Determining protein adducts of fipexide: mass spectrometry based assay for confirming the involvement of its reactive metabolite in covalent binding

Liquid Chromatography-Tandem Mass Spectrometry Analysis of Acetaminophen Covalent Binding to Glutathione S-Transferases

Acetaminophen (APAP)-induced hepatotoxicity is the most common cause of acute liver failure in the Western world. APAP is bioactivated to N-acetyl p-benzoquinone imine (NAPQI), a reactive metabolite, which can subsequently covalently bind to glutathione and protein thiols. In this study, we have used liquid chromatography-tandem mass spectrometry (LC-MS/MS) to characterize NAPQI binding to human glutathione S-transferases (GSTs) in vitro. GSTs play a crucial role in the detoxification of reactive metabolites and therefore are interesting target proteins to study in the context of APAP covalent binding. Recombinantly-expressed and purified GSTs were used to assess NAPQI binding in vitro. APAP biotransformation to NAPQI was achieved using rat liver microsomes or human cytochrome P450 Supersomes in the presence of GSTA1, M1, M2, or P1. Resulting adducts were analyzed using bottom-up proteomics, with or without LC fractionation prior to LC-MS/MS analysis on a quadrupole-time-of-flight instrument with data-dependent acquisition (DDA). Targeted methods using multiple reaction monitoring (MRM) on a triple quadrupole platform were also developed by quantitatively labeling all available cysteine residues with a labeling reagent yielding isomerically-modified peptides following enzymatic digestion. Seven modified cysteine sites were confirmed, including Cys112 in GSTA1, Cys78 in GSTM1, Cys115 and 174 in GSTM2, as well as Cys15, 48, and 170 in GSTP1. Most modified peptides could be detected using both untargeted (DDA) and targeted (MRM) approaches, however the latter yielded better detection sensitivity with higher signal-to-noise and two sites were uniquely found by MRM.

Absolute quantitation of acetaminophen-modified human serum albumin in acute liver failure patients by liquid chromatography/tandem mass spectrometry

RATIONALE

Acetaminophen (APAP) is a well-known analgesic, deemed a very safe over-the-counter medication. However, it is also the main cause of acute liver failure (ALF) in the Western world, via the formation of its reactive metabolite, N-acetyl p-benzoquinone imine (NAPQI), and its covalent attachment to liver proteins. The aim of this study was to develop a sensitive and robust quantitative assay to monitor APAP-protein binding to human serum albumin (HSA) in patient samples.

METHODS

A combination of isotope dilution, peptic digestion and solid-phase extraction coupled to liquid chromatography/multiple reaction monitoring (LC/MRM) was employed. An external calibration curve with surrogate modified protein spiked into blank serum was used for absolute quantitation. Samples were analyzed by LC/MRM to measure the modified active site peptide of HSA. The LC/MRM assay was validated and successfully applied to serum samples from patients suffering from APAP-induced ALF.

RESULTS

Accuracy ranged from 83.8-113.3%, within-run coefficient of variation (CV) ranged from 0.3-6.9%, and total CVs from 1.6-10.6%. Patient samples ranged from 0.12-3.91 nmol/mL NAPQI-HSA; in-between the assay dynamic range of 0.11-50.13 nmol/mL serum. In vivo median concentrations were found to be 0.62 nmol/mL and 0.91 nmol/mL for non-spontaneous survivors (n = 25) and individuals with irreversible liver damage (n = 10), respectively (p-value = 0.028), demonstrating significant potential as a biomarker for ALF outcome.

CONCLUSIONS

A fast and sensitive assay was developed to accurately quantify NAPQI-HSA as a biomarker for APAP-related covalent binding in human serum.

Identification of protein adduction using mass spectrometry: Protein adducts as biomarkers and predictors of toxicity mechanisms

The determination of protein-xenobiotic adducts using mass spectrometry is an emerging area which allows detailed understanding of the underlying mechanisms involved in toxicity. These approaches can also be used to reveal potential biomarkers of exposure or toxic response. The following review covers studies of protein adducts resulting from exposure to a wide variety of xenobiotics including organophosphates, polycyclic aromatic hydrocarbons, acetaminophen, alkylating agents and other related compounds.

Target binding properties and cellular activity of afatinib (BIBW 2992), an irreversible ErbB family blocker

Deregulation of the ErbB (proto-oncogene B of the avian erythroblastosis virus AEV-H strain) receptor network is well recognized as an oncogenic driver in epithelial cancers. Several targeted drugs have been developed, including antibodies and small-molecule kinase inhibitors, each of them characterized by distinct patterns of ErbB receptor interactions. Understanding the precise pharmacological properties of these compounds is important for optimal use in clinical practice. Afatinib [BIBW 2992; N-[4-[(3-chloro-4-fluorophenyl)amino]-7-[[(3S)-tetrahydro-3-furanyl]oxy]-6-quinazolinyl]-4-(dimethylamino)-2-butenamide] is an ATP-competitive anilinoquinazoline derivative harboring a reactive acrylamide group. It was designed to covalently bind and irreversibly block enzymatically active ErbB receptor family members. Here, we show by X-ray crystallography the covalent binding of afatinib to wild-type epidermal growth factor receptor (EGFR) and by mass spectrometry the covalent interaction with EGFR, EGFRL858R/T790M, human epidermal growth factor receptor 2 (HER2), and ErbB-4. Afatinib potently inhibits the enymatic activity of ErbB-4 (EC50=1 nM) and the proliferation of cancer cell lines driven by multiple ErbB receptor aberrations at concentrations below 100 nM. N-[4-[(3-chloro-4-fluorophenyl)amino]-7-[[(3S)-tetrahydro-3-furanyl]oxy]-6-quinazolinyl]-4-(dimethylamino)-2-butanamide (BI 37781), a close analog of afatinib lacking the acrylamide group and thus incapable of covalent bond formation, had similar potency on cells driven by EGFR or EGFRL858R, but less or no detectable activity on cells expressing EGFRL858R/T790M HER2 or ErbB-4. These results stress the importance of the acrylamide group and show that afatinib differs from approved ErbB targeting agents by irreversibly inhibiting the kinase activity of all ErbB family members. They provide a mechanistic rationale for the distinct pharmacological features of this compound and explain the clinical activity seen in some patients who are resistant to antibody or kinase inhibitor therapy because of secondary mutations or ErbB receptor "reprogramming."

Assessing covalent binding of reactive drug metabolites by complete protein digestion and LC–MS analysis

Background: Covalent binding by reactive drug metabolites represents a poorly understood cause of drug toxicity. Currently, assessing protein covalent binding usually entails the use of radioactive drug and therefore has limited applicability in drug discovery. Several marketed drugs are known to form reactive metabolites and have been shown to covalently bind to proteins. Results: In this article, we describe a new method for the analysis of reactive metabolite–protein binding by MS using a strategy of complete digestion of microsomal proteins into free amino acids. Immobilized pronase was found to be the best method for complete digestion in terms of stability of amino acid modifications as well as minimized spectral background. Conclusion: Modified cysteine residues were identified for four tested drug compounds known to form reactive metabolites following in vitro microsomal incubations and accurate mass measurements by LC–MS analysis.

Improved detection of reactive metabolites with a bromine-containing glutathione analog using mass defect and isotope pattern matching

Drug bioactivation leading to the formation of reactive species capable of covalent binding to proteins represents an important cause of drug-induced toxicity. Reactive metabolite detection using in vitro microsomal incubations is a crucial step in assessing potential toxicity of pharmaceutical compounds. The most common method for screening the formation of these unstable, electrophilic species is by trapping them with glutathione (GSH) followed by liquid chromatography/mass spectrometry (LC/MS) analysis. The present work describes the use of a brominated analog of glutathione, N-(2-bromocarbobenzyloxy)-GSH (GSH-Br), for the in vitro screening of reactive metabolites by LC/MS. This novel trapping agent was tested with four drug compounds known to form reactive metabolites, acetaminophen, fipexide, trimethoprim and clozapine. In vitro rat microsomal incubations were performed with GSH and GSH-Br for each drug with subsequent analysis by liquid chromatography/high-resolution mass spectrometry on an electrospray time-of-flight (ESI-TOF) instrument. A generic LC/MS method was used for data acquisition, followed by drug-specific processing of accurate mass data based on mass defect filtering and isotope pattern matching. GSH and GSH-Br incubations were compared to control samples using differential analysis (Mass Profiler) software to identify adducts formed via the formation of reactive metabolites. In all four cases, GSH-Br yielded improved results, with a decreased false positive rate, increased sensitivity and new adducts being identified in contrast to GSH alone. The combination of using this novel trapping agent with powerful processing routines for filtering accurate mass data and differential analysis represents a very reliable method for the identification of reactive metabolites formed in microsomal incubations.

Role of reactive metabolites in drug-induced hepatotoxicity

Drugs are generally converted to biologically inactive forms and eliminated from the body, principally by hepatic metabolism. However, certain drugs undergo biotransformation to metabolites that can interfere with cellular functions through their intrinsic chemical reactivity towards glutathione, leading to thiol depletion, and functionally critical macromolecules, resulting in reversible modification, irreversible adduct formation, and irreversible loss of activity. There is now a great deal of evidence which shows that reactive metabolites are formed from drugs known to cause hepatotoxicity, such as acetaminophen, tamoxifen, isoniazid, and amodiaquine. The main theme of this article is to review the evidence for chemically reactive metabolites being initiating factors for the multiple downstream biological events culminating in toxicity. The major objectives are to understand those idiosyncratic hepatotoxicities thought to be caused by chemically reactive metabolites and to define the role of toxic metabolites.

Development of an automated on-line pepsin digestion-liquid chromatography-tandem mass spectrometry configuration for the rapid analysis of protein adducts of chemical warfare agents

Rapid monitoring and retrospective verification are key issues in protection against and non-proliferation of chemical warfare agents (CWA). Such monitoring and verification are adequately accomplished by the analysis of persistent protein adducts of these agents. Liquid chromatography-mass spectrometry (LC-MS) is the tool of choice in the analysis of such protein adducts, but the overall experimental procedure is quite elaborate. Therefore, an automated on-line pepsin digestion-LC-MS configuration has been developed for the rapid determination of CWA protein adducts. The utility of this configuration is demonstrated by the analysis of specific adducts of sarin and sulfur mustard to human butyryl cholinesterase and human serum albumin, respectively.