The inhibition of thymidylate synthase (TS) by the fluorouracil (5-FU) metabolite 5-fluoro-2′-deoxyuridine-5′-monophosphate (FdUMP) is considered to be one of the main mechanisms of action of 5-FU. The inhibition of TS is mediated by the formation of a ternary complex between TS, FdUMP, and the folate cofactor 5, 10-methylene tetrahydrofolate. The activity of TS, its inhibition by FdUMP, and the binding of FdUMP to TS have been determined in biopsy specimens of colorectal tumors, liver metastases, normal colon mucosa, and liver obtained from patients who never had received chemotherapy, and patients treated with 5-FU or 5-FU with leucovorin (LV). In nontreated patients we observed a large variation in the activity of TS both at 1 μM and 10 μM dUMP (40- to 80-fold difference). In contrast, in normal colonic mucosa this variation was less than 10-fold. FdUMP binding in tumors also varied considerably but was not detectable in normal mucosa. The deviations from normal (ie, as found in mucosa) kinetic patterns of TS may represent a mutant TS form. Thirty-five patients with advanced colorectal cancer received 5-FU (500 mg/m2) at 1 to 48 hours prior to surgery. In biopsy specimens of tumor and normal tissues the residual catalytic activity of TS and the percentage of free-binding sites for FdUMP (TS-free) were determined. After dissociation of FdUMP, total catalytic activity of TS- and total FdUMP-binding sites (TS-tot) were determined. Total and residual catalytic TS activity in primary tumors and metastases showed a large variation. TS-tot and TS-free in tumors also varied considerably. At least eight patients with an undetectable TS-free showed response to subsequent intraarterial treatment with 5-FU. To several patients leucovorin (2-hour infusion of 500 mg/m2) was administered with a 5-FU bolus (500 mg/m2) in the middle of the infusion. Biopsy specimens were obtained about 48 hours after treatment. In these patients inhibition of TS was markedly enhanced compared with patients who did not receive LV. The large variation in TS may be related to the observed variation in clinical response to 5-FU treatment.
|Original language||English (US)|
|Number of pages||10|
|Journal||Seminars in Oncology|
|Issue number||2 SUPPL. 3|
|State||Published - 1992|
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