Melarsomine Dihydrochloride — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	IM (deep intramuscular injection)	2.5 mg/kg	Two doses 24 hours apart (Day 1 and Day 2), followed by a single dose 30 days later (Day 30). Alternative: 2.5 mg/kg once, then 30 days later two doses 24 hours apart.	Duration: Three-dose protocol over 30 days Notes: Administer by deep intramuscular injection in the epaxial muscles (lumbar region). Do not exceed 2.5 mg/kg per injection. For severe cases, consider a split-dose protocol (e.g., 2.5 mg/kg, then 30 days later two doses 24 hours apart).

Clinical Indications & Species Uses

General Indications

Treatment of canine heartworm disease (adulticide)

Dog (Canine)

Treatment of adult heartworm (Dirofilaria immitis) infection

Pharmacology & Mechanism of Action

Drug Class: Arsenical | Pharmacological Group: Antiparasitic (adulticidal for heartworm)

Mechanism of Action: Melarsomine dihydrochloride is an organic arsenical that is rapidly metabolized to melarsen oxide, the active form. It exerts its adulticidal effect by binding to sulfhydryl groups in essential enzymes of the adult heartworm (Dirofilaria immitis), leading to disruption of glucose metabolism and energy production, ultimately causing death of the adult worms. The drug is selectively toxic to the parasite due to differential uptake and metabolism compared to host cells.

Pharmacodynamics: Melarsomine is highly effective against adult heartworms (Dirofilaria immitis) in dogs, with a reported efficacy of >98% when the full three-dose protocol is used. It has minimal activity against microfilariae and immature larvae (L3/L4). The drug causes degeneration and death of adult worms in the pulmonary arteries and heart, which are then cleared by the host's immune system over weeks to months. The drug's toxicity is dose-dependent, and its therapeutic index is narrow, requiring careful dosing and patient monitoring.

⚡ Pharmacokinetics Summary

Absorption: After intramuscular injection, melarsomine is rapidly absorbed into the bloodstream, with peak plasma concentrations occurring within 1-2 hours. The absorption is complete and bioavailability is high.

Distribution: Melarsomine is widely distributed throughout the body, with highest concentrations in the liver, kidneys, and lungs. It penetrates well into tissues, including the pulmonary arteries where adult heartworms reside. It crosses the blood-brain barrier to a limited extent.

Metabolism: Melarsomine is rapidly metabolized in the liver to melarsen oxide, which is the active arsenical species. The metabolism is rapid, and the parent compound is not detectable in plasma after a few hours.

Excretion: The drug and its metabolites are primarily excreted in the feces via biliary excretion, with a smaller amount excreted in the urine. The elimination half-life is approximately 2-3 days in dogs.

Half-Life: Dogs: approximately 2-3 days (for melarsen oxide)

Bioavailability: Intramuscular injection results in near-complete bioavailability (approximately 100%).

Protein Binding: Melarsomine is moderately bound to plasma proteins (approximately 60-70%).

Available Formulations & Strengths

Injectable Solution (lyophilized powder for reconstitution) 50 mg/mL after reconstitution (IM)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to melarsomine or other arsenicals
Concurrent severe hepatic or renal disease
Pregnancy (safety not established)
Use in cats (highly toxic)
Use in dogs with severe pulmonary thromboembolism risk (relative contraindication; use with caution)

Warnings & Clinical Precautions:

For intramuscular use only; do not administer intravenously or subcutaneously.
Use with extreme caution in dogs with advanced heartworm disease (class 3 or 4) due to risk of pulmonary thromboembolism.
Strict cage rest is mandatory for at least 4-6 weeks after treatment to reduce thromboembolic complications.
Pre-treatment with a macrocyclic lactone (e.g., ivermectin) for at least 1-2 months is recommended to eliminate microfilariae and immature stages.
Monitor liver and kidney function before and during treatment.
Do not use in dogs with known arsenic toxicity.
Handle with care; avoid skin contact and inhalation of powder.
Use appropriate aseptic technique when reconstituting and administering.

Adverse Effects & Reactions

Common:

Injection site pain and swelling
Lethargy
Anorexia
Vomiting
Fever

Serious / Severe:

Pulmonary thromboembolism (can be fatal)
Hepatotoxicity
Nephrotoxicity
Local tissue necrosis at injection site
Anaphylaxis (rare)

Rare:

Neurological signs (ataxia, seizures)
Bone marrow suppression
Pancreatitis

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Macrocyclic lactones (ivermectin, milbemycin, etc.)	No direct interaction, but concurrent use is recommended to reduce microfilarial load and prevent reinfection. However, do not administer melarsomine concurrently with high doses of ivermectin (as used for heartworm prevention) due to potential additive toxicity.	Moderate
Corticosteroids	May be used concurrently to reduce inflammation, but may mask signs of thromboembolism. Use with caution.	Mild
Nephrotoxic drugs (e.g., aminoglycosides)	Additive nephrotoxicity; avoid concurrent use.	High
Hepatotoxic drugs (e.g., azathioprine, ketoconazole)	Additive hepatotoxicity; avoid concurrent use.	High

Overdose & Toxicity Management

Signs of Toxicity:

Severe vomiting
Diarrhea (possibly bloody)
Lethargy
Ataxia
Tremors
Seizures
Hepatotoxicity (icterus, elevated liver enzymes)
Nephrotoxicity (azotemia)
Cardiovascular collapse

Emergency Treatment Protocol: There is no specific antidote for arsenic poisoning. Treatment is symptomatic and supportive. Induce emesis if recent oral ingestion (not applicable for IM). Administer activated charcoal if oral. Provide intravenous fluids to maintain hydration and promote diuresis. Use chelating agents such as dimercaprol (BAL) or succimer (DMSA) if available, but efficacy is uncertain. Monitor liver and kidney function, and provide supportive care for organ failure. Control seizures with diazepam or barbiturates. Strict rest and monitoring for thromboembolism.

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 unreconstituted powder at 2-8°C (refrigerate). After reconstitution, use immediately or store at 2-8°C for up to 24 hours. Do not freeze.

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

Handling & Special Conditions: Protect from light. Keep out of reach of children.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved by FDA for use in dogs (Immiticide®).

Extra-Label (Off-Label) Use: In the US, melarsomine is approved for use in dogs only. Extra-label use in other species is prohibited under AMDUCA due to lack of safety and efficacy data. Use in food animals is not permitted.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Melarsomine dihydrochloride is the only FDA-approved adulticide for canine heartworm disease. The three-dose protocol (two doses 24 hours apart, then a third dose 30 days later) is the standard of care and has the highest efficacy and lowest risk of thromboembolism. Strict exercise restriction is critical for 4-6 weeks after treatment to reduce the risk of pulmonary thromboembolism. Pre-treatment with a macrocyclic lactone (e.g., ivermectin) for at least 1-2 months is recommended to eliminate microfilariae and immature stages. Post-treatment, microfilarial testing should be performed at 6 months to confirm clearance. Melarsomine is not safe for cats or other species. Always monitor for adverse effects, especially hepatotoxicity and nephrotoxicity, and provide supportive care as needed.

References & Literature

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