Studies on drug monitoring in thrice and once daily treatment with aminoglycosides

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.

Once-daily netilmicin for neutropenic pyrexia in paediatric oncology

AIM

To establish the safety and efficacy of single daily intravenous netilmicin 6 mg/kg with piperacillin 100 mg/kg every 8 h for empirical, first-line management of children with neutropenic pyrexia following cytotoxic chemotherapy.

METHODS

Observational study of children admitted to a regional oncology unit from October 1999-April 2002. Primary outcome measure was temperature 72 h after commencing antibiotic therapy; secondary measures were mortality, nephrotoxicity, symptomatic ototoxicity and serum netilmicin levels.

RESULTS

280 episodes for 128 patients (median age 7.1 y) were documented, and 248 episodes were evaluated and compared with a previous cohort of 100 episodes for which the only difference was administration of netilmicin three times daily. Twenty-seven per cent of single-dose netilmicin episodes remained febrile at 72 h compared to 32% in the comparator group (difference -4.7%; 95 % CI: -6.8% to 16.2%; p = 0.41). No patients died and we were unable to find evidence of nephrotoxicity or ototoxicity. Eighty-nine per cent of "peak" serum netilmicin levels measured 30 min after infusion were 10 mg/l or greater, and 94% and 86% measured 12-16 h after the first and third dose, respectively, were 1 mg/l or less. Peak serum netilmicin level measurements and 12-16-h measurements after the first dose were abandoned after the first 180 episodes.

CONCLUSIONS

Netilmicin can safely be given as a single daily dose to children with febrile neutropenia who do not have biochemical evidence of nephrotoxicity. Monitoring peak serum levels of netilmicin is unnecessary. Levels taken 12-16 h after the third dose are adequate to monitor therapy if used in conjunction with a therapeutic guideline detailing the response to abnormal serum creatinine and netilmicin levels.

Evaluation of four once-daily aminoglycoside dosing nomograms

STUDY OBJECTIVE

To evaluate the accuracy of four once-daily aminoglycoside dosing nomograms in producing the desired gentamicin peak concentration (Cmax) target of 20 microg/ml in patients with varying degrees of renal function.

DESIGN

Retrospective analysis using prospectively collected pharmacokinetic data.

SETTING

Rural teaching hospital.

PATIENTS

Ninety patients receiving intravenous gentamicin divided into three groups (30 patients each) determined by estimated renal function: group 1, creatinine clearance (Cl(cr),) 60 ml/minute or greater; group 2, Cl(cr) 40-59 ml/minute; group 3, Cl(cr) 20-39 ml/minute. Intervention. Serum gentamicin concentrations were collected for a 2-point (two consecutive infusions and one predose and one postdose concentration sampled during steady state) or 3-point (single infusion and one predose and two postdose concentrations at least 1.5 estimated half-lives apart) pharmacokinetic study for determination of patient-specific pharmacokinetic parameters (elimination rate constant, volume of distribution at steady state, and clearance) after 30-minute infusions of gentamicin 2.8 +/- 1.6 mg/kg.

MEASUREMENTS AND RESULTS

The four nomograms evaluated were from Hartford Hospital, Barnes-Jewish Hospital, University of Rochester, and the Sanford Guide. With a pharmacokinetic analysis program and the patient-specific pharmacokinetic parameters, Cmax and minimum concentration (Cmin) were determined with use of the recommended doses and dosing intervals of the four nomograms. Also, the gentamicin dose and interval needed to achieve a Cmax and Cmin of 20 microg/ml and 0.2 microg/ml, respectively, were determined. Dosing was based on total body weight unless that weight was more than 25% of ideal body weight, in which case, an adjusted body weight was used. In general, the recommended dosages and resultant Cmax produced by the nomograms were significantly less (p < 0.05) than the dosage and Cmax actually needed to achieve a Cmax:minimum inhibitory concentration (MIC) ratio of 10 or greater for bacteria with an MIC of 2 microg/ml.

CONCLUSION

Once-daily aminoglycoside dosing using the four nomograms resulted in inaccurate dosing, and because of the large variability in human pharmacokinetics, dosing nomograms such as these should be abandoned in favor of individualizing dosages with therapeutic drug monitoring.

The therapeutic monitoring of antimicrobial agents

AIMS

To review the basis and optimal use of therapeutic drug monitoring of antimicrobial agents.

METHODS

Antimicrobial agents for which a reasonable case exists for therapeutic drug monitoring were reviewed under the following headings: pharmacokinetics, why monitor, therapeutic range, individualisation of therapy, sampling times, methods of analysis, interpretative problems and cost-effectiveness of monitoring.

RESULTS

There is a strong historical case for monitoring aminoglycosides. The recent move to once-daily dosing means that criteria for therapeutic drug monitoring need to be redefined. Vancomycin has been monitored routinely but many questions remain about the most appropriate approach to this. A case can be made for monitoring teicoplanin, flucytosine and itraconazole in certain circumstances.

CONCLUSIONS

The approach to monitoring aminoglycosides needs to be redefined in the light of once-daily dosing. It is premature to suggest that less stringent monitoring is necessary as toxicity remains a problem with these drugs. The ideal method of monitoring vancomycin remains to be defined although a reasonable case exists for measuring trough concentrations, mainly to ensure efficacy. Teicoplanin is monitored occasionally to ensure efficacy while flucytosine is monitored occasionally to avoid high concentrations associated with toxicity. Itraconazole has various pharmacokinetic problems and monitoring has been suggested to ensure that adequate concentrations are achieved.

Monitoring of serum aminoglycoside levels with once-daily dosing

There is considerable confusion as to how to monitor serum aminoglycoside levels when using once-daily dosing. At least five methods are in use in Australia. We prospectively assessed 100 consecutive once-daily courses of gentamicin or tobramycin, during which 120 pre-dose and 213 sets of immediate post-dose and six hour post-dose levels were taken. By using the six hour post-dose level we were able to compare dosage recommendations made using methods known as ALADDIN, DOSECALC and the Australian Antibiotic Guidelines nomogram (AAGN). There were statistically significant differences in the doses recommended by each method. When comparing each of the three methods, at least 25% of dosage recommendations differed by more than 80 mg per dose. Although we have not been able to determine the clinical significance of these differences, we are concerned that methods used in dosage adjustment of aminoglycosides differ so widely in their recommendations. Presumably the ALADDIN method, which utilises two post-dose levels to determine an area under aminoglycoside concentration-time curve, gives more accurate pharmacokinetic information than methods which rely on a single level. Comparative cost-effectiveness studies of different methods, although in practice difficult to perform, should be undertaken to resolve the optimal management of patients receiving aminoglycosides once-daily.

Laboratory guidelines for monitoring of antimicrobial drugs

Few antimicrobial drugs meet the requirements for therapeutic drug monitoring. Those that are monitored include the aminoglycosides (gentamicin, tobramycin, and amikacin), chloramphenicol, and in some cases, vancomycin. For these drugs, there is evidence of a relationship between serum concentration, efficacy, and/or the incidence of adverse or toxic events. Monitoring begins with the appropriate timing of collection and continues through the analytical process to the integration of all data used to guide the clinician’s next decision.

Pharmacokinetics of gentamicin at traditional versus high doses: implications for once-daily aminoglycoside dosing

Two doses of gentamicin (2 and 7 mg/kg of body weight) were administered to 11 healthy volunteers in a randomized, crossover single-dose study to compare their pharmacokinetics. Doses were infused over 1 h with a syringe infusion pump, and 14 concentrations in sera were obtained over an 8-h period. Concentration in serum versus time data were fitted to a two-compartment pharmacokinetic model. In addition, to mimic the clinical setting, subjects' data were fitted by the Sawchuk-Zaske method. Distributional and postdistributional peak concentrations, along with the last obtained concentration in serum, were utilized to compare the following pharmacokinetic variables: volume of distribution at steady state (Vss), half-life, clearance (CL), and maximum concentration in serum (Cmax). With two-compartment pharmacokinetic fitting, significant differences in distribution half-life (average, 21.8 and 41.6 min [P < or = 0.05]) and gentamicin CL (76.6 +/- 6.6 and 67.2 +/- 4.2 ml/min/1.73 m2 [P < or = 0.001]) were found between traditional-dose and high-dose groups, respectively. When the data for concentrations in sera were fitted to a one-compartment pharmacokinetic model by using either the distributional or the postdistributional Cmax, statistically significant differences (P < or = 0.001) were found between Vss, half-life, CL, and Cmax values for both dosage groups. The results show that the pharmacokinetics of gentamicin at a large dose differ significantly from those at the traditional dose. This information has direct implications for once-daily aminoglycoside (ODA) literature when the Cmax values reported are distributional and therefore show falsely high Cmax/MIC ratio estimates. In addition, ODA nomogram dosing tools developed with distributional Cmax values are probably inaccurate.

Once-daily dosing of aminoglycosides

Since their introduction one or more decades ago, aminoglycosides have generally been administered in multiple daily (i.e. twice- or thrice-daily) dosing regimens. However, nephrotoxicity can be reduced in animal models by administering the same total daily dose as one large dose instead of as multiple small doses. In addition, in vitro and in vivo studies that considered the impact of dosing regimens on efficacy suggest that once-daily dosing is equally or more effective compared to multiple daily dosing. Once-daily versus multiple daily dosing regimens have been compared for amikacin, netilmicin, and gentamicin in 24 randomized, clinical trials including a total of 3,181 patients. An analysis of these studies revealed superior results for once-daily regimens with respect to clinical efficacy (89.5% vs. 84.7%, p < 0.001) as well as bacteriological efficacy (88.6% vs. 83.4%, p < 0.01). No statistically significant differences were noted for toxicity. Nevertheless, both nephrotoxicity and ototoxicity occurred less frequently during once-daily dosing (4.5% vs. 5.5% and 4.2% vs. 5.8%, respectively). Finally, once-daily dosing is more economical, since less nursing time and infusion material are required and the efforts for drug monitoring can be reduced. In conclusion, amikacin, netilmicin, and gentamicin can be administered once a day.

Pharmacokinetics of once-daily dosing of gentamicin in surgical intensive care unit patients with open fractures

OBJECTIVE

To compare the first-dose pharmacokinetic parameters of gentamicin 6 mg/kg and 2 mg/kg in stable, nonobese surgical intensive care unit patients with open extremity fractures receiving gentamicin prophylactically.

METHODS

Serial blood samples were obtained over 8 or 24 hours following the first dose of gentamicin. Serum concentrations of gentamicin were measured using fluorescence polarization immunoassay and analyzed by noncompartmental means.

RESULTS

Eleven patients were enrolled, 7 in the 6 mg/kg group and 4 in the 2 mg/kg group. The median (6 vs. 2 mg/kg) age was 29 versus 28 years; serum creatinine 80 versus 88 mumol/L; and APACHE II score 13 versus 10. The mean +/- SD (micrograms/mL) of concentration at the end of the 30-minute infusion (Cmax), concentration 30 minutes after the end of the infusion (Cpk), and concentration at the end of the dosing interval for 6 versus 2 mg/kg were: 35.0 +/- 19.0 versus 10.1 +/- 1.77; 17.0 +/- 2.7 versus 5.4 +/- 0.4, and 0.45 +/- 0.31 versus 0.69 +/- 0.11, respectively. Area under the curve0-infinity (AUC0-infinity), apparent volume of distribution, and half-life were: 89.0 +/- 28.9 versus 26.1 +/- 1.2 mg.h/L, 0.40 +/- 0.10 versus 0.47 +/- 0.14 L/kg, and 4.0 +/- 1.1 versus 4.3 +/- 1.5 h, respectively.

CONCLUSIONS

The first-dose pharmacokinetics of gentamicin 6 mg/kg resulted in a proportional rise in Cmax, Cpk, and AUC0-infinity compared with gentamicin 2 mg/kg in patients with open fractures, but with greater variability.

What is the evidence for once-daily aminoglycoside therapy?

Aminoglycosides are important antibacterial agents for the treatment of serious infection. Evidence suggests that high peak plasma concentrations must be achieved early in the course of treatment if these agents are to be effective, but prolonged high concentrations may cause ototoxicity and nephrotoxicity. Peak plasma concentrations of 6 to 10 mg/L and trough concentrations of less than 2 mg/L for gentamicin and tobramycin have been traditional goals of therapy. Extensive recent evidence from in vitro, animal and human studies suggests that these target concentrations need revision. Aminoglycosides display concentration-dependent bacterial killing, have a long postantibiotic effect, and induce adaptive resistance in Gram-negative bacteria. All of these factors support the use of larger doses of aminoglycosides that are given less frequently than conventional therapy. Studies in vitro support this approach, showing greater activity when aminoglycosides are given less frequently. Animal studies comparing different dosage intervals have shown varying results, with only a slight bias favouring the longer dosage interval. However, the short elimination half-lives for the drugs in animals limit the applicability of these models to humans. Importantly, there is convincing evidence in animal studies that nephrotoxicity and ototoxicity are both reduced when the same total daily dose is administered in less frequent doses. There have been at least 29 clinical trials comparing once-daily administration of aminoglycosides with conventional administration 2 to 4 times daily. In general, efficacy has not been shown to be different between regimens, although one trial showed an advantage for once-daily administration compared with administration 3 times daily. A small number of trials have shown less nephrotoxicity and ototoxicity with once-daily administration, leading several authors to suggest that there is sufficient evidence to warrant a change to once-daily administration of aminoglycosides. However, once-daily administration has not been well studied in the paediatric population, or in patients with renal failure or endocarditis, and cannot be recommended in these patients as yet. The choice of a 24-hour dosage interval is somewhat arbitrary, and the optimal interval may not necessarily be 24 hours. No studies have included dosage adjustment based on pharmacokinetic modelling methods, and the effect of this on treatment outcome needs to be assessed. The best method of administering aminoglycosides once daily is yet to be determined.