Current insights in the complexities underlying drug-induced cholestasis

Studying the intracellular bile acid concentration and toxicity in drug-induced cholestasis: Comprehensive LC-MS/MS analysis with human liver slices

Drug-induced cholestasis (DIC) is a leading cause of drug-induced liver injury post-drug marketing, characterized by bile flow obstruction and toxic bile constituent accumulation within hepatocytes. This study investigates the toxicity associated with intracellular bile acid (BA) accumulation during DIC development. Using liquid chromatography with tandem mass spectrometry (LC-MS/MS) analysis, we examined intracellular BA concentrations in human precision-cut liver slices (PCLS) following the administration of cyclosporin A and chlorpromazine, both with and without an established BA mixture. Our findings indicate toxicity of cyclosporin A upon BA addition, while chlorpromazine's toxicity remained unaffected. Although neither drug led to the accumulation of all BAs intracellularly, BA mixture addition resulted in the accumulation of unconjugated BAs associated with DIC, such as deoxycholic acid (DCA) and cholic acid (CA). Additionally, cyclosporin A increased taurolithocholic acid (TLCA) concentrations. In the absence of the BA mixture, a decrease in conjugated BAs was observed, suggesting inhibition of BA metabolism by cholestatic drugs and warranting further investigation. The evident increase in CA and DCA for both drugs (and TLCA for cyclosporin A), despite not exacerbating toxicity with chlorpromazine, suggests these increases may be related to DIC development and possible toxicity. In conclusion, the current human PCLS model is appropriate for investigating and detecting essential contributors to DIC and can be used in future studies elucidating DIC ex vivo.

In Vitro-In Silico Models to Elucidate Mechanisms of Bile Acid Disposition and Cellular Aerobics in Human Hepatocytes

Understanding the kinetics of hepatic processes, such as bile acid (BA) handling and cellular aerobic metabolism, is crucial for advancing our knowledge of liver toxicity, particularly drug-induced cholestasis (DiCho). This article aimed to construct interpretable models with parameter estimations serving as reference values when investigating these cell metrics. Longitudinal datasets on BA disposition and oxygen consumption rates were collected using sandwich-cultured human hepatocytes. Chenodeoxycholic acid (CDCA), lithocholic acid (LCA), as well as their amidated and sulfate-conjugated metabolites were quantified with liquid chromatography-mass spectrometry. The bile salt export pump (BSEP) abundance was monitored with targeted proteomics and modelled for activity assessment. Oxygen consumption was measured using Seahorse XFp analyser. Ordinary differential equation-based models were solved in R. The basolateral uptake and efflux clearance of glycine-conjugated CDCA (GCDCA) were estimated at 1.22 µL/min/106 cells (RSE 14%) and 0.11 µL/min/106 cells (RSE 10%), respectively. The GCDCA clearance from canaliculi back to the medium was 2.22 nL/min/106 cells (RSE 17%), and the dissociation constant between (G)CDCA and FXR for regulating BSEP abundance was 25.73 nM (RSE 11%). Sulfation clearance for LCA was 0.19 µL/min/106 cells (RSE 11%). Model performance was further demonstrated by a maximum two-fold deviation of the 95% confidence boundaries from parameter estimates. These in vitro-in silico models provide a quantitative framework for exploring xenobiotic impacts on BA disposition, BSEP activity, and cellular aerobic metabolism in hepatocytes. Model simulations were consistent with reported in vivo data in progressive familial intrahepatic cholestasis type II patients.

Mass Spectrometry Imaging Distinguishes Biliary Toxicants on the Basis of Cellular Distribution

Mass spectrometry imaging (MSI) was used to investigate and provide insights into observed biliary pathology found in dogs and rats after administration of two different compounds. Both compounds were associated with peribiliary inflammatory infiltrates and proliferation of the bile duct epithelium. However, MSI revealed very different spatial distribution profiles for the two compounds: Compound A showed significant accumulation within the bile duct epithelium with a much higher concentration than in the parenchymal hepatocytes, while Compound T exhibited only a slight increase in the bile duct epithelium compared to parenchymal hepatocytes. These findings implicate cholangiocyte uptake and accumulation as a key step in the mechanism of biliary toxicity. In both cases, compounds are shown at the site of toxicity in support of a direct mechanism of toxicity on the biliary epithelium. MSI is a powerful tool for localizing small molecules within tissue sections and improvements in sensitivity have enabled localization down to the cellular level in some cases. MSI was also able to identify biomarker candidates of toxicity by differential analysis of ion profiles comparing treated and control cholangiocytes from tissue sections.

Polygoni Multiflori Radix interferes with bile acid metabolism homeostasis by inhibiting Fxr transcription, leading to cholestasis

Objective: To explore the possible mechanisms of cholestasis induced by Polygoni Multiflori Radix (PM). Methods: Low and high doses of water extract of PM were given to mice by gavage for 8 weeks. The serum biochemical indexes of aspartate aminotransferase (AST), alanine aminotransferase (ALT), glutamyltransferase (GGT) alkaline phosphatase (ALP) and so on were detected in the second, fourth, sixth, and eighth weeks after administration. At the end of the eighth week of administration, the bile acid metabolic profiles of liver and bile were screened by high-performance liquid chromatography tandem triple quadrupole mass spectrometry (HPLC-QQQ-MS/MS). Liver pathological changes were observed by hematoxylin and eosin staining. Real-time quantitative polymerase chain reaction (RT-qPCR) was used to detect the mRNA transcription of the target genes and Western blotting (WB) was used to the detect target protein expression. Results: Biochemical tests results showed the values of ALP and GGT were two and three times greater than the normal values respectively, and the value of R was less than 2. Histopathology also showed that PM caused lymphocyte infiltration, a small amount of hepatocyte necrosis and nuclear fragmentation in mouse liver. The proliferation of bile duct epithelial cells was observed in the high group. These results indicated that PM may lead to cholestatic liver injury. HPLC-QQQ-MS/MS analysis with the multivariate statistical analysis revealed significant alterations of individual bile acids in liver and gallbladder as compared to those of the control group. RT-qPCR showed that the transcription of Fxr, Shp, Bsep, Bacs, Mdr2, and Ugt1a1 were downregulated and that of Cyp7a1, Mrp3, and Cyp3a11 was significantly upregulated in the treatment group. WB demonstrated that PM also markedly downregulated the protein expression of FXR, BSEP, and MDR2, and upregulated CYP7A1. Conclusion: PM inhibited the expression of FXR, which reduced the expression of MDR2 and BSEP, leading to the obstruction of bile acids outflow, and increased the expression of CYP7A1, resulting in an increase of intrahepatic bile acid synthesis, which can lead to cholestasis.

Protective Effect of Rhus chinensis Mill. Fruits on 3,5-Diethoxycarbonyl-1,4-Dihydrocollidine-Induced Cholestasis in Mice via Ameliorating Oxidative Stress and Inflammation

This study focused on the preventive effects of the extracts of Rhus chinensis Mill. (RCM) fruits on cholestasis induced by 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) in mice. The results showed that RCM extracts could significantly ameliorate DDC-induced cholestasis via multiple mechanisms, including (1) alleviating liver damage via enhancing antioxidant capacity, such as increasing the contents of glutathione, superoxide dismutase, and catalase and inhibiting the levels of malondialdehyde; (2) preventing liver inflammation by suppressing NF-κB pathway and reducing proinflammatory cytokines secretion (e.g., tumor necrosis factor-α, interleukin-1β, and interleukin-6); (3) inhibiting liver fibrosis and collagen deposition by regulating the expression of transforming growth factor-β and α-smooth muscle actin; (4) modulating abnormal bile acid metabolism through increasing the expression of bile salt export pump and multidrug resistance-associated protein 2. This study was the first to elucidate the potential preventive effect of RCM extracts on DDC-induced cholestasis in mice from multiple pathways, which suggested that RCM fruits could be considered as a potential dietary supplement to prevent cholestasis.

Hepatic bile acid synthesis and secretion: Comparison of in vitro methods

Reliable hepatic in vitro systems are crucial for the safety assessment of xenobiotics. Certain xenobiotics decrease the hepatic bile efflux, which can ultimately result in cholestasis. Preclinical animal models and the currently available in vitro systems poorly predict a xenobiotic's cholestatic potential. Here, we compared the phenotype and capacity of three liver derived in vitro systems to emulate human functionality to synthesize and secrete bile acids (BAs). To this end, basal BA production of sandwich cultured human hepatocytes (SCHHs), HepaRG cells (HepaRGs) and hepatocyte-like intrahepatic cholangiocyte organoids (ICO-heps) were analysed, and the effect of the known BSEP (Bile Salt Export Pump)-inhibitors bosentan and lopinavir on BA disposition in SCHHs and HepaRGs was quantified. RT-qPCR of selected target genes involved in maturation status, synthesis, transport and conjugation of BAs was performed to mechanistically underpin the observed differences in BA homeostasis. ICO-heps produced a (very) low amount of BAs. SCHHs are a powerful tool in cholestasis-testing due to their high basal BA production and high transporter expression compared to the other models tested. HepaRGs were responsive to both selected BSEP-inhibitors and produced a BA profile that is most similar to the human in vivo situation, making them a suitable and practical candidate for cholestasis-testing.

In Vitro Liver Toxicity Testing of Chemicals: A Pragmatic Approach

The liver is among the most frequently targeted organs by noxious chemicals of diverse nature. Liver toxicity testing using laboratory animals not only raises serious ethical questions, but is also rather poorly predictive of human safety towards chemicals. Increasing attention is, therefore, being paid to the development of non-animal and human-based testing schemes, which rely to a great extent on in vitro methodology. The present paper proposes a rationalized tiered in vitro testing strategy to detect liver toxicity triggered by chemicals, in which the first tier is focused on assessing general cytotoxicity, while the second tier is aimed at identifying liver-specific toxicity as such. A state-of-the-art overview is provided of the most commonly used in vitro assays that can be used in both tiers. Advantages and disadvantages of each assay as well as overall practical considerations are discussed.

Compensatory Transition of Bile Acid Metabolism from Fecal Disposition of Secondary Bile Acids to Urinary Excretion of Primary Bile Acids Underlies Rifampicin-Induced Cholestasis in Beagle Dogs

Drug induced cholestasis (DIC) is complexly associated with dysbiosis of the host-gut microbial cometabolism of bile acids (BAs). Murine animals are not suitable for transitional studies because the murine BA metabolism is quite different from human metabolism. In this work, the rifampicin (RFP) induced cholestasis was established in beagle dogs that have a humanlike BA profile to disclose how RFP affects the host-gut microbial cometabolism of BAs. The daily excretion of BA metabolites in urine and feces was extensively analyzed during cholestasis by quantitative BA profiling along the primary-secondary-tertiary axis. Oral midazolam clearance was also acquired to monitor the RFP-induced enterohepatic CYP3A activities because CYP3A is exclusively responsible for the tertiary oxidation of hydrophobic secondary BAs. RFP treatments caused a compensatory transition of the BA metabolism from the fecal disposition of secondary BAs to the urinary excretion of primary BAs in dogs, resulting in an infantile BA metabolism pattern recently disclosed in newborns. However, the tertiary BAs consistently constituted limitedly in the daily BA excretion, indicating that the detoxification role of the CYP3A catalyzed tertiary BA metabolism was not as strong as expected in this model. Multiple host-gut microbial factors might have contributed to the transition of the BA metabolism, such as inhibition of BA transporters, induction of liver-kidney interplaying detoxification mechanisms, and elimination of gut bacteria responsible for secondary BA production. Transitional studies involving more cholestatic drugs in preclinical animals with a humanlike BA profile and DIC patients may pave the way for understanding the complex mechanism of DIC in the era of metagenomics.

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.

Idiosyncratic Drug-Induced Liver Injury: Mechanistic and Clinical Challenges

Idiosyncratic drug-induced liver injury (IDILI) remains a significant problem for patients and drug development. The idiosyncratic nature of IDILI makes mechanistic studies difficult, and little is known of its pathogenesis for certain. Circumstantial evidence suggests that most, but not all, IDILI is caused by reactive metabolites of drugs that are bioactivated by cytochromes P450 and other enzymes in the liver. Additionally, there is overwhelming evidence that most IDILI is mediated by the adaptive immune system; one example being the association of IDILI caused by specific drugs with specific human leukocyte antigen (HLA) haplotypes, and this may in part explain the idiosyncratic nature of these reactions. The T cell receptor repertoire likely also contributes to the idiosyncratic nature. Although most of the liver injury is likely mediated by the adaptive immune system, specifically cytotoxic CD8+ T cells, adaptive immune activation first requires an innate immune response to activate antigen presenting cells and produce cytokines required for T cell proliferation. This innate response is likely caused by either a reactive metabolite or some form of cell stress that is clinically silent but not idiosyncratic. If this is true it would make it possible to study the early steps in the immune response that in some patients can lead to IDILI. Other hypotheses have been proposed, such as mitochondrial injury, inhibition of the bile salt export pump, unfolded protein response, and oxidative stress although, in most cases, it is likely that they are also involved in the initiation of an immune response rather than representing a completely separate mechanism. Using the clinical manifestations of liver injury from a number of examples of IDILI-associated drugs, this review aims to summarize and illustrate these mechanistic hypotheses.

Tanshinone ⅡA may alleviate rifampin-induced cholestasis by regulating the expression and function of NTCP

The Na⁺-taurocholate cotransporting polypeptide (NTCP) acts as the major hepatic basolateral uptake system, and plays a key role in balancing bile flow. The anti-tuberculosis drugs rifampin (RFP) can affect bile flow causing liver injury, while tanshinone IIA (TAN IIA) has the effect of protecting liver. This study aimed to investigate the effects of RFP and TAN IIA on the NTCP expression and activity, and explore the potential connections. Herein, we established sandwich-cultured primary rat hepatocytes, and quantified mRNA and protein levels of NRF2 and NTCP after treatment with RFP (10, 25, or 50 μM) or co-treatment with TAN IIA (5, 10, or 20 μM) for 12, 24, 48 h (n = 3). NTCP activity was assessed by measuring the initial uptake rates of known substrates taurocholate (TCA) (n = 3) after treatment with different concentrations of RFP, TAN ⅡA for 12, 24 and 48 h. We found that RFP had inhibition effects on NRF2, NTCP mRNA and protein expression, and co-administration of TAN IIA could reverse RFP inhibition. TCA cellular accumulation was significantly decreased by RFP (39.1%), and TAN IIA could significantly induce TCA uptake of NTCP (2.9-fold at 48 h). The TCA uptake activity was correlated with the NTCP mRNA expression, confirming the role of RFP or TAN IIA on NTCP expression and activity is synchronous, and we can predict NTCP activity by detecting its mRNA expression. In conclusion, our work will enrich the significance of NTCP in the liver protection, and provide theoretical basis for TAN IIA to prevent RFP induced cholestatic liver injury.

A Robust, Mechanistically Based In Silico Structural Profiler for Hepatic Cholestasis

Owing to the primary role which it holds within metabolism of xenobiotics, the liver stands at heightened risk of exposure to, and injury from, potentially hazardous substances. A principal manifestation of liver dysfunction is cholestasis-the impairment of physiological bile circulation from its point of origin within the organ to the site of action in the small intestine. The capacity for early identification of compounds liable to exert cholestatic effects is of particular utility within the field of pharmaceutical development, where contribution toward candidate attrition is great. Shortcomings associated with the present in vitro methodologies forecasting cholestasis render their predictivity questionable, permitting scope for the adoption of computational toxicology techniques. As such, the intention of this study has been to construct an in silico profiler, founded upon clinical data, highlighting structural motifs most reliably associated with the end point. Drawing upon a list of >1500 small molecular drugs, compiled and annotated by Kotsampasakou, E. and Ecker, G. F. (J. Chem. Inf. Model. 2017, 57, 608-615), we have formulated a series of 15 structural alerts. These describe fragments intrinsic within distinct pharmaceutical classes including psychoactive tricyclics, β-lactam antimicrobials, and estrogenic/androgenic steroids. Description of the coverage and selectivity of each are provided, alongside consideration of the underlying reactive mechanisms and relevant structure-activity concerns. Provision of mechanistic anchoring ensures that potential exists for framing within the adverse outcome pathway paradigm-the chemistry conveyed through the alert, in particular enabling rationalization at the level of the molecular initiating event.

Drug-induced liver injury after switching from tamoxifen to anastrozole in a patient with a history of breast cancer being treated for hypertension and diabetes

Anastrozole is a selective non-steroidal aromatase inhibitor that blocks the conversion of androgens to estrogens in peripheral tissues. It is used as adjuvant therapy for early-stage hormone-sensitive breast cancer in postmenopausal women. Significant side effects of anastrozole include osteoporosis and increased levels of cholesterol. To date, seven case reports on anastrozole hepatotoxicity have been published. We report the case of an 81-year-old woman with a history of breast cancer, arterial hypertension, type 2 diabetes mellitus, hyperlipidemia, and chronic renal insufficiency. Four days after switching hormone therapy from tamoxifen to anastrozole, icterus developed along with a significant increase in liver enzymes (measured in the blood). The patient was admitted to hospital, where a differential diagnosis of jaundice was made and anastrozole was withdrawn. Subsequently, hepatic functions quickly normalized. The observed liver injury was attributed to anastrozole since other possible causes of jaundice were excluded. However, concomitant pharmacotherapy could have contributed to the development of jaundice and hepatotoxicity, after switching from tamoxifen to anastrozole since several the patient’s medications were capable of inhibiting hepatobiliary transport of bilirubin, bile acids, and metabolized drugs through inhibition of ATP-binding cassette proteins. Telmisartan, tamoxifen, and metformin all block bile salt efflux pumps. The efflux function of multidrug resistance protein 2 is known to be reduced by telmisartan and tamoxifen and breast cancer resistance protein is known to be inhibited by telmisartan and amlodipine. Moreover, the activity of P-glycoprotein transporters are known to be decreased by telmisartan, amlodipine, gliquidone, as well as the previously administered tamoxifen. Finally, the role of genetic polymorphisms of cytochrome P450 enzymes and/or drug transporters cannot be ruled out since the patient was not tested for polymorphisms.

A Two-Tiered In Vitro Approach to De-Risk Drug Candidates for Potential Bile Salt Export Pump Inhibition Liabilities in Drug Discovery

Hepatocellular accumulation of bile salts by inhibition of bile salt export pump (BSEP/ABCB11) may result in cholestasis and is one proposed mechanism of drug-induced liver injury (DILI). To understand the relationship between BSEP inhibition and DILI, we evaluated 64 DILI-positive and 57 DILI-negative compounds in BSEP, multidrug resistance protein (MRP) 2, MRP3, and MRP4 vesicular inhibition assays. An empirical cutoff (5 μM) for BSEP inhibition was established based on a relationship between BSEP IC50 values and the calculated maximal unbound concentration at the inlet of the human liver (fu*Iin,max, assay specificity = 98%). Including inhibition of MRP2-4 did not increase DILI predictivity. To further understand the potential to inhibit bile salt transport, a selected subset of 30 compounds were tested for inhibition of taurocholate (TCA) transport in a long-term human hepatocyte micropatterned co-culture (MPCC) system. The resulting IC50 for TCA in vitro biliary clearance and biliary excretion index (BEI) in MPCCs were compared with the compound's fu*Iin,max to assess potential risk for bile salt transport perturbation. The data show high specificity (89%). Nine out of 15 compounds showed an IC50 value in the BSEP vesicular assay of <5μM, but the BEI IC50 was more than 10-fold the fu*Iin,max, suggesting that inhibition of BSEP in vivo is unlikely. The data indicate that although BSEP inhibition measured in membrane vesicles correlates with DILI risk, that measurement of this assay activity is insufficient. A two-tiered strategy incorporating MPCCs is presented to reduce BSEP inhibition potential and improve DILI risk. SIGNIFICANCE STATEMENT: This work describes a two-tiered in vitro approach to de-risk compounds for potential bile salt export pump inhibition liabilities in drug discovery utilizing membrane vesicles and a long-term human hepatocyte micropatterned co-culture system. Cutoffs to maximize specificity were established based on in vitro data from a set of 121 DILI-positive and -negative compounds and associated calculated maximal unbound concentration at the inlet of the human liver based on the highest clinical dose.

Drug-induced cholestasis assay in primary hepatocytes

Drug-induced cholestasis (DIC) is a major cause of clinical failure of drug candidates. Numerous patients worldwide are affected when exposed to marketed drugs exhibiting a DIC signature. Prospective identification of DIC during early compound development remains challenging. Here we describe the optimized in vitro procedure for early assessment and prediction of an increased DIC risk. Our method is based on three principles:•Exposure of primary human hepatocyte cultures to test compounds in the absence and presence of a physiologically relevant mixture of endogenous bile salts.•Rapid and quantitative assessment of the influence of concomitant bile salt exposure on hepatocyte functionality and integrity after 24 h or 48 h of incubation.•Translation of the in vitro result, expressed as a DIC index (DICI) value, into an in vivo safety margin.Using our historical control data, a new (data driven) DICI cut-off value of 0.78 was established for discerning cholestatic and non-cholestatic compounds. Our DIC assay protocol was further improved by now relying on the principle of the no observable adverse effect level (NOAEL) for determining the highest test compound concentration corresponding to a DICI  ≥  0.78. Predicted safety margin values were subsequently calculated for compounds displaying hepatotoxic and/or cholestatic effects in patients, thus enabling evaluation of the performance of our DIC assay. Of note, this assay can be extended to explore the role of drug metabolites in precipitating DIC.

Cholestasis Differentially Affects Liver Connexins

Connexins are goal keepers of tissue homeostasis, including in the liver. As a result, they are frequently involved in disease. The current study was set up to investigate the effects of cholestatic disease on the production of connexin26, connexin32 and connexin43 in the liver. For this purpose, bile duct ligation, a well-known trigger of cholestatic liver injury, was applied to mice. In parallel, human hepatoma HepaRG cell cultures were exposed to cholestatic drugs and bile acids. Samples from both the in vivo and in vitro settings were subsequently subjected to assessment of mRNA and protein quantities as well as to in situ immunostaining. While the outcome of cholestasis on connexin26 and connexin43 varied among experimental settings, a more generalized repressing effect was seen for connexin32. This has also been observed in many other liver pathologies and could suggest a role for connexin32 as a robust biomarker of liver disease and toxicity.

Use of imaging to assess the activity of hepatic transporters

Introduction: Membrane transporters of the SLC and ABC families are abundantly expressed in the liver, where they control the transfer of drugs/drug metabolites across the sinusoidal and canalicular hepatocyte membranes and play a pivotal role in hepatic drug clearance. Noninvasive imaging methods, such as PET, SPECT or MRI, allow for measuring the activity of hepatic transporters in vivo, provided that suitable transporter imaging probes are available.Areas covered: We give an overview of the working principles of imaging-based assessment of hepatic transporter activity. We discuss different currently available PET/SPECT radiotracers and MRI contrast agents and their applications to measure hepatic transporter activity in health and disease. We cover mathematical modeling approaches to obtain quantitative parameters of transporter activity and provide a critical assessment of methodological limitations and challenges associated with this approach.Expert opinion: PET in combination with pharmacokinetic modeling can be potentially applied in drug development to study the distribution of new drug candidates to the liver and their clearance mechanisms. This approach bears potential to mechanistically assess transporter-mediated drug-drug interactions, to assess the influence of disease on hepatic drug disposition and to validate and refine currently available in vitro-in vivo extrapolation methods to predict hepatic clearance of drugs.