Mechanism of Toxicity of Nitro Compounds Used in the Chemotherapy of Trichomoniasis

Moreno, Silvia N.J.; Docampo, Roberto

doi:10.2307/3430010

Cited by 36 publications

(42 citation statements)

References 53 publications

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“…This activation process is predominant in mammalian cells (27,33). Where oxygen is limited, the nitroso form can be produced either by a further single-electron reduction step or by the interaction of two nitro anions (26). The nitroso molecule is highly reactive and can lead to cellular damage directly or indirectly through the formation a hydroxylamine derivative (16,18).…”

Section: Figmentioning

confidence: 99%

Activation of Benznidazole by Trypanosomal Type I Nitroreductases Results in Glyoxal Formation

Hall

Wilkinson

2012

Antimicrob Agents Chemother

195

180

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Benznidazole, a 2-nitroimidazole, is the front-line treatment used against American trypanosomiasis, a parasitic infection caused by Trypanosoma cruzi. Despite nearly 40 years of use, the trypanocidal activity of this prodrug is not fully understood. It has been proposed that benznidazole activation leads to the formation of reductive metabolites that can cause a series of deleterious effects, including DNA damage and thiol depletion. Here, we show that the key step in benznidazole activation involves an NADH-dependent trypanosomal type I nitroreductase. This catalyzes an oxygen-insensitive reaction with the interaction of enzyme, reductant, and prodrug occurring through a ping-pong mechanism. Liquid chromatography/mass spectrometry (LC/MS) analysis of the resultant metabolites identified 4,5-dihydro-4,5-dihydroxyimidazole as the major product of a reductive pathway proceeding through hydroxylamine and hydroxy intermediates. The breakdown of this product released the reactive dialdehyde glyoxal, which, in the presence of guanosine, generated guanosine-glyoxal adducts. These experiments indicate that the reduction of benznidazole by type I nitroreductase activity leads to the formation of highly reactive metabolites and that the expression of this enzyme is key to the trypanocidal properties displayed by the prodrug.

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Section: Figmentioning

confidence: 99%

Activation of Benznidazole by Trypanosomal Type I Nitroreductases Results in Glyoxal Formation

Hall

Wilkinson

2012

Antimicrob Agents Chemother

195

180

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“…In a variety of microaerophilic and anaerobic protists and bacteria, it has been suggested that drug reduction products are responsible for organism killing; these products include the nitro radical anion generated by electron donation from pyruvate : ferredoxin oxidoreductase (PFOR) via ferredoxin species [e.g. in the hydrogenosomes of trichomonads (Moreno & Docampo, 1985;Yarlett et al, 1986bYarlett et al, , 1987]. Drug free radical generation also leads to the production of reactive oxygen species (ROS) by a redox cycling mechanism.…”

Section: Introductionmentioning

confidence: 99%

Menadione kills trophozoites and cysts of Giardia intestinalis

et al. 2004

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Production of reactive oxygen species by redox cycling in the presence of low levels of oxygen has been studied as a possible approach to anti-protozoal chemotherapeutic strategy. Incubation of the diplomonad flagellate Giardia intestinalis with 2-methy-1,4-naphthoquinone (menadione), under anaerobic conditions, gave UV absorption changes characteristic of reduction to menadiol; partial reversal was observed on admitting O 2 . Under microaerobic conditions, similar to those on the surface of the jejunal mucosa, trophozoites consumed O 2 rapidly in the presence of menadione; reaction products included singlet O 2 (monitored by single photon counting of O 2 -dependent low-level chemiluminescence) and H 2 O 2 (measured by the formation of Complex I of microperoxidase). Trophozoites became swollen and incapable of regulatory volume control; these irreversible responses led to loss of motility, cessation of flagellar activity and cell death. Comparison of the sensitivities of trophozoites to metronidazole and menadione gave LC 50 values (mg ml "1 ) of 1?2 and 0?7, respectively; corresponding values for cysts (measured by in vitro excystation capacities) were >50 and 1?3. Menadione (LD 50 in mice, 0?5 g kg "1 ) is therefore a potentially more useful and general anti-giardial agent than metronidazole, as it is active against cysts as well as trophozoites.

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“…Nitroheterocyclic compounds are generally believed to exert their cytotoxic effects only after activation by single electron reduction of their corresponding nitro anion radicals (27). In bacteria, the toxic effects of metronidazole are accompanied by damage to DNA (25,29) and cells defective in DNA repair become hypersensitive to the action of metronidazole (11).…”

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confidence: 99%

Activity of Megazol, a Trypanocidal Nitroimidazole, Is Associated with DNA Damage

Enanga

Ariyanayagam

Stewart

et al. 2003

Antimicrob Agents Chemother

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DNA damage associated with the trypanocidal activity of megazol [2-amino-5-(1-methyl-5-nitro-2-imidazolyl)-1,3,4-thiadiazole] was shown in experiments in which DNA repair-deficient RAD51؊/؊ Trypanosoma brucei mutants were found to be hypersensitive to the drug. Parasites resistant to megazol were selected and showed modest cross-resistance to other trypanocides, although neither drug efflux nor changes to intracellular thiols correlated with resistance.New drugs are urgently required for treatment of human African trypanosomiasis (22). One compound with promise is megazol (5, 12, 13, 18), a nitroheterocyclic compound that forms a nitro radical anion upon reduction (31). Megazol appears to enter Trypanosoma brucei via passive diffusion (3), but little is known about its mode of action. We determined the effect of megazol in assays that measured oxidative and reductive stress. We also selected parasites resistant to megazol in order to assess cellular changes associated with resistance.Bloodstream forms (17) and procyclic forms (6) of T. brucei brucei (strain 427) were cultivated by using standard techniques. Assays measuring drug sensitivity were carried out to determine the drug concentration producing a 50% decrease in cell proliferation (IC 50 ) and the resistance factor (average ratio of the IC 50 for a resistant clone to the IC 50 for a wild-type clone). The Alamar Blue test (28) was used for bloodstream forms, while cell counting by using an improved Neubauer chamber was required for procyclics. For procyclic cells the IC 50 of megazol was 0.28 Ϯ 0.01 M, and for bloodstream forms the IC 50 was 0.15 Ϯ 0.02 M.To induce megazol resistance in vitro, trypanosomes were exposed to drug concentrations that doubled every 2 weeks, starting at 0.008 and 0.01 M for bloodstream forms and procyclics, respectively. After 6 months of cultivation, procyclic and bloodstream forms that tolerated 10 and 1 M megazol, respectively, were cloned by limiting dilution.The IC 50 for the cloned, resistant, procyclic line was 29.24 Ϯ 3.2 M (105-fold-reduced sensitivity), while the IC 50 for the bloodstream form line was 3.2 Ϯ 0.48 M (21-fold-reduced sensitivity).Cross-resistance to megazol and other trypanocides was also studied (Table 1). Moderate levels of cross-resistance to two nitroheterocyclic trypanocides, the nitrofuran nifurtimox and the nitroimidazole benznidazole, were detected. Significant, albeit low-level, cross-resistance to the melaminophenyl arsenical compounds cymelarsan, melarsen oxide, and melarsoprol, the diamidine compounds pentamidine and berenil, and suramin was also observed.Verapamil (20), PAK-104P (7), and two phenothiazine derivatives (prochlorperazine and trifluoperazine) (14) all inhibit cellular extrusion pumps but, when used at 5 M, failed to reverse megazol resistance in T. brucei. Efflux pumps are thus unlikely to play a significant role in the megazol resistance characterized here.It was recently shown that megazol exposure reduces trypanothione levels in Trypanosoma cruzi (23). Bloodstream form T. b...

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Mechanism of Toxicity of Nitro Compounds Used in the Chemotherapy of Trichomoniasis

Cited by 36 publications

References 53 publications

Activation of Benznidazole by Trypanosomal Type I Nitroreductases Results in Glyoxal Formation

Activation of Benznidazole by Trypanosomal Type I Nitroreductases Results in Glyoxal Formation

Menadione kills trophozoites and cysts of Giardia intestinalis

Activity of Megazol, a Trypanocidal Nitroimidazole, Is Associated with DNA Damage

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