Plasma Phenytoin Concentrations Research Articles

Patient resuscitated after cardiopulmonary arrest exhibits abnormally increased phenytoin metabolic rate due to unknown factors: a case report.

Fosphenytoin (FOS) is a prodrug of phenytoin (PHT) with a metabolism that exhibits Michaelis-Menten-type kinetics. Genetic polymorphisms of the metabolic enzymes of PHT make it challenging to predict its plasma concentrations. High plasma PHT concentrations are typically problematic, and several causes have been elucidated. In contrast, cases of patients with low PHT plasma concentrations that did not increase despite the administration of appropriate PHT doses have been reported, and the causes may include changes in plasma protein-binding rates, genetic mutations, and concomitant use of drugs that induce liver enzymes; however, even these factors do not explain the low PHT plasma concentrations in some cases. We encountered the case of a patient with plasma PHT concentrations that were continuously < 0.7µg/mL after daily use of FOS for seizures that occurred after cardiopulmonary arrest. We analyzed the protein-unbound fraction, urinary metabolites, and related genes to investigate the cause. False negatives due to the measurement method, errors in dosage and administration method, and increased excretion of PHT were excluded. Hepatic metabolic activity of PHT increased to 4.6-6.1 times the normal level. The S/R ratio of 5-(p-hydroxyphenyl)-5-phenylhydantoin-glucuronide, a major PHT metabolite, was normal at 15.2, suggesting increased activities of CYP2C9 and CYP2C19. Furthermore, the protein-unbound fraction of PHT was 5.2-6.9%, CYP2C19*17 was wild type, and there was no concomitant drug use to induce both enzymes. The low PHT plasma concentration in this patient was found to be caused by increased hepatic metabolic activity that could not be explained by known factors. Careful monitoring is necessary to consider the possibility of increased hepatic metabolic activity in similar cases.

Development of UPLC–MS/MS method for the simultaneous quantification of valproic acid and phenytoin in human plasma and application to study pharmacokinetic interaction in epilepsy patients

Valproic acid and phenytoin are two prevalent antiepileptic medications known for their narrow indices and propensity for cardiovascular and respiratory system toxicity. Therefore, therapeutic drug monitoring (TDM) of valproic acid (VAL) and phenytoin (PHE) concentrations in patient plasma is extremely beneficial for improving clinical choices, avoiding adverse reactions, and optimizing treatment for individual patients. In this study, a rapid and sensitive ultra-performance liquid chromatographic tandem mass spectrometer (UPLC-MS/MS) method was developed and validated for the simultaneous quantitative determination of valproic acid (VAL) and phenytoin (PHE) in human plasma. Negative electron spray ionization (ESI-) mode with selective ion recording (SIR) was employed to determine the transitions of m/z 142.98 and m/z 250.93 for VAL and PHE, respectively. The internal standard (IS) betamethasone (BETA) was ionized using positive electron spray ionization (ESI+) and detected by multi-reaction monitoring (MRM) mode to obtain precursor ions and specific fragment ions for quantification, and the MRM transition was chosen to be m/z 393.17 → 355.16. The separation was performed using a Phenomenex Synergi Hydro-RP (4 μm, 250 × 4.6 mm, I.D.) with an isocratic mobile phase consisting of acetonitrile – water (75:25, v/v) at a flow rate of 0.8 mL/min. The column temperature was maintained at 25 °C. The lower limit of quantification of VAL and PHE was 3.6 μg/mL and 0.72 μg/mL, respectively, which resulted in a recovery of more than 85 % for most analytes. According to US-FDA bioanalytical technique validation, the specificity, intra- and inter-day precision and accuracy, matrix effect, carryover, dilution, and stability of all analytes were within acceptable ranges. This analytical method was successful in evaluating the levels of valproic acid and phenytoin in human plasma from epileptic patients.

Simultaneously measure the concentrations of busulfan and phenytoin in human plasma using an HPLC-MS/MS method: Application to the TDM for children underwent hematological stem cell transplantation

AbstractIn this work, a simple and rapid high performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS) method was developed and validated to carry out the simultaneous measurement of busulfan (BU) and phenytoin (PHT) in the plasma of children. In this method, plasma sample could be prepared by one-step protein precipitation using 1 mL of methanol/water (1:1, v/v). After centrifugation (14,500 rpm, 5 min, 4 °C), 10 μL of the supernatant was injected into a Hypersil Gold C18 column (150 × 2.1 mm, 5 μm, Thermo Fisher Scientific) for separation by gradient elution. Quantification was carried out using multiple reactions monitoring (MRM) under positive scan mode. In the method verification, the calibration curves of BU and PHT showed satisfactory linearity (r &gt; 0.99) at the concentration ranging from 0.02 to 20 μg mL−1. The accuracy and precision were tested at four concentration levels (including the LLOQ level) with the relative error (RE) ranging from −0.80% to 11.45% and coefficient of variation (CV) between 0.93% and 7.74%. There was no pronounced matrix effect to interfere with the quantitative analysis. Compared to determine BU and PHT using two individual methods, less pre-treatment process, labor and blood sample volume are required in this proposed method. Finally, this method was successfully applied to the therapeutic drug monitoring of BU and PHT for children underwent hematological stem cell transplantation.

Interaction of Phenytoin with Enteral Formulas, Proteins, and Dietary Fiber in Vitro and in Vivo

Simultaneous administration of enteral formula and phenytoin in the clinical setting is known to reduce the plasma concentration of phenytoin. In this study, we examined the binding of phenytoin with enteral formulas and its components by quantifying the free phenytoin concentration. Furthermore, we investigated the effect of enteral formulas on gastrointestinal absorption of phenytoin in rats. The free phenytoin rate was reduced in vitro when phenytoin and enteral formula or pectin, a dietary fiber in enteral formulas, were co-administered. In vivo, when phenytoin and the enteral formula Mei Balance R® were co-administered, the time to maximum plasma concentration (Tmax) after oral administration was significantly increased. Moreover, the area under the phenytoin concentration-time curve from time zero to 6 h (AUC0-6 h) was significantly increased by co-administration of phenytoin with the enteral formula PG Soft EJ®. These results showed the gastrointestinal absorption of phenytoin differs according to the type of enteral formula. In addition, we found the first time that plasma phenytoin levels increase when combined with enteral formula. Among the components of enteral formulas, in particular, milk protein delayed the absorption of phenytoin. Moreover, milk protein, casein and carrageenan tended to increase AUC0-6 h. These results suggest the change in phenytoin concentration is due not only to the binding of enteral formula but also to the disintegration of components such as protein. Therefore, when co-administrated of phenytoin and enteral formula, phenytoin must be monitored frequently according to the enteral formula interaction.

Therapeutic drug monitoring of phenytoin and valproic acid in critically ill patients at Windhoek Central Hospital, Namibia.

BackgroundPhenytoin and valproic acid, anticonvulsants, have a low therapeutic index and are highly plasma protein bound, mainly to albumin. Hypoalbuminaemia is common in critically ill patients and increases the unbound drug concentration. Thus, monitoring unbound rather than total plasma drug concentrations is recommended to optimise the dosing of these drugs.ObjectiveThis retrospective study determined unbound plasma concentrations of phenytoin and valproic as a more accurate value of drug levels than total plasma drug concentrations.MethodsTotal plasma concentrations were retrieved for 56 Intensive Care Unit patients for phenytoin and 93 for valproic acid. Total drug concentrations were converted to unbound concentrations using a serum albumin-based normalising equation.ResultsTotal phenytoin plasma concentration was below (41.1% of patients), within (46.4%) or above (12.5%) the therapeutic range (10 μg/mL – 20 μg/mL). However, the predicted unbound plasma concentration of phenytoin was above the therapeutic range (1 μg/mL – 2 μg/mL) in the majority of patients (57.1%). For valproic acid, the total plasma concentration of most patients (87.1%) was below the therapeutic range (50 μg/mL – 100 μg/mL); among remaining patients (12.9%), it was within the therapeutic range. In the majority of patients (91.4%), the predicted unbound plasma concentration of valproic acid was between 2.5 μg/mL and 20 μg/mL.ConclusionThe usefulness of monitoring the total phenytoin or valproic acid levels for dose optimisation is limited as it is an inaccurate indicator of a patient’s drug therapeutic state. Thus, the unbound plasma drug concentrations should be quantified experimentally or predicted in resource-limited settings.

Phenytoin Metabolic Ratio, a Marker of CYP2C9 Activity, is Superior to the CYP2C9 Genotype as a Predictor of (S)-Warfarin Clearance.

CYP2C9 is a member of the cytochrome P450 (CYP) superfamily responsible for the metabolism of 16% of drugs that undergo oxidative metabolism. The activity of CYP2C9 exhibits marked inter-individual variability, which translates into prominent differences in the pharmacokinetics of CYP2C9 substrates, some of which are characterized by a narrow therapeutic window. Genetic polymorphisms in the gene encoding for CYP2C9 account for a fraction of the variability in CYP2C9 activity. The phenytoin metabolic ratio (PMR) is a marker of CYP2C9 activity in vivo, which correlates with CYP2C9 genetic polymorphisms. The purpose of the current study was to evaluate the ability of the PMR to predict the oral clearance of (S)-warfarin (SWOCL) and its formation clearance towards its CYP2C9-mediated metabolites (SWCLf) [i.e., 6- and 7-hydroxy-(S)-warfarin]. The study was conducted in 150 healthy non-smoker subjects (segment 1) and 60 patients treated with warfarin (segment 2). In the first segment, the participants received on two separate occasions a single 300-mg dose of phenytoin and at least 7 days later a single dose of warfarin (5 or 10 mg). The same PMR procedure was performed in the second segment, except that it was performed either before warfarin initiation or after the patients had reached stable anticoagulation. The PMR was derived from the ratio of 5-(4-hydroxyphenyl)-5-phenyl-hydantoin content in a 24-hour urine collection to plasma phenytoin concentration 12- (PMR24/12) or 24- (PMR24/24) post-dosing. In segment 1, SWOCL was calculated from the ratio of (S)-warfarin dose to the warfarin area under the plasma concentration-time curve extrapolated to infinity and the SWCLf from the ratio of urine content of 6- and 7-hydroxy-(S)-warfarin to (S)-warfarin area under the (S)-warfarin plasma concentration-time curve until the last measured timepoint. In segment 2, estimated SWOCL was derived from the ratio of (S)-warfarin dose to the mid-interval plasma concentration of (S)-warfarin. The PMR, SWOCL, and SWCLf varied significantly between carriers of different CYP2C9 genotypes in both healthy subjects (p < 0.001) and patients (p < 0.005). However, PMR and SWOCL values exhibited substantial intra-genotypic variability. PMR24/12 and PMR24/24 were significantly correlated with SWOCL both in healthy subjects (r = 0.62 and r = 0.67, respectively, p < 0.001) and in patients (r = 0.57 and r = 0.61, respectively, p < 0.001). In a multiple regression model that included all variables that correlated with SWOCL, PMR was the strongest predictor, explaining 44% and 38% of the variability in SWOCL among healthy subjects and patients, respectively, and accounting for 95.7% (44%/46%) and 90.5% (38%/42%) of the total explained variability in SWOCL among healthy subjects and patients, respectively. The PMR is the strongest predictor of SWOCL, and as such, it exhibits a significant advantage over the CYP2C9 genotype. The inclusion of PMR in future dosing algorithms of CYP2C9 substrates characterized by a narrow therapeutic window should be encouraged and further investigated.

DETERMINATION OF PHARMACOKINETIC PARAMETERS AND FACTORS INFLUENCING THE PHARMACOKINETIC PARAMETERS OF PHENYTOIN IN THAI CHILDREN WITH EPILEPSY

Objective: Phenytoin displays nonlinear pharmacokinetics due to the saturation of its’ metabolizing enzymes, thus making dosing adjustments a challenge in clinical practice. However, data on the pharmacokinetic parameters of phenytoin in pediatric patients with epilepsy in Thailand remain lacking. This study aimed to determine the pharmacokinetic parameters of phenytoin, and the factors influencing them in epileptic Thai children using therapeutic drug monitoring data.&#x0D; Methods: Steady state phenytoin plasma concentrations were collected from 96 Thai children with epilepsy aged ≤ 15 y. For individuals having a single steady-state concentration (Css) on one dosage, the Vmax was determined by fixing Km = 4.5 mg/l. For patients with two values for Css with different dosages, the Ludden method was used to determine Vmax and Km. Influences on Vmax and Km by demographic factors including age, sex, weight, serum albumin, and the use of CYP2C9 inducers (carbamazepine, phenobarbital, folic acid, and vigabatrin) and inhibitors (valproic acid and topiramate) were determined by linear mixed-effects regression.&#x0D; Results: The majority of patients had sub-therapeutic plasma concentrations of phenytoin. The Vmax and Km were estimated to be 9.84 mg/kg/d and 2.32 mg/l, respectively. Age and weight significantly influenced Vmax, whereas Km had no significant influencing factors.&#x0D; Conclusion: The Vmax estimated in this study is different from other populations previously reported. Our results suggest that increased phenytoin dosages may be required to achieve optimal concentrations due to the sub-therapeutic concentrations reported here. The results from this study are beneficial for phenytoin dosage individualization in Thai children.

Factors Influencing Plasma Concentrations of Phenytoin, Phenobarbitone, Carbamazepine and Sodium Valproate in Epileptic Patients Attending a Tertiary Care Hospital

Objective: The traditional anti-epileptic drugs like phenytoin, sodium valproate and carbamazepine continue to be used widely in the low to middle income countries due to their time tested efficacy and low costs. The factors which affect the AED concentrations in non-toxicity indications has not been reported so far. Hence, we studied the factors that could influence the plasma concentrations of these AEDs. Methods: This is a retrospective record based study and a short term prosepective study. Requisition forms of epileptic patients referred for TDM 2012-2014 were compared with the AED concentrations reported. Epileptic patients who were referred during May-June 2015 for TDM were evaluated for their seizure history and anthropometic measurements for calculation of extracellular volume (VECW) and compared with the AED concentrations. Results: Out of the 170 requisitions for TDM, 68.2% were monotherapy and 31.8% received two or more antiepileptic drugs. In 44% of patients, the plasma concentrations of the AED correlated with clinical response. In our study the plasma level of carbamazepine was found to be reduced by phenytoin, sodium valproate concentration was reduced by carbamazepine and phenytoin levels were reduced by sodium valproate. In male, phenytoin levels were significantly lower than in female, Phenytoin decreased carbamazepine levels significantly in female. Carbamazepine significantly decreased sodium valproate concentrations in females. Carbamazepine dose adjusted for weight and VECW was found to show a trend of correlation to the blood levels, Conclusion: TDM for AEDs is an indispensable investigation that guides the clinicians to tailor dose of drugs in individual patients. Factors such as age, gender, concomitant drugs in general and VECW in patients on carbamazepine needs to be considered while interpreting AED plasma concentrations for monitoring therapy.

Estimation of Phenytoin Pharmacokinetic Parameters in Saudi Epileptic Patients

Objective: This study aimed to assess the population pharmacokinetics of phenytoin in Saudi patients and identify factors affecting therapeutic parameters. Method: A retrospective chart review was performed at King Saud University Medical City on patients treated with oral phenytoin. We used Monolix 4.4. for population pharmacokinetic modeling. A base model was developed to investigate several covariates, including age, gender, weight, total daily dose (TTD), and liver function test results. Results: The analysis included a total of 81 phenytoin plasma concentrations from 43 patients (70% male). Patients’ mean (± SD) age was 41 (±18.7) years and body weight was 65.4 (±17.7) kg. The patients received a phenytoin TDD of 330.5 (±104.5) mg/day, resulting in a trough concentration of 11.2 (±10.3) mg/L. The data were sufficiently described by the one-compartment open model with linear absorption and nonlinear elimination processes. Average parameter estimates for phenytoin volume of distribution (V), maximal elimination rate (Vmax), and Michaelis-Menten constant (Km) were 0.61 L/h/kg, 6.12 mg/kg/day, and 5.33 mg/L, respectively. The most significant covariates on phenytoin Vmax and Km were the age and body weight of the patients, along with valproic acid (VPA) cotherapy. Conclusion: The population pharmacokinetic model of phenytoin in Saudi patients found significant interindividual variability between subjects, which was affected by the patients’ age, body weight, and VPA cotherapy as the most significant covariates on phenytoin Vmax and Km. To provide guidance in drug dosage decisions, further studies are required to evaluate all factors that may potentially influence the pharmacokinetics of phenytoin.

Phenytoin Cream for the Treatment for Neuropathic Pain: Case Series.

BACKGROUND: Neuropathic pain can be disabling, and is often difficult to treat. Within a year, over half of all patients stop taking their prescribed neuropathic pain medication, which is most probably due to side effects or disappointing analgesic results. Therefore, new therapies are needed to alleviate neuropathic pain. As such, topical analgesics could be a new inroad in the treatment of neuropathic pain. In 2014, we developed a new topical formulation containing either phenytoin or sodium phenytoin. After optimization of the formulation, we were able to reach a 10% concentration and combine phenytoin with other co-analgesics in the same base cream. OBJECTIVE: To describe a series of 70 neuropathic pain patients who were treated with phenytoin cream. MATERIAL AND METHODS: Cases treated with phenytoin 5% or 10% creams were gathered. The mean onset of pain relief, the duration of effect, and reduction in pain intensity measured on the 11-point numerical rating scale (NRS) were all studied. A single-blind response test with phenytoin 10% and placebo creams was conducted on 12 patients in order to select responders prior to prescribing the active cream. Plasma phenytoin concentrations were measured in 16 patients. RESULTS: Nine patients applied phenytoin 5% cream, and 61 patients used phenytoin 10% cream. After grouping the effects of all of the patients, the mean onset of pain relief was 16.3 min (SD: 14.8), the mean duration of analgesia was 8.1 h (SD: 9.1), and the mean pain reduction on the NRS was 61.2% (SD: 25.0). The mean pain reduction on the NRS while using phenytoin cream was statistically significant compared with the baseline, with a reduction of 4.5 (CI: 4.0 to 5.0, p < 0.01). The 12 patients on whom a single-blind response test was performed experienced a statistically significant reduction in pain in the area where the phenytoin 10% cream was applied in comparison to the area where the placebo cream was applied (p < 0.01). Thirty minutes after the test application, the mean pain reduction on the NRS in the areas where the phenytoin 10% cream and the placebo cream were applied was 3.3 (CI: 2.3 to 4.4, p < 0.01) and 1.1 (CI: 0.4 to 1.9, p < 0.05), respectively. In all 16 patients, the phenytoin plasma levels were below the limit of detection. So far, no systemic side effects were reported. Two patients only reported local side effects: a transient burning aggravation and skin rash. CONCLUSION: In this case series, the phenytoin cream had reduced neuropathic pain considerably, with a fast onset of analgesic effect.

The Sphingosine 1-Phosphate Analogue FTY720 Alleviates Seizure-induced Overexpression of P-Glycoprotein in Rat Hippocampus.

Overexpression of P-glycoprotein (P-gp) in the brain is an important factor leading to drug-resistant epilepsy. Clinical use of P-gp inhibitors is limited by their systemic toxicity. In this study, we tested the hypothesis that FTY720, a sphingosine 1-phosphate (S1P) analogue used for treating multiple sclerosis, modulates the up-regulation of P-gp and improves brain delivery of phenytoin (PHT) through S1P receptor 1 in the hippocampus of a pilocarpine-induced rat model of status epilepticus (SE). We administered vehicle, FTY720 or FTY720+ W146 (an S1P receptor 1 antagonist) to SE rats. Forty-eight hours after SE, we dissected the hippocampus and measured P-gp expression, NF-κB activity and levels of inflammatory mediators (TNF-α and COX-2) by Western blotting and enzyme-linked immunosorbent assay. We also measured hippocampal and plasma concentrations of PHT 30, 60, 90, 120 and 180min. after an intraperitoneal injection of PHT (50mg/kg) 48hr after SE, using microdialysis and high-performance liquid chromatography. FTY720 alleviated the overexpression of hippocampal P-gp in SE rats and reduced NF-κB activity and TNF-α and COX-2 expression, and W146 blocked the effects of FTY720. Furthermore, SE rats that received FTY720 showed significantly greater hippocampal extracellular PHT concentrations than those that received vehicle, and W146 abolished this effect. Our results suggest that FTY720 alleviates seizure-induced overexpression of P-gp by inhibiting S1P receptor 1-mediated inflammation in rat hippocampus and improves PHT delivery to brain. FTY720 shows potential as an adjuvant therapy for drug-resistant epilepsy.

Ataxia, Manifestation of Phenytoin Toxicity: A Case Report

Phenytoin is a choice of drug for the treatment of epilepsy. It is categorized under drugs with narrow therapeutic range. Adverse effects of phenytoin observed in patients are gum hypertrophy, dystonia, myoclonus, abnormal muscle movement and rarely ballismusand ataxia. We report a case of a 32-year-old female, who had chief complaints of ataxia, oscillopsia, dizziness and nausea. She had past medical history of epilepsy and was taking phenytoin from the past 10 years. Treatment was started after tapering the dose of phenytoin, finally discontinuing, she was prescribed with valproic acid as an alternative for phenytoin. Present case report describes the phenytoin toxicity that helps to create awareness among physicians. There should be proper monitoring of plasma concentration of phenytoin to avoid its toxicity in the patients on chronic drug therapy so that drug dosage can be tailored as per the pharmacokinetic needs of the patient. Key words: Ataxia, Phenytoin Toxicity, Oscillopsia, Therapeutic Drug Monitoring, Phenytoin.

SEIZURES WITH ANTI-SEIZURE DRUG

This case study is to report and explore the etiology of a case of refractory seizures due to overdosage of phenytoin tablets. A case report from the Department of Neurology generated through voluntary adverse drug reaction (ADR)reporting stated phenytoin overdosage leading to refractory status epilepticus which did not respond to any of the antiepileptic drugs. A 33-year-old female patient with the history of consumption 15 tablets of phenytoin and a metal ring as part of a suicidal bid a month ago, presented with unconsciousness, persistent seizures, and gangrene of fingers. Magnetic resonance imaging showed generalized atrophic changes of the cerebrum and cerebellum. Electroencephalogram suggested multifocal onset status epilepticus. The patient did not respond to standard emergency treatment of status epilepticus with known antiepileptic drugs and was treated with thiopentone infusion under mechanical ventilation, which controlled her seizures as long as she was maintained under the infusion. Causality analysis using the World Health Organization scale categorizes this ADR as “possible”, as the patient is a known case of seizure disorder with additional cerebral changes. Hence, the disease could have had an influence over the toxic reaction. To conclude, seizures are a rare complication of phenytoin. Seizures can be prevented by evaluating therapeutic plasma concentration of phenytoin. In this case, the patient was on chronic treatment, and due to intentional toxicity, she progressed to a refractory state of seizures. This could have occurred because of the unique kinetic profile of phenytoin, small therapeutic index, genetic variation in drug metabolizing enzymes, and saturated sodium channels.

Changes in the Pharmacokinetics of Phenytoin in Dextran Sulfate Sodium-Induced Ulcerative Colitis in Mice.

We previously demonstrated that the expression levels of drug-metabolizing enzymes, cytochrome P450 (CYP) enzymes, in the liver are significantly decreased in a murine model of ulcerative colitis (UC). In this study, we investigated changes in the pharmacokinetics of phenytoin, a CYP2C substrate drug, in the presence of UC. Colitis was induced by feeding male mice 3.5% dextran sulfate sodium (DSS) dissolved in drinking water for 10 days. The messenger RNA (mRNA) expression of CYP2C29 and CYP2C37 and the protein expression of CYP2C in the liver were evaluated via real-time reverse transcription-polymerase chain reaction and Western blotting, respectively. In DSS-treated animals, both mRNA and protein expression levels of CYP2C in the liver were significantly reduced relative to those in control animals (by 20%-40%). Phenytoin (30 mg/kg) was administered orally in a single dose to mice, and plasma concentrations were measured. Plasma concentrations of phenytoin were higher in the DSS-treated group than in the control group at 12, 24, and 36 hours after administration. Animals given DSS also exhibited a higher area under the plasma concentration-time curve extrapolated to infinity (AUCinf, 315 μg·h/mL), a delayed elimination half-life ( T1/2, 8.1 hours), and a decreased body clearance (CL/F, 3.52 mL/h) compared with that of control animals (AUCinf, 215 μg·h/mL; T1/2, 3.6 h; CL/F, 5.58 mL/h). This study indicated that the presence of UC decreases CYP2C expression levels in the liver, thereby delaying the metabolism of CYP2C substrates, including phenytoin, and increasing blood concentrations of these substrates.

Role of CYP2C9, CYP2C19 and EPHX Polymorphism in the Pharmacokinetic of Phenytoin: A Study on Uruguayan Caucasian Subjects

Phenytoin (PHT) oxidative route leads to its main metabolite p-hydroxyphenytoin (p-HPPH), by means of CYP2C9 and CYP2C19. Formation of p-HPPH proceeds via a reactive arene-oxide intermediate. This intermediate can also be converted into PHT dihydrodiol by microsomal epoxide hydrolase (EPHX). The three enzymes are polymorphically expressed and the genetic variants are responsible for changes in the enzyme activity. In order to evaluate the effect that these polymorphisms have on PHT metabolism, PHT and p-HPPH plasma concentrations were measured and the genotype for the three enzymes was assessed in 50 Uruguayan epileptic patients. 30% of the patients were intermediate and 2% were poor metabolizers for CYP2C9, while 20% were intermediate metabolizers for CYP2C19. 44%, 10%, and 46% of subjects had intermediate, increased and decreased activities of EPHX respectively. CYP2C9 was confirmed to be the main responsible enzyme for PHT biotransformation. CYP2C19 seemed to be preponderant in p-HPPH oxidative metabolism. Apart from being responsible for the production of the dihydrodiol metabolite, EPHX also seemed to contribute to pHPPH formation when its activity is low. PHT might be recovered with a decreased activity of EPHX regardless the activity of CYP2C9.

A prospective study to assess the association between genotype, phenotype and Prakriti in individuals on phenytoin monotherapy

BackgroundGenetic polymorphisms in drug metabolizing enzymes (DMEs) impart distinct drug metabolizing capacity and a unique phenotype to an individual. Phenytoin has large inter-individual variability in metabolism due to polymorphisms in CYP2C9 and CYP2C19. As per Ayurveda, Prakriti imparts a unique phenotype to an individual. ObjectiveTo assess whether Prakriti can substitute phenotyping [therapeutic drug monitoring (TDM)] and genotyping in individualizing therapy with phenytoin in epilepsy patients. Methods and materialsThis was a cross-sectional study conducted over a period of three years. Prakriti was assessed using standardized and validated software. Polymorphisms in CYP2C9 and CYP2C19 were assessed using Polymerase Chain Reaction (PCR)-Restriction fragment length polymorphism (PCR-RFLP). Plasma concentrations of phenytoin (phenotype) were determined using reverse phase-high performance liquid chromatography (RF-HPLC). ResultsTotal 351 patients were enrolled for the study. Kapha vata (KV) (39%) was the predominantly observed Prakriti followed by vata kapha (VK) (20.8%) and vata pitta (VP) (8.83%) among the patients. The CYP2C9 and CYP2C19 genotype distributions were in accordance with Hardy–Weinberg equilibrium. There was no association between Prakriti and genotypes and Prakriti and phenotype (p > 0.05 each). Patients with CYP2C9 *1/*3 genotype were thrice more likely to have toxic plasma concentrations of phenytoin as compared to those with wild-type genotype (*1/*1) (Adjusted odds ratio – 3.36; 95% C.I. 1.61, 7.01). However, no such association was observed between polymorphisms of CYP2C19 and phenotype. ConclusionsWe did not find any association between Prakriti and either phenotype or genotypes suggesting that Prakriti assessment would be of limited utility in individualizing phenytoin therapy in epilepsy patients.

Pharmacokinetic Behavior of Phenytoin in Head Trauma and Cerebrovascular Accident Patients in an Iranian Population.

Objective:Acute brain injury is one of the leading causes of morbidity and mortality worldwide. Phenytoin has been commonly used as an anticonvulsant agent for the treatment or prophylaxis of seizures following acute brain injury. After a severe head injury, several pharmacokinetic changes occur. The aim of this study is the comparative evaluation of phenytoin serum concentration in patients with traumatic and nontraumatic brain injury (TBI).Methods:This prospective observational study was performed on twenty adult brain injury patients who were admitted to an Intensive Care Unit and required phenytoin for the treatment or prophylaxis of postinjury seizures. For all the patients, phenytoin serum concentration was determined in three scheduled time points. Phenytoin serum concentration and pharmacokinetic parameters were compared between patients with TBI and cerebrovascular accident (CVA).Findings:The Vmaxand Kmwere significantly higher in head trauma (HT) patients than the CVA group. The phenytoin concentration (Cp) and the Cp/dose ratio were significantly higher in the CVA group patients during the first sampling (P < 0.05). The Acute Physiology and Chronic Health Evaluation П (APACHE П) score was significantly lower than the baseline at the end of the study in each group of patients (P < 0.05). In addition, no significant correlation was observed between Vmax, Km, Cp, Cp/dose ratio, and APACHE II scores at the time of sampling.Conclusion:Due to significant differences in phenytoin plasma concentration and pharmacokinetic parameters between HT and CVA patients, close attention must be paid to the pharmacokinetic behavior of phenytoin in the efforts to improve the patient's outcome after a severe HT.

CYP2C9, CYP2C19, ABCB1 genetic polymorphisms and phenytoin plasma concentrations in Mexican-Mestizo patients with epilepsy.

We aimed to explore the possible influence of CYP2C9 (*2, *3 and IVS8-109 A>T), CYP2C19 (*2, *3 and *17) and ABCB1 (1236C>T, 2677G>A/T and 3435C>T) on phenytoin (PHT) plasma concentrations in 64 Mexican Mestizo (MM) patients with epilepsy currently treated with PHT in mono- (n=25) and polytherapy (n=39). Genotype and allele frequencies of these variants were also estimated in 300 MM healthy volunteers. Linear regression models were used to assess associations between the dependent variables (PHT plasma concentration and dose-corrected PHT concentration) with independent variables (CYP2C9, CYP2C19 and ABCB1 genotypes, ABCB1 haplotypes, age, sex, weight, and polytherapy). In multivariate models, CYP2C9 IVS8-109 T was significantly associated with higher PHT plasma concentrations (t(64)=2.27; P=0.03). Moreover, this allele was more frequent in the supratherapeutic group as compared with the subtherapeutic group (0.13 versus 0.03, respectively; P=0.05, Fisher's exact test). Results suggest that CYP2C9 IVS8-109 T allele may decrease CYP2C9 enzymatic activity on PHT. More research is needed to confirm findings.

Use of IV fosphenytoin pharmacokinetics to determine the loading dose for a clinical trial of canine status epilepticus.

Canine status epilepticus (CSE) has potential as a translational platform to evaluate the safety and efficacy of novel compounds and inform human status epilepticus trials. The aim of this study was to determine the intravenous dose of fosphenytoin (FOS) needed for dogs in a CSE clinical trial to attain phenytoin (PHT) concentrations similar to those used for human status epilepticus and monitor PHT concentrations. Four healthy dogs were used to characterize PHT pharmacokinetics. Each received either 15 mg/kg or 25 mg/kg of PHT equivalent intravenously. Blood samples were collected and FOS (total) and derived PHT (total and unbound) plasma concentrations were measured using high-performance liquid chromatography-mass spectrometry (HPLC-MS). Noncompartmental pharmacokinetics (PK) parameter values were determined and compartmental PK modeling and simulations were used to select the dose for the clinical trial with a target goal of 1-2 μg/ml unbound PHT at 30-60 min postinfusion. Predicted total and unbound PHT concentrations were compared with concentrations in blood collected from dogs treated for CSE in the clinical trial. Initial estimates suggested that a loading dose of 25 mg/kg would attain unbound concentrations of 1-2 μg/ml; however, this dose produced concentrations above 3-6 μg/ml, which resulted in clinically significant toxicity. A two-compartment model best fit the PHT concentration data with alpha-phase half-life of 2-5 min and elimination half-life of ~5 h. Based on the simulations, a dose of 15 mg/kg was selected and used in the clinical trial and 15 of 16 dogs randomized to the treatment arm had PHT plasma concentrations within the goal range. This study demonstrates that characterization of pharmacokinetics in a small number of dogs is useful in determining dosage regimens designed to attain targeted concentrations in clinical trials. Using this approach, we were able to determine a safe and effective dose of FOS for a clinical trial of CSE.

Canine status epilepticus treated with fosphenytoin: A proof of principle study.

There are a limited number of marketed intravenous antiepileptic drugs (AEDs) available to treat status epilepticus (SE). All were first developed for chronic therapy of epilepsy, not specifically for SE. Epilepsy and canine SE (CSE) occur naturally in dogs, with prevalence, presentation, and percentage of refractory cases similar to human epilepsy. The objective of this study was to determine if CSE treated with fosphenytoin (FOS) results in a similar responder rate as for people. A randomized clinical trial was performed for dogs with CSE. Dogs who presented during a seizure or who had additional seizures after enrolling received intravenous (i.v.) benzodiazepine (BZD) followed immediately by intravenous infusion of 15 mg/kg phenytoin equivalent (PE) of fosphenytoin (FOS) or saline placebo (PBO). If seizures continued, additional AEDs were administered per the standard of care for veterinary patients. Total and unbound plasma phenytoin (PHT) concentrations were measured. Consent was obtained for 50 dogs with CSE. Thirty-one had additional motor seizures and were randomized to the study intervention (22 FOS and 9 PBO). There was a statistically significant difference in the 12 h responder rate, with 63% in the FOS group versus 22% in the placebo group (p = 0.043) having no further seizures. The unbound PHT concentrations at 30 and 60 min were within the therapeutic concentrations for people (1-2 μg/ml) with the exception of one dog. There was mild vomiting in 36% of the FOS group (7/22) within 20 min of FOS administration and none of the placebo group (0/9) (p = 0.064). This proof of concept study provides the first evidence that FOS is tolerated and effective in canine SE at PHT concentrations clinically relevant for human SE. Furthermore, naturally occurring CSE can be utilized as a translational platform for future studies of novel SE compounds.