Reductive metabolism of 4-nitrobiphenyl by rat liver fraction

The numerical probability of carcinogenicity to humans of some antimicrobials: Nitro-monoaromatics (including 5-nitrofurans and 5-nitroimidazoles), quinoxaline-1,4-dioxides (including carbadox), and chloramphenicol

Many substances are already tested in the long-term rodent bioassay (RCB). Nonetheless, statements such as the following are common in the regulatory literature: "the significance of the carcinogenicity findings in rodents relative to the therapeutic use of drugs in humans is unknown." (U.S. FDA prescribing information for nitrofurantoin). In the absence of epidemiological data, chemicals carcinogenic in RCBs are typically classified as either possibly or probably carcinogenic to humans, particularly without the -numerical probability for the carcinogenicity to humans- (PPV) of the classified substance. Through the biostatistics-based and regulatorily pertinent -predictive values approach- (PVA), the present study investigated the PPV of several antimicrobials relevant to human or veterinary medicine. A combination of structure-activity relationship, mutagenicity, and tumor-related histopathology was used to resolve reliable and pertinent PPVs. For 62 specific antimicrobials (e.g., carbadox), a 97.9% (or more) probability of carcinogenicity to humans was estimated. For nitrofurantoin, a 99.9% probability of carcinogenicity to humans was reckoned. Therefore, a risk-benefit evaluation on the in-force authorization of nitrofurantoin for uncomplicated human urinary infections is needed. A discussion was provided on the involved mechanisms of carcinogenic action and some regulatory implications of the findings. Neither this study nor the PVA aimed to encourage indiscriminate animal testing but the contrary, to reduce unnecessary or redundant in vivo testing by powering the predictivity of nonclinical toxicology.

Evidence that 4-aminobiphenyl, benzidine, and benzidine congeners produce genotoxicity through reactive oxygen species

4-Aminobyphenyl (4-Ab), benzidine (Bz), and Bz congeners were evaluated for their ability to induce genotoxicity through an oxidative mechanism. The mutagenicity of these compounds was tested in the presence and absence of Aroclor 1254-induced rat S9 mix using Salmonella typhimurium tester strain TA102, which is sensitive to agents producing reactive oxygen species (ROS). In the presence of S9, 4-Ab, Bz, N-acetyl-benzidine, and 3,3-dimethoxybenzidine were strongly mutagenic in TA102, whereas, 3,3,5,5-tetra-methylbenzidine, 3,3-dimethylbenzidine (O-tolidine), and N,N-diacetylbenzidine were not mutagenic. In addition, 3,3-dichlorobenzidine and 4,4-dinitro-2-biphenylamine were directly mutagenic in TA102. Incorporation of the free radical and metal scavengers, catalase, superoxide dismutase (SOD), butylated hydroxytolune (BHT), and ethylenediamine tetraacetic acid (EDTA) reduced the mutagenic responses of 4-Ab and Bz, whereas heat-inactivated catalase and SOD had no effect. 4-Ab and Bz also induced lipid peroxidation in the presence of S9 mix as shown using the thiobarbituric acid reactive substances assay. The results of this study indicate that 4-Ab and Bz induce mutations through the induction of ROS.

4-Aminobiphenyl-Hemoglobin Adducts and Risk of Smoking-Related Disease in Never Smokers and Former Smokers in the European Prospective Investigation into Cancer and Nutrition Prospective Study

The aim of this study was to evaluate whether biomarkers of environmental tobacco smoke exposure [i.e., 4-aminobiphenyl-hemoglobin (4-ABP-Hb) adducts] were predictive of the risk of tobacco-related cancers and diseases. We did a case-control study nested within the European Prospective Investigation into Cancer and Nutrition, involving 190 controls and 149 cases (incident cancer of the lung, bladder, pharynx, larynx, oral cavity, leukemias, and chronic obstructive pulmonary disease or emphysema deaths). All individuals were never smokers or ex smokers for >10 years. 4-ABP-Hb adducts were analyzed in peripheral blood collected before the onset of the disease (median, 7 years). Overall, 4-ABP-Hb adducts were higher, although not statistically significantly so, in cases (as a whole) than controls. In the control population, high fruit and vegetable consumption significantly lowered the frequency of detectable adducts (Fisher's exact test, P = 0.025). Restricting the analysis to women, 4-ABP-Hb adducts were higher in cases than controls (Mann-Whitney P = 0.036) and the odds ratio (OR) for the presence/absence of adducts was 2.42 [95% confidence interval (95% CI), 1.18-4.98]. Moreover, the association of adducts with the individual cancer types was stronger in women than in the whole study population, although statistically significant only for leukemias (OR, 2.77; 95% CI, 1.06-7.20). The results provide some evidence that women may be more susceptible to environmental tobacco smoke, as suggested by their higher adduct levels. The most important finding of this prospective study is that, at least in women, 4-ABP-Hb adducts may help identify subjects at high risk of cancers related to environmental tobacco smoke exposure.

Bacterial mutagenicity of the three isomeric dicyclopenta-fused pyrenes: the effects of dicyclopenta topology

Cyclopenta[cd]pyrene (1) and its congeners dicyclopenta[cd,mn]- (2), dicyclopenta[cd,fg]- (3), dicyclopenta[cd,jk]pyrene (4), which were all identified as constituents of combustion exhausts, as well as their partially hydrogenated derivatives 3,4-dihydrocyclopenta[cd]- (5), 1,2,4,5-tetrahydrodicyclopenta[cd,mn]- (6), 5,6,7,8-tetrahydrodicyclopenta[cd,fg]- (7) and 1,2,6,7-tetrahydrodicyclopenta[cd,jk]pyrene (8), were assayed for mutagenicity in the Salmonella typhimurium strain TA98 using different concentrations of microsomal protein in the metabolic activation system (S9-mix, with S9-fraction from liver of Aroclor-1254-treated rats: 2, 4 and 10% (v/v), respectively). Whereas a positive mutagenic response is found for 1-4 in the presence of S9-mix, 5-8 exert no mutagenicity either with or without S9-mix. Since for 1-4 the highest response is observed with S9-mix 2% (v/v) instead of the standard 4% (v/v), a one-step activation pathway, i.e. epoxidation of the five-membered ring olefinic bonds, appears to be operational. Surprisingly, 3 and, to a lesser extent, 2 (11.7 versus 4.2 His revertants/nmol) also give a positive response in the absence of S9-mix. Hence, 2 and 3 are expected to contribute to the direct-acting mutagenicity of the non-polar fraction of combustion exhausts. Presumably for the direct-acting mutagenicity one-electron transfer processes play a role in bioactivation. The experimental observations are supported by semi-empirical AM1 calculations on the possible ultimate metabolites, i.e. mono-epoxides (2a-4a), cis-di-epoxides (2b-4b) and trans-di-epoxides (2c-4c) and the related mono-hydroxy carbocations (2d-4d and 2e-4e), and the radical anions 1*(-)-4*(-).

The role of metals in site-specific DNA damage with reference to carcinogenesis

We reviewed the mechanism of oxidative DNA damage with reference to metal carcinogenesis and metal-mediated chemical carcinogenesis. On the basis of the finding that chromium (VI) induced oxidative DNA damage in the presence of hydrogen peroxide (H2O2), we proposed the hypothesis that endogenous reactive oxygen species play a role in metal carcinogenesis. Since then, we have reported that various metal compounds, such as cobalt, nickel, and ferric nitrilotriacetate, directly cause site-specific DNA damage in the presence of H2O2. We also found that carcinogenic metals could cause DNA damage through indirect mechanisms. Certain nickel compounds induced oxidative DNA damage in rat lungs through inflammation. Endogenous metals, copper and iron, catalyzed ROS generation from various organic carcinogens, resulting in oxidative DNA damage. Polynuclear compounds, such as 4-aminobiphenyl and heterocyclic amines, appear to induce cancer mainly through DNA adduct formation, although their N-hydroxy and nitroso metabolites can also cause oxidative DNA damage. On the other hand, mononuclear compounds, such as benzene metabolites, caffeic acid, and o-toluidine, should express their carcionogenicity through oxidative DNA damage. Metabolites of certain carcinogens efficiently caused oxidative DNA damage by forming NADH-dependent redox cycles. These findings suggest that metal-mediated oxidative DNA damage plays important roles in chemical carcinogenesis.

Structure-activity relationships in the deacetylation of O-glucosides of N-hydroxy-N-arylacylamides by mammalian liver microsomes

Structure-activity relationships in the deacylation of O-glucosides of N-hydroxy-N-aryl-acylamides were investigated to provide insights into the metabolic activation of carcinogenic/mutagenic O-glycosides of N-hydroxy-N-arylacylamides. In the subcellular fractions obtained from porcine liver, the deacetylation activity toward O-glucoside of N-hydroxyacetanilide (GAc) was mainly localized in the microsomes. Both the 2-chloro (2ClGAc) and 2-methyl (2MeGAc) derivatives of GAc were not deacetylated by the microsomes. Other compounds having either 3- or 4-substituent (chloro or methyl), however, were deacetylated and showed higher V(max)/K(m) values than that of GAc. 4-Methyl derivative (4MeGAc) was shown to competitively inhibit the deacetylation activity toward GAc, and the K(i) value of 4MeGAc was comparable with its K(m) value obtained in the microsome-catalyzed deacetylation. These apparent K(m) values were shown to correspond to their lipophilicities estimated from retention times on high-performance liquid chromatography (HPLC). As for the effect of acyl groups, the order of V(max)/K(m) values was N-propionyl derivative (GPr)>GAc>N-butyryl derivative (GBu). From the optimized structures and energy levels of the frontier orbitals of these compounds, calculated by the semi-empirical AM1 method, the effects of 2-substituents and acyl groups on the deacetylation activity is thought to be due to a steric factor. From the energy levels of the frontier orbitals of GAc and its 3- or 4-substituted derivatives, the compound having a lower level of LUMO was shown to be deacetylated effectively. There were marked species differences in the microsomal deacetylation activity toward GAc, and the highest activity was found in the rabbit, followed by the porcine, hamster, rat and then bovine liver microsomes.

Detection of metabolites using high-performance liquid chromatography and mass spectrometry

This unit provides a discussion of high-pressure liquid chromatography-atmospheric pressure ionization mass spectrometry (LC-API-MS) for separation, detection, and identification of products from xenobiotic metabolism. It includes an introduction and important technical information for electrospray and atmospheric pressure chemical ionization methods as well as scan modes. Emphasis has been placed on each method as related to phase I and phase II metabolite identification and there are listings of specific applications (and references) to reduce method development time.

Mutagenicity in Salmonella typhimurium TA98 and TA100 of nitroso and respective hydroxylamine compounds

Five aromatic nitroso compounds were prepared and their mutagenicity in Salmonella typhimurium strains TA98 and TA100 compared with that of the corresponding hydroxylamines and the previously studied nitroarenes. A remarkable correspondence of the dose-response curves was observed between the nitroso and the respective hydroxylamine compounds. This effect could be observed in TA98 and TA100. It was only marginally dependent on the metabolical activation by rat liver S9-mix. Even the presence of a bulky alkyl substituent either near to the functional group, or far away from it, previously shown to considerably influence the mutagenic properties of nitroarenes, does not remarkably affect the properties of the nitroso and hydroxylamine species. The similarity between the latter two is likely to be due to a fast reduction of the nitrosoarenes to the hydroxylamine species under the test conditions. It seems that enzymes are not responsible for that reduction step, because sterical crowding near the functional group does not influence that behaviour. The test results of the aromatic hydroxylamines bearing a bulky substituent show that there are at least two ways to influence the mutagenicity of an aromatic nitro compound by such a group. A substituent near the functional group (ortho-position) disturbs the enzymatic reduction of the nitro group, because 3-tert-butyl-4-hydroxylaminobiphenyl and its corresponding nitroso compound are highly mutagenic, whereas 3-tert-butyl-4-nitrobiphenyl was previously shown to be inactive even after addition of S9-mix. In contrast, 4'-tert-butyl-4-hydroxylaminobiphenyl with the tert-butyl group "far away" from the hydroxylamino functionality clearly shows decreased mutagenic activity suggesting a different influence of a substituent in that position. In addition, the substance shows only little cell toxicity even at higher concentrations. Both effects could be due to a reduced effective dose of the hydroxylamine in the cells compared to the non-alkylated compound, caused by a faster degradation of the hydroxylamine or a hindered interaction between that substance and the cells.

Mechanism of oxidative DNA damage induced by carcinogenic 4-aminobiphenyl

DNA adduct formation is thought to be a major cause of DNA damage by carcinogenic aromatic amines. We investigated the ability of an aromatic amine, 4-aminobiphenyl (4-ABP) and its N-hydroxy metabolite (4-ABP(NHOH)) to cause oxidative DNA damage, using (32)P-labeled human DNA fragments from the p53 tumor suppressor gene and the c-Ha-ras-1 protooncogene. 4-ABP(NHOH) was found to cause Cu(II)-mediated DNA damage, especially at thymine residues. Addition of the endogenous reductant NADH led to dramatic enhancement of this process. Catalase and bathocuproine, a Cu(I)-specific chelator, reduced the amount of DNA damage, suggesting the involvement of H(2)O(2) and Cu(I). 4-ABP(NHOH) dose-dependently induced 8-hydroxy-2'-deoxyguanosine (8-OHdG) formation in the presence of Cu(ll) and NADH. 4-ABP(NHOH) conversion to nitrosobiphenyl, as measured by UV-visible spectroscopy, occurred rapidly in the presence of Cu(II), suggesting Cu(II)-mediated autoxidation. Increased amounts of 8-OHdG were found in HL-60 cells compared to the H(2)O(2)-resistant clone HP100 following 4-ABP(NHOH) treatment, further supporting the involvement of H(2)O(2). The present study demonstrates that an N-hydroxy derivative of 4-ABP induces oxidative DNA damage through H(2)O(2) in both a cell-free system and in cultured human cells. We conclude that, in addition to DNA adduct formation, oxidative DNA damage may play an important role in the carcinogenic process of 4-ABP.

From mutagenic to non-mutagenic nitroarenes: effect of bulky alkyl substituents on the mutagenic activity of 4-nitrobiphenyl in Salmonella typhimurium. Part I. Substituents ortho to the nitro group and in 2'-position

Eleven alkyl substituted derivatives of 4-nitrobiphenyl (4NBp) and two corresponding nitroso compounds were synthesised and tested for mutagenic potency in strains TA98 and TA100 of Salmonella typhimurium. The mutagenicity of compounds substituted ortho to the nitro group (3-methyl-, 3-ethyl-, 3-isopropyl-, 3-tertbutyl-, 3, 5-diethyl-, 3,5-diisopropyl-, and 3,5-ditertbutyl-4NBp) decreased with growing steric demand of the alkyl substituents in both tester strains. The most sterically hindered compounds were non-mutagenic even at highest concentrations. This reduction of mutagenicity is correlated with deviations from the coplanar orientation of the nitro group relative to the aromatic plane. Since a comparable decrease of mutagenicity for the nitroso compounds (4NOBp and 3-tertbutyl-4NOBp) was not observed, the first reduction step of non-planar nitro groups must be inhibited. Alkyl groups in the 2'-position of 4NBp (2'-methyl-, 2'-ethyl-, 2'-isopropyl-, and 2',4', 6'-trimethyl-4NBp) also reduced mutagenic activity with increasing size and removed it completely for the most sterically hindered species (2'-isopropyl-, 2',4',6'-trimethyl-4NBp). In this case, the co-planarity of the nitro group is not affected but the twisting of the two aromatic rings, which is associated with a less effective charge delocalisation of the nitrenium ion. The experimental mutagenicities of all nitro compounds were compared to predicted values, that are based on recently developed empirical equations. While there was reasonable correspondence for the parent compound (4NBp), its ortho methylated derivative (3-methyl-4NBp) and two highly hydrophobic dialkylated species (3,5-diisopropyl- and 3, 5-ditertbutyl-4NBp), predictions for all other alkyl substituted compounds were too high. Thus, steric parameters should be included to improve the general validity of predictions by means of quantitative structure-activity relationships (QSAR).