NADPH-dependent covalent binding of [3H]paroxetine to human liver microsomes and S-9 fractions: identification of an electrophilic quinone metabolite of paroxetine

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.

Metabolic bioactivation of antidepressants: advance and underlying hepatotoxicity

Many drugs that serve as first-line medications for the treatment of depression are associated with severe side effects, including liver injury. Of the 34 antidepressants discussed in this review, four have been withdrawn from the market due to severe hepatotoxicity, and others carry boxed warnings for idiosyncratic liver toxicity. The clinical and economic implications of antidepressant-induced liver injury are substantial, but the underlying mechanisms remain elusive. Drug-induced liver injury may involve the host immune system, the parent drug, or its metabolites, and reactive drug metabolites are one of the most commonly referenced risk factors. Although the precise mechanism by which toxicity is induced may be difficult to determine, identifying reactive metabolites that cause toxicity can offer valuable insights for decreasing the bioactivation potential of candidates during the drug discovery process. A comprehensive understanding of drug metabolic pathways can mitigate adverse drug-drug interactions that may be caused by elevated formation of reactive metabolites. This review provides a comprehensive overview of the current state of knowledge on antidepressant bioactivation, the metabolizing enzymes responsible for the formation of reactive metabolites, and their potential implication in hepatotoxicity. This information can be a valuable resource for medicinal chemists, toxicologists, and clinicians engaged in the fields of antidepressant development, toxicity, and depression treatment.

Methysticin Acts as a Mechanism-Based Inactivator of Cytochrome P450 2C9

Methysticin is one of the naturally occurring bioactive constituents extracted from Piper methysticum Forst. In the present study, we intended to investigate the inhibitory effect of methysticin on cytochrome P450 (P450) enzymes. Methysticin exhibited time-, concentration-, and NADPH-dependent inhibition on CYP2C9 using diclofenac as a probe substrate. Approximately 85% of CYP2C9 activity was inhibited by methysticin at 50 μM after a 30 min preincubation with human liver microsomes in the presence of NADPH. The kinetic parameters K_I, k_inact, and t_1/2,inact were 13.32 ± 1.35 μM, 0.054 ± 0.005 min^-1, and 12.83 ± 3.23 min, respectively. Sulfaphenazole (competitive inhibitor of CYP2C9) displayed a significant protective effect on methysticin-induced CYP2C9 inactivation. However, the inclusion of catalase/superoxide dismutase or glutathione (GSH) showed no such protection. A carbene intermediate was postulated to be involved in methysticin-induced CYP2C9 inactivation as K₃Fe(CN)₆ recovered 14.96% of CYP2C9 activity. A methysticin-derived ortho-quinone intermediate dependent on NADPH was trapped by GSH, and this intermediate was believed to be involved in CYP2C9 inactivation. CYP1A2, 2C9, and 3A4 were the major enzymes responsible for methysticin bioactivation. Taken together, the present work demonstrated that methysticin was a mechanism-based inactivator of CYP2C9. Both ortho-quinone and carbene intermediates appeared to be involved in the inactivation of CYP2C9 induced by methysticin.

Metabolic map of the antiviral drug podophyllotoxin provides insights into hepatotoxicity

Podophyllotoxin (POD) is a natural compound with antiviral and anticancer activities. The purpose of the present study was to determine the metabolic map of POD in vitro and in vivo.Mouse and human liver microsomes were employed to identify POD metabolites in vitro and recombinant drug-metabolizing enzymes were used to identify the mono-oxygenase enzymes involved in POD metabolism. All in vitro incubation mixtures and bile samples from mice treated with POD were analysed with ultra-performance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight mass spectrometry.A total of 38metabolites, including six phase-I metabolites and 32 phase-II metabolites, of POD were identified from bile and faeces samples after oral administration, and their structures were elucidated through interpreting MS/MS fragmentation patterns.Nine metabolites, including two phase-I metabolites, five glucuronide conjugates, and two GSH conjugates were detected in both human and mouse liver microsome incubation systems and the generation of all metabolites were NADPH-dependent. The main phase-I enzymes involved in metabolism of POD in vitro include CYP2C9, CYP2C19, CYP3A4, and CYP3A5.POD administration to mice caused hepatic and intestinal toxicity, and the cellular damage was exacerbated when 1-aminobenzotriazole, a broad-spectrum inhibitor of CYPs, was administered with POD, indicating that POD, but not its metabolites, induced hepatic and intestinal toxicities.This study elucidated the metabolic map and provides important reference basis for the safety evaluation and rational for the clinical application of POD.

Geminal Diheteroatomic Motifs: Some Applications of Acetals, Ketals, and Their Sulfur and Nitrogen Homologues in Medicinal Chemistry and Drug Design

Acetals and ketals and their nitrogen and sulfur homologues are often considered to be unconventional and potentially problematic scaffolding elements or pharmacophores for the design of orally bioavailable drugs. This opinion is largely a function of the perception that such motifs might be chemically unstable under the acidic conditions of the stomach and upper gastrointestinal tract. However, even simple acetals and ketals, including acyclic molecules, can be sufficiently robust under acidic conditions to be fashioned into orally bioavailable drugs, and these structural elements are embedded in many effective therapeutic agents. The chemical stability of molecules incorporating geminal diheteroatomic motifs can be modulated by physicochemical design principles that include the judicious deployment of proximal electron-withdrawing substituents and conformational restriction. In this Perspective, we exemplify geminal diheteroatomic motifs that have been utilized in the discovery of orally bioavailable drugs or drug candidates against the backdrop of understanding their potential for chemical lability.

Mechanism-Based Inactivation of Cytochrome P450 Enzymes: Computational Insights

Mechanism-based inactivation (MBI) refers to the metabolic bioactivation of a xenobiotic by cytochrome P450s to a highly reactive intermediate which subsequently binds to the enzyme and leads to the quasi-irreversible or irreversible inhibition. Xenobiotics, mainly drugs with specific functional units, are the major sources of MBI. Two possible consequences of MBI by medicinal compounds are drug-drug interaction and severe toxicity that are observed and highlighted by clinical experiments. Today almost all of these latent functional groups (e.g., thiophene, furan, alkylamines, etc.) are known, and their features and mechanisms of action, owing to the vast experimental and theoretical studies, are determined. In the past decade, molecular modeling techniques, mostly density functional theory, have revealed the most feasible mechanism that a drug undergoes by P450 enzymes to generate a highly reactive intermediate. In this review, we provide a comprehensive and detailed picture of computational advances toward the elucidation of the activation mechanisms of various known groups with MBI activity. To this aim, we briefly describe the computational concepts to carry out and analyze the mechanistic investigations, and then, we summarize the studies on compounds with known inhibition activity including thiophene, furan, alkylamines, terminal acetylene, etc. This study can be reference literature for both theoretical and experimental (bio)chemists in several different fields including rational drug design, the process of toxicity prevention, and the discovery of novel inhibitors and catalysts.

Reactive Metabolites: Generation and Estimation with Electrochemistry Based Analytical Strategy as an Emerging Screening Tool

Background:

Analytical scientists have constantly been in search for more efficient and economical methods for drug simulation studies. Owing to great progress in this field, there are various techniques available nowadays that mimic drug metabolism in the hepatic microenvironment. The conventional in vitro and in vivo studies pose inherent methodological drawbacks due to which alternative analytical approaches are devised for different drug metabolism experiments.

Methods:

Electrochemistry has gained attention due to its benefits over conventional metabolism studies. Because of the protein binding nature of reactive metabolites, it is difficult to identify them directly after formation, although the use of trapping agents aids in their successful identification. Furthermore, various scientific reports confirmed the successful simulation of drug metabolism studies by electrochemical cells. Electrochemical cells coupled with chromatography and mass spectrometry made it easy for direct detection of reactive metabolites. In this review, an insight into the application of electrochemical techniques for metabolism simulation studies has been provided. The sole use of electrochemical cells, as well as their setups on coupling to liquid chromatography and mass spectrometry has been discussed. The importance of metabolism prediction in early drug discovery and development stages along with a brief overview of other conventional methods has also been highlighted.

Conclusion:

To the best of our knowledge, this is the first article to review the electrochemistry based strategy for the analysis of reactive metabolites. The outcome of this ‘first of its kind’ review will significantly help the researchers in the application of electrochemistry based bioanalysis for metabolite detection.

Precision dosing-based optimisation of paroxetine during pregnancy for poor and ultrarapid CYP2D6 metabolisers: a virtual clinical trial pharmacokinetics study

Objective

Paroxetine has been demonstrated to undergo gestation-related reductions in plasma concentrations, to an extent which is dictated by the polymorphic state of CYP 2D6. However, knowledge of appropriate dose titrations is lacking.

Methods

A pharmacokinetic modelling approach was applied to examine gestational changes in trough plasma concentrations for CYP 2D6 phenotypes, followed by necessary dose adjustment strategies to maintain paroxetine levels within a therapeutic range of 20–60 ng/ml.

Key findings

A decrease in trough plasma concentrations was simulated throughout gestation for all phenotypes. A significant number of ultrarapid (UM) phenotype subjects possessed trough levels below 20 ng/ml (73–76%) compared to extensive metabolisers (EM) (51–53%).

Conclusions

For all phenotypes studied, there was a requirement for daily doses in excess of the standard 20 mg dose throughout gestation. For EM, a dose of 30 mg daily in trimester 1 followed by 40 mg daily in trimesters 2 and 3 is suggested to be optimal. For poor metabolisers (PM), a 20 mg daily dose in trimester 1 followed by 30 mg daily in trimesters 2 and 3 is suggested to be optimal. For UM, a 40 mg daily dose throughout gestation is suggested to be optimal.

In vitro metabolism of triclosan studied by liquid chromatography-high-resolution tandem mass spectrometry

Triclosan (TCS) is an antibacterial and antifungal compound found in many hygiene products, including toothpaste, soap, and detergents. However, this molecule can act as an endocrine disruptor and can induce harmful effects on human health and the environment. In this study, triclosan was biotransformed in vitro using human and rat liver fractions, to evaluate oxidative metabolism, the formation of reactive metabolites via the detection of GSH adducts, as well as glucuronide and sulfate conjugates using liquid chromatography coupled to high-resolution tandem mass spectrometry (LC-HRMS/MS). A deuterated analog of triclosan was also employed for better structural elucidation of specific metabolic sites. Several GSH adducts were found, either via oxidative metabolism of triclosan or its cleavage product, 2,4-dichlorophenol. We also detected glucuronide and sulfated conjugates of triclosan and its cleaved product. This study was aimed at understanding the routes of detoxification of this xenobiotic, as well as investigating any potential pathways related to additional toxicity via reactive metabolite formation. Graphical abstract.

Designing around Structural Alerts in Drug Discovery

Cumulative research over several decades has implicated the involvement of reactive metabolites in many idiosyncratic adverse drug reactions (IADRs). Consequently, "avoidance" strategies have been inserted into drug discovery paradigms, which include the exclusion of structural alerts and possible termination of reactive metabolite-positive compounds. Several noteworthy examples where reactive metabolite-related liabilities have been resolved through structure-metabolism studies are presented herein. Considerable progress has also been made in addressing the limitations of the avoidance strategy and further refining the process of managing reactive metabolite issues in drug development. These efforts primarily stemmed from the observation that numerous drugs, which contain structural alerts and/or form reactive metabolites, are devoid of ADRs. The Perspective also dwells into an analysis of the structural alert/reactive metabolite concept with a discussion of risk mitigation tactics to support the progression of reactive metabolite-positive drug candidates.

Characterization of stable and reactive metabolites of piperine formed on incubation with human liver microsomes

Black pepper, though commonly employed as a spice, has many medicinal properties. It consists of volatile oils, alkaloids, pungent resins, etc., of which piperine is a major constituent. Though safe at low doses, piperine causes alteration in the activity of drug metabolising enzymes and transporters at high dose and is known to precipitate liver toxicity. It has a potential to form reactive metabolite(s) (RM) owing to the presence of structural alerts, such as methylenedioxyphenyl (MDP), α, β-unsaturated carbonyl group (Michael acceptor), and piperidine. The present study was designed to detect and characterize stable and RM(s) of piperine formed on in vitro incubation with human liver microsomes. The investigation of RMs was done with the aid of trapping agents, viz, glutathione (GSH) and N-acetylcysteine (NAC). The samples were analysed by ultra-high performance liquid chromatography coupled with high resolution mass spectrometry (UHPLC-HRMS) using Thermo Scientific Q Exactive Plus Orbitrap. Full scan MS followed by data-dependent MS² (Full MS-ddMS² ) mode was used to establish mass spectrometric fragmentation pathways of protonated piperine and its metabolites. In total, four stable metabolites and their isomers (M1a-c, M2a-b, M3a-c, and M4a-b) were detected. Their formation involved removal of carbon (3, M1a-c), hydroxylation (2, M2a-b), hydroxylation with hydrogenation (3, M3a-c), and dehydrogenation (2, M4a-b). Out of these metabolites, M1, M2, and M3 are reported earlier in the literature, but their isomers and two M4 variants are novel. In addition, six novel conjugates of RMs, including three GSH conjugates of m/z 579 and three NAC conjugates of m/z 435, were also observed.

Fluorine and Fluorinated Motifs in the Design and Application of Bioisosteres for Drug Design

The electronic properties and relatively small size of fluorine endow it with considerable versatility as a bioisostere and it has found application as a substitute for lone pairs of electrons, the hydrogen atom, and the methyl group while also acting as a functional mimetic of the carbonyl, carbinol, and nitrile moieties. In this context, fluorine substitution can influence the potency, conformation, metabolism, membrane permeability, and P-gp recognition of a molecule and temper inhibition of the hERG channel by basic amines. However, as a consequence of the unique properties of fluorine, it features prominently in the design of higher order structural metaphors that are more esoteric in their conception and which reflect a more sophisticated molecular construction that broadens biological mimesis. In this Perspective, applications of fluorine in the construction of bioisosteric elements designed to enhance the in vitro and in vivo properties of a molecule are summarized.

Formation and Biological Targets of Quinones: Cytotoxic versus Cytoprotective Effects

Quinones represent a class of toxicological intermediates, which can create a variety of hazardous effects in vivo including, acute cytotoxicity, immunotoxicity, and carcinogenesis. In contrast, quinones can induce cytoprotection through the induction of detoxification enzymes, anti-inflammatory activities, and modification of redox status. The mechanisms by which quinones cause these effects can be quite complex. The various biological targets of quinones depend on their rate and site of formation and their reactivity. Quinones are formed through a variety of mechanisms from simple oxidation of catechols/hydroquinones catalyzed by a variety of oxidative enzymes and metal ions to more complex mechanisms involving initial P450-catalyzed hydroxylation reactions followed by two-electron oxidation. Quinones are Michael acceptors, and modification of cellular processes could occur through alkylation of crucial cellular proteins and/or DNA. Alternatively, quinones are highly redox active molecules which can redox cycle with their semiquinone radical anions leading to the formation of reactive oxygen species (ROS) including superoxide, hydrogen peroxide, and ultimately the hydroxyl radical. Production of ROS can alter redox balance within cells through the formation of oxidized cellular macromolecules including lipids, proteins, and DNA. This perspective explores the varied biological targets of quinones including GSH, NADPH, protein sulfhydryls [heat shock proteins, P450s, cyclooxygenase-2 (COX-2), glutathione S-transferase (GST), NAD(P)H:quinone oxidoreductase 1, (NQO1), kelch-like ECH-associated protein 1 (Keap1), IκB kinase (IKK), and arylhydrocarbon receptor (AhR)], and DNA. The evidence strongly suggests that the numerous mechanisms of quinone modulations (i.e., alkylation versus oxidative stress) can be correlated with the known pathology/cytoprotection of the parent compound(s) that is best described by an inverse U-shaped dose-response curve.

Comparison of the Reactivity of Trapping Reagents toward Electrophiles: Cysteine Derivatives Can Be Bifunctional Trapping Reagents

Trapping reagents are powerful tools to detect unstable reactive metabolites. There are a variety of trapping reagents based on chemical reactivity to electrophiles, and we investigated the reactivity of thiol and amine trapping reagents to metabolically generated electrophiles and commercially available electrophilic compounds. Glutathione (GSH) and N-acetylcysteine (Nac) trapped soft electrophiles, and amine derivatives such as semicarbazide (SC) and methoxyamine (MeA) reacted as hard nucleophiles to trap aldehydes as imine derivatives. Cysteine (Cys) and homocysteine (HCys) captured both soft electrophiles and hard electrophilic aldehydes. There were no qualitative differences in trapping soft electrophiles among Cys, HCys, GSH, and Nac, although quantitative reactivity to trap soft electrophiles varied likely depending on the pKa values of their thiol group. In the reactivity with aldehydes, Cys and HCys showed relatively lower reactivity as compared with SC and MeA. Nonetheless, they can trap aldehydes, and the resulting conjugates were stable and detected easily because their amino group formed imines after reaction with aldehydes, which are successively attacked by the intramolecular thiol group to form stable ring structures. This report demonstrated that Cys and HCys are advantageous to evaluate the formations of both soft electrophiles and aldehyde-type derivatives from a lot of drug candidates at early drug discovery by their unique structural characteristics.

Detecting reactive drug metabolites for reducing the potential for drug toxicity

INTRODUCTION

A number of withdrawn drugs are known to undergo bioactivation by a range of drug metabolizing enzymes to chemically reactive metabolites that bind covalently to protein and DNA resulting in organ toxicity and carcinogenesis, respectively. An important goal in drug discovery is to identify structural sites of bioactivation within discovery molecules for providing strategic modifications that eliminate or minimize reactive metabolite formation, while maintaining target potency, selectivity and desired pharmacokinetic properties leading to the development of efficacious and nontoxic drugs.

AREAS COVERED

This review covers experimental techniques currently used to detect reactive drug metabolites and provides recent examples where information from mechanistic in vitro studies was successfully used to redesign candidate drugs leading to blocked or minimized bioactivation. Reviewed techniques include in vitro radiolabeled drug covalent binding to protein and reactive metabolite trapping with reagents such as glutathione, cyanide, semicarbazide and DNA bases. Case studies regarding reactive metabolite detection using a combination of varied techniques, including liquid chromatography-tandem mass spectrometry and NMR analyses and subsequent structural modification are discussed.

EXPERT OPINION

Information derived from state-of-art mechanistic drug metabolism studies can be used successfully to direct medicinal chemistry towards the synthesis of candidate drugs devoid of bioactivation liabilities, while maintaining desired pharmacology and pharmacokinetic properties.

Discovery of (1R,2S)-2-{[(2,4-Dimethylpyrimidin-5-yl)oxy]methyl}-2-(3-fluorophenyl)-N-(5-fluoropyridin-2-yl)cyclopropanecarboxamide (E2006): A Potent and Efficacious Oral Orexin Receptor Antagonist

The orexin/hypocretin receptors are a family of G protein-coupled receptors and consist of orexin-1 (OX1) and orexin-2 (OX2) receptor subtypes. Orexin receptors are expressed throughout the central nervous system and are involved in the regulation of the sleep/wake cycle. Because modulation of these receptors constitutes a promising target for novel treatments of disorders associated with the control of sleep and wakefulness, such as insomnia, the development of orexin receptor antagonists has emerged as an important focus in drug discovery research. Here, we report the design, synthesis, characterization, and structure-activity relationships (SARs) of novel orexin receptor antagonists. Various modifications made to the core structure of a previously developed compound (-)-5, the lead molecule, resulted in compounds with improved chemical and pharmacological profiles. The investigation afforded a potential therapeutic agent, (1R,2S)-2-{[(2,4-dimethylpyrimidin-5-yl)oxy]methyl}-2-(3-fluorophenyl)-N-(5-fluoropyridin-2-yl)cyclopropanecarboxamide (E2006), an orally active, potent orexin antagonist. The efficacy was demonstrated in mice in an in vivo study by using sleep parameter measurements.

Predicting toxicities of reactive metabolite-positive drug candidates

Because of the inability to predict and quantify the risk of idiosyncratic adverse drug reactions (IADRs) and because reactive metabolites (RMs) are thought to be responsible for the pathogenesis of some IADRs, the potential for RM formation within new chemical entities is routinely examined with the ultimate goal of eliminating or reducing the liability through iterative design. Likewise, avoidance of structural alerts is almost a standard practice in drug design. However, the perceived safety concerns associated with the use of structural alerts and/or RM screening tools as standalone predictors of toxicity risks may be overexaggerated. Numerous marketed drugs form RMs but do not cause idiosyncratic toxicity. In this review article, we present a critique of the structural alert/RM concept as applied in drug discovery and evaluate the evidence linking structural alerts and RMs to observed toxic effects. Pragmatic risk mitigation strategies to aid the advancement of drug candidates that carry a RM liability are also discussed.

Discovery of 5''-chloro-N-[(5,6-dimethoxypyridin-2-yl)methyl]-2,2':5',3''-terpyridine-3'-carboxamide (MK-1064): a selective orexin 2 receptor antagonist (2-SORA) for the treatment of insomnia

The field of small-molecule orexin antagonist research has evolved rapidly in the last 15 years from the discovery of the orexin peptides to clinical proof-of-concept for the treatment of insomnia. Clinical programs have focused on the development of antagonists that reversibly block the action of endogenous peptides at both the orexin 1 and orexin 2 receptors (OX1 R and OX2 R), termed dual orexin receptor antagonists (DORAs), affording late-stage development candidates including Merck's suvorexant (new drug application filed 2012). Full characterization of the pharmacology associated with antagonism of either OX1 R or OX2 R alone has been hampered by the dearth of suitable subtype-selective, orally bioavailable ligands. Herein, we report the development of a selective orexin 2 antagonist (2-SORA) series to afford a potent, orally bioavailable 2-SORA ligand. Several challenging medicinal chemistry issues were identified and overcome during the development of these 2,5-disubstituted nicotinamides, including reversible CYP inhibition, physiochemical properties, P-glycoprotein efflux and bioactivation. This article highlights structural modifications the team utilized to drive compound design, as well as in vivo characterization of our 2-SORA clinical candidate, 5''-chloro-N-[(5,6-dimethoxypyridin-2-yl)methyl]-2,2':5',3''-terpyridine-3'-carboxamide (MK-1064), in mouse, rat, dog, and rhesus sleep models.

Metabolic map and bioactivation of the anti-tumour drug noscapine

BACKGROUND AND PURPOSE

Noscapine is a promising anti-tumour agent. The purpose of the present study was to describe the metabolic map and investigate the bioactivation of noscapine.

EXPERIMENTAL APPROACH

Ultra-performance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight mass spectrometry-based metabolomics was used to analyse the in vitro incubation mixtures, urine and faeces samples from mice treated with noscapine. Recombinant drug-metabolizing enzymes were employed to identify those involved in noscapine metabolism. Hepatic GSH levels and serum biochemistry were also carried out to determine reactive metabolites of noscapine.

KEY RESULTS

Several novel phase I metabolites of noscapine were detected after oral gavage of mice, including an N-demethylated metabolite, two hydroxylated metabolites, one metabolite undergoing both demethylation and cleavage of the methylenedioxy group and a bis-demethylated metabolite. Additionally, several novel glucuronides were detected, and their structures were elucidated through MS/MS fragmentology. Recombinant enzymes screening showed the involvement of several cytochromes P450, flavin-containing mono-oxygenase 1 and the UDP-glucuronosyltransferases UGT1A1, UGT1A3, UGT1A9 and UGT2B7, in noscapine metabolism. In vitro glutathione trapping revealed the existence of an ortho-quinone reactive intermediate formed through further oxidation of a catechol metabolite. However, this bioactivation process of noscapine does not occur in vivo. Similar to this result, altered glutathione levels in liver and serum biochemistry revealed no evidence of hepatic damage, thus indicating that, at least in mice, noscapine does not induce hepatotoxicity through bioactivation.

CONCLUSIONS AND IMPLICATIONS

A comprehensive metabolic map and bioactivation evaluation provides important information for the development of noscapine as an anti-tumour drug.

Assessment of hepatotoxicity potential of drug candidate molecules including kinase inhibitors by hepatocyte imaging assay technology and bile flux imaging assay technology

Kinases are members of a major protein family targeted for drug discovery and development. Given the ubiquitous nature of many kinases as well as the broad range of pathways controlled by these enzymes, early safety assessments of small molecule inhibitors of kinases are crucial in identifying new molecules with sufficient therapeutic window for clinical development. Failure or attrition of drug candidates in late-stage pipelines due to hepatotoxicity is a significant challenge in the drug development field. Herein we provide detailed methods for the hepatocyte imaging assay technology (HIAT) and the bile flux imaging assay technology (BIAT) to evaluate drug-induced liver injury (DILI) potentials for drug candidates. Optimized culturing methods for primary human hepatocytes, both freshly isolated and prequalified cryopreserved cells, are also presented. The applications of these high-content cellular imaging technologies in the evaluation of p38 and Her2 kinase inhibitors are highlighted to illustrate the usefulness of the research methodology in a compound screening as well as mechanistic investigative setting.

High-resolution chromatography/time-of-flight MSE with in silico data mining is an information-rich approach to reactive metabolite screening

Electrophilic reactive metabolite screening by liquid chromatography/mass spectrometry (LC/MS) is commonly performed during drug discovery and early-stage drug development. Accurate mass spectrometry has excellent utility in this application, but sophisticated data processing strategies are essential to extract useful information. Herein, a unified approach to glutathione (GSH) trapped reactive metabolite screening with high-resolution LC/TOF MS(E) analysis and drug-conjugate-specific in silico data processing was applied to rapid analysis of test compounds without the need for stable- or radio-isotope-labeled trapping agents. Accurate mass defect filtering (MDF) with a C-heteroatom dealkylation algorithm dynamic with mass range was compared to linear MDF and shown to minimize false positive results. MS(E) data-filtering, time-alignment and data mining post-acquisition enabled detection of 53 GSH conjugates overall formed from 5 drugs. Automated comparison of sample and control data in conjunction with the mass defect filter enabled detection of several conjugates that were not evident with mass defect filtering alone. High- and low-energy MS(E) data were time-aligned to generate in silico product ion spectra which were successfully applied to structural elucidation of detected GSH conjugates. Pseudo neutral loss and precursor ion chromatograms derived post-acquisition demonstrated 50.9% potential coverage, at best, of the detected conjugates by any individual precursor or neutral loss scan type. In contrast with commonly applied neutral loss and precursor-based techniques, the unified method has the advantage of applicability across different classes of GSH conjugates. The unified method was also successfully applied to cyanide trapping analysis and has potential for application to alternate trapping agents.

Metabolic activation in drug-induced liver injury

It is generally believed that metabolic bioactivation of drug molecules to form reactive metabolites, followed by their covalent binding to endogenous macromolecules, is one of the mechanisms that can lead to hepatotoxicity or idiosyncratic adverse drug reactions (IADRs). Although the role of bioactivation in drug-induced liver injury has been reasonably well established and accepted, and methodologies (e.g., structural alerts, reactive metabolite trapping, and covalent binding) continue to emerge in an attempt to detect the occurrence of bioactivation, the challenge remains to accurately predict the likelihood for idiosyncratic liver toxicity. Recent advances in risk-assessment methodologies, such as by the estimate of total body burden of covalent binding or by zone classification, taking the clinical dose into consideration, are positive steps toward improving risk assessment. The ability to better predict the potential of a drug candidate to cause IADRs will further be dependent upon a better understanding of the pathophysiological mechanisms of such reactions. Until a thorough understanding of the relationship between liver toxicity and the formation of reactive metabolites is achieved, it appears, at present, that the most practical strategy in drug discovery and development to reduce the likelihood of idiosyncratic liver toxicity via metabolic activation is to minimize or eliminate the occurrence of bioactivation and, at the same time, to maximize the pharmacological potency (to minimze the clinical dose) of the drug of interest.

Structural alert/reactive metabolite concept as applied in medicinal chemistry to mitigate the risk of idiosyncratic drug toxicity: a perspective based on the critical examination of trends in the top 200 drugs marketed in the United States

Because of a preconceived notion that eliminating reactive metabolite (RM) formation with new drug candidates could mitigate the risk of idiosyncratic drug toxicity, the potential for RM formation is routinely examined as part of lead optimization efforts in drug discovery. Likewise, avoidance of "structural alerts" is almost a norm in drug design. However, there is a growing concern that the perceived safety hazards associated with structural alerts and/or RM screening tools as standalone predictors of toxicity risks may be over exaggerated. In addition, the multifactorial nature of idiosyncratic toxicity is now well recognized based upon observations that mechanisms other than RM formation (e.g., mitochondrial toxicity and inhibition of bile salt export pump (BSEP)) also can account for certain target organ toxicities. Hence, fundamental questions arise such as: When is a molecule that contains a structural alert (RM positive or negative) a cause for concern? Could the molecule in its parent form exert toxicity? Can a low dose drug candidate truly mitigate metabolism-dependent and -independent idiosyncratic toxicity risks? In an effort to address these questions, we have retrospectively examined 68 drugs (recalled or associated with a black box warning due to idiosyncratic toxicity) and the top 200 drugs (prescription and sales) in the United States in 2009 for trends in physiochemical characteristics, daily doses, presence of structural alerts, evidence for RM formation as well as toxicity mechanism(s) potentially mediated by parent drugs. Collectively, our analysis revealed that a significant proportion (∼78-86%) of drugs associated with toxicity contained structural alerts and evidence indicating that RM formation as a causative factor for toxicity has been presented in 62-69% of these molecules. In several cases, mitochondrial toxicity and BSEP inhibition mediated by parent drugs were also noted as potential causative factors. Most drugs were administered at daily doses exceeding several hundred milligrams. There was no obvious link between idiosyncratic toxicity and physicochemical properties such as molecular weight, lipophilicity, etc. Approximately half of the top 200 drugs for 2009 (prescription and sales) also contained one or more alerts in their chemical architecture, and many were found to be RM-positive. Several instances of BSEP and mitochondrial liabilities were also noted with agents in the top 200 category. However, with relatively few exceptions, the vast majority of these drugs are rarely associated with idiosyncratic toxicity, despite years of patient use. The major differentiating factor appeared to be the daily dose; most of the drugs in the top 200 list are administered at low daily doses. In addition, competing detoxication pathways and/or alternate nonmetabolic clearance routes provided suitable justifications for the safety records of RM-positive drugs in the top 200 category. Thus, while RM elimination may be a useful and pragmatic starting point in mitigating idiosyncratic toxicity risks, our analysis suggests a need for a more integrated screening paradigm for chemical hazard identification in drug discovery. Thus, in addition to a detailed assessment of RM formation potential (in relationship to the overall elimination mechanisms of the compound(s)) for lead compounds, effects on cellular health (e.g., cytotoxicity assays), BSEP inhibition, and mitochondrial toxicity are the recommended suite of assays to characterize compound liabilities. However, the prospective use of such data in compound selection will require further validation of the cellular assays using marketed agents. Until we gain a better understanding of the pathophysiological mechanisms associated with idiosyncratic toxicities, improving pharmacokinetics and intrinsic potency as means of decreasing the dose size and the associated "body burden" of the parent drug and its metabolites will remain an overarching goal in drug discovery.

Handling reactive metabolite positives in drug discovery: What has retrospective structure-toxicity analyses taught us?

Because of the inability to predict and quantify the risk of idiosyncratic adverse drug reactions (IADRs) and because reactive metabolites (RMs) as opposed to the parent molecules from which they are derived are thought to be responsible for the pathogenesis of some IADRs, procedures (RM trapping/covalent binding) are being incorporated into the discovery screening funnel early-on to assess the risk of RM formation. Utility of the methodology in structure-toxicity relationships and scope in abrogating RM formation at the lead optimization stage are discussed in this article. Interpretation of the output from RM assessment assays, however, is confounded by the fact that many successfully marketed drugs are false positives. Therefore, caution must be exercised in deprioritizing a compound based on a positive result, so that the development of a useful and potentially profitable compound won't be unnecessarily halted. Risk mitigation strategies (e.g., competing detoxication pathways, low daily dose, etc.) when selecting RM positives for clinical development are also reviewed.

Nimesulide-induced electrophile stress activates Nrf2 in human hepatocytes and mice but is not sufficient to induce hepatotoxicity in Nrf2-deficient mice

Nimesulide is a widely prescribed nitroaromatic sulfoanilide-type nonsteroidal anti-inflammatory drug that, despite its favorable safety profile, has been associated with rare cases of idiosyncratic drug-induced liver injury (DILI). Because reactive metabolites have been implicated in DILI, we aimed at investigating whether hepatic bioactivation of nimesulide produces a protein-reactive intermediate in hepatocytes. Also, we explored whether nimesulide can activate the transcription factor Nrf2 that would protect from drug-induced hepatocyte injury. We found that [(14)C]-nimesulide covalently bound to human liver microsomes (<50 pmol/mg under standard conditions) or immortalized human hepatocytes in a sulfaphenazole-sensitive, rifampicin-inducible manner; yet the overall extent of binding was modest. Although exposure of hepatocytes to nimesulide was not associated with increased net levels of superoxide anion, nimesulide (100 microM, 24 h) caused nuclear translocation of Nrf2 in a sulfaphenazole-sensitive manner, indicating a role of electrophilic metabolites. However, knockdown of Nrf2 with siRNA did not make the cells more sensitive to nimesulide-induced cell injury. Similarly, exposure of wild-type C57BL/6x129 Sv mice to nimesulide (100 mg/kg/day, po, for 5 days) was associated with nuclear translocation of immunoreactive Nrf2 in a small number of hepatocytes and induced >2-fold the expression levels of the Nrf2-target gene Nqo1 in wild-type but not Nrf2-null mice. Nimesulide administered to Nrf2(-/-) knockout mice did not cause increases in serum ALT activity or any apparent histopathological signs of liver injury. In conclusion, these data indicate that nimesulide is bioactivated by CYP2C to a protein-reactive electrophilic intermediate that activates the Nrf2 pathway even at nontoxic exposure levels.

Can in vitro metabolism-dependent covalent binding data distinguish hepatotoxic from nonhepatotoxic drugs? An analysis using human hepatocytes and liver S-9 fraction

In vitro covalent binding studies in which xenobiotics are shown to undergo metabolism-dependent covalent binding to macromolecules have been commonly used to shed light on the biochemical mechanisms of xenobiotic-induced toxicity. In this paper, 18 drugs (nine hepatotoxins and nine nonhepatotoxins) were tested for their proclivity to demonstrate metabolism-dependent covalent binding to macromolecules in human liver S-9 fraction (9000 g supernatant) or human hepatocytes, as an extension to previous work that used human liver microsomes published in this journal [ Obach et al. ( 2008 ) Chem. Res. Toxicol. 21 , 1814 -1822 ]. In the S-9 fraction, seven out of the nine drugs in each category demonstrated some level of metabolism-dependent covalent binding. Inclusion of reduced glutathione, cofactors needed by conjugating enzymes, and other parameters (total daily dose and fraction of total intrinsic clearance comprised by covalent binding) improved the ability of the system to separate hepatotoxins from nonhepatotoxins to a limited extent. Covalent binding in human hepatocytes showed that six out of the nine hepatotoxins and four out of eight nonhepatotoxins demonstrated covalent binding. Taking into account estimates of total daily body burden of covalent binding from the hepatocyte data showed an improvement over other in vitro systems for distinguishing hepatotoxins from nonhepatotoxins; however, this metabolism system still displayed some false positives. Combined with the previous study using liver microsomes, these findings identify the limitations of in vitro covalent binding data for prospective prediction of hepatotoxicity for new drug candidates and highlight the need for a better understanding of the link between drug bioactivation, covalent adduct formation, and toxicity outcomes. Directly relating covalent binding to hepatotoxicity is likely an oversimplification of the process whereby adduct formation ultimately leads to toxicity. Understanding underlying complexities (e.g., which macromolecules are important covalent binding targets, interindividual differences in susceptibility, etc.) will be essential to any understanding of the problem of metabolism-dependent hepatotoxicity and predicting toxicity from in vitro experiments.

Can in vitro metabolism-dependent covalent binding data in liver microsomes distinguish hepatotoxic from nonhepatotoxic drugs? An analysis of 18 drugs with consideration of intrinsic clearance and daily dose

In vitro covalent binding assessments of drugs have been useful in providing retrospective insights into the association between drug metabolism and a resulting toxicological response. On the basis of these studies, it has been advocated that in vitro covalent binding to liver microsomal proteins in the presence and the absence of NADPH be used routinely to screen drug candidates. However, the utility of this approach in predicting toxicities of drug candidates accurately remains an unanswered question. Importantly, the years of research that have been invested in understanding metabolic bioactivation and covalent binding and its potential role in toxicity have focused only on those compounds that demonstrate toxicity. Investigations have not frequently queried whether in vitro covalent binding could be observed with drugs with good safety records. Eighteen drugs (nine hepatotoxins and nine nonhepatotoxins in humans) were assessed for in vitro covalent binding in NADPH-supplemented human liver microsomes. Of the two sets of nine drugs, seven in each set were shown to undergo some degree of covalent binding. Among hepatotoxic drugs, acetaminophen, carbamazepine, diclofenac, indomethacin, nefazodone, sudoxicam, and tienilic acid demonstrated covalent binding, while benoxaprofen and felbamate did not. Of the nonhepatotoxic drugs evaluated, buspirone, diphenhydramine, meloxicam, paroxetine, propranolol, raloxifene, and simvastatin demonstrated covalent binding, while ibuprofen and theophylline did not. A quantitative comparison of covalent binding in vitro intrinsic clearance did not separate the two groups of compounds, and in fact, paroxetine, a nonhepatotoxin, showed the greatest amount of covalent binding in microsomes. Including factors such as the fraction of total metabolism comprised by covalent binding and the total daily dose of each drug improved the discrimination between hepatotoxic and nontoxic drugs based on in vitro covalent binding data; however, the approach still would falsely identify some agents as potentially hepatotoxic.

Cellular imaging predictions of clinical drug-induced liver injury

Drug-induced liver injury (DILI) is the most common adverse event causing drug nonapprovals and drug withdrawals. Using drugs as test agents and measuring a panel of cellular phenotypes that are directly linked to key mechanisms of hepatotoxicity, we have developed an in vitro testing strategy that is predictive of many clinical outcomes of DILI. Mitochondrial damage, oxidative stress, and intracellular glutathione, all measured by high content cellular imaging in primary human hepatocyte cultures, are the three most important features contributing to the hepatotoxicity prediction. When applied to over 300 drugs and chemicals including many that caused rare and idiosyncratic liver toxicity in humans, our testing strategy has a true-positive rate of 50-60% and an exceptionally low false-positive rate of 0-5%. These in vitro predictions can augment the performance of the combined traditional preclinical animal tests by identifying idiosyncratic human hepatotoxicants such as nimesulide, telithromycin, nefazodone, troglitazone, tetracycline, sulindac, zileuton, labetalol, diclofenac, chlorzoxazone, dantrolene, and many others. Our findings provide insight to key DILI mechanisms, and suggest a new approach in hepatotoxicity testing of pharmaceuticals.