Population pharmacokinetics of gentamicin in hospitalized patients receiving once-daily dosing

Multiple Blood Culture Sampling, Proper Antimicrobial Choice, and Adequate Dose in Definitive Therapy Supported by the Antimicrobial Stewardship Team Could Decrease 30-Day Sepsis Mortality Rates

Objective

This study aimed to identify factors that should be focused on by the antimicrobial stewardship team for treating patients with sepsis, by investigating the mortality of patients with sepsis within 30 days and the mortality-related factors in our hospital over a 10-year period from the perspective of appropriate antimicrobial use.

Methods

Factors associated with 30-day mortality were investigated using hierarchical multiple logistic regression in 1406 patients with pathogen-identified sepsis in Hirosaki University Hospital. These factors were clinical data, microbiological data, antimicrobials used in empiric and definitive therapies, presence/absence of ineffective use, underdosing as evaluated using Monte Carlo simulation, and practice of de-escalation.

Results

The ineffective use of antimicrobials in empiric therapy and the underdosing and ineffective use in definitive therapy were significantly associated with 30-day mortality (odds ratio [OR] = 2.70, 3.72, and 3.65, respectively). Multiple blood culture sampling was inversely associated with these inappropriate antimicrobial uses. Every year, the 30-day mortality rate has been decreasing, in line with the increase in multiple blood culture sampling and de-escalation; the inappropriate use of antimicrobials has also decreased.

Conclusion

Multiple blood culture sampling, proper choice of antimicrobial, and using an adequate dose in definitive therapy could decrease the 30-day mortality rate in patients with sepsis and these factors could be supported by the antimicrobial stewardship team.

Use of a common spreadsheet program to demonstrate the ability of Bayesian forecasting to estimate the pharmacokinetic parameters of antibiotics

OBJECTIVES

Recent guidelines for vancomycin have incorporated the use of Bayesian forecasting, reinforcing the need to inform students in pharmacy and clinical pharmacology of its use in therapeutic drug monitoring. The goal was to devise a PharmD research project that could demonstrate to students through simulation and data generation the utility of the Bayesian approach in estimating the pharmacokinetics of gentamicin and vancomycin.

METHODS

A series of steps were devised using Microsoft Excel to simulate patient data based on study-derived means and variances, pharmacokinetic modelling, random selection of sparse blood samples, introduce random error into the selected concentrations based on assay variability measure, and finally, inputting of the information into an add-in computer program to find the pharmacokinetic estimates using Bayesian forecasting.

KEY FINDINGS

Excellent correlations were seen between Bayesian estimates and true clearances. Lower assay variability tended to provide better estimates than larger assay variability for gentamicin, and for vancomycin, selecting a sample during the distribution phase and near the trough values tended to provide estimates with less bias and greater precision.

CONCLUSIONS

The approach used was able to demonstrate all aspects involved in Bayesian forecasting, and the results supported its use for these antibiotics.

Hemoperfusion with Seraph 100 Microbind Affinity Blood Filter Unlikely to Require Increased Antibiotic Dosing: A Simulations Study Using a Pharmacokinetic/Pharmacodynamic Approach

INTRODUCTION

The Seraph® 100 Microbind® Affinity Blood Filter (Seraph 100) is a hemoperfusion device that can remove pathogens from central circulation. However, the effect of Seraph 100 on achieving pharmacodynamic (PD) targets is not well described. We sought to determine the impact of Seraph 100 on ability to achieve PD targets for commonly used antibiotics.

METHODS

Estimates of Seraph 100 antibiotic clearance were obtained via literature. For vancomycin and gentamicin, published pharmacokinetic models were used to explore the effect of Seraph 100 on ability to achieve probability of target attainment (PTA). For meropenem and imipenem, the reported effect of continuous kidney replacement therapy (CKRT) on achieving PTA was used to extrapolate decisions for Seraph 100.

RESULTS

Seraph 100 antibiotic clearance is likely less than 0.5 L/h for most antibiotics. Theoretical Seraph 100 clearance up to 0.5 L/h and 2 L/h had a negligible effect on vancomycin PTA in virtual patients with creatinine clearance (CrCl) = 14 mL/min and CrCl >14 mL/min, respectively. Theoretical Seraph 100 clearance up to 0.5 L/h and 2 L/h had a negligible effect on gentamicin PTA in virtual patients with CrCl = 120 mL/min and CrCl <60 mL/min, respectively. CKRT intensity resulting in antibiotic clearance up to 2 L/h generally does not require dose increases for meropenem or imipenem. As Seraph 100 is prescribed intermittently and likely contributes far less to antibiotic clearance, dose increases would also not be required.

CONCLUSION

Seraph 100 clearance of vancomycin, gentamicin, meropenem, and imipenem is likely clinically insignificant. There is insufficient evidence to recommend increased doses. For aminoglycosides, we recommend extended interval dosing and initiating Seraph 100 at least 30 min to 1 h after completion of infusion to avoid the possibility of interference with maximum concentrations.

Population pharmacokinetics of apramycin from first-in-human plasma and urine data to support prediction of efficacious dose

Background

Apramycin is under development for human use as EBL-1003, a crystalline free base of apramycin, in face of increasing incidence of multidrug-resistant bacteria. Both toxicity and cross-resistance, commonly seen for other aminoglycosides, appear relatively low owing to its distinct chemical structure.

Objectives

To perform a population pharmacokinetic (PPK) analysis and predict an efficacious dose based on data from a first-in-human Phase I trial.

Methods

The drug was administered intravenously over 30 min in five ascending-dose groups ranging from 0.3 to 30 mg/kg. Plasma and urine samples were collected from 30 healthy volunteers. PPK model development was performed stepwise and the final model was used for PTA analysis.

Results

A mammillary four-compartment PPK model, with linear elimination and a renal fractional excretion of 90%, described the data. Apramycin clearance was proportional to the absolute estimated glomerular filtration rate (eGFR). All fixed effect parameters were allometrically scaled to total body weight (TBW). Clearance and steady-state volume of distribution were estimated to 5.5 L/h and 16 L, respectively, for a typical individual with absolute eGFR of 124 mL/min and TBW of 70 kg. PTA analyses demonstrated that the anticipated efficacious dose (30 mg/kg daily, 30 min intravenous infusion) reaches a probability of 96.4% for a free AUC/MIC target of 40, given an MIC of 8 mg/L, in a virtual Phase II patient population with an absolute eGFR extrapolated to 80 mL/min.

Conclusions

The results support further Phase II clinical trials with apramycin at an anticipated efficacious dose of 30 mg/kg once daily.

Clinical Pharmacokinetics of Gentamicin in Various Patient Populations and Consequences for Optimal Dosing for Gram-Negative Infections: An Updated Review

Gentamicin is an aminoglycoside antibiotic with a small therapeutic window that is currently used primarily as part of short-term empirical combination therapy. Gentamicin dosing schemes still need refinement, especially for subpopulations where pharmacokinetics can differ from pharmacokinetics in the general adult population: obese patients, critically ill patients, paediatric patients, neonates, elderly patients and patients on dialysis. This review summarizes the clinical pharmacokinetics of gentamicin in these patient populations and the consequences for optimal dosing of gentamicin for infections caused by Gram-negative bacteria, highlighting new insights from the last 10 years. In this period, several new population pharmacokinetic studies have focused on these subpopulations, providing insights into the typical values of the most relevant pharmacokinetic parameters, the variability of these parameters and possible explanations for this variability, although unexplained variability often remains high. Both dosing schemes and pharmacokinetic/pharmacodynamic (PK/PD) targets varied widely between these studies. A gentamicin starting dose of 7 mg/kg based on total body weight (or on adjusted body weight in obese patients) appears to be the optimal strategy for increasing the probability of target attainment (PTA) after the first administration for the most commonly used PK/PD targets in adults and children older than 1 month, including critically ill patients. However, evidence that increasing the PTA results in higher efficacy is lacking; no studies were identified that show a correlation between estimated or predicted PK/PD target attainment and clinical success. Although it is unclear if performing therapeutic drug monitoring (TDM) for optimization of the PTA is of clinical value, it is recommended in patients with highly variable pharmacokinetics, including patients from all subpopulations that are critically ill (such as elderly, children and neonates) and patients on intermittent haemodialysis. In addition, TDM for optimization of the dosing interval, targeting a trough concentration of at least < 2 mg/L but preferably < 0.5–1 mg/L, has proven to reduce nephrotoxicity and is therefore recommended in all patients receiving more than one dose of gentamicin. The usefulness of the daily area under the plasma concentration–time curve for predicting nephrotoxicity should be further investigated. Additionally, more research is needed on the optimal PK/PD targets for efficacy in the clinical situations in which gentamicin is currently used, that is, as monotherapy for urinary tract infections or as part of short-term combination therapy.

Efficacy of EBL-1003 (apramycin) against Acinetobacter baumannii lung infections in mice

OBJECTIVES

Novel therapeutics are urgently required for the treatment of carbapenem-resistant Acinetobacter baumannii (CRAB) causing critical infections with high mortality. Here we assessed the therapeutic potential of the clinical-stage drug candidate EBL-1003 (crystalline free base of apramycin) in the treatment of CRAB lung infections.

METHODS

The genotypic and phenotypic susceptibility of CRAB clinical isolates to aminoglycosides and colistin was assessed by database mining and broth microdilution. The therapeutic potential was assessed by target attainment simulations on the basis of time-kill kinetics, a murine lung infection model, comparative pharmacokinetic analysis in plasma, epithelial lining fluid (ELF) and lung tissue, and pharmacokinetic/pharmacodynamic (PKPD) modelling.

RESULTS

Resistance gene annotations of 5451 CRAB genomes deposited in the National Database of Antibiotic Resistant Organisms (NDARO) suggested >99.9% of genotypic susceptibility to apramycin. Low susceptibility to standard-of-care aminoglycosides and high susceptibility to EBL-1003 were confirmed by antimicrobial susceptibility testing of 100 A. baumannii isolates. Time-kill experiments and a mouse lung infection model with the extremely drug-resistant CRAB strain AR Bank #0282 resulted in rapid 4-log CFU reduction both in vitro and in vivo. A single dose of 125 mg/kg EBL-1003 in CRAB-infected mice resulted in an AUC of 339 h × μg/mL in plasma and 299 h × μg/mL in ELF, suggesting a lung penetration of 88%. PKPD simulations suggested a previously predicted dose of 30 mg/kg in patients (creatinine clearance (CLCr) = 80 mL/min) to result in >99% probability of -2 log target attainment for MICs up to 16 μg/mL.

CONCLUSIONS

This study provides proof of concept for the efficacy of EBL-1003 in the treatment of CRAB lung infections. Broad in vitro coverage, rapid killing, potent in vivo efficacy, and a high probability of target attainment render EBL-1003 a strong therapeutic candidate for a priority pathogen for which treatment options are very limited.

Model-Informed Drug Development for Antimicrobials: Translational PK and PK/PD Modeling to Predict an Efficacious Human Dose for Apramycin

Apramycin represents a subclass of aminoglycoside antibiotics that has been shown to evade almost all mechanisms of clinically relevant aminoglycoside resistance. Model-informed drug development may facilitate its transition from preclinical to clinical phase. This study explored the potential of pharmacokinetic/pharmacodynamic (PK/PD) modeling to maximize the use of in vitro time-kill and in vivo preclinical data for prediction of a human efficacious dose (HED) for apramycin. PK model parameters of apramycin from four different species (mouse, rat, guinea pig, and dog) were allometrically scaled to humans. A semimechanistic PK/PD model was developed from the rich in vitro data on four Escherichia coli strains and subsequently the sparse in vivo efficacy data on the same strains were integrated. An efficacious human dose was predicted from the PK/PD model and compared with the classical PK/PD index methodology and the aminoglycoside dose similarity. One-compartment models described the PK data and human values for clearance and volume of distribution were predicted to 7.07 L/hour and 26.8 L, respectively. The required fAUC/MIC (area under the unbound drug concentration-time curve over MIC ratio) targets for stasis and 1-log kill in the thigh model were 34.5 and 76.2, respectively. The developed PK/PD model predicted the efficacy data well with strain-specific differences in susceptibility, maximum bacterial load, and resistance development. All three dose prediction approaches supported an apramycin daily dose of 30 mg/kg for a typical adult patient. The results indicate that the mechanistic PK/PD modeling approach can be suitable for HED prediction and serves to efficiently integrate all available efficacy data with potential to improve predictive capacity.

Population Pharmacokinetic Analyses for Arbekacin after Administration of ME1100 Inhalation Solution

ME1100, an inhalation solution of arbekacin, an aminoglycoside, is being developed for the treatment of hospital-acquired and ventilator-associated bacterial pneumonia. The objective of these analyses was to develop a population pharmacokinetic model to describe the arbekacin concentration-time profile in plasma and epithelial lining fluid (ELF) following ME1100 administration. Data were obtained from a postmarketing study for an intravenous (i.v.) formulation of arbekacin, a phase 1 study of ME1100 in healthy volunteers, and a phase 1b study of ME1100 in mechanically ventilated subjects with bacterial pneumonia. Data from the postmarketing study were utilized to develop a population pharmacokinetic model following i.v. administration, and this model was subsequently utilized as the foundation for development of the model characterizing arbekacin disposition following inhalation of ME1100. The final model utilized two compartments for both plasma and ELF disposition, with movement of arbekacin between the ELF and plasma parameterized using linear first-order rate constants. A bioavailability term was included for the inhalational route of administration, which was estimated to be 19.5% for a typical subject. The model included normalized creatinine clearance (CLcrn) and weight as covariates on arbekacin clearance: CL = (weight/52.2)^0.855·[(CLcrn-77)·0.0289 + 2.32]. The model simultaneously described arbekacin concentrations following both i.v. and inhaled administration and provided acceptable fits to the plasma and ELF data (r ² of 0.922 and 0.557 for observed versus fitted concentrations, respectively). The developed model will be useful for conducting future analyses to support ME1100 dose selection.

Population Pharmacokinetic Modeling of Gentamicin in Pediatrics

The primary objective of this work was to characterize the pharmacokinetics (PK) of gentamicin across the whole pediatric age spectrum from premature neonates to young adults with a single model by identifying significant clinical predictors. A nonlinear mixed-effect population PK model was developed with retrospective therapeutic drug-monitoring data. A total of 6459 drug concentration measurements from 3370 hospitalized patients were collected for model building (n = 2357) and evaluation (n = 1013). In agreement with previously reported models, a 2-compartment model with first-order elimination best described the drug PK. Patient-specific factors significantly impacting gentamicin clearance included fat-free mass, postmenstrual age, and serum creatinine (SCr). Based on our model, the deviation of the individual SCr from the age-dependent expected mean SCr value (SCrM) can result in a 40% lower clearance in a patient with renal impairment than that in a patient with normal kidney function, with SCrM:SCr ratios between 0.16 and 3.2 in this study. Consistent with the known age-dependent changes of the proportion of extracellular water in body weight, the inclusion of the impact of extracellular water maturation on the central volume of distribution was found to improve the model fitting significantly. In comparison with other published models, model evaluation suggested the developed model was the least biased and physiologically most representative. These results will be used to inform individualized initial dosing strategies and serve as a prior PK model for Bayesian updating and forecasting as individual clinical observations become available.

Pharmacokinetic/pharmacodynamic analysis as a tool for surveillance of the activity of antimicrobials against Pseudomonas aeruginosa strains isolated in critically ill patients

INTRODUCTION

To evaluate the changes in the susceptibility of Pseudomonas aeruginosa over time (2000-2017) against antimicrobials used in an intensive care unit of a Spanish tertiary hospital, and to compare them with the antimicrobial activity considering theoretical pharmacokinetic/pharmacodynamic (PK/PD) criteria. The influence of the method for handling duplicate isolates to quantify susceptibility rates was also evaluated.

METHODS

The susceptibility was studied considering the Clinical and Laboratory Standards Institute (CLSI) breakpoints. Monte Carlo simulations were conducted to calculate the cumulative fraction of response (CFR). Linear regression analysis was applied to determine the trends in susceptibility and in the CFR.

RESULTS

A significant decrease in the susceptibility to gentamicin and imipenem was observed, and more recently the highest percentages of susceptible strains were found for amikacin, cephalosporins and piperacillin/tazobactam (>80%). The probability of success of an empiric treatment or CFR for most of the evaluated antimicrobials was lower than 70% during the last two-year period. Only meropenem provided high probabilities (>90%) to achieve the PK/PD target. Cephalosporins provided moderate probabilities (>80%) although for ceftazidime, the highest dose (2g/8h) was required. Moreover, a significant decrease in the CFR trend for ciprofloxacin, imipenem and levofloxacin was observed.

CONCLUSIONS

Both susceptibility rates and CFR values have to be considered together to optimize the antimicrobial dose regimen for clinical making-decisions. They are complementary tools and, they should be used jointly in surveillance programmes. In fact, susceptibility data are not always useful to detect changes in the CFR. No relevant differences were observed among the methods for handling repeated isolates.

Therapeutic Drug Monitoring of Gentamicin Peak Concentrations in Critically Ill Patients

BACKGROUND

Adequate gentamicin peak concentrations (Cmax) are important for optimal clinical efficacy. Within a critically ill patient, substantial variability in Cmax can occur over time, hampering the usefulness of therapeutic drug monitoring (TDM). The aim of this study was to evaluate the effect of gentamicin dosing based on Cmax after the first dose on gentamicin target attainment in critically ill patients.

METHODS

From gentamicin-treated critically ill patients, dosing information, clinical parameters, and serum concentrations were collected prospectively. A population pharmacokinetic model was developed using nonlinear mixed-effects modeling to estimate Cmax after each dose. To evaluate the usefulness of routine TDM, percentages of Cmax within (%Cther, 15-20 mg/L), above (>20 mg/L), and below (%Csubther, <15 mg/L) the therapeutic range after the first and second doses were compared. In addition, simulations were performed to evaluate the impact of TDM.

RESULTS

Four hundred sixteen measurements from 59 patients receiving 130 gentamicin doses were included. In the 30 patients who received >1 dose, TDM increased %Cther from 40% after a first median dose of 5.0 mg/kg to 50% after the second dose, and decreased %Csubther from 47% to 30%. Simulations using a 5 mg/kg starting dose revealed %Cther after the second dose of 28.4% without and 36.8% with TDM and %Csubther of 56.9% and 29.3%, respectively. Increasing the simulated starting dose to 6 mg/kg increased %Cther after the first dose from 27.7% to 33.5% and decreased %Csubther from 58.6% to 35.6%. TDM after a first dose of 6 mg/kg had no substantial effect on %Cther or %Csubther after the second dose.

CONCLUSIONS

Gentamicin dosing based on Cmax after the first dose increased %Cther and decreased %Csubther, but did not result in therapeutic Cmax in half of the patients. When simulating a higher starting dose, %Csubther after the first dose decreased, and TDM showed no additional influence. These data suggest that a starting dose of 6 mg/kg should be considered and that repeated Cmax measurements are not of added value.

Predictive performance of a gentamicin population pharmacokinetic model in two western populations of critically ill patients

External validation of population pharmacokinetic (PK) models is warranted before they can be clinically applied to aid in antibiotic dose selection. The primary objective of this study was to assess the predictive performance of a gentamicin population PK model in intensive care unit (ICU) patients in two independent western populations of critically ill patients.

METHODS

Data were collected from the ICU where the model was developed (Academic Medical Centre, Amsterdam [AMC]) and from the Centre Hospitalier Universitaire de Nîmes (CHU Nîmes). Primary endpoints were bias and accuracy. The model was regarded as valid if bias was not significantly different from 0 and accuracy was equal to or less than 2.5 mg/L. Non-linear mixed-effects modelling (NONMEM) was used for data analysis.

RESULTS

The AMC validation dataset consisted of 192 samples from 66 ICU patients and the CHU Nîmes dataset of 230 gentamicin samples from 50 ICU patients. The structural model predicted the gentamicin plasma concentrations in the AMC population with a non-significant bias (0.35, 95%CI: -0.11-0.81) and a sufficient accuracy of 2.5 mg/L (95%CI: 2.3-2.8). The gentamicin plasma concentrations were overpredicted in the CHU Nîmes population with a significant bias of 4.8 mg/L (95%CI: 4.00-5.62) and an accuracy of 5.5 mg/L (95%CI: 4.7-6.2).

CONCLUSION

The model is valid for use in the AMC ICU population but not in the CHU Nîmes ICU population. This illustrates that caution is needed when using a population PK model in an external population.

Population pharmacokinetic modelling, Monte Carlo simulation and semi-mechanistic pharmacodynamic modelling as tools to personalize gentamicin therapy

Population pharmacokinetic modelling, Monte Carlo simulation and semi-mechanistic pharmacodynamic modelling are all tools that can be applied to personalize gentamicin therapy. This review summarizes and evaluates literature knowledge on the population pharmacokinetics and pharmacodynamics of gentamicin and identifies areas where further research is required to successfully individualize gentamicin therapy using modelling and simulation techniques. Thirty-five studies have developed a population pharmacokinetic model of gentamicin and 15 studies have made dosing recommendations based on Monte Carlo simulation. Variability in gentamicin clearance was most commonly related to renal function in adults and body weight and age in paediatrics. Nine studies have related aminoglycoside exposure indices to clinical outcomes. Most commonly, efficacy has been linked to a Cmax/MIC ≥7-10 and a AUC24/MIC ≥70-100. No study to date has shown a relationship between predicted achievement of exposure targets and actual clinical success. Five studies have developed a semi-mechanistic pharmacokinetic/pharmacodynamic model to predict bacteria killing and regrowth following gentamicin exposure and one study has developed a deterministic model of aminoglycoside nephrotoxicity. More complex semi-mechanistic models are required that consider the immune response, use of multiple antibiotics, the severity of illness, and both efficacy and toxicity. As our understanding grows, dosing of gentamicin based on sound pharmacokinetic/pharmacodynamic principles should be applied more commonly in clinical practice.

Scaling clearance in paediatric pharmacokinetics: All models are wrong, which are useful?

Linked Articles

This article is commented on in the editorial by Holford NHG and Anderson BJ. Why standards are useful for predicting doses. Br J Clin Pharmacol 2017; 83: 685–7. doi: 10.1111/bcp.13230

Aim

When different models for weight and age are used in paediatric pharmacokinetic studies it is difficult to compare parameters between studies or perform model‐based meta‐analyses. This study aimed to compare published models with the proposed standard model (allometric weight^0.75 and sigmoidal maturation function).

Methods

A systematic literature search was undertaken to identify published clearance (CL) reports for gentamicin and midazolam and all published models for scaling clearance in children. Each model was fitted to the CL values for gentamicin and midazolam, and the results compared with the standard model (allometric weight exponent of 0.75, along with a sigmoidal maturation function estimating the time in weeks of postmenstrual age to reach half the mature value and a shape parameter). For comparison, we also looked at allometric size models with no age effect, the influence of estimating the allometric exponent in the standard model and, for gentamicin, using a fixed allometric exponent of 0.632 as per a study on glomerular filtration rate maturation. Akaike information criteria (AIC) and visual predictive checks were used for evaluation.

Results

No model gave an improved AIC in all age groups, but one model for gentamicin and three models for midazolam gave slightly improved global AIC fits albeit using more parameters: AIC drop (number of parameters), –4.1 (5), –9.2 (4), –10.8 (5) and –10.1 (5), respectively. The 95% confidence interval of estimated CL for all top performing models overlapped.

Conclusion

No evidence to reject the standard model was found; given the benefits of standardised parameterisation, its use should therefore be recommended.

Population pharmacokinetics of gentamicin and dosing optimization for infants

The aim of this study was to characterize and validate the population pharmacokinetics of gentamicin in infants and to determine the influences of clinically relevant covariates to explain the inter- and intraindividual variabilities associated with this drug. Infants receiving intravenous gentamicin and with routine therapeutic drug monitoring were consecutively enrolled in the study. Plasma concentration and time data were retrospectively collected from 208 infants (1 to 24 months old) of the Hospital Universitario Severo Ochoa (Spain), of whom 44% were males (mean age [± standard deviation], 5.8 ± 4.8 months; mean body weight, 6.4 ± 2.2 kg). Data analysis was performed with NONMEM 7.2. One- and two-compartment open models were analyzed to estimate the gentamicin population parameters and the influences of several covariates. External validation was carried out in another population of 55 infants. The behavior of gentamicin in infants exhibits two-compartment pharmacokinetics, with total body weight being the covariate that mainly influences central volume (Vc) and clearance (CL); this parameter was also related to creatinine clearance. Both parameters are age related and different from those reported for neonatal populations. On the basis of clinical presentation and diagnosis, a once-daily dosage regimen of 7 mg/kg of body weight every 24 h is proposed for intravenous gentamicin, followed by therapeutic drug monitoring in order to avoid toxicity and ensure efficacy with minimal blood sampling. Gentamicin pharmacokinetics and disposition were accurately characterized in this pediatric population (infants), with the parameters obtained being different from those reported for neonates and children. These differences should be considered in the dosing and therapeutic monitoring of this antibiotic.

Predictive performance of gentamicin dosing nomograms

Background

Several nomograms have been proposed to facilitate the determination of initial gentamicin dosing regimens in clinical settings. This study aimed to assess the predictive performance of these nomograms in Korean patients.

Methods

Gentamicin concentrations were determined in 84 patients with infective endocarditis (IE) and in 95 patients with other infections. All patients underwent therapeutic drug monitoring in Seoul National University Hospital from 2006 to 2012. Individual pharmacokinetic parameters were estimated using a Bayesian method, which predicted steady state peak and trough serum concentrations. Six nomograms were evaluated in patients with “other” infections: the Thomson guidelines, Hull-Sarubbi table, and Rule of Eights, for multiple daily dosing; and the Hartford nomogram, Barnes-Jewish Hospital nomogram, and Sanford Guide, for extended-interval dosing. In IE patients, synergistic combination dosing nomograms, based on the American Heart Association dosing interval guidelines, were evaluated.

Results

Gentamicin dosing nomograms performed poorly in attaining the target peak serum concentrations. Multiple-daily dosing nomograms predicted peak serum gentamicin concentrations better than did the extended-interval dosing nomograms (31.9%–72.3% vs 4.3%–45.7%, respectively). Similarly, in patients with IE, the once-daily dosing nomogram resulted in a significantly lower percentage of patients achieving target peak gentamicin concentrations than that associated with the thrice-daily dosing nomogram (P=0.0015). All of the multiple-daily dosing, extended-interval dosing, and synergistic combination dosing nomograms predicted the nontoxic target trough concentrations in >80% of patients.

Conclusion

Gentamicin dosing nomograms performed poorly in achieving the target peak serum concentrations. New gentamicin nomograms may be required in patients with IE, particularly for once-daily dosing. Therapeutic drug monitoring is highly recommended for gentamicin to ensure that the target concentrations are achieved.

The influence of acute kidney injury on antimicrobial dosing in critically ill patients: are dose reductions always necessary?

Optimal dosing of antimicrobial therapy is pivotal to increase the likelihood of survival in critically ill patients with sepsis. Drug exposure that maximizes bacterial killing, minimizes the development of antimicrobial resistance, and avoids concentration-related toxicities should be considered the target of therapy. However, antimicrobial dosing is problematic as pathophysiological factors inherent to sepsis that alter may result in reduced concentrations. Alternatively, sepsis may evolve to multiple-organ dysfunction including acute kidney injury (AKI). In this case, decreased clearance of renally cleared drugs is possible, which may lead to increased concentrations that may cause drug toxicities. Consequently, when dosing antibiotics in septic patients with AKI, one should consider factors that may lead to underdosing and overdosing. Drug-specific pharmacokinetic and pharmacodynamic data may be helpful to guide dosing in these circumstances. Yet, because of the high interpatient variability in pharmacokinetics of antibiotics during sepsis, this issue remains a significant challenge.

Pharmacokinetic/ pharmacodynamic evaluation of anti-infective agents

Pharmacokinetic/pharmacodynamic modeling has become an extremely important tool in evaluating and optimizing anti-infective therapy. By systematically linking the pharmacokinetic and pharmacodynamic properties of the anti-infective agent, it is possible to make educated decisions about the correct drug to be used, correct dosing regimen and to estimate the probability of success with the selected dose regimen. This article gives an overview of the current pharmacokinetic/pharmacodynamic approaches for anti-infective agents and discusses their use in optimizing drug therapy.

Integrating pharmacokinetics, pharmacodynamics and MIC distributions to assess changing antimicrobial activity against clinical isolates of Pseudomonas aeruginosa causing infections in Canadian hospitals (CANWARD)

OBJECTIVES

To study antimicrobial pharmacodynamic (PD) activity over time against clinical isolates of Pseudomonas aeruginosa in Canadian hospitals.

METHODS

Integrated pharmacokinetic (PK)/PD analyses with Monte Carlo simulations were used to study cefepime, meropenem, piperacillin/tazobactam, ciprofloxacin, amikacin, gentamicin and colistin. Profiles of P. aeruginosa infections were modelled using CANWARD data from January 2007 to June 2012 inclusive. The probability of target attainment (PTA) was the proportion of cases achieving a %ƒT>MIC ≥ 50% for cefepime, meropenem and piperacillin/tazobactam, an ƒAUC/MIC ≥ 90 for ciprofloxacin and the aminoglycosides, and a total AUC/MIC ≥ 60 for colistin.

RESULTS

Some 2126 P. aeruginosa isolates were identified. There were no significant trends over time in the PTA for cefepime (0.93-1.0), meropenem (0.89-0.92) or piperacillin/tazobactam (0.74-0.79) (data shown for the highest recommended doses). The PD activity for ciprofloxacin (PTA 0.48-0.64) was variable. There were notable improvements in the PTA for amikacin (from 0.21 to 0.55, P = 0.027), gentamicin (from 0.10 to 0.51, P = 0.035) and colistin (from 0.04 to ~0.20, P = 0.05), which were not reliably detected by MIC indices. There was a decline over time in the PTA for piperacillin/tazobactam from 0.73 to 0.61 against P. aeruginosa isolated from intensive care units (ICUs) (Pearson correlation coefficient -0.99, P = 0.003). Neither MIC50 nor MIC90 detected this reduction in PD activity.

CONCLUSIONS

The overall PD activity against P. aeruginosa was stable from 2007 to 2012 for cefepime, meropenem and piperacillin/tazobactam, was variable and unreliable for ciprofloxacin, and improved significantly but remained relatively low for the aminoglycosides and colistin. There was a progressive reduction over time in the PD activity of piperacillin/tazobactam against ICU isolates, which was not detected by simply assessing MIC indices.

Dosing of Gentamicin in Patients With End-Stage Renal Disease Receiving Hemodialysis

The aim of this study was to evaluate dosing schedules of gentamicin in patients with end-stage renal disease and receiving hemodialysis. Forty-six patients were recruited who received gentamicin while on hemodialysis. Each patient provided approximately 4 blood samples at various times before and after dialysis for analysis of plasma gentamicin concentrations. A population pharmacokinetic model was constructed using NONMEM (version 5). The clearance of gentamicin during dialysis was 4.69 L/h and between dialysis was 0.453 L/h. The clearance between dialysis was best described by residual creatinine clearance (as calculated using the Cockcroft and Gault equation), which probably reflects both lean mass and residual clearance mechanisms. Simulation from the final population model showed that predialysis dosing has a higher probability of achieving target maximum concentration (Cmax) concentrations (> 8 mg/L) within acceptable exposure limits (area under the concentration-time curve [AUC] values > 70 and < 120 mg x h/L per 24 hours) than postdialysis dosing.

Comparison of four renal function estimation equations for pharmacokinetic modeling of gentamicin in geriatric patients

Most aminoglycoside pharmacokinetic models include an index of renal function, such as creatinine clearance, to describe drug clearance. However, the best clinical descriptor of renal function for the pharmacokinetic modeling of aminoglycosides has not been established. This analysis was based on 412 gentamicin concentrations from 92 geriatric patients who received intravenous gentamicin for various infectious diseases. Four two-compartment population models were fitted to gentamicin concentrations in a learning set of 64 patients using the nonparametric adaptive grid (NPAG) algorithm. Each model included an index of renal function, namely, the Cockcroft-Gault (CG), Jelliffe (JEL), modification of diet in renal disease (MDRD), or modified MDRD (MDRDm; adjusted to individual body surface area) equation as a covariate influencing gentamicin serum clearance. Goodness of fit and predictive performance of the four models were compared using standard criteria in both the learning set and in a validation set of 28 patients. A final analysis was performed to estimate the population pharmacokinetic parameter values of the entire 92-patient group. In the learning set, the CG-based model best fit the data, followed by JEL-, MDRD-, and MDRDm-based models, with relative reductions of the Akaike information criterion of 29.4, 20.2, 14.2, and 4.2, respectively. Bias and precision of population predictions were significantly different among the four models. In the validation set, individual predictions from the four models showed marginally different biases. The final estimation confirmed the previous results. Specifically, the CG-based model showed predictive performance that was comparable to or better than that of the MDRD-based model at each stage of the analysis. This study shows that methods used to estimate renal function should not be considered interchangeable for the model-based estimation of gentamicin concentrations.

Factors Affecting Gentamicin Penetration in Lower Extremity Ischemic Tissues With Ulcers

The aims of the study were to analyze the penetration of gentamicin in foot ulcers in patients with different severities of peripheral arterial disease (PAD) and to determine significant parameters affecting lower limb tissue concentrations. Patients undergoing debridement of a wound or an amputation procedure were included. All patients received a 120 mg or 240 mg intravenous dose of gentamicin prior to the procedure. Patients were classified according to the degree of PAD. Tissue and serum samples were collected at the time of intervention, and gentamicin concentrations were determined by fluorescence polarization immunoassay. Blood and tissue samples were taken from 61 patients, 41 males and 20 females with a mean age of 66 years. Nineteen patients had nil or borderline PAD, 9 patients had mild or moderate PAD, and 26 patients had severe PAD. Forty-eight patients had type 2 diabetes, 8 patients had type 1 diabetes, and 5 patients were nondiabetic. The concentration of gentamicin in peripheral skeletal muscle tissue was dependent on the serum concentration, degree of PAD, gender, and age. For patients with ischemic lower extremity wounds (patients with mild, moderate, and severe PAD), the concentration of gentamicin was significantly lower ( P = .010) than the concentration in nonischemic wounds, and the concentration in female patients was also significantly lower than in male patients ( P = .047). The concentration in peripheral subcutaneous tissue was 0.663 times the concentration in skeletal muscle tissue ( P < .00001). Gentamicin showed greatest penetration in male patients without PAD. For patients with severe PAD, higher doses of gentamicin may be required to achieve the same effect.

Antibiotic pharmacokinetic and pharmacodynamic considerations in patients with kidney disease

Although pharmacokinetic changes occurring in kidney disease are well described, pharmacodynamics in kidney disease is rarely considered. Knowledge of pharmacodynamic principles can allow a clinician to maximize an antibiotic's effectiveness while minimizing adverse effects and antibacterial resistance. An antibiotic's pharmacokinetic and pharmacodynamic profiles should drive dose adjustment decisions in patients with kidney disease. For example, although the half-lives of beta-lactams and aminoglycosides are both prolonged in these patients, beta-lactams exhibit time-dependent antibacterial activity; consequently, maintenance doses should be smaller but given at the same interval. In contrast, aminoglycosides are concentration-dependent antibiotics; hence prolongation of the dosing interval while using larger doses may be advantageous. The timing of drug administration in relation to hemodialysis may be used to achieve specific pharmacodynamic goals. Aminoglycosides given before hemodialysis generate high peaks, whereas subsequent dialytic drug removal minimizes the area under the serum concentration-time curve, potentially decreasing the risk of developing toxicity. Furthermore, new dialysis prescribing patterns (eg, automated peritoneal dialysis, nocturnal dialysis) affect pharmacokinetic and pharmacodynamic parameters in ways not appreciated by clinicians. Studies quantifying the often considerable drug removal with these therapies, as well as efforts to identify pharmacodynamic targets in patients with kidney disease are essential. This paper reviews pharmacodynamic as well as pharmacokinetic issues that should be considered when prescribing antibiotics to treat infections in this population.

Pharmacokinetics of gentamicin in critically ill patients: pilot study evaluating the first dose

Gentamicin is extensively used in the treatment of severe Gram-negative bacterial infections. A loading dose of 7 mg/kg is recommended to achieve a maximum concentration (C(max)) above 16 mg/L. We studied gentamicin pharmacokinetic data from patients treated between January 2006 and June 2008 in two intensive-care units. The Sawchuk and Zaske one-compartment pharmacokinetic model was used to estimate the gentamicin volume of distribution (the 32 patients had a median age of 68 years (23 men)). The median volume of distibution (V(d)) per kilogram of body weight (V(d)/kg) was 0.41 L/kg (interquartile range of 0.36-0.46 L/kg), with no correlations with age, Charlson comorbidity score, sequential organ failure assessment (SOFA) score and creatinine serum level (r(2) = 0.016, 0.058, 0.037, and 0.067, respectively). Women had a significantly higher median V(d)/kg (0.50 vs. 0.40 L/kg, p 0.002) and lower C(max) (15.2 vs. 18.5 mg/L, p 0.016), despite similar dose/kg. In a logistic regression model, only sex (female: OR 0.032; 95% CI 0.03-0.387) and dose/kg (per mg/kg: OR 3.21; 95% CI 1.17-8.79) were significantly associated with the achievement of C(max) above 16 mg/L. Gentamicin clearance was 57 mL/min (interquartile range of 44.7-78 mL/min) and decreased with age (r(2) = 0.178, p 0.016), SOFA score (r(2) = 0.199, p 0.011) and creatinine clearance (r(2) = 0.258, p 0.003). Gentamicin V(d) was increased in critically ill patients, particularly in women. Therefore, high gentamicin loading doses should be given to all patients, especially women, independently of organ failure. Gentamicin clearance decreases with age, SOFA score, and renal failure.

Use of Pharmacodynamic Principles to Optimise Dosage Regimens for Antibacterial Agents in the Elderly

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