1,3-butadiene: i. review of metabolism and the implications to human health risk assessment

Contents

Abstract �� 1Abbreviations�� 11� Introduction �� 22� Species differences in BD metabolism �� 2 2�1� In vitro/in situ studies �� 2 2�2� Metabolite levels in tissue �� 4 2�3� Hemoglobin adduct levels�� 4 2�4� Urinary excretion of metabolites �� 4 2�5� Summary of species differences �� 53� Sources of nonlinear toxicokinetics �� 6

(Received 03 February 2010; revised 15 June 2010; accepted 05 July 2010)

ISSN 1040-8444 print/ISSN 1547-6898 online © 2010 Informa Healthcare USA, Inc.DOI: 10.3109/10408444.2010.507181 http://www.informahealthcare.com/txc

R E V I E W A R T I C L E

1,3-Butadiene: I. Review of metabolism and the implications to human health risk assessment

Christopher R� Kirman1,2, Richard J� Albertini3, Lisa M� Sweeney *4,6, and Michael L� Gargas5,6

1Summit Toxicology, Orange, Ohio, USA, 2The Sapphire Group, Orange, Ohio, USA, 3Pathology Department, College of Medicine, University of Vermont, Burlington, Vermont, USA, 4Toxicology Excellence for Risk Assessment, Cincinnati, Ohio, USA, 5Naval Health Research Center, Wright Patterson Air Force Base, Ohio, USA, and 6The Sapphire Group, Dayton, Ohio, USA

Abstract1,3-Butadiene (BD) is a multisite carcinogen in laboratory rodents following lifetime exposure, with mice demon-strating greater sensitivity than rats. In epidemiology studies of men in the styrene-butadiene rubber industry, leukemia mortality is associated with butadiene exposure, and this association is most pronounced for high-intensity BD exposures. Metabolism is an important determinant of BD carcinogenicity. BD is metabolized to sev-eral electrophilic intermediates, including epoxybutene (EB), diepoxybutane (DEB), and epoxybutane diol (EBD), which differ considerably in their genotoxic potency (DEB >> EB > EBD). Important species differences exist with respect to the formation of reactive metabolites and their subsequent detoxification, which underlie observed species differences in sensitivity to the carcinogenic effects of BD. The modes of action for human leukemia and for the observed solid tumors in rodents are both likely related to the genotoxic potencies for one or more of these metabolites. A number of factors related to metabolism can also contribute to nonlinearity in the dose-response relationship, including enzyme induction and inhibition, depletion of tissue glutathione, and saturation of oxidative metabolism. A quantitative risk assessment of BD needs to reflect these species differences and sources of nonlinearity if it is to reflect the current understanding of the disposition of BD.

Keywords: 1,3-Butadiene; 1,2-dihydroxy-3; 4-epoxybutane; 1;2;3; 4-diepoxybutane; 1;2-epoxy-3-butene; metabolism; nonlinearity; species differences

Abbreviations: ADH, alcohol dehydrogenase; AUC, area under the curve; BD, 1,3-butadiene; B-diol, buten-ediol; DEB, diepoxybutane; EB, epoxybutene; EBD, epoxybutane diol; ECOD, 7-ethoxycoumarin O-deethylation; EH, epoxide hydrolase; GSH, glutathione; GST, glutathione S-transferase; HBVal, N-(2-hydroxy-3-butenyl)-valine; HMVK, hydroxymethylvinyl ketone; M1, 1,2-dihydroxy-4-(N-acetylcysteinyl)-butane; M2, 1-(N-acetylcysteinyl)-2-hydroxy-3-butene; P450, cytochrome P450; PBPK, physiologically based pharmacokinetic; PROD; 7-pentoxyre-sorufin O-dealkylation; pyrVal, N,N-(2,3-dihydroxy-1,4-butadiyl)-valine; THBVal, N-(2,3,4-trihydroxybutyl)-valine.

Critical Reviews in Toxicology, 2010; 40(S1): 1–11Critical Reviews in Toxicology

2010

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03 February 2010

15 June 2010

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© 2010 Informa Healthcare USA, Inc�

10�3109/10408444�2010�507181

* Current employer is TERA. Address for Correspondence: Christopher R. Kirman, Summit Toxicology, 29449 Pike Drive, Orange Village, OH 44022, USA. E-mail: [email protected]

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http://informahealthcare.com/doi/abs/10.3109/10408444.2010.507181

mailto:[email protected]

2 C. R. Kirman et al.

1.Introduction

1,3-Butadiene (BD; CAS 106-99-0) is a multisite carcinogen in laboratory rodents following lifetime exposure (NTP, 1984, 1993; Owen et al�, 1987)� Exposure to high concen-trations of BD (high-intensity exposure events exceeding 100 ppm for short periods of time) and other substances in the styrene-butadiene rubber (SBR) manufacturing proc-ess are associated with an increase in leukemia mortality in SBR workers (Cheng et al�, 2007; Delzell et al�, 1996, 2001, 2006; Graff et al�, 2005; Macaluso et al�, 2004)� The carci-nogenicity of BD is generally attributed to genotoxicity of one or more reactive metabolites� As such, metabolism is an important determinant of the carcinogenicity of BD� This article, which reviews the metabolism of BD, is the first of a three-part series on butadiene� The other two articles of the series include a review of BD genotoxicity (Albertini et al�, 2010) and a review of the mode of action for BD (Kirman et al�, 2010)�

The metabolism of BD to reactive intermediates has been well studied (Figure 1) (reviewed in, e�g�, Albertini et al�, 2003; Himmelstein et al�, 1997)� The parent compound is initially oxidized to the 1,2-epoxy-3-butene (EB), a reaction mediated primarily by cytochrome P450 (CYP) isozyme CYP2E1, although other isozymes such as CYP2A6 have also been shown to be involved (Deuscher and Elfarra, 1994)� Further oxidation of EB produces the 1,2:3,4-diepoxybutane (DEB)� Detoxification of EB proceeds by conjugation with glutathione (GSH) (mediated by glutathione S-transferase or GST) or by hydrolysis (mediated by epoxide hydrolase or EH), the latter producing the 1,2-dihydroxy-3-butene (butenediol or B-diol) metabolite� Both DEB and B-diol undergo further conversions in vivo, the former by EH hydrolysis and the latter by CYP2E1 oxidation, to produce the 1,2-dihydroxy-3,4-epoxybutane (epoxybutane diol or EBD) metabolite� B-diol can also be metabolized by alcohol dehydrogenase (ADH) and CYP2E1 to form hydroxymeth-ylvinylketone (HMVK)� The epoxide metabolites of BD can be detoxified via conjugation with glutathione via glutath-ione S-transferase� EB, DEB, and EBD (and perhaps HMVK, which has not been as well studied), as reactive electrophilic compounds, are capable of reacting with DNA, resulting in one or more genotoxicity events likely relevant to the carcinogenic mode of action (MOA) for BD; however, their genotoxic potencies are remarkably different (DEB >> EB > EBD) (reviewed in Albertini et al�, 2010)� The potency of

DEB is likely attributed to its ability to serve as a bifunctional alkylating agent, capable of binding to two cellular macro-molecules (e�g�, DNA-protein cross-links; Loeber et al�, 2006) or to the same molecule twice (e�g�, DNA cross-links; Goggin et al�, 2007, 2009), whereas the other three metabolites are all monofunctional agents� DNA cross-links are relatively poorly repaired when compared to DNA damage produced by monofunctional agents (Kligerman and Hu, 2007; Vock et al�, 1999)�

In human health risk assessment, two assumptions are typically made: (1) humans are as sensitive as, if not more sensitive than, the most sensitive test species; and (2) for substances acting by genotoxic modes of action, the can-cer dose-response relationship at low doses (i�e�, below the point of departure for tumor formation) is linear� To assess the validity of these two assumptions for BD risk assessment, two important aspects of metabolism need to be consid-ered: (1) species differences, which can impact interspecies extrapolation; and (2) sources of nonlinear kinetics, which can impact low-dose extrapolation� Each of these aspects of BD metabolism is discussed below�

2.Species differences in BD metabolism

2.1.In vitro/in situ studiesAlthough the overall metabolic scheme is qualitatively the same, there are considerable quantitative differences between species as to the rates of reaction through a given pathway� In vitro studies using tissue microsomal fractions have shown mice to be more efficient than rats in oxidiz-ing BD to EB (Csanady et al�, 1992; Schmidt and Loeser, 1985)� Crystallographic modeling studies have indicated that this is due to structural conformational differences in CYP2E1 (Lewis et al�, 1997)� The second oxidation step to DEB, which may be the critical step for genotoxicity and carcinogenicity, has been well characterized for isolated hepatic microsomes from mice, rats, and humans (Krause and Elfarra, 1997; Seaton et al�, 1995)� Both oxidation steps follow apparent Michaelis-Menten kinetics� The V

max/K

M

ratio (for the conversion of EB to DEB) for mice was shown to be 3�3-fold greater than for rats, and from 2�4- to 61-fold greater than for individual humans (Seaton et al�, 1995)� Coupling these results with V

max/K

M values for the dif-

ferent EB detoxification pathways (EH hydrolysis, which predominates in humans; and GST, which predominates in mice) (Bond et al�, 1993; Csanady et al�, 1992; Kreuzer

3�1� Enzyme induction/inhibition �� 6 3�2� Glutathione depletion �� 6 3�3� Saturable metabolism �� 7 3�4� Summary of nonlinear toxicokinetics �� 84� Physiologically based pharmacokinetic (PBPK) models �� 85� Discussion and conclusions �� 9Acknowledgments �� 10Declaration of interest �� 10References �� 10

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1,3-Butadiene: Metabolism 3

et al�, 1991) has shown mice to have a significantly higher ratio of EB activation to detoxification than either rats or humans (Seaton et al�, 1995)�

These in vitro studies in rodents have been extended by studies of first-pass metabolism of BD in once-through perfused livers of mice and rats where marked species dif-ferences were observed in the formation of the different epoxide metabolites (Filser et al�, 2001, 2010)� In the effluent of mouse livers perfused with BD, all three epoxides (EB, DEB, and EBD) and B-diol were observed while in efflu-ents from rat livers perfused with BD only EB and B-diol were found (DEB was not detected, whereas EBD was not quantifiable because of an interfering peak)� At very high BD perfusion concentrations (0�240–0�330 mM), the EB con-centrations in mouse effluent were 8�5-fold greater than EB concentrations in rat effluent (Filser et al�, 2001)� When the livers were perfused with EB, B-diol, EBD, and DEB were formed, with B-diol predominating in both species (Filser et al�, 2010)� DEB formation was greater in mouse than in rat livers (Filser et al�, 2010)� When the livers were perfused with DEB, EBD was the primary metabolite formed in both species� In the B-diol–perfused rat liver, but not mouse liver, a small amount of EBD was formed (approximately 4% of the administered B-diol)�

Both the in vitro studies and the investigations in perfused livers predict that higher EB and DEB levels will be present in the blood and tissues of mice than in blood and tissues of rats and presumably humans based upon relative rates of formation and detoxification�

Complicating the species differences in the formation of reactive metabolites from BD are reports of species differ-ences in the stereochemistry of these metabolites, which in turn can impact their genotoxic and cytotoxic potency (reviewed in Albertini et al�, 2010)� With respect to the for-mation of EB, mouse liver microsomes produce slightly more (S)-EB than (R)-EB, whereas rat liver microsomes initially produce more (S)-EB than (R)-EB, but the ratio of S:R falls below 1�0 after 30 minutes of incubation (Nieusma et al�, 1997)� With respect to hydrolysis of EB, rat liver microsomes displayed a small, but significant, preference for (S)-EB that was not seen in mouse liver microsomes� With respect to DEB formation, in rats the formation was greater when starting with (R)-EB than (S)-EB, whereas the opposite was seen with mouse liver microsomes� The cytotoxicity of the enantiomers in rat hepatocytes differ with (R)-EB > (S)-EB for the monoepoxides; and with (meso)-DEB > (S,S)-DEB > (R,R)-DEB for the diepoxides (Nieusma et al�, 1997)�

BD

H2C

CH2

CH2

CH2

CH2

P-450

P450

P450

P450

GST

GST

GST

GST

EH

EH

EH

Hb Adducts(HBVal)

Hb Adducts(THBVal)

Hb Adducts(pyrVal)

EB*

DEB**

EBD*

GSTconjugates

(M2)

GSTconjugates

(M1)

GSTconjugates

GSTconjugates OH

OH

HO

HO

HO

O

O

O

O

OADH HMVK*

B-diol

Erythritol

Figure 1. Metabolism of 1,3-butadiene. BD = 1,3-butadiene; EB = epoxybutene; DEB = diepoxybutane; B-diol = butenediol; HMVK = hydroxymethylvi-nyl ketone; EBD = epoxybutane diol; * = monofunctional alkylating agent; ** = bifunctional alkylating agent; P450 = cytochrome P450; GST = glutathione S-transferase; EH = epoxide hydrolase; ADH = alcohol dehydrogenase; HBVal = N-(2-hydroxy-3-butenyl)-valine; M1 = 1,2-dihydroxy-4-(N-acetylcysteinyl)-butane; M2 = 1-(N-acetylcysteinyl)-2-hydroxy-3-butene; pyrVal = N,N-(2,3-dihydroxy-1,4-butadiyl)-valine; THBVal = N-(2,3,4-trihydroxybutyl)-valine. Boxes indicate biomarkers of exposure that have been measured in exposed workers (Albertini et al., 2003).

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2.2.Metabolite levels in tissueThe prediction from in vitro/in situ studies that mice will have significantly higher blood and tissue concentrations of BD epoxides than rats (and subhuman primates) has been confirmed from in vivo studies� The magnitude of the species difference is both dose and time dependent� At low concentrations (below metabolic saturation and GSH depletion), EB levels in mice tissues are approximately 2- to 3-fold higher than corresponding levels in similarly exposed rats� At high concentrations (above metabolic sat-uration and GSH depletion), EB levels in blood and tissues have been observed up to 15-fold higher in mice than in rats when exposed to similar concentrations (Himmelstein et al�, 1994, 1995; Thornton-Manning et al�, 1995a, 1995b)� For BD exposures ranging from 1 to 1250 ppm, EB levels in blood were reported to be approximately 2- to 8-fold higher in mice than in rats, with the lower end of the range cor-responding to the lowest test concentration (Filser et al�, 2007)� Across this broad range of BD exposures, EBD blood levels were fairly similar in both species, whereas maximal levels of B-diol (occurring at slightly different BD exposures in both species) were approximately 4-fold higher in mice than in rats (Filser et al�, 2007)� DEB differences appear to be even greater in these two species, reportedly over 100-fold greater in mice (Filser et al�, 2007; Thornton-Manning et al�, 1995a, 1995b)� However, such a large difference is not supported by the hemoglobin adduct data (see below)� For comparison, an earlier study of total concentrations of radiolabeled BD metabolites (undifferentiated) following inhalation exposures revealed them to be 5- to 50-fold higher in the blood of mice and 4- to 14-fold higher lower than in blood of rats when compared to levels in subhuman primates (Dahl et al�, 1991)�

2.3.Hemoglobin adduct levelsQuantitative differences in the in vivo production of BD metabolites are also reflected in their in vivo accumulations as hemoglobin adducts (Tables 1 and 2)� N-(2-hydroxy-3-butenyl)-valine (HBVal) adducts formed by EB are greater in concentration in mice than in rats exposed to BD by inhalation (Osterman-Golkar et al�, 1998)� The species dif-ference exhibited some concentration dependence, yielding approximately 4-fold greater concentrations in mice than rats when exposed to 100 ppm BD for 4 weeks, but drop-ping to an approximate 2-fold difference at concentrations of 10 ppm� The N-(2,3,4-trihydroxybutyl)-valine (THBVal) hemoglobin adducts, although potentially formed by either EBD or DEB, have been shown to be produced almost entirely by the EBD (Boysen et al�, 2004; Koivisto et al�, 1999; Perez et al�, 1997)� As seen for the HBVal adducts, the THBVal adduct concentrations in mice are greater than they are in rats� Furthermore, THBVal adduct concentrations are also greater than HBVal adduct concentrations in both species, with the concentration disparity between the two adducts being greater in mice than rats� These large THBVal adduct concentrations indicate that EBD is the most abun-dant electrophilic metabolite of in vivo BD metabolism in

both mice and rats, with more present in mice than rats (Koc et al�, 1999; Koivisto et al�, 1999; Perez et al�, 1997; Swenberg et al�, 2000)�

More recent studies have measured HBVal and THBVal adduct concentrations at even lower external exposure con-centrations and, importantly have also measured the critical DEB specific hemoglobin adduct N,N-(2,3-dihydroxy-1,4-butadiyl)-valine (pyrVal), providing new insights into spe-cies and exposure differences in BD metabolism (Boysen et al�, 2004)� Female mice formed similar amounts of HBVal and pyrVal adducts, both of which increased with BD expo-sure levels between 3�0 and 62�5 ppm (Table 1)� Female rats formed much lower concentrations of either at these expo-sure levels (Table 2)� Both species, however, formed greater quantities of the THBVal adducts than either HBVal or pyr-Val adducts� The formation of pyrVal adducts has been stud-ied in male and female mice and rats exposed to 1�0 ppm by inhalation for 6 hours/day for 4 weeks (Swenberg et al�, 2007)� At this low exposure concentration, although clearly detectible, the adduct concentrations for male and female rats were only 0�9 ± 0�1 and 0�7 ± 0�1 pmol/g, respectively—more than 30-fold lower than the corresponding values in mice� However, data from Boysen et al� (2004) suggest that at high concentrations, and shorter exposure dura-tions (10 days), the difference between species becomes smaller (approximately 12-fold at 3 ppm, and only 3-fold at 62�5 ppm)� These observations suggest that BD is prima-rily metabolized via the B-diol pathway in both mice and rats, but that mice are much more efficient in producing EB and DEB, especially DEB, at low BD exposure levels� The formation of hemoglobin adduct pyrVal was com-pared in mice and rats exposed to 1 ppm BD 6 hours/ day for 4 weeks, and in humans occupationally exposed to 0�18–0�37 ppm BD for 4 months (Swenberg et al�, 2007)� pyrVal levels showed strong species differences, with approximately 30 times higher levels found in mice than in rats at low BD exposures (1 ppm)� pyrVal adducts were not quantifiable in exposed humans� Using one half the detection limit (0�3 pmol/g) and normalizing the data for differences in cumulative exposures, human levels were estimated to be approximately 4- to 9-fold lower than found in rats and approximately 120- to 320-fold lower than found in mice; recent unpublished studies from the same labora-tory with a lower detection limit indicate that the pyrVal levels in humans were approximately 3-fold lower than detected in rats (Swenberg, 2008, personal communica-tion)� These findings are in accord with and extend the direct metabolic studies in rodents�

2.4.Urinary excretion of metabolitesDetoxification of BD metabolites produces different urinary excretion products depending on the pathway followed� Direct GST-mediated conjugation of EB with GSH leads to 1-hydroxy-2-(N-acetylcysteinyl)-3-butene as a racemic mixture with 1-(N-acetylcysteinyl)-2-hydroxy-3-butene (also known as the urinary M2 compound), which is a biomarker for this pathway� The GST-mediated conjugation of HMVK

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with GSH leads to the production of 1,2-dihydroxy-4-(N-acetylcysteinyl)-butane (also known as the urinary M1 com-pound)� M1 is a biomarker of the hydrolytic pathway because EH-mediated hydrolysis of EB is the initial step, followed by reaction with alcohol dehydrogenase and cytochrome P450 to form HMVK� The ratio M1/(M1 + M2) in urine provides an estimate of the relative importance of hydrolysis versus conjugation in detoxification of EB (Bechtold et al�, 1994; reviewed in Henderson et al�, 1996)� M1/(M1 + M2) ratios in urine for mice and rats exposed to BD by inhalation indi-cate that conjugation detoxification predominates in mice but that hydrolysis is more important in rats and humans� The relative detoxification metabolic pathways in the three species as assessed by the ratio of M1/(M1 + M2) in urine are 24% hydrolysis in mice, 51% hydrolysis in rats, and 99% hydrolysis in humans (Albertini, 2004)�

2.5.Summary of species differencesData collected from in vitro studies, measurements of metabolites in tissues, measurements of hemoglobin adduct biomarkers, and measurements of metabolites in urine describe a consistent pattern of species differences between mice, rats, and humans� Mice are more efficient in the production of epoxide metabolites of BD (especially DEB), whereas rats and humans are more efficient in hydro-lytic detoxification of these metabolites� Blood and tissue concentrations and accumulations of all three electrophilic metabolites are greater in mice than in rats—in some cases (depending on test concentration and metabolite) much greater� Of importance are the higher concentrations of the reactive metabolites EB, EBD, and DEB, especially DEB� BD total metabolite concentrations determined by exposures to radiolabeled BD are even lower in subhuman primates than

Table 2. Recent studies of hemoglobin adducts of EB, DEB, and EBD in BD-exposed rats.

ReferenceStrain (Sex) Exposurea

Concentration (ppm)

Averaging time for adduct

production (days)

Adduct level (pmol/g globin)

Adduct production efficiency (pmol/g globin per ppm-hr)

HBVal THBVal pyrVal HBVal THBVal pyrVal

Swenberg et al., 2007

F344 (M) 4 weeks 1 16.2b ND ND 0.9 ± 0.1 ND ND 0.0093 ± 0.0010


F344 (F) 4 weeks 1 16.2b ND ND 0.7 ± 0.1 ND ND 0.0072 ± 0.0010

Boysen et al., 2004

F344 (F) 2 weeks 3 9.17b 13 ± 2.4 339 ± 41 3.9 ± 0.8 0.079 ± 0.014 2.05 ± 0.248 0.024 ± 0.0048

Boysen et al., 2004

F344 (F) 2 weeks 62.5 9.17b 87 ± 7.6 3202 ± 302 38.3 ± 1.2 0.025 ± 0.0022 0.93 ± 0.088 0.011 ± 0.00035

Boysen et al., 2004

Crl:CD (F) 90 days 1000 21.8c 8690 ± 930 24066 ± 9292 58.1 ± 17.3 0.067 ± 0.0071 0.18 ± 0.071 0.00044 ± 0.00013

Boysen et al., 2004

Crl:CD (M) 90 days 1000 21.8c 5480 ± 2880 12095 ± 3712 16.7 ± 6.6 0.042 ± 0.022 0.093 ± 0.028 0.00013 ± 0.000051

aAnimals exposed for 6 hours/day, 5 days/week.bAveraging time is the number of exposure days, adjusted for turnover of red blood cells between exposure and blood sample collection. For F344 rats, the average red blood cell life span was 66 days (Derelanko, 1987). For example, of the adducts formed on the first day of exposure (day 1), only 41/66 will remain on day 26; from day 2, 42/66 will remain on day 26, etc.cThe exposure duration exceeded the red blood cell life span. Therefore the averaging time is average age of a red blood cell for Sprague-Dawley rats (life span of 61 days, divided by 2 = average age of 30.5 days; Derelanko, 1987) adjusted for 5 days/week exposure.

Table 1. Recent studies of hemoglobin adducts of EB, DEB, and EBD in BD-exposed mice.

Reference Strain (Sex) Exposurea

Concentration (ppm)

Averaging time for adduct

production (days)

Adduct level (pmol/g globin)

Adduct production efficiency (pmol/g globin per ppm-hour)

HBVal THBVal pyrVal HBVal THBVal pyrVal


B6C3F1 (M) 4 weeks 1 11.1b ND ND 30.8 ± 4.6 ND ND 0.46 ± 0.069


B6C3F1 (F) 4 weeks 1 11.1b ND ND 23.5 ± 3.1 ND ND 0.35 ± 0.047

Boysen et al., 2004

B6C3F1 (F) 2 weeks 3 8.04b 53 ± 7.6 452 ± 38 48.7 ± 3.23 0.37 ± 0.053 3.1 ± 0.26 0.34 ± 0.022

Boysen et al., 2004

B6C3F1 (F) 2 weeks 62.5 8.04b 137 ±12 3410 ± 177 130.4 ± 64 0.045 ± 0.0040 1.1 ± 0.059 0.043 ± 0.021

Boysen et al., 2004

B6C3F1 (F) 90 days 1250 8.04b 7143 ± 537 13755 ± 1651 2487 ± 426 0.12 ± 0.0089 0.23 ± 0.027 0.041 ± 0.0071

aAnimals exposed for 6 hours/day, 5 days/week.bAveraging time is the number of exposure days, adjusted for turnover of red blood cells between exposure and blood sample collection with an average red blood cell life span of 28 days for mice.

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in either mice or rats, whereas the hydrolytic detoxification of these metabolites in primates is greater� Hemoglobin adduct data indicate that the levels of DEB in humans are lower than levels observed in rats, and much lower than levels observed in mice�

3.Sources of nonlinear toxicokinetics

Potential sources of nonlinear toxicokinetics for BD include enzyme induction and enzyme inhibition, GSH depletion, and saturable metabolism� Each of these sources is discussed below�

3.1.Enzyme induction/inhibitionBond et al� (1988) exposed male Sprague-Dawley rats to 7600 ppm BD and male B6C3F1 mice to 740 ppm BD for 6 hours/day for 5 days� Microsomes were prepared from the livers, lungs, and nasal tissue (rats only) at the end of the final exposure� The rate of BD metabolism, as determined by the disappearance of BD from the reaction flasks, was determined in microsomes from control and exposed ani-mals� The authors concluded that BD metabolism in liver (both species) and nasal tissue microsomes (rats only) was unaffected by the 5-day exposure to BD� However, BD metabolism in lung microsomes was decreased by 50% in both species�

Elovaara et al� (1994) exposed male Wistar rats to 500 ppm BD 6 hours/day for 5 days and prepared microsomes from the liver and lungs of the exposed animals for in vitro metabo-lism studies� No change in styrene oxidation (an indicator of CYP2E1 metabolism), 7-ethoxycoumarin O-deethylation (ECOD; an indicator of CYP1A1/1A2/2B metabolism), or 7-pentoxyresorufin O-dealkylation (PROD; an indicator of CYP2B1/2B2 metabolism) in lung microsomes was found� In liver, a 50% increase in styrene 7,8-oxidation (at 0�087 mM) was observed, as were smaller increases in ECOD, epox-ide hydrolase (EH; with styrene oxide as substrate), and N-nitrosodimethylamine N-demethylation, and increased expression of CYP2E1 was indicated in Western blots� The dif-ferences between Bond et al� (1988) and Elovaara et al� (1994) may reflect strain differences or differences in methodology—for example, Bond et al� (1988) measured BD metabolism whereas Elovaara et al� (1994) assessed styrene metabolism�

The reactive metabolites of BD are capable of binding cel-lular macromolecules, including the enzymes responsible for BD metabolism� EB has been shown to covalently bind to histidine and tyrosine residues of CYP2E1, which may reduce its activity (Boysen et al�, 2007); however, such an inhibition has not been observed from in vivo studies�

Perhaps more important than changes in enzyme expres-sion/activity are the resulting changes in blood and tissue concentrations of parent compound or metabolites when ani-mals are exposed repeatedly to BD� Thornton-Manning et al� (1997) exposed female Sprague-Dawley rats and B6C3F1 mice to 62�5 ppm BD for 6 hours or 6 hours/day, 5 days/week for 2 weeks� No statistically significant changes in blood concen-trations of EB and DEB at the end of exposure were produced

by repeated exposure� However, some statistically significant changes in tissue concentrations were noted, although their biological significance remains unclear� The tissue DEB con-centration differences between high and low test exposures most likely reflect interexperimental variability, suggesting that repeated exposures do not have a significant impact on the blood and tissue levels of DEB in rats at toxicologically relevant concentrations�

3.2.Glutathione depletionSeveral studies have examined GSH depletion in male Sprague-Dawley rats and B6C3F1 mice� Kreiling et al� (1988) exposed animals under conditions of saturated BD metabo-lism to concentrations (2000–4000 ppm BD) for up to 15 hours and measured hepatic GSH� Deutschman and Laib (1989) likewise exposed animals for 7 hours, but considered a range of concentrations (10–2000 ppm) and extrahepatic tissues (lung and heart)� Himmelstein et al� (1995) exposed mice and rats for 3 or 6 hours to 62�5, 625, 1250, or 8000 ppm BD (rats only) and measured pulmonary and hepatic GSH� Consistent with noted species differences in metabolism (mice > rats), thresholds for significant pulmonary GSH depletion were lower in mice than rats, and pulmonary GSH depletion in mice was much more extensive and is consistent with conju-gation being the major pathway in this species� These species differences are described in more detail below�

MiceKreiling et al� (1988) exposed mice under conditions of satu-rated BD metabolism to concentrations of 2000–4000 ppm BD for up to 15 hours and measured hepatic GSH only� The extent of depletion at 7 hours was much greater than in rats (reduced to 20% of control), and GSH levels further declined to 4% of control upon continued exposure up to 15 hours� Deutschman and Laib (1989) found that after 7 hours of exposure, hepatic GSH depletion progressed steadily over the range from 100 to 2000 ppm BD� The threshold for lung GSH depletion was approximately the same as for liver� Cardiac GSH decreased slowly as exposure increased from 10 to 1000 ppm, then sharply decreased between 1000 and 2000 ppm from 75% to 30% of control� The pulmonary and hepatic results of Himmelstein et al� (1995) were similar those of Deutschman and Laib (1989)�

RatsKreiling et al� (1988) exposed rats under conditions of satu-rated BD metabolism concentrations (2000–4000 ppm BD) for up to 15 hours and measured hepatic GSH� At 7 hours, GSH was moderately reduced in rats (80% of control remaining), and additional exposure (to 15 hours) produced no further decline� Deutschman and Laib (1989) evaluated a range of concentrations (10–2000 ppm) and extrahepatic tissues (lung and heart)� In the rat, cardiac GSH depletion was minimal; pulmonary GSH depletion was moderate, reaching ∼25% depletion at 2000 ppm� The hepatic GSH depletion in the rat observed by Deutschman and Laib at 2000 ppm (40% of control remaining) was greater than that observed by Kreiling

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et al� (1988)� A distinct drop in hepatic GSH levels was noted as dose increased from 1000 to 2000 ppm� Exposure of rats to 500 ppm BD (6 hours/day, 5 days) had no effect on GSH levels in blood, liver, or lung; however, when co-treated with acetone in drinking water, this resulted in a significant decrease (∼13% lower) in hepatic GSH (Elovaara et al�, 1994)� The results of Himmelstein et al� (1995) gave transition points and extent of GSH depletion similar to that of Deutschman and Laib (1989), and confirmed that elevated BD exposure (8000 ppm) did not further exacerbate the extent of GSH depletion in rats�

3.3.Saturable metabolismBecause the oxidation of BD and its metabolites obey appar-ent Michaelis-Menten kinetics, their metabolism can become saturated at high concentrations� Nonlinear kinetics due to the saturable metabolism of BD differs between species and between BD metabolites, as discussed below�

MiceThe blood DEB concentrations in mice appear to increase linearly up to 625 ppm� Himmelstein et al� (1995) reported blood DEB concentrations of 1�9 μM at 625 ppm and 2�5 μM at 1250 ppm� It appears that DEB production in mice approaches saturation in the range of 625–1250 ppm� However, more recent data using hemoglobin adducts to reflect metabolite production (Table 1) indicate that there is no saturation of DEB even up to 1250 ppm BD exposures lev-els (Boysen et al�, 2004; Swenberg et al�, 2007)� At BD expo-sure levels of 3�0 ppm for 4 weeks in female B6C3F1 mice, EB levels as determined by HBVal adducts was 53 pmol/g globin� At exposure concentrations of 62�5 and 1250 ppm these adduct concentrations rose to 137 and 7143 pmol/g respectively� Following exposure to 3, 62�5, and 1250 ppm BD in mice, THBVal adduct concentrations, reflecting EBD levels, rose from 452 to 3410 to 13,755 pmol/g, indicating a decline in production efficiency with increasing BD concentration� pyrVal adduct concentrations, reflecting DEB levels, were 23�5 and 30�8 pmol/g in female and male B6C3F1 mice, respectively, when exposed to 1�0 ppm BD for 4 weeks� In female mice, these concentrations then rose from 48�7 to 130�4 to 2487 pmol/g with efficiencies of 16�2, 2�1, and 2�0 adducts per ppm at exposure levels of 3�0, 62�5 and 1250 ppm, respectively� Levels of all three metabolites continued to rise in a supralinear manner with increasing external BD exposure concentrations� Production of all three metabolites was most efficient at low exposure levels� Mice are more efficient in producing all three metabolites than are rats, with the difference being most pronounced for adducts resulting from oxidative metabolism (i�e�, EB and DEB production)� The differences between mice and rats are most pronounced for DEB with no decline in the production efficiency in mice up to high exposure levels (1250 ppm)�

Although in vivo kinetic studies of BD and its metabo-lites are the best indicator of nonlinearities, insights can also be drawn from the many in vitro studies (summarized

in Himmelstein et al�, 1997)� Nonlinearities in kinetics are most likely to result from saturation of epoxidation steps (Figure 1)� Identified K

M values of the GSH conjugation

reactions of EB and DEB were in the range of 2 to 37 mM and the K

M values of the hydrolysis reactions were in the

range of 0�2 to 12 mM (summarized by Himmelstein et al�, 1997; Krause et al�, 1997), and therefore saturation is not expected to occur with in vivo exposures� The epoxidation reactions, however, may have much lower K

M values� For

example, Csanady et al� (1992) identified KM

values in the range of 0�002–0�008 mM for the epoxidation of BD and Dahl and Henderson (2000) identified a K

M of 0�0007 mM in

human microsomes for this reaction� Csanady et al� (1992) and Seaton et al� (1995) determined K

M values of 0�016 and

0�14–0�9 mM, respectively, for EB epoxidation in liver micro-somes, whereas K

M values for EB hydrolysis were reported

to range from 0�5 to 1�5 mM in liver microsomes (Kreuzer et al�, 1991)� In contrast, Duescher and Elfarra (1994) found larger K

M values for BD epoxidation (0�12–0�16 mM) and EB

epoxidation (1–35 mM)� Similarly, in rat liver S9 fraction, K

M values for the oxidation reactions ranged from 0�61 to

1�6 mM, whereas KM

values for hydrolysis reactions ranged from 0�14 to 0�77 mM (Motwani et al�, 2009)�

Another metabolic step with the potential for nonlinearity is the oxidation of B-diol to HMVK� This reaction appeared to be mediated by both high affinity (K

M = 0�05–0�08 mM)

and lower affinity (KM

= 0�6–1�8 mM) enzymes (Krause et al�, 2001)�

RatsFemale Sprague-Dawley rats were exposed to 62�5 or 8000 ppm BD under single (6-hour) and multiple expo-sure (6 hours/day, 5 days/week for 2 weeks) regimens (Thornton-Manning et al�, 1997, 1998)� For the repeat-exposure study, blood DEB concentrations at 8000 ppm were essentially identical to those measured at 62�5 ppm� In contrast, blood DEB concentrations were lower at 8000 ppm than at 62�5 ppm in the single-exposure study� As noted above, when the tissue DEB concentrations for repeated high and repeated low exposures were compared, some differences were reported� These differences likely reflect interexperimental variability and indicate that DEB con-centrations in rat blood and tissues plateau at relatively low BD exposure concentrations� EB concentrations in blood and tissues of rats are clearly higher following repeated exposures to 8000 ppm BD than to 62�5 ppm BD, so the limiting step must be the metabolism of EB to DEB rather than BD to EB�

The conclusions from tissue metabolite studies are in accord with recent studies where BD exposures by inhala-tion have been taken to much lower levels with metabolite accumulations being measured as hemoglobin adduct concentrations (Boysen et al�, 2004; Swenberg et al�, 2007) (Table 2)� EB levels in female F344 rats as detected by HBVal concentrations rose from 13 to 87 pmol/g globin at 3�0 and 62�5 ppm for 10 days, respectively� However, when female and male Crl:CD rats were exposed to 1000 ppm

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for 90 days, the HBVal production efficiencies (expressed as pmol adduct/g globin per ppm-hour of exposure) were intermediate between the efficiencies observed at 3�0 and 62�5 ppm� It is unclear if this reflects a strain difference in EB formation or metabolism� In rats exposed to 2–100 ppm BD for 1 or 4 weeks (6 hours/day, 5 days/week), a decrease in adduct production efficiency was observed at 10 and 100 ppm, compared to that observed at lower concentra-tions (2 ppm) (Osterman-Golkar et al�, 1998)� EBD pro-duction in these studies was measured by THBVal adduct concentrations� These were 339 and 3202 pmol/g globin at the 3�0 and 62�5 ppm exposure levels in female F344 rats for 10 days, respectively� At 1000 ppm exposure for 90 days in the female and male Crl:CD rats, the THBVal adduct con-centrations were 24,066 and 12,095 pmol/g for the females and males, respectively, indicative of lower THBVal pro-duction efficiency with increasing BD concentration� Of importance, DEB production as detected by pyrVal adduct concentrations have now been directly measured at these low exposure concentrations, including measurements at 1�0 ppm (Swenberg et al�, 2007)� These concentrations in female and male F334 rats were 0�7 and 0�9 pmol/g in females and males, respectively, when exposed to 1�0 ppm by inhalation for 4 weeks� Three and 62�5 ppm exposures in female F344 rats for 10 days produced pyrVal adduct concentrations of 3�9 and 38�3 pmol/g at these two expo-sure levels, respectively� Of note, the female and male Crl:CD rats exposed to BD levels of 1000 ppm for 90 days produced pyrVal adduct concentrations of 58�1 and 16�7 for the females and males, respectively� Therefore, EB production appears to rise with exposure level as does EBD production, although the latter rise is supralinear� DEB production shows almost a plateau effect with great reduction in efficiency per ppm BD under high, prolonged exposure conditions� The rat production of DEB appears saturated at the 62�5 ppm exposure level� This is likely related to a competitive inhibition of the EBD-producing cyrochrome P450 by BD at high concentrations, which was more extensive in rats than mice (Filser et al�, 2007)�

3.4.Summary of nonlinear toxicokineticsAlthough some evidence is present from in vitro studies of enzyme inhibition possibly due to metabolite binding, in vivo studies with measurements of circulating metabolites sug-gest any effects are minimal� GSH depletion occurs following exposures to fairly high concentrations of BD (>1000 ppm)� EBD concentrations in blood do not increase linearly with BD exposure, with concentrations in blood reaching peak levels at 150 ppm BD in rats and 300 ppm BD in mice, with lower concentrations achieved in both species at higher exposures to BD� In rats, concentrations of DEB in tissues appear to plateau at moderate exposure levels (62�5 ppm), whereas in mice DEB levels reach a plateau at much higher exposure levels (625–1250 ppm)� No information is avail-able regarding nonlinear kinetics of BD in humans; but with respect to BD metabolism, humans are more similar to rats than to mice�

4.Physiologically based pharmacokinetic (PBPK) models

The species differences in BD metabolism and sources of nonlinear kinetics are best addressed within the context of a PBPK model� Multiple PBPK models have been developed for BD, as summarized below�

• Johanson and Filser (1996)—The authors developed a PBPK model for BD and EB in mice, rats, and humans� The model includes compartment for lung, liver, fat, and vessel-rich group, and incorporated three important features: (1) reduced alveolar ventilation; (2) intrahe-patic first-pass hydrolysis of EB; and (3) a two-substrate Michealis-Menten kinetic description of EB conjugation with GSH� The model was validated against a number of published experimental observations� Model predic-tions for the relative internal doses of EB (area under the curve [AUC] in mixed venous blood) are mouse 1�6, rat 1�0, human 0�3� The authors concluded that the relatively small difference between mice and rats in internal EB doses can only partly explain the marked species differ-ence in cancer response between mice and rats exposed to BD�

• Csanady et al. (1996)—A PBPK model was developed to describe disposition and metabolism of BD and EB in rats, mice, and humans� In addition, DEB disposi-tion and metabolism was also described in mice� The model describes the formation of EB and DEB, intra-hepatocellular first-pass hydrolysis of EB, conjugation of EB with GSH and GSH turnover in the liver� Tissue:air partition coefficients were determined experimentally� Model predictions for hemoglobin adducts of EB in rodents following exposure to BD were compared with published data� The authors reported that species dif-ferences in EB dosimetry were not sufficiently large to explain the observed species differences in carcino-genic potency for BD, suggesting that additional factors are involved�

• Kohn and Melnick (2001)—Earlier PBPK models of BD disposition were able to reproduced uptake of the gas from closed chambers, but overpredicted steady-state circulating concentrations of EB and DEB� To correct this overprediction, the model was revised to include a transient complex between cytochrome P450 and micro-somal EH� This proximity effect corresponds to the notion that epoxides produced in situ have privileged access to EH� The model was further enhanced by the addition of equations for the production and detoxication of EBD in the liver, lungs, and kidneys� The model for mice and rats includes compartments for lungs, liver, fat, kidneys, gastrointestinal tract, other rapidly perfused tissues, and other slowly perfused tissues� Parameters were estimated to fit data for chamber uptake of BD and EB, steady-state blood concentrations of EB and DEB, and the fractions of

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the inhaled BD that appears as various excreted metabo-lites� The model predicts accumulation of EBD, which is consistent with observations that most of the DNA adducts in animals exposed to butadiene arise from EBD�

• Sweeney et al. (1996, 1997, 2001)—In vitro and in vivo BD metabolism data from laboratory animals were integrated into a PBPK model in rats and mice with flow- and diffusion-limited compartments (Sweeney et al�, 1996, 1997)� The model describes experimental data from closed chamber inhalation and nose-only flow-through inhalation exposures� An isolated tissue model based on rate parameters determined in vitro predicted the decrease in epoxide concentrations in animals during the time lag between exsanguina-tion and tissue removal for tissues capable of epoxide biotransformation, providing a better indication of in vivo dosimetry� Improved simulation of blood epoxide concentrations was achieved by addition of first-order metabolism in the slowly perfused tissues� Blood con-centrations of BD were accurately predicted for mice and rats exposed by inhalation to constant concentra-tions of BD� However, blood concentrations of EB were initially overpredicted� By assuming that only a fraction of BD metabolism produces EB, due to a competing metabolic pathway supported by in vitro and in vivo studies, blood concentrations of EB could be predicted over a range of BD exposure concentrations for both species� A preliminary human model was developed that accurately predicted published data on exhaled breath BD concentrations in a human volunteer exposed to BD by inhalation (Sweeney et al�, 2001)� The fit was relatively insensitive to the rate constant for BD epoxidation� Using a range of published rate constants, human blood DEB was found to be sensitive to rates of epoxidation of EB to DEB and hydrolysis of EB and DEB, but not BD epoxidation�

• Brochot et al. (2007)—A PBPK model for BD was devel-oped to estimate its metabolic rate to EB, using exhaled breath BD concentrations in human volunteers exposed by inhalation� The model was extended to describe the kinetics of its four major metabolites EB, DEB, B-diol, and EBD� Global sensitivity analyses were conducted to evaluate the relative importance of the model parame-ters on model predictions for short-term exposure� Three model parameters influence numerous outputs: (1) the blood:air partition coefficient for BD; (2) the metabolic rate of BD to EB; and (3) the volume of the well-perfused tissues� Other influential parameters include other meta-bolic rates, some partition coefficients, and parameters driving the gas exchanges (in particular, for BD outputs)� The sensitivity analysis showed that (1) the impact of the metabolic rate of BD to EB on the BD concentra-tions in exhaled air is greatly increased if a few of the model’s important parameters (blood:air partition coef-ficient for BD) are measured experimentally; (2) all the

transformation pathways may not be estimable if only data on the studied outputs are collected; and (3) data on a specific output for a chemical may not inform all the transformations involving that chemical�

5.Discussion and conclusions

Because the metabolism is an important determinant of the carcinogenicity of BD (i�e�, via the formation of reactive metabolites), review of the available information from in vivo and in vitro studies reveal two important implications of metabolism to the quantitative dose-response assessment of BD cancer risk, as discussed below�

First, the species differences in the metabolism of BD (Section 2) suggest that use of rodent tumor data expressed in terms of external concentration (i�e�, ignoring species dif-ferences in metabolism) will overestimate the potential risks to human populations� Assuming that humans and rodents are similarly susceptible to the effects of BD metabolites (e�g�, DEB), the degree of overestimation would likely be approximately 2 orders of magnitude if based on mice, and 1 order of magnitude if based on rats� If the rodent data are to be used quantitatively in human health risk assess-ment (note: epidemiology data are available to serve as the preferred basis for risk assessment), then the use of a PBPK model (Section 4) would be helpful in addressing the species differences in BD metabolite tissue dose� A recent analysis by Fred et al� (2008) suggests that internal dose of metabolites (AUC for EB, DEB, EBD) when combined with the relative genotoxic potency of the metabolites is capa-ble of reconciling differences between rats and mice with respect to carcinogenic potency�

Second, the sources of nonlinear toxicokinetics (Section 3) likely contribute to nonlinearity in the dose-response rela-tionship for BD-induced rodent tumors in the range of observation� Because of these sources of nonlinearity, the risk associated with high-intensity exposures (>100 ppm) associated with past occupational exposures to BD may not be predictive, at least not in a linear manner, of the risks associated with low-intensity exposures (<100 ppm) to BD� In mice, nonlinear kinetics can become important at exposures of 600 ppm BD or higher (e�g�, decrease in EBD concentrations)� In rats (and presumably in humans), non-linear kinetics can become important at lower exposures (e�g�, plateau for DEB levels for exposures above 62�5 ppm; inhibition of EBD formation at 300 ppm BD or higher)� This may be an important consideration in using the epidemiol-ogy data for risk assessment, since high-intensity exposures exceeding 100 ppm BD were frequently encountered in the past by styrene-butadiene rubber workers (Delzell et al�, 2001; Macaluso et al�, 2004)� Again, a PBPK model (Section 4) is useful for addressing nonlinear toxicokinetics associated with high-intensity BD exposures� When conducting a can-cer dose-response assessment for BD, care needs to be taken to ensure that (1) sources of nonlinearity above the point of departure are addressed because they can affect the spacing

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between dose groups (and, therefore, dose-response model fits to the data) when assessed in terms of internal dose; and (2) these sources of nonlinear kinetics do not occur below the point of departure, since their presence would invalidate the default assumption of low-dose linearity used in risk assessment�

Acknowledgments

The authors would like to acknowledge the Ms� Leigh Carson for her valuable assistance in preparing the manuscript and the support of Dr� Robert Tardiff of The Sapphire Group, Inc�

Declaration of interest

This work was funded by the Olefins Panel of the American Chemistry Council� The authors’ affiliations are as shown on the first page� The authors have sole responsibility for the writing and content of the paper� Dr� Albertini, Dr� Gargas, Dr� Sweeney, and Mr� Kirman are consultants to the Olefins Panel�

ReferencesAlbertini R. (2004). Mechanistic insights from biomarker studies: Somatic muta-

tions and rodent/human comparisons following exposure to a potential carcinogen. In: Mechanisms of Carcingenesis. IARC Scientific Publications No. 157. Geneva: IARC, 153–177.

Albertini RJ, Sram RJ, Vacek PM, Lynch J, Nicklas JA, van Sittert NJ, Boogaard PJ, Henderson RF, Swenberg JA, Tates AD, Ward JB Jr, Wright M, Ammenheuser MM, Binkova B, Blackwell W, de Zwart F.A, Krako D, Krone J, Megens H, Musilova P, Rajska G, Ranasinghe A, Rosenblatt JI, Rossner P, Rubes J, Sullivan L, Upton P, Zwinderman AH. (2003a). Biomarkers in Czech Workers exposed to 1,3-butadiene: A transitional epidemiologic study. Res Rep Health Effects Inst (116):1–141.

Albertini RA, Carson ML, Kirman CR, Gargas ML. (2010). 1,3-Butadiene: II. Review of mutagenicity of its relevance for cancer potency. Crit Rev Toxicol 40(Suppl 1):this issue.

Bechtold WE, Strunk MR, Chang IY, Ward JB Jr Henderson RF. (1994). Species differences in urinary butadiene metabolites: Comparisons of metabo-lite ratios between mice, rats, and humans. Toxicol Appl Pharmacol 127:44–49.

Bond JA, Martin OS, Birnbaum LS, Dahl AR, Melnick RL, Henderson RF. (1988). Metabolism of 1,3-butadiene by lung and liver microsomes of rats and mice repeatedly exposed by inhalation to 1,3-butadiene. Toxicol Lett 44:143–151.

Bond JA, Csanady GA, Leavens T, Medinsky MA. (1993). Research strategy for assessing target tissue dosimetry of 1,3-butadiene in laboratory animals and humans. IARC Sci Publ (127):45–55.

Boysen G, Georgieva NI, Upton PB, Jayaraj K, Li Y, Walker VE, Swenberg JA. (2004). Analysis of diepoxide-specific cyclic N-terminal globin adducts in mice and rats after inhalation exposure to 1,3-butadiene. Cancer Res 64:8517–8520.

Boysen G, Scarlett CO, Temple B, Combs TP, Brooks NL, Borchers CH, Swenberg JA. (2007). Identification of covalent modifications in P450 2E1 by 1,2-epoxy-3-butene in vitro. Chem Biol Interact 166:170–175.

Brochot C, Smith TJ, Bois FY. (2007). Development of a physiologically based toxicokinetic model for butadiene and four major metabolites in humans: Global sensitivity analysis for experimental design issues. Chem Biol Interact 167:168–183.

Cheng H, Sathiakumar N, Graff J, Matthews R, Delzell E. (2007). 1,3-Butadi-ene and leukemia among synthetic rubber industry workers: Exposure-response relationships. Chem Biol Interact 166:15–24.

Csanady GA, Guengerich FP, Bond JA. (1992). Comparison of the biotrans-formation of 1,3-butadiene and its metabolite, butadiene monoepox-ide, by hepatic and pulmonary tissues from humans, rats and mice. Carcinogenesis 13:1143–1153.

Csanády GA, Kreuzer PE, Baur C, Filser JG. (1996). A physiological toxi-cokinetic model for 1,3-butadiene in rodents and man: Blood concen-trations of 1,3-butadiene, its metabolically formed epoxides, and of haemoglobin adducts—Relevance of glutathione depletion. Toxicology 113:300–305.

Dahl AR, Sun JD, Birnbaum LS, Bond JA, Griffith WC, Jr, Mauderly JL, Muggenburg BA, Sabourin PJ, Henderson RF. (1991). Toxicokinetics of inhaled 1,3-butadiene in monkeys: Comparison to toxicokinetics in rats and mice. Toxicol Appl Pharmacol 110:9–19.

Dahl AR, Henderson RF. (2000). Comparative metabolism of low concentra-tions of butadiene and its monoepoxide in human and monkey hepatic microsomes. Inhal Toxicol 12:439–451.

Delzell E, Sathiakumar N, Hovinga M, Macaluso M, Julian J, Larson R, Cole P, Muir DC. (1996). A follow-up study of synthetic rubber workers. Toxicology 113:182–189.

Delzell E, Macaluso M, Sathiakumar N, Matthews R. (2001). Leukemia and exposure to 1,3-butadiene, styrene and dimethyldithiocarbamate among workers in the synthetic rubber industry. Chem Biol Interact 135–136:515–534.

Delzell E, Sathiakumar N, Graff J, Macaluso M, Maldanado G, Matthews R. (2006). An updated study of mortality among North American synthetic rubber industry workers. Res Rep Health Effects Inst (132):1–63.

Derelanko MJ. (1987). Determination of erythrocyte life span in F-344, Wistar, and Sprague-Dawley rats using a modification of the [3H]diisopropylfluorophosphate ([3H]DFP) method. Fundam Appl Toxicol 9:271–276.

Deutschmann S, Laib RJ. (1989). Concentration-dependent depletion of non-protein sulfhydryl (NPSH) content in lung, heart and liver tissue of rats and mice after acute inhalation exposure to butadiene. Toxicol Lett 45:175–183.

Duescher RJ, Elfarra AA. (1994). Human liver microsomes are efficient catalysts of 1,3-butadiene oxidation: Evidence for major roles by cytochromes P450 2A6 and 2E1. Arch Biochem Biophys 311:342–349.

Elovaara E, Osterman-Golkar S, Nakajima T, Gelboin H.V, Riihimaki V, Sorsa M. (1994). 1,3-Butadiene exposure of rats: Hemoglobin adducts of 12-epoxy-butene and cytochrome P450 related changes in styrene metabolism. In: Lechner MC, ed. Cytochrome P450: Biochemistry, Biophysics and Molecular Biology. 8th International Conference. Lisbon: Gelbenkian Institute of Science, 567–570.

Filser JG, Faller TH, Bhowmik S, Schuster A, Kessler W, Putz C, Csanady GA. (2001). First-pass metabolism of 1,3-butadiene in once-through perfused livers of rats and mice. Chem Biol Interact 135–136:249–265.

Filser JG, Hutzler C, Meischner V, Veereshwarayya V, Csanady GA. (2007). Metabolism of 1,3-butadiene to toxicologically relevant metabolites in single-exposed mice and rats. Chem Biol Interact 166:93–103.

Filser JG, Bhowmik S, Faller TH, Hutzler C, Kessler W, Midpanon S, Putz C, Schuster A, Semder B, Veereshwarayya V, Csanady GA. (2010). Quantitative investigation on the metabolism of 1,3-butadiene and of its oxidized metabolites in once-through perfused livers of mice and rats. Toxicol Sci 114:25–37.

Fred C, Tornqvist M, Granath F. (2008). Evaluation of cancer tests of 1,3-buta-diene using internal dose, genotoxic potency, and a multiplicative risk model. Cancer Res 68:8014–8021.

Goggin M, Loeber R, Park S, Walker V, Wickliffe J, Tretyakova N. (2007). HPLC-ESI+-MS/MS analysis of N7-guanine-N7-guanine DNA cross-links in tissues of mice exposed to 1,3-butadiene. Chem Res Toxicol 20:839–847.

Goggin M, Swenberg JA, Walker VE, Tretyakova N. (2009). Molecular dosim-etry of 1,2,3,4-diepoxybutane-induced DNA-DNA cross-links in B6C3F1 mice and F344 rats exposed to 1,3-butadiene by inhalation. Cancer Res 69:2479–2486.

Graff JJ, Sathiakumar N, Macaluso M, Maldonado G, Matthews R, Delzell E. (2005). Chemical exposures in the synthetic rubber industry and lympho-hematopoietic cancer mortality. J Occup Environ Med 47:916–932.

Henderson RF, Thornton-Manning JR, Bechtold WE, Dahl AR. (1996). Metabolism of 1,3-butadiene: Species differences. Toxicology 113:17–22.

Himmelstein MW, Turner MJ, Asgharian B, Bond JA. (1994). Comparison of blood concentrations of 1,3-butadiene and butadiene epoxides in mice and rats exposed to 1,3-butadiene by inhalation. Carcinogenesis 15:1479–1486.

Himmelstein MW, Asgharian B, Bond JA. (1995). High concentrations of buta-diene epoxides in livers and lungs of mice compared to rats exposed to 1,3-butadiene. Toxicol Appl Pharmacol 132:281–288.

Himmelstein MW, Acquavella JF, Recio L, Medinsky MA, Bond JA. (1997). Toxicology and epidemiology of 1,3-butadiene. Crit Rev Toxicol 27:1–108.

Cri

tical

Rev

iew

s in

Tox

icol

ogy

Dow

nloa

ded

from

info

rmah

ealth

care

.com

by

Uni

vers

ity o

f T

oron

to o

n 10

/29/

14Fo

r pe

rson

al u

se o

nly.


Huff JE, Melnick RL, Solleveld HA, Haseman JK, Powers M, Miller RA. (1985). Multiple organ carcinogenicity of 1,3-butadiene in B6C3F1 mice after 60 weeks of inhalation exposure. Science 227:548–549.

Johanson G, Filser JG. (1993). A physiologically based pharmacokinetic model for butadiene and its metabolite butadiene monoxide in rat and mouse and its significance for risk extrapolation. Arch Toxicol 67:151–163.

Johanson G, Filser JG. (1996). PBPK model for butadiene metabolism to epoxides: Quantitative species differences in metabolism. Toxicology 113:40–47.

Kirman CR, Albertini RA, Gargas ML. (2010). 1,3-Butadiene: III. Assessing car-cinogenic modes of action. Crit Rev Toxicol 40(Supp 1):this issue.

Kligerman AD, Hu Y. (2007). Some insights into the mode of action of butadi-ene by examining the genotoxicity of its metabolites. Chem Biol Interact. 166:132–139.

Koc H, Tretyakova NY, Walker VE, Henderson RF, Swenberg JA. (1999). Molecular dosimetry of N-7 guanine adduct formation in mice and rats exposed to 1,3-butadiene. Chem Res Toxicol 12:566–574.

Kohn MC, Melnick RL. (2001). Physiological modeling of butadiene disposition in mice and rats. Chem Biol Interact 135–136:285–301.

Koivisto P, Kilpelainen I, Rasanen I, Adler ID, Pacchierotti F, Peltonen K. (1999). Butadiene diolepoxide- and diepoxybutane-derived DNA adducts at N7-guanine: A high occurrence of diolepoxide-derived adducts in mouse lung after 1,3-butadiene exposure. Carcinogenesis 20:1253–1259.

Krause RJ, Elfarra AA. (1997). Oxidation of butadiene monoxide to meso- and (+/−)-diepoxybutane by cDNA-expressed human cytochrome P450s and by mouse, rat, human liver microsomes: Evidence for preferential hydra-tion of meso-diepoxybutane in rat and human liver microsomes. Arch Biochem Biophys 337:176–184.

Krause RJ, Sharer JE, Elfarra AA. (1997). Epoxide hydrolase-dependent metab-olism of butadiene monoxide to 3-butene-1,2-diol in mouse, rat, and human liver. Drug Metab Dispos. 25:1013–1015.

Krause RJ, Kemper RA, Elfarra AA. (2001). Hydroxymethylvinyl ketone: A reac-tive Michael acceptor formed by the oxidation of 3-butene-1,2-diol by cDNA-expressed human cytochrome P450s and mouse, rat, and human liver microsomes. Chem Res Toxicol 14:1590–1595.

Kreiling R, Laib RJ, Bolt HM. (1988). Depletion of hepatic non-protein sulfhy-dryl content during exposure of rats and mice to butadiene. Toxicol Lett 41:209–214.

Kreuzer PE, Kessler W, Welter HF, Baur C, Filser JG. (1991). Enzyme specific kinetics of 1,2-epoxybutene-3 in microsomes and cytosol from livers of mouse, rat and man. Arch Toxicol 65:59–67.

Lewis DFV, Bird MG, Parke DV. (1997). Molecular modeling of CYP 2EI enzymes from mouse rat and man: An explanation for species difference in butadi-ene metabolism and potential carcinogenicity and rationalization of CYP 2EI substrate specificity. Toxicology 118:93–113.

Loeber R, Rajesh M, Fang Q, Pegg AE, Tretyakova N. (2006). Cross-linking of the human DNA repair protein O6-alkylguanine DNA alkyltransferase to DNA in the presence of 1,2,3,4-diepoxybutane. Chem Res Toxicol 19:645–54.

Macaluso M, Larson R, Lynch J, Lipton S, Delzell E. (2004). Historical estima-tion of exposure to 1,3-butadiene, styrene, and dimethyldithiocarbamate among synthetic rubber workers. J Occup Environ Hyg 1:371–390.

Melnick RL, Huff J, Chou BJ, Miller RA. (1990). Carcinogenicity of 1,3-butadiene in C57BL/6 × C3H F1 mice at low exposure concentrations. Cancer Res 50:6592–6599.

Motwani HV, Fred C, Haglund J, Golding BT, Tornqvist M. (2009). Cob(I)alamin for trapping butadiene epoxides in metabolism with rat S9 and for determining associated kinetic parameters. Chem Res Toxicol 22:1509–1516.

National Toxicology Program (NTP). (1984). National Toxicology Program Toxicology and carcinogenesis studies of 1,3-butadiene in B6C3F1 mice (inhalation studies). NTP Technical Reports 288.

National Toxicology Program (NTP). (1993). National Toxicology Program Toxicology and carcinogenesis studies of 1,3-butadiene in B6C3F1 mice (inhalation studies). NTP Technical Reports 434.

Nieusma JL, Claffey DJ, Maniglier-Poulet C, Imiolczyk T, Ross D, Ruth JA. (1997). Stereochemical aspects of 1,3-butadiene metabolism and toxicity in rat and mouse liver microsomes and freshly isolated rat hepatocytes. Chem Res Toxicol 10:450–456.

Osterman-Golkar SM, Moss O, James A, Bryant MS, Turner M, Bond JA. (1998) Epoxybutene-hemoglobin adducts in rats and mice: Dose response for formation and persistence during and following long-term low-level exposure to butadiene. Toxicol Appl Pharmacol 150:166–173.

Owen PE, Glaister JR, Gaunt IF, Pullinger DH. (1987). Inhalation toxicity studies with 1,3-butadiene. 3. Two year toxicity/carcinogenicity study in rats. Am Ind Hyg Assoc J 48:407–413.

Perez HL, Lahdetie J, Landin H, Kilpelainen I, Koivisto P, Peltonen K, Osterman-Golkar S. (1997). Haemoglobin adducts of epoxybutanediol from exposure to 1,3-butadiene or butadiene epoxides. Chem Biol Interact 105:181–198.

Schmidt U, Loeser E. (1985). Species differences in the formation of butadiene monoxide from 1,3-butadiene. Arch Toxicol 57:222–225.

Seaton MJ, Follansbee MH, Bond JA. (1995). Oxidation of 12-epoxy-3-butene to 1,2:3,4-diepoxybutane by cDNA-expressed human cytochromes P450 2E1 and 3A4 and human mouse and rat liver microsomes. Carcinogenesis 16:2287–2293.

Sweeney LM, Himmelstein MW, Schlosser PM, Medinsky MA. (1996). Physiologically based pharmacokinetic modeling of blood and tissue epoxide measurements for butadiene. Toxicology 113:318–321.

Sweeney LM, Schlosser PM, Medinsky MA, Bond JA. (1997). Physiologically based pharmacokinetic modeling of 1,3-butadiene, 1,2-epoxy-3-butene, and 1,2:3,4-diepoxybutane toxicokinetics in mice and rats. Carcinogenesis 18:611–625.

Sweeney LM, Himmelstein MW, Gargas ML. (2001). Development of a prelimi-nary physiologically based toxicokinetic (PBTK) model for 1,3-butadiene risk assessment. Chem Biol Interact 135–136:303–322.

Swenberg JA, Christova-Gueorguieva NI, Upton PB, Ranasinghe A, Scheller N, Wu KY, Yen TY, Hayes R. (2000). 1,3-Butadiene: Cancer, mutations, adducts. Part V: Hemoglobin adducts as biomarkers of 1,3-butadiene exposure and metabolism. Res Rep Health Effects Inst (92):191–210.

Swenberg JA, Boysen G, Georgieva N, Bird MG, Lewis RJ. (2007). Future direc-tions in butadiene risk assessment and the role of cross-species internal dosimetry. Chem Biol Interact 166:78–83.

Swenberg JA. (2008). Personal communication.Thornton-Manning JR, Dahl AR, Bechtold WE, Griffith WC Jr, Henderson RF.

(1995a). Disposition of butadiene monoepoxide and butadiene diepoxide in various tissues of rats and mice following a low-level inhalation expo-sure to 1,3-butadiene. Carcinogenesis 16:1723–1731.

Thornton-Manning JR, Dahl AR, Bechtold WE, Griffith WC Jr, Pei L, Henderson RF. (1995b). Gender differences in the metabolism of 1,3-butadiene in Sprague-Dawley rats following a low level inhalation exposure. Carcinogenesis 16:2875–2878.

Thornton-Manning JR, Dahl AR, Bechtold WE, Griffith WC Jr, Henderson RF. (1997). Comparison of the disposition of butadiene epoxides in Sprague-Dawley rats and B6C3F1 mice following a single and repeated exposures to 1,3-butadiene via inhalation. Toxicology 123:125–134.

Thornton-Manning JR, Dahl AR, Allen ML, Bechtold WE, Griffith WC Jr, Henderson RF. (1998). Disposition of butadiene epoxides in Sprague-Dawley rats follow-ing exposures to 8000 ppm 1,3-butadiene: Comparisons with tissue epoxide concentrations following low-level exposures. Toxicol Sci 41:167–173.

Vock EH, Lutz WK, Llinskaya O, Vamvakas S. (1999). Discrimination between genotoxicity and cytotoxicity for the induction of DNA double-strand breaks in cells treated with aldehydes and diepoxides. Mutat Res 441:85–93.

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1,3-butadiene: i. review of metabolism and the implications to human health risk assessment

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