Effect of caffeine on cytotoxicity and sister chromatid exchange induction in sensitive and resistant rat brain tumor cells treated with 1,3-bis(2-chloroethyl)-1-nitrosourea.

Treatment of 9L and 9L-2 cells, which are, respectively, sensitive and resistant to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), with various concentrations of BCNU followed by treatment with 1 mM caffeine potentiated BCNU cytotoxicity by 10-fold with dose modification factors of 1.5 to 1.7. The synergistic effect of caffeine on cellular toxicity diminished when caffeine was added 6 to 24 h after treatment of BCNU. The number of sister chromatid exchanges (SCEs) induced by treatment with BCNU in both cell lines showed a good correlation with cytotoxicity; the number of SCEs induced in 9L cells is 6-fold higher than the number induced in 9L-2 cells. Caffeine potentiated BCNU induction of SCEs in both 9L or 9L-2 cells by the same amount. Caffeine potentiation of BCNU-induced SCEs was also time dependent and was eliminated by a delay of 6 h between BCNU treatment and addition of caffeine. Caffeine had no effect on the formation and removal of DNA cross-links in either 9L or 9L-2 cells after BCNU treatment as determined with the alkaline elution assay. Eighteen to 24 h after BCNU treatment there was an accumulation of 9L cells in late-S-G2-M phase of the cell cycle which diminished with time. Caffeine treatment potentiated the BCNU-induced accumulation of cells in late-S-G2-M phase of the cell cycle. Our results suggest that caffeine potentiates BCNU cytotoxicity and induction of SCEs by a mechanism that is independent of repair of alkylation products, but that may depend on alterations of cellular replication in BCNU-treated cells.