Unbound Phenytoin Concentrations Research Articles

Doxorubicin alters the disposition of phenytoin by reducing its metabolic elimination and binding affinity to serum albumin in rats.

We investigated the pharmacokinetic interaction of doxorubicin (DOX) with phenytoin (PHT) and the underlying mechanism in rats to clarify why the serum PHT concentration decreases despite the impaired PHT metabolic capacity in patients receiving DOX. Rats were administered 15 mg/kg of DOX or saline alone. The pharmacokinetic disposition of intravenously administered PHT was examined 4 days after DOX exposure. Enzyme kinetics of CYP2C-dependent PHT p-hydroxylation were analysed using hepatic microsomes. The unbound PHT concentration in serum was measured by the ultrafiltration method, and the relationship between the unbound fraction (fu) and serum albumin level was assessed. The total clearance (CLtot) of PHT was significantly increased by DOX, but the activity of PHT p-hydroxylation conversely decreased. The unbound serum PHT concentration and its fu were significantly higher in the DOX group than in the control group, and the CLtot/fu, a measure of intrinsic clearance, significantly decreased. An increase in the fu was observed even when using a serum sample with an albumin concentration equal to that in the control group. DOX treatment increases the unbound serum PHT concentration by depressing the metabolic capacity and alters the total PHT by reducing serum albumin and its affinity to PHT.

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.

Response to “Characterization of Unbound Phenytoin Concentrations in Neurointensive Care Unit Patients Using a Revised Winter-Tozer Equation” by Sean P. Kane, Adam P. Bress, and Eljim P. Tesoro

Response to “Characterization of Unbound Phenytoin Concentrations in Neurointensive Care Unit Patients Using a Revised Winter-Tozer Equation” by Sean P. Kane, Adam P. Bress, and Eljim P. Tesoro

The Quantitative Effect of Serum Albumin, Serum Urea, and Valproic Acid on Unbound Phenytoin Concentrations in Children

Dosing of phenytoin is difficult in children because of its variable pharmacokinetics and protein binding. Possible covariates for this protein binding have mostly been univariately investigated in small, and often adult, adult populations. We conducted a study to identify and quantify these covariates in children. We extracted data on serum phenytoin concentrations, albumin, triglycerides, urea, total bilirubin and creatinine concentrations and data on coadministration of valproic acid or carbamazepine in 186 children. Using nonlinear mixed effects modeling the effects of covariates on the unbound phenytoin fraction were investigated. Serum albumin, serum urea concentrations, and concomitant valproic acid use significantly influenced the unbound phenytoin fraction. For clinical practice, we recommend that unbound phenytoin concentrations are measured routinely. However, if this is impossible, we suggest to use our model to calculate the unbound concentration. In selected children, close treatment monitoring and dose reductions should be considered to prevent toxicity.

Predicting Unbound Phenytoin Concentrations in the Critically Ill Neurosurgical Patient

To assess the correlation of measured unbound phenytoin concentration (dphF) to estimated unbound concentration (dphEF) using the Sheiner-Tozer equation in critically ill patients in the neurosurgical intensive care unit. The dphF and total phenytoin (dphT) trough serum concentrations were measured during the first week of therapy in 17 consecutive patients with albumin concentrations less than 3.5 g/dL. Serum albumin concentrations were measured within 24 hours of serum phenytoin concentration measurement. A university-affiliated urban teaching hospital. The study population consisted of 17 neurosurgical patients who were at least 18 years old. The predictability of the Sheiner-Tozer equation was tested by measuring dphF, dphT, and serum albumin concentrations. Measured phenytoin concentrations were compared with phenytoin concentrations calculated from the Sheiner-Tozer equation. To estimate correlation between variables linear regression was calculated. Mean absolute value of error and mean error were estimated to assess precision and bias between measures, respectively. The mean +/- SD dphT was 13.05 +/- 5.15 mu g/mL. The measured dphF was 1.89 +/- 0.80 compared with 2.00 +/- 0.8 mu g/mL for the dphEF (NS). Regression analysis for dphEF versus dphF revealed a significant correlation (r2 = 0.94, p = 0.001). The mean absolute value of error for the Sheiner-Tozer equation to predict dphEF was 0.167, which was 9% of the mean value of dphEF (1.89). These results indicate that, in this population, the Sheiner-Tozer estimate of dphEF provides an unbiased, precise clinical estimate of dphF in patients for whom measured dphF is unavailable or impractical.

Predicting Unbound Phenytoin Concentrations In Patients Receiving Valproic Acid: A Comparison of Two Prediction Methods

To compare the predictive performance of 2 equations that estimate unbound (free) phenytoin plasma concentrations when valproic acid (VPA) and phenytoin are administered concurrently. Eighty-eight adults receiving VPA and phenytoin concurrently were included in the study. Steady-state plasma concentration measurements of total phenytoin, total VPA, and unbound phenytoin were collected prospectively in the inpatient group (group 1) and retrospectively in the outpatient group (group 2). Using the equations developed by Haidukewych and May, unbound phenytoin concentrations were calculated. The mean predicted unbound phenytoin concentrations then were compared with mean actual unbound phenytoin concentrations measured in the laboratory. Identical assays were performed to measure unbound phenytoin, total phenytoin, and VPA from each patient group. Antiepileptic drug concentration measurements were collected from 43 inpatients (mean age 34.8 y) from the epilepsy unit at Abbott Northwestern Hospital and 45 outpatients (mean age 37.3 y) at the MINCEP Epilepsy Care clinic, Minneapolis, MN. Mean prediction error (MPE) and mean squared error (MSE) were calculated to determine which equation was the least biased and most precise in predicting unbound phenytoin when total VPA and phenytoin concentrations are known. Linear regression of predicted unbound phenytoin on measured unbound phenytoin values determined the correlation coefficients (r). A paired Student's t-test also was performed comparing mean predicted unbound phenytoin concentration with mean actual unbound phenytoin concentrations in both groups. The MPE from May's equation was -0.49 and -0.45 for groups 1 and 2, respectively; using the Haidukewych equation, MPE was -0.02 and 0.08 for groups 1 and 2, respectively. The MSE using May's equation was 0.34 for both groups. Using the Haidukewych equation, group 1 MSE was 0.07, and for group 2, 0.12. Correlation coefficients were more than 0.91 (p < 0.001) from each equation in both patient groups. In group 1, mean actual unbound phenytoin concentration was 2.02 micrograms/mL; May's equation predicted 1.52 micrograms/mL (p < 0.001) and the Haidukewych equation predicted 2.00 micrograms/mL (p = 0.64). In group 2, mean actual unbound phenytoin concentration was 2.10 micrograms/mL; May's equation predicted 1.65 micrograms/mL (p < 0.001) and the Haidukewych equation predicted 2.18 micrograms/mL (p = 0.11). Haidukewych's equation predicts unbound phenytoin concentrations with the least bias and most precision with statistical significance. May's equation consistently underpredicted unbound phenytoin concentrations. Because unbound phenytoin fraction is not constant (and usually more than the expected 10%) in patients comedicated with VPA, unbound phenytoin concentrations cannot be predicted even though total VPA and phenytoin concentrations are known. If unbound phenytoin concentrations cannot be readily measured, Haidukewych's equation is a reliable predictor of unbound phenytoin concentrations.

Nomogram for the Prediction of Unbound Phenytoin Concentrations in Patients on a Combined Treatment of Phenytoin and Valproic Acid

The interpretation of the total phenytoin (PT) concentration can be problematic if valproic acid (VPA) is given as a comedication, because VPA displaces PT from the protein binding sites and can increase the free fraction of PT. In order to estimate the free or unbound PT concentration (PTf) from the total PT concentration (PTt) and VPA concentration, a nomogram was constructed and evaluated. Data of 84 patients on combined therapy with PT and VPA were used in drawing up the regression equation (PTf = 0.0792.PTt + 0.000636.PTt.VPA) from which the nomogram was constructed. The predictions were checked using another 33 patients whose serum concentrations were determined in the morning and in addition several times during the day. The results show that using this method the PTf concentrations can be accurately estimated from the PTt and VPA concentrations.

Fluctuations of Unbound and Total Phenytoin Concentrations During the Day in Epileptic Patients on Valproic Acid Comedication

The influence of daily fluctuations in the concentration of valproic acid (VPA) on the unbound and total concentration of phenytoin (PT) was examined in a prospective study. The serum concentrations of 28 patients with epilepsy (group PT + VPA) who were treated with PT and concurrently with VPA and 15 patients (group PT) who were treated with PT but without VPA comedication were determined at 8.00, 11.00, 14.00, 17.00, and in part at 20.00 h. The results show that there are significantly greater fluctuations in the total PT concentration among patients on VPA than those not on VPA. The fluctuations in the total PT concentration during the day correlated with the fluctuations in the VPA concentration. On the other hand, the fluctuations in the concentrations of unbound PT of patients on VPA were comparable with those not on VPA. In the absence of VPA, the diurnal fluctuations of the total PT concentration correlated highly significantly with the fluctuations of the unbound PT concentration. This is not the case in patients on VPA. Our data are a further indication of the smaller significance of the total PT concentration as compared with the unbound PT concentration in the combined treatment of PT and VPA.

A New Approach to Establishing Individualized Maintenance Doses of Phenytoin

A new method for arriving at individualized doses of phenytoin (PHT) is described. This method, designated the eta method, operates on two measurements of unbound plasma PHT. One plasma sample is collected 48 h after a single, oral PHT dose (approximately 4.5 mg/kg). From the single dose, single sample measurements of plasma unbound PHT individual estimates of intrinsic unbound PHT clearance (CLuint) can be made. With the use of CLuint and weight-adjusted mean population values of Vmax predicted plasma concentrations of unbound PHT for 11 subjects exhibited a mean prediction error (mpe) of 0.07 mg/L with 95% confidence limits of -0.11 to 0.26 mg/L and an error coefficient of variation [root mean squared error (rmse)/mean unbound concentration at steady state (Cuss)] of 0.4. The second plasma unbound PHT is collected at steady state. From CLuint and one Cuss measurement, Vmax and Km can be directly calculated. Doses calculated by the eta method to produce plasma unbound PHT concentrations of approximately 1.0 mg/L were identical to those calculated by the Bayesian method to yield plasma total PHT concentrations of approximately 10 mg/L. When those doses were administered to six subjects, the plasma unbound PHT concentrations predicted by the eta method exhibited a mpe of -0.13 mg/L, with 95% confidence limits ranging from -0.33 to 0.06 mg/L and an error coefficient of variation of 0.2. The plasma PHT concentrations predicted by the Bayesian method exhibited an mpe of -1.30 mg/L, with 95% confidence limits of -4.02 to 1.42 mg/L and an error coefficient of variation of 0.3. Values of Vmax and doses of PHT calculated to produce a Cuss of 1.0 mg/L calculated by the eta method were nearly identical to Vmax values and doses of PHT calculated by Bayesian double feedback (based on two different Css values) to produce a Css of 10.0 mg/L.

Use of plasma unbound drug concentrations in adjusting phenytoin doses.

The application of a Bayesian feedback algorithm for phenytoin (PHT) dosing adjustments was modified empirically to take advantage of measurements of plasma unbound PHT concentrations in the estimation of Km values. Doses were calculated to achieve steady-state unbound PHT nadirs of 1.2 mg/L with an allowable range of 0.9-1.5 mg/L. The success of this approach was compared with that of a routine strategy encompassing the use of monitored total plasma PHT concentrations and no estimates of kinetic parameters. Allowing up to a double feedback of monitored values, the Bayesian approach was better than the routine procedure in predicting doses that led to PHT concentration within the targeted range (p less than 0.005). The approach to optimizing PHT dosing described herein offers the advantage of rapidly converging on near-optimal therapy for those patients who are already on PHT but whose seizures are not suitably controlled, and may be useful even for patients who exhibit abnormal PHT plasma protein binding, such as those who also must take valproic acid.

Plasma concentrations of unbound phenytoin in the management of epilepsy.

In 46 epileptic patients the range of the unbound fraction of phenytoin in plasma measured by ultrafiltration (at 37 degrees C) and tracer-labelling with [14C]-phenytoin was 6.7%-33.3% with a median of 11.9%. The total and unbound phenytoin plasma concentrations were significantly correlated (r = 0.93, P less than 0.001), but in six patients the unbound concentration fell on or outside the 90% predictability limits for a single value. In all patients the unbound concentration reflected the clinical status of the patient equally or better than the total concentration. An inverse relationship was found between the plasma albumin concentration (within the normal reference range) and the phenytoin unbound fraction (r = -0.83, P less than 0.001) indicating that plasma albumin concentration is one of the important overall determinants of phenytoin protein binding. Saliva and plasma unbound phenytoin concentrations were significantly correlated (r = 0.98, P less than 0.001) but both collection of plasma samples and preparation of plasma ultrafiltrate using the Amicon micropartition system are simpler than collection and processing saliva, and interpretation of plasma unbound concentration does not require allowance for potential contamination. The additional value of the unbound phenytoin concentration in a clinically significant number of individuals would justify routine measurement of unbound phenytoin concentration in monitoring therapy, once available simplified methodology has been adequately characterised.

Importance of unbound phenytoin serum levels in head trauma patients.

Total (bound + unbound) and unbound phenytoin serum concentrations were measured in ten male comatose head trauma patients and ten male epileptic patients. Serum biochemistry and complete blood cell counts were normal for both groups, except albumin concentrations were below normal in the head trauma patients for nutritional reasons. Total phenytoin concentrations were 6.8 +/- 1.8 mcg/ml for the head trauma patients and 14.5 +/- 3.0 mcg/ml for the epileptic patients (p less than 0.0002) even though phenytoin doses were similar. However, unbound phenytoin concentrations were within the therapeutic range of 1 to 2 mcg/ml for both groups and were not significantly different. Had only the total concentrations been measured, phenytoin doses might have been increased inappropriately in the head trauma patients. The reason head trauma patients had therapeutic unbound concentrations despite lower total concentrations was that the unbound per cent of phenytoin was higher in these patients (21 +/- 3.2%) than in the epileptic patients (10 +/- 1.3%, p less than 0.0002).