5-Fluorouridine

5-Fluorouridine (FUrd) is a fluoropyrimidine nucleoside analogue: uridine with a fluorine atom at C-5 of the base. It is a cell-permeable, modified RNA precursor.

Inside the cell. Uridine–cytidine kinase converts 5-fluorouridine to 5-fluorouridine monophosphate (FUMP). Further phosphorylation gives the diphosphate (FUDP) and the triphosphate (FUTP), which is incorporated into RNA. Through ribonucleotide reductase, part of the FUDP is converted to 5-fluoro-2′-deoxyuridine monophosphate (FdUMP), an inhibitor of thymidylate synthase.

Antiproliferative activity. 5-Fluorouridine is antiproliferative towards cancer cells and is used in comparison studies with 5-fluorouracil and related fluoropyrimidines. In human colorectal (SW480) and cervical (HeLa) cancer cells, uridine and uridine monophosphate reversed its effect, but thymidine did not. It inhibited thymidylate synthase about 100 times less strongly than 5-fluoro-2′-deoxyuridine (FdUrd). Its effect therefore arises mainly through RNA, that of FdUrd mainly through DNA.

Labelling active transcription sites. Porcine fetal fibroblasts were incubated with 2.5 mM 5-fluorouridine for 30 minutes, fixed and stained with an antibody against 5-bromo-2′-deoxyuridine (anti-BrdU), which detects the incorporated label. The label appeared as dots throughout the nucleus and in the nucleolus. Flavopiridol, an inhibitor of transcription by RNA polymerase II, abolished the signal.

Immunocytochemistry in human cell lines. A 5-minute pulse of 100 µM 5-fluorouridine, detected with anti-BrdU, labels transcription in the nucleoli. The nucleolar signal correlated with the amounts of RNA polymerase I and of fibrillarin, a nucleolar protein. Actinomycin D (0.05 µg/mL for 10 minutes) blocked the incorporation.