Impact of Body Habitus on Phenytoin Levels Following Fosphenytoin Loading Dose in Pediatric Patients

A Quality Initiative to Improve Appropriate Medication Dosing in Pediatric Patients with Obesity

Introduction

Emerging evidence supports the use of alternative dosing weights for medications in patients with obesity. Pediatric obesity presents a particular challenge because most medications are dosed based on patient weight. Additionally, building system-wide pediatric obesity safeguards is difficult due to pediatric obesity definitions of body mass index-percentile-for-age via the Center for Disease Control growth charts. We describe a quality initiative to increase appropriate medication dosing in inpatients with obesity. The specific aim was to increase appropriate dosing for 7 high-risk medications in inpatients with obesity ≥2 years old from 37% to >74% and to sustain for 1 year.

Methods

The Institute for Healthcare Improvement model for improvement was used to plan interventions and track outcomes progress. Interventions included a literature review to establish internal dosing guidance, electronic health record (EHR) functionality to identify pediatric patients with obesity, a default selection for medication weight with an opt-out, and obtaining patient heights in the emergency department.

Results

Appropriate dosing weight use in medication ordered for patients with obesity increased from 37% to 83.4% and was sustained above the goal of 74% for 12 months.

Conclusions

Implementation of EHR-based clinical decision support has increased appropriate evidence-based dosing of medications in pediatric and adult inpatients with obesity. Future studies should investigate the clinical and safety implications of using alternative dosing weights in pediatric patients.

Dosing Common Medications in Hospitalized Pediatric Patients with Obesity: A Review

Medication management in children and adolescents with obesity is challenging because both developmental and pathophysiological changes may impact drug disposition and response. Evidence to date indicates an effect of obesity on drug disposition for certain drugs used in this population. This work identified published studies evaluating drug dosing, pharmacokinetics (PK), and effect in pediatric patients with obesity, focusing on 70 common medications used in a pediatric network of 42 US medical centers. A PubMed search revealed 33 studies providing PK and/or effectiveness data for 23% (16 of 70) of medications, 44% of which have just one study and can be considered exploratory. This work appraising 4 decades of literature shows several promising approaches: greater use of PK models applied to prospective clinical studies, dosing recommendations derived from both PK and safety, and multiyear effectiveness data on drugs for chronic conditions (e.g., asthma). Most studies make dose recommendations but are weakened by retrospective study design, small study populations, and no controls or historic controls. Dosing decisions continue to rely on extrapolating knowledge, including targeting systemic drug exposure typically achieved in adults. Optimal weight-based dosing strategies vary by drug and warrant prospective, controlled studies incorporating PK and modeling and simulation to complement clinical assessment.

Fosphenytoin Population Pharmacokinetics in the Acutely Ill Pediatric Population

OBJECTIVE

The purpose of this study is to describe the pharmacokinetics of phenytoin in pediatric patients receiving fosphenytoin.

DESIGN

Retrospective, population pharmacokinetic analysis.

SETTING

Emergency department or PICU of a large tertiary care children's hospital.

PATIENTS

Patients less than 19 years old who received fosphenytoin in the PICU or emergency center for treatment of seizures from January 2011 to June 2017 were included.

INTERVENTIONS

Population pharmacokinetic analysis was performed with NONMEM v7.3 (Icon Plc, Dublin, Ireland). Simulation was performed to determine optimal loading dose and maintenance dosing regimens.

MEASUREMENTS AND MAIN RESULTS

A total of 536 patients (55.4% male; median age, 3.4 yr [interquartile range, 0.92-8.5 yr]) met study criteria. Fosphenytoin was administered at median 15.1 mg/kg/dose (interquartile range, 6.3-20.7 mg/kg/dose). Mean serum concentrations of 17.5 ± 7.8 mg/L were at a median 4.2 hours (interquartile range, 2.5-7.8 hr) after a dose. A pharmacokinetic model with two compartments, allometrically scaled fat-free mass on all parameters, and serum creatinine and concomitant phenobarbital use on clearance had the best fit. Simulation demonstrated that a 20 mg/kg loading dose followed by 6 mg/kg/dose every 8 hours had the greatest percentage of concentrations in the 10-20 mg/L range, with reduced doses to achieve therapeutic in patients with reduced kidney function.

CONCLUSIONS

A loading dose of 20 mg/kg followed by 6 mg/kg/dose every 8 hours based on fat-free mass is a reasonable empiric strategy for attainment and maintenance of therapeutic trough concentrations. Concomitant phenobarbital use may increase clearance of phenytoin and fosphenytoin dose reductions should occur in patients with reduced kidney function.

Impact of Obesity on Fosphenytoin Volume of Distribution in Pediatric Patients

The impact of body habitus on fosphenytoin pharmacokinetics is poorly understood in pediatric patients. This retrospective, single-center review examined differences in fosphenytoin volume of distribution (V_D) between children with normal and obese body habitus. From 2013 to 2015, patients 2 to 18 years of age who received a loading dose of fosphenytoin were identified. Thirty-seven patients met inclusion criteria. Mean total serum phenytoin concentration was 25.3 ± 6.5 μg/mL in the nonobese group and 29.5 ± 7.6 μg/mL in the obese group ( P = .09). V_D was not significantly different between obese and nonobese groups, 0.92 ± 0.26 L/kg and 0.97 ± 0.48 L/kg ( P = .76), respectively. In contrast to adult studies, these data suggest that fosphenytoin dose adjustments for obese children may be unnecessary.

IV fosphenytoin in obese patients: Dosing strategies, safety, and efficacy

Background

Previous studies evaluated the disposition of IV phenytoin loading doses and found that obese patients had increased drug distribution into excess body weight, larger volumes of distribution, and longer half-lives when compared to their nonobese counterparts. We assess the safety and efficacy of fosphenytoin loading doses in patients with different body mass indices (BMIs).

Methods

A retrospective chart review was conducted in 410 patients who received fosphenytoin. Patients were divided into 2 groups: BMI <30 (nonobese) and BMI ≥30 (obese). Patient demographics, fosphenytoin dose administered in mg/kg body weight, renal and liver function tests, fosphenytoin drug levels, and pre- and post-fosphenytoin administration vital signs were collected to assess for adverse events. Necessity of additional antiepileptic loading doses was used as a surrogate for clinical efficacy.

Results

The median dose of fosphenytoin administered was 19 mg/kg (interquartile range 15-20). The most frequently encountered adverse event was hypotension, which occurred in 39% of the cohort. Using a Bonferroni adjustment for multiple comparisons, there were no differences in adverse events between the 2 groups. The need for additional antiepileptic loading doses was not different between the 2 groups (p = 0.07).

Conclusions

The incidence of adverse events and the need for repeat loading antiepileptic medications was similar between the 2 groups. From our findings, the patients in our study did not receive empiric loading dose adjustments and the current method of loading fosphenytoin achieves similar outcomes, regardless of the patient's BMI.