Pharmacokinetics and administration regimens of vancomycin in neonates, infants and children

Ontogeny of hepatic and renal systemic clearance pathways in infants: part II

Maturation of drug systemic clearance mechanisms during the postnatal period produces dramatic and rapid changes in an infant's capacity to eliminate drugs. A tentative general mathematical model describing the ontogeny of hepatic cytochrome P450 (CYP) enzyme-mediated clearance and renal clearance due to glomerular filtration in the first 6 months of life was elaborated from age-specific in vitro hepatic microsomal activity data (normalised to amount of hepatic microsomal protein) for enzyme-specific probe substrates and in vivo probe substrate data for glomerular filtration (normalised to bodyweight), respectively. The model predicts an age- and clearance pathway-specific Infant Scaling Factor (ISF) for the first 6 months of life. The ISF reflects functional maturation of a specific clearance pathway (normalised to bodyweight) relative to adult values. Therefore, the ISF directly correlates adult clearance values with an infant's capacity to eliminate drugs. Substitution of appropriate model parameter estimates and the age of the infant into the model provides an estimated ISF value, which may then be used to predict the contribution of a particular clearance pathway to total systemic clearance in the infant when adult systemic clearance values are known. The model was tested for its ability to predict infant systemic clearance of drugs whose elimination is principally mediated by a single CYP enzyme or by glomerular filtration. The model performed reasonably well for CYP1A2 and CYP3A4, but poorer predictions were obtained for CYP2D6 and CYP2C because of lack of model complexity and/or inadequate hepatic microsomal activity data to fully describe the maturational process of functional enzyme. For renal clearance due to glomerular filtration, data normalised to bodyweight (kg) showed a limited maturational trend, suggesting that adult renal clearances normalised to bodyweight might reasonably predict infant renal clearances in the first 6 months of life. The model provided reasonable predictions of renal clearance due to glomerular filtration in the infant.

Ontogeny of hepatic and renal systemic clearance pathways in infants: part I

Dramatic developmental changes in the physiological and biochemical processes that govern drug pharmacokinetics and pharmacodynamics occur during the first year of life. These changes may have significant consequences for the way infants respond to and deal with drugs. The ontogenesis of systemic clearance mechanisms is probably the most critical determinant of a pharmacological response in the developing infant. In recent years, advances in molecular techniques and an increased availability of fetal and infant tissues have afforded enhanced insight into the ontogeny of clearance mechanisms. Information from these studies is reviewed to highlight the dynamic and complex nature of developmental changes in clearance mechanisms in infants during the first year of life. Hepatic and renal elimination mechanisms constitute the two principal clearance pathways of the developing infant. Drug metabolising enzyme activity is primarily responsible for the hepatic clearance of many drugs. In general, when compared with adult activity levels normalised to amount of hepatic microsomal protein, hepatic cytochrome P450-mediated metabolism and the phase II reactions of glucuronidation, glutathione conjugation and acetylation are deficient in the neonate, but sulfate conjugation is an efficient pathway at birth. Parturition triggers the dramatic development of drug metabolising enzymes, and each enzyme demonstrates an independent rate and pattern of maturation. Marked interindividual variability is associated with their developmental expression, making the ontogenesis of hepatic metabolism a highly variable process. By the first year of life, most enzymes have matured to adult activity levels. When compared with adult values, renal clearance mechanisms are compromised at birth. Dramatic increases in renal function occur in the ensuing postpartum period, and by 6 months of age glomerular filtration rate normalised to bodyweight has approached adult values. Maturation of renal tubular functions exhibits a more protracted time course of development, resulting in a glomerulotubular imbalance. This imbalance exists until adult renal tubule function values are approached by 1 year of age. The ontogeny of hepatic biliary and renal tubular transport processes and their impact on the elimination of drugs remain largely unknown. The summary of the current understanding of the ontogeny of individual pathways of hepatic and renal elimination presented in this review should serve as a basis for the continued accruement of age-specific information concerning the ontogeny of clearance mechanisms in infants. Such information can only help to improve the pharmacotherapeutic management of paediatric patients.

Dose regimen for vancomycin not needing serum peak levels?

AIM

To determine the safety, efficacy, and need to measure peak serum vancomycin concentrations in a neonatal population using a standard vancomycin dosage regimen.

METHOD

A total of 101 infants who were admitted to a regional neonatal intensive care unit and received vancomycin (15 mg/kg every 12 or 18 hours depending on postnatal age) were studied retrospectively. Infants who had been started on vancomycin before they were transferred to the unit were excluded. The proportion of infants was measured whose serum vancomycin concentrations were within a conservative therapeutic range of trough 5-10 mg/l, peak 20-40 mg/l, and a less conservative, but still safe, range of trough 5-12 mg/l, peak 15-60 mg/l.

RESULTS

Trough concentrations of 5-10 mg/l were achieved by 46.5% of infants, and 5-12 mg/l by 55.4%. Peak concentrations of 20-40 mg/l were found in 83.2% of infants, and 15-60 mg/l in 99.0%. Highest peak concentration was 47.2 mg/l. Some 89.4% of infants with trough concentrations of 5-10 mg/l had a peak concentration of 20-40 mg/l.

CONCLUSIONS

The vancomycin dosage regimen used in this study produces acceptable therapeutic serum vancomycin concentrations. Peak serum vancomycin concentrations do not need to be measured in neonates using this dosage regimen.

Monitoring the treatment of sepsis with vancomycin in term newborn infants

A prospective study was conducted to determine if standardized vancomycin doses could produce adequate serum concentrations in 25 term newborn infants with sepsis.

PURPOSE

The therapeutic response of neonatal sepsis by Staphylococcus sp. treated with vancomycin was evaluated through serum concentrations of vancomycin, serum bactericidal titers (SBT), and minimum inhibitory concentration (MIC).

METHOD

Vancomycin serum concentrations were determined by the fluorescence polarization immunoassay technique, SBT by the macro-broth dilution method, and MIC by diffusion test in agar.

RESULTS

Thirteen newborn infants (59.1%) had adequate peak vancomycin serum concentrations (20 - 40 mg/mL) and one had peak concentration with potential ototoxicity risk (>40 microg/mL). Only 48% had adequate trough concentrations (5 - 10 mg/mL), and seven (28%) had a potential nephrotoxicity risk (>10 microg/mL). There was no significant agreement regarding normality for peak and trough vancomycin method (McNemar test : p = 0.7905). Peak serum vancomycin concentrations were compared with the clinical evaluation (good or bad clinical evolution) of the infants, with no significant difference found (U=51.5; p=0.1947). There was also no significant difference between the patients' trough concentrations and good or bad clinical evolution (U = 77.0; p=0.1710). All Staphylococcus isolates were sensitive to vancomycin according to the MIC. Half of the patients with adequate trough SBT (1/8), also had adequate trough vancomycin concentrations and satisfactory clinical evolution.

CONCLUSIONS

Recommended vancomycin schedules for term newborn infants with neonatal sepsis should be based on the weight and postconceptual age only to start antimicrobial therapy. There is no ideal pattern of vancomycin dosing; vancomycin dosages must be individualized. SBT interpretation should be made in conjunction with the patient's clinical presentation and vancomycin serum concentrations. Those laboratory and clinical data favor elucidation of the probable cause of patient's bad evolution, which would facilitate drug adjustment and reduce the risk of toxicity or failing to achieve therapeutic doses.

Vancomycin pharmacokinetics and Bayesian estimation in pediatric patients

The vancomycin pharmacokinetic profile was characterized in six pediatric patients and the potential of nonlinear mixed effects modeling and Bayesian forecasting for vancomycin monitoring was explored using NONMEM V (1.1). Based on steady state serial vancomycin concentrations, the estimates of mean t1/2, Vd, and Cl derived by the Sawchuk and Zaske method (1) were 3.52 hours, 0.57 L/kg, and 0.12 L/h per kg, respectively. NONMEM analysis demonstrated that a weight-adjusted two-compartment model described individual patients' data better than a comparable one-compartment model. The two-compartment estimates of mean t1/2alpha, t1/2beta, Vss, and Cl were 0.80 hour, 5.63 hours, 0.63 L/kg, and 0.11 L/h per kg, respectively. The relatively long mean t1/2alpha suggests that peak vancomycin concentrations measured earlier than 4 hours postdose do not reflect postdistributional serum concentrations. NONMEM population modeling revealed that a weight-adjusted two-compartment model provided a better fit than a comparable one-compartment model. The resulting population parameters and variances were fixed in NONMEM to obtain Bayesian predictions of individual vancomycin serum concentrations. Bayesian estimation with either a single midinterval or trough sample has the potential to provide accurate and precise predictions of vancomycin concentrations. This should be evaluated using a vancomycin population pharmacokinetic model based on a larger sample of pediatric patients.

Dosage adjustments for antibacterials in obese patients: applying clinical pharmacokinetics

Obesity is associated with physiological changes that can alter the pharmacokinetic parameters of many drugs. Vancomycin and the aminoglycosides are the only antibacterials that have been extensively investigated in the obese population. The apparent volume of distribution (Vd) and total body clearance of vancomycin are increased in obese patients and have a better correlation with total bodyweight (TBW) than with ideal bodyweight (IBW). The Vd of aminoglycosides is increased in obesity and can be estimated from an adjusted bodyweight that accounts for a fraction of the excess bodyweight (TBW - IBW). These observed changes in pharmacokinetic parameters of vancomycin and aminoglycosides in obese patients may necessitate a deviation from the commonly recommended dosages administered to non-obese individuals. There are limited data regarding the pharmacokinetics of other antibacterial classes in obese patients. The available information for cephalosporins suggests that dosages may need to be increased in obese patients in order to obtain similar serum and tissue concentrations as in non-obese patients. Additional pharmacokinetic studies of other antibacterial classes are required in this special patient population.

Vancomycin dosage requirements among pediatric intensive care unit patients with normal renal function

PURPOSE

The purpose of this study was to determine a vancomycin dosage regimen among pediatric intensive care unit (PICU) patients with normal renal function resulting in desired peak and trough serum concentration and to determine the predictability of vancomycin peak concentrations based on reported trough concentrations.

MATERIALS AND METHODS

The medical records of all PICU patients who received vancomycin over a 12-month period were identified through a hospital computer search and were obtained from the hospital's Department of Medical Records. Demographic and laboratory data as well as the patient's vancomycin dosing history were recorded. Patients who lacked appropriately timed vancomycin peak and trough concentrations or who exhibited renal dysfunction were excluded from the study population. The optimal vancomycin dose and the predictability of peak concentrations based on trough concentrations were assessed.

RESULTS

A total of 135 patients were identified as having received vancomycin therapy during their PICU hospitalization between June 1997 and June 1998. Fifty-nine patients were excluded due to renal dysfunction or inappropriate vancomycin concentrations resulting in 76 patients representing our study population. The initial mean dose of vancomycin was 47 mg/kg/day resulting in a mean peak and trough serum concentration of 19 and 6 microg/mL, respectively. A mean of 2.2 (range, 1 to 5) and 2.1 (range, 1 to 5) peak and trough serum concentrations were reported for each patient, respectively. A mean of 1.1 (range, 0 to 4) dosing changes per patient was noted resulting in a final mean dose of 60 mg/kg/day corresponding to a mean peak and trough serum concentration of 26 and 8 microg/mL, respectively. A vancomycin trough concentration >5 microg/mL was highly predictive for a corresponding peak concentration >20 microg/mL (P > .0001). Eighty percent of the trough concentrations <5 microg/mL were associated with peak concentrations <20 microg/mL, whereas 81% of trough concentrations >5 microg/mL were associated with corresponding peak concentrations >20 microg/mL.

CONCLUSIONS

PICU patients required higher doses of vancomycin than are typically prescribed to achieve conventionally accepted peak and trough vancomycin serum concentrations. In the absence of renal impairment, we recommend an initial dosage regimen of 60 mg/kg/day divided every 8 hours. Vancomycin trough concentrations are highly predictive of corresponding peak concentrations and therefore may negate the need to obtain routine peak concentrations.

A population pharmacokinetic model for vancomycin in pediatric patients and its predictive value in a naive population

The objectives of this study were to (i) construct a population pharmacokinetic (PK) model able to describe vancomycin (VAN) concentrations in serum in pediatric patients, (ii) determine VAN PK parameters in this population, and (iii) validate the predictive ability of this model in a naive pediatric population. Data used in this study were obtained from 78 pediatric patients (under 18 years old). PK analyses were performed using compartmental methods. The most appropriate model was chosen based on the evaluation of pertinent graphics and calculation of the Akaike information criterion test. The population PK analysis was performed using an iterative two-stage method. A two-compartment PK model using age, sex, weight, and serum creatinine as covariates was determined to be the most appropriate one to describe serum VAN concentrations. The quality of fit was very good, and the distribution of weighted residuals was found to be homoscedastic (Wilcoxon signed rank test). Fitted population PK parameters (mean +/- standard deviation) were as follows: central clearance (0.1 +/- 0.05 liter/h/kg), central volume of distribution (0.27 +/- 0.07 liter/kg), peripheral volume of distribution (0.16 +/- 0.07 liter/kg), and distributional clearance (0.16 +/- 0.07 liter/kg). The predictive ability of the developed model (including the above-mentioned covariates) was evaluated in a naive population of 19 pediatric patients. The predictability was very good. Precision (+/-95% confidence interval [CI]) (peak, 4.1 [+/-1.4], and trough, 2.2 [+/-0.7]) and bias (+/-95% CI) (peak, -0.58 [+/-2.2], and trough, 0.63 [+/-1.1] mg/liter) were significantly (P < 0.05) superior to those obtained using a conventional method (precision [+/-95% CI]: peak, 8.03 [+/-2. 46], and trough, 2.7 [+/-0.74]; bias: peak, -7.1 [+/-2.9], and trough, -1.35 [+/-1.2] mg/liter). We propose the use of this population PK model to optimize VAN clinical therapies in our institution and others with similar patient population characteristics.he object

Pharmacokinetics and dose requirements of vancomycin in neonates

AIMS

To design and evaluate dosing guidelines for vancomycin based on data collected during routine use of the drug.

METHODS

Following the observation that 66% of neonatal vancomycin trough concentrations were outside the target range, new dose guidelines were developed using a population pharmacokinetic approach. NONMEM (non-linear mixed effects model) was used to analyse dose histories and 347 concentration measurements collected during routine therapeutic drug monitoring in 59 neonates.

RESULTS

Postconceptual ages in the patient group ranged from 26-45 weeks, weights from 0. 57-4.23 kg, and creatinine concentrations from 18-172 micromol/l. The population estimate of vancomycin clearance (l/h/kg) was 3. 56/creatinine concentration (micromol/l) with an interpatient coefficient of variation (CV) of 22% and volume of distribution 0.67 l/kg with a CV of 18%. Residual error was 4.5 mg/l. When the new recommendations on dosing were used prospectively in a separate group of neonates the proportion of acceptable troughs increased from 33% to 72%.

CONCLUSIONS

The pharmacokinetics of vancomycin in neonates and young infants depend on weight and serum creatinine. Preliminary results from the new guidelines indicate an improvement on previous practice, but also an ongoing need to monitor concentrations.

Basis of anti-infective therapy: pharmacokinetic-pharmacodynamic criteria and methodology for dual dosage individualisation

Antimicrobial therapy should be designed on the basis of microbiological, as well as pharmacokinetic, criteria; microbiological parameters provide information about the susceptibility of the pathogen responsible for the infectious process while pharmacokinetic parameters give information about the potential ability of the drug in question to reach and remain at the sites of infection in the body. Microbiological parameters such as the minimum inhibitory concentration, minimum bactericidal concentration, bacterial titre, bactericidal rate and 'post-antibiotic effect' (PAE) must be considered. Among the pharmacokinetic parameters, the maximum serum concentration at steady state (CmaxSS), area under the concentration-time curve (AUC) and length of time that the serum concentrations exceed a particular value are the most useful in this context. Different relationships between these parameters, known as efficacy indices, have been established to predict the potential efficacy of antibacterial therapy. Antimicrobial dosage individualisation should be based on the optimisation of the efficacy index that best correlates with patient response. It seems appropriate to establish the degree of correlation among the different efficacy indices and clinical response observed in patients by means of a correlation analysis. This type of analysis can be either retrospective or prospective and may be based on linear or maximum response models. Simulation of the plasma concentration curves obtained with the particular regimen administered offers a methodology which is easy to apply and provides the pharmacokinetic information necessary to calculate the different efficacy indices. Information about the susceptibility of the pathogen to the antibacterial in question and about the response to the treatment used is also necessary for the correlation analysis. This type of analysis determines which of the indices is best correlated with efficacy and, hence, is the index to be optimised when attempting to individualise antibacterial therapy for different situations.

Antibiotics in neonatal infections: a review

The bacteria most commonly responsible for early-onset (materno-fetal) infections in neonates are group B streptococci, enterococci, Enterobacteriaceae and Listeria monocytogenes. Coagulase-negative staphylococci, particularly Staphylococcus epidermidis, are the main pathogens in late-onset (nosocomial) infections, especially in high-risk patients such as those with very low birthweight, umbilical or central venous catheters or undergoing prolonged ventilation. The primary objective of the paediatrician is to identity all potential cases of bacterial disease quickly and begin antibacterial treatment immediately after the appropriate cultures have been obtained. Combination therapy is recommended for initial empirical treatment in the neonate. In early-onset infections, an effective first-line empirical therapy is ampicillin plus an aminoglycoside (duration of treatment 10 days). An alternative is ampicillin plus a third-generation cephalosporin such as cefotaxime, a combination particularly useful in neonatal meningitis (mean duration of treatment 14 to 21 days), in patients at risk of nephrotoxicity and/or when therapeutic monitoring of aminoglycosides is not possible. Another potential substitute for the aminoglycoside is aztreonam. Triple combination therapy (such as amoxicillin plus cefotaxime and an aminoglycoside) could also be used for the first 2 to 3 days of life, followed by dual therapy after the microbiological results. In late-onset infections the combination oxacillin plus an aminoglycoside is widely recommended. However, vancomycin plus ceftazidime (+/- an aminoglycoside for the first 2 to 3 days) may be a better choice. Teicoplanin may be a substitute for vancomycin. However, the initial approach should always be modified by knowledge of the local bacterial epidemiology. After the microbiological results, treatment should be switched to narrower spectrum agents if a specific organism has been identified, and should be discontinued if cultures are negative and the neonate is in good clinical condition. Penicillins and third-generation cephalosporins are generally well tolerated in neonates. There is controversy regarding whether therapeutic drug monitoring of aminoglycosides will decrease toxicity (particularly renal damage) in neonates, and on the efficacy and safety of a single daily dose versus multiple daily doses of these drugs. Toxic effects caused by vancomycin are uncommon, but debate still exists over the need for therapeutic drug monitoring of this agent. When antibacterials are used in neonates, accurate determination of dosage is required, particularly for compounds with a low therapeutic index and in patients with renal failure. Very low birthweight infants are also particularly prone to antibacterial-induced toxicity.

Non steady state and steady state PKS Bayesian forecasting and vancomycin pharmacokinetics in ICU adult patients

The pharmacokinetics of vancomycin was investigated in adult ICU patients after the first administration and at steady state. Then the predictive performance of a two-compartment Bayesian forecasting program was assessed in these patients by using population-based parameters and three non steady state vancomycin concentrations as feedback information. Finally a prospective investigation was carried out to search potential covariates. At steady state, a significant decrease (around 30%) in clearance (CL) was observed, while creatinine clearance (CLcr) was stable and a significant increase (around 30%) in volume of distribution (V(SS)) was observed. A two-fold increase in elimination half-life was found. CL was weakly correlated with CLcr at onset of therapy and at steady state. The Bayesian program tended to overpredict vancomycin peak and trough concentrations. A larger mean prediction error and a poorer precision were observed when population-based parameter estimates were used (no feedback) compared to feedback prediction, but the differences were not significant. Mechanical ventilation and concurrent opioid therapy may be pertinent covariates of vancomycin pharmacokinetics. The current work has shown that vancomycin pharmacokinetics in ICU patients displayed a significant variability and a significant change in both clearance and distribution during the course of therapy. Further investigation is necessary to clarify these findings. Moreover, the use of the Bayesian forecasting PKS program in our patients led to a prediction with low bias but rather poor precision. This outcome highlights the need to implement a population modeling approach, to determine the vancomycin pharmacokinetic parameters and covariates in our ICU patients, and to apply this information to provide more accurate concentration predictions.

Population pharmacokinetics of vancomycin in Japanese pediatric patients

The population pharmacokinetic profile of vancomycin (VCM) in Japanese pediatric patients infected with methicillin-resistant Staphylococcus aureus was analyzed using 181 samples of serum concentration data from 49 patients obtained in routine drug monitoring. The one-compartment linear model was adopted, where the VCM clearance (CL) and the distribution volume (Vd) were correlated with covariates such as postnatal age (AGE) and body weight (BWT). The population pharmacokinetic analysis program NONMEM with the first-order conditional estimation method was used. The results showed that the population mean clearance normalized by BWT increases with AGE up to 1 year of age [CL(L/hour per kg) = 0.1 19 + 0.0619 x (AGE - 1)] and decreases with age over 1 year old [CL(L/hour per kg) = 0.119 + 0.00508 x (1 - AGE)]. The population mean of the distribution volume normalized by BWT was independent of AGE (Vd (L/kg) = 0.522). The interindividual variability of CL was 39.6%, and that of Vd was 18.8%. The intraindividual, residual variability was 34.6%. These results were compared with those in other articles, and a guideline for dosage adjustment in VCM therapy is discussed.

The kinetic profile of vancomycin in neonates

The pharmacokinetic parameters of vancomycin in a neonatal population have been characterized to enable development of optimum dosage guidelines for neonatal intensive-care units and to examine the relationship between these pharmacokinetic parameters and various demographic, developmental and clinical factors which might be associated with changes in the kinetic profile of vancomycin. Forty-four infants (twenty-five males and nineteen females) with suspected or proven Gram-positive infection and who received intravenous vancomycin between October 1993 and December 1996 were included in this retrospective analysis. Gestational age ranged from 25 to 40 weeks and postconceptional age at the time of the study ranged from 28 to 45 weeks. Sixty case-studies were obtained from the forty-four patients, with one period of study corresponding to one week or one cycle of therapy. Vancomycin pharmacokinetic parameters were determined by use of a one-compartment model. By regression analysis the current weight (g) was shown to be the stronger covariate, and both vancomycin clearance (L h(-1)) and volume of distribution (L) had to be normalized. The vancomycin volume of distribution depended on the postconceptional age with a cut-off at 32 weeks, whereas vancomycin clearance depended on the presence or absence of concomitant treatment with indomethacin or of mechanical ventilation, or both. On the basis of the pharmacokinetic parameters obtained we suggest initial dosage guidelines for vancomycin ranging from 10 mg kg(-1) every 8 h to 10 mg kg(-1) every 12 h, depending on the demographic and clinical characteristics of the patients. The results obtained enabled application of better a priori and a posteriori dosage schedules to infants in neonatal intensive-care units by use of the Bayesian approach, although further prospective study is recommended before direct extrapolation to patients in other settings.