Comparison of two techniques for detecting tumour hypoxia: a fluorescent immunochemical method and an in vitro colony assay

The aim of this study was to compare the percentage of hypoxic cells obtained with two methods: an in vitro colony assay and a new method based on immunodetection of a marker for hypoxic cells (NITP) which could be used in patients. These studies have been carried out using one rodent tumour EMT6 (a mammary carcinoma) and one human tumour HRT18 (a rectal adenocarcinoma). The hypoxic cell fraction was assessed in control mice and in mice receiving two treatments: 250 mg/kg nicotinamide + carbogen, and 250 mg/kg nicotinamide + carbogen + 4 ml/kg perflubron emulsion. The two treatments increased the radiosensitivity of the two cell lines, nicotinamide plus carbogen plus perflubron emulsion having the greatest radiosensitising effect. For untreated and treated tumours, the percentage of hypoxic cells obtained with the in vitro colony assay were comparable to those obtained with immunodetection using NITP. Whatever the treatment, NITP detection was a convenient test to detect the hypoxic cell fraction in the two solid tumours we have studied.

Importance of thiols in the reductive binding of 2-nitroimidazoles to macromolecules

Reductive activation of 2-nitroimidazoles in the presence of bovine serum albumin (BSA) led to binding of the nitroheterocycles to the protein. The binding was most efficient to BSA in which protein disulfides had been reduced to thiol groups. Protein thiols were at least twenty times more efficient than other protein, RNA or DNA nucleophiles in binding the reductively-activated nitroheterocycles. This result is of practical importance in the development of immunoassays for 2-nitroimidazoles as hypoxia markers in normal and tumor tissue.

A comparison of the techniques of alkaline filter elution and alkaline sucrose sedimentation used to assess DNA damage induced by 2-nitroimidazoles

The induction of DNA single-strand breaks (DNA-SSB) in Chinese hamster V79-379A lung fibroblasts by misonidazole or RSU-1069 under both aerobic and hypoxic conditions was examined following incubations for up to 4 hr at 310 degrees K using the technique of alkaline filter elution. Incubation with RSU-1069 induces DNA-SSB under both hypoxic and aerobic conditions, whereas incubation with misonidazole induces DNA-SSB only under hypoxia. The yield of breaks is dependent on both agent concentration and contact time. Following identical treatments with these agents, the yield of DNA-SSB (expressed in radiation dose equivalents) determined by alkaline filter elution is about one order of magnitude less than that previously determined by alkaline sucrose gradient sedimentation. In contrast to radiation induced DNA-SSB, alkaline elution is less sensitive than alkaline sucrose gradient sedimentation when determining DNA-SSB induced by RSU-1069 and misonidazole. During the filter elution assay, either increasing cell lysis from 2 to 4 hr, the pH of the lysing buffer from pH 8.7 to 12.5 or the elution buffer from pH 12.2 to 12.5 does not significantly effect the yield of DNA-SSB. Increasing the pH of the lysing or elution buffers to greater than pH 13 however results in considerable degradation of the DNA, whereby 50-85% of the total DNA passes through the filter with the lysing solution. This effect was similar for DNA from both control and chemically insulted cells. In conclusion, it is apparent that incubation with these agents results in the induction of DNA damage which is expressed as a DNA-SSB only after prolonged treatment under alkaline conditions. Further, the use of alkaline elution to study DNA-SSB damage induced chemically must be treated with caution in the light of these findings.

Tissue distribution of 14C- and 3H-labelled misonidazole in the tumor-bearing mouse

The retention of labelled misonidazole (MISO) was measured in a range of normal tissues in the mouse 24 hr after the intravenous injection of [14C]MISO (ring labelled) and [3H]-MISO (side-arm labelled). For [14C]MISO the 24 hr tissue retention, in order of the highest to the lowest levels (excluding pathways of excretion), was esophageal epithelium, liver, foot pad, eyelid, lung, subcutaneous lung tumor (A110), esophageal wall, uterus, eye ball, blood, salivary gland, spleen, voluntary muscle, pancreas, inguinal fat. It was assumed that the 14C represented MISO metabolite(s) bound to macromolecules. An approximately similar pattern was observed for [3H]MISO, but a higher percentage of the injected activity per gram of tissue was retained, probably due to the presence of tritiated water in the tissues. It has generally been assumed that significant levels of MISO binding are restricted to hypoxic tissues, for example tumors, but the present results show that significant levels of binding can also occur in apparently normoxic tissues. The explanation is put forward that this binding may be due to local high levels of nitroreductase capacity.

Metabolic binding of misonidazole to mouse tissues. Comparison between labels on the ring and side chain, and the production of tritiated water

The 2-nitroimidazole, misonidazole, is of current interest as an imaging agent for hypoxic regions in tumors and in vascular disease such as stroke. The basis of this technique is the reductive activation and binding of nitroheterocycles which is much more efficient in the absence of oxygen. The appropriate molecular location for an active isotope on the nitroheterocyclic probe depends on the nature of the metabolites retained in tissues after the parent drug has been cleared. Previous studies with tumor cells in vitro indicated that a ring label (2-14C) and a side-chain label (3H) were retained equally efficiently in the acid-insoluble fraction, whereas 1.5 to 3 times more side-chain label was retained in the total pool (acid soluble plus acid insoluble) of metabolites in several normal murine tissues. We show here that the excess side-chain label in six normal tissues, plasma and EMT6 tumors was found entirely in the acid-soluble fraction as a volatile component. This volatile component was tentatively identified as tritiated water. It appeared that, in general, molecular products of misonidazole metabolism were retained in mouse tissues, with the exceptions that a small excess of ring label was found in liver and heart and that tritiated water appeared in the acid-soluble fraction of all tissues. Tritiated water would not be important in imaging studies but could be a factor in studies in which scintillation counting of tritiated nitroheterocyles is used.

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.

Flash photolysis of misonidazole and metronidazole

Laser flash photolysis at 355 nm of misonidazole or metronidazole in aqueous solutions produced the relatively long-lived nitro radical anion as the only observable transient species. When 266 nm excitation was used, a small yield of solvated electron was observed. It is suggested that the nitroimidazole first undergoes photoionization and the photoelectrons are scavenged by ground state nitroimidazole molecules to produce the nitro radical anion. Alternatively, added EDTA or carbonate ion acted as an electron donor to the excited state nitroimidazole molecule, thereby increasing the yield of nitro radical anion. The transient yield from metronidazole was about half that from misonidazole, while the phosphorescence intensity of metronidazole in an ethanol glass was about 20 times that of misonidazole. The misonidazole n, pi* triplet state is more easily reduced than that of metronidazole and, in the presence of an electron donor, the radical anion is postulated to result from electron transfer to the triplet state of the nitroimidazole.

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.

Generation of radical anions from metronidazole, misonidazole and azathioprine by photoreduction in the presence of EDTA

Ultraviolet irradiation of the nitroimidazole derivatives metronidazole, misonidazole, azathioprine and 1-methyl-4-nitroimidazole in aqueous solution with various reductants produced the respective nitro radical anions, as detected by electron spin resonance spectroscopy. The most effective reductant, yielding high concentrations of the radical anions, was EDTA at pH 10. NADH, NADPH, formaldehyde glutathione and methanol were also tested but were less efficient as reductants.

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.

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

Radiation-chemical reductions have been carried out with several nitroimidazoles. Reduction of these drugs in the presence of single-stranded phi chi 174 DNA causes extensive lethal damage. However, relatively stable (end) products, do not contribute to the damage, although glyoxal is potentially toxic. This demonstrates that a short-lived intermediate in the reduction process is responsible. Further, the quantity of damage in the DNA depends on both dose (reduction)-rate and also the nature of the drug.

Misonidazole and other hypoxia markers: metabolism and applications

A substantial effort is being devoted to developing markers for hypoxia in tumors. Most of the work to date has been performed on misonidazole (MISO), which is selectively metabolized by hypoxic cells to reactive products that bind covalently to cellular constituents. This paper attempts to review the metabolism of MISO as it relates to binding, to summarize several of the properties of the binding of MISO to cells and tissues which appear to be directly relevant to the characteristics of the reactive species involved, and to evaluate the potential of MISO and other nitroheterocycles as markers for hypoxia. Four roles for a hypoxic marker are considered. MISO labeled with 3H or 14C is a good marker for local radiobiological hypoxia in autoradiograms of tumor sections, but more work is required to investigate factors other than oxygen concentration that conceivably might affect the binding process. In quantitating hypoxic fraction in tumors using non-destructive techniques, which has been modelled by correlating surviving fraction with 14C-misonidazole uptake, non-specific binding to aerobic and necrotic tissue limits the accuracy of the estimate, but useful clinical applications can still be envisaged. For quantitation of a change in the hypoxic fraction of an individual tumor using serial assays, preliminary data suggest that MISO binding should be a sensitive assay. Fluorescent nitroheterocycles have a great deal of potential as markers to enable the sorting of tumor cell suspensions into portions derived from the hypoxic and aerobic regions, but better compounds are needed.

Misonidazole retention by normal tissues: a distinction between label on the ring and side chain

The retention of misonidazole by mouse normal tissues and EMT6/Ed tumors was studied 24 hr after injection of ring labelled (2-14C) and side chain-labelled (3H) drug. Slightly more 3H than 14C was retained by tumors, but this was considered to be within experimental error of no difference. The ring label (14C) activity retained by tumors was 5-12 times greater than that retained by heart, kidney, brain, muscle and spleen. However, in the same animals, the side chain label (3H) was retained to an appreciably greater extent by the normal tissues, so that the ratio of activities retained in tumors and normal tissues was 3 to 4. This difference in discrimination between tumors and normal tissues implies that gamma-emitting or NMR-active analogues of misonidazole will detect hypoxia in tumors in situ more efficiently, if the active isotope is situated on the ring. The data also indicate that fragmentation products of the metabolism of misonidazole, which contain the side chain and exclude the 2-carbon, are responsible for 50-70% of the misonidazole retained by normal tissues when a side chain-label is used.

Electrolytic reduction of nitroheterocyclic drugs leads to biologically important damage in DNA

The effect of electrolytic reduction of nitroimidazole drugs on biologically active DNA was studied. The results show that reduction of the drugs in the presence of DNA affects inactivation for both double-stranded (RF) and single-stranded phi X174 DNA. However, stable reduction products did not make a significant contribution to the lethal damage in DNA. This suggests that probably a short-lived intermediate of reduction of nitro-compounds is responsible for damage to DNA.

Some reactions and properties of nitro radical-anions important in biology and medicine

Nitroaromatic compounds, ArNO2 have widespread actual or potential use in medicine and cancer therapy. There is direct proof that free-radical metabolites are involved in many applications, and an appreciation of the conceptual basis for their therapeutic differential; however, an understanding of the detailed mechanisms involved is lacking. Redox properties control most biological responses of nitro compounds, and the characteristics of the one-electron couple: ArNO2/ArNO2- are detailed. The "futile metabolism" of nitroaryl compounds characteristic of most aerobic nitroreductase systems reflects competition between natural radical-decay pathways and a one-electron transfer reaction to yield superoxide ion, O2-. Prototropic properties control the rate of radical decay, and redox properties control the rate of electron transfer to O2 or other acceptors. There are clear parallels in the chemistry of ArNO2- and O2-. While nitro radicals have frequently been invoked as damaging species, they are very unreactive (except as simple reductants). It seems likely that reductive metabolism of nitroaryl compounds, although generally involving nitro radical-anions as obligate intermediates (and this is required for therapeutic selectivity towards anaerobes), results in biological damage via reductive metabolites of higher reduction order than the one-electron product.

Reactions of nitroimidazoles with free radicals--enhancement of reaction by u.v. irradiation

The free-radical reactivity of the nitroimidazole derivatives metronidazole, misonidazole, benznidazole and ornidazole was investigated by observing their effect on the polymerization of acrylamide in aqueous solution. Free-radical polymerization was initiated by the thermal decomposition of potassium peroxydisulphate at 50 degrees C. Measurement of the polymerization rate showed an inhibitory effect of the nitroimidazoles which was greatly enhanced when the system was irradiated with u.v. light near their absorption maximum of 320-325 nm. Analysis of the competitive kinetics of the system enabled calculation of the rate constant for reaction of the ground state and photoexcited nitroimidazole with the polyacrylamide free radicals. No significant difference between the various nitroimidazoles could be found in the dark reaction, but in the u.v.-irradiated system the order of reactivity (misonidazole greater than benznidazole greater than metronidazole approximately equal to ornidazole) was the same as the reported relative mutagenic, cytotoxic and radiosensitizing potency of the compounds. These results imply that the excited states of the nitroimidazoles are important to their activity in radical-radical reactions.

Misonidazole-glutathione conjugates in CHO cells

Misonidazole, after reduction to the hydroxylamine derivative, reacts with glutathione (GSH) under physiological conditions. The reaction product has been identified as a mixture of two isomeric conjugates. When water soluble extracts of CHO cells exposed to misonidazole under hypoxic conditions are subjected to HPLC analysis, misonidazole derivatives, having the same chromatographic properties as the GSH-MISO conjugates, were detected. The identity of the synthetic and cellular products was further confirmed by identification of the amine derivative of misonidazole after desulfurization with Raney Nickel. When CHO cells were incubated with misonidazole in the presence of added GSH, a substantial increase in the amount of the conjugate was detected. When extracts of CHO cells exposed to misonidazole under hypoxia were subsequently exposed to GSH, an increased formation of the conjugate was observed. A rearrangement product of the hydroxylamine derivative of misonidazole is postulated as the reactive intermediate responsible for the formation of the conjugate.

Detection of a reactive metabolite of misonidazole in human urine

Chemical studies have indicated that, following reduction of misonidazole to the hydroxylamine derivative, reaction with guanosine leads to the formation of a 2-carbon addition product of guanosine. In this study, the formation of the guanosine product is used to detect the presence of a reactive metabolite of misonidazole in the urine of patients treated with misonidazole. Urine samples were incubated with [14C]guanosine and the guanosine product was separated by HPLC analysis. The quantities of product vary as much as 10-fold from patient to patient and it is suggested that the assay might be useful as a predictor of patients susceptible to the development of peripheral neuropathy or other effects of misonidazole.