Mechanisms of anti-tumor action of 5-fluorouracil (5-FU) are presumed to inhibit DNA synthesis and RNA function, and the balance of these mechanisms is presumed to depend on the modalities of administration. On the other hand, variability of 5-FU sensitivity of the tumors is also presumed to depend on the enzymes of 5-FU metabolism (e.g. dihydropyrimidine dehydrogenase; DPD, rate limiting enzyme of catabolism) and action target (e.g. thymidylate synthase; TS). We studied the effects of modalities of administration and enzyme activities related to metabolism and target of 5-FU on the mechanism of anti-tumor action in patients with colorectal cancer. Thirty-eight patients who were diagnosed at stage II to IV preoperatively were enrolled. Patients were randomly assigned to receive 24-h protracted IV infusion of 5-FU at 320 mg/m2/day for 5 days (CIV group: 18 patients) or 10-min bolus IV infusion of 5-FU at the same dosage for 5 days (BIV group: 20 patients) administered from the 5th preoperative day. Specimens from the tumor and non-tumor regions were obtained by operation. F-RNA (fraudulent-RNA, or 5-FU in RNA) concentration, an indicator for action of 5-FU to RNA, and the enzyme activities of DPD and TS, an indicator for action of 5-FU to DNA, in the collected specimens were measured by GC-MS or RI-HPLC. F-RNA concentration (ng/mg-RNA) in the tumor and non-tumor region in the CIV group was 100.58±16.88 and 50.11±6.03, respectively, with a significant difference between them (P<0.05), and in the tumor and non-tumor region in the BIV group was 195.32±16.26 and 121.05±10.62, respectively, with a significant difference between them (P<0.01). F-RNA concentration in the tumor and non-tumor regions in the BIV group was significantly higher than those in the CIV group (P<0.05). DPD activity and TS activity were not significantly different between the CIV and the BIV groups in the tumor and non-tumor region, respectively. F-RNA concentration was negatively correlated to DPD activity (r=-0.540, P<0.05) in the tumor region in the CIV group. F-RNA was not correlated to DPD activity in the non-tumor region in the CIV group or in the tumor and non-tumor region in the BIV group. F-RNA was not correlated to TS activity in the tumor or non-tumor region of the two groups. DPD activity was not correlated to TS activity in the tumor or non-tumor region of the two groups. BIV inhibited RNA function more potently than CIV and this was not dependent on TS or DPD activity. As for the inhibition of DNA synthesis, other indicators should be considered further.

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