Comparative DNA strand breakage induced by FUra and FdUrd in human ileocecal adenocarcinoma (HCT-8) cells: relevance to cell growth inhibition.

Although several mechanisms of 5-fluorouracil (FUra) and 5-fluoro-2'-deoxyuridine (FdUrd) cytotoxicity have been postulated, their effects on cellular DNA integrity have not been fully investigated. Cytotoxicity and the induction of DNA single and double strand breaks (SSB and DSB) were evaluated in the human ileocecal adenocarcinoma cell line, HCT-8, following 2 hr of exposure to these agents. Alkaline and neutral elution techniques were utilized to quantitate the amounts of DNA SSB and DSB induced by FdUrd and FUra. FdUrd, but not FUra, induced a high level of DNA SSB and DSB. These effects were concentration and time dependent, reaching a maximum at 10 microM FdUrd and at 12 hr post-treatment. Minimal amounts of DNA damage were observed in HCT-8 cells exposed to FUra, even at a concentration of 300 microM that produced greater than 99% inhibition of cell growth. In order to delineate the mechanisms associated with the effects observed following FUra and FdUrd treatment of HCT-8 cells, the effects of thymidine and leucovorin were evaluated. DNA SSB and DSB induced by FdUrd were reversed by thymidine. The effects of thymidine were time dependent. Complete reversal of DNA damage and cytotoxicity was achieved when 10 microM thymidine was added at up to 12 hr after treatment, the time of appearance of maximum DNA damage. These results suggest that the extensive DNA damage induced by FdUrd (but not by FUra) was an important determinant of FdUrd cytotoxicity in HCT-8 cells.