From pediatric covariate model to semiphysiological function for maturation: part I-extrapolation of a covariate model from morphine to Zidovudine

Application of Modelling and Simulation Approaches to Predict Pharmacokinetics of Therapeutic Monoclonal Antibodies in Pediatric Population

Ethical regulations and limited paediatric participants are key challenges that contribute to a median delay of 6 years in paediatric mAb approval. To overcome these barriers, modelling and simulation methodologies have been adopted to design optimized paediatric clinical studies and reduce patient burden. The classical modelling approach in paediatric pharmacokinetic studies for regulatory submissions is to apply body weight-based or body surface area-based allometric scaling to adult PK parameters derived from a popPK model to inform the paediatric dosing regimen. However, this approach is limited in its ability to account for the rapidly changing physiology in paediatrics, especially in younger infants. To overcome this limitation, PBPK modelling, which accounts for the ontogeny of key physiological processes in paediatrics, is emerging as an alternative modelling strategy. While only a few mAb PBPK models have been published, PBPK modelling shows great promise demonstrating a similar prediction accuracy to popPK modelling in an Infliximab paediatric case study. To facilitate future PBPK studies, this review consolidated comprehensive data on the ontogeny of key physiological processes in paediatric mAb disposition. To conclude, this review discussed different use-cases for pop-PK and PBPK modelling and how they can complement each other to increase confidence in pharmacokinetic predictions.

An update on the use of allometric and other scaling methods to scale drug clearance in children: towards decision tables

INTRODUCTION

When pediatric data are not available for a drug, allometric and other methods are applied to scale drug clearance across the pediatric age-range from adult values. This is applied when designing first-in-child studies, but also for off-label drug prescription.

AREAS COVERED

This review provides an overview of the systematic accuracy of allometric and other pediatric clearance scaling methods compared to gold-standard PBPK predictions. The findings are summarized in decision tables to provide a priori guidance on the selection of appropriate pediatric clearance scaling methods for both novel drugs for which no pediatric data are available and existing drugs in clinical practice.

EXPERT OPINION

While allometric scaling principles are commonly used to scale pediatric clearance, there is no universal allometric exponent (i.e., 1, 0.75 or 0.67) that can accurately scale clearance for all drugs from adults to children of all ages. Therefore, pediatric scaling decision tables based on age, drug elimination route, binding plasma protein, fraction unbound, extraction ratio, and/or isoenzyme maturation are proposed to a priori select the appropriate (allometric) clearance scaling method, thereby reducing the need for full PBPK-based clearance predictions. Guidance on allometric scaling when estimating pediatric clearance values is provided as well.

Evaluating risk detection methods to uncover ontogenic-mediated adverse drug effect mechanisms in children

BACKGROUND

Identifying adverse drugs effects (ADEs) in children, overall and within pediatric age groups, is essential for preventing disability and death from marketed drugs. At the same time, however, detection is challenging due to dynamic biological processes during growth and maturation, called ontogeny, that alter pharmacokinetics and pharmacodynamics. As a result, methodologies in pediatric drug safety have been limited to event surveillance and have not focused on investigating adverse event mechanisms. There is an opportunity to identify drug event patterns within observational databases for evaluating ontogenic-mediated adverse event mechanisms. The first step of which is to establish statistical models that can identify temporal trends of adverse effects across childhood.

RESULTS

Using simulation, we evaluated a population stratification method (the proportional reporting ratio or PRR) and a population modeling method (the generalized additive model or GAM) to identify and quantify ADE risk at varying reporting rates and dynamics. We found that GAMs showed improved performance over the PRR in detecting dynamic drug event reporting across child development stages. Moreover, GAMs exhibited normally distributed and robust ADE risk estimation at all development stages by sharing information across child development stages.

CONCLUSIONS

Our study underscores the opportunity for using population modeling techniques, which leverage drug event reporting across development stages, as biologically-inspired detection methods for evaluating ontogenic mechanisms.

Comparison between physiologically based pharmacokinetic and population pharmacokinetic modelling to select paediatric doses of gepotidacin in plague

Aims

To develop physiologically based pharmacokinetic (PBPK) and population pharmacokinetic (PopPK) models to predict effective doses of gepotidacin in paediatrics for the treatment of pneumonic plague (Yersinia pestis).

Methods

A gepotidacin PBPK model was constructed using a population‐based absorption, distribution, metabolism and excretion simulator, Simcyp®, with physicochemical and in vitro data, optimized with clinical data from a dose‐escalation intravenous (IV) study and a human mass balance study. A PopPK model was developed with pooled PK data from phase 1 studies with IV gepotidacin in healthy adults.

Results

For both the PopPK and PBPK models, body weight was found to be a key covariate affecting gepotidacin clearance. With PBPK, ~90% of the predicted PK for paediatrics fell between the 5th and 95th percentiles of adult values except for subjects weighing ≤5 kg. PopPK‐simulated paediatric means for C_max and AUC_(0‐τ) were similar to adult exposures across various weight brackets. The proposed dosing regimens were weight‐based for subjects ≤40 kg and fixed‐dose for subjects >40 kg. Comparison of observed and predicted exposures in adults indicated that both PBPK and PopPK models achieved similar AUC and C_max for a given dose, but the C_max predictions with PopPK were slightly higher than with PBPK. The two models differed on dose predictions in children <3 months old. The PopPK model may be suboptimal for low age groups due to the absence of maturation characterization of drug‐metabolizing enzymes involved with clearance in adults.

Conclusions

Both PBPK and PopPK approaches can reasonably predict gepotidacin exposures in children.

Recent advances in the ontogeny of drug disposition

Developmental changes that occur throughout childhood have long been known to impact drug disposition. However, pharmacokinetic studies in the paediatric population have historically been limited due to ethical concerns arising from incorporating children into clinical trials. As such, much of the early work in the field of developmental pharmacology was reliant on difficult-to-interpret in vitro and in vivo animal studies. Over the last 2 decades, our understanding of the mechanistic processes underlying age-related changes in drug disposition has advanced considerably. Progress has largely been driven by technological advances in mass spectrometry-based methods for quantifying proteins implicated in drug disposition, and in silico tools that leverage these data to predict age-related changes in pharmacokinetics. This review summarizes our current understanding of the impact of childhood development on drug disposition, particularly focusing on research of the past 20 years, but also highlighting select examples of earlier foundational research. Equally important to the studies reviewed herein are the areas that we cannot currently describe due to the lack of research evidence; these gaps provide a map of drug disposition pathways for which developmental trends still need to be characterized.

Enzyme Kinetics of Uridine Diphosphate Glucuronosyltransferases (UGTs)

Glucuronidation, catalyzed by uridine diphosphate glucuronosyltransferases (UGTs), is an important process for the metabolism and clearance of many lipophilic chemicals, including drugs, environmental chemicals, and endogenous compounds. Glucuronidation is a bisubstrate reaction that requires the aglycone and the cofactor, UDP-GlcUA. Accumulating evidence suggests that the bisubstrate reaction follows a compulsory-order ternary mechanism. To simplify the kinetic modeling of glucuronidation reactions in vitro, UDP-GlcUA is usually added to incubations in large excess. Many factors have been shown to influence UGT activity and kinetics in vitro, and these must be accounted for during experimental design and data interpretation. While the assessment of drug-drug interactions resulting from UGT inhibition has been challenging in the past, the increasing availability of UGT enzyme-selective substrate and inhibitor "probes" provides the prospect for more reliable reaction phenotyping and assessment of drug-drug interaction potential. Although extrapolation of the in vitro intrinsic clearance of a glucuronidated drug often underpredicts in vivo clearance, careful selection of in vitro experimental conditions and inclusion of extrahepatic glucuronidation may improve the predictivity of in vitro-in vivo extrapolation. Physiologically based pharmacokinetic (PBPK) modeling has also shown to be of value for predicting PK of drugs eliminated by glucuronidation.

The Predictive Value of Glomerular Filtration Rate-Based Scaling of Pediatric Clearance and Doses for Drugs Eliminated by Glomerular Filtration with Varying Protein-Binding Properties

INTRODUCTION

For drugs eliminated by glomerular filtration (GF), clearance (CL) is determined by GF rate (GFR) and the unbound fraction of the drug. When predicting CL of GF-eliminated drugs in children, instead of physiologically based pharmacokinetic (PBPK) methods that consider changes in both GFR and protein binding, empiric bodyweight-based methods are often used. In this article, we explore the predictive value of scaling using a GFR function, and compare the results with linear and allometric scaling methods for drugs with different protein-binding properties.

METHODS

First, different GFR maturation functions were compared to identify the GFR function that would yield the most accurate GFR predictions across the pediatric age range compared with published pediatric inulin/mannitol CL values. Subsequently, the accuracy of pediatric CL scaling using this GFR maturation function was assessed and compared with PBPK CL predictions for hypothetical drugs binding, to varying extents, to serum albumin or α-acid glycoprotein across the pediatric age range. Additionally, empiric bodyweight-based methods were assessed.

RESULTS

The published GFR maturation functions yielded comparable maturation profiles, with the function reported by Salem et al. leading to the most accurate predictions. On the basis of this function, GFR-based scaling yields reasonably accurate (percentage prediction error ≤ 50%) pediatric CL values for all drugs, except for some drugs highly bound to AGP in neonates. Overall, this method was more accurate than linear or 0.75 allometric bodyweight-based scaling.

CONCLUSION

When scaling CL and dose by GFR function, maturational changes in plasma protein concentrations impact GF minimally, making this method a superior alternative to empiric bodyweight-based scaling.

The Utility of Pharmacometric Models in Clinical Pharmacology Research in Infants

Purpose of commentary

Acquiring knowledge on drug disposition and action in infant is challenging because of the problem of sparse and unbalanced data obtained for each individual infant due to the limited blood volume as well as the issue of extensive inter-subject and intra-subject variability in drug exposure and response due to the fast growth and dynamic maturation changes in infants. This commentary highlights the importance of using population-based pharmacometric models to improve knowledge on drug disposition and action in infants.

Recent findings

Pharmacometric modeling remains to be critical in clinical pharmacology research in infants. Many pediatric covariate models developed for scaling of drug clearance use a combination of allometric weight scaling to account for size change and a sigmoid function of antenatal development and postnatal maturation to characterize the age-related maturation. To expedite the development of safe and effective dosing regimens in infants, a number of strategies have been proposed recently, including the use of pediatric covariate model obtained from one drug for extrapolation to other drugs undergoing similar elimination pathways, as well as the combination of opportunistic clinical studies and population-based pharmacometrics models.

Summary

Population-based pharmacometric modeling plays a pivotal role in clinical pharmacology research in infants. Most of the covariate models reported so far focus on antibiotics undergoing renal elimination. Novel modeling strategies have been proposed recently to facilitate clinical pharmacology research and expedite the dose optimization process in infants.

State-of-the-Art Review on Physiologically Based Pharmacokinetic Modeling in Pediatric Drug Development

Physiologically based pharmacokinetic modeling and simulation is an important tool for predicting the pharmacokinetics, pharmacodynamics, and safety of drugs in pediatrics. Physiologically based pharmacokinetic modeling is applied in pediatric drug development for first-time-in-pediatric dose selection, simulation-based trial design, correlation with target organ toxicities, risk assessment by investigating possible drug-drug interactions, real-time assessment of pharmacokinetic-safety relationships, and assessment of non-systemic biodistribution targets. This review summarizes the details of a physiologically based pharmacokinetic modeling approach in pediatric drug research, emphasizing reports on pediatric physiologically based pharmacokinetic models of individual drugs. We also compare and contrast the strategies employed by various researchers in pediatric physiologically based pharmacokinetic modeling and provide a comprehensive overview of physiologically based pharmacokinetic modeling strategies and approaches in pediatrics. We discuss the impact of physiologically based pharmacokinetic models on regulatory reviews and product labels in the field of pediatric pharmacotherapy. Additionally, we examine in detail the current limitations and future directions of physiologically based pharmacokinetic modeling in pediatrics with regard to the ability to predict plasma concentrations and pharmacokinetic parameters. Despite the skepticism and concern in the pediatric community about the reliability of physiologically based pharmacokinetic models, there is substantial evidence that pediatric physiologically based pharmacokinetic models have been used successfully to predict differences in pharmacokinetics between adults and children for several drugs. It is obvious that the use of physiologically based pharmacokinetic modeling to support various stages of pediatric drug development is highly attractive and will rapidly increase, provided the robustness and reliability of these techniques are well established.

Pharmacokinetics and Pharmacodynamics of Drugs in Obese Pediatric Patients: How to Map Uncharted Clinical Territories

Clinicians are increasingly faced with challenges regarding the pharmacological treatment of obese pediatric patients. To provide guidance for these treatments, a better understanding of the impact of obesity on pharmacological processes in children is needed. Results on pharmacological studies in adults show however ambiguous patterns regarding the impact of obesity on ADME processes or on drug pharmacodynamics. Additionally, based on the limited research performed in obese pediatric patients, it becomes clear that findings from obese adults cannot be expected to always translate directly to similar findings in obese children. To improve knowledge on drug pharmacology in obese pediatric patients, studies should focus on quantifying the impact of maturation, obesity, and other relevant variables on primary pharmacological parameters and on disentangling systemic (renal and/or hepatic) and presystemic (gut and/or first-pass hepatic) clearance. For this, data is required from well-designed clinical trials that include patients with not only a wide range in age but also a range in excess body weight, upon oral and intravenous dosing. Population modelling approaches are ideally suitable for this purpose and can also be used to link the pharmacokinetics to pharmacodynamics and to derive drug dosing regimens. Generalizability of research findings can be achieved by including mechanistic aspects in the data analysis, for instance, using either extrapolation approaches in population modelling or by applying physiologically based modelling principles. It is imperative that more and smarter studies are performed in obese pediatric patients to provide safe and effective treatment for this special patient population.

A Pediatric Covariate Function for CYP3A-Mediated Midazolam Clearance Can Scale Clearance of Selected CYP3A Substrates in Children

Recently a framework was presented to assess whether pediatric covariate models for clearance can be extrapolated between drugs sharing elimination pathways, based on extraction ratio, protein binding, and other drug properties. Here we evaluate when a pediatric covariate function for midazolam clearance can be used to scale clearance of other CYP3A substrates. A population PK model including a covariate function for clearance was developed for midazolam in children aged 1–17 years. Commonly used CYP3A substrates were selected and using the framework, it was assessed whether the midazolam covariate function accurately scales their clearance. For eight substrates, reported pediatric clearance values were compared numerically and graphically with clearance values scaled using the midazolam covariate function. For sildenafil, clearance values obtained with population PK modeling based on pediatric concentration-time data were compared with those scaled with the midazolam covariate function. According to the framework, a midazolam covariate function will lead to systemically accurate clearance scaling (absolute prediction error (PE) < 30%) for CYP3A substrates binding to albumin with an extraction ratio between 0.35 and 0.65 when binding < 10% in adults, between 0.05 and 0.55 when binding > 90%, and with an extraction ratio ranging between these values when binding between 10 and 90%. Scaled clearance values for eight commonly used CYP3A substrates were reasonably accurate (PE < 50%). Scaling of sildenafil clearance was accurate (PE < 30%). We defined for which CYP3A substrates a pediatric covariate function for midazolam clearance can accurately scale plasma clearance in children. This scaling approach may be useful for CYP3A substrates with scarce or no available pediatric PK information.

Evaluation of a System-Specific Function To Describe the Pharmacokinetics of Benzylpenicillin in Term Neonates Undergoing Moderate Hypothermia

The pharmacokinetic (PK) properties of intravenous (i.v.) benzylpenicillin in term neonates undergoing moderate hypothermia after perinatal asphyxia were evaluated, as they have been unknown until now. A system-specific modeling approach was applied, in which our recently developed covariate model describing developmental and temperature-induced changes in amoxicillin clearance (CL) in the same patient study population was incorporated into a population PK model of benzylpenicillin with a priori birthweight (BW)-based allometric scaling. Pediatric population covariate models describing the developmental changes in drug elimination may constitute system-specific information and may therefore be incorporated into PK models of drugs cleared through the same pathway. The performance of this system-specific model was compared to that of a reference model. Furthermore, Monte-Carlo simulations were performed to evaluate the optimal dose. The system-specific model performed as well as the reference model. Significant correlations were found between CL and postnatal age (PNA), gestational age (GA), body temperature (TEMP), urine output (UO; system-specific model), and multiorgan failure (reference model). For a typical patient with a GA of 40 weeks, BW of 3,000 g, PNA of 2 days (TEMP, 33.5°C), and normal UO (2 ml/kg/h), benzylpenicillin CL was 0.48 liter/h (interindividual variability [IIV] of 49%) and the volume of distribution of the central compartment was 0.62 liter/kg (IIV of 53%) in the system-specific model. Based on simulations, we advise a benzylpenicillin i.v. dose regimen of 75,000 IU/kg/day every 8 h (q8h), 150,000 IU/kg/day q8h, and 200,000 IU/kg/day q6h for patients with GAs of 36 to 37 weeks, 38 to 41 weeks, and ≥42 weeks, respectively. The system-specific model may be used for other drugs cleared through the same pathway accelerating model development.

Drugs Being Eliminated via the Same Pathway Will Not Always Require Similar Pediatric Dose Adjustments

For scaling drug plasma clearance (CLp) from adults to children, extrapolations of population pharmacokinetic (PopPK) covariate models between drugs sharing an elimination pathway have enabled accelerated development of pediatric models and dosing recommendations. This study aims at identifying conditions for which this approach consistently leads to accurate pathway specific CLp scaling from adults to children for drugs undergoing hepatic metabolism. A physiologically based pharmacokinetic (PBPK) simulation workflow utilizing mechanistic equations defining hepatic metabolism was developed. We found that drugs eliminated via the same pathway require similar pediatric dose adjustments only in specific cases, depending on drugs extraction ratio, unbound fraction, type of binding plasma protein, and the fraction metabolized by the isoenzyme pathway for which CLp is scaled. Overall, between‐drug extrapolation of pediatric covariate functions for CLp is mostly applicable to low and intermediate extraction ratio drugs eliminated by one isoenzyme and binding to human serum albumin in children older than 1 month.

Inflammation and Organ Failure Severely Affect Midazolam Clearance in Critically Ill Children

RATIONALE

Various in vitro, animal, and limited human adult studies suggest a profound inhibitory effect of inflammation and disease on cytochrome P-450 3A (CYP3A)-mediated drug metabolism. Studies showing this relationship in critically ill patients are lacking, whereas clearance of many CYP3A drug substrates may be decreased, potentially leading to toxicity.

OBJECTIVES

To prospectively study the relationship between inflammation, organ failure, and midazolam clearance as a validated marker of CYP3A-mediated drug metabolism in critically ill children.

METHODS

From 83 critically ill children (median age, 5.1 mo [range, 0.02-202 mo]), midazolam plasma (n = 532), cytokine (e.g., IL-6, tumor necrosis factor-α), and C-reactive protein (CRP) levels; organ dysfunction scores (Pediatric Risk of Mortality II, Pediatric Index of Mortality 2, Pediatric Logistic Organ Dysfunction); and number of failing organs were prospectively collected. A population pharmacokinetic model to study the impact of inflammation and organ failure on midazolam pharmacokinetics was developed using NONMEM 7.3.

MEASUREMENTS AND MAIN RESULTS

In a two-compartmental pharmacokinetic model, body weight was the most significant covariate for clearance and volume of distribution. CRP and organ failure were significantly associated with clearance (P < 0.01), explaining both interindividual and interoccasional variability. In simulations, a CRP of 300 mg/L was associated with a 65% lower clearance compared with 10 mg/L, and three failing organs were associated with a 35% lower clearance compared with one failing organ.

CONCLUSIONS

Inflammation and organ failure strongly reduce midazolam clearance, a surrogate marker of CYP3A-mediated drug metabolism, in critically ill children. Hence, critically ill patients receiving CYP3A substrate drugs may be at risk of increased drug levels and associated toxicity.

Development of Human Membrane Transporters: Drug Disposition and Pharmacogenetics

Membrane transporters play an essential role in the transport of endogenous and exogenous compounds, and consequently they mediate the uptake, distribution, and excretion of many drugs. The clinical relevance of transporters in drug disposition and their effect in adults have been shown in drug–drug interaction and pharmacogenomic studies. Little is known, however, about the ontogeny of human membrane transporters and their roles in pediatric pharmacotherapy. As they are involved in the transport of endogenous substrates, growth and development may be important determinants of their expression and activity. This review presents an overview of our current knowledge on human membrane transporters in pediatric drug disposition and effect. Existing pharmacokinetic and pharmacogenetic data on membrane substrate drugs frequently used in children are presented and related, where possible, to existing ex vivo data, providing a basis for developmental patterns for individual human membrane transporters. As data for individual transporters are currently still scarce, there is a striking information gap regarding the role of human membrane transporters in drug therapy in children.

Predictive Performance of Physiologically Based Pharmacokinetic and Population Pharmacokinetic Modeling of Renally Cleared Drugs in Children

Predictive performance of physiologically based pharmacokinetic (PBPK) and population pharmacokinetic (PopPK) models of drugs predominantly eliminated through kidney in the pediatric population was evaluated. After optimization using adult clinical data, the verified PBPK models can predict 33 of 34 drug clearance within twofold of the observed values in children 1 month and older. More specifically, 10 of 11 of predicted clearance values were within 1.5‐fold of those observed in children between 1 month and 2 years old. The PopPK approach also predicted 19 of 21 drug clearance within twofold of the observed values in children. In summary, our analysis demonstrated both PBPK and PopPK adult models, after verification with additional adult pharmacokinetic (PK) studies and incorporation of known ontogeny of renal filtration, could be applied for dosing regimen recommendation in children 1 month and older for renally eliminated drugs in a first‐in‐pediatric study.

Drug-induced renal damage in preterm neonates: state of the art and methods for early detection

Only a small fraction of drugs widely used in neonatal intensive care units (NICU) are specifically authorized for this population. Even if unlicensed or off-label use is necessary, it is associated with increased adverse drug reactions, which must be carefully weighed against expected benefits. In particular, renal damage is frequent among preterm babies, and is considered a predisposing factor for the development of chronic kidney disease in adulthood. Apart from specific conditions affecting premature neonates (e.g. respiratory distress syndrome, perinatal asphyxia), drugs play an important role in impairing renal function because of well-known nephrotoxicity and/or interaction with renal developmental factors. From a review of the available studies on drug use in NICU patients, we identified and described the most commonly administered drugs that are correlated to renal damage. Early detection of kidney injury is becoming an essential aspects for clinicians because of the limited number of biomarkers applicable in the neonatal population. Postnatal changes of biochemical processes that influence pharmacokinetic and pharmacodynamic aspects need to be further investigated in order to better understand the mechanisms of drug toxicity in this population. The most promising strategies for dose adjustment and therapeutic schemes are discussed. The purpose of this review was to describe current knowledge on drug use among premature babies and their implication in kidney injury development, as well as to highlight available strategies for early detection of renal damage.

Paracetamol pharmacokinetics and metabolism in young women

Background

There is relevant between individual variability in paracetamol clearance in young women. In this pooled study, we focused on the population pharmacokinetic profile of intravenous paracetamol metabolism and its covariates in young women.

Methods

Population PK parameters using non-linear mixed effect modelling were estimated in a pooled dataset of plasma and urine PK studies in 69 young women [47 at delivery, 8/47 again 10–15 weeks after delivery (early postpartum), and 7/8 again 1 year after delivery (late postpartum), 22 healthy female volunteers with or without oral contraceptives].

Results

Population PK parameters were estimated based on 815 plasma samples and 101 urine collections. Compared to healthy female volunteers (reference group) not on oral contraceptives, being at delivery was the most significant covariate for clearance to paracetamol glucuronide (Factor = 2.03), while women in early postpartum had decreased paracetamol glucuronidation clearance (Factor = 0.55). Women on contraceptives showed increased paracetamol glucuronidation clearance (Factor = 1.46). The oestradiol level did not further affect this model. Being at delivery did not prove significant for clearance to paracetamol sulphate, but was higher in pregnant women who delivered preterm (<37 weeks, Factor = 1.34) compared to term delivery and non-pregnant women. Finally, clearance of unchanged paracetamol was dependent on urine flow rate.

Conclusions

Compared to healthy female volunteers not on oral contraceptives, urine paracetamol glucuronidation elimination in young women is affected by pregnancy (higher), early postpartum (lower) or exposure to oral contraceptives (higher), resulting in at least a two fold variability in paracetamol clearance in young women.

Population pharmacokinetic modeling of Thymoglobulin(®) in children receiving allogeneic-hematopoietic cell transplantation: towards improved survival through individualized dosing

BACKGROUND AND OBJECTIVES

To prevent graft-versus-host disease and rejection in hematopoietic cell transplantation (HCT), children receive Thymoglobulin(®), a polyclonal antibody acting mainly by depleting T cells. The therapeutic window is critical as over-exposure may result in delayed immune reconstitution of donor T cells. In this study, we describe the population pharmacokinetics of Thymoglobulin(®) as a first step towards an evidence-based dosing regimen of Thymoglobulin(®) in pediatric HCT.

METHODS

Serum active Thymoglobulin(®) concentrations were measured in all pediatric HCTs performed between 2004 and 2012 in two pediatric HCT centers in The Netherlands. Population pharmacokinetic analysis was performed using NONMEM(®) version 7.2.

RESULTS

A total of 3,113 concentration samples from 280 pediatric HCTs were analyzed, with age ranging from 3 months to 23 years old. The cumulative Thymoglobulin(®) dose was 10 mg/kg in 94 % of the patients given in 4 consecutive days. A model incorporating parallel linear and concentration-dependent clearance of Thymoglobulin(®) was identified. Body weight [for linear clearance (CL) and central volume of distribution] as well as lymphocyte count pre-Thymoglobulin(®) infusion (for CL) were important covariates. As such, the current dosing regimen results in higher exposure in children with a higher bodyweight and/or a lower lymphocyte count pre-Thymoglobulin(®) infusion.

CONCLUSION

This model can be used to develop an individual dosing regimen for Thymoglobulin(®), based on both body weight and lymphocyte counts, once the therapeutic window has been determined. This individualized regimen may contribute to a better immune reconstitution and thus outcome of allogeneic HCT.

Human Ontogeny of Drug Transporters: Review and Recommendations of the Pediatric Transporter Working Group

The critical importance of membrane-bound transporters in pharmacotherapy is widely recognized, but little is known about drug transporter activity in children. In this white paper, the Pediatric Transporter Working Group presents a systematic review of the ontogeny of clinically relevant membrane transporters (e.g., SLC, ABC superfamilies) in intestine, liver, and kidney. Different developmental patterns for individual transporters emerge, but much remains unknown. Recommendations to increase our understanding of membrane transporters in pediatric pharmacotherapy are presented.

Drug disposition in obesity: toward evidence-based dosing

Obesity and morbid obesity are associated with many physiological changes affecting pharmacokinetics, such as increased blood volume, cardiac output, splanchnic blood flow, and hepatic blood flow. In obesity, drug absorption appears unaltered, although recent evidence suggests that this conclusion may be premature. Volume of distribution may vary largely, but the magnitude and direction of changes seem difficult to predict, with extrapolation on the basis of total body weight being the best approach to date. Changes in clearance may be smaller than in distribution, whereas there is growing evidence that the influence of obesity on clearance can be predicted on the basis of reported changes in the metabolic or elimination pathways involved. For obese children, we propose two methods to distinguish between developmental and obesity-related changes. Future research should focus on the characterization of physiological concepts to predict the optimal dose for each drug in the obese population.

Pediatric pharmacology: current efforts and future goals to improve clinical practice

Interest in pediatric pharmacology has increased over the past two decades. With few exceptions, research efforts are currently, however, still limited to pharmacokinetic (PK) queries on single drugs in a limited number of subjects. It is now time to move forward and integrate and generalize the PK information that is currently available more efficiently across different drugs and different populations. Additionally, for pediatric patients to truly benefit from pharmacological research efforts, the knowledge that is obtained in these studies needs to be translated into dosing recommendations that are subsequently prospectively evaluated in adequately powered randomized clinical trials. Finally, as drug effects and safety are the result of both PK and pharmacodynamic (PD) processes and as developmental changes may occur in both processes, it is essential for PK studies to be followed-up by PD studies when dose-adjustments based on PKs alone have been proven insufficient. In this report, examples illustrating this approach are provided. As PD studies in children are generally more complicated to perform than PK studies, this is where a big challenge in pediatric pharmacological research still lies.

Neonatal medicines research: challenges and opportunities

INTRODUCTION

The key feature of the newborn is its fast age-dependent maturation, resulting in extensive variability in pharmacokinetics and -dynamics, further aggravated by newly emerging covariates like treatment modalities, environmental issues or pharmacogenetics. This makes clinical research in neonates relevant and needed, but also challenging.

AREAS COVERED

To improve this knowledge, tailoring research tools as well as building research networks and clinical research skills for neonates are urgently needed. Tailoring of research tools is illustrated using the development of dried blood spot techniques and the introduction of micro-dosing and -tracer methodology in neonatal drug studies. Both techniques can be combined with sparse sampling techniques through population modeling. Building research networks and clinical research skills is illustrated by the initiatives of agencies to build and integrate knowledge on neonatal pharmacotherapy through dedicated working groups.

EXPERT OPINION

Challenges relating to neonatal medicine research can largely be overcome. Tailored tools and legal initiatives, combined with clever trial design will result in more robust information on neonatal pharmacotherapy. This necessitates collaborative efforts between clinical researchers, sponsors, regulatory authorities, and last but not least patient representatives and society.

Drug metabolism for the paediatrician

Drug metabolism importantly determines drug concentrations. The efficacy and safety of many drugs prescribed for children are, therefore, dependent on intraindividual and interindividual variation in drug-metabolising enzyme activity. During growth and development, changes in drug-metabolising enzyme activity result in age-related differences in drug disposition, most pronounced in preterm infants and young infants. The shape of the developmental trajectory is unique to the drug-metabolising enzyme involved in the metabolism of individual drugs. Other factors impacting drug metabolism are underlying disease, drug-drug interactions and genetic variation. The interplay of age with these other factors may result in unexpected variation in drug metabolism in children of different ages. Extrapolation of adult data to guide drug dosing in children should be done with caution. The younger the child, the less reliable is the extrapolation. This review aims to identify the primary sources of variability of drug metabolism in children, the knowledge of which can ultimately guide the practitioner towards effective and safe drug therapy.

Tailored tools to improve pharmacotherapy in infants

INTRODUCTION

Extensive within-population variability is the essence of neonatal pharmacology. Despite this, infants remain one of the last therapeutic orphans. Together with additional legal initiatives, tailoring of already available tools (modeling, covariates, pharmacovigilance) may significantly improve pharmacotherapy in infants.

AREAS COVERED

Modeling approaches that hold the promise to improve pharmacotherapy in infants are between-compound extrapolation for compounds that undergo the same route of elimination and integration of time-varying physiology to adapt for the fast maturational changes. Besides these maturational covariates (size, age), newly emerging covariates relate to novel treatment modalities (extracorporeal circulation, hypothermia), environmental issues (microbiome, critical illness) or pharmacogenetics. All these covariates interact with the maturational variation. Finally, pharmacovigilance also needs to be tailored to the characteristics of this population. This relates to preventive strategies, signal detection and assessment of causality.

EXPERT OPINION

Knowledge on pharmacotherapy in infants is lagging. Tailoring available tools to the specific characteristics (maturation) and clinical needs (newly emerging covariates) of infants is feasible but needs creativity and a multidisciplinary collaboration between modelers, academia, clinical researchers and, obviously, the public, including parents.

Simultaneous pharmacokinetic modeling of gentamicin, tobramycin and vancomycin clearance from neonates to adults: towards a semi-physiological function for maturation in glomerular filtration

PURPOSE

Since glomerular filtration rate (GFR) is responsible for the elimination of a large number of water-soluble drugs, the aim of this study was to develop a semi-physiological function for GFR maturation from neonates to adults.

METHODS

In the pharmacokinetic analysis (NONMEM VI) based on data of gentamicin, tobramycin and vancomycin collected in 1,760 patients (age 1 day-18 years, bodyweight 415 g-85 kg), a distinction was made between drug-specific and system-specific information. Since the maturational model for clearance is considered to contain system-specific information on the developmental changes in GFR, one GFR maturational function was derived for all three drugs.

RESULTS

Simultaneous analysis of these three drugs showed that maturation of GFR mediated clearance from preterm neonates to adults was best described by a bodyweight-dependent exponent (BDE) function with an exponent varying from 1.4 in neonates to 1.0 in adults (ClGFR = Cldrug*(BW/4 kg)(BDE) with BDE = 2.23*BW(-0.065)). Population clearance values (Cldrug) for gentamicin, tobramycin and vancomycin were 0.21, 0.28 and 0.39 L/h for a full term neonate of 4 kg, respectively.

DISCUSSION

Based on an integrated analysis of gentamicin, tobramycin and vancomycin, a semi-physiological function for GFR mediated clearance was derived that can potentially be used to establish evidence based dosing regimens of renally excreted drugs in children.

Towards evidence-based dosing regimens in children on the basis of population pharmacokinetic pharmacodynamic modelling

When growing up, the pharmacokinetic (PK) and pharmacodynamic (PD) profiles of drugs change, which may alter the effect of drugs. To ensure optimal drug efficacy and safety in paediatric care, PK and PD relationships of drugs need to be explored in children. This article presents an outline on performing a population PK/PD study and translating these results into rational dosing regimens, with the development and prospective evaluation of PK/PD derived evidence-based dosing regimen being discussed. Examples on amikacin, morphine and busulfan are provided, showing how PK(/PD) modelling not only led to optimization and individualization in paediatric clinical care for the specific drugs but also to insight in maturation of organ systems involved. It is shown that the latter results can subsequently be used as a basis for dosing of other drugs eliminated through the same pathway. Ultimately, these efforts should lead to predictable drug efficacy and safety across all age groups.

Developmental changes in morphine clearance across the entire paediatric age range are best described by a bodyweight-dependent exponent model

BACKGROUND AND OBJECTIVE

Morphine clearance has been successfully scaled from preterm neonates to 3-year-old children on the basis of a bodyweight-based exponential (BDE) function and age younger or older than 10 days. The aim of the current study was to characterize the developmental changes in morphine clearance across the entire paediatric age range.

METHODS

Morphine and morphine-3-glucuronide (M3G) concentration data from 358 (pre)term neonates, infants, children and adults, and morphine concentration data from 117 adolescents were analysed using NONMEM 7.2. Based on available data, two models were developed: I. using morphine data; II. using morphine and M3G data.

RESULTS

In model I, morphine clearance across the paediatric age range was very well described by a BDE function in which the allometric exponent decreased in a sigmoidal manner with bodyweight (BDE model) from 1.47 to 0.88, with half the decrease in exponent reached at 4.01 kg. In model II, the exponent for the formation and elimination clearance of M3G was found to decrease from 1.56 to 0.89 and from 1.06 to 0.61, with half the decrease reached at 3.89 and 4.87 kg, respectively. Using the BDE model, there was no need to use additional measures for size or age.

CONCLUSION

The BDE model was able to scale both total morphine clearance and glucuronidation clearance through the M3G pathway across all age ranges between (pre)term neonates and adults by allowing the allometric exponent to decrease across the paediatric age range from values higher than 1 for neonates to values lower than 1 for infants and children.

Clinical pharmacology in neonates: small size, huge variability

Drug therapy is a powerful tool for improving neonatal outcome. Despite this, neonatologists still routinely prescribe off-label compounds developed for adults and extrapolate doses from those used for children or adults. Knowledge integration through pharmacokinetic modeling is a method that could improve the current situation. Such predictive models may convert neonatal pharmacotherapy from explorative to confirmatory. This can be illustrated by research projects related to the prediction of neonatal renal clearance and neonatal glucuronidation. This type of model will also improve the current knowledge of neonatal (patho)physiology. In the meanwhile, the fields of clinical pharmacology (e.g. pharmacokinetic/pharmacodynamic modeling and pharmacogenetics) and neonatology (e.g. whole-body cooling and the lower limit of viability) have both matured, resulting in new research topics. However, in order for the modeling and the newly emerging topics to become effective tools, they need to be tailored to the specific characteristics of neonates. Consequently, the field of neonatal pharmacotherapy needs dedicated neonatologists who continue to raise the awareness that off-label practices, eminence-based dosing regimens and the absence of neonatal drug formulations all reflect suboptimal care.

A neonatal amikacin covariate model can be used to predict ontogeny of other drugs eliminated through glomerular filtration in neonates

PURPOSE

Recently, a covariate model characterizing developmental changes in clearance of amikacin in neonates has been developed using birth bodyweight and postnatal age. The aim of this study was to evaluate whether this covariate model can be used to predict maturation in clearance of other renally excreted drugs.

METHODS

Five different neonatal datasets were available on netilmicin, vancomycin, tobramycin and gentamicin. The extensively validated covariate model for amikacin clearance was used to predict clearance of these drugs. In addition, independent reference models were developed based on a systematic covariate analysis.

RESULTS

The descriptive and predictive properties of the models developed using the amikacin covariate model were good, and fairly similar to the independent reference models (goodness-of-fit plots, NPDE). Moreover, similar clearance values were obtained for both approaches. Finally, the same covariates as in the covariate model of amikacin, i.e. birth bodyweight and postnatal age, were identified on clearance in the independent reference models.

CONCLUSIONS

This study shows that pediatric covariate models may contain physiological information since information derived from one drug can be used to describe other drugs. This semi-physiological approach may be used to optimize sparse data analysis and to derive individualized dosing algorithms for drugs in children.

From Pediatric Covariate Model to Semiphysiological Function for Maturation: Part II-Sensitivity to Physiological and Physicochemical Properties

To develop a maturation function for drug glucuronidation in children, that can be used in population and physiologically based modeling approaches, the physiological and physicochemical basis of a semiphysiological glucuronidation function for children was untangled using Simcyp. The results show that using the currently available in vitro data, in vivo morphine and zidovudine clearances were under predicted by the physiologically based model in Simcyp. The maturation profile was similar to the clinically observed profile except for the first 2 weeks of life, and liver size and UGT2B7 ontogeny are the physiological drivers of the maturation of glucuronidation. Physicochemical drug parameters did not affect this maturation profile, although log P and pKa influenced the absolute value of clearance. The results suggest that the semiphysiological glucuronidation function for young children can be used to predict the developmental clearance profile of other UGT2B7 substrates, though scenarios with nonlinear kinetics and high-extraction ratios require further investigation.CPT: Pharmacometrics & Systems Pharmacology (2012) 1, e10; doi:10.1038/psp.2012.12; advance online publication 10 October 2012.