Busulfan — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	2-4 mg/m² (body surface area) once daily	q24h	Duration: Indefinite until remission or toxicity; adjust based on hematologic monitoring Notes: Dose adjustments are necessary based on white blood cell counts. Monitor CBC weekly initially.
Cat	PO	2-3 mg/m² once daily	q24h	Duration: Indefinite until remission or toxicity; adjust based on hematologic monitoring Notes: Cats may be more sensitive to myelosuppression; start at lower end of dose range.

Clinical Indications & Species Uses

General Indications

Treatment of chronic myelogenous leukemia and other myeloproliferative diseases in dogs and cats

Dog (Canine)

Chronic myelogenous leukemia (CML)
Myeloproliferative disorders
Bone marrow ablation prior to stem cell transplantation (experimental)

Cat (Feline)

Chronic myelogenous leukemia (CML)
Myeloproliferative disorders

Pharmacology & Mechanism of Action

Drug Class: Alkylating agent | Pharmacological Group: Antineoplastic agent

Mechanism of Action: Busulfan is a bifunctional alkylating agent that cross-links DNA, primarily at guanine residues, leading to DNA fragmentation and inhibition of DNA replication and transcription. It is cell cycle phase-nonspecific, affecting both dividing and resting cells. Its cytotoxic effects are most pronounced on rapidly proliferating tissues, particularly bone marrow and lymphoid tissues.

Pharmacodynamics: Busulfan exhibits potent myelosuppressive activity, especially on myeloid progenitors. It suppresses bone marrow function, leading to decreased production of granulocytes, platelets, and erythrocytes. It also has immunosuppressive properties. In veterinary oncology, it is used for its cytotoxic effect on neoplastic cells, particularly in chronic myelogenous leukemia and other myeloproliferative disorders.

⚡ Pharmacokinetics Summary

Absorption: Busulfan is well absorbed after oral administration, with peak plasma concentrations occurring within 1-2 hours. Bioavailability is approximately 80-90% in humans; similar data in animals are limited but assumed comparable.

Distribution: Busulfan is widely distributed throughout the body. It crosses the blood-brain barrier and enters cerebrospinal fluid. It is extensively bound to plasma proteins (approximately 30-50%). It distributes into tissues, with high concentrations in the liver, spleen, and bone marrow.

Metabolism: Busulfan is extensively metabolized in the liver, primarily via conjugation with glutathione (spontaneous and glutathione S-transferase-mediated) to form tetrahydrothiophene derivatives. It also undergoes oxidative metabolism via cytochrome P450 enzymes (CYP3A4).

Excretion: Metabolites are excreted primarily in the urine; less than 2% of the parent drug is excreted unchanged. Biliary excretion is minimal.

Half-Life: The elimination half-life in dogs is approximately 2-3 hours; in cats, it is approximately 1.5-2 hours. In humans, it is about 2.5 hours.

Bioavailability: Oral bioavailability is approximately 80-90% in humans; in dogs, it is reported to be around 70-80%.

Protein Binding: Approximately 30-50% bound to plasma proteins.

Available Formulations & Strengths

Oral Tablet 2 mg, 4 mg, 10 mg (PO)

Injectable Solution 6 mg/mL (for IV use, often compounded) (IV)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to busulfan or any component
Severe bone marrow depression (unless intended for myeloablation)
Pregnancy (teratogenic)
Lactating animals (excreted in milk)

Warnings & Clinical Precautions:

Myelosuppression is dose-limiting; monitor CBC frequently.
May cause pulmonary fibrosis with prolonged use.
Use with caution in animals with hepatic or renal impairment.
May cause infertility; advise owners of breeding animals.
Carcinogenic potential; handle with care.
Extravasation may cause tissue necrosis if IV administered.

Adverse Effects & Reactions

Common:

Myelosuppression (leukopenia, thrombocytopenia, anemia)
Gastrointestinal disturbances (nausea, vomiting, diarrhea)
Alopecia (especially in dogs)

Serious / Severe:

Severe bone marrow aplasia
Pulmonary fibrosis
Hepatic veno-occlusive disease
Secondary malignancies (long-term)

Rare:

Seizures (at high doses)
Cardiac fibrosis
Addison-like syndrome
Hyperpigmentation

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Other myelosuppressive agents (e.g., cyclophosphamide, chlorambucil)	Additive myelosuppression	High
Itraconazole or other CYP3A4 inhibitors	May increase busulfan exposure and toxicity	Moderate
Phenytoin or other CYP3A4 inducers	May decrease busulfan efficacy	Moderate
Acetaminophen	May reduce busulfan clearance via glutathione depletion	Moderate

Overdose & Toxicity Management

Signs of Toxicity:

Severe and prolonged myelosuppression
Bone marrow aplasia
Gastrointestinal toxicity
Hepatic veno-occlusive disease
Seizures (at very high doses)

Emergency Treatment Protocol: There is no specific antidote. Treatment is supportive: hospitalization, aggressive supportive care, including blood transfusions, antibiotics for infections, and hematopoietic growth factors (e.g., G-CSF). In severe cases, stem cell transplantation may be considered. Monitor organ function closely.

Food Animal Withdrawal Times

Not approved for food animals; not to be used in animals intended for food production.

Storage, Handling & Regulatory Information

Storage Temperature: Store at room temperature (20-25°C)

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

Handling & Special Conditions: Protect from light and moisture. Keep in tightly closed container.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; used extra-label in companion animals.

Extra-Label (Off-Label) Use: Extra-label use in animals is permitted under AMDUCA for non-food animals only. Use in food animals is prohibited.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Busulfan is an effective agent for managing chronic myelogenous leukemia and other myeloproliferative disorders in dogs and cats. It is typically used when other agents (e.g., hydroxyurea) are ineffective or not tolerated. Due to its potent myelosuppressive effects, strict hematologic monitoring is essential. Dosage adjustments should be based on white blood cell counts and platelet counts. The drug has a narrow therapeutic index; therefore, precise dosing and careful monitoring are critical. Owners should be counseled on the potential for severe side effects and the need for regular veterinary visits. In veterinary medicine, busulfan is often compounded into smaller doses for accurate administration.

References & Literature

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