INDIRECT MECHANISMS CONTRIBUTE TO BIOLOGICAL EFFECTS PRODUCED BY DECAY OF DNA-INCORPORATED I-125 IN MAMMALIAN-CELLS IN-VITRO - CLONOGENIC SURVIVAL

We have examined whether mammalian cells in vitro can be protected aga inst the lethal effects of irradiation by Auger electrons emitted from DNA-incorporated I-125. Chinese hamster V79 lung fibroblasts were cul tivated in the presence of 5-[I-125]iodo-2'-deoxyuridine ((125)IdU) fo r 18 h and resuspended in ice-cold medium in the presence or absence o f 10% dimethyl sulfoxide (DMSO). DNA-incorporated I-125 activity was m easured 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 t o 48 h, after which the cells were plated to determine survival. Stora ge on ice up to 48 h without radioactivity reduced plating efficiency from 67 +/- 4% (SEM) to 20 +/- 1%. DMSO had a protective effect on col ony formation, as the respective cloning efficiencies were 83 +/- 3% a nd 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 absen ce of DMSO and 195 +/- 20 decays per cell in the presence of DMSO. Thu s 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 ra ys. These results indicate that the radiotoxicity of Auger electrons f rom I-125 decay in mammalian cells is caused mainly by an indirect mec hanism(s). (C) 1998 by Radiation Research Society.