Chapter 35  ·  Module 7  ·  Antibacterial Agents
Macrolide Antibiotics
Mechanism, pharmacokinetics, adverse effects, and resistance at a glance
Abbreviations: rRNA = ribosomal RNA  ·  CYP3A4 = cytochrome P450 3A4  ·  QTc = corrected QT interval  ·  hERG = human ether-a-go-go-related gene  ·  CAP = community-acquired pneumonia  ·  MAC = Mycobacterium avium complex  ·  MLSB = macrolide-lincosamide-streptogramin B  ·  CDC = Centers for Disease Control and Prevention
Agent Comparison
AgentBioavailabilityHalf-LifeCYP3A4 InhibitionGI TolerabilityKey Niche
Erythromycin 35-65% (variable; acid-labile) ~2 h; 4x/day Greatest (mechanism-based, irreversible) Worst (motilin agonist) Prototype; gastroparesis (prokinetic)
Clarithromycin ~50-55% (acid-stable) 3-7 h; twice-daily Significant (intermediate) Moderate H. pylori; MAC treatment; CAP
Azithromycin ~37% (extensive tissue uptake) Serum ~11 h; tissue ~68 h; once-daily Negligible Best CAP; chlamydia; pertussis; MAC prophylaxis; polypharmacy patients
Key Adverse Effects & Clinical Indications
Key Adverse Effects
Safety Profile by Category
  • GI intolerance: erythromycin > clarithromycin > azithromycin (motilin agonism)
  • QTc prolongation via hERG blockade: azithromycin and erythromycin greatest risk
  • Cholestatic hepatitis: erythromycin estolate (largely withdrawn)
  • Ototoxicity: reversible at high IV doses (unlike aminoglycosides)
  • CYP3A4 interactions: erythromycin and clarithromycin only
  • Colchicine + clarithromycin or erythromycin: potentially fatal in renal impairment
Clinical Indications
Agent-Matched Indications
  • CAP outpatient (macrolide monotherapy if resistance < 25%)
  • Chlamydia: doxycycline preferred (non-pregnant); azithromycin in pregnancy
  • Pertussis: azithromycin preferred (5-day course)
  • H. pylori: clarithromycin triple therapy (if local resistance < 15-20%)
  • MAC prophylaxis: azithromycin 1200 mg weekly (CD4 < 50)
  • MAC treatment: clarithromycin + ethambutol ± rifabutin; never monotherapy
Resistance Mechanisms
MechanismGeneHow It WorksClinical Note
MLSB (erm methylase) erm genes (plasmid/transposon) Methylates A2058 on 23S rRNA — reduces macrolide, clindamycin, and streptogramin B binding Constitutive or inducible. D-zone test detects inducible form — positive = clindamycin resistant
M phenotype (mef efflux) mef gene Proton-dependent efflux pump specific to macrolides No clindamycin cross-resistance. Low-level resistance. Prevalent in Streptococcus pneumoniae (North America)
23S rRNA point mutation Chromosomal (positions 2063/2064) Structural change at macrolide binding site — same site as erm methylation Macrolide-resistant Mycoplasma pneumoniae (> 90% in Asia). Use doxycycline or fluoroquinolone
Key Rules

Use azithromycin in polypharmacy patients — negligible CYP3A4 inhibition. Colchicine + clarithromycin or erythromycin is contraindicated in renal/hepatic impairment (potentially fatal). Never use macrolide monotherapy for MAC (rapid resistance emergence).

D-zone test: erythromycin-resistant, clindamycin-susceptible — test before using clindamycin. Positive D-zone = inducible MLSB resistance = report as clindamycin-resistant.