## Distinguishing Gentamicin from Amikacin ### Structural and Enzymatic Basis **Key Point:** Amikacin is a semisynthetic derivative of kanamycin with an L-(-)-γ-amino-butyric acid side chain that confers resistance to most aminoglycoside-modifying enzymes (AMEs), whereas gentamicin is susceptible to enzymatic inactivation by multiple AMEs (phosphotransferases, nucleotidyltransferases, acetyltransferases). ### Comparative Pharmacology | Feature | Gentamicin | Amikacin | |---------|-----------|----------| | **AME Resistance** | Inactivated by ~80% of resistant strains | Resistant to most AMEs; effective against ~90% of resistant gram-negatives | | **Half-life** | 2–3 hours (similar to amikacin) | 2–3 hours (similar to gentamicin) | | **Peak Levels** | 4–8 μg/mL (standard dosing) | 15–30 μg/mL (standard dosing) | | **CSF Penetration** | ~10% of serum levels | ~10% of serum levels (similar) | | **Spectrum** | Broader against gram-negatives; used first-line | Reserved for resistant organisms | **High-Yield:** Amikacin's structural modification makes it the aminoglycoside of choice for infections caused by organisms resistant to gentamicin due to aminoglycoside-modifying enzyme production. ### Clinical Pearl Gentamicin remains first-line for susceptible gram-negative infections (e.g., *Pseudomonas aeruginosa*, *Enterobacteriaceae*) due to cost and efficacy. Amikacin is reserved for documented or suspected resistance patterns, particularly in nosocomial infections with extended-spectrum β-lactamase (ESBL)–producing organisms or multidrug-resistant gram-negatives. **Mnemonic:** **AMIKA** = **A**minoglycoside **M**odifying enzyme **I**nhibitor/**K**anamy**c**in **A**nalogue — resistant to enzymatic degradation.
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