Radiosensitization of solid tumors by nitroimidazoles

Selective potentiation of the hypoxic cytotoxicity of tirapazamine by its 1-N-oxide metabolite SR 4317

Tirapazamine (TPZ), a bioreductive drug with selective toxicity for hypoxic cells in tumors, is currently in Phase III clinical trials. It has been suggested to have a dual mechanism of action, both generating DNA radicals and oxidizing these radicals to form DNA breaks; whether the second (radical oxidation) step is rate-limiting in cells is not known. In this study we exploit the DNA radical oxidizing ability of the 1-N-oxide metabolite of TPZ, SR 4317, to address this question. SR 4317 at high, but nontoxic, concentrations potentiated the hypoxic (but not aerobic) cytotoxicity of TPZ in all four of the human tumor cell lines tested (HT29, SiHa, FaDu, and A549), thus providing a 2-3-fold increase in the hypoxic cytotoxicity ratio. In potentiating TPZ, SR 4317 was 20-fold more potent than the hypoxic cell radiosensitizers misonidazole and metronidazole but was less potent than misonidazole as a radiosensitizer, suggesting that the initial DNA radicals from TPZ and radiation are different. SR 4317 had favorable pharmacokinetic properties in CD-1 nude mice; coadministration with TPZ provided a large increase in the SR 4317 plasma concentrations relative to that for endogenous SR 4317 from TPZ. It also showed excellent extravascular transport properties in oxic and anoxic HT29 multicellular layers (diffusion coefficient 3 x 10(-6) cm(2)s(-1), with no metabolic consumption). Coadministration of SR 4317 (1 mmol/kg) with TPZ at a subtherapeutic dose (0.133 mmol/kg) significantly enhanced hypoxic cell killing in HT29 tumor xenografts without causing oxic cell killing, and the combination at its maximum tolerated dose was less toxic to hypoxic cells in the retina than was TPZ alone at its maximum tolerated dose. This study demonstrates that benzotriazine mono-N-oxides have potential use for improving the therapeutic utility of TPZ as a hypoxic cytotoxin in cancer treatment.

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.

Radiosensitizing effect of camphor on transplantable mammary adenocarcinoma in mice

The comparative radiosensitizing effects of camphor and metronidazole on murine transplantable mammary adenocarcinoma are reported. Male C3H/Jax mice bearing transplanted mammary tumours were treated with camphor (0.5 microM/body wt) or metronidazole (0.5 microM/g body wt) 45 min before subjecting to local X-irradiation at the dose levels of 30, 80, 100 or 120 Gy. Sequential in situ measurement of the tumour volumes during the follow-up period of 45 days revealed that the maximum enhancement ratios of tumour growth delay for camphor and metronidazole were 4.8 and 2.5, respectively. This suggests that camphor can be a potential radiosensitizing agent in cancer radiotherapy.

Retreatment of pediatric brain tumors with radiation and misonidazole. Results of a CCSG/RTOG phase I/II study

Twenty-nine patients with recurrent pediatric brain tumors after full-dose treatment with radiation were retreated with misonidazole (9 gm/m2) and whole-brain irradiation (300 rad X 10). Seventeen of 29 patients (59%) experienced nausea and vomiting with misonidazole administration and 6 patients (21%) developed peripheral neuropathy. Serum levels of misonidazole were similar to those reported for adults. One patient died of trauma shortly after retreatment. For the remaining 28 patients, the median time-to-progression was 5.5 months. Median survival was 13 months. Six patients (21%) developed radiation toxicity, two of whom died from the toxicity. Whereas the results on this study are similar to reported series of patients receiving salvage chemotherapy for recurrent pediatric brain tumors, more effective treatment clearly is needed.

Determination of misonidazole and desmethylmisonidazole in plasma by high-performance liquid chromatography with reductive electrochemical detection

A reversed-phase high-performance liquid chromatographic method with reductive mode electrochemical detection was developed for the determination of misonidazole and desmethylmisonidazole in plasma. A thin-layer amperometric detector with glassy carbon working electrode was used to detect the nitroimidazoles at a potential of -0.60 V. The calibration curves were linear. The within-day and day-to-day coefficients of variation were below 3% for plasma misonidazole concentrations of 6-60 mg/l and 1-15 mg/l for desmethylmisonidazole. Electrochemical detection limits were between 2 and 4 pg, which is about 10-20 times lower than that obtained by detection at 323 nm. Limits of quantitation of the nitroimidazoles in plasma were in the order of 1-2 micrograms/l. Under the described conditions other nitroimidazoles and nitro compounds can also be detected with ultimate sensitivity by reductive mode electrochemical detection.

Effects of radiomodifiers on high and low LET responses in vivo

The radioprotector WR-2721 and the radiosensitizer misonidazole have been studied for their influence on the response of mouse skin and tumours when irradiated with either single or fractionated doses of 240 kV X-rays or 3 MeV neutrons. The modification of radiosensitivity to neutrons was significantly less than that for X-rays, both for tumours and skin. When the two drugs were given in combination before single neutron doses, competitive interaction was observed.

Effect of misonidazole on the radiosensitivity and repair of potentially lethal damage of L5178Y ascites tumor cells

The radiosensitizing effect of low concentrations of misonidazole was investigated by using L5178Y cells growing as an ascites tumor in DBA-2 mice. The cells were irradiated in vivo with graded doses of X-rays in the presence or absence of 0.1-0.5 mg/g body weight of misonidazole. Then cell survival was assayed in vitro by plating cells in soft agar medium. By analyzing the X-ray survival curves with or without misonidazole, the dose-modifying effects were determined. The results indicated that the slope and shoulder of the survival curves were greatly modified by the treatment with misonidazole. The dose-modifying factor in terms of the D0 ratio between the drug-treated and untreated control cells was increased as the drug concentration was increased. Further, it was revealed that the isoeffect dose ratios, estimated by the linear quadratic equation of Chadwick and Leenhouts, were higher at a low-radiation dose range. This is due to the suppression of the shoulder region of survival curves for the drug-treated cells. The inhibition of the X-ray-induced repair of potentially lethal damage was apparent with 0.1 mg/g body weight of misonidazole. The inhibition became more effective as the drug concentration increased.

Cancer 1980: achievements, challenges, and prospects

This review article touches on various categories of research that have been expanded or made possible predominantly by funding through the National Cancer Program of the National Cancer Institute and the American Cancer Society. Under diagnosis, categories mentioned are biological markers, chromosome banding techniques, fluorescent-activated cell sorter identification of cell surface antigens, ultrastructural studies with electron microscopy, histochemical, radiologic, ultrasonographic, thermographic, angiographic techniques, nuclear magnetic resonance imaging, radioactively labeled compounds that attach to specific tumor cell surface receptors, and other agents that are preferentially taken up by tumor tissues. Predictive tests include human tumor stem cell assays, sister chromatid exchange assay, and hormone receptor assays. The techniques listed under therapy include hyperthermia, immunotherapy, chemotherapy, radiosensitizing compounds, and the supportive measures of hyperalimentation and other nutritional manipulations, psychological reinforcement, rehabilitative efforts, bone marrow transplants, blood component therapy, protective "germ-free" environments, and pain control. Drug delivery systems, animal and cell culture models, and prevention of carcinogenesis are also mentioned.

The penetration of misonidazole into a mature structural cartilage

Mature cartilage may be expected to contain populations of hypoxic cells as a result of the tissues lack of direct vascularization and structure; it may therefore be at risk from possible radiosensitization. The hypoxic-cell radiosensitizing drug misonidazole Ro-07-0582 (MISO) was administered i.v. to mature New Zealand White rabbits at a dose of 100 mg/kg, and the resulting drug concentrations in both blood and ear-cartilage samples measured by HPLC. Samples were taken at regular intervals up to 4 h after administration of MISO. Blood concentrations of MISO rose rapidly to 240 microgram/ml within 5 min of administration, before falling steadily, with a t1/2 of 45 min. Cartilage levels reached a peak of 70% of the blood levels approximately 30 min after administration. The levels of MISO then fell, with a t1/2 of 44 min.

Misonidazole as a radiosensitizer in the radiotherapy of glioblastomas and oesophageal cancer. Pharmacokinetic and clinical studies

Since May 1978 the hypoxic-cell radiosensitizer, misonidazole (MIS), has been under clinical investigation in a phase III trial with multiple doses of the drug in 11 patients with brain tumours (seven glioblastomas, four recurrent brain tumours) and three patients with oesophageal carcinoma. The doses of MIS administered were usually well tolerated but the principal toxicities observed were peripheral neuropathy as well as nausea and vomiting was completely reversible. The incidence of neuropathy was not related to the pharmacological parameters of plasma level or half-life. Pharmacological assessment by high-pressure liquid chromatography included assays of plasma, urine and cerebrospinal fluid. The demethylated product, Ro-05-9963, was detected as the major metabolite. Peak plasma levels were obtained one to four hours after administration of MIS, with a half-life of five to ten hours. Cerebrospinal fluid levels of MIS correlated well with those of the plasma. MIS was mainly excreted as the demethylated metabolite, but less than 40% of the given dose could be recovered. The results obtained suggest that the present MIS dosage for glioblastoma patients results in a low plasma level with no observable therapeutic effect.

Effect of acute and chronic misonidazole administration on peripheral-nerve electrophysiology in mice

I.p. administration at several dose levels over periods of up to 12 weeks, or continuous i.v. infusion of high doses of misonidazole (MISO) for 15 h, produced no significant change in peripheral nerve conduction velocity (NCV) and did not prevent the normal increase in NCV as the animals matured from 12 to 24 weeks of age. Peripheral NCV (sural nerve) was reduced in both MISO-treated and control mice with hind-limb tumour implants, presumably owing to physical pressure due to tumour growth. In addition, neither the medial nerves nor the tibial nerve in the normal limbs of the tumour-implanted, drug-treated animals showed any change. Consequently our earlier and present studies do not confirm the recent reports of changes in NCV following either acute or chronic MISO administration to mice.

Effect of misonidazole and hyperthermia on the radiosensitivity of a C3H mouse mammary carcinoma and its surrounding normal tissue

Both misonidazole (MISO) and hyperthermia are known to enhance the radiation response of hypoxic cells, and to be selectively cytotoxic against cells in a hypoxic and acidic environment. The ability of these conditions to modify the effect of irradiation and their individual relationship was studied in a C3H mammary carcinoma and its surrounding skin. Simultaneous treatment with MISO, hyperthermia and radiation increased the radiation effect, with enhancement ratios (ER) of up to about 15 (1 mg/g MISO and 43.5 degrees C for 60 min.). However, such treatment also caused a smaller hyperthermic radiosensitization of the normal tissue, so that the therapeutic ratio was only increased by a factor of about 3 compared to radiation alone. Simultaneous MISO and radiation followed by hyperthermia 4 h later gave a moderate enhancement, with ER up to 3 in the tumour, but with no enhancement of the normal tissue, so that there is a similar 3-fold increase in therapeutic gain. The mechanism by which MISO and hyperthermia enhanced the radiation response may be explained as an independent action of the hypoxic radiosensitization of MISO and the selective hyperthermic cytotoxicity against acidic and chronic hypoxic cells; simultaneous hyperthermia added a further heat-induced general radiosensitization. Surprisingly, no MISO cytotoxicity could be detected in this tumour system, with or without simultaneous hyperthermia. The results indicate that in the proper treatment schedule, MISO may be a valuable addition to a combined hyperthermia and radiation treatment.