Assessing the clinical impact of CYP2C9 pharmacogenetic variation on phenytoin prescribing practice and patient response in an integrated health system

Sources of pharmacokinetic and pharmacodynamic variability and clinical pharmacology studies of antiseizure medications in the pediatric population

Multiple treatment options exist for children with epilepsy, including surgery, dietary therapies, neurostimulation, and antiseizure medications (ASMs). ASMs are the first line of therapy, and more than 30 ASMs have U.S. Food and Drug Administration (FDA) approval for the treatment of various epilepsy and seizure types in children. Given the extensive FDA approval of ASMs in children, it is crucial to consider how the physiological and developmental changes throughout childhood may impact drug disposition. Various sources of pharmacokinetic (PK) variability from different extrinsic and intrinsic factors such as patients' size, age, drug-drug interactions, and drug formulation could result in suboptimal dosing of ASMs. Barriers exist to conducting clinical pharmacological studies in neonates, infants, and children due to ethical and practical reasons, limiting available data to fully characterize these drugs' disposition and better elucidate sources of PK variability. Modeling and simulation offer ways to circumvent traditional and intensive clinical pharmacology methods to address gaps in epilepsy and seizure management in children. This review discusses various physiological and developmental changes that influence the PK and pharmacodynamic (PD) variability of ASMs in children, and several key ASMs will be discussed in detail. We will also review novel trial designs in younger pediatric populations, highlight the role of extrapolation of efficacy in epilepsy, and the use of physiologically based PK modeling as a tool to investigate sources of PK/PD variability in children. Finally, we will conclude with current challenges and future directions for optimizing the efficacy and safety of these drugs across the pediatric age spectrum.

Exome sequencing allows detection of relevant pharmacogenetic variants in epileptic patients

Beyond the identification of causal genetic variants in the diagnosis of Mendelian disorders, exome sequencing can detect numerous variants with potential relevance for clinical care. Clinical interventions can thus be conducted to improve future health outcomes for patients and their at-risk relatives, such as predicting late-onset genetic disorders accessible to prevention, treatment or identifying differential drug efficacy and safety. To evaluate the interest of such pharmacogenetic information, we designed an "in house" pipeline to determine the status of 122 PharmGKB (Pharmacogenomics Knowledgebase) variant-drug combinations in 31 genes. This pipeline was applied to a cohort of 90 epileptic patients who had previously an exome sequencing (ES) analysis, to determine the frequency of pharmacogenetic variants. We performed a retrospective analysis of drug plasma concentrations and treatment efficacy in patients bearing at least one relevant PharmGKB variant. For PharmGKB level 1A variants, CYP2C9 status for phenytoin prescription was the only relevant information. Nineteen patients were treated with phenytoin, among phenytoin-treated patients, none were poor metabolizers and four were intermediate metabolizers. While being treated with a standard protocol (10-23 mg/kg/30 min loading dose followed by 5 mg/kg/8 h maintenance dose), all identified intermediate metabolizers had toxic plasma concentrations (20 mg/L). In epileptic patients, pangenomic sequencing can provide information about common pharmacogenetic variants likely to be useful to guide therapeutic drug monitoring, and in the case of phenytoin, to prevent clinical toxicity caused by high plasma levels.

Analyzing Precision Medicine Utilization with Real-World Data: A Scoping Review

Precision medicine (PM), specifically genetic-based testing, is currently used in over 140,000 individual tests to inform the clinical management of disease. Though several databases (e.g., the NIH Genetic Testing Registry) demonstrate the availability of these sequencing-based tests, we do not currently understand the extent to which these tests are used. There exists a need to synthesize the body of real-world data (RWD) describing the use of sequencing-based tests to inform their appropriate use. To accomplish this, we performed a scoping review to examine what RWD sources have been used in studies of PM utilization between January 2015 and August 2021 to characterize the use of genome sequencing (GS), exome sequencing (ES), tumor sequencing (TS), next-generation sequencing-based panels (NGS), gene expression profiling (GEP), and pharmacogenomics (PGx) panels. We abstracted variables describing the use of these types of tests and performed a descriptive statistical analysis. We identified 440 articles in our search and included 72 articles in our study. Publications based on registry databases were the most common, followed by studies based on private insurer administrative claims. Slightly more than one-third (38%) used integrated datasets. Two thirds (67%) of the studies focused on the use of tests for oncological clinical applications. We summarize the RWD sources used in peer-reviewed literature on the use of PM. Our findings will help improve future study design by encouraging the use of centralized databases and registries to track the implementation and use of PM.

Cervical and ocular vestibular evoked myogenic potentials in epileptic patients

Background

Vertigo and dizziness are very common complaints that may be related to epilepsy. The purpose of this study was to assess vestibulo-spinal and linear vestibulo-ocular function in epileptic patients in the inter ictal period. The current observational study was carried out in audio-vestibular unit Menoufia University. Subjects in the current study were divided into two groups: The control group included 30 normal individuals not complaining from any dizzy symptoms and the epileptic cases group included 30 epileptic patients. All subjects in the study were submitted to cervical and ocular vestibular evoked myogenic potential.

Results

There was no significant difference between the control and epileptic group regarding the age and sex distribution. Sixty-seven percent of epileptic cases had dizzy symptoms. There was statistically significant difference in the latency and amplitude of c and o VEMP between the control and the epileptic group, 39/60 ears (65%) in the study group had cVEMP abnormalities, 32/60 ears (53%) had oVEMP abnormalities. Abnormal c and o VEMP were reported in 28/60 ears (46.7%). There was statistically significant relationship between VEMP abnormalities and duration of seizures, frequency of epileptic attacks, and type of therapy.

Conclusion

Vestibular abnormalities were frequently reported in epileptic patients in the current study which may be related to the severity and control of epilepsy.

Treating Status Epilepticus: Phenytoin Versus Levetiracetam

For decades, phenytoin has been the drug of choice for the treatment of epilepsy but also the second-line treatment for status epilepticus (SE). However, newer antiepileptic drugs (AEDs) have emerged as safer alternatives for the suppression of seizures. Consequently, phenytoin has recently fallen under scrutiny in the research world, prompting many studies to compare its efficacy to these other drugs, most notably levetiracetam. Levetiracetam is a second-generation AED, which is gaining wide clinical use as the second-line agent in treating SE patients. This review focuses on several clinical studies that have directly compared the effectiveness of phenytoin and levetiracetam in suppressing SE seizure activity. Additionally, this review highlights several advantages of using levetiracetam over phenytoin in this clinical context.

A meta-analysis of effects of CYP2C9 and CYP2C19 polymorphisms on phenytoin pharmacokinetic parameters

Aim: Phenytoin is metabolized through CYP2C9 and CYP2C19. Polymorphisms of CYP2C9 and CYP2C19 may increase plasma concentration and side effects. Materials & methods: Systematic review and meta-analysis were performed to evaluate the effects of CYP2C9 and CYP2C19 polymorphism on pharmacokinetic parameters. PubMed, Science Direct, Cochrane library, and Thai databases were systematically searched. Results: Eight observational studies, comprising a total of 633 patients were included. Michaelis-Menten constant was significantly higher in the polymorphism of CYP2C9IM/CYP2C19EM and CYP2C9IM/CYP2C19IM groups as compared with the control groups (CYP2C9EM/CYP2C19EM) at 2.16 and 1.55 mg/l (p < 0.00001, p < 0.0001). The maximum rate of action was significantly lower in the control groups as compared with the polymorphism of CYP2C9IM/CYP2C19EM and CYP2C9IM/CYP2C19IM groups at 3.10 and 3.53 mg/kg/day (p = 0.00001, <0.0001). Conclusion: The dosage regimen for patients in the CYP2C9IM group to achieve phenytoin therapeutic levels was 2.1-3.4 mg/kg/day.

Participation of Monocarboxylate Transporter 8, But Not P-Glycoprotein, in Carrier-Mediated Cerebral Elimination of Phenytoin across the Blood-Brain Barrier

PURPOSE

In this study, we investigated in detail the transport of phenytoin across the blood-brain barrier (BBB) to identify the transporter(s) involved in BBB-mediated phenytoin efflux from the brain.

METHODS

We evaluated the brain-to-blood efflux transport of phenytoin in vivo by determining the brain efflux index (BEI) and uptake in brain slices. We additionally conducted brain perfusion experiments and BEI studies in P-glycoprotein (P-gp)-deficient mice. In addition, we determined the mRNA expression of monocarboxylate transporter (MCT) in isolated brain capillaries and performed phenytoin uptake studies in MCT-expressing Xenopus oocytes.

RESULTS

[¹⁴C]Phenytoin brain efflux was time-dependent with a half-life of 17 min in rats and 31 min in mice. Intracerebral pre-administration of unlabeled phenytoin attenuated BBB-mediated phenytoin efflux transport, suggesting carrier-mediated phenytoin efflux transport across the BBB. Pre-administration of P-gp substrates in rats and genetic P-gp deficiency in mice did not affect BBB-mediated phenytoin efflux transport. In contrast, pre-administration of MCT8 inhibitors attenuated phenytoin efflux. Moreover, rat MCT8-expressing Xenopus oocytes exhibited [¹⁴C]phenytoin uptake, which was inhibited by unlabeled phenytoin.

CONCLUSION

Our data suggest that MCT8 at the BBB participates in phenytoin efflux transport from the brain to the blood.

Associations of CYP2C9 and CYP2C19 Pharmacogenetic Variation with Phenytoin-Induced Cutaneous Adverse Drug Reactions

The role of cytochrome P450 (CYP)2C9 and CYP2C19 genetic variation in risk for phenytoin‐induced cutaneous adverse drug events is not well understood independently of the human leukocyte antigen B (HLA‐B)*15:02 risk allele. In the multi‐ethnic resource for Genetic Epidemiology Research on Adult Health and Aging (GERA) cohort, we identified 382 participants who filled a phenytoin prescription between 2005 and 2017. These participants included 21 people (5%) who self‐identified as Asian, 18 (5%) as black, 29 (8%) as white Hispanic, and 308 (81%) as white non‐Hispanic. We identified 264 (69%) CYP2C9*1/*1, 77 (20%) CYP2C9*1/*2, and 29 (8%) CYP2C9*1/*3. We also determined CYP2C19 genotypes, including 112 with the increased activity CYP2C19*17 allele. Using electronic clinical notes, we identified 32 participants (8%) with phenytoin‐induced cutaneous adverse events recorded within 100 days of first phenytoin dispensing. Adjusting for age, sex, daily dose, and race/ethnicity, participants with CYP2C9*1/*3 or CYP2C9*2/*2 genotypes were more likely to develop cutaneous adverse events compared with CYP2C9*1/*1 participants (odds ratio 4.47; 95% confidence interval 1.64–11.69; P < 0.01). Among participants with low‐intermediate and poor CYP2C9 metabolizer genotypes, eight (22%) who also had extensive and rapid CYP2C19 metabolizer genotypes experienced cutaneous adverse events, compared with none of those who also had intermediate CYP2C19 metabolizer genotypes (P = 0.17). Genetic variation reducing CYP2C9 metabolic activity may increase risk for phenytoin‐induced cutaneous adverse events in the absence of the HLA‐B*15:02 risk allele.

Emerging strategies to bridge the gap between pharmacogenomic research and its clinical implementation

The genomic inter-individual heterogeneity remains a significant challenge for both clinical decision-making and the design of clinical trials. Although next-generation sequencing (NGS) is increasingly implemented in drug development and clinical trials, translation of the obtained genomic information into actionable clinical advice lags behind. Major reasons are the paucity of sufficiently powered trials that can quantify the added value of pharmacogenetic testing, and the considerable pharmacogenetic complexity with millions of rare variants with unclear functional consequences. The resulting uncertainty is reflected in inconsistencies of pharmacogenomic drug labels in Europe and the United States. In this review, we discuss how the knowledge gap for bridging pharmacogenomics into the clinics can be reduced. First, emerging methods that allow the high-throughput experimental characterization of pharmacogenomic variants combined with novel computational tools hold promise to improve the accuracy of drug response predictions. Second, tapping of large biobanks of therapeutic drug monitoring data allows to conduct high-powered retrospective studies that can validate the clinical importance of genetic variants, which are currently incompletely characterized. Combined, we are confident that these methods will improve the accuracy of drug response predictions and will narrow the gap between variant identification and its utilization for clinical decision-support.