Flucytosine, also known as 5-fluorocytosine (5-FC), is an antifungal medication. It is specifically used, together with amphotericin B, for serious Candida infections and cryptococcosis. It may be used by itself or with other antifungals for chromomycosis. Flucytosine is used by mouth and by injection into a vein.Common side effects include bone marrow suppression, loss of appetite, diarrhea, vomiting, and psychosis. Anaphylaxis and other allergic reactions occasionally occur. It is unclear if use in pregnancy is safe for the baby. Flucytosine is in the fluorinated pyrimidine analogue family of medications. It works by being converted into fluorouracil inside the fungus, which impairs its ability to make protein.Flucytosine was first made in 1957. It is on the World Health Organizations List of Essential Medicines. As of 2016, in the United States the medication cost about US$2,000 per day while in the United Kingdom it is about US$22 per day. It is not available in much of the developing world. Medical uses
Flucytosine by mouth is used for the treatment of serious infections caused by susceptible strains of Candida or Cryptococcus neoformans. It can also be used for the treatment of chromomycosis (chromoblastomycosis), if susceptible strains cause the infection. Flucytosine must not be used as a sole agent in life-threatening fungal infections due to relatively weak antifungal effects and fast development of resistance, but rather in combination with amphotericin B and/or azole antifungals such as fluconazole or itraconazole. Minor infections such as candidal cystitis may be treated with flucytosine alone.
Interactions
Flucytosine may increase the toxicity of amphotericin B and vice versa, although the combination may be life-saving and should be used whenever indicated (e.g., cryptococcal meningitis). The cytostatic cytarabine inhibits the antimycotic activity of flucytosine. Overdose
Symptoms and their severities are unknown, because flucytosine is used under close medical supervision, but expected to be an excess of the usually encountered side effects on the bone marrow, gastrointestinal tract, liver and kidney function. Vigorous hydration and hemodialysis may be helpful in removing the drug from the body. Hemodialysis is particular useful in patients with impaired renal function. Pharmacology
Mechanisms of action
Two major mechanisms of action have been elucidated:

Flucytosine is intrafungally converted into the cytostatic fluorouracil which undergoes further steps of activation and finally interacts as 5-fluorouridinetriphosphate with RNA biosynthesis thus disturbing the building of certain essential proteins. Flucytosine also undergoes conversion into 5-fluorodeoxyuridinemonophosphate which inhibits fungal DNA synthesis. Spectrum of susceptible fungi and resistance
Flucytosine is active in vitro as well as in vivo against some strains of Candida and Cryptococcus. Limited studies demonstrate that flucytosine may be of value against infections with Sporothrix, Aspergillus, Cladosporium, Exophiala, and Phialophora. Resistance is quite commonly seen as well in treatment-naive patients and under current treatment with flucytosine. In different strains of Candida resistance has been noted to occur in 1 to 50% of all specimens obtained from patients. Pharmacokinetic data
Flucytosine is well absorbed (75 to 90%) from the gastrointestinal tract. Intake with meals slows the absorption, but does not decrease the amount absorbed.