Flucytosine

Flucytosine capsules are indicated only for serious fungal infections caused by susceptible strains of Candida or Cryptococcus. Supported uses include:

• Candida septicemia (bloodstream infection) — supported by both Ancobon and generic Flucytosine capsules
• Candida endocarditis (heart valve infection) — supported by both Ancobon and generic Flucytosine capsules
• Candida urinary tract infections — supported by both Ancobon and generic Flucytosine capsules
• Candida pulmonary (lung) infections — limited trial data support use with both Ancobon and generic Flucytosine capsules
• Cryptococcal meningitis (brain and spinal cord lining infection) — supported by both Ancobon and generic Flucytosine capsules
• Cryptococcal pulmonary infections — supported by both Ancobon and generic Flucytosine capsules
• Cryptococcal septicemia and urinary tract infections — limited data, but good responses have been reported with both Ancobon and generic Flucytosine capsules
• Both Ancobon and generic Flucytosine capsules are used in combination with amphotericin B for systemic candidiasis and cryptococcosis to reduce the risk of resistance

Flucytosine gets into fungal cells using a transport protein called cytosine permease. Once inside, the fungal cell converts it into fluorouracil (a compound that damages the fungus's ability to function). That compound is then turned into active metabolites that either get falsely built into the fungus's RNA — disrupting protein production — or block an enzyme called thymidylate synthetase, which the fungus needs to replicate its DNA. The end result: the fungus can't grow or reproduce.

Resistance can develop when fungi lose the enzymes needed to take up or convert flucytosine, or when they overproduce competing compounds. This is why flucytosine is almost always paired with amphotericin B — together, they are harder for the fungus to resist.

Flucytosine capsules are taken by mouth, divided into doses given every 6 hours throughout the day. To ease stomach upset, it helps to take the capsules a few at a time spread over about 15 minutes rather than all at once. Your exact dose will be determined by your prescriber based on your weight and kidney function — never adjust it on your own.

• Both Ancobon and generic Flucytosine capsules follow the same every-6-hours schedule
• If you have any sign of reduced kidney function, your starting dose will be lower and will be monitored closely
• Always used in combination with amphotericin B for systemic candidiasis or cryptococcosis

Flucytosine can cause a range of side effects affecting several organ systems. Common ones include:

• Nausea, vomiting, diarrhea, or abdominal pain
• Loss of appetite or dry mouth
• Rash, itching, or skin sensitivity to sunlight
• Headache, dizziness, or sedation
• Confusion or hallucinations
• Fatigue or weakness
• Elevated liver enzymes or bilirubin on lab tests

More serious reactions include bone marrow suppression (low blood counts), liver damage, kidney failure, seizures, and in rare cases cardiac or respiratory arrest — contact your doctor immediately if you notice any of these.

• Kidney impairment: Flucytosine is cleared almost entirely by the kidneys. Reduced kidney function causes dangerous drug buildup — close monitoring and dose adjustment are mandatory.
• Bone marrow suppression: Flucytosine can severely reduce blood cell production. This can be irreversible and potentially fatal, especially in people who are immunosuppressed or already have bone marrow problems.
• Liver toxicity: Serious liver injury, including liver cell death, has been reported — particularly in debilitated patients. Regular liver function tests are required.
• DPD enzyme deficiency: People with complete dihydropyrimidine dehydrogenase (DPD) deficiency cannot safely break down a key metabolite of flucytosine and face a much higher risk of severe toxicity including mucositis, severe diarrhea, dangerously low white blood cell counts, and nerve damage. Flucytosine is contraindicated in complete DPD deficiency.
• Radiation or immunosuppressive therapy: People receiving treatments that already suppress bone marrow face even greater risk and require especially careful monitoring.

Flucytosine has a small but important list of known interactions to be aware of:

• Cytosine arabinoside (cytarabine): This chemotherapy drug can competitively block flucytosine's antifungal activity, potentially making treatment ineffective — tell your oncology and infectious disease teams if you are on both.
• Drugs that reduce kidney filtration: Any medication that impairs how well the kidneys filter blood (glomerular filtration) can cause flucytosine to stay in the body longer and reach toxic levels, since the drug is eliminated almost entirely unchanged by the kidneys.

Not a complete list. The label covers the interactions the manufacturer studied or reported, and our own interaction database does not cover every medicine either. Bring your full medication list, including vitamins and herbal products, to your pharmacist.

• Do not take flucytosine if you have a known allergy or hypersensitivity to it
• Do not take flucytosine if you have complete DPD (dihydropyrimidine dehydrogenase) enzyme deficiency — this is a contraindication due to serious toxicity risk
• Tell your doctor if you have kidney disease or any reduced kidney function — dose adjustment and monitoring are required
• Tell your doctor if you have any blood disorder or bone marrow condition
• Tell your doctor if you are currently receiving or have received radiation therapy or bone marrow-suppressing drugs
• Tell your doctor about all medications you take, especially cancer treatments like cytarabine
• Tell your prescriber if you are pregnant, planning to become pregnant, or breastfeeding
• Special monitoring is needed in neonates and infants, who clear the drug more slowly than adults and can show significant variation in blood levels

Taking too much flucytosine can cause intensified versions of its known side effects — particularly severe diarrhea, nausea, and vomiting; dangerous drops in white blood cells and platelets; and liver inflammation. Blood levels above 100 mcg/mL are associated with a higher risk of these serious effects.

If overdose is suspected, the label recommends prompt steps to empty the stomach and maintaining good fluid intake. Blood counts, liver function, and kidney function must be monitored closely. Because hemodialysis (a kidney-filtering treatment) can rapidly lower flucytosine levels in the blood, it may be used in severe cases.

Flucytosine works selectively against fungal organisms by being converted inside fungal cells into compounds that disrupt two essential processes: building proteins (via false incorporation into RNA) and copying DNA (via inhibition of thymidylate synthetase). Resistance can arise when fungi develop mutations that prevent uptake or conversion of the drug, or when they overproduce competing pyrimidine compounds — which is why combination therapy with amphotericin B is standard practice.

Flucytosine is well absorbed after oral dosing, with 78–89% of the dose reaching the bloodstream. It is not significantly broken down by the body and passes into the cerebrospinal fluid (the fluid around the brain and spinal cord) at levels high enough to treat meningitis. More than 90% of the drug is excreted unchanged in the urine through kidney filtration.

In people with normal kidney function, the drug clears the body in roughly 2.4 to 4.8 hours (half-life). In people with severe kidney impairment or no kidney function, the half-life can stretch to an average of 85 hours — making drug monitoring and dose adjustment critical in this group. In neonates, the half-life is roughly double that seen in adults, and blood levels can vary widely between infants.

Store at 25°C (77°F); excursions permitted to 15°C - 30°C (59°F - 86°F).