Enhancement by thymidine of 5-fluorouracil's inhibition of thymidylate synthetase in human tumors.

The effects of thymidine (TdR) on the conversion of 5-fluorouracil (5-FU) to 5-fluoro-2'-deoxyuridine-5'-monophosphate and the resulting inhibition of thymidylate synthetase was determined in crude, human tissue extracts. TdR alone has no effect but it augmented the inhibition of thymidylate synthetase produced by 5-FU. TdR augmentation was probably mediated either by increasing 5-FU conversion to 5-fluoro-2'-deoxyuridine-5'-monophosphate or by inhibiting 5-FU catabolism.