Clarithromycin

Overview

Clarithromycin (Biaxin®) is a semi-synthetic macrolide antibiotic used in veterinary medicine to treat a variety of bacterial infections, particularly those caused by intracellular or atypical organisms. It has been used in dogs, cats, ferrets, and foals, and may be beneficial in the treatment of atypical mycobacterial infections, Helicobacter spp infections, and Rhodococcus equi infections in foals.

The drug has a spectrum of activity similar to erythromycin, but it also demonstrates activity against several organisms that are difficult to treat with other antibiotics. Clarithromycin is active against many gram-positive bacteria, some gram-negative organisms such as Pasteurella, and several atypical pathogens including Mycoplasma, Chlamydia, Rickettsia, Bartonella, and certain Mycobacterium species. However, most Enterobacteriaceae such as Escherichia coli, Klebsiella, and Pseudomonas are resistant.

Mechanism of Action (MOA): Clarithromycin penetrates susceptible bacterial cells and binds to the 50S ribosomal subunit, inhibiting bacterial protein synthesis. Macrolides are typically bacteriostatic, although they may exert bactericidal activity at higher concentrations against susceptible organisms.

Clarithromycin is highly lipid-soluble and distributes widely throughout the body, achieving therapeutic concentrations in many tissues and body fluids, including the eye and prostate. It is well absorbed after oral administration and has an elimination half-life of approximately 4.6–5.9 hours in dogs. The drug may also have immunomodulatory properties, although the clinical significance of these effects remains uncertain.

Indications

Clarithromycin is used in veterinary medicine for the treatment of infections caused by susceptible bacteria, particularly intracellular and atypical pathogens. Because of its good tissue penetration and activity against several difficult-to-treat organisms, it may be useful in a variety of infectious conditions in small animals and foals.

• Atypical mycobacterial infections: Frequently used as part of multidrug therapy for infections caused by non-tuberculous mycobacteria.
• Helicobacter infections: May be included in treatment protocols for Helicobacter spp infections affecting the gastrointestinal tract.
• Respiratory tract infections: Useful against susceptible pathogens, including organisms such as Mycoplasma and other atypical respiratory bacteria.
• Skin and soft tissue infections: May be used for mild to moderate infections caused by susceptible bacteria.
• Rhodococcus equi infections (foals): Often used in combination with other antimicrobials, such as rifampin, in the treatment of Rhodococcus equi pneumonia.

Dosage (Reference)

Dog

In dogs, clarithromycin is used for the treatment of susceptible bacterial infections, particularly those caused by atypical or intracellular organisms. Dosing recommendations are largely empirical because limited pharmacokinetic data are available in veterinary patients.

Cat

In cats, clarithromycin may be used to treat susceptible bacterial infections, including atypical organisms. As in dogs, most dosing recommendations are based on limited reports and clinical experience.

Warnings & Precautions

Clarithromycin should be used carefully in veterinary patients due to potential species sensitivities, hepatic metabolism, and the possibility of significant drug interactions. Appropriate patient selection and monitoring can help reduce the risk of adverse effects.

• Hypersensitivity: Contraindicated in animals with known hypersensitivity to clarithromycin or other macrolide antibiotics.
• Hindgut fermenters: Use cautiously in hindgut fermenting species such as adult horses, rabbits, guinea pigs, and similar species, as macrolide antibiotics may disrupt intestinal flora and lead to severe diarrhea or enteritis.
• Hepatic disease: Clarithromycin undergoes hepatic metabolism; use cautiously in animals with liver dysfunction and monitor patients for signs of hepatotoxicity.
• Renal impairment: Dose reduction may be required in animals with decreased renal function to prevent drug accumulation.
• QT interval prolongation: Rare cardiac rhythm disturbances have been reported in humans; caution is advised in animals receiving other drugs that may prolong the QT interval.
• Pregnancy and lactation: Safety in pregnant animals has not been fully established. Use only when the potential benefits outweigh the possible risks to the fetus or neonates.

Drug Interactions

Clarithromycin can interact with many medications because it inhibits the cytochrome P450 (CYP3A) enzyme system. These interactions may increase the blood concentration of certain drugs or alter their therapeutic effects. Careful monitoring and dose adjustment may be necessary when clarithromycin is used with other medications.

• Cyclosporine: Clarithromycin may significantly increase cyclosporine blood concentrations in dogs and cats, which may require reducing the cyclosporine dose by approximately 30–35%.
• Anticoagulants (e.g., warfarin, rivaroxaban): Clarithromycin may enhance the anticoagulant effects, increasing the risk of bleeding.
• Digoxin: May increase serum digoxin concentrations and raise the risk of digoxin toxicity.
• Rifampin: Can significantly reduce clarithromycin oral bioavailability, potentially decreasing its therapeutic effectiveness.
• Colchicine: Concurrent use may increase the risk of colchicine toxicity, particularly in patients with renal or hepatic impairment.
• Drugs that prolong QT interval (e.g., amiodarone, cisapride, procainamide, sotalol): Concurrent administration may increase the risk of cardiac arrhythmias.
• Azole antifungals (e.g., ketoconazole, itraconazole): May increase concentrations of both drugs and increase the risk of QT prolongation.
• Benzodiazepines (e.g., diazepam, midazolam): Clarithromycin may reduce their metabolism and increase sedative effects.
• Calcium channel blockers (e.g., amlodipine, diltiazem): Blood concentrations may increase due to reduced metabolism.
• Opioid analgesics (e.g., fentanyl, morphine, methadone): Clarithromycin may increase drug exposure and enhance pharmacologic effects.
• Theophylline or aminophylline: Clarithromycin may increase serum concentrations, requiring monitoring for toxicity.

Side Effects & Overdose

Side Effects

Clarithromycin is generally well tolerated in dogs, cats, ferrets, and foals. As with many orally administered antibiotics, most adverse effects involve the gastrointestinal tract and are usually mild and self-limiting.

• Gastrointestinal effects: Inappetence, vomiting, diarrhea, and abdominal discomfort may occur.
• Skin reactions (cats): Redness of the ear pinnae or generalized erythema has occasionally been reported.
• Skin staining: Orange discoloration of the skin may occur in some animals.
• Anhidrosis in foals: Decreased sweating may occur, although less frequently than with erythromycin.

Overdose

Clarithromycin overdoses are usually associated with gastrointestinal signs and are rarely life-threatening. Large ingestions may result in more pronounced digestive disturbances.

• Gastrointestinal signs: Nausea, vomiting, abdominal discomfort, or diarrhea are the most common findings.
• Severe ingestion: Very large doses may lead to gastrointestinal irritation and bleeding.
• Management: Treatment is typically supportive. Activated charcoal with a cathartic may be administered to reduce further absorption if ingestion was recent.
• Drug removal: Procedures such as forced diuresis, peritoneal dialysis, or hemodialysis are not effective in removing clarithromycin from the body.

Key Notes

Practical clinical points that may help optimize the safe and effective use of clarithromycin in veterinary patients:

• Intracellular penetration: Clarithromycin is highly lipid-soluble and penetrates tissues and cells effectively, making it useful against intracellular pathogens.
• Administration timing: Drug absorption may be improved when administered on an empty stomach because oral bioavailability is higher in fasted animals.
• Alkaline activity: Antibacterial activity may be enhanced in slightly alkaline environments.
• Combination protocols: Clarithromycin is frequently used as part of multidrug antimicrobial protocols for certain infections, particularly atypical or mycobacterial diseases.
• Tissue distribution: The drug distributes widely in the body and can achieve therapeutic concentrations in many tissues and body fluids, including difficult-to-penetrate sites.