Age- and Bodyweight-dependent Allometric Exponent Model for Scaling Clearance and Maintenance Dose of Theophylline From Neonates to Adults

All You Need to Know About Allometric Scaling: An Integrative Review on the Theoretical Basis, Empirical Evidence, and Application in Human Pharmacology

Scaling approaches are used to describe or predict clearance for paediatric or obese populations from normal-weight adult values. Theoretical allometry assumes the existence of a universal bodyweight-based scaling relationship. Although theoretical allometry is highly disputed, it is commonly applied in pharmacological data analyses and clinical practice. The aim of the current review is to (1) increase pharmacologists' understanding of theoretical allometry to better understand the (implicit) assumptions and (dis)advantages and (2) highlight important methodological considerations with the application of this methodology. Theoretical allometry originated in an empirical, and later debated, observation by Kleiber of a scaling exponent of 0.75 between basal metabolic rate and body mass of mammals. The mathematical framework of West, Brown, and Enquist provides one possible explanation for this value. To date, multiple key assumptions of this framework have been disputed or disproven, and an increasing body of evidence is emerging against the existence of one universal allometric exponent. The promise of ease and universality of use that comes with theoretical approaches may be the reason they are so strongly sought after and defended. However, ecologists have suggested that the theory should move from a 'Newtonian approach', in which physical explanations are sought for a universal law and variability is of minor importance, to a 'Darwinian approach', in which variability is considered of primary importance for which evolutionary explanations can be found. No scientific support was found for the application of allometry for within-species scaling, so the application of basal metabolic rate-based scaling principles to clearance scaling remains unsubstantiated. Recent insights from physiologically based modelling approaches emphasise the interplay between drugs with different properties and physiological variables that underlie drug clearance, which drives the variability in the allometric scaling exponent in the field of pharmacology. To deal with this variability, drug-specific or patient-specific adaptations to theoretical allometric scaling are proposed, that introduce empiric elements and reduce the universality of the theory. The use of allometric scaling with an exponent of 0.75 may hold empirical merit for paediatric populations, except for the youngest individuals (aged ≤ 5 years). Nevertheless, biological interpretations and extrapolation potential attributed to models based on 0.75 allometric scaling are theoretically unfounded, and merits of the empirical application of this function should, as for all models, always be supported by appropriate model validation procedures. In this respect, it is not the value of the allometric exponent but the description and prediction of individual clearance values and drug concentrations that are of primary interest.

Dose Evaluation and Optimization of Amoxicillin in Children Treated for Lyme Disease

Amoxicillin is commonly used to treat erythema migrans in the first stage of Lyme disease in children, with a recommended dose of 50 mg/kg/day, administered three times a day (q8h). This model-based simulation study aimed to determine whether splitting the same daily dose into two administrations (q12h) would provide comparable drug exposure. A pharmacokinetic model suitable for a pediatric population (age: 1 month to 18 years, weight: 4-80 kg) was selected through a literature review. Simulations were performed with 15,000 virtual patients receiving 16.67 mg/kg/dose q8h, 25 mg/kg/dose q12h, or other q12h dosing variations. The target therapeutic level was defined by the percentage of time that the unbound drug concentration remained above the minimum inhibitory concentration (% fT > MIC) specific to Borrelia burgdorferi, with MICs of 0.06, 0.25, 1, 2, and 4 mg/L, requiring at least 40% and 50% of time for effective treatment. Probability of target attainment (PTA) was considered acceptable if it exceeded 50%, allowing for comparison of dosing schedules. Results indicated that the 50 mg/kg/day divided q12h regimen provided similar drug exposure to the q8h regimen for MICs below 2 mg/L (PTAs >50%). For a MIC of 2 mg/L, PTA was achieved with a higher dose of 30 mg/kg/dose q12h. However, for a MIC of 4 mg/L, the PTA criterion was not met. These findings suggest that a twice-daily dosing of 25 mg/kg/dose provides comparable bactericidal activity to the thrice-daily regimen for MICs between 0.06 and 1 mg/L. This simplified regimen may improve adherence and treatment implementation in children.

Model-informed pediatric dose selection of marzeptacog alfa (activated): An exposure matching strategy

Marzeptacog alfa (activated) (MarzAA) is an activated recombinant human rFVII variant intended for subcutaneous (s.c.) administration to treat or prevent bleeding in individuals with hemophilia A (HA) or B (HB) with inhibitors, and other rare bleeding disorders. The s.c. administration provides benefits over i.v. injections. The objective of the study was to support the first-in-pediatric dose selection for s.c. MarzAA to treat episodic bleeding episodes in children up through 11 years in a registrational phase III trial. Assuming the same exposure-response relationship as in adults, an exposure matching strategy was used with a population pharmacokinetics model. A sensitivity analysis evaluating the impact of doubling in absorption rate and age-dependent allometric exponents on dose selection was performed. Subsequently, the probability of trial success, defined as the number of successful trials for a given pediatric dose divided by the number of simulated trials (n = 1000) was studied. A successful trial was defined as outcome where four, three, or two out of 24 pediatric subjects per trial were allowed to fall outside the adult exposures after s.c. administration of 60 μg/kg. A dose of 60 μg/kg in children with HA/HB was supported by the clinical trial simulations to match exposures in adults. The sensitivity analyses further supported selection of the 60 μg/kg dose level in all age groups. Moreover, the probability of trial success evaluations given a plausible design confirmed the potential of a 60 μg/kg dose level. Taken together, this work demonstrates the utility of model-informed drug development and could be helpful for other pediatric development programs for rare diseases.

A General Biphasic Bodyweight Model for Scaling Basal Metabolic Rate, Glomerular Filtration Rate, and Drug Clearance from Birth to Adulthood

The objective of this study is to propose a unified, continuous, and bodyweight-only equation to quantify the changes of human basal metabolic rate (BMR), glomerular filtration rate (GFR), and drug clearance (CL) from infancy to adulthood. The BMR datasets were retrieved from a comprehensive historical database of male and female subjects (0.02 to 64 years). The CL datasets for 17 drugs and the GFR dataset were generated from published maturation and growth models with reported parameter values. A statistical approach was used to simulate the model-generated CL and GFR data for a hypothetical population with 26 age groups (from 0 to 20 years). A biphasic equation with two power-law functions of bodyweight was proposed and evaluated as a general model using nonlinear regression and dimensionless analysis. All datasets universally reveal biphasic curves with two distinct linear segments on log-log plots. The biphasic equation consists of two reciprocal allometric terms that asymptotically determine the overall curvature. The fitting results show a superlinear scaling phase (asymptotic exponent >1; ca. 1.5-3.5) and a sublinear scaling phase (asymptotic exponent <1; ca. 0.5-0.7), which are separated at the phase transition bodyweight ranging from 5 to 20 kg with a mean value of 10 kg (corresponding to 1 year of age). The dimensionless analysis generalizes and offers quantitative realization of the maturation and growth process. In conclusion, the proposed mixed-allometry equation is a generic model that quantitatively describes the phase transition in the human maturation process of diverse human functions.

Allometric Scaling in Pharmacokinetic Studies in Anesthesiology

A clinical review is presented of basic allometric scaling theory and its application to pharmacokinetic models in anesthesia and other fields in the biologic sciences.

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.

Sufentanil Disposition and Pharmacokinetic Model-Based Dosage Regimen for Sufentanil in Ventilated Full-Term Neonates

INTRODUCTION

Sufentanil is a potent synthetic opioid used for analgesia in neonates; however, data concerning drug disposition of sufentanil and dosage regimen are sparse in this population. Therefore, the aim of the study was to explore sufentanil disposition and to propose optimal loading and maintenance doses of sufentanil in ventilated full-term neonates.

METHODS

Individual sufentanil pharmacokinetic parameters were calculated based on therapeutic drug monitoring data using a 2-compartmental model. Linear regression models were used to explore the covariates.

RESULTS

The median (IQR) central volume of distribution (Vdc) and clearance (CL) for sufentanil were 4.7 (4.1-5.4) L/kg and 0.651 (0.433-0.751) L/h/kg, respectively. Linear regression models showed relationship between Vdc (L) and GA (r2 = 0.3436; p = 0.0452) as well as BW (r2 = 0.4019; p = 0.0268). Median optimal sufentanil LD and MD were 2.13 (95% CI: 1.78-2.48) μg/kg and 0.29 (95% CI: 0.22-0.37) μg/kg/h, respectively. Median daily COMFORT-B (IQR) scores ranged from 6 to 23 while no significant relationship between pharmacokinetic parameters and COMFORT-B scores was found.

DISCUSSION/CONCLUSION

Body weight and gestational age were found as weak covariates for sufentanil distribution, and the dosage regimen was developed for a prospective trial.

Simulated Assessment of Pharmacokinetically Guided Dosing for Investigational Treatments of Pediatric Patients With Coronavirus Disease 2019

IMPORTANCE

Children of all ages appear susceptible to severe acute respiratory syndrome coronavirus 2 infection. To support pediatric clinical studies for investigational treatments of coronavirus disease 2019 (COVID-19), pediatric-specific dosing is required.

OBJECTIVE

To define pediatric-specific dosing regimens for hydroxychloroquine and remdesivir for COVID-19 treatment.

DESIGN, SETTING, AND PARTICIPANTS

Pharmacokinetic modeling and simulation were used to extrapolate investigated adult dosages toward children (March 2020-April 2020). Physiologically based pharmacokinetic modeling was used to inform pediatric dosing for hydroxychloroquine. For remdesivir, pediatric dosages were derived using allometric-scaling with age-dependent exponents. Dosing simulations were conducted using simulated pediatric and adult participants based on the demographics of a white US population.

INTERVENTIONS

Simulated drug exposures following a 5-day course of hydroxychloroquine (400 mg every 12 hours × 2 doses followed by 200 mg every 12 hours × 8 doses) and a single 200-mg intravenous dose of remdesivir were computed for simulated adult participants. A simulation-based dose-ranging study was conducted in simulated children exploring different absolute and weight-normalized dosing strategies.

MAIN OUTCOMES AND MEASURES

The primary outcome for hydroxychloroquine was average unbound plasma concentrations for 5 treatment days. Additionally, unbound interstitial lung concentrations were simulated. For remdesivir, the primary outcome was plasma exposure (area under the curve, 0 to infinity) following single-dose administration.

RESULTS

For hydroxychloroquine, the physiologically based pharmacokinetic model analysis included 500 and 600 simulated white adult and pediatric participants, respectively, and supported weight-normalized dosing for children weighing less than 50 kg. Geometric mean-simulated average unbound plasma concentration values among children within different developmental age groups (32-35 ng/mL) were congruent to adults (32 ng/mL). Simulated unbound hydroxychloroquine concentrations in lung interstitial fluid mirrored those in unbound plasma and were notably lower than in vitro concentrations needed to mediate antiviral activity. For remdesivir, the analysis included 1000 and 6000 simulated adult and pediatric participants, respectively. The proposed pediatric dosing strategy supported weight-normalized dosing for participants weighing less than 60 kg. Geometric mean-simulated plasma area under the time curve 0 to infinity values among children within different developmental age-groups (4315-5027 ng × h/mL) were similar to adults (4398 ng × h/mL).

CONCLUSIONS AND RELEVANCE

This analysis provides pediatric-specific dosing suggestions for hydroxychloroquine and remdesivir and raises concerns regarding hydroxychloroquine use for COVID-19 treatment because concentrations were less than those needed to mediate an antiviral effect.

Age-Associated Theophylline Metabolic Activity Corresponds to the Ratio of 1,3-Dimethyluric Acid to Theophylline in Mice

Age is known as one of influencing factor for theophylline (TP)-metabolizing capacity. In a previous our study, the ratio of TP and its major metabolite 1,3-dimethyluric acid (DMU) in serum (DMU/TP) is a useful index to estimate TP-metabolizing capacity, and this value markedly increased by influencing factor, such as the history of smoking. However, it is unknown whether DMU/TP values in serum reflect age-associated changes of TP-metabolizing capacity. In this study, the effect of age on the DMU/TP values in serum were investigated using mice of different age due to the limited blood sampling in human. The concentrations of TP and its metabolites in mouse serum were simultaneously measured using HPLC. As observed in human serum, serum TP concentrations were closely correlated with DMU concentration in mice, which indicates that the DMU/TP ratio is a good indicator of TP metabolic ability in mice. When TP was administered subcutaneously in 2-28-week-old mice, age-associated changes in the DMU/TP ratio in mice were observed. In conclusion, age-associated changes in TP-metabolizing capacity can be estimated by the DMU/TP ratio in serum.

Scaling Drug Clearance from Adults to the Young Children for Drugs Undergoing Hepatic Metabolism: A Simulation Study to Search for the Simplest Scaling Method

Previous research showed that scaling drug clearance from adults to children based on body weight alone is not accurate for all hepatically cleared drugs in very young children. This study systematically assesses the accuracy of scaling methods that, in addition to body weight, also take age-based variables into account for drugs undergoing hepatic metabolism in children younger than five years, namely scaling with (1) a body weight-based function using an age-dependent exponent (ADE) and (2) a body weight-based function with fixed exponent of 0.75 (AS0.75) combined with isoenzyme maturation functions (MFPBPK) similar to those implemented in physiologically based pharmacokinetic (PBPK) models (AS0.75 + MFPBPK). A PBPK-based simulation workflow was used, including hypothetical drugs with a wide range of properties and metabolized by different isoenzymes. Adult clearance values were scaled to seven typical children between one day and four years. Prediction errors of ± 50% were considered reasonably accurate. Isoenzyme maturation was found to be an important driver of changes in hepatic metabolic clearance in children younger than five years, which prevents the systematic accuracy of ADE scaling. AS0.75 + MFPBPK, when accounting for maturation of isoenzymes and microsomal protein per gram of liver (MPPGL), can reasonably accurately scale hepatic metabolic clearance for all low and intermediate extraction ratio drugs except for drugs binding to alpha-1-acid glycoprotein in neonates. As differences in the impact of changes in system-specific parameters on drugs with different properties yield differences in clearance ontogeny, it is unlikely that for the remaining drugs, scaling methods that do not take drug properties into account will be systematically accurate.