Penetration of AF‐2 and 4NQO into multicell spheroids

The mutagenicity of 4NQO and AF-2 was evaluated in a multicell system, Chinese hamster V79 spheroids, in order to determine the effects of drug delivery and metabolism on toxicity and mutagenicity. Both 4NQO and AF-2 undergo metabolic reduction of the nitro group to produce toxic intermediates. However, 4NQO is metabolized under oxic as well as hypoxic conditions, while AF-2 is reduced predominantly under hypoxia. This difference is apparently responsible for the observed pattern of toxicity and mutagenicity towards cells of large spheroids. Fluorescence microscopy revealed that 4NQO fluorescence localizes primarily in external (oxic) cells while AF-2 can penetrate further into the hypoxic region of spheroids. Although 4NQO is a far more potent single cell mutagen than AF-2 for equimolar exposures of single cells, the internal cells of spheroids exposed to 7.5 micrograms/ml of either agent under hypoxia were more readily mutated by AF-2 than by 4NQO.