Tumor sensitizing effect by misonidazole in a clinically relevant radiation dose range.

The survival of cells from two murine fibrosarcoma (FSa) subpopulations after exposure to radiation only or radiation in combination with misonidazole (0.2 mg/g/fraction) was determined using a lung colony assay. FSa tumors grown in the hind legs of C3Hf/Kam pathogen-free mice were irradiated in situ when the tumors were 8 to 10 mm in diameter. Single cell suspensions prepared from excised tumors were separated on linear density gradients of Renografin, and the clonogenicity of predominantly oxic Band 2 (density 1.08 g/cm3) and predominantly hypoxic Band 4 (density 1.14 g/cm3) cells were measured. The surviving fraction of cells after doses of 1, 2, and 3 Gy, alone or preceded 30 minutes earlier with an i.p. injection of misonidazole (0.2 mg/g) was estimated from that measured after total radiation doses of 5 Gy = 5 X 1 Gy, 10 Gy = 5 X 2 Gy, and 15 Gy = 5 X 3 Gy, with the misonidazole-treated groups receiving a total drug dose of 1 mg/g, under the assumption of an equal effect per fraction. Under these conditions the initial slopes of Band 2 cells following irradiation only or irradiation plus misonidazole were 1D0 = 3.6 Gy and 2.74 Gy, respectively, giving rise to a sensitizer enhancement ratio of 1.3. Band 4 cells exhibited a 1D0 of 5.15 Gy to radiation only and 2.75 Gy to radiation plus misonidazole (SER of 1.9). In addition, misonidazole when administered alone in a single dose or up to 5 fractions of 0.2 mg/g each separated by 4-hour intervals, was effective in killing 50% of the Band 4 cells. The target population at risk appeared to remain constant regardless of the number of dose fractions administered. In contrast, Band 2 cells were not affected by the cytotoxic action of misonidazole. These data suggest that misonidazole is effective in sensitizing hypoxic cells in the clinical dose range, and that it is directly cytotoxic to hypoxic tumor cells.