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.

Analysis of Factors Affecting Concentrations and Concentration-To-Dose Ratios of Trazodone

BACKGROUND

Trazodone is prescribed for several clinical conditions. Multiple factors may affect trazodone to reach its therapeutic reference range. The concentration-to-dose (C/D) ratio can be used to facilitate the therapeutic drug monitoring of trazodone. The study aimed to investigate factors on the concentrations and C/D ratio of trazodone.

METHODS

This study analyzed the therapeutic drug monitoring electronic case information of inpatients in the First Hospital of Hebei Medical University from October 2021 to July 2023. Factors that could affect the concentrations and C/D ratio of trazodone were analyzed, including body mass index, sex, age, smoking, drinking, drug manufacturers, and concomitant drugs.

RESULTS

A total of 255 patients were analyzed. The mean age was 52.44 years, and 142 (55.69%) were women. The mean dose of trazodone was 115.29 mg. The mean concentration of trazodone was 748.28 ng/mL, which was in the therapeutic reference range (700-1000 ng/mL). 50.20% of patients reached the reference range, and some patients (36.86%) had concentrations below the reference range. The mean C/D ratio of trazodone was 6.76 (ng/mL)/(mg/d). A significant positive correlation was found between daily dose and trazodone concentrations (r 2 = 0.2885, P < 0.001). Trazodone concentrations were significantly affected by dosage, sex, smoking, drinking, and concomitant drugs of duloxetine or fluoxetine. After dosage emendation, besides the above factors, it was influenced by age ( P < 0.05, P < 0.01, or P < 0.001).

CONCLUSIONS

This study identified factors affecting trazodone concentrations and C/D ratio. The results can help clinicians closely monitor patients on trazodone therapy and maintain concentrations within the reference range.

Solriamfetol and m-chlorophenylpiperazine cause false positive amphetamine results on urine drug screening

Urine drug screening by immunoassay is a common method to quickly identify drug exposures in the emergency setting and to detect unexpected drug exposures in a variety of patient care and occupational health settings. Although they provide rapid results, immunoassays are susceptible to cross-reactivity with other medications and metabolites. Herein we evaluate the performance of the Thermo Scientific DRI Amphetamines immunoassay for reactivity with trazodone, aripiprazole, atomoxetine, solriamfetol and relevant metabolites. Each of these compounds were spiked into drug-free urine across a range of concentrations and assessed for positivity on amphetamine screen. We demonstrate that the Thermo Scientific DRI assay is susceptible to interferences from m-chlorophenylpiperazine (mCPP), the main metabolite of trazodone, and solriamfetol. Characterization of assay-specific interferences in toxicology screening is instrumental for accurate interpretation of toxicology results, evaluation of patients in emergent settings and supporting patient care.

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.

In vitro evaluation of fenfluramine and norfenfluramine as victims of drug interactions

Fenfluramine (FFA) has potent antiseizure activity in severe, pharmacoresistant childhood‐onset developmental and epileptic encephalopathies (e.g., Dravet syndrome). To assess risk of drug interaction affecting pharmacokinetics of FFA and its major metabolite, norfenfluramine (nFFA), we conducted in vitro metabolite characterization, reaction phenotyping, and drug transporter−mediated cellular uptake studies. FFA showed low in vitro clearance in human liver S9 fractions and in intestinal S9 fractions in all three species tested (t_1/2 > 120 min). Two metabolites (nFFA and an N‐oxide or a hydroxylamine) were detected in human liver microsomes versus six in dog and seven in rat liver microsomes; no metabolite was unique to humans. Selective CYP inhibitor studies showed FFA metabolism partially inhibited by quinidine (CYP2D6, 48%), phencyclidine (CYP2B6, 42%), and furafylline (CYP1A2, 32%) and, to a lesser extent (<15%), by tienilic acid (CYP2C9), esomeprazole (CYP2C19), and troleandomycin (CYP3A4/5). Incubation of nFFA with rCYP1A2, rCYP2B6, rCYP2C19, and rCYP2D6 resulted in 10%−20% metabolism and no clear inhibition of nFFA metabolism by any CYP‐selective inhibitor. Reaction phenotyping showed metabolism of FFA by recombinant human cytochrome P450 (rCYP) enzymes rCYP2B6 (10%–21% disappearance for 1 and 10 µM FFA, respectively), rCYP1A2 (22%−23%), rCYP2C19 (49%−50%), and rCYP2D6 (59%−97%). Neither FFA nor nFFA was a drug transporter substrate. Results show FFA metabolism to nFFA occurs through multiple pathways of elimination. FFA dose adjustments may be needed when administered with strong inhibitors or inducers of multiple enzymes involved in FFA metabolism (e.g., stiripentol).

Uptake and metabolism of the antidepressants sertraline, clomipramine, and trazodone in a garden cress (Lepidium sativum) model

Environmental contamination with pharmaceuticals has received growing attention in recent years. Several studies describe the presence of traces of drugs in water bodies and soils and their impacts on nontarget organisms including plants. Due to these facts investigations of the uptake and metabolism of pharmaceuticals in organisms is an emerging research area. The present study demonstrates the analysis of three selected antidepressants (sertraline, clomipramine, and trazodone) as well as metabolites and transformation products in a cress model (Lepidium sativum). Cress was treated with tap water containing 10 mg/L of the parent drugs. Employing an analytical approach based on high performance liquid chromatography coupled with quadrupole time of flight or Orbitrap mass spectrometry in MS and MS² modes, in total 14 substances were identified in the cress extracts. All three parent drugs were taken up by the cress and translocated from the roots to the leaves in specific patterns. In addition to this, eleven metabolite species were identified. They were generated by hydroxylation, demethylation, conjugation with amino acids, or combinations of these mechanisms. Finally, the inclusion of control cultures in the experimental setup allowed for a differentiation of "true" metabolites generated by the cress and transformation products generated by plant-independent mechanisms.

Alleviating the hepatotoxicity of trazodone via supramolecular encapsulation

In order to develop a novel strategy to alleviate the inherent hepatotoxicity of antidepressant trazodone (TZ), Cucurbit[7]uril (CB[7]) was adopted as pharmaceutical excipients and was studied for its capability to reduce the hepatotoxicity of TZ via supramolecular encapsulation. CB[7] was found to form strong 1:1 host-guest complexes with TZ and its metabolite m-chlorophenyl piperazine (mCPP), with binding constants of 1.50 (±0.13) × 10⁶ M^-1 and 6.90 (±0.49) × 10⁵ M^-1, respectively. The supramolecular complexations were examined by ¹H NMR and UV-visible spectroscopic titrations, ESI-MS and ITC. In the presence of 0.5 mM CB[7], the IC₅₀ values of TZ and mCPP on a human normal liver cell line L02 were increased from 215.5 ± 3.3 μM to 544.1 ± 51.2 μM, and from 166.8 ± 3.8 μM to 241.7 ± 6.8 μM, respectively. Evaluation on a zebrafish model demonstrated that CB[7] (0.1 mM) significantly alleviated the TZ induced liver toxicity, as shown by the level of liver degeneration, liver size and yolk sac retention. Our study may provide a supramolecular strategy to alleviate the hepatotoxicity induced by TZ and its metabolite mCPP, and this strategy may be extendable to other drugs that have inherent hepatotoxicity or other adverse effects.

Pharmacogenetics of trazodone in healthy volunteers: association with pharmacokinetics, pharmacodynamics and safety

Aim: The aim was to evaluate the effect of polymorphisms in metabolizing enzymes and transporters on the pharmacokinetics, pharmacodynamics and adverse effects of trazodone in healthy volunteers. Materials & methods: 36 healthy volunteers receiving a single 100-mg oral dose of trazodone were genotyped for 11 variants in CYP3A4, CYP3A5, CYP2D6 and ABCB1 by real-time PCR. Plasma concentrations were measured using liquid chromatography-tandem mass spectrometry method. Results & conclusion: Sex affected the pharmacokinetics of trazodone with higher clearance in women. Polymorphisms in ABCB1, but not in CYP3A or CYP2D6, influenced trazodone pharmacokinetics. Trazodone decreased blood pressure and prolonged the corrected QT interval interval. CYP2D6 and ABCB1 polymorphisms were associated with the incidence of dizziness and prolonged corrected QT interval, respectively. Subjects with adverse drug reactions had lower concentrations of trazodone suggesting its metabolite (m-chlorophenylpiperazine) could be responsible for these effects.

Development of a cell viability assay to assess drug metabolite structure-toxicity relationships

Many adverse drug reactions are caused by the cytochrome P450 (CYP)-dependent activation of drugs into reactive metabolites. In order to reduce attrition due to metabolism-induced toxicity and to improve the safety of drug candidates, we developed a simple cell viability assay by combining a bioactivation system (human CYP3A4, CYP2D6 and CYP2C9) with Hep3B cells. We screened a series of drugs to explore structural motifs that may be responsible for CYP450-dependent activation caused by reactive metabolite formation, which highlighted specific liabilities regarding certain phenols and anilines.

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

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

Sucralose, a synthetic organochlorine sweetener: overview of biological issues

Sucralose is a synthetic organochlorine sweetener (OC) that is a common ingredient in the world's food supply. Sucralose interacts with chemosensors in the alimentary tract that play a role in sweet taste sensation and hormone secretion. In rats, sucralose ingestion was shown to increase the expression of the efflux transporter P-glycoprotein (P-gp) and two cytochrome P-450 (CYP) isozymes in the intestine. P-gp and CYP are key components of the presystemic detoxification system involved in first-pass drug metabolism. The effect of sucralose on first-pass drug metabolism in humans, however, has not yet been determined. In rats, sucralose alters the microbial composition in the gastrointestinal tract (GIT), with relatively greater reduction in beneficial bacteria. Although early studies asserted that sucralose passes through the GIT unchanged, subsequent analysis suggested that some of the ingested sweetener is metabolized in the GIT, as indicated by multiple peaks found in thin-layer radiochromatographic profiles of methanolic fecal extracts after oral sucralose administration. The identity and safety profile of these putative sucralose metabolites are not known at this time. Sucralose and one of its hydrolysis products were found to be mutagenic at elevated concentrations in several testing methods. Cooking with sucralose at high temperatures was reported to generate chloropropanols, a potentially toxic class of compounds. Both human and rodent studies demonstrated that sucralose may alter glucose, insulin, and glucagon-like peptide 1 (GLP-1) levels. Taken together, these findings indicate that sucralose is not a biologically inert compound.

Antidepressant drugs

Therapeutic Reviews aim to provide essential independent information for health professionals about drugs used in palliative and hospice care. Additional content is available on www.palliativedrugs.com. Country-specific books (Hospice and Palliative Care Formulary USA, and Palliative Care Formulary, British and Canadian editions) are also available and can be ordered from www.palliativedrugs.com. The series editors welcome feedback on the articles (hq@palliativedrugs.com).

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.

Bioactivation of glafenine by human liver microsomes and peroxidases: identification of electrophilic iminoquinone species and GSH conjugates

Glafenine (Privadol; 2,3-dihydroxypropyl 2-[(7-chloro-4-quinolinyl) amino]benzoate) is a non-narcotic analgesic agent widely used for the treatment of pains of various origins. Severe liver toxicity and a high incidence of anaphylaxis were reported in patients treated with glafenine, eventually leading to its withdrawal from the market in most countries. It is proposed that bioactivation of glafenine and subsequent binding of reactive metabolite(s) to critical cellular proteins play a causative role. The study described herein aimed at characterizing pathways of glafenine bioactivation and the metabolic enzymes involved. Two GSH conjugates of glafenine were detected in human liver microsomal incubations using liquid chromatography tandem mass spectrometry. The structures of detected conjugates were determined as GSH adducts of 5-hydroxyglafenine (M3) and 5-hydroxy glafenic acid (M4), respectively. GSH conjugation took place with a strong preference at C6 of the benzene ring of glafenine, ortho to the carbonyl moiety. These findings are consistent with a bioactivation sequence involving initial cytochrome P450-catalyzed 5-hydroxylation of the benzene ring of glafenine, followed by two electron oxidations of M3 and M4 to form corresponding para-quinone imine intermediates that react with GSH to form GSH adducts M1 and M2, respectively. Formation of M1 and M2 was primarily catalyzed by heterologously expressed recombinant CYP3A4 and to a lesser extent, CYP2C19 and CYP2D6. We demonstrated that M3 can also be bioactivated by peroxidases, such as horseradish peroxidase and myeloperoxidase. In summary, these findings have significance in understanding the bioactivation pathways of glafenine and their potential link to mechanisms of toxicity of glafenine.

Developing structure-activity relationships for the prediction of hepatotoxicity

Drug-induced liver injury is a major issue of concern and has led to the withdrawal of a significant number of marketed drugs. An understanding of structure-activity relationships (SARs) of chemicals can make a significant contribution to the identification of potential toxic effects early in the drug development process and aid in avoiding such problems. This process can be supported by the use of existing toxicity data and mechanistic understanding of the biological processes for related compounds. In the published literature, this information is often spread across diverse sources and can be varied and unstructured in quality and content. The current work has explored whether it is feasible to collect and use such data for the development of new SARs for the hepatotoxicity endpoint and expand upon the limited information currently available in this area. Reviews of hepatotoxicity data were used to build a structure-searchable database, which was analyzed to identify chemical classes associated with an adverse effect on the liver. Searches of the published literature were then undertaken to identify additional supporting evidence, and the resulting information was incorporated into the database. This collated information was evaluated and used to determine the scope of the SARs for each class identified. Data for over 1266 chemicals were collected, and SARs for 38 classes were developed. The SARs have been implemented as structural alerts using Derek for Windows (DfW), a knowledge-based expert system, to allow clearly supported and transparent predictions. An evaluation exercise performed using a customized DfW version 10 knowledge base demonstrated an overall concordance of 56% and specificity and sensitivity values of 73% and 46%, respectively. The approach taken demonstrates that SARs for complex endpoints can be derived from the published data for use in the in silico toxicity assessment of new compounds.

Dose proportionality of once-daily trazodone extended-release caplets under fasting conditions

An extended-release trazodone HCl formulation, Trazodone Contramid OAD (TzCOAD), was developed as scored 150-mg and 300-mg caplets for once-daily administration. Dose proportionality of intact and bisected caplets (dose range, 75-375 mg) was evaluated in a single-dose, randomized, 5-way crossover study. Plasma trazodone and m-chlorophenylpiperazine (mCPP) levels were determined using a validated liquid chromatography-tandem mass spectroscopy method. Dose proportionality was assessed based on confidence intervals for logarithmically transformed, dose-normalized maximum plasma concentration (C(max)), area under the plasma concentration versus time data pairs (AUC(0-t)), and area under the curve from time 0 to infinity (AUC(0-∞)) in relation to the acceptance range of 80% to 125% (bioequivalence approach). The power method, combined with confidence interval criteria, was also used to assess proportionality. The conclusion of dose proportionality was generally supported using the bioequivalence approach. Based on the power model, values of the slope and corresponding 90% confidence interval for trazodone C(max), AUC(0-t), and AUC(0-∞) were 0.948 (0.899-0.997), 0.920 (0.875-0.964), and 0.913 (0.867-0.958), respectively. All were within the acceptance interval (0.861-1.139). Results for mCPP also fell within the acceptance interval. TzCOAD exhibits linear pharmacokinetics over doses ranging from 75 to 375 mg and maintains its controlled-release properties when the caplets are bisected along the score line.

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

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

Mass defect filter technique and its applications to drug metabolite identification by high-resolution mass spectrometry

Identification of drug metabolites by liquid chromatography/mass spectrometry (LC/MS) involves metabolite detection in biological matrixes and structural characterization based on product ion spectra. Traditionally, metabolite detection is accomplished primarily on the basis of predicted molecular masses or fragmentation patterns of metabolites using triple-quadrupole and ion trap mass spectrometers. Recently, a novel mass defect filter (MDF) technique has been developed, which enables high-resolution mass spectrometers to be utilized for detecting both predicted and unexpected drug metabolites based on narrow, well-defined mass defect ranges for these metabolites. This is a new approach that is completely different from, but complementary to, traditional molecular mass- or MS/MS fragmentation-based LC/MS approaches. This article reviews the mass defect patterns of various classes of drug metabolites and the basic principles of the MDF approach. Examples are given on the applications of the MDF technique to the detection of stable and chemically reactive metabolites in vitro and in vivo. Advantages, limitations, and future applications are also discussed on MDF and its combinations with other data mining techniques for the detection and identification of drug metabolites.

Analytical strategies for the screening and evaluation of chemically reactive drug metabolites

BACKGROUND

Metabolic activation leading to formation of chemically reactive drug metabolites is a long-standing issue for drug development inasmuch as some, but not all, reactive intermediates play a role as mediators of drug-induced toxicities. The risk assessment profile/decision-making guide requires a comprehensive understanding of bioactivation mechanism(s), quantitative magnitude and cellular consequences of this principal and continued safety attrition.

OBJECTIVE

To evaluate analytical methodologies with improved sensitivity, selectivity and throughput for the analysis of reactive metabolites.

CONCLUSIONS

Identification and quantification of short-lived electrophilic intermediates through appropriate trapping experiments have become relatively straightforward. Minimizing the bioactivation potential of drug candidates during the discovery/lead optimization phase has been adopted as a default strategy. Together with advances of proteomics, metabolomics and toxicogenomics, an integrated multitier approach possibly provides a deeper insight into mechanistic aspects of drug-induced toxicities, and contributes to bridging the relationships between metabolic activation, drug-protein adduct formation and their toxicological consequences.

Screening for cytochrome p450 reactivity by harnessing catalase as reporter enzyme

Cytochrome P450 enzymes are known to catalyze a variety of reactions that are difficult to perform by standard organic synthesis, such as the oxidation of unactivated C--C bonds. Cytochrome P450 enzymes can also be used in artificial systems in which organic peroxides act as cosubstrates. To find substrates that are converted by a certain P450 catalyst in the presence of an organic peroxide, various screening assays have been established, however, most of them are limited to one or only a few specific substrates. Here, we report a simple and rapid screening assay that works independently of the nature of the substrate and utilizes a previously undescribed reactivity of catalase as reporter enzyme. In an initial demonstration of this assay, we screened 180 enzyme/peroxide/substrate combinations for potential bioconversions. As shown by subsequent verification of the screening results with liquid chromatography/multistage mass spectrometry (LC/MS(n)), we were able to identify three new substrates for the enzyme CYP152A1 and at least two previously undescribed conversions by the enzyme CYP119.

Characterization of dasatinib and its structural analogs as CYP3A4 mechanism-based inactivators and the proposed bioactivation pathways

Dasatinib was approved in 2006 for the treatment of imatinib-resistant chronic myelogenous leukemia and functions primarily through the inhibition of BCR-ABL and Src kinase. Dasatinib is extensively metabolized in humans by CYP3A4. In this study, we report that the bioactivation of dasatinib by CYP3A4 proceeds through a reactive intermediate that leads to CYP3A4 inactivation with K(I) = 6.3 microM and k(inact) = 0.034 min(-1). The major mechanism of inactivation proceeds through hydroxylation at the para-position of the 2-chloro-6-methylphenyl ring followed by further oxidation, forming a reactive quinone-imine, similar to the reactive intermediates formed by acetaminophen and diclofenac. Formation of a reactive imine-methide was also detected but appears to be a minor pathway. When glutathione was added to human liver microsomal incubations, dasatinib-glutathione adducts were detected. Numerous dasatinib analogs were synthesized in an effort to understand what modifications would block the formation of reactive intermediates during dasatinib metabolism. It is interesting to note that blocking the site of hydroxylation with a methyl group was not effective because a reactive imine-methide was formed, nor was blocking the site with fluorine because the fluorine was removed through an oxidative defluorination mechanism and the reactive quinone-imine was still formed. Numerous analogs are presented that did effectively block the formation of glutathione adducts and prevent the inactivation of CYP3A4.

Application of a trazodone-selective electrode to pharmaceutical quality control and urine analyses

A trazodone-selective electrode for application in pharmaceutical quality control and urine analysis was developed. The electrode is based on incorporation of a trazodone-tetraphenylborate ion exchanger in a poly(vinyl chloride) membrane. The electrode showed a fast, stable and Nernstian response over a wide trazodone concentration range (5 x 10(-5)-1 x 10(-2) M) with a mean slope of 59.3 +/- 0.9 mV/dec of concentration, a mean detection limit of 1.8 x 10(-5) +/- 2.2 x 10(-6) M, a wide working pH range (5-7.5) and a fast response time (less than 20 s). The electrode also showed good accuracy, repeatability, reproducibility and selectivity with respect to some inorganic and organic compounds, including the main trazodone metabolite. The electrode provided good analytical results in the determination of trazodone in pharmaceuticals and spiked urine samples; no extraction steps were necessary. Dissolution testing of trazodone tablets, in different conditions of pH and particle size, based on a direct potentiometric determination with the new selective electrode is presented as well.

Rapid screening and characterization of drug metabolites using a multiple ion monitoring-dependent MS/MS acquisition method on a hybrid triple quadrupole-linear ion trap mass spectrometer

A novel LC/MS/MS method that uses multiple ion monitoring (MIM) as a survey scan to trigger the acquisition of enhanced product ions (EPI) on a hybrid quadrupole-linear ion trap mass spectrometer (Q TRAP) was developed for drug metabolite identification. In the MIM experiment, multiple predicted metabolite ions were monitored in both Q1 and Q3. The collision energy in Q2 was set to a low value to minimize fragmentation. Results from analyzing ritonavir metabolites in rat hepatocytes demonstrate that MIM-EPI was capable of targeting a larger number of metabolites regardless of their fragmentation and retained sensitivity and duty cycle similar to multiple reaction monitoring (MRM)-EPI. MIM-based scanning methods were shown to be particularly useful in several applications. First, MIM-EPI enabled the sensitive detection and MS/MS acquisition of up to 100 predicted metabolites. Second, MIM-MRM-EPI was better than MRM-EPI in the analysis of metabolites that undergo either predictable or unpredictable fragmentation pathways. Finally, a combination of MIM-EPI and full-scan MS (EMS), as an alternative to EMS-EPI, was well suited for routine in vitro metabolite profiling. Overall, MIM-EPI significantly enhanced the metabolite identification capability of the hybrid triple quadrupole-linear ion trap LC/MS.