Nitroimidazoles labeled with technetium-99m are being investigated as non-invasive markers of tumor hypoxia. They are bioreductive compounds that require enzymatic reduction for retention in hypoxic cells, but little is known about the cellular factors affecting their accumulation in hypoxic cells. If the absolute accumulation of
hypoxia markers is affected by
enzyme levels, an inaccurate assessment of the hypoxic cell fraction in
tumors may occur. BRU59-21, (99m)Tc-oxo[[3,3,9, 9-tetramethyl-6-[(2-nitro-1H-imidazol-1-yl)methyl]5-oxa-4, 8-diazadioximato]-(3-)-N,N',N",N"']
technetium (V), a technetium-99m-nitroimidazole that is being studied as a potential marker of tumor hypoxia, was used in the present study to evaluate the effect of
NADPH:cytochrome P450 reductase (EC 1.6.2.4) levels on BRU59-21 accumulation and metabolism. Metabolism of BRU59-21 in hypoxic cellular lysates derived from Chinese hamster ovary cells overexpressing
NADPH:cytochrome P450 reductase was 8-fold greater than in control cells. This effect required the presence of exogenous
NADPH. The increased metabolism of BRU59-21 in lysates overexpressing
NADPH:cytochrome P450 reductase was inhibited at 4 degrees and by the addition of
NADPH:cytochrome P450 reductase inhibitors. The addition of inhibitors of other
nitroreductase enzymes had no effect on BRU59-21 metabolism in these lysates. When the accumulation and metabolism of BRU59-21 were studied in stirred
suspension cultures, it was found that cells overexpressing
NADPH:cytochrome P450 reductase exhibited about a 3-fold increase in both the hypoxic metabolism and the accumulation of BRU59-21. These findings suggest that
NADPH:cytochrome P450 reductase is an important
enzyme in BRU59-21 metabolism in model systems of tumor hypoxia.