Dacarbazine

Alkylating agent (antineoplastic) Triazene derivative

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog IV 200-250 mg/m² q24h for 5 days, repeated every 3 weeks Duration: Cycles of 5 days; repeat as per protocol
Notes: Administer as a slow IV infusion over 30-60 minutes. Premedicate with antiemetics. Monitor for myelosuppression.
Dog IV 800-1000 mg/m² Single dose every 3-4 weeks Duration: Repeat cycles as per protocol
Notes: High-dose protocol; ensure adequate hydration and renal function.
Cat IV 200 mg/m² q24h for 5 days, repeated every 3 weeks Duration: Cycles of 5 days; repeat as per protocol
Notes: Use with caution; cats may be more sensitive to myelosuppression.

Clinical Indications & Species Uses

General Indications
  • Malignant melanoma
  • Hodgkin's lymphoma (in humans; off-label in animals)
  • Soft tissue sarcoma
Dog (Canine)
  • Malignant melanoma
  • Lymphoma (as part of combination protocols)
  • Soft tissue sarcoma
  • Mast cell tumor (rescue therapy)
Cat (Feline)
  • Lymphoma (as part of combination protocols)
  • Mammary adenocarcinoma (limited efficacy)

Pharmacology & Mechanism of Action

Drug Class: Alkylating agent (antineoplastic) | Pharmacological Group: Triazene derivative

Mechanism of Action: Dacarbazine is a prodrug that undergoes metabolic activation in the liver via N-demethylation to form 5-(3-methyl-1-triazeno)imidazole-4-carboxamide (MTIC), which then alkylates DNA at the O6 and N7 positions of guanine. This leads to DNA cross-linking, strand breaks, and inhibition of DNA, RNA, and protein synthesis, ultimately causing cell death. It is cell-cycle nonspecific but most active in the S phase.

Pharmacodynamics: Dacarbazine exhibits antitumor activity against a range of neoplasms, particularly malignant melanoma, Hodgkin's lymphoma, and soft tissue sarcomas. It is also used as a component of multi-agent chemotherapy protocols. Its cytotoxic effects are dose-dependent and may be schedule-dependent, with fractionated dosing often better tolerated. It has minimal immunosuppressive effects compared to other alkylating agents.

⚡ Pharmacokinetics Summary

Absorption: Dacarbazine is poorly absorbed orally and is therefore administered intravenously. After IV administration, peak plasma concentrations are achieved immediately.
Distribution: Dacarbazine is widely distributed throughout the body. It crosses the blood-brain barrier to a limited extent. It is not extensively bound to plasma proteins (approximately 5%).
Metabolism: Dacarbazine is extensively metabolized in the liver by cytochrome P450 enzymes (CYP1A1, CYP1A2, CYP2E1) to its active metabolite MTIC. It also undergoes renal metabolism to some extent.
Excretion: Dacarbazine and its metabolites are primarily excreted via the kidneys. Approximately 40-50% of the administered dose is excreted unchanged in the urine within 6 hours. Biliary excretion is minimal.
Half-Life: The elimination half-life in dogs is approximately 5 hours; in humans it is about 5-6 hours. In other species, data are limited.
Bioavailability: Oral bioavailability is poor (<5%) and erratic; therefore, it is only administered intravenously.
Protein Binding: Approximately 5% bound to plasma proteins.

Available Formulations & Strengths

Injectable Solution (lyophilized powder for reconstitution) 100 mg, 200 mg, 500 mg vials (IV)

Contraindications & Clinical Warnings

Contraindications:
  • Known hypersensitivity to dacarbazine
  • Severe bone marrow depression
  • Severe hepatic or renal impairment (use with caution)
  • Pregnancy (teratogenic)
  • Lactation (do not use in nursing animals)
Warnings & Clinical Precautions:
  • Dacarbazine is a vesicant; extravasation can cause severe tissue necrosis. Administer via a secure IV catheter.
  • Myelosuppression is dose-limiting; monitor complete blood counts regularly.
  • Hepatotoxicity may occur; monitor liver enzymes.
  • Renal function should be assessed before and during therapy.
  • Use with caution in animals with pre-existing cardiac disease.
  • Immunosuppression may increase risk of infections.
  • Handle with care; wear protective gloves and clothing. Dacarbazine is carcinogenic and mutagenic.

Adverse Effects & Reactions

Common:

  • Myelosuppression (neutropenia, thrombocytopenia, anemia)
  • Nausea and vomiting
  • Anorexia
  • Diarrhea
  • Lethargy

Serious / Severe:

  • Severe neutropenia with sepsis
  • Hepatotoxicity
  • Renal toxicity
  • Extravasation injury
  • Anaphylaxis (rare)

Rare:

  • Photosensitivity
  • Alopecia
  • Neurotoxicity
  • Secondary malignancies (long-term use)

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Other myelosuppressive agents (e.g., doxorubicin, cyclophosphamide) Additive bone marrow suppression High
Hepatotoxic drugs (e.g., azathioprine, ketoconazole) Increased risk of hepatotoxicity Moderate
Phenobarbital or other CYP inducers May increase metabolism of dacarbazine, reducing efficacy Moderate
Allopurinol May increase risk of bone marrow suppression Mild

Overdose & Toxicity Management

Signs of Toxicity:

  • Severe myelosuppression
  • Nausea and vomiting
  • Diarrhea
  • Hepatotoxicity
  • Renal failure
  • Seizures (in severe cases)

Emergency Treatment Protocol: There is no specific antidote. Treatment is supportive and symptomatic. Hospitalize the animal, provide IV fluids, antiemetics, and broad-spectrum antibiotics if neutropenic. Consider hematopoietic growth factors (e.g., filgrastim) for severe neutropenia. Monitor organ function closely. In cases of recent overdose, consider activated charcoal if oral ingestion (unlikely) or hemodialysis (not effective).

Food Animal Withdrawal Times

Not approved for use in food animals. Do not use in animals intended for food production.

Storage, Handling & Regulatory Information

Storage Temperature: Store at 2-8°C (refrigerate) and protect from light.

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

Handling & Special Conditions: Reconstituted solution is stable for 8 hours at room temperature or 72 hours under refrigeration. Protect from light.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for human use; not approved for veterinary use.

Extra-Label (Off-Label) Use: In the US, dacarbazine is not FDA-approved for veterinary use; however, it may be used legally under the Animal Medicinal Drug Use Clarification Act (AMDUCA) in non-food animals. Extra-label use in food animals is prohibited.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Dacarbazine is an effective chemotherapeutic agent for canine malignant melanoma and lymphoma. It is typically used in multi-drug protocols. Because of its vesicant nature, careful IV administration is essential. Premedication with antiemetics (e.g., maropitant) is recommended to reduce nausea. Monitor CBC and biochemistry profiles before each cycle. Dose adjustments may be necessary based on hematologic toxicity. In cats, use with caution due to increased sensitivity to myelosuppression. Dacarbazine is not recommended for food animals.

References & Literature

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