Important role of population pharmacokinetic/pharmacodynamic modeling in pediatric therapeutics

A Population Pharmacokinetic Model of Pentobarbital for Children with Status Epilepticus and Severe Traumatic Brain Injury

BACKGROUND

Pentobarbital pharmacokinetics (PK) remain elusive and the therapeutic windows narrow. Administration is frequent in critically ill children with refractory status epilepticus (SE) and severe traumatic brain injury (sTBI).

OBJECTIVES

To investigate pentobarbital PK in SE and sTBI patients admitted to the paediatric intensive care unit (PICU) with population-based PK (PopPK) modelling and dosing simulations.

METHODS

Develop a PopPK model with non-linear mixed-effects modelling (NONMEM®) with retrospective data (n = 36; median age 1.3 years; median weight 10 kg; 178 blood samples) treated with continuous intravenous pentobarbital. An independent dataset was used for external validation (n = 9). Dosing simulations with the validated model evaluated dosing regimens.

RESULTS

A one-compartment PK model with allometrically scaled weight on clearance (CL; 0.75) and volume of distribution (Vd; 1) captured data well. Typical CL and Vd values were 3.59 L/70 kg/h and 142 L/70 kg, respectively. Elevated creatinine and C-reactive protein (CRP) levels significantly correlated to decreased CL, explaining 84% of inter-patient variability, and were incorporated in the final model. External validation using stratified visual predictive checks showed good results. Simulations demonstrated patients with elevated serum creatinine and CRP failed to achieve steady state yet progressed to toxic levels with current dosing regimens.

CONCLUSIONS

The one-compartment PK model of intravenous pentobarbital described data well whereby serum creatinine and CRP significantly correlated with pentobarbital CL. Dosing simulations formulated adjusted dosing advice in patients with elevated creatinine and/or CRP. Prospective PK studies with pharmacodynamic endpoints, are imperative to optimise pentobarbital dosing in terms of safety and clinical efficacy in critically ill children.

Population Pharmacokinetic and Exposure-Response Analyses of Prasugrel in Pediatric Patients with Sickle Cell Anemia

BACKGROUND AND OBJECTIVE

Prasugrel, a P2Y₁₂ adenosine diphosphate (ADP) receptor antagonist, inhibits ADP-mediated platelet activation and aggregation in patients with sickle cell anemia (SCA). We developed a population pharmacokinetic (popPK) model in pediatric patients from 2 to <18 years of age with SCA, and performed exposure-response evaluations to characterize the effects of prasugrel in a subset of these patients who weighed 19 kg or more and experienced at least two episodes of vaso-occlusive crises (VOC) in the past year.

METHODS

A three-compartment popPK model adapted from that used in adults with acute coronary syndrome was used to describe the relationship between plasma concentrations of prasugrel's active metabolite (Pras-AM) and time using data from phase II and III clinical studies in children. A VOC event rate model was developed from the phase III study to explore the exposure-response relationship between Pras-AM exposure and VOC, and included evaluation of covariates.

RESULTS

The final popPK model for children with SCA provided a reasonable fit to Pras-AM plasma concentrations over time, with estimates of apparent clearance (CL/F) (172 L/h) and apparent volume of distribution (Vd/F) (51.7 L) that were comparable to previous studies in adults. The final model included weight as a covariate on both CL/F and Vd/F, and age as a covariate on CL/F. Analyses of safety (bleeding events requiring medical intervention) and efficacy (VOC event rate) variables showed no apparent relationship to model-predicted Pras-AM exposure quartiles, and no statistically significant effects of intrinsic or extrinsic factors on the VOC event rate were identified in the VOC event rate model. The effect of post hoc exposures on the VOC event rate did not reach statistical significance.

CONCLUSIONS

A popPK model was developed that provided reasonable parameter estimates, goodness-of-fit diagnostics, and visual predictive checks when applied to Pras-AM plasma concentrations in pediatric patients with SCA. Post hoc exposures obtained from this model did not correlate with measures of VOC or bleeding events in this population.

Population pharmacokinetics of vancomycin and AUC-guided dosing in Chinese neonates and young infants

OBJECTIVES

To develop a population pharmacokinetic (PK) model for vancomycin in Chinese neonates and infants less than 2 months of age (young infants) with a wide gestational age range, in order to determine the appropriate dosing regimen for this population.

METHODS

We performed a retrospective chart review of patients from the neonatal intensive care unit (NICU) at Children's Hospital of Fudan University to identify neonates and young infants treated with vancomycin from May 2014 to May 2017. Vancomycin concentrations and covariates were utilized to develop a one-compartment model with first-order elimination. The predictive performance of the final model was assessed by both internal and external evaluation, and the relationship between trough concentration and AUC_0-24 was investigated. Monte Carlo simulations were performed to design an initial dosing schedule targeting an AUC_0-24 ≥ 400.

RESULTS

The analysis included a total of 330 concentration-time data points from 213 neonates and young infants with gestational age (GA) and body weight of 25-42 weeks and 0.88-5.1 kg, respectively. Body weight, postmenstrual age (PMA) and serum creatinine level were found to be important factors explaining the between-subject variability in vancomycin PK parameters for this population. Both internal and external evaluation supported the prediction of the final vancomycin PK model. The typical population parameter estimates of clearance and distribution volume for an infant weighing 2.73 kg with a PMA of 39.8 weeks and serum creatinine of 0.28 mg/dL were 0.103 L/h/kg and 0.58 L/kg, respectively. Although vancomycin serum trough concentrations were predictive of the AUC, considerable variability was observed in the achievement of an AUC_0-24/MIC of ≥400. For MIC values of ≤0.5 mg/L, AUC_0-24/MIC ≥400 was achieved for 95% of the newborn infants with vancomycin troughs of 5-10 mg/L. When the MIC increased to 1 mg/L, only 15% of the patients with troughs of 5-10 mg/L achieved AUC_0-24/MIC ≥400. For MIC values of 2 mg/L, no infants achieved the target. Simulations predicted that a dose of at least 14 and 15 mg/kg every 12 h was required to attain the target AUC_0-24 ≥ 400 in 90% of infants with a PMA of 30-32 and 32-34 weeks, respectively. This target was also achieved in 93% of simulated infants in the oldest PMA groups (36-38 and 38-40 weeks, respectively) when the dosing interval was extended to 8 h. For infants with a PMA ≥44 weeks, a dose increase to 18 mg/kg every 8 h was needed. The trough concentrations of 5-15 mg/L were highly predictive of an AUC_0-24 of ≥400 when treating invasive MRSA infections with an MIC of ≤1 mg/L.

CONCLUSIONS

The PK parameters for vancomycin in Chinese infants younger than 2 months of age were estimated using the model developed herein. This model has been used to predict individualized dosing regimens in this vulnerable population in our hospital. A large external evaluation of our model will be conducted in future studies.

Development of a pharmacokinetic-guided dose individualization strategy for hydroxyurea treatment in children with sickle cell anaemia

AIMS

Hydroxyurea has emerged as the primary disease-modifying therapy for patients with sickle cell anaemia (SCA). The laboratory and clinical benefits of hydroxyurea are optimal at maximum tolerated dose (MTD), but the current empirical dose escalation process often takes up to 12 months. The purpose of this study was to develop a pharmacokinetic-guided dosing strategy to reduce the time required to reach hydroxyurea MTD in children with SCA.

METHODS

Pharmacokinetic (PK) data from the HUSTLE trial (NCT00305175) were used to develop a population PK model using non-linear mixed effects modelling (nonmem 7.2). A D-optimal sampling strategy was developed to estimate individual PK and hydroxyurea exposure (area under the concentration-time curve (AUC)). The initial AUC target was derived from HUSTLE clinical data and defined as the mean AUC at MTD.

RESULTS

PK profiles were best described by a one compartment with Michaelis-Menten elimination and a transit absorption model. Body weight and cystatin C were identified as significant predictors of hydroxyurea clearance. The following clinically feasible sampling times are included in a new prospective protocol: pre-dose (baseline), 15-20 min, 50-60 min and 3 h after an initial 20 mg kg(-1) oral dose. The mean target AUC(0,∞) for initial dose titration was 115 mg l(-1)  h.

CONCLUSION

We developed a PK model-based individualized dosing strategy for the prospective Therapeutic Response Evaluation and Adherence Trial (TREAT, ClinicalTrials.gov NCT02286154). This approach has the potential to optimize the dose titration of hydroxyurea therapy for children with SCA, such that the clinical benefits at MTD are achieved more quickly.

Summary of the National Institute of Child Health and Human Development-best pharmaceuticals for Children Act Pediatric Formulation Initiatives Workshop-Pediatric Biopharmaceutics Classification System Working Group

BACKGROUND

The Biopharmaceutics Classification System (BCS) allows compounds to be classified based on their in vitro solubility and intestinal permeability. The BCS has found widespread use in the pharmaceutical community to be an enabling guide for the rational selection of compounds, formulation for clinical advancement, and generic biowaivers. The Pediatric Biopharmaceutics Classification System (PBCS) Working Group was convened to consider the possibility of developing an analogous pediatric-based classification system. Because there are distinct developmental differences that can alter intestinal contents, volumes, permeability, and potentially biorelevant solubilities at different ages, the PBCS Working Group focused on identifying age-specific issues that need to be considered in establishing a flexible, yet rigorous PBCS.

OBJECTIVE

We summarized the findings of the PBCS Working Group and provided insights into considerations required for the development of a PBCS.

METHODS

Through several meetings conducted both at The Eunice Kennedy Shriver National Institute of Child Health, Human Development-US Pediatric Formulation Initiative Workshop (November 2011) and via teleconferences, the PBCS Working Group considered several high-level questions that were raised to frame the classification system. In addition, the PBCS Working Group identified a number of knowledge gaps that need to be addressed to develop a rigorous PBCS.

RESULTS

It was determined that for a PBCS to be truly meaningful, it needs to be broken down into several different age groups that account for developmental changes in intestinal permeability, luminal contents, and gastrointestinal (GI) transit. Several critical knowledge gaps were identified, including (1) a lack of fully understanding the ontogeny of drug metabolizing enzymes and transporters along the GI tract, in the liver, and in the kidney; (2) an incomplete understanding of age-based changes in the GI, liver, and kidney physiology; (3) a clear need to better understand age-based intestinal permeability and fraction absorbed required to develop the PBCS; (4) a clear need for the development and organization of pediatric tissue biobanks to serve as a source for ontogenic research; and (5) a lack of literature published in age-based pediatric pharmacokinetics to build physiologically- and population-based pharmacokinetic (PBPK) databases.

CONCLUSIONS

To begin the process of establishing a PBPK model, 10 pediatric therapeutic agents were selected (based on their adult BCS classifications). These agents should be targeted for additional research in the future. The PBCS Working Group also identified several areas where greater emphasis on research was needed to enable the development of a PBCS.