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Importance of repair of potentially lethal damage in assays of cytotoxic agents.

Most in vitro assays expose growing cells to cytotoxic agents for a fixed period of time. However, these assays fail to account for repair of potentially lethal damage, the enhancement in survival that occurs when cells are maintained under nongrowth conditions after treatment with cytotoxic agents. This study extends previous observations of the repair of potentially lethal damage in human melanoma cells to parallel in vitro and in vivo experiments with the F10 subline of B16 melanoma. An in vivo (in C57BL/6 mice) and in vitro (in plastic plates) radiation dose-response relationship for the F10 subline of B16 melanoma was determined (200 to 1100 rads). Time delay until explant, allowing repair of potentially lethal damage, resulted in a significant and progressive enhancement of survival, five- to sixfold, both in vivo and in vitro. Survival with 700 rads and 24-hour delay was equivalent to that after treatment with 300 rads and no delay. Repair of potentially lethal damage, demonstrated in human cells in vitro and in animal preparations in vitro and in vivo, may account for the lack of clinical efficacy of some cytotoxic agents. Our results suggest that repair of potentially lethal damage should be taken into consideration in the design of in vitro chemotherapy and radiation therapy assays.