The effects of X rays on BCNU-induced DNA crosslinking.

We have used the technique of alkaline elution to study DNA interstrand crosslinking in 9L rat brain tumor cells treated with combinations of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and X rays. Irradiation with doses as low as 50 rad of X rays immediately or 6 hr after a 1-hr treatment with 60, 80, or 100 microM BCNU produced a significant increase in BCNU-induced DNA interstrand crosslinking. If cells were irradiated before BCNU treatment, the amount of crosslinking was not affected compared with BCNU alone. Cell survival experiments using 600 rad of X rays and 1-hr treatments with 0-30 microM BCNU were also performed. As found in the crosslinking studies, irradiation immediately or 6 hr after the BCNU treatment produced enhanced cell kill, but irradiation 6 hr before BCNU treatment did not produce enhanced cell kill. Therefore, the X-ray-mediated increase in BCNU-induced DNA interstrand crosslinking may be the mechanism through which cell kill is increased by combination treatment with the agents.