NEW MECHANISM OF ACTION OF THE CANCER CHEMOTHERAPEUTIC AGENT 5-FLUOROURACIL IN HUMAN-CELLS

5-Fluorouracil (5-FIUra), a cancer chemotherapeutic agent used in the treatment of colon, breast, ovarian and prostate cancer, is incorporat ed into DNA as a result of its utilization as 5-FldUTP during DNA synt hesis. This promutagenic DNA lesion is excised by the base excision re pair enzyme uracil DNA glycosylase (UDG). In this report we describe f or the first time a mechanism by which 5-FIUra as the free base specif ically binds in vivo to the UDG in noncycling human cells, thereby inh ibiting its activity. By using 5-FIUra concentrations which did not el icit demonstrable cell toxicity, a dose-dependent decrease in UDG acti vity was detected which approached 30% of that observed in control cel ls. In contrast, exposure of cells to equivalent concentrations of ura cil, 5-fluorodeoxyuridine or 5-bromouracil had no effect on UDG activi ty. Subsequent studies demonstrated a reversible binding of 5-FIUra to the glycosylase. Kinetic analysis using nonlinear regression analysis demonstrated a competitive mode of inhibition and indicated a tight b inding of 5-FIUra to UDG in vivo, although the 5-FIUra-UDG complex was easily dissociated in vitro. These findings describe a potentially ne w and novel mechanism of action of 5-FIUra in a nonproliferating human cell population. The potential relevance of these findings to the uti lity of 5-FIUra as a cancer chemotherapeutic agent is considered.