Enhancement of radiation-induced tumor cell killing by the hypoxic cell toxin SR 4233

SR 4233 (3-amino-1,2,4-benzotriazine 1,4-dioxide) is the lead compound in a series of benzotriazine di-N-oxides which exhibit high selective killing of hypoxic mammalian cells in vitro. Drug concentrations to produce equivalent levels of cell killing of SCC VII murine carcinoma cells under hypoxia were nearly 200-fold lower than under aerobic conditions. Following a one hour hypoxic incubation with drug, 20 microM SR 4233 killed 99.9% of SCC VII cells. The hypoxia-specific cytotoxicity of SR 4233 is due to bioreductive metabolism. For in vivo studies, pharmacokinetic measurements showed that drug concentrations well in excess of 20 microM were achievable in SCC VII tumors in mice for approximately one hour after a single injection of SR 4233. Under these conditions, cell killing was considerably enhanced in SCC VII tumors when SR 4233 was combined with a single X-ray dose of 20 Gy. The enhancement was seen whether SR 4233 was given for up to 2 h before or for up to an hour after radiation, and was comparable to the enhanced cell killing achievable with a single large dose of the radiosensitizer misonidazole. While this finding is consistent with the selective killing of at least some subset of hypoxic tumor cells by SR 4233, other interactions between the drug and radiation damage may contribute to the overall effect observed.