Hypoxic cells and in situ chemopotentiation of the nitrosoureas by misonidazole

Intracerebral (i.c.) and subcutaneous (s.c.) 9L tumours were treated simultaneously with various doses of the nitrosoureas, BCNU or CCNU, and 2.5 mmol kg-1 of misonidazole (MISO). After 24 h, tumours were removed, dissociated into single cell suspensions and the cells plated for colony formation. In both i.c. and s.c. tumours, no cell kill was observed after exposure to MISO alone, and no additional cell kill was observed when MISO was combined with either nitrosourea. If s.c. 9L tumours were clamped 30 min after i.p. injection of 2.5 mmol kg-1 MISO, then 2 h later the clamps were removed and the nitrosourea injected, an increase in cell kill was observed. This increase in cell kill was statistically significant (P less than 0.01) for each dose of BCNU administered, but not statistically significant (P greater than 0.05) for the moderate dose of CCNU administered. Clamping did not alter the colony forming efficiency of cells from untreated 9L s.c. tumours or from those treated with each drug alone. These data demonstrate that hypoxic cells are required for misonidazole to potentiate the cell-killing effects of the nitrosoureas and that s.c. 9L tumours contain no such cells.