Severe intoxication after phenytoin infusion: a preventable pharmacogenetic adverse reaction

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.

Pharmacogenomics and epilepsy: the road ahead

Epilepsy is one of the most common, serious neurological disorders, affecting an estimated 50 million people worldwide. The condition is typically treated using antiepileptic drugs of which there are 16 in widespread use. However, there are many different syndrome and seizure types within epilepsy and information guiding clinicians on the most effective drug and dose for individual patients is lacking. Further, all of the antiepileptic drugs have associated adverse reactions, some of which are severe and life-threatening. Here, we review the pharmacogenomic work to date in the context of these issues and comment on key aspects of study design that are required to speed up the identification of clinically relevant genetic factors.

Influence of CYP2C9 genetic polymorphism and undernourishment on plasma-free phenytoin concentrations in epileptic patients

The objective of this study was to study the effect of CYP2C9 genetic polymorphism and undernourishment on free phenytoin concentrations in epileptic patients. The study was done in 70 patients who were taking phenytoin therapy for the treatment of epileptic seizures. Genotyping of CYP2C9 (*2 and *3) was determined by the polymerase chain reaction-restriction fragment length polymorphism method. Bound and free plasma phenytoin was separated using equilibrium dialysis technique. Total and free phenytoin concentrations were measured by the reverse-phase high-performance liquid chromatography method. Patients were broadly classified into well-nourished and undernourished and further subclassified by CYP2C9 genotypes. In well-nourished groups (G1 to G3 group), free phenytoin concentrations were significantly higher in the heterozygous poor metabolizer of CYP2C9 genotype (G2) group (3.1 ± 0.62 μg/mL) and homozygous poor metabolizer of CYP2C9 genotype (G3) group (4.3 ± 1.76 μg/mL) when compared with patients with the wild-type CYP2C9 (G1) group (1.1 ± 0.72 μg/mL). Similarly, in undernourished patient groups (G4-G6 group), free phenytoin concentrations were significantly higher in the wild-type CYP2C9 (G4) group (2.5 ± 0.52 μg/mL), heterozygous poor metabolizer of CYP2C9 genotype (G5) group (4.3 ± 1.76 μg/mL), and homozygous poor metabolizer of CYP2C9 genotype (G6) group (8.2 ± 1.08 μg/mL) when compared with well-nourished patients with the wild-type CYP2C9 (G1) group (1.1 ± 0.72 μg/mL). The percentage increase in free phenytoin concentration by undernourishment, CYP2C9 allelic variants, and undernourishment cum CYP2C9 allelic variants were 127%, 290%, and 472%, respectively, compared with well-nourished patients with the wild-type CYP2C9 genotype (G1) group. The contribution of undernourishment and genetic factors (CYP2C9 allelic variant) for developing phenytoin toxicity was calculated to have an odds ratio of 37.3 (P < 0.0001). Undernourishment and variant CYP2C9 alleles elevate free phenytoin concentrations individually and in combination show additive effects.

Application of TDM, pharmacogenomics and biomarkers for neurological disease pharmacotherapy: focus on antiepileptic drugs

Anticonvulsants, or antiepileptic drugs (AEDs), are a vital tool in the therapeutic management of epilepsy patients. However, many AEDs are commonly used in the management of nonepileptic conditions, such as chronic pain, migraine headaches and psychiatric disorders. It is well documented that serum drug levels are an important data tool for the management of patients taking these drugs. As we move toward the personalized optimization of pharmacotherapy, drug level data will not be sufficient. This article will review tools for therapeutic drug management of AEDs including pharmacogenetics and biomarkers, in addition to traditional serum drug levels.

Phenytoin poisoning

Phenytoin toxicity may result from intentional overdose, dosage adjustments, drug interactions, or alterations in physiology. Intoxication manifests predominantly as nausea, central nervous system dysfunction (particularly confusion, nystagmus, and ataxia), with depressed conscious state, coma, and seizures occurring in more severe cases. Cardiac complications such as arrhythmias and hypotension are rare in cases of phenytoin ingestion, but they may be seen in parenteral administration of phenytoin or fosphenytoin. Deaths are unlikely after phenytoin intoxication alone. A greatly increased half-life in overdose due to zero-order pharmacokinetics can result in a prolonged duration of symptoms and thus prolonged hospitalization with its attendant complications. The mainstay of therapy for a patient with phenytoin intoxication is supportive care. Treatment includes attention to vital functions, management of nausea and vomiting, and prevention of injuries due to confusion and ataxia. There is no antidote, and there is no evidence that any method of gastrointestinal decontamination or enhanced elimination improves outcome. Activated charcoal should be considered if the patient presents early; however, the role of multiple-dose activated charcoal is controversial. Experimental studies have proven increased clearance rates, but this effect has not been translated into clinical benefit. There is no evidence that any invasive method of enhanced elimination (such as plasmapheresis, hemodialysis, or hemoperfusion) provides any benefit. This article provides an overview of phenytoin pharmacokinetics and the clinical manifestations of toxicity, followed by a detailed review of the various treatment modalities.