Mechanism of Action and Spectrum of Activity
Like other beta-lactams, the drug exhibits a bactericidal effect. Due to its hydrophilicity, the amoxicillin molecule easily penetrates through the porin channels of the outer membrane of Gram-negative ($G-$) bacteria, providing an expanded spectrum of activity compared to natural penicillins (which are hydrophobic and do not cross $G-$ porins).
Spectrum of activity:
- Highly active against Streptococcus pneumoniae (including strains resistant to natural penicillins).
- Active against Helicobacter pylori.
- In combination with clavulanic acid, it acts against Haemophilus influenzae.
- Less active against Shigella species (compared to ampicillin).
Important: Amoxicillin is not resistant to beta-lactamases. To overcome resistance from bacteria producing these enzymes, combination (protected) drugs are used, where the antibiotic is paired with a $\beta$-lactamase inhibitor (e.g., Amoxicillin + Clavulanate).
Pharmacokinetics
Amoxicillin has several fundamental differences from its precursor, ampicillin:
- Absorption: The drug is acid-stable. When taken orally, it is absorbed 2–2.5 times better than ampicillin (bioavailability reaches 90–95%).
- Food interaction: Absorption is independent of food intake, ensuring stable plasma concentrations.
- GI distribution: Because the drug is almost completely and very rapidly absorbed in the upper gastrointestinal tract, its concentration in the lower bowel remains low.
- Route of administration: Administered exclusively orally (per os).
- Excretion: Excreted primarily unchanged by the kidneys.
Indications for Use
The drug is prescribed for the following conditions:
- Treatment of uncomplicated upper respiratory tract and ENT infections (ear, nose, throat).
- Eradication of Helicobacter pylori in peptic ulcer disease. Since monotherapy is insufficient, amoxicillin is used in combination regimens (along with clarithromycin, metronidazole, and proton pump inhibitors such as lansoprazole, rabeprazole, or pantoprazole).
- Prophylaxis of infective endocarditis during dental procedures in high-risk patients.
- As a protected combination (Amoxicillin + Clavulanate) for pneumonia and meningitis caused by Haemophilus influenzae.
Administration Considerations and Risks
Impact of renal pathology: Because amoxicillin is excreted unchanged by the kidneys, renal impairment is critical for its pharmacokinetics. In renal failure, the half-life ($t_{1/2}$) increases sharply. Using standard dosing regimens in the presence of such pathology leads to accumulation (drug buildup) in the body, significantly increasing the risk of toxicity.
Gastroenterology limitations: Despite high efficacy against gastric Helicobacter infections, amoxicillin is not used for intestinal infections (such as shigellosis or salmonellosis). This is due to its pharmacokinetics: because 95% is absorbed in the upper GI tract, it simply does not reach the lower intestine in therapeutic concentrations.
Formulations and Prescription
The reference guides list the following oral formulations:
- Tablets: 0.25, 0.5, and 0.75 g.
- Capsules: 0.25 and 0.5 g.
- Suspension in bottles: 0.125 and 0.25 g per 5 mL (60 mL volume).
Standard dosing regimen: orally 0.5–1.0 g 3 times daily.
Example prescription for 500 mg (0.5 g) tablets: Rp.: Tab. Amoxicillini 0.5 D.t.d. N. 20 S. Take 1 tablet orally 3 times daily.