Mechanism of Action and Spectrum of Activity
Vancomycin exhibits a potent bactericidal effect. Its spectrum of activity is strictly limited to gram-positive bacteria (cocci and bacilli).
A key characteristic of the drug is its natural (intrinsic) resistance in gram-negative bacteria. This phenomenon is due to the innate refractoriness of these microorganisms to the antibiotic. The large molecular size of the glycopeptide physically prevents it from passing through the lipopolysaccharide outer membrane of gram-negative pathogens. Consequently, the drug is completely ineffective against them.
Resistance: MRSA and VRE
Despite its high efficacy, acquired resistance strains emerge through genetic mechanisms:
- Chromosomal mutations: Affect genes encoding target proteins or transport systems, transmitted vertically (from mother to daughter cells during division).
- Horizontal gene transfer: Acquisition of genetic material from other bacteria.
Horizontal gene transfer led to the emergence of vancomycin-resistant enterococci (VRE). Meanwhile, vancomycin remains a primary weapon against MRSA (methicillin-resistant Staphylococcus aureus). MRSA develops resistance to oxacillin and methicillin and causes severe nosocomial and community-acquired infections.
Pharmacokinetics: Why Capsules Only Work in the Gut
When administered orally, vancomycin exhibits extremely poor gastrointestinal absorption. The drug practically does not enter the systemic circulation; instead, it passes through the intestinal lumen and is excreted in the feces.
This pharmacokinetic property is utilized clinically for localized therapy directly within the intestinal lumen (decontamination, treatment of specific infections). The duration of systemic action is approximately 6 hours.
Clinical Indications
Vancomycin is a classic reserve antibiotic. It is prescribed for severe staphylococcal and streptococcal infections when standard therapy with penicillins (including antistaphylococcal penicillins like oxacillin and nafcillin) and cephalosporins fails or is contraindicated. In immediate life-threatening situations, vancomycin may be used empirically as a first-line agent.
Intravenous Administration:
- Drug of choice for sepsis and infective endocarditis caused by MRSA.
- Used in combination with aminoglycosides for enterococcal endocarditis (vancomycin synergistically enhances their efficacy).
Oral Administration:
- Capsules are indicated for gastrointestinal infections, primarily pseudomembranous colitis associated with Clostridium difficile.
Formulations and Administration Guidelines
The drug is available in two main forms:
- Powder in vials of 0.5 g and 1.0 g (for parenteral use).
- Capsules of 125 mg and 250 mg (for oral use).
For systemic effects, vancomycin must be administered via slow intravenous infusion.
- Dosage: 0.5 g every 6 hours or 1.0 g every 12 hours. The total daily dose is 2 g.
- Reconstitution: Solutions must be prepared ex tempore (immediately prior to use). The powder is dissolved at a ratio of 1.0 g of the drug per 10 mL of 0.9% sodium chloride (NaCl) solution.
- Critical rule: The rate of intravenous infusion must not exceed 10 mg per minute (0.01 g/min) to prevent severe adverse reactions.