An integrated approach to model hepatic drug clearance

PBPK model-based gender-specific risk assessment of N-nitrosodimethylamine (NDMA) using human biomonitoring data

Despite several screening levels for NDMA reported in water, soil, air, and drugs, the human risk assessment using biomonitoring concentrations has not been performed. In this study, gender-specific exposure guidance values were determined in humans, then biomonitoring measurements in healthy Korean subjects (32 men and 40 women) were compared to the exposure guidance values to evaluate the current exposure level to NDMA. For the human risk assessment of NDMA, the gender-specific physiologically based pharmacokinetic (PBPK) model was developed in humans using proper physiological parameters, partition coefficients, and biochemical parameters. Using the PBPK model, a Monte Carlo simulation was performed to describe the magnitudes of inter-individual variability and uncertainty on the single model predictions. The PBPK modeling and Monte Carlo simulation allowed the estimation of the relationship between external dose and blood concentration for the risk assessment. The procedure for the human risk assessment was summarized as follows: (1) estimating a steady-state blood concentration (Cavg) corresponding to the daily no observed adverse effect level (NOAEL) administration in rats; (2) applying uncertainty factors (UFs) for deriving the human Cavg; (3) determining the exposure guidance values as screening criteria; (4) interpreting the human biomonitoring measurements by forward and reverse dosimetry approaches. Using the biomonitoring concentrations, current daily exposures to NDMA were estimated to be 3.95 μg/day/kg for men and 10.60 μg/day/kg for women, respectively. The result of the study could be used as a basis for implementing further risk management and regulatory decision-making for NDMA.

Hepatic OATP1B zonal distribution: Implications for rifampicin-mediated drug-drug interactions explored within a PBPK framework

OATP1B facilitates the uptake of xenobiotics into hepatocytes and is a prominent target for drug-drug interactions (DDIs). Reduced systemic exposure of OATP1B substrates has been reported following multiple-dose rifampicin; one explanation for this observation is OATP1B induction. Non-uniform hepatic distribution of OATP1B may impact local rifampicin tissue concentrations and rifampicin-mediated protein induction, which may affect the accuracy of transporter- and/or metabolizing enzyme-mediated DDI predictions. We incorporated quantitative zonal OATP1B distribution data from immunofluorescence imaging into a PBPK modeling framework to explore rifampicin interactions with OATP1B and CYP substrates. PBPK models were developed for rifampicin, two OATP1B substrates, pravastatin and repaglinide (also metabolized by CYP2C8/CYP3A4), and the CYP3A probe, midazolam. Simulated hepatic uptake of pravastatin and repaglinide increased from the periportal to the pericentral region (approximately 2.1-fold), consistent with OATP1B distribution data. Simulated rifampicin unbound intracellular concentrations increased in the pericentral region (1.64-fold) compared to simulations with uniformly distributed OATP1B. The absolute average fold error of the rifampicin PBPK model for predicting substrate maximal concentration (Cmax) and area under the plasma concentration-time curve (AUC) ratios was 1.41 and 1.54, respectively (nine studies). In conclusion, hepatic OATP1B distribution has a considerable impact on simulated zonal substrate uptake clearance values and simulated intracellular perpetrator concentrations, which regulate transporter and metabolic DDIs. Additionally, accounting for rifampicin-mediated OATP1B induction in parallel with inhibition improved model predictions. This study provides novel insight into the effect of hepatic OATP1B distribution on site-specific DDI predictions and the impact of accounting for zonal transporter distributions within PBPK models.

Physiologically-based pharmacokinetic model for evaluating gender-specific exposures of N-nitrosodimethylamine (NDMA)

N-nitrosodimethylamine (NDMA) is classified as a human carcinogen and could be produced by both natural and industrial processes. Although its toxicity and histopathology have been well-studied in animal species, there is insufficient data on the blood and tissue exposures that can be correlated with the toxicity of NDMA. The purpose of this study was to evaluate gender-specific pharmacokinetics/toxicokinetics (PKs/TKs), tissue distribution, and excretion after the oral administration of three different doses of NDMA in rats using a physiologically-based pharmacokinetic (PBPK) model. The major target tissues for developing the PBPK model and evaluating dose metrics of NDMA included blood, gastrointestinal (GI) tract, liver, kidney, lung, heart, and brain. The predictive performance of the model was validated using sensitivity analysis, (average) fold error, and visual inspection of observations versus predictions. Then, a Monte Carlo simulation was performed to describe the magnitudes of inter-individual variability and uncertainty of the single model predictions. The developed PBPK model was applied for the exposure simulation of daily oral NDMA to estimate blood concentration ranges affecting health effects following acute-duration (≤ 14 days), intermediate-duration (15-364 days), and chronic-duration (≥ 365 days) intakes. The results of the study could be used as a scientific basis for interpreting the correlation between in vivo exposures and toxicological effects of NDMA.

Utility of Minimal Physiologically Based Pharmacokinetic Models for Assessing Fractional Distribution, Oral Absorption, and Series-Compartment Models of Hepatic Clearance

Minimal physiologically based pharmacokinetic (mPBPK) models are physiologically relevant, require less information than full PBPK models, and offer flexibility in pharmacokinetics (PK). The well-stirred hepatic model (WSM) is commonly used in PBPK, whereas the more plausible dispersion model (DM) poses computational complexities. The series-compartment model (SCM) mimics the DM but is easier to operate. This work implements the SCM and mPBPK models for assessing fractional tissue distribution, oral absorption, and hepatic clearance using literature-reported blood and liver concentration-time data in rats for compounds mainly cleared by the liver. Further handled were various complexities, including nonlinear hepatic binding and metabolism, differing absorption kinetics, and sites of administration. The SCM containing one to five (n) liver subcompartments yields similar fittings and provides comparable estimates for hepatic extraction ratio (ER), prehepatic availability (Fg ), and first-order absorption rate constants (ka ). However, they produce decreased intrinsic clearances (CLint ) and liver-to-plasma partition coefficients (Kph ) with increasing n as expected. Model simulations demonstrated changes in intravenous and oral PK profiles with alterations in Kph and ka and with hepatic metabolic zonation. The permeability (PAMPA P) of the various compounds well explained the fitted fractional distribution (fd ) parameters. The SCM and mPBPK models offer advantages in distinguishing systemic, extrahepatic, and hepatic clearances. The SCM allows for incorporation of liver zonation and is useful in assessing changes in internal concentration gradients potentially masked by similar blood PK profiles. Improved assessment of intraorgan drug concentrations may offer insights into active moieties driving metabolism, biliary excretion, pharmacodynamics, and hepatic toxicity. SIGNIFICANCE STATEMENT: The minimal physiologically based pharmacokinetic model and the series-compartment model are useful in assessing oral absorption and hepatic clearance. They add flexibility in accounting for various drug- or system-specific complexities, including fractional distribution, nonlinear binding and saturable hepatic metabolism, and hepatic zonation. These models can offer improved insights into the intraorgan concentrations that reflect physiologically active moieties often driving disposition, pharmacodynamics, and toxicity.

Shared learning from a physiologically based pharmacokinetic modeling strategy for human pharmacokinetics prediction through retrospective analysis of Genentech compounds

The quantitative prediction of human pharmacokinetics (PK) including the PK profile and key PK parameters are critical for early drug development decisions, successful phase I clinical trials, and the establishment of a range of doses to enable phase II clinical dose selection. Here, we describe an approach employing physiologically based pharmacokinetic (PBPK) modeling (Simcyp) to predict human PK and to validate its performance through retrospective analysis of 18 Genentech compounds for which clinical data are available. In short, physicochemical parameters and in vitro data for preclinical species were integrated using PBPK modeling to predict the in vivo PK observed in mouse, rat, dog, and cynomolgus monkey. Through this process, the in vitro to in vivo extrapolation (IVIVE) was determined and then incorporated into PBPK modeling in order to predict human PK. Overall, the prediction obtained using this PBPK-IVIVE approach captured the observed human PK profiles of the compounds from the dataset well. The predicted Cmax was within 2-fold of the observed Cmax for 94% of the compounds while the predicted area under the curve (AUC) was within 2-fold of the observed AUC for 72% of the compounds. Additionally, important IVIVE trends were revealed through this investigation, including application of scaling factors determined from preclinical IVIVE to human PK prediction for each molecule. Based upon the analysis, this PBPK-based approach now serves as a practical strategy for human PK prediction at the candidate selection stage at Genentech.

In silico modeling for the hepatic circulation and transport: From the liver organ to lobules

The function of the liver depends critically on its blood supply. Numerous in silico models have been developed to study various aspects of the hepatic circulation, including not only the macro-hemodynamics at the organ level, but also the microcirculation at the lobular level. In addition, computational models of blood flow and bile flow have been used to study the transport, metabolism, and clearance of drugs in pharmacokinetic studies. These in silico models aim to provide insights into the liver organ function under both healthy and diseased states, and to assist quantitative analysis for surgical planning and postsurgery treatment. The purpose of this review is to provide an update on state-of-the-art in silico models of the hepatic circulation and transport processes. We introduce the numerical methods and the physiological background of these models. We also discuss multiscale frameworks that have been proposed for the liver, and their linkage with the large context of systems biology, systems pharmacology, and the Physiome project. This article is categorized under: Metabolic Diseases > Computational Models Metabolic Diseases > Biomedical Engineering Cardiovascular Diseases > Computational Models.

The impact of the intestinal microbiota and the mucosal permeability on three different antibiotic drugs

BackgroundThe totality of bacteria, protozoa, viruses and fungi that lives in the human body is called microbiota. Human microbiota specifically colonizes the skin, the respiratory and urinary tract, the urogenital tract and the gastrointestinal system. This study focuses on the intestinal microbiota to explore the drug-microbiota relationship and, therefore, how the drug bioavailability changes in relation to the microbiota biodiversity to identify more personalized therapies, with the minimum risk of side effects. MethodsTo achieve this goal, we developed a new mathematical model with two compartments, the intestine and the blood, which takes into account the colonic mucosal permeability variation - measured by Ussing chamber system on human colonic mucosal biopsies - and the fecal microbiota composition, determined through microbiota 16S rRNA sequencing analysis. Both of the clinical parameters were evaluated in a group of Irritable Bowel Syndrome patients compared to a group of healthy controls. Key ResultsThe results show that plasma drug concentration increases as bacterial concentration decreases, while it decreases as intestinal length decreases too. ConclusionsThe study provides interesting data since in literature there are not yet mathematical models with these features, in which the importance of intestinal microbiota, the "forgotten organ", is considered both for the subject health state and in the nutrients and drugs metabolism.

A computational model for hepatotoxicity by coupling drug transport and acetaminophen metabolism equations

The spatial distributions of cytochrome P450 (CYP450) and glutathione (GSH) in liver lobules determine the heterogeneous hepatotoxicity of acetaminophen (APAP). Their interplay in conjunction with blood flow is not well understood. In this paper, we integrate a cellular APAP metabolism model with a sinusoidal blood flow to simulate the temporal-spatial patterns of APAP-induced hepatotoxicity. The heterogeneous distribution of CYP450 and GSH is modeled by linearly varying their reaction rates along the portal triad to the central vein axis of a sinusoid. We found that the spatial distribution of GSH, glutathione S-transferases (GSTs), and CYP450 all contributes to the high acetaminophen protein adduct formation at zone 3 of the lobules. The reversed spatial gradients of CYP450 and GSH cause quick depletion of GSH, which is further accelerated by the distribution of GST. The hepatic flow congestion and hyperperfusion however do not seem to play a significant role in the zonal hepatotoxicity. The simulation results may be useful for understanding the APAP-induced hepatotoxicity and associated pharmaceutical treatment.

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.

The use of PBPK modeling across the pediatric age range using propofol as a case

The project SAFEPEDRUG aims to provide guidelines for drug research in children, based on bottom-up and top-down approaches. Propofol, one of the studied model compounds, was selected because it is extensively metabolized in liver and kidney, with an important role for the glucuronidation pathway. Besides, being a lipophilic molecule, it is distributed into fat tissues, from where it redistributes into the systemic circulation. In the past, both bottom-up (Physiologically based pharmacokinetic, PBPK) and top-down approaches (population pharmacokinetic, popPK) were applied to describe its pharmacokinetics (PK). In this work, a combination of the two was used to check their performance to describe PK in children and neonates (both term and preterm) using propofol as a case compound. First, in vitro data was generated in human liver microsomes and recombinant enzymes and used to develop an adult PBPK model in Simcyp^®. Activity adjustment factors (AAFs) were calculated to account for differences between in vitro and in vivo enzyme activity. Clinical data were analyzed using a 3-compartment model in NONMEM. These data were used to construct a retrograde PBPK model and for qualification of the PBPK models. Once an accurate in vivo clearance was obtained accounting for the contribution of the different metabolic pathways, the resulting PBPK models were challenged with new data for qualification. After that, the constructed adult PPBK model for propofol was extrapolated to the pediatric population. Both the default built-in and in vivo derived ontogeny functions were used to do so. The models were qualified by comparing their predicted PK parameters to published values, and by comparison of predicted concentration-time profiles to available clinical data. Clearance values were predicted well, especially when compared with values obtained from trials where long-term sampling was applied, whereas volume of distribution was lower compared to the most common popPK model predictions. Concentration-time profiles were predicted well up until and including the preterm neonatal population. In this work, it was thus shown that PBPK can be used to predict the PK up to and including the preterm neonatal population without the use of pediatric in vivo data. This work adds weight to the need for further development of PBPK models, especially regarding distribution modeling and the use of in vivo derived ontogeny functions.

Potential use of metabolic breath tests to assess liver disease and prognosis: has the time arrived for routine use in the clinic?

The progression of liver disease may be unique among organ system diseases in that progressive fibrosis compromises not only the sufficiency of hepatocyte mass but also impairs blood flow to the liver, resulting in porto-systemic shunting. Although liver biopsy as an assessment of fibrosis has become the key biomarker of and target for new therapies, it is invasive and subject to sampling error, and cannot quantify metabolic function or porto-systemic shunting. Measurement of the hepatic venous pressure gradient accommodates some of the deficiencies of biopsy but requires expertise not widely available and misses minor changes in hepatocellular mass and thereby information about metabolic function. Thus, an unmet need in clinical hepatology remains unfulfilled: a noninvasive biomarker which quantitates both the hepatocellular insufficiency and porto-systemic shunting inherent in progressive hepatic fibrosis. Ideally, such a biomarker should correlate with clinical endpoints including liver-related survival and cirrhotic complications, be performed at the point-of-care, and be affordable and easy to use. This review, an expert opinion, summarizes background and recent data suggesting that metabolic breath tests may now meet these requirements and have a valid place in clinical hepatology to supplant the time-honoured assessment of hepatic fibrosis.

Hepatic drug transporters: the journey so far

INTRODUCTION

The key role of transporter biology in both the manifestation and treatment of disease is now firmly established. Experiences of sub-optimal drug exposure due to drug-transporter interplay have supported incorporation of studies aimed at understanding the interactions between compounds and drug transporters much earlier in drug discovery. While drug transporters can impact the most pivotal pharmacokinetic parameter with respect to human dose and exposure projections, clearance, at a renal or hepatobiliary level, the latter will form the focus of this perspective.

AREAS COVERED

A synopsis of guidelines on which transporters to study together with an overview of the currently available toolkit is presented. A perspective on when to conduct studies with various hepatic transporters is also provided together with structural "alerts" which should prompt early investigation.

EXPERT OPINION

Great progress has been made in individual laboratories and via consortia to understand the role of drug transporters in disease, drug disposition, drug-drug interactions and toxicity. A systematic analysis of the value posed by the available approaches and an inter-lab comparison now seems warranted. The emerging ability to use physico-chemical properties to guide future screening cascades promises to revolutionise the efficiency of early drug discovery.

Expression of indocyanine green-related transporters in hepatocellular carcinoma

BACKGROUND

Indocyanine green (ICG), an organic anion used in liver function tests, is known to accumulate in hepatocellular carcinoma (HCC) tissues after an intravenous injection. Because the intratumoral expression of transporters for chemical agents influences the behaviors of some malignant tumors, we investigated whether the expression of ICG-related transporters influenced the clinicopathologic features of HCC.

MATERIALS AND METHODS

ICG accumulation patterns were examined using near-infrared spectroscopy and the protein expression of ICG-related transporters was assessed using immunohistochemistry and immunoblotting in 40 resected HCC specimens. We also determined whether the intratumor expression of these transporters affected intratumor lipid composition by imaging mass spectrometry.

RESULTS

Immunoblot analysis revealed that the expression of organic anion transporting polypeptide 1B3 (OATP1B3) and multidrug resistance p-glycoprotein (MDR)-3, as influx and efflux transporters, respectively, were significantly higher in ICG-accumulated HCC (ICG-high HCC) than in ICG-low HCC. ICG was fluorescently observed in the pseudoglands and bile canaliculi abundantly expressing MDR3. An immunohistochemical examination revealed significantly worse disease-free and overall survival rates in patients with MDR3-negative HCC, in which the intratumoral accumulation of some phosphatidylcholine species was observed under imaging mass spectrometry.

CONCLUSIONS

The intratumoral expression of MDR3, a key efflux transporter of ICG, affected the prognosis of patients with HCC, presumably by altering the lipid composition of the lipid bilayers.

Computational approaches to analyse and predict small molecule transport and distribution at cellular and subcellular levels

Quantitative structure-activity relationship (QSAR) studies and mechanistic mathematical modeling approaches have been independently employed for analysing and predicting the transport and distribution of small molecule chemical agents in living organisms. Both of these computational approaches have been useful for interpreting experiments measuring the transport properties of small molecule chemical agents, in vitro and in vivo. Nevertheless, mechanistic cell-based pharmacokinetic models have been especially useful to guide the design of experiments probing the molecular pathways underlying small molecule transport phenomena. Unlike QSAR models, mechanistic models can be integrated from microscopic to macroscopic levels, to analyse the spatiotemporal dynamics of small molecule chemical agents from intracellular organelles to whole organs, well beyond the experiments and training data sets upon which the models are based. Based on differential equations, mechanistic models can also be integrated with other differential equations-based systems biology models of biochemical networks or signaling pathways. Although the origin and evolution of mathematical modeling approaches aimed at predicting drug transport and distribution has occurred independently from systems biology, we propose that the incorporation of mechanistic cell-based computational models of drug transport and distribution into a systems biology modeling framework is a logical next step for the advancement of systems pharmacology research.

The effect of lipophilicity on the hepatobiliary properties of iminodiacetic acid derivatives in the conditions of hyperbilirubinemia

The partition coefficients (log P) of theoretically possible alkyliodinated iminodiacetic acid (IDA) derivatives and commercial IDA derivatives were calculated using two computer programs: ChemSketch Log P and ChemOffice Ultra. Newly synthesized ligands (DIETHYLIODIDA and DIISOPROPYLIODIDA) with the highest calculated log P were labeled with technetium-99m. The biodistribution and the influence of bilirubin on their biokinetics were investigated in rats and compared to corresponding results for commercial (99m)Tc-BROMIDA. Log P of (99m)Tc-complexes of synthesized ligands were determined experimentally as well as the protein binding. In comparison to (99m)Tc-BROMIDA, (99m)Tc-DIETHYLIODIDA has: (a) better biliary excretion (2.76±0.15%ID/g versus 1.83±0.10%ID/g); (b) faster hepatic clearance (2.90±0.21%ID/g versus 7.47±0.70%ID/g) and decreased biliary excretion (for 14% versus 22%) in conditions of hyperbilirubinemia after 15min. It is proved that (99m)Tc-DIISOPROPYLIODIDA has a prolonged hepatic transit time and decreased biliary excretion.

Systems toxicology from genes to organs

This unique overview of systems toxicology methods and techniques begins with a brief account of systems thinking in biology over the last century. We discuss how systems biology and toxicology continue to leverage advances in computational modeling, informatics, large-scale computing, and biotechnology. Next, we chart the genesis of systems toxicology from previous work in physiologically based models, models of early development, and more recently, molecular systems biology. For readers interested in further details this background provides useful linkages to the relevant literature. It also lays the foundations for new ideas in systems toxicology that could translate laboratory measurements of molecular responses from xenobiotic perturbations to adverse organ level effects in humans. By providing innovative solutions across disciplinary boundaries and highlighting key scientific gaps, we believe this chapter provides useful information about the current state, and valuable insight about future directions in systems toxicity.

Continuous 13C-methacetin breath test differentiates biliary atresia from other causes of neonatal cholestasis

BACKGROUND AND AIM

Distinguishing biliary atresia (BA) from other causes of neonatal cholestasis (NC) is challenging. Continuous BreathID C-methacetin breath test (MBT) is a novel method that determines liver function. Methacetin is metabolized uniquely by the liver and CO2 is measured passively, through a nasal cannula in the exhaled breath. The aim of this study was to assess the ability of MBT to differentiate BA from other causes of NC.

METHODS

MBT was performed in infants with NC before any invasive procedure. Percent dose recovered (PDR) peak and time to peak (TTPP) of C recovered were correlated with blood test results and degree of fibrosis on liver biopsy.

RESULTS

Fifteen infants were enrolled in the study. Eight were eventually diagnosed as having BA. MBT showed that infants with NC from various causes reached the PDR peak after 44.5 ± 6.7 minutes, whereas infants with BA reached the PDR peak value after 54.7 ± 4.3 minutes (P < 0.005). This suggested low cytochrome P450 1A2 activity in the BA group. The area under the curve (AUC) was 0.95 (95% confidence interval [CI] 0.83-1), sensitivity of 88%, and specificity of 100%.

CONCLUSIONS

This pilot study shows that MBT can differentiate between BA and other causes of NC by time to peak of methacetin metabolism. The results suggest that MBT may be used as part of the diagnostic algorithm in infants with liver disease. Larger-scale studies should be conducted to confirm these initial observations.

Primovist, Eovist: what to expect?

Gadolinium ethoxybenzyl dimeglumine (Gd-EOB-DTPA, Primovist in Europe and Eovist in the USA) is a liver-specific magnetic resonance imaging contrast agent that has up to 50% hepatobiliary excretion in the normal liver. After intravenous injection, Gd-EOB-DTPA distributes into the vascular and extravascular spaces during the arterial, portal venous and late dynamic phases, and progressively into the hepatocytes and bile ducts during the hepatobiliary phase. The hepatocyte uptake of Gd-EOB-DTPA mainly occurs via the organic anion transporter polypeptides OATP1B1 and B3 located at the sinusoidal membrane and biliary excretion via the multidrug resistance-associated proteins MRP2 at the canalicular membrane. Because of these characteristics, Gd-EOB-DTPA behaves similarly to non-specific gadolinium chelates during the dynamic phases, and adds substantial information during the hepatobiliary phase, improving the detection and characterization of focal liver lesions and diffuse liver disease. This information is particularly relevant for the detection of metastases, and for the detection and characterization of nodular lesions in liver cirrhosis, including early hepatocellular carcinomas. Finally, GD-EOB-DTPA-enhanced magnetic resonance imaging may provide quantitative assessment regarding liver perfusion and hepatocyte function in diffuse liver diseases. The full potential of GD-EOB-DTPA-enhanced magnetic resonance imaging has to be established further. It is already clear that GD-EOB-DTPA-enhanced magnetic resonance imaging provides anatomic and functional information in the setting of focal and diffuse liver disease that is unattainable with magnetic resonance imaging enhanced with non-specific contrast agents.

Transporters involved in the hepatic uptake of (99m)Tc-mebrofenin and indocyanine green

BACKGROUND & AIMS

(99m)Tc-mebrofenin hepatobiliary scintigraphy (HBS) and the indocyanine green (ICG) clearance test are used for the assessment of hepatic function before and after liver surgery. The hepatic uptake of (99m)Tc-mebrofenin and ICG is considered similar to the uptake of organic anions such as bilirubin and bile acids. Little is known about hepatic uptake mechanisms of both compounds and recent evidence suggests that the hepatic transporters for ICG and (99m)Tc-mebrofenin are distinct. The aim of this study was to identify the specific human hepatic transporters of (99m)Tc-mebrofenin and ICG.

METHODS

The uptake of (99m)Tc-mebrofenin was investigated in cRNA-injected Xenopus laevis oocytes expressing human OATP1B1, OATP1B3, OATP2B1, or NTCP. Chinese hamster ovary (CHO) cells stably expressing OATP1B1, OATP1B3, OATP2B1, or NTCP were used as a mammalian expression system. ICG transport into CHO cells was additionally imaged with confocal microscopy.

RESULTS

We demonstrated that OATP1B1 and OATP1B3 are involved in the transport of (99m)Tc-mebrofenin. OATP1B1 showed an approximately 1.5-fold higher affinity for (99m)Tc-mebrofenin compared to OATP1B3. ICG is transported by OATP1B3 and NTCP.

CONCLUSIONS

The transporter specificity of (99m)Tc-mebrofenin and ICG partially overlaps as both compounds are transported by OATP1B3. (99m)Tc-mebrofenin is also taken up by OATP1B1, whereas ICG is additionally transported by NTCP.

Physiologically-based pharmacokinetic modeling for absorption, transport, metabolism and excretion

The seminal paper on the liver physiologically-based pharmacokinetic (PBPK) model by Rowland et al. (J Pharmacokinet Biopharm 1:123-136, 1973) that described the influence of blood flow, intrinsic clearance, and binding on hepatic clearance had inspired further development of PBPK modeling of the liver, kidney and intestine as well as whole body. Shortly thereafter, a series of papers from Pang and Rowland compared the well-stirred and parallel-tube liver models and sparked further development on clearance concepts in the liver, including those described by the dispersion model. From 2005 onwards, several seminal papers by Rodgers and Rowland, in their recognition of the binding of molecules to tissue acidic and neutral phospholipids, improved the methodology in providing estimates of the tissue-to-plasma coefficient and rendering easy calculation of these hard-to-get constants. The improvement has strongly consolidated the basic premise on PBPK modeling and simulations and these basics have allowed scientists to focus on other important variables: membrane barriers, and transporter and enzyme and their heterogeneities that further impact drug disposition. In particular, the PBPK models have delved into sequential metabolism and futile cycling to illustrate how transporters and enzymes could affect the metabolism of drugs and metabolites. PBPK models that are especially pertinent to metabolite kinetics are being utilized in drug studies and risk assessment. These types of PBPK modeling reveal differences in kinetics between the formed vs. preformed metabolite, showing special considerations for membrane barriers, and the influence of competing pathways and competing organs.

A fourth dimension in decision making in hepatology

Today, the assessment of liver function in patients suffering from acute or chronic liver disease is based on liver biopsy and blood tests including synthetic function, liver enzymes and viral load, most of which provide only circumstantial evidence as to the degree of hepatic impairment. Most of these tests lack the degree of sensitivity to be useful for follow-up of these patients at the frequency that is needed for decision making in clinical hepatology. Accurate assessment of liver function is essential to determine both short- and long-term prognosis, and for making decisions about liver and non-liver surgery, TIPS, chemoembolization or radiofrequency ablation in patients with chronic liver disease. Liver function tests can serve as the basis for accurate decision-making regarding the need for liver transplantation in the setting of acute failure or in patients with chronic liver disease. The liver metabolic breath test relies on measuring exhaled (13) C tagged methacetin, which is metabolized only by the liver. Measuring this liver-specific substrate by means of molecular correlation spectroscopy is a rapid, non-invasive method for assessing liver function at the point-of-care. The (13) C methacetin breath test (MBT) is a powerful tool to aid clinical hepatologists in bedside decision-making. Our recent findings regarding the ability of point-of-care (13) C MBT to assess the hepatic functional reserve in patients with acute and chronic liver disease are reviewed along with suggested treatment algorithms for common liver disorders.

Circadian rhythms in gene expression: Relationship to physiology, disease, drug disposition and drug action

Circadian rhythms (24h cycles) are observed in virtually all aspects of mammalian function from expression of genes to complex physiological processes. The master clock is present in the suprachiasmatic nucleus (SCN) in the anterior part of the hypothalamus and controls peripheral clocks present in other parts of the body. Components of this core clock mechanism regulate the circadian rhythms in genome-wide mRNA expression, which in turn regulate various biological processes. Disruption of circadian rhythms can be either the cause or the effect of various disorders including metabolic syndrome, inflammatory diseases and cancer. Furthermore, circadian rhythms in gene expression regulate both the action and disposition of various drugs and affect therapeutic efficacy and toxicity based on dosing time. Understanding the regulation of circadian rhythms in gene expression plays an important role in both optimizing the dosing time for existing drugs and in the development of new therapeutics targeting the molecular clock.

Simulating microdosimetry in a virtual hepatic lobule

The liver plays a key role in removing harmful chemicals from the body and is therefore often the first tissue to suffer potentially adverse consequences. To protect public health it is necessary to quantitatively estimate the risk of long-term low dose exposure to environmental pollutants. Animal testing is the primary tool for extrapolating human risk but it is fraught with uncertainty, necessitating novel alternative approaches. Our goal is to integrate in vitro liver experiments with agent-based cellular models to simulate a spatially extended hepatic lobule. Here we describe a graphical model of the sinusoidal network that efficiently simulates portal to centrilobular mass transfer in the hepatic lobule. We analyzed the effects of vascular topology and metabolism on the cell-level distribution following oral exposure to chemicals. The spatial distribution of metabolically inactive chemicals was similar across different vascular networks and a baseline well-mixed compartment. When chemicals were rapidly metabolized, concentration heterogeneity of the parent compound increased across the vascular network. As a result, our spatially extended lobule generated greater variability in dose-dependent cellular responses, in this case apoptosis, than were observed in the classical well-mixed liver or in a parallel tubes model. The mass-balanced graphical approach to modeling the hepatic lobule is computationally efficient for simulating long-term exposure, modular for incorporating complex cellular interactions, and flexible for dealing with evolving tissues.

Virtual tissues are emerging as a powerful tool for computational biology. By encoding known biology into a simulation of tissue function, gaps in knowledge can be identified. As a simulation of tissue function, in silico experiments can be performed inexpensively and rapidly. There are over 6000 chemicals produced in large quantities that may be present in our environment, many of which have not been thoroughly examined for human toxicity. Traditional toxicity testing is expensive, lengthy, and relies heavily upon the use of animals. For this reason in vitro toxicity testing techniques are being developed. However, techniques are needed to relate in vitro results to in vivo conditions. The liver is often the first tissue to show signs of toxicity and therefore a predictive liver toxicity simulator would be a powerful tool to reduce the financial and animal cost of toxicity testing. As a first step, we have developed a model for relating environmental exposure to cell-level concentrations; a model for virtual tissue microdosimetry. We identify regimes in which this approach is equivalent to previous techniques, as well as regimes where large cell-to-cell variability exists. This variability should have consequences both for normal liver function and the onset of injury.

Tracing multiscale mechanisms of drug disposition in normal and diseased livers

Hepatic drug disposition is different in normal and diseased livers. Different disease types alter disposition differently. What are the responsible micromechanistic changes and how do they influence drug movement within the liver? We provide plausible, concrete answers for two compounds, diltiazem and sucrose, in normal livers and two different types of cirrhotic rat livers: chronic pretreatment of rats with carbon tetrachloride (CCl(4)) and alcohol caused different types of cirrhosis. We started with simulated disposition data from normal, multilevel, physiologically based, object-oriented, discrete event in silico livers (normal ISLs) that validated against diltiazem and sucrose disposition data from normal livers. We searched the parameter space of the mechanism and found three parameter vectors that enabled matching the three wet-lab data sets. They specified micromechanistic transformations that enabled converting the normal ISL into two different types of diseased ISLs. Disease caused lobular changes at three of six levels. The latter provided in silico disposition data that achieved a prespecified degree of validation against wet-lab data. The in silico transformations from normal to diseased ISLs stand as concrete theories for disease progression from the disposition perspective. We also developed and implemented methods to trace objects representing diltiazem and sucrose during disposition experiments. This allowed valuable insight into plausible disposition details in normal and diseased livers. We posit that changes in ISL micromechanistic details may have disease-causing counterparts.

Interplay of transporters and enzymes in drug and metabolite processing

This review highlights the "interplay" between enzymes and transporters, essential components of eliminating organs for drug removal. The understanding of the interplay is important in terms of deciphering the change of one eliminatory pathway on compensatory mechanisms in drug disposal, and, ultimately, their importance in drug-drug interactions. Controversy existed on the explanation underlying the interplay between transporters and enzymes in the Caco-2 cell monolayer or cell culture systems, but less so on eliminating organs such as the intestine and liver. For the Caco-2 system, the increase in the mean residence time (MRT) accompanying increased secretion had been construed as the basis for increased metabolism. We hold the opposite view and assert that increased secretion should evoke a decrease in metabolism due to the competition between the enzyme and apical efflux transporter for the drug within the cell. To illustrate this point, simulations on the MRT, fraction of dose metabolized (f(met)) and the extraction ratio (ER) as defined by various investigators under linear and nonlinear metabolic conditions were compared to observed data and the trends upon induction/inhibition of secretion. The conclusion is that the f(met) is the more appropriate index to reflect the extent of metabolism in transporter-enzyme interplay, since the parameter captures drug metabolism in the cell when its contents in the apical, cell, and basolateral compartments or the entire dose is considered to be available for metabolism. This parameter for metabolism (f(met)) bears a reciprocal relationship to the secretory intrinsic clearance and is in concordance with the notion that both the enzyme and apical transporter compete for the cellular substrate within. For the liver and intestine, several physiologically based pharmacokinetic (PBPK) models that contain transporters and enzymes were utilized, together with the solved equations for the area under the curve (AUC), metabolic, excretory, and total clearance (CL) to shed meaningful insight of how the inhibition of one pathway can result in a higher AUC and therefore a reduced total clearance for drug, but a higher apparent clearance of the alternate pathway; induction of the same pathway would lead to an increased total clearance but decreased drug AUC, and reduced clearance of the alternate pathway. The use of an increased MRT to explain increased extents of metabolism upon increased apical excretion is not tenable in these organs or "open systems" since the MRT of drug in the cell is reduced with irreversible loss from biliary excretion or hastened gastrointestinal transit of the secreted drug in the lumen. Data in the literature for the Caco-2 system, knockout animals and organ perfusion systems were discussed in relation to these concepts on clearance based on fundamental, pharmacokinetic theory. The shortcomings in data interpretation were discussed. The general conclusion is that a reciprocal relationship exists between the clearances related to enzymes and apical transporters due to their competition for the substrate within the cell, and is a relationship independent of the MRT of drug in the system.

Formed and preformed metabolites: facts and comparisons

The administration of metabolites arising from new drug entities is often employed in drug discovery to investigate their associated toxicity. It is expected that administration of metabolites can predict the exposure of metabolites originating from the administration of precursor drug. Whether exact and meaningful information can be obtained from this has been a topic of debate. This communication summarizes observations and theoretical relationships based on physiological modelling for the liver, kidney and intestine, three major eliminating organs/tissues. Theoretical solutions based on physiological modelling of organs were solved, and the results suggest that deviations are expected. Here, examples of metabolite kinetics observed mostly in perfused organs that did not match predictions are provided. For the liver, discrepancies in fate between formed and preformed metabolites may be explained by the heterogeneity of enzymes, the presence of membrane barriers and whether transporters are involved. For the kidney, differences have been attributed to glomerular filtration of the preformed but not the formed metabolite. For the intestine, the complexity of segregated flows to the enterocyte and serosal layers and differences in metabolism due to the route of administration are addressed. Administration of the metabolite may or may not directly reflect the toxicity associated with drug use. However, kinetic data on the preformed metabolite will be extremely useful to develop a sound model for modelling and simulations; in-vitro evidence on metabolite handling at the target organ is also paramount. Subsequent modelling and simulation of metabolite data arising from a combined model based on both drug and preformed metabolite data are needed to improve predictions on the behaviours of formed metabolites.

Characterization of cardamonin metabolism by P450 in different species via HPLC-ESI-ion trap and UPLC-ESI-quadrupole mass spectrometry

AIM

To characterize the metabolism of cardamonin by the P450 enzymes in human and animal liver microsomes.

METHODS

Cardamonin was incubated with both human and animal liver microsomal incubation systems containing P450 reaction factors. High performance liquid chromatography coupled with ion trap mass spectrometry was used to identify the metabolites. Serial cardamonin dilutions were used to perform a kinetic study in human liver microsomes. Selective inhibitors of 7 of the major P450 isozymes were used to inhibit cardamonin hydroxylation to identify the isozymes involved in cardamonin metabolism. The cardamonin hydroxylation metabolic capacities of human and various other animals were investigated using the liver microsomal incubation system.

RESULTS

Two metabolites generated by the liver microsome system were detected and identified as hydroxylated cardamonin. The Km and Vmax values for cardamonin hydroxylation were calculated as 32 micromol/L and 35 pmol x min(-1) x mg(-1), respectively. Furafylline and clomethiazole significantly inhibited cardamonin hydroxylation. Guinea pigs showed the highest similarity to humans with respect to the metabolism of cardamonin.

CONCLUSION

CYP 1A2 and 2E1 were identified as the P450 isozymes involved in the metabolism of cardamonin in human liver microsomes. Furthermore, our research suggests that guinea pigs could be used in the advanced pharmacokinetic studies of cardamonin in vivo.

Presystemic elimination of trichloroethylene in rats following environmentally relevant oral exposures

1,1,2-Trichloroethylene (TCE), a volatile organic contaminant (VOC) of drinking water in the Unites States, is frequently present in trace amounts. TCE is currently classified by the International Agency for Research on Cancer and the U.S. Environmental Protection Agency as a probable human carcinogen, because it produces tumors in some organs of certain strains of mice or rats in chronic, high-dose bioassays. Previous studies (Toxicol Appl Pharmacol 60:509-526, 1981; Regul Toxicol Pharmacol 8:447-466, 1988) used physiological modeling principles to reason that the liver should remove virtually all of a well metabolized VOC, such as TCE, as long as concentrations in the portal blood were not high enough to saturate metabolism. To test this hypothesis, groups of unanesthetized male Sprague-Dawley rats received intravenous injections of 0.1, 1.0, or 2.5 mg TCE/kg as an aqueous emulsion. Other rats were gavaged with 0.0001, 0.001, 0.01, 0.1, 1, 2.5, 5, or 10 mg TCE/kg b.wt. Serial microblood samples were taken via an indwelling carotid artery cannula, to generate blood TCE versus time profiles. Headspace solid-phase microextraction gas chromatography with negative chemical ionization mass spectrometry (limit of quantitation = 25 pg/ml) was used to quantify TCE. TCE was undetectable in rats given 0.0001 mg/kg, but it exhibited linear kinetics from 0.1 to 5.0 mg/kg. Bioavailability was consistent over this dosage range, ranging from 12.5 to 16.4%. The presence of these limited amounts of TCE in the arterial blood disprove the aforementioned hypothesis, yet demonstrate that first-pass hepatic and pulmonary elimination in the rat afford its extrahepatic organs protection from potential adverse effects by the majority of the low levels of TCE absorbed from drinking water.

Point-of-care continuous 13C-methacetin breath test improves decision making in acute liver disease: Results of a pilot clinical trial

AIM: To assess the role of the ¹³C-methacetin breath test (MBT) in patients with acute liver disease.

METHODS: Fifteen patients with severe acute liver disease from diverse etiologies were followed-up with ¹³C-MBT during the acute phase of their illnesses (range 3-116 d after treatment). Patients fasted for 8 h and ingested 75 mg of methacetin prior to the MBT. We compared results from standard clinical assessment, serum liver enzymes, synthetic function, and breath test scores.

RESULTS: Thirteen patients recovered and two patients died. In patients that recovered, MBT parameters improved in parallel with improvements in lab results. Evidence of consistent improvement began on day 3 for MBT parameters and between days 7 and 9 for blood tests. Later convergence to normality occurred at an average of 9 d for MBT parameters and from 13 to 28 d for blood tests. In both patients that died, MBT parameters remained low despite fluctuating laboratory values.

CONCLUSION: The ¹³C-MBT provides a rapid, non-invasive assessment of liver function in acute severe liver disease of diverse etiologies. The results of this pilot clinical trial suggest that the MBT may offer greater sensitivity than standard clinical tests for managing patients with severe acute liver disease.

Relationship between drug/metabolite exposure and impairment of excretory transport function

The quantitative impact of excretory transport modulation on the systemic exposure to xenobiotics and derived metabolites is poorly understood. This article presents fundamental relationships between exposure and loss of a specific excretory process that contributes to overall clearance. The mathematical relationships presented herein were explored on the basis of hepatic excretory data for polar metabolites formed in the livers of various transporter-deficient rodents. Experimental data and theoretical relationships indicated that the fold change in exposure is governed by the relationship, 1/(1 - f(e)), where f(e) is the fraction excreted by a particular transport protein. Loss of function of a transport pathway associated with f(e) < 0.5 will have minor consequences (<2-fold) on exposure, but exposure will increase exponentially in response to loss of function of transport pathways with f(e) > 0.5. These mathematical relationships may be extended to other organs, such as the intestine and kidney, as well as to systemic drug exposure. Finally, the relationship between exposure and f(e) is not only applicable to complete loss of function of a transport pathway but also can be extended to partial inhibition scenarios by modifying the equation with the ratio of the inhibitor concentration and inhibition constant.

Interplay of conjugating enzymes with OATP uptake transporters and ABCC/MRP efflux pumps in the elimination of drugs

BACKGROUND

Biliary excretion is a major elimination route of many drugs and their metabolites. Hepatobiliary elimination is a vectorial process involving uptake transporters in the basolateral hepatocyte membrane, possibly Phase I and Phase II metabolizing enzymes, and ATP-dependent efflux pumps in the apical hepatocyte membrane.

OBJECTIVES

Because many drugs and their metabolites are anions, this review focuses on transporters involved in their hepatocellular uptake (members of the organic anion transporting polypeptide (OATP) family) and biliary elimination (apical conjugate efflux pump ABCC2/MRP2).

METHODS

The molecular and functional characteristics of the human OATP and ABCC/MRP transporters are presented, including a detailed overview of endogenous and drug substrates. Examples illustrate the interplay of transporters with Phase II conjugating enzymes. Model systems to study the vectorial transport of organic anions are also discussed.

RESULTS/CONCLUSIONS

OATP uptake transporters, conjugating enzymes, and ABCC2/MRP2 work in concert to enable the hepatobiliary elimination of anionic drugs and their metabolites. It is increasingly important to understand how genetic variants of these transporters and enzymes influence the interindividual variability of drug elimination.

Safety testing of metabolites: Expectations and outcomes

Metabolites arising from chemical entities, old or new, are often mediators of toxicity. Frequently, metabolites are investigated in test animals, with the expectation that the resultant toxicity or activity will mimic the exposure of their formed counterparts. This communication described observations that showed discrepant kinetics between formed and preformed metabolites in the liver, intestine, and kidney, major drug removal organs. Differences in the observed areas under the curve (AUCs) or the extraction ratios (Es) of formed and preformed metabolites in the liver had been attributed to zonal, enzyme heterogeneity, membrane barriers, or transporters. Preformed and formed metabolite also differed in their handling by the kidney; only the preformed and not the formed metabolite would be filtered. In the intestine, differences in the absorption of the precursor and the metabolite and the flow pattern in the intestine would bring about discrepancy in the time-courses of the formed vs. preformed metabolites. Analytical solutions of the AUCs of the metabolites and extraction ratios, based on physiological modeling of the liver, kidney, and intestine, showed that the AUC of the preformed, administered metabolite was dependent only on metabolite parameters, whereas the AUC of the formed metabolite was modulated additionally by the metabolic, secretory and intestinal absorptive intrinsic clearances of the precursor drug. Hence, administration of the synthetic metabolite would not reflect the toxicity associated with the metabolite formed via bioactivation. However, data on preformed metabolite may be used for simultaneous fitting by a combined model of drug and metabolite. Such a strategy is shown to be successful in risk assessment of environmental chemicals. Upon refinement of the resultant model with data on metabolite transport and handling by modeling and simulations, the resultant model would be more robust to provide improved predictions on metabolite toxicity pursuant to drug administration.

Review article: the assessment of liver function using breath tests

BACKGROUND

Hepatologists have long sought to develop a test for assessing liver function, but this aim has been stalled by the complexity of the liver and its diverse functions. Results of metabolic tests, including breath tests, correlate with clinical and histological parameters of patients with liver disorders; however, these tests tend to be cumbersome and impractical for everyday use. The recent development of a real-time, point-of-care liver function breath test has made it straightforward to assess the metabolic function of the liver.

AIM

To review the available data on the use of breath tests for assessing liver reserve in various conditions and their application in various clinical hepatology settings.

RESULTS

The (13)C-methacetin breath test enables accurate follow-up of patients with acute or chronic liver damage, where overall hepatic function is significantly suppressed by known causes of liver disorders, including acute, sub-acute or chronic conditions. The metabolic breath test can detect both gradual and spontaneous improvements in liver function and the effects of treatment.

CONCLUSIONS

Breath testing that provides continuous quantification of methacetin metabolism may be a sensitive tool for the diagnosis and follow-up of patients with liver disorders.