Cytotoxic properties of hydroxylamino- and amino-misonidazole, possible metabolic products of misonidazole, in hypoxic HeLa S3 cells.

Misonidazole, a derivative of 2-nitroimidazole, has selective cytotoxic activity on hypoxic cells in addition to its radiosensitizing activity. This cytotoxicity is considered to be due to metabolic reduction of the drug. A possible metabolite seems to be hydroxylaminomisonidazole, an intermediate product derived via reduction of the nitro group. Authentic samples of hydroxylamino- and aminomisonidazole (a final reduction product) were synthesized and their cytotoxicity towards HeLa S3 cells was compared with that of misonidazole. After a 3-hr exposure to 1mM hydroxylaminomisonidazole under aerobic and hypoxic conditions, the surviving cell fractions were 0.18 and 0.0056, respectively. This represents a cytotoxicity five and 125 times greater, respectively, than that of misonidazole. Under the same conditions, aminomisonidazole showed no apparent cytotoxicity.