Amoxicillin: A Broad-Spectrum Antibiotic for Respiratory and Other Infections
Amoxicillin is a semisynthetic, beta-lactam antibiotic with a broad spectrum of bactericidal activity against many gram-positive and gram-negative microorganisms, making it a first-line treatment for various common infections including sinusitis, pneumonia, and otitis media. 1
Chemical Structure and Classification
Amoxicillin is an aminopenicillin with the chemical formula C₁₆H₁₉N₃O₅S·3H₂O. It is structurally similar to ampicillin but contains an additional hydroxyl group that enhances its oral absorption. Chemically, it is (2S,5R,6R)-6-[(R)-(-)-2-amino-2-(p-hydroxyphenyl)acetamido]-3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid trihydrate. 1
Mechanism of Action
Amoxicillin works by inhibiting bacterial cell wall biosynthesis, specifically targeting the peptidoglycan layer, which leads to bacterial cell death. Like other beta-lactam antibiotics, it binds to penicillin-binding proteins (PBPs) that are essential for bacterial cell wall synthesis. 1
Pharmacokinetic Properties
- Absorption: Amoxicillin is stable in gastric acid and rapidly absorbed from the gastrointestinal tract, with superior bioavailability compared to other oral penicillins
- Distribution: It diffuses readily into most body tissues and fluids except cerebrospinal fluid (unless meninges are inflamed)
- Protein binding: Approximately 20% protein-bound in blood serum
- Half-life: 61.3 minutes
- Excretion: About 60% of an oral dose is excreted unchanged in urine within 6-8 hours 1
Clinical Applications
Respiratory Tract Infections
Acute Sinusitis: First-line treatment with amoxicillin for 10-14 days is recommended
- Initial dosing for 3-5 days to assess improvement
- If no improvement, switch to amoxicillin-clavulanate or cefuroxime axetil 2
Community-Acquired Pneumonia:
- In children under 3 years: Amoxicillin 80-100 mg/kg/day in three daily doses is the initial choice for pneumococcal pneumonia
- Treatment duration: 10 days for pneumococcal pneumonia 2
Dosing Considerations
- Children: Dosing should be modified in pediatric patients 12 weeks or younger due to incompletely developed renal function 1
- Elderly: No specific dosage adjustment based on age alone, but renal function should be monitored as the drug is primarily excreted by the kidneys 1
- Renal Impairment: Dosage adjustment required in patients with severe renal impairment (GFR less than 30 mL/min) 1
Advantages Over Other Penicillins
Amoxicillin offers several advantages over other penicillins, particularly penicillin G:
- Superior oral bioavailability
- Broader spectrum of activity against gram-negative organisms
- More convenient dosing (2-3 times daily versus 3-4 times daily for oral penicillin G)
- Enhanced activity against Haemophilus influenzae (70-85% coverage) 3
Resistance Patterns
- Group A Streptococcus has never developed resistance to penicillins, making amoxicillin still effective for streptococcal pharyngitis
- Approximately 75% of E. coli urinary isolates are resistant to amoxicillin, limiting its use as monotherapy for urinary tract infections 3
Adverse Effects
Common adverse effects include:
- Gastrointestinal disturbances (diarrhea, nausea)
- Hypersensitivity reactions (rash, urticaria)
- Amoxicillin may be better tolerated orally than penicillin V due to improved taste, especially in pediatric populations 3
Important Clinical Considerations
- Complete the full course: Patients should be instructed to complete the entire course of antibiotics (typically 10-14 days) to ensure complete eradication of the organism and prevent relapse 2
- Monitoring: If symptoms don't improve within 3-5 days of treatment initiation, reevaluation and potential switch to a different antibiotic is recommended 2
- Combination therapy: Amoxicillin-clavulanate should be considered when beta-lactamase-producing organisms are suspected 2
Contraindications
Amoxicillin is contraindicated in patients with a history of allergic reactions to penicillins or cephalosporins. Cross-reactivity between penicillins and cephalosporins occurs in approximately 10% of patients with penicillin allergy.
Note on "Extencillin"
It's important to note that "Extencillin" is not amoxicillin. Extencillin is actually benzathine penicillin G, a long-acting form of penicillin G used primarily for treating syphilis and for prophylaxis of rheumatic fever. 4 It should not be confused with amoxicillin, which is an entirely different penicillin derivative with distinct pharmacological properties and clinical applications.