Aminoglycoside dosage regimens after therapeutic drug monitoring

Evaluation of a Once/Day Tobramycin Regimen to Achieve Target Concentrations in Adult Patients with Cystic Fibrosis

STUDY OBJECTIVE

To evaluate the success of an initial tobramycin dosing regimen to achieve target peak and trough concentrations in adult patients with pulmonary exacerbations of cystic fibrosis (CF).

DESIGN

Retrospective single-center medical record review.

SETTING

Large tertiary care academic medical center.

PATIENTS

A total of 186 patient encounters where 112 patients with CF were treated for acute pulmonary exacerbations with 10 mg/kg/day of tobramycin between January 1, 2009, and December 5, 2014.

MEASUREMENTS AND MAIN RESULTS

Baseline demographics, clinical characteristics, tobramycin data, and pharmacokinetic variables were collected. The primary outcome evaluated the success of the initial tobramycin dosing regimen in attaining the target peak concentration. Secondary end points were achievement of the target trough concentration, achievement of combined peak and trough targets, and incidence of nephrotoxicity. Bivariate and multivariate analyses were performed to evaluate factors associated with achieving target concentrations. Of the 186 patient encounters, 41% achieved the target peak with the first dosing regimen, 62% achieved a target trough, and 23% achieved the target peak and trough. Nephrotoxicity occurred in 10% of patient encounters. A body mass index (BMI) of 18.5-24.9 kg/m(2) was associated with higher odds of meeting the target peak compared with a BMI lower than 18.5 kg/m(2) (odds ratio [OR] 24.5; 95% confidence interval [CI] 5.2-117.2). Conversely, a BMI of 18.5-24.9 kg/m(2) was associated with lower odds of attaining the target trough compared with a BMI lower than 18.5 kg/m(2) (OR 0.16; 95% CI 0.05-0.56). Higher volume of distribution and elimination rate constants (Kel ) were associated with significantly lower odds of achieving the target peak. In addition, higher Kel values were associated with significantly higher odds of achieving the target trough.

CONCLUSIONS

The current initial tobramycin regimen did not achieve target serum tobramycin concentrations reliably. Optimization of the initial CF tobramycin dosing regimen is warranted.

Therapeutic drug monitoring of once daily tobramycin in cystic fibrosis—caution with trough concentrations

BACKGROUND

: Once daily intravenous aminoglycoside dosing (ODD) is widely used to treat acute Pseudomonas aeruginosa exacerbations in patients with cystic fibrosis. Controversy exists as to what is the most appropriate method of therapeutic drug monitoring (TDM) of such therapy with recommendations including trough plasma concentrations of <1 mg/L or <2 mg/L, area under curve (AUC) and various nomograms. This study aimed to compare the exposures to ODD of tobramycin in adults and children with cystic fibrosis using the AUC and trough TDM approaches.

METHODS

: Using a mono-exponential software program to calculate AUC from 2 plasma concentrations, AUCs were determined in 22 adults with pre-dose tobramycin concentrations <1 mg/L. The exposure of 5 children with reduced tobramycin clearances was simulated at the usual recommended dose of 10 mg/kg/daily but retaining a trough <1 mg/L.

RESULTS

: A tobramycin dose of 10 mg/kg of tobramycin in these patients with normal serum creatinine and a trough concentration <1 mg/L resulted in exposures in excess of those associated with conventional 8-hourly dosing.

CONCLUSIONS

: The TDM approach of a trough <1 mg/L, as used with conventional 8-hourly tobramycin dosing, is not relevant to ODD.

Use of pharmacodynamic principles to optimise dosage regimens for antibacterial agents in the elderly

Throughout most of the world we are witnessing an ever increasing number of aged people as a percentage of the general population. In the coming years, the unique spectrum of infections presented by an elderly population, particularly those in long-term care facilities, will challenge our ability to maintain an effective battery of antibacterials. The pharmacokinetic parameters of most antibacterial agents are altered when assessed in the elderly due in part to non-pathological physiological changes. The inability to clear a drug from the body due to declining lung, kidney/bladder, gastrointestinal and circulatory efficiency can cause accumulation in the body of drugs given in standard dosages. While this may have the potential benefit of achieving therapeutic concentrations at a lower dose, there is also a heightened risk of attaining toxic drug concentrations and an increased chance of unfavourable interactions with other medications. Pharmacodynamic issues in the elderly are related to problems that arise from treating elderly patients who may have a history of previous antibacterial treatment and exposure to resistant organisms from multiple hospitalisations. Furthermore, the elderly often acquire infections in tandem with other common disease states such as diabetes mellitus and heart disease. Thus, it is essential that optimised dosage strategies be designed specifically for this population using pharmacodynamic principles that take into account the unique circumstances of the elderly. Rational and effective dosage and administration strategies based on pharmacodynamic breakpoints and detailed understanding of the pharmacokinetics of antibacterials in the elderly increase the chances of achieving complete eradication of an infection in a timely manner. In addition, this strategy helps prevent selection of drug-resistant bacteria and minimises the toxic effects of antibacterial therapy in the elderly patient.

Quantitative justification for target concentration intervention--parameter variability and predictive performance using population pharmacokinetic models for aminoglycosides

AIMS

[1] To quantify the random and predictable components of variability for aminoglycoside clearance and volume of distribution [2] To investigate models for predicting aminoglycoside clearance in patients with low serum creatinine concentrations [3] To evaluate the predictive performance of initial dosing strategies for achieving an aminoglycoside target concentration.

METHODS

Aminoglycoside demographic, dosing and concentration data were collected from 697 adult patients (> or =20 years old) as part of standard clinical care using a target concentration intervention approach for dose individualization. It was assumed that aminoglycoside clearance had a renal and a nonrenal component, with the renal component being linearly related to predicted creatinine clearance.

RESULTS

A two compartment pharmacokinetic model best described the aminoglycoside data. The addition of weight, age, sex and serum creatinine as covariates reduced the random component of between subject variability (BSVR) in clearance (CL) from 94% to 36% of population parameter variability (PPV). The final pharmacokinetic parameter estimates for the model with the best predictive performance were: CL, 4.7 l h(-1) 70 kg(-1); intercompartmental clearance (CLic), 1 l h(-1) 70 kg(-1); volume of central compartment (V1), 19.5 l 70 kg(-1); volume of peripheral compartment (V2) 11.2 l 70 kg(-1).

CONCLUSIONS

Using a fixed dose of aminoglycoside will achieve 35% of typical patients within 80-125% of a required dose. Covariate guided predictions increase this up to 61%. However, because we have shown that random within subject variability (WSVR) in clearance is less than safe and effective variability (SEV), target concentration intervention can potentially achieve safe and effective doses in 90% of patients.

Effect of once-daily dosing vs. multiple daily dosing of tobramycin on enzyme markers of nephrotoxicity

OBJECTIVE

To determine the incidence of nephrotoxicity of once-daily dosing (ODD) and multiple daily dosing (MDD) regimens of tobramycin in critically ill patients.

DESIGN

Randomized, prospective clinical trial.

SETTING

: Adult intensive care units at two university hospitals.

PATIENTS

Fifty-eight critically ill patients with a suspected or documented aerobic Gram-negative infection.

INTERVENTIONS

Patients were randomized to receive tobramycin by ODD (7 mg/kg) or MDD. Baseline urine aliquots and 24-hr urine collections were collected on days 3, 7, and 11 during therapy and on days 3, 7, and 11 following discontinuation of therapy for measurement of alanine aminopeptidase (AAP), N-acetyl-beta-d-glucosaminidase (NAG), and creatinine.

MEASUREMENTS AND MAIN RESULTS

Fifty-four patients were evaluable (ODD n = 25; MDD n = 29). The groups were similar with regard to demographic and clinical variables. The tobramycin dose was higher in the ODD group compared with the MDD group (425 +/- 122.5 mg vs. 312.8 +/- 116.6 mg, p <.001). Patients in the MDD group received a mean of 3.89 +/- 1.14 mg.kg(-1)day(-1) at intervals of 11.92 +/- 3.12 hrs. In the ODD group, patients had a higher measured creatinine clearance at the end of therapy compared with MDD group (70 +/- 18.6 vs. 64.8 +/- 17.5 mL/min, p =.047). Fewer patients in the ODD group developed nephrotoxicity than the MDD group (5 vs. 12, p =.142). Although there were increases in urinary enzymes in both treatment groups (AAP, 8.7 +/- 2.9 vs. 5.2 +/- 2.1 units/24 hrs, p <.01 MDD vs. ODD; NAG, 14.7 +/- 4.9 vs. 6.8 +/- 3.1, p <.01 MDD vs. ODD), the increases in the ODD group were significantly lower than in the MDD group.

CONCLUSIONS

: The ODD tobramycin regimen appeared to be less nephrotoxic than the MDD regimen despite significantly higher doses. Tobramycin administered by ODD may be the preferred dosing method in selected critically ill medical patients to reduce the incidence and extent of renal damage.