Industrial Approach to Determine the Relative Contribution of Seven Major UGT Isoforms to Hepatic Glucuronidation.

The pharma industry designs increasingly less cytochrome P450 dependent and more metabolically stable drugs, and consequently UGT-metabolism becomes more frequently involved. This study compares 2 glucuronidation RAF-scaling approaches, product formation and substrate depletion, regarding their potential for prediction of in vivo DDI and the relative contribution of UGT-mediated phase II reactions in an industrial setting. RAFs were developed for UGT1A1, 1A3, 1A4, 1A6, 1A9, 2B7 and 2B15 recombinant UGT isoforms and a large 150-donor pooled human liver microsome batch. The RAF-values ranged from small values of 0.06 (UGT1A3), over 0.24 and 0.48 (UGT1A9 and UGT1A4), to values around 1 (1.11 for UGT2B7, 1.14 for UGT1A1), and high RAFs of 4.8 (UGT1A6) and 6.57 (UGT2B15). Both approaches identified the same primarily involved isoforms (≥75% relative contribution) of 5 clinical reference compounds (raloxifene, haloperidol, laropiprant, telmisartan and naloxone), in concordance with reported in vitro (R2 = 0.65) and clinical results for UGT1A1, 1A3, 1A4, 1A9, 2B7 and 2B15. This study is distinctive in that it is reporting the glucuronide formation in addition to substrate depletion. The product formation approach proved more sensitive and enables UGT phenotyping of slowly metabolized drugs, additionally it allows identification of structurally different glucuronides.

Integrated in vitro models for hepatic safety and metabolism: evaluation of a human Liver-Chip and liver spheroid.

Drug-induced liver injury remains a frequent reason for drug withdrawal. Accordingly, more predictive and translational models are required to assess human hepatotoxicity risk. This study presents a comprehensive evaluation of two promising models to assess mechanistic hepatotoxicity, microengineered Organ-Chips and 3D hepatic spheroids, which have enhanced liver phenotype, metabolic activity and stability in culture not attainable with conventional 2D models. Sensitivity of the models to two hepatotoxins, acetaminophen (APAP) and fialuridine (FIAU), was assessed across a range of cytotoxicity biomarkers (ATP, albumin, miR-122, α-GST) as well as their metabolic functionality by quantifying APAP, FIAU and CYP probe substrate metabolites. APAP and FIAU produced dose- and time-dependent increases in miR-122 and α-GST release as well as decreases in albumin secretion in both Liver-Chips and hepatic spheroids. Metabolic turnover of CYP probe substrates, APAP and FIAU, was maintained over the 10-day exposure period at concentrations where no cytotoxicity was detected and APAP turnover decreased at concentrations where cytotoxicity was detected. With APAP, the most sensitive biomarkers were albumin in the Liver-Chips (EC50 5.6 mM, day 1) and miR-122 and ATP in the liver spheroids (14-fold and EC50 2.9 mM, respectively, day 3). With FIAU, the most sensitive biomarkers were albumin in the Liver-Chip (EC50 126 µM) and miR-122 (15-fold) in the liver spheroids, both on day 7. In conclusion, both models exhibited integrated toxicity and metabolism, and broadly similar sensitivity to the hepatotoxicants at relevant clinical concentrations, demonstrating the utility of these models for improved hepatotoxicity risk assessment.

Use of HµREL Human Coculture System for Prediction of Intrinsic Clearance and Metabolite Formation for Slowly Metabolized Compounds.

Design of slowly metabolized compounds is an important goal in many drug discovery projects. Standard hepatocyte suspension intrinsic clearance (CLint) methods can only provide reliable CLint values above 2.5 µL/min/million cells. A method that permits extended incubation time with maintained performance and metabolic activity of the in vitro system is warranted to allow in vivo clearance predictions and metabolite identification of slowly metabolized drugs. The aim of this study was to evaluate the static HµREL coculture of human hepatocytes with stromal cells to be set up in-house as a standard method for in vivo clearance prediction and metabolite identification of slowly metabolized drugs. Fourteen low CLint compounds were incubated for 3 days, and seven intermediate to high CLint compounds and a cocktail of cytochrome P450 (P450) marker substrates were incubated for 3 h. In vivo clearance was predicted for 20 compounds applying the regression line approach, and HµREL coculture predicted the human intrinsic clearance for 45% of the drugs within 2-fold and 70% of the drugs within 3-fold of the clinical values. CLint values as low as 0.3 µL/min/million hepatocytes were robustly produced, giving 8-fold improved sensitivity of robust low CLint determination, over the cutoff in hepatocyte suspension CLint methods. The CLint values of intermediate to high CLint compounds were at similar levels both in HµREL coculture and in freshly thawed hepatocytes. In the HµREL coculture formation rates for five P450-isoform marker reactions, paracetamol (CYP1A2), 1-OH-bupropion (CYP2B6), 4-OH-diclofenac (CYP2C9), and 1-OH-midazolam (3A4) were within the range of literature values for freshly thawed hepatocytes, whereas 1-OH-bufuralol (CYP2D6) formation rate was lower. Further, both phase I and phase II metabolites were detected and an increased number of metabolites were observed in the HµREL coculture compared to hepatocyte suspension. In conclusion, HµREL coculture can be applied to accurately estimate intrinsic clearance of slowly metabolized drugs and is now utilized as a standard method for in vivo clearance prediction of such compounds in-house.

Significantly Different Covalent Binding of Oxidative Metabolites, Acyl Glucuronides, and S-Acyl CoA Conjugates Formed from Xenobiotic Carboxylic Acids in Human Liver Microsomes.

Xenobiotic carboxylic acids may be metabolized to oxidative metabolites, acyl glucuronides, and/or S-acyl-CoA thioesters (CoA conjugates) in vitro, e.g., in hepatocytes, and in vivo. These metabolites can potentially be reactive species and bind covalently to tissue proteins and are generally considered to mediate adverse drug reactions in humans. Acyl glucuronide metabolites have been the focus of reactive metabolite research for decades, whereas drug-CoA conjugates, which have been shown to be up to 40-70 times more reactive, have been given much less attention. In an attempt to dissect the contribution of different pathways to covalent binding, we utilized human liver microsomes supplemented with NADPH, uridine 5'-diphosphoglucuronic acid (UDPGA), or CoA to evaluate the reactivity of each metabolite separately. Seven carboxylic acid drugs were included in this study. While ibuprofen and tolmetin are still on the market, ibufenac, fenclozic acid, tienilic acid, suprofen, and zomepirac were stopped before their launch or withdrawn. The reactivities of the CoA conjugates of ibuprofen, ibufenac, fenclozic acid, and tolmetin were higher compared to those of their corresponding oxidative metabolites and acyl glucuronides, as measured by the level of covalent binding to human liver microsomal proteins. The highest covalent binding was observed for ibuprofenyl-CoA and ibufenacyl-CoA, to levels of 1000 and 8600 pmol drug eq/mg protein, respectively. In contrast and in agreement with the proposed P450-mediated toxicity for these drug molecules, the reactivities of oxidative metabolites of suprofen and tienilic acid were higher compared to the reactivities of their conjugated metabolites, with NADPH-dependent covalent binding of 250 pmol drug eq/mg protein for both drugs. The seven drugs all formed UDPGA-dependent acyl glucuronides, but none of these resulted in covalent binding. This study shows that, unlike studies with hepatocytes or in vivo, human liver microsomes provide an opportunity to investigate the reactivity of individual metabolites.

Serum-free culture of primary human hepatocytes in a miniaturized hollow-fibre membrane bioreactor for pharmacological in vitro studies.

Primary human hepatocytes represent an important cell source for in vitro investigation of hepatic drug metabolism and disposition. In this study, a multi-compartment capillary membrane-based bioreactor technology for three-dimensional (3D) perfusion culture was further developed and miniaturized to a volume of less than 0.5 ml to reduce demand for cells. The miniaturized bioreactor was composed of two capillary layers, each made of alternately arranged oxygen and medium capillaries serving as a 3D culture for the cells. Metabolic activity and stability of primary human hepatocytes was studied in this bioreactor in the presence of 2.5% fetal calf serum (FCS) under serum-free conditions over a culture period of 10 days. The miniaturized bioreactor showed functions comparable to previously reported data for larger variants. Glucose and lactate metabolism, urea production, albumin synthesis and release of intracellular enzymes (AST, ALT, GLDH) showed no significant differences between serum-free and serum-supplemented bioreactors. Activities of human-relevant cytochrome P450 (CYP) isoenzymes (CYP1A2, CYP3A4/5, CYP2C9, CYP2D6, CYP2B6) analyzed by determination of product formation rates from selective probe substrates were also comparable in both groups. Gene expression analysis showed moderately higher expression in the majority of CYP enzymes, transport proteins and enzymes of Phase II metabolism in the serum-free bioreactors compared to those maintained with FCS. In conclusion, the miniaturized bioreactor maintained stable function over the investigated period and thus provides a suitable system for pharmacological studies on primary human hepatocytes under defined serum-free conditions.

Metabolism of xenobiotic carboxylic acids: focus on coenzyme A conjugation, reactivity, and interference with lipid metabolism.

While xenobiotic carboxylic acids (XCAs) have been studied extensively with respect to their enzymatic conversion to potentially reactive acyl glucuronides with implications to drug induced hepatotoxicity, the formation of xenobiotic-S-acyl-CoA thioesters (xenobiotic-CoAs) have been much less studied in spite of data indicating that such conjugates may be equally or more reactive than the corresponding acyl glucuronides. This review addresses enzymes and cell organelles involved in the formation of xenobiotic-CoAs, the reactivity of such conjugates toward biological macromolecules, and in vitro and in vivo methodology to assess consequences of such reactivity. Further, the propensity of xenobiotic-CoAs to interfere with endogenous lipid metabolism, e.g., inhibition of ß-oxidation or depletion of the CoA or carnitine pools, adds to the complexity of the potential contribution of XCAs to hepatotoxicity by a number of mechanisms in addition to those in common with the corresponding acyl glucuronides. On the basis of our review of the literature on xenobiotic-CoA conjugates, there appear to be a number of gaps in our understanding of the bioactivation of XCA both with respect to the mechanisms involved and the experimental approaches to distinguish between the role of acyl glucuronides and xenobiotic-CoA conjugates. These aspects are focused upon and described in detail in this review.

Evaluation of organic anion-transporting polypeptide 1B1 and CYP3A4 activities in primary human hepatocytes and HepaRG cells cultured in a dynamic three-dimensional bioreactor system.

The long-term stability of liver cell functions is a major challenge when studying hepatic drug transport, metabolism, and toxicity in vitro. The aim of the present study was to investigate organic anion-transporting polypeptide (OATP) 1B1 and CYP3A4 activities in fresh primary human hepatocytes and differentiated cryopreserved HepaRG cells when cultured in a three-dimensional (3D) bioreactor system. OATP1B1 activity was determined by loss from media experiments of [(3)H]estradiol-17ß-D-glucuronide and atorvastatin acid (ATA) for up to 7 days in culture. ATA metabolite formation was determined at days 3 to 4 to evaluate CYP3A4 activity. Overall, the results showed that freshly isolated human hepatocytes inoculated in the bioreactor retained OATP1B1 activity for at least 7 days, whereas in HepaRG cells no OATP1B1 activity was observed beyond day 2. The activity data were in agreement with immunohistochemical stainings, which showed that OATP1B1 protein expression was preserved for at least 9 days in fresh human hepatocytes, whereas OATP1B1 was expressed markedly lower in HepaRG cells after 9 days in culture. Fresh human hepatocytes and HepaRG cells exhibited similar CYP3A4 activity in bioreactor culture, and immunohistochemical stainings supported these findings. Activity and mRNA expression of OATP1B1 and CYP3A4 in primary human hepatocytes compared with HepaRG cells in fresh suspensions were in agreement with data obtained in bioreactor culture. In conclusion, freshly isolated human hepatocytes cultured in a 3D bioreactor system preserve both OATP1B1 and CYP3A4 activities, allowing long-term in vitro studies on drug disposition and toxicity.

In vitro evaluation of major in vivo drug metabolic pathways using primary human hepatocytes and HepaRG cells in suspension and a dynamic three-dimensional bioreactor system.

Major human specific metabolites, not detected during in vivo and in vitro preclinical studies, may cause unexpected drug interactions and toxicity in human and delays in clinical programs. Thus, reliable preclinical tools for the detection of major human metabolites are of high importance. The aim of this study was to compare major drug metabolic pathways in HepaRG cells, a human hepatoma cell line, to fresh human hepatocytes, cryopreserved human hepatocytes, and human in vivo data. Furthermore, the maintenance of cytochrome P450 (P450) and UDP-glucuronosyltransferase (UGT) activities in a dynamic three-dimensional (3D) bioreactor were evaluated over time by using HepaRG cells and human hepatocytes. (14)C-diclofenac and a candidate from AstraZeneca's drug development program, (14)C-AZD6610, which are metabolized by P450 and UGT in vivo, were used as model substrates. The proportion of relevant biotransformation pathways of the investigated drug was clearly different in the various cell systems. The hydroxylation route was favored in primary human hepatocytes, whereas the glucuronidation route was favored in HepaRG cells. The human in vivo metabolite profile of AZD6610 was best represented by human hepatocytes, whereas all major diclofenac metabolites were detected in HepaRG cells. Moreover, the metabolite profiles in cryopreserved and fresh human hepatocytes were essentially the same. The liver bioreactor using both fresh human hepatocytes and HepaRG cells retained biotransformation capacity over 1 week. Thus, the incubation time can be increased from a few hours in suspension to several days in 3D cultures, which opens up for detection of metabolites from slowly metabolized drugs.

An ex vivo perfusion system emulating in vivo conditions in noncirrhotic and cirrhotic human liver.

Various models are used for investigating human liver diseases and testing new drugs. However, data generated in such models have only limited relevance for in vivo conditions in humans. We present here an ex vivo perfusion system using human liver samples that enables the characterization of parameters in a functionally intact tissue context. Resected samples of noncirrhotic liver (NC; n = 10) and cirrhotic liver (CL; n = 12) were perfused for 6-h periods. General and liver-specific parameters (glucose, lactate, oxygen, albumin, urea, and bile acids), liver enzymes (aspartate aminotransferase, alanine aminotransferase, lactate dehydrogenase, glutamate dehydrogenase, and γ-glutamyl transferase), overall (M65) and apoptotic (M30) cell-death markers, and indicators of phase-I/phase-II biotransformations were analyzed. The measurement readings closely resembled (patho)physiological characteristics in patients with NC and CL. Mean courses of glucose levels reflected the CLs' reduced glycogen storage capability. Furthermore, CL samples exhibited significantly stronger increases in lactate, bile acids, and the M30/M65 ratio than NC specimens. Likewise, NC samples exhibited more rapid phase-I transformations of phenacetin, midazolam, and diclofenac and phase-I to phase-II turnover rates of the respective intermediates than CL tissue. Collectively, these findings reveal the better hepatic functionality in NC. Perfusion of human liver tissue with this system emulates in vivo conditions and clearly discriminates between noncirrhotic and cirrhotic tissue. This highly reliable device for investigating basic hepatic functionality and testing safety/toxicity, pharmacokinetics/pharmacodynamics and efficacies of novel therapeutic modalities promises to generate superior data compared with those obtained via existing economic perfusion systems.

Cytochrome P450-dependent metabolism in HepaRG cells cultured in a dynamic three-dimensional bioreactor.

Reliable and stable in vitro cellular systems maintaining specific liver functions important for drug metabolism and disposition are urgently needed in preclinical drug discovery and development research. The cell line HepaRG exhibits promising properties such as expression and function of drug-metabolizing enzymes and transporter proteins, which resemble those found in freshly isolated human hepatocytes. In this study, HepaRG cells were cultured up to 68 days in a three-dimensional multicompartment capillary membrane bioreactor, which enables high-density cell culture under dynamic conditions. The activity of drug-metabolizing cytochrome P450 (P450) enzymes was investigated by a cocktail of substrates for CYP1A1/2 (phenacetin), CYP2C9 (diclofenac), CYP2B6 (bupropion), and CYP3A4 (midazolam). The model P450 substrates, which were introduced to the bioreactor system mimicking in vivo bolus doses, showed stable metabolism over the entire experimental period of several weeks with the exception of bupropion hydroxylase, which increased over time. Ketoconazole treatment decreased the CYP3A4 activity by 69%, and rifampicin induced the CYP3A4- and CYP2B6-dependent activity 6-fold, which predicts well the magnitude of changes observed in vivo. Moreover, polarity of transporter expression and formation of tissue-like structures including bile canaliculi were demonstrated by immune histochemistry. The long-lasting bioreactor system using HepaRG cells thus provides a promising and stable liver-like in vitro model for continuous investigations of the hepatic kinetics of drugs and of drug-drug interactions, which well predict the situation in vivo in humans.

Scaling down of a clinical three-dimensional perfusion multicompartment hollow fiber liver bioreactor developed for extracorporeal liver support to an analytical scale device useful for hepatic pharmacological in vitro studies.

Within the scope of developing an in vitro culture model for pharmacological research on human liver functions, a three-dimensional multicompartment hollow fiber bioreactor proven to function as a clinical extracorporeal liver support system was scaled down in two steps from 800 mL to 8 mL and 2 mL bioreactors. Primary human liver cells cultured over 14 days in 800, 8, or 2 mL bioreactors exhibited comparable time-course profiles for most of the metabolic parameters in the different bioreactor size variants. Major drug-metabolizing cytochrome P450 activities analyzed in the 2 mL bioreactor were preserved over up to 23 days. Immunohistochemical studies revealed tissue-like structures of parenchymal and nonparenchymal cells in the miniaturized bioreactor, indicating physiological reorganization of the cells. Moreover, the canalicular transporters multidrug-resistance-associated protein 2, multidrug-resistance protein 1 (P-glycoprotein), and breast cancer resistance protein showed a similar distribution pattern to that found in human liver tissue. In conclusion, the down-scaled multicompartment hollow fiber technology allows stable maintenance of primary human liver cells and provides an innovative tool for pharmacological and kinetic studies of hepatic functions with small cell numbers.

CYP3A4 catalytic activity is induced in confluent Huh7 hepatoma cells.

Drug-induced hepatotoxicity is an important cause for disapproval, limitations of use, or withdrawal of drugs, and there is a high need for reproducible in vitro systems that can predict such toxicity. In this study, we show that confluent growth of the human hepatoma cell line Huh7 up to 5 weeks results in increased gene expression of several cytochromes P450 (P450s), UDP-glucuronosyltransferases, transporters, transcription factors, and several liver-specific genes, as measured by low-density array. The most striking effect was seen for CYP3A4 expression. Western blot analysis revealed increased amounts of CYP3A4 together with increased levels of NADPH-P450 reductase, cytochrome b(5), and albumin with prolonged time of confluence. By using the CYP3A4-specific substrates luciferin 6' benzyl ether, testosterone, and midazolam, we could confirm that the increased CYP3A4 gene expression also was accompanied by a similar increase in catalytic activity, inhibitable by the CYP3A4-selective inhibitor ketoconazole. The CYP3A4 activity in confluent cells was also inducible by rifampicin. Finally, the cell system could support the CYP3A4-dependent hepatotoxic activation of aflatoxin B(1), which was effectively inhibited by ketoconazole. Our results show that Huh7 cells grown confluent differentiate into a more metabolically competent cell line, especially with regard to CYP3A4.

Investigation of the involvement of P-glycoprotein and multidrug resistance-associated protein 2 in the efflux of ximelagatran and its metabolites by using short hairpin RNA knockdown in Caco-2 cells.

Liver and bile secretion can be an important first-pass and clearance route for drug compounds and also the site of several drug-drug interactions. In the clinical program for ximelagatran development, an unexpected effect of erythromycin on the pharmacokinetics of the direct thrombin inhibitor ximelagatran and its metabolites was detected. This interaction was believed to be mediated by inhibition of drug transporters, which normally extrude the drug into the bile. Previous Caco-2 cell experiments indicated the involvement of an active efflux mechanism for ximelagatran, hydroxy-melagatran, and melagatran possibly mediated by P-glycoprotein (P-gp). However, the inhibitors used may not have been specific enough and the possibility that transporters other than P-gp were important in the Caco-2 cell assay cannot be excluded. In this study we used RNA interference, a post-transcriptional gene silencing mechanism in which mRNA is degraded in a sequence-specific manner, to specifically knock down P-gp or multidrug resistance-associated protein 2 (MRP2) transporters in Caco-2 cells. The data obtained from bidirectional transport studies in these cells indicate a clear involvement of P-gp but not of MRP2 in the transport of ximelagatran, hydroxy-melagatran, and melagatran across the apical cell membrane. The present study shows that short hairpin RNA Caco-2 cells are a valuable tool to investigate the contribution of specific transporters in the transcellular transport of drug molecules and to predict potential sites of pharmacokinetic interactions. The results also suggest that inhibition of hepatic P-gp is involved in the erythromycin-ximelagatran interaction seen in clinical studies.

Lipopolysaccharide induced-in vivo increases in beta-defensins of the rat parotid gland.

Antimicrobial beta-defensins are thought to protect epithelial surfaces. Their mobilization in response to inflammation was studied in the rat parotid gland using an ELISA assay. Bacterial lipopolysaccharide (LPS), injected into the parotid duct on one side, induced a marked local inflammatory response in the parotid gland as judged by several fold increases in myeloperoxidase activity and, in histological sections, infiltration of neutrophils. Three hours after the injection, beta-defensin 1 and 3 were increased (by 41% and 15%, respectively, P<0.01) as compared to the contralateral gland. Though still elevated 6h after the injection, the percentage figures for beta-defensin 1 were, at this time, somewhat lower (30%) compared to the situation at 3h, while those for defensin 3 were significantly higher 65% (P<0.01); neither at the early nor at the late time of observation were any changes in the level of beta-defensin 2 observed. The beta-defensins under study were not detected in submandibular and sublingual glands, neither were they detected in the inflamed submandibular gland, showing also here several fold increases in myeloperoxidase activity and, in addition, the presence of inflammatory cells, following ductal injection of LPS towards the gland.