BACKGROUND: Busulfan (BUS) is a highly toxic antineoplastic bifunctional-alkylating agent and has a narrow therapeutic window. Our previous study revealed a narrow dose-range of BUS, which causes a sudden dose-dependent transition from early- to late-expressing micronucleus induction and from a non-cytotoxic to a cytotoxic effect. In the present study, the kinetics of DNA-damaged cell induction by BUS and its dose-effect relationship were established. METHODS: This was achieved by using the kinetics of DNA-damaged cell induction, determined by the comet assay in murine peripheral blood leukocytes of mice, after the intraperitoneal exposure to 16, 30, 45, 60 or 80 micromol/kg of BUS. RESULTS: Doses of 15 or 30 micromol/kg of BUS were able to increase DNA-damaged cell frequency, but doses of 45 micromol/kg body weight or higher caused a sudden drop in this frequency. CONCLUSIONS: This suggests that higher doses cause lesions that inhibit the expression of damage as comets, i.e., DNA-protein or interstrand crosslinks. The latter could be explained by sudden monoadduct-to-crosslink transformation due to a DNA conformational change induced by monoadduct accumulation that facilitates crosslink formation. This narrow dose-dependent transition could contribute to the narrow therapeutic window of BUS.
BACKGROUND:Busulfan (BUS) is a highly toxic antineoplastic bifunctional-alkylating agent and has a narrow therapeutic window. Our previous study revealed a narrow dose-range of BUS, which causes a sudden dose-dependent transition from early- to late-expressing micronucleus induction and from a non-cytotoxic to a cytotoxic effect. In the present study, the kinetics of DNA-damaged cell induction by BUS and its dose-effect relationship were established. METHODS: This was achieved by using the kinetics of DNA-damaged cell induction, determined by the comet assay in murine peripheral blood leukocytes of mice, after the intraperitoneal exposure to 16, 30, 45, 60 or 80 micromol/kg of BUS. RESULTS: Doses of 15 or 30 micromol/kg of BUS were able to increase DNA-damaged cell frequency, but doses of 45 micromol/kg body weight or higher caused a sudden drop in this frequency. CONCLUSIONS: This suggests that higher doses cause lesions that inhibit the expression of damage as comets, i.e., DNA-protein or interstrand crosslinks. The latter could be explained by sudden monoadduct-to-crosslink transformation due to a DNA conformational change induced by monoadduct accumulation that facilitates crosslink formation. This narrow dose-dependent transition could contribute to the narrow therapeutic window of BUS.