Flucytosine

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 Organization's List of Essential Medicines, the safest and most effective medicines needed in a health system. 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. In some countries, treatment with slow intravenous infusions for no more than a week is also a therapeutic option, particular if the disease is life-threatening.
Serious fungal infections may occur in those who are immunocompromised. These people benefit from combination therapy including flucytosine, but the incidence of side-effects of a combination therapy, particular with amphotericin B, may be higher.
Pregnancy and breastfeeding.
In animal models (rats), flucytosine has been found to be teratogenic. Sufficient human data does not exist. Pregnant women should be given flucytosine only if the potential benefits exceed the potential harm to the fetus.
It is not known if flucytosine is distributed in human breast milk. Given the potential risk to the child, the patient should not breastfeed during treatment with flucytosine.
Children.
The efficacy and safety in patients under 18 years of age has not been determined.
Side effects.
It is not known if flucytosine is a human carcinogen. The issue has been raised because traces of 5-fluorouracil, which is a known carcinogen, are found in the colon resulting from the metabolization of flucytosine.
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:
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. Following an oral dose of 2 grams peak serum levels are reached after approximately 6 hours. The time to peak level decreases with continued therapy. After 4 days peak levels are measured after 2 hours. The drug is eliminated renally. In normal patients flucytosine has reportedly a half-life of 2.5 to 6 hours. In patients with impaired renal function higher serum levels are seen and the drug tends to accumulate. The drug is mainly excreted unchanged in the urine (90% of an oral dose) and only traces are metabolized and excreted in the feces. Therapeutic serum levels range from 25 to 100 μg/ml. Serum levels in excess of 100 μg are associated with a higher incidence of side effects. Periodic measurements of serum levels are recommended for all patients and are a must in patients with renal damage.
Economics.
Although a generic, off patent medication in the U.S., as of January 2016, there was only one FDA-approved pharmaceutical supplier, Valeant Pharmaceuticals. Due to this monopoly, the cost per 250 mg tablet was $70.46 per tablet for a daily treatment cost of ~$2110/day for a 75 kg adult (165 pounds) adult and $29,591 for a two-week treatment course as of December 2015. This cost of flucytosine is more than 100-fold higher in the U.S. than in the United Kingdom and Europe via Meda AB Pharmaceuticals. Flucytosine tablets are available in India for US$2.00 per tablet via Jolly Healthcare Pvt. Ltd. and are available in 100 tablet packs.
Other animals.
In some countries, such as Switzerland, flucytosine has been licensed to treat cats, dogs and birds (in most cases together with amphotericin B) for the same indications as in humans.