Background: Conventional therapy for renal cell carcinoma using interferon (IFN) has shown limited antitumor action. The purpose of our study was to investigate synergistic antitumor effects of IFN and 5-fluorouracil (5-FU), and to elucidate the mechanisms of interaction between the 2 agents in mice.
Methods: Antitumor effects and biochemical modulation of murine IFN-alpha/beta and 5-FU were determined against the murine renal cell carcinoma cell line, Renca, in vivo. The activity of thymidylate synthetase and thymidine kinase was measured using cytosolic extracts of the tumors.
Results: Combination treatment with IFN-alpha/beta and 5-FU produced a significant enhancement of growth inhibition against Renca tumor. Treatment with 5-FU resulted in a 2.7-fold increase in the total amount of thymidylate synthetase and an 11.6-fold increase in the thymidylate synthetase inhibition rate, while the administration of IFN-alpha/beta did not significantly reduce the 5-FU-induced increase in thymidylate synthetase. The administration of IFN-alpha/beta decreased thymidine kinase activity to 65.5% maximally, compared with that in the control mice or the mice treated with 5-FU.
Conclusions: The reduction of thymidine kinase caused by treating the mice with IFN-alpha/beta changes the utilization of exogenous thymidine for DNA synthesis, and may represent the mechanism of the additive antitumor effect of the 2 agents, through the suppression of the salvage pathway for deoxythymidine monophosphate induction.