Fluorouracil

Fluorouracil (5-FU) is a fluorinated pyrimidine analog that disrupts nucleic acid synthesis through multiple intracellular mechanisms. Following cellular uptake, 5-FU is metabolically activated to several cytotoxic nucleotide derivatives that interfere with both DNA and RNA metabolism. One major active metabolite, 5-fluoro-2′-deoxyuridine-5′-monophosphate (FdUMP), forms a stable ternary complex with thymidylate synthase (TS) and the folate cofactor 5,10-methylenetetrahydrofolate. This complex inhibits TS activity, leading to depletion of deoxythymidine monophosphate (dTMP) and subsequently reduced dTTP pools. The resulting imbalance in deoxynucleotide levels causes DNA replication stress, stalled replication forks, and DNA damage. In parallel, 5-FU is converted to 5-fluorouridine triphosphate (FUTP), which can be incorporated into RNA in place of uridine. This incorporation disrupts RNA processing, ribosomal assembly, and translation, impairing protein synthesis. Additionally, formation of 5-fluoro-2′-deoxyuridine triphosphate (FdUTP) allows misincorporation into DNA, triggering base excision repair pathways and further genomic instability. Collectively, these effects lead to cell cycle arrest, activation of DNA damage response pathways, and induction of apoptosis, particularly in rapidly proliferating cells.