The effect of misonidazole in situ on the radiation response of selected tumor subpopoulations.

The effects of misonidazole on fibrosarcome FSa) tumor cells irradiated in situ were determined for tumor cell subpopulations isolated by density gradient centrifugation. Mice (C3HF/Kam) bearing FSa tumors in the flank were injected i.p. with misonidazole (1 mg/g) 20 minutes before irradiation with various doses from a 137Cs gamma source. Immediately after irradiation single-cell suspensions were prepared and tumor cell subpopulations were separated by density gradient centrifugation. Clonogenicity for selected tumor cell populations was determined using the lung colony assay. It has been reported previously that following in situ irradiation, the cell population collected at a density of 1.08g/cm3 contains a relatively large fraction of sensitive (oxic) cells. It has also been reported that the cell population collected at 1.14 g/cm3 behaves as a chronically hypoxic cell population with respect to radiation response (Do = 400 rads; n = 5.2). Following injection of misonidazole, both the Do and the n of the resultant survival curve were reduced (Do = 306 rads; n = 0.5) indicating that the drug was effective in radiosensitizing this chronically hypoxic cell population. It also effectively sensitized the small fraction of resistant cells present in the cell population banding at 1.08 g/cm3. This was evidenced by a change in the survival curve parameters calculated for the terminal region of the curve: drug, Do = 279 rads, n = 0.3; no drug, Do = 459 rads, n = 1.2. Similar sensitization was found on unseparated control cells (USC). No cytotoxic effects from misonidazole were observed due to the short time interval in which the FSa was exposed to the sensitizer.