Properties of 2-hydroxylaminoimidazoles and their implications for the biological effects of 2-nitroimidazoles

Major benznidazole metabolites in patients treated for Chagas disease: Mass spectrometry-based identification, structural analysis and detoxification pathways

Benznidazole is the drug of choice for the treatment of Chagas disease, but its metabolism in humans is unclear. Here, we identified and characterized the major benznidazole metabolites and their biosynthetic mechanisms in humans by analyzing the ionic profiles of urine samples from patients and untreated donors through reversed-phase UHPLC-ESI-QTOF-MS and UHPLC-ESI-QqLIT-MS. A strategy for simultaneous detection and fragmentation of characteristic positive and negative ions was employed using information-dependent acquisitions (IDA). Selected precursor ions, neutral losses, and MS³ experiments complemented the study. A total of six phase-I and ten phase-II metabolites were identified and structurally characterized in urine of benznidazole-treated patients. Based on creatinine-corrected ion intensities, nitroreduction to amino-benznidazole (M1) and its subsequent N-glucuronidation to M5 were the main metabolic pathways, followed by imidazole-ring cleavage, oxidations, and cysteine conjugations. This extensive exploration of benznidazole metabolites revealed potentially toxic structures in the form of glucuronides and glutathione derivatives, which may be associated with recurrent treatment adverse events; this possibility warrants further exploration in future clinical trials. Incorporation of this knowledge of the benznidazole metabolic profile into clinical pharmacology trials could lead to improved treatments, facilitate the study of possible drug-drug interactions, and even mitigation of adverse drug reactions.

Click chemistry-facilitated comprehensive identification of proteins adducted by antimicrobial 5-nitroimidazoles for discovery of alternative drug targets against giardiasis

Giardiasis and other protozoan infections are major worldwide causes of morbidity and mortality, yet development of new antimicrobial agents with improved efficacy and ability to override increasingly common drug resistance remains a major challenge. Antimicrobial drug development typically proceeds by broad functional screens of large chemical libraries or hypothesis-driven exploration of single microbial targets, but both strategies have challenges that have limited the introduction of new antimicrobials. Here, we describe an alternative drug development strategy that identifies a sufficient but manageable number of promising targets, while reducing the risk of pursuing targets of unproven value. The strategy is based on defining and exploiting the incompletely understood adduction targets of 5-nitroimidazoles, which are proven antimicrobials against a wide range of anaerobic protozoan and bacterial pathogens. Comprehensive adductome analysis by modified click chemistry and multi-dimensional proteomics were applied to the model pathogen Giardia lamblia to identify dozens of adducted protein targets common to both 5'-nitroimidazole-sensitive and -resistant cells. The list was highly enriched for known targets in G. lamblia, including arginine deiminase, α-tubulin, carbamate kinase, and heat shock protein 90, demonstrating the utility of the approach. Importantly, over twenty potential novel drug targets were identified. Inhibitors of two representative new targets, NADP-specific glutamate dehydrogenase and peroxiredoxin, were found to have significant antigiardial activity. Furthermore, all the identified targets remained available in resistant cells, since giardicidal activity of the respective inhibitors was not impacted by resistance to 5'-nitroimidazoles. These results demonstrate that the combined use of click chemistry and proteomics has the potential to reveal alternative drug targets for overcoming antimicrobial drug resistance in protozoan parasites.

Mass Spectrometry Imaging of the Hypoxia Marker Pimonidazole in a Breast Tumor Model

Although tumor hypoxia is associated with tumor aggressiveness and resistance to cancer treatment, many details of hypoxia-induced changes in tumors remain to be elucidated. Mass spectrometry imaging (MSI) is a technique that is well suited to study the biomolecular composition of specific tissue regions, such as hypoxic tumor regions. Here, we investigate the use of pimonidazole as an exogenous hypoxia marker for matrix-assisted laser desorption/ionization (MALDI) MSI. In hypoxic cells, pimonidazole is reduced and forms reactive products that bind to thiol groups in proteins, peptides, and amino acids. We show that a reductively activated pimonidazole metabolite can be imaged by MALDI-MSI in a breast tumor xenograft model. Immunohistochemical detection of pimonidazole adducts on adjacent tissue sections confirmed that this metabolite is localized to hypoxic tissue regions. We used this metabolite to image hypoxic tissue regions and their associated lipid and small molecule distributions with MALDI-MSI. We identified a heterogeneous distribution of 1-methylnicotinamide and acetylcarnitine, which mostly colocalized with hypoxic tumor regions. As pimonidazole is a widely used immunohistochemical marker of tissue hypoxia, it is likely that the presented direct MALDI-MSI approach is also applicable to other tissues from pimonidazole-injected animals or humans.

Identification of novel 4-anilinoquinazoline derivatives as potent EGFR inhibitors both under normoxia and hypoxia

A novel series of 4-anilinoquinazoline derivatives (19a-19t) were designed and synthesized through incorporation of the 2-nitroimidazole moiety into the 4-anilinoquinazoline scaffold of EGFR inhibitors. The most promising compound 19h displayed potent EGFR inhibitory activity with the IC50 value of 0.47 nM. It also strongly suppressed the proliferation of A549 and HT-29 cells with sub-micromolar IC50 values both under normoxia and hypoxia, which were several folds more potent than gefitinib and erlotinib. Further reductive mimic investigation revealed that 19h could be reductive activated under hypoxia and was fully consistent with the results of cell apoptotic assay and in vitro metabolism evaluation. Our results suggest that the incorporation of hypoxia-activated moiety into EGFR inhibitor scaffold might be a tractable strategy to overcome the tumor hypoxia.

Activation of benznidazole by trypanosomal type I nitroreductases results in glyoxal formation

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.

Imaging tumor hypoxia by magnetic resonance methods

Tumor hypoxia results from the negative balance between the oxygen demands of the tissue and the capacity of the neovasculature to deliver sufficient oxygen. The resulting oxygen deficit has important consequences with regard to the aggressiveness and malignancy of tumors, as well as their resistance to therapy, endowing the imaging of hypoxia with vital repercussions in tumor prognosis and therapy design. The molecular and cellular events underlying hypoxia are mediated mainly through hypoxia-inducible factor, a transcription factor with pleiotropic effects over a variety of cellular processes, including oncologic transformation, invasion and metastasis. However, few methodologies have been able to monitor noninvasively the oxygen tensions in vivo. MRI and MRS are often used for this purpose. Most MRI approaches are based on the effects of the local oxygen tension on: (i) the relaxation times of (19)F or (1)H indicators, such as perfluorocarbons or their (1)H analogs; (ii) the hemodynamics and magnetic susceptibility effects of oxy- and deoxyhemoglobin; and (iii) the effects of paramagnetic oxygen on the relaxation times of tissue water. (19)F MRS approaches monitor tumor hypoxia through the selective accumulation of reduced nitroimidazole derivatives in hypoxic zones, whereas electron spin resonance methods determine the oxygen level through its influence on the linewidths of appropriate paramagnetic probes in vivo. Finally, Overhauser-enhanced MRI combines the sensitivity of EPR methodology with the resolution of MRI, providing a window into the future use of hyperpolarized oxygen probes.

Modeling of the anticancer action for radical derivatives of nitroazoles: quantitative structure-activity relationship (QSAR) study

A QSAR analysis of the anti-tumor, anti-metastasis and anti-colony formation (for metastatic colonies) activities of eighteen nitroazoles (including metronidazole and hypoxic radiosensitizers RP-170, KU-2285 and sanazole (drug AK-2123)) and their nitro and nitroso anion radical derivatives against melanoma B16 in mice has been performed. The QSAR models were built with the use of the frontal polygon method. This approach has features of different 3D QSAR methodologies and belongs to the group of "indirect" methods. The procedure allows to build robust models with high predictive ability even in series of diverse and conformationally flexible compounds. Key atomic characteristics accompany the geometrical requirements in the analysis of local 3D molecular similarity. By variation of weight coefficients for hydrophobicity, refraction increments, and partial charge it is possible to achieve better quality of QSAR and evaluate the importance of each characteristic for biological activity under consideration. It was found that hydrophobicity, molar refraction and charge characteristics of nitro anion radical derivatives are more significant for interaction with molecular targets than those of the parent compounds and of the nitroso anion radical derivatives. Size and hydrophobic properties of substituents in nitro anion radicals play significant role for ligand-target interaction in the processes of inhibition of metastatic spreading and growth of metastatic colonies by nitroazoles. A scheme of competitive interaction of parent nitroazoles and their anion radicals with a target in organism is suggested. It can be concluded that the step of one-electron reduction of nitroazoles can be important for anticancer activity of these drugs.

Nitroimidazoles and imaging hypoxia

Decreased tissue oxygen tension is a component of many diseases. Although hypoxia can be secondary to a low inspired pO2 or a variety of lung disorders, the commonest cause is ischemia due to an oxygen demand greater than the local oxygen supply. In tumors, low tissue pO2 is often observed, most often due to a blood supply inadequate to meet the tumor's demands. Hypoxic tumor tissue is associated with increased resistance to therapy. In the heart tissue hypoxia is often observed in persistent low-flow states, such as hibernating myocardium. In patients with stroke, hypoxia has been associated with the penumbral region, where an intervention could preserve function. Despite the potential importance of oxygen levels in tissue, difficulty in making this measurement in vivo has limited its role in clinical decision making. A class of compounds known to undergo different intracellular metabolism depending on the availability of oxygen in tissue, the nitroimidazoles, have been advocated for imaging hypoxic tissue. When a nitroimidazole enters a viable cell the molecule undergoes a single electron reduction, to form a potentially reactive species. In the presence of normal oxygen levels the molecule is immediately reoxidized. This futile shuttling takes place for some time, before the molecule diffuses out of the cell. In hypoxic tissue the low oxygen concentration is not able to effectively compete to reoxidize the molecule and further reduction appears to take place, culminating in the association of the reduced nitroimidazole with various intracellular components. The association is not irreversible, since these agents clear from hypoxic tissue over time. Initial development of nitroimidazoles for in vivo imaging used radiohalogenated derivatives of misonidazole, such as fluoromisonidazole, some of which have recently been employed in patients. Two major problems with fluoromisonidazole are its relatively low concentration within the lesion and the need to wait several hours to permit clearance of the agent from the normoxic background tissue (contrast between lesion and background typically < 2:1 at about 90 min after injection). Even with high-resolution positron emission tomographic imaging, this combination of circumstances makes successful evaluation of hypoxic lesions a challenge.(ABSTRACT TRUNCATED AT 400 WORDS)

DNA damage induced in HT-29 colon cancer cells by exposure to 1-methyl-2-nitrosoimidazole, a reductive metabolite of 1-methyl-2-nitroimidazole

Exposure of HT-29 colon carcinoma cells to 1-methyl-2-nitrosoimidazole (INO), a reductive metabolite of a model 2-nitroimidazole, induced concentration-dependent DNA damage detectable by conventional alkaline (single-strand breaks) and neutral (double-strand breaks) filter elution techniques. Elution of DNA from the filters under alkaline conditions was distinctly biphasic. No evidence of DNA damage was detected when cellular DNA was incubated directly with INO prior to filter elution. DNA damage was enhanced markedly in HT-29 cells incubated with buthionine sulfoximine to deplete intracellular glutathione levels prior to INO treatment. The biphasic shape of the elution profiles was not attributable to loss of labeled thymidine mononucleotides or to the formation of DNA-protein crosslinks. Rather, the data suggest the existence of two subpopulations of cells differing in sensitivity to the DNA-damaging effects of INO exposure. Based upon differential adherence, two populations of cells, differing with respect to the rate and extent of elution from the filters during alkaline elution assays, were detected, although they could not be purified sufficiently by this technique to permit biochemical characterization. The results suggest that the nitroso intermediate is either an active metabolite, or a proximate form of the ultimate DNA-reactive species, responsible for DNA damage in cells exposed to 2-nitroimidazoles under reducing conditions.

Enhancement of melphalan (L-PAM) toxicity by reductive metabolites of 1-methyl-2-nitroimidazole, a model nitroimidazole chemosensitizing agent

Chemosensitization of bifunctional alkylators by misonidazole (MISO) and related nitroimidazoles in vitro has been shown to require hypoxic exposures. Presumably, reductive metabolism of the nitroimidazole under hypoxic conditions results in generation of a chemosensitizing intermediate(s) in a manner analogous to that described for the hypoxic toxicity of these compounds. In an attempt to identify these intermediates, we examined the ability of reductive metabolites of a model 2-nitroimidazole compound, 1-methyl-2-nitroimidazole (INO2), to enhance the toxicity of melphalan (t-PAM) in HT-29 human colon cancer cells. INO2 was a modest chemosensitizing agent, enhancing L-PAM only under hypoxic conditions. The 2-electron reduction product, 1-methyl-2-nitrosoimidazole (INO), was a potent chemosensitizer, enhancing L-PAM toxicity at micromolar concentrations under either aerobic or hypoxic conditions. In contrast, the 4- and 6-electron reduction products, 1-methyl-2-[hydroxylamino]imidazole and 1-methyl-2-aminoimidazole, respectively, failed to modify cell kill by L-PAM even at millimolar concentration. These results suggest that nitrosoimidazoles may be the active chemosensitizing species generated upon the reductive metabolism of nitroimidazoles.

Cytotoxicity and glutathione depletion by 1-methyl-2-nitrosoimidazole in human colon cancer cells

The biological effects of 1-methyl-2-nitrosoimidazole (INO), the 2 electron reduction product of biologically active 1-methyl-2-nitroimidazole, were examined in HT-29 human colon cancer cells by clonogenic assay and glutathione (GSH) determination. INO was very toxic towards HT-29 cells and was equally toxic under aerobic and hypoxic conditions. Cytotoxicity was highly dependent on cell concentration, decreasing as cell concentration increased. INO also resulted in an initial dose-dependent depletion of intracellular GSH which plateaued when the GSH content of INO-treated cells reached approximately 8% of control levels. As was the case for cytotoxicity, the magnitude of GSH depletion by any given INO dose was highly dependent on cell concentration, being greatest at low cell densities. Both toxicity and GSH depletion were more pronounced when cells were exposed in culture medium without the reducing agent, ascorbate. HT-29 cells preincubated with the GSH synthesis inhibitor, buthionine sulfoximine (BSO), to reduce GSH levels to approximately 10% of control levels were more sensitive to subsequent INO exposure. These data suggest that the nitroso- reduction product of 2-nitroimidazoles may be responsible for cytotoxicity and glutathione depletion associated with hypoxic exposure to 2-nitroimidazoles.

Toxicity of RSU-1069 for KHT cells treated in vivo or in vitro: evidence for a diffusible toxic product

RSU-1069 is a highly effective hypoxic cell cytotoxin in KHT sarcomas treated in vivo. However, relative to the hypoxic cells, the oxic cells in the tumor appear more sensitive to the drug than would have been predicted on the basis of results with CHO (AA8-4) cells treated in vitro with the drug under oxic and hypoxic conditions. To examine possible reasons for this difference, suspensions of KHT cells were prepared from tumors growing in vivo, and treated with RSU-1069 in vitro under oxic or hypoxic conditions. The sensitivity of the KHT cells was similar to that of AA8-4 cells, regardless of whether the cells were obtained from untreated tumors or from tumors given 15 Gy in vivo just prior to the preparation of the cell suspension. We observed, however, that the sensitivity of both AA8-4 cells and KHT cells to drug treatment under hypoxic conditions increased with the density of the cells in the treated suspension. This result suggests the possibility that a diffusible toxic product may be released from cells. Such a product could contribute to the toxicity of the drug for oxic cells in tumors in situ.

1-Methyl-2-nitrosoimidazole: cytotoxic and glutathione depleting capabilities

We tested 1-methyl-2-nitrosoimidazole (INO), the two electron reduction product of 1-methyl-2-nitroimidazole (INO2) for its in vitro cytotoxicity and glutathione (GSH) depleting capabilities. The half life of INO was shown to be dependent on cell concentration above 10(5) cells/ml, decreasing with increasing cell concentration up to 2 X 10(6) cells/ml. For a 10-fold decrease in cell concentration, from 10(6) to 10(5) cells/ml, the toxicity curve shifted 10-fold towards lower concentrations. At 10(6) cells/ml, INO depleted GSH, in the range of concentrations where toxicity was observed, down to a plateau of 15% of the control level at a concentration of 100 microM INO. Oxidized glutathione (GSSG) levels were not elevated significantly above control cultures at this concentration. INO2, 1000 microM, did not deplete GSH under similar exposure conditions while 2-hydroxylamino-1-methylimidazole (INHOH) depleted GSH minimally at this same concentration. The nitroso intermediate may play a central role in the toxicity and GSH depleting capabilities of 2-nitroimidazoles in mammalian cells.

Preparation, toxicity and mutagenicity of 1-methyl-2-nitrosoimidazole. A toxic 2-nitroimidazole reduction product

1-Methyl-2-nitrosoimidazole (INO), the 2-electron reduction product of 1-methyl-2-nitroimidazole (INO2), was prepared by electrochemical reduction of INO2 to 2-hydroxylamino-1-methyl-imidazole (INHOH), followed by back oxidation with iodine. Although stable in crystalline form, INO reacted in water, phosphate-buffered saline, and mammalian cell growth medium. Half-lives for decay were determined by UV-visible spectroscopy. INO was found to be highly toxic towards Chinese hamster ovary (CHO) cells, concentrations of 10-60 microM producing significant cytotoxicity. The rate of INO decay was found to be increased in the presence of CHO cells. INO was also toxic and mutagenic towards Salmonella typhimurium TA-100. When compared on a molar basis to the parent nitro compound INO2, and the 4- and 6-electron reduction products INHOH and 2-amino-1-methylimidazole (INH2), INO was by far (two orders of magnitude) the most toxic under aerobic conditions. These results suggest that the nitroso reduction product of 2-nitroimidazoles may be the reduced species responsible for hypoxic cell selective toxicity of 2-nitroimidazoles.

Mechanism of reductive activation of a 5-nitroimidazole by flavoproteins: model studies with dithionite

The flavoprotein nitroreductases NADPH:cytochrome P-450 reductase and xanthine oxidase catalyzed the cofactor-dependent anaerobic nitro group reduction and covalent binding to protein sulfhydryl groups of the 5-nitroimidazole substrate ronidazole [1-methyl-5-nitroimidazole-2-yl)-methyl carbamate). Studies with variously radiolabeled ronidazole molecules demonstrated that the imidazole ring was intact while greater than 80% of the C-4 3H and 2-carbamoyl group were lost from the covalently bound product. The stoichiometry of cofactor consumption during the enzyme-catalyzed reduction of the substrate could not be determined, so a model nitroreductase system which utilized dithionite as the reductant and agarose-immobilized cysteine as the target for alkylation was developed. Two moles of dithionite was consumed per mole of substrate for maximal reduction of uv absorbance due to the nitro group, for maximal release of C-4 3H, and for maximal covalent binding to agarose-immobilized cysteine. These results indicate that four electrons are required for the reductive activation of the substrate, consistent with formation of a hydroxylamine reactive intermediate. Covalent binding of variously radiolabeled substrate molecules after dithionite reduction exhibited the same labeling pattern as flavoprotein-catalyzed covalent binding, suggesting that covalent binding is mediated by the same species in both chemical and biological systems. The data are consistent with a mechanism where the substrate undergoes four-electron reduction to form a hydroxylamine, which is susceptible to nucleophilic attack at C-4. When water attacks C-4, the 2-carbamoyl group can eliminate to form a Michael-like acceptor which adds thiols at the 2-methylene position.

Induction of DNA strand breaks by reduced nitroimidazoles. Implications for DNA base damage

Radiation-reduced 2-nitroimidazoles (misonidazole, RSU-1137, Ro.03-8799 and Ro.03-8800) incubated in air with plasmid DNA (pH 7.0, 310K) induce DNA strand breakage, as revealed following subsequent heat or alkali treatment. Only RSU-1137 resulted in the binding of a [2-14C] fragment and significant yields of heat-labile strand breaks (greater than 20% loss of type-I DNA after 48 hr incubation). RSU-1137 was shown to be greater than 6 times more effective than misonidazole at producing alkali-labile breaks. In fact, the efficiency of alkali-induced strand break production is in the order: misonidazole less than Ro.03-8799 approximately Ro.03-8800 less than RSU-1137. Reaction of these reduced 2-nitroimidazoles with 2'-deoxyguanosine (dG) also results in the formation of a common glyoxal-dG product, with its yield and rate of production being dependent upon the 2-nitroimidazole used. It has been demonstrated that these variations are influenced by the N-1 side-chain of the 2-nitroimidazole. Product yields are approximately 5-6 times greater with misonidazole than with RSU-1137. From the evidence presented, it is apparent that formation of glyoxal (or a glyoxal-like product) is not responsible for the DNA strand breakage seen. It is inferred that these breaks are induced by a nitro-reduction product(s) which remains unidentified.

Stable reduction product of misonidazole

The predominant stable product (greater than 80%) of the anaerobic radiation chemical reduction (pH 7, formate, N2O) of misonidazole (MISO) has been identified as the cyclic guanidinium ion MISO-DDI, a 4,5-dihydro-4,5-dihydroxyimidazolium ion. This cation was prepared as its sulfate salt by the reaction of glyoxal and the appropriate N-substituted guanidinium sulfate. Its formation during MISO reduction was established by NMR spectral comparison and by derivatization as glyoxal bis-oxime, which was formed in 86% yield in fully reduced systems. The toxicity of pure MISO-DDI X sulfate was examined in vivo (C3H mice) and in vitro (CHO cells). This product is less toxic than the parent MISO and free glyoxal. A reactive, short-lived, intermediate is suggested as the agent responsible for the toxicity of MISO under hypoxic conditions.

Identification of a reactive glutathione conjugate as a metabolite of SR-2508 in CHO cells

Reaction between GSH and the hydroxylamine derivative of SR-2508 results in the formation of two stable conjugates identified as 2-amino-4-S-glutathionyl and 2-amino-5-S-glutathionyl imidazoles. These stable conjugates are apparently formed from a reactive derivative of the hydroxylamine that is sufficiently stable to be isolated after HPLC separation. The physical and chemical properties of this derivative are consistent with it being a GSH conjugate in which the glutathionyl residue is attached to the 2-amino nitrogen of the imidazole moiety through sulphur. With excess GSH, under physiological conditions, it forms a mixture of the two stable GSH conjugates. In CHO cells exposed to SR-2508 under hypoxic conditions, this unstable GSH conjugate has been detected and suggests the possibility of GSH functioning as a carrier of a toxic metabolite of 2-nitroimidazoles under certain conditions.