Factors modifying cytotoxicity induced by 5-FU and hydroxyurea

Pretreatment of exponentially growing 9L cells for 24 hours with a nontoxic (0.77 microM) or moderately toxic (7.7 microM) concentration of 5-FU (5-FUra) enhanced cell kill induced by a subsequent 6-hour treatment with 1.3 mM hydroxyurea (HU) by a magnitude of 1 log. In confluent 9L cultures or 9L cells treated in serum-free medium that decreased the cycling population, however, the 5-FUra-HU combination did not enhance the cell kill. No recovery from potentially lethal damage was observed for up to 24 hours after treatment of 9L cells with 7.7 microM 5-FUra alone or with 0.77 or 7.7 microM 5-FUra followed by 1.3 mM HU. These results show that 5-FUra and HU in combination kill cycling cells effectively and that cytotoxicity is fixed on completion of the treatment. Although the number of sister chromatid exchanges per metaphase increased significantly with increasing length of exposure to a nontoxic (0.77 microM) or moderately toxic (7.7 microM) concentration of 5-FUra, subsequent treatment with HU that resulted in cell kills of 70%-80% and 99%, respectively, did not increase the number of sister chromatid exchanges per metaphase beyond that expected from 5-FUra treatment alone. Thus, the enhanced cell kill does not appear to be a result of direct DNA damage.