The radiosensitizing and radioprotective effects of various compounds have been characterized in Chinese hamster fibroblasts growing in vitro and in a model chemical system utilizing DNA as target. The contribution to the lethal action of ionizing radiation in mammalian cells from the indirect effect of OH has been measured. The data are consistent with the “oxygen fixation hypothesis,” whereby target free radicals react either with radical-reducing species, resulting in “chemical repair,” or with radical-oxidizing species, resulting in “fixation” of radical damage to a potentially lethal form. Radical repair–fixation competition has been demonstrated in the in vitro chemical system with a sensitizer other than O2.

Dimethyl sulfoxide is shown to radioprotect in both the cellular and chemical systems by scavenging OH. Cysteamine, on the other hand, is shown to protect primarily by adding to the pool of radical-reducing species, resulting in enhanced repair of free-radical damage in the targets. Electron-affinic compounds are shown to radiosensitize, predominantly, by adding to the pool of radical-oxidizing species, resulting in enhanced fixation of free-radical damage in the targets. Diamide is shown to radiosensitize mammalian cells by a mechanism which is additive to that of the electron-affinic compounds. N-Ethylmaleimide shows radiosensitizing characteristics, in mammalian cells, which, in some respects, are qualitatively similar to those of Diamide. These compounds may radiosensitize cells by chemically and/or biochemically altering the pool of radical-reducing species in the vicinity of the cellular targets, resulting in enhanced fixation of radical damage in the targets by endogenous radical-oxidizing species. The oxygen enhancement ratio for hamster cells is shown to depend upon the intracellular target environment, and can be made to vary between near 1 and 3.3.

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