Effect of radiation-induced reduction of nitroimidazoles on biologically active DNA

Nitropyrrole natural products: isolation, biosynthesis and total synthesis

A review of the isolation, biological activity, biosynthesis and chemical synthesis of nitropyrrole-containing natural products reported to date, including the pyrrolomycins, heronapyrroles and nitropyrrolins.

Current therapeutics, their problems, and sulfur-containing-amino-acid metabolism as a novel target against infections by "amitochondriate" protozoan parasites

The "amitochondriate" protozoan parasites of humans Entamoeba histolytica, Giardia intestinalis, and Trichomonas vaginalis share many biochemical features, e.g., energy and amino acid metabolism, a spectrum of drugs for their treatment, and the occurrence of drug resistance. These parasites possess metabolic pathways that are divergent from those of their mammalian hosts and are often considered to be good targets for drug development. Sulfur-containing-amino-acid metabolism represents one such divergent metabolic pathway, namely, the cysteine biosynthetic pathway and methionine gamma-lyase-mediated catabolism of sulfur-containing amino acids, which are present in T. vaginalis and E. histolytica but absent in G. intestinalis. These pathways are potentially exploitable for development of drugs against amoebiasis and trichomoniasis. For instance, L-trifluoromethionine, which is catalyzed by methionine gamma-lyase and produces a toxic product, is effective against T. vaginalis and E. histolytica parasites in vitro and in vivo and may represent a good lead compound. In this review, we summarize the biology of these microaerophilic parasites, their clinical manifestation and epidemiology of disease, chemotherapeutics, the modes of action of representative drugs, and problems related to these drugs, including drug resistance. We further discuss our approach to exploit unique sulfur-containing-amino-acid metabolism, focusing on development of drugs against E. histolytica.

Treatment of infections caused by metronidazole-resistant Trichomonas vaginalis

Infections with the sexually transmitted protozoan Trichomonas vaginalis are usually treated with metronidazole, a 5-nitroimidazole drug derived from the antibiotic azomycin. Metronidazole treatment is generally efficient in eliminating T. vaginalis infection and has a low risk of serious side effects. However, studies have shown that at least 5% of clinical cases of trichomoniasis are caused by parasites resistant to the drug. The lack of approved alternative therapies for T. vaginalis treatment means that higher and sometimes toxic doses of metronidazole are the only option for patients with resistant disease. Clearly, studies of the treatment and prevention of refractory trichomoniasis are essential. This review describes the mechanisms of metronidazole resistance in T. vaginalis and provides a summary of trichomonicidal and vaccine candidate drugs.

DNA single strand breaks in peripheral blood lymphocytes induced by three nitroimidazole derivatives

Tinidazole (TNZ), ornidazole (ONZ) and metronidazole (MTZ) are antiparasitic drugs (nitroimidazole derivatives) that have proven to be effective against Trichomonas vaginalis, Entoamoeba histolytica, Giardia lamblia and Helicobacter pylori. The reduction of the nitro group and the generation of short-lived reactive intermediates are the basis of its parasiticidal activity. This reduction is associated with its mutagenic activity in bacteria, although in mammalian cells DNA damage seems to be related to the production of reactive oxygen species (ROS). Using alkaline single cell electrophoresis, a significant increase in single strand breaks and alkali labile sites in human peripheral blood lymphocytes (PBL) exposed to MTZ, ONZ and TNZ at 10, 100 and 500 microg/ml is observed. MTZ causes less damage, especially at higher concentrations, when compared with TNZ, the most harmful of the drugs tested. These findings suggest that primary damage is induced under aerobic conditions and confirms that these nitroimidazoles are DNA damaging agents.

Radiation-chemical properties of the hypoxic cell radiosensitizer doranidazole (PR-350)

This study was performed to confirm the radiation-chemical properties of the 2-nitroimidazole derivative doranidazole, (+/-)-(2RS,3SR)-3-[(2-nitroimdazol-1-yl)-methoxy]butane-1,2,4-triol [CAS 137339-64-1], PR-350, which was synthesized as a hypoxic cell radiosensitizer with low toxicity. Radiation-chemical experiments using doranidazole showed that (1) unlike O2, it had high reactivity toward not only hydrated electrons (eaq-), but also hydroxyl radicals (.OH), (2) the reduced intermediates of doranidasole had no ability to induce immediate strand breaks of colE1 plasmid DNA, (3) doranidazole enhanced radiation-induced DNA strand breaks of colE1 plasmid DNA in the aqueous state, whereas it did not enhance the base alteration, such as 8-oxo-deoxyguanosine, (4) it enhanced the radiation-induced formation of strand breaks with 3'-phosophate and 3'-phosphoglycolate termini, and (5) it was bound to DNA after irradiation. These facts revealed that the majority of radiation-chemical properties of doranidazole, except for the high reactivity toward OH, were similar to those of oxygen.

Contrasting effects of SH-compounds on oxidative DNA damage: repair and increase of damage

The non-radical singlet oxygen (1O2) and the OH radical (.OH) are the major damaging oxidative species that can be generated inside cells during normal aerobic metabolism and by processes such as photosensitization. Both reactive oxygen species fulfill essential prerequisites to be a genotoxic agent. Due to their continuous production they represent an ever-present threat to all vital cellular molecules, especially DNA. As might be anticipated from the difference in character between these reactive species (non-radical versus radical) the pattern of DNA modifications caused by singlet oxygen is different from that produced by OH radicals. All cells possess an elaborate defense system against oxidative damage. This paper focuses mainly on the effect of thiols such as glutathione, which are thought to play a role as antioxidants. Under certain conditions thiols can repair chemically, probably by H-donation, some of the DNA damage caused by .OH; for instance breaks can be rather easily prevented in this way. This process will compete with fixation of damage by oxygen. However, there is ample evidence that H-atom donation does not always lead to 'correct' repair. Moreover under aerobic conditions thiyl peroxy radicals might increase DNA damage. Although the repair/fixation process could not be examined in the case of 1O2 yet, it could be demonstrated that reactive species can be formed out of the reaction of thiols with 1O2 capable of enhancing the number of DNA modifications such as 8-oxoguanine and single-strand breaks, probably arising from different pathways. Although it is quite clear that thiols are to some extent excellent antioxidants they possess unexpected properties which, depending on the conditions, can have genotoxic consequences.

DNA damage induced by reduced nitroimidazole drugs

Five nitroimidazole drugs were reduced electrolytically and by gamma-radiolysis at fast (300 mumoles or 100% per hr) and slow (3-9 mumoles or 1-3% per hr) reduction rates in the presence of Escherichia coli DNA and single stranded or double stranded DNA from the bacteriophage phi X174. The degree of DNA damage depends upon the rate of drug reduction, where slow reduction produces more damage than fast reduction. The efficiency of damage produced is in the order metronidazole greater than ornidazole greater than azomycin greater than misonidazole greater than benznidazole which reveals a linear correlation between the one-electron reduction potential (E17) and the negative logarithm of the concentration of reduced drug at which 37% of the original DNA activity remains. Damage is not influenced by the presence of O2 at least between about 1-100 ppm. We suggest the protonated one-electron nitro radical anion as a possible candidate for the active damaging species and explain the basis of the relative cytotoxicity of these drugs under conditions of hypoxia.