What is the evidence for once-daily aminoglycoside therapy?

Improved characterization of aminoglycoside penetration into human lung epithelial lining fluid via population pharmacokinetics

Aminoglycosides are important treatment options for serious lung infections, but modeling analyses to quantify their human lung epithelial lining fluid (ELF) penetration are lacking. We estimated the extent and rate of penetration for five aminoglycosides via population pharmacokinetics from eight published studies. The area under the curve in ELF vs plasma ranged from 50% to 100% and equilibration half-lives from 0.61 to 5.80 h, indicating extensive system hysteresis. Aminoglycoside ELF peak concentrations were blunted, but overall exposures were moderately high.

A Clinical Comparison of Two Formulations of Tobramycin 0.3% Eyedrops in the Treatment of Acute Bacterial Conjunctivitis

Purpose

To compare the safety and efficacy of a new enhanced viscosity ophthalmic formulation of tobramycin, given twice daily (BID), with the existing four times daily (QID) treatment regimen in patients with acute bacterial conjunctivitis.

Methods

This was a 12-day, multicenter, observer-masked, randomized, parallel group study. Patients received one drop of tobramycin 0.3% (3 mg/mL) enhanced viscosity ophthalmic solution BID or tobramycin 0.3% (3 mg/mL) ophthalmic solution QID in the affected eyes for 7 days. The primary efficacy variable was the percentage of patients with sustained cure/presumed bacterial eradication based on clinical judgment at the test-of-cure visit (Day 12). Pretherapy bacterial isolates were obtained and tested for susceptibility to tobramycin by determination of minimum inhibitory concentrations (MIC).

Results

A total of 276 patients were enrolled in the study and 203 of these were culture positive and attended all follow-up examinations. In this group, 98% of those treated with tobramycin enhanced viscosity ophthalmic solution and 99% of those treated with tobramycin 0.3% ophthalmic solution were categorized as having sustained cure/presumed eradication at the test-of-cure visit (p=0.6037). Reported adverse events were not serious, mild to moderate in severity, and generally did not prevent continuation in the study. Several pretreatment pathogens demonstrated tobramycin resistance (MIC > 4 mg/mL). However, therapy with both treatments was effective in the majority of the cases.

Conclusions

Tobramycin enhanced viscosity ophthalmic solution is well tolerated and has equivalent efficacy to the established treatment regimen with a simplified posology. The formulation provides an alternative therapy for acute bacterial conjunctivitis that should improve patient compliance and satisfaction.

Aminoglycoside-induced nephrotoxicity

Aminoglycosides are among the oldest antibiotics available to treat serious infections caused by primarily, Gram-negative bacteria. The most commonly utilized parenteral agents in this class include gentamicin, tobramycin and amikacin. Aminoglycosides are concentration-dependent, bactericidal agents that undergo active transport into the cell where they inhibit protein synthesis on the 30S subunit of the bacterial ribosome. As the use of aminoglycosides became more widespread, the toxic effects of these agents, most notably ototoxicity and nephrotoxicity, became more apparent. When other, safer, antimicrobial agents became available, the use of aminoglycosides sharply declined. The development of multi-drug resistance among bacteria has now lead clinicians to reexamine the role of the aminoglycosides in the treatment of serious infections. This review will revisit the mechanism and risk factors for the development of aminoglycoside-induced nephrotoxicity, as well as strategies to prevent patients from developing nephrotoxicity.

Long-term adherence to a 5 day antibiotic course guideline for treatment of intensive care unit (ICU)-associated Gram-negative infections

OBJECTIVES

To determine long-term adherence to a 5 day antibiotic course guideline for treating intensive care unit (ICU)-acquired Gram-negative bacteria (GNB) infections.

METHODS

Descriptive analysis of patient-level data on all GNB-active antibiotics prescribed from day 3 and all GNB identified in clinical samples in 5350 patients admitted to a 30 bed general ICU between 2002 and 2009.

RESULTS

Four thousand five hundred and eleven of 5350 (84%) patients were treated with one or more antibiotics active against GNB commenced from day 3. Gentamicin was the most frequently prescribed antibiotic (92.2 days of therapy/1000 patient-days). Only 6% of courses spanned >6 days of therapy and 89% of antibiotic therapy days were with a single antibiotic active against GNB. There was no significant difference between gentamicin and meropenem in the number of first courses in which a resistant GNB was identified in blood cultures [11/1177 (0.9%) versus 5/351 (1.4%); P = 0.43] or respiratory tract specimens [59/951 (6.2%) versus 17/246 (6.9%); P = 0.68] at the time of starting therapy.

CONCLUSIONS

This study demonstrates long-term adherence to a 5 day course antibiotic guideline for treatment of ICU-associated GNB infections. This guideline is a potential antibiotic-sparing alternative to currently recommended dual empirical courses extending to ≥7 days.

Development of a Semimechanistic Model to Describe the Pharmacokinetics of Gentamicin in Patients Receiving Hemodialysis

The aim of this study was to ascertain the most suitable dosing schedule for gentamicin in patients receiving hemodialysis. We developed a model to describe the concentration-time course of gentamicin in patients receiving hemodialysis. Using the model, an optimal dosing schedule was evaluated. Various dosing regimens were compared in their ability to achieve maximum concentration (C(max), > or = 8 mg/L) and area under the concentration time-curve (AUC > or = 70 mg x h/L and < or = 120 mg x h/L per 24 hours). The model was evaluated by comparing model predictions against real data collected retrospectively. Simulations from the model confirmed the benefits of predialysis dosing. The mean optimal dose was 230 mg administered immediately before dialysis. The model was found to have good predictive performance when simulated data were compared to data observed in real patients. In summary, a model was developed that describes gentamicin pharmacokinetics in patients receiving hemodialysis. Predialysis dosing provided a superior pharmacokinetic profile than did postdialysis dosing.

Off-label use of medicines in paediatric inpatients at an Australian teaching hospital

AIM

The study aims to investigate the prevalence of off-label prescribing in the general paediatric ward at a major teaching hospital in Tasmania, Australia.

METHOD

The drug charts and medical records from two groups of 150 consecutive paediatric patients, admitted 6 months apart in July 2009 and January 2010, were studied retrospectively. Patients were required to spend at least one night in hospital and be aged less than 12 years. Each prescribed drug was compared with the approved product information to determine if the usage was off-label. Data concerning documented informed consent and adverse drug reactions were also recorded.

RESULTS

Three hundred patients were prescribed a total of 887 medicines. Of these, 31.8% were off-label and 57.3% of children received an off-label medication. There was no significant seasonal variation in patient characteristics or prescriptions. Drugs were most commonly off-label due to their dosage or frequency of administration. Of the 106 different drugs used, the use of 51 was off-label on at least one occasion, and for 30 drugs their use was off-label on more than 75% of occasions. The drugs most commonly used off-label were oxycodone, salbutamol and paracetamol. No informed consent documentation was identified, and two of five recorded adverse drug reactions were associated with off-label drug use.

CONCLUSION

Off-label use of medicines occurred frequently in paediatric inpatients. The available evidence often supported off-label medication use. An improved system for the revision of approved drug information and an Australian guideline for paediatric prescribing are needed.

Current use of aminoglycosides: indications, pharmacokinetics and monitoring for toxicity

The new Australian Therapeutic Guidelines: Antibiotic, version 14 have revised the recommendations for the use and monitoring of aminoglycosides. The guidelines have clear distinctions between empirical and directed therapy as well as revised recommendations about the monitoring of aminoglycosides. This has led many clinicians to review their current practice with regard to the use of aminoglycosides. This review summarizes why aminoglycosides are still a valid treatment option and discusses the rationale for current dosing regimens in Gram-negative infections. In particular it focuses on the various methods for monitoring aminoglycosides that are currently being used. The aminoglycoside monitoring methods can be categorized into three groups: linear regression analysis (one compartment model), population methods and Bayesian estimation procedures. Although the population methods are easy to use and require minimal resources they can recommend clinically inappropriate doses as they have constant pharmacokinetic parameters and are not valid in special population groups, that is, renal impairment. The linear regression and Bayesian methods recommend more accurate dosage regimens; however, they require additional resources, such as information technology and healthcare personnel with background training in pharmacokinetics. The Bayesian methods offer additional advantages, such as calculation of doses based on a single serum concentration and optimization of the patient's previous pharmacokinetic data, in order to determine subsequent dosage regimens. We recommend the Bayesian estimation procedures be used, wherever feasible. However, they require the expertise of healthcare practitioners with a good understanding of pharmacokinetic principles, such as clinical pharmacists/clinical pharmacologists, in order to make appropriate recommendations.

Once-daily gentamicin in infants and children: a prospective cohort study evaluating safety and the role of therapeutic drug monitoring in minimizing toxicity

BACKGROUND

The clinical evidence base for ototoxicity and nephrotoxicity outcomes with once-daily dosing (ODD) of gentamicin in children is suboptimal. Therapeutic drug monitoring (TDM) in once-daily gentamicin regimens is variable and its role in predicting or preventing clinical toxicity is unclear. We aimed to assess the safety of ODD of gentamicin and the usefulness of TDM in a pediatric cohort.

METHODS

Children with suspected sepsis were prospectively enrolled to receive ODD of gentamicin at 7 mg/kg/day. Hearing and renal function were objectively assessed at baseline, during therapy, and after therapy. TDM was performed using an interval-adjusted graphical method (Hartford nomogram).

RESULTS

A total of 79 children (median age: 5.6 years; range: 1 month-16 years) received 106 episodes of therapy. In all, 61% of these episodes were for febrile neutropenia. Evaluation was complete in 88% for ototoxicity and 92% for nephrotoxicity. Two patients (1.88%, 95% confidence interval: 0.10%-7.13%) experienced permanent hearing loss. One patient (0.94%, 95% confidence interval: <0.10%-5.73%) experienced transient nephrotoxicity. No abnormal serum gentamicin values were detected, even in those experiencing toxicity. Children experiencing toxicity were undergoing treatment for malignancies and had received nephrotoxic or ototoxic medicines before gentamicin.

CONCLUSIONS

In this pediatric cohort receiving ODD of gentamicin, nephrotoxicity was uncommon and reversible, but irreversible ototoxicity occurred more frequently. TDM using a nomogram neither predicted nor prevented toxicity, which was only observed in those with risk factors.

Antibiotic dosing in critical illness

Early and effective antibiotic therapy is essential in the management of infection in critical illness. The loading dose is probably the most important dose and is a function of the volume of distribution of the drug and the desired plasma concentration but independent of renal function. Antibiotics are classified in a number of ways that have implications for dosing. Doses of hydrophilic agents such as β-lactams should be increased in the early stages of sepsis as the extravascular space increases. For lipophilic agents such as macrolides, the inflammatory process is less important, although factors such as obesity will affect dosing. Classification can also be based on pharmacodynamic properties. Concentration-dependent antibiotics such as aminoglycosides should be administered by extended interval regimens, which maximize bactericidal effect, minimize nephrotoxicity and allow time between doses for the post-antibiotic effect. The critical factor for time-dependent agents, such as β-lactams, is time above the MIC. Ideally administration of these agents should be continuous, although vascular access availability can restrict infusion time to between 4 and 6 h, which is probably adequate. As well as antibiotic factors, patient factors such as hepatic and renal failure will affect dosing. Hepatic failure will affect antibiotic metabolism, although it is most important in end-stage failure. Renal failure and support will affect drug elimination. Knowledge of these factors is essential. Patient safety and prevention of unnecessary harm is a weighty consideration in critical illness. To ensure effective treatment and minimize adverse effects, therapy should be reviewed daily and adjusted in the light of changes in patient organ function and underlying pathology.

A pilot study of high-dose short duration daptomycin for the treatment of patients with complicated skin and skin structure infections caused by gram-positive bacteria

BACKGROUND

Methicillin-susceptible and -resistant (MRSA) Staphylococcus aureus are significant causes of complicated skin and skin structure infections (cSSSI). The bactericidal antibiotic daptomycin is approved for gram-positive cSSSI at 4 mg/kg/day for 7-14 days, but the optimal dose level and duration of therapy have not been firmly established. This pilot study evaluated the efficacy and safety of daptomycin at 10 mg/kg every 24 h for 4 days [high-dose short duration (HDSD) regimen] vs. standard of care therapy with vancomycin or semi-synthetic penicillin for the treatment of cSSSI.

METHODS

This was a semi-single blind, randomised, multicentre, comparative trial. The primary efficacy end-point was the clinical response 7-14 days posttherapy.

RESULTS

One hundred patients were randomised; 48 in each arm were treated. The treatment groups were well balanced with respect to demographics, comorbidities and the type of infection (75% because of MRSA). Overall, clinical success rates were 75.0% (36/48) for daptomycin and 87.5% (42/48) for comparator (95% confidence interval for the difference: -27.9, 2.9). The median duration of comparator therapy was 8 days. Two comparator patients and no daptomycin patients experienced treatment-related serious adverse events requiring hospitalisation.

CONCLUSION

We found that the HDSD regimen had a safety profile similar to that seen in previous studies. Although the differences were not statistically significant, clinical success rates for comparator were higher than for daptomycin. In post hoc analyses HDSD daptomycin performed better in some subgroups (e.g. outpatients) than in others (e.g. certain MRSA infections). These observations require confirmation in larger trials.

Providing guidelines and education is not enough: an audit of gentamicin use at The Royal Melbourne Hospital

BACKGROUND

Aminoglycoside antibiotics are commonly prescribed for the treatment of Gram-negative infections. Appropriate dosing and therapeutic monitoring of aminoglycosides are important because these agents have a narrow therapeutic index.

AIM

To audit gentamicin use at our hospital, focusing on selection of the initial dose and therapeutic monitoring practices, and to compare the results against recommendations in the existing hospital aminoglycoside guidelines, which had recently been promoted to doctors.

METHODS

This audit included all inpatients receiving gentamicin at The Royal Melbourne Hospital from 1 February to 12 March 2004. The principal researcher checked the drug charts of all inpatients to identify those receiving gentamicin and collected data from the medical records and the pathology database. Doses were considered 'concordant' if the dose given was within the recommended dosing range +/-20 mg.

RESULTS

A total of 132 courses of gentamicin was included in the study. Gentamicin was prescribed for prophylaxis in 31.1% of courses. Thirty-six per cent of patients prescribed gentamicin were more than 65 years of age. Eighty-two per cent of the gentamicin used therapeutically was given as a single daily dose. Sixty-six per cent of gentamicin initial dosing was not in accordance with existing hospital guidelines. Seventy-seven per cent of gentamicin courses requiring therapeutic drug monitoring received such monitoring; however, in only 8.8% of these was the monitoring conducted according to guidelines.

CONCLUSION

Aminoglycoside prescribing practices at our hospital are suboptimal, despite ready access to prescribing guidelines. Provision of a guideline and education sessions with doctors do not necessarily lead to widespread adoption of recommended practices. We suggest that changes to hospital systems related to prescribing and monitoring of aminoglycosides are required.

Antibacterial Dosing in Intensive Care

Treatment of sepsis remains a significant challenge with persisting high mortality and morbidity. Early and appropriate antibacterial therapy remains an important intervention for such patients. To optimise antibacterial therapy, the clinician must possess knowledge of the pharmacokinetic and pharmacodynamic properties of commonly used antibacterials and how these parameters may be affected by the constellation of pathophysiological changes occurring during sepsis. Sepsis, and the treatment thereof, increases renal preload and, via capillary permeability, leads to 'third-spacing', both resulting in higher antibacterial clearances. Alternatively, sepsis can induce multiple organ dysfunction, including renal and/or hepatic dysfunction, causing a decrease in antibacterial clearance. Aminoglycosides are concentration-dependent antibacterials and they display an increased volume of distribution (V(d)) in sepsis, resulting in decreased peak serum concentrations. Reduced clearance from renal dysfunction would increase the likelihood of toxicity. Individualised dosing using extended interval dosing, which maximises the peak serum drug concentration (C(max))/minimum inhibitory concentration ratio is recommended. Beta-lactams and carbapenems are time-dependent antibacterials. An increase in V(d) and renal clearance will require increased dosing or administration by continuous infusion. If renal impairment occurs a corresponding dose reduction may be required. Vancomycin displays predominantly time-dependent pharmacodynamic properties and probably requires higher than conventionally recommended doses because of an increased V(d) and clearance during sepsis without organ dysfunction. However, optimal dosing regimens remain unresolved. The poor penetration of vancomycin into solid organs may require alternative therapies when sepsis involves solid organs (e.g. lung). Ciprofloxacin displays largely concentration-dependent kill characteristics, but also exerts some time-dependent effects. The V(d) of ciprofloxacin is not altered with fluid shifts or over time, and thus no alterations of standard doses are required unless renal dysfunction occurs. In order to optimise antibacterial regimens in patients with sepsis, the pathophysiological effects of systemic inflammatory response syndrome need consideration, in conjunction with knowledge of the different kill characteristics of the various antibacterial classes. In conclusion, certain antibacterials can have a very high V(d), therefore leading to a low C(max) and if a high peak is needed, then this would lead to underdosing. The V(d) of certain antibacterials, namely aminoglycosides and vancomycin, changes over time, which means dosing may need to be altered over time. Some patients with serum creatinine values within the normal range can have very high drug clearances, thereby producing low serum drug levels and again leading to underdosing.

Evaluation of once daily tobramycin dosing in critically ill patients through Bayesian simulation

OBJECTIVE

To evaluate if once-daily dose (ODD) regimens of tobramycin attain pharmacodynamic goals using individualized pharmacokinetic monitoring of critically ill patients with creatinine clearance (Clcr) over 60 mL/min.

METHODS

Fifty-one adult critically ill patients treated with intravenous tobramycin with ODD were included in the study. The effect of dosing using the proposed method was compared with a weight-based (7 mg/kg) dosing method. Pharmacokinetics parameters, peak concentration (Cpeak), minimum concentration (Cmin) and the time below the minimum inhibitory concentration (MIC) were estimated using Bayesian analysis. Pharmacodynamic parameters used to evaluate both dosing regimens were Cpeak/MIC ratio and, secondly, time below MIC (T< MIC).

RESULTS

The median dose of tobramycin administrated in our hospital was too low for achieving pharmacodynamic goals. In contrast, the weight-based (7 mg/kg) method produced an adequate Cpeak/MIC ratio but an increase of the dose would not reduce the secondary pharmacodynamic index T<MIC.

CONCLUSION

The results from the current study explain why weight-based daily dosing of tobramycin in critically ill patients with Clcr>60 mL/min achieved the Cpeak/MIC target values of 10. However in critically ill patients with Clcr>80 mL/min, T<MIC is greater than the aminoglycoside post-antibiotic effect, so these patients do not attain the secondary pharmacodynamic index. These data show the need for rapid pharmacokinetic optimization with individualized aminoglycoside dosing. Additional studies are necessary to better define the best tobramycin regimen for critically ill patients.

Pharmacodynamics and dosing of aminoglycosides

Aminoglycosides are concentration-dependent killing agents whose pharmacodynamic predictors of efficacy are the area-under-the-curve to minimum inhibitory concentration ratio and the peak to minimum inhibitory concentration ratio. Prospective studies have shown that these agents can be given once-daily or less frequently in most clinical settings, with equal efficacy and possible reduced toxicity. Dosages for different clinical settings have been studied and methods are available to monitor once-daily dosing.

Extended interval aminoglycoside dosing: from concept to clinic

Extended-interval aminoglycoside dosing (EIAD), while a relatively recent concept in mainstream clinical practice, actually has its roots in the mid 1970s. Early trial and error approaches of manipulating the dosage regimen to avoid toxicity and improve efficacy have helped to characterize the pharmacodynamic properties of these drugs. The increasing successful use of EIAD and improved understanding of pharmacodynamics has helped this dosing regimen gain acceptance into routine clinical practice. A 1998 United States survey demonstrated that approximately 75% of hospitals have adopted EIAD into routine patient care. However, controversy still exists regarding some aspects of infrequent aminoglycoside administration, such as length of the drug-free interval and patient exclusion criteria. After more than 50 years of experience with the aminoglycosides we continue to learn how to most appropriately use these drugs.

Renal transport of antibiotics and nephrotoxicity: a review

The renal excretion of a drug can essentially be divided schematically into three functional processes: glomerular filtration, tubular reabsorption and tubular secretion. When assessing nephrotoxicity, the tubular secretion system, which allows transport of the drug from the blood to the urine via the tubular cells, is particularly important. Historically, two distinct tubular secretion mechanisms have been described for drugs: one via organic cations and the other via organic anions. More recently, a third tubular secretion mechanism has been identified, mediated by P-glycoprotein. In the present review, a number of examples will be given relating to antibiotic-induced kidney damage determined via the tubular reabsorption mechanism (aminoglycosides, amphotericin B) and via the tubular secretion mechanism (cephalosporins, vancomycin), respectively. Drug transport within the tubular cells is the first fundamental stage in the onset of the nephrotoxic process. Knowledge of these concepts is important for the prevention of iatrogenic kidney damage, particularly in patients with underlying disease receiving concomitant treatment with several potentially nephrotoxic molecules.

[Influence of dose regimen on gentamycin nephrotoxicity in rats]

The acute renal failure (ARF), that still presents a right mortality rate (50%) can be defined as an abrupt decline of the glomerular filtration, resultant of ischemic or toxicity event. The drugs nephrotoxicity is one of the most frequent cause (27%) of ARF and it is suggested that the interval of administration of the drug can interfere in this side effect, however the best administration regimen is not very well established. This study evaluated the renal function of rats that received gentamicin (100 mg/kg) in one dose or in two doses (2 x 50 mg/kg), by intraperitoneal infusion. The results obtained in this research, indicated that the single infusion of gentamicin determined smaller nephrotoxicity by the reduction of serum concentration of this drug in 24 hours, decreasing the intracellular accumulation of this gentamicin, which is one of the main cellular mechanisms of this renal injury. The single dose treatment regime, otherwise, shows advantages not only related to the nephrotoxicity effect, but also it is relevant to the cost and safety, which can be rationable factors in the administration of this drug.

In vitro elution of gentamicin, amikacin, and ceftiofur from polymethylmethacrylate and hydroxyapatite cement

OBJECTIVE

To compare the elution characteristics of ceftiofur and liquid and powdered gentamicin and amikacin from polymethylmethacrylate (PMMA) and from hydroxyapatite cement (HAC).

METHODS

PMMA and HAC beads in triplicate were impregnated with various amounts and formulations of antibiotics. Beads were immersed in 5 mL of phosphate buffered saline that was replaced at 1, 3, 6, and 12 hours, and 1, 2, 3, 5, 7, 10, 14, 18, 22, 26, and 30 days. The eluent was stored at -70 degrees C until assayed within 2 weeks by microbiological assay (gentamicin and amikacin) or capillary electrophoresis (ceftiofur).

RESULTS

Rate of elution for all beads was greatest within the first 24 hours. Cumulative release of total antibiotic dose from beads over 30 days was significantly greater from HAC than PMMA. Antibiotic elution was directly related to the amount of antibiotic incorporated into the cement. Powdered and liquid forms of gentamicin had similar elution rates from PMMA. Elution of amikacin from PMMA beads was greater when the powdered form was used compared with liquid amikacin. Eluent concentrations of ceftiofur were similar to those of the aminoglycosides during the first 3 to 7 days but then decreased precipitously by comparison.

CONCLUSIONS

Elution of antibiotics from HAC was greater than from PMMA. Gentamicin- and amikacin-impregnated PMMA and HAC released bactericidal concentrations of antibiotic for at least 30 days. Ceftiofur-impregnated PMMA or HAC is unlikely to provide long-term bactericidal concentrations.

CLINICAL RELEVANCE

Gentamicin and amikacin elute effectively from PMMA and HAC.

Gentamicin for the practicing urologist: review of efficacy, single daily dosing and "switch" therapy

PURPOSE

We review the literature on gentamicin, including single daily dosing and "switch" therapy.

MATERIALS AND METHODS

We used MEDLINE to search the literature from 1966 to June 1997, and then manually searched bibliographies to identify studies that our initial search might have missed.

RESULTS

Gentamicin has attractive characteristics, including wide spectrum, infrequent resistance, economy and familiarity. Although limited by well known toxicities, gentamicin remains a drug of choice for serious Gram-negative infections. Dosing strategies, such as single daily dosing and switch therapy, have renewed enthusiasm for this time-honored drug.

CONCLUSIONS

Gentamicin remains a valuable drug in urology. Once daily dosing and switch therapy offer the potential to increase effectiveness and convenience while decreasing toxicity and costs.

Population pharmacokinetics of gentamicin in preterm neonates: evaluation of a once-daily dosage regimen

Population pharmacokinetic parameters of gentamicin in preterm neonates on a once-daily dosage regimen of 3.0 mg/kg given intravenously every 24 hours were established prospectively. In 34 preterm neonates with a mean gestational age of 32 +/- 4 (SD), 182 serum gentamicin levels (91 peak/trough pairs) were determined. Individual adjustments of dose or dosage interval were calculated by computer-aided Bayesian forecasting. The parameters Vd, ke, and CL for each patient were obtained by the nonparametric estimation of maximization method. The predictive power of the model was calculated and the pharmacokinetic estimates were statistically analyzed with SPSS/PC. Cluster analysis showed a division into 2 subpopulations (designated 1 and 2) on the basis of postnatal age. The mean +/- SD postnatal age of subpopulation 1 (n = 29) was 6 +/- 2 days (range 1-7) and of subpopulation 2 (n = 5) 15 +/- 4 days (range 12-24). The mean +/- SD gentamicin relative clearances of subpopulation 1 and subpopulation 2 were 0.0515 +/- 0.0128 and 0.1026 +/- 0.0102 L kg(-1) hr(-1), respectively (p < 0.05). The mean +/- SD values for Vd (Lkg(-1)) in both populations 1 and 2 were 0.6916 +/- 0.1670 and 0.7509 +/- 0.1961, respectively (not significantly different). For ke these data were 0.0744 +/- 0.0200 and 0.1366 +/- 0.0522 (p < 0.05). Statistics showed that the data for Vd and ke of subpopulation 1 were normally distributed (Vd and ke skewness 1.61 and 1.46; kurtosis 3.09 and 3.10 respectively). The model yielded a bias of -0.11 mg/L and a precision of 0.36 mg/L. It is recommended that gentamicin be started in a dosage of 3.5 mg/kg intravenously once-daily under close monitoring.

Once-daily aminoglycosides in the treatment of gram-positive endocarditis

OBJECTIVE

To examine the role of once-daily aminoglycosides (ODA) in the treatment of gram-positive endocarditis.

DATA SOURCES

A MEDLINE search was conducted from January 1984 to August 1998, and a Current Contents search was performed from September 1998 to December 1998, using endocarditis or aminoglycoside as key words. In addition, relevant articles were cross-referenced to screen for additional information.

DATA EXTRACTION

Data published in English regarding the use of aminoglycosides in endocarditis are cited. Emphasis was placed on animal and human studies, but in vitro studies and review articles are also included.

DATA SYNTHESIS

Endocarditis and the pharmacology of aminoglycosides are briefly reviewed. ODA is an alternative to conventional dosing in the treatment of endocarditis. Extensive work in endocarditis has been done recently in animals and humans to add to our understanding. Limited clinical data exist to support the theoretical advantages of increased efficacy, reduced toxicity, and potential cost savings versus traditional synergistic aminoglycoside dosing. Optimal monitoring of ODA remains undefined.

CONCLUSIONS

Routine use of ODA for the treatment of endocarditis is not yet advocated. Promising supporting evidence and speculation of success of ODA in gram-positive endocarditis justify well-designed trials to further define its role in therapy.

Comparison of tobramycin nephrotoxicity in young adult and aged female rats

A two week toxicity study was performed in rats to study the possible age-dependent toxicity of tobramycin, an aminoglycoside antibiotic with well known ototoxic and nephrotoxic properties in animals and man. Young adult female Wag/Rij rats aged 12 weeks (n = 10) and old female rats aged 23 to 26 months (n = 14) were treated subcutaneously with 0, 10, 40 or 160 mg tobramycin sulphate/kg/day. Clinical chemistry and urinalysis revealed significant changes in renal function in young adult rats mainly at 160 mg/kg, whereas in old rats significant changes were seen at 10, 40 and 160 mg/kg. Excretion of N-acetyl-beta-glucosaminidase, indicative for tubular dysfunction, was statistically significantly increased only in old animals at 160 mg/kg. Histopathology: At 40 mg/kg, tubular necrosis was increased in old animals and hyaline droplet formation in both age groups. At 160 mg/kg these lesions were increased in both age groups. For tubulonephrosis, interstitial nephritis and tubular regeneration, age-related differences were predominantly reflected in severity, for example, at 40 mg/kg, tubular regeneration in young animals was "moderate" in 7/10 and "marked" in 2/10, while in old animals the scores were 3/14 and 11/14, respectively. Secondary treatment-related lesions (in heart and adrenals) were also more increased in old animals. Chemistry and histopathology revealed the increased sensitivity to the toxic effects of tobramycin in old rats, which is important for the discussion of the most appropriate dosing regimen for aminoglycoside in humans. The once-daily dosing regimen for tobramycin should not be recommended for elderly, because high peak concentrations should be avoided to minimise nephrotoxicity.

Once daily aminoglycoside therapy. Is it less toxic than multiple daily doses and how should it be monitored?

After 50 years of clinical experience with the aminoglycoside agents, there is continuing debate over the most appropriate administration regimen for these drugs. In recent years, once daily administration has been used increasingly, in the hope of both improving efficacy and reducing toxicity. At least 30 controlled clinical trials have compared once versus conventional multiple daily administration. Efficacy was assessed in some, but not all, studies using clinical and/or bacteriological cure. Toxicity was generally determined using rather nonsensitive end-points such as measurement of serum creatinine for nephrotoxicity and clinically detectable hearing loss for ototoxicity. The results of individual clinical trials and subsequent meta-analyses have been variable. However, 5 of 9 meta-analyses found clinical efficacy to be significantly better with once daily administration, and in 3 of the 9 there were significantly less nephrotoxicity with once daily administration. The results were not significant for ototoxicity in any of the meta-analyses. There is debate about how therapeutic drug monitoring should be performed, and whether it is still required with once daily administration. Previous experience with the aminoglycosides, especially in patients with impaired drug clearance caused by renal impairment, suggests that monitoring is still prudent. Results from the once daily administration trials appear to support this. Various methods of monitoring and dose adjustment have been proposed. The most common is to measure a 24-hour trough concentration and to adjust the dose to maintain the trough concentration below a value of 2, 1 or 0.5 mg/L. However, this method allows for greater total aminoglycoside exposure than has been permitted with conventional dosages, increasing the likelihood of toxicity in patients with impaired aminoglycoside clearance. Other methods measure drug concentrations at a time-point or points within the dose interval (when the concentration is still measurable), and adjust the dose according to concentration-time curve nomograms or to a target area under the concentration-time curve. This allows the use of higher doses in those with high drug clearance. Furthermore, in patients with impaired clearance, drug exposure is limited to the same extent as, or less than, that with conventional multiple daily administration. To date no controlled trials have compared methods of dose-individualisation. In summary, in addition to a slight overall improvement in efficacy, once daily administration has resulted in a small reduction in nephrotoxicity. In the studies using more sensitive measures of toxicity, the differences in toxicity were greater, strengthening the case for once daily administration. Therapeutic drug monitoring is probably required with once daily administration. Methods which use mid-dosage interval concentrations to gauge drug exposure would seem to be preferable over trough concentration measurement.

Gentamicin dosage intervals in neonates: longer dosage interval--less toxicity

OBJECTIVES

The aim of this study was to determine the incidence of toxic trough serum gentamicin levels in neonates in the first week of life, with different dosage intervals.

METHODS

This was a retrospective study of neonates born between 01.07.95 and 31.12.95, who received gentamicin. Data were collected on birth weight, gestation, gentamicin dose, the trough level of gentamicin, serum creatinine and urine output. A trough serum gentamicin level of > or =1.5 mg/L was considered toxic.

RESULTS

One hundred and seventy infants met the study criteria. All 21 infants in group one (24-29 weeks) received gentamicin with a dosage interval of 24 h. Sixteen (76%) infants had toxic trough serum gentamicin levels. In group two (30-34 weeks) 8 infants had gentamicin q12hly and all (100%) had toxic trough serum gentamicin levels. Fourteen infants had gentamicin every 18 h and 13 (93%) had toxic trough serum gentamicin levels. Sixty-one infants had gentamicin q24hly and 25 (41%) had toxic trough serum gentamicin levels. The differences in proportions with toxic levels were statistically significant. In group three (> or =35 weeks) 29 infants had gentamicin q12hly and 25 (86%) had toxic trough serum gentamicin levels. Six infants had gentamicin every 18 h and 2 (33%) had toxic trough serum gentamicin levels. Thirty-one infants had gentamicin q24hly and 4 (13%) had toxic trough serum gentamicin levels. The differences in proportions comparing infants having gentamicin q12hly with those having it q24hly were statistically significant.

CONCLUSIONS

A starting gentamicin dosage interval of 12 h in infants of any gestational age, or a starting dosage interval of 24 h for infants of less than 30 weeks gestational age, leads to most having toxic trough serum gentamicin levels. In infants of 30 weeks gestational age or greater, most have safe non-toxic trough serum gentamicin levels if started on a dosage interval of 24 h.

Therapeutic drug monitoring of tobramycin: once-daily versus twice-daily dosage schedules

OBJECTIVE

To evaluate the effect of dosage regimen (once-daily vs. twice-daily) of tobramicyn on steady-state serum concentrations and toxicity.

MATERIALS AND METHODS

Patients undergoing treatment with i.v. tobramycin (4 mg/kg/day) were randomised to two groups. Group OD (n = 22) received a once-daily dose of tobramycin and group TD (n = 21) received the same dose divided into two doses daily. Tobramycin serum concentrations (peak and trough) were measured by enzyme multiplied immunoassay. The renal and auditory functions of the patients were monitored before, during and immediately after treatment.

RESULTS

The two groups were comparable with respect to sex, age, body weight and renal function. No statistically significant differences were found in mean daily dose, duration of treatment, or cumulative dose. Trough concentrations were < 2 g/ml in the two groups (100%). Peak concentrations were > 6 microg/ml in 100% of the OD group and in 67% of the TD group (P< 0.01). Mean peak concentrations were markedly different: 11.00+/-2.89 microg/ml in OD vs. 6.53+/-1.45 microg/ml in TD (P< 0.01). The pharmacokinetics parameters were: Ke, (0.15+/-0.03/h in OD vs. 0.24+/-0.06/h in TD), t1/2, (4.95+/-1.41 h in OD vs. 3.07+/-0.71 h in TD), Vd (0.35+/-0.11 l/kg in OD vs. 0.33+/-0.09 l/kg in TD), Cl (0.86+/-0.29 ml/min/kg in OD vs. 1.28+/-0.33 ml/min/kg in TD). Increased serum creatinine was observed in 73% of patients in OD versus 57% of patients in TD, without evidence of nephrotoxicity. In TD group, three patients developed decreased auditory function, of which one presented with an auditory loss of -30 dB, whereas in the OD group only one patient presented decreased auditory function.

CONCLUSION

This small study suggests that a once-daily dosing regimen of tobramycin is at least as effective as and is no more and possibly less toxic than the twice-daily regimen. Using a single-dose therapy, peak concentration determination is not necessary, only trough samples should be monitored to ensure levels below 2 microg/ml.

Chromatography as an analytical tool for selected antibiotic classes: a reappraisal addressed to pharmacokinetic applications

The first antibiotic discovered, penicillin, appeared on the market just after the Second World War. Intensive research in subsequent years led to the discovery and development of cephalosporins, aminoglycosides, tetracyclines and rifamycin. The chemotherapeutic quinolones and the more recently discovered fluoroquinolones have added promising new therapeutic weapons to fight the microbial challenge. The major role pharmacokinetics has played in developing these compounds should be highlighted. Plasma concentration-time profiles and the therapeutic activity evoked by these compounds allow the therapeutic window, doses and dose turnovers to be appropriately defined as well as possible dose adjustment to be made in renal failure. The pharmacokinetics of antimicrobial agents were initially explored by using microbiological methods, but these lack specificity. The HPLC technique with UV, fluorometric, electrochemical and, in some cases, mass spectrometry detection has satisfactory solved the problem of antimicrobial agent assay for pharmacokinetic, bioavailability and bioequivalence purposes alike. Indeed, in these studies, plasma concentrations of the given analyte must be followed up for a period > or = 3 times the half-life, which calls for specific sensitive assays. In the review, the authors have described the analytical methods employed in the pharmacokinetics of antibiotics, including some chemotherapeutic agents which are used in medical practice as alternatives to antibiotics. The pharmacokinetic characteristics of each class of drugs are also briefly described, and some historical and chemical notes on the various classes are given.

Population analysis of once-daily dosing of gentamicin in patients with neutropenia

BACKGROUND

Aminoglycoside antibiotics are used as first-line therapy in the treatment of patients with febrile neutropenia. However, there is little information on the pharmacokinetics of aminoglycosides in this specific patient group and whether they differ from the accepted population values.

AIM

To determine the population pharmacokinetics of gentamicin for patients with febrile neutropenia, with or without sepsis.

METHODS

Data were collected from 26 patients with febrile neutropenia receiving once-daily gentamicin. Patient age, height, weight, estimated lean body weight (LBW), gender, serum creatinine (SCr), creatinine clearance (Cl(Cr)), and serum drug concentration data were collected. Severity of infection and degree of neutropenia were assessed. An initial two-stage population analysis using a Bayesian dose-individualisation programme was performed to estimate likely population values of the parameters. If these were significantly different from typical values then a true population analysis was to be performed.

RESULTS

Data for 13 female and 13 male patients were collected. Median age was 57 years (range 19 to 87 years), estimated LBW (kg) = 66 (+/- 10), Cl(Cr) (1/hour) = 5.6 (+/- 2.0) (mean +/- SD). Results of the two-stage approach follow [initial population values in square brackets]: Cl (1/hour) = 0.73 (+/- 0.12) x Cl(Cr) + 0.01 x LBW [0.70 (+/- 0.27) x Cl(Cr) + 0.01 x LBW]; Vd (1) = 0.28 (+/- 0.05) x LBW [0.27 (+/- 0.06) x LBW].

CONCLUSIONS

The population values of Cl and Vd from the two-stage approach do not differ significantly from initial population values of Cl and Vd. Patients receiving gentamicin for febrile neutropenia may be considered pharmacokinetically similar to the general population receiving gentamicin.

Pharmacokinetic parameters of netilmicin and protective effect of piperacillin regarding nephrotoxicity caused by netilmicin

The pharmacokinetic interaction of Netilmicin and Piperacillin has been studied as well as the potential protective effect that Piperacillin exert on nephrotoxicity caused by Netilmicin, when both antibiotics are administered to rabbits by single and multiple dosage regimens. Netilmicin was administered at a dose of 7 mg/kg and 12 h interval, which allometrically correspond to 5 mg/kg at 24 h interval for men. Piperacillin was administered at a dose of 280 mg/kg at 12 h interval (the total number of doses of both antibiotics was 20). After single and multiple dose regimens plasma level curves of Netilmicin and renal concentration were determined using an HPLC technique. Besides that, an histologic study was carried out by electronic microscopy to determine the renal damage. A significant variation of some pharmacokinetic parameters of Netilmicin such as Vc and t(1/2) was observed when Netilmicin is administered together with Piperacillin; a similar modification in the renal accumulation and renal damage caused by Netilmicin was shown.

Pharmacokinetics of drugs used in critically ill adults

Critically ill patients exhibit a range of organ dysfunctions and often require treatment with a variety of drugs including sedatives, analgesics, neuromuscular blockers, antimicrobials, inotropes and gastric acid suppressants. Understanding how organ dysfunction can alter the pharmacokinetics of drugs is a vital aspect of therapy in this patient group. Many drugs will need to be given intravenously because of gastrointestinal failure. For those occasions on which the oral route is possible, bioavailability may be altered by hypomotility, changes in gastrointestinal pH and enteral feeding. Hepatic and renal dysfunction are the primary determinants of drug clearance, and hence of steady-state drug concentrations, and of efficacy and toxicity in the individual patient. Oxidative metabolism is the main clearance mechanism for many drugs and there is increasing recognition of the importance of decreased activity of the hepatic cytochrome P450 system in critically ill patients. Renal failure is equally important with both filtration and secretion clearance mechanisms being required for the removal of parent drugs and their active metabolites. Changes in the steady-state volume of distribution are often secondary to renal failure and may lower the effective drug concentrations in the body. Failure of the central nervous system, muscle, the endothelial system and endocrine system may also affect the pharmacokinetics of specific drugs. Time-dependency of alterations in pharmacokinetic parameters is well documented for some drugs. Understanding the underlying pathophysiology in the critically ill and applying pharmacokinetic principles in selection of drug and dose regimen is, therefore, crucial to optimising the pharmacodynamic response and outcome.

A comparison of two amikacin dosing regimens in paediatric surgical patients

This investigation aimed to compare the efficacy and toxicity of two amikacin dosing regimens in seriously ill paediatric surgical patients. Children (0.6-12 years old) received amikacin intravenously either once daily (15 mg/kg, n = 27) or twice daily (7.5 mg/kg, n = 27). Concomitant medication was given as prescribed. Mean (SD) peak serum amikacin levels were significantly different (p < 0.05) between the once and twice daily groups (37.7 (6.9) mg/l and 19.5 (3.7) mg/l, respectively). Cumulative dose and duration of therapy were also significantly higher in the once-daily group. Regimen efficacy (favourable, unfavourable or indeterminate outcome) was assessed by patient temperatures, clinical improvement and white cell counts. Serum creatinine measurements and post-therapy, pure tone air conduction audiometry assessed nephro- and ototoxicity, respectively. No statistically significant differences were found between the groups in terms of outcome (18/24 and 22/25 patients in the once- and twice-daily groups had favourable outcomes; there were no unfavourable outcomes), nephrotoxicity (none of the patients assessed developed nephrotoxicity) or ototoxicity (2/20 and 5/20 patients, respectively, had mild high frequency hearing deficits which were predominantly unilateral and reversible). Although the regimens were similar in this study, other investigations will further clarify the optimal dosing approach in paediatric patients.

Pharmacokinetics of once-daily dosing of gentamicin in neonates

In a prospective, randomized trial of once-daily versus twice-daily intravenous or intramuscular dosing with gentamicin, 11 neonates received 5.0 mg/kg once daily and 15 received 2.5 mg/kg twice daily for 2 ro 3 days. The once-daily intravenous dosing group and the twice-daily intravenous or intramuscular dosing group, respectively, had mean steady-state gentamicin peak concentrations of 10.7 versus 6.6 micrograms/ml (p < 0.05), 6-hour postdosing concentrations of 4.7 versus 2.8 micrograms/ml (p < 0.05), trough concentrations of 1.7 versus 1.7 micrograms/ml, elimination half-life of 8.8 versus 5.4 hours (p < 0.05), and volume of distribution at steady state of 0.67 versus 0.46 L/kg. No nephrotoxic effects were identified in any group. Once-daily gentamicin therapy with 5.0 mg/kg in neonates achieves peak serum levels that are more suitable for optimal bacterial killing than those which traditional regimens achieve. Similar trough levels suggest that even larger doses and longer dosing intervals may be ideal in term neonates.

Drug administration in patients with renal insufficiency. Minimising renal and extrarenal toxicity

Renal insufficiency has been associated with an increased risk of adverse effects with many classes of medications. The risk of some, but not all, adverse effects has been linked to the patient's degree of residual renal function. This may be the result of inappropriate individualisation of those agents that are primarily eliminated by the kidney, or an alteration in the pharmacodynamic response as a result of renal insufficiency. The pathophysiological mechanism responsible for alterations in drug disposition, especially metabolism and renal excretion, is the accumulation of uraemic toxins that may modulate cytochrome P450 enzyme activity and decrease glomerular filtration as well as tubular secretion. The general principles to enhance the safety of drug therapy in patients with renal insufficiency include knowledge of the potential toxicities and interactions of the therapeutic agent, consideration of possible alternatives therapies and individualisation of drug therapy based on patient level of renal function. Although optimisation of the desired therapeutic outcomes are of paramount importance, additional pharmacotherapeutic issues for patients with reduced renal function are the prevention or minimisation of future acute or chronic nephrotoxic insults, as well as the severity and occurrence of adverse effects on other organ systems. Risk factors for the development of nephrotoxicity for selected high-risk therapies (e.g. aminoglycosides, nonsteroidal anti-inflammatory drugs, ACE inhibitors and radiographic contrast media) are quite similar and include pre-existing renal insufficiency, concomitant administration of other nephrotoxins, volume depletion and concomitant hepatic disease or congestive heart failure. Investigations of prophylactic approaches to enhance the safety of these agents in patients with renal insufficiency have yielded inconsistent outcomes. Hydration with saline prior to drug exposure has given the most consistent benefit, while sodium loading and use of pharmacological interventions [e.g. furosemide (frusemide) dopomine/dobutamine, calcium antagonists and mannitol] have resulted in limited success. The mechanisms responsible for altered dynamic responses of some agents (benzodiazepines, theophylline, digoxin and loop diuretics) in renally compromised patients include enhanced receptor sensitivity secondary to the accumulation of endogenous uraemic toxins and competition for secretion to the renal tubular site of action. Application of the pharmacotherapeutic principles discussed into clinical practice will hopefully enhance the safety of these agents and optimise patient outcomes.

Dihydrostreptomycin or streptomycin in combination with penicillin G in dairy cattle therapeutics: A review and re-analysis of published data Part 1: Clinical pharmacology

Combination formulations of penicillin G salts and dihydrostreptomycin were developed during the 1960s and are currently marketed in New Zealand for parenteral and intramammary use in dairy cattle. In this paper, the clinical indications and efficacy, pharmacokinetics and potential for antimicrobial synergy of penicillin and dihydrostreptomycin or streptomycin, when combined, are discussed.

Once-daily gentamicin therapy for experimental Escherichia coli meningitis

In vitro and in vivo studies have demonstrated that the bacteriologic efficacy of once-daily aminoglycoside therapy is equivalent to that achieved with conventional multiple daily dosing. The impact of once-daily dosing for meningitis has not been studied. Using the well-characterized rabbit meningitis model, we compared two regimens of the same daily dosage of gentamicin given either once or in three divided doses for 24 or 72 h. The initial 1 h mean cerebrospinal fluid (CSF) gentamicin concentration for animals receiving a single dose (2.9 +/- 1.7 micrograms/ml) was threefold higher than that for the animals receiving multiple doses. The rate of bacterial killing in the first 8 h of treatment was significantly greater for the animals with higher concentrations in their CSF (-0.21 +/- 0.19 versus -0.03 +/- 0.22 log10 CFU/ml/h), suggesting concentration-dependent killing. By 24h, the mean reduction in bacterial titers was similar for the two regimens. In animals treated for 72 h, no differences in bactericidal activity was noted for 24, 48, or 72 h. Gentamicin at two different dosages was administered intracisternally to a separate set of animals to achieve considerably higher CSF gentamicin concentrations. In these animals, the rate of bacterial clearance in the first 8 h (0.52 +/- 0.15 and 0.58 +/- 0.15 log10 CFU/ml/h for the lower and higher dosages, respectively) was significantly greater than that in animals treated intravenously. In conclusion, there is evidence of concentration-dependent killing with gentamicin early in treatment for experimental E. coli meningitis, and once-daily dosing therapy appears to be at least as effective as multiple-dose therapy in reducing bacterial counts in CSF.

Does once-daily dosing of aminoglycosides affect neuromuscular function?

OBJECTIVE

Aminoglycosides have been reported to produce a curare-like neuromuscular blockade in animals at serum concentrations higher than those obtained with traditional dosing (1-2 mg/kg every 8 h) in humans. Aminoglycoside-induced neuromuscular blockade is rarely, if ever, seen in humans with traditional dosing. The recent adoption of once-daily dosing of aminoglycosides has raised concerns about increased potential for this adverse effect because higher serum concentrations are produced. The objective of this study was to determine if once-daily dosing of aminoglycosides inhibits respiratory muscle function.

METHOD

Nine mechanically ventilated ICU patients on once-daily dosing of gentamicin 6 mg/kg/day were assessed for respiratory muscle strength by measuring maximum inspiratory pressure (MIP). MIP is a measurement of the maximal negative pressure generated by repeated inhalations against an occluded airway over 20 s. This was measured within 1 hour before (MIPpre) and within 1 hour after each aminoglycoside dose (MIPpost).

RESULTS

Mean values for MIPpre and MIPpost were -26.7 cm H2O and -26.5 cm H2O, respectively. The mean difference between MIPpre and MIPpost was -0.2 cm H2O, which was not statistically significant (P > 0.05).

CONCLUSION

The effect of gentamicin (6 mg/kg/day) on respiratory muscle function was not statistically, nor clinically significant, and weaning from mechanical ventilation does not seem to be inhibited by once-daily dosing of aminoglycosides as detectable by measurement of MIP.

Fluctuation of the volume of distribution of amikacin and its effect on once-daily dosage and clearance in a seriously ill patient

OBJECTIVE

The main aim of the trial was to determine the extent to which the volume of distribution of amikacin fluctuates in a seriously ill patient receiving copious quantities of i.v. fluid over an extended term of treatment. The impact of the volume fluctuation on amikacin therapeutic peak concentrations was also assessed.

DESIGN AND SETTING

The case report describes a young, previously healthy male adult admitted to the surgical ICU of a teaching hospital following trauma to the head and central nervous system.

INTERVENTION

The patient received 1 g of amikacin once-daily i.v. for 35 consecutive days as part of an antimicrobial regimen. Blood samples were drawn for routine amikacin concentration determinations on 14 occasions, extending over the entire term of treatment, from which the required pharmacokinetic parameters were determined.

RESULTS

The volume of distribution of amikacin varied extensively from 0.27 to 0.61 l/kg (normal range 0.27 +/- 0.06 1/kg) notwithstanding the fact that amikacin clearance remained satisfactorily high throughout the term of treatment.

CONCLUSIONS

Once-daily therapeutic amikacin concentrations fluctuate extensively and rapidly in the seriously ill patient receiving copious quantities of i.v. fluids, despite competent renal function. The volume expansion seen in our patient is difficult to account for in terms of the extracellular fluid compartment only.

RECOMMENDATIONS

(a) Once-daily regimen amikacin peak concentrations should be frequently monitored in the seriously ill patient; (b) once-daily amikacin regimens are best monitored using blood specimens drawn at 1 and 6-8 h post administration.

Effect of polyaspartic acid on pharmacokinetics of gentamicin after single intravenous dose in the dog

The effects of poly-L-aspartic acid on the pharmacokinetics of gentamicin were examined by using a randomized crossover trial design with the dog. When analyzed according to a three-compartment open model, poly-L-aspartic acid reduced some first-order rate equation constants (A3, lambda 1, and lambda 3), the deep peripheral compartment exit microconstant (k31), the elimination rate constant (k(el)), and the area under the concentration-time curve from 0 to 480 h (AUC0-480) (0.21-, 0.60-, 0.26-, 0.27-, 0.72-, and 0.76-fold, respectively; P < 0.05) but increased the volume of distribution at steady state (Vss), the volume of distribution calculated by the area method (V(area)), the apparent volume of the peripheral compartment (Vp), and all mean time parameters. These results suggested that poly-L-aspartic acid increased the distribution of gentamicin to or binding within the deep peripheral compartment and that poly-L-aspartic acid may have delayed gentamicin transit through the peripheral tissues. In contrast, poly-L-aspartic acid did not alter pharmacokinetic parameters relevant to the central or shallow peripheral compartments to a clinically significant extent. Although gentamicin's pharmacokinetic parameters of relevance to therapeutic drug monitoring were not directly altered, this study has provided pharmacokinetic evidence that poly-L-aspartic acid alters the peripheral distribution of gentamicin. This pharmacokinetic interaction occurred after a single intravenous dose of each drug. Therefore, this interaction should be investigated further, before polyaspartic acid can be considered for use as a clinical nephroprotectant.

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.