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 todays dosing standard, body surface area (BSA). 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 (such as the antiviral drug sorivudine). It may also increase the INR and prothrombin times in patients on warfarin. Fluorouracils 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 (DPD) enzyme is responsible for the detoxifying metabolism of fluoropyrimidines, a class of drugs that includes 5-fluorouracil, capecitabine, and tegafur.
Drug Information Portal. U.S. National Library of Medicine. "Almotriptan malate". Drug Information Portal. U.S. National Library of Medicine.