Fluorouracil has been given systemically for anal, breast, colorectal, oesophageal, stomach, pancreatic and skin cancers. It has also been given topically for actinic keratoses, skin cancers and Bowen's disease and as eye drops for treatment of ocular surface squamous neoplasia. Other uses include ocular injections into a previously created trabeculectomy bleb to inhibit healing and cause scarring of tissue, thus allowing adequate aqueous humor flow to reduce intraocular pressure.
Contraindications
It is contraindicated in patients that are severely debilitated or in patients with bone marrow suppression due to either radiotherapy or chemotherapy. It is likewise contraindicated in pregnant or breastfeeding women. It should also be avoided in patients that do not have malignant illnesses.
Diarrhea is severe and may be dose-limiting and is exacerbated by co-treatment with calcium folinate. Neutropenia tends to peak about 9–14 days after beginning treatment. Thrombocytopenia tends to peak about 7–17 days after the beginning of treatment and tends to recover about 10 days after its peak. Cardiotoxicity is a fairly common side effect, but usually this cardiotoxicity is just angina or symptoms associated with coronary artery spasm, but in about 0.55% of those receiving the drug will develop life-threatening cardiotoxicity. Life-threatening cardiotoxicity includes: arrhythmias, ventricular tachycardia and cardiac arrest, secondary to transmural ischaemia.
There is very little difference between the minimum effective dose and maximum tolerated dose of 5-FU, and the drug exhibits marked individual pharmacokinetic variability. Therefore, an identical dose of 5-FU may result in a therapeutic response with acceptable toxicity in some patients and unacceptable and possibly life-threatening toxicity in others. Both overdosing and underdosing are of concern with 5-FU, although several studies have shown that the majority of colorectal cancer patients treated with 5-FU are underdosed based on today's dosing standard, body surface area. The limitations of BSA-based dosing prevent oncologists from being able to accurately titer the dosage of 5-FU for the majority of individual patients, which results in sub-optimal treatment efficacy or excessive toxicity. Numerous studies have found significant relationships between concentrations of 5-FU in blood plasma and both desirable or undesirable effects on patients. Studies have also shown that dosing based on the concentration of 5-FU in plasma can greatly increase desirable outcomes while minimizing negative side effects of 5-FU therapy. One such test that has been shown to successfully monitor 5-FU plasma levels and which "may contribute to improved efficacy and safety of commonly used 5-FU-based chemotherapies" is the My5-FU test.
Interactions
Its use should be avoided in patients receiving drugs known to modulate dihydropyrimidine dehydrogenase. It may also increase the INR and prothrombin times in patients on warfarin. Fluorouracil's efficacy is decreased when used alongside allopurinol, which can be used to decrease fluorouracil induced stomatitis through use of allopurinol mouthwash.
Pharmacology
Pharmacogenetics
The dihydropyrimidine dehydrogenase enzyme is responsible for the detoxifying metabolism of fluoropyrimidines, a class of drugs that includes 5-fluorouracil, capecitabine, and tegafur. Genetic variations within the DPD gene can lead to reduced or absent DPD activity, and individuals who are heterozygous or homozygous for these variations may have partial or complete DPD deficiency; an estimated 0.2% of individuals have complete DPD deficiency. Those with partial or complete DPD deficiency have a significantly increased risk of severe or even fatal drug toxicities when treated with fluoropyrimidines; examples of toxicities include myelosuppression, neurotoxicity and hand-foot syndrome.
Mechanism of action
5-FU acts in several ways, but principally as a thymidylate synthase inhibitor. Interrupting the action of this enzyme blocks synthesis of the pyrimidine thymidine, which is a nucleoside required for DNA replication. Thymidylate synthase methylates deoxyuridine monophosphate to form thymidine monophosphate. Administration of 5-FU causes a scarcity in dTMP, so rapidly dividing cancerous cells undergo cell death via thymineless death. Calcium folinate provides an exogenous source of reduced folinates and hence stabilises the 5-FU-TS complex, hence enhancing 5-FU's cytotoxicity.
History
In 1954, Abraham Cantarow and Karl Paschkis found liver tumors absorbed radioactive uracil more readily than normal liver cells. Charles Heidelberger, who had earlier found that fluorine in fluoroacetic acid inhibited a vital enzyme, asked Robert Duschinsky and Robert Schnitzer at Hoffmann-La Roche to synthesize fluorouracil. Some credit Heidelberger and Duschinsky with the discovery that 5-fluorouracil markedly inhibited tumors in mice. The original 1957 report in Nature has Heidelberger as lead author, along with N. K. Chaudhuri, Peter Danneberg, Dorothy Mooren, Louis Griesbach, Robert Duschinsky, R. J. Schnitzer, E. Pleven, and J. Scheiner. In 1958, Anthony R. Curreri, Fred J. Ansfield, Forde A. McIver, Harry A. Waisman, and Charles Heidelberger reported the first clinical findings of 5-FU's activity in cancer in humans.
Natural analogues
In 2003, scientists isolated 5-fluorouracil derivatives, closely related compounds, from the marine sponge, Phakellia fusca collected around the Yongxing Island of the Xisha Islands in the South China Sea. This is significant because fluorine-containing organic compounds are rare in nature, and also because manmade anticancer drugs are not frequently found to have analogues in nature.
Interactive pathway map
Names
The name "fluorouracil" is the INN, USAN, USP name, and BAN. The form "5-fluorouracil" is often used; it shows that there is a fluorine atom on the 5th carbon of a uracil ring.
