Flucytosine — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	25-50 mg/kg	q6-8h (every 6-8 hours)	Duration: Weeks to months depending on infection; often 2-6 months for cryptococcosis Notes: Monitor renal function and blood counts. Adjust dose in renal impairment. Combine with amphotericin B or fluconazole for synergy.
Cat	PO	25-50 mg/kg	q6-8h	Duration: Weeks to months; often 2-6 months for cryptococcosis Notes: Cats may be more sensitive to GI side effects. Use with caution in renal disease. Combination therapy recommended.
Horse	PO	25-50 mg/kg	q6-8h	Duration: Variable; based on clinical response Notes: Oral absorption may be erratic. Consider IV formulation if available. Monitor for bone marrow suppression.
Cattle	PO	25-50 mg/kg (extrapolated)	q6-8h	Duration: Variable Notes: Not approved; use with caution. Observe withdrawal times.
Small Ruminants	PO	25-50 mg/kg (extrapolated)	q6-8h	Duration: Variable Notes: Limited data; use with caution.
Rabbit	PO	25-50 mg/kg (extrapolated)	q8-12h	Duration: Variable Notes: Limited data; monitor for GI disturbances.
Bird (pet)	PO	25-50 mg/kg (extrapolated)	q8-12h	Duration: Variable Notes: Limited data; use with caution.
Reptiles	PO	25-50 mg/kg (extrapolated)	q24-48h	Duration: Variable Notes: Limited data; adjust based on species and temperature.

Clinical Indications & Species Uses

General Indications

Treatment of systemic fungal infections caused by susceptible organisms, particularly Cryptococcus neoformans and Candida spp.
Adjunctive therapy in combination with amphotericin B or azoles for severe mycoses.

Dog (Canine)

Cryptococcosis (often in combination with amphotericin B or azoles)
Candidiasis (systemic or urinary tract)
Systemic mycosis caused by susceptible fungi (e.g., aspergillosis, but less common)
Chromomycosis (phaeohyphomycosis) caused by dematiaceous fungi

Cat (Feline)

Cryptococcosis (first-line in combination with amphotericin B or fluconazole)
Candidiasis
Systemic mycosis caused by susceptible fungi

Horse (Equine)

Cryptococcosis (rare)
Mycotic keratitis (topical or systemic, but limited efficacy)
Systemic candidiasis (neonatal foals)

Exotic & Other Species

Reptiles: limited use for systemic mycoses; dosing extrapolated.
Small mammals (ferrets, guinea pigs): limited data; use with caution.

Pharmacology & Mechanism of Action

Drug Class: Antifungal | Pharmacological Group: Pyrimidine analog

Mechanism of Action: Flucytosine (5-fluorocytosine) is a synthetic pyrimidine antimetabolite. It is taken up by susceptible fungal cells via cytosine permease and converted intracellularly to 5-fluorouracil (5-FU) by cytosine deaminase. 5-FU is further metabolized to 5-fluorodeoxyuridine monophosphate (FdUMP), which inhibits thymidylate synthase, thereby blocking DNA synthesis. Additionally, 5-FU is incorporated into RNA, interfering with protein synthesis. The selective toxicity arises because mammalian cells lack cytosine deaminase, so the drug is not converted to its toxic metabolite in host cells.

Pharmacodynamics: Flucytosine is primarily fungistatic, but may be fungicidal at high concentrations. It exhibits activity against susceptible strains of Candida spp., Cryptococcus neoformans, and some dematiaceous fungi (e.g., Cladophialophora, Phialophora). Resistance can develop rapidly when used as monotherapy, especially in Candida and Cryptococcus, due to mutations in cytosine permease or deaminase. Therefore, it is often used in combination with amphotericin B or azoles for synergistic effects.

⚡ Pharmacokinetics Summary

Absorption: Flucytosine is well absorbed after oral administration in most species, with bioavailability exceeding 75-90% in dogs, cats, and humans. Food may delay absorption but does not significantly reduce the extent. In horses, oral absorption is variable and may be less reliable.

Distribution: Flucytosine distributes widely into body tissues and fluids, including the CNS, achieving concentrations in cerebrospinal fluid that are 60-80% of plasma levels. It crosses the blood-brain barrier well. Volume of distribution is approximately 0.6-0.8 L/kg in dogs and cats. Protein binding is minimal (less than 5%).

Metabolism: Flucytosine undergoes minimal hepatic metabolism. The primary metabolic transformation occurs intracellularly in fungal cells to 5-fluorouracil. In mammalian hosts, a small fraction may be converted to 5-FU by gut microbiota, but this is generally negligible.

Excretion: Flucytosine is primarily excreted unchanged by the kidneys via glomerular filtration, with active tubular secretion also contributing. Renal clearance is high, and the elimination half-life is prolonged in patients with renal impairment. In dogs, the half-life is approximately 3-5 hours; in cats, it is about 3-4 hours; in horses, it is around 2-3 hours.

Half-Life: Dogs: 3-5 hours; Cats: 3-4 hours; Horses: 2-3 hours; Humans: 3-6 hours

Bioavailability: Oral: >75-90% in dogs and cats; variable in horses

Protein Binding: Less than 5%

Available Formulations & Strengths

Oral Capsule 250 mg, 500 mg (PO)

Oral Tablet 250 mg, 500 mg (PO)

Oral Suspension (compounded) 10 mg/mL (extemporaneous) (PO)

Injectable Solution (IV) - not commercially available in many countries; may be compounded 10 mg/mL (IV)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to flucytosine or any component
Patients with severe renal impairment (unless dose adjusted and monitored)
Patients with bone marrow depression (e.g., due to other drugs or disease)
Pregnancy (unless benefits outweigh risks; teratogenic in animal studies)
Lactating animals (unless benefits outweigh risks; excreted in milk)

Warnings & Clinical Precautions:

Use with caution in patients with renal insufficiency; dose adjustment and therapeutic drug monitoring recommended.
Monitor complete blood count (CBC) regularly due to risk of bone marrow suppression (leukopenia, thrombocytopenia, anemia).
Monitor liver enzymes as hepatotoxicity may occur.
Use with caution in neonates and debilitated animals.
Resistance can develop rapidly when used as monotherapy; combination therapy is often recommended.
In food animals, observe withdrawal times; not approved for use in food-producing animals in many countries.
Handle with care; flucytosine is potentially teratogenic and mutagenic. Wear gloves when handling broken capsules.

Adverse Effects & Reactions

Common:

Gastrointestinal disturbances (nausea, vomiting, diarrhea, anorexia)
Elevated liver enzymes (ALT, AST)
Skin rash

Serious / Severe:

Bone marrow suppression (leukopenia, thrombocytopenia, anemia) - dose-dependent and more common in renal impairment
Hepatotoxicity (hepatic necrosis)
Severe enterocolitis (due to conversion to 5-FU by gut bacteria)
Teratogenicity (in pregnant animals)

Rare:

Neurological signs (confusion, seizures, ataxia) - more common in renal failure
Eosinophilia
Photosensitivity

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Amphotericin B	Synergistic antifungal effect; however, amphotericin B may cause renal impairment, reducing flucytosine clearance and increasing risk of toxicity. Monitor renal function and blood levels.	Moderate
Nephrotoxic drugs (e.g., aminoglycosides, cyclosporine, NSAIDs)	Increased risk of renal toxicity, leading to reduced flucytosine clearance and increased risk of adverse effects.	High
Myelosuppressive drugs (e.g., azathioprine, chloramphenicol, sulfonamides)	Additive bone marrow suppression.	High
Cytarabine	May antagonize the antifungal effect of flucytosine (competitive inhibition).	Moderate
Probenecid	May reduce renal tubular secretion of flucytosine, increasing plasma levels and toxicity.	Moderate

Overdose & Toxicity Management

Signs of Toxicity:

Severe nausea, vomiting, diarrhea
Bone marrow suppression (leukopenia, thrombocytopenia, anemia)
Hepatotoxicity (elevated liver enzymes, jaundice)
Neurological signs (confusion, seizures, coma)
Renal failure (in severe cases)

Emergency Treatment Protocol: There is no specific antidote. Treatment is supportive and symptomatic. Induce vomiting if recent ingestion and animal is conscious. Administer activated charcoal to reduce absorption. Provide IV fluids to maintain hydration and promote renal excretion. Monitor CBC, liver enzymes, and renal function. In severe cases, consider hemodialysis or peritoneal dialysis to enhance elimination. Discontinue the drug immediately.

Food Animal Withdrawal Times

Flucytosine is not approved for use in food-producing animals in the United States and many other countries. If used off-label, a prolonged withdrawal period (e.g., 30 days for meat and 7 days for milk) is often recommended, but specific withdrawal times are not established. Consult regulatory authorities.

Storage, Handling & Regulatory Information

Storage Temperature: Store at room temperature (15-30°C / 59-86°F).

Handling & Special Conditions: Keep container tightly closed. Protect from moisture. Compounded suspensions should be refrigerated and used within the labeled period (usually 14-30 days).

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use (Ancobon).

Extra-Label (Off-Label) Use: In the United States, flucytosine is not FDA-approved for veterinary use, but it can be prescribed legally under the Animal Medicinal Drug Use Clarification Act (AMDUCA) for extra-label use in animals, provided a valid veterinarian-client-patient relationship exists. For food animals, extra-label use is prohibited if an approved drug is available and effective, and withdrawal times must be followed.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Flucytosine is a valuable antifungal agent, particularly for the treatment of cryptococcosis and candidiasis in dogs and cats. It is often used in combination with amphotericin B or azoles (e.g., fluconazole) to enhance efficacy and reduce the development of resistance. Due to the risk of bone marrow suppression and hepatotoxicity, therapeutic drug monitoring is recommended, especially in patients with renal impairment. Dosage adjustments are necessary in renal disease. Oral administration is preferred, but the drug is not available in all countries; compounding may be required. In horses, oral absorption is variable, and IV formulations may be preferred if available. Flucytosine should be used with caution in pregnant or lactating animals due to potential teratogenicity. For food animals, extra-label use is subject to regulatory restrictions, and withdrawal times must be observed. Overall, flucytosine is a potent antifungal but requires careful monitoring and judicious use.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)