SR-4233 (Tirapazamine) acts as an uncoupler of oxidative phosphorylation in human MCF-7 breast carcinoma cells

SR-4233 (Tirapazamine) is a hypoxic cell selective cytotoxic agent currently in Phase I clinical trial. Although SR-4233 is selectively cytotoxic toward hypoxic cells some cytotoxicity toward normally oxygenated cells also occurs. SR-4233 (500 μM, 1 h) killed about 70% of normally oxygenated and 99% of hypoxic human MCF-7 breast carcinoma cells. Using a polarographic chamber and a Clark O₂ electrode the O₂ consumption of MCF-7 cells was measured in the presence or absence of SR-4233 (500 μM) or other inhibitors or uncouplers of oxidative phosphorylation. MCF-7 cells exhibited increased O₂ consumption in the presence of SR-4233 alone and after treatment with oligomycin but not after treatment with retenone. The pattern of O₂ consumption observed after treatment with SR-4233 was very similar to that seen when the cells were treated with the classical uncoupler FCCP. After 1 h of exposure to SR-4233 (500 μM) the cells were not responsive to treatment with oligomycin or FCCP for at least 3 h, but by 24 h post exposure to SR-4233 the cells had regained responsiveness to both FCCP and oligomycin. These results indicate that in normally oxygenated cells SR-4233 acts as an uncoupler of oxidative phosphorylation so that the cells continue to consume O₂ but no ATP is produced. This condition can lead to ATP depletion especially in respiration intensive tissues and may provide an explanation for the muscle cramping observed in some patients treated with SR-4233.