Aminoglycosides

Intro & MOA - Mighty Microbe Mincers

• Potent bactericidal antibiotics.
• Spectrum: Primarily aerobic Gram-negative bacilli.
• Mechanism of Action (MOA):
  • Irreversibly bind to the bacterial 30S ribosomal subunit.
  • Interfere with protein synthesis:
    • Block initiation complex formation.
    • Cause mRNA misreading, leading to abnormal protein insertion.
    • Block translocation of peptidyl-tRNA.
  • Exhibit concentration-dependent killing (↑ conc. = ↑ killing rate).
  • Long Post-Antibiotic Effect (PAE).
  • Oxygen-dependent uptake (ineffective against anaerobes).
• 📌 Examples: Gentamicin, Neomycin, Amikacin, Tobramycin, Streptomycin (GNATS).

⭐ Aminoglycosides cause misreading of mRNA and block translocation, leading to bactericidal action primarily against aerobic Gram-negative bacteria.

PK/PD Properties - Journey & Jolt

• Pharmacokinetics (Journey - ADME):
  • Absorption: Poor oral (highly polar); IV/IM administration.
  • Distribution: Accumulates in renal cortex & inner ear (⚠️ nephro/ototoxicity risk).
  • Metabolism: Negligible.
  • Excretion: Renal, via glomerular filtration (unchanged). Adjust dose in renal failure.
• Pharmacodynamics (Jolt - Effects):
  • Killing: Concentration-Dependent Killing (CDK) - higher $C_{max}$ = faster kill.
  • Duration: Significant Post-Antibiotic Effect (PAE) - suppression post-MIC drop.
• Dosing Rationale:
  • Once-Daily Dosing (ODD) preferred:
    • Maximizes CDK with high peaks.
    • Utilizes PAE for sustained effect.
    • Minimizes toxicity (drug-free interval for washout from kidney/ear).

⭐ Once-daily dosing regimens leverage concentration-dependent killing and post-antibiotic effect to maximize efficacy and minimize nephrotoxicity.

Clinical Uses & Spectrum - Germ Warfare Gurus

• Core Spectrum: Serious aerobic Gram-negative bacilli infections (Pseudomonas aeruginosa, E. coli, Klebsiella).

• Synergy: With β-lactams/Vancomycin for Gram-positive endocarditis (Enterococcus, Staphylococcus).

• Specific Pathogen Coverage:

  Aminoglycoside	Key Indications
  Streptomycin	Tuberculosis (TB), Plague, Tularemia
  Gentamicin	Plague, Tularemia, Serious Gram-negative sepsis
  Amikacin	TB (MDR), Gentamicin-resistant Gram-negatives
  Neomycin	Bowel sterilization (oral), Topical infections
  Tobramycin	Pseudomonas aeruginosa infections (esp. lungs)

⭐ Aminoglycosides are crucial in combination therapy with cell wall synthesis inhibitors (like penicillin or vancomycin) for synergistic bactericidal activity against certain Gram-positive cocci, especially in endocarditis.

Adverse Effects - Toxic Trio Alert

📌 NNN: Nephrotoxic, Neurotoxic (ototoxic), Neuromuscular blockade.

• Nephrotoxicity:
  • Reversible acute tubular necrosis (ATN).
  • Monitor: Serum creatinine, BUN. Ensure hydration.
• Ototoxicity:
  • Irreversible auditory (cochlear: hearing loss, tinnitus) & vestibular (vertigo, ataxia) damage.
  • Dose-related. Monitor audiometry in high-risk/long therapy.
• Neuromuscular Blockade:
  • Rare; risk of respiratory paralysis.
  • Risk factors: Myasthenia gravis, rapid IV infusion, concurrent neuromuscular blockers, hypocalcemia.
  • Treatment: IV Calcium gluconate or neostigmine.

⭐ Ototoxicity from aminoglycosides can be irreversible and may manifest as hearing loss (cochlear damage) or vertigo/ataxia (vestibular damage).

Resistance & Interactions - Defense Breachers

• Resistance Mechanisms:
  • Enzymatic modification (acetylation, phosphorylation, adenylation) - most common.
  • Impaired entry.
  • Altered ribosomal binding.
• Key Interactions & Cautions:
  • Loop diuretics: ↑ ototoxicity.
  • Vancomycin/Amphotericin B: ↑ nephrotoxicity.
  • Neuromuscular blockers: ↑ blockade.
  • Pregnancy: Category D. ⚠️

⭐ Most common resistance: enzymatic drug inactivation by bacterial enzymes.

High‑Yield Points - ⚡ Biggest Takeaways

• Bactericidal; irreversibly bind 30S ribosome, inhibiting protein synthesis.
• Spectrum: Aerobic Gram-negative bacilli (e.g., Pseudomonas); synergistic with β-lactams.
• Major toxicities: Nephrotoxicity (reversible ATN), Ototoxicity (irreversible; cochlear & vestibular), Neuromuscular blockade.
• Administered IV/IM (poor oral absorption); exhibit significant post-antibiotic effect.
• Resistance: Primarily via enzymatic modification (acetylation, phosphorylation).
• Therapeutic Drug Monitoring (TDM) essential due to narrow therapeutic index.