Covalent Adduction of Nitrogen Mustards to Model Protein Nucleophiles

Fluoride derivatization-enabled sensitive and simultaneous detection of biomarkers for nitrogen mustard in human plasma and urine via gas chromatography tandem mass spectrometry

Methyl-diethanolamine (CAS: 105-59-9), ethyl-diethanolamine (CAS: 139-87-7), and triethanolamine (CAS: 102-71-6) were identified as the degradation products and bio-markers of nitrogen mustard exposure. Sensitive and convenient detection methods for amino alcohol are of great importance to identify nitrogen mustard exposure in forensic analysis. Herein, analytical methods including gas chromatography-tandem mass spectrometry combined with heptafluorobutyryl derivatization and solid phase extraction were established for retrospective detection of the biomarkers in human plasma and urine samples. The efficiency of the method was improved by optimizing the conditions for sample preparation and the GC-MS/MS method. The optimization included the derivatization temperature, reaction time, reagent dosage and solid phase extraction cartridges, eluent and pH of the loading sample. The results indicated that the SCX cartridge resulted in better enrichment and purification effects, and the best recovery could be obtained with pH = 3-4 for the loading samples and an eluent of 2 mL 10% NH4OH/MeOH. The GC-MS/MS parameters were also optimized for better specificity and sensitivity. The established method was fully validated for each analyte both in plasma and urine matrixes. The linear range of analytes in plasma was 1.0-1000 ng mL-1 with a correlation parameter (R2) of ≥0.994, intra-day/inter-day accuracy of 93.7-117%, and relative standard deviation (RSD) of ≤6.5%. Meanwhile the results in urine were 1.0-1000 ng mL-1 with R2 of ≥0.996, intra-day/inter-day accuracy of 94.3-122%, and RSD of ≤6.6%. The detection limit of the analytes was 1.0 ng mL-1. The method was applied for the detection and identification of trace amino alcohols present in urine samples dispatched by the Organization for the Prohibition of Chemical Weapons (OPCW) and the results were confirmed to be correct.

Nitrogen Mustard Alkylates and Cross-Links p53 in Human Keratinocytes

Cytotoxic blistering agents such as sulfur mustard and nitrogen mustard (HN2) were synthesized for chemical warfare. Toxicity is due to reactive chloroethyl side chains that modify and damage cellular macromolecules including DNA and proteins. In response to DNA damage, cells initiate a DNA damage response directed at the recruitment and activation of repair-related proteins. A central mediator of the DNA damage response is p53, a protein that plays a critical role in regulating DNA repair. We found that HN2 causes cytosolic and nuclear accumulation of p53 in HaCaT keratinocytes; HN2 also induced post-translational modifications on p53 including S15 phosphorylation and K382 acetylation, which enhance p53 stability, promote DNA repair, and mediate cellular metabolic responses to stress. HN2 also cross-linked p53, forming dimers and high-molecular-weight protein complexes in the cells. Cross-linked multimers were also modified by K48-linked ubiquitination indicating that they are targets for proteasome degradation. HN2-induced modifications transiently suppressed the transcriptional activity of p53. Using recombinant human p53, HN2 alkylation was found to be concentration- and redox status-dependent. Dithiothreitol-reduced protein was more efficiently cross-linked indicating that p53 cysteine residues play a key role in protein modification. LC-MS/MS analysis revealed that HN2 directly alkylated p53 at C124, C135, C141, C176, C182, C275, C277, H115, H178, K132, and K139, forming both monoadducts and cross-links. The formation of intermolecular complexes was a consequence of HN2 cross-linked cysteine residues between two molecules of p53. Together, these data demonstrate that p53 is a molecular target for mustard vesicants. Modification of p53 likely mediates cellular responses to HN2 including DNA repair and cell survival contributing to vesicant-induced cytotoxicity.

Mechanisms of change in gel water-holding capacity of myofibrillar proteins affected by lipid oxidation: The role of protein unfolding and cross-linking

This work explored the effects of protein unfolding and cross-linking induced by lipid oxidation (linoleic acid, OLA) on the gel water-holding capacity (WHC) of beef myofibrillar proteins (MP). Medium concentration of OLA (≤6 mM) caused the increase of gel WHC from 55.2% to 65.1%, while relative high OLA concentration (>6 mM) decreased the gel WHC. When the OLA concentrations increased from 0 to 10 mM, the population of immobile water of gel decreased from 92.91% to 78.97%, whereas that of free water increased from 6.13% to 19.80%, suggesting that OLA treatment regardless concentration was harmful for gel WHC. However, medium OLA concentrations (≤6 mM) caused the shifting of α-helixes to β-sheets in MP gel, exerting positive effect on gel WHC. Protein unfolding and cross-linking jointly determined the increased gel WHC at moderate oxidative modification. Additionally, the protein aggregation at high OLA concentration resulted in decreased gel WHC.

Gas chromatography-tandem mass spectrometry-based detection of half nitrogen mustards in plasma as a new biomarker of nitrogen mustard exposure

The development and optimization of an analytical method for the detection and identification of reactive metabolite of organochlorine chemical warfare agent nitrogen mustards (NMs), 2-[(2-chloroethyl)(alkyl)amino]ethanol (CEAAE), known as half nitrogen mustard, in blood samples is presented, herein. In this study, half nitrogen mustards in plasma are presented as a new and unambiguous biomarker of NM exposure since the fully hydrolyzed product, i.e., amino alcohols, are common industrial chemicals that can be present as such without getting exposed to NMs. Thus, the detection of half nitrogen mustard as a biomarker holds great significance for verification by the Chemical Weapon Convention (CWC) and will also be helpful in understanding the pharmacokinetics of NM-based chemotherapeutic pro-drugs. To the best of our knowledge, this is the first report on the detection of half nitrogen mustards in any matrice, including plasma. A very simple sample preparation protocol was developed for its extraction from plasma samples. Heptafluorobutyrylation and gas chromatography-tandem mass spectrometry in the positive chemical ionization mode were developed for the detection and identification of halfNMs. The developed method has shown excellent analytical figures of merits such as a wide range of linearity (1.0-50 ng mL^-1), low limit of detection (0.3-0.5 ng mL^-1), and low limit of quantification (1.0 ng mL^-1). The interday and intraday reproducibilities were also less than 15%. The developed method was successfully applied to real-world samples; in vitro human plasma was spiked with ∼1 ng mL^-1 of all the NMs and in vivo studies were done with rats intravenously exposed to 1 × LD₅₀ of bis(2-chloroethyl)methylamine (HN2).

Analysis of nitrogen mustard degradation products via post-pentafluorobenzoylation liquid chromatography-tandem mass spectrometry

A pentafluorobenzoylation (PFBz)-liquid chromatography-tandem mass spectrometry method was developed for qualitative and quantitative analysis of ethanolamines (EAs, nitrogen mustard degradation products). With this method, highly hydrophilic EAs can be sufficiently analyzed with a commonly used reversed phase column (retention times: (PFBz)₂-methyl diethanolamine, 9.1 min; (PFBz)₂-ethyl diethanolamine, 9.8 min; and (PFBz)₃-triethanolamine, 17.6 min). The applicability of the method for real samples was investigated via recovery tests. Methyl diethanolamine and ethyl diethanolamine were detected at concentrations as low as 1 ng/mL in serum and 10 ng/mL in urine, and quantified within the range of 1-1000 ng/mL and 10-1000 ng/mL, respectively.

Mass spectral fragmentation of perfluoroacyl derivatives of half nitrogen mustards for their detection by gas chromatography/mass spectrometry

RATIONALE

Analytical methods for the detection and identification of half nitrogen mustards (halfNMs), i.e., partially hydrolyzed products of nitrogen mustards (pHpNMs), using silyl derivatives are often associated with low sensitivity and selectivity. In order to overcome these limitations, the derivatization of halfNMs was performed using perfluoroacylation.

METHODS

Two efficient derivatization techniques using trifluoroacetyl (TFA) and heptafluorobutyryl (HFB) groups were developed for the unambiguous identification of halfNMs. A mass spectral database was generated by performing gas chromatography/electron ionization mass spectrometry (GC/EI-MS) and gas chromatography/positive chemical ionization mass spectrometry (GC/PCI-MS). The fragmentation pathways were studied by tandem mass spectrometry (MS/MS) in both EI and PCI mode.

RESULTS

The EI-MS spectra of the TFA and HFB derivatives of halfNMs contain intense molecular ions and fragment ions, thus making perfluoroacylation preferable to silylation. In addition, the background-free chromatogram obtained using these derivatives provides unambiguous identification of these compounds in blind samples. The structures of the fragment ions were postulated, and the sources of significant ions were traced by performing MS/MS precursor ion scans. In the PCI-MS spectra, along with the protonated molecule, significant peaks due to neutral losses of HF, HCl, CH₃ Cl and CF₃ COOH were observed.

CONCLUSIONS

This is the first report of the elucidation of the fragmentation pathways of perfluoroacyl derivatives of halfNMs. The complementary GC/PCI-MS and GC/PCI-MS/MS data will be helpful in the identification of unknown metabolites in a fast and reliable fashion.

Mass spectral studies of silyl derivatives of partially hydrolyzed products of nitrogen mustards: Important markers of nitrogen mustard exposure

RATIONALE

Nitrogen mustards (NMs) are vesicant class of chemical warfare agents. From the viewpoint of the Chemical Weapons Convention partially hydrolyzed products of nitrogen mustards (pHpNMs) are considered as important markers of nitrogen mustard exposure. The detection of pHpNMs from biological or environmental samples is highly useful for obtaining forensic evidence of exposure to NMs.

METHODS

Gas chromatography interfaced with tandem mass spectrometry (GC/MS/MS) is a widely used tool for the identification and sensitive detection of metabolites of NMs in complex matrices. The pHpNMs were derivatized using silylating agents as they are highly polar and non-amenable to GC. The mass spectral studies of these silyl derivatives of pHpNMs were performed using GC/MS/MS in both electron ionization (EI) and chemical ionization (CI) mode.

RESULTS

Various approaches have been proposed to assess the fragmentation pathways of the trimethylsilyl (TMS) and tert-butyldimethylsilyl (TBDMS) derivatives of pHpNMs. All the proposed fragmentation pathways were based on the product and/or precursor ion scanning of corresponding ions in both EI and CI mode. In the case of EI, most of the fragmentation pathways involved either α-cleavage or inductive cleavage.

CONCLUSIONS

This is the first report on the MS study of the silyl derivatives of pHpNMs. The study of the two different derivatives of pHpNMs using both EI- and CI-MS provides a reliable, unambiguous identification of pHpNMs in complex environmental and biomedical matrices (such as plasma and urine) during any verification activities.

Advances in covalent kinase inhibitors

This comprehensive review details recent advances, challenges and innovations in covalent kinase inhibition within a 10 year period (2007–2018).

Analysis of degradation products of nitrogen mustards via hydrophilic interaction liquid chromatography-tandem mass spectrometry

A hydrophilic interaction liquid chromatography-tandem mass spectrometry (HILIC-MS/MS) method was developed for qualitative and quantitative analysis of ethanolamines (EAs), which are nitrogen mustard degradation products. With this method, the retention times of the highly hydrophilic EAs on the HILIC column were sufficient (retention times: methyl diethanolamine, 12.2 min; ethyl diethanolamine, 11.2 min; and triethanolamine, 9.5 min) and the EAs were analyzed more efficiently than with reported HILIC-MS/MS methods. The detection limits of methyl diethanolamine and ethyl diethanolamine in serum and urine using this approach were 15-20 ng/mL. The suitability of the method for real samples was evaluated via recovery tests involving urine and serum, and the method was validated. The MS/MS fragmentation of EAs was discussed based on density functional theory calculations.

Acute corneal injury in rabbits following nitrogen mustard ocular exposure

Sulfur mustard (SM), a potent vesicating chemical warfare agent, and its analog nitrogen mustard (NM), are both strong bi-functional alkylating agents. Eyes, skin, and the respiratory system are the main targets of SM and NM exposure; however, ocular tissue is most sensitive, resulting in severe ocular injury. The mechanism of ocular injury from vesicating agents' exposure is not completely understood. To understand the injury mechanism from exposure to vesicating agents, NM has been previously employed in our toxicity studies on primary human corneal epithelial cells and ex vivo rabbit cornea organ culture model. In the current study, corneal toxicity from NM ocular exposure (1%) was analyzed for up to 28 days post-exposure in New Zealand White male rabbits to develop an acute corneal injury model. NM exposure led to conjunctival and eyelid swelling within a few hours after exposure, in addition to significant corneal opacity and ulceration. An increase in total corneal thickness and epithelial degradation was observed starting at day 3 post-NM exposure, which was maximal at day 14 post-exposure and did not resolve until 28 days post-exposure. There was an NM-induced increase in the number of blood vessels and inflammatory cells, and a decrease in keratocytes in the corneal stroma. NM exposure resulted in increased expression levels of cyclooxygenase-2, Interleukin-8, vascular endothelial growth factor and Matrix Metalloproteinase 9 indicating their involvement in NM-induced corneal injury. These clinical, biological, and molecular markers could be useful for the evaluation of acute corneal injury and to screen for therapies against NM- and SM-induced ocular injury.

A quantitative method to detect human exposure to sulfur and nitrogen mustards via protein adducts

Sulfur and nitrogen mustards are internationally banned vesicants listed as Schedule 1 chemical agents in the Chemical Weapons Convention. These compounds are highly reactive electrophiles that form stable adducts to a variety of available amino acid residues on proteins upon exposure. We present a quantitative exposure assay that simultaneously measures agent specific protein adducts to cysteine for sulfur mustard (HD) and three nitrogen mustards (HN1, HN2, and HN3). Proteinase K was added to a serum or plasma sample to digest protein adducts and form the target analyte, the blister agent bound to the tripeptide cysteine-proline-phenylalanine (CPF). The mustard adducted-tripeptide was purified by solid phase extraction and analyzed using isotope dilution LC-MS/MS. Product ion structures were identified using high-resolution product ion scan data for HD-CPF, HN1-CPF, HN2-CPF, and HN3-CPF. Thorough matrix comparison, analyte recovery, ruggedness, and stability studies were incorporated during method validation to produce a robust method. The method demonstrated long term-stability, precision (RSD < 15%), and intra- and inter-day accuracies > 85% across the reportable range of 3.00-200 ng/mL for each analyte. Compared to previously published assays, this method quantitates both sulfur and nitrogen mustard exposure biomarkers, requires only 10 μL of sample volume, and can use either a liquid sample or dried sample spot.

An Engineered Human Fibroblast Growth Factor-1 Derivative, TTHX1114, Ameliorates Short-term Corneal Nitrogen Mustard Injury in Rabbit Organ Cultures

Purpose

Organ cultures of rabbit corneas have been used to ascertain the effectiveness of a human fibroblast growth factor (FGF)-1 derivative (TTHX1114), lacking cysteine residues, to protect against and/or repair epithelial lesions following exposure to nitrogen mustard (NM).

Methods

Rabbit corneas were exposed to NM and cultured for up to 14 days, with or without drug (TTHX1114). At specified times, tissue was examined by histopathology and graded by a novel composite scale. Proliferation was measured by 5-ethynyl-2'-deoxyuridine (EdU) incorporation, and the expression of native FGF-1 and ADAM-17 after NM exposure was determined by immunofluorescence.

Results

Rabbit corneas, exposed to a single dose of NM, showed a nearly complete loss of epithelial cells by day 6 but were significantly regenerated by day 14. When treated continuously with TTHX1114 following vesicant exposure, the losses remained at day 2 levels. The loss of keratocytes in the stroma was not affected by TTHX1114. EdU incorporation over the same time course showed a steady increase in tissue that had not been treated with TTHX1114, while corneas that were treated with the drug showed a higher percent incorporation initially, which then decreased, indicating the strong proliferative response to TTHX1114. ADAM-17 was not significantly altered by TTHX1114 treatment. Corneal epithelial FGF-1 disappeared after only 1 day following exposure to NM.

Conclusions

TTHX1114 is protective against NM-induced damage of the corneal epithelium, possibly by supplying an NM-resistant source of trophic support and by stimulating regeneration of new epithelial cells. These responses underscore the potential value of TTHX1114 as an anti-vesicant therapeutic.

Guidelines for the Synthesis of Small-Molecule Irreversible Probes Targeting G Protein-Coupled Receptors

Irreversible probes have been proven to be useful pharmacological tools in the study of structural and functional features in drug receptor pharmacology. They have been demonstrated to be particularly valuable for the isolation and purification of receptors. Furthermore, irreversible probes are helpful tools for the identification and characterization of binding sites, thereby supporting the advancement of rational drug design. In this Minireview, we provide insight into universal strategies and guidelines to successfully synthesize irreversible probes that target G protein-coupled receptors (GPCRs). We provide an overview of commonly used chemoreactive and photoreactive groups, and make a comparison of their properties and potential applications. Furthermore, there is a particular focus on synthetic approaches to introduce these reactive groups based on commercially available reagents.

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.

Thioredoxin Cross-Linking by Nitrogen Mustard in Lung Epithelial Cells: Formation of Multimeric Thioredoxin/Thioredoxin Reductase Complexes and Inhibition of Disulfide Reduction

The thioredoxin (Trx) system, which consists of Trx and thioredoxin reductase (TrxR), is a major cellular disulfide reduction system important in antioxidant defense. TrxR is a target of mechlorethamine (methylbis(2-chloroethyl)amine; HN2), a bifunctional alkylating agent that covalently binds to selenocysteine/cysteine residues in the redox centers of the enzyme, leading to inactivation and toxicity. Mammalian Trx contains two catalytic cysteines; herein, we determined if HN2 also targets Trx. HN2 caused a time- and concentration-dependent inhibition of purified Trx and Trx in A549 lung epithelial cells. Three Trx cross-linked protein complexes were identified in both cytosolic and nuclear fractions of HN2-treated cells. LC-MS/MS of these complexes identified both Trx and TrxR, indicating that HN2 cross-linked TrxR and Trx. This is supported by our findings of a significant decrease of Trx/TrxR complexes in cytosolic TrxR knockdown cells after HN2 treatment. Using purified recombinant enzymes, the formation of protein cross-links and enzyme inhibition were found to be redox status-dependent; reduced Trx was more sensitive to HN2 inactivation than the oxidized enzyme, and Trx/TrxR cross-links were only observed using reduced enzyme. These data suggest that HN2 directly targets catalytic cysteine residues in Trx resulting in enzyme inactivation and protein complex formation. LC-MS/MS confirmed that HN2 directly alkylated cysteine residues on Trx, including Cys32 and Cys35 in the redox center of the enzyme. Inhibition of the Trx system by HN2 can disrupt cellular thiol-disulfide balance, contributing to vesicant-induced lung toxicity.

Mechanistic understanding of molecular initiating events (MIEs) using NMR spectroscopy

Toxicological risk assessments in the 21^st century are increasingly being driven by the Adverse Outcome Pathways (AOP) conceptual framework in which the Molecular Initiating Event (MIE) is of fundamental importance to pathway progression. For those MIEs that involve covalent chemical reactions, such as protein haptenation, determination of relative rates and mechanisms of reactions is a prerequisite for their understanding. The utility of NMR spectroscopy as an experimental technique for effectively providing reaction rate and mechanistic information for early assessment of likely MIE(s) has been demonstrated. To demonstrate the concept, model systems exemplifying common chemical reactions involved in the covalent modification of proteins were utilized; these involved chemical reactions of electrophilic species (representing different mechanistic classes) with simple amine and thiol nucleophiles acting as surrogates for the reactive groups of lysine and cysteine protein side chains respectively. Such molecular interactions are recognized as critical mechanisms in a variety of chemical and drug toxicities, including respiratory and skin sensitization and liver toxicity as well as being the key mechanism of action for a number of therapeutic agents.

Covalent inhibitors in drug discovery: from accidental discoveries to avoided liabilities and designed therapies

Drugs that covalently bond to their biological targets have a long history in drug discovery. A look at drug approvals in recent years suggests that covalent drugs will continue to make impacts on human health for years to come. Although fraught with concerns about toxicity, the high potencies and prolonged effects achievable with covalent drugs may result in less-frequent drug dosing and in wide therapeutic margins for patients. Covalent inhibition can also dissociate drug pharmacodynamics (PD) from pharmacokinetics (PK), which can result in desired drug efficacy for inhibitors that have short systemic exposure. Evidence suggests that there is a reduced risk for the development of resistance against covalent drugs, which is a major challenge in areas such as oncology and infectious disease.

Cross-linking of thioredoxin reductase by the sulfur mustard analogue mechlorethamine (methylbis(2-chloroethyl)amine) in human lung epithelial cells and rat lung: selective inhibition of disulfide reduction but not redox cycling

Oxidative stress plays a key role in mechlorethamine (methylbis(2-chloroethyl)amine, HN2) toxicity. The thioredoxin system, consisting of thioredoxin reductase (TrxR), thioredoxin, and NADPH, is important in redox regulation and protection against oxidative stress. HN2 contains two electrophilic side chains that can react with nucleophilic sites in proteins, leading to changes in their structure and function. We report that HN2 inhibits the cytosolic (TrxR1) and mitochondrial (TrxR2) forms of TrxR in A549 lung epithelial cells. TrxR exists as homodimers under native conditions; monomers can be detected by denaturing and reducing SDS-PAGE followed by western blotting. HN2 treatment caused marked decreases in TrxR1 and TrxR2 monomers along with increases in dimers and oligomers under reducing conditions, indicating that HN2 cross-links TrxR. Cross-links were also observed in rat lung after HN2 treatment. Using purified TrxR1, NADPH reduced, but not oxidized, enzyme was inhibited and cross-linked by HN2. LC-MS/MS analysis of TrxR1 demonstrated that HN2 adducted cysteine- and selenocysteine-containing redox centers forming monoadducts, intramolecule and intermolecule cross-links, resulting in enzyme inhibition. HN2 cross-links two dimeric subunits through intermolecular binding to cysteine 59 in one subunit of the dimer and selenocysteine 498 in the other subunit, confirming the close proximity of the N- and C-terminal redox centers of adjacent subunits. Despite cross-linking and inhibition of TrxR activity by HN2, TrxR continued to mediate menadione redox cycling and generated reactive oxygen species. These data suggest that disruption of the thioredoxin system contributes to oxidative stress and tissue injury induced by HN2.

Pronounced toxicity differences between homobifunctional protein cross-linkers and analogous monofunctional electrophiles

Bifunctional electrophiles have been used in various chemopreventive, chemotherapeutic, and bioconjugate applications. Many of their effects in biological systems are traceable to their reactive properties, whereby they can modify nucleophilic sites in DNA, proteins, and other cellular molecules. Previously, we found that two different bifunctional electrophiles--diethyl acetylenedicarboxylate and divinyl sulfone--exhibited a strong enhancement of toxicity when compared with analogous monofunctional electrophiles in both human colorectal carcinoma cells and baker's yeast. Here, we have compared the toxicities for a broader panel of homobifunctional electrophiles bearing diverse electrophilic centers (e.g., isothiocyanate, isocyanate, epoxide, nitrogen mustard, and aldehyde groups) to their monofunctional analogues. Each bifunctional electrophile showed at least a 3-fold enhancement of toxicity over its monofunctional counterpart, although in most cases, the differences were even more pronounced. To explain their enhanced toxicity, we tested the ability of each bifunctional electrophile to cross-link recombinant yeast thioredoxin 2 (Trx2), a known intracellular target of electrophiles. The bifunctional electrophiles were capable of cross-linking Trx2 to itself in vitro and to other proteins in cells exposed to toxic concentrations. Moreover, most cross-linkers were preferentially reactive with thiols in these experiments. Collectively, our results indicate that thiol-reactive protein cross-linkers in general are much more potent cytotoxins than analogous monofunctional electrophiles, irrespective of the electrophilic group studied.