Identification of quinone imine containing glutathione conjugates of diclofenac in rat bile

Metabolic Activation and Cytotoxicity of Gramine Mediated by CYP3A in Rats

Gramine (GRM), which occurs in Gramineae plants, has been developed to be a biological insecticide. Exposure to GRM was reported to induce elevations of serum ALT and AST in rats, but the mechanisms of the observed hepatotoxicity have not been elucidated. The present study aimed to identify reactive metabolites that potentially participate in the toxicity. In rat liver microsomal incubations fortified with glutathione or N-acetylcysteine, one oxidative metabolite (M1), one glutathione conjugate (M2), and one N-acetylcysteine conjugate (M3) were detected after exposure to GRM. The corresponding conjugates were detected in the bile and urine of rats after GRM administration. CYP3A was the main enzyme mediating the metabolic activation of GRM. The detected GSH and NAC conjugates suggest that GRM was metabolized to a quinone imine intermediate. Both GRM and M1 showed significant toxicity to rat primary hepatocytes.

Glutathione conjugation and protein modification resulting from metabolic activation of pesticide metalaxyl in vitro and in vivo

Metalaxyl (MTL), a germicidal agent, is widely used in agriculture. Due to the biological amplification effect, MTL entering the ecological environment would result in a threat to human health through the food chain. MTL is reportedly accumulated in liver. The objectives of the study included investigating the metabolic activation of MTL in liver and defining the mechanisms participating in the hepatotoxicity of MTL. The corresponding glutathione (GSH), N-acetylcysteine (NAC) conjugate, and cysteine conjugates were observed in liver microsomes, prepared from liver tissues of mice, containing MTL and GSH, NAC or cysteine. These conjugates were also detected in urine and bile of rats receiving MTL. Apparently, MTL was biotransformed to a quinone imine intermediate dose-dependently attacking the thiols and cysteine residues of protein. The bioactivation of MTL required cytochrome P450 enzymes, and CYP3A dominated the bio-activation of MTL.

Discovery and Optimization of 5-Hydroxy-Diclofenac toward a New Class of Ligands with Nanomolar Affinity for the CaMKIIα Hub Domain

The Ca²⁺/calmodulin-dependent protein kinase II α (CaMKIIα) is a brain-relevant kinase involved in long-term potentiation and synaptic plasticity. We have recently pinpointed the CaMKIIα hub domain as the long-sought-after high-affinity target of γ-hydroxybutyrate ligands substantiated with a high-resolution cocrystal of 5-hydroxydiclofenac (3). Herein, we employed in silico approaches to rationalize and guide the synthesis and pharmacological characterization of a new series of analogues circumventing chemical stability problems associated with 3. The oxygen-bridged analogue 4d showed mid-nanomolar affinity and notable ligand-induced stabilization effects toward the CaMKIIα hub oligomer. Importantly, 4d displayed superior chemical and metabolic stability over 3 by showing excellent chemical stability in phosphate-buffered saline and high resistance to form reactive intermediates and subsequent sulfur conjugates. Altogether, our study highlights 4d as a new CaMKIIα hub high-affinity ligand with enhanced pharmacokinetic properties, representing a powerful tool compound for allosteric regulation of kinase activity with subtype specificity.

Impacts of diphenylamine NSAID halogenation on bioactivation risks

Diphenylamine NSAIDs are highly prescribed therapeutics for chronic pain despite causing symptomatic hepatotoxicity through mitochondrial damage in five percent of patients taking them. Differences in toxicity are attributed to structural modifications to the diphenylamine scaffold rather than its inherent toxicity. We hypothesize that marketed diphenylamine NSAID substituents affect preference and efficiency of bioactivation pathways and clearance. We parsed the FDA DILIrank hepatotoxicity database and modeled marketed drug bioactivation into quinone-species metabolites to identify a family of seven clinically relevant diphenylamine NSAIDs. These drugs fell into two subgroups, i.e., acetic acid and propionic acid diphenylamines, varying in hepatotoxicity risks and modeled bioactivation propensities. We carried out steady-state kinetic studies to assess bioactivation pathways by trapping quinone-species metabolites with dansyl glutathione. Analysis of the glutathione adducts by mass spectrometry characterized structures while dansyl fluorescence provided quantitative yields for their formation. Resulting kinetics identified four possible bioactivation pathways among the drugs, but reaction preference and efficiency depended upon structural modifications to the diphenylamine scaffold. Strikingly, diphenylamine dihalogenation promotes formation of quinone metabolites through four distinct metabolic pathways with high efficiency, whereas those without aromatic halogen atoms were metabolized less efficiently through two or fewer metabolic pathways. Overall metabolism of the drugs was comparable with bioactivation accounting for 4-13% of clearance. Lastly, we calculated daily bioload exposure of quinone-species metabolites based on bioactivation efficiency, bioavailability, and maximal daily dose. The results revealed stratification into the two subgroups; propionic acid diphenylamines had an average four-fold greater daily bioload compared to acetic acid diphenylamines. However, the lack of sufficient study on the hepatotoxicity for all drugs prevents further correlative analyses. These findings provide critical insights on the impact of diphenylamine bioactivation as a precursor to hepatotoxicity and thus, provide a foundation for better risk assessment in drug discovery and development.

Characterization of Stable and Reactive Metabolites of the Anticancer Drug, Ensartinib, in Human Liver Microsomes Using LC-MS/MS: An in silico and Practical Bioactivation Approach

BACKGROUND

Ensartinib (ESB) is a novel anaplastic lymphoma kinase inhibitor (ALK) with additional activity against Abelson murine leukemia (ABL), met proto-oncogene (MET), receptor tyrosine kinase (AXL), and v-ros UR2 sarcoma virus oncogene homolog 1 (ROS1) and is considered a safer alternative for other ALK inhibitors. ESB chemical structure contains a dichloro-fluorophenyl ring and cyclic tertiary amine rings (piperazine) that can be bioactivated generating reactive intermediates.

METHODS

In vitro metabolic study of ESB with human liver microsomes (HLMs) was performed and the hypothesis of generating reactive intermediates during metabolism was tested utilizing trapping agents to capture and stabilize reactive intermediates to facilitate their LC-MS/MS detection. Reduced glutathione (GSH) and potassium cyanide (KCN) were utilized as trapping agents for quinone methide and iminium intermediates, respectively.

RESULTS

Four in vitro ESB phase I metabolites were characterized. Three reactive intermediates including one epoxide and one iminium intermediates were characterized. ESB bioactivation is proposed to occur through unexpected metabolic pathways. The piperazine ring was bioactivated through iminium ions intermediates generation, while the dichloro-phenyl group was bioactivated through a special mechanism that was revealed by LC-MS/MS.

CONCLUSION

These findings lay the foundations for additional work on ESB toxicity. Substituents to the bioactive centers (piperazine ring), either for blocking or isosteric replacement, would likely block or interrupt hydroxylation reaction that will end the bioactivation sequence.

Drug metabolite identification using ultrahigh-performance liquid chromatography-ultraviolet spectroscopy and parallelized scans on a tribrid Orbitrap mass spectrometer

RATIONALE

To capture all metabolites in metabolite identification studies, MS/MS information is required in both positive and negative ionization mode, usually involving several sample injections to gain all information about samples. A high-resolution and high mass accuracy quadrupole/linear trap/Orbitrap tribrid instrument was used to gain this information in a novel single injection 'capture-all' approach to metabolite identification.

METHODS

Diclofenac, a model compound, was incubated in human and rat hepatocytes. These incubated samples were run using an ultrahigh-performance liquid chromatography/ultraviolet (UHPLC-UV) system coupled to a Thermo Fusion tribrid mass spectrometer. Five parallel scans were used: positive and negative ion full scan, data-dependent MS/MS, both high energy dissociation and collision-induced dissociation, and data-independent all ion fragmentation (AIF) spectra were collected in positive and negative ion mode.

RESULTS

Nine metabolites were identified; a metabolite observed in the UV trace, but not positive ion full scan MS, was detected in the same sample injection by negative ion full scan MS. This was identified as a sulphate metabolite, and the corresponding negative ion AIF allowed for some structural elucidation. The use of a photo-diode array (PDA) detector allowed for spectral assessment in case of changes in absorbance spectra, and the subsequent semi-quantification of metabolites.

CONCLUSIONS

This method provided good-quality MS/MS data across the m/z range in both positive and negative ion mode. The addition of both negative ion full scan MS and negative ion MS/MS allowed for the detection and structural elucidation of metabolites not observed in positive ion mode. The use of the PDA detector allowed for the semi-quantification of metabolites.

Phase I metabolic profiling and unexpected reactive metabolites in human liver microsome incubations of X-376 using LC-MS/MS: bioactivation pathway elucidation and in silico toxicity studies of its metabolites

Metabolites of X-376 were characterized by LC-MS/MS. Pyridazine ring and dichloro-phenyl groups were bioactivated by novel pathways.

Identification and Characterization of Efflux Transporters That Modulate the Subtoxic Disposition of Diclofenac and Its Metabolites

In the present work, in vivo transporter knockout (KO) mouse models were used to characterize the disposition of diclofenac (DCF) and its primary metabolites following a single subtoxic dose in mice lacking breast cancer resistance protein (Bcrp) or multidrug resistance-associated protein (Mrp)3. The results indicate that Bcrp acts as a canalicular efflux mediator for DCF, as wild-type (WT) mice had biliary excretion values that were 2.2- to 2.6-fold greater than Bcrp KO mice, although DCF plasma levels were not affected. The loss of Bcrp resulted in a 1.8- to 3.2-fold increase of diclofenac acyl glucuronide (DCF-AG) plasma concentrations in KO animals compared with WT mice, while the biliary excretion of DCF-AG increased 1.4-fold in WT versus KO mice. Furthermore, Mrp3 was found to mediate the basolateral transport of DCF-AG, but not DCF or 4'-hydroxy diclofenac. WT mice had DCF-AG plasma concentrations 7.0- to 8.6-fold higher than Mrp3 KO animals; however, there were no changes in biliary excretion of DCF-AG. Vesicular transport experiments with human MRP3 demonstrated that MRP3 is able to transport DCF-AG via low- and high-affinity binding sites. The low-affinity MRP3 transport had a V _max and K _m of 170 pmol/min/mg and 98.2 µM, respectively, while the high-affinity V _max and K _m parameters were estimated to be 71.9 pmol/min/mg and 1.78 µM, respectively. In summary, we offer evidence that the disposition of DCF-AG can be affected by both Bcrp and Mrp3, and these findings may be applicable to humans.

Accessing Drug Metabolites via Transition-Metal Catalyzed C-H Oxidation: The Liver as Synthetic Inspiration

Can classical and modern chemical C-H oxidation reactions complement biotransformation in the synthesis of drug metabolites? We have surveyed the literature in an effort to try to answer this important question of major practical significance in the pharmaceutical industry. Drug metabolites are required throughout all phases of the drug discovery and development process; however, their synthesis is still an unsolved problem. This Review, not intended to be comprehensive or historical, highlights relevant applications of chemical C-H oxidation reactions, electrochemistry and microfluidic technologies to drug templates in order to access drug metabolites, and also highlights promising reactions to this end. Where possible or appropriate, the contrast with biotransformation is drawn. In doing so, we have tried to identify gaps where they exist in the hope to spur further activity in this very important research area.

Lumiracoxib metabolism in male C57bl/6J mice: characterisation of novel in vivo metabolites

1. The pharmacokinetics and metabolism of lumiracoxib in male C57bl/6J mice were investigated following a single oral dose of 10 mg/kg. 2. Lumiracoxib achieved peak observed concentrations in the blood of 1.26 + 0.51 μg/mL 0.5 h (0.5-1.0) post-dose with an AUC_inf of 3.48 + 1.09 μg h/mL. Concentrations of lumiracoxib then declined with a terminal half-life of 1.54 + 0.31 h. 3. Metabolic profiling showed only the presence of unchanged lumiracoxib in blood by 24 h, while urine, bile and faecal extracts contained, in addition to the unchanged parent drug, large amounts of hydroxylated and conjugated metabolites. 4. No evidence was obtained in the mouse for the production of the downstream products of glutathione conjugation such as mercapturates, suggesting that the metabolism of the drug via quinone-imine generating pathways is not a major route of biotransformation in this species. Acyl glucuronidation appeared absent or a very minor route. 5. While there was significant overlap with reported human metabolites, a number of unique mouse metabolites were detected, particularly taurine conjugates of lumiracoxib and its oxidative metabolites.

A quantitative assay for reductive metabolism of a pesticide in fish using electrochemistry coupled with liquid chromatography tandem mass spectrometry

This is the first study to use electrochemistry to generate a nitro reduction metabolite as a standard for a liquid chromatography-mass spectrometry-based quantitative assay. This approach is further used to quantify 3-trifluoromethyl-4-nitrophenol (TFM) reductive metabolism. TFM is a widely used pesticide for the population control of sea lamprey (Petromyzon marinus), an invasive species of the Laurentian Great Lakes. Three animal models, sea lamprey, lake sturgeon (Acipenser fulvescens), and rainbow trout (Oncorhynchus mykiss), were selected to evaluate TFM reductive metabolism because they have been known to show differential susceptibilities to TFM toxicity. Amino-TFM (aTFM; 3-trifluoromethyl-4-aminophenol) was the only reductive metabolite identified through liquid chromatography-high-resolution mass spectrometry screening of liver extracts incubated with TFM and was targeted for electrochemical synthesis. After synthesis and purification, aTFM was used to develop a quantitative assay of the reductive metabolism of TFM through liquid chromatography and tandem mass spectrometry. The concentrations of aTFM were measured from TFM-treated cellular fractions, including cytosolic, nuclear, membrane, and mitochondrial protein extracts. Sea lamprey extracts produced the highest concentrations (500 ng/mL) of aTFM. In addition, sea lamprey and sturgeon cytosolic extracts showed concentrations of aTFM substantially higher than those of rainbow trout. However, other fractions of lake sturgeon extracts tend to show aTFM concentrations similar to those of rainbow trout but not with sea lamprey. These data suggest that the level of reductive metabolism of TFM may be associated with the sensitivities of the animals to this particular pesticide.

Preparative microfluidic electrosynthesis of drug metabolites

In vivo, a drug molecule undergoes its first chemical transformation within the liver via CYP450-catalyzed oxidation. The chemical outcome of the first pass hepatic oxidation is key information to any drug development process. Electrochemistry can be used to simulate CYP450 oxidation, yet it is often confined to the analytical scale, hampering product isolation and full characterization. In an effort to replicate hepatic oxidations, while retaining high throughput at the preparative scale, microfluidic technology and electrochemistry are combined in this study by using a microfluidic electrochemical cell. Several commercial drugs were subjected to continuous-flow electrolysis. They were chosen for their various chemical reactivity: their metabolites in vivo are generated via aromatic hydroxylation, alkyl oxidation, glutathione conjugation, or sulfoxidation. It is demonstrated that such metabolites can be synthesized by flow electrolysis at the 10 to 100 mg scale, and the purified products are fully characterized.

Potential protective effect of sunitinib after administration of diclofenac: biochemical and histopathological drug-drug interaction assessment in a mouse model

Sunitinib is a tyrosine kinase inhibitor for GIST and advanced renal cell carcinoma. Diclofenac is used in cancer pain management. Coadministration may mediate P450 toxicity. We evaluate their interaction, assessing biomarkers ALT, AST, BUN, creatinine, and histopathological changes in the liver, kidney, heart, brain, and spleen. ICR mice (male, n = 6 per group/dose) were administered saline (group A) or 30 mg/kg diclofenac ip (group B), or sunitinib po at 25, 50, 80, 100, 140 mg/kg (group C) or combination of diclofenac (30 mg/kg, ip) and sunitinib (25, 50, 80, 100, 140 mg/kg po). Diclofenac was administered 15 min before sunitinib, mice were euthanized 4 h post-sunitinib dose, and biomarkers and tissue histopathology were assessed. AST was 92.2 ± 8.0 U/L in group A and 159.7 ± 14.6 U/L in group B (p < 0.05); in group C, it the range was 105.1-152.6 U/L, and in group D, it was 156.0-209.5 U/L (p < 0.05). ALT was 48.9 ± 1.6 U/L (group A), 95.1 ± 4.5 U/L (p < 0.05) in group B, and 50.5-77.5 U/L in group C and 82.3-115.6 U/L after coadministration (p < 0.05). Renal function biomarker BUN was 16.3 ± 0.6 mg/dl (group A) and increased to 29.9 ± 2.6 mg/dl in group B (p < 0.05) and it the range was 19.1-33.3 mg/dl (p < 0.05) and 26.9-40.8 mg/dl in groups C and D, respectively. Creatinine was 5.9 pmol/ml in group A; 6.2 pmol/ml in group B (p < 0.01), and the range was 6.0-6.2 and 6.2-6.4 pmol/ml in groups C and D, respectively (p < 0.05 for D). Histopathological assessment (vascular and inflammation damages) showed toxicity in group B (p < 0.05) and mild toxicity in group C. Damage was significantly lesser in group D than group B (p < 0.05). Spleen only showed toxicity after coadministration. These results suggest vascular and inflammation protective effects of sunitinib, not shown after biomarker analysis.

Inhibition and induction of glutathione S-transferases by flavonoids: possible pharmacological and toxicological consequences

Many studies reviewed herein demonstrated the potency of some flavonoids to modulate the activity and/or expression of glutathione S-transferases (GSTs). Because GSTs play a crucial role in the detoxification of xenobiotics, their inhibition or induction may significantly affect metabolism and biological effects of many drugs, industrials, and environmental contaminants. The effect of flavonoids on GSTs strongly depends on flavonoid structure, concentration, period of administration, as well as on GST isoform and origin. Moreover, the results obtained in vitro are often contrary to the vivo results. Based on these facts, the revelation of important flavonoid-drug or flavonoid-pollutant interaction has been complicated. However, it should be borne in mind that ingestion of certain flavonoids in combination with drugs or pollutants (e.g., acetaminophen, simvastatin, cyclophosphamide, cisplatine, polycyclic aromatic hydrocarbons, chlorpyrifos, acrylamide, and isocyanates), which are GST substrates, could have significant pharmacological and toxicological consequences. Although reasonable consumptions of a flavonoids-rich diet (that may lead to GST induction) are mostly beneficial, the uncontrolled intake of high concentrations of certain flavonoids (e.g., quercetin and catechins) in dietary supplements (that may cause GST inhibition) may threaten human health.

Bioactivation of the cancer chemopreventive agent tamoxifen to quinone methides by cytochrome P4502B6 and identification of the modified residue on the apoprotein

The nonsteroidal antiestrogen tamoxifen was introduced as a treatment for breast cancer 3 decades ago. It has also been approved as a chemopreventive agent and is prescribed to women at high risk for this disease. However, several studies have shown that use of tamoxifen leads to increased risk of endometrial cancer in humans. One potential pathway of tamoxifen toxicity could involve metabolism via hydroxylation to give 4-hydroxytamoxifen (4OHtam), which may be further oxidized to form a quinone methide. CYP2B6 is a highly polymorphic drug-metabolizing enzyme, and it metabolizes a number of clinically important drugs. Earlier studies from our laboratory have shown that tamoxifen is a mechanism-based inactivator of CYP2B6. The aim of the current study was to investigate the possible formation of reactive intermediates through detection of protein covalent binding and glutathione ethyl ester adduct (GSHEE) formation. The incubation of tamoxifen with 2B6 gave rise to an adduct of 4OHtam with glutathione, which was characterized as the 4OHtam quinone methide + GSHEE with an m/z value of 719, and the structure was characterized by liquid chromatography-tandem mass spectrometry. The metabolic activation of tamoxifen in the CYP2B6 reconstituted system also resulted in the formation of an adduct to the P4502B6 apoprotein, which was identified using liquid chromatography mass spectrometry. The site responsible for the inactivation of CYP2B6 was determined by proteolytic digestion and identification of the labeled peptide. This revealed a tryptic peptide ¹⁸⁸FHYQDQE¹⁹⁴ with the site of adduct formation localized to Gln193 as the site modified by the reactive metabolite formed during tamoxifen metabolism.

Covalent protein binding and tissue distribution of houttuynin in rats after intravenous administration of sodium houttuyfonate

AIM

To investigate the potential of houttuynin to covalently bind to proteins in vitro and in vivo and to identify the adduct structures.

METHODS

Male Sprague-Dawley rats were intravenously injected with sodium houttuyfonate (10 mg/kg). The concentrations of houttuynin in blood, plasma and five tissues tested were determined using an LC/MS/MS method. The covalent binding values of houttuynin with hemoglobin, plasma and tissue proteins were measured in rats after intravenous injection of [1-(14)C]sodium houttuyfonate (10 mg/kg, 150 mCi/kg). Human serum albumin was used as model protein to identify the modification site(s) and structure(s) through enzymatic digestion and LC/MS(n) analysis.

RESULTS

The drug was widely distributed 10 min after intravenous injection. The lungs were the preferred site for disposition, followed by the heart and kidneys with significantly higher concentrations than that in the plasma. The extent of covalent binding was correlated with the respective concentrations in the tissues, ranging from 1137 nmol/g protein in lung to 266 nmol/g protein in liver. Houttuynin reacted primarily with arginine residues in human serum albumin to form a pyrimidine adduct at 1:1 molar ratio. The same adduct was detected in rat lungs digested by pronase E.

CONCLUSION

This study showed that the β-keto aldehyde moiety in houttuynin is strongly electrophilic and readily confers covalent binding to tissue proteins, especially lung proteins, by a Schiff's base mechanism. The findings explain partially the idiosyncratic reactions of houttuyniae injection in clinical use.

Application of on-line nano-liquid chromatography/mass spectrometry in metabolite identification studies

Metabolite identification is an important part of the drug discovery and development process. High sensitivity is necessary to identify metabolic products in vitro and in vivo. The most common method utilizes standard high-performance liquid chromatography (4.6  mm i.d. column and 1  mL/min flow rate) coupled to tandem mass spectrometry (HPLC/MS/MS). We have developed a method that utilizes a nano-LC system coupled to a high-resolution tandem mass spectrometer to identify metabolites from in vitro and in vivo samples. Using this approach, we were able to increase the sensitivity of analysis by approximately 1000-fold over HPLC/MS. In vitro samples were analyzed after simple acetonitrile precipitation, centrifugation, and dilution. The significant improvement in sensitivity enabled us to conduct experiments at very low substrate concentrations (0.01  μM), and very low incubation volumes (20  μL). In vivo samples were injected after simple dilution without any pre-purification. All the metabolites identified by conventional HPLC/MS/MS were also identified using the nano-LC method. This study demonstrates a very sensitive approach to identifying phase I and II metabolites with throughput and separation equivalent to the standard HPLC/MS/MS method.

Diclofenac metabolism in the mouse: novel in vivo metabolites identified by high performance liquid chromatography coupled to linear ion trap mass spectrometry

The metabolism of [(14)C]-diclofenac in mice was investigated following a single oral dose of 10 mg/kg. The majority of the drug-related material was excreted in the urine within 24 h of administration (49.7 %). Liquid chromatographic analyses of urine and faecal extracts revealed extensive metabolism to at least 37 components, with little unchanged diclofenac excreted. Metabolites were identified using a hybrid linear ion-trap mass spectrometer via exact mass determinations of molecular ions and subsequent multi-stage fragmentation. The major routes of metabolism identified included: 1) conjugation with taurine; and 2) hydroxylation (probably at the 4'-and 5-arene positions) followed by conjugation to taurine, glucuronic acid or glucose. Ether, rather than acyl glucuronidation, predominated. There was no evidence for p-benzoquinone-imine formation (i.e. no glutathione or mercapturic acid conjugates were detected). A myriad of novel minor drug-related metabolites were also detected, including ribose, glucose, sulfate and glucuronide ether-linked conjugates of hydroxylated diclofenac derivatives. Combinations of these hydroxylated derivatives with acyl conjugates (glucose, glucuronide and taurine) or N-linked sulfation or glucosidation were also observed. Acyl- or amide-linked-conjugates of benzoic acid metabolites and several indolinone derivatives with further hydroxylated and conjugated moieties were also evident. The mechanisms involved in the generation of benzoic acid and indolinone products indicate the formation reactive intermediates in vivo that may possibly contribute to hepatotoxicity.