Adductomics is epidemiology at the molecular level. Untargeted adductomics compares levels of chemical adducts on albumin, hemoglobin, and DNA between healthy and exposed individuals. The goal is to determine a cause-and-effect relationship between chemical exposure and illness. Chemical exposures are not necessarily due to synthetic chemicals but are often due to oxidation products of naturally occurring lipids, for example, 4-hydroxynonenal and acrolein produced by lipid peroxidation of arachidonic and linoleic acids. The preferred method used in adductomics is ultra-high pressure liquid chromatography coupled to with nanoelectrospray tandem mass spectrometry. The mass of the adduct indicates its structure and identifies the chemical. The advantages of molecular epidemiology include information about the many toxicants to which a person is exposed over a period of weeks or months and the relative exposure levels. The disadvantage is the absence of information about the mechanism of toxicity. Untargeted adductomics examines albumin and hemoglobin adducts, which serve as biomarkers of exposure but do not identify the proteins and genes responsible for the toxicity. Targeted adductomics is used when the origin of the toxicity is known. This can be either an adducted protein, such as the butyrylcholinesterase protein modified by nerve agents, or a toxicant, such as acetaminophen. Untargeted adductomics methods have identified potential protein adduct biomarkers of breast cancer, colorectal cancer, childhood leukemia, and lung cancer. Adductomics is a new research area that offers structural insights into chemical exposures and a platform for the discovery of disease biomarkers. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC.

The oldest person ever recorded was a 122-year-old woman who had smoked all her life. Is smoking good for a long life? The statistics are clear that smoking does not promote a long life, rather it causes cancer, stroke, heart failure, and emphysema, thereby shortening the lifespan for most people who smoke. How then has this woman managed to live so long? The answer to the riddle of living a very long life is most likely genetic makeup, with minor roles for diet and other factors. This centenarian is likely missing the genes that convert the chemicals in smoke into toxic metabolites. Unlike the 122-year-old smoker, most people have genes encoding enzymes such as Cytochrome P450, which poison smokers by converting harmless chemicals into toxic metabolites. The toxic metabolites change the ability of proteins to fulfill their functions and maintain life. The toxic metabolites also destroy DNA: they break and bind to DNA, thereby introducing mutations that can lead to cancer.

The fact that most smokers live well beyond adulthood means the body has methods to minimize the destructive effects of chemicals. Innate defense systems capture low levels of chemicals and expel the toxins in urine and feces. But chronic exposure to low levels of poisonous chemicals can cause illness. In addition to those in cigarette smoke, harmful chemical exposures include chemicals in the air from vehicle exhaust and industrial pollution, chemicals in cooked food produced during the cooking process, alcohol, and occupational exposure to benzene and tar, to name a few.

Protein alkylation as a mechanism of chemical toxicity was a novel idea when it was first reported. By now it is an accepted mechanism supported by solid evidence. Inhibition of acetylcholinesterase is one example of protein alkylation that leads to toxicity. The military wanted to understand why nerve agents are lethal; humans died within minutes of exposure. Scientists found that nerve agents, such as sarin and soman, make a covalent bond with acetylcholinesterase, a protein critical for nerve impulse transmission (Millard et al., 1999). The covalent bond with acetylcholinesterase paralyzes the breathing muscles, resulting in death. A nerve agent adduct on a single amino acid, serine in the catalytic triad, completely inhibited acetylcholinesterase activity. Without acetylcholinesterase activity, excess levels of the neurotransmitter, acetylcholine, accumulated at the neuromuscular junction, blocking transmission of nerve signals. Chemists went to work to determine how to reactivate acetylcholinesterase activity. Irwin Wilson found that an oxime restored acetylcholinesterase activity by chemically removing the inhibitor from serine at the active site (Wilson & Ginsburg, 1955). Oximes are now a standard antidote used for treatment of poisoning by nerve agents and organophosphorus pesticides (Thiermann & Worek, 2022).

A second example of protein alkylation is caused by an acetaminophen (Tylenol) overdose, which results in a loss of liver function within a few days and is lethal. The first studies of acetaminophen toxicity assumed that the mechanism of toxicity would follow the pattern established for acetylcholinesterase. It was anticipated that modification of a single protein by acetaminophen would explain liver dysfunction. However, major differences between the mechanisms of nerve agent and acetaminophen toxicity were uncovered (McGill & Hinson, 2020). Acetaminophen is not toxic until it is metabolized to a quinone by cytochrome P450 enzymes. In contrast, nerve agents are active poisons immediately upon exposure. This difference explains why acetaminophen overdose is lethal days after ingestion, but nerve agents are lethal within minutes. The assumption that acetaminophen toxicity could be explained by modification of a single protein was ruled out when it was found that many proteins are irreversibly alkylated by the acetaminophen quinone. A common feature of the acetaminophen-modified proteins was the identity of cysteine as the modified amino acid. The covalent bond was found between the sulfhydryl group of cysteine and the acetaminophen quinone. Prescription level doses of Tylenol are not toxic because low doses of quinones are scavenged by glutathione inside the cell; however, high doses deplete glutathione, allowing the excess acetaminophen quinone to covalently bind to proteins. A parallel process explains why low doses of nerve agents are not toxic. Absence of toxicity is explained by the scavenging activity of butyrylcholinesterase in blood. An understanding of the protective role of glutathione from acetaminophen toxicity, and the fact that toxicity was associated with binding to sulfhydryl groups, led to an antidote for Tylenol overdose (N -acetylcysteine). When administered within hours of Tylenol overdose, N -acetylcysteine protects liver function (McGill & Hinson, 2020). Figure 1 summarizes the mechanisms of toxicity for nerve agents and acetaminophen overdose.

These two examples of protein alkylation show that successful antidotes for specific poisons were developed after the protein adducts had been identified. In the case of acetylcholinesterase, the protein adduct responsible for the toxicity was identified and the adduct could be removed, thereby reversing toxicity. In the case of acetaminophen, the identification of the early steps of protein adduct formation led to a treatment. However, protein adducts themselves are useful only as biomarkers for the diagnosis of exposure. An understanding of the link between protein adducts and organ failure requires years of additional study.

Two centuries before the discovery of DNA, it was known that the high frequency of cancer of the scrotum among chimney sweepers was related to soot exposure. After DNA was shown to be the inherited informational macromolecule, researchers studying mechanisms of chemical carcinogenesis discovered that many known chemical carcinogens are biotransformed to metabolites that covalently bind to DNA (Poirier, 2016). Figure 2 shows the activation of benz(a)pyrene, the carcinogen in soot, into 7β,8α-diol-9α,10α-epoxy-7,8,9,10-tetrahydro-benz(a)pyrene, which then makes a covalent bond with purines in DNA at the exocyclic amino group.

By binding to DNA, chemical carcinogens induce the incorporation of an incorrect base during DNA replication. If the mutation occurs in oncogenes, tumor suppressor genes, and DNA repair genes, the mutation becomes a cancer risk. However, cancer risk is low unless a second strong cancer risk factor is present. For example, chemical exposure to aflatoxin B1 combined with an infection (hepatitis B) increases the risk of liver cancer 60-fold (Poirier, 2016). Chronic exposure to the toxicant and a latent period on the order of years is necessary for sufficient driver mutations to accumulate and tumors to appear (Poirier, 2016).

The search for evidence of chemical exposures is predicated on the hypothesis that some diseases are the consequence of chemical exposure. Lung cancer (tobacco) and cirrhosis (alcohol) are firmly established as diseases triggered by chronic, low level, chemical exposures in combination with a person's genetic makeup, lifestyle, and immune status. It is possible, but not yet clear, that some cases of neurodegenerative and auto-immune diseases may be related to chemical exposure. The disease-inducing chemicals are not exclusively synthetic chemicals but include endogenous chemicals such as those produced by lipid peroxidation during oxidative stress. Researchers hypothesize that knowing the identity and quantity of the chemicals to which a person is exposed will yield biomarkers of disease and provide the missing link between diseases and their environmental causes. This hypothesis led to the field of adductomics.

Adductomics is Epidemiology at the Molecular Level

Untargeted adductomics refers to a search for unknown chemical modifications in specific model proteins. Albumin and hemoglobin are the most popular protein targets of untargeted adductomics because of their high concentrations and long lifetime in human blood (Carlsson, Rappaport, & Tornqvist, 2019). In addition, as blood-proteins, they are readily accessible via minimally invasive procedures. The adducts on albumin and hemoglobin provide a historical record of the chemicals to which a person is exposed over the 20-day lifetime of albumin and the 120-day lifetime of hemoglobin. In 2011, the Rappaport laboratory in Berkeley, California, developed an untargeted strategy for adducts on human albumin (Li et al., 2011) and used untargeted adductomics to compare adducts in pooled plasma obtained from 158 healthy smokers or nonsmokers (Grigoryan et al., 2016). The Tornqvist laboratory in Stockholm, Sweden compared hemoglobin adducts in smokers versus nonsmokers (Carlsson, von Stedingk, Nilsson, & Tornqvist, 2014). Both laboratories agreed that smokers have higher levels of acrylonitrile and ethylene oxide adducts. Tobacco smoke is a known source of acrylonitrile. Both laboratories found dozens of unidentified adducts whose masses do not correspond to known electrophiles. This is consistent with the general observation that humans have a continuous background exposure to electrophiles, both from endogenous and exogenous sources (Carlsson et al., 2019).

Targeted Adductomics

Targeted adductomics refers to a search for covalent adducts formed upon exposure to a specific chemical agent (Nunes et al., 2019). Samples to be searched in a targeted adductomics method are enriched for specific modified proteins before the enriched sample is analyzed by liquid chromatography tandem mass spectrometry. Several strategies have been used to enrich chemically modified proteins: a) Cellular targets of an alkynyl analog of 4-hydroxy-2-nonenal were tagged with biotin using click chemistry (Yang, Tallman, Porter, & Liebler, 2015), followed by binding to immobilized streptavidin; b) Immobilized antibodies specific for diethyl phosphate adducts on tyrosine (Onder et al., 2021) were used to immunopurify proteins modified by chlorpyrifos oxon; c) Histones modified by a carcinogenic furan metabolite were purified by acid precipitation (Nunes et al., 2019); and d) DNA samples were digested to nucleosides. Collision induced dissociation in the mass spectrometer consistently cleaves nucleosides at the glycosidic bond between the 2′-deoxyribose and the nucleobase, regardless of whether the search is for multiple DNA adducts or a specific DNA adduct (Guo & Turesky, 2019). The neutral loss of deoxyribose is a common screening strategy for nucleobases.

Target Tissue Versus Surrogate Tissue

In experimental animal and cell models, one can measure adducts in the target issue (Geib, Moghaddam, Supinski, Golizeh, & Sleno, 2021), but this is not possible in living humans. Autopsy tissues from a fatal poisoning with the nerve agent sarin were studied for sarin adducts on proteins in the bronchus, blood (albumin and butyrylcholinesterase), lung, liver, and kidney of a female victim of chemical warfare in Syria (John et al., 2018). The finding of protein adducts and the detection of sarin hydrolysis products in all tissues including brain, muscle, hair, skin, and heart, provided unambiguous evidence for systemic intoxication and the use of the nerve agent in the conflict (John et al., 2018).

This study of human autopsy tissues supports the conclusion that protein adducts in blood are representative of protein adducts in target tissue for the specific example of the nerve agent sarin. What about other compounds? Aflatoxin in contaminated corn, peanuts, and other foods induces hepatocellular carcinoma. The liver is the primary target organ of aflatoxin adverse effects due to liver proteins CYP1A2 and CYP3A4, which metabolize aflatoxin to aflatoxin-8,9-epoxide (Kensler, Roebuck, Wogan, & Groopman, 2011). The epoxide produces a mutagenic DNA adduct and a long-lived lysine adduct on albumin in blood. Aflatoxin-mercapturic acid detoxification products that are excreted in urine, and aflatoxin-albumin adducts in blood, serve as biomarkers of biologically effective aflatoxin dose (Kensler et al., 2011). The study of acrylamide exposure in factory workers involved in synthesizing acrylamide (Bergmark, Calleman, He, & Costa, 1993; Costa, 2017) provides another example of adducts on the surrogate tissue hemoglobin, which represents adducts on target tissues. Exposure to acrylamide monomers is characterized by peripheral neuropathy including ataxia, skeletal muscle weakness, cognitive impairment, and numbness of the extremities (LoPachin & Gavin, 2012). The most toxicologically relevant targets of acrylamide are the nerve terminal proteins that turnover slowly and are involved in neurotransmitter release, storage, and re-uptake (LoPachin & Gavin, 2012). The peripheral neuropathy in factory workers is accompanied by acrylamide adducts on hemoglobin (Bergmark et al., 1993).

DNA adducts are surrogates only in the sense that the adducted gene is unidentified. The toxic chemical and the modified nucleotides are known. The adduct level is known and correlated with the risk of cancer. For example, depurinating estrogen-DNA adducts are initiators of cancers of the breast, thyroid, ovary, and prostate (Cavalieri & Rogan, 2016).

Ultra-high performance liquid chromatography coupled with high-resolution mass spectrometry is the method of choice both for protein and DNA adductomics (Carlsson et al., 2019; Park et al., 2022). The advantages of liquid chromatography–tandem mass spectrometry over older methods are smaller sample size, higher sensitivity, greater accuracy, and simpler sample preparation procedures. Protein adducts are studied after the protein has been hydrolyzed to peptides. The peptide fragmentation pattern identifies the location and mass of the adduct in a peptide. The peptide sequence identifies the modified protein. DNA adducts are studied after DNA has been digested to individual nucleobases (Park et al., 2022). This strategy identifies the modified nucleobase and the covalently bound chemical but does not identify the modified gene. Studies of chemical carcinogens assume the modified nucleobases are located on a gene frequently mutated in human cancers, such as p53. This assumption was validated by showing that the acrolein-DNA binding pattern is similar to the p53 mutational pattern in human lung cancer (Feng, Hu, Hu, & Tang, 2006). Table 1 summarizes the methods for identifying protein and DNA adducts.

Older methods for the identification of protein adducts involved detaching the adduct from the proteins followed by liquid chromatography–mass spectrometry or gas chromatography–mass spectrometry with or without derivatization. Older methods for the identification of DNA adducts involved 32P-postlabeling, or high-pressure liquid chromatography with electrochemical or fluorescence detection.

Metabolic Activation

Many chemicals are unreactive with proteins and DNA until they are modified by Cytochrome P450 enzymes into alkynes, aldehydes, quinones, or epoxides. Cytochrome P450 enzymes are located within cells. Activated metabolites produced by Cytochrome P450 enzymes are short-lived. Albumin and DNA, which are located in the intracellular space and the nucleus, respectively, can capture short-lived metabolites, whereas hemoglobin inside red blood cell has limited access. The consequence of this accessibility difference is reflected in adducts of benzene. Cytochrome P450 enzymes convert benzene to the carcinogenic metabolites benzene diol epoxide and benzoquinone. Adducts from these compounds on albumin have been identified by mass spectrometry methods (Smith et al., 2021), but hemoglobin adducts have not. Explanations for the absence of these adducts on hemoglobin include the high concentration of glutathione inside the red blood cell that scavenge incoming electrophiles, and the presence of the red blood cell membrane that blocks electrophile entry (Carlsson et al., 2019).

Patients on certain oral medications, including midazolam, are advised not to drink grapefruit juice for 24 hours before swallowing a pill (Bailey, Malcolm, Arnold, & Spence, 1998). The reason for avoiding grapefruit juice is that components in grapefruit juice inhibit the activity of CYP3A4, a Cytochrome P450 enzyme. In the absence of grapefruit juice, CYP3A4 in the small intestine and colon inactivates midazolam, allowing only 15% of an ingested dose to pass into the circulation. However, in the presence of grapefruit juice, up to 30% of an ingested dose passes into the circulation. The grapefruit juice-induced overdose can cause adverse reactions to the medicine. Similarly, cyclosporin is an oral immunosuppressant for transplant patients. Plasma cyclosporin concentrations must be maintained within a narrow range to achieve adequate immunosuppression without nephrotoxicity (Bailey et al., 1998). Cyclosporin taken with grapefruit juice increases the plasma concentration by 160% compared to when taken with water. The activity of more than 85 drugs is affected by grapefruit juice (Bailey, Dresser, & Arnold, 2013).

Diseases Associated with Protein and DNA Adducts

Diseases associated with protein and DNA adducts have been known for decades. The idea of adductomics was introduced when liquid chromatography coupled with electrospray ionization mass spectrometry became broadly available to research laboratories. The older methods had already established that cancer was caused by DNA-modifying chemicals in tar and smoke, liver failure was caused by acetaminophen overdose (Tylenol), liver cirrhosis was caused by the metabolic products of alcohol, suffocation was caused by protein adducts with chemical warfare nerve agents, and Jamaican vomiting sickness was caused by a toxin in unripe lychee fruit. These diseases were studied because people had symptoms that could be traced to chemical exposure. Adductomics aims to show a cause-and-effect relationship between chemical exposure and a disease before a person has symptoms. The adductomics methods are exquisitely sensitive. Adducts are detected when only 3 albumin molecules out of 100,000, or 5-25 hemoglobin chains out of 10,000,000 are modified (Carlsson et al., 2019). DNA adduct levels are typically detected at 1 per 10⁶ to 1 per 10¹¹ bases (Peterson et al., 2020). Table 2 lists diseases associated with chemical adducts.

The review by Carlsson lists adducts on albumin and hemoglobin found in untargeted adductomics studies (Carlsson et al., 2019). A consistent finding is that healthy control groups have a low background level of adducts on Cys34 of albumin and the N-terminal Val of hemoglobin. With few exceptions, the same adducts are found in controls and exposed individuals. Quantitative differences range from 1.11 to 1.20-fold (Grigoryan et al., 2019). Subjects known to have been exposed to various potentially adduct forming compounds have either higher, lower, or identical adduct levels compared to unexposed controls. Of the 34 adducts in a study comparing smokers to nonsmokers (Grigoryan et al., 2016), the levels of 5 adducts differed between smokers and nonsmokers. Levels of ethylene oxide and acrylonitrile adducts on Cys34 of albumin were higher in smokers. Levels of Cys34 sulfinic acid, and S-Cys adducts were lower in smokers. The levels of 29 adducts were statistically the same in smokers and nonsmokers (Grigoryan et al., 2016).

Exposure to tobacco smoke is a risk factor for lung cancer. The albumin and hemoglobin adductomics studies agree that smokers have higher levels of acrylonitrile and ethylene oxide adducts than nonsmokers. These molecules are derived from tobacco smoke. The albumin studies report lower adduct levels of oxidized Cys34 in smokers and attribute this unexpected finding to smoking-induced hypoxia.

Methanethiol is a biomarker of human enteric bacteria. The finding of an increased level of methanethiol adducts on albumin Cys34 in subjects with colon cancer implicates the gut microbiota as a risk factor for colon cancer (Grigoryan et al., 2019).

Occupational exposure to benzene is a risk factor for leukemia. Subjects exposed to benzene have benzene oxide and benzene diolepoxide adducts on Cys34 of albumin (Grigoryan et al., 2018), suggesting these adducts are biomarkers of leukemia risk.

Of the 77 adducts on Cys34 of albumin, 15 are labeled as unknown because their structures have not been determined. Of the 25 adducts on the N-terminal Valine of hemoglobin, 11 are labeled as unknown (Carlsson et al., 2019). The procedure for identifying unknown adducts includes synthesizing potential adducts with the identical molecular weight as the unknown, followed by verification studies to ensure the candidate has the same retention time on the liquid chromatography system and the same fragmentation spectrum in the mass spectrometer as the unknown (Carlsson & Tornqvist, 2016; Degner et al., 2018).

Untargeted adductomics has not revealed exposure to some common chemicals. The herbicide atrazine is an abundant contaminant in ground water. Tylenol is a popular, non-prescription drug for treatment of headache and pain. Malathion pesticide is used to control mosquitoes, lice, ticks, fleas, and insects that attack fruits, vegetables, and shrubs. The added mass for atrazine adducts is 110 and 179 Da (Chu & Letcher, 2021; Dooley et al., 2006), the added mass for Tylenol (acetaminophen) adducts is 151.1 Da (Damsten et al., 2007), and malathion adds 205 and 177 Da to Cys34 of albumin (Yamagishi, Iwase, & Ogra, 2021). No adduct masses corresponding to adducts from atrazine, acetaminophen, or malathion have been found. A 177 Da adduct on Cys34 of albumin that might reflect exposure to malathion is attributed to CysGly (Carlsson et al., 2019), not to malathion.

How to Apply Adductomics Results to an Understanding of Disease

Adduct levels on albumin, hemoglobin and DNA are very low in healthy people. The presence of a biomarker for a particular disease indicates exposure to a toxin, but not the presence of disease. Detoxification systems rid the body of low levels of toxicants. Only when chemical concentrations exceed the body's innate defenses is there a risk for toxic symptoms. Bruce Ames reported that almost all chemicals are carcinogens at high concentrations, but the same chemicals are not toxic at low concentrations (Ames & Gold, 1997).

The path from adduct detection to understanding the underlying mechanism of toxicity requires years of research using a variety of research methods. After 50 years of study, researchers are still searching for a mechanism to explain how an overdose of acetaminophen (Tylenol) damages the liver. Acetaminophen makes adducts on many proteins, but the relevance of acetaminophen adducts to toxicity has only been partially defined (McGill & Hinson, 2020). Acetaminophen becomes toxic after it is activated by Cytochrome P450 enzymes to a quinone. The quinone is detoxified by reaction with glutathione. When high doses deplete glutathione, the quinone binds to protein sulfhydryl groups. Protein binding and toxicity co-localize in hepatocytes high in Cytochrome 2E1. The events leading to hepatocyte necrosis include oxidative stress and mitochondrial dysfunction (McGill & Hinson, 2020), but details of these processes are still under investigation. Figure 3 summarizes the current view of the mechanism of acetaminophen liver toxicity with mitochondrial dysfunction taking center stage.

Prediction of Adverse Drug Reactions for New Drug Candidates

Adverse reactions to drugs are often initiated by bioactivation of the drug to an electrophile that modifies cellular proteins. The protein conjugates can disrupt cellular functions and cause an immune response (Stepan et al., 2011). The screening tools for bioactivated metabolites include glutathione trapping and covalent binding assays. The goal is to make a drug that yields minimal or no bioactivated metabolites. The premise in these safety assessments is that a drug candidate that produces no bioactivated metabolites is safe. The principal bioactivation process in safety assessment studies employs liver Cytochrome P450 enzymes. However, as pointed out by Stepan et al. (2011), in vitro assays with Cytochrome P450 enzymes overlook other bioactivation pathways, for example bioactivation to electrophilic acyl-coenzyme A thioester derivatives.

Challenges for Adductomics

To understand organ toxicity, it is important to identify toxicologically relevant protein targets of the bioactivated metabolites. Adductomics can identify many proteins that form covalent links with metabolites, but only a few of these adducts are likely to be critical for toxicity.

An immune response to prescription drugs can cause organ failure. Adductomics methods could be applied to gain information about the nature of the antigenic determinants that trigger an immune response to covalently modified proteins (Stepan et al., 2011)

Where the Field is Going

Targeted covalent inhibitors for various proteins are continually being developed. The initial, reversible association between inhibitor and protein is followed by the formation of a covalent bond between the drug and the protein (Baillie, 2016). New, targeted covalent inhibitors differ from old, reversible inhibitors in three important ways. First, the new, targeted covalent inhibitors are designed to be highly selective for the target protein (Baillie, 2016). High affinity means that low doses are required for efficacy. A low dose reduces interaction with nontargeted proteins. Second, the targeted covalent inhibitors carry a warhead that acts only after the inhibitor is noncovalently bound to its target. Third, metabolic activation is not involved. The warhead in the drug only requires proximity to the nucleophilic target for a reaction to occur.

Osimertinib and carfilzomib are examples of FDA-approved targeted covalent inhibitors (Baillie, 2016). Osimertinib is an oral, epidermal growth factor receptor tyrosine kinase inhibitor indicated for the treatment of metastatic non-small cell lung cancer, where the tumor is positive for the T790M mutation. Carfilzomib is an injectable proteasome inhibitor for the treatment of multiple myeloma.

The pharmaceutical industry hesitated to develop covalent inhibitors because decades of experience warned that unacceptable, serious, adverse effects are associated with drugs that make covalent protein adducts. The standard practice in drug discovery programs is to screen out at an early stage of development those drugs that undergo metabolism to reactive electrophiles that make covalent protein adducts. This standard practice is still in force to exclude the possibility that a targeted covalent inhibitor will be transformed to an electrophile that randomly attacks nonspecific proteins. The eight FDA-approved, targeted, covalent inhibitors have been shown to be specific and cannot be transformed into random electrophiles. Despite the proven effectiveness and generally well tolerated history of these targeted covalent inhibitors, skepticism remains (Baillie, 2021). The concern is the potential of covalent inhibitors to cause immune-mediated adverse reactions triggered by covalent modification of either target or off-target proteins (Baillie, 2021).

Development of covalent drugs for both oncology and non-oncology indications are projected to expand significantly in the coming years. The high-resolution liquid chromatography mass spectrometry methods developed for identification of chemical adducts in adductomics studies are well suited to the study of drug-protein covalent interactions, and for defining the biological consequences, whether beneficial or adverse. (Baillie, 2021)

ACKNOWLEDGMENTS

The helpful editorial comments of Dr. Lucio Costa are gratefully acknowledged. Funded in part by the Fred & Pamela Buffet Cancer Center Support Grant P30CA036727 from the National Institutes of Health.

AUTHOR CONTRIBUTIONS

Oksana Lockridge : Writing original draft

CONFLICT OF INTEREST

The author declares no conflict of interest. The sponsor was not involved in preparation of this review.

DATA AVAILABILITY STATEMENT

Data sharing is not applicable to this article as no new data were created or analyzed in this study.

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