Macrolides

Mechanism of action

  • Inhibits bacterial RNA-dependent protein synthesis by binding to the 50S subunit of the 70S ribosome, which causes dissociation of t-RNA from the ribosome and prevents protein synthesis
  • Protein synthesis can resume once the antibiotic is removed i.e. bacteriostatic
    • The bacteria are then killed by WBCs
  • Effective against
    • gram-positive aerobic organisms e.g. Strep pneumoniae;
    • some gram-negative aerobic organisms e.g. Legionella, Neisseria gonorrhoea, Moraxella and Bordetella (Enterobacteriaceae are resistant)
    • useful against other intracellular pathogens e.g. mycoplasma, Chlamydia

Pharmacokinetics

  • Absorption via the gut is poor due to destruction by gastric acid and poor water solubility.  (Better absorbed in a fasting state)
  • Excreted mainly by the liver, biliary system and into the gut (little renal excretion)
  • Clarithromycin has a fairly short half life (requires bi-daily dosing) but azithromycin has an extended half-life which means a single dose can often be effective

Adverse effects

  • GI upset is common
  • Prolongation of the QT interval is a potentially serious side effect in patients with either an already prolonged QT interval or WPWS
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