Neurotoxicity with desmethylmisonidazole

Chemical radiosensitizers in cancer therapy

The development of effective low-LET radiation therapy for cancer has been hindered by the lack of consistent differential responses to radiation between tumor and normal tissues. One major difference between many solid tumors and the surrounding normal stroma is the presence of hypoxic foci in solid tumors due to the inadequate supply of nutritional needs as a result of the breakdown of microvasculature. Consequently, failure of conventional radiotherapy and local recurrences are in part attributed to the radioresistant hypoxic cell populations, present in the tumor. Local cure/control rates of a tumor can be increased only by an effective increase in the radiation dose. At the same time, an increase in such a dose would damage the oxic normal stroma, more than the hypoxic tumor cells. Hence, specific modification of tumor radiosensitivity by the use of chemical radiosensitizers, in combination with conventional radiotherapy, is an attractive alternative. Many clinicians and radiotherapists are skeptical about the outcome of using radiosensitizers in patients. Nevertheless, a vast amount of information is currently available regarding the first- and second-generation radiosensitizers both in murine and in human tumors. As a result, it is hoped that eventually a radiosensitizing drug would be discovered/synthesized that will overcome the drawbacks so far encountered in their use in the clinic. In this article, the development of chemical radiosensitizers since the early sixties, the basis for their selection, their mechanism(s) of action, and the results obtained with the various groups of radiosensitizers are reviewed.

Effects on intermediary metabolism in mouse tissues by Ro-03-8799

Glucose and lipid metabolism in the brain, liver and in a transplanted tumour were found to be variously altered within 2 to 3 h of administering single doses of the radiosensitizer Ro-03-8799 to normal and tumour-bearing mice. Hepatic lactate and glycerol-3-phosphate (G3P) levels were decreased but those of the ketone body beta-hydroxybutyrate (beta-HOBu) were raised. However, in the tumour, these levels were all enhanced. The lactate levels in brain remained relatively constant but both beta-HOBu and G3P levels were altered in a manner similar to that in the liver. The levels of glucose were approximately doubled in blood, brain and tumour, but whereas tumour G6P levels increased, those in the brain were lowered to below the limits of detection. Hepatic glucose levels were significantly decreased after 1 h but G6P levels were not affected. These changes could neither be related to inhibitory effects on hepatic glucokinase or brain hexokinase activity nor to limiting amounts of ATP in both tissues. However, the activity of glucose-6-phosphatase (G6P'ase) was distinctly raised in the liver and the hepatic glycogen stores were also rapidly lowered. Overall, the results suggest that Ro-03-8799 exerts a stimulatory effect on glucose production in the liver. In both liver and brain the levels of free fatty acids and phospholipids were increased whereas those of esterified fatty acids were lowered. Most importantly, the changes in metabolite levels affect the cellular redox couples; those of the cytosol (lactate/pyruvate; G3P/dihydroxyacetone phosphate (DAP] are directed towards the oxidised state in the liver but to a more reduced state in the tumour. The mitochondrial couple (beta-HOBu/acetoacetate (AcAc)) in both tissues is shifted towards the reduced state. These metabolic changes may result in an increase in the degree of hypoxia in the tumour and may well play an important role in the development of neuropathies.

Hypoxic cell radiosensitizers: expectations and progress in drug development

When misonidazole (MISO) was introduced into clinical trials there were great expectations that the cure rate of many tumors would be dramatically increased. The lack of efficacy of MISO discouraged further studies with hypoxic cell sensitizers. In recent years superior sensitizers SR 2508 and RO-03-8799 have been introduced into the clinic. SR 2508 is less neurotoxic than MISO, allowing more than three times the total amount of drug to be administered. Furthermore, based on the analysis of a patient's plasma pharmacokinetic profile, neurotoxicity may be largely avoidable. RO-03-8799 is superior in that it produces a higher sensitizer enhancement ratio than MISO for the same administered dose. Unlike with MISO and SR 2508, the dose of RO-03-8799 that can be administered is limited by acute toxicity with no cumulative toxicity having yet been encountered. The lack of overlapping toxicities of RO-03-8799 and SR 2508 may permit their simultaneous use with radiation thereby further increasing the utility of this class of compounds. Study design has improved and the expected clinical benefit from sensitizers has been clarified. Sensitizers, like particle radiation therapy and hyperthermia will, if successful, effect the rate of local tumor control, but cannot improve the cure rate of patients with preexisting metastatic disease. Taking into account the need to optimize reoxygenation, the various reasons for tumor radioresistance other than hypoxia, and the lower oxygen and sensitizer enhancement ratios at 200 cGy per fraction, it is likely that sensitizers will provide some clinical benefit for patients with selected tumor types. Future trials with sensitizers may not only provide clinical benefit but may help answer the question as to the role of hypoxia in clinical radiotherapy.

Chemical sensitizers for hypoxic cells: a decade of experience in clinical radiotherapy

The clinical work with chemical agents to restore the radiosensitivity of hypoxic cells began in 1973 with metronidazole, misonidazole was first given in 1974. The results so far recorded of the clinical trials with misonidazole have been generally disappointing. Only in 5 of 32 studies analyzed have significant benefits been shown to suggest real advantage with the use of misonidazole. Hypoxic cells must exist in all human tumours presenting for treatment and it is, however, probable that the oxygen effect is an important one at all dose fractionation regimes employed in radiotherapy but, after conventional fractionated radiotherapy, hypoxia may be a reason for failure in only a proportion of cases. The most important factor underlying the failure of misonidazole to achieve useful advantage is undoubtedly the low radiosensitizing concentrations achievable with the permitted dose of this neurotoxic drug. New drugs are under development and some have different dose-limiting toxicity. Those showing promise at this time are the Stanford compound, SR-2508, which is being extensively studied in the United States and the Roche compound, Ro 03-8799, which is being studied in the United Kingdom. It is possible that the greatest sensitization with the greatest tolerance will be achieved by a combination of drugs.

Initial report of the phase I trial of the hypoxic cell radiosensitizer SR-2508

From March 15, through August 31, 1983, 37 patients have been entered on the RTOG Phase I trial of SR-2508. The drug was given intravenously three times weekly for three weeks. The starting total dose was 11.7 g/m2 and the highest total dose given was 32 g/m2. The lower lipophilicity of SR-2508 has produced the expected decrease in terminal half-life (5.4 hrs) of drug excretion and increase in total drug excreted unchanged in the urine (71%) compared to misonidazole or desmethylmisonidazole. The maximum single dose (3.7 g/m2) administered was well tolerated. With multiple doses peripheral neuropathy is the dose-limiting toxicity. The lowest cumulative dose producing toxicity was 21.6 g/m2, the highest non-toxic dose was 29.7 g/m2. The use of an individual patient's drug exposure as measured by the area under the curve of drug concentration vs time may be an excellent predictor of toxicity. This may eventually permit individualization of dose and prevention of serious toxicity. A single dose of 2 g/m2 will produce a tumor concentration of drug (approx. 100 micrograms/ml) that will yield a sensitizer enhancement ratio of 1.5 to 1.7. Using a starting dose of 2 g/m2 three times weekly, patients are now being studied on a five week drug schedule to further evaluate predictability of drug toxicity in preparation for clinical trials of drug efficacy.

The effect of nitroimidazoles on the oxygen consumption rate and respiratory control ratio of beef heart mitochondria

The neurotoxic effect of the nitroimidazole radiosensitizers misonidazole (MISO) and desmethylmisonidazole (DMM) has seriously compromised their clinical effectiveness. We compare here the effect of MISO and DMM on oxygen consumption in purified beef heart mitochondria. MISO has been found to significantly increase the oxygen consumption rate and decrease the respiratory control ratio in isolated mitochondria when incubated in the presence of the NAD+ dependent substrate, beta-hydroxybutyrate. DMM has a similar but less pronounced effect than MISO on these respiratory parameters. When mitochondria were incubated in the presence of these radiosensitizers for 8, 15, 30, 45, and 60 minutes, the oxygen consumption rate was decreased when succinate, a FAD dependent substrate, was added following the incubation. This decrease, which is both time and dosage dependent, is equivalent for MISO and DMM.

The penetration of misonidazole and desmethylmisonidazole into brain tumours and other central nervous system tissues in man

The concentration of drug after oral administration of 1 g misonidazole to patients undergoing neurosurgery has been studied in samples taken from intracerebral tumours, normal brain, cerebrospinal fluid and brain tumour cyst fluid. A total of 31 patients yielded samples at various times 2-8 h after drug administration. Results show a considerable range of tumour/plasma percentages but from about 2-5 h after drug administration this averaged between 65-80%. Normal brain and CSF showed similar good penetration but drug entry into cyst fluid was lower (30-50%). A further study with desmethylmisonidazole given to 11 additional patients showed a similar range of tumour penetration but lower concentrations in CSF and cyst fluid (10-20%). Low values were also seen in some but not all of the normal brain tissue samples. Necrotic tumour also had much lower concentrations than macroscopically viable tissue. These results are discussed in terms of their clinical relevance to sensitiser studies on patients with brain tumours with drugs of different lipophilicities.

Radiosensitization in vivo: a study with an homologous series of 2-nitroimidazoles

An homologous series of 1-(omega-morpholino)alkyl-2-nitroimidazoles, previously reported to be more efficient hypoxic cell radiosensitizers than misonidazole (MIS) in vitro, were evaluated in vivo using the murine Lewis Lung carcinoma. When given i.p. the compounds were 3-20 times more acutely toxic (LD50/2d) than MIS and this toxicity increased with both the number of methylene groups (n) in the side chain and the lipophilicity of the compounds. The compounds sensitized the tumour to single doses of X-rays. On the basis of equimolar administered dose, the most effective compounds, n = 2, 4 and 5, were as efficient as MIS. However, on the basis of the measured concentration of drug in the tumour at the optimum time of irradiation the compounds with n = 4 and n = 5 were less efficient than expected from previously published data in vitro. This is attributed to the basicity of the morpholino nitrogen in these compounds such that at physiological pH the compounds are primarily in an ionized form and hence poorly able to penetrate hypoxic cells.

The radiosensitizer Ro 03-8799 and the concentrations which may be achieved in human tumours: a preliminary study

A new hypoxic cell radiosensitizer, Ro 03-8799, has been administered i.v. to 2 normal and 6 patient volunteers. Generally in non-necrotic tumours the concentrations obtained were 3 times greater than in plasma sampled at the same time. These observations added to the reports concerning toxicology in monkeys and rats and radiosensitizing efficiency in the laboratory, suggest that Ro 03-8799 may prove to be much more effective sensitizer than misonidazole in man.

In vivo assessment of basic 2-nitroimidazole radiosensitizers

The radiosensitizing efficiencies of 4 structural analogues of misonidazole (MISO) have been compared with that of the parent compound. Three of these were charged basic compounds, previously shown in vitro to be 10 times more efficient. Enhancement ratios were measured from pairs of tumour growth-delay curves for the mouse fibrosarcoma SA Fab. Two routes of administration and ranges of drug dose and intervals between injection and irradiation were tested. Drug concentrations in blood, brain and tumor were measured using high-performance liquid chromatography. The peak concentration in tumours coincided with the peak in radiosensitization: 20 min after i.v. injection and 40 min after i.p. injection. The concentration in tumours was similar for either route. Comparison of radiosensitizing efficiency on the basic of equal administered dose showed no difference between the 5 compounds, but after equimolar doses the charged compounds achieved lower tumour concentrations. Comparison of sensitizing efficiency on the basis of tumour concentration showed that they were 3 times more potent than MISO, as predicted from their higher electron-affinity. The resultant improvement in radiosensitization at low, clinically relevant, concentrations is so slight that any therapeutic benefit would depend on reduced drug toxicity in man.

Initial pharmacology and toxicology of intravenous desmethylmisonidazole

Since January 1981, 52 patients have entered the Radiation Therapy Oncology Group Phase I trial with intravenous (i.v.) desmethylmisonidazole (DMM). DMM is less lipophilic than misonidazole (MISO) and theoretically will be less neurotoxic due to lower penetration into neural tissue and more rapid elimination. The drug is administered intravenously to achieve the maximum drug concentration in tumor for a given dose. The protocol slowly escalates the total dose of drug administered. At this time the planned dose on the three week schedule is 1g/m2 five times per week to a total of 15g/m2, and on the seven week schedule is 1.25g/m2 twice weekly to a total dose of 17.5g/m2. The preliminary plasma pharmacokinetic data demonstrates high peak plasma levels within five minutes of the end of the drug infusion. Compared to MISO the percent of DMM excreted in the urine is increased, 63% vs 10%, and the elimination half-life is decreased: DMM, i.v. 5.3h; MISO, i.v. 9.3h; MISO, oral 10 to 13h. Neurotoxicity has been observed in approximately 30% of patients given a cumulative dose of greater than 11g/m2. This is in comparison to a 50% incidence in the RTOG Phase I study with oral MISO at doses of 12g/m2. There is not sufficient data to evaluate the relationship between neurotoxicity and drug exposure. Further patient accrual on this study is required to better define the properties of DMM.

The development of an oral prodrug, SR-2545, of the 2-nitroimidazole radiosensitizer SR-2508

SR-2508, a 2-nitroimidazole radiosensitizer, is expected to be clinically superior to desmethylmisonidazole or misonidazole because of its lower lipophilicity with subsequent lower drug levels in neural tissue and more rapid plasma elimination. The intravenous route of administration will be optimal but oral drugs may be necessary. Since decreased lipophilicity will decrease oral absorption we have synthesized, and tested in mice, SR-2545, an acetate ester prodrug of SR-2508. In the liver there is complete first pass metabolism to parent drug with no prodrug detectable in the blood. Compared to an equal dose of oral SR-2508, the prodrug yields a more rapid, reproducible, plasma peak with twice the bioavailability, peak plasma concentration and radiosensitization. If oral preparations of SR-2508 are to be used in the clinic the prodrug, SR-2545, is likely to be superior to oral SR-2508.

Neurotoxicity of radiation sensitizers in the mouse

The neurotoxicity of a homologous series of 1-substituted, 2-nitroimidazole compounds, synthesized in this laboratory, has been studied in mice. This involves measurement of the enzyme beta-glucuronidase in the distal sciatic, tibial and common peroneal nerves. The amount of compound required to give a known neurotoxic response, in terms of elevated beta-glucuronidase (arbitrarily set at 60% increase) has been determined. A correlation between increased number of methylene groups (N) in the side chain and neurotoxicity has been shown. A correlation between increased neurotoxicity and effective octanol; water coefficient, at pH 7.4, was also established. A more soluble version of the n = 4 member of the homologous series, that is the compound RSU 1047 (NSC 328897), and misonidazole also fit this correlation of partition coefficient and dose.

Evaluation of novel radiation sensitizers in vitro and in vivo

In principle, radiation sensitizers with therapeutic ratios greater than that of misonidazole can be obtained either by increasing sensitizing efficiency, decreasing toxicity or preferably both. This paper illustrates, firstly that a 5-nitroimidazole (S73-0662) with an electron affinity close to that of metronidazole shows sensitizing efficiency similar to misonidazole both in vitro and in vivo. The suggestion is made that this compound should receive a detailed toxicological study to ascertain if its toxicity is lower than misonidazole. Secondly, Imuran, a 5-substituted 4-nitroimidazole and one of a series of compounds which show sensitizing efficiencies in vitro much greater than would be predicted from electron affinity considerations, also shows good sensitization in vivo. Compounds in this series are generally metabolically unstable and the positive results with Imuran in vivo provide a direction for future synthesis of novel sensitizers.

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.

Desmethylmisonidazole (Ro 05-9963): clinical pharmacokinetics after multiple oral administration

The O-demethylated metabolite of misonidazole, Ro 05-9963, has been administered orally, prior to irradiation, to over 50 patients with malignant disease in order to assess the effectiveness of this compound as a hypoxic cell radiosensitizer. This paper reports the pharmacokinetic data observed in those patients who received multiple doses to a total of 12 gm-2. The mean time and magnitude of the peak plasma concentration was determined together with the plasma profile and half-life at the start and finish of each regime of 6, 20, 25 or so fractions. Half-life was independent of drug dose while peak plasma levels rose with increasing amount of drug given. The importance of urinary clearance for this polar drug is indicated by the figure of approximately 50% of administered dose excreted by this route over 24 hours, compared to less than 25% for misonidazole. It was also illustrated by the increased half-life shown by one patient who suffered renal failure during treatment. Peak plasma concentration appeared to be slightly later and were more variable than in the more ideal conditions of the normal volunteer study. In addition, this study confirmed the latter study's findings that absorption is rapid following oral administration of Ro 05-9963, yielding peak plasma levels nearly as high as those seen with misonidazole. Administration in capsules rather than aqueous solution lead to slower absorption, but the peak level did not change.

Comparative studies of hypoxic-cell radiosensitization using artificially hypoxic skin in vivo

The survival of epidermal cells in vivo has been used to assess potential radiosensitizers. Mouse skin was made acutely hypoxic for the irradiations, to give radioprotection by a factor of 2.7-3.0. Several concentrations of each drug were used to determine whether any of them were more effective sensitizers than misonidazole. The SER at each concentration was determined from radiobiological dose-response curves. The blood concentration and toxicity of the compounds were also determined. The sensitizing efficiency, assessed in several ways, indicated that only Ro 03-8799 gave significantly greater sensitization than misonidazole, and then only when assessed by comparing the compounds on the basis of equimolar blood concentrations. If the comparison of efficiency was made in terms of LD50 the ranking order change. The need for a more clinically relevant test of peripheral neurotoxicity is stressed.

Inhibition of plant fatty acid synthesis by nitroimidazoles

1. The effect of the addition of a number of nitroimidazoles was tested on fatty acid synthesis by germinating pea seeds, isolated lettuce chloroplasts and a soluble fraction from pea seeds. 2. All the compounds tested had a marked inhibition on stearate desaturation by lettuce chloroplasts and on the synthesis of very-long-chain fatty acids by pea seeds. 3. In contrast, the effect of the drugs on total fatty acid synthesis from [14C]acetate in chloroplasts was related to the compound's electron reduction potentials. 4. Of the compounds used, only metronidazole had a marked inhibition on palmitate elongation in the systems tested. 5. The mechanism of inhibition of plant fatty acid synthesis by nitroimidazoles is discussed and the possible relevance of these findings to their neurotoxicity is suggested.