Increased thymidylate synthetase in 5-fluorodeoxyuridine resistant cultured hepatoma cells

A FdUrd resistant line of cultured mouse hepatoma cells has been obtained. The resistant cell line had 6- to 10-fold higher levels of thymidylate synthetase, but dihydrofolate reductase and thymidine kinase were unchanged. No impairment of FdUrd incorporation by the resistant cell line could be detected. The increased thymidylate synthetase in resistant cells had the same turnover number and I₅₀ for FdUMP as the enzyme found in sensitive cells, making it unlikely that a new gene product had been obtained. Sensitive cells could be completely rescued by the addition of thymidine, suggesting that the primary mode of drug action is to diminish thymidine metabolites. Resistant cells, removed from FdUrd for several generations, did not proliferate immediately upon reintroduction of the drug; however, loss of sensitivity was much more rapid than upon initial exposure. These results are interpreted in terms of a mechanism for resistance.