Fluorouracil — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	Topical	Apply 1% or 5% cream to affected area	Once or twice daily	Duration: 2-4 weeks or as directed Notes: Use with caution; wear gloves to avoid human exposure. Avoid contact with eyes and mucous membranes.
Cat	Topical	Apply 1% or 5% cream to affected area	Once or twice daily	Duration: 2-4 weeks or as directed Notes: Cats are sensitive to fluorouracil; systemic toxicity can occur if ingested or absorbed. Use only under veterinary supervision.
Dog	Intralesional	50 mg/cm³ of tumor volume (off-label)	Every 2 weeks	Duration: Up to 4 treatments Notes: For mast cell tumors; must be performed by a veterinarian. Monitor for local and systemic toxicity.
Cat	Intralesional	50 mg/cm³ of tumor volume (off-label)	Every 2 weeks	Duration: Up to 4 treatments Notes: For sarcoids; use extreme caution due to feline sensitivity.
Horse	Topical	Apply 5% cream to sarcoid lesions	Once daily	Duration: Variable, often several weeks Notes: Off-label use; monitor for local irritation.

Clinical Indications & Species Uses

General Indications

Topical treatment of cutaneous neoplasms and precancerous lesions

Dog (Canine)

Topical treatment of actinic keratosis and superficial basal cell carcinoma
Intralesional treatment of canine mast cell tumors (off-label)
Systemic use for certain carcinomas (off-label, but rarely used due to toxicity)

Cat (Feline)

Topical treatment of actinic keratosis and squamous cell carcinoma in situ (Bowen's disease)
Intralesional treatment of feline sarcoids (off-label)

Pharmacology & Mechanism of Action

Drug Class: Antineoplastic agent | Pharmacological Group: Antimetabolite (pyrimidine analog)

Mechanism of Action: Fluorouracil (5-FU) is a fluorinated pyrimidine antimetabolite that inhibits thymidylate synthase by its active metabolite, 5-fluoro-2'-deoxyuridine-5'-monophosphate (FdUMP). This inhibition blocks the conversion of deoxyuridylic acid to thymidylic acid, leading to depletion of thymidine triphosphate, a necessary precursor for DNA synthesis. Additionally, 5-FU is incorporated into RNA, disrupting RNA processing and protein synthesis. The net effect is inhibition of cell proliferation, particularly in rapidly dividing cells.

Pharmacodynamics: Fluorouracil is cell-cycle specific, primarily affecting cells in the S-phase (DNA synthesis). It exhibits cytotoxic activity against a wide range of tumors, including carcinomas and sarcomas. In veterinary medicine, it is used topically for certain skin conditions and systemically for some neoplasms, though systemic use is limited due to toxicity. The drug's efficacy is schedule-dependent, with prolonged exposure being more effective than a single high dose.

⚡ Pharmacokinetics Summary

Absorption: When administered topically, systemic absorption is minimal but can occur through inflamed or broken skin. After oral administration, absorption is erratic and incomplete; therefore, oral use is not recommended. Intravenous administration provides complete bioavailability.

Distribution: Fluorouracil distributes widely into body tissues and fluids, including cerebrospinal fluid. It crosses the placenta and is excreted in milk. Protein binding is minimal (less than 10%).

Metabolism: The drug is metabolized primarily in the liver by dihydropyrimidine dehydrogenase (DPD) to inactive metabolites. It also undergoes catabolism in other tissues.

Excretion: Excretion is primarily renal, with about 60-80% of the dose excreted as metabolites and unchanged drug in urine within 24 hours. A small amount is excreted in feces and expired CO2.

Half-Life: The plasma half-life is very short, approximately 10-20 minutes in dogs and cats, due to rapid metabolism.

Bioavailability: Oral bioavailability is erratic and low (0-80%); therefore, oral administration is not recommended. Topical absorption is minimal but variable.

Protein Binding: Less than 10%

Available Formulations & Strengths

Topical Cream 1%, 5% (Topical)

Injectable Solution 50 mg/mL (IV (human use; not commonly used in veterinary medicine))

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to fluorouracil or any component of the formulation
Pregnancy and lactation (teratogenic and embryotoxic)
Severe hepatic or renal impairment
Bone marrow suppression (neutropenia, thrombocytopenia)
Active infections
Do not use systemically in cats due to extreme toxicity

Warnings & Clinical Precautions:

Extremely toxic to cats; even small amounts can be fatal if ingested or absorbed through skin. Use only under strict veterinary supervision.
Wear gloves when applying topical formulations to avoid human exposure.
Avoid contact with eyes, mucous membranes, and broken skin.
Do not use in animals intended for breeding.
Use with caution in animals with compromised liver or kidney function.
Monitor complete blood count and liver enzymes during systemic therapy.
Topical application may cause local irritation, erythema, and necrosis.
Keep out of reach of children and other animals.

Adverse Effects & Reactions

Common:

Local skin irritation
Erythema
Pruritus
Pain at application site
Gastrointestinal signs (if ingested): vomiting, diarrhea

Serious / Severe:

Bone marrow suppression (leukopenia, thrombocytopenia, anemia)
Severe gastrointestinal ulceration
Neurotoxicity (ataxia, seizures)
Hepatotoxicity
Nephrotoxicity
Death (especially in cats)

Rare:

Cardiotoxicity
Dermatitis
Alopecia
Hyperpigmentation

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Leucovorin (folinic acid)	Enhances the antitumor activity of fluorouracil by stabilizing the FdUMP-thymidylate synthase complex, but also increases toxicity.	High
Warfarin and other anticoagulants	May increase anticoagulant effect; monitor coagulation parameters.	Moderate
Phenytoin	May increase phenytoin levels; monitor for toxicity.	Moderate
Cimetidine	May increase fluorouracil toxicity by decreasing its metabolism.	Moderate
Metronidazole	May increase fluorouracil toxicity; avoid concurrent use.	High

Overdose & Toxicity Management

Signs of Toxicity:

Severe bone marrow suppression
Gastrointestinal ulceration and hemorrhage
Neurological signs (seizures, ataxia)
Hepatotoxicity
Nephrotoxicity
Cardiac arrhythmias
Death

Emergency Treatment Protocol: There is no specific antidote. Treatment is symptomatic and supportive. Induce emesis if recent ingestion (but avoid in cats due to risk of aspiration). Administer activated charcoal to reduce absorption. Provide intravenous fluids, antiemetics, and gastrointestinal protectants. Monitor blood counts and organ function. Consider granulocyte colony-stimulating factor (G-CSF) for severe neutropenia. In severe cases, hemodialysis may be considered.

Food Animal Withdrawal Times

Fluorouracil is not approved for use in food animals. If used off-label, a prolonged withdrawal period is recommended (at least 30 days for meat and 7 days for milk), but due to its toxicity and lack of data, it should not be used in food-producing animals.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature (20-25°C, 68-77°F). Protect from light.

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Keep container tightly closed. Do not freeze. Store away from heat and moisture.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use. Approved for human use as an antineoplastic agent.

Extra-Label (Off-Label) Use: In the US, extra-label use of fluorouracil in animals is permitted under AMDUCA only with a valid veterinarian-client-patient relationship. It is not approved for veterinary use; therefore, extra-label use requires careful consideration of withdrawal times and safety. Use in food animals is prohibited due to lack of residue data.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Fluorouracil is a potent antineoplastic agent that is rarely used systemically in veterinary medicine due to its narrow therapeutic index and severe toxicity, especially in cats. Its primary use is topical for cutaneous neoplasms and precancerous lesions. When used topically, systemic absorption is minimal, but caution is still required to prevent ingestion or excessive absorption through broken skin. Intralesional administration may be considered for certain tumors, but should only be performed by experienced veterinarians. Due to the risk of severe adverse effects, clients must be thoroughly counseled on handling and application. Alternative therapies should be considered when possible. In cats, even small amounts of fluorouracil can be fatal; therefore, its use in cats is strongly discouraged unless absolutely necessary and under strict supervision. Always monitor for signs of toxicity and provide supportive care as needed.

References & Literature

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