Indirect mechanisms contribute to biological effects produced by decay of DNA-incorporated iodine-125 in mammalian cells in vitro:: Clonogenic survival

We have examined whether mammalian cells in vitro can be protected against the lethal effects of irradiation by Auger electrons emitted from DNA-incorporated I-125. Chinese hamster V79 lung fibroblasts were cultivated in the presence of 5-[I-125]iodo-2'-deoxyuridine ((125)IdU) for 18 h and resuspended in ice-cold medium in the presence or absence of 10% dimethyl sulfoxide (DMSO). DNA-incorporated I-125 activity was measured and the cells were plated for survival. A portion of the cell suspensions were also stored on ice to accumulate I-125 decays for 6 to 48 h, after which the cells were plated to determine survival. Storage on ice up to 48 h without radioactivity reduced plating efficiency from 67 +/- 4% (SEM) to 20 +/- 1%. DMSO had a protective effect on colony formation, as the respective cloning efficiencies were 83 +/- 3% and 72 +/- 12% at 0 and 48 h. The survival curves for (125)IdU-labeled cells are exponential with D-0 = 36 +/- 2 decays per cell in the absence of DMSO and 195 +/- 20 decays per cell in the presence of DMSO. Thus the dose modification factor (DMF) at 37% survival for 10% DMSO is 5.4 +/- 0.6 for DNA-incorporated I-125. In reference experiments, a DMF of 2.5 +/- 0.8 was measured for cells irradiated with Cs-137 gamma rays. These results indicate that the radiotoxicity of Auger electrons from I-125 decay in mammalian cells is caused mainly by an indirect mechanism(s). (C) 1998 by Radiation Research Society.