Mutagenicity of nitro-azabenzo[a]pyrene and its related compounds

Two nitro derivatives of azabenzo[a]pyrene N-oxide: electronic properties and their relation to mutagenic activity

The equilibrium geometries, relative energies, IR and Raman spectra, vertical ionization potentials (IP), vertical electron affinities (EA), dipole moments (μ), electronic dipole polarizabilities (α), and molecular electrostatic potentials (MEP) of two species that show very high mutagenicity, 1-nitro-6-azabenzo[a]pyrene N-oxide (1-N-6-ABPO) and 3-nitro-6-azabenzo[a]pyrene N-oxide (3-N-6-ABPO), are investigated by means of Density Functional Theory (DFT) using B3LYP functional with different basis sets. The 3-N-6-ABPO isomer was estimated to be much more mutagenic in Salmonella typhimurium tester strain TA98 (396,000 revertants/nmol) than 1-N-6-ABPO (36100 revertants/nmol) (Fukuhara et al., 1992). The results show that for both isomers the structural, energetic, and vibrational properties are similar. The orientation of the nitro group with respect to the plane of the aromatic system as well as the nitroreduction and oxidation reaction and polarizability seem not be important for the determination of different mutagenic behavior of these isomers. However, the dipole moment of 3-N-6-ABPO is about 3 times that of 1-N-6-ABPO. The larger dipole moment and the different electronic charge distribution of 3-N-6-ABPO compared to 1-N-6-ABPO imply stronger electrostatic and inductive molecular interactions so that the active site of the enzyme involved in the mutagenic activation can more effectively bind 3-N-6-ABPO compared to 1-N-6-ABPO.

The role of electronic properties to the mutagenic activity of 1,6- and 3,6-dinitrobenzo[a]pyrene isomers

Equilibrium geometries, infrared spectra, vertical first ionization potential (IP), electronic affinity (EA), dipole moment (mu) and electronic dipole polarizability (alpha) of 1,6- and 3,6-dinitrobenzo[a]pyrene isomers (1,6-DNBaP and 3,6-DNBaP) were evaluated by means of Density Functional Theory (DFT) and recent semiempirical PM6 method. Structural, energetic and vibrational properties of DNBaP isomers are substantially similar to each other. Calculated IP, EA and alpha values of these isomers are practically identical, while mu of 3,6-DNBaP (8.2 D at DFT level) is predicted to be ca. 4 times the value of 1,6-DNBaP isomer (1.9 D at DFT level), owing to favorable mutual orientation of the individual nitro group vectors. Higher direct-mutagenic activities of 3,6-DNBaP with respect to 1,6-DNBaP isomer by 1-2 orders of magnitude might be determined by its peculiar electronic charge distribution, which through stronger electrostatic and inductive interactions, can promote much more effectively binding to active-site of enzymes involved in mutagenic pathways. On the other hand, orientation of the nitro substituents relatively to the plane of the aromatic moiety, molecular sizes, as well as nitroreduction and oxidation reactions seem not to have a key role in the determination of the different mutagenic behaviour of these isomers.

Chagas disease

Chagas disease is the clinical condition triggered by infection with the protozoan Trypanosoma cruzi. The infection is transmitted by triatomine insects while blood feeding on a human host. Field studies predict that one third of an estimated 18 million T cruzi-infected humans in Latin America will die of Chagas disease. Acute infections are usually asymptomatic, but the ensuing chronic T cruzi infections have been associated with high ratios of morbidity and mortality: Chagas heart disease leads to unexpected death in 37.5% of patients, 58% develop heart failure and die and megacolon or megaoesophagus has been associated with death in 4.5%. The pathogenesis of Chagas disease appears to be related to a parasite-induced mutation of the vertebrate genome. Currently, treatment is unsatisfactory.

Structural activity relationship between Salmonella-mutagenicity and nitro-orientation of nitroazaphenanthrenes

Nitroazaphenanthrenes (NAphs) and their N-oxides (NAphOs) were synthesized as derivatives with nitrogen atoms in the 1, 4, and 9 positions of phenanthrene rings, and as nitrated derivatives substituted at the 1, 2, 3, 4, 5, 6, 7, and 8 positions of phenanthrene rings. To determine the structure activity relationship of these derivatives, all 19 isomers were bioassayed with Salmonella tester strains. NAphs substituted at the 4, 6, 7 and 8 positions were mutagenic for TA98, and 1-, 2-, and 3-N-9-AphOs, 6-N-1-AphO and 6-N-4-AphO were mutagenic for TA98 and TA100 without the S9 mix, while 5-N-1-AphO and 5-N-9-AphO were non- or weakly mutagenic. Nitrated derivatives, 6-N-4-Aph, 6-N-9-Aph, 6-N-1-AphO, and 6-N-4-AphO, were powerful mutagens for TA98 and TA100. Mutagenicity was enhanced by mutant strains producing nitroreductase, such as YG1021 and 1026, and by those producing O-acetyltransferase, such as YG1024 and 1029. Nitro derivatives substituted at positions 4 and 5 in the phenanthrene rings were perpendicular, while those at positions 2, 3, 6 and 7 were coplanar to the phenanthrene rings. NAphs substituted at the 1 and 8 positions were noncoplanar due to steric hindrance of the aromatic proton at the peri position. On the other hand, 1,5- and 1,8-dinitro-4-azaphenanthrenes showed high mutagenicity for strains TA98 and TA100 in the absence of the S9 mix, and were strongly enhanced by nitroreductase and O-acetyltransferase, over-producing mutants. Therefore, it was found that the mutagenic potency of NAphs and NAphOs was closely associated with the chemical properties and orientation of nitro substitution of aromatic rings.

Mutagenic nitrated benzo[a]pyrene derivatives in the reaction product of benzo[a]pyrene in NO2-air in the presence of O3 or under photoirradiation

In order to clarify the contribution of nitrated products to the direct-mutagenic activity of products of the reactions of benzo[a]pyrene in NO2-air under various conditions, heterogeneous reactions of BaP deposited on filter in the air containing 10 ppm of NO2 have been conducted in dark or under photoirradiation. The reaction products have been analyzed by gas chromatography and mutagenicity of the products fractionated by preparative HPLC was assayed for Salmonella typhimurium strains TA98 and YG1024 in the absence of S9 mix. 3,6-dinitrobenzo[a]pyrene and 1,3-dinitrobenzo[a]pyrene, which are strong direct-acting mutagens, largely contributed to the total direct-acting mutagenicity of the dark reaction products in NO2-air. On the other hand, both the dark reaction in the presence of O3 and the photoreaction in NO2-air resulted in the formation of much smaller amounts of nitrobenzo[a]pyrenes than that observed in the dark reaction in the absence of O3. These results show that the contribution of other direct-acting mutagens to the total direct-acting mutagenicity of the products in these reactions should be considered. Benzo[a]pyrene lactones were identified in a highly mutagenic fraction of the products of the dark reaction in the presence of O3 and photoreaction and a nitrobenzo[a]pyrene lactone was also identified in a highly mutagenic fraction of the dark reaction products in the presence of O3. Nitrated oxygenated benzo[a]pyrene derivatives such as nitrobenzo[a]pyrene lactone were considered to largely contribute to direct-acting mutagenicity of the products of the dark reaction in the presence of O3 and photoreaction.

Chromatographic methods for carcinogenic/mutagenic nitropolycyclic aromatic hydrocarbons

Nitropolycyclic aromatic hydrocarbons (NPAHs), which are known to be carcinogenic and/or mutagenic, are considered to be one of the air pollutants that cause lung cancer. In the last two decades, a number of sensitive and selective methods have been developed for the determination of NPAHs and related compounds in environmental and biological samples. This paper describes the state of the art of the methods and applications.

Persistent infections in chronic Chagas' disease patients treated with anti-Trypanosoma cruzi nitroderivatives

We used a molecular method and demonstrated that treatment of the chronic human Trypanosoma cruzi infections with nitroderivatives did not lead to parasitological cure. Seventeen treated and 17 untreated chronic Chagas' disease patients, with at least two out of three positive serologic assays for the infection, and 17 control subjects formed the study groups. PCR assays with nested sets of T. cruzi DNA primers monitored the efficacy of treatment. The amplification products were hybridized to their complementary internal sequences. Untreated and treated Chagas' disease patients yielded PCR amplification products with T. cruzi nuclear DNA primers. Competitive PCR was conducted to determine the quantity of parasites in the blood and revealed < 1 to 75 T. cruzi/ml in untreated (means 25.83+/-26.32) and < 1 to 36 T. cruzi/ml in treated (means 6.45+/-9.28) Chagas' disease patients. The difference between the means was not statistically significant. These findings reveal a need for precise definition of the role of treatment of chronic Chagas' disease patients with nitrofuran and nitroimidazole compounds.

Substituent effect of a fluorine atom on the mutagenicity of nitroquinolines

Some 16 nitroquinolines (NQs) and their fluorinated derivatives were tested for mutagenicity in Salmonella typhimurium TA100 without S9 mix to investigate the effect of fluorine-substitution on the mutagenicity. These NQs consist of 5-NQs, 5-nitroquinoline N-oxides (5-NQOs), N-methyl-5-nitroquinolinium methanesulfonates (N-Me-5-NQs) and 8-NQs, including three ortho-F-NQs, one meta-F-NQ, four para-F-NQs and four 3-F-NQs. For this purpose, eight F-NQs were newly synthesized. The data indicated that the ratio of the mutagenic activities (revertants/plate/nmol) of fluorinated NQs to those of the corresponding parent non-fluorinated compounds ranged from 0.6- to 119-fold. The fluorine atom located para to the nitro group markedly enhanced the mutagenicity (24-fold and more), while three ortho-fluorinated derivatives showed no significant increase in mutagenicity (enhancement ratio were 0.6, 0.8 and 1.7). With respect to 8-NQs, its meta-fluorinated derivative also had an enhanced mutagenicity over the parent compound (53-fold). In addition, although N-Me-5-NQ was less mutagenic than 5-NQ and 5-NQO, the mutagenicity of N-Me-5-NQ was most significantly enhanced by fluorine-substitution. These results suggest that introduction of a fluorine atom to the molecule in question may be a useful tool to modify their mutagenic potency and to better understand the mechanism of mutation.

Comparative tumorigenicity of 1- and 3-nitrobenzo[a]pyrenes, and 3,6- and 1,6-dinitrobenzo[a]pyrenes in F344/DuCrj rats

Our earlier study revealed that 1- and 3-nitrobenzo[a]pyrene (NBP), 1,6- and 3,6-dinitrobenzo[a]pyrene (DNBP), nitrated derivatives of benzo[a]BP (BP), are present in the environment. These derivatives are potent mutagens for Salmonella tester strains and we have preliminarily reported them to be carcinogenic in F344/DuCrj rats. In this study, the tumorigenic action of 1- and 3-NBP, 1.6- and 3,6-DNBP, and BP induced by subcutaneous injection into rats was found to differ according to the NO2-substitution in the BP structure. The chemicals were suspended in equal volumes of beeswax and tricaprylin, and rats were subcutaneously injected with single doses of 500, 1000, and 2000 microg for 1- and 3-NBP, and of 8, 40, 200, and 1000 microg for 3,6- and 1,6-DNBP, and BP as a positive control. 3,6-DNBP and BP induced tumors in a dose-dependent manner at the injection site. Rats given 1000 microg of 3,6-DNBP (2924 nmol) and BP (3968 nmol) developed subcutaneous tumors at the rate of 70 and 80%, respectively, and those given a minimum dose of 23 nmol for 3.6-DNBP and 32 nmol for BP per rat developed tumors at a rate of 4.8 and 18.2%, respectively. However, rats given 500 and 1000 microg of 1- and 3-NBP did not develop any tumors while those given a high dose, 2000 microg, of each chemical developed tumors at only one of ten animals used. It was concluded, therefore, that these chemicals are weak carcinogens. Histologically, most of the tumors were malignant fibrous histiocytomas. Rats given various doses of 1,6-DNBP did not develop any tumors at the injection site. The failure of 1,6-DNBP to induce tumors may involve its metabolites because of the lower mutagenicity of its reduction products, 1-nitroso-6-NBP and 1-amino-6-NBP. It is suggested, therefore, that tumorigenicities of NBPs and DNBPs differ according to the NO2-substitution on the chemical structure, which may be due to the possible nitroreduction of the chemicals.

Mutagenicity of nitrophenanthrene derivatives for Salmonella typhimurium: effects of nitroreductase and acetyltransferase

To determine the mutagenicity of nitrophenanthrenes, three mononitrophenanthrenes (NPhs), 11 dinitrophenanthrenes (diNPhs) and eight trinitrophenanthrenes (tiNPhs) were synthesized, and their mutagenicity was investigated by using Salmonella typhimurium his- strains TA98, TA100, and TA98NR, nitroreductase-deficient, and TA98/1,8-DNP6, O-acetyltransferase-deficient mutants, and strains YG1021 and YG1026, nitroreductase-overproducing mutants of TA98 and TA100, respectively, and strains YG1024 and YG1029, O-acetyltransferase-overproducing mutants of TA98 and TA100, respectively. 1-, 3- and 9-NPhis induced 329, 620 and 438 revertants per nmol in strain TA100, respectively, and 4,839, 11,309 and 16728 revertants per nmol, respectively, in strain YG1029. Mutagenicity of 1,6-, 2,6-, 2,9-, 2,10-, 3,5-, 3,6- and 3,10-diNPh was elevated in strains YG1021, YG1024, YG1026 and YG1029. Among these derivatives, 1,6-, 2,6-, 3,6- and 3,10-diNPhs were more mutagenic in strains YG1024 and YG1029 than YG1021 and YG1026, and they showed a structure-activity relationship between mutagenicity and NO2-substitution. Nitro derivatives substituted at the 3 and 6 positions of their chemical structure strongly mutated both strains YG1024 and YG1029, whereas those substituted at the 9 and 10 positions showed weak mutagenicity. In addition, nitro substituents at positions 4 and 5 were perpendicular while those on positions 2,3,6 and 7 were nearly coplanar to the aromatic ring. Furthermore, 2,6,9-, 3,6,9- and 1,6,9-trinitrophenanthrenes (triNPhs) were mutagenic for strain TA100, and their mutagenicity was more enhanced in YG1024 and YG1029 than in YG1021 and YG1026. Of the eight triNPhs all except 1,5,10-triNP were mutagenic in TA98 and TA100, and their mutagenicity was more enhanced in YG1024 and YG1029 than in YG1021 and YG1026. These results suggest that these compounds are mutagens that are activated by O-acetyltransferase esterification following nitroreductase. The nitrated derivatives substituted at the 2(7) and 3(6) positions of the phenanthrene ring were highly mutagenic. The relationship between chemical structure and the mutagenicity of NPh derivatives is discussed.

Chagas' disease: carcinogenic activity of the antitrypanosomal nitroarenes in mice

The carcinogenic activity of antitrypanosomal 2-nitroimidazole, 5-nitroimidazole and 5-nitrofuran derivatives was assessed in female Swiss mice of the same age group. A statistically significantly higher incidence of growths was seen in mice into which 2-nitro had been injected than in mice receiving 5-nitro derivatives intraperitoneally. A histologic type of lymphoblastic lymphoma that invades lymph nodes, spleen, liver, lungs and lymphatic tissue elsewhere was frequently found in nitroarene-treated mice. Further, it is shown that the potency of the drug, rather than the duration of its administration, was usually associated with the growth of lymphomas. The 2-nitro derivative which induced the highest incidence of lymphomas significantly decreased the survival of treated mice; this probably occurred because it undergoes enzymatic reduction of the nitro group more efficiently than the 5-nitro compounds used. The differences of incidence of lymphomas in mice receiving any of these nitroarenes and in control mice that received daily injections of 0.15 M saline were statistically significant (alpha = 0.05). The indiscriminate use of these nitroarenes to treat Trypanosoma cruzi infections in man could therefore induce a significant number of lymphomas.