Thymidylate synthase (TS) and ribonucleotide reductase (RNR) may be involved in acquired resistance to 5-fluorouracil (5-FU) in human cancer xenografts in vivo.

A human tumour sub-line resistant to 5-fluorouracil (5-FU) was established by once a day and every 5, with at least 50 administrations of 5-FU to KM12C human colorectal xenografts in nude mice. KM12C tumours treated with 5-FU showed less sensitivity to 5-FU with an inhibition rate (IR) of 7.9%, while non-treated tumours were highly sensitive to 5-FU with an IR of 81.8%. To clarify the mechanism of 5-FU-resistance, the activities of various enzymes and gene expressions involved in the metabolism of 5-FU in both parental and 5-FU-treated KM12C tumours were measured. A 2- to 3-fold increase in thymidylate synthase (TS) activity and 4- to 5-fold decrease in ribonucleotide reductase (RNR) activity were observed in 5-FU-resistant KM12C tumours, while the activities of orotate phosphoribosyltransferase (OPRT) thymidine and uridine phosphorylases (TP,UP) and thymidine kinase (TK) were not markedly changed as a consequence of repeated treatment of KM12C tumours with 5-FU. The expression of TS mRNA was also amplified in accordance with the increased TS activity in a 5-FU-treated tumour sub-line (KM12C/5-FU) compared with that in parental tumours, but changed expressions of both RNR-R1 and RNA-R2 mRNA could not be detected in the 5-FU-resistant tumour sub-line compared with the parental tumours, suggesting possible post-transcriptional regulation of RNR. Moreover, RNR, in addition to TS and OPRT, seemed to be related to the inherent insensitivity to 5-FU in human cancer xenografts. From these results, it may be concluded that RNR activity is one of the acquired or inherent resistant factors, including TS, to 5-FU in human cancer xenografts in vivo.