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.
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