benzylpenicillin
penicillin antibiotic· Brands: Penicillin G, Pfizerpen, CrystapenIMIV
Mechanism of action
Benzylpenicillin is a narrow-spectrum penicillin that inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs), preventing the transpeptidation step in peptidoglycan cross-linking, resulting in a defective cell wall and bacterial lysis. It is bactericidal against actively dividing susceptible organisms.
Pharmacokinetics
Benzylpenicillin is poorly absorbed from the gastrointestinal tract and is therefore administered parenterally. After intramuscular or intravenous injection, it is widely distributed to body tissues and fluids, including the placenta, but penetration into the cerebrospinal fluid is limited unless the meninges are inflamed. It is metabolised to a small extent and is eliminated mainly by the kidneys through glomerular filtration and active tubular secretion; the elimination half-life is approximately 30 to 60 minutes in patients with normal renal function, and dosage adjustment is required in renal impairment. Probenecid can reduce its renal clearance.
Indications & dosing
Pneumococcal pneumonia
For moderate to severe infections.
- adult · IV: 1.2 g every 6 hours · For moderate to severe infections.
Meningococcal meningitis
High-dose regimen for severe infection.
- adult · IV: 2.4 g every 4 hours · High-dose regimen for severe infection.
Infective endocarditis due to streptococci
For sensitive organisms; duration typically 4 weeks.
- adult · IV: 1.2 g every 4 hours · For sensitive organisms; duration typically 4 weeks.
Neurosyphilis
Give for 14 days.
- adult · IV: 2.4 g every 4 hours · Give for 14 days.
Contraindications & precautions
- Hypersensitivity to penicillins · Includes history of immediate hypersensitivity reaction to penicillins or severe beta-lactam allergy.
- Infectious mononucleosis · Increased risk of maculopapular rash; avoid or use caution.
- Severe renal impairment · Requires dose adjustment; risk of neurotoxicity at high doses.
- Use with caution in patients with renal impairment, as dose adjustment may be required to prevent neurotoxicity.
- Avoid in patients with a history of immediate hypersensitivity to beta-lactams; cross-reactivity with cephalosporins may occur.
- Monitor for signs of anaphylaxis during therapy, especially after first dose.
- Prolonged use may lead to overgrowth of non-susceptible organisms including Clostridium difficile.
Adverse effects
- Nausea
- Diarrhoea
- Rash (maculopapular)
- Anaphylaxisserious
- Angioedemaserious
- Clostridium difficile-associated colitisserious
- Convulsions (high doses, renal impairment)serious
- Serum sickness-like reactionserious
Renal & hepatic adjustment
- eGFR 50–300: No dose adjustment normally required.
- eGFR 30–50: Consider reducing the dose or extending the dosing interval if there is a risk of accumulation.
- eGFR 0–30: Reduce the dose to 50% or double the dosing interval. Monitor carefully for toxicity.
Monitoring, pregnancy & toxicity
- Renal function · Baseline and regularly during therapy
- Serum electrolytes · During prolonged therapy
- Hypersensitivity reactions · At each dose
- Complete blood count · During prolonged therapy
- Neurological status · In high doses or renal impairment
- Pregnancy: Benzylpenicillin (penicillin G) is considered safe in pregnancy. It crosses the placenta, but no teratogenic effects have been reported. It is the preferred treatment for infections such as syphilis in pregnancy.
- Toxicity: Hypersensitivity reactions, including rash, urticaria, and anaphylaxis. · Immediate discontinuation of the drug. Management of anaphylaxis with adrenaline, oxygen, and supportive measures.
- Toxicity: Neurotoxicity (e.g., seizures, encephalopathy), particularly with high doses or in renal impairment. · Discontinue the drug, manage seizures, and provide supportive care. Consider reducing subsequent doses.
- Toxicity: Electrolyte disturbances, such as hyperkalaemia (if potassium salt used) or hypernatraemia (if sodium salt used), especially with high doses. · Monitor serum electrolytes regularly. Correct imbalances and consider alternative antibiotic if severe.
Clinical notes
Benzylpenicillin is used for the treatment of infections caused by susceptible gram-positive organisms, including streptococcal pharyngitis, syphilis, and meningococcal meningitis. It is not stable to penicillinase and is therefore ineffective against beta-lactamase-producing staphylococci. Hypersensitivity reactions are common, ranging from rashes to anaphylaxis; skin testing may be considered. Local pain and inflammation can occur at the injection site. Resistance is emerging, and local antibiograms should be consulted. The dose and frequency should be adjusted in renal impairment.
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BNF; local formulary