The role of specific reductases in the intracellular activation and binding 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.

Deep-Penetrating Triple-Responsive Prodrug Nanosensitizer Actuates Efficient Chemoradiotherapy in Pancreatic Ductal Adenocarcinoma Models

Chemoradiotherapy (CRT) is the most accepted treatment for locally advanced pancreatic ductal adenocarcinoma (PDAC) and can significantly improve the R0 resection rate. However, there are few long-term survivors after CRT. Although some polymer nanoparticles have shown potential in alleviating the dose-limiting toxicity and assisting the chemotherapy of PDAC, there are few efficient nanosensitizers (NS) available for CRT of this malignancy, especially in the context of its hypoxic nature. Herein, based on the biological features of PDAC, a γ-glutamyl transpeptidase (GGT)/glutathione (GSH)/hypoxia triple-responsive prodrug NS to overcome the biological barrier and microenvironmental limitations confronted by CRT in PDAC is developed. Due to triple-responsiveness, deep tumor penetration, GSH/hypoxia-responsive drug release/activation, and hypoxia-induced chemoradio-sensitization can be simultaneously achieved with this NS. As a result, tumor shrinkage after CRT with this NS can be observed in both subcutaneous and orthotopic PDAC models, foreshadowing its potential in clinical neoadjuvant CRT.

Nitroimidazoles with a halogen-containing side-chain

Syntheses are reported for: nine 2-nitroimidazoles, the abbreviated names all beginning with E, based on derivation from Etanidazole); five 2-methyl-5-nitroimidazoles (M compounds, derived from Metronidazole); and five 2-methyl-4-nitroimidazoles (labelled 2M4N compounds). The nitroimidazoles all have an amide side-chain at the N1 atom of the imidazole, with 17 of them containing one to five halogen atoms. The aim is to study compounds for comparison with EF5 (the number showing the presence of five F-atoms), a previously reported, pentafluoropropylacetamide derivative of 2-nitroimidazole that is currently used as a hypoxia marker drug to detect cancerous tumours. The new compounds are characterized by standard methods, including X-ray structural data for the fluorinated MF5, 2M4NF5, and 2M4NF1(−1) species, with the “–1” indicating two C-atoms in an alkylamide chain rather than the three C-atoms in the propylacetamide of EF5. Intra- and inter-molecular H-bonding is seen in the solid state structures, likely an important property in biological use; another key property of the nitroimidazoles is their reduction potentials, and the measured CV data confirm that 2NO2Im compounds with longer side-chains and more F-atoms (like EF5) are worth investigating for possible activity as hypoxia-selective, bioreductive agents.

Hypoxia in atherosclerosis and inflammation

PURPOSE OF REVIEW

Hypoxia triggers various cellular processes, both in physiological and pathological conditions, and has recently also been implicated in atherosclerosis. This review summarizes the recent evidence for the presence and the role of hypoxia in atherosclerosis. Additionally, it will elucidate on hypoxic signaling, which is interlinked with inflammatory signaling, and discuss recent advances in imaging of hypoxia in atherosclerosis.

RECENT FINDINGS

Hypoxia is present in atherosclerotic plaques in humans and animal models, and systemic hypoxia promotes atherosclerosis. Hypoxia stimulates proatherosclerotic processes, like deficient lipid efflux, inflammation, interference with macrophage polarization and glucose metabolism. However, the molecular mechanism of hypoxia-mediated atherogenesis remains unclear. Noninvasive imaging directly targeting plaque hypoxia has been applied in animal models of atherosclerosis, but remains to be validated in humans. Meanwhile, the metabolic marker ¹⁸F-fluorodeoxyglucose, used to detect human atherosclerosis in vivo, may serve as an indirect marker of plaque hypoxia due to enhanced glucose uptake in anaerobic metabolism.

SUMMARY

Recent studies underscore the proatherogenic role of hypoxia in macrophage lipid and glucose metabolism, inflammation and polarization. These studies provide new insights into the pathogenesis of atherosclerosis and unravel novel therapeutic targets and new options for noninvasive imaging of human atherosclerotic plaques.

Molecular and cellular pharmacology of the hypoxia-activated prodrug TH-302

TH-302 is a 2-nitroimidazole triggered hypoxia-activated prodrug (HAP) of bromo-isophosphoramide mustard currently undergoing clinical evaluation. Here, we describe broad-spectrum activity, hypoxia-selective activation, and mechanism of action of TH-302. The concentration and time dependence of TH-302 activation was examined as a function of oxygen concentration, with reference to the prototypic HAP tirapazamine, and showed superior oxygen inhibition of cytotoxicity and much improved dose potency relative to tirapazamine. Enhanced TH-302 cytotoxicity under hypoxia was observed across 32 human cancer cell lines. One-electron reductive enzyme dependence was confirmed using cells overexpressing human NADPH:cytochrome P450 oxidoreductase and radiolytic reduction established the single-electron stoichiometry of TH-302 fragmentation (activation). Examining downstream effects of TH-302 activity, we observed hypoxia-dependent induction of γH2AX phosphorylation, DNA cross-linking, and cell-cycle arrest. We used Chinese hamster ovary cell-based DNA repair mutant cell lines and established that lines deficient in homology-dependent repair, but not lines deficient in base excision, nucleotide excision, or nonhomologous end-joining repair, exhibited marked sensitivity to TH-302 under hypoxia. Consistent with this finding, enhanced sensitivity to TH-302 was also observed in lines deficient in BRCA1, BRCA2, and FANCA. Finally, we characterized TH-302 activity in the three-dimensional tumor spheroid and multicellular layer models. TH-302 showed much enhanced potency in H460 spheroids compared with H460 monolayer cells under normoxia. Multicellular layers composed of mixtures of parental HCT116 cells and HCT116 cells engineered to express an oxygen-insensitive bacterial nitroreductase showed that TH-302 exhibits a significant bystander effect.

The human carcinogen aristolochic acid i is activated to form DNA adducts by human NAD(P)H:quinone oxidoreductase without the contribution of acetyltransferases or sulfotransferases

Ingestion of aristolochic acid (AA) is associated with development of urothelial tumors linked with AA nephropathy and is implicated in the development of Balkan endemic nephropathy-associated urothelial tumors. We investigated the efficiency of human NAD(P)H:quinone oxidoreductase (NQO1) to activate aristolochic acid I (AAI) and used in silico docking, using soft-soft (flexible) docking procedure, to study the interactions of AAI with the active site of human NQO1. AAI binds to the active site of NQO1 indicating that the binding orientation allows for direct hydride transfer (i.e., two electron reductions) to the nitro group of AAI. NQO1 activated AAI, generating DNA adduct patterns reproducing those found in urothelial tissues from humans exposed to AA. Because reduced aromatic nitro-compounds are often further activated by sulfotransferases (SULTs) or N,O-acetlytransferases (NATs), their roles in AAI activation were investigated. Our results indicate that phase II reactions do not play a major role in AAI bioactivation; neither native enzymes present in human hepatic or renal cytosols nor human SULT1A1, -1A2, -1A3, -1E, or -2A nor NAT1 or NAT2 further enhanced DNA adduct formation by AAI. Instead under the in vitro conditions used, DNA adducts arise by enzymatic reduction of AAI through the formation of a cyclic hydroxamic acid (N-hydroxyaristolactam I) favored by the carboxy group in peri position to the nitro group without additional conjugation. These results emphasize the major importance of NQO1 in the metabolic activation of AAI and provide the first evidence that initial nitroreduction is the rate limiting step in AAI activation.

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.

A phase I study of the nitroimidazole hypoxia marker SR4554 using 19F magnetic resonance spectroscopy

Background:

SR4554 is a fluorine-containing 2-nitroimidazole, designed as a hypoxia marker detectable with ¹⁹F magnetic resonance spectroscopy (MRS). In an initial phase I study of SR4554, nausea/vomiting was found to be dose-limiting, and 1400 mg m⁻² was established as MTD. Preliminary MRS studies demonstrated some evidence of ¹⁹F retention in tumour. In this study we investigated higher doses of SR4554 and intratumoral localisation of the ¹⁹F MRS signal.

Methods:

Patients had tumours ⩾3 cm in diameter and ⩽4 cm deep. Measurements were performed using ¹H/¹⁹F surface coils and localised ¹⁹F MRS acquisition. SR4554 was administered at 1400 mg m⁻², with subsequent increase to 2600 mg m⁻² using prophylactic metoclopramide. Spectra were obtained immediately post infusion (MRS no. 1), at 16 h (MRS no. 2) and 20 h (MRS no. 3), based on the SR4554 half-life of 3.5 h determined from a previous study. ¹⁹Fluorine retention index (%) was defined as (MRS no. 2/MRS no. 1)*100.

Results:

A total of 26 patients enrolled at: 1400 (n=16), 1800 (n=1), 2200 (n=1) and 2600 mg m⁻² (n=8). SR4554 was well tolerated and toxicities were all ⩽grade 1; mean plasma elimination half-life was 3.7±0.9 h. SR4554 signal was seen on both unlocalised and localised MRS no. 1 in all patients. Localised ¹⁹F signals were detected at MRS no. 2 in 5 out of 9 patients and 4 out of 5 patients at MRS no. 3. The mean retention index in tumour was 13.6 (range 0.6–43.7) compared with 4.1 (range 0.6–7.3) for plasma samples taken at the same times (P=0.001) suggesting ¹⁹F retention in tumour and, therefore, the presence of hypoxia.

Conclusion:

We have demonstrated the feasibility of using ¹⁹F MRS with SR4554 as a potential method of detecting hypoxia. Certain patients showed evidence of ¹⁹F retention in tumour, supporting further development of this technique for detection of tumour hypoxia.

Intermittent hypoxia is a key regulator of cancer cell and endothelial cell interplay in tumours

Solid tumours are complex structures in which the interdependent relationship between tumour and endothelial cells modulates tumour development and metastasis dissemination. The tumour microenvironment plays an important role in this cell interplay, and changes in its features have a major impact on tumour growth as well as on anticancer therapy responsiveness. Different studies have shown irregular blood flow in tumours, which is responsible for hypoxia and reoxygenation phases, also called intermittent hypoxia. Intermittent hypoxia induces transient changes, the impact of which has been underestimated for a long time. Recent in vitro and in vivo studies have shown that intermittent hypoxia could positively modulate tumour development, inducing tumour growth, angiogenic processes, chemoresistance, and radioresistance. In this article, we review the effects of intermittent hypoxia on tumour and endothelial cells as well as its impacts on tumour development.

Applications of nitroimidazole in vivo hypoxia imaging in ischemic stroke

BACKGROUND AND PURPOSE

Nitroimidazole imaging is a promising contender for noninvasive in vivo mapping of brain hypoxia after stroke. However, there is a dearth of knowledge about the behavior of these compounds in the various pathophysiologic situations encountered in ischemic stroke. In this article we report the findings from a systematic review of the literature on the use of the nitroimidazoles to map hypoxia after stroke.

SUMMARY OF REVIEW

We describe the characteristics of nitroimidazoles as imaging tracers, their pharmacology, and results of both animal and clinical studies during and after focal cerebral ischemia. Findings in brain tumors are also presented to the extent that they enlighten results in stroke. Early results from application of kinetic modeling for quantitative measurement of tracer binding are briefly discussed.

CONCLUSIONS

Based on this literature review, nitroimidazole hypoxia imaging agents are of considerable interest in stroke because they appear, both in animal models and in humans, to specifically detect the severely hypoxic viable tissue, but not the reperfused nor the necrotic tissue. To fully realize this potential in stroke, however, formal validation by concurrent measurement of tissue oxygen tension, together with development of novel ligands with faster distribution kinetics, faster clearance from normal tissue, and well-defined trapping mechanisms, are important goals for future investigations.

Metabolic activation of carcinogenic aristolochic acid, a risk factor for Balkan endemic nephropathy

Aristolochic acid (AA), a naturally occurring nephrotoxin and carcinogen, is associated with tumor development in patients suffering from Chinese herbs nephropathy (now termed aristolochic acid nephropathy, AAN) and may also be a cause for the development of a similar type of nephropathy, the Balkan endemic nephropathy (BEN). Major DNA adducts [7-(deoxyadenosin-N6-yl)-aristolactam and 7-(deoxyguanosin-N2-yl)aristolactam] formed from AA after reductive metabolic activation were found in renal tissues of patients with both diseases. Understanding which human enzymes are involved in AA activation and/or detoxication is important in the assessment of an individual's susceptibility to this plant carcinogen. This paper reviews major hepatic and renal enzymes responsible for AA-DNA adduct formation in humans. Phase I biotransformation enzymes play a crucial role in the metabolic activation of AA to species forming DNA adducts, while a role of phase II enzymes in this process is questionable. Most of the activation of AA in human hepatic microsomes is mediated by cytochrome P450 (CYP) 1A2 and, to a lower extent, by CYP1A1; NADPH:CYP reductase plays a minor role. In human renal microsomes NADPH:CYP reductase is more effective in AA activation. Prostaglandin H synthase (cyclooxygenase, COX) is another enzyme activating AA in human renal microsomes. Among the cytosolic reductases, NAD(P)H:quinone oxidoreductase (NQO1) is the most efficient in the activation of AA in human liver and kidney. Studies with purified enzymes confirmed the importance of CYPs, NADPH:CYP reductase, COX and NQO1 in the AA activation. The orientation of AA in the active sites of human CYP1A1, -1A2 and NQO1 was predicted from molecular modeling and explains the strong reductive potential of these enzymes for AA detected experimentally. We hypothesized that inter-individual variations in expressions and activities of enzymes activating AA may be one of the causes responsible for the different susceptibilities to this carcinogen reflected in the development of AA-induced nephropathies and associated urothelial cancer.

Hypoxia: importance in tumor biology, noninvasive measurement by imaging, and value of its measurement in the management of cancer therapy

PURPOSE

The Cancer Imaging Program of the National Cancer Institute convened a workshop to assess the current status of hypoxia imaging, to assess what is known about the biology of hypoxia as it relates to cancer and cancer therapy, and to define clinical scenarios in which in vivo hypoxia imaging could prove valuable.

RESULTS

Hypoxia, or low oxygenation, has emerged as an important factor in tumor biology and response to cancer treatment. It has been correlated with angiogenesis, tumor aggressiveness, local recurrence, and metastasis, and it appears to be a prognostic factor for several cancers, including those of the cervix, head and neck, prostate, pancreas, and brain. The relationship between tumor oxygenation and response to radiation therapy has been well established, but hypoxia also affects and is affected by some chemotherapeutic agents. Although hypoxia is an important aspect of tumor physiology and response to treatment, the lack of simple and efficient methods to measure and image oxygenation hampers further understanding and limits their prognostic usefulness. There is no gold standard for measuring hypoxia; Eppendorf measurement of pO(2) has been used, but this method is invasive. Recent studies have focused on molecular markers of hypoxia, such as hypoxia inducible factor 1 (HIF-1) and carbonic anhydrase isozyme IX (CA-IX), and on developing noninvasive imaging techniques.

CONCLUSIONS

This workshop yielded recommendations on using hypoxia measurement to identify patients who would respond best to radiation therapy, which would improve treatment planning. This represents a narrow focus, as hypoxia measurement might also prove useful in drug development and in increasing our understanding of tumor biology.

Current issues in the utility of 19F nuclear magnetic resonance methodologies for the assessment of tumour hypoxia

It is now well established that uncontrolled proliferation of tumour cells together with the chaotic and poorly regulated blood supply of solid tumours result in tissue hypoxia, and that hypoxic regions of tumours are resistant to radiotherapy and chemotherapy. The development and application of non-invasive methods to rapidly determine the degree and extent of tumour hypoxia in an individual tumour would clearly enhance cancer treatment strategies. This review describes the current status of two (19)F nuclear magnetic resonance (NMR) methodologies that have been exploited to investigate tumour hypoxia, namely: (i) (19)F NMR oximetry following administration of perfluorocarbons, from which tumour p(O)(2) measurements can be made; and (ii) (19)F NMR measurements of the tumour retention of fluorinated 2-nitroimidazoles.

In vivo evaluation of [18F]fluoroetanidazole as a new marker for imaging tumour hypoxia with positron emission tomography

Development of hypoxia-targeted therapies has stimulated the search for clinically applicable noninvasive markers of tumour hypoxia. Here, we describe the validation of [¹⁸F]fluoroetanidazole ([¹⁸F]FETA) as a tumour hypoxia marker by positron emission tomography (PET). Cellular transport and retention of [¹⁸F]FETA were determined in vitro under air vs nitrogen. Biodistribution and metabolism of the radiotracer were determined in mice bearing MCF-7, RIF-1, EMT6, HT1080/26.6, and HT1080/1-3C xenografts. Dynamic PET imaging was performed on a dedicated small animal scanner. [¹⁸F]FETA, with an octanol–water partition coefficient of 0.16±0.01, was selectively retained by RIF-1 cells under hypoxia compared to air (3.4- to 4.3-fold at 60–120 min). The radiotracer was stable in the plasma and distributed well to all the tissues studied. The 60-min tumour/muscle ratios positively correlated with the percentage of pO₂ values <5 mmHg (r=0.805, P=0.027) and carbogen breathing decreased [¹⁸F]FETA-derived radioactivity levels (P=0.028). In contrast, nitroreductase activity did not influence accumulation. Tumours were sufficiently visualised by PET imaging within 30–60 min. Higher fractional retention of [¹⁸F]FETA in HT1080/1-3C vs HT1080/26.6 tumours determined by dynamic PET imaging (P=0.05) reflected higher percentage of pO₂ values <1 mmHg (P=0.023), lower vessel density (P=0.026), and higher radiobiological hypoxic fraction (P=0.008) of the HT1080/1-3C tumours. In conclusion, [¹⁸F]FETA shows hypoxia-dependent tumour retention and is, thus, a promising PET marker that warrants clinical evaluation.

Synthesis of [18F]-labeled EF3 [2-(2-nitroimidazol-1-yl)-N-(3,3,3-trifluoropropyl)-acetamide], a marker for PET detection of hypoxia

[18F]-2-(2-Nitroimidazol-1-yl)-N-(3,3,3-trifluoropropyl)-acetamide ([18F]-EF3) has been prepared, in 65% chemical yield and 5% radiochemical yield, by coupling 2,3,5,6-tetrafluorophenyl 2-(2-nitroimidazol-1-yl) acetate 1 with [18F]-3,3,3-trifluoropropylamine 7. This original radiolabelled key-synthon was obtained in 40% overall chemical yield by oxidative [18F]-fluorodesulfurization of ethyl N-phthalimido-3-aminopropane dithioate 4, followed by deprotection with hydrazine of the resulting [18F]-N-phthalimido-3,3,3-trifluoropropylamine 5. All the process was performed within 90 min, from the [18F]-HF production in the cyclotron to the purification of the final target.

Single-photon emission computed tomography and positron-emission tomography assays for tissue oxygenation

Radiotherapy prescription can now be customized to target the major mechanism(s) of resistance of individual tumors. In that regard, functional imaging techniques should be exploited to identify the dominant mechanism(s). Tumor biology research has identified several mechanisms of tumor resistance that may be unique to radiation treatments. These fall into 3 broad areas associated with (1) tumor hypoxic fraction, (2) tumor growth rate, (3) and the intrinsic radiosensitivity of tumor clonogens. Imaging research has markers in various stages of development for quantifying relevant information about each of these mechanisms, and those that measure tumor oxygenation and predict for radioresistance are the most advanced. Positron-emission tomography (PET) measurement of oxygen 15 has yielded important information, particularly about brain tissue perfusion, metabolism, and function. Indirect markers of tumor hypoxia have exploited the covalent binding of bioreductive intermediates of azomycin-containing compounds whose uptakes are inversely proportional to intracellular oxygen concentrations. Pilot clinical studies with single-photon emission computed tomography (SPECT) and PET detection of radiolabeled markers to tumor hypoxia have been reported. Recently, other studies have attempted to exploit the reduction properties of both technetium and copper chelates for the selective deposition of radioactive metals in hypoxic tissues. A growing number of potentially useful isotopes are now available for labeling several novel chemicals that could have the appropriate specificity and sensitivity. Preclinical studies with "microSPECT" and "microPET" will be important to define the optimal radiodiagnostic(s) for measuring tissue oxygenation and for determining the time after their administration for optimal hypoxic signal acquisition. Radiolabeled markers of growth kinetics and intrinsic radiosensitivity of cells in solid tumors are also being developed. We conclude that radiation oncology is uniquely positioned to benefit from functional imaging markers that identify important mechanisms of tumor radioresistance, since several strategies for overcoming these individual mechanisms have already been identified.

Technetium-99m-cyclam AK 2123: a novel marker for tumor hypoxia

Technetium-99m labeled cyclam N-2'-methoxyethyl-2-(3'-nitro-1'-triazole) acetamide (cyclam AK 2123) has been synthesized, radiolabeled and characterized as a hypoxic tumor imaging agent. Radiochemical purity was greater than 95%. Marker biodistribution was measured in normal Wistar strain rats at different time intervals after intra venous (i.v.) administration. In vivo distribution and scintigraphic imaging studies were performed after i.v. injection into mammary tumor-bearing rats using a gamma camera and associated computer. Intratumor partial oxygen pressure (pO2) and oxygen saturation measurements were performed to estimate the oxygenation status of the tumors. Tumor to muscle ratio (T/M) of 99mTc-cyclam AK 2123 was 8.5 which was compared with other tumor seeking radiopharmaceuticals, viz. 99mTc-(V) DMSA (3.07), 99mTc-citrate (5.29) and 201T1C1 (3.29). T/M ratios were also evaluated in comparison with radioiodinated iodoazomycin galactopyronoside (125I-IAZG). The ratio obtained was 18 for 99mTc-cyclam AK 2123 and 20 for 125I-IAZG, respectively. The increased concentration of radioactivity in these tumors suggests that this agent could be labelling hypoxic cells and have utility as an imaging agent.

NAD(P)H:quinone oxidoreductase 1 (NQO1): chemoprotection, bioactivation, gene regulation and genetic polymorphisms

NAD(P)H:quinone oxidoreductase 1 (NQO1) is an obligate two-electron reductase that is involved in chemoprotection and can also bioactivate certain antitumor quinones. This review focuses on detoxification reactions catalyzed by NQO1 and its role in antioxidant defense via the generation of antioxidant forms of ubiquinone and vitamin E. Bioactivation reactions catalyzed by NQO1 are also summarized and the development of new antitumor agents for the therapy of solid tumors with marked NQO1 content is reviewed. NQO1 gene regulation and the role of the antioxidant response element and the xenobiotic response element in transcriptional regulation is summarized. An overview of genetic polymorphisms in NQO1 is presented and biological significance for chemoprotection, cancer susceptibility and antitumor drug action is discussed.

Role of NADPH:cytochrome P450 reductase in the hypoxic accumulation and metabolism of BRU59-21, a technetium-99m-nitroimidazole for imaging tumor hypoxia

Nitroimidazoles labeled with technetium-99m are being investigated as non-invasive markers of tumor hypoxia. They are bioreductive compounds that require enzymatic reduction for retention in hypoxic cells, but little is known about the cellular factors affecting their accumulation in hypoxic cells. If the absolute accumulation of hypoxia markers is affected by enzyme levels, an inaccurate assessment of the hypoxic cell fraction in tumors may occur. BRU59-21, (99m)Tc-oxo[[3,3,9, 9-tetramethyl-6-[(2-nitro-1H-imidazol-1-yl)methyl]5-oxa-4, 8-diazadioximato]-(3-)-N,N',N",N"'] technetium (V), a technetium-99m-nitroimidazole that is being studied as a potential marker of tumor hypoxia, was used in the present study to evaluate the effect of NADPH:cytochrome P450 reductase (EC 1.6.2.4) levels on BRU59-21 accumulation and metabolism. Metabolism of BRU59-21 in hypoxic cellular lysates derived from Chinese hamster ovary cells overexpressing NADPH:cytochrome P450 reductase was 8-fold greater than in control cells. This effect required the presence of exogenous NADPH. The increased metabolism of BRU59-21 in lysates overexpressing NADPH:cytochrome P450 reductase was inhibited at 4 degrees and by the addition of NADPH:cytochrome P450 reductase inhibitors. The addition of inhibitors of other nitroreductase enzymes had no effect on BRU59-21 metabolism in these lysates. When the accumulation and metabolism of BRU59-21 were studied in stirred suspension cultures, it was found that cells overexpressing NADPH:cytochrome P450 reductase exhibited about a 3-fold increase in both the hypoxic metabolism and the accumulation of BRU59-21. These findings suggest that NADPH:cytochrome P450 reductase is an important enzyme in BRU59-21 metabolism in model systems of tumor hypoxia.

Detection of hypoxic cells with the 2-nitroimidazole, EF5, correlates with early redox changes in rat brain after perinatal hypoxia-ischemia

The hypoxia-dependent activation of nitroheterocyclic drugs by cellular nitroreductases leads to the formation of intracellular adducts between the drugs and cellular macromolecules. Because this covalent binding is maximal in the absence of oxygen, detection of bound adducts provides an assay for estimating the degree of cellular hypoxia in vivo. Using a pentafluorintated derivative of etanidazole called EF5, we studied the distribution of EF5 adducts in seven-day-old rats subjected to different treatments which decrease the level of oxygen in the brain. EF5 solution was administered intraperitoneally 30 min prior to each treatment. The effect of acute and chronic hypoxia on EF5 adduct formation (binding) was studied in the brain of newborn rats exposed to global hypoxia (8% O2 for 30, 90 or 150 min) and in the brain of chronically hypoxic rat pups with congenital cardiac defects (Wistar Kyoto). The effect of combined hypoxia-ischemia was investigated in rat pups subjected to right carotid coagulation and concurrent exposure to 8% O2 for 30, 90 or 150 min. Brains were frozen immediately at the end of each treatment. Using a Cy3-conjugated monoclonal mouse antibody (ELK3-51) raised against EF5 adducts, hypoxic cells within brain regions were visualized by fluorescence immunocytochemistry. Brains from controls or vehicle-injected animals showed no EF5 binding. Notably, brains from animals which were chronically hypoxemic as a result of congenital cardiac defects also showed no EF5 binding. A short exposure (30 min) to hypoxia or to combined hypoxia-ischemia resulted in increased background stain and few scattered cells with low-intensity immunostaining. Acute hypoxia exposure of at least 90-150 min, which in this age animal does not result in frank cellular damage, produced patchy areas of low- to moderate-intensity fluorescence scattered throughout the brain. In contrast, 90-150 min of hypoxia-ischemia was associated with intense immunofluorescence in the hemisphere ipsilateral to the carotid occlusion, with a pattern similar to that reported previously for the histological damage seen in this model. This study provides a sensitive method for the evaluation of the level of oxygen depletion in brain tissue after neonatal hypoxia-ischemia at times much earlier than any method demonstrates apoptotic or necrotic cell death Since the level of in vivo formation of macromolecular adducts of EF5 depends on the degree of oxygen depletion in a tissue, intracellular EF5 binding may serve as a useful marker of regional cellular vulnerability and redox state after brain injury resulting from hypoxia-ischemia.

Bioreductively-activated prodrugs for targeting hypoxic tissues: elimination of aspirin from 2-nitroimidazole derivatives

2-Nitroimidazoles were synthesised substituted with aspirin or salicylic acid, as leaving groups linked through the (imidazol-5-yl)methyl position. Activation of aqueous solutions by CO2*- (a model one-electron reductant) resulted in release of aspirin or salicylate, probably via the 2-hydroxyaminoimidazole. The analogous 2-nitroimidazole with bromide as leaving group eliminated bromide in < 1 ms via the radical-anion.

DT-diaphorase activity in NSCLC and SCLC cell lines: a role for fos/jun regulation

To assess the potential differential lung tumour expression of NAD(P)H:quinone reductase (NQO1), the human (h) NQO1 promoter was characterized in gene transfer studies. A deletion panel of 5' flanking hNQO1 promoter constructs was made and tested in transient transfection assays in NSCLC and SCLC cell lines. The largest hNQO1 construct (-1539/+115) containing the antioxidant response element (ARE), exhibited robust levels of reporter activity in the NSCLC (H460, H520, and A549) cell lines and expression was over 12 to 77-fold higher than the minimal (-259/+115) promoter construct. In contrast, there was little difference in promoter activity between the largest and minimal promoter construct in the SCLC (H146, H82 and H187) cell lines. Deletion of the sites for NFkappaB and AP-2 and the XRE did not significantly affect hNQO1 promoter activity in either the NSCLC or SCLC cell lines. Robust promoter activity in NSCLC lines was mediated by a 359 bp segment of the proximal promoter that contained a canonical AP-1 binding site, TGACTCAG, within the ARE. Gel supershift assays with various specific Fos/Jun antibodies identified Fra1, Fra2 and Jun B binding activity in NSCLC cells to a promoter fragment (-477 to -438) spanning the AP-1 site, whereas SCLC do not appear to express functional Fra or Jun B. These results suggest a possible role for AP-1 activity in the differential expression of hNQO1 in NSCLC.

Evidence of hypoxic areas within the arterial wall in vivo

The anoxemia theory of atherosclerosis states that an imbalance between the demand and supply of oxygen in the arterial wall is a key factor for the development of atherosclerotic lesions. Direct in vitro and in situ measurements have shown that PO2 is decreased in the more deeply situated parts of the media, but the degree of hypoxia in vivo or the distribution of hypoxia along the arterial tree is not known. For this reason, we have developed a method for the detection of hypoxia in the arterial wall in vivo by using a hypoxia marker, 7-(4'-(2-nitroimidazol-1-yl)-butyl)-theophylline, that may be visualized by immunofluorescence. In the present study, we have used this method in rabbits with experimentally induced atherosclerosis. Our results indicate that zones of hypoxia occur at depth in the atherosclerotic plaque. The mechanism was probably an impaired oxygen diffusion capacity due to the thickness of the lesion, together with high oxygen consumption by the foam cells. Thus, we have for the first time demonstrated that hypoxia actually does exist in the arterial wall in vivo, lending support to the anoxemia theory of atherosclerosis.

The relationship between tumour oxygenation determined by oxygen electrode measurements and magnetic resonance spectroscopy of the fluorinated 2-nitroimidazole SR-4554

The relationship between two methods of assessing tumour oxygenation in vivo, namely oxygen electrode measurement and magnetic resonance spectroscopy (MRS) of the fluorinated 2-nitroimidazole SR-4554, was investigated. Using three tumour models (two sites), no linear correlation was observed between 19F retention index and pO2 parameters (r < or = 0.3). Substantial retention of SR-4554 (19F retention index > 0.5) was, however, associated with low tumour pO2 (% pO2 < or = 5 mmHg = 60%). Depending on the pO2 parameters used, SR-4554 administration was shown to produce either a significant or a non-significant increase in tumour oxygenation. We conclude that measurement of SR-4554-related compound(s) by 19F-MRS has the potential to detect clinically relevant levels of tumour hypoxia.

Bioreductive metabolism of the novel fluorinated 2-nitroimidazole hypoxia probe N-(2-hydroxy-3,3,3-trifluoropropyl)-2-(2-nitroimidazolyl) acetamide (SR-4554)

The aim of this work was to study the metabolic characteristics of the novel fluorinated 2-nitroimidazole hypoxia probe N-(2-hydroxy-3,3,3-trifluoropropyl)-2-(2-nitroimidazolyl) acetamide (SR-4554). HPLC and 19F NMR methods were employed to evaluate the rate of reductive metabolism of SR-4554 and the nature of the resulting metabolites, respectively. SR-4554 was enzymatically reduced by mouse liver microsomes (1.1 +/- 0.1 nmol of SR-4554 reduced/min/mg protein), purified rat and human NADPH:cytochrome P450 reductase (17.8 +/- 0.4 and 5.0 +/- 0.5 nmol of SR-4554 reduced/min/mg protein, respectively), and SCCVII tumour homogenates (2.3 +/- 0.3 nmol of SR-4554 reduced/min/g tumour) under nitrogen. NADPH:cytochrome P450 reductase was a major microsomal enzyme involved in the bioreduction of SR-4554 by liver microsomes. In a panel of murine and human tumour xenografts, cytochrome P450 reductase activities were found to be low and only varied by 3-fold between different tumour types, suggesting that enzyme activities within the tumours are unlikely to influence markedly in vivo reductive metabolism. Reduction of SR-4554 by mouse liver microsomes showed a characteristic oxygen dependence with a half-maximal inhibition of 0.48 +/- 0.06%. Thus, the reductive metabolism of SR-4554 can be employed to detect the low oxygen tensions that occur within both murine and human tumours. Soluble, low molecular weight reductive metabolites of SR-4554 were identified by 19F NMR. These metabolite peaks appeared (up to 0.12 ppm) downfield of the parent drug peak. In conclusion, SR-4554 undergoes an oxygen-dependent metabolism that involves NADPH:cytochrome P450 reductase. 19F NMR is capable of identifying reduced metabolites that are undetectable by HPLC.

Anomalous patterns of nitroimidazole binding adjacent to necrosis in human glioma xenografts: possible role of decreased oxygen consumption

In contrast to reports of extensive hypoxia in human gliomas in situ measured by pO2 histography, non-invasive methods of assessing glioma oxygenation, including nitroimidazole binding, have yielded surprisingly contradictory results. In order to investigate the relationship of necrosis, hypoxia, nitroreductase activity and cellular respiration in human gliomas, subcutaneous models using the human glioma cell lines M059K, M006 and M010b were developed in the murine SCID host. Intracranial growth of the M006 line was achieved in nude rats. The nitroreductive capacity of glioma cell lines was assessed and found to be similar to transplanted tumours previously reported in the literature. This suggests that if substantial numbers of viable hypoxic cells were present in situ in gliomas, then nitroimidazole-binding techniques should be capable of identifying them. Inter-tumour variability in the amount of necrosis was seen. M006 xenografts growing in subcutaneous and intracranial sites revealed extensive necrotic regions histologically, some of which were surrounded by cells labelled heavily for [3H]misonidazole, while other areas were lightly labelled. Similar binding patterns were seen for subcutaneous M059K tumours, while subcutaneous M010b tumours display necroses of which almost all were surrounded by heavily labelled cells. The oxygen consumption rates of tumour cell lines grown in vivo, in which venous pO2 concentrations are of the order of 2-5%, were two to sevenfold less than those of the same lines grown as monolayers in vitro under oxygen concentrations of 18%. We postulate that glioma cell lines behave as 'oxygen conformers', in that their rate of oxygen consumption appears to vary with the availability of oxygen. Together with the misonidazole-binding data, the results in this glioma tumour model are consistent with coordinate inhibition or down-regulation of respiration under moderate hypoxia.

A method for the assessment of hypoxia in the arterial wall, with potential application in vivo

According to the anoxemia theory of atherosclerosis, an imbalance between the demand for and supply of oxygen in the arterial wall is a key factor in the development of atherosclerotic lesions. Direct in vitro and in situ measurements have shown that PO2 is decreased in the inner part of the media, but the degree of hypoxia in vivo or the distribution of hypoxia along the arterial tree is not known. We applied a hypoxia marker, 7-(4'-(2-nitroimidazol-1-yl)-butyl)-theophylline (NITP), to develop a method for the detection of hypoxia in the arterial wall. Immunoperoxidase and immunofluorescence were used to detect the marker, and a clearly PO2-dependent staining was observed in the media of rabbit and swine aorta in vitro. The cutoff PO2 level was probably around 2 to 3 mm Hg. In experimental atherosclerotic lesions in the rabbit the marker seemed to bind to foam cells that were already at a higher surrounding PO2, which might reflect a higher local oxygen consumption. The binding of the marker to endothelial cells was not PO2 dependent. One explanation for this finding could be that the marker was metabolized via a non-oxygen-dependent pathway in these cells. We propose that this method may be used to assess arterial wall hypoxia in vivo. Furthermore, the spatial resolution allows the detection of local variations within the arterial tree.

Oxygen dependence of cellular uptake of EF5 [2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl)a cet amide] : analysis of drug adducts by fluorescent antibodies vs bound radioactivity

The present studies were initiated to quantitate the oxygen dependence of bioreductive metabolism-induced binding of EF5, a pentafluorinated derivative of the 2-nitroimidazole, etanidazole. Two different assays were compared: first, radioactive drug incorporation into cell lysates, which provides a direct measure of drug metabolism or uptake; second, monoclonal antibody detection of cellular macromolecular adducts of EF5 after whole cell permeabilisation and fixing. The antibodies (a single clone designated ELK3-51) were conjugated with the fluorescent dye Cy3, with fluorescence determined by fluorescence microscopy and flow cytometry. For the two cell lines tested (V79 Chinese hamster fibroblasts and 9L rat glioma), the oxygen dependence of binding was found to be the same for the two techniques. Using the antibody binding technique, the fluorescence signal was highly reproducible between experiments, resistant to light or chemical bleaching and stable over time following cell or tissue staining. Flow cytometric analysis of cells from rat 9L tumours treated with EF5 in vivo or in vitro showed a distribution of fluorescent signal which was very compatible, on both a relative and absolute basis, with the in vitro results. Our results indicate that immunofluorescent techniques provide a quantitative assay for bioreductive drug adducts, and therefore may be able to measure the absolute oxygen concentration distribution in cell populations and tissues of interest.