Pharmacokinetics of phenytoin in children

Fosphenytoin dosing regimen including optimal timing for the measurement of serum phenytoin concentration in pediatric patients

INTRODUCTION

We aimed to evaluate the pediatric fosphenytoin dosing regimen, including optimal timing for the measurement of total serum phenytoin concentration (C_PHT).

METHODS

We retrospectively investigated pediatric patients with status epilepticus or seizure clusters treated with fosphenytoin between April 2013 and March 2018. Two C_PHT measurements were analyzed, one 2-4 h after the loading dose and another before the second dose. Individual pharmacokinetic parameters were estimated using the Bayesian method and were used to simulate C_PHT.

RESULTS

The present study involved 12 pediatric patients; the loading dose of fosphenytoin was 22.1 (17.2-27.2) mg/kg. The C_PHT was 13.4 (8.6-18.9) μg/mL 2-4 h after the loading dose. The C_PHT estimated from individual pharmacokinetic parameters 12 and 24 h after the loading dose was 9.5 (6.7-14.2) and 5.8 (3.7-10.0) μg/mL, respectively. If fosphenytoin was administered at a loading dose of 22.5 mg/kg and a maintenance dose of 5 or 7.5 mg/kg (administered every 12 h, starting 12 h after the loading dose), then the C_PHT on day 8 was estimated to be 5.74 (2.6-15.4) μg/mL at 5 mg/kg and 13.9 (5.7-31.0) μg/mL at 7.5 mg/kg.

CONCLUSIONS

In pediatric patients, a maintenance dose of fosphenytoin should be started 12 h after the loading dose, and a maintenance dose of 5-7.5 mg/kg/dose every 12 h may be better than every 24 h. We recommend measuring C_PHT at 2 and 12 h after the loading dose to simplify and safely adjust the dosage in clinical practice.

Neurological toxicity after phenytoin infusion in a pediatric patient with epilepsy: influence of CYP2C9, CYP2C19 and ABCB1 genetic polymorphisms

Pharmacogenetic studies have shown that genetic defects in drug-metabolizing enzymes encoded by CYP2C9, CYP2C19 genes and by the transporter ABCB1 gene can influence phenytoin (PTH) plasma levels and toxicity. The patient reported here is a 2-year-old girl with a medical history of cryptogenic (probably symptomatic) epilepsy, who had her first focal seizure with secondary generalization at 13 months of age. She initially received oral valproate treatment and three months later, she was prescribed an oral oxcarbazepine treatment. At 20 months of age, she was admitted to the Emergency Department because of generalized convulsive Status Epilepticus needing to be immediately treated with rectal diazepam (0.5 mg kg(-1)), intravenous diazepam (0.3 mg kg(-1)), and intravenous phenytoin with an initial-loading dose of 15 mg kg(-1). However, two hours after the initial-loading dose of PTH, the patient developed dizziness, nystagmus, ataxia and excessive sedation. Other potential causes of PTH toxicity were excluded such as drug interactions, decreased albumin or lab error. Therefore, to explain the neurological toxicity, PTH plasma levels and CYP2C9, CYP2C19 and ABCB1 genetic polymorphisms were analyzed. Initial plasma PTH levels were higher than expected (69 mg l(-1); normal range: 10-20 mg l(-1)), and the patient was homozygous for the CYP2C9*2 allele, heterozygous for the CYP2C19*4 allele and homozygous for the 3435C and 1236C ABCB1 alleles. Present findings support the previously established relationship between CYP2C9 and CYP2C19 genetic polymorphisms and the increased risk to develop PTH toxicity owing to high plasma concentrations. Nevertheless, although the association of these genes with PTH-induced adverse effects has been well-documented in adult populations, this is the first report examining the influence of these genetic polymorphisms on PTH plasma levels and toxicity in a pediatric patient.

Application of Routine Monitoring Data for Determination of the Population Pharmacokinetics and Enteral Bioavailability of Phenytoin in Neonates and Infants With Seizures

This study investigated the population pharmacokinetics and the enteral bioavailability of phenytoin (PTN) in neonates and infants with seizures. Data from 83 patients were obtained retrospectively from medical records. A 1-compartment model was fitted to the log-transformed concentration data using NONMEM. Between-subject variability and interoccasion variability were modelled exponentially together with a log transform, both-sides exponential residual unexplained variance model. Covariates in nested models were screened for significance. Model robustness was assessed by bootstrapping with replacement (n = 500) from the study data. The parameters of the final pharmacokinetic model were clearance (L/h) = 0.826.[weight (WT, kg) / 70].[1 + 0.0692.(postnatal age (d) - 11)]; volume of distribution (L) = 74.2.[WT (kg) / 70]; absolute enteral bioavailability = 0.76; absorption rate constant (h) = 0.167. The between-subject variability for clearance and volume of distribution was 74.2% and 65.6%, respectively. The interoccasion variability for clearance was 54.4%. The unexplained variability was 51.1%. Final model parameter values deviated from median bootstrap estimates by less than 9%. Phenytoin disposition in neonates and infants can be described satisfactorily by linear pharmacokinetics. The values of allometrically scaled clearance and volume were similar to adult values, suggesting no major kinetic differences between adults and infants on the basis of size alone. Postnatal age independently influenced clearance. Switching from enteral to intravenous routes may require a dosage adjustment. The results of this study provide a basis for more rational prescribing of phenytoin in infants and neonates.

Fosphenytoin: clinical pharmacokinetics and comparative advantages in the acute treatment of seizures

Fosphenytoin is a phosphate ester prodrug developed as an alternative to intravenous phenytoin for acute treatment of seizures. Advantages include more convenient and rapid intravenous administration, availability for intramuscular injection, and low potential for adverse local reactions at injection sites. Drawbacks include the occurrence of transient paraesthesias and pruritus at rapid infusion rates, and cost. Fosphenytoin is highly bound (93-98%) to plasma proteins. Saturable binding at higher plasma concentrations accounts for an increase in its distribution volume and clearance with increasing dose and infusion rate. Fosphenytoin is entirely eliminated through metabolism to phenytoin by blood and tissue phosphatases. The bioavailability of the derived phenytoin relative to intravenous phenytoin is approximately 100% following intravenous or intramuscular administration. The half-life for conversion of fosphenytoin to phenytoin ranges from 7-15 minutes. Faster intravenous infusion rates and competitive displacement of derived phenytoin from plasma protein binding sites by fosphenytoin compensate for the expected conversion-related delay in appearance of phenytoin in the plasma. Unbound phenytoin plasma concentrations achieved with intravenous fosphenytoin loading doses of 100-150 or 50-100mg phenytoin sodium equivalents/min are comparable, and achieved at similar times, to those with equimolar doses of intravenous phenytoin at 50 (maximum recommended rate) or 20-40 mg/min, respectively. The rapid achievement of effective concentrations permits the use of fosphenytoin in emergency situations, such as status epilepticus. Following intramuscular administration, therapeutic phenytoin plasma concentrations are observed within 30 minutes and maximum plasma concentrations occur at approximately 30 minutes for fosphenytoin and at 2-4 hours for derived phenytoin. Plasma concentration profiles for fosphenytoin and total and unbound phenytoin in infants and children closely approximate those in adults following intravenous or intramuscular fosphenytoin at comparable doses and infusion rates. Earlier and higher unbound phenytoin plasma concentrations, and thus an increase in systemic adverse effects, may occur following intravenous fosphenytoin loading doses in patients with a decreased ability to bind fosphenytoin and phenytoin (renal or hepatic disease, hypoalbuminaemia, the elderly). Close monitoring and reduction in the infusion rate by 25-50% are recommended when intravenous loading doses of fosphenytoin are administered in these patients. The potential exists for clinically significant interactions when fosphenytoin is coadministered with other highly protein bound drugs. The pharmacokinetic properties of fosphenytoin permit the drug to serve as a well tolerated and effective alternative to parenteral phenytoin in the emergency and non-emergency management of acute seizures in children and adults.

Optimization of phenytoin therapy in adults with epilepsy in the Western Cape, South Africa

OBJECTIVE

To assess the extent to which adults with epilepsy were optimized and individualized on phenytoin monotherapy in the Western Cape, South Africa and to estimate the average optimized dose and serum phenytoin concentration, and the therapeutic range for this patient group.

METHODS

Patients were considered to be optimized on phenytoin if they were seizure-free or the best compromise was achieved between seizure reduction and side-effects.

RESULTS

538 (233 black and 305 coloured) adult people with epilepsy were treated at nine epilepsy clinics as outpatients. Of these patients, 332 (226 male and 106 female, 149 black and 183 coloured) were included in the data analysis as they were considered to have reliable phenytoin levels. Phenytoin doses and steady-state serum concentrations were predicted using the Michaelis-Menten equation. Patients attended a clinical pharmacokinetic service for 7.7+/-5.3 (range 1-22) months. The average optimized dose was 305.8 (range 100-500) mg/day and the average optimized level was 62.7+/-23.9 (range 15-133) micromol/l. Most patients (61.9%) were optimized in the therapeutic range 40-79 micromol/l; 21.1% were optimized above and 17% below this range. In 1.6% of patients serum concentrations above 120 micromol/l were required. Dosage adjustments were made in 47.0% of patients, increased in 31.9% and reduced in 15.1%.

CONCLUSION

These findings indicate that many patients (47%) attending outpatient clinics were not optimized on phenytoin therapy.

Human variability and noncancer risk assessment- An analysis of the default uncertainty factor

A 10-fold uncertainty factor is used for noncancer risk assessments to allow for possible interindividual differences between humans in the fate of the chemical in the body (kinetics) and target organ sensitivity (dynamics). Analysis of a database on the variability in each of these aspects is consistent with an even subdivision of the 10-fold factor into 10(0.5) (3.16) for kinetics and 10(0.5) (3.16) for dynamics. Analysis of the number of subjects in a normally and log-normally distributed population which would not be covered by factors of 3.16 supports this subdivision and also the use of a 10-fold factor to allow for both aspects. Analysis of kinetic data for subgroups of the population indicates that the standard default value of 3.16 for kinetics will not be adequate for all routes of elimination and all groups of the population. A scheme is proposed which would allow the selection of appropriate default uncertainty factors based on knowledge of the biological fate and effects of the chemical under review. Copyright 1998 Academic Press.

Clinical pharmacokinetics of antiepileptic drugs in paediatric patients. Part II. Phenytoin, carbamazepine, sulthiame, lamotrigine, vigabatrin, oxcarbazepine and felbamate

This article is the second part of a review of the pharmacokinetics of antiepileptic drugs (AEDs) in paediatric patients. It reviews 139 papers published since 1969 on the pharmacokinetics of phenytoin, carbamazepine, sulthiame, lamotrigine (phenyltriazine), vigabatrin, oxcarbazepine and felbamate in this population. The pharmacokinetics of phenytoin are significantly affected by age. The terminal elimination half-life (t1/2z) is relatively long in neonates; it then decreases during the first postnatal month to lower values than in adults, and then progressively increases with age due to an age-dependent decrease in the metabolic rate. Rate of elimination is strongly dose-dependent at all ages. The combination of these factors makes it difficult to predict what plasma concentrations would result from dose per kilogram (dose/kg) adjustments in neonates and children, especially when phenytoin is coadministered with other liver enzyme-inducing drugs, such as phenobarbital and carbamazepine. The concentration of phenytoin in brain and other tissues depends on the unbound/total concentration ratio. For neonates this ratio is higher than that found in adults; it then decreases over the first 3 postnatal months to approach adult values. The fraction of unbound phenytoin is significantly higher in patients also receiving valproic acid. Carbamazepine is almost completely epoxidised to the active metabolite carbamazepine epoxide, which is in turn converted to carbamazepine diol. Metabolic conversion of carbamazepine and renal clearance of carbamazepine diol are much higher in children than in adults; t1/2z of carbamazepine is thus very short in young children, increasing with age. No data are available on the neonatal period. The carbamazepine epoxide/carbamazepine ratio may be significantly increased by metabolic inducers (e.g. phenytoin, phenobarbital and primidone) or by inhibitors of the carbamazepine epoxide to carbamazepine diol conversion (e.g. valproic acid). Macrolides inhibit carbamazepine metabolism, thus increasing carbamazepine plasma concentrations. Drug-induced changes in carbamazepine kinetics are particularly pronounced in children. In children, a higher dose/kg of sulthiame, lamotrigine, oxcarbazepine and felbamate than in adults is required to obtain an effective plasma concentration. The published data do not support the use of a different dose/kg of vigabatrin in children age between 1 month and 15 years. The pharmacokinetic information in the paediatric literature may help in assessing AED prescriptions in childhood to prevent seizures and AED-related adverse effects on the ongoing maturational processes of the brain.

Pharmacokinetics of phenytoin in routine clinic patients in Malaysia

We estimated individual and population Michaelis-Menten pharmacokinetic parameters for phenytoin (DPH) in epileptic patients attending our neurology clinic using the computer programme. OPT. Our results agreed well with literature values but were lower than those we obtained earlier in a smaller number of patients. The Km was independent of age, weight and sex but there was a weak, correlation between Vm and body weight. We conclude that the use of population Vm and Km in normograms could lead to errors in DPH dose estimations as they correlated very poorly with patient characteristics. OPT was easy to use and sufficiently accurate for deriving dose estimates in routine patients. Its use would enable practitioners to generate their patients' own parameters for use in individual dosage adjustments. The estimates can subsequently be updated as more data become available.

Michaelis-Menten pharmacokinetics of phenytoin in adult Malaysian patients

We reviewed our data from 122 records of patients taking phenytoin for the treatment of various types of epilepsy and selected 15 (age range 10-43 years old) who were on phenytoin alone to calculate Michaelis-Menten pharmacokinetic parameters. The average Vm and Km for this age group was found to be 8.45 mg/kg/day and 6.72 mg/litre, respectively. Km was independent of age and weight. Vm correlated well with weight but there was no relationship with age.

Population pharmacokinetics of phenytoin in Singapore Chinese

The pharmacokinetics of phenytoin was studied in 66 epileptic Chinese children and adults. The data were analysed by the population approach, using the non-linear mixed effect model, in the MULTI (ELS) program. There was no age or gender-related effect on either the apparent maximum elimination rate (kmax) or Michaelis-Menten constant (KM). Kmax was related to body weight 0.656. The population pharmacokinetics was similar in children and adults. Kmax and KM were estimated to be 30.72 mg.kg-0.656 day-1 and 2.307 mg.l-1, respectively. Kmax was higher than reported values, and KM was comparable to that reported in a study in Japanese, but was much lower than that reported in studies of European patients. The inter-individual variability of KM (CV 65.58%) was substantially higher than that of kmax (CV 28.49%), and the residual (intra-individual) variability was found 21.33% (CV).

Cutaneous and immunologic reactions to phenytoin

Phenytoin (diphenylhydantoin; Dilantin) is a highly effective and widely prescribed anticonvulsant and antiarrhythmic agent. Since 1938 it has been invaluable in the treatment of grand mal and psychomotor epilepsy. Hydantoin derivatives have been used medicinally for more than a half-century. In recent years dermatologists have broadened the indications for phenytoin use to include recessive dystrophic epidermolysis bullosa, linear scleroderma, and pachyonychia congenita. In spite of widespread use and popularity, it is interesting that the frequency of complications relating to drug therapy remains low, relatively speaking. Nevertheless, a broad spectrum of cutaneous and immunologic reactions to phenytoin have been reported. These range from tissue proliferative syndromes (side effects), drug hypersensitivity syndromes (allergic effects), and a possible linkage with lymphoma (idiosyncratic effects). Therapeutic and toxic reactions to this commonly prescribed drug are comprehensively reviewed, analyzed, and summarized in this monograph.

Phenytoin-isoniazid interaction: a kinetic approach to management

A case of phenytoin-isoniazid drug interaction is reported. Pharmacokinetic evaluation was performed on the basis of two different phenytoin serum concentrations obtained on two different dosage regimens. Whereas Vmax was found to be in the normal range, the patient's Km was increased five-fold. The literature regarding this interaction is reviewed. This case illustrates the usefulness of pharmacokinetic methods in the management of phenytoin-isoniazid interaction.

Phenytoin Michaelis-Menten pharmacokinetics in Caucasian paediatric patients

The Michaelis-Menten pharmacokinetic parameters Vmax and Km were calculated for 135 epileptic paediatric patients receiving phenytoin as their only anticonvulsant therapy. Mean Vmax and Km values were 13.95 mg/kg/day and 6.59 micrograms/ml for 0.5 to 3-year-old patients, 10.93 mg/kg/day and 6.82 micrograms/ml for the 4 to 6 year age group, 10.05 mg/kg/day and 6.51 micrograms/ml for the 7 to 9-year-olds, and 8.25 mg/kg/day and 5.69 micrograms/ml for the 10 to 16 year group. Using analysis of variance, the Vmax values were significantly different (p less than 0.01) but the Km values were not. Linear regression analysis of Vmax versus age revealed a significant decline in Vmax with age (r = -0.554; p less than 0.001). A plot of Km versus age showed a poor correlation (r = -0.170) and a large amount of variability. Based on this data, the youngest age group would require on average 62% more phenytoin/kg/day than the oldest age group in order to maintain a steady-state phenytoin concentration of 15 micrograms/ml. Because of these age-related pharmacokinetic differences, phenytoin dosages may require adjustment as paediatric patients become older.

Steady-state pharmacokinetics of phenytoin from routinely collected patient data

Previously reported routine phenytoin clinical pharmacokinetic data from Japan, England, and Germany were analysed to estimate population pharmacokinetic parameters. There were 780 steady-state phenytoin concentrations and associated dosage rates (mg/day) from 322 patients. The patient group spanned paediatric and adult ages, mean age being 18.4 +/- 17.3 (SD) years; 53% were males. The data were analysed using NONMEM, a computer programme designed for population pharmacokinetic analysis. Estimates of the influence of age, gender, data source, height and weight on the maximum elimination rate (Vm) and Michaelis-Menten constant (Km) were obtained. The Vm and Km of a 70 kg adult male European were estimated to be 415 mg/day and 5.7 mg/L, respectively. Vm is not influenced by gender, age or data source. The parameters of a power function of height and weight were estimated to adjust Vm for body size. The best function adjusts Vm in proportion to weight to the 0.6 power; height contains no useful information. Km is not influenced by gender. The Km for patients less than 15 years old is 43% less than that of older patients. The Km of Japanese patients appears to be 23% less than that for European patients. Even after adjustments for age, etc., apparent Vm and Km vary unpredictably among individuals with a coefficient of variation between 10 to 20% and approximately 50% respectively.