Species differences in urinary butadiene metabolites; identification of 1,2-dihydroxy-4-(N-acetylcysteinyl)butane, a novel metabolite of butadiene

1,3-Butadiene: a ubiquitous environmental mutagen and its associations with diseases

1,3-Butadiene (BD) is a petrochemical manufactured in high volumes. It is a human carcinogen and can induce lymphohematopoietic cancers, particularly leukemia, in occupationally-exposed workers. BD is an air pollutant with the major environmental sources being automobile exhaust and tobacco smoke. It is one of the major constituents and is considered the most carcinogenic compound in cigarette smoke. The BD concentrations in urban areas usually vary between 0.01 and 3.3 μg/m³ but can be significantly higher in some microenvironments. For BD exposure of the general population, microenvironments, particularly indoor microenvironments, are the primary determinant and environmental tobacco smoke is the main contributor. BD has high cancer risk and has been ranked the second or the third in the environmental pollutants monitored in most urban areas, with the cancer risks exceeding 10^-5. Mutagenicity/carcinogenicity of BD is mediated by its genotoxic metabolites but the specific metabolite(s) responsible for the effects in humans have not been determined. BD can be bioactivated to yield three mutagenic epoxide metabolites by cytochrome P450 enzymes, or potentially be biotransformed into a mutagenic chlorohydrin by myeloperoxidase, a peroxidase almost specifically present in neutrophils and monocytes. Several urinary BD biomarkers have been developed, among which N-acetyl-S-(4-hydroxy-2-buten-1-yl)-L-cysteine is the most sensitive and is suitable for biomonitoring BD exposure in the general population. Exposure to BD has been associated with leukemia, cardiovascular disease, and possibly reproductive effects, and may be associated with several cancers, autism, and asthma in children. Collectively, BD is a ubiquitous pollutant that has been associated with a range of adverse health effects and diseases with children being a subpopulation with potentially greater susceptibility. Its adverse effects on human health may have been underestimated and more studies are needed.

An unexpected butadiene diolepoxide-mediated genotoxicity implies alternative mechanism for 1,3-butadiene carcinogenicity

1,3-Butadiene (BD) is abundant in combustion products such as cigarette smoke. While BD has been classified as a known human carcinogen, a long-standing question is the identity of the ultimate carcinogenic metabolite in humans. We hypothesize that 3,4-epoxybutane-1,2-diol (EBD) may play a critical role in human carcinogenesis due to its high bioavailability. We utilized a differential toxicity assay for BD metabolites and newly synthesized EBD analogs in a series of isogenic chicken cells lacking specific DNA repair proteins to address the mode of action of BD genotoxicity and infer a mode of action. Surprisingly, as with the diepoxide 1,2:3,4-diepoxybutane (DEB), the monoepoxide EBD showed remarkable toxicity to cells deficient in Fanconi anemia (FANC) genes. This observation suggests that EBD may be transformed into a bifunctional metabolite and forms interstrand cross-links. EBD and its analog with a hydroxy substituent at C1 were found to be highly toxic to FANCD2-deficient chicken and human cells. The Results suggest that EBD may be transformed to a bifunctional epoxy aldehyde, perhaps by alcohol dehydrogenase, to which the observed FANC sensitivity could be attributed. The implications of this study are very important in considering mechanisms by which EBD may cause leukemia and lymphoma in humans exposed to BD.

Effects of 2-Phenethyl Isothiocyanate on Metabolism of 1,3-Butadiene in Smokers

2-Phenethyl isothiocyanate (PEITC) is a natural product found as a conjugate in cruciferous vegetables. It has been reported to have preventative properties against lung cancer and to inhibit metabolic activation of tobacco carcinogens. In this study, we evaluated the ability of PEITC to influence the metabolism of the human carcinogen 1,3-butadiene in current smokers in a phase II clinical trial with a crossover design. Urinary mercapturic acids of 1,3-butadiene were quantified at baseline and during PEITC treatment. Seventy-nine smokers were randomly assigned to one of two arms: PEITC followed by placebo or placebo followed by PEITC. During the 1-week treatment period, each subject took PEITC (10 mg in 1 mL of olive oil, 4 times per day). There was a 1-week washout period between the PEITC and placebo periods. Oral ingestion of PEITC increased urinary levels of BD-mercapturic acids (MHBMA and DHBMA) by 11.1% and 3.7%, respectively, but these increases were not statistically significant (P = 0.17 and 0.64, respectively). A much stronger effect was observed among subjects with the null genotype of both GSTM1 and GSTT1: in these individuals, PEITC increased urinary levels of MHBMA by 58.7% (P = 0.004) and 90.0% (P = 0.001), respectively, but did not have a significant effect on urinary DHBMA. These results reveal a potentially protective effect of PEITC treatment with respect to the detoxification of 1,3-butadiene in cigarette smokers, specifically in those null for GSTT1, and provide further evidence in support of stronger chemopreventive effects from consumption of dietary isothiocyanates in these individuals.

1,3-Butadiene metabolite 1,2,3,4 diepoxybutane induces DNA adducts and micronuclei but not t(9;22) translocations in human cells

Epidemiological studies of 1,3-butadiene (BD) exposures have reported a possible association with chronic myelogenous leukemia (CML), which is defined by the presence of the t(9;22) translocation (Philadelphia chromosome) creating an oncogenic BCR-ABL fusion gene. Butadiene diepoxide (DEB), the most mutagenic of three epoxides resulting from BD, forms DNA-DNA crosslink adducts that can lead to DNA double-strand breaks (DSBs). Thus, a study was designed to determine if (±)-DEB exposure of HL60 cells, a promyelocytic leukemia cell line lacking the Philadelphia chromosome, can produce t(9;22) translocations. In HL60 cells exposed for 3 h to 0-10 μM DEB, overlapping dose-response curves suggested a direct relationship between 1,4-bis-(guan-7-yl)-2,3-butanediol crosslink adduct formation (R = 0.977, P = 0.03) and cytotoxicity (R = 0.961, P = 0.002). Experiments to define the relationships between cytotoxicity and the induction of micronuclei (MN), a dosimeter of DNA DSBs, showed that 24 h exposures of HL60 cells to 0-5.0 μM DEB caused significant positive correlations between the concentration and (i) the degree of cytotoxicity (R = 0.998, p = 0.002) and (ii) the frequency of MN (R = 0.984, p = 0.016) at 48 h post exposure. To determine the relative induction of MN and t(9;22) translocations following exposures to DEB, or x-rays as a positive control for formation of t(9;22) translocations, HL60 cells were exposed for 24 h to 0, 1, 2.5, or 5 μM DEB or to 0, 2.0, 3.5, or 5.0 Gy x-rays, or treatments demonstrated to yield 0, 20%, 50%, or 80% cytotoxicity. Treatments between 0 and 3.5 Gy x-rays caused significant dose-related increases in both MN (p < 0.001) and t(9;22) translocations (p = 0.01), whereas DEB exposures causing similar cytotoxicity levels did not increase translocations over background. These data indicate that, while DEB induces DNA DSBs required for formation of MN and translocations, acute DEB exposures of HL60 cells did not produce the Philadelphia chromosome obligatory for CML.

Genetic Determinants of 1,3-Butadiene Metabolism and Detoxification in Three Populations of Smokers with Different Risks of Lung Cancer

Background: 1,3-Butadiene (BD) is an important carcinogen in tobacco smoke that undergoes metabolic activation to DNA-reactive epoxides. These species can be detoxified via glutathione conjugation and excreted in urine as the corresponding N-acetylcysteine conjugates. We hypothesize that single nucleotide polymorphisms (SNPs) in BD-metabolizing genes may change the balance of BD bioactivation and detoxification in White, Japanese American, and African American smokers, potentially contributing to ethnic differences in lung cancer risk.Methods: We measured the levels of BD metabolites, 1- and 2-(N-acetyl-L-cysteine-S-yl)-1-hydroxybut-3-ene (MHBMA) and N-acetyl-S-(3,4-dihydroxybutyl)-L-cysteine (DHBMA), in urine samples from a total of 1,072 White, Japanese American, and African American smokers and adjusted these values for body mass index, age, batch, and total nicotine equivalents. We also conducted a genome-wide association study to identify genetic determinants of BD metabolism.Results: We found that mean urinary MHBMA concentrations differed significantly by ethnicity (P = 4.0 × 10-25). African Americans excreted the highest levels of MHBMA followed by Whites and Japanese Americans. MHBMA levels were affected by GSTT1 gene copy number (P < 0.0001); conditional on GSTT1, no other polymorphisms showed a significant association. Urinary DHBMA levels also differed between ethnic groups (P = 3.3 × 10-4), but were not affected by GSTT1 copy number (P = 0.226).Conclusions:GSTT1 gene deletion has a strong effect on urinary MHBMA levels, and therefore BD metabolism, in smokers.Impact: Our results show that the order of MHBMA levels among ethnic groups is consistent with their respective lung cancer risk and can be partially explained by GSTT1 genotype. Cancer Epidemiol Biomarkers Prev; 26(7); 1034-42. ©2017 AACR.

Metabolism of metofluthrin in rats: I. Identification of metabolites

1. Metofluthrin (2,3,5,6-tetrafluoro-4-(methoxymethyl)benzyl (Z/E)-(1R)-trans-2,2-dimethyl-3-(1-propenyl)-cyclopropanecarboxylate) is a novel pyrethroid insecticide, which has E/Z isomers at prop-1-enyl group. 2. Rats were orally dosed with each [¹⁴C]-labelled E/Z isomer, and the excreta were collected for isolation and identification of metabolites. Analysis of the excreta by LC/MS and NMR revealed formation of 33 and 23 (total 42) metabolites from rats dosed with Z-isomer and E-isomer, respectively. 3. Major metabolic reactions were cleavage of ester linkage, O-demethylation, hydroxylation, epoxidation or reduction of double bond, glutathione conjugation and its further metabolism, hydroxylation of epoxide and formation of lactone ring. Notably, the acid side, 2,2-dimethyl-3-(1-propenyl)-cyclopropanecarboxylic acid, was much more variously metabolised compared to chrysanthemic acid, the acid side of the known pyrethroids. 4. Major metabolites for Z-isomer mostly retained ester linkage with 1,2-dihydroxypropyl group and/or 2-methylalcohol of cyclopropane ring, while most of those for E-isomer received hydrolysis of the ester linkage without oxidation at the 1-propenyl group or the gem-methyl groups, suggesting epoxidation and hydroxylation could occur more easily on Z-isomer. 5. As the novel metabolic pathways for pyrethroids, isomerisation of ω-carboxylic acid moiety, reduction or hydration of double bond and cleavage of cyclopropane ring via epoxidation were suggested.

High throughput HPLC-ESI(-)-MS/MS methodology for mercapturic acid metabolites of 1,3-butadiene: Biomarkers of exposure and bioactivation

1,3-Butadiene (BD) is an important industrial and environmental carcinogen present in cigarette smoke, automobile exhaust, and urban air. The major urinary metabolites of BD in humans are 2-(N-acetyl-L-cystein-S-yl)-1-hydroxybut-3-ene/1-(N-acetyl-L-cystein-S-yl)-2-hydroxybut-3-ene (MHBMA), 4-(N-acetyl-L-cystein-S-yl)-1,2-dihydroxybutane (DHBMA), and 4-(N-acetyl-L-cystein-S-yl)-1,2,3-trihydroxybutyl mercapturic acid (THBMA), which are formed from the electrophilic metabolites of BD, 3,4-epoxy-1-butene (EB), hydroxymethyl vinyl ketone (HMVK), and 3,4-epoxy-1,2-diol (EBD), respectively. In the present work, a sensitive high-throughput HPLC-ESI(-)-MS/MS method was developed for simultaneous quantification of MHBMA and DHBMA in small volumes of human urine (200 μl). The method employs a 96 well Oasis HLB SPE enrichment step, followed by isotope dilution HPLC-ESI(-)-MS/MS analysis on a triple quadrupole mass spectrometer. The validated method was used to quantify MHBMA and DHBMA in urine of workers from a BD monomer and styrene-butadiene rubber production facility (40 controls and 32 occupationally exposed to BD). Urinary THBMA concentrations were also determined in the same samples. The concentrations of all three BD-mercapturic acids and the metabolic ratio (MHBMA/(MHBMA+DHBMA+THBMA)) were significantly higher in the occupationally exposed group as compared to controls and correlated with BD exposure, with each other, and with BD-hemoglobin biomarkers. This improved high throughput methodology for MHBMA and DHBMA will be useful for future epidemiological studies in smokers and occupationally exposed workers.

Inhibitory potency of 4-carbon alkanes and alkenes toward CYP2E1 activity

CYP2E1 has been implicated in the bioactivation of many small molecules into reactive metabolites which form adducts with proteins and DNA, and thus a better understanding of the molecular determinants of its selectivity are critical for accurate toxicological predictions. In this study, we determined the potency of inhibition of human CYP2E1 for various 4-carbon alkanes, alkenes and alcohols. In addition, known CYP2E1 substrates and inhibitors including 4-methylpyrazole, aniline, and dimethylnitrosamine were included to determine their relative potencies. Of the 1,3-butadiene-derived metabolites studied, 3,4-epoxy-1-butene was the strongest inhibitor with an IC50 of 110 μM compared to 1700 μM and 6600 μM for 1,2-butenediol and 1,2:3,4-diepoxybutane, respectively. Compared to known inhibitors, inhibitory potency of 3,4-epoxy-1-butene is between 4-methylpyrazole (IC50 = 1.8 μM) and dimethylnitrosamine (IC50 = 230 μM). All three butadiene metabolites inhibit CYP2E1 activity through a simple competitive mechanism. Among the 4-carbon compounds studied, the presence and location of polar groups seems to influence inhibitory potency. To further examine this notion, the investigation was extended to include structurally and chemically similar analogues, including propylene oxide and various butane alcohols. Those results demonstrated preferential recognition of CYP2E1 toward the type and location of polar and hydrophobic structural elements. Taken together, CYP2E1 metabolism may be modified in vivo by exposure to 4-carbon compounds, such as drugs, and nutritional constituents, a finding that highlights the complexity of exposure to mixtures.

Differences in butadiene adduct formation between rats and mice not due to selective inhibition of CYP2E1 by butadiene metabolites

CYP2E1 metabolizes 1,3-butadiene (BD) into genotoxic and possibly carcinogenic 1,2-epoxy-3-butene (EB), 1,2:3,4-diepoxybutane (DEB), and 1,2-epoxy-3,4-butanediol (EB-diol). The dose response of DNA and protein adducts derived from BD metabolites increases linearly at low BD exposures and then saturates at higher exposures in rats, but not mice. It was hypothesized that differences in adduct formation between rodents reflect more efficient BD oxidation in mice than rats. Herein, we assessed whether BD-derived metabolites selectively inhibit rat but not mouse CYP2E1 activity using B6C3F1 mouse and Fisher 344 rat liver microsomes. Basal CYP2E1 activities toward 4-nitrophenol were similar between rodents. Through IC50 studies, EB was the strongest inhibitor (IC50 54μM, mouse; 98μM, rat), BD-diol considerably weaker (IC50 1200μM, mouse; 1000μM, rat), and DEB inhibition nonexistent (IC50>25mM). Kinetic studies showed that in both species EB and BD-diol inhibited 4-nitrophenol oxidation through two-site mechanisms in which inhibition constants reflected trends observed in IC50 studies. None of the reactive epoxide metabolites inactivated CYP2E1 irreversibly. Thus, there was no selective inhibition or inactivation of rat CYP2E1 by BD metabolites relative to mouse Cyp2e1, and it can be inferred that CYP2E1 activity toward BD between rodent species would similarly not be impacted by the presence of BD metabolites. Inhibition of CYP2E1 by BD metabolites is then not responsible for the reported species difference in BD metabolism, formation of BD-derived DNA and protein adducts, mutagenicity and tumorigenesis.

Alcohol dehydrogenase- and rat liver cytosol-dependent bioactivation of 1-chloro-2-hydroxy-3-butene to 1-chloro-3-buten-2-one, a bifunctional alkylating agent

1,3-Butadiene (BD) is an air pollutant whose toxicity and carcinogenicity have been considered primarily mediated by its reactive metabolites, 3,4-epoxy-1-butene and 1,2,3,4-diepoxybutane, formed in liver and extrahepatic tissues by cytochromes P450s. A possible alternative metabolic pathway in bone marrow and immune cells is the conversion of BD to the chlorinated allylic alcohol 1-chloro-2-hydroxy-3-butene (CHB) by myeloperoxidase in the presence of hydrogen peroxide and chloride ion. In the present study, we investigated the in vitro bioactivation of CHB by alcohol dehydrogenases (ADH) under in vitro physiological conditions (pH 7.4, 37 °C). The results provide clear evidence for CHB being converted to 1-chloro-3-buten-2-one (CBO) by purified horse liver ADH and rat liver cytosol. CBO readily reacted with glutathione (GSH) under assay conditions to form three products: two CBO-mono-GSH conjugates [1-chloro-4-(S-glutathionyl)butan-2-one (3) and 1-(S-glutathionyl)-3-buten-2-one (4)] and one CBO-di-GSH conjugate [1,4-bis(S-glutathionyl)butan-2-one (5)]. CHB bioactivation and the ratios of the three GSH conjugates formed were dependent upon incubation time, GSH and CHB concentrations, and the presence of ADH or rat liver cytosol. The ADH enzymatic reaction followed Michaelis-Menten kinetics with a K(m) at 3.5 mM and a k(cat) at 0.033 s(-1). After CBO was incubated with freshly isolated mouse erythrocytes, globin dimers were detected using SDS-PAGE and silver staining, providing evidence that CBO can act as a protein cross-linking agent. Collectively, the results provide clear evidence for CHB bioactivation by ADH and rat liver cytosol to yield CBO. The bifunctional alkylating ability of CBO suggests that it may play a role in BD toxicity and/or carcinogenicity.

A chronic reference value for 1,3-butadiene based on an updated noncancer toxicity assessment

A chronic noncancer toxicity assessment for 1,3-butadiene (BD) has been conducted by the Texas Commission on Environmental Quality (TCEQ) using information not available to the U.S. Environmental Protection Agency (U.S. EPA) in 2002. The TCEQ developed a chronic reference value (ReV) of 33 microg/m3 (15 ppb). The chronic ReV is based on the same animal study and critical endpoint used by U.S. EPA for ovarian atrophy in B6C3F1 mice, but uses mode of action (MOA) information that indicates the diepoxide metabolite is responsible for ovarian atrophy. In addition, diepoxide-specific hemoglobin adduct data in mice, rats, and humans and other experimental data that became available after 2002 were used to support a conservative data-derived toxicokinetic animal-to-human uncertainty factor (UFA) of 0.3. The default toxicodynamic UFA of 3 was used, together with the data-derived toxicokinetic UFA of 0.3, resulting in a total UFA of 1. The necessary experimental data were not available to calculate a chemical-specific adjustment factor, although supporting data suggest the toxicokinetic UFA may range from 0.01 to 0.2. The chronic ReV value, along with a unit risk factor developed by the TCEQ, will be used to evaluate ambient air monitoring data so that the general public is protected against adverse health effects from chronic exposure to BD.

The formation and biological significance of N7-guanine adducts

DNA alkylation or adduct formation occurs at nucleophilic sites in DNA, mainly the N7-position of guanine. Ever since identification of the first N7-guanine adduct, several hundred studies on DNA adducts have been reported. Major issues addressed include the relationships between N7-guanine adducts and exposure, mutagenesis, and other biological endpoints. It became quickly apparent that N7-guanine adducts are frequently formed, but may have minimal biological relevance, since they are chemically unstable and do not participate in Watson Crick base pairing. However, N7-guanine adducts have been shown to be excellent biomarkers for internal exposure to direct acting and metabolically activated carcinogens. Questions arise, however, regarding the biological significance of N7-guanine adducts that are readily formed, do not persist, and are not likely to be mutagenic. Thus, we set out to review the current literature to evaluate their formation and the mechanistic evidence for the involvement of N7-guanine adducts in mutagenesis or other biological processes. It was concluded that there is insufficient evidence that N7-guanine adducts can be used beyond confirmation of exposure to the target tissue and demonstration of the molecular dose. There is little to no evidence that N7-guanine adducts or their depurination product, apurinic sites, are the cause of mutations in cells and tissues, since increases in AP sites have not been shown unless toxicity is extant. However, more research is needed to define the extent of chemical depurination versus removal by DNA repair proteins. Interestingly, N7-guanine adducts are clearly present as endogenous background adducts and the endogenous background amounts appear to increase with age. Furthermore, the N7-guanine adducts have been shown to convert to ring opened lesions (FAPy), which are much more persistent and have higher mutagenic potency. Studies in humans are limited in sample size and differences between controls and study groups are small. Future investigations should involve human studies with larger numbers of individuals and analysis should include the corresponding ring opened FAPy derivatives.

Effects of smoking cessation on eight urinary tobacco carcinogen and toxicant biomarkers

We determined the persistence at various times (3, 7, 14, 21, 28, 42, and 56 days) of eight tobacco smoke carcinogen and toxicant biomarkers in the urine of 17 smokers who stopped smoking. The biomarkers were 1-hydroxy-2-(N-acetylcysteinyl)-3-butene (1) and 1-(N-acetylcysteinyl)-2-hydroxy-3-butene (2) [collectively called MHBMA for monohydroxybutyl mercapturic acid] and 1,2-dihydroxy-4-(N-acetylcysteinyl)butane (3) [DHBMA for dihydroxybutyl mercapturic acid], metabolites of 1,3-butadiene; 1-(N-acetylcysteinyl)-propan-3-ol (4, HPMA for 3-hydroxypropyl mercapturic acid), a metabolite of acrolein; 2-(N-acetylcysteinyl)butan-4-ol (5, HBMA for 4-hydroxybut-2-yl mercapturic acid), a metabolite of crotonaldehyde; (N-acetylcysteinyl)benzene (6, SPMA for S-phenyl mercapturic acid), a metabolite of benzene; (N-acetylcysteinyl)ethanol (7, HEMA for 2-hydroxyethyl mercapturic acid), a metabolite of ethylene oxide; 1-hydroxypyrene (8) and its glucuronides (1-HOP), metabolites of pyrene; and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (9) and its glucuronides (total NNAL), a biomarker of exposure to 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). These biomarkers represent some of the major carcinogens and toxicants in cigarette smoke: 1,3-butadiene, acrolein, crotonaldehyde, benzene, ethylene oxide, polycyclic aromatic hydrocarbons (PAH), and NNK. With the exception of DHBMA, levels of which did not change after cessation of smoking, all other biomarkers decreased significantly after 3 days of cessation (P < 0.001). The decreases in MHBMA, HPMA, HBMA, SPMA, and HEMA were rapid, nearly reaching their ultimate levels (81-91% reduction) after 3 days. The decrease in total NNAL was gradual, reaching 92% after 42 days, while reduction in 1-HOP was variable among subjects to about 50% of baseline. Since DHBMA did not change upon smoking cessation, there appear to be sources of this metabolite other than 1,3-butadiene. The results of this study demonstrate that the tobacco smoke carcinogen/toxicant biomarkers MHBMA, HPMA, HBMA, SPMA, HEMA, 1-HOP, and NNAL are related to smoking and are good indicators of the impact of smoking on human exposure to 1,3-butadiene, acrolein, crotonaldehyde, benzene, ethylene oxide, PAH, and NNK.

The importance of 3,4-epoxy-1,2-butanediol and hydroxymethylvinyl ketone in 3-butene-1,2-diol associated mutagenicity

1,2:3,4-Diepoxybutane is hypothesized to be the main intermediate involved in mutagenicity following exposure to low levels of 1,3-butadiene (BD) in mice, while metabolites of 3-butene-1,2-diol (BD-diol) are thought to become involved in both rats and mice at higher exposures. BD-diol is biotransformed to hydroxymethylvinyl ketone (HMVK), a potentially mutagenic metabolite, and 3,4-epoxy-1,2-butanediol (EB-diol), a known mutagen. To determine the relative importance of HMVK and EB-diol in BD-diol associated mutagenesis, we have examined the dosimetry of a HMVK derived DNA adduct, as well as EB-diol derived DNA and hemoglobin adducts, in rodents exposed to BD-diol. We previously demonstrated similarities in the shapes of the dose-response curves for EB-diol derived DNA adducts, hemoglobin adducts, and Hprt mutant frequencies in BD-diol exposed rodents, indicating that EB-diol was involved in the mutagenic response associated with BD-diol exposure. To examine the role of HMVK in BD-diol mutagenicity, a method to quantify the alpha-regioisomer of HMVK derived 1,N(2)-propanodeoxyguanosine (alpha-HMVK-dGuo) was developed. The method involved enzymatic hydrolysis of DNA, HPLC purification, and adduct measurement by liquid chromatography - tandem mass spectrometry. Intra- and inter-experimental variabilities were determined to be 2.3-18.2 and 4.1%, respectively. The limit of detection was approximately 5 fmol of analyte standard injected onto the column or 5 fmol/200 microg DNA. The method was used to analyze liver DNA from control female F344 rats and female F344 rats exposed to 36 ppm BD-diol. In addition, liver samples from female Sprague-Dawley rats exposed to 1000 ppm BD were analyzed. alpha-HMVK-dGuo was not detected in any of the samples analyzed. Several possible explanations exist for the negative results including the possibility that alpha-HMVK-dGuo may be a minor adduct or may be efficiently repaired. Alternatively, HMVK itself may be readily detoxified by glutathione (GSH) conjugation. While experiments must be conducted to understand the exact mechanism(s), these results, in addition to published EB-diol derived adduct dosimetry and existing HMVK derived mercapturic acid data, suggest that EB-diol is primarily responsible for BD-diol induced mutagenicity in rodents.

Toxicology of 1,3-butadiene, chloroprene, and isoprene

The diene monomers, 1,3-butadiene, chloroprene, and isoprene, respectively, differ only in substitution of a hydrogen, a chlorine, or a methyl group at the second of the four unsaturated carbon atoms in these linear molecules. Literature reviewed in the preceding sections indicates that these chemicals have important uses in synthesis of polymers, which offer significant benefits within modern society. Additionally, studies document that these monomers can increase the tumor formation rate in various organs of rats and mice during chronic cancer bioassays. The extent of tumor formation versus animal exposure to these monomers varies significantly across species, as well among strains within species. These studies approach, but do not resolve, important questions of human risk from inhalation exposure. Each of these diene monomers can be activated to electrophilic epoxide metabolites through microsomal oxidation reactions in mammals. These epoxide metabolites are genotoxic through reactions with nucleic acids. Some of these reactions cause mutations and subsequent cancers, as noted in animal experiments. Significant differences exist among the compounds, particularly in the extent of formation of highly mutagenic diepoxide metabolites, when animals are exposed. These metabolites are detoxified through hydrolysis by epoxide hydrolase enzymes and through conjugation with glutathione with the aid of glutathione S-transferase. Different strains and species perform these reactions with varying efficacy. Mice produce these electrophilic epoxides more rapidly and appear to have less adequate detoxification mechanisms than rats or humans. The weight of evidence from many studies suggests that the balance of activation versus detoxification offers explanation of differing sensitivities of animals to these carcinogenic actions. Other aspects, including molecular biology of the many processes that lead through specific mutations to cancer, are yet to be understood. Melnick and Sills (2001) compared the carcinogenic potentials of these three dienes, along with that of ethylene oxide, which also acts through an epoxide intermediate. From the number of tissue sites where experimental animal tumors were detected, butadiene offers greatest potential for carcinogenicity of these dienes. Chloroprene and then isoprene appear to follow in this order. Comparisons among these chemicals based on responses to external exposures are complicated by differences among studies and of species and tissue susceptibilities. Physiologically based pharmacokinetic models offer promise to overcome these impediments to interpretation. Mechanistic studies at the molecular level offer promise for understanding the relationships among electrophilic metabolites and vital genetic components. Significant improvements in minimization of industrial worker exposures to carcinogenic chemicals have been accomplished after realization that vinyl chloride caused hepatic angiosarcoma in polymer production workers (Creech and Johnson 1974; Falk et al. 1974). Efforts continue to minimize disease, particularly cancer, from exposures to chemicals such as these dienes. Industry has responded to significant challenges that affect the health of workers through efforts that minimize plant exposures and by sponsorship of research, including animal and epidemiological studies. Governmental agencies provide oversight and have developed facilities that accomplish studies of continuing scientific excellence. These entities grapple with differences in perspective, objectives, and interpretation as synthesis of knowledge develops through mutual work. A major challenge remains, however, in assessment of significance of environmental human exposures to these dienes. Such exposure levels are orders of magnitude less than exposures studied in experimental or epidemiological settings, but exposures may persist much longer and may involve unknown but potentially significant sensitivities in the general population. New paradigms likely will be needed for toxicological evaluation of these human exposures, which are ongoing but as yet are not interpreted.

Urinary biomarkers of 1,3-butadiene in environmental settings using liquid chromatography isotope dilution tandem mass spectrometry

Although, 1,3-butadiene is a known human carcinogen emitted from mobile sources, little is known about traffic-related human exposure to this toxicant. This pilot study was designed to characterize traffic-related environmental exposure to 1,3-butadiene and evaluate its urinary mercapturic acids as biomarkers of exposure in these settings. Personal air samples and multiple urine samples were collected on two separate occasions from three groups of individuals that differed by spatial proximity as well as intensity of traffic: (i) toll collectors, (ii) urban-weekday and (iii) suburban-weekend group. Air samples were analyzed using thermal desorption followed by GC/MS and urine samples were analyzed using isotope dilution liquid chromatography tandem mass spectrometry (ID-LC-MS/MS) for two mercapturic acids of 1,3-butadiene: monohydroxy-3-butenyl mercapturic acid (MHBMA) and 1,2-dihydroxybutyl mercapturic acid (DHBMA). Exposure differed between groups (p<0.05) with median values of 2.38, 1.62 and 0.88 microg/m(3) for toll collectors, the urban-weekday group and the suburban-weekend group, respectively. A refined ID-LC-MS/MS method enabled detection of MHBMA, previously detected only in occupational settings, with high frequency. MHBMA and DHBMA were detected in 95 and 100% of urine samples at levels (mean+/-S.D.) of 9.7+/-9.5, 6.0+/-4.3 and 6.8+/-2.6 ng/mL for MHBMA and 378+/-196, 258+/-133 and 306+/-242 ng/mL for DHBMA for the three different groups, respectively. Mean biomarker levels were higher among the toll collectors compared to the other two groups, however, the differences were not statistically significant (p>0.05). This study is the first to evaluate 1,3-butadiene biomarkers for subtle differences in environmental exposures. However, additional research will be required to ascertain whether the lack of statistical association observed here is real or attributable to unexpectedly small differences in exposure between groups (<1 microg/m(3)), non-specificity of the biomarker at low exposure, and/or small sample size.

Quantification of DNA and hemoglobin adducts of 3,4-epoxy-1,2-butanediol in rodents exposed to 3-butene-1,2-diol

1,3-Butadiene (BD) is a confirmed rodent carcinogen and a suspect human carcinogen that forms mutagenic epoxide metabolites during biotransformation. Species differences in the roles of individual DNA reactive intermediates in BD mutagenicity and carcinogenicity are not completely understood. Evidence suggests that 1,2:3,4-diepoxybutane (DEB) is responsible for the mutagenic effect induced by exposures to low concentrations of BD in mice and that metabolites of 3-butene-1,2-diol (BD-diol) are involved in the mutagenicity at high exposures in both mice and rats. Two reactive metabolites, 3,4-epoxy-1,2-butanediol (EB-diol) and hydroxymethylvinyl ketone (HMVK), are formed during the biotransformation of BD-diol and could potentially be involved in BD-diol associated mutagenicity. To examine the role of EB-diol in BD-diol mutagenicity we have evaluated the dosimetry of N7-(2,3,4-trihydroxybutyl)guanine (THB-Gua) and N-(2,3,4-trihydroxybutyl)valine (THB-Val) in female B6C3F1 mice and female F344 rats exposed by inhalation to 0, 6, 18 and 36 p.p.m. BD-diol for 4 weeks (6 h/day x 5 days/week). Results showed higher levels of both THB-Gua and THB-Val in mice than in rats. An evaluation of THB-Gua adducts showed virtually no differences between liver and lung for either species, suggesting that EB-diol is stable and is freely circulated. The data also indicated that THB adduct formation began to plateau around 18 p.p.m. in both species. Most importantly, the shape of the dose-response curve for THB adduct formation mimicked the one observed for hypoxanthine-guanine phosphoribosyltransferase (Hprt) mutation frequency. This showed that THB adducts, which are not thought to be responsible for causing the mutations, are good quantitative indicators of mutagenicity in rodents exposed to BD-diol. Although the potential contribution of HMVK still needs to be evaluated, the data suggest that EB-diol is responsible, at least in part, for BD-diol associated mutagenicity in rodents.

Dose responses for the formation of hemoglobin adducts and urinary metabolites in rats and mice exposed by inhalation to low concentrations of 1,3-[2,3-(14)C]-butadiene

Blood and urine were obtained from male Sprague-Dawley rats and B6C3F1 mice exposed to either a single 6 h or multiple daily (5 x 6 h) nose-only doses of 1,3-[2,3- (14)C]-butadiene at atmospheric concentrations of 1, 5 or 20 ppM. Globin was isolated from erythrocytes of exposed animals and analyzed for total radioactivity and also for N-(1,2,3-trihydroxybut-4-yl)-valine adducts. The modified Edman degradation procedure coupled with GC-MS was used for the adduct analysis. Linear relationships were observed between the exposures to 1,3-[2,3-(14)C]-butadiene and the total radioactivity measured in globin and the level of trihydroxybutyl valine adducts in globin. A greater level of radioactivity (ca. 1.3-fold) was found in rat globin compared with mouse globin. When analyzed for specific amino acid adducts, higher levels of trihydroxybutyl valine adducts were found in mouse globin compared with rat globin. Average levels of trihydroxybutyl valine adduct measured in globin from rats and mice exposed for 5 x 6 h at 1, 5 and 20 ppM 1,3-[2,3-(14)C]-butadiene were, respectively, for rats: 80, 179, 512 pM/g globin and for mice: 143, 351, 1100 pM/g globin. The profiles of urinary metabolites for rats and mice exposed at the different concentrations of butadiene were obtained by reverse phase HPLC analysis on urine collected 24 h after the start of exposure and were compared with results of a previous similar study carried out for 6 h at 200 ppM butadiene. Whilst there were qualitative and quantitative differences between the profiles for rats and mice, the major metabolites detected in both cases were those representing products of epoxide hydrolase mediated hydrolysis and glutathione (GSH) conjugation of the metabolically formed 1,2-epoxy-3-butene. These were 4-(N-acetyl-l-cysteine-S-yl)-1,2-dihydroxy butane and (R)-2-(N-acetyl-l-cystein-S-yl)-1-hydroxybut-3-ene, 1-(N-acetyl-l-cystein-S-yl)-2-(S)-hydroxybut-3-ene, 1-(N-acetyl-l-cystein-S-yl)-2-(R)-hydroxybut-3-ene, (S)-2-(N-acetyl-l-cystein-S-yl)-1-hydroxybut-3-ene, respectively. The former pathway showed a greater predominance in the rat. The profiles of metabolites were similar at exposure concentration in the range 1-20 ppM. There were however some subtle differences compared with results of exposure to the higher 200 ppM concentrations. Overall the results provide the basis for cross species comparison of low exposures in the range of occupational exposures, with the wealth of data available from high exposure studies.

Determination of the major mercapturic acids of 1,3-butadiene in human and rat urine using liquid chromatography with tandem mass spectrometry

The major urinary metabolites of 1,3-butadiene are monohydroxybutenyl-mercapturic acids (MHBMA) and dihydroxy-butyl-mercapturic acid (DHBMA). These metabolites can be used as biomarkers of exposure to this diene. In order to determine the smoking-related exposure to 1,3-butadiene, we have developed a rapid LC-MS/MS method for the determination of MHBMA and DHBMA in urine of humans and rats. The method requires 2-5 ml of urine which is solid phase extracted prior to LC-MS/MS analysis. Precision for MHBMA is < or =11.2% for human and < or = 17% for rat urine. Corresponding values for DHBMA are < or = 7.2 and < or = 19%, respectively. Recovery rates are approximately 100% for both analytes in human urine and about 115% in rat urine. Limits of detection (LOD) are for humans 0.9 and 23 ng/ml and for rats 1.5 and 33 ng/ml for MHBMA and DHBMA, respectively. Application of the method to urine of humans and rats showed a significant effect of tobacco smoke exposure on the urinary excretion of MHBMA and the metabolic ratio DHBMA/(DHBMA + MHBMA).

1,3-Butadiene: exposure estimation, hazard characterization, and exposure-response analysis

1,3-Butadiene has been assessed as a Priority Substance under the Canadian Environmental Protection Act. The general population in Canada is exposed to 1,3-butadiene primarily through ambient air. Inhaled 1,3-butadiene is carcinogenic in both mice and rats, inducing tumors at multiple sites at all concentrations tested in all identified studies. In addition, 1,3-butadiene is genotoxic in both somatic and germ cells of rodents. It also induces adverse effects in the reproductive organs of female mice at relatively low concentrations. The greater sensitivity in mice than in rats to induction of these effects by 1,3-butadiene is likely related to species differences in metabolism to active epoxide metabolites. Exposure to 1,3-butadiene in the occupational environment has been associated with the induction of leukemia; there is also some limited evidence that 1,3-butadiene is genotoxic in exposed workers. Therefore, in view of the weight of evidence of available epidemiological and toxicological data, 1,3-butadiene is considered highly likely to be carcinogenic, and likely to be genotoxic, in humans. Estimates of the potency of butadiene to induce cancer have been derived on the basis of both epidemiological investigation and bioassays in mice and rats. Potencies to induce ovarian effects have been estimated on the basis of studies in mice. Uncertainties have been delineated, and, while there are clear species differences in metabolism, estimates of potency to induce effects are considered justifiably conservative in view of the likely variability in metabolism across the population related to genetic polymorphism for enzymes for the critical metabolic pathway.

Dose-dependent excretion of unconjugated 3-butene-1,2-diol measured in urine with a gc/ms after 1,3-butadiene exposure

1,3-Butadiene is clearly carcinogenic, and it has a complex pathway of metabolic transformation, as it forms several reactive intermediates. We have previously shown that butadiene diolepoxide is a key metabolite in DNA and hemoglobin adduct formation. Here we report the analysis of 3-buten-1,2-diol-the precursor of butadiene diolepoxide-in urine samples from rats exposed to butadiene by means of inhalation. Urine samples were extracted with isopropanol, and extracts were analyzed using a gas chromatograph and mass spectrometer. Selected ion monitoring was performed by using ion 57 m/z; a retention time allowed reliable analysis. The analysis showed a linear excretion of 3-buten-1,2-diol during the 5 days of 6-h exposures and 18-h recovery times between exposures. The daily correlation coefficient (r) values varied from 0.9945 to 0.9999. A 6-h exposure of rats to 1000 ppm 1,3-butadiene resulted in a mean urinary concentration of 3-buten-1,2-diol of approximately 38 mg/L. Urine samples were also collected during the recovery times, and the extracts were analyzed. The linearity of the excretion during the recovery times showed r values ranging from 0.6932 to 0.9813; 0.5 mg/L of 3-butene-1,2-diol was detected in urine samples excreted after exposure to 1000 ppm 1,3-butadiene. The data demonstrated that butadiene monoepoxide was converted into 3-buten-1,2-diol which, to some extent, was excreted in urine in a nonconjugated form. The excretion of 3-buten-1,2-diol was prompt, with about 98% of the compound being excreted during exposure. We acknowledge Mr. Yrjö Peltonen for his very skillful assistance in maintaining a stable BD atmosphere during animal exposure, and we thank Ms. Marja Pihlaja for the animal care.

Species differences in the metabolism of olefins: implications for risk assessment

Olefinic compounds are commercially valuable because they form useful polymeric substances. The same chemical property (presence of double bonds) that makes the olefins useful may also cause them to be toxic in the body. The double bonds of olefins can be oxidized by cytochrome P450 enzymes to epoxides, which are electrophiles that can react with DNA and may cause alterations in the genetic information carried by that macromolecule. Epoxides can be rendered inactive toward DNA by binding to proteins, by hydrolysis to diols through epoxide hydrolase enzymes (EHs), or by forming conjugates with glutathione via glutathione S-transferase (GST) activities. The balance between the oxidizing enzymatic activities and the hydrolyzing or conjugating enzymatic activities in the livers of different species can influence the potential toxicity of the olefins. The location of the enzymes and the potential for concerted reactions in which epoxides are inactivated immediately after formation will also influence the potential toxicity of the olefins. Cytochrome P450 enzymes and EHs are in microsomes located in the rough endoplasmic reticulum surrounding the nucleus where the DNA is located. GST is in the cytoplasm of the cell. In the case of 1,3-butadiene (BD), such enzymatic differences may strongly influence the toxicity in different species. The mouse, in which BD is a potent multi-site carcinogen, has the lowest microsomal EH activity of any species. This allows the monoepoxides formed in the microsomes by cytochrome P450 enzymes to be further oxidized to the highly genotoxic diepoxide (DEB), and both epoxides can either be released into the blood for distribution throughout the body or can react with DNA in the nucleus. The rat, in which BD is a weak carcinogen, has much higher levels of microsomal EH, and only trace amounts of DEB enter the bloodstream. Major BD metabolites in primates suggest that the hydrolysis pathway is even more prominent in primates than in rats. Data suggest that BD will be much less toxic in primates than in mice. Considering these quantitative differences in metabolism may help to reduce the uncertainties in extrapolating animal data on olefin toxicity to health risk assessments for humans exposed to the compounds.

Urinary metabolites and haemoglobin adducts as biomarkers of exposure to 1,3-butadiene: a basis for 1,3-butadiene cancer risk assessment

Since 1,3-butadiene (BD) is a suspected human carcinogen, exposure to BD should be minimised and controlled. This study aimed at comparing the suitability of biomarkers for low levels of exposure to BD, and at exploration of the relative pathways of human metabolism of BD for comparison with experimental animals. Potentially sensitive biomarkers for BD are its urinary metabolites 1,2-dihydroxybutyl mercapturic acid (DHBMA, also referred to as MI) and 1- and 2-monohydroxy-3-butenyl mercapturic acid (MHBMA, also referred to as MII) and its haemoglobin (Hb) adducts 1- and 2-hydroxy-3-butenyl valine (MHBVal). In two field studies in BD-workers, airborne BD, MHBMA, DHBMA and MHBVal were determined. MHBMA proved more sensitive than DHBMA for monitoring recent exposures to BD and could measure 8-h time weighted average exposures as low as 0.13 ppm (0.29 mg/m(3)). The sensitivity of DHBMA was restricted by relatively high natural background levels in urine, of which the origin is currently unknown. MHBVal proved a sensitive method for monitoring cumulative exposures to BD at or above 0.35 ppm (0.77 mg/m(3)). Statistically significant relationships were found between either MHBMA or DHBMA and 8-h airborne BD levels, and between MHBVal adducts and average airborne BD levels over 60 days. The data showed a much higher rate of hydrolytic metabolism of BD in humans compared to animals, which was reflected in a much higher DHBMA/(MHBMA+DHBMA) ratio, and in much lower levels of MHBVal in humans, confirming in vitro results. Assuming a genotoxic mechanism, the data of this study coupled with our recent data on DNA and Hb binding in rodents, suggest that the cancer risk for humans from exposure to BD will be less than for the rat, and much less than for the mouse.

Health risk assessment of 1,3-butadiene as a Priority Substance in Canada

1,3-Butadiene was included in the second list of Priority Substances to be assessed under the Canadian Environmental Protection Act. Potential hazards to human health were characterized on the basis of critical examination of available data on health effects in experimental animals and occupationally exposed human populations, as well as information on mode of action. Based on consideration of all relevant data identified as of April 1998, butadiene was considered highly likely to be carcinogenic to humans, and likely to be a somatic and germ cell genotoxicant in humans. In addition, butadiene may also be a reproductive toxicant in humans. Estimates of the potency of butadiene to induce these effects have been derived on the basis of quantitation of observed exposure-response relationships for the purposes of characterization of risk to the general population in Canada exposed to butadiene in the ambient environment.

The influence of co-exposure to dimethyldithiocarbamate on butadiene metabolism

Treatment of rats and mice with a single oral dose of dimethyldithiocarbamate (DMDTC; 250 mg/kg) had a marked effect on hepatic CYP2E1 and aldehyde dehydrogenase activities, measured in vitro, for up to 24 h after dosing. The same treatment did not affect CYP2A6, glutathione S-transferase, epoxide hydrolase, alcohol dehydrogenase activities or hepatic glutathione levels. As a consequence of the loss of CYP2E1 activity, butadiene metabolism in liver fractions from DMDTC treated rats and mice was markedly reduced, as was the metabolism of the mono-epoxide to the di-epoxide in mouse liver. The conversion of the mono-epoxide to the diol by epoxide hydrolases was not affected by DMDTC treatment. Urinary excretion of radioactivity, following dosing with DMDTC and exposure to 200 ppm C-14 butadiene for 6 h, was markedly reduced in rats, but increased in mice. The profiles of urinary metabolites were qualitatively similar from mice exposed to butadiene to those exposed after dosing with DMDTC. In the rat, pre-dosing with DMDTC resulted in the formation of three additional urinary metabolites following exposure to butadiene. Overall, DMDTC appears to impact qualitatively and quantitatively on the metabolism of butadiene. The nature and full significance of these changes has yet to be characterised.

Cellular and molecular basis for species, sex and tissue differences in 1,3-butadiene metabolism

Species differences in 1,3-butadiene (BD) bioactivation and detoxication have been implicated in the greater sensitivity of mice to the carcinogenic effects of BD compared to rats, but the molecular basis for species differences in BD metabolism is not well understood. Previous and recent work conducted in this laboratory has examined the relative rates of BD oxidation to epoxybutene (EB) in male and female B6C3F1 mouse tissues, characterized the major cytochrome P450 enzymes involved in BD bioactivation in these tissues, and determined the potential utility of the freshly isolated hepatocyte model to investigate species differences in metabolism of BD and related compounds. Collectively, the results suggest a role for P450s 2E1, 2A5, and 4B1 in sex and tissue differences in BD bioactivation in the mouse. When coordinated metabolism of EB was investigated in male B6C3F1 mouse and Sprague-Dawley rat hepatocytes, the hepatocytes from both species were found to catalyze EB oxidation to meso- and (+/-)-diepoxybutane (DEB), EB hydrolysis to 3-butene-1,2-diol (BDD), and EB conjugation to form GSH conjugates (GSEB). The metabolite area under the curve (AUC) exhibited dependence on the EB concentration used. However, the EB activation/detoxication ratios with the mouse hepatocytes were much higher than the ratios obtained with the rat hepatocytes. These results illustrate the potential utility of the hepatocyte model for estimating flux through competing metabolic pathways and predicting in-vivo metabolism of EB. Collectively, the results may allow a better understanding of the molecular and kinetic basis of species differences in BD metabolism and may lead to a more accurate assessment of human risk.

To what extent are biomonitoring data available in chemical risk assessment?

1. Chemical risk assessment integrates the identification of hazards and the human exposure levels which can be established from external and/or internal exposure data. 2. The availability of biomonitoring and metabolism animal data, the skin penetration ability, and the existence of atmospheric threshold limit values were examined for twelve substances of the European first list of priority existing substances. This investigation was focused on workplace exposures and on urinary biomarkers of exposure. Appropriate biomonitoring data appeared to be available for two substances: styrene and trichloroethylene. Some biomonitoring research has been conducted on acrylonitrile, buta-1,3-diene, cyclohexane, 1,4-dichlorobenzene, hydrogen fluoride, 2-(2-methoxyethoxy)ethanol, however additional studies could be usefully carried out. No biomonitoring data are available for alkanes, C10-13, chloro; benzene, C10-13-alkyl derivatives; bis(pentabromophenyl)ether; diphenylether, octabromo-derivative. 3. It was concluded that in some cases, biomonitoring data are either lacking or scarce. This is rather surprising since the selection of the substances of the priority list was based on high tonnage, widespread use, extent of human exposure, and toxicological concern. The development of biomonitoring information could be helpful in assessing individual or population chemical exposure whatever the source and route, and would result in both more realistic and more accurate risk assessments.

Butadiene: species comparison for metabolism and genetic toxicology

The synthetic monomer 1,3-butadiene and its metabolites have been reviewed in various in vitro and in vivo metabolic studies and in genetic toxicology assays. The species differences have been compared.

Epoxybutene-hemoglobin adducts in rats and mice: dose response for formation and persistence during and following long-term low-level exposure to butadiene

Measurement of specific adducts to hemoglobin can be used to establish the dosimetry of electrophilic compounds and metabolites in experimental animals and in humans. The purpose of this study was to investigate the dose response for adduct formation and persistence in rats and mice during long-term low-level exposure to butadiene by inhalation. Adducts of 3,4-epoxy-1-butene, the primary metabolite of butadiene, with N-terminal valine in hemoglobin were determined in male B6C3F1 mice and male Sprague-Dawley rats following exposure to 0, 2, 10, or 100 ppm of 1,3-butadiene, 6 h/day, 5 days/week for 1, 2, 3, or 4 weeks. Blood samples were collected from groups of five mice and three rats at the end of each week during the 4 weeks of exposure and weekly for 3 weeks following the end of the 4-week exposure period. The increase and decrease, respectively, of the adduct levels during and following the end of the 4-week exposure followed closely the theoretical curve for adduct accumulation and removal for rats and mice, thereby demonstrating that the adducts are chemically stable in vivo and that the elimination follows the turnover of the red blood cells. The adduct level increased linearly with butadiene exposure concentration in the mice, whereas a deviation from linearity was observed in the rats. For example, after exposure to 100 ppm butadiene, the epoxybutene-hemoglobin adduct levels were about four times higher in mice than in rats; at lower concentrations of butadiene, the species difference was less pronounced. Blood concentrations of epoxybutene, estimated from hemoglobin adduct levels, were in general agreement with reported concentrations in mice and rats exposed by inhalation to 62.5 ppm. These studies show that adducts of epoxybutene with N-terminal valine in hemoglobin can be used to predict blood concentration of epoxybutene in experimental animals.

Haemoglobin adducts of epoxybutanediol from exposure to 1,3-butadiene or butadiene epoxides

Epoxybutanediol is one of the reactive metabolites of butadiene. It is formed via hydrolysis followed by oxidation of the primary metabolite of butadiene, epoxybutene, or via hydrolysis of diepoxybutane, a secondary metabolite of butadiene. Groups of male Sprague Dawley rats were treated by intraperitoneal injection of epoxybutene, epoxybutanediol or diepoxybutane. N-(2,3,4-Trihydroxybutyl)valine adducts in haemoglobin, formed from epoxybutanediol in its reaction with N-terminal valine, were measured using the N-alkyl Edman method followed by acetylation of the Edman derivatives and analysis by gas chromatography mass spectrometry. The same adducts were also measured in male Wistar rats exposed to butadiene by inhalation and in a few workers with occupational exposure to butadiene. Haemoglobin binding indexes, HBI, (pmol adduct/g per mumol of alkylating agent, or, for butadiene, per ppm x h), were calculated. The HBI for epoxybutanediol (about 10) is comparable to that of ethylene oxide in the rat demonstrating a similar capacity of the two compounds to alkylate nucleophilic sites in vivo. The HBI of diepoxybutane (about 8) for epoxybutanediol adduct formation is approximately the same as that of epoxybutanediol itself. Epoxybutanediol adduct formation was nonlinearly related to exposure in butadiene exposed rats. The epoxybutanediol-haemoglobin adduct levels were substantially higher than those of epoxybutene in both butadiene-exposed rats and humans suggesting an important role of epoxybutanediol in the toxicity of butadiene. Adducts of epoxybutanediol are probably useful for biomonitoring of human exposure to butadiene.

Metabolism of 1,3-butadiene: species differences

Species differences in the metabolism of 1,3-butadiene (BD) have been studied in an effort to explain the major differences observed in the responses of mice, the sensitive species, and rats, the resistant species, to the toxicity of inhaled BD. BD is metabolized by the same metabolic pathways in all species studied, but there are major species differences in the quantitative aspects of those pathways. Of the species studied, mice are the most efficient at metabolizing BD to the initial metabolite, the monoepoxide (BDO). Mice either convert most of the BDO to the diepoxide (BDO2), the most mutagenic of the BD metabolites, or form conjugates of the BDO with glutathione (GSH). Rats, on the other hand, are less active at forming BDO, oxidize very little of the BDO to BDO2, and form GSH conjugates with either the BDO or its hydrolysis product, butenediol. Primates convert even less of inhaled BD to BDO and hydrolyze most of the BDO to the butenediol. The extent to which primates form BDO2 is unknown. Because of the association of high levels of the highly mutagenic BDO2 with the sensitive rodent strain, it is important to determine the production of this metabolite in primates, particularly humans.

Pharmacokinetic model describing the disposition of butadiene and styrene in mice

Coexposure to 1,3-butadiene (BD) and styrene occurs in the workplace of many polymer industries. The reactive epoxide metabolites of both compounds are responsible for their genotoxicity. A physiologically based pharmacokinetic (PBPK) model was developed to describe the simultaneous disposition of BD and styrene in mice coexposed by inhalation. A model with one oxidative pathway and competition between BD and styrene was compared with a model with two oxidation pathways for both BD and styrene. The different PBPK models were used to simulate the observed rate of BD metabolism and blood concentration of styrene from 8-h inhalation exposures of mice to mixtures of BD and styrene. The model with two oxidative pathways more accurately simulated the observed inhibition of BD uptake in coexposed mice.

Effects of the structure of a toxicokinetic model of butadiene inhalation exposure on computed production of carcinogenic intermediates

A flow-limited physiologically based toxicokinetic model was constructed for uptake, metabolism, and clearance of butadiene (BD) and its principal metabolite 1,2-epoxy-3-butene (EB), using physiological and biochemical parameters from the literature where available. The model includes compartments for blood, liver, lung, fat, GI tract, other rapidly perfused tissues, and slowly perfused tissues. The blood was distributed among compartments for arterial plus venous blood and subcompartments for vascular spaces associated with each of the tissue compartments. The lung contained a subcompartment for the alveolar space. Metabolic activation of BD by cytochrome P450-catalyzed epoxidation was modeled as occurring in liver, lung, and the rapidly perfused tissue compartments. The detoxication of EB catalyzed by epoxide hydrolase and glutathione S-transferase (GST) was modeled as occurring in liver, lung, and the rapidly perfused tissues compartments and by blood GST activity. The model also includes depletion of glutathione (GSH) by GST-catalyzed conjugation of EB and 3-butene-1,2-diol and resynthesis of GSH from cysteine. Values of biochemical parameters that were unavailable in the literature were estimated by iteratively reweighted least squares optimization to reproduce data for uptake of BD and EB by rats and mice in closed chambers. The resulting model also reproduced the depletion of GSH in liver and lung in flow-through systems. It reproduced the concentrations of expired EB produced from BD in closed chambers but overpredicted separately measured blood EB concentrations in flow-through systems, indicating an inconsistency between these two experiments that cannot be resolved by this model or an inadequacy in the model. Equilibration of chamber gases with the alveolar space and alveolar gas with lung capillary blood results in much less dilution of the inhaled gas in the blood compared with the predictions of models in which chamber gas equilibrates directly with the total circulation. The production of EB predicted by the present model was found to be sensitive to a number of physiological and biochemical parameters. A valid and useful toxicokinetic model must have reliable physiological and enzymological data for BD biotransformation before it can be credibly used for human risk assessment.