Mechanisms of Action
Fluorouracil and its metabolites possess a number of different mechanisms of action. In vivo, fluoruracil is converted to the active metabolite 5-fluoroxyuridine monophosphate (F-UMP); replacing uracil, F-UMP incorporates into RNA and inhibits RNA processing, thereby inhibiting cell growth. Another active metabolite, 5-5-fluoro-2'-deoxyuridine-5'-O-monophosphate (F-dUMP), inhibits thymidylate synthase, resulting in the depletion of thymidine triphosphate (TTP), one of the four nucleotide triphosphates used in the in vivo synthesis of DNA. Other fluorouracil metabolites incorporate into both RNA and DNA; incorporation into RNA results in major effects on both RNA processing and functions. 
It is used topically for actinic keratosis and superficial basal cell carcinoma; these indications are approved by the Food and Drug Administration. Other applications include actinic gheilitis, carcinoma in situ of the glans penis, oral leukoplakia and radiodermatitis.
Approved parenteral indications are colorectal cancer, localized and metastatic gastric cancer, metastatic breast carcinoma, and pancreatic carcinoma. Additional uses include malignancy of the biliary tract, localized breast carcinoma, localized tumor of the anus, carcinoid syndrome, bladder cancer malignancy, malignancy esophageal cancer, head and neck tumor, neuroendocrine carcinoma and bladder cancer.
It is often used in combined chemotherapy regimens, and also in concert with radiotherapy.
- "fluorouracil", Drug Dictionary, National Cancer Institute
- James M Spencer & Michelle Henry (23 July 2010), "Actinic Keratosis", Medscape
- Maria P Brizzi et al. (2009), "Continuous 5-fluorouracil infusion plus long acting octreotide in advanced well-differentiated neuroendocrine carcinomas. A phase II trial of the Piemonte Oncology Network", BMC Cancer 9: 388, DOI:10.1186/1471-2407-9-388