Pharmacology  ·  Antibacterial Agents

Aminoglycosides

Mechanism, dosing strategy, toxicity, synergy, and resistance

Abbreviations: MIC = minimum inhibitory concentration  ·  Cmax = peak drug concentration  ·  PAE = post-antibiotic effect  ·  EID = extended-interval dosing  ·  TDM = therapeutic drug monitoring  ·  AME = aminoglycoside-modifying enzyme  ·  HLAR = high-level aminoglycoside resistance  ·  CF = cystic fibrosis  ·  rRNA = ribosomal RNA  ·  LPS = lipopolysaccharide

Mechanism of Action
Concentration-Dependent Bactericidal Killing
Self-Amplifying Membrane Disruption → 30S Ribosome Misreading → Lysis
Step 1
Outer Membrane Binding
Polycationic drug displaces Mg²⁺/Ca²⁺ from LPS — initial membrane disruption
→
Step 2
Active Uptake
Proton motive force drives drug across inner membrane — requires electron transport chain (anaerobes: resistant)
→
Step 3
30S Ribosome Binding
Binds decoding site of 16S rRNA — mRNA misreading, aberrant proteins produced
→
Result
Self-Amplifying Lysis
Aberrant proteins insert into inner membrane → more drug entry → rapid, irreversible, concentration-dependent killing
Key Agents — Spectrum, Toxicity, and Clinical Niche
Agent Spectrum Highlight Principal Toxicity Key Clinical Use
Gentamicin Broad gram-negative; enterococcal synergy Vestibulotoxicity; nephrotoxicity Gram-negative bacteremia; enterococcal endocarditis synergy (low-level dose); HLAR eliminates synergy
Tobramycin Pseudomonas (2–4× more potent than gentamicin) Cochleotoxicity; nephrotoxicity Pseudomonas bacteremia / pneumonia; inhaled suppression therapy in CF
Amikacin Broadest — 1-N-acyl group resists most AMEs Cochleotoxicity; nephrotoxicity Gentamicin/tobramycin-resistant gram-negatives; MDR organisms; reserved agent
Streptomycin M. tuberculosis; Brucella; Yersinia pestis Vestibulotoxicity (severe); nephrotoxicity Drug-resistant tuberculosis; plague; tularemia; brucellosis
Neomycin Gram-negative GI decontamination; topical Most cochleotoxic — systemic use absolutely contraindicated Bowel prep; hepatic encephalopathy (oral, non-absorbed); topical wound care only
Dosing Strategy and Toxicity
Extended-Interval Dosing (EID)
Once-Daily Dosing Strategy
  • Entire daily dose as single infusion — maximizes Cmax/MIC (>8–10 target)
  • Drug-free trough period limits proximal tubular accumulation → reduced nephrotoxicity vs. multiple-daily dosing
  • Post-antibiotic effect (PAE) provides continued bacterial suppression during trough
  • Hartford nomogram: single level drawn 6–14 hr post-infusion determines 24/36/48 hr interval
  • Not validated for: neonates, pregnancy, significant burns, ascites
  • TDM essential — individualize interval based on renal function and level
Nephrotoxicity
Proximal Tubular Accumulation
  • Non-oliguric acute kidney injury after 5–10 days of therapy
  • Serum creatinine rise lags tubular injury by 24–48 hours
  • Risk factors: volume depletion, vancomycin co-administration, prolonged duration, baseline renal impairment
  • Vancomycin + aminoglycoside = highest nephrotoxicity combination — monitor renal function daily
  • EID reduces risk (drug-free trough allows tubular clearance)
  • Generally reversible if recognized and drug stopped promptly
Ototoxicity
Irreversible Hair Cell Destruction
  • Cochlear: high-frequency hearing loss first (above speech range) — amikacin, tobramycin predominant
  • Vestibular: oscillopsia, chronic imbalance, disequilibrium — gentamicin, streptomycin predominant
  • Neomycin most cochleotoxic — never systemic
  • A1555G mitochondrial variant: severe irreversible hearing loss possible after a single dose — genetic screening in appropriate populations
  • Not reliably reversed — prevention through TDM and shortest effective course
Resistance Mechanisms
AMEs and rRNA Methyltransferases
  • AMEs (acetyltransferases, nucleotidyltransferases, phosphotransferases) — modify drug, abolish ribosomal binding; encoded on mobile plasmids/transposons
  • Amikacin resists most AMEs via 1-N-acyl group — use when AME resistance suspected
  • 16S rRNA methyltransferases — methylate ribosomal binding site → high-level resistance to all aminoglycosides including amikacin
  • Co-located with NDM and other carbapenemase genes on mobile plasmids — pan-resistant phenotypes; genotypic testing required
  • HLAR (gentamicin MIC ≥500 mcg/mL) eliminates enterococcal synergy for endocarditis

Clinical Rules: Synergy, Monitoring, and Toxicity Avoidance

For enterococcal endocarditis, aminoglycoside synergy with ampicillin or penicillin requires confirmed low-level susceptibility — gentamicin MIC below 500 mcg/mL and streptomycin MIC below 2000 mcg/mL. High-level aminoglycoside resistance eliminates the synergistic bactericidal effect entirely. Ampicillin-ceftriaxone double beta-lactam therapy is now an equivalent alternative for Enterococcus faecalis endocarditis and avoids aminoglycoside toxicity.

Therapeutic drug monitoring is mandatory for all patients receiving systemic aminoglycosides. For EID, use the Hartford nomogram (single level at 6–14 hours post-infusion) to individualize the dosing interval. Monitor serum creatinine every 48 hours minimum; daily if vancomycin is co-administered or if the patient has baseline renal impairment.

Suggested References

Author / Source Title Publication
Katzung BG, ed. Basic and Clinical Pharmacology, 15th ed. — Chapter 44: Aminoglycosides and Spectinomycin McGraw-Hill, 2021
Brunton LL, Knollmann BC, eds. Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. — Chapter 51: Protein Synthesis Inhibitors and Miscellaneous Antibacterial Agents McGraw-Hill, 2023
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