Amoxicillin/Clavulanic Acid

Mechanism

Pharmacological Class and Group

Association of a broad-spectrum beta-lactam antibiotic (Amoxicillin, aminopenicillin) with an irreversible inhibitor of plasmid-transmitted beta-lactamases (Clavulanic Acid).

Mechanism of Action

This drug combination directly restores the potency of amoxicillin against Gram-positive and Gram-negative bacterial strains that express destructive beta-lactamase enzymes:

• Amoxicillin: It binds covalently and irreversibly to the penicillin-binding proteins (PBP-1 and PBP-3) located on the inner surface of the bacterial cell membrane. These enzymes catalyze the transpeptidation reaction necessary for the cross-linking of glycopeptide chains in the synthesis of cell wall peptidoglycan. Its inhibition stops the synthesis and assembly of the active bacterial cell wall, inducing autolysis of bacteria in the exponential growth phase (bactericidal).
• Clavulanic Acid: It is a beta-lactam molecule that acts as an irreversible "suicide" type inhibitor. It binds with high affinity to the active site of bacterial beta-lactamases of the Ambler classification group of type class A (penicillinases and cephalosporinases of plasmid transmission common in Haemophilus influenzae and Moraxella catarrhalis). By binding covalently, it irreversibly inactivates the enzyme, sacrificing its own structure and protecting the beta-lactam ring of amoxicillin from hydrolytic degradation.

Mechanism of Suicidal Action and Inhibition

Amoxicillin-PBP Transpeptidation block Bacterial osmotic lysis Clavulanate-Beta-lactamase Irreversible inactive covalent complex (Suicide inhibitor)

This association is fundamental in otorhinolaryngology for the empirical control of infections of the upper airway and ear, where the prevalence of beta-lactamase-producing bacteria (especially Haemophilus influenzae and Moraxella catarrhalis) exceeds 30-40% of clinical isolates.

Pharmacokinetics

High Resolution Pharmacokinetics

• Absorption and Bioavailability: Both drugs are absorbed quickly and completely through the upper digestive tract. The bioavailability of amoxicillin is 90%, and that of clavulanate is approximately 60-70%. Administration together with the start of a light meal optimizes the absorption of clavulanate and significantly reduces irritative digestive disorders.
• Distribution: Excellent penetration into the tissues of the nasal passage, secretions of the paranasal sinuses, mucosa of the Eustachian tube and fluid of the middle ear, where effective bactericidal concentrations are achieved. Low binding to circulating plasma proteins (18% for amoxicillin; 22% for clavulanate). They do not cross healthy meninges in any relevant way.
• Metabolism: Amoxicillin is barely metabolized in the liver (less than 10%); Clavulanic acid undergoes partial hepatic metabolism (approximately 50%).
• Excretion: Preferential renal glomerular excretion by filtration and active tubular secretion (amoxicillin is recovered unchanged in urine by 60-70% after oral dosing). Probenecid prolongs the renal excretion of amoxicillin but not that of clavulanate.
• Half-life (t_1/2): Elimination half-life of approximately 1 hour for both compounds in patients with normal renal function.

Indicators and dose

Clinical Indications and Off-Label Uses

• Acute Otitis Media (AOM): Indicated in pediatric and adult patients who have severe AOM or who do not respond to the standard initial dose of amoxicillin alone, or who have risk factors for beta-lactamase-producing pathogens.
• Acute Bacterial Rhinosinusitis (RSAB): First-line treatment of choice in patients with persistent or severe symptoms (for more than 10 days or with high fever and intolerable facial pain) according to EPOS guidelines.
• Acute Recurrent or Complicated Pharyngotonsillitis: Management of infections caused by Streptococcus pyogenes (EGBHA) refractory to penicillin or suspected superinfection by bacterial anaerobes that produce beta-lactamases.
• Cellulitis or Periatonsillar Abscess: Supportive systemic antibacterial adjuvant after performing emergency surgical drainage.

Dosage and Clinical Adjustment

Schedule in Adults and Adolescents (Based on amoxicillin doses):

• Mild to Moderate Infections: 875 mg / 125 mg every 12 hours orally, or 500 mg / 125 mg every 8 hours.
• Severe Infections / Acute Chronic Rhinosinusitis or suspected resistance: High-dose formulation: 1000 mg / 62.5 mg prolonged release twice daily orally.
• Duration: 5 to 7 days in bacterial pharyngitis and uncomplicated AOM; 10 to 14 days in severe bacterial rhinosinusitis.

Scheme in Children (Dose calculated on the basis of amoxicillin):

• Standard Dose: 40 mg/kg/day to 80 mg/kg/day divided into 3 doses per day (using the 8:1 amoxicillin/clavulanate suspension formulation).
• Dose in Severe Acute Otitis Media (due to decreased susceptibility pneumococcus): 80-90 mg/kg/day divided every 12 hours orally (using formulations with low dose of 14:1 type clavulanate to avoid severe diarrhea in infants).

Adjustment for Mandatory Renal Failure (Based on Amoxicillin dose):

• eGFR 30 mL/min/1.73 m²: No adjustment required.
• eGFR 10 to 29 mL/min/1.73 m²: Administer a maximum of 500 mg / 125 mg every 12 hours orally. Do not use the 1000 mg extended-release presentation.
• eGFR < 10 mL/min/1.73 m²: Administer a maximum of 500 mg / 125 mg every 24 hours orally.
• Hemodialysis: Administer 500 mg / 125 mg every 24 hours plus an identical full additional dose after the dialysis session.

Security

Absolute and Relative Contraindications

Absolute Contraindications

• Documented history of immediate hypersensitivity (anaphylaxis or angioedema) to beta-lactam antibiotics (penicillins or cephalosporins).
• Personal history of cholestatic jaundice or severe hepatic dysfunction induced by the coadministration of amoxicillin/clavulanate.
• Suspected or confirmed infection with Epstein-Barr Virus (Infectious Mononucleosis): The use of aminopenicillins in patients with mononucleosis causally induces a generalized non-allergic maculo-papular skin rash (in more than 80-90% of those exposed).

Relative Contraindications

• Chronic kidney disease with severe clearance (requires proportional dose reduction according to eGFR).
• Moderate liver failure of cholestatic origin.

Adverse Effects (ADR) and Specific Toxicity

• Very common (>10%): Dose-dependent gastrointestinal disorders (osmotic diarrhea due to direct irritation of clavulanate in the enterocytic mucosa; it is attenuated by using formulations with a lower ratio of clavulanate, such as 8:1 or 14:1, and administering it with meals).
• Frequent (1%-10%): Mucocutaneous candidiasis (oral thrush, opportunistic vaginitis due to alteration of the natural bacterial microbiota), nausea, vomiting.
• Uncommon (0.1%-1%): Moderate elevation of transaminases of hepatic origin (TGP/AST), mild urticarial skin rashes.
• Rare (0.01%-0.1%): Hepatic Cholestatic Jaundice (ADR directly linked to the clavulanate component, presenting in a delayed manner up to 2-4 weeks after stopping treatment, usually self-limited), pseudomembranous colitis due to proliferation of Clostridioides difficile.

Critical Toxicity: Cholestatic Hepatitis due to Clavulanate

Clavulanic acid-induced cholestatic hepatitis is an idiosyncratic immunoallergic reaction. The patient presents generalized pruritus, dark urine (choluria), pale stools (acolia) and progressive conjunctival jaundice after ingesting the antibiotic. The drug must be immediately suspended, analytical determinations of bilirubin and alkaline phosphatase must be carried out, and the prohibition on the use of formulations with clavulanic acid must be definitively recorded in the patient's clinical record.

Drug Interactions of Clinical Relevance

• Oral Anticoagulants (Warfarin): Pharmacodynamic synergism. The elimination of the vitamin K-producing bacterial intestinal microbiota by the broad-spectrum antibiotic alters coagulation homeostasis, raising the INR value and the risk of severe bleeding.
• Allopurinol: Coadministration significantly increases the incidence of non-allergic maculo-papular skin rashes and exanthems in the patient.
• Methotrexate: Penicillins decrease active renal tubular secretion of methotrexate, increasing the risk of bone marrow toxicity of the immunosuppressant.

Safety in Pregnancy and Breastfeeding

• Pregnancy: Compatible. It is considered one of the first-choice broad-spectrum treatments in pregnancy, with no teratogenic effects having been demonstrated. However, in patients with premature rupture of uterine membranes, the use of amoxicillin/clavulanate may be associated with an increased risk of necrotizing enterocolitis in the newborn, so it should be prescribed under strict medical indication.
• Breastfeeding: Compatible. It is excreted in minimal quantities in human milk; The possible appearance of transient diarrhea or rash in the term infant should be monitored due to alteration of the intestinal flora.