PROTECTION AGAINST SR-4233 (TIRAPAZAMINE) AEROBIC CYTOTOXICITY BY THEMETAL CHELATORS DESFERRIOXAMINE AND TIRON

Purpose: Metal chelating agents and antioxidants were evaluated as pot ential protectors against aerobic SR 4233 cytotoxicity in Chinese hams ter V79 cells. The differential protection of aerobic and hypoxic cell s by two metal chelators, desferrrioxamine and Tiron, is discussed in the context of their potential use in the on-going clinical trials wit h SR 4233. Methods and Materials: Cytotoxicity was evaluated using clo nogenic assay. SR 4233 exposure was done in glass flasks as a function of time either alone or in the presence of the following agents: supe roxide dismutase, catalase, 5,5-dimethyl-1-pyrroline, Trolox, ICRF-187 , desferrioxamine, Tiron (1,2-dihydroxybenzene-3,5-disulfonate), and a scorbic acid. Experiments done under hypoxic conditions were carried o ut in specially designed glass flasks that were gassed with humidified nitrogen/carbon dioxide mixture and with a side-arm reservoir from wh ich SR 4233 was added to cell media after hypoxia was obtained. Electr on paramagnetic resonance studies were also performed. Results: Electr on paramagnetic resonance and spectrophotometry experiments suggest th at under aerobic conditions SR 4233 undergoes futile redox cycling to produce superoxide. Treatment of cells during aerobic exposure to SR 4 233 with the enzymes superoxide dismutase and catalase, the spin trapp ing agent DMPO, the water-soluble vitamin E analog Trolox, and the met al chelator ICRF-187 provided little or no protection against aerobic SR 4233 cytotoxicity. However, two other metal chelators, desferrioxam ine and Tiron, afforded significant protection against aerobic SR 4233 cytotoxicity (protection factors at 50% survival were 3.8 and 3.1, re spectively), while exhibiting minimal protection to hypoxic cells trea ted with SR 4233. Conclusions: One potential mechanism of aerobic cyto toxicity is redox cycling of SR 4233 with molecular oxygen resulting i n several potentially toxic oxidative species that overburden the intr insic intracellular detoxification systems such as superoxide dismutas e, catalase, and glutathione peroxidase. This study identifies two met al chelating agents, desferrioxamine and Tiron, that were able to prot ect against aerobic but not hypoxic SR 4233 cytotoxicity.