Macrolide Antibiotics Used in Pediatric Age
Classification of Macrolide Antibiotics
Macrolide antibiotics are a class of drugs characterized by a large macrocyclic lactone ring. They are broadly classified into three generations:
- First Generation:
- Erythromycin
- Second Generation:
- Clarithromycin
- Azithromycin
- Third Generation:
- Ketolides (e.g., Telithromycin) - not approved for pediatric use
In pediatric practice, the most commonly used macrolides are erythromycin, azithromycin, and clarithromycin.
Mechanism of Action
Macrolides exert their antimicrobial effect through the following mechanisms:
- Protein Synthesis Inhibition: They bind reversibly to the 50S subunit of bacterial ribosomes, specifically to the 23S rRNA molecule. This inhibits peptidyl transferase activity, preventing peptide bond formation and subsequent protein synthesis.
- Translocation Inhibition: Macrolides interfere with the translocation of tRNA from the A-site to the P-site on the ribosome, further disrupting protein synthesis.
- Anti-inflammatory Effects: In addition to their antimicrobial properties, macrolides have been shown to have immunomodulatory and anti-inflammatory effects, which can be beneficial in certain respiratory conditions.
The antimicrobial spectrum of macrolides includes:
- Gram-positive bacteria (e.g., Streptococcus pneumoniae, Streptococcus pyogenes)
- Some gram-negative bacteria (e.g., Haemophilus influenzae, Moraxella catarrhalis)
- Atypical pathogens (e.g., Mycoplasma pneumoniae, Chlamydophila pneumoniae, Legionella pneumophila)
Indications in Pediatrics
Macrolides are used in various pediatric infections, including:
- Respiratory Tract Infections:
- Community-acquired pneumonia (particularly atypical pneumonia)
- Acute otitis media (as an alternative in penicillin-allergic patients)
- Streptococcal pharyngitis (in penicillin-allergic patients)
- Pertussis (treatment and prophylaxis)
- Skin and Soft Tissue Infections:
- Impetigo
- Cellulitis (in penicillin-allergic patients)
- Sexually Transmitted Infections:
- Chlamydia trachomatis infections in adolescents
- Mycobacterial Infections:
- Prophylaxis and treatment of Mycobacterium avium complex (MAC) in HIV-infected children
- Other Indications:
- Gastroparesis (erythromycin as a prokinetic agent)
- Chronic inflammatory lung diseases (e.g., cystic fibrosis, bronchiectasis) - for their anti-inflammatory properties
Pharmacokinetics in Pediatrics
Understanding the pharmacokinetics of macrolides is crucial for their appropriate use in children:
- Absorption:
- Erythromycin: Variably absorbed, acid-labile
- Clarithromycin: Well-absorbed, not affected by food
- Azithromycin: Well-absorbed, decreased by food
- Distribution:
- All macrolides distribute widely throughout the body
- Achieve high intracellular concentrations
- Penetrate well into respiratory tissues and fluids
- Metabolism:
- Erythromycin and Clarithromycin: Extensively metabolized by CYP3A4
- Azithromycin: Minimal hepatic metabolism
- Elimination:
- Erythromycin: Primarily biliary excretion
- Clarithromycin: Both renal and biliary excretion
- Azithromycin: Primarily biliary excretion
- Half-life:
- Erythromycin: 1.5-2 hours
- Clarithromycin: 3-4 hours
- Azithromycin: 68 hours (tissue half-life)
The long tissue half-life of azithromycin allows for once-daily dosing and shorter treatment courses in many indications.
Dosing in Pediatrics
Dosing of macrolides in children varies based on the specific drug, indication, and patient factors:
- Erythromycin:
- 30-50 mg/kg/day divided every 6-8 hours (max 2 g/day)
- For pertussis: 40-50 mg/kg/day divided every 6 hours for 14 days
- Clarithromycin:
- 15 mg/kg/day divided every 12 hours (max 1 g/day)
- For MAC prophylaxis in HIV: 7.5 mg/kg (max 500 mg) twice daily
- Azithromycin:
- Acute otitis media: 30 mg/kg as a single dose or 10 mg/kg once daily for 3 days
- Community-acquired pneumonia: 10 mg/kg (max 500 mg) on day 1, followed by 5 mg/kg (max 250 mg) once daily for 4 days
- Pertussis: 10 mg/kg (max 500 mg) on day 1, then 5 mg/kg (max 250 mg) once daily for 4 days
Note: Dosing may need adjustment in renal impairment, particularly for clarithromycin.
Adverse Effects
While generally well-tolerated, macrolides can cause various adverse effects:
- Gastrointestinal:
- Nausea, vomiting, abdominal pain, diarrhea (most common)
- Erythromycin is associated with more GI side effects than newer macrolides
- Hepatotoxicity:
- Rare but can occur, particularly with erythromycin estolate
- Monitor liver function tests in prolonged therapy
- Cardiac:
- QT interval prolongation (risk varies among macrolides)
- Torsades de pointes (rare but serious)
- Ototoxicity:
- Reversible hearing loss (rare, usually with high doses or prolonged use)
- Allergic Reactions:
- Rash, urticaria, anaphylaxis (rare)
Azithromycin generally has the most favorable side effect profile among macrolides.
Contraindications and Precautions
Macrolides should be used with caution or avoided in certain situations:
- Hypersensitivity: Known allergy to any macrolide antibiotic
- Hepatic Impairment: Use with caution, monitor liver function
- Myasthenia Gravis: May exacerbate symptoms
- Long QT Syndrome: Increased risk of arrhythmias
- Pregnancy: Erythromycin preferred if a macrolide is necessary
- Concurrent Use of QT-prolonging Medications: Increased risk of arrhythmias
Always consider the risk-benefit ratio, especially in patients with pre-existing conditions or those on multiple medications.
Drug Interactions
Macrolides, especially erythromycin and clarithromycin, have numerous drug interactions due to their inhibition of CYP3A4:
- CYP3A4 Substrates:
- Increased levels of: statins, benzodiazepines, cyclosporine, tacrolimus
- Potential for increased toxicity
- QT-prolonging Drugs:
- Antiarrhythmics, antipsychotics, antihistamines
- Increased risk of arrhythmias
- Anticoagulants:
- May increase INR with warfarin
- P-glycoprotein Substrates:
- May increase levels of digoxin
Azithromycin has fewer drug interactions compared to erythromycin and clarithromycin, making it a preferred choice in patients on multiple medications.
Antibiotic Resistance
Macrolide resistance is an increasing concern in pediatric practice:
- Mechanisms of Resistance:
- Target site modification (e.g., methylation of 23S rRNA)
- Efflux pumps
- Enzymatic inactivation (rare)
- Prevalence:
- Varies geographically and by pathogen
- High rates in some Streptococcus pneumoniae and Streptococcus pyogenes strains
- Cross-resistance:
- Resistance to one macrolide often confers resistance to others
- Prevention Strategies:
- Judicious use of macrolides
- Adherence to local antibiotic guidelines
- Consideration of local resistance patterns
Monitoring local resistance patterns is crucial for effective empiric therapy with macrolides in pediatric infections.
Erythromycin
Erythromycin is a first-generation macrolide antibiotic, discovered in 1952.
Key Characteristics:
- Available in various salt forms (e.g., estolate, ethylsuccinate, stearate)
- Relatively short half-life (1.5-2 hours)
- More prone to gastrointestinal side effects than newer macrolides
Pediatric Uses:
- Treatment of pertussis (whooping cough)
- Alternative for penicillin-allergic patients in streptococcal pharyngitis
- Treatment of atypical pneumonia
- Prophylaxis for rheumatic fever
- Prokinetic agent in gastroparesis
Dosing in Children:
- General dosing: 30-50 mg/kg/day divided every 6-8 hours (max 2 g/day)
- Pertussis treatment: 40-50 mg/kg/day divided every 6 hours for 14 days
Advantages:
- Long history of use and well-understood safety profile
- Inexpensive compared to newer macrolides
- Effective against many atypical pathogens
Disadvantages:
- Higher incidence of gastrointestinal side effects
- More frequent dosing required due to short half-life
- More drug interactions due to strong CYP3A4 inhibition
- Increasing resistance among some pathogens
Azithromycin
Azithromycin is a second-generation macrolide antibiotic, introduced in the 1980s.
Key Characteristics:
- Long half-life (68 hours) allows for once-daily dosing and short treatment courses
- Better tissue penetration compared to erythromycin
- Fewer drug interactions due to minimal CYP3A4 inhibition
Pediatric Uses:
- Community-acquired pneumonia
- Acute otitis media
- Treatment and prophylaxis of pertussis
- Streptococcal pharyngitis in penicillin-allergic patients
- Mycoplasma genitalium infections in adolescents
- Traveler's diarrhea
Dosing in Children:
- Acute otitis media: 30 mg/kg as a single dose or 10 mg/kg once daily for 3 days
- Community-acquired pneumonia: 10 mg/kg (max 500 mg) on day 1, followed by 5 mg/kg (max 250 mg) once daily for 4 days
- Pertussis: 10 mg/kg (max 500 mg) on day 1, then 5 mg/kg (max 250 mg) once daily for 4 days
Advantages:
- Convenient once-daily dosing
- Short treatment courses (often 3-5 days)
- Better gastrointestinal tolerability than erythromycin
- Fewer drug interactions
Disadvantages:
- Higher cost compared to erythromycin
- Concerns about potential for inducing antimicrobial resistance due to long half-life
- Some concerns about cardiovascular safety, though risk appears low in children
Clarithromycin
Clarithromycin is a second-generation macrolide antibiotic, introduced in the 1990s.
Key Characteristics:
- Better acid stability than erythromycin
- Forms an active metabolite (14-hydroxyclarithromycin) with additional antimicrobial activity
- Intermediate half-life (3-4 hours) allows for twice-daily dosing
Pediatric Uses:
- Community-acquired pneumonia
- Acute otitis media
- Pharyngitis/tonsillitis in penicillin-allergic patients
- Skin and soft tissue infections
- Mycobacterium avium complex (MAC) prophylaxis in HIV-infected children
- H. pylori eradication (as part of combination therapy)
Dosing in Children:
- General dosing: 15 mg/kg/day divided every 12 hours (max 1 g/day)
- MAC prophylaxis in HIV: 7.5 mg/kg (max 500 mg) twice daily
Advantages:
- Better gastrointestinal tolerability than erythromycin
- Twice-daily dosing improves compliance compared to erythromycin
- Active metabolite enhances effectiveness against certain pathogens (e.g., H. influenzae)
- Good tissue penetration, especially in respiratory tract
Disadvantages:
- More drug interactions than azithromycin due to CYP3A4 inhibition
- Higher cost than erythromycin
- Taste disturbances more common than with other macrolides
- Not approved for children under 6 months of age