The therapeutic efficacy of FUra has been attributed to its incorporation into cellular RNA and, to its inhibition of thymidylate synthase, leading to potent inhibition of DNA synthesis and DNA damage. Studies of cell lines in vitro and model systems in vivo have demonstrated that although mechanisms of sensitivity and resistance to FUra are multifactorial, in the presence of citrovorum factor (LV, CF, 5-formyltetrahydrofolate) the site of action of FUr a becomes predominantly the pronounced and prolonged inhibition of thymidylate synthase. This action is the result of stabilization of the...