Differences in the toxicity and metabolism of the 2-nitroimidazole misonidazole (Ro-07-0582) in HeLa and Chinese hamster ovary cells.

The 2.nitroimidazolemisonidazole(Ro-07-0582)is a hy poxic cell radiosensitizer currently undergoing clinical investigation.In additionto its radiosensitizingabilities,it showsan Increasedtoxicitytowardhypoxiccomparedto aerobic cells. Human HeLa cells are 2 to 3 times more sensitive to misonidazole in terms of the time required to reduce survival to 0.1% after hypoxic incubations in 5 mis drugthan are Chinese hamsterovary cells. The basis of thisdifferencein sensitivitywas investigatedwiththe use of [†̃4C]misonidazole. There is an increasedaccumulation of radioactivityin hypoxiccells over that seen in aerobic cells that remain in equilibriumwith externaldrug levels. In the presenceof 5 mislabeled drug, hypoxicHeLa cells accumulateradioactivityat 2 to 3 timesthe rateof hypoxic Chinesehamsterovarycells.Thisaccumulationhasbeen measuredas a functionof drugconcentrationandoxygen concentration.At 1% oxygen both cell lines accumulate radioactivity at greater than one-half the maximum rate seen at the very lowest oxygen tensions studied, <0.001%. A significant increase in drug toxicity in terms of lossof colony-formingability is also seen for bothcell lines at 1% oxygencomparedto air. Chromatographyof soluble cell extracts indicated that the intracellular accu mulation of radioactivityduring hypoxic incubations is due to a retentionof at least somemisonidazolemetabo lites, but there is no net accumulationof the unaltered drug.As well, severalmetabolitesof misonidazolecan be detected in the medium in which hypoxic cells are incu bated compared to aerobic incubationswhere only the unaltered drug and an 0-demethylation derivative are seen. These resultssuggestthat differencesin the sensi tivity of HeLa and Chinese hamster ovary cells to misoni dazole are due to differences in the rate at which they metabolize the drug.