The aminoglycosides are the mainstay in the treatment of serious gram-negative systemic infections. A disadvantage of the aminoglycosides is their association with nephrotoxicity and ototoxicity, both of which are associated with elevated trough levels and sustained elevated peak levels.
Aminoglycosides have bactericidal activity against most gram-negative bacteria including Acinetobacter, Citrobacter, Enterobacter, E. Coli, Klebsiella, Proteus, Providencia, Pseudomanas, Salmonella, Serratia and Shigella. The MIC's of gram negative bacteria are usually less than 2 mcg/ml for gentamicin and tobramycin and 8 mcg/ml for amikacin.
Aminoglycosides are active against most strains of Staphylococcus aureus and S. epidermidis. Most strains of enterococcus are resistant to aminoglycosides alone, however when used in combination with penicillins they are often effective in enterococcal endocarditis due to synergistic antimicrobial mechanisms. Anaerobic bacteria are universally resistant because aminoglycoside transport into cells is oxygen-dependent.
Aminoglycoside nephrotoxicity manifests clinically as nonoliguric renal failure, with a slow rise in serum creatinine and a hypoosmolar urinary output developing after several days of therapy. The reported incidence of nephrotoxicity varies substantially between studies, averaging 6% to 10%. Nephrotoxicity rates do not vary significantly among the different aminoglycosides. Factors associated with nephrotoxicity include duration of treatment, increasing age, compromised renal function, volume depletion, elevated peak and trough levels, concurrent nephrotoxic drugs (i.e., vancomycin) and previous exposure to aminoglycosides.
Aminoglycosides can cause permanent vestibular and/or auditory ototoxicity. Overt otoxicity occurs in 2% to 10% of patients treated with aminoglycosides. Factors associated with otoxicity include increasing age, duration of therapy, elevated peak and trough levels, concurrent loop diuretics or vancomycin, underlying disease states and previous exposure to aminoglycosides.
Vestibulotoxicity is difficult to diagnose and there is no reliable monitoring process. Recent studies indicate a genetic predisposition to aminoglycoside auditory ototoxicity due to a mutation of mitochondrial DNA.23,24 However, this genetic component does not appear to influence aminoglycoside vestibular ototoxicity. Gentamicin toxicity is the most common single known cause of bilateral vestibulopathy, accounting for 15-50% of all cases.25 A web site, Wobblers Anonymous presents personal testimony from patients who have suffered from this disabling ADE.
For gentamicin, tobramycin and netilmicin, the risk of ototoxicity and nephrotoxicity is increased if peak levels are consistently maintained above 12 to 14 mcg/ml or trough levels consistently exceed 2 mcg/ml. For amikacin, peak levels above 32 to 34 mcg/ml or trough levels greater than 10 mcg/ml have been associated with a higher risk of ototoxicity and nephrotoxicity.
The pharmacodynamic properties of aminoglycosides are:
Aminoglycosides eliminate bacteria quickest when their concentration is appreciably above the MIC for an organism, this is referred to as concentration dependent activity. The aminoglycosides also exhibit a significant post-antibiotic effect (PAE). PAE is the persistent suppression of bacterial growth following antibiotic exposure. Practically speaking this means that trough levels can drop below the MIC of targeted bacteria for a sustained period without decreasing efficacy.
For AG's the ideal dosing regimen would maximize concentration, because the higher the concentration, the more extensive and the faster is the degree of bacteriocide. Therefore, the Peak/MIC ratio is an important predictor of efficacy. It has been shown that aminoglycosides eradicate bacteria best when they achieve a Peak/MIC ratio of at least 8-10. Therefore it is important to give a large enough dose to produce a peak level 8 to 10 times greater than the MIC.
Aminoglycoside Pharmacodynamics in Vivo
|Initial serum peak level||Died||Survived|
When given by IV infusion over 30 minutes, aminoglycosides follow a 3-compartment pharmacokinetic model; alpha (distribution), ß (elimination), and gamma (tissue release). When infused over one hour, the distribution phase is usually not observed. The gamma phase begins approximately sixteen hours post infusion, drug that was tissue bound to various organs is released. The amount released from tissue is very small, but does accumulate over time, contributing to AG toxicity.
Although this model accurately represents the time course of AG serum levels, it cannot be used clinically because of its complexity. Therefore, the simpler one compartment model is widely used, and does, in fact, accurately predict serum AG levels.
Volume of distribution
The average Vd of AG's in otherwise healthy adults is 0.26 L/kg (range: 0.2-0.3). Although AG's do not distribute into adipose tissue, they do enter the extracellular fluid contained therein. Therefore, obese patients require a correction in the weight used for Vd calculation: LBW + 40% of weight above LBW. Patients with cystic fibrosis have a markedly increased Vd of 0.35 L/kg due to increases in extracellular fluid brought about by the disease process. Patients with ascites have additional extracellular fluid because of accumulation of ascitic fluid, which increases the Vd to approximately 0.32 L/kg. ICU patients may have a Vd 25-50% above normal.
AG elimination is closely correlated with creatinine clearance, the average value for the slope is between 0.0024 and 0.0029 and the y-intercept is typically 0.01 to 0.015. Cystic fibrosis patients show a 50% increase in elimination rate. A major body burn increases the basal metabolic rate resulting in a marked increase in AG elimination. ICU patients are often hyper metabolic and therefore eliminate AG's more rapidly.
Achieving therapeutic serum levels of aminoglycosides early in the course of treatment is critical to therapeutic success. Dosing error on the high side is preferable to the risks of under-treatment. An adequate loading dose is critical for rapid attainment of therapeutic peak levels.
The method of Sarrubi and Hull4 utilizes serum creatinine, lean body weight, age, and sex to estimate creatinine clearance. This method considers more patient variables, which may improve the estimation of aminoglycoside elimination. Lesar et al found that the Sarubbi and Hull nomogram achieved therapeutic concentrations in 78% of patients.9 Tsubaki and Chandler evaluated 5 methods for determining initial dosing requirements for gentamicin. They concluded that the Sarubbi and Hull method was the most accurate.22 However, dosing nomograms are initial guidelines only. They can produce substantial variations in serum concentrations and should be subsequently adjusted based on serum level determinations and clinical response.
Dosage regimens necessary to achieve therapeutic aminoglycoside serum concentrations can be quantitatively determined by using simple pharmacokinetic principles. Individualized pharmacokinetic parameters are determined from the patient's serum concentration versus time data. Sawchuk and Zaske have described a method for establishing multiple infusion regimens based on individually calculated pharmacokinetic parameters.3 Lesar, et al found that this individualized method achieved therapeutic concentrations in 90% of patients.9
For evaluation of serum level data, methods incorporating Bayesian principles appear to give the best overall predictive performance compared with traditional methods of vancomycin dosage adjustment. The Bayesian approach combines both population and patient-specific information (i.e., serum level data) in predicting dosage requirements.
Extended-interval aminoglycoside dosing has gained popularity in recent years. This simplified dosing method is appropriate in young, otherwise healthy patients with sepsis. However, there are many patient groups who are not candidates for this dosing methodology: the elderly, CrCl less than 30, dialysis, pregnancy, endocarditis, cystic fibrosis, ascites, neutropenia, infants, 20% or greater BSA burns, history of hearing loss or vestibular dysfunction, gram positive infections (when aminoglycoside is used for synergy), or mycobacterial infections.
The trough sample should be obtained 30 minutes prior to the dose. Measure the peak level 15 to 30 minutes after completion of the IV infusion to avoid the distributive phase. Measure the peak level 90 minutes after an IM injection. Drawing the peak too soon will result in inaccurate analysis.
Drawing at exactly the right time is not as important as having the lab note the exact times that the samples were drawn. Also, have the nurse note the exact times that the sample infusion was started and when it ended. Please be aware of the widespread policy of nursing personnel to record a dose as having been given exactly as ordered if it is given within 30 minutes of the recorded time. This will lead to significant errors in analysis, have everyone record the exact times.
This issue cannot be stressed enough. Inaccurate recording of drug administration times and lab draw times are the greatest source of calculation error, having a greater effect than pharmacy preparation error or lab assay error.
In general the Bayesian approach to the determination of individual drug-dosage requirements performs better than other approaches. However, outlying patients in a population (ie, those patients whose pharmacokinetic parameters lie outside of the 95th percentile of the population) may be put at risk. As is always the case, the computerized algorithms outlined below can only assist in the decision-making process and should never become a substitute for rational thought or informed judgement.
Before calculating an initial dose or analyzing serum level data, enter the target peak and trough levels and the standard length of infusion.
The program supports six different serum level analysis methods for the one-compartment model:
The aminoglycoside model is not hard-coded into the program. The parameters are found in the drug model database and are fully user-editable. You can tailor each drug model to fit your patient population, or you can create your own models.
An initial dose, prior to serum level measurement, is based solely on the population model As stated above, the pk models supplied with the program may be edited, also multiple models of the same drug may be added to reflect different patient populations.
Patient specific pharmacokinetic parameters are calculated using the proven pharmacokinetic method of Sawchuk and Zaske.3
The Bayesian method uses population-derived pharmacokinetic parameters (ie., Vd and kel) as a starting point and then adjusts those parameters based on the serum level results, taking into consideration the variability of the population-derived parameters and the variability of the drug assay procedure. To achieve that end, the least squares method based on the Bayesian algorithm estimates the parameters which minimize the following function:11
|60 and above||24 hours|
|40 to 59||36 hours|
|30 to 39||48 hours|
|Less than 30||Use traditional dosing method|
The three interval break points on the Hartford interval adjustment nomogram (illustrated below) are the approximate decay curves from a 7mg/kg gentamicin dose. These decay curves were calculated using a one compartment model with a volume of distribution of 0.25 L/kg and an elimination rate calculated from creatinine clearances of 25, 40, and 60 ml/min for 48, 36, and 24 hour intervals respectively. The authors of the Hartford nomogram then flattened these decay curves to simplify the nomogram. To use the Hartford nomogram for 15mg/kg doses of amikacin, multiply the drug-level scale by a factor of two. 5
It is important to note that the Hartford interval adjustment nomogram is only valid for a 7mg/kg dose. A nomogram for the less aggressive dose of 5mg/kg was developed by a consensus panel (illustrated below). The consensus panel argues that the 48 hour interval should be abandoned and that patients with a CrCl less than 40ml/min should be dosed by traditional pharmacokinetic methods. To use the consensus nomogram for 15mg/kg doses of amikacin, multiply the drug-level scale by a factor of three. 20
The consensus panel also suggests that younger patients with excellent renal function may require Q 12 hour dosing. A 5mg/kg dosing algorithm for this subpopulation has been proposed by Urban and Craig (illustrated below). To use the Urban and Craig nomogram for 15mg/kg doses of amikacin, multiply the drug-level scale by a factor of three.21
Some have questioned the validity of all ODA nomograms because they are based on one-compartment parameters derived from traditional dosing methods. Some pk studies have shown that the pharmacokinetics of aminoglycosides at high doses differ significantly from those at traditional doses. Therefore, it is argued that nomograms based on an assumption of similar kinetics are invalid. 22