In vivo interaction between radiation and 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea in the absence or presence of misonidazole in mice.

The effect of combining the radiosensitizer misonidazole (MISO) with 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) prior to radiation on the response of various KHT sarcoma cell subpopulations was evaluated. Centrifugal elutriation was used to obtain homogeneous populations of cells with respect to their cell cycle position from suspensions prepared directly from treated solid KHT tumors. Survival in these subpopulations was determined using an in vivo to in vitro cloning assay. In situ treatment consisted of either (a) CCNU (3.0 mg/kg) plus MISO (1.0 mg/g), (b) radiation (15 Gy) alone, or (c) CCNU plus MISO preceding the radiation by 24 hr. In the latter protocol, one treatment effectively eliminated those cells being preferentially spared by the other. The CCNU-MISO followed by radiation combination not only reduced the treatment resistance due to hypoxia and cell cycle position but also left all cell subpopulations in the tumor equally sensitive to the treatment. To determine the nature of the interaction between these agents in detail, the extent of cell killing following such a treatment regimen was evaluated using isoeffect plot (isobologram) analysis. KHT sarcoma-bearing C3H mice were treated with various doses of CCNU either alone or in simultaneous combination with a 1.0-mg/g dose of MISO 24 hr prior to receiving a range of radiation doses. Combining CCNU and radiation led to significantly enhanced tumor cell killing. When evaluated by isoeffect plot analysis, the data points resulting from this combination fell well below the envelope of additivity, indicating that a supraadditive interaction between these two agents had occurred in the tumor cells. An even greater interaction was observed in the KHT sarcomas when MISO was combined with CCNU prior to irradiation. The findings of effective elimination of treatment-resistant subpopulations along with a supraadditive tumor cell kill offer, at least in part, an explanation for the previously described therapeutic advantage observed for the CCNU-MISO-radiation protocol.