It has been shown that exposure to triacetoneamine-N-oxyl (TAN) can enhance the damage produced in Chinese hamster cells by ionizing radiation even if the drug is not present during irradiation. Preirradiation exposure to TAN gives a dose modifying factor (DMF) of 1.3 while postirradiation treatment with TAN can be nearly as effective as TAN present at the time of irradiation (DMF = 1.5). Postirradiation sensitization is strongly dependent on temperature and is only observed if TAN is added within a few minutes after irradiation. The mode of action of TAN in mammalian cells is therefore not the same as in bacterial cells where no postirradiation sensitization occurs. Electron spin resonance (ESR) measurements indicate that TAN is entering the cells but that at high cell concentrations there is a gradual decay of free spins which is accompanied by a small loss in sensitization. A much larger loss in sensitization, however, occurs very rapidly at these high cell concentrations (centrifugation pellets) without any apparent loss in spins.

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