Macrolides are a class of antibiotics that end in the suffix “-thromycin”: including azithromycin, clarithromycin, and erythromycin. Macrolides work by inhibiting the 50S subunit of bacterial ribosomes, stopping protein production to inhibit bacterial growth. Clinically, macrolides are used for a wide variety of bacterial infections, including the treatment of atypical pneumonias from organisms like Legionella or Mycoplasma, as well as treatment of sexually-transmitted infections like chlamydia and gonorrhoea. Important adverse effects to know include hepatitis, GI upset, and QT prolongation, with a risk of causing Torsades de Pointes. It’s also extremely important to remember that macrolides can cause drug interactions by inhibiting CYP enzymes in the liver.

Key Points

  • Macrolides
    • Drug Names (-thromycin ending)
      • Azithromycin
      • Clarithromycin
      • Erythromycin
    • Mechanism
      • Bind to 50S subunit of bacterial ribosomes
        • Bind to the 23S rRNA of the 50S ribosomal subunit, to blocking transpeptidation, translocation, and chain elongation
        • Overall effect is a blocking of bacterial protein synthesis
      • Bacteriostatic
    • Clinical Use
      • Atypical pneumonias (Mycoplasma, Chlamydia, Legionella)
      • STIs (Chlamydia, N. gonorrhoae)
      • Bordetella pertussis
      • Gram-positive cocci (streptococcal) in patients allergic to penicillin
    • Adverse Effects
      • Inhibits CYP450 Enzymes
        • Leads to drug interactions (e.g. warfarin, theophylline)
      • GI Upset
        • All macrolides act as agonists at motilin receptors in the GI tract, increasing GI motility
      • Prolonged QT interval (Torsades de Pointes risk)
      • Acute cholestatic hepatitis
      • Rash
      • Eosinophilia
    • Mechanism of Resistance
      • Methylation of 23S rRNA-binding site prevents binding of drug