Vancomycin pharmacokinetics and Bayesian estimation in pediatric patients

Dose optimization and target attainment of vancomycin in children

Vancomycin is a glycopeptide antibiotic that has been adopted in clinical practice to treat gram-positive infections for more than 70 years. Despite vancomycin's long history of therapeutic use, optimal dose adjustments and pharmacokinetic/pharmacodynamic (PK/PD) target attainment in children are still under debate. Therapeutic drug monitoring (TDM) has been widely integrated into pediatric clinical practice to maximize efficacy and safety of vancomycin treatment. Area under the curve (AUC)-guided TDM has been recently recommended instead of trough-only TDM to ensure PK/PD target attainment of AUC0-24h/minimal inhibitory concentration (MIC) > 400 to 600 and minimize acute kidney injury risk. Bayesian forecasting in pediatric patients allows estimation of population PK to accurately predict individual vancomycin concentrations over time, and consequently total vancomycin exposure. AUC-guided TDM for vancomycin, preferably with Bayesian forecasting, is therefore suggested for all pediatric age groups and special pediatric populations. In this review we aim to analyze the current literature on the pediatric use of vancomycin and summarize the current knowledge on dosing optimization for target attainment in special patient populations.

The Significance of Bayesian Pharmacokinetics in Dosing for Critically Ill Patients: A Primer for Clinicians Using Vancomycin as an Example

Antibiotic use is becoming increasingly challenging with the emergence of multidrug-resistant organisms. Pharmacokinetic (PK) alterations result from complex pathophysiologic changes in some patient populations, particularly those with critical illness. Therefore, antibiotic dose individualization in such populations is warranted. Recently, there have been advances in dose optimization strategies to improve the utilization of existing antibiotics. Bayesian-based dosing is one of the novel approaches that could help clinicians achieve target concentrations in a greater percentage of their patients earlier during therapy. This review summarizes the advantages and disadvantages of current approaches to antibiotic dosing, with a focus on critically ill patients, and discusses the use of Bayesian methods to optimize vancomycin dosing. The Bayesian method of antibiotic dosing was developed to provide more precise predictions of drug concentrations and target achievement early in therapy. It has benefits such as the incorporation of personalized PK/PD parameters, improved predictive abilities, and improved patient outcomes. Recent vancomycin dosing guidelines emphasize the importance of using the Bayesian method. The Bayesian method is able to achieve appropriate antibiotic dosing prior to the patient reaching the steady state, allowing the patient to receive the right drug at the right dose earlier in therapy.

Two Innovative Approaches to Optimize Vancomycin Dosing Using Estimated AUC after First Dose: Validation Using Data Generated from Population PK Model Coupled with Monte-Carlo Simulation and Comparison with the First-Order PK Equation Approach

The revised consensus guidelines for optimizing vancomycin doses suggest that maintaining the area under the concentration-time curve to minimal inhibitory concentration ratio (AUC/MIC) of 400–600 mg·h/L is the target pharmacokinetic/pharmacodynamic (PK/PD) index for efficacy. AUC-guided dosing approach uses a first-order pharmacokinetics (PK) equation to estimate AUC using two samples obtained at steady state and one-compartment model, which can cause inaccurate AUC estimation and fail to achieve the effective PK/PD target early in therapy (days 1 and 2). To achieve an efficacy target from the third or fourth dose, two innovative approaches (Method 1 and Method 2) to estimate vancomycin AUC at steady state (AUCSS) using two-compartment model and three or four levels after the first dose are proposed. The feasibility of the proposed methods was evaluated and compared with another published dosing algorithm (Method 3), which uses two samples and a one-compartment approach. Monte Carlo simulation was performed using a well-established population PK model, and concentration-time profiles for virtual patients with various degrees of renal function were generated, with 1000 subjects per group. AUC extrapolated to infinity (AUC0–∞) after the first dose was estimated using the three methods, whereas reference AUC (AUCref) was calculated using the linear-trapezoidal method at steady state after repeated doses. The ratio of AUC0–∞: AUCref and % bias were selected as the indicators to evaluate the accuracy of three methods. Sensitivity analysis was performed to examine the influence of change in each sampling time on the estimated AUC0–∞ using the two proposed approaches. For simulated patients with various creatinine clearance, the mean of AUC0–∞: AUCref obtained from Method 1, Method 2 and Method 3 ranged between 0.98 to 1, 0.96 to 0.99, and 0.44 to 0.69, respectively. The mean bias observed with the three methods was −0.10% to −2.09%, −1.30% to −3.59% and −30.75% to −55.53%, respectively. The largest mean bias observed by changing sampling time while using Method 1 and Method 2 were −4.30% and −10.50%, respectively. Three user-friendly and easy-to-use excel calculators were built based on the two proposed methods. The results showed that our approaches ensured sufficient accuracy and achieved target PK/PD index early and were superior to the published methodologies. Our methodology has the potential to be used for vancomycin dose optimization and can be easily implemented in clinical practice.

Establishment and application of population pharmacokinetics model of vancomycin in infants with meningitis

BACKGROUND

To establish a population pharmacokinetics (PPK) model of vancomycin (VCM) for dose individualization in Chinese infants with meningitis.

METHODS

We collected the data of 82 children with meningitis in hospital from July 2014 to June 2016. The initial vancomycin dosage regimen for children was 10 or 15 mg/kg for q12 h, q8 h or q6 h. Serum concentrations were determined by Viva-E Analyzer before and after the fifth administration. The PPK model was developed by nonlinear mixed-effect model software, assessed by the bootstrap method and then tested in 20 infant patients.

RESULTS

The VCM clearance (CL) was increased by body weight (WT) and decreased by blood urea nitrogen (BUN). Pharmacokinetic parameters of VCM were not influenced by co-administered drugs. The trough concentrations of VCM were accurately predicted by the PPK model, with the prediction errors less than 32%.

CONCLUSION

A new individual strategy for VCM regimens was proposed and validated by the PPK model.

Population Pharmacokinetics and Dosing Optimization of Vancomycin in Pediatric Liver Transplant Recipients

Methicillin-resistant Staphylococcus aureus infections are a significant cause of morbidity and mortality in pediatric liver transplant (LT) recipients. Physiological changes following LT may affect vancomycin pharmacokinetics; however, appropriate dosing to achieve sufficient drug exposure (i.e., 24-h area under the concentration-time curve [AUC₂₄]/MIC ≥ 400) in pediatric LT recipients has not been reported. This retrospective pharmacokinetics study of LT recipients aged <18 years utilized data on patient characteristics with vancomycin concentrations and dosing information obtained from electronic medical records. Population pharmacokinetics analysis was conducted by nonlinear mixed-effects modeling with the Phoenix NLME software. Potential covariates were screened with univariate and multivariate analysis. Monte Carlo simulations were performed using the final model to explore appropriate dosing. The study included 270 pharmacokinetics profiles encompassing 1,158 concentrations measured in 161 patients. The median age was 13.3 (interquartile range, 7.6 to 53.5) months, serum creatinine (sCr) was 0.16 (0.12 to 0.23) mg/dl, and days from LT (DFLT) was 17 (6 to 31). Multivariate analysis demonstrated that lower sCr and shorter DFLT were associated with higher clearance. By post hoc estimation, the average clearance and volume of distribution were 0.18 liters/h/kg and 1.01 liters/kg, respectively. The Monte Carlo simulations revealed that only 16% of patients achieved an AUC₂₄/MIC of ≥400 with the assumed vancomycin MIC of 1 μg/ml. DFLT and sCr were significant covariates for vancomycin clearance in pediatric LT recipients. Standard vancomycin dosing may be insufficient, and higher or more frequent dosing may be required to achieve an AUC₂₄/MIC of ≥400 in pediatric LT recipients with normal renal function.

IMPORTANCE We evaluated vancomycin pharmacokinetics in pediatric LT recipients and developed a population pharmacokinetics model by considering various factors that might account for alterations in vancomycin pharmacokinetics. Our analyses revealed that lower serum creatinine levels and a shorter duration from the day of LT were associated with higher vancomycin clearance and led to subtherapeutic drug exposure. We also performed Monte Carlo simulations to determine the appropriate dosing strategy in pediatric LT recipients, which revealed that a standard vancomycin dosing might be insufficient and that higher or more frequent dosing might be necessary to achieve an AUC₂₄/MIC of ≥400 in pediatric LT recipients with normal renal function. To the best of our knowledge, this is the first study to assess vancomycin pharmacokinetics in pediatric LT recipients by population pharmacokinetics analysis.

Population Pharmacokinetic Models of Vancomycin in Paediatric Patients: A Systematic Review

BACKGROUND

Vancomycin is commonly used to treat gram-positive bacterial infections in the paediatric population, but dosing can be challenging. Population pharmacokinetic (popPK) modelling can improve individualization of dosing regimens. The primary objective of this study was to describe popPK models of vancomycin and factors that influence pharmacokinetic (PK) variability in paediatric patients.

METHODS

Systematic searches were conducted in the Cochrane Central Register of Controlled Trials, MEDLINE, EMBASE, International Pharmaceutical Abstracts and the grey literature without language or publication status restrictions from inception to 17 August 2020. Observational studies that described the development of popPK models of vancomycin in paediatric patients (< 18 years of age) were included. Risk of bias was assessed using the National Heart, Lung and Blood Institute Study Quality Assessment Tool for Case Series Studies.

RESULTS

Sixty-four observational studies (1 randomized controlled trial, 13 prospective studies and 50 retrospective studies of 9019 patients with at least 25,769 serum vancomycin concentrations) were included. The mean age was 2.5 years (range 1 day-18 years), serum creatinine was 47.1 ± 33.6 µmol/L, and estimated creatinine clearance was 97.4 ± 76 mL/min/1.73m². Most studies found that vancomycin PK was best described by a one-compartment model (71.9%). There was a wide range of clearance and volume of distribution (V_d) values (range 0.014-0.27 L/kg/h and 0.43-1.46 L/kg, respectively) with interindividual variability as high as 49.7% for clearance and 136% for V_d, proportional residual variability up to 37.5% and additive residual variability up to 17.5 mg/L. The most significant covariates for clearance were weight, age, and serum creatinine or creatinine clearance, and weight for V_d. Variable dosing recommendations were suggested.

CONCLUSION

Numerous popPK models of vancomycin were derived, however external validation of suggested dosing regimens and analyses in subgroup paediatric populations such as dialysis patients are still needed before a popPK model with best predictive performance can be applied for dosing recommendations. Significant intraindividual and interindividual PK variability was present, which demonstrated the need for ongoing therapeutic drug monitoring and derivation of PK models for vancomycin for certain subgroup populations, such as dialysis patients.

Contribution of Population Pharmacokinetics of Glycopeptides and Antifungals to Dosage Adaptation in Paediatric Onco-hematological Malignancies: A Review

The response to medications in children differs not only in comparison to adults but also between children of the different age groups and according to the disease. This is true for anti-infectives that are widely prescribed in children with malignancy. In the absence of pharmacokinetic/pharmacodynamic paediatric studies, dosage is frequently based on protocols adapted to adults. After a short presentation of the drugs, we reviewed the population pharmacokinetic studies available for glycopeptides (vancomycin and teicoplanin, n = 5) and antifungals (voriconazole, posaconazole, and amphotericin B, n = 9) currently administered in children with onco-hematological malignancies. For each of them, we reported the main study characteristics including identified covariates affecting pharmacokinetics and proposed paediatric dosage recommendations. This review highlighted the very limited amount of data available, the lack of consensus regarding PK/PD targets used for dosing optimization and regarding dosage recommendations when available. Additional PK studies are urgently needed in this specific patient population. In addition to pharmacokinetics, efficacy may be altered in immunocompromised patients and prospective clinical evaluation of new dosage regimen should be provided as they are missing in most cases.

Population Pharmacokinetic Analysis and Dose Regimen Optimization in Japanese Infants with an Extremely Low Birth Weight

Vancomycin is a synthetic antibiotic effective against Gram-positive pathogens. Although the clinical applicability of vancomycin for infants has been increasing, the pharmacokinetic data for vancomycin in extremely low-birth-weight infants are limited. The aim of this study was to construct a population pharmacokinetics model for vancomycin in extremely-low-birth-weight infants and establish an optimal dosage regimen. We enrolled children aged less than 1 year with a birth weight of less than 1,000 g and body weight at vancomycin prescription of less than 1,500 g. Pharmacokinetic data from 19 patients were analyzed, and a population pharmacokinetics model was developed using nonlinear mixed-effects modeling software. Goodness-of-fit plots, a nonparametric bootstrap analysis, and a prediction-corrected visual predictive check were employed to evaluate the final model. The dosage regimen was optimized based on the final model. The pharmacokinetic data fit a one-compartment model with first-order elimination, and body weight and estimated serum creatinine level were used as significant covariates. In a simulation using the final model, the optimal dosage regimen, especially when the serum creatinine level (>0.6 mg/dl) was high, was 5.0 to 7.5 mg/kg of body weight twice a day every 12 h; this was required to reduce the dosage compared with that in previous studies. The recommended doses based on the current target time course concentration curves may not be appropriate for extremely-low-birth-weight infants.

Population Pharmacokinetic Study of Vancomycin in Chinese Pediatric Patients with Hematological Malignancies

STUDY OBJECTIVES

Vancomycin is a primary antibiotic for the treatment of severe infections in children with malignant hematological disease. However, precise dosing of vancomycin is difficult in children because of high interindividual variability and limited data of pharmacokinetic profiles. The present study aims to develop a population pharmacokinetic (PPK) model for vancomycin in Chinese pediatric patients with hematological malignancies.

DESIGN

This was a retrospective pharmacokinetic study.

SETTING

The setting for this study was a tertiary-care children's hospital.

PATIENTS

This study included 92 pediatric patients with hematological malignancies who received vancomycin and experienced therapeutic drug monitoring from February 2017 to December 2018.

MEASUREMENTS AND MAIN RESULTS

A PPK model was generated with a nonlinear mixed effects model. In addition, required doses to achieve target therapeutic concentrations were simulated based on the final model. A one-compartment model with first-order elimination fit the concentration data best. Actual body weight (BW) and glomerular filtration rate (GFR) were the significant influential factors on the clearance (CL) of vancomycin. The final PPK model for CL was CL (L/h) = 4.18  × GFR / 145 0.741 × BW / 25 K , K =  BW - 0.856 / BW - 0.856 + 6 . 53 - 0.856 , and the volume of distribution was 22.3 L. The model proved to be robust and reliable. Reference dosing regimens were proposed based on the final model.

CONCLUSIONS

A PPK model of vancomycin was established for Chinese pediatric patients with hematological malignancies using a nonlinear mixed effects model, which provided a reference for the clinical application of vancomycin.

Population Pharmacokinetics and Pharmacodynamics of Norvancomycin in Children With Malignant Hematological Disease

Knowledge of pharmacokinetic (PK) behavior of norvancomycin (NVCM) in pediatric patients is lacking, which leads to empirical therapy in clinical practice. This study developed a population PK model of children aged 0-15 years; 112 opportunistic samples in total from 90 children were analyzed. The stability and prediction of the final model were evaluated by goodness-of-fit plots, nonparametric bootstrap, visual predictive check, and normalized prediction distribution errors. The PKs of NVCM in children was described by a 2-compartment model with first-order elimination along with body weight and estimated glomerular filtration rate as significant covariates on clearance. The population typical values of the PK parameters were as follows: clearance 0.12 L/kg/h, central compartment distribution volume 0.17 L/kg, peripheral compartment distribution volume 0.38 L/kg, and intercompartmental clearance 0.35 L/kg/h. Logistic analysis showed that the ratio of area under the concentration-time curve over 24 hours (AUC_0-24 ) to minimum inhibitory concentration (MIC) had the strongest correlation with clinical efficacy, and at least 80% clinical efficiency could be achieved when AUC_0-24 /MIC ≥ 221.06 was defined as the target. Monte Carlo simulation results suggested that a higher dose was required for this pediatric population in order to reach the target. The dosing regimen was optimized based on the final model. A population PK model of NVCM was first characterized in children with hematologic malignancy, and an evidence-based approach for NVCM dosage individualization was provided.

Desired vancomycin trough concentration to achieve an AUC0-24 /MIC ≥400 in Chinese children with complicated infectious diseases

A vancomycin steady-state trough concentration (C_min ) of 15-20 mg/L is recommended for achieving a ratio of the 24-hour area under the curve to the minimum inhibitory concentration (AUC_0-24 /MIC) of ≥400 in adults. Since few paediatric data are available, our objectives were to (a) measure the pharmacokinetic indices of vancomycin and (b) determine the correlation between C_min and AUC_0-24 /MIC in paediatric patients. Population-based pharmacokinetic modelling was performed for paediatric patients to estimate the individual parameters. The relationship between C_min and the calculated AUC_0-24 /MIC was explored using linear regression and a probabilistic framework. A sensitivity analysis was also conducted using Monte Carlo simulations. Body-weight significantly influenced the pharmacokinetics of vancomycin. Based on real data and simulations, C_min ranges of 5.0-5.9 and 9.0-12.9 mg/L were associated with AUC_0-24 /MIC ≥400 for MIC values of ≤0.5 and ≤1 mg/L, respectively. Vancomycin regimens of 10 and 15 mg/kg every 6 hours achieved a C_min of 5.0-5.9 mg/L and AUC_0-24 /MIC ≥400 in >90% of the children when MIC was ≤0.5 mg/L. At a MIC of ≤1 mg/L, vancomycin at 15 mg/kg every 6 hours achieved C_min of 9.0-12.9 mg/L and AUC_0-24 /MIC ≥400 in 2.0- and 1.6-fold as many children compared to a dose of 10 mg/kg every 6 hours, respectively. Vancomycin C_min values of 5.0-12.9 mg/L were strongly predictive of achieving AUC_0-24 /MIC ≥400, and rational dosing regimens of 10-15 mg/kg q6h were required in paediatric patients, depending on the pathogen.

Handling interoccasion variability in model-based dose individualization using therapeutic drug monitoring data

AIMS

This study aims to assess approaches to handle interoccasion variability (IOV) in a model-based therapeutic drug monitoring (TDM) context, using a population pharmacokinetic model of coagulation factor VIII as example.

METHODS

We assessed 5 model-based TDM approaches: empirical Bayes estimates (EBEs) from a model including IOV, with individualized doses calculated based on individual parameters either (i) including or (ii) excluding variability related to IOV; and EBEs from a model excluding IOV by (iii) setting IOV to zero, (iv) summing variances of interindividual variability (IIV) and IOV into a single IIV term, or (v) re-estimating the model without IOV. The impact of varying IOV magnitudes (0-50%) and number of occasions/observations was explored. The approaches were compared with conventional weight-based dosing. Predictive performance was assessed with the prediction error percentiles.

RESULTS

When IOV was lower than IIV, the accuracy was good for all approaches (50^th percentile of the prediction error [P50] <7.4%), but the precision varied substantially between IOV magnitudes (P97.5 61-528%). Approach (ii) was the most precise forecasting method across a wide range of scenarios, particularly in case of sparse sampling or high magnitudes of IOV. Weight-based dosing led to less precise predictions than the model-based TDM approaches in most scenarios.

CONCLUSIONS

Based on the studied scenarios and theoretical expectations, the best approach to handle IOV in model-based dose individualization is to include IOV in the generation of the EBEs but exclude the portion of unexplained variability related to IOV in the individual parameters used to calculate the future dose.

Vancomycin therapeutic drug monitoring and population pharmacokinetic models in special patient subpopulations

Vancomycin is a fundamental antibiotic in the management of severe Gram-positive infections. Inappropriate vancomycin dosing is associated with therapeutic failure, bacterial resistance and toxicity. Therapeutic drug monitoring (TDM) is acknowledged as an important part of the vancomycin therapy management, at least in specific patient subpopulations, but implementation in clinical practice has been difficult because there are no consensus and agglutinator documents. The aims of the present work are to present an overview of the current knowledge on vancomycin TDM and population pharmacokinetic (PPK) models relevant to specific patient subpopulations. Based on three published international guidelines (American, Japanese and Chinese) on vancomycin TDM and a bibliographic review on available PPK models for vancomycin in distinct subpopulations, an analysis of evidence was carried out and the current knowledge on this topic was summarized. The results of this work can be useful to redirect research efforts to address the detected knowledge gaps. Currently, TDM of vancomycin presents a moderate level of evidence and practical recommendations with great robustness in neonates, pediatric and patients with renal impairment. However, it is important to investigate in other subpopulations known to present altered vancomycin pharmacokinetics (eg neurosurgical, oncological and cystic fibrosis patients), where evidence is still unsufficient.

Use of Individual Pharmacokinetics to Improve Time to Therapeutic Vancomycin Trough in Pediatric Oncology Patients

OBJECTIVE

Optimization of vancomycin dosing is difficult in children, given rapid drug clearance and patient heterogeneity. We sought to evaluate the impact of dosing using individual pharmacokinetic parameters on time to goal trough concentration in pediatric oncology patients.

METHODS

A retrospective review was conducted to assess vancomycin dosing in the pediatric oncology unit at Loma Linda University Children's Hospital between January 2013 and August 2013 (standard dosing group [SDG]). These patients were compared to those in a prospective arm that used pharmacokinetic dosing (pharmacokinetic dosing group [PKG]) between March 2014 and May 2015. Outcomes included percent of patients reaching a target trough by the specified time points, number of dose adjustments, number of serum concentrations drawn, and number of patients with supratherapeutic troughs.

RESULTS

Of 35 patients meeting inclusion criteria for the SDG, 2 (5.7%) reached goal trough concentration by 48 hours, compared with 14 of 16 patients (87%) in the PKG (p = 0.0001). Significantly more patients reached their goal trough at each time point in the PKG. There was no difference in number of dose adjustments, but significantly more concentrations were drawn on average in the PKG (mean, 4.6 versus 3.1, p = 0.02). In the SDG and PKG, respectively, 1 patient and 3 patients had supratherapeutic trough concentrations (p = 0.09).

CONCLUSIONS

Dosing using individual pharmacokinetic parameters led to a significant reduction in time to attain the desired vancomycin trough concentration in our pediatric oncology patients. Given the wide variation in dose requirements in this and other studies, application of patient-specific pharmacokinetics is essential to optimize vancomycin dosing in pediatric patients.

Vancomycin AUC/MIC and Corresponding Troughs in a Pediatric Population

OBJECTIVES

Adult guidelines suggest an area under the curve/minimum inhibitory concentration (AUC/MIC) > 400 corresponds to a vancomycin trough serum concentration of 15 to 20 mg/L for methicillin-resistant Staphylococcus aureus infections, but obtaining these troughs in children are difficult. The primary objective of this study was to assess the likelihood that 15 mg/kg of vancomycin every 6 hours in a child achieves an AUC/MIC > 400.

METHODS

This retrospective chart review included pediatric patients >2 months to <18 years with a positive S aureus blood culture and documented MIC who received at least two doses of vancomycin with corresponding trough. Patients were divided into two groups: group 1 initially receiving ≥15 mg/kg every 6 hours, and group 2 initially receiving any other dosing ranges or intervals. AUCs were calculated four times using three pharmacokinetic methods.

RESULTS

A total of 36 patients with 99 vancomycin trough serum concentrations were assessed. Baseline characteristics were similar between groups. For troughs in group 1 (n = 55), the probability of achieving an AUC/MIC > 400 ranged from 16.4% to 90.9% with a median trough concentration of 11.4 mg/L, while in group 2 (n = 44) the probability of achieving AUC/MIC > 400 ranged from 15.9% to 54.5% with mean trough concentration of 9.2 mg/L. The AUC/MICs were not similar between the different pharmacokinetic methods used; however, a trapezoidal equation (Method A) yielded the highest correlation coefficient (r² = 0.59). When dosing every 6 hours, an AUC/MIC of 400 correlated to a trough serum concentration of 11 mg/L.

CONCLUSIONS

The probability of achieving an AUC/MIC > 400 using only a trough serum concentration and an MIC with patients receiving 15 mg/kg every 6 hours is variable based on the method used to calculate the AUC. An AUC/MIC of 400 in children correlated to a trough concentration of 11 mg/L using a trapezoidal Method to calculate AUC.

Population pharmacokinetics and dosing optimization of vancomycin in children with malignant hematological disease

An increase in vancomycin dose has been proposed in adults with malignant hematological disease. As pediatric data are limited, our aim was to evaluate the population pharmacokinetics of vancomycin in order to define the appropriate dosing regimen in children with malignant hematological disease. Vancomycin concentrations were collected prospectively during therapeutic monitoring. Population pharmacokinetic analysis was performed using NONMEM software. Seventy children (age range, 0.3 to 17.7 years) were included. With the current recommended dosing regimen of 40 to 60 mg/kg/day, 53 children (76%) had subtherapeutic steady-state trough concentrations (Css/min of <10 mg/liter). A one-compartment model with first-order elimination was developed. Systematic covariate analysis identified that weight significantly influenced clearance (CL) and volume of distribution (V) with power functions of 0.677 for CL and 0.838 for V. Vancomycin CL also significantly increased with increases in creatinine clearance and seemed to be higher in children with malignant hematological disease than in the general pediatric population. The model was validated internally. Its predictive performance was further confirmed in an external validation by Bayesian estimation. A patient-tailored dosing regimen was developed based on the final pharmacokinetic model and showed that a higher proportion of patients reached the target Css/min than with the traditional mg/kg-basis dose (60% versus 49%) and that the risks associated with underdosing or overdosing were reduced. This is the first population pharmacokinetic study of vancomycin in children with malignant hematological disease. An optimized dosing regimen, taking into account patient weight, creatinine clearance, and susceptibility of the pathogens involved, could routinely be used to individualize vancomycin therapy in this vulnerable population.

Desired vancomycin trough serum concentration for treating invasive methicillin-resistant Staphylococcal infections

Vancomycin area under the curve/minimal inhibitory concentration (AUC/MIC) >400 best predicts the outcome when treating invasive methicillin-resistant Staphylococcus aureus infection; however, trough serum concentrations are used clinically to assess the appropriateness of dosing. We used pharmacokinetic modeling and simulation to examine the relationship between vancomycin trough values and AUC/MIC in children receiving vancomycin 15 mg/kg every 6 hours and methicillin-resistant Staphylococcus aureus MIC of 1 μg/mL. A trough of 7-10 μg/mL predicted achievement of AUC/MIC >400 in >90% of children.

Vancomycin and gentamicin pharmacokinetic alterations in an adolescent amputee

A 14-year-old male with bilateral above-the-knee amputations presented to our hospital for treatment of a skin and soft-tissue infection. We report the experience of vancomycin and gentamicin therapy in this patient. Because these medications require weight-based dosages and pharmacokinetic monitoring of serum levels, it was necessary to obtain peak and trough levels of the two drugs in order to determine the pharmacokinetic differences in this patient compared to those in an adolescent male without amputations. To our knowledge, this is the first report describing pharmacokinetic differences in an adolescent amputee.

Improved vancomycin dosing in children using area under the curve exposure

BACKGROUND

: Our objectives were to (1) determine the pharmacokinetic indices of vancomycin in pediatric patients; and (2) compare attainment of 2 target exposures: area under curve (AUC) / minimum inhibitory concentration (MIC) ≥400 and trough concentration ≥15 mcg/mL.

METHODS

: The population-based pharmacokinetic modeling was performed using NONMEM 7.2 for children ≥3 months old who received vancomycin for ≥48 hours from 2003 to 2011. A 1-compartment model with first-order kinetics was used to estimate clearance, volume of distribution and AUC. Empiric Bayesian post hoc individual parameters and Monte Carlo simulations (N = 11,000) were performed.

RESULTS

: Analysis included 702 patients with 1660 vancomycin serum concentrations. Median age was 6.6 (interquartile range 2.2-13.4) years, weight 22.7 (12.6-46) kg and baseline serum creatinine 0.40 (0.30-0.60) mg/dL. Final model pharmacokinetic indices were clearance (L/h) = 0.248 * Wt * (0.48/serum creatinine) * (ln(age)/7.8) and volume of distribution (L) = 0.636 * Wt. Using these parameters and the observed MIC distribution, Monte Carlo simulation indicated that the initial median dose of 44 (39-52) mg/kg/day was inadequate in most subjects. Regimens of 60 mg/kg/day for subjects ≥12 years old and 70 mg/kg/day for those <12 years old achieved target AUC/MIC in ~75% and trough concentrations ≥15 in ~45% of virtual subjects. An AUC/MIC ~400 corresponded to trough concentration ~8 to 9 mcg/mL.

CONCLUSIONS

: Targeted exposure using vancomycin AUC/MIC, compared with trough concentrations, is a more realistic target in children. Depending on age, serum creatinine and MIC distribution, vancomycin in a dosage of 60 to 70 mg/kg/day was necessary to achieve AUC/MIC ≥ 400 in 75% of patients.

Vancomycin: a review of population pharmacokinetic analyses

Despite nearly five decades of clinical use, vancomycin has retained a significant and uncontested niche in the antibacterial arsenal because of its consistent activity against almost all Gram-positive bacteria. Nevertheless, major vancomycin toxicities have been reported in the literature - in particular, nephrotoxicity and ototoxicity. Vancomycin pharmacokinetics have been described in numerous studies for 25 years. This review presents a synthesis of the reported population pharmacokinetic models of vancomycin. The objective was to determine if there was a consensus on a structural model and which covariates were identified. A literature search was conducted from the PubMed database, from its inception through December 2010, using the following terms: 'vancomycin', 'pharmacokinetic(s)', 'population', 'model(ling)' and 'nonlinear mixed effect'. Articles were excluded if they were not pertinent. The reference lists of all selected articles were also evaluated. Twenty-five articles were included in this review: 15 models concerned paediatric patients and ten models were conducted in adults. In neonates and infants, the pharmacokinetics of vancomycin were mainly described by a one-compartment model, whereas in adults, a two-compartment model was preferentially used. Various covariates were tested but only three (age, creatinine clearance [CL(CR)] and body weight) were included in almost all of the described models. After inclusion of these covariates, the mean (range) values of the interindividual variability in the clearance and volume of distribution were 30% (15.6-45%) and 23% (12.6-48%), respectively. The mean (range) value of the residual variability was 20% (7-39.6%). This review highlights the numerous population pharmacokinetic models of vancomycin developed in recent decades and concludes with relevant information for clinicians and researchers. To optimize vancomycin dosage, this review points out the relevant covariates according to the target population. In adults, dosage optimization depends on CL(CR) and body weight, while in children, it depends on age, body weight and CL(CR). For future population pharmacokinetic studies, a sensitive liquid chromatography-tandem mass spectrometry method could be used and new covariates such as cardiac output or possible renal transporters could be tested. Finally, we suggest that external evaluation should be the first step in a pharmacokinetic analysis of vancomycin rather than describing a new model.

Impact of a hospitalwide increase in empiric pediatric vancomycin dosing on initial trough concentrations

STUDY OBJECTIVE

To evaluate the impact of a hospitalwide increase in the recommended vancomycin starting dose from 45 to 60 mg/kg/day on initial vancomycin trough concentrations in children suspected of having an invasive methicillin-resistant Staphylococcus aureus (MRSA) infection.

DESIGN

Retrospective medical record review.

SETTING

Dedicated children's hospital located in a tertiary care, academic medical center.

PATIENTS

A total of 182 children aged 1 month-12 years with normal renal function who had suspected MRSA infections treated with vancomycin during two different starting dose recommendation periods: 45 mg/kg/day divided every 8 hours during July 2006-June 2007 (low-dose group [88 children]) and 60 mg/kg/day divided every 6 hours during July 2008-June 2009 (high-dose group [94 children]).

MEASUREMENT AND MAIN RESULTS

Data on patient demographics, vancomycin doses, and initial vancomycin trough concentrations were collected. No significant demographic differences were noted between patients in the low-dose and high-dose groups. The mean ± SD initial vancomycin trough level increased from 7 ± 5 μg/ml in the low-dose group to 9 ± 5 μg/ml in the high-dose group (p<0.001). The percentage of patients with an initial trough level less than 5 μg/ml declined from 38% (33/88 children) in the low-dose group to 17% (16/94 children) in the high-dose group (p<0.001), whereas the percentage of patients with an initial trough concentration in the potentially adverse range (> 20 μg/ml) did not change between the two groups (2% vs 2%, p=0.9). Less than 14% (13/94 children) achieved a trough level in the range of 15-20 μg/ml in the high-dose group.

CONCLUSION

An increase in the recommended vancomycin starting dose to 60 mg/kg/day decreased the likelihood of an initial low vancomycin trough level (< 5 μg/ml), with no increase in the proportion of patients with trough levels in a potentially toxic range. The 60-mg/kg/day dose did not consistently achieve a vancomycin trough of 15-20 μg/ml, a goal suggested by some experts for adults. Comparative effectiveness studies are needed to directly evaluate vancomycin dosing regimens and clinical outcomes for children with invasive MRSA infections.

Prediction of vancomycin pharmacodynamics in children with invasive methicillin-resistant Staphylococcus aureus infections: a Monte Carlo simulation

BACKGROUND

Due to the emergence of community-associated strains, the prevalence of invasive methicillin-resistant Staphylococcus aureus (MRSA) infections has increased substantially in pediatric patients. A vancomycin AUC(0-24)/MIC index >400 best predicts treatment outcomes for invasive MRSA infection in adults. Data on whether recommended vancomycin doses in children achieve this break point are lacking.

OBJECTIVE

This study aimed to assess the likelihood that currently recommended vancomycin doses in children achieve AUC(0-24)/MIC >400.

METHODS

Vancomycin AUC(0-24)/MIC predictions were conducted across a range of dosages (40-70 mg/kg/d) using a Monte Carlo simulation (n = 5000). AUC(0-24) was calculated as daily dose divided by vancomycin clearance, and daily dose was fixed for a given simulation. Three literature-reported estimates in children were used to define vancomycin clearance and its variance. For the MIC distribution of MRSA isolates, susceptibility data were obtained from the University of California, San Francisco Children's Hospital, San Francisco, California (n = 180; 40% < or =0.5 mg/L; 59% = 1 mg/L; and 1% = 2 mg/L).

RESULTS

Using the recommended empiric dosage of 40 mg/kg/d, 58% to 66% of children were predicted to achieve AUC(0-24)/MIC >400. Increasing the vancomycin dosage to 60 mg/kg/d substantially increased the likelihood (88%-98%) of achieving this pharmacodynamic target. On sensitivity analysis, a dosage of 40 mg/kg/d was more strongly influenced by small changes in MIC compared with 60 mg/kg/d.

CONCLUSIONS

Recommended empiric vancomycin dosing in children (40 mg/kg/d) was not predicted to consistently achieve the pharmacodynamic target of AUC(0-24)/MIC >400 for invasive MRSA infections. A vancomycin dosage of 60 mg/kg/d was predicted to optimize achievement of this target in children.

Optimizing pediatric dosing: a developmental pharmacologic approach

Many physiologic differences between children and adults can result in age-related differences in pharmacokinetics. Understanding the effects of age on bioavailability, volume of distribution, protein binding, hepatic metabolic isoenzymes, and renal elimination can provide insight into optimizing doses for pediatric patients. We performed a search of English-language literature using the MEDLINE database regarding age and pharmacokinetics (1979-July 2008). We then evaluated the literature with an emphasis on drugs with one primary elimination pathway, such as renal clearance or a pathway involving a single metabolic isoenzyme. Our mechanistic-based analysis revealed that children need weight-corrected doses that are substantially higher than adult doses for drugs that are metabolically eliminated solely by the specific cytochrome P450 (CYP) isoenzymes CYP1A2, CYP2C9, and CYP3A4. In contrast, weight-corrected doses for drugs eliminated by renal excretion or metabolism involving CYP2C19, CYP2D6, N-acetyltransferase 2, or uridine diphosphate glucuronosyltransferases are similar in children and adults. In children, bioavailability of drugs with high first-pass metabolism is decreased for drugs metabolized by CYP1A2, CYP2C9, and CYP3A4. Limited data suggest that by age 5 years, bioavailability of drugs affected by efflux transporters should be equivalent to that of adults. Using a pharmacokinetics-based approach, rational predictions can be made for the effects of age on drugs that undergo similar pathways of elimination, even when specific pharmacokinetic data are limited or unavailable.

Current recommended dosing of vancomycin for children with invasive methicillin-resistant Staphylococcus aureus infections is inadequate

BACKGROUND

Vancomycin area-under-the-concentration-time-curve (AUC) for 24 hours divided by the minimum inhibitory concentration (MIC) (AUC24/MIC) >400 optimally treats invasive methicillin-resistant Staphylococcus aureus (MRSA) infections in adults. It is unknown whether recommended vancomycin dosing regimens for children achieve this value.

METHODS

AUC24/MIC was calculated in children using vancomycin doses of 40 and 60 mg/kg/d. AUC24 was calculated as daily dose/vancomycin clearance. Vancomycin clearance in children was estimated by 2 approaches: (1) previously literature-reported vancomycin clearance, and (2) calculated vancomycin clearance using previously derived predictor models and a hypothetical population of healthy children. Representative MIC of hospital MRSA isolates was used (0.5, 1.0, and 2.0 microg/mL).

RESULTS

The MIC50/90 for pediatric MRSA isolates in the previous year was 1.0 microg/mL. With a dose of 40 mg/kg/d, both approaches consistently predicted AUC24/MIC <400 when MIC was 1.0 microg/mL. At 60 mg/kg/d, AUC24/MIC >400 was more readily achieved when MIC was 1.0 microg/mL, however, an MIC of 2.0 microg/mL resulted in AUC24/MIC <400 for both dosing regimens.

CONCLUSIONS

A vancomycin dose of 40 mg/kg/d in children is unlikely to achieve the recommended pharmacodynamic target of AUC24/MIC >400 for invasive MRSA infections even when MIC is 1.0 microg/mL. A starting dose of 60 mg/kg/d should be used in settings where isolates with MIC of 1.0 are common. Alternatives to vancomycin should strongly be considered for patients with MIC > or =2.0 microg/mL.

Evaluation of Bayesian predictability of vancomycin concentration using population pharmacokinetic parameters in pediatric patients

The objective of this study was to evaluate the Bayesian predictability of vancomycin (VCM) pharmacokinetics in Japanese pediatric patients using one-compartment population pharmacokinetic (PPK) parameters, which we reported previously. The validity of the PPK model was evaluated by bootstrap method and cross validation method, and the Bayesian predictive performance was examined. The predictive performance of the PPK model for premature patients was also examined. The cross validation method showed the predictability to be acceptable for practical use, especially for predicting trough concentration using other trough data. However, for the external premature patient data, this PPK model did not seem to be adequate. A theoretical approach using a simulation technique was also examined to evaluate the predictive performance. The results suggested that the predictability at the peak was not necessarily good at all sampling times and the predictability at the trough was better when a later time point was used. The optimal sampling time for prediction of VCM concentration in pediatric patients is discussed.

Population pharmacokinetic studies in pediatrics: issues in design and analysis

The current review addresses the following 3 frequently encountered challenges in the design and analysis of population pharmacokinetic studies in pediatrics: (1) body size adjustments during the development of pharmacostatistical models, (2) design and validation of limited sampling strategies, and (3) the integration of historical priors in data analysis and trial simulation. Size adjustments with empiric approaches based on body weight or body surface area have frequently proven as a pragmatic tool to overcome large size differences in a pediatric study population. Allometric size adjustments, however, provide a more mechanistic, physiologically based approach that, if used a priori, allows delineation of the effect of size from that of other covariates that show a high degree of collinearity. The frequent lack of dense data sets in pediatric clinical pharmacology because of ethical and logistic constraints in study design can be overcome with the application of D-optimality-based limited sampling schemes in combination with Bayesian and nonlinear mixed-effects modeling approaches. Empirically based dose selection and clinical trial designs for pediatric clinical pharmacology studies can be improved by applying clinical trial simulation techniques, especially if they integrate adult and pediatric in vitro and/or in vivo data as historic priors. Although integration of these concepts and techniques in population pharmacokinetic analyses is not only limited to pediatric research, their application allows researchers to overcome some major hurdles frequently encountered in pharmacokinetic studies in pediatrics and, thus, provides the basis for additional clinical pharmacology research in this previously insufficiently studied fraction of the general population.

Aminoglycoside and vancomycin serum concentration monitoring and mortality due to neonatal sepsis in Saudi Arabia

OBJECTIVE

To assess the effectiveness of monitoring of serum concentration of aminoglycosides in neonates.

METHOD

A retrospective evaluation of serum concentration monitoring of aminoglycosides (gentamicin and amikacin) and vancomycin in neonates treated for sepsis in a maternity and children hospital in Jeddah, Saudi Arabia, over the period 1998-2000.

RESULTS

The total number of requests for monitoring increased sixfold in 1999 and 12-fold in 2000 relative to 1998. For aminoglycosides, the incidence of both subtherapeutic peak and toxic trough serum levels decreased significantly (P < 0.05) in 1999 and 2000 compared with 1998. Furthermore, the rate of neonatal mortality caused by sepsis showed reduction in both 1999 (34%) and 2000 (35%) in comparison with 1998 (45%). Vancomycin trough (effective) concentration monitoring revealed no change in the incidence (30%) of levels at subtherapeutic values (<5.0 microg/mL) between the compared years. Furthermore, the rate of toxic levels (>10 microg/mL) increased in both 1999 (31%) and 2000 (39%) relative to 1998 (25%).

CONCLUSION

Therapeutic drug monitoring of vancomycin needs re-evaluation in the hospital to explain why existing methods are ineffective.

Forecasting of Blood Tacrolimus Concentrations Based on the Bayesian Method in Adult Patients Receiving Living-Donor Liver Transplantation

OBJECTIVE

To evaluate Bayesian prediction of blood tacrolimus concentrations in adult patients receiving living-donor liver transplantation (LDLT) using previously obtained population pharmacokinetic parameters.

PATIENTS AND METHODS

Data were retrospectively collected from 47 adult patients receiving LDLT who were not included in the estimation of population pharmacokinetic parameters. Blood tacrolimus concentrations were predicted without or with the empirical Bayesian method using sparse samples obtained in the previous week. Predictive performance of the concentrations was evaluated by the mean prediction error (ME), mean absolute prediction error (MAE) and root mean square error (RMSE) as well as the percentage of successful predictions (percentage of absolute prediction error less than 3 microg/L, %PRED3).

RESULTS

Concentrations predicted by the population mean pharmacokinetic parameter values coincided well with observed concentrations during the period of tacrolimus infusion immediately after the operation. For concentrations during subsequent oral therapy with tacrolimus, predictability by the population mean pharmacokinetic parameter values alone was not satisfactory. Bayesian forecasting using one or two blood concentrations obtained in the previous week significantly decreased (p<0.05) MAE and RMSE compared with predictions based on the population mean pharmacokinetic parameters on postoperative days 21 and 28, but not on day 14. During postoperative days 15-21, %PRED3 was increased to 68.6% or 71.2% with the Bayesian method using one or two blood concentrations, respectively, from 44.9% with the population mean pharmacokinetic parameter values.

CONCLUSION

The present study demonstrated the applicability of the Bayesian method with use of one or two samples for prediction of blood tacrolimus concentrations in adult patients receiving LDLT.