Benzylpenicillins and Isoxazolylpenicillins

Cell Wall Inhibitors

Natural penicillins and penicillins stable to staphylococcal penicillinases (isoxazolylpenicillins) represent the original therapeutic pillars of beta-lactam therapy. Its bactericidal action requires active bacterial replication.

Mechanism

💊 Generic and Commercial Names

Penicillin G sodium, benzathine penicillin G (Penibenzatina), cloxacillin (Anaclosil), oxacillin.

🔬 Pharmacological Group

Beta-lactams. Natural penicillins and isoxazolyl penicillins resistant to narrow-spectrum beta-lactamases.

Mechanism of Action

Its molecular target is the final transpeptidation step of cell wall synthesis:

Covalent Binding and Inhibition of PBPs: They are structural analogues of the terminal D-alanyl-D-alanine residue in nascent peptidoglycan. The beta-lactam ring is attacked by the active-site serine residue of PBPs (especially PBP1 and PBP3), forming an inactive penicilloyl–enzyme ester complex that dissociates extremely slowly. Blocking transpeptidation stops the cross-linking of peptidoglycan chains.
Autolysin Activation: Inhibition of cell wall synthesis causes secondary release of endogenous bacterial autolysins (peptidoglycan hydrolases), inducing osmotic cell lysis of the bacteria.
Cloxacillin Resistance: It has an isoxazole ring with bulky substituents that provide steric hindrance, protecting the amide bond of the beta-lactam ring from hydrolysis by the penicillinases of Staphylococcus aureus (but it is vulnerable to ESBL and carbapenemases).

Pharmacokinetics

Key Pharmacokinetics

Parameter	Quantitative Pharmacokinetic Profile
Routes of Administration	Penicillin G Sodium: IV; Penicillin G Benzathine: IM (slow deposition); Cloxacillin: IV and Oral (poor oral bioavailability, ~30-50%, severely affected by food).
Absorption and Onset	Penicillin G Benzathine IM generates very low but persistent plasma concentrations for 14-21 days (useful in prophylaxis of rheumatic fever and syphilis). IV Cloxacillin reaches peak levels within 30 minutes after completing the infusion.
Volume of Distribution (V_d)	Low (0.15 - 0.25 L/kg). Mainly confined to the extracellular space. It presents poor tissue penetration in an intact blood-brain barrier, but it increases in a clinically significant manner in the presence of active meningeal inflammation. Protein binding: Penicillin G ~60%; Extremely high cloxacillin ~90-95%.
Metabolism	Minimal hepatic metabolism (10-20% to inactive metabolites of penicilloic acid).
Excretion and Half-Life	Predominant renal excretion (70-90% unchanged): Through glomerular filtration and active tubular secretion through the organic anion transporter OAT1/OAT3. Plasma half-life (t_1/2) very short: 30 to 60 minutes in healthy adults with preserved kidney function. Renal clearance decreases drastically with age and in neonates.

Antimicrobial Spectrum

Penicillin G: Excellent activity against Streptococcus pyogenes (Group A), Streptococcus pneumoniae (susceptible to penicillin), Neisseria meningitidis, Treponema pallidum, Clostridium perfringens and other anaerobes of the oral cavity (except Bacteroides fragilis).
Cloxacillin / Oxacillin: Specifically designed for the management of methicillin-sensitive Staphylococcus aureus (MSSA) and sensitive coagulase-negative staphylococci. It lacks useful activity against Gram-negative or enterococci.

Indicators and dose

Dosage and Adjustment

Penicillin G Sodium (Adults): 12 to 24 million IU/day IV, divided every 4 hours (or administered as a continuous infusion after loading dose).
Penicillin G Benzathine: 1.2 to 2.4 million IU single dose via deep IM in the upper outer quadrant of the gluteus.
Cloxacillin: 1 to 2 g IV every 4 to 6 hours (maximum dose 12 g/day for severe endovascular infections such as bacterial endocarditis).
Adjustment in Kidney Failure:
- Cl_cr > 30 mL/min: No change.
- Cl_cr 10-30 mL/min: Space Penicillin G every 8 hours or reduce the daily dose by 50%. Cloxacillin does not require routine dose adjustment due to its high excretion and secondary compensatory biliary metabolism.
- Cl_cr < 10 mL/min: Space Penicillin G every 12 hours.

Security

Contraindications and Precautions

Absolute: Demonstrated immediate type I hypersensitivity (anaphylaxis, angioedema, bronchospasm) to penicillins or other beta-lactams in a cross-linked manner.
Relative: Severe renal failure (systemic accumulation that can induce neurotoxicity due to blockade of cortical GABAergic receptors); active epilepsy that is difficult to control.

Adverse Effects (ADR)

Immunoallergic (Common): Maculopapular rash, urticaria, drug fever. Immediate type reactions (anaphylactic shock <0.05% of cases). Immune-mediated acute interstitial nephritis (fever, eosinophilia, hematuria).
Neurotoxicity (Dose-dependent): Myoclonus, mental confusion, generalized tonic-clonic seizures (favored by massive IV doses of sodium penicillin in patients with uncorrected renal failure).
Hematological: Reversible neutropenia in prolonged treatments (>2 weeks of cloxacillin); hemolytic anemia with positive direct Coombs test.

Risk of Intravascular Injection of Penicillin G Benzathine

Penicillin G Benzathine is a viscous, insoluble depot formulation designed exclusively for deep intramuscular administration. Its accidental injection into the arterial or venous stream can cause catastrophic distal microvascular occlusion, focal tissue necrosis, gangrene and permanent neurological damage (Nicolau syndrome). Aspiration should always be performed prior to injection and avoid the vicinity of major neurovascular bundles.

Epistemis is educational review material. It is not a medical device, does not diagnose or prescribe treatment, and does not replace formal medical training, current clinical guidelines, or professional clinical judgment.

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System: Antimicrobial and Infectious
Group: Cell Wall Inhibitors

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