Formation of potentially toxic metabolites of drugs in reactions catalyzed by human drug-metabolizing enzymes

Data are presented on the formation of potentially toxic metabolites of drugs that are substrates of human drug metabolizing enzymes. The tabular data lists the formation of potentially toxic/reactive products. The data were obtained from in vitro experiments and showed that the oxidative reactions predominate (with 96% of the total potential toxication reactions). Reductive reactions (e.g., reduction of nitro to amino group and reductive dehalogenation) participate to the extent of 4%. Of the enzymes, cytochrome P450 (P450, CYP) enzymes catalyzed 72% of the reactions, myeloperoxidase (MPO) 7%, flavin-containing monooxygenase (FMO) 3%, aldehyde oxidase (AOX) 4%, sulfotransferase (SULT) 5%, and a group of minor participating enzymes to the extent of 9%. Within the P450 Superfamily, P450 Subfamily 3A (P450 3A4 and 3A5) participates to the extent of 27% and the Subfamily 2C (P450 2C9 and P450 2C19) to the extent of 16%, together catalyzing 43% of the reactions, followed by P450 Subfamily 1A (P450 1A1 and P450 1A2) with 15%. The P450 2D6 enzyme participated in an extent of 8%, P450 2E1 in 10%, and P450 2B6 in 6% of the reactions. All other enzymes participate to the extent of 14%. The data show that, of the human enzymes analyzed, P450 enzymes were dominant in catalyzing potential toxication reactions of drugs and their metabolites, with the major role assigned to the P450 Subfamily 3A and significant participation of the P450 Subfamilies 2C and 1A, plus the 2D6, 2E1 and 2B6 enzymes contributing. Selected examples of drugs that are activated or proposed to form toxic species are discussed.

Sophorin mitigates flutamide-induced hepatotoxicity in wistar rats

Flutamide is frequently used in the management of prostate cancer, hirsutism, and acne. It is a non-steroidal anti-androgenic drug and causes hepatotoxicity. The current study's objective is to evaluate sophorin's hepatoprotective effectiveness against flutamide-induced hepatotoxicity in Wistar rats. Sophorin is a citrus flavonoid glycoside, also known as rutin, which is a low molecular weight polyphenolic compound with natural antioxidant properties and reported to have promising hepatoprotective efficacy. In this study, sophorin was used at a dose of 100 mg/kg body weight in purified water via oral route for 4 week daily whereas, flutamide was used at a dose of 100 mg kg/b.wt for 4 weeks daily in 0.5% carboxy methyl cellulose (CMC) through the oral route for the induction of hepatotoxicity. Flutamide administration leads to enhanced reactive oxygen species (ROS) generation, an imbalance in redox homeostasis and peroxidation of lipid resulted in reduced natural antioxidant level in liver tissue. Our result demonstrated that sophorin significantly abrogate flutamide induced lipid peroxidation, protein carbonyl (PC), and also significantly increasesed in enzymatic activity/level of tissue natural antioxidant such as reduced glutathione(GSH), glutathione reductase(GR), catalase, and superoxide dismutase(SOD). Additionally, sophorin reduced the activity of cytochrome P450 3A1 in liver tissue which was elevated due to flutamide treatment. Furthermore, sophorin treatment significantly decreased the pro-inflammatory cytokines (TNF-α and IL-6) level. Immunohistochemical analysis for the expression of inflammatory proteins (iNOS and COX-2) in hepatic tissue was decreased after sophorin treatment against flutamide-induced hepatotoxicity. Moreover, sophorin suppressed the infiltration of mast cells in liver tissue which further showed anti-inflammatory potential of sophorin. Our histological investigation further demonstrated sophorin's hepatoprotective function by restoring the typical histology of the liver. Based on the aforementioned information, we are able to come to the conclusion that sophorin supplementation might benefit wistar rats with flutamide-induced hepatic damage by reducing oxidative stress and hepatocellular inflammation.

High-throughput AR dimerization assay identifies androgen disrupting chemicals and metabolites

Introduction: Analysis of streamlined computational models used to predict androgen disrupting chemicals revealed that assays measuring androgen receptor (AR) cofactor recruitment/dimerization were particularly indispensable to high predictivity, especially for AR antagonists. As the original dimerization assays used to develop the minimal assay models are no longer available, new assays must be established and evaluated as suitable alternatives to assess chemicals beyond the original 1,800+ supported by the current data. Here we present the AR2 assay, which is a stable, cell-based method that uses an enzyme complementation approach.

Methods: Bipartite domains of the NanoLuc luciferase enzyme were fused to the human AR to quantitatively measure ligand-dependent AR homodimerization. 128 chemicals with known endocrine activity profiles including 43 AR reference chemicals were screened in agonist and antagonist modes and compared to the legacy assays. Test chemicals were rescreened in both modes using a retrofit method to incorporate robust cytochrome P450 (CYP) metabolism to assess CYP-mediated shifts in bioactivity.

Results: The AR2 assay is amenable to high-throughput screening with excellent robust Z’-factors (rZ’) for both agonist (0.94) and antagonist (0.85) modes. The AR2 assay successfully classified known agonists (balanced accuracy = 0.92) and antagonists (balanced accuracy = 0.79–0.88) as well as or better than the legacy assays with equal or higher estimated potencies. The subsequent reevaluation of the 128 chemicals tested in the presence of individual human CYP enzymes changed the activity calls for five compounds and shifted the estimated potencies for several others.

Discussion: This study shows the AR2 assay is well suited to replace the previous AR dimerization assays in a revised computational model to predict AR bioactivity for parent chemicals and their metabolites.

Bulbine natalensis (currently Bulbine latifolia) and select bulbine knipholones modulate the activity of AhR, CYP1A2, CYP2B6, and P-gp

Bulbine natalensis, an emerging medicinal herb on the global market with androgenic properties, is often formulated in dietary supplements that promote perceived sexual enhancement. However, to date, comprehensive safety studies of B. natalensis are lacking, particularly those related to its herb-drug interaction potential. The purpose of this study was to assess the inductive and inhibitory effects of extracts and pure compounds of B. natalensis on human cytochrome P-450 isozymes in vitro. Our findings demonstrated that both water and methanolic extracts of B. natalensis as well as knipholone, bulbine-knipholone, and 6'-O-methylknipholone dose-dependently increased mRNA expression encoded by CYP2B6, CYP1A2, and ABCB1 genes. Functional analyses showed that water (60 to 2.20 µg/mL) and methanolic (30 to 3.75 µg/mL) extracts and knipholones (10 to 0.33 µM) increased CYP2B6 and CYP1A2 activity in a dose-dependent manner. Additionally, water extract (60 µg/mL), methanolic extract (30 µg/mL), and knipholone (10 µM) caused activation of the aryl hydrocarbon receptor up to 11.1 ± 0.7, 8.9 ± 0.6, and 7.1 ± 2.0-fold, respectively. Furthermore, inhibition studies revealed that methanolic extract attenuated the activity of metabolically active CYP1A2 (IC₅₀, 22.6 ± 0.4 µg/mL) and CYP2B6 (IC₅₀, 34.2 ± 6.6 µg/mL) proteins, whereas water extracts had no inhibitory effect on either isoform. These findings suggest that chronic consumption of B. natalensis may affect normal homeostasis of select CYPs with subsequent risks for HDIs when concomitantly ingested with conventional medications that are substrates of CYP2B6 and CYP1A2. However, more in-depth translational studies are required to validate our current findings and their clinical relevance.

Next Generation Risk Assessment of the Anti-Androgen Flutamide Including the Contribution of Its Active Metabolite Hydroxyflutamide

In next generation risk assessment (NGRA), non-animal approaches are used to quantify the chemical concentrations required to trigger bioactivity responses, in order to assure safe levels of human exposure. A limitation of many in vitro bioactivity assays, which are used in an NGRA context as new approach methodologies (NAMs), is that toxicokinetics, including biotransformation, are not adequately captured. The present study aimed to include, as a proof of principle, the bioactivity of the metabolite hydroxyflutamide (HF) in an NGRA approach to evaluate the safety of the anti-androgen flutamide (FLU), using the AR-CALUX assay to derive the NAM point of departure (PoD). The NGRA approach applied also included PBK modelling-facilitated quantitative in vitro to in vivo extrapolation (QIVIVE). The PBK model describing FLU and HF kinetics in humans was developed using GastroPlus™ and validated against human pharmacokinetic data. PBK model-facilitated QIVIVE was performed to translate the in vitro AR-CALUX derived concentration-response data to a corresponding in vivo dose-response curve for the anti-androgenicity of FLU, excluding and including the activity of HF (-HF and +HF, respectively). The in vivo benchmark dose 5% lower confidence limits (BMDL₀₅) derived from the predicted in vivo dose-response curves for FLU, revealed a 440-fold lower BMDL₀₅ when taking the bioactivity of HF into account. Subsequent comparison of the predicted BMDL₀₅ values to the human therapeutic doses and historical animal derived PoDs, revealed that PBK modelling-facilitated QIVIVE that includes the bioactivity of the active metabolite is protective and provides a more appropriate PoD to assure human safety via NGRA, whereas excluding this would potentially result in an underestimation of the risk of FLU exposure in humans.

Nitroreduction of flutamide by Cunninghamella elegans NADPH: Cytochrome P450 reductase

The microbial model of mammalian drug metabolism, Cunninghamella elegans, has three cytochrome P450 reductase genes in its genome: g1631 (CPR_A), g4301 (CPR_B), and g7609 (CPR_C). The nitroreductase activity of the encoded enzymes was investigated via expression of the genes in the yeast Pichia pastoris X33. Whole cell assays with the recombinant yeast demonstrated that the reductases converted the anticancer drug flutamide to the nitroreduced metabolite that was also produced from the same substrate when incubated with human NADPH: cytochrome P450 reductase. The nitroreductase activity extended to other substrates such as the related drug nilutamide and the environmental contaminants 1-nitronaphthalene and 1,3-dinitronaphthalene. Comparative experiments with cell lysates of recombinant yeast were conducted under aerobic and reduced oxygen conditions and demonstrated that the reductases are oxygen sensitive.

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Three cytochrome P450 reductase genes from Cunninghamella elegans were heterologously expressed in Pichia pastoris.

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TThe enzymes displayed nitroreductase activity towards flutamide, which is analogous to human cytochrome P450 reductase.

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The enzymes are oxygen sensitive, which is also a property shared with the human enzyme.

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Other nitro-containing substrates can be reduced by the fungal enzymes.

Potentiation of flutamide-induced hepatotoxicity in mice by Xian-Ling-Gu-Bao through induction of CYP1A2

ETHNOPHARMACOLOGICAL RELEVANCE

Xian-Ling-Gu-Bao (XLGB) Fufang is herbal formula widely used to treat osteoporosis and other bone disorders. Because of its commonality in the clinical use, there is a safety concern over the use of XLGB combined with other androgen deprivation therapy (ADT) drugs such as flutamide (FLU) that is associated with reduced bone density. To date, there have been no evaluations on the side effects of the drug-drug interaction between XLGB and FLU.

AIM OF THE STUDY

The present study was designed to investigate the hepatotoxicity in the context of the combined treatment of XLGB and FLU in a mouse model, and to determine whether the metabolic activation of FLU through induction of CYP1A2 plays a role in the increased hepatoxicity caused by the combination of XLGB and FLU.

MATERIALS AND METHODS

C57 mice were administered with either XLGB (6,160 mg/kg), FLU (300 mg/kg), or with the combination of the two drugs. Animals were treated with XLGB for 5 days before the combined administration of XLGB and FLU for another 4 days. The serum of mice from single or the combined administration groups was collected for biochemical analysis. The mouse liver was collected to examine liver morphological changes, evaluate liver coefficient, as well as determine the mRNA expression of P450 isozymes (Cyp1a2, Cyp3a11 and Cyp2c37). For metabolism analysis, mice were treated with XLGB, FLU, or the combination of XLGB and FLU for 24 h. The urine samples were collected for the analysis of FLU-NAC conjugate by UPLC-Q-Orbitrap MS. The liver microsomes were prepared from fresh livers to determine the activity of metabolizing enzyme CYP1A2.

RESULTS

The combined treatment of XLGB and FLU caused loss of mice body weight and elicited significant liver toxicity as evidenced by an increased liver coefficient and serum lactate dehydrogenase (LDH) activity as well as pathological changes of fatty lesion of liver tissue. FLU increased hepatic expression of Cyp1a2 mRNA that was further elevated in the liver of mice when administered with both FLU and XLGB. Treatment of FLU resulted in an increase in the expression of Cyp3a11 mRNA that was negated when mice were co-treated with FLU and XLGB. No significant difference in Cyp2c37 mRNA expression was observed among the different treatment groups as compared to the control. Analysis of metabolic activity showed that the combined administration caused a synergic effect in elevating the activity of the CYP1A2 enzyme. Mass spectrometry analysis identified the presence of FLU reactive metabolite derived FLU-NAC conjugate in the urine of mice treated with FLU. Strikingly, about a two-fold increase of the FLU-NAC conjugate was detected when treated with both FLU and XLGB, indicating an elevated amount of toxic metabolite produced from FLU in the present of XLGB.

CONCLUSION

FLU and XLGB co-treatment potentiated FLU-induced hepatoxicity. This increased hepatoxicity was mediated through the induction of CYP1A2 activity which in turn enhanced bioactivation of FLU leading to over production of FLU-NAC conjugate and oxidative stress. These results offer warnings about serious side effects of the FLU-XLGB interaction in the clinical practice.

5Flavoenzyme-catalyzed single-electron reduction of nitroaromatic antiandrogens: implications for their cytotoxicity

The therapeutic action of nitroaromatic antiandrogens nilutamide and flutamide may be complicated by their cytotoxicity, whose mechanisms are still incomprehensively understood. In particular this concerns the enzymatic redox cycling of flutamide and its metabolites, and its impact on their cytotoxicity. In this work, we examined the single-electron reduction of nilutamide, flutamide, its metabolites 2-hydroxyflutamide and 4-nitro-3-trifluorormethyl-phenylamine, and a topical antiandrogen (3-amino-2-hydroxy-2-methyl-N-(4-nitro-3-trifluoromethyl)-phenyl) propanamide by NADPH:cytochrome P-450 reductase and adrenodoxin reductase/adrenodoxin. The obtained steady-state bimolecular rate constants of oxidant reduction (k_cat/K_m) enabled to establish single-electron reduction midpoint potentials (E¹₇) of compounds, -0.377 - -0.413 V, which were in line with enthalpies of formation of their free radicals, obtained by quantum mechanical calculations. Using murine hepatoma MH22a cells, the obtained cytotoxicity vs. E¹₇ correlation based on the data of model nitroaromatic compounds shows that redox cycling and oxidative stress could be the main factor of cytotoxicity of nitroaromatic antiandrogens. Other minor cytotoxicity factors could be their redox metabolism involving NAD(P)H:quinone oxidoreductase (NQO1) and cytochromes P-450.

Human Family 1-4 cytochrome P450 enzymes involved in the metabolic activation of xenobiotic and physiological chemicals: an update

This is an overview of the metabolic activation of drugs, natural products, physiological compounds, and general chemicals by the catalytic activity of cytochrome P450 enzymes belonging to Families 1-4. The data were collected from > 5152 references. The total number of data entries of reactions catalyzed by P450s Families 1-4 was 7696 of which 1121 (~ 15%) were defined as bioactivation reactions of different degrees. The data were divided into groups of General Chemicals, Drugs, Natural Products, and Physiological Compounds, presented in tabular form. The metabolism and bioactivation of selected examples of each group are discussed. In most of the cases, the metabolites are directly toxic chemicals reacting with cell macromolecules, but in some cases the metabolites formed are not direct toxicants but participate as substrates in succeeding metabolic reactions (e.g., conjugation reactions), the products of which are final toxicants. We identified a high level of activation for three groups of compounds (General Chemicals, Drugs, and Natural Products) yielding activated metabolites and the generally low participation of Physiological Compounds in bioactivation reactions. In the group of General Chemicals, P450 enzymes 1A1, 1A2, and 1B1 dominate in the formation of activated metabolites. Drugs are mostly activated by the enzyme P450 3A4, and Natural Products by P450s 1A2, 2E1, and 3A4. Physiological Compounds showed no clearly dominant enzyme, but the highest numbers of activations are attributed to P450 1A, 1B1, and 3A enzymes. The results thus show, perhaps not surprisingly, that Physiological Compounds are infrequent substrates in bioactivation reactions catalyzed by P450 enzyme Families 1-4, with the exception of estrogens and arachidonic acid. The results thus provide information on the enzymes that activate specific groups of chemicals to toxic metabolites.

High-Content Screening for the Detection of Drug-Induced Oxidative Stress in Liver Cells

Drug-induced liver injury (DILI) remains a major cause of drug development failure, post-marketing warnings and restriction of use. An improved understanding of the mechanisms underlying DILI is required for better drug design and development. Enhanced reactive oxygen species (ROS) levels may cause a wide spectrum of oxidative damage, which has been described as a major mechanism implicated in DILI. Several cell-based assays have been developed as in vitro tools for early safety risk assessments. Among them, high-content screening technology has been used for the identification of modes of action, the determination of the level of injury and the discovery of predictive biomarkers for the safety assessment of compounds. In this paper, we review the value of in vitro high-content screening studies and evaluate how to assess oxidative stress induced by drugs in hepatic cells, demonstrating the detection of pre-lethal mechanisms of DILI as a powerful tool in human toxicology.

The atypical neuroleptics iloperidone and lurasidone inhibit human cytochrome P450 enzymes in vitro. Evaluation of potential metabolic interactions

Background

The present study aimed at examining the inhibitory effect of two atypical neuroleptics iloperidone and lurasidone on the main human cytochrome P450 (CYP) enzymes in pooled human liver microsomes and cDNA-expressed CYP enzymes (supersomes).

Methods

The activity of these enzymes was determined by the following CYP-specific reactions: caffeine 3-N-demethylation/CYP1A2, diclofenac 4′-hydroxylation/CYP2C9, perazine N-demethylation/CYP2C19, bufuralol 1′-hydroxylation/CYP2D6 and testosterone 6β-hydroxylation/CYP3A4, respectively, using HPLC.

Results

Iloperidone inhibited the activity of CYP3A4 via a noncompetitive mechanism (K_i = 0.38 and 0.3 µM in liver microsomes and supersomes, respectively) and CYP2D6 via a competitive mechanism (K_i = 2.9 and 10 µM in microsomes and supersomes). Moreover, iloperidone attenuated the activity of CYP1A2 (K_i = 45 and 31 µM in microsomes and supersomes) and CYP2C19 via a mixed mechanism (K_i = 6.5 and 32 µM in microsomes and supersomes) but did not affect CYP2C9. Lurasidone moderately inhibited CYP1A2 (K_i = 12.6 and 15.5 µM in microsomes and supersomes), CYP2C9 (K_i = 18 and 3.5 µM in microsomes and supersomes) and CYP2C19 via a mixed mechanism (K_i = 18 and 18.4 µM in microsomes and supersomes), and CYP3A4 via a competitive mechanism (K_i = 29.4 and 9.1 µM in microsomes and supersomes). Moreover, lurasidone competitively, though weakly diminished the CYP2D6 activity (K_i = 37.5 and 85 µM in microsomes and supersomes).

Conclusion

The examined neuroleptics showed inhibitory effects on different CYP enzymes. The obtained results indicate that metabolic/pharmacokinetic interactions with iloperidone (involving mainly CYP3A4 and CYP2D6) and possibly with lurasidone (involving CYP1A2, CYP2C9 or CYP2C19) may occur during combined therapy.

Selenium and Glutathione-Depleted Rats as a Sensitive Animal Model to Predict Drug-Induced Liver Injury in Humans

Drug-induced liver injury (DILI) is one of the most serious and frequent drug-related adverse events in humans. Selenium (Se) and glutathione (GSH) have a crucial role for the hepatoprotective effect against reactive metabolites or oxidative damage leading to DILI. The hepatoprotective capacity related to Se and GSH in rodents is considered to be superior compared to the capacity in humans. Therefore, we hypothesize that Se/GSH-depleted rats could be a sensitive animal model to predict DILI in humans. In this study, Se-deficiency is induced by feeding a Se-deficient diet and GSH-deficiency is induced by l-buthionine-S,R-sulfoxinine treatment via drinking water. The usefulness of this animal model is validated using flutamide, which is known to cause DILI in humans but not in intact rats. In the Se/GSH-depleted rats from the present study, decreases in glutathione peroxidase-1 protein expression and GSH levels and an increase in malondialdehyde levels in the liver are observed without any increase in plasma liver function parameters. Five-day repeated dosing of flutamide at 150 mg/kg causes hepatotoxicity in the Se/GSH-depleted rats but not in normal rats. In conclusion, Se/GSH-depleted rats are the most sensitive for detecting flutamide-induced hepatotoxicity in all the reported animal models.

Long-term and mechanistic evaluation of drug-induced liver injury in Upcyte human hepatocytes

Drug-induced liver injury (DILI) constitutes one of the most frequent reasons of restricted-use warnings as well as withdrawals of drugs in postmarketing and poses an important concern for the pharmaceutical industry. The current hepatic in vivo and in vitro models for DILI detection have shown clear limitations, mainly for studies of long-term hepatotoxicity. For this reason, we here evaluated the potential of using Upcytes human hepatocytes (UHH) for repeated-dose long-term exposure to drugs. The UHH were incubated with 15 toxic and non-toxic compounds for up to 21 days using a repeated-dose approach, and, in addition to conventional examination of effects on viability, the mechanisms implicated in cell toxicity were also assessed by means of high-content screening. The UHH maintained the expression and activity levels of drug-metabolizing enzymes for up to 21 days of culture and became more sensitive to the toxic compounds after extended exposures, showing inter-donor differences which would reflect variability among the population. The assay also allowed to detect the main mechanisms implicated in the toxicity of each drug as well as identifying special susceptibilities depending on the donor. UHH can be used for a long-term repeated detection of DILI at clinically relevant concentrations and also offers key mechanistic features of drug-induced hepatotoxicity. This system is therefore a promising tool in preclinical testing of human relevance that could help to reduce and/or replace animal testing for drug adverse effects.

Cell-based assay using glutathione-depleted HepaRG and HepG2 human liver cells for predicting drug-induced liver injury

Immortalized liver cells have been used for evaluating the toxicity of compounds; however, excessive glutathione is considered to lessen cytotoxicity. In this study, we compared the effects of glutathione depletion on cytotoxicities of drugs using HepaRG and HepG2 cells, which express and lack drug-metabolizing enzymes, respectively, for predicting drug-induced liver injury (DILI) risks. These cells were pre-incubated with L-buthionine-S,R-sulfoximine (BSO) and then exposed to 34 test compounds with various DILI risks for 24 h. ATP level exhibited the highest predictability of DILI among tested parameters. BSO treatment rendered cells susceptible to drug-induced cytotoxicity when evaluated by cell viability and caspase 3/7 activity with the sensitivity of cell viability from 50% in non-treated HepaRG cells to 71% in BSO-treated HepaRG cells. These results indicate that cytotoxicity assays using GSH-depleted HepaRG cells improve the predictability of DILI risks. However, HepaRG cells were not always superior to HepG2 cells when assessed by ATP level. The combination of HepG2 and HepaRG cells index produced the best prediction in the cases of caspase 3/7 acitivity and ATP level. In conclusions, the developed highly sensitive cell-based assay using GSH-reduced cells would be useful for predicting potential DILI risks at an early stage of drug development.

Investigating the application of a nitroreductase-expressing transgenic zebrafish line for high-throughput toxicity testing

Nitroreductase enzymes are responsible for the reduction of nitro functional groups to amino functional groups, and are found in a range of animal models, zebrafish (Danio rerio) excluded. Transgenic zebrafish models have been developed for tissue-specific cell ablation, which use nitroreductase to ablate specific tissues or cell types following exposure to the non-toxic pro-drug metronidazole (MTZ). When metabolized by nitroreductase, MTZ produces a potent cytotoxin, which specifically ablates the tissue in which metabolism occurs. Uses, beyond tissue-specific cell ablation, are possible for the hepatocyte-specific Tg(l-fabp:CFP-NTR)^s891 zebrafish line, including investigations of the role of nitroreductase in the toxicity of nitrated compounds. The hepatic ablation characteristics of this transgenic line were explored, in order to expand its potential uses. Embryos were exposed at 48, 72, or 96 hours post fertilization (hpf) to a range of MTZ concentrations, and the ablation profiles were compared. Ablation occurred at a 10-fold lower concentration than previously reported. Embryos were exposed to a selection of other compounds, with and without MTZ, in order to investigate alternative uses for this transgenic line. Test compounds were selected based on: their ability to undergo nitroreduction, known importance of hepatic metabolism to toxicity, and known pharmaceutical hepatotoxins. Selected compounds included nitrated polycyclic aromatic hydrocarbons (nitro-PAHs), the PAHs retene and benzo[a]pyrene, and the pharmaceuticals acetaminophen and flutamide. The results suggest a range of potential roles of the liver in the toxicity of these compounds, and highlight the additional uses of this transgenic model in toxicity testing.

Customised in vitro model to detect human metabolism-dependent idiosyncratic drug-induced liver injury

Drug-induced liver injury (DILI) has a considerable impact on human health and is a major challenge in drug safety assessments. DILI is a frequent cause of liver injury and a leading reason for post-approval drug regulatory actions. Considerable variations in the expression levels of both cytochrome P450 (CYP) and conjugating enzymes have been described in humans, which could be responsible for increased susceptibility to DILI in some individuals. We herein explored the feasibility of the combined use of HepG2 cells co-transduced with multiple adenoviruses that encode drug-metabolising enzymes, and a high-content screening assay to evaluate metabolism-dependent drug toxicity and to identify metabolic phenotypes with increased susceptibility to DILI. To this end, HepG2 cells with different expression levels of specific drug-metabolism enzymes (CYP1A2, CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, GSTM1 and UGT2B7) were exposed to nine drugs with reported hepatotoxicity. A panel of pre-lethal mechanistic parameters (mitochondrial superoxide production, mitochondrial membrane potential, ROS production, intracellular calcium concentration, apoptotic nuclei) was used. Significant differences were observed according to the level of expression and/or the combination of several drug-metabolism enzymes in the cells created ad hoc according to the enzymes implicated in drug toxicity. Additionally, the main mechanisms implicated in the toxicity of the compounds were also determined showing also differences between the different types of cells employed. This screening tool allowed to mimic the variability in drug metabolism in the population and showed a highly efficient system for predicting human DILI, identifying the metabolic phenotypes associated with increased DILI risk, and indicating the mechanisms implicated in their toxicity.

Role of Cytochrome P450 2C8 in Drug Metabolism and Interactions

During the last 10-15 years, cytochrome P450 (CYP) 2C8 has emerged as an important drug-metabolizing enzyme. CYP2C8 is highly expressed in human liver and is known to metabolize more than 100 drugs. CYP2C8 substrate drugs include amodiaquine, cerivastatin, dasabuvir, enzalutamide, imatinib, loperamide, montelukast, paclitaxel, pioglitazone, repaglinide, and rosiglitazone, and the number is increasing. Similarly, many drugs have been identified as CYP2C8 inhibitors or inducers. In vivo, already a small dose of gemfibrozil, i.e., 10% of its therapeutic dose, is a strong, irreversible inhibitor of CYP2C8. Interestingly, recent findings indicate that the acyl-β-glucuronides of gemfibrozil and clopidogrel cause metabolism-dependent inactivation of CYP2C8, leading to a strong potential for drug interactions. Also several other glucuronide metabolites interact with CYP2C8 as substrates or inhibitors, suggesting that an interplay between CYP2C8 and glucuronides is common. Lack of fully selective and safe probe substrates, inhibitors, and inducers challenges execution and interpretation of drug-drug interaction studies in humans. Apart from drug-drug interactions, some CYP2C8 genetic variants are associated with altered CYP2C8 activity and exhibit significant interethnic frequency differences. Herein, we review the current knowledge on substrates, inhibitors, inducers, and pharmacogenetics of CYP2C8, as well as its role in clinically relevant drug interactions. In addition, implications for selection of CYP2C8 marker and perpetrator drugs to investigate CYP2C8-mediated drug metabolism and interactions in preclinical and clinical studies are discussed.

The antiandrogen flutamide is a novel aryl hydrocarbon receptor ligand that disrupts bile acid homeostasis in mice through induction of Abcc4

Flutamide (FLU), an oral, nonsteroidal antiandrogen drug used in the treatment of prostate cancer, is associated with idiosyncratic hepatotoxicity that sometimes causes severe liver damage, including cholestasis, jaundice, and liver necrosis. To understand the mechanism of toxicity, a combination of aryl hydrocarbon receptor (Ahr)-deficient (Ahr^-/-) mice, primary hepatocytes, luciferase reporter gene assays, in silico ligand docking and ultra-performance chromatography-quadrupole time-of-flight mass spectrometry-based metabolomics was used. A significant increase of liver weights, and liver and serum bile acid levels was observed after FLU treatment, indicating hepatomegaly and disrupted bile acid homeostasis. Expression of the AhR gene battery was markedly increased in livers of wild-type mice Ahr^+/+ treated with FLU, while no change was noted in Ahr^-/- mice. Messenger RNAs encoded by AhR target genes were induced in primary mouse hepatocytes cultured with FLU, which confirmed the in vivo results. Ligand-docking analysis further predicted that FLU is an AhR agonist ligand which was confirmed by luciferase reporter gene assays. Multivariate data analysis showed that bile acids were responsible for the separation of vehicle- and FLU-treated Ahr^+/+ mice, while there was no separation in Ahr^-/- mice. Expression of mRNA encoding the bile acid transporter ABCC4 was increased and farnesoid X receptor signaling was inhibited in the livers of Ahr^+/+ mice, but not in Ahr^-/- mice treated with FLU, in agreement with the observed downstream metabolic alterations. These findings provide new insights into the mechanism of liver injury caused by FLU treatment involving activation of AhR and the alterations of bile acid homeostasis, which could guide clinical application.

High-throughput and combinatorial gene expression on a chip for metabolism-induced toxicology screening

Differential expression of various drug-metabolizing enzymes (DMEs) in the human liver may cause deviations of pharmacokinetic profiles, resulting in interindividual variability of drug toxicity and/or efficacy. Here, we present the 'Transfected Enzyme and Metabolism Chip' (TeamChip), which predicts potential metabolism-induced drug or drug-candidate toxicity. The TeamChip is prepared by delivering genes into miniaturized three-dimensional cellular microarrays on a micropillar chip using recombinant adenoviruses in a complementary microwell chip. The device enables users to manipulate the expression of individual and multiple human metabolizing-enzyme genes (such as CYP3A4, CYP2D6, CYP2C9, CYP1A2, CYP2E1 and UGT1A4) in THLE-2 cell microarrays. To identify specific enzymes involved in drug detoxification, we created 84 combinations of metabolic-gene expressions in a combinatorial fashion on a single microarray. Thus, the TeamChip platform can provide critical information necessary for evaluating metabolism-induced toxicity in a high-throughput manner.

Oxidative stress/reactive metabolite gene expression signature in rat liver detects idiosyncratic hepatotoxicants

Previously we reported a gene expression signature in rat liver for detecting a specific type of oxidative stress (OS) related to reactive metabolites (RM). High doses of the drugs disulfiram, ethinyl estradiol and nimesulide were used with another dozen paradigm OS/RM compounds, and three other drugs flutamide, phenacetin and sulindac were identified by this signature. In a second study, antiepileptic drugs were compared for covalent binding and their effects on OS/RM; felbamate, carbamazepine, and phenobarbital produced robust OS/RM gene expression. In the present study, liver RNA samples from drug-treated rats from more recent experiments were examined for statistical fit to the OS/RM signature. Of all 97 drugs examined, in addition to the nine drugs noted above, 19 more were identified as OS/RM-producing compounds-chlorpromazine, clozapine, cyproterone acetate, dantrolene, dipyridamole, glibenclamide, isoniazid, ketoconazole, methapyrilene, naltrexone, nifedipine, sulfamethoxazole, tamoxifen, coumarin, ritonavir, amitriptyline, valproic acid, enalapril, and chloramphenicol. Importantly, all of the OS/RM drugs listed above have been linked to idiosyncratic hepatotoxicity, excepting chloramphenicol, which does not have a package label for hepatotoxicity, but does have a black box warning for idiosyncratic bone marrow suppression. Most of these drugs are not acutely toxic in the rat. The OS/RM signature should be useful to avoid idiosyncratic hepatotoxicity of drug candidates.

In vivo and in vitro metabolism of a novel β2-adrenoceptor agonist, trantinterol: metabolites isolation and identification by LC-MS/MS and NMR

Trantinterol is a novel β(2)-adrenoceptor agonist used for the treatment of asthma. The aim of this study is to identify the metabolites of trantinterol using liquid chromatography tandem mass spectrometry (LC-MS/MS), to isolate the main metabolites, and confirm their structures by nuclear magnetic resonance (NMR). Urine, feces, bile, and blood samples of rats were obtained and analyzed. Reference standards of six metabolites were achieved with the combination of chemical synthesis, microbial transformation, and the model systems of rats. Moreover, in order to investigate the phase I metabolism of trantinterol in humans and to study the species differences between rats and humans, incubations with liver microsomes were performed. The biotransformation by a microbial model Cunninghamella blakesleana AS 3.970 was also studied. A total of 18 metabolites were identified in vivo and in vitro together, 13 of which were newly detected. Three phase I metabolites were detected in vivo and in vitro as well as in the microbial model, including the arylhydroxylamine (M1), the tert-butyl hydroxylated trantinterol (M2) and the 1-carbonyltrantinterol (M3). Another important pathway in rats is glutathione conjugation and further catabolism and oxidation to form consecutive derivatives (M4 through M10). Other metabolites include glucuronide, glucoside, and sulfate conjugates. The results of in vitro experiments indicate no species difference exists among rats, humans, and C. blakesleana AS 3.970 on the phase I metabolism of trantinterol. Our study provided the most comprehensive picture for trantinterol in vivo and in vitro metabolism to this day, and may predict its metabolism in humans.

Cytochrome P450 enzymes in drug metabolism: regulation of gene expression, enzyme activities, and impact of genetic variation

Cytochromes P450 (CYP) are a major source of variability in drug pharmacokinetics and response. Of 57 putatively functional human CYPs only about a dozen enzymes, belonging to the CYP1, 2, and 3 families, are responsible for the biotransformation of most foreign substances including 70-80% of all drugs in clinical use. The highest expressed forms in liver are CYPs 3A4, 2C9, 2C8, 2E1, and 1A2, while 2A6, 2D6, 2B6, 2C19 and 3A5 are less abundant and CYPs 2J2, 1A1, and 1B1 are mainly expressed extrahepatically. Expression of each CYP is influenced by a unique combination of mechanisms and factors including genetic polymorphisms, induction by xenobiotics, regulation by cytokines, hormones and during disease states, as well as sex, age, and others. Multiallelic genetic polymorphisms, which strongly depend on ethnicity, play a major role for the function of CYPs 2D6, 2C19, 2C9, 2B6, 3A5 and 2A6, and lead to distinct pharmacogenetic phenotypes termed as poor, intermediate, extensive, and ultrarapid metabolizers. For these CYPs, the evidence for clinical significance regarding adverse drug reactions (ADRs), drug efficacy and dose requirement is rapidly growing. Polymorphisms in CYPs 1A1, 1A2, 2C8, 2E1, 2J2, and 3A4 are generally less predictive, but new data on CYP3A4 show that predictive variants exist and that additional variants in regulatory genes or in NADPH:cytochrome P450 oxidoreductase (POR) can have an influence. Here we review the recent progress on drug metabolism activity profiles, interindividual variability and regulation of expression, and the functional and clinical impact of genetic variation in drug metabolizing P450s.

Metabolomics-on-a-chip of hepatotoxicity induced by anticancer drug flutamide and Its active metabolite hydroxyflutamide using HepG2/C3a microfluidic biochips

We used the recently introduced "metabolomics-on-a-chip" approach to test secondary drug toxicity in bioartificial organs. Bioartificial organs cultivated in microfluidic culture conditions provide a beneficial environment, in which the cellular cytoprotective mechanisms are enhanced, compared with Petri dish culture conditions. We investigated the metabolic response of HepG2/C3a cells exposed to flutamide, an anticancer prodrug, and hydroxyflutamide (HF), its active metabolite, in a microfluidic biochip. The cellular response was analyzed by (1)H nuclear magnetic resonance spectroscopy to identify cell-specific molecule-response markers. The metabolic response to flutamide results in a disruption of glucose homeostasis and in mitochondrial dysfunctions. This flutamide-specific metabolic response was illustrated by a reduction of the extracellular glucose and fructose consumptions and a general reduction of the tricarboxylic acid cycle activity leading to the reduction of the consumption of several amino acids. We also found a higher production of 3-hydroxybutyrate and lactate, and the reduction of the albumin production compared with controls. The toxic metabolic signature associated with the active metabolite HF was illustrated by a high-energy demand and an increase in several amino acid metabolism. Finally, for both molecules, the hepatotoxicity was correlated to the glutathione (GSH) metabolism illustrated by the levels of the 2-hydroxybutyrate and pyroglutamate productions and the increase of the glutamate and glycine productions. Thus, the entire set of results contributed to extract specific mechanistic toxic signatures and their relation to hepatotoxicity, which appeared consistent with literature reports. As new finding of HepG2/C3a cells hepatotoxicity, we propose a metabolic network with a related list of metabolite variations to describe the GSH depletion when followed by a cell death for the HepG2/C3a cells cultivated in our polydimethylsiloxane microfluidic biochips. Our findings illustrate the potential of metabolomics-on-a-chip as an in vitro alternative method for predictive toxicology.

Metabolism of gambogic acid in rats: a rare intestinal metabolic pathway responsible for its final disposition

Gambogic acid (GA) is a promising natural anticancer candidate. Although the anticancer activity of GA has been well demonstrated, information regarding the metabolic fate of GA is limited. Previous studies suggested that GA is mainly excreted into intestinal tract in rats through bile after intravenous administration, whereas only traces appeared in the feces, suggesting that GA is metabolized extensively in the intestine. However, there has been no report about the intestinal metabolism of GA either in animals or humans. In this study, large amounts of two sulfonic acid metabolites of GA were found in the feces samples of rats after intravenous administration, and their structures were identified as 10-α sulfonic acid GA and 10-β sulfonic acid GA by comparison of the retention times and spectral data with those of synthesized reference substances using liquid chromatography-diode array detector-tandem mass spectrometry. This rare intestinal metabolic pathway mainly involves Michael addition of the sulfite ion to the 9,10 carbon-carbon double bond of α,β-unsaturated ketone. In addition, a more detailed metabolic profile in rats is proposed, according to the results of in vitro and in vivo studies. It was found that GA can be metabolized by a variety of routes, including monooxidation, hydration, glutathionylation, glucuronidation, and glucosidation in the liver of rats. These findings provide information on the major metabolic soft spot of GA in the intestine and liver of rats, which is not only useful in the future human metabolic study of this compound but also of value in the metabolic studies of GA analogs.

Embryonic expression of the cysteine rich protein 61 (CYR61) gene: A candidate for the development of human epispadias

BACKGROUND

Isolated epispadias is the mildest phenotype of the exstrophy-epispadias complex, a urogenital birth defect of variable severity. The androgen receptor antagonist flutamide (FLU) is known to cause malformations in the rat genital and reproductive tract, and single-dose prenatal FLU exposure can induce epispadias in rat offspring. The Cyr61 gene exhibited the highest response to FLU in rat fetal testis, and we suggested it a promising candidate gene for epispadias in humans, because its protein product promotes proliferation, migration, and adhesion of endothelial cells and fibroblasts.

METHODS

We used whole mount in situ analysis in mice to investigate ventrocaudal expression of the Cyr61 transcript at gestational days 9.5 to 11.5, which is the equivalent of human gestational weeks 4 to 6 (postulated time of epispadias organogenesis in humans). We also performed mutational analysis of the CYR61 gene in 11 patients with isolated epispadias and in additional eight patients with the related classic bladder exstrophy phenotype.

RESULTS

Expression of Cyr61 was detected in endothelial cells of vessels surrounding the cloaca and the umbilical cord on gestational days 10 and 11.5. The mutation screening, however, revealed no alterations in the coding region of human CYR61.

CONCLUSIONS

The spatiotemporal expression pattern observed suggests a role for Cyr61 in the development of the external genitalia. Our mutation screening study, however, could not confirm that mutations affecting the CYR61 gene are a frequent cause of epispadias or classic bladder exstrophy, although rare mutations might be detectable in larger patient samples.

Production of human metabolites of the anti-cancer drug flutamide via biotransformation in Cunninghamella species

Fungi belonging to the genus Cunninghamella have enzymes similar to those employed by mammals for the detoxification of xenobiotics, thus they are useful as models of mammalian drug metabolism, and as a source for drug metabolites. We report the transformation of the anti-cancer drug flutamide in Cunninghamella sp. The most predominant phase I metabolites present in the plasma of humans, 2-hydroxyflutamide and 4-nitro-3-(trifluoromethyl)aniline, were also produced in Cunninghamella cultures. Other phase I and phase II metabolites were also detected using a combination of HPLC, GC-MS and (19)F-NMR.

Pharmacogenetics of drug-induced liver injury

Recent progress in research on drug-induced liver injury (DILI) has been determined by key developments in two areas. First, new technologies allow the identification of genetic risk factors with improved sensitivity, specificity, and efficiency. Second, new mechanistic concepts of DILI emphasize the importance of unspecific "downstream" events following drug-specific initial "upstream" hepatocyte injury and of complex interactions between environmental and genetic risk factors. The integration of genetic and mechanistic concepts is essential for current research approaches, and genetic studies of DILI now focus on targets that affect the function and transcriptional regulation of genes relating not only to drug metabolism but also to human leukocyte antigens (HLAs), cytokines, oxidative stress, and hepatobiliary transporters. Risk factors affecting unspecific downstream mechanisms may be identified using pooled DILI cases caused by various drugs. The power to detect variants that confer a low risk can be increased by recruitment of strictly selected cases through large networks, whereas controls may also be obtained from genotyped reference populations. The first genomewide studies of DILI identified HLA variants as risk factors for hepatotoxicity associated with flucloxacillin and ximelagatran, and their design has defined a new standard for pharmacogenetic studies. From a clinical and regulatory point of view, there is a need for genetic tests that identify patients at increased hepatotoxic risk. However, DILI is a rare complex disease, and pharmacogenetic studies have so far not been able to identify interactions of several risk factors defining a high population-attributable risk and clinically relevant absolute risk for DILI.

Screening strategy for the rapid detection of in vitro generated glutathione conjugates using high-performance liquid chromatography and low-resolution mass spectrometry in combination with LightSight software for data processing

The knowledge of drug metabolism in the early phases of the drug discovery process is vital for minimising compound failure at later stages. As chemically reactive metabolites may cause adverse drug reactions, it is generally accepted that avoiding formation of reactive metabolites increases the chances of success of a molecule. In order to generate this important information, a screening strategy for the rapid detection of in vitro generated reactive metabolites trapped by glutathione has been developed. The bioassay incorporated the use of native glutathione and its close analogue the glutathione ethyl ester. The generic conditions for detecting glutathione conjugates that undergo constant neutral loss of 129 Da were optimised using a glutathione-based test mix of four compounds. The final liquid chromatography/tandem mass spectrometry constant neutral loss method used low-resolution settings and a scanning window of 200 amu. Data mining was rapidly and efficiently performed using LightSight software. Unambiguous identification of the glutathione conjugates was significantly facilitated by the analytical characteristics of the conjugate pairs formed with glutathione and glutathione ethyl ester, i.e. by chromatographic retention time and mass differences. The reliability and robustness of the screening strategy was tested using a number of compounds known to form reactive metabolites. Overall, the developed screening strategy provided comprehensive and reliable identification of glutathione conjugates and is well suited for rapid routine detection of trapped reactive metabolites. This new approach allowed the identification of a previously unreported diclofenac glutathione conjugate.

Comparison of in vitro bioactivation of flutamide and its cyano analogue: evidence for reductive activation by human NADPH:cytochrome P450 reductase

Flutamide (FLU), a nonsteroidal antiandrogen drug widely used in the treatment of prostate cancer, has been associated with idiosyncratic hepatotoxicity in patients. It is proposed that bioactivation of FLU and subsequent binding of reactive metabolite(s) to cellular proteins play a causative role. A toxicogenomic study comparing FLU and its nitro to cyano analogue (CYA) showed that the nitroaromatic group of FLU enhanced cytotoxicity to hepatocytes, indicating that reduction of the nitroaromatic group may represent a potential route of FLU-induced hepatotoxicity [Coe et al. (2007) Chem. Res. Toxicol. 20, 1277-1290]. In the current study, we compared in vitro bioactivation of FLU and CYA in human liver microsomes and cryopreserved human hepatocytes. A nitroreduction metabolite FLU-6 was formed in liver microsomal incubations of FLU under atmospheric oxygen levels and, to a greater extent, under anaerobic conditions. Seven glutathione (GSH) adducts of FLU, FLU-G1-7, were tentatively identified in human liver microsomal incubations using liquid chromatography-tandem mass spectrometry (LC/ MS/MS), while CYA formed only four corresponding GSH adducts, CYA-G1-4, under the same conditions. Of particular interest was the formation of FLU-G5-7 from FLU, where the nitroaromatic group of FLU was reduced to an amino group. A tentative pathway is that upon nitroreduction, the para-diamines undergo cytochrome P450 (P450)-catalyzed two-electron oxidations to form corresponding para-diimine intermediates that react with GSH to form GSH adducts FLU-G5-7, respectively. The identities of FLU-G5-7 were further confirmed by LC/MS/MS analyses of microsomal incubations of a synthesized standard FLU-6. In an attempt to identify enzymes involved in the nitroreduction of FLU, NADPH:cytochrome P450 reductase (CPR) was shown to reduce FLU to FLU-6 under both aerobic and anaerobic conditions. Furthermore, the formation of FLU-G5-7 was completely blocked by the addition of a reversible CPR inhibitor, alpha-lipoic acid, to the incubations of FLU under aerobic conditions. In summary, these results clearly demonstrate that nitroreduction of FLU by CPR contributes to bioactivation and potentially to hepatotoxicity of FLU.