Misonidazole neurotoxicity in the mouse: evaluation of functional, pharmacokinetic, electrophysiologic and morphologic parameters

Evaluation of a new 2-nitroimidazole nucleoside analogue, RK-28 as a radiosensitizer for clinical use

The experimental data previously reported on RK-28, a hypoxic cell sensitizer which is now being tested in a phase I clinical trial, are confusing. Some data indicate superiority of RK-28 over misonidazole (MISO), whereas others do not. This paper presents our experimental data on the efficacy, toxicity, and pharmacokinetics of RK-28, in comparison with those of MISO, and also summarizes the data of other investigators. In our experiments, RK-28 had a 1.5-2.5 times higher sensitizing activity in vitro on EMT6 and SCCVII cells than MISO, and the difference was larger when the pre-irradiation incubation time was longer. The latter was considered to be due to the time-dependent cellular uptake and reactivity of RK-28 with non-protein sulphydryls. In vivo, RK-28 was almost as efficient as or slightly inferior to MISO against SCCVII and EMT6 tumours when assayed with an in vivo/in vitro assay and a growth delay time assay. The LD50/7 by a single injection of RK-28 was half that of MISO, but when 60% of LD50/7 was injected into mice every day, the total dose that could be given was higher for RK-28 than for MISO. Pharmacokinetic studies using mice, rats, rabbits, and a dog showed that RK-28 was rapidly eliminated from the blood and various tissues. From our results it was concluded that the possible success of the clinical trial of RK-28 depends on its low cumulative toxicity.

Effects of nitroimidazoles on neuronal cells in vitro

Peripheral neuropathy is a significant dose-limiting side effect of the nitroimidazole drugs in vivo. We have thus undertaken a study on the mechanisms of nitroimidazole neurotoxicity in the cultured neuronal cell lines, PC-12 (rat pheochromocytoma) and NB4183 (mouse neuroblastoma). Cells were differentiated with either nerve growth factor or dibutyryl cAMP and then were exposed to misonidazole and SR 2508. Cells underwent extensive morphological changes following exposure to nitroimidazole drugs, including loss of differentiated neurite projections. Loss of neurites appeared to correlate with changes in neurofilament proteins. Immunoblot analysis of the neurofilament proteins revealed a loss of the major parent proteins and the appearance of lower molecular weight (degradation) fragments. Our preliminary data in cultured neuronal cell lines suggest that nitroimidazoles cause disruption and degradation of the neurofilament lattice with subsequent degeneration of dendritic projections, and provide an in vitro model for studying the cellular and biochemical mechanisms of drug-induced neurotoxicity.

Scanning electron microscopy and transmission electron microscopy of the ciliated cells of the trachea of the rabbit treated with misonidazole alone and in combination with ionizing radiation

The trachea is often located in the treatment volume when irradiating malignant tumours in the thorax. In order to evaluate possible synergism between misonidazole and irradiation on this tissue, the following studies were made. Fifty rabbits were treated with daily injections of 100 mg misonidazole given i.p. on consecutive days from 1 to 10 days. Morphological investigations of the trachea were made with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and light microscopy (LM). Physiological examinations were performed with recording of the ciliary beat frequency. The results were compared with those from a group of 100 rabbits given misonidazole in a similar manner and exposed to irradiation (2 Gy) 15-30 min after each injection. Ten rabbits were used as controls. The results are compared to the effect of fractionated irradiation alone with 2 Gy/day. Fractionated irradiation of the ciliary epithelium in the trachea of the rabbit has shown dose-dependent physiological and morphological effects. Misonidazole potentiates these effects of radiation with a more pronounced change of the ciliary beat frequency and an increased metabolic activity as could be visualized on TEM. The combination of drug and irradiation also induced a hyperplasia of the ciliary epithelium. Misonidazole itself had no effect on the ciliary beat frequency, but caused a hypoplasia of the ciliary epithelium.

Effects of vincristine, maytansine, andcis-platinum on behavioral and electrophysiological indices of neurotoxicity in the rat

The effects of the antineoplastic drugs vincristine, maytansine and cis-platinum were studied to determine the appropriateness of a behavioral and electrophysiological test battery for characterizing the neurotoxicity of such therapeutic compounds. Single- and repeated-dose studies in rats were performed initially, to establish doses for the subchronic neurobehavioral study. Measurements obtained in the subchronic study included body weight, rectal temperature, forelimb and hindlimb grip strengths; performance of a multisensory conditional avoidance response task and other behavioral tests; and a series of evoked responses (ventral caudal nerve action potential, brainstem auditory response and responses from other modalities). The drugs were injected intraperitoneally 5 days per week for 7 weeks. The rats were tested weekly during baseline, treatment and recovery phases. Each drug caused a different pattern of effects, but they all altered body weight, rectal temperature, peripheral nerve conduction velocity, the somatosensory evoked potential and undifferentiated motor activity. cis-Platinum was the most toxic, and maytansine was the least toxic. The results indicated that some elements of the test battery were useful for evaluating the neurotoxicity of anticancer drugs. However, other tests - notably, a test of negative geotaxis and the cortical auditory evoked response - were unreliable.

Misonidazole neurotoxicity in mice decreased by administration with pyridoxine

A series of toxicological and pharmacological experiments was performed to test the hypothesis that alterations of pyridoxine (Vitamin B6) metabolism may play an important role in the development of misonidazole (MISO) neurotoxicity. The formation of a Schiff's base between the final reduction product of MISO, 2-amino MISO (NH2-MISO), and pyridoxal-HCl in ethanol was demonstrated. Mice receiving daily intraperitoneal injections of MISO suffered significantly less toxicity (as determined by survival, weight gain and neurological tests) when large doses of pyridoxine-HCl (PYR) were delivered concomitantly, and consequently were able to tolerate administration of more than twice as many MISO injections. PYR did not alter the pharmacokinetics of MISO, either when given simultaneously or when given by multiple repeated daily injections prior to MISO. The administration of PYR also did not alter the radiosensitization by MISO in an in vivo-in vitro cloning assay with the EMT6 tumor in BALB/c mice. If depletion or altered metabolism of pyridoxine by reduced metabolites is also responsible for the neurotoxic effects of nitroimidazoles in humans, then concomitant administration of pyridoxine (in doses greater than the molar quantity of NH2-MISO formed) should inhibit the development of such symptoms and allow administration of larger doses of MISO than are currently clinically employable.

Effect of misonidazole on the tolerance of the rat spinal cord to daily and multiple fractions per day of X rays

The effect of misonidazole on the induction of early and late delayed radiation damage in the rat cervical spinal cord has been determined for single doses, daily, and multiple fractions per day of X rays. Paralysis occurred in two separate waves, which could be attributed to histologically different types of damage. Administration of misonidazole before irradiation did not modify the early and late delayed radiation response of the spinal cord. This suggested that the targets for misonidazole and radiation toxicity in the central nervous system are different. Comparison of different types of anaesthesia, Nembutal and Ethrane, with or without breathing oxygen, indicated that hypoxia was not induced in the spinal cord by the experimental conditions. Irradiation with two or three fractions a day showed a reduction in spinal cord tolerance, but this reduction became less with decreasing doses per fraction.

Interaction of misonidazole and WR-2721--II. Modification of tumour radiosensitization

Two types of mouse tumour have been used to study the radiomodifying actions of Misonidazole (MISO) and WR-2721 when used alone and in combination with each other. Single dose studies were performed in both of the tumours and fractionated studies were performed on the anaplastic carcinoma, CA MT. Radioprotection with WR-2721 was seen in both tumours, being most marked at low X-ray doses. The protection was more obvious and the sensitization by MISO less in the fractionated experiment. The combination of MISO and WR-2721 gave an intermediate response compared with either drug used alone, resulting in some sensitization with single doses and an overall protection with repeated small doses. An interactive toxicity of the 2 drugs was also observed, suggesting an additive effect when assessed in terms of lethality. These studies indicate that the effects of both MISO and WR-2721 are dependent upon the oxygen status of the cells in the tumour, and that MISO can act in an oxygen-mimetic manner to modify the radioprotection observed with WR-2721.

Assessment of the effects of acrylamide, methylmercury, and 2,5-hexanedione on motor functions in mice

Neurotoxic effects of acrylamide, methylmercury, and 2,5-hexanedione were studied in forth female BALB/c mice. The chemicals were dissolved in distilled water and administered via light-tight drinking bottles. Three control groups were used. The first received distilled water, the second received concentrated saccharin solution to assess the effects of reduced water intake, and the third was maintained on a reduced food diet. Motor functions were quantified by measuring landing foot-spread and rotarod performance. Baseline data were collected before dosing started. Mice were placed, twice weekly, on an accelerating rotarod, and their retention time was recorded. In the landing foot-spread test, the experimenter dropped mice from 15 cm onto a flat, smooth surface once a week. The hindlimb splay was then measured by the examiner. Both experimenter and examiner were unaware of the identity of each group (except of the food deprived group, in the case of the experimenter) during the first exposure. Decreased retention time and increased hindlimb splay were observed in mice after 12 d of exposure to acrylamide. Recovery followed treatment cessation. Increased hindlimb splay preceded an obvious decline of rotarod performance in the group receiving the 10 ppm of methylmercury solution. Mice receiving the 20 and 40 ppm of methylmercury solutions did not display any change in these tests before overt signs of toxicity. 2,6-Hexanedione produced a small decline in performance to a constant level after 85 d of exposure. After dosing termination, performance returned to baseline values. Control groups showed no change in performance on either the rotarod or the landing foot-spread test. Our data show that the rotarod and hindlimb splay tests in mice are about equal in sensitivity to the effects of the neurotoxic chemicals tested.

Effect of clinical levels of misonidazole on the response of tumour and normal tissues in the mouse to alkylating agents

Experiments were carried out to determine whether the enhancement of alkylating-agent cytotoxicity seen after large single doses of misonidazole (MISO) in mouse tumours can also be achieved by prolonged exposure to low MISO levels similar to those which can be tolerated clinically. The level in mouse blood plasma could be maintained at about 100 micrograms/ml for 7 h by injecting small doses of MISO every 1/2 h. The effect of this treatment in combination with cyclophosphamide (CY) or melphalan (L-PAM) was studied in the RIF-1 tumour, using regrowth delay and cell-survival cloning assays. In each case, prolonged exposure to low levels of MISO gave enhancement ratios very close to those obtained with a large single dose. ERs of 1.6-2.0 were obtained with CY and 1.8-2.2 with L-PAM over the range of alkylating-agent doses used. In experiments with CY the response of 2 normal-tissue systems, marrow and WBC count, was also studied. No significant enhancement of CY damage occurred in either case. In the L-PAM experiments the LD50/30 and WBC counts were determined as normal-tissue end points. Multiple MISO had no effect. Our results show that levels of MISO which can be achieved safely in man yield good enhancement of the tumour cytotoxicity of 2 widely used chemotherapeutic agents without increasing the damage to normal tissues.

Quantitative cytochemical assessment of the neurotoxicity of misonidazole in the mouse

A quantitative, cytochemical assay for measuring lysosomal enzymes in the peripheral nerves of mice has been developed. That the time course of lysosomal enzyme changes after misonidazole (MISO) treatment reflects the degree of neurotoxicity of this agent in the mouse, has been confirmed by the use of two known neurotoxic compounds: methyl mercury and acrylamide. This effect is specific to the peripheral nerves and was not found in liver, kidney, heart or cerebral cortex. Enzyme activities varied with mouse strain and sex, as did the response to MISO treatment. Of the mice studied, female C57 gave the greatest increase in beta-glucuronidase activity. With the MISO dose of 0.6 mg/g/dose the increased enzyme activity was independent of the route of administration and appeared to approach a plateau after 5 daily doses.

Mechanisms of hypoxic cell radiosensitization and the development of new sensitizers

Some of the mechanisms by which drugs can potentiate the radiation response of tumors and cells in culture are discussed. Emphasis is placed on the action of nitroaromatic and heterocyclic compounds as hypoxic cell radiosensitizers, and some potential successors to misonidazole (MISO) are described. These include desmethylmisonidazole and SR 2508, selected because of their low toxicity in experimental systems. Groups of compounds, more efficient sensitizers than would be predicted from electron affinity correlations, have been examined and the use of Ro-03-8799 or RSU 1047 is proposed. Finally, ortho-substituted nitroimidazoles and electron-affinic compounds with alkylating groups are described. The latter group, in particular, holds promise for the development of compounds much superior to MISO.

Fatal misonidazole-induced encephalopathy: an RTOG case report

A case of fatal encephalopathy following administration of misonidazole (MISO) is reported. Anorexia and dehydration preceded signs of encephalopathy after a cumulative dose of 18 g (11 g/m2) of MISO in five weekly doses. Serum MISO levels four hours after weekly administrations were markedly elevated. Postmortem examination revealed focal cortical neuronal necrosis and hemorrhage with focal demyelination. The possible relationship of dehydration or altered drug metabolism to the development of MISO central nervous system toxicity is noted.

In vivo response of KHT sarcomas to combination chemotherapy with radiosensitizers and BCNU

Female C3H/HeJ mice bearing intramuscularly transplanted KHT sarcomas were treated with a single dose of 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU, 30 mg/kg, i.p.) alone or in combination with a single dose of misonidazole (MISO, 1.0 mg/g, i.p.) or its desmethylated metabolite Ro-05-9963 (2.0 mg/g, i.p.). The effectiveness of drug therapy was assessed by a tumour growth-delay assay (i.e. measuring the median time required for tumours to grow to treatment size x 4). The relative efficacy of administering the nitroimidazoles in various schedules ranging from 12 h before to 12 h after BCNU administration also was evaluated. Untreated control KHT tumours grew to the initial size x 4 in a median time of 4 days. No significant growth delay was seen in mice treated with either nitroimidazole alone, whilst treatment with BCNU alone produced a median growth delay of 7 days. Combination chemotherapy with 9963 administration 3 h after BCNU significantly increased the median tumour growth delay to 9 days. However, no significant growth delay was produced in any of the other combinations of these agents. The median growth delay was significantly reduced to 5 days when MISO was administered 3 h before BCNU, whereas MISO administered simultaneously 3,6, or 12 h after BCNU significantly enhanced delays ( 9 days). These results indicate that both MISO and 0063 may be combined with conventional therapeutic agents, in this particular case a nitrosourea, to produce an enhanced tumour response. The production of such a response appears to be nitroimidazole as well as schedule dependent.

A biochemical method for assessing the neurotoxic effects of misonidazole in the rat

A proven biochemical method for assessing chemically induced neurotoxicity has been applied to the study of the toxic effects of misonidazole (MISO) in the rat. This involves the fluorimetric measurement of beta-glucuronidase and beta-galactosidase activities in homogenates of rat nervous tissue. The tissues analysed were sciatic/posterior tibial nerve (SPTN) cut into 4 sections, trigeminal ganglia and cerebellum. MISO administered i.p. to Wistar rats in doses greater than 300 mg/kg/day for 7 consecutive days produced maximal increases in both beta-glucuronidase and beta-galactosidase activities in th SPTN at 4 weeks (140-180% of control values). The highest increases were associated with the most distal secretion of the nerve. Significant enzyme-activity changes were also found in the trigeminal ganglia and cerebellum of MISO-dosed rats. The greatest activity occurred 4-5 weeks after dosing, and was dose-related. It is concluded that, in the rat, MISO can produce biochemical changes consistent with a dying-back peripheral neuropathy, and biochemical changes suggestive of cerebellar damage. This biochemical approach would appear to offer a convenient quantitative method for the detection of neurotoxic effects of other potential radio-sensitizing drugs.

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.