Nitroimidazoles as hypoxic cell radiosensitizers and hypoxia probes: misonidazole, myths and mistakes

Nitroimidazoles have been extensively explored as hypoxic cell radiosensitizers but have had limited clinical success, with efficacy restricted by toxicity. However, they have proven clinically useful as probes for tumour hypoxia. Both applications, and probably much of the dose-limiting toxicities, reflect the dominant chemical property of electron affinity or ease of reduction, associated with the nitro substituent in an aromatic structure. This single dominant property affords unusual, indeed extraordinary flexibility in drug or probe design, suggesting further development is possible in spite of earlier limitations, in particular building on the benefit of hindsight and an appreciation of errors made in earlier studies. The most notable errors were: the delay in viewing cellular thiol depletion as a likely common artefact in testing in vitro; slow recognition of pH-driven concentration gradients when compounds were weak acids and bases; and a failure to explore the possible involvement of pH and ascorbate in influencing hypoxia probe binding. The experience points to the need to involve a wider range of expertise than that historically involved in many laboratories when studying the effects of chemicals on radiation response or using diagnostic probes.

Time scale of radiation damage by 2,4-dinitrophenyl-aziridine (CB 1954) in Chinese hamster cells: a rapid-mix study

A rapid-mix device was used to study the time-scale of radiation sensitization of hypoxic cells by CB 1954, a monofunctional alkylating agent. It has an electron affinity (E1(7)-385 mV) similar to that of misonidazole but its effectiveness as a sensitizer occurs at a five-fold lower concentration under stationary-state conditions. In the rapid-mix study, the enhancement ratio (e.r.) value of 1 mmol dm-3 CB 1954 rapidly rises to 1.75 within 120 ms, with no further rise by 500 ms. The e.r. obtained is lower than that observed under stationary-state conditions for a similar concentration. The data suggests that CB 1954 sensitizes by at least two independent mechanisms.

Structure-activity relationships in the development of hypoxic cell radiosensitizers. III. Effects of basic substituents in nitroimidazole sidechains

The effects of substituting 2-nitroimidazoles with groups carrying basic functions were studied. Prototropic, redox, lipophilicity and protein-binding properties were compared with the efficiency in radiosensitizing hypoxic Chinese hamster V79-379A cells in vitro and the cytotoxicities of the compounds after chronic aerobic exposure. Seventeen compounds were (2-nitro-1-imidazolyl)alkylamines in which the effects of changes in the terminating base and of alkyl chain length were investigated. About an order of magnitude increase in sensitization efficiency could be observed in some compounds without any increase in the aerobic cytotoxicity compared to simple, uncharged 2-nitroimidazoles such as misonidazole. The behaviour of five hydrazones was similar to that of uncharged analogues. The methiodide quaternary salts of two of the (2-nitro-1-imidazolyl)alkylamines showed that quaternization considerably reduced sensitization efficiency. (Nitro-1-imidazolyl)alkylamines appear worthy of further investigation as hypoxic cell radiosensitizers in vivo.

Time effects in molecular radiation biology

Radiation action occurs over a broad timescale which extends from the very early physical processes associated with energy absorption to the very late biological effects, such as carcinogenesis which may not become apparent until many years later. The various temporal stages of radiation action are classified and their interrelationships described. Experimental projects in cellular radiation chemistry, including pulse radiolysis, are discussed, together with some applications of the techniques in this general area. The paper also deals with some aspects of the oxygen effect in radiobiology and the mechanisms of its action. Various studies employing fast response techniques have been useful in verifying the role of fast free radical reactions in the oxygen effects and examples are given of some applications. Investigations with other hypoxic cell sensitizers, the electron affinic agents, are also briefly discussed, with an account of how studies of the timescale of radiation sensitization can be valuable in understanding mechanisms. Possible mechanisms of action of radiation sensitization by oxygen and other agents, including radical fixation and direct action processes, are considered in the light of evidence from some model systems.

The effect of the electron affinic sensitizers Ro 07-0582 and Ro 03-6156 on the survival of several E. coli mutants

Concentration versus radiosensitizing effect curves have been determined for four E. coli strains with two nitroaromatic sensitizers. The data are consistent with a simple competition kinetic model of sensitization, and K values for Ro 07-0582 are reported. An empirical relationship, DMF=alpha o.e.r., relating the dose-modifying factor produced by a given concentration of sensitizer to the oxygen-enhancement ratio of a bacterium was found to hold for all the mutants tested for values of alpha greater than 1/o.e.r. The limiting value of alpha at high concentrations of sensitizer was 0.75. Some implications of this relationship and the limiting value of alpha are discussed.