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
References
Pharm 101 700

Pharmacology 101

Top 200 drugs

MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner

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