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 Emergency Medicine Trainee with interests in medical education, ECG interpretation, and the use of point-of-care ultrasound in the undifferentiated patient. Co-author of the LITFL ECG Library | Twitter

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