Mechanism-based concepts of size and maturity in pharmacokinetics

Thiopentone elimination in newborn infants: exploring Michaelis-Menten kinetics

BACKGROUND

Thiopentone elimination has been described using Michaelis-Menten pharmacokinetics in adults after prolonged infusion or overdose, but there are few reports of elimination in neonates.

METHODS

Time-concentration profiles for neonates (n=37) given single-dose thiopentone were examined using both first-order (constant clearance) and mixed-order (Michaelis-Menten) elimination processes using nonlinear mixed effects models. These profiles included a 33-week post-menstrual age (PMA) neonate given an overdose. A two-compartment mamillary model was used to fit data. Parameter estimates were standardized to a 70 kg person using allometric models.

RESULTS

There were 197 observations available for analysis from neonates with a mean post-menstrual age of 35 (SD 4.5) weeks and a mean weight of 2.5 (SD 0.9) kg. They were given a mean thiopentone dose of 3 (SD 0.4) mg/kg as a rapid bolus. Clearance at 26 weeks PMA was 0.015 l/min/70 kg and increased to 0.119 l/min/70 kg by 42 weeks PMA. The maximum rate of elimination (V(max)) at 26 weeks PMA was 0.22 mg/min/70 kg and increased to 4.13 mg/min/70 kg by 42 weeks PMA. These parameter estimates are approximately 40% adult values at term gestation. The Michaelis constant (K(m)) was 28.3 [between subject variability (BSV) 46.4%, 95% confidence interval (CI) 4.49-99.2] mg/l; intercompartment clearance was 0.44 (BSV 97.5%, 95% CI 0.27-0.63) l/min/70 kg; central volume of distribution was 46.4 (BSV 29.2%, 95% CI 41.7-59.8) l/70 kg; peripheral volume of distribution was 95.7 (BSV 70.3%, 95% CI 61.3-128) l/70 kg.

CONCLUSIONS

Both first-order and mixed-order processes satisfactorily described elimination. First-order elimination adequately described the time-concentration profile in the premature neonate given an overdose. Clearance is immature in the pre-term neonate although there is rapid maturation around 40 weeks PMA, irrespective of post-natal age.

Systematic evaluation of the descriptive and predictive performance of paediatric morphine population models

PURPOSE

A framework for the evaluation of paediatric population models is proposed and applied to two different paediatric population pharmacokinetic models for morphine. One covariate model was based on a systematic covariate analysis, the other on fixed allometric scaling principles.

METHODS

The six evaluation criteria in the framework were 1) number of parameters and condition number, 2) numerical diagnostics, 3) prediction-based diagnostics, 4) η-shrinkage, 5) simulation-based diagnostics, 6) diagnostics of individual and population parameter estimates versus covariates, including measurements of bias and precision of the population values compared to the observed individual values. The framework entails both an internal and external model evaluation procedure.

RESULTS

The application of the framework to the two models resulted in the detection of overparameterization and misleading diagnostics based on individual predictions caused by high shrinkage. The diagnostic of individual and population parameter estimates versus covariates proved to be highly informative in assessing obtained covariate relationships. Based on the framework, the systematic covariate model proved to be superior over the fixed allometric model in terms of predictive performance.

CONCLUSIONS

The proposed framework is suitable for the evaluation of paediatric (covariate) models and should be applied to corroborate the descriptive and predictive properties of these models.

Indications for a ceftriaxone dosing regimen in Japanese paediatric patients using population pharmacokinetic/pharmacodynamic analysis and simulation

OBJECTIVES

The objective of this study was to build a ceftriaxone population pharmacokinetic model for Japanese paediatric patients and to examine the dosing regimen of ceftriaxone based on pharmacokinetic/pharmacodynamic (PK/PD) analysis.

METHODS

The population pharmacokinetic analysis using NONMEM was based on published serum concentrations of ceftriaxone. A Monte Carlo simulation was examined to evaluate the time above the minimum inhibitory concentration (TAM) in 20 and 60 mg/kg body weight dose regimen using the population pharmacokinetic parameters.

KEY FINDINGS

The time course of the serum concentration of ceftriaxone in paediatric patients was fitted to a two-compartment model and body weight was incorporated to pharmacokinetic parameters as the covariate. Based on the percent TAM estimated from the final population pharmacokinetic model and the minimum inhibitory concentration (MIC) of ceftriaxone in 2004, we have predicted that the once daily administration of 20 mg/kg ceftriaxone would be effective on various infecting organisms.

CONCLUSIONS

A population pharmacokinetic model of ceftriaxone was built for Japanese paediatric patients based on the available data. The estimated PK/PD result confirmed the appropriateness of once daily dose of 20 mg/kg. In some patients for whom no efficacy was observed at 20 mg/kg, an increase to 60 mg/kg may be required.

Developmental pharmacology

Understanding the pharmacokinetics and pharmacodynamics of drugs used in psychopharmacology across the pediatric age spectrum from infants to adolescents represents a major challenge for clinicians. In pediatrics, treatment protocols use either standard dose reductions for these drugs for children below a certain age or use less conventional parameters such as weight for allometric dosing. The rationale behind this, however, is often lacking. In this review current available data on the developmental changes in absorption, distribution, metabolism, and elimination of drugs are presented with a specific focus on metabolic pathways, indicating significant differences in the development of enzymes involved in the biotransformation of drugs used in psychopharmacology. Major emphasis will be given to the genetic variation in CYP2D6 activity, the most important enzyme for the metabolism of many psychotropic medications. Finally, this review will summarize the role of the developmental pharmacologist in ensuring rational use of drugs in children with developmental disabilities and in translating pharmacological concepts from the bench to the clinic.

Buprenorphine TTS for children--a review of the drug's clinical pharmacology

OBJECTIVE

The transdermal therapeutic system (TTS) with buprenorphine is currently being used 'off-label' to treat chronic pediatric pain. We compiled available pharmacokinetic (PK), pharmacodynamic (PD), and clinical pediatric data on buprenorphine to rationalize treatment regimens.

METHODS

We conducted a systematic biomedical literature review focusing on pediatric buprenorphine data.

RESULTS

There are few relevant pediatric buprenorphine data, particularly in children suffering chronic pain. There are no pediatric PK and PD data for children with chronic pain given sublingual or TTS formulations. Children given single dose buprenorphine have increased drug clearance referenced to bodyweight with a possible paradoxical longer duration of action. Buprenorphine metabolism is independent of renal function, which is advantageous in renal insufficiency. The risk of respiratory depression after buprenorphine is difficult to quantify. A concentration-response relationship for respiratory effects has not been described and it is unknown whether children have a ceiling effect similar to that described in healthy adult volunteers.

CONCLUSIONS

Buprenorphine is of interest in pediatric postoperative pain and cancer pain control because of its multiple administration routes, long duration of action, and metabolism largely independent of renal function. There is little reason to expect buprenorphine effects in children out of infancy are fundamentally different to those in adults. From the limited pediatric data available, it appears that buprenorphine has no higher adverse potential than the more commonly used opioids. There is an urgent need for focused PK, PD, and safety studies in children before use in children becomes more widespread.

Tips and traps analyzing pediatric PK data

Pharmacokinetic (PK) and pharmacodynamic (PD) modeling has elucidated aspects of developmental pharmacology of value to the anesthetic community. The increasing sophistication of modeling techniques is associated with pitfalls that may not be readily apparent to readers or investigators. While size and age are considered primary covariates for PK models, the impact of birth on clearance maturation is poorly documented, dose in obese children is poorly investigated, pharmacologic implications of physiologic changes poorly portrayed, disease progression on drug response poorly depicted and the impact of metabolites on effect poorly illustrated. This review identifies some of these pitfalls and suggests ideas to circumvent or investigate these hazards.

Circadian rhythms and their development in children: implications for pharmacokinetics and pharmacodynamics in anesthesia

The influence of time-of-day on the action and toxicity of drugs may be an important factor in the design of pharmacokinetic (PK) and pharmacodynamic (PD) studies, and the interpretation of data resulting from these studies. Time-of-day can have a profound influence on the action of drugs. In some settings (e.g. cancer chemotherapy), the timing of drug administration has been utilized to maximize therapeutic effect and minimize toxicity. Time-of-day variation in the action of anesthetic drugs has been clearly demonstrated in adults. For example, local anesthetic action is longest during the afternoon, and neuromuscular blockade by rocuronium lasts one-third longer in the morning than the afternoon. Circadian rhythms develop over the first months and years of life. Robust rhythms in hormone production (e.g. melatonin and cortisol) are seen at approximately 3 months of age, but it remains unclear as to when daily rhythms in drug PK and PD first appear. Here, we review the evidence for time-of-day effects in anesthetic drugs in adults and children and outline the potential influence this has on pediatric anesthesia.

A maturation model for midazolam clearance

BACKGROUND

Physiological-based pharmacokinetic models have been used to describe midazolam clearance (CL) maturation. There are no maturation descriptors of CL from neonate to adulthood based on reported estimates at different ages.

METHODS

Published CL estimates after intravenous administration from time-concentration profiles were used to construct a maturation model based on size and age. Curve fitting was performed using nonlinear mixed effects models.

RESULTS

There were 16 publications reporting an estimate of CL after intravenous administration in children, although few estimates were available from 44-80 weeks postmenstrual age (PMA). CL maturation, standardized to a 70 -kg person was described using the Hill equation. Mature CL was 523 (CV 32%, 95%CI 469, 597) ml·min(-1) ·70 kg(-1) . The maturation half-time was 73.6 (95%CI 59.4, 80.0) weeks PMA and the Hill coefficient 3 (95%CI 2.2, 4.1). Predicted CL changes with age based on this model were in close agreement with physiologically based pharmacokinetic (PBPK) models. A comparison with a published PBPK model predictions revealed a root mean squared prediction error (precision) of 4.0% (95%CI 1.1, 5.8) and bias was -0.9% (95%CI -4.3, 2.6).

CONCLUSIONS

Previously published pharmacokinetic parameters can be used to develop maturation models that address gaps in current knowledge regarding the influence of age on a drug's disposition. If a midazolam sedation target concentration of 0.1 mg·l(-1) , similar to that given to adults, is assumed, then we might anticipate steady-state infusion rates of 0.014 mg·kg(-1) ·h(-1) in neonates, 0.05 mg·kg(-1) ·h(-1) in a 1-year-old, 0.06 mg·kg(-1) ·h(-1) in a 5-year-old and 0.05 mg·kg(-1) ·h(-1) in a 12-year-old child. Age-related pharmacodynamic differences that will affect dose and the impact of active metabolites on response are not yet quantified.

Predicting weight using postmenstrual age--neonates to adults

OBJECTIVES

To describe the pattern and variability of body weight with postmenstrual age (PMA) using nonlinear mixed effect modeling and to create a single mathematical function that can be used from prematurity to adulthood.

BACKGROUND

PMA has been shown to predict functional properties of humans such as glomerular filtration rate and drug clearance. Widely used growth charts use postnatal age to predict weight in an idealized population and are not available as a mathematical function.

METHODS

We modeled 7164 body weight and PMA observations from a pooled database of 5031 premature neonates, infants, children, and adults. All subjects were participants in pharmacokinetic or renal function studies. PMA ranged from 23 weeks to 82 years. A mixed effect model was used to describe fixed (PMA, sex) and random between-subject variability.

RESULTS

A model based on the sum of three sigmoid hyperbolic and one exponential functions described the data. Females were typically 12% lighter in weight. Part of the between-subject variability in weight decreased exponentially with a half-life of 3.5 PMA years, while the remainder stayed a constant fraction of the weight asymptote for each of the four functions.

CONCLUSIONS

The change of weight with PMA and sex can be described with a simple equation. This is suitable for simulation of typical weight-age distributions and may be useful for evaluation of appropriate weight for age in children requiring medical treatment.

Developmental pharmacology of tramadol during infancy: ontogeny, pharmacogenetics and elimination clearance

AIMS AND OBJECTIVES

To illustrate the complex interaction between ontogeny, i.e., age-dependent maturation, genetic polymorphisms and renal elimination clearance during infancy, based on developmental disposition of intravenous tramadol during infancy.

BACKGROUND

Tramadol (M) is metabolized by O-demethylation (cytochrome P450 [CYP] 2D6) to the pharmacodynamic active metabolite O-demethyl tramadol (M1). This metabolite is subsequently eliminated by renal route while M1 formation will in part depend on ontogeny, i.e., age-dependent activity and CYP2D6 polymorphisms. However, these pathways do not mature simultaneously.

METHODS

A pooled pharmacokinetic analysis of earlier reported time-concentration profiles in neonates and infants was performed with subsequent simulation of the impact of ontogeny, polymorphisms and renal elimination clearance during infancy.

RESULTS

Tramadol plasma time-concentration profile changes with postmenstrual age. The highest metabolite concentrations occur in the 52-week infant, where M1 formation clearance (hepatic, CYP2D6) is already mature but metabolite elimination clearance (through glomerular filtration rate) is immature.

DISCUSSION

The phenotypic observations might in part explain unanticipated (side-)effects of tramadol. In addition to the compound-specific clinical implications, it is important to stress that the maturational trends in the elimination processes described can be considered for other compounds (e.g., codeine) that undergo similar elimination routes.

Oral bioavailability of clonidine in children

BACKGROUND

Oral clonidine is used as premedication in children. The bioavailability of clonidine given orally in adults is 75-100% but is unknown in children.

METHODS

Children (3-10 years) undergoing adenotonsillectomy were administered oral clonidine 4 mcg·kg(-1) mixed with apple fruit drink as premedication. Intravenous plasma was assayed for clonidine concentration at 5, 15, 30, 45 min and 1, 2, 4, 6, 12, 18 h after administration. Clonidine plasma concentrations were determined by liquid chromatography-mass spectroscopy, and pharmacokinetic parameters were calculated using nonlinear effects mixed-effects models. Current data were pooled with published time-concentration profiles from children (n = 49) administered intravenous clonidine to determine oral bioavailability.

RESULTS

There were eight children studied (age 3-10 years, weight 10.5-36 kg). A two-compartment model with first-order absorption and elimination was used to describe time-concentration profiles. Population parameter estimates (CV%; 95% CI), standardized to a 70-kg person, were absorption half-life (Tabs), 0.45 (85.1; 0.221-0.884) h, absorption lag time (Tlag), 0.148 (91.2; 0.002-0.316) h, Clearance (CL) 17.9 (30.3; 16-20.3) l·h(-1) per 70 kg, between compartment clearance (Q) 121 (44.3; 80.1-165) l·h(-1) per 70 kg, central volume (V1) 81.2 (71.5; 60.7-105) l·70 kg(-1), peripheral volume of distribution (V2) 113 (33.9; 91-131) l·70 kg(-1). The oral bioavailability was 55.4% (CV 6.4%; 95% CI 0.469, 0.654).

CONCLUSIONS

Clonidine administered with an apple fruit drink displays a variable and relatively slow absorption after oral administration (T(max) 1.04 h, C(max) 0.77 mcg·l(-1)). The oral bioavailability was 55.4%, which is less than reported in adults. Consequently, higher oral doses of clonidine (per kg) are required when this formulation is used to achieve concentrations similar to those reported in adults.

Diclofenac pharmacokinetic meta-analysis and dose recommendations for surgical pain in children aged 1-12 years

BACKGROUND

Diclofenac is an effective, opiate-sparing analgesic for acute pain in children, which is commonly used in pediatric surgical units. Recently, a Cochrane review concluded the major knowledge gap in diclofenac use is dosing information. A pharmacokinetic meta-analysis has been undertaken with the aim of recommending a dose for children aged 1-12 years.

METHODS

Studies containing diclofenac pharmacokinetic data were identified during a Cochrane systematic review, and authors were asked to provide raw data. A pooled population analysis was undertaken in NONMEM to define the pharmacokinetics of intravenous, oral, and rectal diclofenac in children. Simulations were performed to recommend a dose yielding an equivalent area under diclofenac concentration-time curve (AUC) to a 50-mg dispersible tablet in adults.

RESULTS

Data from 111 children aged 1-14 years consisting of 375 samples following intravenous, oral suspension, and suppositories were used. Adult dispersible tablet and suspension data were added to provide a reference AUC and support the absorption modeling, respectively. A three-compartment model described disposition, a dual-absorption compartment model was used for suspension and dispersible tablet data, and single-absorption compartment model for suppositories. The estimate of clearance was 16.5 l·h(-1) ·70 kg(-1) and bioavailabilities were 0.36, 0.63, and 0.35 for suspension, suppository, and dispersible tablets, respectively.

CONCLUSIONS

Single doses of 0.3 mg·kg(-1) for intravenous, 0.5 mg·kg(-1) for suppositories, and 1 mg·kg(-1) for oral diclofenac in children aged 1-12 years are recommended as they yield a similar AUC to 50 mg in adults.

Developmental pharmacogenetics of immunosuppressants in pediatric organ transplantation

Cyclosporine, tacrolimus, sirolimus, and mycophenolate mofetil are the primary immunosuppressants used on pediatric organ transplantation. Therapeutic drug monitoring is used in daily practice, because their clinical use is hampered by a narrow therapeutic index and large variability. Tailoring immunosuppressive therapy to the individual patient to optimize efficacy and minimize toxicity is therefore essential. Because research in pharmacogenetics already identified polymorphisms impacting their pharmacokinetic parameters in adults, developmental pharmacogenetics of immunosuppressants holds promises for optimizing dosage regimens and improving clinical outcome in children. In this review, we focus on the impact of age and pharmacogenetics on these immunosuppressants in children undergoing organ transplantation.

Plasma and cerebrospinal fluid pharmacokinetics of flurbiprofen in children

AIMS

This study was designed to characterize paediatric pharmacokinetics and central nervous system exposure of flurbiprofen.

METHODS

The pharmacokinetics of flurbiprofen were studied in 64 healthy children aged 3 months to 13 years, undergoing surgery with spinal anaesthesia. Children were administered preoperatively a single dose of flurbiprofen intravenously as prodrug (n= 27) or by mouth as syrup (n= 37). A single cerebrospinal fluid (CSF) sample (n= 60) was collected at the induction of anaesthesia, and plasma samples (n= 304) before, during and after the operation (up to 20 h after administration). A population pharmacokinetic model was built using the NONMEM software package.

RESULTS

Flurbiprofen concentrations in plasma were well described by a three compartment model. The apparent bioavailability of oral flurbiprofen syrup was 81%. The estimated clearance (CL) was 0.96l h(-1) 70 kg(-1) . Age did not affect the clearance after weight had been included as a covariate. The estimated volume of distribution at steady state (V(ss) ) was 8.1 l 70 kg(-1) . Flurbiprofen permeated into the CSF, reaching concentrations that were seven-fold higher compared with unbound plasma concentrations.

CONCLUSIONS

Flurbiprofen pharmacokinetics can be described using only weight as a covariate in children above 6months, while more research is needed in neonates and in younger infants.

Population pharmacokinetics of tamsulosin hydrochloride in paediatric patients with neuropathic and non-neuropathic bladder

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

Tamsulosin is available on prescription as a modified release capsule in the US (Flomax), and in most European countries for the treatment of the signs and symptoms of benign prostatic hyperplasia (BPH). The pharmacokinetics of tamsulosin hydrochloride (HCl) have been extensively studied in adults, but no pharmacokinetic data for paediatrics have been published to date.

WHAT THIS STUDY ADDS

A population pharmacokinetic model of tamsulosin HCl was developed in paediatric patients. Covariate analysis revealed that body weight and alpha(1)-acid glycoprotein influenced both the apparent clearance and the apparent volume of distribution. This study confirms that there is no major difference in the pharmacokinetics of tamsulosin HCl between paediatrics (age range 2-16 years) and adults when the effect of body weight is taken into consideration.

AIMS

The main objective of this study was to characterize the population pharmacokinetics of tamsulosin hydrochloride (HCl) in paediatric patients with neuropathic and non-neuropathic bladder. A secondary objective was to compare the pharmacokinetics in paediatric patients and adults.

METHODS

Tamsulosin HCl plasma concentrations in 1082 plasma samples from 189 paediatric patients (age range 2-16 years) were analyzed with NONMEM, applying a one compartment model with first-order absorption. Based on the principles of allometry, body weight was incorporated in the base model, along with fixed allometric exponents. Covariate analysis was performed by means of a stepwise forward inclusion and backward elimination procedure. Simulations based on the final model were used to compare the pharmacokinetics with those in adults.

RESULTS

Beside the priori-implemented body weight, only alpha(1)-acid glycoprotein had an effect on both apparent clearance and apparent volume of distribution. No other investigated covariates, including gender, age, race, patient population and concomitant therapy with anti-cholinergics, significantly affected the pharmacokinetics of tamsulosin HCl (P < 0.001). The results of simulations indicated that the exposure in 12.5 kg paediatric patients was 3.5-4.3 fold higher than that in 70.0 kg adults. After a weight-based dose administration, the exposure in paediatric patients was comparable with that in healthy adults.

CONCLUSIONS

A population pharmacokinetic model of tamsulosin HCl in paediatric patients was established and it described the data well. There was no major difference in the pharmacokinetics of tamsulosin HCl between paediatric patients (age range 2-16 years) and adults when the effect of body weight was taken into consideration.

Pharmacokinetic properties of conventional and double-dose sulfadoxine-pyrimethamine given as intermittent preventive treatment in infancy

Intermittent preventive treatment in infancy (IPTi) entails routine administration of antimalarial treatment doses at specified times in at-risk infants. Sulfadoxine-pyrimethamine (SDX/PYR) is a combination that has been used as first-line IPTi. Because of limited pharmacokinetic data and suggestions that higher milligram/kilogram pediatric doses than recommended should be considered, we assessed SDX/PYR disposition, randomized to conventional (25/1.25 mg/kg of body weight) or double (50/2.5 mg/kg) dose, in 70 Papua New Guinean children aged 2 to 13 months. Blood samples were drawn at baseline, 28 days, and three time points randomly selected for each infant at 4 to 8 h or 2, 5, 7, 14, or 21 days. Plasma SDX, PYR, and N(4)-acetylsulfadoxine (NSX, the principal metabolite of SDX) were assayed by high-performance liquid chromatography (HPLC). Using population modeling incorporating hepatic maturation and cystatin C-based renal function, two-compartment models provided best fits for PYR and SDX/NSX plasma concentration profiles. The area under the plasma concentration-time curve from 0 h to infinity (AUC(0-∞)) was greater with the double dose versus the conventional dose of PYR (4,915 versus 2,844 μg/day/liter) and SDX (2,434 versus 1,460 mg/day/liter). There was a 32% reduction in SDX relative bioavailability with the double dose but no evidence of dose-dependent metabolism. Terminal elimination half-lives (15.6 days for PYR, 9.1 days for SDX) were longer than previously reported. Both doses were well tolerated without changes in hemoglobin or hepatorenal function. Five children in the conventional and three in the double-dose group developed malaria during follow-up. These data support the potential use of double-dose SDX/PYR in infancy, but further studies should examine the influence of hepatorenal maturation in very young infants.

Developmental pharmacokinetics

The advances in developmental pharmacokinetics during the past decade reside with an enhanced understanding of the influence of growth and development on drug absorption, distribution, metabolism, and excretion (ADME). However, significant information gaps remain with respect to our ability to characterize the impact of ontogeny on the activity of important drug metabolizing enzymes, transporters, and other targets. The ultimate goal of rational drug therapy in neonates, infants, children, and adolescents resides with the ability to individualize it based on known developmental differences in drug disposition and action. The clinical challenge in achieving this is accounting for the variability in all of the contravening factors that influence pharmacokinetics and pharmacodynamics (e.g., genetic variants of ADME genes, different disease phenotypes, disease progression, and concomitant treatment). Application of novel technologies in the fields of pharmacometrics (e.g., in silico simulation of exposure-response relationships; disease progression modeling), pharmacogenomics and biomarker development (e.g., creation of pharmacodynamic surrogate endpoints suitable for pediatric use) are increasingly making integrated approaches for developmentally appropriate dose regimen selection possible.

Pediatric Dosing and Body Size in Biotherapeutics

Although pediatric doses for biotherapeutics are often based on patients' body weight (mg/kg) or body surface area (mg/m²), linear body size dose adjustment is highly empirical. Growth and maturity are also important factors that affect the absorption, distribution, metabolism and excretion (ADME) of biologics in pediatrics. The complexity of the factors involved in pediatric pharmacokinetics lends to the reconsideration of body size based dose adjustment. A proper dosing adjustment for pediatrics should also provide less intersubject variability in the pharmacokinetics and/or pharmacodynamics of the product compared with no dose adjustment. Biological proteins and peptides generally share the same pharmacokinetic principle with small molecules, but the underlying mechanism can be very different. Here, pediatric and adult pharmacokinetic parameters are compared and summarized for selected biotherapeutics. The effect of body size on the pediatric pharmacokinetics for these biological products is discussed in the current review.

Pharmacokinetics, safety, and biologic effects of azithromycin in extremely preterm infants at risk for ureaplasma colonization and bronchopulmonary dysplasia

Ureaplasma spp. respiratory tract colonization is a significant risk factor for bronchopulmonary dysplasia (BPD), a chronic lung disorder in preterm infants. As an initial step preparatory to future clinical trials to evaluate the clinical efficacy of azithromycin to prevent BPD, the authors characterized the pharmacokinetics, safety, and biological effects of a single intravenous dose of azithromycin (10 mg/kg) in preterm neonates (n = 12) 24 to 28 weeks gestation at risk for Ureaplasma infection and BPD. A 2-compartment structural model with the clearance and volume of peripheral compartment (V2) allometrically scaled on body weight (WT) best described the pharmacokinetics of azithromycin in preterm neonates. The estimated parameters were clearance [0.18 L/h × WT(kg)(0.75)], intercompartmental clearance [1.0 L/h], volume of distribution of central compartment [0.93 L], and V2 [14.2 L × WT(kg)]. There were no serious adverse events attributed to azithromycin. A single dose of azithromycin did not suppress inflammatory cytokines or myeloperoxidase activity in tracheal aspirates. These results demonstrated the safety of azithromycin and developed a pharmacokinetic model that is useful for future simulation-based clinical trials for eradicating Ureaplasma and preventing BPD in preterm neonates.

Population pharmacokinetics of high-dose carboplatin in children and adults

INTRODUCTION

Although for conventional dosing of carboplatin, several strategies are available to individualize the dose based on renal function measurements, such approaches are still rare for high-dose regimens with autologous stem cell support. The purpose of this study was to investigate the influence of patient- and regimen-specific factors on the pharmacokinetics of carboplatin and to compare the performance of different dose individualization strategies (Calvert, Newell, Chatelut, flat dosing, dosing according to body surface area).

PATIENTS AND METHODS

A combined data set with 69 patients from five studies, including 13 pediatric patients, was subject to a population pharmacokinetic analysis using NONMEM.

RESULTS

The covariate analysis revealed that carboplatin clearance after high-dose chemotherapy was highly related to creatinine clearance, body height, and infusion duration. After inclusion of these covariates, the population parameter variability of clearance decreased from 53% in the base model to 21% in the final model. We also developed an alternative model that related body weight, instead of height, to clearance by means of allometric scaling. An evaluation of the predictive performance of existing a priori dose individualization strategies revealed that flat dosing is appropriate for adult patients with normal renal function receiving a 1-hour infusion. For a long-term infusion schedule, a dose increase may be necessary. The existing dosing strategies performed poorly for the children in our data set. For this subpopulation, our allometric scaling model may be most appropriate.

CONCLUSION

The two population pharmacokinetic models presented can be applied to estimate individual carboplatin clearance in high-dose chemotherapy based on body weight or height, infusion duration, and creatinine clearance. After prospective evaluation, our models may be suitable for dose adjustments.

Pharmacokinetic design optimization in children and estimation of maturation parameters: example of cytochrome P450 3A4

The aim of this work was to determine whether optimizing the study design in terms of ages and sampling times for a drug eliminated solely via cytochrome P450 3A4 (CYP3A4) would allow us to accurately estimate the pharmacokinetic parameters throughout the entire childhood timespan, while taking into account age- and weight-related changes. A linear monocompartmental model with first-order absorption was used successively with three different residual error models and previously published pharmacokinetic parameters ("true values"). The optimal ages were established by D-optimization using the CYP3A4 maturation function to create "optimized demographic databases." The post-dose times for each previously selected age were determined by D-optimization using the pharmacokinetic model to create "optimized sparse sampling databases." We simulated concentrations by applying the population pharmacokinetic model to the optimized sparse sampling databases to create optimized concentration databases. The latter were modeled to estimate population pharmacokinetic parameters. We then compared true and estimated parameter values. The established optimal design comprised four age ranges: 0.008 years old (i.e., around 3 days), 0.192 years old (i.e., around 2 months), 1.325 years old, and adults, with the same number of subjects per group and three or four samples per subject, in accordance with the error model. The population pharmacokinetic parameters that we estimated with this design were precise and unbiased (root mean square error [RMSE] and mean prediction error [MPE] less than 11% for clearance and distribution volume and less than 18% for k(a)), whereas the maturation parameters were unbiased but less precise (MPE < 6% and RMSE < 37%). Based on our results, taking growth and maturation into account a priori in a pediatric pharmacokinetic study is theoretically feasible. However, it requires that very early ages be included in studies, which may present an obstacle to the use of this approach. First-pass effects, alternative elimination routes, and combined elimination pathways should also be investigated.

Nonlinear pharmacokinetics of piperacillin in healthy volunteers--implications for optimal dosage regimens

AIMS

(i) To describe the first-order and mixed-order elimination pathways of piperacillin, (ii) to determine the between occasion variability (BOV) of pharmacokinetic parameters and (iii) to propose optimized dosage regimens.

METHODS

We performed a five-period replicate dose study in four healthy volunteers. Each subject received 4g piperacillin as a single 5min intravenous infusion in each study period. Drug analysis was performed by HPLC. We used NONMEM and S-ADAPT for population pharmacokinetic analysis and Monte Carlo simulation to predict the probability of target attainment (PTA) with a target time of non-protein bound concentration above MIC >50% of the dosing interval.

RESULTS

A model with first-order nonrenal elimination and parallel first-order and mixed-order renal elimination had the best predictive performance. For a 70kg subject we estimated 4.40lh(-1) for nonrenal clearance, 5.70lh(-1) for first-order renal clearance, 170mgh(-1) for V(max) , and 49.7mgl(-1) for K(m) for the mixed-order renal elimination. The BOV was 39% for V(max) , 117% for K(m) , and 8.5% for total clearance. A 30min infusion of 4g every 6h achieved robust (≥90%) PTAs for MICs ≤12mgl(-1) . As an alternative mode of administration, a 5h infusion of 6g every 8h achieved robust PTAs for MICs ≤48mgl(-1) .

CONCLUSIONS

Part of the renal elimination of piperacillin is saturable at clinically used doses. The BOV of total clearance and volume of distribution were low. Prolonged infusions achieved better PTAs compared with shorter infusions at similar daily doses. This benefit was most pronounced for MICs between 12 and 48mgl(-1) .

Population pharmacokinetics and pharmacodynamics in anesthesia, intensive care and pain medicine

PURPOSE OF REVIEW

Population modeling is a relatively new pharmacological discipline, the development of which has largely been stimulated by the need for accurate models for the pharmacokinetics and dynamics of anesthetic agents.

RECENT FINDINGS

Population-based modeling is now considered superior to older, more traditional modeling methods. Nonlinear mixed-effect modeling - a commonly used population-based modeling approach - estimates intraindividual and interindividual variability, limits the influence of outlying samples and individuals through the use of Bayesian statistical analysis, and provides a potential means of optimizing drug delivery regimens, especially when used to define pharmacokinetic-dynamic models for target-controlled infusion systems. In addition to being used for pharmacokinetic modeling, in which the influence of factors such as age, weight and illness can be studied, it is a powerful tool for the study of the influence of multiple factors on drug pharmacodynamics.

SUMMARY

Nonlinear mixed-effect population-based modeling has become the gold standard method of pharmacokinetic and pharmacodynamic analysis during new drug development and during subsequent pharmacological studies. Population-based modeling techniques have been applied to numerous aspects of drug delivery in anesthesia, intensive care and pain medicine.

Determination of appropriate dosing of influenza drugs in pediatric patients

Dose-finding studies of influenza antiviral drugs are challenging because it is difficult to enroll subjects in pediatric interventional studies and also because of the lack of concentration (or toxicity)–response relationships, the short duration of antiviral therapy, and the continually developing metabolic profiles of infants and young children. The evaluation of influenza antiviral agents in premature infants adds even more complexity. Recent advances in exposure-targeted study designs and modeling and simulations have aided in addressing some of these challenges.

A multicenter, randomized, open-label, pharmacokinetics and safety study of pantoprazole tablets in children and adolescents aged 6 through 16 years with gastroesophageal reflux disease

Children with gastroesophageal reflux disease (GERD) may benefit from gastric acid suppression with proton pump inhibitors such as pantoprazole. Effective treatment with pantoprazole requires correct dosing and understanding of the drug's kinetic profile in children. The aim of these studies was to characterize the pharmacokinetic (PK) profile of single and multiple doses of pantoprazole delayed-release tablets in pediatric patients with GERD aged 6 to 11 years (study 1) and 12 to 16 years (study 2). Patients were randomly assigned to receive pantoprazole 20 or 40 mg once daily. Plasma pantoprazole concentrations were obtained at intervals through 12 hours after the single dose and at 2 and 4 hours after multiple doses for PK evaluation. PK parameters were derived by standard noncompartmental methods and examined as a function of both drug dose and patient age. Safety was also monitored. Pantoprazole PK was dose independent (when dose normalized) and similar to PK reported from adult studies. There was no evidence of accumulation with multiple dosing or reports of serious drug-associated adverse events. In children aged 6 to 16 years with GERD, currently available pantoprazole delayed-release tablets can be used to provide systemic exposure similar to that in adults.

Betamethasone in pregnancy: influence of maternal body weight and multiple gestation on pharmacokinetics

OBJECTIVE

The goals of the study were to estimate the pharmacokinetic parameters of standard dose betamethasone in a large obstetrics population and evaluate the effect of maternal body size and multiple gestation on the pharmacokinetic parameters and their observed variability.

STUDY DESIGN

This was a prospective pharmacokinetic study. Liquid chromatography mass spectrometry was used to measure betamethasone plasma concentrations. Pharmacokinetic parameters and significant clinical covariates were estimated with mixed effect modeling. Bootstrap analysis confirmed validity of the model.

RESULTS

Two hundred seventy-four blood samples from 77 patients were obtained. The greatest effect on pharmacokinetic variability was observed with maternal lean body weight (LBW). The relationship between the pharmacokinetic parameters and LBW remained linear over a wide range of maternal body sizes. Multiple gestations did not affect the pharmacokinetic parameters.

CONCLUSION

Individualization of betamethasone dosing by maternal LBW reduces variability in drug exposure. Mutiple gestations do not require betamethasone dosing adjustment, because pharmacokinetics are the same as singleton gestations.

Influence of obesity on propofol pharmacokinetics: derivation of a pharmacokinetic model

BACKGROUND

The objective of this study was to develop a pharmacokinetic (PK) model to characterize the influence of obesity on propofol PK parameters.

METHODS

Nineteen obese ASA II patients undergoing bariatric surgery were studied. Patients received propofol 2 mg kg(-1) bolus dose followed by a 5-20-40-120 min, 10-8-6-5 mg kg(-1) h(-1) infusion. Arterial blood samples were withdrawn at 1, 3, 5 min after induction, every 10-20 min during propofol infusion, and every 10-30 min for 2 h after stopping the propofol infusion. Arterial samples were processed by high-performance liquid chromatography. Time-concentration data profiles from this study were pooled with data from two other propofol PK studies available at http://www.opentci.org. Population PK modelling was performed using non-linear mixed effects model.

RESULTS

The study involved 19 obese adults who contributed 163 observations. The pooled analysis involved 51 patients (weight 93 sd 24 kg, range 44-160 kg; age 46 sd 16 yr, range 25-81 yr; BMI 33 sd 9 kg m(-2), range 16-52 kg m(-2)). A three-compartment model was used to investigate propofol PK. An allometric size model using total body weight (TBW) was superior to all other models investigated (linear TBW, free fat mass, lean body weight, normal fat mass) for all clearance parameters. Variability in V2 and Q2 was reduced by a function showing a decrease in both parameters with age.

CONCLUSIONS

We have derived a population PK model using obese and non-obese data to characterize propofol PK over a wide range of body weights. An allometric model using TBW as the size descriptor of volumes and clearances was superior to other size descriptors to characterize propofol PK in obese patients.

A combined pharmacokinetic model for the hypoxia-targeted prodrug PR-104A in humans, dogs, rats and mice predicts species differences in clearance and toxicity

BACKGROUND

PR-104 is a phosphate ester that is systemically converted to the corresponding alcohol PR-104A. The latter is activated by nitroreduction in tumours to cytotoxic DNA cross-linking metabolites. Here, we report a population pharmacokinetic (PK) model for PR-104 and PR-104A in non-human species and in humans.

METHODS

A compartmental model was used to fit plasma PR-104 and PR-104A concentration-time data after intravenous (i.v.) dosing of humans, Beagle dogs, Sprague-Dawley rats and CD-1 nude mice. Intraperitoneal (i.p.) PR-104 and i.v. PR-104A dosing of mice was also investigated. Protein binding was measured in plasma from each species. Unbound drug clearances and volumes were scaled allometrically.

RESULTS

A two-compartment model described the disposition of PR-104 and PR-104A in all four species. PR-104 was cleared rapidly by first-order (mice, rats, dogs) or mixed-order (humans) metabolism to PR-104A in the central compartment. The estimated unbound human clearance of PR104A was 211 L/h/70 kg, with a steady state unbound volume of 105 L/70 kg. The size equivalent unbound PR-104A clearance was 2.5 times faster in dogs, 0.78 times slower in rats and 0.63 times slower in mice, which may reflect reported species differences in PR-104A O-glucuronidation.

CONCLUSIONS

The PK model demonstrates faster size equivalent clearance of PR-104A in dogs and humans than rodents. Dose-limiting myelotoxicity restricts the exposure of PR-104A in humans to approximately 25% of that achievable in mice.

Population pharmacokinetics and optimal design of paediatric studies for famciclovir

AIMS

To develop a population pharmacokinetic model for penciclovir (famciclovir is a prodrug of penciclovir) in adults and children and suggest an appropriate dose for children. Furthermore, to develop a limited sampling design based on sampling windows for three different paediatric age groups (1-2, 2-5 and 5-12 years) using an adequate number of subjects for future pharmacokinetic studies.

METHODS

Penciclovir plasma data from six different adult and paediatric studies were supplied by Novartis. Population pharmacokinetic modelling was undertaken in NONMEM version VI. Simulations in MATLAB were used to select an oral paediatric dose that gives similar exposure to 500 mg in adults. Optimal sampling times and sampling windows were obtained in MATLAB and simulations in NONMEM were used to select adequate sample sizes for three paediatric age groups.

RESULTS

A two-compartment, first-order absorption model with an absorption lag time, allometric weight models on V(1), V(2) and Q, and an allometric weight model, age and creatinine clearance as covariates on CL adequately describe the pharmacokinetics of penciclovir in adults and children. Estimated CL (l h(-1) 70 kg(-1)) and V(ss) (l.70 kg(-1)) were 31.2 and 83.1, respectively. An oral dose of 10 mg kg(-1) body weight in children was predicted to give similar exposure as 500 mg in adults. A single sampling windows design (0.25-0.4, 0.5-1, 1.25-1.75, 2.75-3.5 and 7.25-8 h) for five samples per subject and 10 subjects in each of the paediatric age groups is recommended for future studies.

CONCLUSIONS

A population pharmacokinetic model of penciclovir in adults and children has been developed. A prospective study design, including dose adjustment, cohort size and blood sampling design has been recommended.

Proposals for model-based paediatric medicinal development within the current European Union regulatory framework

The new paediatric European Union (EU) regulation and the consequent demand for paediatric studies on one hand and the ethical need for minimizing the burden of studies in children on the other hand necessitate optimal techniques in the assessment of safety/efficacy and use of drugs in children. Modelling and simulation (M&S) is one way to circumvent some difficulties in developing medicinal products in children. M&S allows the quantitative use of sparse sampling, characterization and prediction of pharmacokinetics/pharmacodynamics (PK/PD), extrapolation from adults to children, interpolation between paediatric age subsets, optimal use of scientific literature and in vitro/preclinical data. Together, industry, academia and regulators recognize the usefulness of modelling and simulation in this setting. However, even if M&S is an emerging science, its integration in the EU regulatory decision making is for the time being deficient and M&S expertise is concentrated in big pharmaceutical companies and academic institutions. The European Medicines Agency, acknowledging all the above conditions, organized and hosted a Workshop on Modelling in Paediatric Medicines. The article presents the personal views of the authors on the issues presented and discussed in the workshop.We attempt to identify the regulatory framework for the use of M&S in paediatric medicinal development and to make proposals for model-based paediatric medicinal development. The objective is to open the discussion between industry, academia, paediatricians and regulators on the optimal use of M&S in paediatric medicinal development.

Prediction of propofol clearance in children from an allometric model developed in rats, children and adults versus a 0.75 fixed-exponent allometric model

For propofol clearance, allometric scaling has been applied successfully for extrapolations between species (rats and humans) and within the human bodyweight range (children and adults). In this analysis, the human bodyweight range is explored to determine for which range an allometric model with a fixed or estimated exponent can be used to predict propofol clearance, without correction for maturation. The predictive value of the allometric equation, clearance (CL) is equal to 0.071 x bodyweight in kg0.78, which was developed from rats, children and adults, and the predictive value of a fixed exponent allometric model derived from the basal metabolic rate, CL is equal to CL standardized to a 70 kg adult x (bodyweight in kg standardized to a 70 kg adult)0.75, were evaluated across five independent patient groups including (i) 25 (pre)term neonates with a postmenstrual age of 27-43 weeks; (ii) 22 postoperative infants aged 4-18 months; (iii) 12 toddlers aged 1-3 years; (iv) 14 adolescents aged 10-20 years; and (v) 26 critically ill adults sedated long term. The median percentage error of the predictions was calculated using the equation %error = (CL(allometric) - CL(i))/CL(i) x 100, where CL(allometric) is the predicted propofol clearance from the allometric equations for each individual and CL(i) is the individual-predicted (post hoc) propofol clearance value derived from published population pharmacokinetic models. In neonates, the allometric model developed from rats, children and adults, and the fixed-exponent allometric model, systematically overpredicted individual propofol clearance, with median percentage errors of 288% and 216%, respectively, whereas in infants, both models systematically underpredicted individual propofol clearance, with median percentage errors of -43% and -55%, respectively. In toddlers, adolescents and adults, both models performed reasonably well, with median percentage errors of -12% and -32%, respectively, in toddlers, 16% and -14%, respectively, in adolescents, and 12% and -18%, respectively, in adults. Both allometric models based on bodyweight alone may be of use to predict propofol clearance in individuals older than 2 years. Approaches that also incorporate maturation are required to predict clearance under the age of 2 years.

Is the recommended once-daily dose of lamivudine optimal in West African HIV-infected children?

We aimed in this study to describe lamivudine concentration-time courses in treatment-naïve children after once-daily administration, to study the effects of body weight and age on lamivudine pharmacokinetics, and to simulate an optimized administration scheme. For this purpose, lamivudine concentrations were measured in 49 children after at least 2 weeks of didanosine-lamivudine-efavirenz treatment. A total of 148 plasma lamivudine concentrations were measured, and a population pharmacokinetic model was developed with NONMEM. The influence of individual characteristics was tested using a likelihood ratio test. Children were divided into two groups, according to their pharmacokinetic parameters, thanks to tree regression analysis. For each patient, the area under the curve was derived from estimated individual pharmacokinetic parameters. Different once-daily doses were simulated in each group, to obtain the same exposure in children as the mean effective exposure in adults (8.9 mg/liter.h). A two-compartment model in which the slope of distribution is assumed to be equal to the absorption rate constant adequately described the data. Parameter estimates were standardized for a mean standard body weight using an allometric model. Children were then divided into 2 groups according to body weight: CL/F was significantly higher in children weighing less than 17 kg (1.12 liters/h/kg) than in children over 17 kg (0.95 liters/h/kg; P=0.01). The target mean AUC of 8.9 mg/liters.h was obtained with a 10-mg/kg once-daily lamivudine (3TC) dose for children below 17 kg; the recommended dose of 8 mg/kg seems to be sufficient in children weighing more than 17 kg. These assumptions should be prospectively confirmed.

Clonidine clearance matures rapidly during the early postnatal period: a population pharmacokinetic analysis in newborns with neonatal abstinence syndrome

The population pharmacokinetic (PK) profile of oral clonidine was characterized in newborns with neonatal abstinence syndrome, and significant covariates affecting its PK parameters were identified. Plasma clonidine concentration data were obtained from a clinical trial in which 36 newborns, aged 1 to 25 days (postnatal age, PNA) and weighing 2.1 to 3.9 kg, were enrolled to take multiple oral doses of clonidine. The population PK model of clonidine was developed by NONMEM, and significant covariates were identified, followed by nonparametric bootstraps (2000 replicates) and simulation experiments. A 1-compartment open linear PK model was chosen to describe plasma concentrations of clonidine, and body weight and PNA were significant covariates for apparent clearance (CL/F) as follows: CL/F (L/h) = 15.2 × [body weight (kg)/70](0.75) × [PNA (day)(0.441)/(4.06(0.441) + PNA (day)(0.441))]. Furthermore, CL/F of clonidine increased rapidly with PNA during the first month of life after body weight was adjusted. Any optimal dosage regimen for clonidine in term neonates should be based on infant's age and body weight, and 1.5 µg/kg every 4 hours is proposed starting the second week of life based on the simulation results.

Population pharmacokinetic modeling of pantoprazole in pediatric patients from birth to 16 years

The population pharmacokinetics of pantoprazole was characterized in pediatric patients from birth to 16 years using NONMEM and evaluated via bootstrap and predictive check. Data were described using a 2-compartment model with a typical parameterized in terms of clearance (CL) (95% CI) of 1.93 L per hour (1.53, 2.61), given the reference covariates (female, full term, extensive/unknown CYP2C19 metabolizer status, non-African American, 10 kg weight, intravenous or tablet administration). Pantoprazole pharmacokinetic parameters appear to be similar in pediatric patients compared to adults when allometrically scaled. The effect of age on allometrically scaled CL was best described by a sigmoid Emax model with the age effect reaching an asymptote approximately equal to the adult CL by 1 year. CYP2C19 poor metabolizers exhibited reduced CL with the point estimate and 95% CI more than 70% lower than the typical value. Simulations from the final model indicated that the 1.2-mg/kg dose provides the best comparison to adults.