Penicillin G, also known as benzylpenicillin, is a natural penicillin widely used for its bactericidal activity. Due to its rapid clearance and acid instability, penicillin G is administered parenterally, often in depot forms to prolong its action. Two important depot formulations of penicillin G are Procaine Penicillin and Benzathine Penicillin .
1. Procaine Penicillin
Composition
- Combination of one mole of procaine (a local anesthetic) with one mole of benzylpenicillin .
- Procaine serves to reduce pain on injection and delays absorption of penicillin, providing a prolonged effect.
Pharmacokinetics
- Administered via intramuscular injection.
- Slowly absorbed from the muscle over several hours.
- Procaine is hydrolyzed by plasma esterases into non-toxic metabolites: para-aminobenzoic acid (PABA) and dimethylaminoethanol.
- Prolonged duration of penicillin G action for approximately 12 to 24 hours .
- Provides higher peak plasma penicillin levels compared to benzathine penicillin but shorter duration.
Clinical Uses
- Treatment of infections caused by penicillin-sensitive bacteria such as:
- Septicemia
- Pneumonia
- Endocarditis
- Empyema
- Pericarditis
- Commonly used for once-daily dosing via intramuscular injection (gluteal or mid-lateral thigh sites).
2. Benzathine Penicillin
Composition
- Combination of one mole of benzathine (an ammonium salt) with two moles of benzylpenicillin .
- Benzathine is poorly soluble, resulting in very slow absorption.
Pharmacokinetics
- Administered via intramuscular injection.
- Very slow absorption with low but sustained plasma penicillin concentrations.
- Duration of action lasts 2 to 4 weeks .
- Due to low solubility, serum levels remain low but prolonged.
- Insufficient cerebrospinal fluid (CSF) penetration — not suitable for neurosyphilis treatment .
Clinical Uses
- Long-acting treatment for infections requiring prolonged low-level penicillin exposure such as:
- Syphilis (early and latent stages)
- Rheumatic fever prophylaxis
- Yaws
- Eradication of group A beta-hemolytic streptococci (prevention of post-streptococcal complications)
- Some upper respiratory tract infections caused by streptococci
3. Key Differences Between Procaine and Benzathine Penicillin
| Feature | Procaine Penicillin | Benzathine Penicillin |
|---|---|---|
| Duration of action | ~12–24 hours | 2–4 weeks |
| Peak plasma levels | Higher | Lower, sustained |
| Solubility | More soluble | Poorly soluble (insoluble) |
| Onset of action | Faster | Slower |
| Use | Acute bacterial infections requiring higher plasma levels | Long-term prophylaxis, chronic infections needing prolonged therapy |
| Pain on injection | Reduced due to procaine anesthetic | May be painful due to low solubility and depot effect |
NB:
- Administration: Both formulations are only given via intramuscular injection ; intravenous use of depot forms is contraindicated.
- Probenecid interaction: Co-administration of probenecid reduces renal tubular secretion of penicillin, increasing and prolonging serum penicillin levels.
- Rapid IV injection caution: Rapid intravenous injection of penicillin G (not depot forms) can cause cardiac arrhythmias due to potassium content and high plasma levels.
- Depot effect: Both procaine and benzathine penicillins provide incomplete but prolonged release of penicillin into circulation.
- Patient monitoring: Especially important in patients with renal impairment or cardiac conditions.
