in situ metabolism in modulation of BSEP activity. Hepatocyte-based assays that use the SCH format provide results that are generally more physiologically relevant than membrane assays. The SCH model is useful in detailed studies of the biliary excretion of drugs and BSEP inhibition, but due to the complexity of SCH preparation, this model is used primarily for determining biliary clearance and BSEP inhibition in a limited number of compounds. The newly developed hepatocyte in suspension assay avoids many of the complexities of the SCH method. The use of pooled cryopreserved hepatocytes in suspension minimizes genetic variance and individual differences in BSEP activity and also provides the opportunity for higher throughput screening and cross-species comparisons.

Comparative Proteomic Analysis of Human Liver Tissue and Isolated Hepatocytes with a Focus on Proteins Determining Drug Exposure

Freshly isolated human hepatocytes are considered the gold standard for in vitro studies of liver functions, including drug transport, metabolism, and toxicity. For accurate predictions of the in vivo outcome, the isolated hepatocytes should reflect the phenotype of their in vivo counterpart, i.e., hepatocytes in human liver tissue. Here, we quantified and compared the membrane proteomes of freshly isolated hepatocytes and human liver tissue using a label-free shotgun proteomics approach. A total of 5144 unique proteins were identified, spanning over 6 orders of magnitude in abundance. There was a good global correlation in protein abundance. However, the expression of many plasma membrane proteins was lower in the isolated hepatocytes than in the liver tissue. This included transport proteins that determine hepatocyte exposure to many drugs and endogenous compounds. Pathway analysis of the differentially expressed proteins confirmed that hepatocytes are exposed to oxidative stress during isolation and suggested that plasma membrane proteins were degraded via the protein ubiquitination pathway. Finally, using pitavastatin as an example, we show how protein quantifications can improve in vitro predictions of in vivo liver clearance. We tentatively conclude that our data set will be a useful resource for improved hepatocyte predictions of the in vivo outcome.

Verapamil hepatic clearance in four preclinical rat models: towards activity-based scaling

The current study was designed to cross-validate rat liver microsomes (RLM), suspended rat hepatocytes (SRH) and the isolated perfused rat liver (IPRL) model against in vivo pharmacokinetic data, using verapamil as a model drug. Michaelis-Menten constants (Km), for the metabolic disappearance kinetics of verapamil in RLM and SRH (freshly isolated and cryopreserved), were determined and corrected for non-specific binding. The 'unbound' Km determined with RLM (2.8 µM) was divided by the 'unbound' Km determined with fresh and cryopreserved SRH (3.9 µM and 2.1 µM, respectively) to calculate the ratio of intracellular to extracellular unbound concentration (Kpu,u). Kpu,u was significantly different between freshly isolated (0.71) and cryopreserved (1.31) SRH, but intracellular capacity for verapamil metabolism was maintained after cryopreservation (200 vs. 191 µl/min/million cells). Direct comparison of intrinsic clearance values (Clint) in RLM versus SRH, yielded an activity-based scaling factor (SF) of 0.28-0.30 mg microsomal protein/million cells (MPPMC). Merging the IPRL-derived Clint with the MPPMC and SRH data, resulted in scaling factors for MPPGL (80 and 43 mg microsomal protein/g liver) and HPGL (269 and 153 million cells/g liver), respectively. Likewise, the hepatic blood flow (61 ml/min/kg b.wt) was calculated using IPRL Clint and the in vivo Cl. The scaling factors determined here are consistent with previously reported CYP450-content based scaling factors. Overall, the results show that integrated interpretation of data obtained with multiple preclinical tools (i.e. RLM, SRH, IPRL) can contribute to more reliable estimates for scaling factors and ultimately to improved in vivo clearance predictions based on in vitro experimentation.

Meta-analysis of expression of hepatic organic anion-transporting polypeptide (OATP) transporters in cellular systems relative to human liver tissue

Organic anion-transporting polypeptide (OATP)1B1, OATP1B3, and OATP2B1 transporters play an important role in hepatic drug disposition. Recently, an increasing number of studies have reported proteomic expression data for OATP transporters. However, systematic analysis and understanding of the actual differences in OATP expression between liver tissue and commonly used cellular systems is lacking. In the current study, meta-analysis was performed to assess the protein expression of OATP transporters reported in hepatocytes relative to liver tissue and to identify any potential correlations in transporter expression levels in the same individual. OATP1B1 was identified as the most abundant uptake transporter at 5.9 ± 8.3, 5.8 ± 3.3, and 4.2 ± 1.7 fmol/μg protein in liver tissue, sandwich-cultured human hepatocytes (SCHH), and cryopreserved suspended hepatocytes, respectively. The rank order in average expression in liver tissue and cellular systems was OATP1B1 > OATP1B3 ≈ OATP2B1. Abundance levels of the OATP transporters investigated were not significantly different between liver and cellular systems, with the exception of OATP2B1 expression in SCHH relative to liver tissue. Analysis of OATP1B1, OATP1B3, and OATP2B1 liver expression data in the same individuals (n = 86) identified weak (OATP1B1-OATP2B1) to moderately (OATP1B3-OATP2B1) significant correlations. A significant weak correlation was noted between OATP1B1 abundance and age of human donors, whereas expression of the OATPs investigated was independent of sex. Implications of the current analysis on the in vitro-in vivo extrapolation of transporter-mediated drug disposition using physiologically based pharmacokinetic models are discussed.

Case study 3. Application of basic enzyme kinetics to metabolism studies: real-life examples

An appreciation of the principles of enzyme kinetics can be applied in a number of drug metabolism applications. The concept for this chapter arose from a simple discussion on selecting appropriate time points to most efficiently assess metabolite profiles in a human Phase 1a clinical study (Subheading 4). By considering enzyme kinetics, a logical approach to the issue was derived. The dialog was an important learning opportunity for the participants in the discussion, and we have endeavored to capture this experience with other questions related to determination of K m and V max parameters, a consideration of the value of hepatocytes versus liver microsomes and enzyme inhibition parameters.

Primary liver cells cultured on carbon nanotube substrates for liver tissue engineering and drug discovery applications

Here, we explore the use of two- and three-dimensional scaffolds of multiwalled-carbon nanotubes (MWNTs) for hepatocyte cell culture. Our objective is to study the use of these scaffolds in liver tissue engineering and drug discovery. In our experiments, primary rat hepatocytes, the parenchymal (main functional) cell type in the liver, were cultured on aligned nanogrooved MWNT sheets, MWNT yarns, or standard 2-dimensional culture conditions as a control. We find comparable cell viability between all three culture conditions but enhanced production of the hepatocyte-specific marker albumin for cells cultured on MWNTs. The basal activity of two clinically relevant cytochrome P450 enzymes, CYP1A2 and CYP3A4, are similar on all substrates, but we find enhanced induction of CYP1A2 for cells on the MWNT sheets. Our data thus supports the use of these substrates for applications including tissue engineering and enhancing liver-specific functions, as well as in in vitro model systems with enhanced predictive capability in drug discovery and development.

Bridging in vitro and in vivo metabolism and transport of faldaprevir in human using a novel cocultured human hepatocyte system, HepatoPac

An increased appreciation of the importance of transporter and enzyme interplay in drug clearance and a desire to delineate these mechanisms necessitates the utilization of models that contain a full complement of enzymes and transporters at physiologically relevant activities. Additionally, the development of drugs with longer half-lives requires in vitro systems with extended incubation times that allow characterization of metabolic pathways for low-clearance drugs. A recently developed coculture hepatocyte model, HepatoPac, has been applied to meet these challenges. Faldaprevir is a drug in late-stage development for the treatment of hepatitis C. Faldaprevir is a low-clearance drug with the somewhat unique characteristic of being slowly metabolized, producing two abundant hydroxylated metabolites (M2a and M2b) in feces (∼40% of the dose) without exhibiting significant levels of circulating metabolites in humans. The human HepatoPac model was investigated to characterize the metabolism and transport of faldaprevir. In human HepatoPac cultures, M2a and M2b were the predominant metabolites formed, with extents of formation comparable to in vivo. Direct glucuronidation of faldaprevir was shown to be a minor metabolic pathway. HepatoPac studies also demonstrated that faldaprevir is concentrated in liver with active uptake by multiple transporters (including OATP1B1 and Na(+)-dependent transporters). Overall, human HepatoPac cultures provided valuable insights into the metabolism and disposition of faldaprevir in humans and demonstrated the importance of enzyme and transporter interplay in the clearance of the drug.

Prediction of in vivo rat biliary drug clearance from an in vitro hepatocyte efflux model

Well-established techniques are available to predict in vivo hepatic uptake and metabolism from in vitro data, but predictive models for biliary clearance remain elusive. Several studies have verified the expression and activity of ATP-binding cassette (ABC) efflux transporters central to biliary clearance in freshly isolated rat hepatocytes, raising the possibility of predicting biliary clearance from in vitro efflux measurements. In the present study, short-term plated rat hepatocytes were evaluated as a model to predict biliary clearance from in vitro efflux measurements before major changes in transporter expression known to take place in long-term hepatocyte cultures. The short-term cultures were carefully characterized for their uptake and metabolic properties using a set of model compounds. In vitro efflux was studied using digoxin, fexofenadine, napsagatran, and rosuvastatin, representing compounds with over 100-fold differences in efflux rates in vitro and 60-fold difference in measured in vivo biliary clearance. The predicted biliary clearances from short-term plated rat hepatocytes were within 2-fold of measured in vivo values. As in vitro efflux includes both basolateral and canalicular effluxes, pronounced basolateral efflux may introduce errors in predictions for some compounds. In addition, in vitro rat hepatocyte uptake rates corrected for simultaneous efflux predicted rat in vivo hepatic clearance of the biliary cleared compounds with less than 2-fold error. Short-term plated hepatocytes could thus be used to quantify hepatocyte uptake, metabolism, and efflux of compounds and considerably improve the prediction of hepatic clearance, especially for compounds with a large biliary clearance component.

Generating an in vitro-in vivo correlation for metabolism and liver enrichment of a hepatitis C virus drug, faldaprevir, using a rat hepatocyte model (HepatoPac)

Hepatocytes provide an integrated model to study drug metabolism and disposition. As a result of a loss of polarity or a significant decrease in the expression of enzymes and transporters, suspended and sandwich-cultured hepatocytes have limitations in determining hepatocellular drug concentrations. Underprediction of the extent of glucuronidation is also a concern for these hepatocyte models. Faldaprevir is a hepatitis C virus protease inhibitor in late-stage development that has demonstrated significant liver enrichment in in vivo rat models based on quantitative whole-body autoradiography (QWBA) and liver-to-plasma area under-the-curve ratio. In bile duct cannulated rats, the primary biliary metabolite was a glucuronide. Owing to ethical concerns, it is difficult to assess liver enrichment in humans, and a lack of in vitro and in vivo correlation of glucuronidation has been reported. The current study was conducted to verify whether a hepatocyte model, rat HepatoPac, could overcome some of these limitations and provide validity for follow-up studies with human HepatoPac. With rat HepatoPac, liver enrichment values averaged 34-fold and were consistent with rat QWBA (26.8-fold) and in vivo data (42-fold). In contrast, liver enrichment in suspended hepatocytes was only 2.8-fold. Furthermore, the extent of faldaprevir glucuronidation in HepatoPac studies was in agreement with in vivo results, with glucuronidation as the major pathway (96%). Suspended rat hepatocytes did not generate the glucuronide or two key hydroxylated metabolites that were observed in vivo. Overall, our studies suggest that HepatoPac is a promising in vitro model to predict in vivo liver enrichment and metabolism, especially for glucuronidation, and has demonstrated superiority over suspended hepatocytes.