DNA adduct formation by 7-methoxy-2-nitro-naphtho[2,1-b]furan (R7000), an extremely potent mutagen

Synthesis, reactions and biological evaluation of some new naphtho[2,1-b]furan derivatives bearing a pyrazole nucleus

Vilsmeier formylation of 2-(1-phenylhydrazonoethyl)naphtho[2,1-b]furan (2) gave 3-naphtho[2,1-b]furan-2-yl-1-phenyl-1H-pyrazole-4-carbaldehyde (3), which was reacted with C- and N-nucleophiles to afford naphthofuranpyrazol derivatives 4-8. Treatment of 2-[(3-(naphtho[2,1-b]furan-2-yl)-1-phenyl-1H-pyrazol-4-yl)methylene]-malononitrile (4a) with reactants having active hydrogen and Et₃N gave the corresponding pyrazoline, pyran and chromene addition product derivatives 10, 12 and 13, consisting of three different connected heterocyclic moieties. Reaction of 1-((3-(naphtho[2,1-b]furan-2-yl)-1-phenyl-1H-pyrazol-4-yl) methylene)-2-phenylhydrazone (6b) with AcONa and ethyl bromoacetate or chloroacetone afforded the thiazolidinone and methylthiazole derivatives 14 and 15, respectively. In addition, intramolecular cyclization of 6d with Ac₂O afford the corresponding 1,3,4-thiadiazol-2-yl acetamide derivative 16. The structures of the synthesized compounds were confirmed by IR, ¹H-NMR/¹³C-NMR and mass spectral studies. Compound 14 showed promising effects against the tested Gram positive and negative bacteria and fungi.

Genotoxicity of 2-nitro-7-methoxy-naphtho[2,1-b]furan (R7000): a case study with some considerations on nitrofurantoin and nifuroxazide

Two nitrofurans present broad-spectrum antimicrobial properties and some of them are used in human and veterinary medicine. Most of these molecules are mutagens and some of them were reported as carcinogens. Due to its extreme mutagenic potency in bacteria, the nitronaphtho derivative 2-nitro-7-methoxy-naphtho[2,1-b]furan (R7000) was used as a tool to analyze the mechanism of the genotoxic action of this family of chemicals. In the present paper, we review essential data on the genotoxicity of R7000 and briefly discuss the case of nitrofurantoin and nifuroxazide, two nitrofurans, still in use as urinary and gastrointestinal disinfectants.

Mutagenic properties of a nitrofuran, 7-methoxy-2-nitronaphtho[2, 1-b]furan (R7000), in lacI transgenic mice

The in vivo mutagenic properties of a 5-nitrofuran, the 7-methoxy-2-nitronaphtho[2,1-b]furan (R7000), already well known in bacteria, was evaluated in lacI transgenic mice (Big Blue). The mutation frequency was determined in various organs of i.p. - treated mice and the nature of induced mutations was determined for the target organs in which mutation induction was significant. It was found that R7000 is mutagenic in mice, although, on the basis of the number of induced mutants per unit mass in comparison with other known mutagenic chemicals, R7000 appears to be considerably less mutagenic in mice than in bacteria. The most affected organs, small intestine, caecum and colon organs belong to the digestive apparatus. The distribution of R7000-induced mutations in the lacI gene recovered from small intestine of transgenic mice was very similar to that which had been found in E. coli. The difference between mouse and E. coli in the R7000 induced mutational spectra are mainly in the proportion of single base frameshifts versus base substitutions. Since R7000 induced mutations seemed to arise in the population of stem cells and that the stem cells are important for carcinogenesis, our results are compatible with a possible carcinogenic effect of R7000 and other nitrofurans.

DNA damage induced in vivo by 7-methoxy-2-nitronaphtho[2,1-b]-furan (R7000) in the lacI gene of Escherichia coli

DNA adducts that block replication, induced in vivo by the 5-nitrofuran derivative R7000 (7-methoxy-2-nitronaphtho[2, 1-b]-furan) were mapped, at nucleotide resolution, in a region of the lacI gene of Escherichia coli, using a reiterative primer extension assay [D. Chandrasekhar, B. Van Houten, High resolution mapping of UV-induced photoproducts in the Escherichia coli lacI gene: inefficient repair of the non-transcribed strand correlates with high mutation frequency, J. Mol. Biol., 1994, Vol. 238, pp. 319-332]. It was found that R7000 induced a broad spectrum of low frequency replication blocks rather than particular hot spots in a limited number of particular targets. Most of these replication blocks were observed at G nucleotides, and most of G nucleotides present in the DNA sequence, if not all, constituted a possible target for the chemical attack of the compound. In addition, a large part of replication blocks observed at A, C or T could also reflect a replication block at the 3' or 5' nucleotide flanking a guanosine-DNA adduct. Only a very small number of replication blocks could be observed at A, C or T nucleotides non-adjacent to a G. These results show that, guanosine-DNA adducts are the main DNA lesions that block replication induced by R7000 in E. coli and suggests a strong reactivity of the genotoxic species generated in vivo by R7000 with the G nucleotidic targets. From 26 R7000-induced mutations previously mapped in this region [E. Touati, E. Krin, P. Quillardet, M. Hofnung, 7-methoxy-2-nitronaphto[2,1-b]furan (R7000)-induced mutation spectrum in the lacI gene of Escherichia coli: influence of SOS mutagenesis, Carcinogenesis, 1996, Vol. 17, pp. 2543-2550.], 22 (85%) occurred at GC base pairs at which termination products were observed. The other mutagenic events involved AT base pairs adjacent to a G nucleotide forming a replication block. Thus all mutagenic events occurred at, or adjacent to, a G nucleotide forming a replication block. Although it could not be excluded that some mutagenic events are due to undetected DNA lesions that do not block replication, these results strongly suggest that guanosine-DNA adducts that block DNA replication are responsive for a large part of the mutagenic events generated by R7000. The powerful capacity of R7000 to form adducts at most of the guanosine residues in a DNA sequence may account for at least part of its very potent genotoxic properties.

Influence of the uvr-dependent nucleotide excision repair on DNA adducts formation and mutagenic spectrum of a potent genotoxic agent: 7-methoxy-2-nitronaphtho[2,1-b]furan (R7000)

The influence of the uvr-dependent excision repair system on the lethal action, mutagenic specificity, SOS induction and DNA adducts formation of 7-methoxy-2-nitronaphtho[2,1-b]furan (R7000), a potent genotoxic nitrofuran, were examined in Escherichia coli. Binding measurements of 3H-labelled R7000 to DNA indicated that R7000-DNA adducts can be removed by excision repair soon after the action of the chemical: 50% of the DNA adducts were removed within 10 min of treatment. After 1 h of incubation the level of excision reached 70%. This result was confirmed using the postlabelling technique. We found that R7000 yielded at least 10 different DNA adducts. Each of the adducts detected could be removed by excision repair. The rates of excision appeared different from one to the other. In addition, using a lacZ reversion system that is able to detect each type of base substitution mutations [1], we found that in uvrA bacteria deficient in excision repair, R7000 can induce 5 out of the 6 possible mutational events: GC-->TA, AT-->TA, GC-->CG, AT-->CG and GC-->AT. The transition AT-->GC was not observed. Only 3 transversions: GC-->TA, AT-->TA and GC-->CG could be detected in repair proficient uvr+ bacteria. The differences between the mutagenic spectra obtained in either uvr+ bacteria or uvrA mutants indicate that some potentially mutagenic DNA adducts induced by R7000 can be removed by excision repair, thus lowering the mutagenic potency of the chemical and modifying the mutagenic spectrum detected.

Global response of Escherichia coli cells to a treatment with 7-methoxy-2-nitronaphtho[2,1-b]furan (R7000), an extremely potent mutagen

We have studied the global changes which are induced in exponentially growing cultures of Escherichia coli during treatment with 7-methoxy-2-nitronaphtho[2,1-b]furan (R7000), a very potent mutagen and inducer of the SOS response. A two-dimensional analysis of the proteins synthesized in the presence of this agent was performed. In a strain deficient in SOS induction, the expression of 24 polypeptides was found to be affected by this treatment: 14 corresponded to proteins known to be implicated in different stress responses, particularly chaperones such as DnaK and GroEL. The variation of another protein was detected for the first time in this study: transketolase I, an enzyme of intermediary metabolism which was characterized in the present work by microsequencing. In parallel, the expression of 9 other proteins, still unidentified, was modified by this nitrofuran. They represent potential candidates involved in metabolic and DNA lesions repair specific of R7000 action.

Free radical production during photo-assisted NADPH reduction of four alpha-nitroarenofurans

Generation of radical anions during NADPH reduction of four mutagenic and genotoxic alpha-nitroarenofurans was examined. ESR showed that free radicals were generated during reduction solely in the presence of light. Computer simulations of ESR spectra were in good agreement with the experimental ones.