Activation of misonidazole by rat liver microsomes and purified NADPH-cytochrome c reductase

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 imaging agents labeled with positron emitters

Imaging hypoxia using positron emission tomography (PET) is of great importance for therapy of cancer. [(18)F]Fluoromisonidazole (FMISO) was the first PET agent for hypoxia imaging, and various radiolabeled nitroimidazole derivatives such as [(18)F]fluoroerythronitroimidazole (FETNIM), [(18)F]1-α-D: -(2-deoxy-2-fluoroarabinofuranosyl)-2-nitroimidazole (FAZA), [(18)F]2-(2-nitro-1H-imidazol-1-yl)-N-(2,2,3,3,3-pentafluoropropyl) acetamide (EF-5), and [(18)F]fluoroetanidazole (FETA) have been developed successively. To overcome the high cost of cyclotron installation, (68)Ga-labeled nitroimidazole derivatives also have been developed. Another important hypoxia imaging agent is (64)Cu-diacetyl-bis(N (4)-methylthiosemicarbazone) ((64)Cu-ATSM), which can distribute in cancer tissue rapidly due to high lipophilicity. However, its application is limited due to high cost of radionuclide production. Although various hypoxia imaging agents have been reported and tested, hypoxia PET images still have to be improved, because of the low blood flow in hypoxic tissues and resulting low uptake of the agents.

Prospects for bioreductive drug development

Bioreductive drugs are inactive prodrugs that are converted into potent cytotoxins under conditions of either low oxygen tension or in the presence of high levels of specific reductases. The biochemical basis for selectivity relies on the ability of oxygen to reverse the activation process and the presence of elevated reductase levels in some tumour types. Key criteria for an ideal bioreductive drug should include poor activity against aerobic cells, activation over a broad range of oxygen tensions and, penetration through the aerobic fraction of cells. In addition, the active drug should be capable of killing non-proliferating cells. Numerous compounds are currently at various stages of drug development but Mitomycin C, which is generally considered to be the prototype bioreductive drug, is the only one in clinical use today. Of the drugs currently being evaluated clinically, tirapazamine has definite clinical activity against a variety of solid tumours when used in combination with cisplatin. Other drugs, such as EO9 and various nitroimidazoles, have not been impressive in the clinic and further development is required to improve properties such as drug delivery in the case of indoloquinones. A novel approach to exploiting tumour hypoxia is the development of a gene-directed enzyme prodrug therapy (GDEPT) strategy, where a gene encoding for a prodrug activating enzyme has been placed under the control of a hypoxia responsive promoter sequence. It is generally recognised that bioreductive drugs must be directed towards patients whose tumours have hypoxic regions or have appropriate enzymological characteristics. In terms of identifying tumour hypoxia, there has been considerable progress in the development of nitroimidazole based hypoxia markers that can be detected either via non-invasive or invasive procedures. Another strategy currently undergoing preclinical evaluation is the use of agents that modulate tumour blood flow and synergistic effects have been reported between bioreductive drugs and photodynamic therapy or inhibitors of nitric oxide synthase for example. The development of clinically useful bioreductive drugs depends therefore on the expertise of scientists and clinicians with varying backgrounds. The purpose of this review is to describe and critically assess recent developments in this field, with particular emphasis being placed on drug development and strategies aimed at optimising bioreductive drug activity.

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.

The novel fluorinated 2-nitroimidazole hypoxia probe SR-4554: reductive metabolism and semiquantitative localisation in human ovarian cancer multicellular spheroids as measured by electron energy loss spectroscopic analysis

The novel fluorinated 2-nitroimidazole SR-4554 is undergoing preclinical development as a magnetic resonance spectroscopy and imaging probe for hypoxic tumour cells. We have used electron energy loss spectroscopic analysis (EELS) to show selective reduction and differential subcellular localisation of SR-4554 in human ovarian multicellular spheroids. SR-4554 was demonstrated to be metabolised by these A2780 cells under hypoxic but not under normal aerobic cell culture conditions. The EELS technique illustrated that the relative amount of drug within the cytoplasm of cells from both the inner region (150-160 microns from edge) and outer edge of the spheroid did not differ significantly after an initial 3 h incubation with drug. In contrast, an 8-fold differential between the amount of drug retained in the cytoplasm (primarily ribosomes and endoplasmic reticulum) of cells from the inner vs outer regions of the spheroids was observed following a subsequent 2 h 'chase' culture in drug-free medium. Within cells from the hypoxic region of the spheroid, SR-4554 was mainly associated with the endoplasmic reticulum, nucleus and the cytoplasmic side of intracellular vesicles and also to a lesser extent with the nuclear periphery. Interestingly, the drug was only weakly associated with the mitochondria and plasma membrane of the cells. The characteristics of cellular and subcellular distribution of SR-4554 are consistent with the hypothesis that 2-nitroimidazole compounds undergo hypoxia-mediated enzymatic reduction to reactive species. These reactive species are selectively retained in the cells in which they are metabolised through covalent association with subcellular components. These findings provide additional support for the clinical development of the drug as a non-invasive probe for tumour hypoxia and at the same time illustrate the utility of the EELS technique for examining the heterogeneity of drug distribution both between and within cells.

Drug residue formation from ronidazole, a 5-nitroimidazole. VIII. Identification of the 2-methylene position as a site of protein alkylation

Ronidazole protein-bound adducts were generated by the in vitro anaerobic incubation of [2-methylene-14C]ronidazole with microsomes from the livers of male rats. Acid hydrolysis of the protein adducts yielded an imidazole ring fragment bearing the radiolabel and an amino acid residue derived from the proteins. This fragment has been identified as carboxymethylcysteine by co-chromatography of the amino acid and its dansyl derivative with known standards under a variety of conditions. The carboxymethylcysteine was estimated to represent at least 15% of the radioactivity derived from the protein-bound adducts and provides unequivocal evidence that nucleophilic attack by protein cysteine thiols occurred at the 2-methylene position of ronidazole.

NAD(P)H nitroblue tetrazolium reductase levels in apparently normoxic tissues: a histochemical study correlating enzyme activity with binding of radiolabelled misonidazole

Hack and Helmy's method for the histochemical identification of NAD(P)H nitroblue tetrazolium reductase activity was employed to pinpoint reductase activity in certain cells in the mouse. High activity was observed in the following: lower airway epithelium, liver (centrilobular zone), eyelid (meibomian and sebaceous glands), vulval gland and parotid gland (striated cells of intralobular ducts). All of these cells had previously been identified as sites of binding of the reactive metabolites formed from the enzymic reduction of misonidazole (MISO) (Cobb et al., 1989). It had previously been thought that MISO binding would only take place in significant amounts in hypoxic tissues (tumour and possibly liver) since in normoxic tissues oxygen should reverse the initial one electron enzymic reduction, thus preventing progressive reduction to reactive species. We suggest that the very high levels of reductase in the above listed, probably normoxic, tissues contribute significantly to the accumulation of bound reactive MISO metabolite(s).

Misonidazole binding in murine liver tissue: a marker for cellular hypoxia in vivo

The hepatic microcirculation is believed to cause variable cellular oxygenation within the organ. In this study a marker of cellular hypoxia was used to demonstrate liver oxygen tension gradients in vivo. Covalent binding of misonidazole adducts to cellular macromolecules is enhanced by hypoxia. Autoradiographs of liver from mice treated with radiolabeled misonidazole demonstrated enhanced binding of adducts within hepatocytes surrounding hepatic veins. Livers from both hypoxic and normal mice had characteristic autoradiographic grain patterns reflecting regional oxygen tension variation in vivo. Differential binding of misonidazole adducts formed in hypoxic cells could have an application in studies of liver physiology and biochemistry.

Biodistribution of misonidazole and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) in rats bearing unclamped and clamped 9L subcutaneous tumors

The biodistribution of misonidazole (MISO) and 1,3bis(2-chloroethyl)-1-nitrosourea (BCNU) was studied using the subcutaneous (s.c.) 9L tumor model in male Fisher 344 rats. A transient hypoxia in these tumors was created by clamping the blood supply to the tumor. Reoxygenation occurred upon release of the clamp. The plasma and tumor concentrations of MISO and BCNU were quantitated by high pressure liquid chromatography. When 12 mg/kg of BCNU was given i.p. without MISO, the peak plasma concentration was about 6 micrograms/ml, and the elimination half-time was about 16 min. When 2.5 mmole/kg of MISO was given i.p. 150 min before the BCNU, the peak plasma concentration of BCNU increased by approximately 33%, and the plasma elimination half-time increased by approximately 57%. Clamping the tumor for 120 min did not significantly change the BCNU concentration in plasma, but in tumors the time to reach the peak level was delayed slightly, and the peak concentration was reduced when compared to that in the unclamped tumors. MISO pretreatment decreased the BCNU peak concentration in both unclamped and clamped tumors, but the decrease was more pronounced in the unclamped tumors. In both unclamped and clamped tumors, the BCNU concentration and its rate of disappearance were identical about 30 min after BCNU administration, with or without MISO pretreatment. The elimination half-time of MISO from the plasma (approximately 142 min) was identical for rats with unclamped or clamped tumors. The half-time for the disappearance of MISO from unclamped tumors was about 98 min. BCNU had no effect on the MISO concentration in plasma and unclamped tumors. MISO disappeared in the clamped tumors with a half-time of about 40 min. When the clamp was released, the MISO concentration returned to the level in the unclamped tumors after about 45 min. BCNU delayed the return of the MISO concentration to the unclamped tumor level by about 60 min. Two conclusions can be drawn from this study. First, the pharmacokinetics of each drug changed when the two drugs were combined. Second, the data indicate that alterations in the tumor BCNU pharmacokinetics are not the major mechanism responsible for the chemopotentiation previously measured in s.c. 9L tumors.

Role of nonprotein thiols in enzymatic reduction of 2-nitroimidazoles

The role of nonprotein thiols (NPSH) in the enzymatic reduction of the nitro function in 2-nitroimidazoles (2-NI) has been investigated. The addition of NPSH has been shown previously to protect cells from the hypoxic cytotoxicity of 2-NI, whereas depletion of NPSH enhances the hypoxic cytotoxicity. In this report, we have investigated the effects of thiol depleting agents, N-ethylmaleimide (NEM) and diethyl maleate (DEM), on the enzymatic reduction of the nitro group. Cytosolic and microsomal fractions of rat hepatic tissue and xanthine oxidase were employed as sources of nitro reductases. Addition of NPSH caused an enhancement in the reduction of the nitro group of 2-NI; cysteine was significantly more effective than glutathione (GSH) in stimulating the enzymatic reduction. The reduction of the nitro function was decreased markedly in the presence of NEM or DEM. Addition of cysteine or GSH reversed the inhibition with NEM. Both NEM and DEM also attenuated the enhancement of reduction observed after the addition of NPSH. These results suggest that the addition of NPSH facilities the reduction of the nitro function to the reduced intermediates that may be inactivated by an excess of NPSH, whereas the depletion of NPSH allows the accumulation of the toxic nitro radicals causing increased cytotoxicity.

Effects of substituted 2-nitroimidazoles and related compounds on unscheduled DNA synthesis in rat hepatocytes and in non-transformed (BL8) and transformed (JB1) rat liver epithelial derived cell lines

1. Using unscheduled DNA synthesis as an index, the possible interaction of a number of substituted nitroimidazoles, e.g. misonidazole, with cellular DNA has been investigated. Transformed (JB1), non-transformed (BL8) rat liver epithelial derived cell lines and freshly prepared rat hepatocytes have been used. 2. Under anaerobic or aerobic conditions, relative to cells exposed to a nitroquinoline-N-oxide standard, misonidazole and related nitroimidazoles were very poor at stimulating unscheduled DNA synthesis in JB1 or BL8 cells or in hepatocytes, even at the highest concentrations tested (10 mM). Under anaerobic conditions, metabolic activation did occur as judged from the time-dependent depletion of cellular reduced glutathione in all three cell types. 3. It was concluded that in hypoxic cells an important mode of action of such nitroimidazoles as chemotherapeutic sensitisers may be by their interaction with cellular thiols rather from their interaction with DNA. 4. Functionalisation of the nitroimidazole ring with a side chain containing an aziridine function, e.g. RSU-1069 (1-(2-nitro-1-imidazolyl)-3-(1-aziridinyl)-2-propanol), results in the induction of unscheduled DNA synthesis in cells exposed under both aerobic and anaerobic conditions. On a molar basis, however, this induction was not so great as that caused by the simple monofunctional alkylating agent 1-aziridineethanol itself. Methyl-substitution of the aziridine ring in RSU-1069 reduced the extent of unscheduled DNA synthesis. 5. With all the compounds tested, unscheduled DNA synthesis was greater in JB1 cells than in BL8s or in hepatocytes.

Nitroimidazole bioreductive metabolism. Quantitation and characterisation of mouse tissue benznidazole nitroreductases in vivo and in vitro

We have investigated the nitroreduction of the 2-nitroimidazole benznidazole (BENZO) to its corresponding amine by murine normal tissues and tumours. In vivo concentrations of BENZO and its amine metabolite were measured by HPLC 3 hr after BENZO, 2.5 mmoles kg-1 i.p. This gave plasma and tissue BENZO concentrations of 96-160 micrograms ml-1 or g-1. Mouse plasma, KHT and RIF-1 tumour BENZO amine concentrations were very low (0.3-1.4 micrograms g-1); kidney and EMT6 tumours had intermediate levels; and liver contained very high amine levels (approximately 50 micrograms g-1). Three per cent of the BENZO dose was recovered as amine in the 24 hr urine, compared to 5% for the parent compound. Nitroreduction to the amine was demonstrated with liver and tumour preparations under N2 in vitro. The reaction was highly dependent on NADPH, and inhibited extensively in air. With liver microsomes and whole homogenates 2 and 3 moles respectively of BENZO were consumed per mole of amine formed. Inhibitor studies showed that NADPH: cytochrome P-450 (cytochrome c) reductase and cytochrome P-450 were both involved in BENZO reduction, predominantly at early and late reduction steps respectively. Aldehyde oxidase contributed to the cytosolic nitroreduction. Purified buttermilk xanthine oxidase also reduced BENZO to its amine under anaerobic conditions in vitro, but very inefficiently. The apparent Km and Vmax for BENZO amine production by whole liver homogenates were 0.148 mM and 1.45 nmole min-1 mg-1 protein respectively. Tumour homogenates were less active than liver; e.g. Vmax for the KHT tumour was 6-10-fold lower.

Inhibition of glutathione peroxidase and glutathione transferase in mouse liver by misonidazole

The mechanisms of toxicity and sensitization by the radiosensitizer misonidazole [1-(2-nitro-1-imidazolyl)-3-methoxy-2-propanol] are not well understood. We report here on the inhibition of total glutathione peroxidase (GSHPx), selenium-dependent glutathione peroxidase (selenium-GSHPx) and glutathione transferase (GSHTx) activities by misonidazole. Mouse liver cytosol GSHPx and selenium-GSHPx were inhibited in vitro with 0.5 mM misonidazole. On administration of the drug intraperitoneally (800 mg/kg) to mice, it was found that GSHPx, selenium-GSHPx, and GSHTx were inhibited in homogenate, cytosol, and microsomal fractions of mouse liver. GSHPx was depressed in all fractions up to 60-70% of control values, with maximum depression occurring in the cytosol and homogenate fractions in less than 2 hr. Recovery of activity was slower in the microsomes. In general, the pattern of depression of selenium-GSHPx was parallel to that of GSHPx except in microsomes, where GSHPx is minimal. Quantitatively, selenium-GSHPx was least affected. GSHTx was inhibited 70-80% of control values in cytosol and homogenate with recovery by 24 hr, whereas a second period of depression occurred at 24 hr in the microsomes. The inhibition of peroxide-metabolizing enzymes may lead to elevation of intracellular peroxide levels, contributing to the radiosensitizing effect and/or toxicity of misonidazole.

Distribution and tumor penetration properties of a radiosensitizer 2-[14C] misonidazole (Ro 07-0582), in mice and rats as studied by whole-body autoradiography

The hypoxic cell radiosensitizer 2-[14C] misonidazole: 1-(2-nitro-1-imidazolyl)-3-methoxy-2-propanol (Ro 07-0582) MISO was administered to mice, control rats, and rats bearing chemically induced rhadbdomyosarcoma. The dose injected was 250 mg/kg and the delivered activity was 100 microCi/kg. Whole-body autoradiography was performed in all animals. We noted the highest uptake of radioactivity in the liver and the kidney. In the liver there was an accumulation of [14C] from 5 min to the 2 hour after treatment, followed by a decrease; this observation is probably related to the metabolic pathway of the drug. The radioactivity was also concentrated in the renal medulla (30 min after injection); this organ is the excretion route for most of the misonidazole or its metabolites. Fecal excretion is also important following biliary elimination. Radioactivity is present in the central nervous system in the first hours after dosage. [14C] Tumor activity was lowest 5 min after IP treatment. By contrast, 12 h after administration of labeled compound the highest activity was detected in this tissue.

Mechanism of DNA strand breaks by mitonafide, an imide derivative of 3-nitro-1,8-naphthalic acid

The metabolism and the mechanism of action of 5-nitro-2-(2-dimethylaminoethyl)-benzo(de) isoquinoline-1,3-dione (mitonafide), a nitro-containing antitumor drug, have been studied. Incubation of mitonafide under anaerobic conditions with rat liver microsomes and NADPH formed the fully reduced amine metabolite, 5-aminomitonafide. The formation of the amine metabolite was not inhibited by SKF-525A, metyrapone or piperonyl butoxide, indicating that the cytochrome P-450 was not involved in this reduction. Incubation of mitonafide with rat liver microsomes and NADPH under aerobic conditions stimulated oxygen consumption; piperonyl butoxide, SKF-525A, superoxide dismutase and catalase had no effect on this stimulation. Both mitonafide and 5-aminomitonafide were found to bind to DNA in a similar manner. However, in inducing single-stand breaks in the DNA of L1210 cells mitonafide was 10-fold more potent than 5-aminomitonafide. These results suggest that metabolic activation of mitonafide to species other than that of the amine metabolite may play a significant role in the induction of DNA damage and the biological activity of the drug.

Enhanced production of ethylene from methional by iron chelates and heme containing proteins in the system consisting of quinone compounds and NADPH-cytochrome P-450 reductase

The addition of iron chelates or heme containing proteins to the systems consisting of NADPH-cytochrome P-450 reductase and quinone compounds, such as vitamin K3 (menadione), adriamycin, tetrahydropyranyladriamycin, daunomycin, aclacinomycin A, carbazilquinone, and mitomycin C, showed the enhanced production of ethylene from methional. In the vitamin K3 system, the effective iron chlates were Fe(II)-EDTA, Fe(II)-ADP, Fe(II)-bleomycin A2, and hemin, and the effective iron containing proteins were methemoglobin, myoglobin, ferredoxin, and partially purified cytochromes P-450, P-420, and b5, and the reversed effects were observed by horse radish peroxidase and sulfite reductase from yeast. In the system consisting of aclacinomycin A and methemoglobin, the ethylene production was potently inhibited by radical scavengers, such as Tiron, Tris, thiourea, and KI, and weakly inhibited by some other scavengers. In the system containing vitamin K3 and methemoglobin, the ethylene production was potently inhibited by catalase, but partially by superoxide dismutase, KCN, and NaN3. In this system, the absorption spectrum of methemoglobin was immediately changed to oxyform and quenched with time, and catalase protected the decrement of the spectrum. The addition of hydrogen peroxide or cumene hydroperoxide to methemoglobin also produced ethylene from methional.

Neurotoxicity of Adriamycin and misonidazole in the mouse

The neurotoxicity of the anticancer drug adriamycin was investigated in the peripheral nerve of the mouse. Injection of adriamycin into the sciatic nerve resulted in biochemical and morphologic signs of severe axonal degeneration. The biochemical evidence was based on marked increases in lysosomal enzyme activity. Acid protease and N-acetylglucosaminidase remained elevated for more than 70 days after injecting the drug. In contrast, local injection of the radiation-sensitizing drug misonidazole into the peripheral nerve failed to increase lysosomal enzyme activity.

Metabolism and excretion of [3H]misonidazole by hypoxic rat liver

Our investigation was initiated to determine if misonidazole's biological activity is related to hypoxia-dependent, reductive biotransformation to form reactive metabolites. This study was facilitated by the synthesis of [3H]misonidazole and by use of the isolated perfused rat liver as a model system for hypoxic tissue. The perfused rat liver was verified as an appropriate model system to study misonidazole (MISO) metabolism. This was done by demonstrating that the perfused liver produced the same metabolites as those isolated from rats given MISO, albeit reductive metabolism was much less in rats. Reductive metabolism of MISO by perfused livers was enhanced (estimated by measuring the rate of 1-[2-aminoimidazol-1-yl]-3-methoxy-2-propanol production) by hypoxic conditions. Formation of a MISO-derived glutathione conjugate (MISO-GSH) and covalent binding of MISO-derived radioactivity to tissue protein was also enhanced by hypoxia. Depletion of hepatic GSH with diethyl maleate increased the extent of covalent binding to protein under both aerobic and hypoxic conditions, and greatly diminished the formation of MISO-GSH. These results support the hypothesis that hypoxic conditions facilitate reductive metabolism of MISO to an alkylating agent, and that GSH plays an intervening role in the alkylation reaction.

Drug residue formation from ronidazole, a 5-nitroimidazole. V. Cysteine adducts formed upon reduction of ronidazole by dithionite or rat liver enzymes in the presence of cysteine

When ronidazole (1-methyl-5-nitroimidazole-2-methanol carbamate) is reduced by either dithionite or rat liver microsomal enzymes in the presence of cysteine, ronidazole-cysteine adducts can be isolated. Upon reduction with dithionite ronidazole can react with either one or two molecules of cysteine to yield either a monosubstituted ronidazole-cysteine adduct substituted at the 4-position or a disubstituted ronidazole-cysteine adduct substituted at both the 4-position and the 2-methylene position. In both products the carbamoyl group of ronidazole has been lost. The use of rat liver microsomes to reduce ronidazole led to the formation of the disubstituted ronidazole-cysteine adduct. These data indicate that upon the reduction of ronidazole one or more reactive species can be formed which can bind covalently to cysteine. The proposed reactive intermediates formed under these conditions may account for the observed binding of ronidazole to microsomal protein and the presence of intractable drug residues in the tissues of animals treated with this compound. They may also account for the mutagenicity of this compound in bacteria.

Influence of hypoxia on the metabolism and excretion of misonidazole by the isolated perfused rat liver--a model system

The isolated perfused rat liver was evaluated as a model system for the characterization of misonidazole metabolism under hypoxic conditions. Misonidazole metabolism by livers perfused under aerobic conditions was also examined. The clearance of misonidazole was more than three times greater under anaerobic compared to aerobic conditions (4.94 +/- 1.56 vs 1.27 +/- 0.22 ml/min; means +/- S.D., N = 3). Misonidazole metabolites were detected only in the bile. Analysis of these metabolites by reverse-phase high performance liquid chromatography (HPLC) demonstrated that misonidazole metabolism was also qualitatively changed when anaerobic conditions were employed. Misonidazole beta-glucuronide was the major metabolite detected under aerobic conditions, but it was a minor metabolite in anaerobically perfused livers. The three major metabolites produced under anaerobic conditions were not characterized, but desmethyl misonidazole (RO-07-9963) and the 2-amino-imidazole derivative of misonidazole (1-[2-aminoimidazol-1-yl]-3-methoxy-2-propanol) were excluded as possible structures.

Guanethidine N-oxide formation as a measure of cellular flavin-containing monooxygenase activity

The usefulness of guanethidine N-oxide formation as a measure of cellular flavin-containing monooxygenase activity was assessed using the purified hog liver enzyme, rat liver microsomes and hepatocytes. The apparent Km and Vmax for this reaction in hepatocytes were 0.30 +/- 0.20 mM and 0.81 +/- 0.36 nmole per 10(6) cells min-1 respectively. The Km for the purified enzyme was 0.31 mM and the Vmax was 0.56 nmole per microgram enzyme min-1. 2-Diethylaminoethyl-2,2-diphenyl valerate (SKF-525A) at a concentration of 0.5 mM had no effect on guanethidine N-oxide formation by either rat liver microsomes or the purified enzyme. In contrast 2,4-dichloro-6-phenylphenoxyethylamine (DPEA) at the same concentration caused greater than a 100% increase in the microsomal production of guanethidine N-oxide. The tertiary amines imipramine, chloropromazine and methylpyrilene inhibited N-oxide formation by both hepatocytes and the purified enzyme. These data indicate that guanethidine N-oxide formation can be used as a measure of cellular flavin-containing monooxygenase activity.