Protection of Survival and Hematopoiesis in Irradiated Mice by 5‐FIuorouracil

The effects of whole‐body irradiation on survival and the hematopoietic system were studied in mice treated with 5‐fluorouracil (5‐FU). Animals (ddY‐SLC male mice, 8–10 weeks old) were injected with 5‐FU (i.p.) as a single dose (150 mg/kg) at various times before or after irradiation with X‐rays at graded doses (4.8 to 7.6 Gy). The treatment of mice with 5‐FU 5 days before irradiation was the most effective for the reduction of radiation lethality, having a radioprotective effect. The dose reduction factor (DRF) was 1.24. However, treatment with 5‐FU at 1 day and 2 hours before, or at various times after irradiation significantly increased the radiation lethality compared to the untreated controls, creating a radiosensitizing effect. The decrease or the increase of radiation lethality exhibited by 5‐FU was similar to. the radiation‐dose relationship pattern shown by endogenous and exogenous CFU‐S. The pattern of change of thrombocyte counts in the circulating blood after irradiation was greatly modified by pretreatment with 5‐FU 5 days before irradiation, effectively lessening the radiation‐induced depression. In contrast, the post‐irradiation patterns of leukocyte and erythrocyte variation did not show any significant change due to pretreatment with 5‐FU.