Gamithromycin — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Cattle (beef and non-lactating dairy)	Subcutaneous (SC)	6 mg/kg (3 mL/100 kg body weight)	Single dose	Duration: One-time administration Notes: Administer subcutaneously in the neck. Do not inject more than 10 mL per injection site. For use in cattle only.

Clinical Indications & Species Uses

General Indications

Treatment of respiratory infections caused by susceptible organisms in cattle

Cattle (Bovine)

Treatment of bovine respiratory disease (BRD) associated with Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, and Mycoplasma bovis

Pharmacology & Mechanism of Action

Drug Class: Macrolide antibiotic | Pharmacological Group: Azalide (subclass of macrolides)

Mechanism of Action: Gamithromycin is a semi-synthetic macrolide antibiotic of the azalide subclass. It exerts its antibacterial effect by binding reversibly to the 50S ribosomal subunit of susceptible bacteria, specifically interacting with the 23S rRNA and ribosomal proteins. This binding inhibits the translocation step of protein synthesis, thereby preventing bacterial growth and replication. Gamithromycin is generally bacteriostatic, but at higher concentrations it may be bactericidal against certain organisms. It has a prolonged post-antibiotic effect and concentrates in phagocytes, which aids in targeted delivery to sites of infection.

Pharmacodynamics: Gamithromycin demonstrates activity against a range of Gram-positive and some Gram-negative bacteria, as well as mycoplasmas. In veterinary medicine, it is primarily used for the treatment of bovine respiratory disease (BRD) associated with Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, and Mycoplasma bovis. It exhibits time-dependent killing with a long half-life, allowing for single-dose therapy. Its efficacy is enhanced by its ability to accumulate in neutrophils and alveolar macrophages, achieving high concentrations at the site of infection. Gamithromycin also has immunomodulatory effects, such as reducing inflammation and modulating cytokine production, which may contribute to its clinical efficacy.

⚡ Pharmacokinetics Summary

Absorption: After subcutaneous administration in cattle, gamithromycin is rapidly and well absorbed, with peak plasma concentrations reached within 1-2 hours. The absolute bioavailability is approximately 100% following subcutaneous injection. Oral absorption is poor and not used clinically.

Distribution: Gamithromycin is extensively distributed throughout the body, with high tissue concentrations, particularly in the lungs, where levels exceed plasma concentrations. It accumulates in phagocytic cells (neutrophils and macrophages) and is transported to sites of infection. The volume of distribution is large, indicating extensive tissue penetration. Protein binding is low to moderate, approximately 26% in cattle.

Metabolism: Gamithromycin undergoes minimal metabolism in the liver. The primary metabolic pathway involves N-demethylation, but the majority of the drug is excreted unchanged. The metabolites are not considered to contribute significantly to the pharmacological activity.

Excretion: The primary route of excretion is biliary/fecal, with a smaller amount excreted in urine. In cattle, the elimination half-life is approximately 2-3 days, allowing for a single-dose treatment regimen. The prolonged half-life is due to extensive tissue binding and enterohepatic recirculation.

Half-Life: Cattle: approximately 2-3 days (subcutaneous).

Bioavailability: Approximately 100% after subcutaneous administration in cattle.

Protein Binding: Approximately 26% in cattle.

Available Formulations & Strengths

Injectable Solution 150 mg/mL (Subcutaneous)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to gamithromycin or other macrolides
Do not use in animals with known hypersensitivity to macrolide antibiotics
Do not use in dairy cattle 20 months of age or older (or in lactating dairy cattle) due to milk withdrawal concerns
Do not use in calves to be processed for veal
Do not use in horses or other equids, as severe adverse reactions may occur

Warnings & Clinical Precautions:

For use in cattle only. Not for use in humans.
Avoid use in animals with severe hepatic or renal impairment.
Use with caution in animals with a history of seizures or neurological disorders.
Subcutaneous injection may cause local tissue reactions; administer in the neck to minimize carcass damage.
Do not administer intravenously or intramuscularly.
Keep out of reach of children.
Wash hands after handling.
Avoid contact with skin and eyes; in case of contact, rinse thoroughly with water.
Do not use in animals that are to be used for breeding purposes unless advised by a veterinarian.
Use extra-label with caution in non-target species; safety and efficacy have not been established.

Adverse Effects & Reactions

Common:

Local injection site reactions (swelling, pain, inflammation)
Transient decrease in feed intake
Soft stools or diarrhea

Serious / Severe:

Anaphylactic reactions (rare)
Severe local tissue necrosis (if injected improperly)
Neurological signs (e.g., ataxia, tremors) in overdose or in sensitive species

Rare:

Hepatotoxicity
Nephrotoxicity
Ototoxicity (with high doses)

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Other macrolides or lincosamides (e.g., clindamycin, lincomycin)	Potential additive effects or increased risk of adverse effects; may compete for binding sites on the ribosome.	Moderate
Chloramphenicol or florfenicol	May antagonize the antibacterial effect due to competition at the ribosomal level.	Moderate
Neuromuscular blocking agents	Macrolides may enhance neuromuscular blockade; use with caution.	Mild
Drugs metabolized by CYP450 enzymes (e.g., theophylline)	Macrolides may inhibit hepatic metabolism, increasing levels of these drugs; monitor for toxicity.	Moderate

Overdose & Toxicity Management

Signs of Toxicity:

In cattle, overdose may cause transient swelling at injection site, diarrhea, and reduced feed intake.
At very high doses, neurological signs such as ataxia, tremors, and seizures may occur.
In non-target species (e.g., horses), severe reactions including cardiovascular collapse and death have been reported.

Emergency Treatment Protocol: There is no specific antidote. Treatment is symptomatic and supportive. Monitor vital signs, provide fluid therapy if needed, and manage neurological signs with anticonvulsants if necessary. In cases of accidental human injection, seek medical attention immediately.

Food Animal Withdrawal Times

🥩 Meat: 35 days

Not approved for use in lactating dairy cattle or in dairy cattle 20 months of age or older. Withdrawal period for meat is 35 days in cattle. Do not use in calves to be processed for veal.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20°C to 25°C (68°F to 77°F), with excursions permitted between 15°C and 30°C (59°F and 86°F).

Handling & Special Conditions: Protect from freezing. Keep container tightly closed. Store in a dry place.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved by the FDA for subcutaneous use in cattle for the treatment of bovine respiratory disease (BRD).

Extra-Label (Off-Label) Use: In the United States, gamithromycin is approved for use in cattle only. Extra-label use in other species is permitted under AMDUCA (Animal Medicinal Drug Use Clarification Act) only with a valid veterinary-client-patient relationship and must comply with established withdrawal times. Use in food animals requires observance of withdrawal periods to avoid violative residues.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Gamithromycin is a valuable antimicrobial for the control and treatment of bovine respiratory disease complex, a major cause of morbidity and mortality in feedlot cattle. Its long half-life allows for a single-dose regimen, reducing handling stress and labor. It is important to accurately diagnose BRD and confirm susceptibility of the causative pathogens. As with all antibiotics, judicious use is essential to minimize the development of antimicrobial resistance. Gamithromycin should be used as part of a comprehensive herd health program including vaccination, good nutrition, and low-stress handling. It is not approved for use in other species, and extra-label use should be avoided due to potential severe adverse reactions, especially in horses. Always adhere to withdrawal times to ensure food safety.

References & Literature

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