Flucytosine
Flucytosine, also known as 5-fluorocytosine, is an antifungal medication. It is specifically used, together with amphotericin B, for serious [Candida infection|Candida infection]s 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. 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, 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, 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
- Patients treated with drugs compromising bone marrow function should be treated carefully. Blood cell counts should be taken very frequently.
- Patients with renal disease should receive flucytosine cautiously and in reduced doses. Guidelines for proper dosing exist. Serum level determinations are mandatory for these patients.
- All patients receiving flucytosine should be under strict medical supervision.
- Hematological, renal and liver function studies should be done frequently during therapy.
- Patients with preexisting bone marrow depression and liver impairment should be treated with caution.
- Antiproliferative actions on bone marrow and GI tissue: Due to the drug's preference for rapidly proliferating tissues, bone marrow depression may occur. Aplastic anemia has also been seen. Bone marrow toxicity can be irreversible and may cause death, particularly in immunocompromised patients. GI toxicity may be severe or rarely fatal and consists of anorexia, abdominal bloating, abdominal pain, diarrhea, dry mouth, duodenal ulcer, GI hemorrhage, nausea, vomiting, and ulcerative colitis.
- Liver function: Elevations of liver enzymes and bilirubin, hepatic dysfunction, jaundice and, in one patient, liver necrosis have all been seen. Some fatal cases have been reported; however, the majority of cases was reversible.
- Renal function: Increased BUN and serum creatinine have been noted. Crystalluria and acute [kidney injury] have also been seen.
- Adverse central nervous system effects are frequent and include confusion, hallucinations, psychosis, ataxia, hearing loss, headache, paresthesia, parkinsonism, peripheral neuropathy, vertigo and sedation.
- Skin reactions: Rash, pruritus, and photosensitivity have all been noticed. Toxic epidermal necrolysis may also be encountered and may be life-threatening.
- Anaphylaxis: Sometimes cases of anaphylaxis consisting of diffuse erythema, pruritus, conjunctival injection, fever, abdominal pain, edema, hypotension and bronchospastic reactions are observed.
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. 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.