Characterization of radiolabeled fluoromisonidazole as a probe for hypoxic cells

Radiolabeled fluoromisonidazole has been characterized as a probe for hypoxic cells in vitro and in vivo. The uptake and retention of [³H]fluoromisonidazole and [³H]misonidazole were compared in V-79 cell monolayers and spheroids by varying incubation time and O₂ levels in contact with the medium. The two labeled drugs were retained similarly in cell populations isolated from different depths in spheroids, and the amount of each drug bound in cells at the spheroid periphery increased with decreasing O₂ level. The labeling patterns in autoradiographs were similar for spheroids incubated with the two labeled drugs, with most silver grains located over a zone of viable and presumed hypoxic cells intermediate between the necrotic center and the periphery of the spheroid. Biodistribution of the two tritiated drugs was compared in C3H mice bearing KHT tumors with 15% radiobiologically hypoxic cells. Tumor:blood and tumor:muscle ratios greater than 5.0 were achieved in mice sacrificed 4 h after the last of three injections of 5 or 20 μmol/kg of [³H]fluoromisonidazole. These ratios are compatible with imaging and are higher than those obtained with 50 μmol/kg misonidazole in a similar administration protocol. TLC analysis of plasma from mice injected with [³H]fluoromisonidazole indicated that the drug was stable in vivo for up to 2 h and that the metabolites formed were too polar to be dehalogenation products. Fluoromisonidazole labeled with ¹⁸F at the end of the alkyl side chain would retain the label on metabolites that bind in hypoxic cells in vivo. Fluoromisonidazole binds stably in the same populations of hypoxic cells as does misonidazole, and we conclude that [¹⁸F]fluoromisonidazole has potential use as a hypoxia imaging agent in vivo.