Identification of adducts derived from reactions of (1-chloroethenyl)oxirane with nucleosides and calf thymus DNA

Alkylation of complementary ribonucleotides in nanoreactors

The aim of the present study was to provide experimental evidence that base pairing, commonly occurring between nucleic bases in more complex supramolecular arrangements, may affect the reaction pathways associated with the alkylation of bases themselves. In pursuit of this aim, dilute aqueous solutions of Cytidine- (CMP) and Guanosine-Mono-Phosphate (GMP) as single reactants or in an equimolar mixture were treated with the electrophilic alkylating agent 1,2-Dodecyl-Epoxide (DE), which was preventively dispersed into micellar solutions prepared with the cationic surfactant hexadecyltrimethylammonium bromide (CTAB). In the early stage of the reaction, CTAB micelles acted as micro-heterogeneous nanoreactors, but as the reaction progressed the systems evolved toward the formation of polydisperse aggregates, whose size and surface-charge properties were monitored as a function of reaction time. From mass spectrometry analyses, it was found that the deamination of cytosine, a side reaction related to the alkylation of the amino group of CMP, was reduced when both the complementary ribonucleotides were present in the same reaction mixture. The involvement of specific sites able to establish C:G interactions (possibly via H-bonding or π-π stacking) could explain the reduced reactivity occurring at the level of some of the nucleophilic centers responsible for molecular recognition.

Cytosine to uracil conversion through hydrolytic deamination of cytidine monophosphate hydroxy-alkylated on the amino group: a liquid chromatography--electrospray ionization--mass spectrometry investigation

A novel pathway for cytosine to uracil conversion performed in a micellar environment, leading to the generation of uridine monophosphate (UMP), was evidenced during the alkylation reaction of cytidine monophosphate (CMP) by dodecyl epoxide. Liquid chromatography-electrospray ionization - ion trap - mass spectrometry was used to separate and identify the reaction products and to follow their formation over time. The detection of hydroxy-amino-dodecane, concurrently with free UMP, in the reaction mixture suggested that, among the various alkyl-derivatives formed, CMP alkylated on the amino group of cytosine could undergo tautomerization to an imine and hydrolytic deamination, generating UMP. Interestingly, no evidence for this peculiar conversion pathway was obtained when guanosine monophosphate (GMP), the complementary ribonucleotide of CMP, was also present in the reaction mixture, due to the fact that NH(2)-alkylated CMP was not formed in this case. The last finding emphasized the role played by CMP-GMP molecular interactions, mediated by a micellar environment, in hindering the alkylation reaction at the level of the cytosine amino group.

In vivo formation of N7-guanine DNA adduct by safrole 2',3'-oxide in mice

Safrole, a naturally occurring product derived from spices and herbs, has been shown to be associated with the development of hepatocellular carcinoma in rodents. Safrole 2',3'-oxide (SFO), an electrophilic metabolite of safrole, was shown to react with DNA bases to form detectable DNA adducts in vitro, but not detected in vivo. Therefore, the objective of this study was to investigate the formation of N7-(3-benzo[1,3]dioxol-5-yl-2-hydroxypropyl)guanine (N7γ-SFO-Gua) resulting from the reaction of SFO with the most nucleophilic site of guanine in vitro and in vivo with a newly developed isotope-dilution high performance liquid chromatography electrospray ionization tandem mass spectrometry (HPLC-ESI-MS/MS) method. N7γ-SFO-Gua and [(15)N(5)]-N7-(3-benzo[1,3]dioxol-5-yl-2-hydroxypropyl)guanine ([(15)N(5)]-N7γ-SFO-Gua) were first synthesized, purified, and characterized. The HPLC-ESI-MS/MS method was developed to measure N7γ-SFO-Gua in calf thymus DNA treated with 60 μmol of SFO for 72 h and in urine samples of mice treated with a single dose of SFO (30 mg/kg body weight, intraperitoneally). In calf thymus DNA, the level of N7γ-SFO-Gua was 2670 adducts per 10(6)nucleotides. In urine of SFO-treated mice, the levels of N7γ-SFO-Gua were 1.02±0.14 ng/mg creatinine (n=4) on day 1, 0.73±0.68 ng/mg creatinine (n=4) on day 2, and below the limit of quantitation on day 3. These results suggest that SFO can cause in vivo formation of N7γ-SFO-Gua, which may then be rapidly depurinated from the DNA backbone and excreted through urine.

Kinetic modeling of β-chloroprene metabolism: Probabilistic in vitro-in vivo extrapolation of metabolism in the lung, liver and kidneys of mice, rats and humans

β-Chloroprene (chloroprene) is carcinogenic in inhalation bioassays with B6C3F1 mice and Fischer rats, but the potential effects in humans have not been adequately characterized. In order to provide a better basis for evaluating chloroprene exposures and potential effects in humans, we have explored species and tissue differences in chloroprene metabolism. This study implemented an in vitro-in vivo extrapolation (IVIVE) approach to parameterize a physiologically based pharmacokinetic (PBPK) model for chloroprene and evaluate the influence of species and gender differences in metabolism on target tissue dosimetry. Chloroprene metabolism was determined in vitro using liver, lung and kidney microsomes from male or female mice, rats, and humans. A two compartment PK model was used to estimate metabolism parameters for chloroprene in an in vitro closed vial system, which were then extrapolated to the whole body PBPK model. Two different strategies were used to estimate parameters for the oxidative metabolism of chloroprene: a deterministic point-estimation using the Nelder-Mead nonlinear optimization algorithm and probabilistic Bayesian analysis using the Markov Chain Monte Carlo technique. Target tissue dosimetry (average amount of chloroprene metabolized in lung per day) was simulated with the PBPK model using the in vitro-based metabolism parameters. The model-predicted target tissue dosimetry, as a surrogate for a risk estimate, was similar between the two approaches; however, the latter approach provided a measure of uncertainty in the metabolism parameters and the opportunity to evaluate the impact of that uncertainty on predicted risk estimates.

Alkylating potential of α,β-unsaturated compounds

Alkylation reactions of the nucleoside guanosine (Guo) by the α,β-unsaturated compounds (α,β-UC) acrylonitrile (AN), acrylamide (AM), acrylic acid (AA) and acrolein (AC), which can act as alkylating agents of DNA, were investigated kinetically. The following conclusions were drawn: i) The Guo alkylation mechanism by AC is different from those brought about the other α,β-UC; ii) for the first three, the following sequence of alkylating potential was found: AN > AM > AA; iii) A correlation between the chemical reactivity (alkylation rate constants) of AN, AM, and AA and their capacity to form adducts with biomarkers was found. iv) Guo alkylation reactions for AN and AM occur through Michael addition mechanisms, reversible in the first case, and irreversible in the second. The equilibrium constant for the formation of the Guo-AN adduct is K(eq) (37 °C) = 5 × 10(-4); v) The low energy barrier (≈10 kJ mol(-1)) to reverse the Guo alkylation by AN reflects the easy reversibility of this reaction and its possible correction by repair mechanisms; vi) No reaction was observed for AN, AM, and AA at pH < 8.0. In contrast, Guo alkylation by AC was observed under cellular pH conditions. The reaction rate constants for the formation of the α-OH-Guo adduct (the most genotoxic isomer), is 1.5-fold faster than that of γ-OH-Guo. vii) a correlation between the chemical reactivity of α,β-UC (alkylation rate constants) and mutagenicity was found.

DNA interstrand cross-linking activity of (1-Chloroethenyl)oxirane, a metabolite of beta-chloroprene

With the goal of elucidating the molecular and cellular mechanisms of chloroprene toxicity, we examined the potential DNA cross-linking of the bifunctional chloroprene metabolite, (1-chloroethenyl)oxirane (CEO). We used denaturing polyacrylamide gel electrophoresis to monitor the possible formation of interstrand cross-links by CEO within synthetic DNA duplexes. Our data suggest interstrand cross-linking at deoxyguanosine residues within 5'-GC and 5'-GGC sites, with the rate of cross-linking depending on pH (pH 5.0 > pH 6.0 > pH 7.0). A comparison of the cross-linking efficiencies of CEO and the structurally similar cross-linkers diepoxybutane (DEB) and epichlorohydrin (ECH) revealed that DEB > CEO > or = ECH. Furthermore, we found that cytotoxicity correlates with cross-linking efficiency, supporting a role for interstrand cross-links in the genotoxicology of chloroprene.

Reactions of 4-[Bis(2-chloroethyl)amino]benzenebutanoic acid (chlorambucil) with DNA

4-[Bis(2-chloroethyl)amino]benzenebutanoic acid (=chlorambucil, 1; 2.5 mM) was allowed to react with single- and double-stranded calf thymus DNA at physiological pH (cacodylic acid, 50% base) at 37 degrees . The DNA-chlorambucil adducts were identified by analyzing the DNA hydrolysates by NMR, UV, HPLC, LC/ESI-MS/MS techniques as well as by spiking with authentic materials. ssDNA was more reactive than dsDNA, and the order of reactivity in ssDNA was Ade-N1>Gua-N7>Cyt-N3>Ade-N3. The most reactive site in dsDNA was Ade-N3. The Gua-N7 and Ade-N3 adducts were hydrolytically labile. Ade-N7 adduct could not be identified in the hydrolysates of ssDNA or dsDNA. The adduct Gua-N7,N7, which consists of two units of Gua bound together with a unit derived from chlorambucil, is a cross-linking adduct, and it was detected in the hydrolysates of ssDNA and dsDNA. Also several other adducts were detected which could be characterized by spiking with previously isolated authentic adducts or tentatively by MS. The role of chlorambucil-DNA adducts on the cytotoxicity and mutagenity of 1 is also discussed.

Characterization of adducts formed in reactions of acrolein with thymidine and calf thymus DNA

Acrolein, an important industrial chemical and environmental contaminant, has been shown to interact with nucleic acids in vitro and in vivo. In this study, we examined the reactivity of acrolein towards thymidine and calf-thymus double- and single-stranded DNA in aqueous buffered solutions. LC-MS Analyses of the reaction mixture of acrolein with thymidine showed the formation of five structurally different adducts. The structures of the products were determined on the basis of mass spectrometry, UV absorbance, and (1)H- and (13)C-NMR spectroscopy. The adducts were identified as 3-(3-oxopropyl)thymidine (dT1), 3-[(tetrahydro-2,4-dihydroxypyran-3-yl)methyl]thymidine (dT2), 2-(hydroxymethyl)-5-(thymidin-3-yl)pent-2-enal (dT3), 3-hydroxy-2-methylidene-5-(thymidin-3-yl)pentanal (dT4), and 2-[(thymidin-3-yl)methyl]penta-2,4-dienal (dT5). The adducts dT2-dT5 were formed in reaction of dT1 with acrolein. In the reaction of acrolein with calf-thymus DNA, dT1 was the only adduct detected in the DNA hydrolysate.

Reactions of malonaldehyde and acetaldehyde with calf thymus DNA: formation of conjugate adducts

Our previous work has shown that treatment of nucleosides with malonaldehyde simultaneously with acetaldehyde affords stable conjugate adducts. In the present study we demonstrate that conjugate adducts are also formed in calf thymus DNA when incubated with the aldehydes. The adducts were identified in the DNA hydrolysates by their positive ion electrospray MS/MS spectra, by coelution with the 2'-deoxynucleoside standards, and, in the case of adducts exhibiting fluorescent properties, also by LC using a fluorescence detector. In the hydrolysates of double-stranded DNA (ds DNA), two deoxyguanosine and two deoxyadenosine conjugate adducts were detected and in single-stranded DNA (ss DNA) also, the deoxycytidine conjugate adduct was observed. The guanine base was the major target for the malonaldehyde-acetaldehyde conjugates and 2'-deoxyguanosine adducts were produced in ds DNA at levels of 100-500 adducts/10(5) nucleotides (0.7-3 nmol/mg DNA).

Synthesis and characterization of DNA containing an N1-2'-deoxyinosine-ethyl-N3-thymidine interstrand cross-link: a structural mimic of the cross-link formed by 1,3-bis-(2-chloroethyl)-1-nitrosourea

Interstrand cross-links, which are generated by chemotherapeutic treatment with bis-alkylating agents, exert their therapeutic effect by connecting the nucleobases of adjacent DNA strands together and represent some of the most threatening forms of damage suffered by genomic DNA. However, one of the reasons for treatment failure using these agents is due to enhanced repair of this DNA damage. The pursuit of understanding the repair of interstrand cross-links by repair systems has necessitated the synthesis of sufficient quantities of such damaged DNA. We report the synthesis of a site-specific interstrand cross-linked duplex containing an ethylene-bridged N (1)-2'-deoxyinosine- N (3)-thymidine base pair prepared by solution and solid-phase synthesis as a mimic for the lesion formed by the therapeutic agent 1,3-bis-(2-chloroethyl)-1-nitrosourea using both a phosphoramidite and a bis-phosphoramidite approach. UV thermal denaturation experiments revealed that this cross-linked duplex was stabilized by 52 degrees C relative to the noncross-linked control, and circular dichroism studies indicated little deviation from a B-form structure compared to a duplex that contained a G-C base pair at the same position. Molecular models of the cross-linked duplex that were geometry optimized using the AMBER forcefield also suggest that this lesion induces minimal distortion in B-form DNA. This modified oligonucleotide will be useful for studies related to the investigation of interstrand cross-linked DNA repair.

Reaction of glycidamide with 2'-deoxyadenosine and 2'-deoxyguanosine--mechanism for the amide hydrolysis

2'-Deoxyadenosine (dA) and 2'-deoxyguanosine (dG) were reacted with mutagenic epoxide glycidamide (GA, Scheme 1). The reactions yielded three GA-dA adducts (N1-GA-dA, N6-GA-dA and N1-GA-dI) and two GA-dG adducts (N1-GA-dG I and N1-GA-dG II) (Scheme 2). The structures of the adducts were characterized by spectroscopic and spectrometric methods (1H-, 13C, and 2D NMR, MS, UV). The mechanism of the amide hydrolysis taking place during formation of the adducts N1-GA-dA and N1-GA-dG I was studied. We propose a mechanism where a transamidation is the key step in the hydrolysis of the amide function of GA.

The metabolism and molecular toxicology of chloroprene

Chloroprene (2-chloro-1,3-butadiene, 1) is oxidised by cytochrome P450 enzymes in mammalian liver microsomes to several metabolites, some of which are reactive towards DNA and are mutagenic. Much less of the metabolite (1-chloroethenyl)oxirane (2a/2b) was formed by human liver microsomes compared with microsomes from Sprague-Dawley rats and B6C3F1 mice. Epoxide (2a/2b) was a substrate for mammalian microsomal epoxide hydrolases, which showed preferential hydrolysis of the (S)-enantiomer (2b). The metabolite 2-chloro-2-ethenyloxirane (3a/3b) was rapidly hydrolysed to 1-hydroxybut-3-en-2-one (4) and in competing processes rearranged to 1-chlorobut-3-en-2-one (5) and 2-chlorobut-3-en-1-al (6). The latter compound isomerised to (Z)-2-chlorobut-2-en-1-al (7). In microsomal preparations from human, rat and mouse liver, compounds 4, 5 and 7 were conjugated by glutathione both in the absence and presence of glutathione transferases. There was no evidence for the formation of a chloroprene diepoxide metabolite in any of the microsomal systems. The major adducts from the reaction of (1-chloroethenyl)oxirane (2a/2b) with calf thymus DNA were identified as N7-(3-chloro-2-hydroxy-3-buten-1-yl)-guanine (20) and N3-(3-chloro-2-hydroxy-3-buten-1-yl)-2'-deoxyuridine (23), with the latter being derived by alkylation at N-3 of 2'-deoxycytidine, followed by deamination. Adducts in DNA were identified by comparison with those derived from individual deoxyribonucleosides. The metabolite (Z)-2-chlorobut-2-en-1-al (7) formed principally two adducts with 2'-deoxyadenosine which were identified as a pair of diastereoisomers of 3-(2'-deoxy-beta-d-ribofuranosyl)-7-(1-hydroxyethyl)-3H-imidazo[2,1-i]purine (25). The chlorine atom of chloroprene thus leads to different intoxication and detoxication profiles compared with those for butadiene and isoprene. The results infer that in vivo oxidations of chloroprene catalysed by cytochrome P450 are more important in rodents, whereas hydrolytic processes catalysed by epoxide hydrolases are more pronounced in humans. The reactivity of chloroprene metabolites towards DNA is important for the toxicology of chloroprene, especially when detoxication is incomplete.

Analyses of (1-chloroethenyl)oxirane headspace and hemoglobin N-valine adducts in erythrocytes indicate selective detoxification of (1-chloroethenyl)oxirane enantiomers

Chloroprene (2-chloro-1,3-butadiene, CAS 126-99-8, CP) is a colorless volatile liquid used in manufacture of polychloroprene, a synthetic rubber polymer. National Toxicology Program inhalation studies of CP in rats and mice gave clear evidence of carcinogenic activity. CP is metabolized by CYP2E1 to electrophilic epoxides, including R- and S-(1-chloroethenyl)oxirane (CEO), which form adducts with nucleic acids and other nucleophiles including glutathione and hemoglobin. As detection of these epoxide metabolites in vivo is technically challenging, measurements of CEO-Hb adducts may provide biomarkers of exposure to bioactivated metabolites of CP. The present studies involved exposure of C57BL/6 mouse erythrocytes (RBC) in vitro to pure enantiomers of CEO. Headspace analysis of CEO using Cyclodex-B capillary GC/MS with selected ion monitoring enabled separation, specific detection, and quantification of CEO enantiomers as reactions proceeded in vitro with RBC. These analyses indicated that R-CEO was much more persistent when incubated in vitro with RBC, while S-CEO disappeared rapidly. After periods of exposure of RBC to various concentrations of R- or S-CEO, erythrocytes were lysed and globin isolated. Covalent adducts, formed by reaction of CEO with N-terminal valine in Hb, were analyzed following Edman cleavage and trimethylsilylation. SIM-GC/MS analyses using a 5%-phenyl-dimethylsiloxane capillary column enabled quantification of CEO-Hb adducts. These analyses produced two chromatographic peaks of CEO-valine adduct derivatives, which were tentatively identified from mass spectra, reaction, and abundance data to be 1-(3-chloro-2-trimethylsilyloxybut-3-en-1-yl)-5-isopropyl-3-phenyl-2-thiohydantoin and 1-[2-chloro-1-(trimethylsilyloxymethyl)prop-2-en-1-yl]-5-isopropyl-3-phenyl-2-thiohydantoin. Analyses quantified significantly greater levels of adducts formed from R-CEO than from S-CEO. Studies involving pretreatment of RBC with glutathione-depleting diethyl maleate diminished the selective detoxification of S-CEO, and suggest enantiomeric selectivity of mouse glutathione-S-transferase as a mechanism of differential detoxification of CEO enantiomers. These results indicate more rapid detoxification of S-CEO by mouse RBC in vitro, while R-CEO may persist to react with cellular nucleophiles.

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.

Evaluation of genetic alterations in cancer-related genes in lung and brain tumors from B6C3F1 mice exposed to 1,3-butadiene or chloroprene

1,3-Butadiene and chloroprene are multisite carcinogens in B6C3F1 mice with the strongest tumor response being the induction of lung neoplasms in females. Incidence of brain tumors in mice exposed to 1,3-butadiene was equivocal. This article reviews the efforts of our laboratory and others to uncover the mechanisms of butadiene and chloroprene induced lung and brain tumor responses in the B6C3F1 mouse. The formation of lung tumors by these chemicals involved mutations in the K-ras cancer gene and loss of heterozygosity in the region of K-ras on distal chromosome 6, while alterations in p53 and p16 were implicated in brain tumorigenesis.

Reaction of epichlorohydrin with adenosine, 2'-deoxyadenosine and calf thymus DNA: identification of adducts

Epichlorohydrin (a probable human carcinogen) was allowed to react with adenosine and the adducts were characterized by NMR and UV spectroscopy, and mass spectrometry. The adduct initially formed was 1-(3-chloro-2-hydroxypropyl)-adenosine, which subsequently ring closures to 1,N(6)-(2-hydroxypropyl)-adenosine at neutral and basic conditions. At acid conditions, the N-1 adduct undergoes a slow deamination to yield 1-(3-chloro-2-hydroxypropyl)-inosine. Minor adducts identified were 7-(3-chloro-2-hydroxypropyl)-adenosine and 3-(3-chloro-2-hydroxypropyl)-adenosine which are easily deglycosylated, and an adduct where the epichlorohydrin residue was attached to the sugar moiety of adenosine. A diadduct, 1,N(6)-(2-hydroxypropyl)-N(6)-(3-chloro-2-hydroxypropyl)-adenosine was also identified. The reaction of epichlorohydrin with calf thymus DNA gave 1,N(6)-(2-hydroxypropyl)-deoxyadenosine and 3-(3-chloro-2-hydroxypropyl)-adenine (major adduct).

Chemically induced renal tubule tumors in the laboratory rat and mouse: review of the NCI/NTP database and categorization of renal carcinogens based on mechanistic information

The incidence of renal tubule carcinogenesis in male and female rats or mice with 69 chemicals from the 513 bioassays conducted to date by the NCI/NTP has been collated, the chemicals categorized, and the relationship between carcinogenesis and renal tubule hyperplasia and exacerbation of the spontaneous, age-related rodent disease chronic progressive nephropathy (CPN) examined. Where information on mechanism or mode of action exists, the chemicals have been categorized based on their ability to directly or indirectly interact with renal DNA, or on their activity via epigenetic pathways involving either direct or indirect cytotoxicity with regenerative hyperplasia, or exacerbation of CPN. Nine chemicals were identified as directly interacting with DNA, with six of these producing renal tubule tumors at high incidence in rats of both sexes, and in some cases also in mice. Ochratoxin A was the most potent compound in this group, producing a high tumor incidence at very low doses, often with metastasis. Three chemicals were discussed in the context of indirect DNA damage mediated by an oxidative free radical mechanism, one of these being from the NTP database. A third category included four chemicals that had the potential to cause DNA damage following conjugation with glutathione and subsequent enzymatic activation to a reactive species, usually a thiol-containing entity. Two chemicals were allocated into the category involving a direct cytotoxic action on the renal tubule followed by sustained compensatory cell proliferation, while nine were included in a group where the cell loss and sustained increase in renal tubule cell turnover were dependent on lysosomal accumulation of the male rat-specific protein, alpha2mu-globulin. In a sixth category, morphologic evidence on two chemicals indicated that the renal tumors were a consequence of exacerbated CPN. For the remaining chemicals, there were no pertinent data enabling assignment to a mechanistic category. Accordingly, these chemicals, acting through an as yet unknown mechanism, were grouped as either being associated with an enhancement of CPN (category 7, 16 chemicals), or not associated with enhanced CPN (category 8, 4 chemicals). A ninth category dealt with 11 chemicals that were regarded as producing increases in renal tubule tumors that did not reach statistical significance. A 10th category discussed 6 chemicals that induced renal tumors in mice but not in rats, plus 8 chemicals that produced a low incidence of renal tubule tumors in mice that did not reach statistical significance. As more mechanistic data are generated, some chemicals will inevitably be placed in different groups, particularly those from categories 7 and 8. A large number of chemicals in the series exacerbated CPN, but those in category 7 especially may be candidates for inclusion in category 6 when further information is gleaned from the relevant NTP studies. Also, new data on specific chemicals will probably expand category 5 as cytotoxicity and cell regeneration are identified as obligatory steps in renal carcinogenesis in more cases. Additional confirmatory outcomes arising from this review are that metastases from renal tubule tumors, while encountered with chemicals causing DNA damage, are rare with those acting through an epigenetic pathway, with the exception being fumonisin B1; that male rats and mice are generally more susceptible than female rats and mice to chemical induction of renal tubule tumors; and that a background of atypical tubule hyperplasia is a useful indicator reflecting a chemically associated renal tubule tumor response. With respect to renal tubule tumors and human risk assessment, chemicals in categories 1 and 2, and possibly 3, would currently be judged by linear default methods; chemicals in category 4 (and probably some in category 3) as exhibiting a threshold of activity warranting the benchmark approach; and those in categories 5 and 6 as representing mechanisms that have no relevance for extrapolation to humans.

Kinetic Modeling of β-Chloroprene Metabolism: II. The Application of Physiologically Based Modeling for Cancer Dose Response Analysis Portions of this research were conducted at the National Health and Environmental Effects Laboratory (NHEERL). The research in this article has been reviewed by NHEERL and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the agency, nor does mention of a trade name or commercial products constitute endorsement or recommendation for use.2Data for 2002 from International Institute of Synthetic Rubber Producers, Houston, TX

beta-Chloroprene (2-chloro-1,3-butadiene; CD), which is used in the synthesis of polychloroprene, caused significant incidences of several tumor types in B6C3F1 mice and Fischer rats, but not in Wistar rats or Syrian hamsters. This project investigates the relevance of the bioassay lung tumor findings to human health risk by developing a physiologically based toxicokinetic (PBTK) model and exploring a tissue specific exposure-dose-response relationship. Key steps included identification of the plausible genotoxic mode of action, experimental quantification of tissue-to-air partition coefficients, scaling of in vitro parameters of CD metabolism for input into the PBTK model, comparing the model with in vivo experimental gas uptake data, selecting an appropriate tissue dosimetric, and predicting a corresponding human exposure concentration. The total daily milligram amount of CD metabolized per gram of lung was compared with the animal bioassay response data, specifically combined bronchiolar adenoma/carcinoma. The faster rate of metabolism in mouse lung agreed with the markedly greater incidence of lung tumors compared with the other rodent species. A lung tissue dose was predicted for the combined rodent lung tumor bioassay data at a 10% benchmark response. A human version of the PBTK model predicted that the lung tissue dose in humans would be equivalent to continuous lifetime daily exposure of 23 ppm CD. PBTK model sensitivity analysis indicated greater dependence of model predictions of dosimetry on physiological than biochemical parameters. The combined analysis of lung tumor response across species using the PBTK-derived internal dose provides an improved alternative to default pharmacokinetic interspecies adjustments for application to human health risk assessment.