2-NITROIMIDAZOLE (EF5) BINDING PREDICTS RADIATION-RESISTANCE IN INDIVIDUAL 9L SC TUMORS

The presence of hypoxic tumor cells is known to be an important cause of radiation treatment resistance in vivo, The ability to predict the presence and extent of hypoxic cells in individual tumors would allow the addition of specific ''antihypoxia''-based treatment regimes. Hypo xia can be monitored by measuring the binding of 2-nitroimidazoles. We have tested the hypothesis that binding of EF5, a fluorinated derivat ive of the 2-nitroimidazole, Etanidazole, can predict radioresistance in individual tumors. Fischer rats bearing 9L s.c. tumors were given i njections i,v, with EF5 3 h before irradiation and tumor harvest. Tumo r cells were dissociated for flow cytometric analysis and plating effi ciency studies, EF5 binding was detected via monoclonal antibodies con jugated to the orange emitting dye, Cy3. In air breathing rats, for a given radiation dose, a large amount of variation in plating efficienc y was seen, However, there was minimal variability of the plating effi ciency for tumors irradiated in euthanized animals (hypoxic tumors; co rrelation coefficient for the fitted curve = 0.93) and in cells dissoc iated from tumors and irradiated in suspension (correlation coefficien t for the fitted curve = 0.99), suggesting that varying sensitivity to the cell disaggregation technique was not responsible, In contrast, a good correlation between the relative radiation resistance or hypoxic survival and EF5 binding of ''moderately'' hypoxic cells in air breat hing rats was identified using these techniques. In these 9L s,c, tumo rs, intertumor variation in oxygenation accounted for most of the rang e in individual tumor radiation response, and this was found to be ind ependent of tumor size. This study provides evidence for the applicati on of EF5 binding with monoclonal antibody detection as an in vivo pre dictive assay of individual tumor hypoxia and resultant therapy resist ance.