Radiosensitization, Thiol Oxidation, and Inhibition of DNA Repair by SR 4077

The mechanism of radiosensitization by diazenedicarboxylic acid bis(N),N-piperidide (SR 4077), a less toxic analog of diamide, was studied using Chinese hamster ovary cells. SR 4077 gave an average SER of 1.58 for postirradiation incubations of 0.5, 1.0, or 2.0 h. Intracellular GSH and protein thiols decreased rapidly following drug addition and GSSG increased. The GSH/GSSG ratio shifted to 1/1.6 after SR 4077 addition but returned to greater than 10/1 between 0.5 and 1.0 h. After 4 h, total intracellular GSH was only 58% of pretreatment level and extracellular GSSG increased. Protein thiols decreased to 18% of pretreatment values, recovered most rapidly between 0.5 and 1.0 h, and reached 87% of pretreatment level after 4 h. A decrease in DNA single-strand break repair as measured by alkaline filter elution rate over 0.5 h was seen, and the initial rate of rapair was slower than in cells not treated with SR 4077. DNA double-strand break repair as measured by neutral filter elution rate was delayed during the first hour after irradiation when cells were treated with SR 4077. The times for maximum radiosensitization, GSH and protein thiol oxidation and recovery, and DNA strand break repair kinetics were closely linked. We propose that a protein thiol(s) required in repair processes was reversibly oxidized during SR 4077 treatment.