Once-daily aminoglycoside therapy

The economic potential of dual individualisation methodologies

Cost-effective treatment of patients with bacterial infections can best be accomplished by facilitating a rapid response. Achieving a more rapid cure of the infection should result in reduced utilisation of healthcare resources. An innovative means of achieving a more rapid response to antibacterial therapy has been termed dual individualisation. Dual individualisation allows for the simultaneous consideration of the pharmacokinetic interaction between an antibacterial agent and a patient, with the pharmacodynamic interaction between the antibacterial agent and the bacterial pathogen. Integrating the pharmacokinetic parameter of area under the curve (AUC), with the pharmacodynamic measure of minimum inhibitory concentration (MIC) yields a ratio called the area under the inhibitory curve (AUIC). Clinical studies using dual individualisation to achieve a target AUIC24h of 125-250 have been performed with cephalosporins and fluoroquinalones. Economic evaluation of the results demonstrate that dual individualisation is cost-effective. Dual individualisation can be implemented in most practice setting using existing clinical data. Use of a computer model allows for cost-effective calculation of AUIC24h without having to measure serum drug concentrations of bacterial MIC. By adjusting antibacterial regimens to achieve a target AUIC, optimisation of antibacterial therapy can be achieved with resultant economic benefits.

Rational prescribing of antibacterials in hospitalised children

The subject of this review is the rational prescribing of antimicrobial agents for the therapy of serious community-acquired infections in hospitalised infants and children. First, cost-containment strategies such as streamlining of antibacterial therapy, outpatient parenteral antibacterial therapy and sequential ('stepdown') therapy with parenteral followed by oral therapy are reviewed. In most of these areas, paediatric studies are scant or lacking. Then specific paediatric aspects of the choice of parenteral antibacterials such as penicillins, cephalosporins, aminoglycosides, macrolides and other agents are discussed. With particular reference to cost containment, rational treatment strategies for some serious bacterial infections such as meningitis, occult bacteraemia, endocarditis, osteomyelitis, arthritis, pyelonephritis, Lyme borreliosis (advanced stages) and pneumonia are proposed. In most of these disease, there is potential for cutting treatment costs and studies that compare these newer strategies with traditional treatment regimens are urgently needed.

Efficacy and safety of once-daily amikacin in combination with ceftazidime in critically ill adults with severe gram-negative infections

Forty critically ill adult patients with severe Gram-negative infection were treated with once-daily amikacin combined with ceftazidime. The mean age was 56.6 +/- 19 years and mean APACHE II score was 22.7 +/- 6.6. Forty percent of patients required mechanical ventilation. The mean creatinine clearance at onset of therapy was 59.4 +/- 28 ml/min. All bacterial isolates were sensitive to amikacin. Fixed doses of amikacin 15 mg/kg, 12 mg/kg, and 8 mg/kg body weight were given once daily to patients with estimated creatinine clearance of > 80 ml/min., 50-80 ml/min., and < 50 ml/min, respectively. Forty-two causative gram-negative bacteria were isolated from 40 patients. The most common bacteria were Pseudomonas aeruginosa (18), and Escherichia coli (10). Overall clinical success and bacteriological eradication occurred in 85% and 87.5% of patients; 78.9% and 79% of patients with hospital-acquired infections; 90.5% and 95.2% of patients with community-acquired infections; and 62.5% and 81.3% of patients requiring mechanical ventilation, respectively. Therapeutic failure was documented in 15% of patients. Death due to infection was scored in two patients. The remaining were all due to persistence of the initial causative bacteria in patients with hospital-acquired infections. Persistence was documented with Ps. aeruginosa (2), Serratia spp. (1), and Acinetobacter spp. (1). Overall mortality occurred in 22.5% patients. Death unrelated to infection occurred in 7 patients. There was no clinical evidence of ototoxicity in any of our patients, however, nephrotoxicity was documented in 5%. In conclusion, once-daily amikacin combined with ceftazidime is practical, efficacious and probably safe in critically ill infected patients.

Postantibiotic effect of ceftriaxone and gentamicin alone and in combination on Klebsiella pneumoniae, Pseudomonas aeruginosa and Streptococcus viridans

A persistent suppression of bacterial growth following limited exposure to an antimicrobial agent, the postantibiotic effect (PAE), has been described for a variety of antibiotics and microorganisms. In this study the PAE of ceftriaxone and gentamicin was determined in vitro on three strains each of Klebsiella pneumoniae, Pseudomonas aeruginosa and Streptococcus viridans. The strains were exposed to the substances for 2 h at varying concentrations. Ceftriaxone was used at the minimal inhibitory concentration (MIC) and 1/2 MIC and gentamicin at 1/2 MIC, 1/4 MIC, and 1/8 MIC, each alone and in combination. Antibiotic concentrations were reduced by 1,000-fold dilution, bacterial regrowth was consequently monitored by viable count. The PAE of ceftriaxone alone reached up to 145 min (MIC) and 50 min (1/2 MIC), that of gentamicin alone up to 170 min (1/2 MIC), 135 min (1/4 MIC) and 70 min (1/8 MIC), depending on the bacterial species. Combinations of the antibiotics produced longer PAEs than one substance alone; the longest PAE was produced by the combination of ceftriaxone (MIC) and gentamicin (1/2 MIC) lasting up to 320 min (S. viridans). It may be important to take the PAE into account when evaluating dosing intervals.

Tolerance, pharmacokinetics and efficacy of once daily amikacin for treatment of Pseudomonas aeruginosa pulmonary exacerbations in cystic fibrosis patients

Twenty cystic fibrosis patients aged 1.8-22 years (mean +/- SD: 9.6 +/- 4.8 years) with Pseudomonas aeruginosa pulmonary exacerbations were treated with amikacin (AM) (35 mg/kg/day in one daily 30 min infusion) associated with either ceftazidime (200 mg/kg/day in 3 i.v. injections) (n = 19) or imipenem (n = 1) at the same dose. Glomerular and tubular functions (creatinine clearance, 24-h proteinuria, beta 2 microglobulinuria, lysozymuria) and audiometry remained within normal ranges from day 0 to day 14. A peak concentration of AM of 83 +/- 19 mg/l and a trough concentration of 0.8 +/- 0.5 mg/l were observed in blood while AM levels in sputum were above the minimal inhibitory concentration 50 from 30 min to 16 h. No serum accumulation of AM was observed during the treatment. From day 0 to day 14, the following changes were observed: weight/height ratio: 96%-100% (P < 0.001); daily energy intake: 111%-128% of RDA (P < 0.001); prealbumin: 195-290 mg/l (P < 0.001); forced vital capacity (FVC): 66%-81% (P < 0.01); forced expiratory volume in 1 s: 60%-75% (P < 0.01); forced expiratory flow between 25% and 75% of FVC: 42%-56% (P < 0.01); nocturnal SaO2 also improved significantly; cardiac rate decreased from 89 +/- 18/min to 76 +/- 16/min (P < 0.001); respiratory rate decreased from 31 +/- 15/min to 26 +/- 10/min (P < 0.05); inflammatory parameters (white blood cells, polymorphonuclear cells, erythrocyte sedimentation rate) also improved.

CONCLUSION

Once daily amikacin administration associated with ceftazidime is well tolerated for the treatment of Pseudomonas aeruginosa pulmonary exacerbations in cystic fibrosis patients. Serum peak levels and diffusion in sputum are higher than with a conventional schedule.

The economic impact of once-daily versus conventional administration of gentamicin and tobramycin

This retrospective, observational study was designed to compare once-daily with conventional aminoglycoside administration for costs while determining equivalency in efficacy and toxicity. 100 consecutive patients who had been treated with once-daily aminoglycosides after 1st August 1993, were evaluated via retrospective chart review. For comparison, 100 consecutive patients who were treated with conventional regimens of aminoglycosides, over the same calender period 1 year earlier (beginning on 1st August 1992), were evaluated in a similar manner. Aminoglycoside antibacterials, excluding amikacin, were administered as a single daily dose of 6 mg/kg. 89 patients were cured or improved with once-daily administration versus 90 patients with conventional administration. One patient in each group developed definite aminoglycoside-induced renal toxicity. The total cost [in 1993 Canadian dollars ($Can)] per patient for once-daily and conventionally administered aminoglycosides was $Can97.62 and $ Can199.43, respectively. Thus, once-daily administration of aminoglycosides is as effective and well tolerated, while considerably less expensive than, aminoglycoside treatment utilising conventional regimens.

Synergy between amoxicillin and gentamicin in combination against a highly penicillin-resistant and -tolerant strain of Streptococcus pneumoniae in a mouse pneumonia model

In vivo synergy with beta-lactam antibiotics and aminoglycosides has been studied only with penicillin-susceptible Streptococcus pneumoniae strains. We evaluated the interaction between amoxicillin (AMX) and gentamicin (GEN) on the basis of in vitro checkerboard and time-kill curves and of findings in a mouse model of acute bacteremic pneumonia due to a highly penicillin-resistant and -tolerant S. pneumoniae strain of serotype 19 (penicillin and AMX MICs of 4 micrograms/ml; gentamicin MIC of 16 micrograms/ml). Checkerboard results at 18 h of incubation showed indifference. With regard to AMX alone, in vitro time-kill curves demonstrated synergy between AMX (1 microgram/ml) and GEN (16 micrograms/ml) at 5 and 8 h of incubation and for AMX (16 micrograms/ml) in combination with GEN (16 micrograms/ml) at 3, 5, and 8 h of incubation. In leukopenic mice, pulmonary killing curves after a single drug injection demonstrated that AMX (100 mg/kg of body weight) with GEN (16 mg/kg) was more effective than AMX alone (P = 10(-4). With repeated-dose treatment, a synergy was apparent at 8 h after four injections with AMX (100 mg/kg) in combination with GEN (8 or 16 mg/kg) (P < or = 0.05). The cumulative survival rate with AMX (100 mg/kg) every 8 h, combined with GEN (4 or 8 mg/kg) every 8, 12, or 24 h, was better than with AMX alone. Combined use of AMX and GEN may be a valuable therapeutic alternative for pneumococcal pneumonia due to highly penicillin-resistant S. pneumoniae strains.

Once-daily aminoglycoside dosing: impact on requests and costs for therapeutic drug monitoring

Recently, much interest has focused on the use of once-daily aminoglycosides (ODA) in the medical literature. In late 1992, we implemented a hospital-wide ODA program for adult patients at our 850-bed community-teaching hospital. In the first phase of implementation, therapeutic drug monitoring (TDM) was accomplished with the use of a random serum concentration and a nomogram that had been developed at our institution. In the second phase, serum drug concentrations were eliminated on patients with normal renal function. The fully implemented program resulted in a 40% decrease in the request for gentamicin and tobramycin serum concentrations as compared with historic ordering patterns for conventional aminoglycoside dosing regimens. In addition, the incidence of nephrotoxicity was also reduced from 3 to 5% with conventional aminoglycoside dosing, to 1.2 and 1.3% for phases 1 and 2, respectively. Furthermore, the elimination of TDM requests totaling 300 for gentamicin and 50 for tobramycin per month is expected to result in an annual institutional savings of > $100,000.

Pharmacodynamics of antimicrobial therapy in surgery

INTRODUCTION

"Pharmacodynamics" refers to the relationship of drug concentrations in serum or tissues to effects on biologic systems. Concepts used to describe antimicrobial pharmacodynamics include the minimal inhibitory concentration (MIC), the minimal bactericidal concentration (MBC), and serum bactericidal titers (SBT), as well as post-antibiotic effect.

METHODS

Pertinent published literature was identified through a MEDLINE search.

RESULTS

Aminoglycosides have a concentration-dependent effect on bacteria killing and possess a relatively long postantibiotic effect. Given these characteristics, single-daily dosing, where the total daily dose with a traditional aminoglycoside regimen is given as one dose, may be more efficacious compared with more frequent dosing. For beta-lactam antimicrobials, bacterial killing is related to the duration of time that the free drug concentration exceeds the bacterial MIC. Beta-lactam antimicrobials have been shown to have no, or a short postantibiotic effect. Beta-lactam antimicrobials may be more effective when administered as continuous intravenous infusions.

CONCLUSIONS

Pharmacodynamic variation may result from differences in drug sensitivity among individuals and the nature of the interaction between antimicrobials and microorganisms. Proper use of pharmacokinetic and pharmacodynamic principles can result in more effective and less toxic antimicrobial regimens.

Kill kinetics and regrowth patterns of Pseudomonas aeruginosa exposed to concentration-time profiles of tobramycin simulating in vivo infusion and bolus dosing

Pseudomonas aeruginosa ATCC 27853 was exposed to tobramycin concentration-time profiles modelling in vivo bolus and infusion dosing. Dependence of bactericidal and bacteriostatic activity on the initial profile of peak concentration (bolus effect > infusion) and area under the antibiotic concentration-time curve was observed at peak concentration/MIC ratios of 10 or below.

Minimisation of aminoglycoside toxicity in patients with cystic fibrosis

BACKGROUND

There is evidence that administration of higher doses of aminoglycosides given less frequently improves the bactericidal effect and reduces the potential to cause side effects. To investigate this, a prospectively randomised open label therapeutic trial was undertaken in stratified groups of patients with cystic fibrosis to examine the efficacy and toxic potential of an aminoglycoside dosing regimen designed to generate high peak drug concentrations at 12 hourly intervals compared with conventional dosing at eight hourly intervals.

METHODS

Patients in group A received tobramycin eight hourly using a dose aimed at generating a peak concentration of 10 mg/l with trough concentrations below 2 mg/l, and those in group B received the total daily dose required to achieve eight hourly target concentrations administered as two equal 12 hourly doses. Clinical outcomes measured and assessed included vestibular symptoms, hearing and renal function, length of hospital stay, readmission rate, and mortality.

RESULTS

Twenty nine patients were recruited during a six month period, 20 to group A and nine to group B. The average peak tobramycin level was higher in group B (12.5 (2.2) mg/l) than in group A (7.9 (1.9) mg/l), whilst the average trough level was higher in group A (0.8 (0.3) mg/l) than in group B (0.5 (0.2) mg/l). There was a difference in the number of ototoxic events between patients in group A (seven of 18, 38.9%) and group B (none of eight), but no difference was found in other outcome measures assessed.

CONCLUSIONS

These results suggest that 12 hourly high peak aminoglycoside dosing may be less toxic than equivalent eight hourly dosing, without any apparent difference in efficacy.

Effects of fasting on temporal variations in nephrotoxicity of gentamicin in rats

Evidence for temporal variations in the nephrotoxicity of low doses of aminoglycosides were recently shown by using specific and sensitive parameters of renal toxicity. The aim of the present study was to evaluate the effect of a short period of fasting on the temporal variations in the renal toxicity of gentamicin. Twenty-eight normally fed (i.e., food and water were available ad libitum throughout the experiment) female Sprague-Dawley rats (weight, 175 to 220 g) and 28 fasted rats (i.e., only water was available during a 12-h fast before and a 24-h fast after gentamicin injection) were used. The animals were synchronized on a 14-h light, 10-h dark cycle (lights on at 0600 h) for 1 week before gentamicin administration. In July 1993, each group of animals was treated with a single intraperitoneal injection of saline (NaCl, 0.9%) or gentamicin (150 mg/kg of body weight) at either the peak (1400 h) or the trough (0200 h) of the previously determined toxicity. On day 1, the 24-h urinary excretion of beta-galactosidase, N-acetyl-beta-D-glucosaminidase, and gamma-glutamyltransferase was significantly higher in normally fed animals treated with gentamicin at 1400 h than in their time-matched controls and in normally fed animals treated at 0200 h (P < 0.01), which had normal levels of these enzymes. By contrast, the urinary excretion of these enzymes was significantly higher in both groups of gentamicin-treated, fasted rats than in their time-matched control groups (P < 0.01), reaching levels similar to those measured in normally fed rats treated at 1400 h. The accumulation of gentamicin was significantly lower in the renal cortex of normally fed rats treated at 0200 h than in rats treated at 1400 h (P < 0.05), but this time-dependent difference was not found in fasted rats treated at 0200 and 1400 h. Immunogold labeling done on ultrathin sections and observed by electron microscopy showed a similar subcellular localization of gentamicin in normally fed and fasted rats treated at either 1400 or 0200 h. These results suggest that the feeding period is of crucial importance in the temporal variations of the nephrotoxicity of gentamicin in rats.

Alteration of postantibiotic effect during one dosing interval of tobramycin, simulated in an in vitro pharmacokinetic model

The kinetics of the postantibiotic effect (PAE) during one dosing interval of tobramycin against Staphylococcus aureus and Pseudomonas aeruginosa was investigated. We determined the PAE at different time points during this dosing interval of 12 h in an in vitro pharmacokinetic model simulating human pharmacokinetics in which the half-life of tobramycin was adjusted to 2.4 +/- 0.2 h. Using an enzymatic method to inactivate tobramycin, we determined PAEs in samples extracted from the model at 1, 5, 8, and 12 h, corresponding with tobramycin concentrations of 20, 5, 2, and 1 times the MIC for the test organism. The PAE decreased significantly from 2.5 h at 1 h to 0 h at 12 h. No change in MIC was observed for the strains during the experiments. We conclude that the PAE decreases with decreasing tobramycin concentrations during a 12-h dosing interval and completely disappears after the concentration has reached the MIC for the test organism. On the basis of these observations, the emphasis that is placed on the PAE in discussions about the optimal dosing interval in aminoglycoside therapy is questionable.

Single or multiple daily doses of aminoglycosides: a meta-analysis

OBJECTIVE

To assess relative efficacy and toxicity of aminoglycosides given by single daily dose compared with multiple daily doses.

DESIGN

Meta-analysis of 21 randomised trials identified through MEDLARS (1966 to January 1995). Data were overviewed with fixed effects and random effects models and with meta-regression analysis.

SUBJECTS

Total of 3091 patients with bacterial infection, most without pre-existing renal disease.

INTERVENTIONS

Patients were randomized to receive aminoglycosides once daily or multiple times daily with similar total daily dose.

MAIN OUTCOME MEASURES

Clinical failure of treatment, nephrotoxicity, ototoxicity, and mortality.

RESULTS

Single daily dose regimen produced a non-significant decrease in risk of antibiotic failures (random effects risk ratio 0.83 (95% confidence interval 0.57 to 1.21)). Benefit of once daily dosing was greater when the percentage of pseudomonas isolates in a trial was larger. Once daily administration reduced risk of nephrotoxicity (fixed effects risk ratio 0.74 (0.54 to 1.00)). Similar trends were noted for patients with febrile neutropenia and for children. There was no significant difference in ototoxicity between the two dosing regimens, but the power of the pooled trials to detect a meaningful difference was low. There was no significant difference in mortality.

CONCLUSIONS

Once daily administration of aminoglycosides in patients without pre-existing renal impairment is as effective as multiple daily dosing, has a lower risk of nephrotoxicity, and no greater risk of ototoxicity. Given the additional convenience and reduced cost, once daily dosing should be the preferred mode of administration.

Aminoglycoside dosing: one, two or three times a day?

The safety and efficacy of conventional aminoglycoside dosing regimens have been proven in clinical trials. Higher doses at longer intervals may be more effective if they result in higher peak serum levels of the drug, but few trials of "once-a-day" dosing have shown improved clinical outcome. The clinical safety of allowing trough serum levels to fall below the minimum inhibitory concentration is not established. Literal "once-a-day" dosing will result in drug accumulation and toxicity in patients with reduced renal clearance, and in potential lack of efficacy and the emergence of antibiotic-resistant organisms in those with increased renal clearance. However, modified "once-a-day" dosing, with the interval determined by the individual's renal clearance rate (hence avoiding subtherapeutic trough levels), will avoid these problems.

Inpatient High-Dose Chemotherapy with Outpatient Autologous Stem Cell Support: A Model of Continuous Pharmaceutical Care

The adult autologous bone marrow transplant (ABMT) program at Duke University Medical Center has developed an innovative outpatient approach to managing patients with solid tumors with ABMT during their postchemotherapy period of myelosuppression and recovery. The use of colony-stimulating factors in combination with peripheral blood progenitor cells (PBPCs) and bone marrow for hematologic support plus implementation of prophylactic antibiotics with sequential once-daily empiric antibiotic modification regimen were key to the success of this approach. This program reduces the duration of hospitalization, and thus the costs associated with a traditionally complex and difficult therapy. More than 400 patients have been treated in the outpatient setting since May 1992. Patients are initially admitted for 5 days of hospitalization to receive combination high-dose chemotherapy with continuous hydration and antiemetic support. Twenty-four hours after completion of chemotherapy, patients are discharged on oral prophylactic antibiotics, electrolyte supplements, and antiemetic agents. Close to half of the patients are readmitted to the inpatient unit during their period of ambulatory supportive care. Continuous provision of drug therapy and clinical pharmacy services require ongoing communication between pharmacy and both outpatient and inpatient personnel. Months of planning with nurse, physician, and pharmacist input were required to develop a program that meets the changing needs of patients undergoing a complex and potentially toxic therapy. Reliable 7-day pharmaceutical services was one of the essential components required for operation of such an outpatient program. This article reviews the basic principles and procedures used in treating patients in the Duke Outpatient ABMT program, with a focus on the complexity of supportive care issues that occur in the ambulatory setting, and the development of integrated pharmacy services for the program will be discussed.

Pharmacoeconomic evaluation of once-daily aminoglycoside treatment

A special topic in pharmacoeconomics concerns antimicrobial therapy. The cost of antimicrobial therapy and an economic evaluation of aminoglycoside antibiotics in the last 30 years are reviewed. Some innovative approaches have been found to be effective in the control of the use of aminoglycosides and those are: 1) selecting the appropriate aminoglycoside, 2) therapeutic drug monitoring and, 3) once-daily administration. The practical advantages of once-daily aminoglycoside dosing are discussed and the conclusion is that combination therapy continues to be a mainstay in several serious Gram-negative infections. Concerns about breakthrough infection with extended aminoglycoside dosing intervals can be resolved by combination with a betalactam antibiotic. The lower costs associated with once-daily aminoglycoside dosing are the consequence of a straightforward dosage calculation, a guaranteed peak serum concentration in the therapeutic range, potential reduction in treatment period, easier quality control of preparation and administration, decreased personnel time, and fewer assays. However, some practical considerations remain unanswered.

Once-daily dosing regimen for aminoglycoside plus betalactam combination therapy of serious lower respiratory tract infections

Aminoglycosides are important antibacterial agents for treatment of serious gram-negative bacillary infections including lower respiratory tract infection. Once-daily aminoglycosides result in higher peak and lower trough plasma concentrations than conventional multiple daily dosing regimens; once-daily aminoglycoside therapy is equally effective, generally less toxic and much less expensive and therefore this regimen is more and more frequently used for treatment of suspected or confirmed gram-negative bacillary infections and of febrile episodes in neutropenic patients, in particular in combination with an appropriate betalactam antibiotic. Despite the lack of studies on this topic, once-daily aminoglycosides in combination with a betalactam agent can be used in subjects with lower respiratory tract infection, including patients with cystic fibrosis, in which tobramycin appears to be the aminoglycoside antibiotic of choice.

Preclinical and clinical evaluation of once-daily aminoglycoside chemotherapy

The rationale for and effectiveness of extended dosage intervals for aminoglycosides are reviewed. Aminoglycoside antibiotics have a prolonged postantibiotic effect against a variety of common Gram-negative and Gram-positive organisms: higher serum aminoglycoside levels are associated with a longer postantibiotic effect and increased bactericidal activity. Moreover once daily aminoglycoside administration may reduce the potential for adaptive postexposure resistance by allowing less contact time between bacteria and antibiotic. A longer dosage interval may decrease the risk of nephro- and ototoxicity. At least 33 published clinical trials suggest that once-daily administration of aminoglycosides and conventional regimens involving shorter dosage intervals are equally effective in patients with normal renal function and Gram-negative infections: besides, once-daily administration may reduce the frequency of aminoglycoside toxicity or delay it.

The place of tobramycin in lower respiratory tract infections (LRTI)

The Author provides a review of clinical experience with tobramycin as therapy for lower respiratory tract infections, in comparison to other aminoglycosides, including the pharmacokinetics and toxicity, dwelling on oto- and nephrotoxicity. The article includes a discussion of various dosing regimens of the aminoglycosides, focussing on efficacy and toxicity arising from once-daily administration. The Author then provides a more detailed description of tobramycin's pharmacokinetics, indications for its use, and the possibilities of once-daily dosing, concluding that toxicity is favorably influenced by a single daily administration as well as efficacy, and that patient compliance and reduced hospital costs are other advantages of this regimen.

Aminoglycosides

Despite their nephrotoxic and ototoxic side effects, AG remain useful antibiotics because of their major, rapid, and dose-dependent bactericidal effects. Combination therapy with an AG appears particularly important in neutropenic and other high-risk patients to provide broad-spectrum bactericidal activity, synergism, and reduction of emergence of resistant pathogens. OD AG therapy is associated with high peak levels in serum that maintain efficacy and low-to-undetectable trough levels in serum that attenuate the risk of toxicity. Administration of short-term OD AG therapy to patients not at risk without renal impairment may not absolutely require dosing monitoring. This therapeutic strategy has been proved useful in clinical trials, now including febrile episodes in neutropenic patients, but it should be avoided during infections in which antimicrobial synergism is required, such as enterococcal endocarditis.

Antibiotic dosing in renal failure

The Dosing in Renopathy by Easy-To-Use Multipliers (DREM) System is a simple method for dose adjustments of anti-infectives in renal insufficiency. The simple 2-step method involves: (1) estimating creatinine clearance (CLcr) from age, sex, and serum creatinine, and (2) calculating the adjusted dose or dosing interval with the use of multipliers. By multiplying the normal dose or dosing interval with the dose (CLcr/100) or interval (100'CLcr) multiplier, the adjusted dose or dosing interval is obtained, respectively. Dose estimates with this method are reasonably accurate and compare favorably with previously published methods of correction.

Easier monitoring of aminoglycoside therapy with once-daily dosing schedules

Little has been reported on serum levels attained using once-daily aminoglycoside regimens and their relation to dosage administered and renal function. Consecutive patients with serious infections were randomized to receive gentamicin 4 mg/kg q 24h i.v. (n = 69), gentamicin 1.33 mg/kg q 8h i.v. (n = 46) or netilmicin 5.5 mg/kg q 24h i.v. (n = 59) (with dose reduction in case of renal dysfunction). In the three groups, median first serum trough levels were 0.4, 1.0 and 0.4 mg/l, respectively, and median first serum peak levels were 9.5, 4.7 and 12.2 mg/l (p < 0.01 once-daily vs. thrice-daily regimens). Dose adjustment because of first trough concentrations of > 2 mg/l and/or peak concentrations of < 6 mg/l was required in 6%, 78% and 12% of patients, respectively. Second trough and peak concentrations were significantly higher in the thrice-daily gentamicin group; serum levels remained constant in the other two groups. The six patients in the once-daily groups who developed elevated trough levels later in therapy were characterized in most cases by a decline in renal function.

Experience of once-daily aminoglycoside dosing using a target area under the concentration-time curve

BACKGROUND

Many centres are changing to once-daily aminoglycoside administration. However, proposed methods for this practice often have theoretical and practical difficulties. We have developed a method in which a target area under the concentration-time curve (AUC) is used instead of traditional peak and trough serum concentrations.

AIMS

To analyse our experience with the target AUC method in the first 100 courses of once-daily aminoglycoside administration in the Christchurch, New Zealand hospitals.

METHODS

Following a starting dose of 5-7 mg/kg, administered by 30-minute infusion, the AUC was calculated using two serum aminoglycoside concentrations taken at one and six-14 hours after the start of the infusion. Dose adjustment was made to correct for any difference between the calculated AUC and a target AUC (72-101 mg.1(-1).h). The method was assessed for practicality and precision in 100 courses of treatment. The incidence of aminoglycoside toxicity was documented.

RESULTS

The mean final dose of 6.68 mg/kg, and AUC of 92.8 mg.1(-1).h, were significantly different from the mean starting dose and AUC of 5.67 mg/kg and 74.0 mg.1(-1).h, respectively. The method appeared to be more precise than empirical dosing at achieving the target AUC even though the final recommended dose had more variability than the starting dose. Although the study was uncontrolled, observed nephrotoxicity (2%) and ototoxicity (up to 6.9%) were no greater than expected from the results of other studies. There were no deaths related to antibiotic failure.

CONCLUSIONS

The AUC method was practical, and more appropriate for once-daily dosing than the conventional method of aiming for target peak and trough concentrations. Dose adjustment can be made before the next dose.

A suggested approach to once-daily aminoglycoside dosing

1. Once-daily aminoglycoside dosing has many advantages and has been widely advocated. However, existing guidelines for methods of administration and monitoring are non-specific and may lead to excessive dosing. 2. The traditional approach of aiming for target peak and trough concentrations is not appropriate for once-daily dosing. 3. A method is proposed which uses a target area under the concentration-time curve (AUC) for the aminoglycoside based on the 24 h AUC that would result with conventional dosing. This method requires measurement of two drug concentrations, one approximately 0.5 h after the end of the infusion and another at a later time (6-22 h) depending on renal function. 4. A simpler, graphical method is also proposed for patients with normal renal function, which requires the measurement of a single concentration at a time between 6 and 14 h. 5. Both methods are likely to be safer than existing guidelines.

Different patterns of bacterial DNA synthesis during postantibiotic effect

Studies on bacterial metabolism during the postantibiotic effect (PAE) period are limited but might provide insight into the nature of the PAE. We evaluated the rate of DNA synthesis in bacteria during the PAE period after a 1-h exposure of organisms in the logarithmic growth phase to various antibiotics. Staphylococcus aureus ATCC 25923 was exposed to vancomycin, dicloxacillin, rifampin, and ciprofloxacin; Escherichia coli ATCC 25922 was exposed to gentamicin, tobramycin, rifampin, imipenem, and ciprofloxacin; and Pseudomonas aeruginosa ATCC 25783 was exposed to imipenem, tobramycin, and ciprofloxacin. DNA synthesis was determined by measuring the rate of [3H]thymidine incorporation in S. aureus and E. coli and [3H]adenine incorporation in P. aeruginosa. DNA synthesis in S. aureus was suppressed during the PAE phase with vancomycin, dicloxacillin, and rifampin, it was suppressed in E. coli with rifampin, and it was suppressed in P. aeruginosa after exposure to tobramycin. Conversely, DNA synthesis was relatively enhanced in the gram-negative bacilli after exposure to imipenem and in all three species after exposure to ciprofloxacin. However, DNA synthesis in E. coli was only minimally affected after exposure to tobramycin and gentamicin. The differences in DNA synthesis observed after exposure to various antimicrobial agents suggest multiple mechanisms for the PAE.

Antibiotic kinetics and dynamics for the clinician

In recent decades considerable data have been acquired concerning the complex interactions of the organism-antimicrobial-host triad. Using the current understanding of drug disposition in the body, pharmacokinetics, and in vitro microbiologic activity, optimal dosing strategies can be derived to maximize clinical efficacy and minimize potential toxicities. This article discusses pharmacokinetics, pharmadynamics, and the application of this principle in clinical practice.

Antibiotic formulary selection

Physicians worldwide are being forced to consider economics in the care of patients. In most hospitals, antibiotics are a steadily increasing segment of the pharmacy budget. This article discusses how the authors used drug pharmacokinetics, restriction policies, and cost benefit analysis to design a cost-effective antibiotic formulary.

Considerations in antimicrobial prescribing

Soon after antimicrobial drugs became available, it was recognized that they were being overused and misused. Reasons for concern about the inappropriate use of antimicrobials include the emergence of resistant nosocomial bacteria, which have been identified in every hospital nationwide. The presence of resistant nosocomial bacteria presents the physician with a clinical problem and increases the cost of therapy. It is clear that methods need to be implemented to help physicians improve prescribing of antimicrobial agents. As health care practitioners in all fields strive to make patient care more cost-effective, one observation has become evident: The successfully treated patient consumes fewer resources and subsequently costs the hospital less than an otherwise similar, unsuccessfully treated patient. The goal in the 1990s is to provide optimal, cost-effective care for patients without compromising quality. This goal can be achieved by collaborative efforts of physicians, pharmacists, and microbiologists working together as a team to promote quality patient care.

The disposition of gentamicin in equine plasma, synovial fluid and lymph

Plasma (P), synovial fluid (SF) and lymph (L) concentrations of gentamicin were studied in two trials. A lymph vessel in the hindlimb was cannulated. The day after surgery (trial A), P and L samples were collected for 12 h after intravenous injection of gentamicin sulphate at 2.2 mg/kg dose rate. Approximately 48 h after surgery (trial B), the fetlock joint of the cannulated hindlimb was catheterized and P, SF and L samples collected for 12 h after a similar intravenous injection. The kinetic parameters were similar to those in other reports and did not differ between trials (P < 0.05). The P, L and SF disposition profiles were similar. The 95% confidence interval for P & L concentrations overlapped 2-3 h after injection. Thereafter, parallelism between L and P concentrations was observed, but L concentrations were on average 60% higher than P concentrations, and elimination from L was slower than from P. The mean L/SF and P/SF ratios were 1.54 +/- 0.2 and 1.25 +/- 0.2, 2-4 h after injection. Gentamicin elimination from SF appeared to be slower than from L and P. Lymph cannulation is a viable technique for antibiotic disposition studies. A sample of any of the fluids 3 h after injection was representative of the others. While SF concentrations were of limited value for predicting tissue fluid (L) concentrations 3-8 after injection, P concentrations were a useful index.

Ototoxicity and nephrotoxicity of gentamicin vs netilmicin in patients with serious infections. A randomized clinical trial

In the treatment of serious infection by aminoglycoside antibiotics multiple daily treatment with netilmicin is considered to be the least toxic. Studies comparing netilmicin with gentamicin using the less toxic once-daily schedule are lacking. A randomized prospective study was designed to evaluate the efficacy and toxicity of once-daily netilmicin with gentamicin treatment in patients with serious infections. Consecutive patients with serious infections were randomized between gentamicin 4 mg/kg q24h iv or netilmicin 5.5 mg/kg q24h iv. Exclusion criteria were neutropenia or severe renal failure. A good clinical response was observed in 50 of the 54 evaluable patients (92.6%) treated with gentamicin and in 48/52 (92.3%) netilmicin treated patients. Nephrotoxicity developed in 5/72 (6.9%) gentamicin patients and in 10/69 (14.5%) treated with netilmicin. Audiometry was performed with high-frequency audiometry when possible; no significant differences were found between the two aminoglycosides. We conclude that with once-daily treatment no benefit of netilmicin over gentamicin regarding nephro- or ototoxicity could be demonstrated.

Experience with a once-daily aminoglycoside program administered to 2,184 adult patients

Once-daily aminoglycoside (ODA) regimens have been instituted to maximize bacterial killing by optimizing the peak concentration/MIC ratio and to reduce the potential for toxicity. We initiated an ODA program at our institution that utilizes a fixed 7-mg/kg intravenous dose with a drug administration interval based on estimated creatinine clearance: > or = 60 ml/min every 24 h (q24h), 59 to 40 ml/min q36h, and 39 to 20 ml/min q48h. Subsequent interval adjustments are made by using a single concentration in serum and a nomogram designed for monitoring of ODA therapy. Since initiation of the program, 2,184 patients have received this ODA regimen. The median dose was 450 (range, 200 to 925) mg, while the median length of therapy was 3 (range, 1 to 26) days. The median age of the population was 46 (range, 13 to 97) years. Gentamicin accounted for 94% of the aminoglycoside use, and the majority (77%) of patients received the drug q24h. The 36-, 48-, and > 48-h intervals were used for 15, 6, and 2% of this population, respectively. Three patients exhibited clinically apparent ototoxicity. Twenty-seven patients (1.2%) developed nephrotoxicity (the Hartford Hospital historical rate is approximately 3 to 5%) after a median of 7 (range, 3 to 19) days of therapy. On the basis of a prospective evaluation of 58 patients and follow-up of additional patients via clinician reports, we have noted no apparent alterations in clinical response with our ODA program. This ODA program appears to be clinically effective, reduces the incidence of nephrotoxicity, and provides a cost-effective method for administration of aminoglycosides by reducing ancillary service time and serum aminoglycoside determinations.

Aminoglycosides: single or multiple daily dosing? A meta-analysis on efficacy and safety

A number of randomised studies indicate that a single high dose of aminoglycoside every 24 h may be more efficient and less toxic than the same dose divided into multiple daily doses. In the meta-analysis of 16 studies described here, which included more than 1200 patients, the relative chance (i.e. the relative risk, RR) of cure of the single-dose regime compared with the multiple-dose regime was 1.027, indicating that the single daily dose regime had a 2.7% higher cure rate (NS). The RR of avoiding nephrotoxicity was 1.001 (NS) and the RR of avoiding ototoxicity was 1.001 (NS). It is concluded that there is no difference concerning efficacy and safety between single-dose and multiple-dose regimes for administration of aminoglycosides.

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.

Intravenous administration of gentamicin once daily versus thrice daily in adults

The efficacy and safety of intravenous gentamicin administered once daily versus thrice daily was evaluated in adults. Patients over 16 years of age with a suspected or documented gram-negative infection were randomly divided into two groups: one group received gentamicin intravenously 4.5 mg/kg once daily (n = 48), and the other received 1.5 mg/kg every eight hours (n = 52). Baseline characteristics were comparable in the two groups. The mean peak level of gentamicin in the once daily group was significantly higher than that in the thrice daily group, 8.7 +/- 2.3 mg/l versus 4.6 +/- 1.2 mg/l (p < 0.005), and the trough level lower, 0.7 +/- 0.3 mg/l versus 1.1 +/- 0.9 mg/l (p < 0.005). The clinical cure rate was significantly higher in the once daily group, 42 of 48 (87.5%) versus 36 of 52 (69.2%). The microbiological cure rate was also better in the once daily group than in the thrice daily group (31 of 36 versus 28 of 38 patients evaluated), although this difference was insignificant. Nephrotoxicity was not observed in either group, but ototoxicity was present in three of the patients treated thrice daily. A once daily dosing regimen of gentamicin is more effective and less ototoxic than a thrice daily regimen.

Amikacin levels in bronchial secretions of 10 pneumonia patients with respiratory support treated once daily versus twice daily

In this study, concentrations of amikacin in blood and bronchial secretions of 10 patients with mechanical ventilation for acute respiratory failure due to pneumonia were measured. One-half of the patients received amikacin twice daily, and the others received once-daily administration. Concentrations in bronchial secretions of the patients treated twice daily ranged from 3 to 4 mg/liter, i.e., they were similar to those in previously published reports. Peak concentrations in bronchial secretions occurred between 3 and 4 h after the onset of infusion, and they reached 4.8 +/- 2.6 mg/liter on day 1 and 4.0 +/- 2.7 mg/liter on day 3. For the patients treated with amikacin once daily, concentrations in bronchial secretions were more than twofold higher, above 8 mg/liter for 12 h. Peak concentrations in bronchial secretions occurred between 3 and 4 h after the onset of infusion and reached 13.6 +/- 9.3 mg/liter on day 1 and 10.4 +/- 3.5 mg/liter on day 3. These concentrations are higher than the MICs for less sensitive bacterial strains, such as Acinetobacter spp. and Pseudomonas aeruginosa.

Pharmacokinetic contributions to postantibiotic effects. Focus on aminoglycosides

The postantibiotic effect (PAE) refers to a period of time after complete removal of an antimicrobial during which there is no growth of the target organism. The PAE appears to be a feature of most antimicrobial agents and has been documented with a variety of common bacterial pathogens. Various factors influence the presence or duration of the PAE including the type of organism, type of antimicrobial, concentration of antimicrobial, duration of antimicrobial exposure, antimicrobial combinations, and inoculum and medium used. beta-Lactams demonstrate a PAE against Gram-positive cocci, but produce only a short PAE with Gram-negative bacilli. Antimicrobial agents that inhibit RNA or protein synthesis have a PAE against Gram-positive cocci and Gram-negative bacilli. In vivo studies of aminoglycosides suggest that area under the plasma concentration-time curve is the pharmacokinetic parameter that best correlates with clinical efficacy. This is thought to be due to the concentration-dependent killing and PAE possessed by these antimicrobials. Animal and human studies have reported that once-daily administration of aminoglycoside is as effective as, or more effective than, and possibly less toxic than traditional multiple daily administration.