[The mechanism of biochemical modulation in methotrexate/5-fluorouracil sequential chemotherapy].

5-fluorouracil (5-FU)/methotrexate (MTX) sequential chemotherapy is one of the standard regimen for the treatment of gastric cancer. Though this combination therapy is highly effective, the mechanism is still unclear. In this study, we investigated the relationship between antitumor activity and the inhibition of thymidylate synthase and between antitumor activity and incorporation of 5-FU into RNA (F-RNA) in mice bearing Sarcoma-180 treated by tegafur alone (100 mg/kg) or by the combination of MTX (42 mg/kg) and tegafur (100 mg/kg). A new method for the determination of F-RNA of tumor tissue samples was used. Briefly, RNA fractions containing incorporated 5-FU were extracted by KOH hydrolysis. FUra in RNA fractions was released by HCl hydrolysis and its levels were determined by GC-MS. The results showed that the F-RNA levels were significantly elevated by the pretreatment of MTX. According to this result, it is suggested that the measurement of F-RNA levels together with the determination of FUra concentration and the inhibition rate of thymidylate synthase were considered as a good means for the evaluation of antitumor efficacy of FUra. We concluded that the impairment of RNA metabolism played a major role in the antitumor activity of MTX/5-FU combination.