Role of classical nitroreductase and O-acetyltransferase on the mutagenicity of nifurtimox and eight derivatives in Salmonella typhimurium

Antiparasitic prodrug nifurtimox: revisiting its activation mechanism

EVALUATION OF: Hall BS, Bot C, Wilkinson SR. Nifurtimox activation by trypanosomal type I nitroreductases generates cytotoxic nitrile metabolites. J. Biol. Chem. 286, 13088-13095 (2011). The prodrug nifurtimox has been one of the pharmacologic alternatives to treat Chagas disease and currently forms part of a combinational therapy to treat West African trypanosomiasis. Despite this, nifurtimox's mechanism of action is only partially understood and has been related to induction of oxidative stress in the target cell. An alternative mechanism involving reductive activation by a eukaryotic type I nitroreductase has been described. Bloodstream form Trypanosoma brucei overexpressing enzymes, proposed to metabolize nifurtimox, were generated and only cells with elevated levels of the nitroreductase displayed altered susceptibility to the drug, implying that it has a key role in drug action. Reduction of nifurtimox by trypanosomal type I nitroreductases was shown to be insensitive to oxygen and yielded a product characterized by liquid chromatography/mass spectrometry as an unsaturated open chain nitrile. This nitrile inhibited both parasite and mammalian cell growth at equivalent concentrations, in marked contrast to the parental prodrug. These studies indicated that nifurtimox selectivity against T. brucei could be the result of the expression of a parasite-encoded type I nitroreductase.

A core in vitro genotoxicity battery comprising the Ames test plus the in vitro micronucleus test is sufficient to detect rodent carcinogens and in vivo genotoxins

In vitro genotoxicity testing needs to include tests in both bacterial and mammalian cells, and be able to detect gene mutations, chromosomal damage and aneuploidy. This may be achieved by a combination of the Ames test (detects gene mutations) and the in vitro micronucleus test (MNvit), since the latter detects both chromosomal aberrations and aneuploidy. In this paper we therefore present an analysis of an existing database of rodent carcinogens and a new database of in vivo genotoxins in terms of the in vitro genotoxicity tests needed to detect their in vivo activity. Published in vitro data from at least one test system (most were from the Ames test) were available for 557 carcinogens and 405 in vivo genotoxins. Because there are fewer publications on the MNvit than for other mammalian cell tests, and because the concordance between the MNvit and the in vitro chromosomal aberration (CAvit) test is so high for clastogenic activity, positive results in the CAvit test were taken as indicative of a positive result in the MNvit where there were no, or only inadequate data for the latter. Also, because Hprt and Tk loci both detect gene-mutation activity, a positive Hprt test was taken as indicative of a mouse-lymphoma Tk assay (MLA)-positive, where there were no data for the latter. Almost all of the 962 rodent carcinogens and in vivo genotoxins were detected by an in vitro battery comprising Ames+MNvit. An additional 11 carcinogens and six in vivo genotoxins would apparently be detected by the MLA, but many of these had not been tested in the MNvit or CAvit tests. Only four chemicals emerge as potentially being more readily detected in MLA than in Ames+MNvit--benzyl acetate, toluene, morphine and thiabendazole--and none of these are convincing cases to argue for the inclusion of the MLA in addition to Ames+MNvit. Thus, there is no convincing evidence that any genotoxic rodent carcinogens or in vivo genotoxins would remain undetected in an in vitro test battery consisting of Ames+MNvit.

Genotoxicity revaluation of three commercial nitroheterocyclic drugs: nifurtimox, benznidazole, and metronidazole

Nitroheterocyclic compounds are widely used as therapeutic agents against a variety of protozoan and bacterial infections. However, the literature on these compounds, suspected of being carcinogens, is widely controversial. In this study, cytotoxic and genotoxic potential of three drugs, Nifurtimox (NFX), Benznidazole (BNZ), and Metronidazole (MTZ) was re-evaluated by different assays. Only NFX reduces survival rate in actively proliferating cells. The compounds are more active for base-pair substitution than frameshift induction in Salmonella; NFX and BNZ are more mutagenic than MTZ; they are widely dependent from nitroreduction whereas microsomal fraction S9 weakly affects the mutagenic potential. Comet assay detects BNZ- and NFX-induced DNA damage at doses in the range of therapeutically treated patient plasma concentration; BNZ seems to mainly act through ROS generation whereas a dose-dependent mechanism of DNA damaging is suggested for NFX. The lack of effects on mammalian cells for MTZ is confirmed also in MN assay whereas MN induction is observed for NFX and BNZ. The effects of MTZ, that shows comparatively low reduction potential, seem to be strictly dependent on anaerobic/hypoxic conditions. Both NFX and BNZ may not only lead to cellular damage of the infective agent but also interact with the DNA of mammalian cells.

Genotoxicity testing of the furylethylene derivative 1-(5-bromofur-2-yl)-2-bromo-2-nitroethene in cultured human lymphocytes

The genotoxic potential of the compound 1-(5-bromofur-2-yl)-2-bromo-2-nitroethene (G-1) was evaluated in peripheral blood lymphocytes cultured in vitro, at concentrations ranging from 1 to 20 microg/ml. Micronuclei (MN) and sister-chromatid exchanges (SCE) were scored as biomarkers of genotoxic effects. To detect the role of metabolic enzymes on the genotoxicity of this furylethylenic derivative, cultures for MN and SCE demonstrations were treated for 3 h with and without the S9 microsomal fraction as well as for 48 h without S9. Under the conditions of the study, the test agent did not induce significant increases in the frequency of micronucleated cells, irrespective of the presence/absence of the metabolic fraction. Nevertheless, a slight/moderate increase in the SCE frequency was observed in those cultures treated without the S9 mix. In addition, cytotoxic/cytostatic effects of the G-1 compound were observed mainly in cultures without S9 fraction, as indicated by the reduction of cell proliferation measured by the cytokinesis block proliferation index (CBPI) and the proliferative rate index (PRI).

Genotoxic evaluation of the furylethylene derivative 1-(5-bromofur-2-yl)-2-nitroethene in cultured human lymphocytes

The genotoxicity of the 2-furylethylene derivative 1-(5-bromofur-2-yl)-2-nitroethene (2-betaNF) has been evaluated in cultured human peripheral blood lymphocytes at concentrations ranging from 0.5 to 15microg/ml. The frequencies of micronuclei (MN) and sister-chromatid exchanges (SCEs) were used and scored as indicators of genetic damage. To asses the role of the metabolism mediated by the enzymes present in the S9 mix, over the possible genotoxic potential of the test agent, the cultures for MN and SCE demonstrations were treated for 3h in presence and in absence of rat liver microsomal fraction. The results indicate that, under the experimental conditions used, the test agent does not induce significant increases in the frequency of micronucleated cells, irrespective of the presence/absence of metabolic fraction. Nevertheless, a slight increase in the SCE frequency was observed in those cultures treated without the S9 mix; although this slight increase disappeared in the experiments carried out with the microsomal fraction. In addition, cytotoxic/cytostatic effects of (2-betaNF) were observed mainly in the cultures treated without the S9 fraction.

Genotoxic evaluation of the furylethylene derivative 2-furyl-1-nitroethene in cultured human lymphocytes

The compound 2-furyl-1-nitroethene (G-0) was evaluated for genotoxicity in cultured human peripheral blood lymphocytes, at concentrations ranging from 1 to 15microg/ml. Micronuclei (MN) and sister-chromatid exchanges (SCEs) were scored as genetic endpoints. In order to detect the role of metabolic enzymes on the genotoxicity of this furylethylenic derivative, the cultures for MN and SCE demonstrations were also treated with S9 microsomal fraction. The results indicate that, under the conditions of the study, the test agent does not seem to induce significant increases in the frequency of micronucleated cells, irrespective of the presence of metabolic activation. Nevertheless, a slight increase in the SCE frequency was observed in those cultures treated without the S9 mix; although this increase disappeared in presence of the microsomal fraction. In addition, cytostatic effects of 2-furyl-1-nitroethene were observed mainly in cultures without S9 fraction, as indicated by the reduction of cell proliferation.