Phenytoin Toxicity Due to Genetic Polymorphism

Dual Therapeutic Effects of C-10068, a Dextromethorphan Derivative, Against Post-Traumatic Nonconvulsive Seizures and Neuroinflammation in a Rat Model of Penetrating Ballistic-Like Brain Injury

Post-traumatic seizures can exacerbate injurious outcomes of severe brain trauma, yet effective treatments are limited owing to the complexity of the pathology underlying the concomitant occurrence of both events. In this study, we tested C-10068, a novel deuterium-containing analog of (+)-N-methyl-3-ethoxymorphinan, in a rat model of penetrating ballistic-like brain injury (PBBI) and evaluated the effects of C-10068 on PBBI-induced nonconvulsive seizures (NCS), acute neuroinflammation, and neurofunctional outcomes. NCS were detected by electroencephalographic monitoring. Neuroinflammation was evaluated by immunohistochemical markers, for example, glial fibrillary acidic protein and major histocompatibility complex class I, for activation of astrocytes and microglia, respectively. Neurofunction was tested using rotarod and Morris water maze tasks. Three infusion doses of C-10068 (1.0, 2.5, and 5.0 mg/kg/h × 72 h) were tested in the antiseizure study. Neuroinflammation and neurofunction were evaluated in animals treated with 5.0 mg/kg/h × 72 h C-10068. Compared to vehicle treatment, C-10068 dose dependently reduced PBBI-induced NCS incidence (40-50%), frequency (20-70%), and duration (30-82%). The most effective antiseizure dose of C-10068 (5.0 mg/kg/h × 72 h) also significantly attenuated hippocampal astrocyte activation and perilesional microglial reactivity post-PBBI. Within C-10068-treated animals, a positive correlation was observed in reduction in NCS frequency and reduction in hippocampal astrocyte activation. Further, C-10068 treatment significantly attenuated astrocyte activation in seizure-free animals. However, C-10068 failed to improve PBBI-induced motor and cognitive functions with the dosing regimen used in this study. Overall, the results indicating that C-10068 exerts both potent antiseizure and antiinflammatory effects are promising and warrant further investigation.

Implications of pharmacogenetics for the therapeutic use of antiepileptic drugs

INTRODUCTION

Epilepsy is a chronic neurological disease manifesting as recurrent seizures. Despite the availability of numerous antiepileptic drugs (AEDs), one-third of the patients are not responsive to treatment. Such inter-individual variability in the response to AEDs may be partly explained by genetic differences. This review summarizes the pharmacogenetics (PGx) of AEDs. In addition, a model-based approach is presented that enables the integration of PGx data with other relevant sources of variability, such as demographic characteristics and co-medications.

AREAS COVERED

A comprehensive overview is provided of the data available in the literature on the evidence for correlations between genetic mutations and pharmacokinetic (PK) and/or pharmacodynamics (PD) of AEDs. This information is then used in an integrated manner in the second part, where PGx differences are parameterized as covariates in PK and PKPD models.

EXPERT OPINION

Polymorphisms are profuse in the PK and PD of AEDs. However, understanding of their clinical implication remains limited due to the lack of methodologies that discriminate the contribution of other sources of variability in CNS exposure to drugs. A model-based approach, in which other intrinsic (e.g., demographic covariates) and extrinsic (e.g., drug-drug interactions) factors are evaluated concurrently is needed to ensure optimization and individualization of treatment in epileptic patients.

From evidence based medicine to mechanism based medicine. Reviewing the role of pharmacogenetics

AIM OF THE REVIEW

The translation of evidence based medicine to a specific patient presents a considerable challenge. We present by means of the examples nortriptyline, tramadol, clopidogrel, coumarins, abacavir and antipsychotics the discrepancy between available pharmacogenetic information and its implementation in daily clinical practice.

METHOD

Literature review.

RESULTS

A mechanism based approach may be helpful to personalize medicine for the individual patient to which pharmacogenetics may contribute significantly. The lack of consistency in what we accept in bioequivalence and in pharmacogenetics of drug metabolising enzymes is discussed and illustrated with the example of nortriptyline. The impact of pharmacogenetics on examples like tramadol, clopidogrel, coumarins and abacavir is described. Also the present status of the polymorphisms of 5-HT2A and C receptors in antipsychotic-induced weight gain is presented as a pharmacodynamic example with until now a greater distance to clinical implementation.

CONCLUSION

The contribution of pharmacogenetics to tailor-made pharmacotherapy, which especially might be of value for patients deviating from the average, has not yet reached the position it seems to deserve.

Intravenous immunoglobulin in the treatment of drug rash eosinophilia and systemic symptoms caused by phenytoin

A 32 year old Asian female on 300 mg per day of phenytoin following meningioma excision developed a fever with a diffuse maculopapular rash, lymphadenopathy and splenomegaly after12 days. A diagnosis of DRESS (Drug Rash Eosinophilia and Systemic Symptoms) syndrome was made. Patient was started on prednisolone at a dose of 1 mg/kg but since there was further deterioration in her condition, intravenous immunoglobulin was started. Clinical and blood parameters began to improve by the next day with liver functions returning to normal by the third week. DRESS syndrome is a drug hypersensitivity syndrome which can be fatal and therefore needs to be recognized early for the appropriate treatment to be started. The use of Intravenous immunoglobulins is anecdotal and the dramatic improvement noted in this case indicates that it is another treatment choice. The case and a brief review of the literature are discussed.

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.

Cost-utility analysis of levetiracetam and phenytoin for posttraumatic seizure prophylaxis

BACKGROUND

The standard for early posttraumatic brain injury (TBI) seizure prophylaxis is phenytoin. Despite its effectiveness, some argue for the use of newer antiepileptics (e.g., levetiracetam) because phenytoin requires close monitoring to maintain its therapeutic window and is associated with rare cutaneous hypersensitivity reactions. The purpose of this study was to evaluate whether phenytoin or levetiracetam would be more cost-effective in preventing early post-TBI seizures and reducing their negative impact on TBI outcomes.

METHODS

Cost-effectiveness analysis with the following base case assumptions: (1) phenytoin patients receive 1.0 g fosphenytoin load + 3 days of 100 mg three times a day (TID), have level drawn on day 3, "therapeutic" patients receive 100 mg TID on days 4 to 7, and "subtherapeutic" patients receive 200 mg TID on days 4 to 7; (2) levetiracetam patients receive 500 mg load + 7 days of 500 mg two times a day. Glasgow Outcome Scale (GOS) scores 4 to 5 represent good outcome, and GOS scores 2 to 3 represent poor outcome. Patients who develop early seizures: 40% good outcome, 50% poor outcome, and 10% death. Those who do not develop seizures: 75% good outcome, 20% poor outcome, and 5% death. Quality of life outcomes by GOS: good = 0.7, poor = 0.3, and death = 0.0. Severe adverse events and those impacting costs are rare for each agent. Assumptions were obtained through hospital query and exhaustive literature review.

RESULTS

The cost of a 7-day course of fosphenytoin, phenytoin, and free phenytoin level was $37.50, whereas the cost of a 7-day course of levetiracetam was $480.00. Literature review noted phenytoin to be as effective as levetiracetam in preventing early post-TBI seizures (and more effective in subclinical seizures). Quality-adjusted life years (QALY) were 23.6 for phenytoin and 23.2 for levetiracetam. As a result, the cost/effectiveness ratios were $1.58/QALY for phenytoin and $20.72/QALY for levetiracetam. All sensitivity analyses favored phenytoin unless levetiracetam prevented 100% of seizures and cost <$400 for 7-day course.

CONCLUSIONS

Phenytoin is more cost-effective than levetiracetam at all reasonable prices and at all clinically plausible reductions in post-TBI seizure potential.

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.

Gene polymorphisms and their role in epilepsy treatment and prognosis

The human genome carries an enormous number of genetic variants, many of them of functional consequence. In epilepsy, they are likely to be involved in drug-specific treatment efficacy, unwanted or even toxic drug reactions, teratogenic risks in pregnancy as well as in the long-term prognosis of patients with epilepsy. As in many other disorders with a complex genetic background, the associated genetic variants that could be verified successfully in replication studies are still only a few. However, new techniques and improved research strategies are likely to increase their number in the foreseeable future, although at a much slower pace as initially expected.