Dactinomycin

Antineoplastic antibiotic Actinomycin antibiotics

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog IV 0.5-1 mg/m² Every 3 weeks Duration: As per protocol (e.g., 4-6 cycles)
Notes: Administer as a slow IV bolus through a freely flowing IV line to avoid extravasation. Premedicate with antiemetics if needed.
Cat IV 0.5 mg/m² Every 3 weeks Duration: As per protocol
Notes: Use with caution; cats may be more sensitive to myelosuppression.
Horse Intralesional 0.1-0.2 mg per lesion Every 2 weeks Duration: Until resolution
Notes: Limited data; use only by experienced veterinarians.

Clinical Indications & Species Uses

General Indications
  • Antineoplastic agent for various tumors in companion animals
Dog (Canine)
  • Treatment of lymphoma (as part of combination protocols)
  • Treatment of transmissible venereal tumor (TVT)
  • Treatment of soft tissue sarcomas
  • Treatment of mammary carcinoma (adjuvant therapy)
Cat (Feline)
  • Treatment of lymphoma (limited efficacy)
  • Treatment of fibrosarcoma (adjuvant therapy)

Pharmacology & Mechanism of Action

Drug Class: Antineoplastic antibiotic | Pharmacological Group: Actinomycin antibiotics

Mechanism of Action: Dactinomycin (actinomycin D) is a cytotoxic antibiotic that inhibits DNA-dependent RNA synthesis. It intercalates between adjacent guanine-cytosine base pairs of DNA, thereby preventing RNA polymerase from transcribing DNA. This leads to inhibition of protein synthesis and cell death. It is cell-cycle nonspecific but most active in the G1 phase of the cell cycle.

Pharmacodynamics: Dactinomycin exhibits potent antitumor activity against a variety of neoplasms. It also has immunosuppressive and antibacterial properties, but its clinical use is primarily as a chemotherapeutic agent. It is a vesicant and causes tissue damage if extravasated.

⚡ Pharmacokinetics Summary

Absorption: Dactinomycin is poorly absorbed orally and is therefore administered intravenously. After IV administration, it is rapidly distributed to tissues.
Distribution: It is widely distributed throughout the body, with high concentrations in bone marrow, tumor tissue, and other rapidly dividing cells. It does not cross the blood-brain barrier significantly.
Metabolism: Dactinomycin undergoes minimal hepatic metabolism. It is primarily eliminated unchanged in the bile and urine.
Excretion: Approximately 30% of the drug is excreted in the urine and 50% in the feces within 24 hours. The remainder is slowly eliminated over several weeks.
Half-Life: The terminal half-life in dogs is approximately 36 hours; in humans it is about 36 hours as well.
Bioavailability: Oral bioavailability is negligible; therefore, it is only administered intravenously.
Protein Binding: Dactinomycin is approximately 5% bound to plasma proteins.

Available Formulations & Strengths

Injectable Solution (lyophilized powder for reconstitution) 0.5 mg/vial (IV)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to dactinomycin
  • Severe bone marrow suppression
  • Active infections
  • Pregnancy (teratogenic)
  • Lactation (excreted in milk)
Warnings & Clinical Precautions:
  • Extremely vesicant; avoid extravasation. If extravasation occurs, apply cold compresses and consider local administration of dimethyl sulfoxide (DMSO).
  • Myelosuppression is dose-limiting; monitor complete blood count (CBC) regularly.
  • Immunosuppression may predispose to infections.
  • Hepatotoxicity may occur; monitor liver enzymes.
  • Use with caution in animals with impaired renal or hepatic function.
  • Wear protective gloves when handling; avoid inhalation of powder.
  • Teratogenic; avoid use in pregnant animals.

Adverse Effects & Reactions

Common:

  • Myelosuppression (leukopenia, thrombocytopenia, anemia)
  • Nausea, vomiting
  • Anorexia
  • Alopecia (especially in dogs)
  • Phlebitis at injection site

Serious / Severe:

  • Severe bone marrow suppression leading to sepsis or bleeding
  • Hepatotoxicity
  • Extravasation causing tissue necrosis
  • Anaphylaxis (rare)

Rare:

  • Cardiotoxicity
  • Pulmonary fibrosis
  • 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
Live vaccines Reduced vaccine efficacy and increased risk of infection Moderate

Overdose & Toxicity Management

Signs of Toxicity:

  • Severe myelosuppression
  • Gastrointestinal ulceration
  • Hepatotoxicity
  • Nephrotoxicity
  • Neurological signs (seizures, ataxia)

Emergency Treatment Protocol: There is no specific antidote. Treatment is supportive and symptomatic: hospitalization, IV fluids, antiemetics, broad-spectrum antibiotics for infections, blood transfusions for severe anemia/thrombocytopenia, and monitoring of organ function. Consider granulocyte colony-stimulating factor (G-CSF) for neutropenia.

Food Animal Withdrawal Times

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

Storage, Handling & Regulatory Information

Storage Temperature: Store at room temperature (15-30°C) in original packaging.

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

Handling & Special Conditions: Protect from light. Reconstituted solution should be used immediately; discard unused portion.

Dispensing Status: Prescription Required (Rx Only)

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

Extra-Label (Off-Label) Use: In the US, dactinomycin is not FDA-approved for veterinary use; use is extra-label under AMDUCA. It is a hazardous drug; follow safe handling guidelines.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Dactinomycin is a potent chemotherapeutic agent used primarily in dogs for lymphoma and TVT. It is often used in combination protocols (e.g., with vincristine, cyclophosphamide, doxorubicin). Due to its vesicant nature, strict attention to IV administration is required. Myelosuppression is the most significant adverse effect, so CBC monitoring is essential. It is contraindicated in pregnant animals due to teratogenicity. Use in cats is limited due to increased toxicity. Always handle with gloves and in a biological safety cabinet. Prognosis depends on tumor type and stage; response rates vary. Consult a veterinary oncologist for specific protocols.

References & Literature

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