Radiation sensitization of mammalian cells by metal chelators

The cell cycle effects, alteration in radiation response, and inherent cyto toxicity of the metal chelators mimosine, desferrioxamine (DFO), N,N'-bis(o -hydroxybenzyl)-ethylenediamine-N,N'-diacetic acid (HBED), and deferiprone (L1) were studied in exponentially growing Chinese hamster V79 cells. Incub ation of cells with 200-1000 muM mimosine for 12 h reduced clonogenic survi val to 50-60%, while incubation for 24 h reduced survival further to 0.5%. Mimosine treatment resulted in cell cycle blocks at the G(1)/S-phase border and in S phase. Pulse labeling with 5-bromodeoxyuridine indicated that the S-phase cells ceased to actively replicate DNA after only 2 h of mimosine treatment and were unable to replicate DNA for extended periods. Treatment of V79 cells with 600 muM minosine for 12 h resulted in radiosensitization, yielding a sensitizer enhancement ratio (SER) of 2.7 +/- 0.3 at the 10% su rvival level. To study the kinetics of the sensitization, V79 cells were in cubated with mimosine for various times up to 12 h and irradiated with a si ngle 10-Gy dose of X rays. It was found that the radiosensitization increas ed continually up to 8 h (from a 3- to a 100-fold difference in survival) a nd then reached a plateau after 8 h. Mimosine also equally radiosensitized human lung cancer cells having either a normal or mutated TP53 gene, sugges ting a TP53-independent mechanism. To test whether iron binding by mimosine was responsible for the observed radiosensitization, additional experiment s were performed using the iron chelators DFO, HEED and L1. V79 cells treat ed with 500 muM of these agents for 8 h followed by various doses of X rays gave SERs similar to that for mimosine (2.0-2.7). These studies indicate t hat metal chelators are potent radiosensitizers in V79 and human cells. Imp ortantly, when the DFO was preloaded together with Fe3+ [Fe(III)-DFO], the radiosensitizing effect was lost. These preliminary findings warrant furthe r studies for the possible application of metal chelators as radiation sens itizers in radiation oncology. (C) 2001 by Radiation Research Society.