Quantitative comparison of carcinogenicity, mutagenicity and electrophilicity of 10 direct-acting alkylating agents and of the initial O6:7-alkylguanine ratio in DNA with carcinogenic potency in rodents

The quantitative relationship between carcinogenicity in rodents and mutagenicity in Salmonella typhimurium was examined, by using 10 monofunctional alkylating agents, including N-nitrosamides, alkyl methanesulfonates, epoxides, β-propiolactone and 1,3-propane sultone. The compounds were assayed for mutagenicity in two S. typhimurium strains (TA1535 and TA100) and in plate and liquid assays. The mutagenic activity of the agents was compared with their alkylating activity towards 4-(4′-nitrobenzyl)pyridine and with their half-lives (solvolysis constants) in an aqueous medium. No correlations between these variables were found, nor was mutagenic activity correlated with estimates of carcinogenicity in rodents.

There was a positive relationship between carcinogenicity and the initial ratios of 7-: O⁶-alkylguanine formed or expected after their reaction with double-stranded DNA in vitro. The results suggest that alkylation of guanine at position O⁶ (or at other O atoms of DNA bases) may be a critical DNA-base modification that determines the overall carcinogenicity of these alkylating agents in rodents.     

Quantitative comparison of carcinogenicity, mutagenicity and electrophilicity of 10 direct-acting alkylating agents and of the initial O: 7-alkylguanine ratio in DNA with carcinogenic potency in rodents

The quantitative relationship between carcinogenicity in rodents and mutagenicity in Salmonella typhimurium was examined, by using 10 monofunctional alkylating agents, including N-nitrosamides, alkyl methanesulfonates, epoxides, β-propiolactone and 1,3-propane sultone. The compounds were assayed for mutagenicity in two S. typhimurium strains (TA1535 and TA100) and in plate and liquid assays. The mutagenic activity of the agents was compared with their alkylating activity towards 4-(4′-nitrobenzyl)pyridine and with their half-lives (solvolysis constants) in an aqueous medium. No correlations between these variables were found, nor was mutagenic activity correlated with estimates of carcinogenicity in rodents.There was a positive relationship between carcinogenicity and the initial ratios of 7-: O⁶-alkylguanine formed or expected after their reaction with double-stranded DNA in vitro. The results suggest that alkylation of guanine at position O⁶ (or at other O atoms of DNA bases) may be a critical DNA-base modification that determines the overall carcinogenicity of these alkylating agents in rodents.     

DNA adducts derived from administration of acrylamide and glycidamide to mice and rats

Acrylamide (AA) is an important industrial chemical that is neurotoxic, mutagenic to somatic and germ cells, and carcinogenic in chronic rodent bioassays. Recent findings of AA in many common starchy foods have sparked renewed interest in determining toxic mechanisms and in understanding the cancer, neurotoxicity, and reproductive risks from typical human exposures. Dosing mice and rats with AA (50 mg/kg) led to presence of glycidamide (GA) in serum and tissues. Furthermore, GA-derived DNA adducts of adenine and guanine were formed in all tissues examined, including both target tissues identified in rodent carcinogenicity bioassays and in non-target tissues. Dosing rats and mice with an equimolar amount of GA typically produced higher levels of DNA adducts than observed with AA. Kinetics of DNA adduct formation and accumulation were measured following oral administration of a single dose of AA (50 mg/kg) or from repeat dosing (1 mg/kg/day), respectively. The formation of these DNA adducts is consistent with previously reported mutagenicity of AA and GA in vitro, which involved reaction of GA with adenine and guanine bases. These results provide strong support for a genotoxic mechanism of AA carcinogenicity in rodents. The kinetic/biomarker approaches described here may represent a meaningful way to extrapolate cancer risks to actual human exposures from food, which are much lower.     

DNA adducts derived from administration of acrylamide and glycidamide to mice and rats

Acrylamide (AA) is an important industrial chemical that is neurotoxic, mutagenic to somatic and germ cells, and carcinogenic in chronic rodent bioassays. Recent findings of AA in many common starchy foods have sparked renewed interest in determining toxic mechanisms and in understanding the cancer, neurotoxicity, and reproductive risks from typical human exposures. Dosing mice and rats with AA (50 mg/kg) led to presence of glycidamide (GA) in serum and tissues. Furthermore, GA-derived DNA adducts of adenine and guanine were formed in all tissues examined, including both target tissues identified in rodent carcinogenicity bioassays and in non-target tissues. Dosing rats and mice with an equimolar amount of GA typically produced higher levels of DNA adducts than observed with AA. Kinetics of DNA adduct formation and accumulation were measured following oral administration of a single dose of AA (50 mg/kg) or from repeat dosing (1 mg/kg/day), respectively. The formation of these DNA adducts is consistent with previously reported mutagenicity of AA and GA in vitro, which involved reaction of GA with adenine and guanine bases. These results provide strong support for a genotoxic mechanism of AA carcinogenicity in rodents. The kinetic/biomarker approaches described here may represent a meaningful way to extrapolate cancer risks to actual human exposures from food, which are much lower.     

Effect of carcinogenic acrolein on DNA repair and mutagenic susceptibility

Acrolein (Acr), a ubiquitous environmental contaminant, is a human carcinogen. Acr can react with DNA to form mutagenic α- and γ-hydroxy-1, N(2)-cyclic propano-2'-deoxyguanosine adducts (α-OH-Acr-dG and γ-OH-Acr-dG). We demonstrate here that Acr-dG adducts can be efficiently repaired by the nucleotide excision repair (NER) pathway in normal human bronchial epithelia (NHBE) and lung fibroblasts (NHLF). However, the same adducts were poorly processed in cell lysates isolated from Acr-treated NHBE and NHLF, suggesting that Acr inhibits NER. In addition, we show that Acr treatment also inhibits base excision repair and mismatch repair. Although Acr does not change the expression of XPA, XPC, hOGG1, PMS2 or MLH1 genes, it causes a reduction of XPA, XPC, hOGG1, PMS2, and MLH1 proteins; this effect, however, can be neutralized by the proteasome inhibitor MG132. Acr treatment further enhances both bulky and oxidative DNA damage-induced mutagenesis. These results indicate that Acr not only damages DNA but can also modify DNA repair proteins and further causes degradation of these modified repair proteins. We propose that these two detrimental effects contribute to Acr mutagenicity and carcinogenicity.     

Comparison of the DNA-alkylating properties and mutagenic responses of a series of S-(2-haloethyl)-substituted cysteine and glutathione derivatives

The mutagenicity of 1,2-dibromoethane is highly dependent upon its conjugation to glutathione by the enzyme glutathione S-transferase. The conjugates thus formed can react with DNA and yield almost exclusively N7-guanyl adducts. We have synthesized the S-haloethyl conjugates of cysteine and glutathione, as well as selected methyl ester and N-acetyl derivatives, and compared them for ability to produce N7-guanyl adducts with calf thymus DNA. The cysteine compounds were found to be more reactive toward calf thymus DNA and yielded higher adduct levels than did the glutathione compounds. Adduct levels tended to be suppressed when there was a net charge on the compound and were not affected by substitution of bromine for chlorine, as expected for a mechanism known to involve an intermediate episulfonium ion. Sequence-selective alkylation of fragments of pBR322 DNA was investigated. The compounds produced qualitatively similar patterns of alkylation, with higher levels of alkylation at runs of guanines. The compounds were also tested for their ability to act as direct mutagens in Salmonella typhimurium TA98 and TA100. None of the compounds caused mutations in the TA98 frameshift mutagenesis assay. In the strain TA100, where mutation of a specific guanine by base-pair substitution produces reversion, all compounds were found to produce mutations, but the levels of mutagenicity did not correlate at all with the levels of DNA alkylation. The ratio of mutations to adducts varied at least 14-fold among the various N7-guanyl adducts examined.(ABSTRACT TRUNCATED AT 250 WORDS)     

Determining efficacy of cancer chemopreventive agents using a cell-free system concomitant with DNA adduction

The large (>2000) and expanding number of natural and synthetic agents with potential cancer chemopreventive properties renders it economically and physically impossible to test each of these agents for their efficacy in the widely accepted 2-year animal bioassay and clinical trials. Therefore, there is a growing need for relevant short-term screening tests to study these compounds such that only the most efficacious ones undergo extensive long-term studies. We have previously reported in a pilot study that the use of a microsome-mediated test system concomitant with DNA adduction is a pertinent and relevant model for rapidly studying the efficacy and mechanisms of cancer chemopreventive agents. We have extended this study to investigate 26 additional agents for their potential chemopreventive abilities by studying their effects on microsome-mediated benzo[a]pyrene (BP)-DNA adduction. These agents had differential effects on the two major adducts of BP-DNA, i.e., BP-7,8-diol-9,10-epoxide (BPDE)-deoxyguanosine (dG) and 9-OH-BP-dG-derived adducts. These agents were therefore categorized into five classes. Three test agents (ellagic acid, genistein and oltipraz) were strong inhibitors of both adducts. These agents diminished BP-DNA adduction by 65-95% and were categorized as Class I agents. Six other agents (benzyl isocyanate, R(+)-1-phenylethyl isocyanate, linoleic acid ethyl ester, (+)-biotin, indole-3-carboxylic acid and beta-carotene) moderately inhibited both BP-DNA adducts (25-64%); these compounds were identified as Class II agents. Six additional test agents inhibited only one adduct selectively and nine others were ineffective; these agents were categorized as Class III and Class IV, respectively. Interestingly, seven test agents enhanced BPDE-dG or 9-OH-BP-dG or both adducts and were categorized as Class V agents. Four of these Class V agents concomitantly inhibited BPDE-dG while enhancing 9-OH-BP-dG. This emphasizes the importance of studying individual DNA adducts in contrast to total DNA binding. In conclusion, Class I and Class II agents may be good candidates for further chemoprevention studies.     

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

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

Oxidative dealkylation DNA repair mediated by the mononuclear non-heme iron AlkB proteins

DNA can be damaged by various intracellular and environmental alkylating agents to produce alkylation base lesions. These base damages, if not repaired promptly, may cause genetic changes that lead to diseases such as cancer. Recently, it was discovered that some of the alkylation DNA base damage can be directly removed by a family of proteins called the AlkB proteins that utilize a mononuclear non-heme iron(II) and alpha-ketoglutarate as cofactor and cosubstrate. These proteins activate dioxygen and perform an unprecedented oxidative dealkylation of the alkyl adducts on DNA heteroatoms. This review summarizes the discovery of this activity and the recent research advances in studying this unique DNA repair pathway. The focus is placed on the chemical mechanism and function of these proteins.     

The uses of carcinogen-DNA adduct measurement in establishing mechanisms of mutagenesis and in chemoprevention

DNA adducts generated by carcinogenic chemicals reflects human exposure and DNA adducts are related to tumor formation. Most chemical carcinogens require activation to reactive intermediates that bind to nucleophilic centers in proteins and nucleic acids thereby forming covalent adducts. Also, many of the chemicals considered carcinogenic for humans form covalent DNA adducts. Therefore, such DNA damage is generally considered to be causative and linked to tumor formation. In this article we have summarized the work done for many years on the role of DNA adduct formation as an indicator of their carcinogenicity. We have also addressed the important role for measurement of DNA adducts in studies with potential chemopreventive agents for which it is central to have a marker that can be measured more rapidly than changes in cancer incidence.     

Genotoxicity and mutagenicity of chromium(VI)/ascorbate-generated DNA adducts in human and bacterial cells

Reduction of carcinogenic Cr(VI) by vitamin C generates ascorbate-Cr(III)-DNA cross-links, binary Cr(III)-DNA adducts, and can potentially cause oxidative DNA damage by intermediate reaction products. Here, we examined the mutational spectrum and the importance of different forms of DNA damage in genotoxicity and mutagenicity of Cr(VI) activated by physiological concentrations of ascorbate. Reduction of Cr(VI) led to a dose-dependent formation of both mutagenic and replication-blocking DNA lesions as detected by propagation of the pSP189 plasmids in human fibroblasts. Disruption of Cr-DNA binding abolished mutagenic responses and normalized the yield of replicated plasmids, indicating that Cr-DNA adducts were responsible for both mutagenicity and genotoxicity of Cr(VI). The absence of DNA breaks and abasic sites confirmed the lack of a significant production of hydroxyl radicals and Cr(V)-peroxo complexes in Cr(VI)-ascorbate reactions. Ascorbate-Cr(III)-DNA cross-links were much more mutagenic than smaller Cr(III)-DNA adducts and accounted for more than 90% of Cr(VI) mutagenicity. Ternary adducts were also several times more potent in the inhibition of replication than binary complexes. The Cr(VI)-induced mutational spectrum consisted of an approximately equal number of deletions and G/C-targeted point mutations (51% G/C --> T/A and 30% G/C --> A/T). In Escherichia coli cells, Cr(VI)-induced DNA adducts were only highly genotoxic but not mutagenic under either normal or SOS-induced conditions. Lower toxicity and high mutagenicity of ascorbate-Cr(III)-DNA adducts in human cells may result from the recruitment of an error-prone bypass DNA polymerase(s) to the stalled replication forks. Our results suggest that phosphotriester-type DNA adducts could play a more important role in human than bacterial mutagenesis.     

Repair of DNA lesions: mechanisms and relative repair efficiencies

DNA is frequently damaged by endogenous agents inside the cells. Some exogenous agents such as polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the environment and may thus contribute to the 'background' DNA damage in humans. DNA lesions are normally removed by various repair mechanisms. The major repair mechanisms for various DNA lesions are summarized. In contrast to the extensively studied repair mechanisms, much less is known about the relative repair efficiencies of various DNA lesions. Since DNA repair is a crucial defense against carcinogenesis, it may constitute an important factor affecting the carcinogenicity of DNA damaging agents. We have adopted a human cell-free system for measuring relative DNA repair efficiencies based on the concept of repair competition between acetylaminofluorene adducts and other DNA lesions of interest. Using this in vitro system, we determined the relative repair efficiencies of PAH adducts induced by: anti-(+/-)-benzo[a]pyrene-trans-7,8-dihydrodiol-9,10-epoxide (BPDE), anti-(+/-)-benz[a]anthracene-trans-3,4-dihydrodiol-1,2-epoxide (BADE-I), anti-(+/-)-benz[a]anthracene-trans-8,9-dihydrodiol-10, 11-epoxide (BADE-II), anti-(+/-)-benzo[b]fluoranthene-trans-9, 10-dihydrodiol-11,12-epoxide (BFDE), anti-(+/-)-chrysene-trans-1, 2-dihydrodiol-3,4-epoxide (CDE), and anti-(+/-)-dibenzo[a, l]pyrene-trans-11,12-dihydrodiol-13,14-epoxide (DBPDE). While damage by BPDE, DBPDE, CDE, and BFDE were repaired by nucleotide excision repair as efficiently as AAF adducts, the repair of BADE-I and BADE-II adducts were significantly slower in human cell extracts. Damage by DBPDE at 3 microM in vitro yielded approximately 5-fold higher DNA adducts than BPDE as determined by quantitative PCR. This potent DNA reactivity may account in part for the potent carcinogenicity of dibenzo[a,l]pyrene. The correlation of these results to the carcinogenic properties of the PAH compounds is discussed. Furthermore, we show that NER plays a role in AP site repair in vivo in the eukaryotic model organism yeast.     

Efficient translesion replication past oxaliplatin and cisplatin GpG adducts by human DNA polymerase eta

Platinum anticancer agents form bulky DNA adducts which are thought to exert their cytotoxic effect by blocking DNA replication. Translesion synthesis, one of the pathways of postreplication repair, is thought to account for some resistance to DNA damage and much of the mutagenicity of bulky DNA adducts in dividing cells. Oxaliplatin has been shown to be effective in cisplatin-resistant cell lines and less mutagenic than cisplatin in the Ames assay. We have shown that the eukaryotic DNA polymerases yeast pol zeta, human pol beta, and human pol gamma bypass oxaliplatin-GG adducts more efficiently than cisplatin-GG adducts. Human pol eta, a product of the XPV gene, has been shown to catalyze efficient translesion synthesis past cis, syn-cyclobutane pyrimidine dimers. In the present study we compared translesion synthesis past different Pt-GG adducts by human pol eta. Our data show that, similar to other eukaryotic DNA polymerases, pol eta bypasses oxaliplatin-GG adducts more efficiently than cisplatin-GG adducts. However, pol eta-catalyzed translesion replication past Pt-DNA adducts was more efficient and less accurate than that seen for previously tested polymerases. We show that the efficiency and fidelity of translesion replication past Pt-DNA adducts appear to be determined by both the structure of the adduct and the DNA polymerase active site.     

IPCS guidelines for the monitoring of genotoxic effects of carcinogens in humans. International Programme on Chemical Safety

The purpose of these guidelines is to provide concise guidance on the planning, performing and interpretation of studies to monitor groups or individuals exposed to genotoxic agents. Most human carcinogens are genotoxic but not all genotoxic agents have been shown to be carcinogenic in humans. Although the main interest in these studies is due to the association of genotoxicity with carcinogenicity, there is also an inherent interest in monitoring human genotoxicity independently of cancer as an endpoint.The most often studied genotoxicity endpoints have been selected for inclusion in this document and they are structural and numerical chromosomal aberrations assessed using cytogenetic methods (classical chromosomal aberration analysis (CA), fluorescence in situ hybridisation (FISH), micronuclei (MN)); DNA damage (adducts, strand breaks, crosslinking, alkali-labile sites) assessed using bio-chemical/electrophoretic assays or sister chromatid exchanges (SCE); protein adducts; and hypoxanthine-guanine phosphoribosyltransferase (HPRT) mutations. The document does not consider germ cells or gene mutation assays other than HPRT or markers of oxidative stress, which have been applied on a more limited scale.     

DNA adducts in humans after exposure to methylating agents

Human exposure to methylating agents appears to be widespread, as indicated by the frequent occurrence of methylated DNA adducts in human DNA. The high incidence of methylated DNA adducts even in humans thought not to have suffered extensive exposure to environmental methylating agents implies that chemicals of endogenous origin, probably N-nitroso compounds such as the strongly carcinogenic N-nitrosodimethylamine (NDMA), may be primarily responsible for their formation and raises the question of the carcinogenic risks associated with such exposure. In addition to accumulation of DNA damage, other factors (such as induced cell proliferation) appear to be important in determining the probability of induction of mutation or cancer by NDMA, implying that high to low dose risk extrapolations should not be based on the assumption of dose- or even adduct-linearity. Comparative studies of the accumulation and repair of methylated adducts in humans and animals treated with methylating cytostatic drugs do not reveal significant species differences. Based on this and the dosimetry of adduct accumulation in rats chronically exposed to very low doses of NDMA, it is suggested that the exposure needed to account for the levels of adducts found in human DNA may be of the order of hundreds of micrograms NDMA (or equivalent) per day, a level of exposure which may well represent a significant carcinogenic hazard for man.     

Diepoxybutane induces the formation of DNA-DNA rather than DNA-protein cross-links, and single-strand breaks and alkali-labile sites in human hepatocyte L02 cells

1,3-Butadiene (BD) is an air pollutant and a known carcinogen. 1,2,3,4-Diepoxybutane (DEB), one of the major in vivo metabolites of BD, is considered the ultimate culprit of BD mutagenicity/carcinogenicity. DEB is a bifunctional alkylating agent, being capable of inducing the formation of monoalkylated DNA adducts and DNA cross-links, including DNA-DNA and DNA-protein cross-links (DPC). In the present study, we investigated DEB-caused DNA cross-links and breaks in human hepatocyte L02 cells using comet assay. With alkaline comet assay, it was observed that DNA migration increased with the increase of DEB concentration at lower concentrations (10-200μM); however, at higher concentrations (200-1000μM), DNA migration decreased with the increase of DEB concentration. This result indicated the presence of cross-links at >200μM, which was confirmed by the co-treatment experiments using the second genotoxic agents, tert-butyl hydroperoxide and methyl methanesulfonate. At 200μM, which appeared as a threshold, the DNA migration-retarding effect of cross-links was just observable by the co-treatment experiments. At <200μM, the effect of cross-links was too weak to be detected. The DEB-induced cross-links were determined to be DNA-DNA ones rather than DPC through incubating the liberated DNA with proteinase K prior to unwinding and electrophoresis. However, at the highest DEB concentration tested (1000μM), a small proportion of DPC could be formed. In addition, the experiments using neutral and weakly alkaline comet assays showed that DEB did not cause double-strand breaks, but did induce single-strand breaks (SSB) and alkali-labile sites (ALS). Since SSB and ALS are repaired more rapidly than cross-links, the results suggested that DNA-DNA cross-links, rather than DPC, were probably responsible for mutagenicity/carcinogenicity of DEB.     

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 closed to or 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.     

Biomonitoring of tobacco smoke carcinogenicity by dosimetry of DNA adducts and genotyping and phenotyping of biotransformational enzymes: a review on polycyclic aromatic hydrocarbons

In this review article, we summarize the data on tobacco smoke carcinogenicity in relation to DNA adduct dosimetry and genotyping and phenotyping of biotransformational enzymes. A major class of carcinogens, polycyclic aromatic hydrocarbons, present in substantial quantities in tobacco smoke, is discussed. The historical background and an overview of the metabolic pathways are given. The epidemiological and biological data in particular on dosimetry of the representative DNA adducts and genotyping and phenotyping of the respective activating and detoxifying enzymes are presented. The salient findings are highlighted, the uncertainties are underlined and, finally, recommendations for future research are made.     

Biomonitoring of tobacco smoke carcinogenicity by dosimetry of DNA adducts and genotyping and phenotyping of biotransformational enzymes: a review on polycyclic aromatic hydrocarbons

In this review article, we summarize the data on tobacco smoke carcinogenicity in relation to DNA adduct dosimetry and genotyping and phenotyping of biotransformational enzymes. A major class of carcinogens, polycyclic aromatic hydrocarbons, present in substantial quantities in tobacco smoke, is discussed. The historical background and an overview of the metabolic pathways are given. The epidemiological and biological data in particular on dosimetry of the representative DNA adducts and genotyping and phenotyping of the respective activating and detoxifying enzymes are presented. The salient findings are highlighted, the uncertainties are underlined and, finally, recommendations for future research are made.