Responses of tumor cell subpopulations to single modality and combined modality therapies

By centrifugal elutriation, viable cell subpopulations were isolated directly from KHT sarcomas treated with 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea, mitomycin (MIT C), cyclophosphamide (CTX), or radiation. Survival in these subpopulations was determined by a clonogenic cell survival assay either immediately (radiation) or 24 hours after treatment with drugs. Following irradiation, late S and G2/M cells were found to be sensitive, but G1 cells were preferentially spared. For all chemotherapeutic agents evaluated, survival of cells at the G1/S boundary 24 hours after drug exposure was at a nadir, whereas cells in the late S and G2/M at this time were always drug resistant. The response of cells in early G1 24 hours after drug treatment ranged from sensitive (MIT C) to resistant (CTX). Because of these results, the combination of MIT C and radiation was investigated in detail. When MIT C was administered 24 hours before tumor irradiation, the chemotherapeutic agent significantly reduced the radiation-resistant subpopulations, whereas the radiation treatment effectively killed cells spared by the chemotherapeutic agent. Isobologram analysis further indicated that this treatment combination led to "supra-additive" cell kill in the tumor.