Nucleophilic selectivity as a determinant of carcinogenic potency (TD50) in rodents: a comparison of mono- and bi-functional alkylating agents and vinyl chloride metabolites

A new LC-MS/MS method for the quantification of endogenous and vinyl chloride-induced 7-(2-Oxoethyl)guanine in sprague-dawley rats

Vinyl chloride (VC) is an industrial chemical that is known to be carcinogenic to animals and humans. VC primarily induces hepatic angiosarcomas following high exposures (≥50 ppm). VC is also found in Superfund sites at ppb concentrations as a result of microbial metabolism of trichloroethylene and perchloroethylene. Here, we report a new sensitive LC-MS/MS method to analyze the major DNA adduct formed by VC, 7-(2-oxoethylguanine) (7-OEG). We used this method to analyze tissue DNA from both adult and weanling rats exposed to 1100 ppm [(13)C(2)]-VC for 5 days. After neutral thermal hydrolysis, 7-OEG was derivatized with O-t-butyl hydroxylamine to an oxime adduct, followed by LC-MS/MS analysis. The limit of detection was 1 fmol, and the limit of quantitation was 1.5 fmol on the column. The use of stable isotope VC allowed us to demonstrate for the first time that endogenous 7-OEG was present in tissue DNA. We hypothesized that endogenous 7-OEG was formed from lipid peroxidation and demonstrated the formation of [(13)C(2)]-7-OEG from the reaction of calf thymus DNA with [(13)C(18)]-ethyl linoleate (EtLa) under peroxidizing conditions. The concentrations of endogenous 7-OEG in liver, lung, kidney, spleen, testis, and brain DNA from adult and weanling rats typically ranged from 1.0 to 10.0 adducts per 10(6) guanine. The exogenous 7-OEG in liver DNA from adult rats exposed to 1100 ppm [(13)C(2)]-VC for 5 days was 104.0 ± 23.0 adducts per 10(6) guanine (n = 4), while concentrations in other tissues ranged from 1.0 to 39.0 adducts per 10(6) guanine (n = 4). Although endogenous concentrations of 7-OEG in tissues in weanling rats were similar to those of adult rats, exogenous [(13)C(2)]-7-OEG concentrations were higher in weanlings, averaging 300 adducts per 10(6) guanine in liver. Studies on the persistence of [(13)C(2)]-7-OEG in adult rats sacrificed 2, 4, and 8 weeks postexposure to [(13)C(2)]-VC demonstrated a half-life of 7-OEG of 4 days in both liver and lung.

Vinyl Chloride—A Classical Industrial Toxicant of New Interest

The carcinogenicity of vinyl chloride in humans was recognized in 1974 based on observations of hepatic angiosarcomas in highly exposed workers. A multiplicity of endpoints has been demonstrated. The primary target organ, the liver, displays differential susceptibilities of hepatocytes and sinusoidal cells, which are modified by factors of age and dose. There is consistency in organotropism between experimental animals and humans. Vinyl chloride is a pluripotent carcinogen, predominantly directed toward hepatic endothelial (sinusoidal) cells, and second toward the parenchymal cells of the liver. The similarity of results between experimental animals and humans is a solid basis of an amalgamation of experimental and epidemiological risk estimates. Vinyl chloride requires metabolic activation for carcinogenicity and mutagenicity, and toxicokinetics are a key to interpret the dose response. Practically the entire initial metabolism of vinyl chloride is oxidative. At higher exposure concentrations this is nonlinear, and metabolic saturation of metabolism in rats is reached at about 250 ppm. This is consistent with the plateau of hepatic angiosarcoma incidence in rat bioassays. Physiologically based pharmacokinetic/toxicokinetic (PBPK) models have been developed and successfully applied within the frame of human cancer risk assessments. The major DNA adduct induced by vinyl chloride (approximately 98% of total adducts in rats), 7-(2-oxoethyl)guanine, is almost devoid of promutagenic activity. The clearly promutagenic "etheno" adducts N2,3-ethenoguanine and 3,N4-ethenocytosine each represent approximately 1% of the vinyl chloride DNA adducts in rats, and 1,N6-ethenoadenine is found at even lower concentrations. Etheno adducts appear to have a long persistence and are repaired by glycosylases. Vinyl chloride represents a human carcinogen for which a series of mechanistic events connects exposure with the carcinogenic outcome. These include (1) metabolic activation (to form chloroethylene oxide), (2) DNA binding of the reactive metabolite (to exocyclic etheno adducts), (3) promutagenicity of these adducts, and (4) effects of such mutations on protooncogenes/tumor suppressor genes at the gene and gene product levels. In rat hepatocytes, a further event is a biomarker response. Cancer prestages (enzyme-altered foci), as quantitative biomarkers, provide a tool to study dose response even within low dose ranges where a carcinogenic risk cannot be seen in cancer bioassays directly. Such biomarker responses support a linear nonthreshold extrapolation for low-dose assessment of carcinogenic risks due to vinyl chloride. Published risk estimates based on different sets of data (animal experiments, epidemiological studies) appear basically consistent, and on this basis an angiosarcoma risk of approximately 3 x 10(-4) has been deduced by extrapolation, for exposure to 1 ppm vinyl chloride over an entire human working lifetime. An important point that should be considered in regulatory standard settings is the presence of a physiological background of those etheno DNA adducts, which are also produced by vinyl chloride. Likely reasons for this background are oxidative stress and lipid peroxidation. In essence, fundamentals of the hepatocarcinogenicity of vinyl chloride appear now well established, providing a solid scientific basis for regulatory activities.

Residues of genotoxic alkyl mesylates in mesylate salt drug substances: Real or imaginary problems?

Mesylate esters of short-chain (n = 1-3) alcohols are reactive, direct-acting, genotoxic and possibly carcinogenic alkylating agents. Their chemical and biological properties appear to correlate well with Swain-Scott s constants; for example, high S(N)1 character (low s value) is associated with enhanced carcinogenic potential, but also a rapid hydrolysis rate. Concerns over the possible formation of such esters during the preparation of mesylate salt drug substances, by addition of methane sulfonic acid (MSA) to the free base dissolved in an alcoholic solvent, have led regulatory agencies to require applicants to demonstrate that the synthetic method employed does not lead to the presence of detectable levels of alkyl mesylates. Mechanistic considerations, relating mainly to the extremely low nucleophilicity of the mesylate anion, and experimental data, both indicate that alkyl mesylates should not be formed (except from MSA impurities) during mesylate salt synthesis. Mechanistic arguments also predict that residues of alkyl halides (possibly formed in the preparation of amine hydrochlorides or hydrobromides) could represent a similar or greater potential hazard than alkyl mesylates. The perceived risk of alkyl mesylate formation seems to rely on mistaken assumptions and so the concerns appear unjustified. Further reassurance could be achieved however by applying a variety of strategies during synthesis, including pH control, and use of high-purity MSA or of a non-hydroxylic reaction solvent.

Reaction-kinetic parameters of glycidamide as determinants of mutagenic potency

Values for reaction-kinetic parameters of electrophiles can be used to predict mutagenic potency. One approach employs the Swain-Scott relationship for comparative kinetic studies of electrophilic agents reacting with nucleophiles. In this way glycidamide (GA), the putatively mutagenic/carcinogenic metabolite of acrylamide, was assessed by determining the rates of reaction with different nucleophiles. The rate constants (kNu) were determined using the "supernucleophile" cob(I)alamin [Cbl(I)] as an analytical tool. The Swain-Scott parameters for GA were compared with those of ethylene oxide (EO). The substrate constants, s values, for GA and for EO were found to be 1.0 and 0.93, respectively. The reaction rates at low values of nucleophilic strength (n=1-3), corresponding to oxygens in DNA, were determined to be 2-3.5 times higher for GA compared to EO. GA was also more reactive than EO towards other nucleophiles (n=0-6.4). The mutagenic potency of GA was determined in Chinese hamster ovary cells (hprt mutations in CHO-AA8 cells per dose unit with gamma-radiation as reference standard). The potency of GA was estimated to be about three mutations per 10(5) cells and mMh corresponding to about 40 rad-equ./mMh. A preliminary comparison of the mutagenic potency (per mMh and as rad-equivalents) of GA and EO shows an approximately seven times higher potency for GA. A higher mutagenic potency of GA compared to EO is compatible with expectation from reaction-kinetic data of the two compounds. The data confirmed that GA is not a strong mutagen, which is in line with what is expected for simple oxiranes. The present study shows the value of cob(I)alamin for the determination of reaction-kinetic parameters and their use for prediction of mutagenic potency.

Modified immunoenriched (32)P-HPLC assay for the detection of O(4)-ethylthymidine in human biomonitoring studies

Increased excretion of ethylated DNA bases has been reported in the urine of cigarette smokers. To study DNA ethylation in the target organs of smokers, an immunoenriched (32)P-postlabeling assay for O(4)-ethylthymidine (O(4)-etT) was developed. O(4)-etT-3'-monophosphate (O(4)-etT-3'P) was synthesized, purified, and characterized by LC-MS, ESI-MS, and NMR. DNA was enzymatically digested to 2'-deoxynucleoside-3'-monophosphate followed by immunoprecipitation of O(4)-etT-3'P using specific monoclonal antibodies. The immunoconjugate was washed by filtration, and O(4)-etT-3'P was recovered by ethanol treatment. The enriched O(4)-etT-3'P was labeled with [gamma-(32)P]ATP in the presence of T4-polynucleotide kinase at pH 6.8 to yield its 5'-labeled monophosphate and was subsequently resolved on RP-HPLC and detected with online detection of radioactivity. Adduct recovery was >80%, and the detection limit was approximately 500 amol. To further validate the method, O(4)-etT levels were determined in calf thymus DNA treated with N-ethyl-N-nitrosourea, and a dose-dependent formation of O(4)-etT was observed. Furthermore, O(4)-etT was found to be present in the cells obtained from the lower respiratory tract by sputum induction of two out of four smokers but not in three nonsmokers. O(4)-etT is a poorly repaired promutagenic DNA lesion; thus, it could be of potential use for biomonitoring smoking-related DNA damage. Our improved assay was found to be sufficiently sensitive and specific to detect O(4)-etT in surrogate cells from cigarette smoke exposed humans.

QSAR of the inhibition of angiogenesis by TNP-470 and ovalicin analogues: another example of an allosteric interaction

QSAR have been formulated for variations of TNP-470 and Ovalicin on various cell lines. In the examples of mouse lymphocyte cells and bovine endothelial cells the results suggest an allosteric interaction. These results are compared with the binding of nitrobenzene to hemoglobin in rats in vivo. Such a reaction does not occur with methionine aminopeptidase.

The importance of distinct metabolites of N-nitrosodiethylamine for its in vivo mutagenic specificity

Although N-nitrosodiethylamine (NDEA) is a potent carcinogen in rodents and a probable human carcinogen, little attempts were made to characterize its mutation spectrum in higher eukaryotes. We have compared forward mutation frequencies at multiple (700) loci with the mutational spectrum induced at the vermilion gene of Drosophila, after exposure of post- and pre-meiotic male germ cells to NDEA. Among 30 vermilion mutants collected from post-meiotic stages were 12 G:C-->A:T transitions (40%), 8 A:T-->T:A transversions (27%), and 4 structural rearrangements (13%). The remainder were three A:T-->G:C transitions, two G:C-->C:G transversions and one G:C-->T:A transversion. The results show that although NDEA induces predominantly transitions (40% G:C-->A:T and 10% A:T-->G:C), the frequencies of transversions (37%, of which 27% of A:T-->T:A transversions) and especially of rearrangements (13%) are remarkably high. This mutation spectrum differs significantly from that produced by the direct-ethylating agent N-ethylnitrosourea (ENU), although the relative distribution of ethylated DNA adducts is similar for both carcinogens. These differences, in particular the occurrence of rearrangements, are most likely the result of the requirement of NDEA for bioactivation. Since all four rearrangements were collected from non-metabolizing spermatozoa (or late spermatids), it is hypothesized that they derived from acetaldehyde, a stable metabolite of NDEA. Due to its cytotoxicity, attempts to isolate vermilion mutants from NDEA-exposed pre-meiotic cells were largely unsuccessful, because only two mutants (one A:T-->G:C transition and one 1bp insertion) were collected from those stages. Our results show that NDEA is capable of generating carcinogenic lesions other than base pair substitutions.

Genetic damage by bifunctional agents in repair-active pre-meiotic stages of Drosophila males

Most of our understanding of germline mutagenesis in Drosophila is based on the DNA repair-inactive, haploid post-meiotic stages. The diploid, repair-active pre-meiotic stages are more relevant to the situation encountered in somatic cells. DNA mono-adducts induced by agents like methyl methanesulphonate (MMS) and ethylene oxide (EO) are well repaired in the pre-meiotic cell stages, and these agents show therefore, no or considerable lower mutagenic activity in these stages. In contrast, in this study the two bifunctional nitrogen mustards chlorambucil (CAB) and mechlorethamine (MEC) show significantly elevated mutant frequencies of both post- and pre-meiotic germ cells. Results were similar for the X-chromosomal and the autosomal (2nd) recessive lethal (RL) test. CAB and MEC were also active in stem cells, but in comparison with post-stem cell stages they seem to be better protected. The germ cell specific response in post- and pre-meiotic cell stages was for both nitrogen mustards comparable to mutagenic activity patterns observed in the specific locus test in the mouse. It was reported that for diepoxybutane (DEB), another cross-linking agent, the ratio of the RL frequency for the 2nd- and the X-chromosome was increased from 2.1 for post-meiotic stages to 9.5 for pre-meiotic stages. In own experiments aiming to confirm this observation, a high ratio was indeed found. The induction of large deletions by DEB could be the reason for this difference, since such lesions might include both a sex-linked lethal and a vital gene required for the development of spermatocytes into mature sperm. Similar differences were expected for CAB and MEC since they are also inducers of large deletions. But unexpectedly, no differences in 2nd/X RL ratio between post- and pre-meiotic cell stages were found for the nitrogen mustards. Possible causes such as distinct proportions of multi-locus deletions (MLDs), mitotic recombination and the formation of persistent lesions, are discussed.

Biological effect markers for exposure to carcinogenic compound and their relevance for risk assessment

In this review data are summarized on biomarkers that are used for biological effect monitoring of human populations exposed to genotoxic carcinogens. The biomarkers are DNA and protein adducts and cytogenetic effects. Most of these biomarkers are relevant for the process of carcinogenesis. Emphasis is on providing information on the properties of the biomarkers and on their relevance for predicting cancer risk. Overviews are presented of: (1) studies on effects of exposure in target tissues of human origin obtained by surgical biopsies or autopsies, (2) epidemiological studies on healthy (cancer-free) individuals, correlating the putative occupational, lifestyle or environmental exposure with increased levels of biomarkers in blood cells, and (3) studies with animal models on the relation between biomarkers and cancer. Finally, on the basis of epidemiological data the possibilities were explored to use biomarker data to estimate the risk of death due to cancer. For several biomarkers the increment of the cancer mortality risk was calculated on the basis of a lifetime doubling of the biomarker level.

Heritable and cancer risks of exposures to anticancer drugs: inter-species comparisons of covalent deoxyribonucleic acid-binding agents

In the past years, several methodologies were developed for potency ranking of genotoxic carcinogens and germ cell mutagens. In this paper, we analyzed six sub-classes of covalent deoxyribonucleic acid (DNA) binding antineoplastic drugs comprising a total of 37 chemicals and, in addition, four alkyl-epoxides, using four approaches for the ranking of genotoxic agents on a potency scale: the EPA/IARC genetic activity profile (GAP) database, the ICPEMC agent score system, and the analysis of qualitative and quantitative structure-activity and activity-activity relationships (SARs, AARs) between types of DNA modifications and genotoxic endpoints. Considerations of SARs and AARs focused entirely on in vivo data for mutagenicity in male germ cells (mouse, Drosophila), carcinogenicity (TD50s) and acute toxicity (LD50s) in rodents, whereas the former two approaches combined the entire database on in vivo and in vitro mutagenicity tests. The analysis shows that the understanding and prediction of rank positions of individual genotoxic agents requires information on their mechanism of action. Based on SARs and AARs, the covalent DNA binding antineoplastic drugs can be divided into three categories. Category 1 comprises mono-functional alkylating agents that primarily react with N7 and N3 moieties of purines in DNA. Efficient DNA repair is the major protective mechanism for their low and often not measurable genotoxic effects in repair-competent germ cells, and the need of high exposure doses for tumor induction in rodents. Due to cell type related differences in the efficiency of DNA repair, a strong target cell specificity in various species regarding the potency of these agents for adverse effects is found. Three of the four evaluation systems rank category 1 agents lower than those of the other two categories. Category 2 type mutagens produce O-alkyl adducts in DNA in addition to N-alkyl adducts. In general, certain O-alkyl DNA adducts appear to be slowly repaired, or even not at all, which make this kind of agents potent carcinogens and germ cell mutagens. Especially the inefficient repair of O-alkyl-pyrimidines causes the high mutational response of cells to these agents. Agents of this category give high potency scores in all four expert systems. The major determinant for the high rank positions on any scale of genotoxic of category 3 agents is their ability to induce primarily structural chromosomal changes. These agents are able to cross-link DNA. Their high intrinsic genotoxic potency appears to be related to the number of DNA cross-links per target dose unit they can induce. A confounding factor among category 3 agents is that often the genotoxic endpoints occur close to or at toxic levels, and that the width of the mutagenic dose range, i.e., the dose area between the lowest observed effect level and the LD50, is smaller (usually no more than 1 logarithmic unit) than for chemicals of the other two categories. For all three categories of genotoxic agents, strong correlations are observed between their carcinogenic potency, acute toxicity and germ cell specificity.

A nine-year follow up study of a population occupationally exposed to vinyl chloride monomer

The presence of structural chromosome aberrations and sister chromatid exchange frequencies (SCE) were studied in lymphocytes from 28 male subjects occupationally exposed to vinyl chloride monomer for a period of 9 years. A significant increase in chromosomal damages and elevated SCE frequencies was detected during the third and fourth year of the follow-up study. During the last 2 years, all examined parameters are approaching control values. This is a result of decreasing vinyl chloride monomer concentrations in the working environment without interrupting the working process.

DNA damage and repair in mutagenesis and carcinogenesis: implications of structure-activity relationships for cross-species extrapolation

Previous studies on structure-activity relationships (SARs) between types of DNA modifications and tumour incidence revealed linear positive relationships between the log TD50 estimates and s-values for a series of mostly monofunctional alkylating agents. The overall objective of this STEP project was to further elucidate the mechanistic principles underlying these correlations, because detailed knowledge on mechanisms underlying the formation of genotoxic damage is an absolute necessity for establishing guidance values for exposures to genotoxic agents. The analysis included: (1) the re-calculation and further extension of TD50 values in mmol/kg body weight for chemicals carcinogenic in rodents. This part further included the checking up data for Swain-Scott s-values and the use of the covalent binding index (CBI); (2) the elaboration of genetic toxicity including an analysis of induced mutation spectra in specific genes at the DNA level, i.e., the vermilion gene of Drosophila, a plasmid system (pX2 assay) and the HPRT gene in cultured mammalian cells (CHO-9); and (3) the measurement of specific DNA alkylation adducts in animal models (mouse, rat, hamster) and mammalian cells in culture. The analysis of mechanisms controlling the expression of mammalian DNA repair genes (alkyltransferases, glycosylases) as a function of the cell type, differentiation stage, and cellular microenvironment in mammalian cells. The 3 classes of genotoxic carcinogens selected for the project were: (1) chemicals forming monoalkyl adducts upon interaction with DNA; (2) genotoxins capable of forming DNA etheno-adducts; and (3) N-substituted aryl compounds forming covalent adducts at the C8 position of guanine in DNA. In general, clear SARs and AARs (activity-activity relationships) between physiochemical parameters (s-values, O6/N7-alkylguanine ratios, CBI), carcinogenic potency in rodents and several descriptors of genotoxic activity in germ cells (mouse, Drosophila) became apparent when the following descriptors were used: TD50 estimates (lifetime doses expressed in mg/kg b.wt. or mmol/kg b.wt.) from cancer bioassays in rodents; the degree of germ-cell specificity, i.e., the ability of a genotoxic agent to induce mutations in practically all cell stages of the male germ-cell cycle of Drosophila (this project) and the mouse (literature search), as opposed to a more specific response in postmeiotic stages of both species; the Mexr-/Mexr+ hypermutability ratio, determined in a repair assay utilizing Drosophila germ cells; mutation spectra induced at single loci (the 7 loci used in the specific-locus test of the mouse (published data), and the vermilion gene of Drosophila); and doubling doses (DD) in mg/kg (mmol/kg) for specific locus test results on mice. By and large, the TD50 values, the inverse of which can be considered as measures of carcinogenic potency, were shown to be predictable from knowledge of the in vivo doses associated with the absorbed amounts of the investigated alkylators and with the second-order constant, kc, reaction at a critical nucleophilic strength, nc. For alkylating agents kc can be expressed as the second-order rate constant for hydrolysis, kH2O, and the substrate constant s:kH2OTD50 is a function of a certain accumulated degree of alkylation, here given as the (average) daily increment, ac, for 2 years exposure of the rodents. The TD*50 in mmol/kg x day) could then be written: [formula: see text] This expression would be valid for monofunctional alkylators provided the reactive species are uncharged. This is the case for most SN2 reagents. Although it appears possible to predict carcinogenic potency from measured in vivo doses and from detailed knowledge of reaction-kinetic parameter values, it is at present not possible to quantify the uncertainty of such predictions. One main reason for this is the complication due to uneven distribution in the body, with effects on the dose in target tissues. The estimation can be impro

DNA adducts in human carcinogenesis: etiological relevance and structure-activity relationship

Sensitive methods for quantifying DNA adducts from (i) benzo[a]pyrene (BP), (ii) alkylation exposure, and (iii) etheno(epsilon)-DNA adduct-forming chemicals were developed and applied to humans and animal models. The aims were to identify hitherto unknown sources and mechanisms of exogenous and endogenous DNA damage, to examine the effect of drug polymorphism on BP adduct levels, and to develop QSAR between tumorigenic potency, heritable genetic damage and structural elements of alkylating carcinogens (Vogel and Nivard (1994) Mutation Res., 395, 13-32). (i) BP-DNA adducts: An HPLC/fluorimetry assay suitable for measuring (+)-anti-BP-diol-epoxide (BPDE) adducts in human tissues and white blood cells (WBC) was developed (Alexandrov et al. (1992) Cancer Res., 52, 6248-6253). In smokers, a positive correlation was found between pulmonary CYP1A1-related catalytic activity (AHH) and the level of lung BPDE-DNA adducts. In coke oven workers, an enhancing effect of smoking on BPDE-adduct levels in WBC was demonstrated (Rojas et al. (1995) Carcinogenesis, 16, 1373-1376). (ii) 3-Alkyladenines (3-alkAde): Alkylating carcinogens form 3-alkAde adducts in DNA which depurinate to yield 3-alkAde in urine, for which a detection method was developed (Friesen et al. (1991) Chem. Res. Toxicol., 4, 102-106; Prevost et al. (1990) Carcinogenesis, 11, 1747-1751), using immunoaffinity purification and GC-MS analysis. The usefulness of 3-alkAde analysis for the determination of the whole-body dose of alkylating agents derived from exogenous and endogenous sources was demonstrated. (iii) Etheno-DNA adduct-forming agents: Etheno(epsilon)-DNA base adducts (epsilon A, epsilon dC, epsilon dG) are promutagenic DNA lesions that are formed by occupational (vinyl halides) and environmental (urethane) carcinogens. An ultrasensitive detection method was developed (Nair et al. (1995) Carcinogenesis, 16, 613-617), based on immunoaffinity purification and 32P-postlabelling of epsilon-nucleoside 3'-monophosphates. Liver DNA from unexposed rats, mice and from human samples contained background levels of epsilon dA and epsilon dC (Bartsch et al. (1994) Drug. Metab. Rev., 26, 349-371). As formation of epsilon dA and epsilon dC adducts by lipid peroxidation products was demonstrated (El Ghissassi et al. (1995) Chem. Res. Toxicol., 8, 278-283), they may serve as markers for oxidative stress. Results from testing this hypothesis are presented.

N-acetylcysteine protects lymphocytes from nitrogen mustard-induced apoptosis

The ability of the antioxidant N-acetylcysteine to prevent apoptosis induced in lymphocytes by nitrogen mustard (HN2) was investigated. HN2 caused a concentration-dependent induction of apoptosis on C3H murine spleen cells, as identified by two criteria: morphological features revealed by microscopical observations and DNA fragmentation visualized by the characteristic "ladder" pattern observed upon agarose gel electrophoresis, as well as by hypodiploid DNA-containing cells revealed by the flow cytometric analysis of propidium iodide labelled cells. The antioxidant N-acetylcysteine (NAC) was found to markedly reduce the occurrence of HN2-induced apoptosis in these cells. This protective effect will still obtained when NAC was added 30 min after HN2. In contrast, the pretreatment of spleen cells with this antioxidant did not provide any significant protection. We also showed that lymphocytes protected by NAC are still able to respond to a mitogenic stimulation. To gain some insight into the mechanisms underlying the cytoprotective action of NAC against HN2, we tested whether or not poly(ADP-ribose) polymerase (PARP, EC 2.4.2.30), a nuclear enzyme that participates in the triggering of apoptosis induced by alkylating agents, is involved. We report that 6(5H)-phenanthridinone, a potent PARP inhibitor, did not affect the ability of NAC to prevent HN2-induced apoptosis under our experimental conditions. Thus, the exact mechanism by which NAC protects lymphocytes from HN2 cytotoxicity has yet to be determined.

Analysis of mitotic recombination induced by several mono- and bifunctional alkylating agents in the Drosophila wing-spot test

Mitotic recombination induced by six alkylating agents has been studied in the wing-spot test of Drosophila melanogaster. The model mutagens chosen have different models of action at the DNA level. These are: the direct-acting small alkylating agent methylmethanesulfonate (MMS), the small promutagens N-dimethylnitrosamine (DMN) and N-diethylnitrosamine (DEN), the bifunctional cross-linking alkylating agents mitomycin C (MMC), chlorambucil (CLA) and monocrotaline (MCT). Flies of the standard cross (flr3 / TM3, Bds females and mwh males) were used to produce the larvae to be treated. Three-day old Drosophila larvae were exposed by chronic feeding for 48 h to three different concentrations of all six alkylating agents. Acute feeding for only 2 h was used in addition with DEN and MMC. Wings of the marker-heterozygous (mwh+ / + flr3) as well as of the balancer-heterozygous (mwh+ / TM3, Bds) progeny were analysed. The ranking of the compounds with respect to their genotoxic potency, based on mwh clone formation frequency in marker-heterozygous wings was: MMS > MNC > DMN > CLA approximately MCT > DEN. The ranking with respect to the induction of twin spots, which are produced by mitotic recombination exclusively, was: MMS > DMN > MMC > MCT > CLA > DEN. The quantitative determination of recombinagenic activity, based on mwh clone formation frequencies obtained in both types of wings, gave the following values: MMS, 93%; MCT, 87%; CLA, 80%; MMC, 73%; DMN, 67%; DEN, 22%. A clear relationship exists between the extent of N-alkylation of DNA and the efficiency of the monofunctional agents MMS and DMN as well of the bifunctional agents MCT, CLA and MMC to induce mitotic recombination. This contrasts with the ethylation of base oxygen atoms and the resulting lower efficiency of DEN to produce mitotic recombination.

DNA damage and repair in somatic and germ cells in vivo

Alkylation-induced germ cell mutagenesis in the mouse versus Drosophila is compared based on data from forward mutation assays (specific-locus tests in the mouse and in Drosophila and multiple-locus assays in the latter species) but not including assays for structural chromosome aberrations. To facilitate comparisons between mouse and Drosophila, forward mutation test results have been grouped into three categories. Representatives of the first category are MMS (methyl methanesulfonate) and EO (ethylene oxide), alkylating agents with a high s value which predominantly react with ring nitrogens in DNA. ENU (N-ethyl-N-nitrosourea), MNU (N-methyl-N-nitrosourea), PRC (procarbazine), DEN (N-nitrosodiethylamine), and DMN (N-nitrosodimethylamine) belong to the second category. These agents have in common a considerable ability for modification at oxygens in DNA. Cross-linking agents (melphalan, chlorambucil, hexamethylphosphoramide) form the third category. The most unexpected, but encouraging outcome of this study is the identification of common features for three vastly different experimental indicators of genotoxicity: hereditary damage in Drosophila males, genetic damage in male mice, and tumors (TD50 estimates) in rodents. Based on the above three category classification scheme the following tentative conclusions are drawn. Monofunctional agents belonging to category 1, typified by MMS and EO, display genotoxic effects in male germ cell stages that have passed meiotic division. This phenomenon seems to be the consequence of a repair deficiency during spermiogenesis for a period of 3-4 days in Drosophila and 14 days in the mouse. We suggest that the reason for the high resistance of premeiotic stages, and the generally high TD50 estimates observed for this class in rodents, is the efficient error-free repair of N-alkylation damage. If we accept this hypothesis, then the increased carcinogenic potential in rodents, seen when comparing category 2 (ENU-type mutagens) to category 1 (MMS-type mutagens), along with the ability of category 2 genotoxins to induce genetic damage in premeiotic stages, must presumably be due to their enhanced ability for alkylations at oxygens in DNA; it is this property that actually distinguishes the two groups from each other. In contrast to category 1, examination of class 2 genotoxins (ENU and DEN) in premeiotic cells of Drosophila gave no indication for a significant role of germinal selection, and also removal by DNA repair was less dramatic compared to MMS. Thus category 2 mutagens are expected to display activity in a wide range of both post- and premeiotic germ cell stages. A number of these agents have been demonstrated to be among the most potent carcinogens in rodents.(ABSTRACT TRUNCATED AT 400 WORDS)

Relationship between locations of chromosome breaks induced by vinyl chloride monomer and lymphocytosis

The distribution of vinyl chloride monomer (VCM)-induced chromosome breaks was studied in cultured lymphocytes of subjects occupationally exposed to this gas. In the examined subjects, the mean group value of chromosome aberrations is 6.5% and for sister chromatid exchange (SCE) frequencies, the mean value per cell is 7.9. These values are significantly higher than in the control population. Occupational exposure to VCM caused lymphocytosis together with disturbances of mitogenic activity in lymphocytes stimulated by phytohaemagglutinin. The results of G-banding showed that sites of chromosome breakpoints caused by VCM can be related to the lymphatic tissue disorders.

Metabolism and genotoxicity of the halogenated alkyl compound tris(2,3-dibromopropyl)phosphate

1. The genotoxicity of the flame retardant tris(2,3-dibromopropyl)phosphate (Tris-BP) was studied in vivo. Results showed that Tris-BP was highly clastogenic, but it could only initiate a low number of preneoplastic foci in the rat liver in vivo. In Drosophila, Tris-BP could be classified as a cross-linking agent, because it was more clastogenic than mutagenic. The use of completely deuterated Tris-BP as a metabolic probe revealed that cytochrome P450 and most likely the formation of 2-bromoacrolein (2BA) from Tris-BP is important for the observed genotoxic effects. 2. In contrast to the high mutagenicity of Tris-BP and 2BA in Salmonella typhimurium, we were unable to detect an increase in mutation frequency of 2BA on the hprt locus of human TK6 cell line. In another system, using a shuttle vector modified with 2BA:DNA-adducts, also no increase in mutation frequency could be detected in human cells. This low mutagenicity of 2BA corresponds with its low mutagenicity in Drosophila and its low induction of preneoplastic foci in the rat liver. 3. Several DNA adducts of 2BA have been identified, including an unstable 3-(bromooxypropyl)thymidine adduct which has the potential to form cross-links and a cyclic 3,N4-(bromo)propeno-deoxycytidine adduct which can possibly be involved in the clastogenicity of Tris-BP. 4. Taken together, these data indicate that Tris-BP and 2BA may not effectively induce gene mutations in eukaryotic systems, but rather be potent clastogens. Risk assessment of these and related compounds should therefore be based on the knowledge of clastogens rather than mutagens.

The sensitivity of the micronucleus assay for the detection of occupational exposure to vinyl chloride monomer

The micronucleus assay was performed in the peripheral lymphocytes of 32 subjects occupationally exposed to vinyl chloride monomer (VCM) divided into two groups according to years of employment. Blood samples were taken in the period from 24 h to 90 days following a transitory exposure to elevated VCM concentrations of 300 ppm due to the technological process. In subjects with a longer period of employment micronucleus frequencies decreased in proportion to the length of the interval after the last exposure to VCM. The results confirm that the micronucleus assay can serve as a suitable indicator of the time elapsed after last exposure to elevated concentrations of environmental mutagen. It can be assumed that duration of employment may contribute to the occurrence of the cumulative effect produced by exposure to elevated VCM concentrations.

Alkylation of DNA by melphalan with special reference to adenine derivatives and adenine-guanine cross-linking

Alkylation of DNA by melphalan gave four principal products, derived by mono-alkylation of adenine at N-3 and guanine at N-7, and by cross-linking of adenine N-3 to guanine N-7, or of guanine N-7 to guanine N-7. Adenine-guanine cross-linking was unexpected because the two principal nucleophilic centres, N-7 of guanine and N-3 of adenine, are situated in the 'wide' and 'narrow' grooves of the DNA double helix, respectively. These products could be isolated by their hydrolysis from DNA at neutral pH, followed by chromatography of the hydrolysate in an ion-pair ODS system using a solvent containing tetrabutylammonium hydroxide as ion-pairing reagent; this gave better separation than the previously described method using SP-Sephadex.

Expression in mammalian cells of the Escherichia coli O6 alkylguanine-DNA-alkyltransferase gene ogt reduces the toxicity of alkylnitrosoureas

V79 Chinese hamster cells expressing either the O6-alkylguanine-DNA-alkyltransferase (ATase) encoded by the E. coli ogt gene or a truncated version of the E. coli ada gene have been exposed to various alkylnitrosoureas to investigate the contribution of ATase repairable lesions to the toxicity of these compounds. Both ATases are able to repair O6-alkylguanine (O6-AlkG) and O4-alkylthymine (O4-AlkT) but the ogt ATase is more efficient in the repair of O4-methylthymine (O4-MeT) and higher alkyl derivatives of O6-AlkG than is the ada ATase. Expression of the ogt ATase provided greater protection against the toxic effects of the alkylating agents then the ada ATase particularly with N-ethyl-N-nitrosourea (ENU) and N-butyl-N-nitrosourea (BNU) to which the ada ATase expressing cells were as sensitive as parent vector transfected cells. Although ogt was expressed at slightly higher levels than the truncated ada in the transfected cells, this could not account for the differential protection observed. For-N-methyl-N-nitrosourea (MNU) the increased protection in ogt-transfected cells is consistent with O4-MeT acting as a toxic lesion. For the longer chain alkylating agents and chloroethylating agents, the protection afforded by the ogt protein may be a consequence of the more efficient repair of O6-AlkG, O4-AlkT or both of these lesions in comparison with the ada-encoded ATase.

Mutagenicity to Salmonella, Drosophila and the mouse bone marrow of the human antineoplastic agent fotemustine: prediction of carcinogenic potency

The antineoplastic agent fotemustine is shown to be a base-pair mutagen to Salmonella. Activity is more marked in the uvrB-proficient strain G46 than in the repair-deficient strain TA1535. This is consistent with its ability to cross-link DNA. Potent activity as a somatic and germ-cell mutagen to Drosophila was also observed. A potent clastogenic response was given by fotemustine in the mouse bone marrow following either oral gavage or intraperitoneal injection of a single dose of 5 mg/kg. In each of these respects it is shown to be indistinguishable from the structurally related antineoplastic agent and human carcinogen MeCCNU. It is concluded that fotemustine should be regarded as having clear potential to induce cancer in humans. Based on these data, including the preponderance of chromosome breakages over recessive lethal mutations in Drosophila, an estimated rodent carcinogenic potency (TD50) of between 15-150 mg/kg is suggested for fotemustine.

Characterization of the genotoxic action of three structurally related 1,2-dihaloalkanes in Drosophila melanogaster

The genetic activity profiles of three structurally related dihaloalkanes, 1,2-dibromoethane (DBE), 1,2-dichloroethane (DCE) and 1-bromo-2-chloroethane (BCE), were compared in germ cells and somatic tissue of Drosophila melanogaster. The two genotoxicity indices estimated after mutagen exposure of male germ cells were (i) the hypermutability index fexr-/fexr+, measured by the increased frequency of induced recessive lethals (RL) in a strain defective in DNA excision repair (exr-), as compared to the wild type (exr+); (ii) the relative clastogenicity index CL/RL, expressed by the ratio of chromosomal aberrations (CL; ring-X loss) to RL determined in exr+ strains. The fexr-/fexr+ index for DBE was 4-5 times higher than those for DCE and BCE, suggesting a difference in the types of premutagenic lesions produced by DBE in comparison to DCE and BCE. The relative clastogenicity indices for BCE (CL/RL = 0.29) and DCE (0.41) are similar to the value of 0.37 estimated for DBE in an earlier study, all indicating that the three compounds or their metabolites are incapable of forming DNA crosslinks. In somatic cells, after inhalation treatment of female larvae, the effectiveness for the induction of interchromosomal recombination decreased in the order BCE > or = DBE > DCE. It is concluded that in accordance with other studies also in Drosophila the glutathione-mediated pathway is the major cause of genotoxicity caused by DBE, DCE and BCE.

Genetic method for pre-classification of genotoxins into monofunctional or cross-linking agents

To characterize environmental carcinogens, there is a need to distinguish monofunctional genotoxic agents from those having cross-linking potential, because chemicals which can cross-link DNA are among the most potent carcinogens in rodents [Barbin and Bartsch, 1989] and humans [Allen et al., 1988; Kaldor et al., 1988]. Here we provide a genetic method for a pre-classification of genotoxins with respect to their functionality--monofunctional versus cross-linking. The procedure is based on the determination of relative clastogenic efficiency by a two-endpoint comparison in Drosophila: (i) induction of chromosome loss (CL), (ii) incidence of recessive lethal mutations (RL). Analysis of CL/RL ratios of 53 genotoxins, all mutagens in Drosophila, permitted distinction of 45 into two major categories: (i) 21 monofunctional agents with CL/RL indices generally < or = 1; (ii) 24 agents with ratios > 2 exhibiting DNA cross-linking properties. Within the group of monofunctional agents, CL/RL ratios tend to be low for SN1 agents, i.e., for N-ethyl-N-nitrosourea, N-ethyl-N'-nitro-N-nitrosoguanidine, and for N-nitrosodiethylamine. With cross-linking agents, the number of reactive groups appeared of minor importance as bi-, tri-, and tetrafunctional agents showed no significant differences in their CL/RL indices. Among 8 chemicals which could not be grouped into one of the two categories are two (adriamycin, daunomycin) regarded as intercalating agents. It is concluded that this two-endpoint analysis in Drosophila has prognostic value and can assist in the characterization of genotoxic agents with unknown mode of action.

The reaction of melphalan with deoxyguanosine and deoxyguanylic acid

The reaction of melphalan (phenylalanine mustard, I) with 2'-deoxyguanosine, followed by removal of the sugar in acid, yielded two products. The major product was identified as 4-(N-(2-guanin-7-ylethyl)-N-(2-hydroxyethyl)amino)phenyl- alanine (II) by ultra-violet absorption, mass and NMR spectroscopy. The minor product has already been identified as the corresponding bis-guaninyl adduct III (Tilby et al., Chem.-Biol. Interact., 73 (1990) 183-194). The reaction of melphalan with 5'-deoxyguanylic acid yielded the deoxyribonucleotide of II and products resulting from reaction with the phosphate group. The initial products, which were formed with a half-life of approximately 40 min at 37 degrees, still had a reactive chloroethyl group; this was displaced more slowly, by reaction with water or with another molecule of dGMP. The products of reaction of melphalan with DNA were released by treatment with acid (0.1 M HCl, 70 degrees, 30 min) and separated from each other on a cation exchange column. They were identified as II, III and an adenine adduct, in a ratio of approximately 3:1:2.

The persistence of sister-chromatid exchange frequencies in men occupationally exposed to vinyl chloride monomer

The persistence of sister-chromatid exchange frequencies in a population occupationally exposed to the well known chemical mutagen vinyl chloride monomer was studied. It was shown that increased values of sister-chromatid exchange frequencies were still present in the lymphocytes of workers who had not been exposed for 8-120 days and retired persons for 5-10 years after exposure. The possible ability of vinyl chloride monomer alkylating metabolites to cause long-lasting damage of the DNA molecule is discussed.

Embryotoxicity induced by alkylating agents: 6. DNA adduct formation induced by methylnitrosourea in mouse embryos

Formation of DNA adducts in 11-day-old mouse embryos was studied by measuring the initial alkylation rates of the methylated purine bases 7-methylguanine, O6-methylguanine, and 3-methyladenine. In the first part of the studies the adduct rates were measured in the teratogenic dose range (ED10-ED90, 2.7-5.6 mg/kg). These results were compared with similar data obtained from studies with ethylmethanesulfonate and acetoxymethyl-methylnitrosamine. For the three investigated substances a correlation was found between the initial adduct rate of O6-alkylguanine in the DNA of the embryos and the teratogenic potency. In the second part of the study the rate of adduct formation was measured in the sub-teratogenic dose range. These data will be used for molecular dosimetry in a risk assessment of low doses.

Nucleophilic selectivity and reaction kinetics of chloroethylene oxide assessed by the 4-(p-nitrobenzyl)pyridine assay and proton nuclear magnetic resonance spectroscopy

The nucleophilic selectivity (Swain-Scott's constant s) of chloroethylene oxide (CEO), an ultimate carcinogenic metabolite of vinyl chloride, was determined to be 0.71 using the 4-(p-nitrobenzyl)pyridine (NBP) assay (Spears method). The molar extinction coefficient of the adduct formed between NBP and CEO was measured; and the second-order rate constants for the reactions of CEO with NBP and with thiosulfate were estimated at three temperatures. The disappearance of CEO and the formation of chloroacetaldehyde (CAA) and glycolaldehyde (GCA) were followed in D2O or a mixture of D2O/hexadeuterated acetone (acetone-d6), using Fourier transform proton nuclear magnetic resonance spectroscopy (1H-FTNMR). Evidence was obtained that CEO reacts with chloride ions to yield CAA at a rate constant of about 17 M-1 h-1 in D2O/acetone-d6 (1 : 1, v/v) at 280 K. Under the same conditions, the first-order rate constant kr for the thermal rearrangement of CEO into CAA was estimated to be approximately 0.41 h-1. These data suggest that the isomerization of CEO may be a minor reaction in physiological saline. These chemical properties of CEO are discussed in relation to the mechanism of vinyl chloride-induced carcinogenesis.