Aminoglycosides

Aminoglycosides are not absorbed orally and must be administered intravenously or intramuscularly. The volume of distribution is approximately 0.25–0.3 L/kg in euvolemic adults, reflecting distribution predominantly in extracellular fluid. This increases substantially in patients with third-space fluid accumulation (sepsis, ascites, edema) and requires weight-based dosing adjustments using ideal body weight corrected for obesity. Aminoglycosides are eliminated essentially unchanged by glomerular filtration, with a plasma half-life of approximately 2–3 hours in patients with normal renal function. Dose adjustment is required in renal impairment.

Extended-interval dosing administers the entire daily aminoglycoside dose as a single infusion, generating a high peak concentration that optimizes the Cmax/MIC ratio while allowing a drug-free trough period that limits proximal tubular accumulation and adaptive resistance development. This strategy has been demonstrated to be equivalent or superior to multiple-daily dosing for most gram-negative infections, with a consistent trend toward reduced nephrotoxicity.

The Hartford nomogram is widely used to individualize extended-interval dosing intervals. A single serum level drawn 6–14 hours after the infusion is plotted against sampling time; the result determines whether the next dose should be given at 24, 36, or 48 hours based on the patient's clearance. The nomogram is not validated for neonates, pregnancy, significant burns, or ascites.

Therapeutic drug monitoring (TDM) is mandatory for aminoglycosides because of their narrow therapeutic index and the direct relationship between drug exposure and toxicity. For extended-interval dosing, a single level drawn 6–14 hours post-infusion is plotted on the Hartford nomogram. For multiple-daily dosing (used in endocarditis synergy regimens and neonates), both peak levels and trough levels are monitored: target troughs for gentamicin and tobramycin are below 2 mcg/mL (ideally below 1 mcg/mL) to minimize nephrotoxicity; target peaks are 6–10 mcg/mL for gram-negative infections and 3–5 mcg/mL for synergy regimens.

Current evidence does not support the routine addition of an aminoglycoside to a beta-lactam for gram-negative bacteremia caused by susceptible organisms in non-immunocompromised patients, as combination therapy does not improve mortality compared to beta-lactam monotherapy while substantially increasing nephrotoxicity. However, aminoglycosides retain a role in initial empiric coverage of septic shock when multidrug-resistant gram-negative organisms are suspected, with the aminoglycoside typically discontinued once susceptibility data and clinical stability are established.

Tobramycin is preferred over gentamicin for Pseudomonas aeruginosa infections due to superior intrinsic potency. Inhaled tobramycin (tobramycin inhalation solution or tobramycin inhalation powder) is approved for chronic suppression of Pseudomonas aeruginosa in patients with cystic fibrosis, achieving high airway concentrations with minimal systemic exposure and reducing exacerbation frequency without the toxicity burden of parenteral dosing.

Low-dose aminoglycosides exploit synergy with cell wall-active agents to enhance bactericidal killing against enterococci and streptococci. For enterococcal endocarditis caused by gentamicin-susceptible Enterococcus faecalis, the combination of ampicillin plus ceftriaxone has demonstrated equivalent efficacy to ampicillin plus gentamicin with substantially less nephrotoxicity and is now preferred at many centers. Gentamicin synergy remains guideline-supported for streptococcal endocarditis where short-course combination therapy (two weeks) has established benefit. High-level aminoglycoside resistance (defined as gentamicin minimum inhibitory concentration at or above 500 mcg/mL) eliminates synergistic activity and must be tested before relying on this approach.

The dominant resistance mechanism is enzymatic inactivation by aminoglycoside-modifying enzymes (AMEs) that chemically modify the drug, abolishing ribosomal binding. AMEs are encoded on mobile genetic elements (plasmids, transposons, integrons), enabling horizontal transfer among Enterobacteriaceae, Pseudomonas aeruginosa, and Acinetobacter baumannii. Amikacin resists most AMEs, making it the agent of choice when other aminoglycosides have failed.

A second resistance mechanism involves ribosomal RNA methyltransferases that methylate the aminoglycoside binding site on the ribosome, conferring high-level resistance to all aminoglycosides including amikacin. These enzymes are increasingly co-located with carbapenemase genes (including New Delhi metallo-beta-lactamase) on mobile plasmids, creating organisms resistant to essentially all conventional antibiotics. Detection requires genotypic testing.