Cellular pharmacology of 5-fluorouracil in a human colon adenocarcinoma cell line selected for thymidine kinase deficiency

A human colon adenocarcinoma cell line (GC₃TK^-) was selected for thymidine kinase (TK) deficiency from cloned parental cells (GC₃C₁) by exposure to 5-bromodeoxyuridine (BrdUrd). The cellular pharmacology of 5-fluorouracil (FUra) and the influence of physiological concentrations of thymidine (dThd; 0.1 to 1μM) on FUra cytotoxicity during brief exposure in both cell lines were examined. The uptake of FUra during a 1-hr drug exposure, its metabolism to ribo- and deoxyribonucleotides, incorporation into RNA, and inhibition of thymidylate synthase were similar in GC₃C₁ and GC₁TK^- cells as were the ic₅₀ values for FUra (26 and 23 μM respectively). TK deficiency did not reduce the intracellular concentrations of FdUMP generated from FUra. In GC₃C₁, at FUra concentrations up to 100 μM, cytotoxicity was prevented by co-administration of dThd (0.1 to 20 μM). The relationship between cell survival and thymidylate synthase inhibition was close under these conditions. At higher drug concentrations, less dThd protection was observed, and none was detected in GC₃TK^- cells. Thus, the metabolism of FUra did not appear to be altered substantially in GC₃C₁ cells selected for TK deficiency. Also in these cells, at concentrations of FUra < 100 μM, FUra cytotoxicity appeared to be mediated via the inhibition of thymidylate synthase.