Enhanced misonidazole binding in the presence of AF-2

The binding of misonidazole (MISO) in Chinese hamster V79 spheroids was increased several-fold in the presence of an equimolar concentration of AF-2 (2-[2-furyl]-3-[5-nitro-2-furyl]-acrylamide) or several other electron-affinic drugs. This results was unexpected since AF-2 is much more electron affinic than MISO and should therefore compete more effectively for electrons (i.e., AF-2 should inhibit MISO binding). Enhanced MISO binding by AF-2 occurred for both ring-labeled and side chain-labeled MISO, was proportional to AF-2 concentration, and was greatest under anoxia. However, as the ratio of MISO to AF-2 increased, the effectiveness of AF-2 in enhancing MISO binding decreased. In contrast, MISO, even at high concentrations, did not affect AF-2 binding. Since AF-2 enhanced MISO binding significantly only when both drugs were present simultaneously, electron transfer processes are implicated. These results suggest that the sensitivity of MISO as a hypoxia probe may be significantly improved by the simultaneous administration of AF-2 or other nitrofurans.