Vancomycin and other glycopeptides

Mechanism of Action - The Wall Blocker

• Bactericidal Glycopeptide: Directly inhibits bacterial cell wall synthesis by binding to the building blocks of the wall.
• Target: High-affinity binding to the D-Ala-D-Ala terminus of nascent peptidoglycan pentapeptides.
• Inhibition:
  • Prevents transglycosylation (polymerization/elongation of the peptidoglycan chain).
  • This steric hindrance also blocks transpeptidation (cross-linking).
• Result: A weakened, defective cell wall, leading to osmotic lysis and cell death.

⭐ Vancomycin is a large molecule, which prevents its passage through the porin channels of most Gram-negative bacteria, limiting its spectrum primarily to Gram-positive organisms.

Spectrum & Resistance - Gram‑Positive Guru

• Spectrum: Narrow, targeting Gram-positive aerobes and anaerobes. It has no activity against Gram-negative bacteria.

  • Key Targets: MRSA, S. epidermidis, Streptococci (including penicillin-resistant strains), and Enterococci (but not VRE).
  • Niche Use: Oral formulation is used for Clostridioides difficile colitis.

• Resistance Mechanisms:

  • Enterococci (VRE): The primary mechanism involves bacterial enzymes modifying the peptidoglycan precursor binding site from $D-Ala-D-Ala$ to $D-Ala-D-Lac$, which prevents vancomycin from binding effectively.
  • S. aureus (VISA/VRSA):
    • VISA: Features a thickened cell wall with numerous "decoy" $D-Ala-D-Ala$ targets that trap the drug.
    • VRSA: Acquires resistance genes (e.g., vanA) from VRE, leading to the same target site alteration.

⭐ VRE resistance classically stems from altering the drug's target site. The terminal $D-Ala-D-Ala$ of peptidoglycan precursors is changed to $D-Ala-D-Lac$, which dramatically lowers vancomycin's binding affinity.

Clinical Use & PK - The IV Powerhouse

• Primary Use: Serious infections by Gram-positive bacteria, especially Methicillin-resistant Staphylococcus aureus (MRSA).
  • Empiric therapy for sepsis, endocarditis, osteomyelitis, and hospital-acquired pneumonia.
  • Also covers Enterococcus species (if susceptible) and Streptococcus pneumoniae.
• Administration & Distribution:
  • IV for systemic infections due to poor GI absorption.
  • PO (oral) is only for Clostridioides difficile colitis (acts locally in the gut).
• Pharmacokinetics (PK):
  • Time-dependent killing.
  • Elimination: >90% renal; requires dose adjustment in renal impairment.
  • Therapeutic Drug Monitoring (TDM) is crucial: monitor trough levels (target 10-20 mg/L).

⭐ High-Yield: Oral vancomycin is not absorbed and cannot treat systemic infections. Its use is restricted to the gastrointestinal lumen for C. difficile infections.

Adverse Effects - Not So VANilla

• Nephrotoxicity

• Ototoxicity

• Thrombophlebitis (at infusion site)

• Red Man Syndrome (RMS)

  • Histamine-mediated infusion reaction (NOT a true allergy).
  • Causes flushing, erythema, pruritus on face, neck, upper torso.
  • Prevent by slowing infusion rate & pre-treating with antihistamines.

• DRESS Syndrome (Drug Reaction with Eosinophilia and Systemic Symptoms)

⭐ Exam Favorite: Red Man Syndrome is a rate-dependent reaction caused by direct mast cell degranulation, not an IgE-mediated hypersensitivity. Always differentiate from anaphylaxis.

High‑Yield Points - ⚡ Biggest Takeaways

• Inhibits cell wall synthesis by directly binding to D-Ala-D-Ala oligopeptides.
• A key agent for multidrug-resistant Gram-positive infections, including MRSA.
• Resistance emerges from the modification of the binding site to D-Ala-D-Lac.
• Red Man Syndrome is a characteristic infusion-related reaction, not a true allergy.
• Major dose-limiting toxicities include nephrotoxicity and ototoxicity.
• Administered IV for systemic infections; the oral formulation is exclusively for C. difficile colitis.
• Requires therapeutic drug monitoring (trough levels).