Influence of cellular environment on toxicity of nitroheterocycles

The preferential sensitivity of hypoxic cells to nitroheteroxycles is thought to result from the actions of toxic intermediates of drug reduction produced under hypoxic conditions. However, a lack of oxygen also alters the biochemical state of the cell and may indirectly enhance the sensitivity, of hypoxic cells to these drugs. This hypothesis was tested by 'conditioning' mouse L-929 cells in oxygen-free buffer, then exposing the cells to nitrofurazone under both aerobic and anaerobic conditions. After conditioning, the rate of cell inactivation by nitrofurazone was equal in air or nitrogen-equilibrated buffer. Pretreatment of cells in 1 muM rotenone or 0.5 mM 2,4-dinitrophenol for one hour under aerobic conditions increased the sensitivity of the cells to nitrofurazone under aerobic conditions. Similar rates of cell killing were obtained when mouse L-cells were heated in buffer for 30 min at 43 degrees before incubation with nitrofurazone in either air or nitrogen. Also, incubation of cells with nitrofurazone in the presence of 0.1% glucose, or at a cell density less than 10(5) cells/ml significantly enhanced cell killing, especially under aerobic conditions. Thus, the intracellular state of the cell, manipulated by altering the cellular environment, influenced the cellular sensitivity to nitrofurazone. Similar results were not, however, obtained with the nitroimidazoles, dimetronidazole and misonidazole; pretreatment for 2 h in buffer under anaerobic conditions did not increase the sensitivity of L cells to subsequent drug treatment in air-equilibrated buffer.

The role of radiosensitizing drugs in the management of cancer

This paper discusses the role of radiosensitizers not only as adjuncts in the radiotherapy of local neoplastic disease but also in the potential management of metastases by virtue of their direct cytotoxic effects upon hypoxic cells.

Lung damage in C57B1 mice following thoracic irradiation: enhancement by chemotherapy

C57Bl mice were treated with thoracic irradiation to doses in the range 12.0 to 18.9 gray. Few deaths were observed in the period 80--160 days after irradiation (an end-point of lung damage used by other investigators) and the median survival times ranged from 200 to 310 days. CBA mice treated under identical conditions predominantly died between 80--160 days and it is therefore concluded that C57Bl mice show unusually prolonged survival following this treatment. Six chemotherapeutic agents were given to C57Bl mice together with thoracic irradiation, in most cases two weeks beforehand. Adriamycin, bleomycin and cyclophosphamide enhanced the mortality of the mice. Most agents had little effect on radiation-induced skin damage.

Release from the Crabtree effect by hypoxic cell radiosensitizers

The Crabtree effect can be observed when the O2 consumption of tumour cells or of mammalian cells grown in culture is measured in physiological medium containing glucose. The effect of 2 hypoxic cell radiosensitizers, misonidazole and NDPP, on the O2 consumption of Ehrlich ascites tumour cells was compared in media with and without glucose. A stimulatory effect on O2 consumption was found for 5--20mM misonidazole as well as for 0.5mM NDPP, both in media containing 10(-2)M glucose. Thus glucose induced a Crabtree effect in Ehrlich tumour cells, expressed as 38--45% inhibition of O2 consumption relative to that in the same medium without glucose. The stimulatory effect of misonidazole and NDPP on O2 utilization in medium with glucose undoubtedly appeared as a release from the Crabtree effect.

Inhibition of DNA synthesis by nitroheterocycles. II. Mechanisms of cytotoxicity

Nitroheterocycles have been shown to inhibit the incorporation of 3H-TdR by cultured L-929 cells, and the degree of inhibition is related to their electron-affinity. On the basis of their chemical reactivity and potential clinical utility, nitrofurazone, misonidazole and metronidazole were selected for more detailed studies of the mechanism of inhibition of DNA synthesis. Double-isotope labelling in conjunction with hydroxyapatite chromatography allowed the evaluation of drug effects on initiation of DNA replicons, DNA chain elongation and DNA damage (single-strand breaks), and their correlation with eventual cell viability. Partial inhibition of initiation of DNA synthesis generally preceded other measurable effects, and was not reversed by incubation in the absence of drug. In the absence of DNA strand breaks (at low drug doses or after a repair interval) the rate of elongation was similar in both treated and untreated cell populations. Measurable DNA damage (strand breaks) was predictive for cytotoxicity. At lower drug doses, or under aerobic conditions, DNA synthesis was not always associated with a decrease in plating efficiency (cytotoxicity) but was reflected in decreased colony size (growth rate) of the cells. Thus the aerobic "toxicity" previously reported for chronic exposure to these agents may be better described as a "cytostatic" effect. Under anaerobic conditions (where cell killing is much greater) inhibition of initiation plays a less important role, and the nitroheterocycles are metabolically reduced to intermediates which are truly cytotoxic.

Inhibition of DNA synthesis by nitroheterocycles. I. Correlation with half-wave reduction potential

Twenty-one nitroheterocycles, including metronidazole, misonidazole and AF-2, were tested for their ability to inhibit DNA synthesis in mouse L-929 cells growing in culture. All those tested inhibited the rate of incorporation of 3H-thymidine into L cells following drug treatment for 4 h under aerobic conditions. Only 4 drugs reached their limits of solubility before the uptake of 3H-thymidine was inhibited by 50% or more. For the remaining 17, the log of the concentration producing 50% inhibition of incorporation was directly correlated with the half-wave reduction potential of the compound.

Evaluation of the anticholinesterase activity of metronidazole and misonidazole

Contrary to an earlier report that metronidazole exhibited anticholinesterase activity on isolated organ preparations, a direct spectrophotometric assay was unable to demonstrate any significant inhibition of either true or pseudocholinesterase activity in the presence of this drug. The related nitroimidazole, misonidazole, was also found to be inactive when tested in this system. This report demonstrates that the clinical neurotoxicity of these two compounds cannot be attributed to any direct effect on the cholinesterase enzymes.

Radiosensitizing effect of misonidazole in fractionated gamma-ray treatment of a human malignant melanoma grown in athymic nude mice

The effect of the hypoxic cell radiosensitizer misonidazole (Ro-07-0582) has been tested on a human malignant melanoma grown subcutaneously in the athymic mutant nude mouse. A 60Co therapy unit was used for local irradiation of the animals. The tumours were exposed daily to 3.75 Gy (375 rad) over four consecutive days, to a total dose of 15.0 Gy. Misonidazole in doses of 500 mg/kg bodyweight was administered intraperitoneally in the mice 45 minutes before each irradiation. The time necessary for the tumours to regrow to their original sizes was compared with that of tumours treated with four daily fractions of 3.75, 4.50 and 5.25 Gy without sensitizer. The enhancement ratio of misonidazole was found to be 1.05.

Misonidazole and MTDQ in combination: cytotoxic and radiosensitizing properties in hypoxic mammalian cells

A combination of misonidazole and MTDQ (6,6'-methylene-bis-2,2,4 trimethyl-1,2-dihydroquinoline) has been tested for its radiation-sensitizing properties and cytotoxicity, using Chinese hamster V79 cells cultured in vitro. Both compounds sensitize hypoxic cells to the effects of X-rays, and when used in combination their sensitizing properties are additive. By contrast, the presence of MTDQ completely inhibits the cytotoxicity that misonidazole exhibits towards hypoxic cells. These experiments shed some light on the mechanism of action of electron-affinic hypoxic cell sensitizers, and the combination of radiosensitizers suggested may have an application in human cancer radiotherapy by eliminating the neurotoxicity experienced by patients receiving misonidazole during radiotherapy.

Distribution and tumour cytotoxicity of the radiosensitizer misonidazole (Ro-07-0582) in C57 mice

The distribution and clearance of misonidazole (MIS = Ro-07-0582) were studied in C57 mouse tissues and in transplants of Lewis lung tumour. The half life of the drug in blood after a dose of 1 mg/g i.p. was 3 h. Some tissues, such as liver, were found to have consistently low MIS levels, and this was found to be due to degradation of the drug after removal of the tissues from the host. The in vivo cytotoxicity of MIS to Lewis lung tumour cells was studied using an in-vitro colony assay. After half of the tumours had been irradiated with 10 Gy to kill most of the oxic cells, the mice received i.p. injections of MIS. To simulate the longer drug exposure of human tumour cells (due to the longer half life in man) a repeated injection regime was used in some mice. There was no significant cell kill after a single dose, but with a prolonged exposure to the drug in the multiply injected animals, cell survival was reduced to 50% of control in both the irradiated and unirradiated tumours. Since the hypoxic fraction of the unirradiated tumour is probably not more than 30%, it would appear that MIS is not selectively cytotoxic to hypoxic cells. However, MIS had a much greater cytotoxic effect upon hypoxic Lewis lung tumour cells in vitro, with very little or no effect on cells grown in air. This would support the theory that the presence of hypoxic cells is essential for the expression of MIS cytotoxicity.

Electroneurographic investigations of misonidazole polyneuropathy

13 patients with malignant tumors were treated by the radiosensitizer misonidazole (Ro 07-0582), total dosage 20-29 g. The electrophysiological investigations showed (1) an early increase of distal latency, the motor nerve conduction velocity (NCV) of the peroneal nerve and the NCV of the sural nerve remaining normal or only signlty reduced, and in a few cases a marked reduction of the compound action potential or of the nerve action potential (NAP), indicating a primary axonal neuropathy; (2) greater changes in the parameters of sensory nerves (n. suralis) than of motor nerves; (3) the distal latency is a good indicator of subclinical neuropathies; (4) the electrophysiological parameters showed a normalization 6 months after the end of therapy. The mechanisms possibly responsible for the misonidazole neuropathy are discussed.

Influence of misonidazole on the incidence of radiation-induced intestinal tumours in mice

C57BL mice were given local irradiation to 2 cm2 of the lower abdomen in the dose range 16--24 Gy. There were some early deaths, but mice dying between 50--240 days predominantly developed invasive adenocarcinomas of the intestine. When the radiosensitizer misonidazole was given in a single dose shortly before irradiation the proportion of mice developing tumours was higher, but the difference was not statistically significant. However, there was a significant increase in the incidence of multiple tumours, largely attributable to tumours arising in the rectum.