Thymidylate synthase inhibition after administration of fluorouracil with or without leucovorin in colon cancer patients: Implications for treatment with fluorouracil

Purpose: To determine the time-dependence of fluorouracil (5FU)-induced thymidylate synthase (TS) inhibition in colon cancer patients, the effect of leucovorin (LV), and the relation to response. Patients and Methods: A 5FU injection (500 mg/m²) was given to 47 patients with advanced colorectal cancer; tumor biopsy specimens were obtained 1 to 72 hours after laparotomy. Eleven patients received LV (2-hour infusion of 500 mg/m²) with 5FU midinfusion; biopsies were obtained after 45 hours. TS inhibition was evaluated by comparing the number of total and free 5-fluoro-2'-deoxy- uridine-5'-monophosphate (UMP) (FdUMP) binding sites and the total and residual catalytic activity of TS. Results: The total catalytic TS activity varied from 0 to 621 pmol/h/mg protein and the total number of FdUMP binding sites varied from 0 to 976 fmol/mg protein. The residual catalytic TS activity after 2, 23, and 45 hours was 41%, 65%, and 74% of the total catalytic activity; the number of free FdUMP binding sites was 12%, 27%, and 49% of the total number, respectively. LV enhanced TS inhibition after 45 hours; the residual catalytic activity decreased from 74% to 49%, and the number of free FdUMP binding sites from 49% to 24%. Eleven of 19 patients treated with hepatic arterial infusion of 5FU had a partial response (PR). In the nonresponding patients, total TS activity was significantly higher (P <.05) than in responding patients. A high TS activity with a poor inhibition correlated with no response. Conclusion: Residual and total TS activity are predictive for response to 5FU. The findings may be applicable for treatment of patients with advanced disease and TS should be evaluated as a prognostic factor in adjuvant chemotherapy studies.