Twelve 4- and 5-nitroimidazole derivatives, including metronidazole and two of its metabolites, tinidazole, dimetridazole, and nimorazole, were tested for antitrichomonad action on Tritrichomonas foetus (KV(1)) and Trichomonas vaginalis (ATCC 30001) for mutagenicity on a nitroreductase-positive (TA 100) and a nitroreductase-deficient (TA 100-FR(1)) strain of Salmonella typhimurium, as well as for the reducibility of the nitro group by T. foetus homogenates. Compounds with activity <1% of that of metronidazole are regarded as inactive. All antitrichomonad compounds induce mutations and can be reduced. S. typhimurium TA 100 gave mutations under both aerobiosis and anaerobiosis; TA 100-FR(1), however, gave mutations only under anaerobiosis. Certain compounds that are reducible, and the nonreducible derivatives, were inactive. Metronidazole and its inactive 4-nitro analogue were reduced in a four-electron process in ferredoxin- or methyl viologen-mediated reactions with the same velocity. The results underscore the role of the reduction of the nitro group in the antitrichomonad and in the mutagenic activity of nitroimidazoles.

Antitrichomonad Action, Mutagenicity, and Reduction of Metronidazole and Other Nitroimidazoles

Incubation of Chinese hamster ovary cells and KHT murine fibrosarcoma tumor cells in the absence of oxygen with 1-[2-14C]nitro-1-imidazolyl)-3-methoxy-2-propanol, one of the most effective radiation sensitizers of hypoxic cells, results in the preferential reduction of 1-[2-14C]nitro-1-imidazolyl)-3-methoxy-2-propanol. The radioactivity associated with the acid-insoluble precipitate from cells incubated in nitrogen is about four times higher than that of cells incubated in air. When aqueous extracts of tissues of a C3H mouse bearing the KHT tumor, after i.p. injection with 1-[2-14C]nitro-1-imidazolyl)-3-methoxy-2-propanol, are analyzed, a reduction product is found in relatively higher yields in the tumor than in normal tissues. The relative radioactivity in the pellet from the tumor homogenate is also high in comparison with those of most normal tissues. These results provide suggestive evidence for a higher degree of hypoxic in the tumor than in most normal tissues. The formation of reduction products and their subsequent binding to macromolecules may explain the preferential toxicity of nitro compounds to mammalian cells under hypoxia conditions. These results suggest that some nitro compounds may be useful for the treatment of tumors having a high fraction of hypoxic cells even in the absence of radiation.

https://cancerres.aacrjournals.org/content/canres/36/10/3761.full.pdf

Hypoxia-dependent Reduction of 1-(2-Nitro-1-imidazolyl)-3-methoxy-2-propanol by Chinese Hamster Ovary Cells and KHT Tumor Cells in Vitro and in Vivo

Publisher Summary This chapter reviews the developments in imidazole chemistry and discusses condensed imidazoles (such as benzimidazole, purines, etc), and compounds containing reduced imidazole rings The methods for the syntheses of the imidazole ring system are also presented Physical properties of imidazole and its derivatives such as dipole moments; melting points, boiling points, and solubilities; acid and basic strength; crystal structure; UV spectra; IR and Raman spectra; NMR spectra; mass spectrometry; electron spin resonance; optical rotatory dispersion and polarography; chromatography; thermodynamic data; and quantum mechanical calculations are described In addition, the chemical properties such as structure; aromatic character and reactivity; electrophilic substitution reactions; nucleophilic attack; reactions involving radicals; reactions of substituents on ring carbon atoms; reactions of substituents on ring nitrogen atoms; and miscellaneous properties of imidazole and its derivatives are illustrated Most of the reactions and properties of 1-substituted imidazoles have been discussed Apart from the utilization of the readily removable benzyl substituent in synthetic procedures leading to 2-substituted imidazoles, perhaps the most exciting advances have stemmed from the reactions of the 1 -acylimidazoles (imidazolides), which are extremely reactive in such nucleophilic reactions as hydrolysis and alcoholysis

Advances in Imidazole Chemistry

1.

When metronidazole (1-β-hydroxyethyl-2-methyl-5-nitroimidazole) was added at 50 μg/ml to a liquid (35°C) culture of Clostridium acetobutylicum (0.5 mg dry wt of organisms/ml), it inhibited growth for only 2 h; thereafter, growth resumed at a slower and arithmetic rate. Consumption of glucose and excretion of acetate plus butyrate continued, though at diminished rates, during the period of bacteriostasis, and the ATP content of the organisms did not decline.




2.

Metronidazole inhibited production of H2 by a culture of Cl. acetobutylicum, yet it only temporarily halted formation of H2 from pyruvate by cell extracts, and had no deleterious effect on the simultaneous production of acetyl phosphate and CO2. After a relatively short time no residual metronidazole was detectable in this cell-free system and H2 production resumed at its former rate.




3.

The drug did not inhibit the hydrogenases of either Cl. acetobutylicum or Cl. pasteurianum, but having an E′0 at pH 7 of approx. -0.23 V (on the hydrogen scale), it acted as a preferential acceptor of electrons from reduced ferredoxin (or reduced viologen dyes). Reduction of metronidazole was a 6 electron process compatible with complete reduction of its 5-nitro group. Yet, although the expected decrease in absorbance at 320 nm was observed, the 5-amino derivative did not accumulate (probably due to its intrinsic lability).




4.

Since many aryl nitro compounds are similarly able to accept electrons from reduced ferredoxin, it seems that additional reasons might have to be sought to explain the unique effectiveness of metronidazole as an antimicrobial agent.

J.E. Stambaugh

Papers

The characterization of substituted nitroimidazoles on paper and thin layer chromatography by colorimetric reactions