Procarbazine Hydrochloride — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	50 mg/m²	q24h	Duration: Typically administered for 14 days, then 7 days rest in a 28-day cycle (as part of MOPP protocol). Notes: Dose may be adjusted based on hematologic toxicity. Administer with food to reduce GI upset.
Cat	PO	50 mg/m²	q24h	Duration: Typically administered for 14 days, then 7 days rest in a 28-day cycle (as part of MOPP protocol). Notes: Cats may be more sensitive to myelosuppression; monitor CBC closely.

Clinical Indications & Species Uses

General Indications

Treatment of lymphoma and other neoplasms in small animals, particularly as a rescue agent in resistant cases.

Dog (Canine)

Lymphoma (as part of combination chemotherapy protocols, e.g., MOPP or LOPP)
Relapsed or refractory lymphoma
Mast cell tumors (less common)

Cat (Feline)

Lymphoma (as part of combination chemotherapy protocols, e.g., MOPP or LOPP)
Relapsed or refractory lymphoma

Pharmacology & Mechanism of Action

Drug Class: Antineoplastic agent | Pharmacological Group: Alkylating-like agent / Monoamine oxidase inhibitor

Mechanism of Action: Procarbazine is a prodrug that undergoes metabolic activation to form reactive species that alkylate DNA, RNA, and proteins, leading to inhibition of DNA, RNA, and protein synthesis. It also inhibits the methylation of nucleic acids and has weak monoamine oxidase (MAO) inhibitory activity. The drug is cell-cycle phase-specific, primarily acting in the S phase of the cell cycle.

Pharmacodynamics: Procarbazine exerts cytotoxic effects by causing DNA strand breaks and chromosomal aberrations. It also inhibits the synthesis of nucleic acids and proteins. Its MAO inhibition can lead to accumulation of catecholamines and serotonin, contributing to potential drug interactions and adverse effects. The drug is used in combination chemotherapy protocols for lymphoma and other neoplasms.

⚡ Pharmacokinetics Summary

Absorption: Procarbazine is well absorbed after oral administration in most species. Peak plasma concentrations are typically reached within 1-2 hours.

Distribution: Procarbazine is widely distributed throughout the body, including the central nervous system, as it crosses the blood-brain barrier. It is distributed into tissues and body fluids, including cerebrospinal fluid.

Metabolism: Procarbazine is extensively metabolized in the liver, primarily via oxidation and demethylation. The active metabolites are responsible for the cytotoxic effects. Metabolism involves cytochrome P450 enzymes and may be affected by concurrent medications.

Excretion: The metabolites of procarbazine are excreted primarily in the urine. A small portion is excreted in the feces. The elimination half-life is relatively short, but the active metabolites may persist longer.

Half-Life: Dogs: approximately 1-2 hours; humans: approximately 1 hour (parent drug).

Bioavailability: Oral bioavailability is high, estimated at >80% in dogs and humans.

Protein Binding: Procarbazine is minimally bound to plasma proteins (approximately 10-20%).

Available Formulations & Strengths

Capsule 50 mg (PO)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to procarbazine or any component of the formulation
Severe bone marrow suppression
Concurrent use of sympathomimetic drugs, tricyclic antidepressants, or other MAO inhibitors
Pregnancy (teratogenic)
Lactation (may be excreted in milk)

Warnings & Clinical Precautions:

Procarbazine is a potent cytotoxic agent; handle with care (use gloves, avoid inhalation).
May cause severe myelosuppression; monitor complete blood count regularly.
Has MAO inhibitor activity; avoid foods high in tyramine (e.g., aged cheese, cured meats) and concurrent use of sympathomimetic agents.
May cause hepatotoxicity; monitor liver enzymes.
May cause neurotoxicity; monitor for neurological signs.
Use with caution in patients with renal or hepatic impairment.
May be carcinogenic and mutagenic; use appropriate safety precautions.
In cats, monitor for signs of anorexia and weight loss.

Adverse Effects & Reactions

Common:

Myelosuppression (leukopenia, thrombocytopenia, anemia)
Gastrointestinal effects (nausea, vomiting, diarrhea, anorexia)
Lethargy
Hepatotoxicity (elevated liver enzymes)

Serious / Severe:

Severe bone marrow suppression leading to infection or bleeding
Neurotoxicity (peripheral neuropathy, ataxia, seizures)
Pulmonary toxicity (interstitial pneumonitis)
Secondary malignancies (long-term use)

Rare:

Allergic reactions (rash, urticaria)
Hemolytic anemia
Photosensitivity
Teratogenic effects

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Sympathomimetic agents (e.g., ephedrine, phenylpropanolamine)	Increased risk of hypertensive crisis due to MAO inhibition.	High
Tricyclic antidepressants (e.g., amitriptyline)	Increased risk of serotonin syndrome and neurotoxicity.	High
Other MAO inhibitors (e.g., selegiline)	Additive MAO inhibition, increased risk of severe adverse effects.	High
Foods high in tyramine	Hypertensive crisis due to MAO inhibition.	High
Corticosteroids (e.g., prednisone)	May increase risk of gastrointestinal ulceration and immunosuppression.	Moderate
Hepatotoxic drugs (e.g., azathioprine, methotrexate)	Additive hepatotoxicity.	Moderate

Overdose & Toxicity Management

Signs of Toxicity:

Severe myelosuppression (pancytopenia)
Gastrointestinal toxicity (vomiting, diarrhea)
Neurological signs (seizures, ataxia, coma)
Hepatotoxicity
Pulmonary toxicity

Emergency Treatment Protocol: There is no specific antidote. Treatment is supportive and symptomatic. Induce vomiting if recent ingestion and patient is stable. Administer activated charcoal to reduce absorption. Provide intravenous fluids, antiemetics, and supportive care. Monitor CBC, liver enzymes, and neurological status. Consider hospitalization for severe toxicity. In cases of severe myelosuppression, use of colony-stimulating factors (e.g., filgrastim) may be considered.

Food Animal Withdrawal Times

Not approved for use in food animals. Withdrawal times are not established. Do not use in animals intended for food.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature (20-25°C, excursions permitted to 15-30°C).

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

Handling & Special Conditions: Protect from light and moisture. Keep in original container. Store away from heat and open flame.

Dispensing Status: Prescription Required (Rx Only)

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

Extra-Label (Off-Label) Use: In the US, procarbazine is not FDA-approved for veterinary use; use in animals is extra-label. Must follow AMDUCA regulations: require a valid veterinarian-client-patient relationship, and for food animals, ensure extended withdrawal times and avoid residues.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Procarbazine is an oral antineoplastic agent used primarily as a rescue therapy for lymphoma in dogs and cats, often in combination with mechlorethamine (or cyclophosphamide), vincristine, and prednisone (MOPP or LOPP protocols). It is well absorbed orally and has good CNS penetration. Due to its MAO inhibitor activity, dietary restrictions and drug interactions are important. Myelosuppression is dose-limiting; therefore, regular CBC monitoring is essential. Administer with food to reduce GI upset. Use with caution in patients with hepatic or renal impairment. As a cytotoxic drug, handle with care. Prognosis depends on the stage of disease and prior treatments. Always consult a veterinary oncologist for dosing and protocol guidance.

References & Literature

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