Free phenytoin assessment in patients: measured versus calculated blood serum levels

Estimation of Free Phenytoin Concentration in Critically Ill Patients with Hypoalbuminemia: Direct Measurement vs Traditional Equations

Background

In critically ill patients with low albumin, dose individualization of phenytoin is a challenge. The currently used Sheiner–Tozer equation does not accurately predict the free phenytoin concentration in serum and can result in incorrect dose modifications. The best measure to advocate in these patients is the direct-measurement of free phenytoin concentration.

Aims and objectives

Phenytoin exhibits complex pharmacokinetics, requiring careful therapeutic drug monitoring. This study aimed to compare the accuracy of the established Sheiner–Tozer calculation method against the direct-measurement of free phenytoin concentration in serum by high performance liquid chromatography in critically ill patients with low albumin.

Materials and methods

Blood specimens for direct-measurement of both total and free phenytoin concentration were obtained from 57 patients with hypoalbuminemia monitored in the intensive care unit.

Results

The median [inter-quartile range (IQR)] for Sheiner–Tozer equation calculated total phenytoin concentration and direct-measured total was 17.14 (10.63–24.53) and 9.82 (6.02–13.85) μg mL⁻¹, respectively. Approximately 53 and 5% of patients were found to be subtherapeutic and supratherapeutic for direct-measured total phenytoin concentrations, respectively. In contrast, on applying the Sheiner–Tozer calculation, 23 and 40% had subtherapeutic and supratherapeutic concentrations, respectively, for total phenytoin concentration. The median (IQR) for direct-measured, routine and Sheiner–Tozer equation calculated free phenytoin concentration were 1.92 (1.06–2.76), 0.98 (0.60–1.39), and 1.71 (1.06–2.45) μg mL⁻¹, respectively. Only 45.7% of patients were in agreement with respect to the therapeutic category when direct-measured free was compared against routine calculation free.

Conclusion

In patients with low albumin, free phenytoin concentration based on the Sheiner–Tozer corrected equation accurately classified patients based on their therapeutic category of free phenytoin in 73.7% of patients. Hence, for individualization of phenytoin dosage in critically ill patients with low albumin, we recommend direct-measurement of free phenytoin concentration.

How to cite this article

Wilfred PM, Mathew S, Chacko B, Prabha R, Mathew BS. Estimation of Free Phenytoin Concentration in Critically Ill Patients with Hypoalbuminemia: Direct-measurement vs Traditional Equations. Indian J Crit Care Med 2022;26(6):682–687.

Pharmacokinetic Drug-Drug Interactions among Antiepileptic Drugs, Including CBD, Drugs Used to Treat COVID-19 and Nutrients

Anti-epileptic drugs (AEDs) are an important group of drugs of several generations, ranging from the oldest phenobarbital (1912) to the most recent cenobamate (2019). Cannabidiol (CBD) is increasingly used to treat epilepsy. The outbreak of the SARS-CoV-2 pandemic in 2019 created new challenges in the effective treatment of epilepsy in COVID-19 patients. The purpose of this review is to present data from the last few years on drug–drug interactions among of AEDs, as well as AEDs with other drugs, nutrients and food. Literature data was collected mainly in PubMed, as well as google base. The most important pharmacokinetic parameters of the chosen 29 AEDs, mechanism of action and clinical application, as well as their biotransformation, are presented. We pay a special attention to the new potential interactions of the applied first-generation AEDs (carbamazepine, oxcarbazepine, phenytoin, phenobarbital and primidone), on decreased concentration of some medications (atazanavir and remdesivir), or their compositions (darunavir/cobicistat and lopinavir/ritonavir) used in the treatment of COVID-19 patients. CBD interactions with AEDs are clearly defined. In addition, nutrients, as well as diet, cause changes in pharmacokinetics of some AEDs. The understanding of the pharmacokinetic interactions of the AEDs seems to be important in effective management of epilepsy.

The Effect of Plasma Protein Binding on the Therapeutic Monitoring of Antiseizure Medications

Epilepsy is a widely diffused neurological disorder including a heterogeneous range of syndromes with different aetiology, severity and prognosis. Pharmacological treatments are based on the use, either in mono- or in polytherapy, of antiseizure medications (ASMs), which act at different synaptic levels, generally modifying the excitatory and/or inhibitory response through different action mechanisms. To reduce the risk of adverse effects and drug interactions, ASMs levels should be closely evaluated in biological fluids performing an appropriate Therapeutic Drug Monitoring (TDM). However, many decisions in TDM are based on the determination of the total drug concentration although measurement of the free fraction, which is not bound to plasma proteins, is becoming of ever-increasing importance since it correlates better with pharmacological and toxicological effects. Aim of this work has been to review methodological aspects concerning the evaluation of the free plasmatic fraction of some ASMs, focusing on the effect and the clinical significance that drug-protein binding has in the case of widely used drugs such as valproic acid, phenytoin, perampanel and carbamazepine. Although several validated methodologies are currently available which are effective in separating and quantifying the different forms of a drug, prospective validation studies are undoubtedly needed to better correlate, in real-world clinical contexts, pharmacokinetic monitoring to clinical outcomes.

Method for Simultaneous Determination of Free Concentration, Total Concentration, and Plasma Binding Capacity in Clinical Samples

Most quantitative research methods are based on measuring either the total or the free concentration of an analyte in a sample. However, this is often insufficient for the study of complex biological systems. The main objective of this research was to develop new methods for providing more information from samples: the free concentration (C_f), the total concentration (C_t), and the plasma binding capacity (PBC). Samples were processed using microextraction and ultrafiltration. For each of these techniques, two quantification procedures were used: addition of isotopically labeled standard and repeated analysis of the same sample. The new methods were validated by analyzing clinical samples and samples with known concentrations. Methods based on addition of labeled compound were found to be the fastest, and most reproducible. For analysis of clinical samples, methods based on microextraction were more sensitive and more accurate than those based on ultrafiltration. For analysis of pooled plasma samples, the overall accuracy of all approaches to determine PBC, testosterone C_f, and testosterone C_t was between 94 and 109%, 87-113%, and 94-122% respectively. The new approach goes beyond a simple concentration measurement, giving more information from clinical samples, with great potential for personalizing drug dosage and therapy to the needs of individual patients.

Evaluation of Fosphenytoin Therapeutic Drug Monitoring in the Neurocritical Care Unit

OBJECTIVE

The aim of this study was to determine whether the current method of calculating a fosphenytoin reloading dose results in a therapeutic free phenytoin level on subsequent days.

METHODS

Medical records of patients receiving fosphenytoin in the neurocritical care unit between July 2017 and June 2018 were screened. Included patients were those who had received at least three doses of fosphenytoin and required reloading doses according to concentrations obtained through therapeutic drug monitoring. Free phenytoin levels were categorized based on the prespecified patient-specific target range, generally between 1.5 and 2.5 mcg/mL.

RESULTS

Of the fosphenytoin reloading doses administered, 48% (73/152) resulted in a therapeutic free phenytoin concentration on the subsequent day, with the remaining 52% resulting in nontherapeutic levels (39% subtherapeutic, 13% supratherapeutic). Our evaluation of reloading dose calculation strategies indicated that patients were two times as likely to obtain a therapeutic level when a modified pharmacokinetic equation omitting the use of volume of distribution or salt formulation was used (58%, n = 39) than they were with doses calculated using the current pharmacokinetic model (41%, n = 20) or doses based on provider preference (39%, n = 14).

CONCLUSION

The current method of calculating a fosphenytoin reloading dose in the critically ill population does not consistently result in therapeutic concentrations. With multiple factors affecting the pharmacokinetics of critically ill patients, the creation of a new pharmacokinetic model with less emphasis on volume of distribution may more consistently result in therapeutic concentrations.

Development and Validation of an LC-MS/MS Method and Comparison with a GC-MS Method to Measure Phenytoin in Human Brain Dialysate, Blood, and Saliva

Phenytoin (PHT) is one of the most often used critical dose drugs, where insufficient or excessive dosing can have severe consequences such as seizures or toxicity. Thus, the monitoring and precise measuring of PHT concentrations in patients is crucial. This study develops and validates an LC-MS/MS method for the measurement of phenytoin concentrations in different body compartments (i.e., human brain dialysate, blood, and saliva) and compares it with a formerly developed GC-MS method that measures PHT in the same biological matrices. The two methods are evaluated and compared based on their analytical performance, appropriateness to analyze human biological samples, including corresponding extraction and cleanup procedures, and their validation according to ISO 17025/FDA Guidance for Industry. The LC-MS/MS method showed a higher performance compared with the GC-MS method. The LC-MS/MS was more sensitive, needed a smaller sample volume (25 µL) and less chemicals, was less time consuming (cleaning up, sample preparation, and analysis), and resulted in a better LOD (<1 ng/mL)/LOQ (10 ng/mL). The calibration curve of the LC-MS/MS method (10-2000 ng/mL) showed linearity over a larger range with correlation coefficients r2 > 0.995 for all tested matrices (blood, saliva, and dialysate). For larger sample numbers as in pharmacokinetic/pharmacodynamic studies and for bedside as well as routine analyses, the LC-MS/MS method offers significant advantages over the GC-MS method.

Levetiracetam versus phenytoin for seizure prophylaxis in brain injured patients: a systematic review and meta-analysis

Background The onset of early and/or late seizures in brain injured patients is associated with worse outcome. So far, phenytoin is the most commonly used antiepileptic drug to prevent seizures in this group of patients. Objective In the current metaanalysis, we aimed to compare the efficacy and safety of phenytoin versus levetiracetam for seizure prophylaxis in brain injured patients. Methods A systematic search was conducted in PubMed and Cochrane Library Database by 2 investigators. Four randomized controlled trials (RCTs) were included (295 patients). Data were extracted and the quality of each RCT was assessed. Results Levetiracetam was found to be more effective than phenytoin in seizure prophylaxis (OR = 0.23; CI 95% [0.09-0.56]; Q test p value = 0.18 and I² = 38%). A trend toward less serious side effects was also found in patients treated with levetiracetam (OR = 0.27; CI 95% [0.07-1.07]; Q test p value = 0.72 and I² = 0%). Conclusion Levetiracetam is more effective and safer than phenytoin for seizure prophylaxis in brain injured patients.