Resistance to 5-fluorouracil and 5-fluoro-2'-deoxyuridine mechanisms and clinical implications.

Preliminary to studies on biopsy specimens from colorectal carcinomas, we have investigated the mechanisms of resistance to 5-fluorouracil (FUra) and 5-fluoro-2'-deoxyuridine (FdUrd) on a human colon carcinoma cell line, HCT-8 in vitro. After obtaining a high level of resistance, extracts were made from sensitive and resistant HCT-8 cells and the enzymes responsible for the activation or catabolism of fluoropyrimidines were assayed using a thin-layer chromatographic method. The activity of thymidine-kinase, thymidine phosphorylase, uridine-kinase, uridine phosphorylase, orotate phosphoribosyl transferase as well as the activity of the target enzyme thymidylate synthase were not significantly different in sensitive, FUra resistant or FdUrd resistant cells. The activities of these enzymes in sensitive HCT-8 cells were respectively 15.6, 3.4, 40.8, 2.1, 6.9 and 30.2 nmol/mg protein/h. Direct evidence that impaired transport was the mechanism of resistance to FdUrd was obtained using a short term "oil stop" technique, whereas indirect evidence suggests that reduced inhibition of thymidylate synthase is likely to be the mechanism of resistance to FUra. Cells seven hundred-fold resistant to FdUrd were in fact still sensitive to FUra (ED50 value after continuous exposure 2.1 microM) while FUra resistant cells were fully cross-resistant to FdUrd (ED50 value after continuous exposure 0.003 microM). The clinical relevance of these data is discussed in the light of the extensive literature about the potentiation of fluoropyrimidines by leucovorin.