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Penicillin: Mechanism of Action, Side Effects and Dosage

Penicillin is the naturally occurring β-lactam antibiotic, which is produced by molds and was discovered by Alexander Fleming (Nobel Prize 1945).

• Penicillin G: naturally occurring penicillin (benzylpenicillin), only suitable for parenteral use.
• Penicillin V: the partially synthesized penicillin (phenoxymethylpenicillin) is acid-stable and, therefore, suitable for oral administration.
• Benzathine penicillin G: the salt of penicillin G with benzathine is poorly soluble in water and is used intramuscularly as a depot penicillin.

Mechanism of Action:

Penicillins inhibit the peptidoglycan synthesis of the bacterial wall: they bind to so-called penicillin-binding proteins (peptidoglycan synthetases) and inhibit the polymerization of the peptidoglycan and covalent cross-linking of the bacterial wall.

Antibiotic Spectrum of Penicillin:

Penicillins have a bactericidal effect on streptococci, meningococci, pneumococci, diphtheria bacteria, and treponema pallidum. Gonococci became increasingly resistant to penicillin.

Urological Indications for Penicillin:

Treatment of syphilis with benzathine penicillin G, treatment of neurosyphilis with high-dose penicillin G i.v.

Pharmacokinetics of Penicillin:

• Penicillin G is suitable for intravenous and intramuscular applications. The half-life of penicillin G is 30–60 min, 90% renal excretion.
• The depot preparation benzathine penicillin must be administered intramuscularly; the maximum serum concentrations are measured after 24 hours. The depot penicillin is slowly absorbed and ensures an effective serum level for about seven days in susceptible bacteria.

Side Effects of Penicillin:

• Allergy (0.5–2%), especially with repeated use. Very rarely severe cutaneous skin reactions.
• At very high doses: neurotoxic reactions with epilepsy, inhibition of platelets, hemolytic anemia and neutropenia are possible.
• Gastrointestinal side effects (nausea, vomiting, diarrhea), pseudomembranous enterocolitis.
• Jarisch-Herxheimer reaction caused by the treatment of syphilis: fever, muscle pain, and headache due to sudden pathogen decay under antibiotic therapy.

Drug Interactions:

Probenicid inhibits renal tubular excretion and increases the serum concentration of penicillins.

Contraindications:

Allergy to penicillins. Intolerance to lidocaine with benzathine penicillin.

Dosage of penicillin:

One million international units (IU) correspond to 600 mg of penicillin.

• Mild infections with streptococci: 1 million IU p.o. or i.v. every 6 hours; half the dosage in children.
• Early syphilis: Benzathine penicillin G 2.4 million IU i.m. once, usually two gluteal injections left and right with 1.2 million IU each are administered.
• Late syphilis: 3× benzathine penicillin G 2.4 million IU i.m. on days 1,8 and 15.
• Neurosyphilis: penicillin G 3–4 million IU i.v. every 4 hours over 14 days, dose reduction and interval extension in renal insufficiency depending on GFR.

References

H. R. Brodt, A. Hörauf, M. Kresken, W. Solbach, and T. Welte, Infektionstherapie: Antibiotika, Virostatika, Antimykotika, Antiparasitäre Wirkstoffe. Thieme, 2023.

  Deutsche Version: Pharmakologie und Nebenwirkungen von Penicillin

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