Aminoglycosides

Gentamicin can cure a life-threatening infection and permanently destroy hearing or kidney function in the same course of therapy. The difference is how you monitor.

Aminoglycosides (gentamicin, tobramycin, amikacin) carry two dose-limiting toxicities: nephrotoxicity and ototoxicity. Nephrotoxicity is usually reversible if caught early — monitor serum creatinine and BUN at baseline and throughout therapy. A rising creatinine signals accumulation. Ototoxicity can be irreversible and affects both cochlear (hearing loss, tinnitus) and vestibular (vertigo, ataxia) branches of cranial nerve VIII. Hearing loss typically starts in high-frequency ranges before the patient notices it, so report any tinnitus or balance changes immediately. Peak levels (drawn 30 minutes after IV infusion ends) confirm the dose reaches therapeutic concentration. Trough levels (drawn just before the next dose) confirm the drug is clearing adequately — an elevated trough is the strongest predictor of toxicity. For gentamicin and tobramycin, the traditional trough target is less than 2 mcg/mL. Maintain hydration to support renal clearance, track intake and output, and avoid stacking nephrotoxic drugs (NSAIDs, vancomycin, amphotericin B, contrast dye) without close monitoring.

Key Distinctions

Don't confuse peak and trough timing — peak is drawn after infusion (efficacy check), trough is drawn before the next dose (toxicity check). Students mix up ototoxicity and nephrotoxicity reversibility: nephrotoxicity is often reversible, ototoxicity frequently is not. Vancomycin also causes nephro- and ototoxicity, but aminoglycoside monitoring relies on both peak and trough levels, whereas current vancomycin practice increasingly uses AUC/MIC-guided dosing.

Clinical Pearl

Trough predicts toxicity, peak predicts efficacy. If the trough won't come down, the kidneys can't keep up — hold the dose and call the provider.

MOA & Use

Aminoglycosides (gentamicin, tobramycin, amikacin, neomycin, streptomycin) work by irreversibly binding to the 30S ribosomal subunit of bacteria, causing misreading of mRNA and production of aberrant proteins that damage the bacterial cell membrane. Unlike bacteriostatic drugs that merely slow growth, aminoglycosides are bactericidal — they directly kill bacteria. Their killing power is concentration-dependent: the higher the peak concentration above the minimum inhibitory concentration, the greater the bacterial kill. This is why extended-interval (once-daily) dosing is preferred over traditional divided dosing — a single large dose achieves a high peak that maximizes killing, then allows a drug-free interval to reduce toxicity. Primary indications include serious gram-negative infections: Pseudomonas aeruginosa, E. coli, Klebsiella, and Serratia. They're commonly used for sepsis, complicated UTIs, pneumonia from gram-negative organisms, and as synergy with beta-lactams or vancomycin for endocarditis. Aminoglycosides have poor oral absorption — they must be given IV or IM for systemic infections. Oral neomycin is an exception, used specifically for bowel sterilization before surgery or in hepatic encephalopathy because it stays in the gut lumen.

Key Distinctions

Don't confuse the 30S target of aminoglycosides with the 50S target of macrolides — both affect ribosomes but at different subunits. Students mix up bactericidal (aminoglycosides kill) with bacteriostatic (tetracyclines inhibit growth at the same 30S site). Concentration-dependent killing means higher peaks matter; this is opposite of time-dependent drugs like penicillins, where duration above MIC drives efficacy.

Clinical Pearl

Think 'A-MEAN-o-glycoside' — mean to gram-negatives, mean to ribosomes (30S, irreversible), and mean to the body if you don't respect the dosing.