2-Hydroxylaminoimidazoles--unstable intermediates in the reduction of 2-nitroimidazoles

An unstable 2-hydroxylaminoimidazole (2-hydroxylamino-1-methylimidazole) was prepared by the reaction of 2-fluoro-1-methylimidazole with hydroxylamine. This substance was sufficiently stable (half-life of 1-2 days) in acid solutions to be observed and characterized by NMR spectroscopy; decomposition at neutrality was, however, rapid (half-life of 1-10 min). Radiochemical and electrochemical reduction experiments were carried out at pH 4 and pH 7 with 2-nitro-1-methylimidazole and misonidazole [1-(3'-methoxy-2'-hydroxypropyl)-2-nitroimidazole]. A four electron stoichiometry was found in every case. The pH 4 reduced product was identified as the 2-hydroxylamino derivative (greater than 80% yield). The pH 7 reduced solutions, on the other hand, showed no aromatic 1H NMR signals, suggesting that a simple imidazole ring was no longer present. A shift to pH 7 of the hydroxylamine produced at pH 4, however, resulted in very similar NMR spectra. The conclusion, therefore, is that the hydroxylamine was produced initially on reduction of the nitroimidazole, but it was not stable.

Activity of mitomycin C for aerobic and hypoxic cells in vitro and in vivo

We have observed the selective toxicity of mitomycin C toward hypoxic as compared to aerobic cells in vitro for three established cell lines (Chinese hamster ovary, Chinese hamster V-79, and human HeLa) and for cells from the transplantable KHT murine tumor. The magnitude of the selective toxicity was cell line dependent. We have studied the in vivo effects of mitomycin C against aerobic and hypoxic cells of two transplantable murine tumors: the KHT fibrosarcoma and the 16/C mammary carcinoma. Either mitomycin C was given with radiation to kill most of the aerobic cells, or it was given alone. Endpoints of response were cell survival assessed by lung colony assay for the KHT tumor, and growth delay for the 16/C tumor. In some experiments, mitomycin C appeared more effective when used with radiation than when used alone, but the results of combined treatment fell just within the range of additivity as defined by isobologram analysis. The effects of combined treatment were independent of the order in which drug and radiation were given. Mitomycin C was also used in combination with Adriamycin to treat the 16/C tumor, since we have found previously that Adriamycin spares hypoxic cells in this tumor. In three of four experiments, combined drug effects were slightly greater than predicted by an additive relationship. We conclude that mitomycin C is active against hypoxic cells in two murine tumors, but that it has at most minor specificity for hypoxic as compared to aerobic cells in vivo.

Failure of 2-deoxy-D-glucose and 5-thio-D-glucose to kill hypoxic cells of two murine tumors

We have administered the glycolysis inhibitors 2-deoxy-D-glucose and 5-thio-D-glucose to C3H/HeJ mice bearing KHT or 16/C transplantable tumors to seek evidence for hypoxic cell toxicity in vivo. The drugs were given (a) with or without insulin, (b) as large single doses or as multiple hourly injections, and (c) alone or immediately after the tumors had received radiation to kill most of the aerobic cell population. Tumor response was assessed by growth delay or by lung colony assay. Limiting toxicity of 2-deoxy-D-glucose and 5-thio-D-glucose was neurological, leading to seizures and/or death, and this toxicity was increased by insulin. The drugs had at most minimal effects on the growth of either untreated or irradiated tumors at maximal tolerated doses. Despite the known selective toxicity of these glucose analogues for hypoxic cells in tissue culture, we have found them to be ineffective in killing hypoxic cells of two murine tumors.

A phase I study of misonidazole and pelvic irradiation in patients with carcinoma of cervix

A Phase I study of oral daily misonidazole (MISO) with conventional pelvic irradiation, has been conducted in patients with carcinoma of the cervix Stages IB, IIB, IIIB and IVA. MISO was administered in daily dosages to sequential groups of patients at doses of 0.15 g/m2, 0.30 g/m2 or 0.45 g/m2 for 22 days over 5 weeks. Sixteen patients were assigned to each dose level. Using a double-blind randomization, they received either placebo (3/16) or MISO (13/16). The major dose-limiting toxicity was peripheral neuropathy (PN). None of the 13 patients receiving 0.15 g/m2 or the 13 receiving 0.3 g/m2 developed PN. However, 6/13 at the 0.45 g/m2 level (total dose less than or equal to 9.9 g/m2) developed PN. Additional patients were entered at this level and a total of 13/26 developed PN, which was considered of clinically significant severity in 9. Symptoms of PN have persisted from 1 week to 10 months, and have been completely reversed in 9/13 patients. Pharmacological parameters were examined for correlation with clinically evident toxicities. Although peak plasma MISO levels and half-lives did not correlate significantly with PN, there was a significant correlation between the calculated "area under the curve" (AUC) and PN. No correlation exists between PN and total urinary excretion of MISO or the O-demethylation product. A daily dose of 0.45 g/m2; MISO (total dose less than or equal to 9.9 g/m2) is considered to produce an acceptable level of toxicity for this patient population.

Neurotoxic radiosensitizers and head and neck cancer patients--how many will benefit?

The causes for the exclusion of patients from a Phase I dose tolerance study of metronidazole as a hypoxic cell sensitizing agent in patients receiving radiation therapy for head and neck cancer have been reviewed. One hundred and fifty nine consecutive patients were assessed between October 1979 and December 1980 according to eligibility criteria decided upon prior to the study. Only 26 (23%) of 111 patients treated with radical radiation therapy entered the Phase I study. The major reasons for exclusion were a history of prior nervous system abnormality, age over 70, and refusal by some patients to participate in the study. The criteria used for patient selection for studies of hypoxic cell sensitizers (and other investigational agents) must be known so that data from each study can be assessed appropriately before being extrapolated to the general patient population. Clinical trials should be designed to take into account factors that might influence patient entry.

The effects of reduction products of misonidazole on Chinese hamster ovary cells

Electrolytic reduction of the hypoxic cell sensitizer, misonidazole, was performed under hypoxic conditions at a controlled potential of -800 mV relative to a standard calomel electrode, High performance liquid chromatography (HPLC) separation of the reduction product(s) of [14C] misonidazole yielded two radioactive peaks, a major one containing 70 to 80% of the total activity and a minor one, containing 20 to 30%. A six hour hypoxic exposure of Chinese hamster ovary cells at 37 degrees C to a 1 mM equivalent of unseparated reduction product(s) resulted in less toxicity than a similar exposure to misonidazole alone. The corresponding cellular uptake and binding of reduced misonidazole was the same under aerobic or hypoxic conditions, but up to an order of magnitude lower compared to a hypoxic misonidazole exposure.

Selective toxicity of 5-(3,3-dimethyl-1-triazeno)imidazole-4-carboxamide toward hypoxic mammalian cells

The chemotherapeutic agent 5-(3,3-dimethyl-1-triazeno)-imidazole-4-carboxamide (DTIC) is used in the treatment of malignant melanoma where response rates of 15 to 30% have been reported. Some current interest exists in combining DTIC chemotherapy with localized high-dose (800 rads)-per-fraction radiotherapy in the treatment of unresectable metastatic melanoma. The present work investigates the radiosensitizing and chemotherapeutic properties of DTIC in an in vitro system using Chinese hamster ovary or HeLa cells and in vivo, using the KHT transplantable murine tumor. No evidence of a radiosensitizing effect of DTIC was found towards hypoxic or aerobic cells either in vitro in vivo. In vitro, high drug concentrations (1 mg/ml) were approximately 5 times more effective in killing hypoxic Chinese hamster ovary or HeLa cells than in killing aerobic cells over exposure times of 0 to 12 hr. The degree of toxicity was drug dose and temperature dependent but was not highly dependent on cell number or cell type. In vivo plasma levels of DTIC were measured with high-pressure liquid chromatography after i.p. injection of drug into C3H mice. At the highest drug doses tested, near the 50% lethal dose in mice for DTIC (0.5 mg/g), the drug was toxic to both aerobic and hypoxic tumor cells with some evidence of increased toxicity towards hypoxic cells. The present work suggests that DTIC may be more efficiently activated under hypoxic conditions as compared to aerobic conditions. The increased toxicity of DTIC under hypoxic versus aerobic conditions may prove to be a feature of this drug that can be exploited in its clinical use and in the design of new analogs of DTIC.

Sulphydryls, ascorbate and oxygen as modifiers of the toxicity and metabolism of misonidazole in vitro

Equimolar concentrations of cysteamine and reduced glutathione protected against the cytotoxicity of 5 mM misonidazole (MISO), whereas 5mM ascorbate enhanced its toxicity to hypoxic CHO and HeLa cells in vitro. Protection (reappearance of a shoulder region) could also be seen when cysteamine was added at later incubation times. These changes in toxicity were accompanied by changes in drug metabolism, as evidenced by radiochromatograms of cell extracts obtained after treatment with 14C-labelled MISO. In contrast, radiochromatograms obtained from cells treated with toxic levels of MISO (75 mM) under aerobic conditions indicated no drug metabolism. Both toxicity and drug metabolism could be immediately halted by introducing O2 during hypoxic exposure to MISO. These observations are discussed in terms of a possible model for the metabolism-mediated toxicity of MISO and the roles which sulphydryls and O2 may play.

The effect of amino acids on the temperature sensitive phenotype of the mammalian leucyl-tRNA synthetase mutant tsHl and its revertants

The temperature sensitive leucyl-tRNA synthetase mutant tsHl and two revertants have been compared to the parental Chinese hamster ovary cells with respect to the effects of amino acid concentrations in the medium on growth. Elevating the leucine concentration 30- or 100-fold allowed tsHl to grow exponentially at 38.5 degrees C, normally the nonpermissive temperature. Partial revertants that had recovered some enzyme activity required smaller supplements for growth. Measurements of the leucine pools indicated that they respond directly to the extracellular leucine concentration and may mediate the effect. Use of combinations of amino acids confirmed that isoleucine has a similar though weaker effect on tsHl and identified an even weaker protection by valine. The triple combination of leucine, isoleucine and valine was a much more efficient medium supplement and three times normal concentrations of these amino acids supported growth of tsHl at 38.5 degrees C. It is postulated that they are acting at their respective aminoacyl-tRNA synthetases to help stabilize a complex which also contains the mutant leucyl-tRNA synthetase. The pool size measurements also showed that the leucine pools of tsHl and a revertant increased 2-fold more in a response to increased temperature than those of WT. It is suggested that this is a regulatory response to low leucyl-tRNA synthetase activity and is important in determining growth phenotypes.

Isolation and characterization of revertants of the mammalian temperature sensitive leucyl-tRNA synthetase mutant tsHl

Nine spontaneous and seven ethyl methanesulfonate induced revertants of the Chinese hamster ovary cell line mutant (tsHl), which possesses a temperature sensitive leucyl-tRNA synthetase, were isolated and characterized with respect to growth rate, leucyl-tRNA synthetase activity and thermolability, intracellular leucine pool size, and rRNA content. Although most revertants had increased leucyl-tRNA synthetase activity, and of those tested, all but one had increased thermostability, each appears to be unique. One revertant may be an intergenic suppressor since it appears to contain an elevated level of tsHl-like synthetase. There was no evidence for any of the revertants having increased rRNA and tRNA contents, however, many showed leucine pools two to three times larger than wild type cells. Since similar increases have been observed in tsHl cells they are believed to result from regulation of leucine pool size by the leucyl-tRNA synthetase and are of a magnitude sufficient to affect significantly the growth of revertants at 38.5 degrees C.

Screening for the mutagenicity of nitro-group containing hypoxic cell radiosensitizers using Salmonella typhimurium strains TA 100 and TA98

A series of sixteen 2-, 4- and 5-nitroimidazoles, four nitrobenzenes, five nitrofurans, and a nitropyrrole, most of which have been studied previously as hypoxic cell specific radiosensitizers, have been screened for their mutagenicity using the Salmonella typhimurium strains TA 100 and TA 98 developed by Ames and co-workers. Most of these compounds were mutagenic and had a one to two order of magnitude greater mutagenicity towards TA 100 (base-pair substitution sensitive) than TA 98 (frame-shift sensitive). The spectrum of mutagenic efficiencies for the drugs which was observed could be correlated to some extent with the electron affinity of these compounds. Exceptions to this correlation may indicate drugs of interest for further studies both as mutagens and hypoxic cell radiosensitizers.

Testing of hypoxic cell radiosensitizers in vivo

Use has been made of the transplantable KHT sarcoma in C3H mice to test the in vivo effectiveness of some 2-, 4-, and 5-nitroimidazoles as hypoxic cell radiosensitizers. A comparison of the in vivo versus the in vitro sensitizing ability of misonidazole and metronidazole indicates some differences, probably due to drug delivery problems in vivo. The relative sensitizing abilities of eight 2-nitroimidazoles, two 4-nigroimidazoles and two 5-nitroimidazoles are compared on the basis of the amount of drug injected and the plasma levels obtained.

An investigation into the potential of a mammalian temperature sensitive leucyl-tRNA synthetase mutant for mutagenesis studies

Reversion of the mammalian temperature sensitive leucyl-tRNA synthetase mutant CHO tsH1 [33] has been investigated to determine its potential for mutagenesis studies. The protocol for a mutation assay using the new class of temperature sensitive conditional lethal mutants of somatic cells is presented. While it requires careful control of temperature during revertent selection it promises to provide a system complementary to the existing mutation assays. UV and EMS mutagenesis of tsH1 has confirmed that approx. 3TD of expression time are sufficient for complete expression of induced mutants at low mutagen doses (80% survival following UV and 40-50% with EMS). At a higher UV dose resulting in only 10% survival much longer expression times were required which cannot be explained by growth delays alone. While the reason for this is unknown it suggests that care must be taken in studies which require high mutagen doses. Representative revertants which were isolated show a range of phenotypes between those of tsH1 and WT. They appear to be a promising source of extragenic suppressor mutants [33] with alterations in functions affecting protein synthesis.

Increased cell killing by metronidazole and nitrofurazone of hypoxic compared to aerobic mammalian cells

Nitromidazole and nitrofuran derivatives comprise a large family of compounds, some of which have been shown to be hypoxic cell specific radiosensitizers in vivo and in vitro. The effects of metronidazole (2-methyl-5-nitroimidazole-1-ethanol) and nitrofurazone (5-nitro-2-furaldehyde semicarbazone) were studied on cell viability in vitro in the presence of air or nitrogen in the absence of radiation. Exponential-phase Chinese hamster ovary cells were placed in suspension culture in complete medium in the presence of air, made hypoxic by flowing nitrogen (less then 0.001% oxygen), and exposed to various concentrations of these drugs. As a function of time, aliquots were removed and plated to determine cell viability. After 8 hr of incubation of Chinese hamster ovary cells in 29 mM metronidazole or 500 muM nitrofurazone, the absolute plating efficiency remains relatively constant (80 to 40%) in the presence of air. In contrast, under hypoxic conditions the plating efficiency of the cells dropped to 1% after 6 hr of incubation in 29 mM metronidazole or 500 muM nitrofurazone. This phenomenon of hypoxic cell specific toxicity was found to be dependent upon cell type, concentration of drug, temperature of incubation, and oxygen concentration. The results of these experiments indicate an increased toxicity of these drugs under hypoxic conditions and suggest that further investigation into the mechanism and specificity of these effects is warranted.

Effect of leucine on the temperature sensitive phenotype of a mammalian leucyl-tRNA synthetase mutant

The concentration of leucine in the growth medium has been found to influence the expression of the temperature sensitive phenotype of a mutant of Chinese hamster ovary cells with an altered leucyl-tRNA synthetase. Plating efficiency and growth studies showed that increasing the leucine concentration allows cells to survive at normally non-permissive high temperatures and conversely decreasing the leucine concentration enhances the adverse effectsof high temperature. A similar but smaller effect was noted with isoleucine. It is suggested that this observation may form the basis of a rapid test, useful in directing the investigation of the lesion in similar mutants to pathways involving specific amino acids.

In vivo testing of hypoxic radiosensitizers using the KHT murine tumour assayed by the lung-colony technique

The KHT transplantable tumour of C3H mice has been used as a model tumour for the invivo study of hypoxic cell sensitizers. Eleven sensitizers comprising four nitrofuran five nitrobenzene and two nitroimidazole derivatives, which have been shown to be effective on hypoxic mammalian cells in vitro, have been investigated. Two of these compounds, metronidazole (2-methyl-5-nitroimidazole-1 ethanol) and tinidazole (ethyl [2-(2'-methyl-5'-nitro-1'-imidazolyl) ehtyl] sulfone), showed signs of hypoxic cell-sensitization in vivo when given systemically by intraperitoneal injections. In addition, preliminary testing of the nitrobenzene NDPP (P-NITRO-3-DIMETHYL-PROPRIOPHENONE HYDROCHLORIDE) INDICATED THAT WHEN IT WAS INJECTED DIRECTLY INTO THE TUMOUR AND IRRADIATION WAS COMPLETED WITHIN TEN MINUTES AFTER INJECTION, APPRECIABLE SENSITIZATION WAS OBTAINED. More detailed studies indicated that both metronidazole at 1,500 mg/kg and tinidazole at 750 mg/kg given intraperitoneally gave an enhancement ratio of 1-5 for a chronically hyopix cell population in this solid tumour in air-breathing mice. Measures of plasma levels of metronidazole and enhancement ratios obtained in the present in vivo system seem in relative agreement with the in vitro and in vivo results of others.