Metabolic activation and cytochrome P450 inhibition of piperlonguminine mediated by CYP3A4

Piperlonguminine (PLG) is a major alkaloid found in Piper longum fruits. It has been shown to possess a variety of biological activities, including anti-tumor, anti-hyperlipidemic, anti-renal fibrosis and anti-inflammatory properties. Previous studies have reported that PLG inhibits various CYP450 enzymes. The main objective of this study was to identify reactive metabolites of PLG in vitro and assess its ability to inhibit CYP450. In rat and human liver microsomal incubation systems exposed to PLG, two oxidized metabolites (M1 and M2) were detected. Additionally, in microsomes where N-acetylcysteine was used as a trapping agent, N-acetylcysteine conjugates (M3, M4, M5 and M6) of four isomeric O-quinone-derived reactive metabolites were found. The formation of metabolites was dependent on NADPH. Inhibition and recombinant CYP450 enzyme incubation experiments showed that CYP3A4 was the primary enzyme responsible for the metabolic activation of PLG. This study characterized the O-dealkylated metabolite (M1) through chemical synthesis. The IC50 shift assay showed time-dependent inhibition of CYP3A4, 2C9, 2E1, 2C8 and 2D6 by PLG. This research contributes to the understanding of PLG-induced enzyme inhibition and bioactivation.

Integrating network pharmacology and drug side-effect data to explore mechanism of liver injury-induced by tyrosine kinase inhibitors

Tyrosine kinase inhibitors (TKIs) are highly efficient small-molecule anticancer drugs. Despite the specificity and efficacy of TKIs, they can produce off-target effects, leading to severe liver toxicity, and even some of them are labeled as black box hepatotoxicity. Thus, we focused on representative TKIs associated with severe hepatic adverse events, namely lapatinib, pazopanib, regorafenib, and sunitinib as objections of study, then integrated drug side-effect data from United State Food and Drug Administration (U.S. FDA) and network pharmacology to elucidate mechanism underlying TKI-induced liver injury. Based on network pharmacology, we constructed a specific comorbidity module of high risk of serious adverse effects and created drug-disease networks. Enrichment analysis of the networks revealed the depletion of all-trans-retinoic acid and the involvement of down-regulation of the HSP70 family-mediated endoplasmic reticulum (ER) stress as key factors in TKI-induced liver injury. These results were further verified by transcription data. Based on the target prediction results of drugs and reactive metabolites, we also shed light on the association between toxic metabolites and severe hepatic adverse reactions, and thinking HSPA8, HSPA1A, CYP1A1, CYP1A2 and CYP3A4 were potential therapeutic or preventive targets against TKI-induced liver injury. In conclusion, our research provides comprehensive insights into the mechanism underlying severe liver injury caused by TKIs, offering a better understanding of how to enhance patient safety and treatment efficacy.

Electrochemistry/mass spectrometry (EC/MS) for fast generation and identification of novel reactive metabolites of two unsymmetrical bisacridines with anticancer activity

The development of a new drug requires knowledge about its metabolic fate in a living organism, regarding the comprehensive assessment of both drug therapeutic activity and toxicity profiles. Electrochemistry (EC) coupled with mass spectrometry (MS) is an efficient tool for predicting the phase I metabolism of redox-sensitive drugs. In particular, EC/MS represents a clear advantage for the generation of reactive drug transformation products and their direct identification compared to biological matrices. In this work, we focused on the characterization of novel electrochemical products of two representative unsymmetrical bisacridines (C-2028 and C-2045) with demonstrated high anticancer activity. The electrochemical thin-layer flow-through cell μ-PrepCell 2.0 (Antec Scientific) was used here for the effective metabolite electrosynthesis. The electrochemical simulation of C-2028 reductive and C-2045 oxidative metabolism resulted in the generation of new products that were not observed before. The formation of nitroso [M-O+H]+ and azoxy [2M-3O+H]+ species from C-2028, as well as a series of hydroxylated and/or dehydrogenated products, including possible quinones [M-2H+H]+ and [M+O-2H+H]+ from C-2045, was demonstrated. For the latter, a glutathione S-conjugate (m/z 935.3130) was also obtained in measurements supplemented with the excess of reduced glutathione. For the identification of the products of interest, structural confirmation based on MS/MS fragmentation experiments was performed. Novel products of electrochemical conversions of unsymmetrical bisacridines were discussed in the context of their possible biological effect on the human organism.

Chemical metabolite synthesis and profiling: Mimicking in vivo biotransformation reactions

Biotransformation was previously viewed as merely the structural characterization of drug metabolites, and it was performed only when drug candidates entered clinical development. The synthesis of drug metabolites is crucial to the drug development process because it generates either pharmacologically active, inactive, or reactive molecules and hence their characterization and comprehensive pharmacological evaluation is necessary. The chemical metabolite synthesis is very challenging due to the complex structures of many drug molecules, presence of multiple stereocenters, poor reaction yields, and the formation of unwanted by-products. Drug metabolites and their chemical synthesis have immense significance in the drug discovery process. The chemical synthesis of metabolites facilitates on- or off-target pharmacological and toxicological evaluations at the easiest. In a broader view metabolite could be a target lead molecule for drug design, toxic reactive metabolites, pharmaceutical standards for bioanalytical methods, etc. Collectively these metabolite information dossiers will aid regulatory agencies such as the EMA and FDA in maintaining strict vigilance over drug manufacturers with regard to the safety of NCE's and their hidden metabolites. Herein, we are presenting a systematic compilation of chemical and biocatalytic strategies reported to date for pharmaceutical drug metabolite synthesis. This review report is very useful for the laboratory synthesis of new drug metabolites, and their preclinical biological evaluation could aid in the detection of early threats (alerts) in drug discovery, eliminate the toxicity profile, explore newer pharmacology, and delivering a promising and safe drug candidate to humankind.

Preclinical evaluation of chemically reactive metabolites and mitigation of bioactivation in drug discovery

The formation of reactive metabolites (RMs) is thought to be one of the pathogeneses for some idiosyncratic adverse drug reactions (IADRs) which are considered one of the leading causes of some drug attritions and/or recalls. Minimizing or eliminating the formation of RMs via chemical modification is a useful tactic to reduce the risk of IADRs and time-dependent inhibition (TDI) of cytochrome P450 enzymes (CYPs). The RMs should be carefully handled before making a go-no-go decision. Herein, we highlight the role of RMs in the occurrence of IADRs and CYP TDI, the risk of structural alerts, the approaches of RM assessment at the discovery stage and strategies to minimize or eliminate RM liability. Finally, some considerations for developing a RM-positive drug candidate are suggested.

The endoplasmic reticulum participated in drug metabolic toxicity

Covalent binding of reactive metabolites formed by drug metabolic activation with biological macromolecules is considered to be an important mechanism of drug metabolic toxicity. Recent studies indicate that the endoplasmic reticulum (ER) could play an important role in drug toxicity by participating in the metabolic activation of drugs and could be a primarily attacked target by reactive metabolites. In this article, we summarize the generation and mechanism of reactive metabolites in ER stress and their associated cell death and inflammatory cascade, as well as the systematic modulation of unfolded protein response (UPR)-mediated adaptive pathways.

Drug-drug interactions induced by Linderane based on mechanism-based inactivation of CYP2C9 and the molecular mechanisms

Linderane (LDR) is a main furan-containing sesquiterpenoid of the common herbal medicine Lindera aggregata (Sims) Kosterm. Our early study indicated that LDR led to mechanism-based inactivation (MBI) of CYP2C9 in vitro, implying possible drug-drug interactions (DDIs) in clinic. In the present study, influence of LDR on the pharmacokinetics of the corresponding hydroxylated metabolites of CYP2C9 substrates in rats was investigated. Pharmacokinetic studies revealed that pretreatment with LDR at 20 mg/kg for 15 days inhibited the metabolism of both tolbutamide and warfarin catalyzed by CYP2C9. As for 4-hydroxytolbutamide, the C_max was decreased, the t_1/2z was prolonged, and the Vz/F was increased, all with significant difference. As for 7-hydroxywarfarin, the AUC_0-t/AUC_0-∞ and CLz/F were significantly decreased and increased, respectively. Furthermore, the underlying molecular mechanisms based on MBI of CYP2C9 by LDR were revealed. Two reactive metabolites of LDR, furanoepoxide and γ-ketoenal intermediates were identified in CYP2C9 recombinant enzyme incubation systems. Correspondingly, covalent modifications of lysine and cysteine residues of CYP2C9 protein were discovered in the CYP2C9 incubation system treated with LDR. The formation of protein adducts exhibited obvious time- and dose-dependence, which is consistent with the trend of enzyme inhibition caused by LDR in vitro. In addition to the apoprotein of CYP2C9, the heme content was significantly reduced after co-incubation with LDR. These data revealed that modification of both apoprotein and heme of CYP2C9 by reactive metabolites of LDR led to MBI of CYP2C9, therefore resulting in the inhibition of biotransformation of CYP2C9 substrates to their corresponding metabolites in vivo.

Bio-activation of simeprevir in liver microsomes and characterization of its glutathione conjugates by liquid chromatography coupled to ultrahigh-resolution quadrupole time-of-flight mass spectrometry

Liquid chromatography coupled to a triple quadrupole and, alternatively, to an ultrahigh-resolution quadrupole time-of-flight (UHR-QqTOF) mass spectrometers was used to collect qualitative and quantitative information from incubations of the anti-hepatitis C drug simeprevir with human and rat liver microsomes, respectively, supplemented with NADPH and glutathione. For this, different chromatographic methods using two different chromatographic columns, Kinetex® 2.6 µm C₁₈ (50 × 3 mm) and Atlantis T3 (100 Å, 3 µm, 4.6 mm × 150 mm), have been employed. For determination and structural characterization of the reactive metabolites, we used information obtained from high-resolution mass spectrometry, namely accurate mass data to calculate the elemental composition, accurate MS/MS fragmentation patterns for confirmation of structural proposals, and the high mass spectral resolution to eliminate false-positive peaks. In this study, the use of high-resolution mass spectrometry (HR-MS) enabled the identification of 19 simeprevir metabolites generated by O- respectively N-demethylation, oxidation, dehydrogenation, hydrolysis, and formation of glutathione conjugates. The in silico study provides insights into the sites of simeprevir most amenable to reactions involving cytochrome P₄₅₀. The developed methods have been successfully applied to analyze simeprevir and its metabolites simultaneously; based on this data, potential metabolic pathways of simeprevir are discussed. In general, the obtained results demonstrate that simeprevir is susceptible to form reactive simeprevir-glutathione adducts and cyclopropansulfonamide, which may explain the implication of simeprevir in idiosyncratic adverse drug reactions (IADRs) or hepatotoxicity.

Discovery and SAR studies of 3-amino-4-(phenylsulfonyl)tetrahydrothiophene 1,1-dioxides as non-electrophilic antioxidant response element (ARE) activators

The transcription factor NRF2 controls resistance to oxidative insult and is thus a key therapeutic target for treating a number of disease states associated with oxidative stress and aging. We previously reported CBR-470-1, a bis-sulfone which activates NRF2 by increasing the levels of methylglyoxal, a metabolite that covalently modifies NRF2 repressor KEAP1. Here, we report the design, synthesis, and structure activity relationship of a series of bis-sulfones derived from this unexplored chemical template. We identify analogs with sub-micromolar potencies, 7f and 7g, as well as establish that efficacious NRF2 activation can be achieved by non-toxic analogs 7c, 7e, and 9, a key limitation with CBR-470-1. Further efforts to identify non-covalent NRF2 activators of this kind will likely provide new insight into revealing the role of central metabolism in cellular signaling.

Synthesis and pharmacological evaluation of 11-(1,6-dimethyl-1,2,3,6-tetrahydropyridin-4-yl)-5H-dibenzo[b,e][1,4]diazepines with clozapine-like receptor occupancy at dopamine D1/D2 receptor

Clozapine-like compound without agranulocytosis risk is need to cure the treatment resistant schizophrenia (TRS). We discovered (S)-3 as Clozapine-like dopamine D₂/D₁ receptor selectivity and improved reactive metabolites formation profile by the modification of piperazine moiety in Clozapine. The optimization of (S)-3 gave compound 5 to be best compound (approximately 10-fold stronger affinity for D₂/D₁ receptor and similar D₂/D₁ selectivity ratio with Clozapine). Clozapine-like D₂/D₁ receptor occupancy profile was proved by in vivo evaluation. In addition, the reactive metabolites derived agranulocytosis risk of compound 5 was considered to be lower than Clozapine. The pharmacology detail of compound 5 is being investigated to develop it for TRS treatment.

Evaluation of covalent binding of flutamide and its risk assessment using 19F-NMR

The formation of reactive metabolites (RMs) is a problem in drug development that sometimes results in severe hepatotoxicity. As detecting RMs themselves is difficult, a covalent binding assay using expensive radiolabelled tracers is usually performed for candidate selection. This study aimed to provide a practical approach toward the risk assessment of hepatotoxicity induced by covalent binding before candidate selection. We focused on flutamide because it contains a trifluoromethyl group that shows a strong singlet peak by ¹⁹F nuclear magnetic resonance (NMR) spectrometry. The covalent binding of flutamide was evaluated using quantitative NMR and its risk for hepatotoxicity was assessed by estimating the RM burden, an index that reflects the body burden associated with RM exposure by determining the extent of covalent binding, clinical dose and in vivo clearance. The extent of covalent binding and RM burden was 296 pmol/mg/h and 37.9 mg/day, respectively. Flutamide was categorised as high risk with an RM burden >10 mg/day consistent with its clinical hepatotoxicity. These results indicate that a combination of covalent binding assay using ¹⁹F-NMR and RM burden is useful for the risk assessment of RMs without using radiolabelled compounds.

Roles of diclofenac and its metabolites in immune activation associated with acute hepatotoxicity in TgCYP3A4/hPXR-humanized mice

Diclofenac (DCF) is a widely used nonsteroidal anti-inflammatory drug, but it comes with a high risk of drug-induced liver injury (DILI). Despite the quinone-imine adduct pathways, the immunotoxicity is recently considered as another factor for DILI. However, such immune responses are still elusive. In the present study, investigation of the immune response in the acute hepatotoxicity model of TgCYP3A4/hPXR-humanized mice was conducted by administration of DCF and DCF metabolites, respectively. In a single dose intraperitoneal injection of 80 mg/kg DCF, the pharmacokinetic results showed the major DCF metabolites, including 4'-hydroxy-diclofenac (4'-OH-DCF), 5-hydroxy-diclofenac (5-OH-DCF) and diclofenac glucuronide (DCF-G) were generated after DCF treatment. Not only DCF, but those DCF metabolites could also directly cause different degrees of acute liver injury as significantly increased the serum ALT levels in a short time period in the TgCYP3A4/hPXR-humanized mice. Furthermore, the three DCF metabolites could directly stimulate the significant elevation of serum immune-related factors in varying degrees. Transcriptome analysis revealed the differentially expressed genes in the liver of DCF-G treated mice were mostly involved with the "immune system process" and "cell death" and related to "IL-17 signaling pathway" and "TNF-α signaling pathway", but 5-OH-DCF had little effect on the expressions of those genes. These results indicate that the metabolite DCF-G plays an important role in the activation of the hepatic immune system, which might be involved in the pathogenesis of DCF-induced acute liver injury.

Molecular mechanisms and systemic targeting of NRF2 dysregulation in cancer

NF-E2-related factor 2 (NRF2) is a master regulator of redox homeostasis and provides cellular protection against oxidants and electrophiles by inducing the expression of a wide array of phase II cytoprotective genes. Until now, a number of NRF2 activators have been developed for treatment of chronic diseases and some are under evaluation in the clinical studies. On the other hand, accumulating evidence indicates that NRF2 confers chemoresistance and radioresistance, and its expression is correlated with poor prognosis in cancer patients. Studies in the last decade demonstrate that diverse mechanisms such as somatic mutations, accumulation of KEAP1 binding proteins, transcriptional dysregulation, oncogene activation, and accumulation of reactive metabolites contribute to NRF2 activation in cancer. In the present review, we illustrate the molecular mechanisms governing the function of NRF2 and explain how they are hijacked in cancer. We also provide some examples of NRF2 inhibitors together with a brief explanation of their mechanisms of action.

Characterization of covalent protein modification by triclosan in vivo and in vitro via three-dimensional liquid chromatography-mass spectrometry: New insight into its adverse effects

Triclosan (TCS), an antimicrobial agent widely used in personal care products and ubiquitously exists in environment, has drawn increasing concern due to its potential to exert multiple adverse effects, ranging from endocrine disruption to carcinogenesis. However, the mechanism of these adverse effects is still not fully elucidated. More and more studies have shown that chemical reactive metabolites (RMs) covalently binding to proteins is a possible reason for these adverse effects, but there is still a lack of appropriate methods to predict or evaluate these adverse effects due to the extremely low abundance of the modified proteins in complex biological samples. In this study, we attempted to address this problem and investigate the possible mechanism of TCS adverse effects by a shotgun proteomics approach based on three-dimensional-liquid chromatography-mass spectrometry (3D-LC-MS). First, the in vitro incubation with model amino acids and protein in microsomes showed that TCS could react with cysteine residue of proteins through 3 types of RMs. Then, a 3D-LC-MS approach was developed to sensitively determine the low abundant modified proteins, which resulted in the identification of 45 TCS-modified proteins, including albumin, haptoglobin and NR1I2, in rats. STRING analysis indicated that these modified proteins mainly were involved in reproductive and development system, endocrine and immune system, and carcinogenesis, which were in accord with the main reported TCS-induced adverse effects and suggested that the covalent modification of TCS RMs for proteins might affect their activities and functions, thus inducing serious adverse effects. This study provided a new insight into the mechanism of TCS adverse effects and may serve as a valuable method to predict or evaluate adverse effects of ubiquitous chemicals.

The Adductomics of Isolevuglandins: Oxidation of IsoLG Pyrrole Intermediates Generates Pyrrole⁻Pyrrole Crosslinks and Lactams

Isoprostane endoperoxides generated by free radical-induced oxidation of arachidonates, and prostaglandin endoperoxides generated through enzymatic cyclooxygenation of arachidonate, rearrange nonenzymatically to isoprostanes and a family of stereo and structurally isomeric γ-ketoaldehyde seco-isoprostanes, collectively known as isolevuglandins (isoLGs). IsoLGs are stealthy toxins, and free isoLGs are not detected in vivo. Rather, covalent adducts are found to incorporate lysyl ε-amino residues of proteins or ethanolamino residues of phospholipids. In vitro studies have revealed that adduction occurs within seconds and is uniquely prone to cause protein–protein crosslinks. IsoLGs accelerate the formation of the type of amyloid beta oligomers that have been associated with neurotoxicity. Under air, isoLG-derived pyrroles generated initially are readily oxidized to lactams and undergo rapid oxidative coupling to pyrrole–pyrrole crosslinked dimers, and to more highly oxygenated derivatives of those dimers. We have now found that pure isoLG-derived pyrroles, which can be generated under anoxic conditions, do not readily undergo oxidative coupling. Rather, dimer formation only occurs after an induction period by an autocatalytic oxidative coupling. The stable free-radical TEMPO abolishes the induction period, catalyzing rapid oxidative coupling. The amine N-oxide TMAO is similarly effective in catalyzing the oxidative coupling of isoLG pyrroles. N-acetylcysteine abolishes the generation of pyrrole–pyrrole crosslinks. Instead pyrrole-cysteine adducts are produced. Two unified single-electron transfer mechanisms are proposed for crosslink and pyrrole-cysteine adduct formation from isoLG-pyrroles, as well as for their oxidation to lactams and hydroxylactams.

Compensatory mechanisms for methylglyoxal detoxification in experimental & clinical diabetes

OBJECTIVES

The deficit of Glyoxalase I (Glo1) and the subsequent increase in methylglyoxal (MG) has been reported to be one the five mechanisms by which hyperglycemia causes diabetic late complications. Aldo-keto reductases (AKR) have been shown to metabolize MG; however, the relative contribution of this superfamily to the detoxification of MG in vivo, particularly within the diabetic state, remains unknown.

METHODS

CRISPR/Cas9-mediated genome editing was used to generate a Glo1 knock-out (Glo1-/-) mouse line. Streptozotocin was then applied to investigate metabolic changes under hyperglycemic conditions.

RESULTS

Glo1-/- mice were viable and showed no elevated MG or MG-H1 levels under hyperglycemic conditions. It was subsequently found that the enzymatic efficiency of various oxidoreductases in the liver and kidney towards MG were increased in the Glo1-/- mice. The functional relevance of this was supported by the altered distribution of alternative detoxification products. Furthermore, it was shown that MG-dependent AKR activity is a potentially clinical relevant pathway in human patients suffering from diabetes.

CONCLUSIONS

These data suggest that in the absence of GLO1, AKR can effectively compensate to prevent the accumulation of MG. The combination of metabolic, enzymatic, and genetic factors, therefore, may provide a better means of identifying patients who are at risk for the development of late complications caused by elevated levels of MG.

Reactive intermediates and bioactivation pathways characterization of avitinib by LC-MS/MS: In vitro metabolic investigation

Avitinib (AC0010) is a third generation inhibitor of the EGFR (epidermal growth factor receptor) that was permitted parallel phase I clinical trials in the US and in 2014. It is estimated to enter in market within two years. In the current study, eight in vitro metabolites were detected and their chemical structures were postulated. The main in vitro phase-I metabolic reaction was N-oxidation in piperazine moiety. The generation of reactive metabolites in avitinib metabolism was investigated using rat liver microsomes while adding capturing agents, viz potassium cyanide for reactive iminium intermediates, GSH for iminoquinones and methoxylamine for aldehyde forming stable adducts which are identifiable by LC-MS/MS. Ten reactive intermediates (four iminoquinones, three iminium and three aldehydes) were characterized. The three capturing agents used resulted in proposing four different bioactivation pathways. Upon literature examination, no former articles were found for avitinib metabolism including the produced reactive metabolites.

The metabolic fate of fenclozic acid in chimeric mice with a humanized liver

The metabolic fate of the human hepatotoxin fenclozic acid ([2-(4-chlorophenyl)-1,3-thiazol-4-yl]acetic acid) (Myalex) was studied in normal and bile-cannulated chimeric mice with a humanized liver, following oral administration of 10 mg/kg. This in vivo animal model was investigated to assess its utility to study “human” metabolism of fenclozic acid, and in particular to explore the formation of electrophilic reactive metabolites (RMs), potentially unique to humans. Metabolism was extensive, particularly involving the carboxylic acid-containing side chain. Metabolism resulted in the formation of a large number of metabolites and involved biotransformation via both oxidative and conjugative routes. The oxidative metabolites detected included a variety of hydroxylations as well as cysteinyl-, N-acetylcysteinyl-, and cysteinylglycine metabolites. The latter resulted from the formation of glutathione adducts/conjugates providing evidence for the production of RMs. The production of other classes of RMs included acyl-glucuronides, and the biosynthesis of acyl carnitine, taurine, glutamine, and glycine conjugates via potentially reactive acyl-CoA intermediates was also demonstrated. A number of unique “human” metabolites, e.g., those providing evidence for side-chain extension, were detected in the plasma and excreta of the chimeric liver-humanized mice that were not previously characterised in, e.g., the excreta of rat and C57BL/6 mice. The different pattern of metabolism seen in these chimeric mice with a humanized liver compared to the conventional rodents may offer clues to the factors that contributed to the drug-induced liver injury seen in humans.

The pharmacokinetics and metabolism of diclofenac in chimeric humanized and murinized FRG mice

The pharmacokinetics of diclofenac were investigated following single oral doses of 10 mg/kg to chimeric liver humanized and murinized FRG and C57BL/6 mice. In addition, the metabolism and excretion were investigated in chimeric liver humanized and murinized FRG mice. Diclofenac reached maximum blood concentrations of 2.43 ± 0.9 µg/mL (n = 3) at 0.25 h post-dose with an AUC_inf of 3.67 µg h/mL and an effective half-life of 0.86 h (n = 2). In the murinized animals, maximum blood concentrations were determined as 3.86 ± 2.31 µg/mL at 0.25 h post-dose with an AUC_inf of 4.94 ± 2.93 µg h/mL and a half-life of 0.52 ± 0.03 h (n = 3). In C57BL/6J mice, mean peak blood concentrations of 2.31 ± 0.53 µg/mL were seen 0.25 h post-dose with a mean AUC_inf of 2.10 ± 0.49 µg h/mL and a half-life of 0.51 ± 0.49 h (n = 3). Analysis of blood indicated only trace quantities of drug-related material in chimeric humanized and murinized FRG mice. Metabolic profiling of urine, bile and faecal extracts revealed a complex pattern of metabolites for both humanized and murinized animals with, in addition to unchanged parent drug, a variety of hydroxylated and conjugated metabolites detected. The profiles in humanized mice were different to those of both murinized and wild-type animals, e.g., a higher proportion of the dose was detected in the form of acyl glucuronide metabolites and much reduced amounts as taurine conjugates. Comparison of the metabolic profiles obtained from the present study with previously published data from C57BL/6J mice and humans revealed a greater, though not complete, match between chimeric humanized mice and humans, such that the liver humanized FRG model may represent a model for assessing the biotransformation of such compounds in humans.

High resolution mass spectrometry-based methodologies for identification of Etravirine bioactivation to reactive metabolites: In vitro and in vivo approaches

Drug bioactivation to reactive metabolites capable of covalent adduct formation with bionucleophiles is a major cause of drug-induced adverse reactions. Therefore, elucidation of reactive metabolites is essential to unravel the toxicity mechanisms induced by drugs and thereby identify patient subgroups at higher risk. Etravirine (ETR) was the first second-generation Non-Nucleoside Reverse Transcriptase Inhibitor (NNRTI) to be approved, as a therapeutic option for HIV-infected patients who developed resistance to the first-generation NNRTIs. Additionally, ETR came into market aiming to overcome some adverse effects associated with the previously used efavirenz (neurotoxicity) and nevirapine (hepatotoxicity) therapies. Nonetheless, post-marketing reports of severe ETR-induced skin rash and hypersensitivity reactions have prompted the U.S. FDA to issue a safety alert on ETR. Taking into consideration that ETR usage may increase in the near future, due to the possible use of the drug for coinfection with malaria and HIV, the development of reliable prognostic tools for early risk/benefit estimations is urgent. In the current study, high resolution mass spectrometry-based methodologies were integrated with MS³ experiments for the identification of reactive ETR metabolites/adducts: 1) in vitro incubation of the drug with human and rat liver S9 fractions in the presence of Phase I and II co-factors, including glutathione, as a trapping bionucleophile; and 2) in vivo, using urine samples from HIV-infected patients on ETR therapy. We obtained evidence for multiple bioactivation pathways leading to the formation of covalent adducts with glutathione and N-acetyl-L-cysteine. These results suggest that similar reactions may occur with cysteine residues of proteins, supporting a role for ETR bioactivation in the onset of the toxic effects elicited by the drug. Additionally, ETR metabolites stemming from amine oxidation, with potential toxicological significance, were identified in vitro and in vivo. Also noteworthy is the fact that new metabolic conjugation pathways of glucuronide metabolites were demonstrated for the first time, raising questions about their potential toxicological implications. In conclusion, these results represent not only a contribution towards the elucidation of new metabolic pathways of drugs in general but also an important step towards the elucidation of potentially toxic ETR pathways, whose understanding may be crucial for reliable risk/benefit estimations of ETR-based regimens.

[Hepatotoxicity of tyrosine kinase inhibitors: Mechanisms involved and practical implications]

Tyrosine kinase inhibitors (TKIs) are used for the targeted treatment of solid cancers. TKIs produce a variable incidence of liver adverse events (5-25%) which can progress to severe liver injury in a minority of patients if treatment is maintained despite ongoing injury. This risk requires careful patient management to maintain treatment benefit without harm. This review highlights the various mechanisms of idiosyncratic hepatotoxicity, the formation of reactive metabolites and how this leads to toxicity. These critical events depend of the drug-specific characteristics of each TKI and the patient risk factors, especially genetic characterization. With improved understanding of the mechanisms leading to hepatotoxicity, several strategies have been adopted to prevent or treat this side effect. Recommendations on liver function liver test monitoring have been proposed according to each TKI.

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.

TRPC proteins contribute to development of diabetic retinopathy and regulate glyoxalase 1 activity and methylglyoxal accumulation

OBJECTIVE

Diabetic retinopathy (DR) is induced by an accumulation of reactive metabolites such as ROS, RNS, and RCS species, which were reported to modulate the activity of cation channels of the TRPC family. In this study, we use Trpc1/4/5/6^-/- compound knockout mice to analyze the contribution of these TRPC proteins to diabetic retinopathy.

METHODS

We used Nanostring- and qPCR-based analysis to determine mRNA levels of TRPC channels in control and diabetic retinae and retinal cell types. Chronic hyperglycemia was induced by Streptozotocin (STZ) treatment. To assess the development of diabetic retinopathy, vasoregression, pericyte loss, and thickness of individual retinal layers were analyzed. Plasma and cellular methylglyoxal (MG) levels, as well as Glyoxalase 1 (GLO1) enzyme activity and protein expression, were measured in WT and Trpc1/4/5/6^-/- cells or tissues. MG-evoked toxicity in cells of both genotypes was compared by MTT assay.

RESULTS

We find that Trpc1/4/5/6^-/- mice are protected from hyperglycemia-evoked vasoregression determined by the formation of acellular capillaries and pericyte drop-out. In addition, Trpc1/4/5/6^-/- mice are resistant to the STZ-induced reduction in retinal layer thickness. The RCS metabolite methylglyoxal, which represents a key mediator for the development of diabetic retinopathy, was significantly reduced in plasma and red blood cells (RBCs) of STZ-treated Trpc1/4/5/6^-/- mice compared to controls. GLO1 is the major MG detoxifying enzyme, and its activity and protein expression were significantly elevated in Trpc1/4/5/6-deficient cells, which led to significantly increased resistance to MG toxicity. GLO1 activity was also increased in retinal extracts from Trpc1/4/5/6^-/- mice. The TRPCs investigated here are expressed at different levels in endothelial and glial cells of the retina.

CONCLUSION

The protective phenotype in diabetic retinopathy observed in Trpc1/4/5/6^-/- mice is suggestive of a predominant action of TRPCs in Müller cells and microglia because of their central position in the retention of a proper homoeostasis of the neurovascular unit.

Hormesis enables cells to handle accumulating toxic metabolites during increased energy flux

Energy production is inevitably linked to the generation of toxic metabolites, such as reactive oxygen and carbonyl species, known as major contributors to ageing and degenerative diseases. It remains unclear how cells can adapt to elevated energy flux accompanied by accumulating harmful by-products without taking any damage. Therefore, effects of a sudden rise in glucose concentrations were studied in yeast cells. This revealed a feedback mechanism initiated by the reactive dicarbonyl methylglyoxal, which is formed non-enzymatically during glycolysis. Low levels of methylglyoxal activate a multi-layered defence response against toxic metabolites composed of prevention, detoxification and damage remission. The latter is mediated by the protein quality control system and requires inducible Hsp70 and Btn2, the aggregase that sequesters misfolded proteins. This glycohormetic mechanism enables cells to pre-adapt to rising energy flux and directly links metabolic to proteotoxic stress. Further data suggest the existence of a similar response in endothelial cells.

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Low-dose MG induces tolerance towards toxic levels of MG and ROS in yeast cells.

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This preconditioning effect is mediated via a multi-layered defence mechanism.

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The hormetic defence is composed of prevention, detoxification and damage remission.

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Low MG induces the PQS including protein sorting and handling via HSPs.

Glyoxalase 1 expression is associated with an unfavorable prognosis of oropharyngeal squamous cell carcinoma

Background

Glyoxalase 1 is a key enzyme in the detoxification of reactive metabolites such as methylglyoxal and induced Glyoxalase 1 expression has been demonstrated for several human malignancies. However, the regulation and clinical relevance of Glyoxalase 1 in the context of head and neck squamous cell carcinoma has not been addressed so far.

Methods

Argpyrimidine modification as a surrogate for methylglyoxal accumulation and Glyoxalase 1 expression in tumor cells was assessed by immunohistochemical staining of tissue microarrays with specimens from oropharyngeal squamous cell carcinoma patients (n = 154). Prognostic values of distinct Glyoxalase 1 staining patterns were demonstrated by Kaplan-Meier, univariate and multivariate Cox proportional hazard model analysis. The impact of exogenous methylglyoxal or a Glyoxalase 1 inhibitor on the viability of two established tumor cell lines was monitored by a colony-forming assay in vitro.

Results

Glyoxalase 1 expression in tumor cells of oropharyngeal squamous cell carcinoma patients was positively correlated with the presence of Argpyrimidine modification and administration of exogenous methylglyoxal induced Glyoxalase 1 protein levels in FaDu and Cal27 cells in vitro. Cal27 cells with lower basal and methylglyoxal-induced Glyoxalase 1 expression were more sensitive to the cytotoxic effect at high methylgyoxal concentrations and both cell lines showed a decrease in colony formation with increasing amounts of a Glyoxalase 1 inhibitor. A high and nuclear Glyoxalase 1 staining was significantly correlated with shorter progression-free and disease-specific survival, and served as an independent risk factor for an unfavorable prognosis of oropharyngeal squamous cell carcinoma patients.

Conclusions

Induced Glyoxalase 1 expression is a common feature in the pathogenesis of oropharyngeal squamous cell carcinoma and most likely represents an adaptive response to the accumulation of cytotoxic metabolites. Oropharyngeal squamous cell carcinoma patients with a high and nuclear Glyoxalase 1 staining pattern have a high risk for treatment failure, but might benefit from pharmacological targeting Glyoxalase 1 activity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-017-3367-5) contains supplementary material, which is available to authorized users.

Mechanisms of Drug-Induced Hepatotoxicity

Drug-induced hepatotoxicity (DIH) is a significant cause of acute liver failure and liver transplantation. Diagnosis is challenging due to the idiosyncratic nature, its presentation in the form of other liver disease, and the lack of a definite diagnostic criteria. Generation of reactive metabolites, oxidative stress, and mitochondrial dysfunction are common mechanisms involved in DIH. Certain risk factors associated with a drug and within an individual further predispose patients to DIH.

Acute liver effects, disposition and metabolic fate of [14C]-fenclozic acid following oral administration to normal and bile-cannulated male C57BL/6J mice

The distribution, metabolism, excretion and hepatic effects of the human hepatotoxin fenclozic acid were investigated following single oral doses of 10 mg/kg to normal and bile duct-cannulated male C57BL/6J mice. Whole body autoradiography showed distribution into all tissues except the brain, with radioactivity still detectable in blood, kidney and liver at 72 h post-dose. Mice dosed with [¹⁴C]-fenclozic acid showed acute centrilobular hepatocellular necrosis, but no other regions of the liver were affected. The majority of the [¹⁴C]-fenclozic acid-related material recovered was found in the urine/aqueous cage wash, (49%) whilst a smaller portion (13%) was eliminated via the faeces. Metabolic profiles for urine, bile and faecal extracts, obtained using liquid chromatography and a combination of mass spectrometric and radioactivity detection, revealed extensive metabolism of fenclozic acid in mice that involved biotransformations via both oxidation and conjugation. These profiling studies also revealed the presence of glutathione-derived metabolites providing evidence for the production of reactive species by mice administered fenclozic acid. Covalent binding to proteins from liver, kidney and plasma was also demonstrated, although this binding was relatively low (less than 50 pmol eq./mg protein).

Characterizing novel metabolic pathways of melatonin receptor agonist agomelatine using metabolomic approaches

Agomelatine (AGM), an analog of melatonin, is a potential agonist at melatonin receptors 1/2 and a selective antagonist at 5-hydroxytryptamine 2C receptors. AGM is widely used for the treatment of major depressive episodes in adults. However, multiple adverse effects associated with AGM have been reported in clinical practice. It is little known about AGM metabolism in vitro and in vivo, although metabolism plays a pivotal role in its efficacy and safety. To elucidate metabolic pathways of AGM, we systemically investigated AGM metabolism and its bioactivation in human liver microsomes (HLM) and mice using metabolomic approaches. We identified thirty-eight AGM metabolites and adducts, among which thirty-two are novel. In HLM, we uncovered five GSH-trapped adducts and two semicarbazide-trapped aldehydes. Moreover, we characterized three N-acetyl cysteine conjugated-AGM adducts in mouse urine and feces, which were formed from the degradation of AGM_GSH adducts. Using recombinant CYP450 isoenzymes and chemical inhibitors, we demonstrated that CYP1A2 and CYP3A4 are primary enzymes contributing to the formation of AGM_GSH adducts and AGM_hydrazones. This study provided a global view of AGM metabolism and identified the novel pathways of AGM bioactivation, which could be utilized for further understanding the mechanism of adverse effects related to AGM and possible drug-drug interactions.

Characterization of glutathione conjugates derived from reactive metabolites of bakuchiol

Bakuchiol belongs to a family of monoterpene phenols occurring in plant Psoralea corylifolia L., a traditional herbal medicine. Bakuchiol has also demonstrated multiple pharmacologic activities. However, metabolism of bakuchiol had never been investigated. The major objective of the present study was to study the metabolic pathways of bakuchiol in order to identify potential reactive metabolites. A total of five glutathione (GSH) conjugates (M1-M5) were detected in rat/human liver microsomes containing NADPH, GSH, and bakuchiol. M1 and M2 resulted from GSH conjugated on the phenol ring. M3, M4, and M5 were derived from GSH adducted on the side chain. The results displayed that bakuchiol can be bioactivated by oxidation of the phenol moiety to the corresponding ortho-quinone and by epoxidation of the aliphatic side chain to epoxide metabolites. No bakuchiol-derived GSH conjugates were detected in urine of rats given bakuchiol, but six corresponding cysteinylglycine (Cys-Gly) conjugates and mercapturic acids were observed instead. A 2'-iodoxybenzoic acid-mediated oxidation reaction of bakuchiol in the presence of GSH produced M1 and M2, and m-chloroperoxybenzoicacid-mediated oxidation of bakuchiol trapped with GSH generated M3 and M4. The four synthetic metabolites were detected in microsomal incubations. In addition, recombinant P450 enzyme incubations showed that CYP 1A2 was the predominant P450 responsible for the metabolism of bakuchiol. In summary, our results demonstrated that bakuchiol can be bioactivated to quinone and epoxide metabolites. These findings facilitate the understanding of the mechanisms of bakuchiol-induced cytotoxicity.

Cytotoxicity of pyrrolizidine alkaloid in human hepatic parenchymal and sinusoidal endothelial cells: Firm evidence for the reactive metabolites mediated pyrrolizidine alkaloid-induced hepatotoxicity

Pyrrolizidine alkaloids (PAs) widely distribute in plants and can cause hepatic sinusoidal obstruction syndrome (HSOS), which typically presents as a primary sinusoidal endothelial cell damage. It is well-recognized that after ingestion, PAs undergo hepatic cytochromes P450 (CYPs)-mediated metabolic activation to generate dehydropyrrolizidine alkaloids (DHPAs), which are hydrolyzed to dehydroretronecine (DHR). DHPAs and DHR are reactive metabolites having same core pyrrole moiety, and can bind proteins to form pyrrole-protein adducts, which are believed as the primary cause for PA-induced HSOS. However, to date, the direct evidences supporting the toxicity of DHPAs and DHR in the liver, in particular in the sinusoidal endothelial cells, are lacking. Using human hepatic sinusoidal endothelial cells (HSEC) and HepG2 (representing hepatic parenchymal cells), cells that lack CYPs activity, this study determined the direct cytotoxicity of dehydromonocrotaline, a representative DHPA, and DHR, but no cytotoxicity of the intact PA (monocrotaline) in both cell lines, confirming that reactive metabolites mediate PA intoxication. Comparing with HepG2, HSEC had significantly lower basal glutathione (GSH) level, and was significantly more susceptible to the reactive metabolites with severer GSH depletion and pyrrole-protein adducts formation. The toxic potency of two reactive metabolites was also compared. DHPA was more reactive than DHR, leading to severer toxicity. In conclusion, our results unambiguously provided the first direct evidence for the critical role of DHPA and DHR in the reactive metabolites-mediated PA-induced hepatotoxicity, which occurs predominantly in HSEC due to severe GSH depletion and the significant formation of pyrrole-protein adducts in HSEC.

Comparison of trapping profiles between d-peptides and glutathione in the identification of reactive metabolites

Qualitative trapping profile of reactive metabolites arising from six structurally different compounds was tested with three different d-peptide isomers (Peptide 1, gly–tyr–pro–cys–pro–his-pro; Peptide 2, gly–tyr–pro–ala–pro–his–pro; Peptide 3, gly–tyr–arg–pro–cys–pro–his–lys–pro) and glutathione (GSH) using mouse and human liver microsomes as the biocatalyst. The test compounds were classified either as clinically “safe” (amlodipine, caffeine, ibuprofen), or clinically as “risky” (clozapine, nimesulide, ticlopidine; i.e., associated with severe clinical toxicity outcomes). Our working hypothesis was as follows: could the use of short different amino acid sequence containing d-peptides in adduct detection confer any add-on value to that obtained with GSH? All “risky” agents’ resulted in the formation of several GSH adducts in the incubation mixture and with at least one peptide adduct with both microsomal preparations. Amlodipine did not form any adducts with any of the trapping agents. No GSH and peptide 2 and 3 adducts were found with caffeine, but with peptide 1 one adduct with human liver microsomes was detected. Ibuprofen produced one Peptide 1-adduct with human and mouse liver microsomes but not with GSH. In conclusion, GSH still remains the gold trapping standard for reactive metabolites. However, targeted d-peptides could provide additional information about protein binding potential of electrophilic agents, but their clinical significance needs to be clarified using a wider spectrum of chemicals together with other safety estimates.

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.

Reactive metabolites and antioxidant gene polymorphisms in Type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM), by definition is a heterogeneous, multifactorial, polygenic syndrome which results from insulin receptor dysfunction. It is an outcome of oxidative stress caused by interactions of reactive metabolites (RMs) interactions with lipids, proteins and other mechanisms of human body. Production of RMs mainly superoxide (O2−) has been found in a variety of predominating cellular enzyme systems including NAD(P)H oxidase, xanthine oxidase (XO), cyclooxygenase (COX), uncoupled endothelial nitric oxide synthase (eNOS) and myeloperoxidase (MPO). The four main RM related molecular mechanisms are: increased polyol pathway flux; increased advanced glycation end-product (AGE) formation; activation of protein kinase C (PKC) isoforms and increased hexosamine pathway flux which have been implicated in glucose-mediated vascular damage. Superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione-S-transferase (GST), nitric oxide synthase (NOS) are antioxidant enzymes involved in scavenging RMs in normal individuals. Functional polymorphisms of these antioxidant enzymes have been reported to be involved in pathogenesis of T2DM individuals. The low levels of antioxidant enzymes or their non-functionality results in excessive RMs which initiate stress related pathways thereby leading to insulin resistance and T2DM. An attempt has been made to review the role of RMs and antioxidant enzymes in oxidative stress resulting in T2DM.

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Four main molecular mechanisms are implicated in glucose-mediated vascular damage.

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Impaired antioxidant defense contributes to T2DM and related complications.

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SNPs in antioxidant enzymes are associated with pathogenesis of type 2 diabetes.

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Genotyping of gene variants in populations will help identify individuals at risk.

Mechanisms of drug-induced liver injury

Drug-induced liver injury (DILI) represents a broad spectrum of liver manifestations. However, the most common manifestation is hepatocyte death following drug intake. DILI can be predictable and dose dependent with a notable example of acetaminophen toxicity. Idiosyncratic DILI occurs in an unpredictable fashion at low frequencies, implying that environmental and genetic factors alter the susceptibility of individuals to the insult (drugs).