Population clinical pharmacology of children: general principles

Practical Considerations For Developing Nucleoside Reverse Transcriptase Inhibitors

Nucleoside reverse transcriptase inhibitors (NRTI) remain a cornerstone of current antiretroviral regimens in combinations usually with a non-nucleoside reverse transcriptase inhibitor (NNRTI), a protease inhibitor (PI), or an integrase inhibitor (INI). The antiretroviral efficacy and relative safety of current NRTI results from a tight and relatively specific binding of their phosphorylated nucleoside triphosphates (NRTI-TP) with the HIV-1 reverse transcriptase which is essential for replication. The intracellular stability of NRTI-TP produces a sustained antiviral response, which makes convenient dosing feasible. Lessons learned regarding NRTI pharmacology screening, development, and use are discussed. NRTI and prodrugs currently under clinical development are outlined.

Pediatric analgesic clinical trial designs, measures, and extrapolation: report of an FDA scientific workshop

Analgesic trials pose unique scientific, ethical, and practical challenges in pediatrics. Participants in a scientific workshop sponsored by the US Food and Drug Administration developed consensus on aspects of pediatric analgesic clinical trial design. The standard parallel-placebo analgesic trial design commonly used for adults has ethical and practical difficulties in pediatrics, due to the likelihood of subjects experiencing pain for extended periods of time. Immediate-rescue designs using opioid-sparing, rather than pain scores, as a primary outcome measure have been successfully used in pediatric analgesic efficacy trials. These designs maintain some of the scientific benefits of blinding, with some ethical and practical advantages over traditional designs. Preferred outcome measures were recommended for each age group. Acute pain trials are feasible for children undergoing surgery. Pharmacodynamic responses to opioids, local anesthetics, acetaminophen, and nonsteroidal antiinflammatory drugs appear substantially mature by age 2 years. There is currently no clear evidence for analgesic efficacy of acetaminophen or nonsteroidal antiinflammatory drugs in neonates or infants younger than 3 months of age. Small sample designs, including cross-over trials and N of 1 trials, for particular pediatric chronic pain conditions and for studies of pain and irritability in pediatric palliative care should be considered. Pediatric analgesic trials can be improved by using innovative study designs and outcome measures specific for children. Multicenter consortia will help to facilitate adequately powered pediatric analgesic trials.

Assessment of lopinavir pharmacokinetics with respect to developmental changes in infants and the impact on weight band-based dosing

Improved antiretroviral therapies are needed for the treatment of HIV-infected infants, given the rapid progression of the disease and drug resistance resulting from perinatal exposure to antiretrovirals. We examined longitudinal pharmacokinetics (PK) data from a clinical trial of lopinavir/ritonavir (LPV/r) in HIV-infected infants in whom therapy was initiated at less than 6 months of age. A population PK analysis was performed using NONMEM to characterize changes in lopinavir (LPV) PK relating to maturational changes in infants, and to assess dosing requirements in this population. We also investigated the relationship between LPV PK and viral dynamic response. Age and ritonavir concentrations were the only covariates found to be significant. Population PK of LPV was characterized by high apparent clearance (CL/F) in young infants, which decreased with increasing age. Although younger infants had lower LPV concentrations, the viral dynamics did not correlate with initial LPV exposure. Monte Carlo simulations demonstrated that WHO weight band-based dosing recommendations predicted therapeutic LPV concentrations and provided drug exposure levels comparable to those resulting from US Food and Drug Administration (FDA)-suggested dosing regimens.

Bioavailability of inhaled fluticasone propionate via chambers/masks in young children

We determined lung bioavailability of a fluticasone propionate (FP) pressurised metred-dose inhaler (Flovent HFA; GlaxoSmithKline, Research Triangle Park, NC, USA) administered via AeroChamber Plus (Monaghan Medical, Plattsburgh, NY, USA) with Facemask and Babyhaler (GlaxoSmithKline) valved holding chambers (VHCs) using a population pharmacokinetic approach. Children from 1 to <4 yrs of age with stable asthma but a clinical need for inhaled corticosteroid therapy were administered 88 μg FP hydrofluoroalkane (2 × 44 μg) twice daily delivered through the two devices in an open-label, randomised crossover manner for 8 days each. Patients were randomised to one of three sparse sampling schedules for blood collection throughout the 12-h dosing interval on the 8th day of each treatment for pharmacokinetic analysis. The area under the FP plasma concentration-time curve (AUC) was determined for each regimen. 17 children completed the study. The population mean AUC following FP with AeroChamber Plus with Facemask was 97.45 pg·h·mL(-1) (95% CI 85.49-113.32 pg·h·mL(-1)) and with Babyhaler was 51.55 pg·h·mL(-1) (95% CI 34.45-64.46 pg·h·mL(-1)). The relative bioavailability (Babyhaler/AeroChamber Plus) was 0.53 (95% CI 0.30-0.75). Clinically significant differences in lung bioavailability were observed between the devices. VHCs are not interchangeable, as differences in drug delivery to the lung may occur. A population pharmacokinetic approach can be used to determine lung bioavailability of FP.

Optimal study design for pioglitazone in septic pediatric patients

The objective was to demonstrate the methodology and process of optimal sparse sampling pharmacokinetics (PK). This utilized a single daily dose of pioglitazone for pediatric patients with severe sepsis and septic shock based upon adult and minimal adolescent data. Pioglitazone pharmacokinetics were modeled using non-compartment analysis WinNonlin Pro (version 5.1) and population kinetics using NONMEM (version 7.1) with first order conditional estimation method (FOCE) with interaction. The initial model was generated from single- and multiple-dose pioglitazone PK data (15 mg, 30 mg, and 45 mg) in 36 adolescents with diabetes. PK models were simulated and overlaid upon original data to provide a comparison best described by a single compartment, first order model. The optimal design was based on the simulated oral administration of pioglitazone to three groups of pediatric patients, age 3.8 (2-6 years), weight 14.4 (7-28 kg); age 9.6 (6.1-11.9 years), weight 36.5 (28.1-48 kg) and age 15.5 (12-17 years,) weight 61.6 (48.1-80 kg). PFIM (version 3.2) was used to evaluate sample study size. Datasets were compiled using simulation for each dose (15, 30 and 45 mg) for the potential age/weight groups. A target dose of 15 mg daily in the youngest and middle groups was considered appropriate with area under the curve exposure levels (AUC) comparable to studies in adolescents. The final optimal design suggested time points of 0.5, 2, 6 and 21 h for 24 h dosing. This methodology provides a robust method of utilizing adult and limited adolescent data to simulate allometrically scaled, pediatric data sets that allow the optimal design of a pediatric trial. The pharmacokinetics of pioglitazone were described adequately and simulated data estimates were comparable to literature values. The optimal design provided clinically attainable sample times and windows.

The role of population PK-PD modelling in paediatric clinical research

Children differ from adults in their response to drugs. While this may be the result of changes in dose exposure (pharmacokinetics [PK]) and/or exposure response (pharmacodynamics [PD]) relationships, the magnitude of these changes may not be solely reflected by differences in body weight. As a consequence, dosing recommendations empirically derived from adults dosing regimens using linear extrapolations based on body weight, can result in therapeutic failure, occurrence of adverse effect or even fatalities. In order to define rational, patient-tailored dosing schemes, population PK-PD studies in children are needed. For the analysis of the data, population modelling using non-linear mixed effect modelling is the preferred tool since this approach allows for the analysis of sparse and unbalanced datasets. Additionally, it permits the exploration of the influence of different covariates such as body weight and age to explain the variability in drug response. Finally, using this approach, these PK-PD studies can be designed in the most efficient manner in order to obtain the maximum information on the PK-PD parameters with the highest precision. Once a population PK-PD model is developed, internal and external validations should be performed. If the model performs well in these validation procedures, model simulations can be used to define a dosing regimen, which in turn needs to be tested and challenged in a prospective clinical trial. This methodology will improve the efficacy/safety balance of dosing guidelines, which will be of benefit to the individual child.

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.

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.

Modelling and simulation as research tools in paediatric drug development

Purpose

Although practical and ethical constraints impose special requirements for the evaluation of treatment safety and efficacy in children, the main issue remains the empirical basis for patient stratification and dose selection at the early stage of the development of new chemical and biological entities. The aim of this review is to highlight the advantages and limitations of modelling and simulation (M&S) in supporting decision making during paediatric drug development.

Methods

A literature search on Pubmed’s database Medical Subject Headings (MeSH) has been performed to retrieve relevant publications on the use of model-based approaches in paediatric drug development and therapeutics.

Results

M&S enable the assessment of the impact of different regimens as well as of different populations on a drug’s safety and efficacy profile. It has been widely used in the last two decades to support pre-clinical and early clinical drug development. In fact, M&S have been applied to drug development as decision tools, as study optimization tools and as data analysis tools. In particular, this approach can be used to support dose adjustment in specific subgroups of a population. M&S may therefore allow the individualisation of drug therapy in children, improving the risk–benefit ratio in this population.

Conclusions

The lack of consensus on how to assess the impact of developmental factors on pharmacokinetics, pharmacodynamics, efficacy and safety has so far prevented a broader use of M&S. This problem is compounded by the limited collaboration between stakeholders, which prevents data sharing in this field. In this article, we emphasise the need for a concerted effort to promote the effective use of this technology in paediatric drug development and avoid unnecessary exposure of children to clinical trials.

Reversing the Myths Obstructing the Determination of Optimal Age- and Disease-Based Drug Dosing in Pediatrics

The need for critical, well-designed comprehensive clinical pharmacology research in pediatrics that encompasses the age continuum, from the most premature infant through adolescence, may be more important today than ever. New drug regimens often require greater adherence to specific dose guidelines to maximize efficacy and minimize toxic potential. The climate that allowed the propagation of the “therapeutic orphan” concept is now mostly of historical perspective. Nevertheless, the negative impact of this concept continues to linger due to continued propagation of many, now outdated myths surrounding the effective study of optimal drug dosing in pediatrics. Advances in clinical medicine combined with the advances in study design, sampling, and analysis has dramatically improved the paradigm for clinical pharmacology research in infants and children. Capitalizing upon and thoughtfully using these many advances while dispelling these myths will result in greater research focused on optimal drug therapy in pediatric practice.

Drug Development for Pediatric Populations: Regulatory Aspects

Pediatric aspects are nowadays integrated early in the development process of a new drug. The stronger enforcement to obtain pediatric information by the regulatory agencies in recent years resulted in an increased number of trials in children. Specific guidelines and requirements from, in particular, the European Medicines Agency (EMA) and the Food and Drug Administration (FDA) form the regulatory framework. This review summarizes the regulatory requirements and strategies for pediatric drug development from an industry perspective. It covers pediatric study planning and conduct, considerations for first dose in children, appropriate sampling strategies, and different methods for data generation and analysis to generate knowledge about the pharmacokinetics (PK) and pharmacodynamics (PD) of a drug in children. The role of Modeling and Simulation (M&S) in pediatrics is highlighted-including the regulatory basis-and examples of the use of M&S are illustrated to support pediatric drug development.

Tolerance and withdrawal from prolonged opioid use in critically ill children

OBJECTIVE

After prolonged opioid exposure, children develop opioid-induced hyperalgesia, tolerance, and withdrawal. Strategies for prevention and management should be based on the mechanisms of opioid tolerance and withdrawal.

PATIENTS AND METHODS

Relevant manuscripts published in the English language were searched in Medline by using search terms "opioid," "opiate," "sedation," "analgesia," "child," "infant-newborn," "tolerance," "dependency," "withdrawal," "analgesic," "receptor," and "individual opioid drugs." Clinical and preclinical studies were reviewed for data synthesis.

RESULTS

Mechanisms of opioid-induced hyperalgesia and tolerance suggest important drug- and patient-related risk factors that lead to tolerance and withdrawal. Opioid tolerance occurs earlier in the younger age groups, develops commonly during critical illness, and results more frequently from prolonged intravenous infusions of short-acting opioids. Treatment options include slowly tapering opioid doses, switching to longer-acting opioids, or specifically treating the symptoms of opioid withdrawal. Novel therapies may also include blocking the mechanisms of opioid tolerance, which would enhance the safety and effectiveness of opioid analgesia.

CONCLUSIONS

Opioid tolerance and withdrawal occur frequently in critically ill children. Novel insights into opioid receptor physiology and cellular biochemical changes will inform scientific approaches for the use of opioid analgesia and the prevention of opioid tolerance and withdrawal.

Population pharmacokinetics of tacrolimus in pediatric hematopoietic stem cell transplant recipients: new initial dosage suggestions and a model-based dosage adjustment tool

The population pharmacokinetics of tacrolimus was described in 22 pediatric hematopoietic stem cell transplant recipients, and a model-based dosage adjustment tool that may assist with therapy in new patients was developed. Patients received tacrolimus by continuous intravenous (IV) infusion (0.03 mg x kg(-1) x d(-1)) starting 2 days before transplantation, with conversion to oral therapy 2-3 weeks after transplant. Population pharmacokinetic analysis was performed using NONMEM. A Bayesian dosage adjustment tool that searches for individual parameter estimates to describe concentration measurements, counterbalanced by the final population model, was created in Excel. Typical clearance was 106 mL x h(-1) x kg(-0.75), typical distribution volume was 3.71 L/kg, and typical bioavailability was 15.7%. Tacrolimus clearance decreased with increasing serum creatinine, and bioavailability decreased with postoperative day. A Bayesian dosage adjustment tool capable of suggesting an initial infusion rate based on patient covariate values and devising a further individualized dosage regimen as drug concentration measures become available was developed. Predictions from the model showed that current IV dose recommendations of 0.03 mg x kg(-1) x d(-1) may potentially produce toxic drug concentrations in this patient population, whereas current oral conversion of 4 times the adjusted IV dose may lead to subtherapeutic concentrations. A more suitable infusion rate to obtain a steady state concentration of 12 ng/mL was predicted to be 0.035 mg x kg(-0.75) x (-1)d. An additional loading dose of 0.07 mg x kg(-1) x d(-1) (total dose: 0.07 mg x kg(-1) x d(-1) + 0.035 mg x kg(-0.75) x d(-1)) during the first 24 hours of therapy should allow rapid achievement of steady state concentrations. A conversion factor of 6 from IV to enteric therapy may be more suitable. Such dosage recommendations may be site specific. The appropriateness of targets was not investigated in this study. The Bayesian dosing adjustment tool and suggested dose recommendations need to be evaluated in a prospective study before they can be applied in the clinical setting.

Refinement of the population pharmacokinetic model for the monoclonal antibody matuzumab: external model evaluation and simulations

OBJECTIVES

A developed population pharmacokinetic model of the humanized monoclonal antibody (mAb) matuzumab was evaluated by external evaluation. Based on the estimates of the final model, simulations of different dosing regimens and the covariate effect were performed.

METHODS

The development dataset included 90 patients, and the evaluation dataset included 81 patients; the two sets of patients were from three different studies. In all studies, the patients had different types of advanced carcinoma - mainly colon, rectal and pancreatic cancer. They received matuzumab as multiple 1-hour intravenous infusions in a wide range of dosing regimens (development dataset: from 400 mg every 3 weeks to 2000 mg in the first week followed by 1600 mg weekly; evaluation dataset: from 100 mg weekly to 800 mg weekly). In addition to 1256 serum mAb concentrations for model development, there were 1124 concentrations available for model evaluation. Serum concentration-time data were simultaneously fitted using NONMEM software. The developed two-compartment model - with the parameters central volume of distribution (V(1)) and peripheral volume of distribution (V(2)), intercompartmental clearance and linear clearance (CLL), an additional nonlinear elimination pathway (Michaelis-Menten constant: the concentration with the half-maximal elimination rate and V(max): the maximum elimination rate) and covariate relations - was evaluated by an external dataset. Different simulation scenarios were performed to demonstrate the impact of the incorporated covariate effect and the influence of different dosing regimens and dosing strategies on the concentration-time profiles.

RESULTS

The developed model included the covariate fat-free mass (FFM) on V(1) and on CLL. The evaluation did not support the covariate FFM on V(1) and, after deletion of this covariate, the model parameters of the refined model were estimated. The model showed good precision for all parameters: the relative standard errors (RSEs) were <42% for the development dataset and < or = 51% for the evaluation dataset (excluding the higher RSEs for the correlation between V(2) and V(max) and the interindividual variability on V(2) for the evaluation dataset). The model showed good robustness for the ability to estimate highly precise parameters for the combined dataset of 171 patients (RSE <29%). Simulations revealed that variability in concentration-time profiles for minimum and maximum steady-state concentrations was reduced to a marginal extent by a proposed dose adaptation.

CONCLUSION

The population pharmacokinetic model for matuzumab was improved by evaluation with an external dataset. The new model obtained precise parameter estimates and demonstrated robustness. After correlation with efficacy data simulation results in particular could serve as a tool to guide dose selection for this 'targeted' cancer therapy.

Population pharmacokinetics of intravenous pantoprazole in paediatric intensive care patients

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

The use of intravenous pantoprazole, a proton pump inhibitor, has been increasing in the paediatric intensive care unit. Despite this increased use, data on the disposition of intravenous pantoprazole in paediatric intensive care patients are very scarce.

WHAT THIS STUDY ADDS

Our population approach has determined the pharmacokinetic parameters of intravenous pantoprazole in paediatric intensive care patients and the relative importance of factors influencing its disposition. Pantoprazole clearance was significantly influenced by developmental changes and by the presence of systemic inflammatory syndrome, hepatic dysfunction and CYP2C19 inhibitors.

AIMS

To characterize the pharmacokinetics of intravenous pantoprazole in a paediatric intensive care population and to determine the influence of demographic factors, systemic inflammatory response syndrome (SIRS), hepatic dysfunction and concomitantly used CYP2C19 inducers and inhibitors on the drug's pharmacokinetics.

METHODS

A total of 156 pantoprazole concentration measurements from 20 patients (10 days to 16.4 years of age) at risk for or with upper gastrointestinal bleeding, who received pantoprazole doses ranging from 19.9 to 140.6 mg/1.73 m(2)/day, were analysed using a population pharmacokinetic approach (nonmem program).

RESULTS

The best structural model for pantoprazole was a two-compartment model with zero order infusion and first-order elimination. Body weight, SIRS, age, hepatic dysfunction and presence of CYP2C19 inhibitors were significant covariates affecting clearance (CL), accounting for 75% of interindividual variability. Only body weight significantly influenced central volume of distribution (V(c)). In the final population model, the estimated CL and V(c) were 5.28 l h(-1) and 2.22 l, respectively, for a typical 5-year-old child weighing 20 kg. Pantoprazole CL increased with weight and age, whereas the presence of SIRS, CYP2C19 inhibitors and hepatic dysfunction, when present separately, significantly decreased pantoprazole CL by 62.3, 65.8 and 50.5%, respectively. For patients aged between 6 months and 5 years without SIRS, CYP2C19 inhibitor or hepatic dysfunction, the predicted pantoprazole CL is faster than that reported in adults.

CONCLUSION

These results provide important information for physicians regarding selection of a starting dose and dosing regimens of pantoprazole for paediatric intensive care patients based on factors frequently encountered in this population.

A Pharmacokinetic and Pharmacogenetic Study of Efavirenz in Children: Dosing Guidelines can Result in Subtherapeutic Concentrations

Background

Our main objectives were to study the population pharmacokinetics of efavirenz and to explore the adequacy of dosing guidelines.

Methods

A total of 33 HIV-1-infected patients were recruited from the Emma Children's Hospital (Amsterdam, the Netherlands). Gender, age, drug formulation, the presence of the c.516G>T polymorphism in the CYP2B6 gene and the quantitation of liver enzymes alanine aminotransferase and aspartate aminotransferase at baseline were collected. A non-linear mixed effect pharmacokinetic model was developed.

Results

CYP2B6 genotype and drug formulation significantly influenced efavirenz pharmacokinetics. Clearance was 29.7% lower in children carrying the CYP2B6-516-G/T genotype compared with children carrying the G/G genotype. Relative bioavailiability of the oral liquid compared with tablets or capsules was 46.6%. Children carrying the CYP2B6-516-G/G genotype had a 50–70% probability of developing a subtherapeutic trough level of efavirenz and only 1–3% probability of developing a trough level >4 mg/l. To reduce the probability of developing a subtherapeutic trough concentration, we propose to give an adult efavirenz dose to children weighing ≥25 kg and to allometrically scale doses for other weight levels a priori. The dose of the oral solution should be twice the dose of capsules.

Conclusions

Population pharmacokinetics of efavirenz in children were adequately described. Current dosing guidelines can result in subtherapeutic concentrations in children carrying the CYP2B6-516-G/G genotype and with the liquid formulation. A priori dose adaptations in the paediatric population seem feasible and need prospective validation.

Pharmacotherapy during neonatal extracorporeal membrane oxygenation: toward an evidence-based approach

Van der Vorst and coworkers recently illustrated the large variability in furosemide regimens used in their unit. This finding at least suggests that we need more data on the pharmacokinetics and pharmacodynamics of this drug in neonates during treatment with extracorporeal membrane oxygenation, in order to ensure quality of care and safety, and to promote evidence-based prescription. The implementation of population pharmacokinetic models can further increase both the feasibility of such studies and the relevance of the results generated.