Identification of a genotoxic mechanism for the carcinogenicity of the environmental pollutant and suspected human carcinogen o-anisidine

Co-Exposure to Aristolochic Acids I and II Increases DNA Adduct Formation Responsible for Aristolochic Acid I-Mediated Carcinogenicity in Rats

The plant extract aristolochic acid (AA), containing aristolochic acids I (AAI) and II (AAII) as major components, causes aristolochic acid nephropathy (AAN) and Balkan endemic nephropathy (BEN), unique renal diseases associated with upper urothelial cancer. Recently (Chemical Research in Toxicology 33(11), 2804–2818, 2020), we showed that the in vivo metabolism of AAI and AAII in Wistar rats is influenced by their co-exposure (i.e., AAI/AAII mixture). Using the same rat model, we investigated how exposure to the AAI/AAII mixture can influence AAI and AAII DNA adduct formation (i.e., AA-mediated genotoxicity). Using ³²P-postlabelling, we found that AA-DNA adduct formation was increased in the livers and kidneys of rats treated with AAI/AAII mixture compared to rats treated with AAI or AAII alone. Measuring the activity of enzymes involved in AA metabolism, we showed that enhanced AA-DNA adduct formation might be caused partially by both decreased AAI detoxification as a result of hepatic CYP2C11 inhibition during treatment with AAI/AAII mixture and by hepatic or renal NQO1 induction, the key enzyme predominantly activating AA to DNA adducts. Moreover, our results indicate that AAII might act as an inhibitor of AAI detoxification in vivo. Consequently, higher amounts of AAI might remain in liver and kidney tissues, which can be reductively activated, resulting in enhanced AAI DNA adduct formation. Collectively, these results indicate that AAII present in the plant extract AA enhances the genotoxic properties of AAI (i.e., AAI DNA adduct formation). As patients suffering from AAN and BEN are always exposed to the plant extract (i.e., AAI/AAII mixture), our findings are crucial to better understanding host factors critical for AAN- and BEN-associated urothelial malignancy.

Mechanistic insights of the degradation of an O-anisidine carcinogenic pollutant initiated by OH radical attack: theoretical investigations

O-Anisidine (O-AND) is one of the amino organic compounds that harm human health, and is considered as a carcinogenic chemical.

Comparative γ-H2AX analysis for assessment of the genotoxicity of six aromatic amines implicated in bladder cancer in human urothelial cell line

Recently, it was reported that ten cases of bladder cancer occurred among employees, who handled several kinds of aromatic amines, at a Japanese chemical plant. The common aromatic amines were identified as ortho-toluidine, para-toluidine, aniline, ortho-chloroaniline, ortho-anisidine, and 2,4-dimethylaniline. All of these aromatic amines, except ortho-chloroaniline, have been found to be carcinogenic in animals and/or humans. Genotoxic events are known to be crucial steps in the initiation of cancer; information on the genotoxicity of these aromatic amines is insufficient and consistent results have not been obtained. In this study, we examined the genotoxicity of the six different aromatic amines associated with bladder cancer by assessing phosphorylated histone H2AX (γ-H2AX) in a cultured human urothelial cell line, 1T1. We showed that all six aromatic amines generated γ-H2AX. In addition, the γ-H2AX-inducing potential of these six aromatic amines was distinctly different; ortho-chloroaniline and 2,4-dimethylaniline showed particularly high potential, followed by ortho-toluidine, ortho-anisidine, para-toluidine ≒ aniline. The findings of this study may provide important information for the risk assessment of chemicals and for interpreting epidemiological studies on occupational bladder cancer.

Formation of Covalent DNA Adducts by Enzymatically Activated Carcinogens and Drugs In Vitro and Their Determination by 32P-postlabeling

Covalent DNA adducts formed by chemicals or drugs with carcinogenic potency are judged as one of the most important factors in the initiation phase of carcinogenic processes. This covalent binding, which is considered the cause of tumorigenesis, is now evaluated as a central dogma of chemical carcinogenesis. Here, methods are described employing the reactions catalyzed by cytochrome P450 and additional biotransformation enzymes to investigate the potency of chemicals or drugs for their activation to metabolites forming these DNA adducts. Procedures are presented describing the isolation of cellular fractions possessing biotransformation enzymes (microsomal and cytosolic samples with cytochromes P450 or other biotransformation enzymes, i.e., peroxidases, NADPH:cytochrome P450 oxidoreductase, NAD(P)H:quinone oxidoreductase, or xanthine oxidase). Furthermore, methods are described that can be used for the metabolic activation of analyzed chemicals by these enzymes as well as those for isolation of DNA. Further, the appropriate methods capable of detecting and quantifying chemical/drug-derived DNA adducts, i.e., different modifications of the ³²P-postlabeling technique and employment of radioactive-labeled analyzed chemicals, are shown in detail.

Evaluating Metabolite-Related DNA Oxidation and Adduct Damage from Aryl Amines Using a Microfluidic ECL Array

Damage to DNA from the metabolites of drugs and pollutants constitutes a major human toxicity pathway known as genotoxicity. Metabolites can react with metal ions and NADPH to oxidize DNA or participate in SN2 reactions to form covalently linked adducts with DNA bases. Guanines are the main DNA oxidation sites, and 8-oxo-7,8-dihydro-2-deoxyguanosine (8-oxodG) is the initial product. Here we describe a novel electrochemiluminescent (ECL) microwell array that produces metabolites from test compounds and measures relative rates of DNA oxidation and DNA adduct damage. In this new array, films of DNA, metabolic enzymes, and an ECL metallopolymer or complex assembled in microwells on a pyrolytic graphite wafer are housed in dual microfluidic chambers. As reactant solution passes over the wells, metabolites form and can react with DNA in the films to form DNA adducts. These adducts are detected by ECL from a RuPVP polymer that uses DNA as a coreactant. Aryl amines also combine with Cu2+ and NADPH to form reactive oxygen species (ROS) that oxidize DNA. The resulting 8-oxodG was detected selectively by ECL-generating bis(2,2'-bipyridine)-(4-(1,10-phenanthrolin-6-yl)-benzoic acid)Os(II). DNA/enzyme films on magnetic beads were oxidized similarly, and 8-oxodG determined by LC/MS/MS enabled array standardization. The array limit of detection for oxidation was 720 8-oxodG per 106 nucleobases. For a series of aryl amines, metabolite-generated DNA oxidation and adduct formation turnover rates from the array correlated very well with rodent 1/TD50 and Comet assay results.

Hepatic co-cultures in vitro reveal suitable to detect Nrf2-mediated oxidative stress responses on the bladder carcinogen o-anisidine

The azo dye o-anisidine is known as an industrial and environmental pollutant. Metabolites of o-anisidine remain in the liver for >24h. However, the toxicological impact of o-anisidine on the liver and its individual cell types, e.g., hepatocytes and immune cells, is currently poorly understood. A novel co-culture system, composed of HepG2 or Huh-7 cells, and differentiated THP-1 cells was used to study the metabolic capacity towards o-anisidine, and compared to primary murine hepatocytes which express high enzyme activities. As model compounds the carcinogenic arylamine o-anisidine and its non-carcinogenic isomer, p-anisidine, as well as caffeine were used. Global proteome analysis revealed an activation of eIF2 and Nrf2-mediated oxidative stress response pathways only in co-cultures after treatment with o-anisidine. This was confirmed via detection of reactive oxygen species. In addition, the mitochondrial membrane potential decreased already after 3h treatment of cells, which correlated with a decrease of ATP levels (R²>0.92). In the supernatant of co-cultured, but not single-cultured HepG2 and Huh-7 cells, o-anisidine caused increases of damage-associated proteins, such as HMGB1 (high mobility group box-1) protein. In summary, only co-cultures of HepG2 and THP-1 cells predict o-anisidine induced stress responsive pathways, since the system has a higher sensitivity compared to single cultured cells.

The urinary bladder carcinogen propoxur does not produce genotoxic effects in the urinary bladder of Wistar male rats

Propoxur (PPX) is a carbamate insecticide which induced urinary bladder cancer in Wistar rats when fed at 5000ppm in Altromin 1321 diet (1321). In the present investigation, PPX was studied for induction of several key events related to modes of action (MOA) of carcinogenicity in urinary bladders (UBs). Wistar rats were administered the compound for 28 days at 8000ppm in Provini Liba SA 3883 diet, which is similar to the 1321 diet. o-Anisidine HCl (AH) was used as a genotoxic UB carcinogenic comparator, and trisodium nitrilotriacetate (NTA) as an epigenetic UB carcinogen comparator. Along with the non-dosed control and three test substance groups (PPX, AH, NTA), four more groups were additionally fed 2% ammonium chloride (AC) in the diet to acidify the urine, since 1321 was reported to increase urinary pH. AC did acidify the urine, as expected, although the 3883 diet itself did not increase pH values above 8. In the alkaline comet assay, AH produced DNA single strand breaks (SSBs) in the UB urothelium (UBU) irrespective of AC administration, whereas PPX and NTA did not. In the nucleotide (32)P-postlabeling assay (NPL), AH produced DNA adducts irrespective of AC administration, whereas PPX and NTA did not. Routine (H&E) histopathology evaluation of the UBU did not reveal any hyperplasia or evidence of luminal microprecipitates or calculi in any of the groups. Assessment of UBU proliferation as measured by immunohistochemistry of proliferating cell nuclear antigen, revealed that NTA and NTA plus AC increased the replicating fraction (RF). Also AH plus AC, but not AH alone, increased the RF of UBU, whereas PPX groups were not significantly different from controls. Thus, the results reveal no evidence for DNA SSBs, binding, or alteration of DNA synthesis in the UBU by PPX, while demonstrating UBU DNA damage by AH and showing that NTA does not damage DNA, but causes increased UBU proliferation. The findings are in accord with a genotoxic MOA for AH, and an epigenetic MOA for NTA. The MOA of PPX does not involve genotoxicity and may be specific to the 1321 diet.

Comet assay evaluation of six chemicals of known genotoxic potential in rats

As a part of an international validation of the in vivo rat alkaline comet assay (comet assay) initiated by the Japanese Center for the Validation of Alternative Methods (JaCVAM) we examined six chemicals for potential to induce DNA damage: 2-acetylaminofluorene (2-AAF), N-nitrosodimethylamine (DMN), o-anisidine, 1,2-dimethylhydrazine dihydrochloride (1,2-DMH), sodium chloride, and sodium arsenite. DNA damage was evaluated in the liver and stomach of 7- to 9-week-old male Sprague Dawley rats. Of the five genotoxic carcinogens tested in our laboratory, DMN and 1,2-DMH were positive in the liver and negative in the stomach, 2-AAF and o-anisidine produced an equivocal result in liver and negative results in stomach, and sodium arsenite was negative in both liver and stomach. 1,2-DMH and DMN induced dose-related increases in hedgehogs in the same tissue (liver) that exhibited increased DNA migration. However, no cytotoxicity was indicated by the neutral diffusion assay (assessment of highly fragmented DNA) or histopathology in response to treatment with any of the tested chemicals. Therefore, the increased DNA damage resulting from exposure to DMN and 1,2-DMH was considered to represent a genotoxic response. Sodium chloride, a non-genotoxic non-carcinogen, was negative in both tissues as would be predicted. Although only two (1,2-DMH and DMN) out of five genotoxic carcinogens produced clearly positive results in the comet assay, the results obtained for o-anisidine and sodium arsenite in liver and stomach cells are consistent with the known mode of genotoxicity and tissue specificity exhibited by these carcinogens. In contrast, given the known genotoxic mode-of-action and target organ carcinogenicity of 2-AAF, it is unclear why this chemical failed to convincingly increase DNA migration in the liver. Thus, the results of the comet assay validation studies conducted in our laboratory were considered appropriate for five out of the six test chemicals.

JaCVAM-organized international validation study of the in vivo rodent alkaline comet assay for detection of genotoxic carcinogens: II. Summary of definitive validation study results

The in vivo rodent alkaline comet assay (comet assay) is used internationally to investigate the in vivo genotoxic potential of test chemicals. This assay, however, has not previously been formally validated. The Japanese Center for the Validation of Alternative Methods (JaCVAM), with the cooperation of the U.S. NTP Interagency Center for the Evaluation of Alternative Toxicological Methods (NICEATM)/the Interagency Coordinating Committee on the Validation of Alternative Methods (ICCVAM), the European Centre for the Validation of Alternative Methods (ECVAM), and the Japanese Environmental Mutagen Society/Mammalian Mutagenesis Study Group (JEMS/MMS), organized an international validation study to evaluate the reliability and relevance of the assay for identifying genotoxic carcinogens, using liver and stomach as target organs. The ultimate goal of this exercise was to establish an Organisation for Economic Co-operation and Development (OECD) test guideline. The study protocol was optimized in the pre-validation studies, and then the definitive (4th phase) validation study was conducted in two steps. In the 1st step, assay reproducibility was confirmed among laboratories using four coded reference chemicals and the positive control ethyl methanesulfonate. In the 2nd step, the predictive capability was investigated using 40 coded chemicals with known genotoxic and carcinogenic activity (i.e., genotoxic carcinogens, genotoxic non-carcinogens, non-genotoxic carcinogens, and non-genotoxic non-carcinogens). Based on the results obtained, the in vivo comet assay is concluded to be highly capable of identifying genotoxic chemicals and therefore can serve as a reliable predictor of rodent carcinogenicity.

Construction of a CYP2E1-template system for prediction of the metabolism on both site and preference order

We have constructed an in silico system for the prediction of CYP2E1-mediated reaction using a two-dimensional template derived from substrate structures. Although CYP2E1 prefers small-size molecules for the substrates, the enzyme mediates oxidations of large-size molecules, such as benzo[a]pyrene. Overlays of these substrates, to assemble their sites of oxidation into a specific area, suggested a range of regions frequently occupied. The region, having a benzo[a]pyrene-like shape, was thus used as a CYP2E1 template. In this system, atoms in substrates, except for hydrogen atoms, were placed on corners of honeycomb structures of the template after having expanded the structures. Using published data for the metabolism on more than 80 substrates of CYP2E1, the core template was further refined to verify the adjacent area and to define the relative contribution of template positions for the catalysis. The positions on the template were classified into four different point (0-3) groups, depending on relative usage. In addition, we set independent points (-5 to 3) for specific positions to incorporate three-dimensional or functional information. Total scores from both position-occupancy and -function points were calculated for all the orientations of possible conformers of test substrates, and the scores were found to predict the relative abundance (i.e., order) as well as the regioselectivity of human CYP2E1 reactions with high fidelities.

Contributions of human enzymes in carcinogen metabolism

Considerable support exists for the roles of metabolism in modulating the carcinogenic properties of chemicals. In particular, many of these compounds are pro-carcinogens that require activation to electrophilic forms to exert genotoxic effects. We systematically analyzed the existing literature on the metabolism of carcinogens by human enzymes, which has been developed largely in the past 25 years. The metabolism and especially bioactivation of carcinogens are dominated by cytochrome P450 enzymes (66% of bioactivations). Within this group, six P450s--1A1, 1A2, 1B1, 2A6, 2E1, and 3A4--accounted for 77% of the P450 activation reactions. The roles of these P450s can be compared with those estimated for drug metabolism and should be considered in issues involving enzyme induction, chemoprevention, molecular epidemiology, interindividual variations, and risk assessment.

Formation, persistence, and identification of DNA adducts formed by the carcinogenic environmental pollutant o-anisidine in rats

2-Methoxyaniline (o-anisidine) is an industrial and environmental pollutant causing tumors of urinary bladder in rodents. Here, we investigated the formation and persistence of DNA adducts in the Wistar rat. Using the (32)P-postlabeling method, three o-anisidine-derived DNA adducts were found in several organs of rats treated with a total dose of 0.53 mg o-anisidine/kg body wt (0.15, 0.18, and 0.2 mg/kg body wt ip in the first, second, and third day, respectively), of which the urinary bladder had the highest levels. At four posttreatment times (1 day, 13 days, 10 weeks, and 36 weeks), DNA adducts in bladder, liver, kidney, and spleen of rats were analyzed to study their persistence. In all time points, the highest total adduct levels were found in urinary bladder (39 adducts per 10(7) nucleotides after 1 day and 15 adducts per 10(7) nucleotides after 36 weeks) where 39% adducts remained. In contrast to the urinary bladder, no persistence was detected in other organs. All three DNA adducts were identified as deoxyguanosine adducts. When deoxyguanosine was reacted with the oxidative metabolite of o-anisidine, N-(2-methoxyphenyl)hydroxylamine, three adducts could be separated by high-performance liquid chromatography (HPLC) and were identified by mass spectroscopy and/or nuclear magnetic resonance spectrometry. All adducts are products of the nitrenium/carbenium ions, the reactive species generated from N-(2-methoxyphenyl)hydroxylamine. The major adduct was identified to be N-(deoxyguanosin-8-yl)-2-methoxyaniline. Using cochromatography on HPLC, this adduct was found to be identical to the major adduct generated by activation of o-anisidine in vitro and in vivo.

Cytochrome P450-mediated metabolism of N-(2-methoxyphenyl)-hydroxylamine, a human metabolite of the environmental pollutants and carcinogens o-anisidine and o-nitroanisole

N-(2-methoxyphenyl)hydroxylamine is a human metabolite of the industrial and environmental pollutants and bladder carcinogens 2-methoxyaniline (o-anisidine) and 2-methoxynitrobenzene (o-nitroanisole). Here, we investigated the ability of hepatic microsomes from rat and rabbit to metabolize this reactive compound. We found that N-(2-methoxyphenyl)hydroxylamine is metabolized by microsomes of both species mainly to o-aminophenol and a parent carcinogen, o-anisidine, whereas 2-methoxynitrosobenzene (o-nitrosoanisole) is formed as a minor metabolite. Another N-(2-methoxyphenyl)hydroxylamine metabolite, the exact structure of which has not been identified as yet, was generated by hepatic microsomes of rabbits, but its formation by those of rats was negligible. To evaluate the role of rat hepatic microsomal cytochromes P450 (CYP) in N-(2-methoxyphenyl)hydroxylamine metabolism, we investigated the modulation of its metabolism by specific inducers of these enzymes. The results of this study show that rat hepatic CYPs of a 1A subfamily and, to a lesser extent those of a 2B subfamily, catalyze N-(2-methoxyphenyl)hydroxylamine conversion to form both its reductive metabolite, o-anisidine, and o-aminophenol. CYP2E1 is the most efficient enzyme catalyzing conversion of N-(2-methoxyphenyl)hydroxylamine to o-aminophenol.

Genotoxic mechanisms for the carcinogenicity of the environmental pollutants and carcinogens o-anisidine and 2-nitroanisole follow from adducts generated by their metabolite N-(2-methoxyphenyl)-hydroxylamine with deoxyguanosine in DNA

An aromatic amine, o-anisidine (2-methoxyaniline) and its oxidative counterpart, 2-nitroanisole (2-methoxynitrobenzene), are the industrial and environmental pollutants causing tumors of the urinary bladder in rats and mice. Both carcinogens are activated to the same proximate carcinogenic metabolite, N-(2-methoxyphenyl)hydroxylamine, which spontaneously decomposes to nitrenium and/or carbenium ions responsible for formation of deoxyguanosine adducts in DNA in vitro and in vivo. In other words, generation of N-(2-methoxyphenyl)hydroxylamine is responsible for the genotoxic mechanisms of the o-anisidine and 2-nitroanisole carcinogenicity. Analogous enzymes of human and rat livers are capable of activating these carcinogens. Namely, human and rat cytochorme P4502E1 is the major enzyme oxidizing o-anisidine to N-(2-methoxyphenyl)hydroxylamine, while xanthine oxidase of both species reduces 2-nitroanisole to this metabolite. Likewise, O-demethylation of 2-nitroanisole, which is the detoxication pathway of its metabolism, is also catalyzed by the same human and rat enzyme, cytochorme P450 2E1. The results demonstrate that the rat is a suitable animal model mimicking the fate of both carcinogens in humans and suggest that both compounds are potential carcinogens also for humans.

Rat liver microsomal metabolism of o-aminophenol and N-(2-methoxyphenyl)hydroxylamine, two metabolites of the environmental pollutant and carcinogen o-anisidine in humans

o-Aminophenol and N-(2-methoxyphenyl)hydroxylamine are human metabolites of the industrial and environmental pollutant and bladder carcinogen 2-methoxyaniline (o-anisidine). The latter one is also a human metabolite of another pollutant and bladder carcinogen, 2-methoxynitrobenzene (o-nitroanisole). Here, we investigated the ability of rat hepatic micro- somes to metabolize these metabolites. N-(2-methoxyphenyl)hydroxylamine is metabolized by rat hepatic microsomes to o-aminophenol and predominantly o-anisidine, the parent carcinogen from which N-(2-methoxyphenyl)hydroxylamine is formed. In addition, two N-(2-methoxyphenyl)hydroxylamine metabolites, whose exact structures have not been identified as yet, were generated. On the contrary, no metabolites were found to be formed from o-aminophenol by rat hepatic microsomes. Whereas N-(2-methoxyphenyl)hydroxylamine is responsible for formation of three deoxyguanosine adducts in DNA, o-aminophenol seems to be a detoxication metabolite of N-(2-methoxyphenyl)hydroxylamine and/or a parental carcinogen, o-anisidine; no o-aminophenol-derived DNA adducts were found after its reaction with microsomal cytochromes P450 and peroxidases.

Creating context for the use of DNA adduct data in cancer risk assessment: II. Overview of methods of identification and quantitation of DNA damage

The formation of deoxyribonucleic acid (DNA) adducts can have important and adverse consequences for cellular and whole organism function. Available methods for identification of DNA damage and quantification of adducts are reviewed. Analyses can be performed on various samples including tissues, isolated cells, and intact or hydrolyzed (digested) DNA from a variety of biological samples of interest for monitoring in humans. Sensitivity and specificity are considered key factors for selecting the type of method for assessing DNA perturbation. The amount of DNA needed for analysis is dependent upon the method and ranges widely, from <1 microg to 3 mg. The methods discussed include the Comet assay, the ligation-mediated polymerase reaction, histochemical and immunologic methods, radiolabeled ((14)C- and (3)H-) binding, (32)P-postlabeling, and methods dependent on gas chromatography (GC) or high-performance liquid chromatography (HPLC) with detection by electron capture, electrochemical detection, single or tandem mass spectrometry, or accelerator mass spectrometry. Sensitivity is ranked, and ranges from approximately 1 adduct in 10(4) to 10(12) nucleotides. A brief overview of oxidatively generated DNA damage is also presented. Assay limitations are discussed along with issues that may have impact on the reliability of results, such as sample collection, processing, and storage. Although certain methodologies are mature, improving technology will continue to enhance the specificity and sensitivity of adduct analysis. Because limited guidance and recommendations exist for adduct analysis, this effort supports the HESI Committee goal of developing a framework for use of DNA adduct data in risk assessment.

Redox cycling in the metabolism of the environmental pollutant and suspected human carcinogen o-anisidine by rat and rabbit hepatic microsomes

We investigated the ability of hepatic microsomes from rat and rabbit to metabolize 2-methoxyaniline (o-anisidine), an industrial and environmental pollutant and a bladder carcinogen for rodents. Using HPLC combined with electrospray tandem mass spectrometry, we determined that o-anisidine is oxidized by microsomes of both species to N-(2-methoxyphenyl)hydroxylamine, o-aminophenol, and one additional metabolite, the exact structure of which has not been identified as yet. N-(2-Methoxyphenyl)hydroxylamine is either further oxidized to 2-methoxynitrosobenzene (o-nitrosoanisole) or reduced to parental o-anisidine, which can be oxidized again to produce o-aminophenol. To define the role of microsomal cytochromes P450 (P450) in o-anisidine metabolism, we investigated the modulation of this metabolism by specific inducers and selective inhibitors of these enzymes. The results of the studies suggest that o-anisidine is a promiscuous substrate of P450s of rat and rabbit liver; because P450s of 1A, 2B, 2E, and 3A subfamilies metabolize o-anisidine in hepatic microsomes of both studied species. Using purified enzymes of rat and rabbit (P450s 1A1, 1A2, 2B2, 2B4, 2E1, 2C3, 3A1, and 3A6), reconstituted with NADPH:P450 reductase, the ability of P450s 1A1, 1A2, 2B2, 2B4, 2E1, and 3A6 to metabolize o-anisidine was confirmed. In the reconstituted P450 system, rabbit P450 2E1 was the most efficient enzyme metabolizing o-anisidine. The data demonstrate the participation of different rat and rabbit P450s in o-anisidine metabolism and indicate that both experimental animal species might serve as suitable models to mimic the fate of o-anisidine in human.

Hemoglobin adducts of the human bladder carcinogen o-toluidine after treatment with the local anesthetic prilocaine

Prilocaine, a widely used local anesthetic, is metabolized to o-toluidine which is classified as human carcinogen. We aimed to assess the impact of prilocaine-treatment on hemoglobin adducts from o-toluidine. Blood samples were obtained before and 24h after receiving prilocaine local anesthesia (Xylonest, 100mg) from 20 head and neck surgery patients and 6 healthy volunteers. Hemoglobin adducts of o-toluidine and 4-aminobiphenyl were determined by gas chromatography/mass spectrometry. Hemoglobin adducts of o-toluidine were significantly increased 24h after 100mg prilocaine-treatment by 21.6+/-12.8ng/g hemoglobin (mean+/-S.D., N=26; P<0.0001). This corresponds to a 6-360-fold increase of o-toluidine adduct levels in 25 patients from 0.54+/-0.95ng/g before treatment to 22.0+/-13.2ng/g 24h after surgery (mean+/-S.D.). Because of an extremely high background level the increase was only 1.6-fold in one patient (40.9ng/g before and 64.4ng/g 24h after prilocaine injection). Current smoking had no influence on background values and on the increase of o-toluidine adducts. No treatment-related differences were seen in mean hemoglobin adduct levels of 4-aminobiphenyl which were significantly higher in smokers, 0.149+/-0.096ng/g (mean+/-S.D., N=8) as compared to nonsmokers 0.036+/-0.035ng/g (mean+/-S.D., N=16; P<0.01). In conclusion, prilocaine anesthesia leads to a massive increase of hemoglobin adducts of the carcinogenic arylamine o-toluidine. This implies a carcinogenic risk which should be taken into account in preventive hazard minimization.

Carcinogenic pollutants o-nitroanisole and o-anisidine are substrates and inducers of cytochromes P450

2-Methoxyaniline (o-anisidine) and 2-methoxynitrobenzene (o-nitroanisole) are important pollutants and potent carcinogens for rodents. o-Anisidine is oxidized by microsomes of rats and rabbits to N-(2-methoxyphenyl)hydroxylamine that is also formed as the reduction metabolite of o-nitroanisole. o-Anisidine is a promiscuity substrate of rat and rabbit cytochrome P450 (CYP) enzymes, because CYPs of 1A, 2B, 2E and 3A subfamilies oxidize o-anisidine. Using purified CYP enzymes, reconstituted with NADPH: CYP reductase, rabbit CYP2E1 was the most efficient enzyme oxidizing o-anisidine, but the ability of CYP1A1, 1A2, 2B2, 2B4 and 3A6 to participate in o-anisidine oxidation was also proved. Utilizing Western blotting and consecutive immunoquantification employing chicken polyclonal anti bodies raised against various CYPs, the effect of o-anisidine and o-nitroanisole on the expression of the CYP enzymes was investigated. The expression of CYP1A1/2 was found to be strongly induced in rats treated with either compounds. In addition, 7-ethoxyresorufin O-deethylation, a marker activity for both CYP1A1 and 1A2, was significantly increased in rats treated with either carcinogen. The data demonstrate the participation of different rat and rabbit CYP enzymes in o-anisidine oxidation and indicate that both experimental animal species might serve as suitable models to mimic the o-anisidine oxidation in human. Furthermore, by induction of rat hepatic and renal CYP1A1/2, both o-nitroanisole and o-anisidine influence their carcinogenic effects, modifying their detoxification and/or activation pathways.