Radioimmunotherapy is hindered by the slow penetration of antibody molecules into tumors. Cells that are poorly targeted by antibody, because of their distance from feeding blood vessels, receive the lowest radiation dose, and this problem is compounded if there are radioresistant hypoxic cells present. It would be desirable to combine radioimmunotherapy with an agent that is preferentially toxic to these cells. SR 4233 is a potent hypoxic cytotoxin, and it was combined with${}^{131}{\rm I}\text{-}{\rm NR}\text{-}{\rm LU}\text{-}10$ to treat LS174T human colon adenocarcinoma multicell spheroids and nude mouse xenografts for these studies. Under conditions of severe hypoxia$(<0.01\%\ {\rm O}_{2})$, 2 h of pretreatment or 18 h of simultaneous treatment with SR 4233 did not significantly enhance the effectiveness of${}^{131}{\rm I}\text{-}{\rm NR}\text{-}{\rm LU}\text{-}10$ in spheroids. However, under aerobic conditions with a 10% fraction of hypoxic cells, there was more toxicity than would be predicted from simple additivity. Xenografts treated with${}^{131}{\rm I}\text{-}{\rm NR}\text{-}{\rm LU}\text{-}10+{\rm SR}$ 4233 had a growth delay that was significantly longer than that achieved with${}^{131}{\rm I}\text{-}{\rm NR}\text{-}{\rm LU}\text{-}10$ alone. In both spheroids and xenografts, combined treatment produced about 10 times more cell killing than${}^{131}{\rm I}\text{-}{\rm NR}\text{-}{\rm LU}\text{-}10$ alone. The lack of enhancement in spheroids under complete hypoxia suggests that SR 4233 does not sensitize hypoxic cells to radiation damage. The results with aerobic spheroids and in vivo, where a portion of the cells were hypoxic, could be explained by the targeting of different cell populations (hypoxic and aerobic) by each therapeutic modality. This effect should also be enhanced by reoxygenation and reestablishment of the hypoxic fraction during treatment, thus allowing more than the initially hypoxic fraction of cells to be killed by the SR 4233.

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