Persistence of N7-(2,3,4-trihydroxybutyl)guanine adducts in the livers of mice and rats exposed to 1,3-butadiene

Kinetics of in Vitro Guanine- N7-Alkylation in Calf Thymus DNA by (2 S,3 S)-1,2-Epoxybutane-3,4-diol 4-methanesulfonate and (2 S,3 S)-1,2:3,4-Diepoxybutane: Revision of the Mechanism of DNA Cross-Linking by the Prodrug Treosulfan

Prodrug treosulfan, originally registered for treatment of ovarian cancer, has gained a use in conditioning prior to hematopoietic stem cell transplantation. Treosulfan converts nonenzymatically to the monoepoxide intermediate (EBDM), and then to (2 S,3 S)-1,2:3,4-diepoxybutane (DEB). The latter alkylates DNA forming mainly (2' S,3' S)- N-7-(2',3',4'-trihydroxybut-1'-yl)guanine (THBG) and (2 S,3 S)-1,4-bis(guan-7'-yl)butane-2,3-diol cross-link (bis-N7G-BD) via the intermediate epoxide adduct (EHBG). It is believed that DNA cross-linking by DEB is a primary mechanism for the anticancer and myeloablative properties of treosulfan, but clear evidence is lacking. Recently, we have proved that EBDM alkylates DNA producing (2' S,3' S)- N-7-(2',3'-dihydroxy-4'-methylsulfonyloxybut-1'-yl)-guanine (HMSBG) and that free HMSBG converts to EHBG. In this paper, we investigated the kinetics of HMSBG, bis-N7G-BD, and THBG in DNA in vitro to elucidate the contribution of EBDM and DEB to treosulfan-dependent DNA-DNA cross-linking. Calf thymus DNA was exposed to ( A) 100 μM treosulfan, ( B) 200 μM treosulfan, and ( C) DEB at a concentration 100 μM, exceeding that produced by 200 μM treosulfan. Following mild acid thermal hydrolysis of DNA, ultrafiltration, and off-line HPLC purification, the guanine adducts were quantified by LC-MS/MS. Both bis-N7G-BD and THBG reached highest concentrations in the DNA in experiment B. Ratios of the maximal concentration of bis-N7G-BD and THBG to DEB (adduct C_max/DEB C_max) in experiments A and B were 1.7-3.0-times greater than in experiment C. EHBG converted to the bis-N7G-BD cross-link at a much higher rate constant (0.20 h^-1) than EBDM and DEB initially alkylated the DNA (1.8-3.4 × 10^-5 h^-1), giving rise to HMSBG and EHBG, respectively. HMSBG decayed unexpectedly slowly (0.022 h^-1) compared with the previously reported behavior of the free adduct (0.14 h^-1), which revealed the inhibitory effect of the DNA environment on the adduct epoxidation to EHBG. A kinetic simulation based on the obtained results and the literature pharmacokinetic parameters of treosulfan, EBDM, and DEB suggested that in patients treated with the prodrug, EBDM could produce the vast majority of EHBG and bis-N7G-BD via HMSBG. In conclusion, EBDM can produce DNA-DNA lesions independently of DEB, and likely plays a greater role in DNA cross-linking after in vivo administration of treosulfan than DEB. These findings compel revision of the previously proposed mechanism of the pharmacological action of treosulfan and contribute to better understanding of the importance of EBDM for biological effects.

Liquid chromatography-tandem mass spectrometry method for simultaneous determination of three N-7-guanine adducts of the active epoxides of prodrug treosulfan in DNA in vitro

Prodrug treosulfan undergoes a pH and temperature-dependent activation to the monoepoxide intermediate (EBDM) and (2S,3S)-1,2:3,4-diepoxybutane (DEB). The latter DNA cross-linker is presently believed to mainly account for the pharmacological action of treosulfan. However, neither respective monoadducts nor cross-links have been isolated from treosulfan-treated DNA, and the exact alkylation mechanism of the treosulfan epoxides is unclear. In this paper, liquid chromatography method with tandem mass spectrometry detection (LC-MS/MS) for simultaneous determination of the N-7-guanine adducts of EBDM and DEB - (2'S,3'S)-N-7-(2'3'-dihydroxy-4'-methylsulfonyloxybut-1'-yl)guanine (HMSBG), N-7-(2',3',4'-trihydroxybut-1'-yl)guanine (THBG), and 1,4-bis(N-7-guanyl)butane-2,3-diol cross-link (bis-N7G-BD) - in calf-thymus DNA has been developed and validated for the first time. The mixture of drug-free nucleic acid with the analytes and ¹⁵N-isotope labeled internal standards underwent a mild acid thermal hydrolysis and ultrafiltration (cut-off 10 kDa). Following offline LC purification, the analytes and internal standards were determined in the LC-MS/MS system with an electrospray interface. Complete resolution of THBG, HMSBG, and bis-N7G-BD was accomplished on a Zorbax Eclipse C18 column using gradient elution with a mobile phase composed of 0.1% formic acid and acetonitrile. Calibration curves were linear in the ranges: THBG 0.2-200 pmol, HMSBG 0.2-20 pmol, and bis-N7G-BD 0.4-40 pmol. The limits of quantitation allowed to determine the adducts at concentration of 330 or 660 per 10⁹ DNA nucleotides. The LC-MS/MS method was adequately precise (coefficient of variation ≤ 16.7%) and accurate (relative error ≤ 17.7%). Calibration standards were stable for 14 days at -25 °C. The validated method enabled determination of THBG, HMSBG, and bis-N7G-BD in calf thymus DNA treated with treosulfan at pH 7.2 and 37 °C, which constitutes a novel bioanalytical application. To the authors' best knowledge, the quantification of THBG and bis-N7G-BD in one analytical run is also reported for the first time.

NanoLC/ESI+ HRMS3 quantitation of DNA adducts induced by 1,3-butadiene

Human exposure to 1,3-butadiene (BD) present in automobile exhaust, cigarette smoke, and forest fires is of great concern because of its potent carcinogenicity. The adverse health effects of BD are mediated by its epoxide metabolites such as 3,4-epoxy-1-butene (EB), which covalently modify genomic DNA to form promutagenic nucleobase adducts. Because of their direct role in cancer, BD-DNA adducts can be used as mechanism-based biomarkers of BD exposure. In the present work, a mass spectrometry-based methodology was developed for accurate, sensitive, and precise quantification of EB-induced N-7-(1-hydroxy-3-buten-2-yl) guanine (EB-GII) DNA adducts in vivo. In our approach, EB-GII adducts are selectively released from DNA backbone by neutral thermal hydrolysis, followed by ultrafiltration, offline HPLC purification, and isotope dilution nanoLC/ESI(+)-HRMS(3) analysis on an Orbitrap Velos mass spectrometer. Following method validation, EB-GII lesions were quantified in human fibrosarcoma (HT1080) cells treated with micromolar concentrations of EB and in liver tissues of rats exposed to sub-ppm concentrations of BD (0.5-1.5 ppm). EB-GII concentrations increased linearly from 1.15 ± 0.23 to 10.11 ± 0.45 adducts per 10(8) nucleotides in HT1080 cells treated with 0.5-10 μM EB. EB-GII concentrations in DNA of laboratory rats exposed to 0.5, 1.0, and 1.5 ppm BD were 0.17 ± 0.05, 0.33 ± 0.08, and 0.50 ± 0.04 adducts per 10(8) nucleotides, respectively [corrected]. We also used the new method to determine the in vivo half-life of EB-GII adducts in rat liver DNA (2.20 ± 0.12 d) and to detect EB-GII in human blood DNA. To our knowledge, this is the first application of nanoLC/ESI(+)-HRMS(3) Orbitrap methodology to quantitative analysis of DNA adducts in vivo.

Capillary HPLC-accurate mass MS/MS quantitation of N7-(2,3,4-trihydroxybut-1-yl)-guanine adducts of 1,3-butadiene in human leukocyte DNA

1,3-Butadiene (BD) is a high volume industrial chemical commonly used in polymer and rubber production. It is also present in cigarette smoke, automobile exhaust, and urban air, leading to widespread exposure of human populations. Upon entering the body, BD is metabolized to electrophilic epoxides, 3,4-epoxy-1-butene (EB), diepoxybutane (DEB), and 3,4-epoxy-1,2-diol (EBD), which can alkylate DNA nucleobases. The most abundant BD epoxide, EBD, modifies the N7-guanine positions in DNA to form N7-(2, 3, 4-trihydroxybut-1-yl) guanine (N7-THBG) adducts, which can be useful as biomarkers of BD exposure and metabolic activation to DNA-reactive epoxides. In the present work, a capillary HPLC-high resolution ESI⁺-MS/MS (HPLC-ESI⁺-HRMS/MS) methodology was developed for accurate, sensitive, and reproducible quantification of N7-THBG in cell culture and in human white blood cells. In our approach, DNA is subjected to neutral thermal hydrolysis to release N7-guanine adducts from the DNA backbone, followed by ultrafiltration, solid-phase extraction, and isotope dilution HPLC-ESI⁺-HRMS/MS analysis on an Orbitrap Velos mass spectrometer. Following method validation, N7-THBG was quantified in human fibrosarcoma (HT1080) cells treated with micromolar concentrations of DEB and in DNA isolated from blood of smokers, nonsmokers, individuals participating in a smoking cessation program, and occupationally exposed workers. N7-THBG concentrations increased linearly from 31.4 ± 4.84 to 966.55 ± 128.05 adducts per 10⁹ nucleotides in HT1080 cells treated with 1-100 μM DEB. N7-THBG amounts in leukocyte DNA of nonsmokers, smokers, and occupationally exposed workers were 7.08 ± 5.29, 8.20 ± 5.12, and 9.72 ± 3.80 adducts per 10⁹ nucleotides, respectively, suggesting the presence of an endogenous or environmental source for this adduct. The availability of sensitive HPLC-ESI⁺-HRMS/MS methodology for BD-induced DNA adducts in humans will enable future population studies of interindividual and ethnic differences in BD bioactivation to DNA-reactive epoxides.

1,3-Butadiene: II. Genotoxicity profile

1,3-Butadiene’s (BD’s) major electrophilic metabolites 1,2-epoxy-3-butene (EB), 1,2-dihydroxy-3,4-epoxybutane (EBD), and 1,2,3,4-diepoxybutane (DEB) are responsible for both its mutagenicity and carcinogenicity. EB, EBD, and DEB are DNA reactive, forming a variety of adducts. All three metabolites are genotoxic in vitro and in vivo, with relative mutagenic potencies of DEB >> EB > EBD. DEB also effectively produces gene deletions and chromosome aberrations. BD’s greater mutagenicity and carcinogenicity in mice over rats as well as its failure to induce chromosome-level mutations in vivo in rats appear to be due to greater production of DEB in mice. Concentrations of EB and DEB in vivo in humans are even lower than in rats. Although most studies of BD-exposed humans have failed to find increases in gene mutations, one group has reported positive findings. Reasons for these discordant results are examined. BD-related chromosome aberrations have never been demonstrated in humans except for the possible production of micronuclei in lymphocytes of workers exposed to extremely high levels of BD in the workplace. The relative potencies of the BD metabolites, their relative abundance in the different species, and the kinds of mutations they can induce are major considerations in BD’s overall genotoxicity profile.

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.

Quantitative analysis of amyloid beta peptides in cerebrospinal fluid of Alzheimer's disease patients by immunoaffinity purification and stable isotope dilution liquid chromatography/negative electrospray ionization tandem mass spectrometry

The 40 and 42 amino-acid residue forms of amyloid beta (Abeta(1-40) and Abeta(1-42)) in cerebrospinal fluid (CSF) have been proposed as potential biomarkers of Alzheimer's disease (AD). Quantitative analyses of Abeta peptides in CSF have relied almost exclusively on the use of immunoassay-based assays such as the enzyme-linked immunosorbent assay (ELISA) procedure. However, due to the ability of the Abeta peptides to readily self-aggregate or bind to other proteins and glassware, such analyses are extremely challenging. Analyses are further complicated by the potential of the peptides to undergo post-translational modifications and the possibilities for cross-reaction in the ELISA assays with endogenous components of the CSF. An approach based on liquid chromatography/tandem mass spectrometry (LC/MS/MS) has now been developed which overcomes these methodological issues. The key steps in implementing this new approach involved immunoaffinity purification coupled with the use of [15N]-labeled Abeta peptides as internal standards, a basic LC mobile phase, negative ion electrospray ionization, and a basic solvent for dissolving the peptides and washing the injection needle to prevent carryover of analytes during multiple injections on the LC/MS system. The validated method had limits of quantitation of 44 fmol/mL (200 pg/mL) for Abeta(1-42) and 92 fmol/mL (400 pg/mL) for Abeta(1-40). An excellent correlation was found between the LC/MS/MS assay and an ELISA assay for Abeta(1-42) in human CSF (r2 = 0.915), although less correlation was observed for Abeta(1-40) (r2 = 0.644). Mean CSF Abeta(1-42) concentrations for samples collected 2 weeks apart from a limited number of AD patients provided additional confidence in the reproducibility of the LC/MS/MS assay. Concentrations for duplicate samples from AD patients were slightly higher than most previously reported values (mean 1.06 +/- 0.25 ng/mL; n = 7). Abeta(1-40) concentrations in duplicate samples obtained from AD patients were also reproducible but were found to be slightly lower than most previously reported values (mean 6.36 +/- 3.07 ng/mL; n = 7). Consistent with literature reports, mean Abeta(1-42) concentrations were found to be lower in AD patients compared with the normal subjects (mean 1.49 +/- 0.59 ng/mL; n = 7), whereas there was no difference in Abeta(1-40) concentrations between AD patients and normal subjects (mean 5.88 +/- 3.03 ng/mL; n = 7). The accuracy and precision of the LC/MS assay mean that it will be a useful complement to existing ELISA assays for monitoring therapeutic interventions designed to modulate CSF Abeta(1-42) concentrations in individual AD patients. Moreover, the introduction of stable isotope labeled internal standards offers the potential to achieve a more rigorous account of the influence of methodological effects related to sample collection and processing.

Reaction of 1,2,3,4-diepoxybutane with 2'-deoxyguanosine: initial products and their stabilities and decomposition patterns under physiological conditions

1,2,3,4-Diepoxybutane (DEB), an in vivo metabolite of 1,3-butadiene (BD), is a carcinogen and a potent mutagen. Previously, DEB was shown to react with 2'-deoxyguanosine (dG) under physiological conditions to produce seven major nucleoside adducts resulting from alkylation at the N1- (P8 and P9), N7- (P5 and P5'), and both the N1- and the N2-positions of dG to form six-membered (P4-1 and P4-2) and seven-membered fused ring systems (P6), respectively [Zhang, X.-Y., and Elfarra, A. A. (2003) Chem. Res. Toxicol. 16, 1606. Zhang, X.-Y., and Elfarra, A. A. (2004) Chem. Res. Toxicol. 17, 521]. In the present study, the stabilities and decomposition products of the seven adducts under in vitro physiological conditions (phosphate buffer containing KCl, pH 7.4, 37 degrees C) were investigated. The results showed that P4-1, P4-2, and P6 were stable, whereas P5, P5', P8, and P9 were labile with half-lives of 2.6, 2.7, 16, and 16 h, respectively. P5 and P5' decomposed initially by the loss of the deoxyribose moiety to yield the corresponding guanine adduct P5D, which exhibited a half-life of 33 h and decomposed through opening of the remaining oxirane ring by dihydrogen phosphate ion, water, or chloride ion. Decomposition of P8 yielded P4-1, P6, and nucleoside products resulting from opening of the oxirane ring by dihydrogen phosphate ion, water, or chloride ion. Similarly, decomposition of P9 led to the formation of P4-2, P6, and nucleoside products resulting from opening of the oxirane ring by dihydrogen phosphate ion, water, or chloride ion. These results indicate that the initial products of the reaction of DEB with dG are P5, P5', P8, and P9, whereas P4-1, P4-2, and P6 are secondary products. The results may also facilitate development of useful biomarkers of exposure to DEB.

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.

DNA adducts in rats and mice following exposure to [4-14C]-1,2-epoxy-3-butene and to [2,3-14C]-1,3-butadiene

1,3-Butadiene (BD) is a major industrial chemical and a rodent carcinogen, with mice being much more susceptible than rats. Oxidative metabolism of BD, leading to the DNA-reactive epoxides 1,2-epoxy-3-butene (BMO), 1,2-epoxy-3,4-butanediol (EBD) and 1,2:3,4-diepoxybutane (DEB), is greater in mice than rats. In the present study the DNA adduct profiles in liver and lungs of rats and mice were determined following exposure to BMO and to BD since these profiles may provide qualitative and quantitative information on the DNA-reactive metabolites in target tissues. Adducts detected in vivo were identified by comparison with the products formed from the reaction of the individual epoxides with 2'-deoxyguanosine (dG). In rats and mice exposed to [4-14C]-BMO (1-50 mg/kg, i.p.), DNA adduct profiles were similar in liver and lung with N7-(2-hydroxy-3-butenyl)guanine (G1) and N7-(1-(hydroxymethyl)-2-propenyl)guanine (G2) as major adducts and N7-2,3,4-trihydroxybutylguanine (G4) as minor adduct. In rats and mice exposed to 200 ppm [2,3-14C]-BD by nose-only inhalation for 6 h, G4 was the major adduct in liver, lung and testes while G1 and G2 were only minor adducts. Another N7-trihydroxybutylguanine adduct (G3), which could not unambiguously be identified but is either another isomer of N7-2,3,4-trihydroxybutylguanine or, more likely, N7-(1-hydroxymethyl-2,3-dihydroxypropyl)guanine, was present at low concentrations in liver and lung DNA of mice, but absent in rats. The evidence indicates that the major DNA adduct formed in liver, lung and testes following in vivo exposure to BD is G4, which is formed from EBD, and not from DEB.

Dose responses for DNA adduct formation in tissues of rats and mice exposed by inhalation to low concentrations of 1,3-[2,3-[(14)C]-butadiene

Male Sprague-Dawley rats and B6C3F1 mice were exposed to either a single 6h or a multiple (5) daily (6h) nose-only dose of 1,3-[2,3-(14)C]-butadiene at exposure concentrations of nominally 1, 5 or 20 ppm. The aim was to compare the results with those from a similar previous study at 200 ppm. DNA isolated from liver, lung and testis of exposed rats and mice was analysed for the presence of butadiene related adducts, especially the N7-guanine adducts. Total radioactivity present in the DNA from liver, lung and testis was quantified and indicated more covalent binding of radioactivity for mouse tissue DNA than rat tissue DNA. Following release of the depurinating DNA adducts by neutral thermal hydrolysis, the liberated depurinated DNA adducts were measured by reverse phase HPLC coupled with liquid scintillation counting. The guanine adduct G4, assigned as N7-(2,3,4-trihydroxybutyl)- guanine, was the major adduct measured in liver, lung and testis DNA in both rats and mice. Higher levels of G4 were detected in all mouse tissues compared with rat tissue. The dose-response relationship for the formation of adduct G4 was approximately linear for all tissues studied for both rats and mice exposed in the 1-20 ppm range. The formation of G4 in liver tissue was about three times more effective for mouse than rat in this exposure range. Average levels of adduct G4 measured in liver DNA of rats and mice exposed to 5 x 6 h 1, 5 and 20 ppm 1,3-[2,3-(14)C]-butadiene were, respectively, for rats: 0.79 +/- 0.30, 2.90 +/- 1.19, 16.35 +/- 4.8 adducts/10(8) nucleotides and for mice: 2.23 +/- 0.71, 12.24 +/- 2.15, 48.63 +/- 12.61 adducts/10(8) nucleotides. For lung DNA the corresponding values were for rats: 1.02 +/- 0.44, 3.12 +/- 1.06, 17.02 +/- 4.07 adducts/10(8) nucleotides, and for mice: 3.28 +/- 0.32, 14.04 +/- 1.55, 42.47 +/- 13.12 adducts/10(8) nucleotides. Limited comparative data showed that the levels of adduct G4 formed in liver and lung DNA of mice exposed to a single exposure to butadiene in the present 20 ppm study and earlier 200 ppm study were approximately directly proportional across dose, but this was not observed in the case of rats. From the available evidence it is most likely that adduct G4 was formed from a specific isomer of the diol-epoxide metabolite, 3,4-epoxy-1,2-butanediol rather than the diepoxide, 1,2,3,4-diepoxybutane. Another adduct G3, possibly a diastereomer of N7-(2,3,4-trihydroxybutyl)-guanine or most likely the regioisomer N7-(1-hydroxymethyl-2,3-dihydroxypropyl)-guanine, was also detected in DNA of mouse tissues but was essentially absent in DNA from rat tissue. Qualitatively similar profiles of adducts were observed following exposures to butadiene in the present 20 ppm study and the previous 200 ppm study. Overall the DNA adduct levels measured in tissues of both rats and mice were very low. The differences in the profiles and quantity of adducts seen between mice and rats were considered insufficient to explain the large difference in carcinogenic potency of butadiene to mice compared with rats.

Determination of the platinum drug cis-amminedichloro(2-methylpyridine)platinum(II) in human urine by liquid chromatography-tandem mass spectrometry

A validated stable isotope dilution liquid chromatography-tandem mass spectrometry assay for the novel platinum drug cis-amminedichloro(2-methylpyridine)platinum(II) (ZD0473) in human urine has been developed. This method uses selected reaction monitoring on the transition of m/z 393 [M+NH(4)](+) to m/z 304 [M+NH(4)-NH(3)-2 x H(35)Cl](+) for ZD0473, and m/z 400 [M+NH(4)](+) to m/z 310 [M+NH(4)-NH(3)-H(35)Cl-(2)H(35)Cl](+) for the internal standard [(2)H(7)]ZD0473. Standard curves were prepared over the range from 0.15 to 50 microg/ml. The lower limit of quantitation was 0.2 microg/ml for 100 microl of urine. This simple, rapid, reliable, and sensitive method of quantitation displayed acceptable accuracy and precision over the 3 days of assay validation. A novel platinum adduct was formed during the storage of ZD0473 in human urine. The adduct did not correspond to any of the typical sulfhydryl adducts that have been identified previously for platinum drugs. Formation of the adduct was prevented by the addition of 50% (w/v) sodium chloride to the urine. The assay can be used to quantify intact ZD0473 in the urine of subjects dosed with this new platinum drug.

Applications of mass spectrometry for quantitation of DNA adducts

DNA adducts are formed when electrophilic molecules or free radicals attack DNA. 32P-postlabeling has been the most commonly used assay for quantitation of DNA adducts due mainly to its excellent sensitivity that allows quantitation at concentrations as low as approximately 1 adduct per 10(9) normal bases. Such methods, however, do not have the specificity desired for accurate and reliable quantitation, and are prone to produce false positives and artifacts. In the last decade, mass spectrometry in combination with liquid and gas chromatography has presented itself as a good alternative to these techniques since it can satisfy the need for specificity and reliability through the use of stable isotope-labeled internal standards and highly specific detection modes such as selected reaction monitoring and high-resolution mass spectrometry. In this article, the contribution of mass spectrometry to the quantitation of DNA adducts is reviewed with special emphasis on unique applications of mass spectrometry in the area of DNA adduct quantitation and recent applications with improvements in sensitivity.

N7-guanine adducts of the epoxy metabolites of 1,3-butadiene in mice lung

Epoxy metabolites of 1,3-butadiene are electrophilic and can bind to nucleophilic sites in DNA forming DNA adducts. In this study, guanine N7 adducts of epoxy butene and guanine N7 adducts of epoxy butanediol were measured in lung tissues of mice inhalation exposed to various concentrations of 1,3-butadiene. 32P-postlabeling of DNA adducts were used to demonstrate that the DNA adducts derived from epoxybutene and epoxybutanediol were formed in a dose dependent manner. More than 98% of all adducts detected were formed from epoxybutanediol. Enantiomeric distribution of the adducts formed in vivo differs from that of in vitro experiments demonstrated before. In the case of epoxybutene most of the adducts were formed to the terminal carbon of the S-epoxybutene enantiomer. Most of the adducts derived from epoxybutanediol were formed from the 2S-3R enantiomer. The data demonstrates that enzymatic processes involved with activation and/or detoxification of the metabolites are enantiospecific and/or DNA repair machinery repairs the damage with stereochemical considerations. These are the crucial factors if interspecies differences in tumor sensitiveness is concerned.

Micronuclei and gene mutations in transgenic big Blue((R)) mouse and rat fibroblasts after exposure to the epoxide metabolites of 1, 3-butadiene

1,3-Butadiene (BD) is a commodity compound and by-product in the manufacture of synthetic rubber that elicits a differential carcinogenic response in rodents after chronic exposure. Mice are up to approximately 1000-fold more sensitive to the tumorigenicity of inhaled BD than rats, thereby confounding human risk assessment analyses. Rodent transgenic in vivo and in vitro models have been recently utilized for generating genetic toxicology data in support of risk assessment studies. However, studies have not been extended to investigate multiple endpoints of genetic damage using in vitro transgenic models. The goal of this study was to evaluate possible differences in the production of genetic damage in transgenic Big Blue((R)) mouse (BBM1) and rat (BBR1) fibroblasts exposed to three predominant epoxide metabolites of BD. Analyses of cytotoxicity, micronucleus (MN) formation, cII mutant frequency (MF) and apoptosis were assessed after in vitro exposure of BBM1 and BBR1 cells exposed to various concentrations of butadiene monoepoxide (BMO), diepoxybutane (DEB) and butadiene diolepoxide (BDE). Both BMO and DEB reduced cell survival in BBM1 and BBR1 cells. However, BDE decreased cell survival only in BBM1 cells at the concentrations evaluated. Concentration-dependent increases in the formation of MN was observed in both BBM1 and BBR1 cells, with DEB being the most potent followed by BDE and then BMO. The dose-response for mutations induced at the cII locus was essentially equal after DEB exposure of BBM1 and BBR1 fibroblasts. In contrast, the cII MF was significantly increased only in BBM1 cells after exposure to either BMO or BDE. These data demonstrate a differential genetic response for gene mutations but not for MN formation in transgenic BBM1 and BBR1 fibroblasts and suggest a rodent species-specific difference in the persistence of DNA damage that results in gene mutations. In addition, apoptosis was observed in BBR1 cells but not in BBM1 cells when treated with any of the three BD epoxide metabolites. This response may partially explain the differential response to mutations induced by BMO and BDE. These data offer insight into specific differences in mouse and rat cells with respect to their response to BD epoxide metabolites. Such data may help to explain the different tumorigenicity results observed in rodent BD carcinogenicity studies.

Specific DNA adducts induced by some mono-substituted epoxides in vitro and in vivo

Alkyl epoxides are important intermediates in the chemical industry. They are also formed in vivo during the detoxification of alkenes. Alkyl epoxides have shown genotoxicity in many toxicology assays which has been associated with their covalent binding to DNA. Here aspects of the formation and properties of DNA adducts, induced by some industrially important alkenes and mono-substituted epoxides are discussed. These include propylene oxide, epichlorohydrin, allyl glycidyl ether and the epoxy metabolites of styrene and butadiene. The major DNA adducts formed by epoxides are 7-substituted guanines, 1- and 3-substituted adenines and 3-substituted cytosines. In addition, styrene oxide and butadiene monoepoxide are able to modify exocyclic sites in the DNA bases, the sites being in the case of styrene oxide N(2)- and O(6)-positions of guanine, N(6)-adenine as well as N(4)-and O(2)-cytosine. In vivo the main adduct is the 7-substituted guanines. The 1-substituted adenines have also shown marked levels, and these adducts should also be targets in biomonitoring of human exposures. Due to its low mutagenicity, 7-substituted guanines are considered as a surrogate marker for other mutagenic lesions, e.g. those of 1-adenine or 3-uracil adducts.

A review of the genetic and related effects of 1,3-butadiene in rodents and humans

In this paper, the metabolism and genetic toxicity of 1,3-butadiene (BD) and its oxidative metabolites in humans and rodents is reviewed with attention to newer data that have been published since the latest evaluation of BD by the International Agency for Research on Cancer (IARC). The oxidative metabolism of BD in mice, rats and humans is compared with emphasis on the major pathways leading to the reactive intermediates 1,2-epoxy-3-butene (EB), 1,2:3, 4-diepoxybutane (DEB), and 3,4-epoxy-1,2-butanediol (EBdiol). Results from recent studies of DNA and hemoglobin adducts indicate that EBdiol may play a more significant role in the toxicity of BD than previously thought. All three metabolites are capable of reacting with macromolecules, such as DNA and hemoglobin, and have been shown to induce a variety of genotoxic effects in mice and rats as well as in human cells in vitro. DEB is clearly the most potent of these genotoxins followed by EB, which in turn is more potent than EBdiol. Studies of mutations in lacI and lacZ mice and of the Hprt mutational spectrum in rodents and humans show that mutations at G:C base pairs are critical events in the mutagenicity of BD. In-depth analyses of the mutational spectra induced by BD and/or its oxidative metabolites should help to clarify which metabolite(s) are associated with specific mutations in each animal species and which mutational events contribute to BD-induced carcinogenicity. While the quantitative relationship between exposure to BD, its genotoxicity, and the induction of cancer in occupationally exposed humans remains to be fully established, there is sufficient data currently available to demonstrate that 1,3-butadiene is a probable human carcinogen.

Liquid chromatography-mass spectrometry in the study of the metabolism of drugs and other xenobiotics

The application of liquid chromatography-mass spectrometry (LC/MS) to the study of metabolism of drugs and other xenobiotics is reviewed. Original research papers covering the period from 1998 to early 2000 and concerning the use of LC/MS in the study of xenobiotic metabolism in humans and other mammalian species are reviewed. LC/MS interfaces, sample preparation steps, column types, mobile phases and additives, and the type of metabolites detected are summarized and discussed in an attempt to identify the current and future trends in the use of LC/MS for metabolism studies. Applications are listed according to the parent xenobiotic type and include substances used in therapeutics, drug candidates, compounds being evaluated in clinical trials, environmental pollutants, adulterants and naturally occurring substances.

Comparison of the mutations at Hprt exon 3 of T-lymphocytes from B6C3F1 mice and F344 rats exposed by inhalation to 1,3-butadiene or the racemic mixture of 1,2:3,4-diepoxybutane

Experiments were conducted to define the spectra of mutations occurring in Hprt exon 3 of T-cells isolated from spleens of female B6C3F1 mice and F344 rats exposed by inhalation to 1,3-butadiene (BD) or its reactive metabolite, (+/-)-diepoxybutane (DEB). Hprt mutant frequencies (Mfs) in BD-exposed (1250 ppm for 2 weeks or 625 ppm for 4 weeks; 6 h/day, 5 days/week) and DEB-exposed (2 or 4 ppm for 4 weeks or 5 ppm for 6 weeks; 6 h/day, 5 days/week) mice and rats were significantly increased over concurrent control values. Mutant T-cell colonies from control and treated animals were screened for mutations in Hprt exon 3 using PCR amplification of genomic DNA and denaturing gradient gel electrophoresis, followed by sequence analysis. Exon 3 mutations were found at the following frequencies: 20/394 (5%) in control mice, 56/712 (8%) in BD-exposed mice, 59/1178 (5%) in BD-exposed rats, 66/642 (10%) in DEB-exposed mice, and 51/732 (7%) in DEB-exposed rats. Mutations in exposed animals included base substitutions, small deletions (1 to 74 bp), and small insertions (1 to 8 bp), with base substitutions predominating. Among the types of base substitutions observed in mice, the proportions of G.C-->A.T transitions (p=0.035, Fisher's Exact Test) and G.C-->C.G transversions (p=0.05) were significantly different in control vs. BD-exposed animals. Given the small number of exon 3 mutants analyzed, there was a high degree of overlap in the mutational spectra between BD-exposed mice and rats, between BD- and DEB-exposed mice, and between BD- and DEB-exposed rats in terms of the sites with base substitutions, the mutations found at those mutated sites, the relative occurrence of the most frequently observed base substitutions, and the occurrence of a consistent strand bias for the most frequently observed base substitutions. The spectra data suggest that adduction of both G.C and A.T bps is important in the induction of in vivo mutations by BD metabolites in exposed mice and rats.

Mutagenicity of the racemic mixtures of butadiene monoepoxide and butadiene diepoxide at the Hprt locus of T-lymphocytes following inhalation exposures of female mice and rats

The purpose of this study was to determine if Hprt mutant frequency (Mf) data from rodents exposed directly to individual epoxy metabolites of 1,3-butadiene (BD) can be used to identify the relative significance of each intermediate in the mutagenicity of BD in mice vs. rats. To this end, the relative contributions of the racemic mixtures of BD monoepoxide (BDO) and BD diepoxide (BDO(2)) to BD-induced mutagenicity was investigated by exposing mice and rats to selected concentrations of BDO and BDO(2) (i.e., 2.5 and 4.0 ppm, respectively) and comparing the mutagenic potency of each intermediate to that of BD (at 62.5 ppm) when comparable blood levels of metabolites are achieved (in the mouse). Female B6C3F1 mice and F344 rats (4-5 weeks old) were exposed to rac-BDO (0, 2.5, or 25 ppm) or (+/-)-BDO(2) (0, 2, 4 ppm) by inhalation for 4 weeks (6 h/day, 5 days/week), and then groups of control and exposed animals (n=3-12/group) were necropsied at multiple time points post-exposure for measuring Hprt Mfs in splenic lymphocytes (via the T-cell cloning assay) and estimating mutagenic potencies (represented by the difference in the areas under the mutant T-cell 'manifestation' curves of treated vs. control animals). The resulting Mf data, along with the extant metabolism data, suggest that at lower BD exposures (</=62.5 ppm) (+/-)-BDO(2) is a major contributor to the mutagenicity of BD in mice, whereas other metabolites and stereochemical configurations are responsible for mutations in BD-exposed rats and for the incremental mutagenic effects at higher BD exposures in mice. These studies indicate that additional work is needed to determine more definitively the relative contributions of these and other metabolites and stereochemical forms to BD-induced mutagenicity. Also, the novel approach of measuring mutagenic potencies as the change in Hprt Mfs over time in T-cells of exposed vs. control animals, as used in this study, can be valuable for predicting the potential role of these intermediates in each species.