Pharm 101: Erythromycin

Class

Macrolide antibiotic

Pharmacodynamics

• Inhibits bacterial protein synthesis by binding to 50S ribosomal RNA
• Antibacterial action:
  • Bacteriostatic at low concentrations
  • Bactericidal at higher concentrations
• Resistance is plasmid-encoded, mechanisms include:
  • Reduced membrane permeability or active efflux
  • Esterase production
  • Modification of ribosomal binding site

Pharmacokinetics

• Administration: PO, IV
• Destroyed by gastric acid, must be enteric coated
• Distributed widely except to brain/CSF
• Crosses placenta
• Half-life 1.5 hours normally, 5 hours in anuria
• Biliary excretion, only 5% excreted in urine
  • Adjustment for renal failure not necessary
• CYP3A4 enzyme inhibitor (see adverse effects)

Antimicrobial activity

• Gram positive organisms e.g. pneumococci, strep, staph
• Mycoplasma pneumoniae, L pneumophila, Chlamydophila pneumoniae, Listeria monocytogenes
• Gram negative organisms e.g. neisseria, bordatella pertussis, campylobacter
• Treponema pallidum

Adverse effects

• Acute cholestatic hepatitis
• Gastrointestinal upset (nausea/vomiting, anorexia, diarrhoea)
• Allergic reactions e.g. fever, eosinophilia, rash
• Drug interactions:
  • Increases concentration of warfarin, methylprednisolone due to CYP inhibition
  • Combination with terfenadine (H1 antagonist) causes lethal ventricular arrhythmia
  • Increases digoxin concentration in oral administration due to increased bioavailability

Further reading

• Buttner R. Pharm 101: Azithromycin. LITFL

References

• Katzung BG. Basic and Clinical Pharmacology. 14e. 2018: 819-820, 824