Effect of serum separator tubes on free and total phenytoin and carbamazepine serum concentrations

The effect of serum separator tubes (SSTs) on free and total serum phenytoin and carbamazepine concentrations was determined by comparing standard no-additive tubes with SSTs (Becton Dickinson SST and Terumo Autosep). The influence of time prior to centrifugation, sample volume, and initial drug concentration on the effects were also studied. Results were analyzed using repeated measures two-way analysis of variance with tube type and either time, sample volume, or concentration as main effects. The most significant reductions noted were with Becton Dickinson SSTs in free and total serum phenytoin and total carbamazepine concentrations, where all reductions were less than 10%. The only factor to significantly influence extent of reduction was the effect of time on total serum phenytoin concentration in Becton Dickinson SSTs. Terumo Autosep tubes caused no major reductions in free or total phenytoin or carbamazepine serum concentrations. Autosep tubes should provide accurate measurements of total and free serum phenytoin and carbamazepine concentrations. With Becton Dickinson SSTs, the reductions noted in free and total phenytoin and total carbamazepine concentrations were not large enough to preclude their clinical use. Becton Dickinson SSTs should not be used for determining free or total phenytoin or total carbamazepine concentrations for purposes of research.     

Adaptation of total phenytoin reagent pack for measuring free phenytoin levels with the Abbott AxSYM immunoassay analyzer

One hundred and four serum samples submitted to our laboratory for total phenytoin levels were also assayed for free phenytoin levels using the Abbott TDx and AxSYM immunochemistry analyzers (Abbott Laboratories; Abbott Park, IL, U.S.A.). Free phenytoin levels were assessed using ultrafiltrates obtained with the Centrifree Microfiltration system (Amicon, Inc.; Beverly, MA, U.S.A.). There was an acceptable correlation of free phenytoin levels between the two instruments (r = 0.994; mean value of free fraction was TDx = 1.7 microg/mL and AxSYM = 1.6 microg/mL). The mean free phenytoin values correlated well with previously reported values in the literature. Acceptable recovery (88.8%-101.5%) and precision (CV = 2.3%-11.0%) were also obtained. The results of this study indicated that free phenytoin levels can be obtained with acceptable accuracy and precision using the total phenytoin reagents.     

Serum phenytoin concentration and clinical response in patients with epilepsy.

1 Patients with poorly controlled epilepsy were cautiously transferred from multiple drug therapy to treatment with phenytoin sodium alone. One patient suffered more severe seizures and the initial treatment was restarted. The remainder showed no deterioration. 2 The daily dose of phenytoin was then increased by a small increment at intervals of 2 or more months. The serum phenytoin concentration (total and free) was measured regularly and response was assessed by records of seizure frequency and tests of speech, handwriting, short-term memory and coordination. 3 Patients (n = 11) with partial seizures showed no consistent improvement with increased phenytoin concentration within the range 15 mg/l (60 mumol/l) to the individual threshold for intoxication, greater than or equal to 35 mg/l (140 mumol/l). Patients (n = 4) with generalized seizures however were consistently improved at higher concentrations. 4 Tolerance to phenytoin varied, the threshold for symptomatic intoxication ranging from 35-60 mg/l (140-240 mumol/l) total and 2.7-5.2 mg/l (10.8-20.8 mumol/l) free. Ataxia was the commonest symptom and in some cases this was manifest by worsening of performance on the test of coordination (pursuit rotor). Even at lower phenytoin concentrations the patients performed less well on this test than control subjects. Other tests of performance showed no evidence of impairment at higher phenytoin concentrations. 5 The same daily dose of phenytoin tended to give higher serum drug concentrations after intoxication than before.     

Factors affecting the free plasma fraction of phenytoin in patients with epilepsy

The variability and possible factors modifying the free plasma fraction of phenytoin were investigated in 134 patients with epilepsy undergoing long-term treatment. The total and free plasma concentrations of phenytoin were determined using fluorescent polarisation immunoassay. Concentrations of albumin, bilirubin and creatinine were also obtained. The free plasma concentration was separated by ultrafiltration, at 25 degrees C, using Centrifree((R)) filters. Factors related to the free plasma fraction of phenytoin (free plasma concentration/total plasma concentration) were gender, age, dose, therapeutic regimen, total plasma concentration and the biochemical parameters mentioned. The mean of free plasma fraction was 9.1% with a very high variability (between 3.3 and 37%). No significant relationship was found between the free plasma fraction and dose, age, gender, total plasma concentration or the biochemical data. The only variable with a significant effect (p < 0.05) on the free plasma fraction was polytherapy with valproic acid. The variability in the free plasma fraction of phenytoin was high in epileptic patients, and was poorly related to the clinical or analytical variables studied. In the absence of pathologies that modify phenytoin binding (uraemia, hypoalbuminaemia), the only factor predictive of a possible alteration in the binding of phenytoin to plasma proteins was polytherapy with valproic acid.     

In vivo binding characteristics of phenytoin to serum proteins in monotherapy pediatric patients with epilepsy

AIM: The aim of the present study was to determine the binding characteristics of phenytoin (PHT) to serum proteins in the pediatric population. Binding parameters of PHT to serum proteins in our study were conducted to compare with in vivo or in vitro binding parameters of PHT to serum proteins in adult subjects reported by other investigators. SUBJECTS AND MATERIALS: Serum samples in the study were obtained from 40 pediatric patients (16 male, 24 female) receiving PHT monotherapy. Their age ranged from 1 to 15 years (9.2 +/- 3.6 years, mean +/- SD). The in vivo population binding parameters of PHT to serum proteins and theoretical minimal unbound serum PHT fraction (fu) were determined using an equation derived from the Scatchard equation. RESULTS: The association constant (Ka) was 0.014 l/micromol, while the total concentration of binding sites (n(Pt)) was 747 micromol/l. The number of binding sites per albumin molecule (n) was 1.13, while binding ability (n x Ka) was 0.0161/micromol. The fu was 0.087. The n x Ka is approximately 1.2 times higher in PHT monotherapy adult patients of Pospisil et al. [1992] (i.e. 0.0191 l/micromol) than in all our patients. The association constant is approximately 1.3 times higher in the in vitro study of Monks et al. [1978] (i.e. 0.0186 l/micromol) than in our study, while n is similar between the two studies. The fu in our pediatric patients is similar to the unbound serum PHT fraction in adult patients receiving PHT therapy reported by Richens [1979] (i.e. 0.1). CONCLUSION: Our results suggest that there may be small differences in the binding characteristics of PHT to serum proteins between Japanese pediatric and non-Japanese adult subjects. The unbound serum fraction of PHT in pediatric patients with epilepsy can be assumed to be relatively constant in the therapeutic concentration range of PHT.     

A correlation between the levels of phenobarbital, phenytoin and valproic acid in the blood serum and in saliva from children treated due to epilepsy

Blood serum and saliva levels of phenobarbital, phenytoin and valproic acid were estimated in children treated due to epilepsy. The ratio between the levels of the above drugs in saliva and blood serum can be of importance in calculating approximated levels of the drugs without a need of inconvenient inserting of needle and taking blood in children.     

Prediction of phenytoin dosage in relation to the variability of phenytoin plasma concentration.

1 In a model study the influence of some variables on the accuracy of predictions of phenytoin doses which should produce desired plasma concentrations were studied. All predictions were extrapolations from low test doses with concurrent observations of plasma concentrations. 2 The variables in introduced were:--Variability in observed plasma concentration;--Variability in applied test doses;--Variability in patient characteristics as expressed in Km and Vmax values. 3 The model study generated the same sort of result as a previous one, executed in practice (Driessen. Van der Velde + Höppener, 1980), i.e. predictions are often inaccurate and of the method tested (Richens & Dunlop, 1975; Ludden, Hawkins, Allen & Hoffman, 1976; Martin, Tozer, Sheiner & Riegelman, 1977; Rambeck, Boenigk, Dunlop, Mullen, Wadsworth & Richens, 1980), Richens' first nomogram (Richens & Dunlop, 1975) is to be preferred.     

Temperature effects on the estimation of free levels of phenytoin carbamazepine and phenobarbitone

Serum samples from patients on phenytoin (PHT), carbamazepine (CBZ), or phenobarbitone (PB) monotherapy were filtered at 15, 25, and 37 degrees C and the free concentrations measured by high-performance liquid chromatography. The mean apparent dissociation constants at each temperature were calculated, and were used to predict free drug levels from a further series of patients' samples in which total drug and albumin concentrations only were known. The correlation coefficients (r) between these predicted free levels and experimental results obtained by analysis of the same samples for PHT, CBZ, or PB were 0.977, 0.968, and 0.998, respectively, at 25 degrees C; at 37 degrees C, the corresponding values of r were 0.975, 0.961, and 0.997, respectively. We then determined free fractions (alpha) of PHT, CBZ, and PB at 25 and 37 degrees C and used these values to derive theoretical target ranges for free levels for each of the three drugs. We discuss the implication of these results for patient care, with special reference to the need to specify temperature and quote the appropriate target range when analyses of free levels of AEDs are carried out.     

Comparison of serum phenytoin levels in epileptic patients who swallowed their phenytoin tablets with or without previous chewing

Summary This cross-over study was conducted to compare serum phenytoin levels after chronic ingestion of phenytoin tablets with or without previous chewing. The phenytoin therapy was administered as 50 mg chewable Infatabs® tablets in a single morning dose of 200 mg. There was no significant difference between the two modes of ingestion as regards serum phenytoin levels measured at various times after ingestion of the phenytoin tablets. Moreover, the area under the curve did not differ significantly during the 24 h interval. Minor changes between two Dilantin® formulations, however, could influence drug availability.     

Effects of various drugs on serum free and total tryptophan levels and brain tryptophan metabolism in rats.

Various drugs known to bind to serum albumin were examined to determine whether or not they influenced the level of free tryptophan in serum in vitro and in vivo. Possible relationships between the serum free tryptophan level and serotonin (5-HT) synthesis in the brain and the hypothermic effects of these drugs were investigated. Of the drugs examined, sodium salicylate, sodium benzoate and indomethacin caused a significant increase in the concentration of serum free tryptophan and stimulated the synthesis of 5-HT in the brain. Hypothermia induced by salicylate and indomethacin was potentiated by pretreatment with pargyline, a monoamine oxidase inhibitor. Administration of benzoate did not cause any change in body temperature, but after pargyline a hypothermia did occur. However, pretreatment with parachlorophenylalanine, an inhibitor of 5-HT synthesis, did not influence the hypothermia induced by salicylate and indomethacin. Relationship between the hypothermic effect and the increase of 5-HT synthesis in the brain after a large dose of salicylate and indomethacin is discussed.     

Clinical responses in relation to blood acetaldehyde levels.

A study was undertaken to examine the relationship between blood acetaldehyde levels and clinical responses in volunteers receiving the anti-alcohol drugs disulfiram and calcium cyanamide. In the first part of this study volunteers received different doses of disulfiram (125 mg and 500 + 250 mg), of calcium cyanamide (25 mg, 50 mg and 100 mg) and of ethanol (0.2 g/kg orally and 0.5 g/kg intravenously). The ensuing interactions ranged from no reaction at all to an intense hypotensive cyanamide-ethanol reaction (CER). A blood acetaldehyde concentration-effect relationship was suggested. In the second part of this study seven subjects received 50 mg of calcium cyanamide 4 hr prior to an intravenous ethanol dose of 0.2 g/kg. The maximum blood level of acetaldehyde ranged from 16 to 241 microM. Aversive interactions started to occur at acetaldehyde levels around 40-60 microM. Changes in flushing reaction and diastolic blood pressure appeared best to reflect changing blood acetaldehyde levels. As a rule, however, the expected cyanamide-ethanol and disulfiram-ethanol reactions are more clearly registered as an increase in acetaldehyde levels than as the ensuing physiological responses.     

Factors influencing simultaneous concentrations of total and free carbamazepine and carbamazepine-10, 11-epoxide in serum of children with epilepsy

Various factors that may influence the simultaneous concentration of total and free carbamazepine (CBZ) and carbamazepine-10,11-epoxide (CBZ-E) in serum of 68 children (mean age 11.8 +/- 4.5 years) with epilepsy were assessed. Separation of free and bound drug fractions was achieved by ultrafiltration, and CBZ and CBZ-E concentrations were determined using a sensitive high pressure liquid chromatographic technique. Thirty children were on CBZ monotherapy. Both total CBZ and CBZ-E serum concentrations correlated significantly with their respective free serum concentrations. CBZ was 81 +/- 3% and CBZ-E 63 +/- 9% bound. There was no correlation between the CBZ dose and either CBZ total or free serum concentrations. A statistically significant correlation was, however, observed between CBZ dose and simultaneous CBZ-E total and free concentrations. CBZ total and free concentrations correlated significantly with those of total CBZ-E. A significant negative correlation was observed between age and total (r = -0.49, p less than 0.01) and free (r = -0.43, p less than 0.025) CBZ-E/CBZ ratios. Concomitant drug therapy (phenytoin, phenobarbitone, and sodium valproate) significantly elevated CBZ-E/CBZ ratios.     

Temperature effect on serum protein binding kinetics of phenytoin in monotherapy patients with epilepsy.

The effects of temperature on the binding kinetics of phenytoin (PHT) to serum proteins were determined in patients with epilepsy. Serum samples examined in the study were obtained from 59 patients (31 male, 28 female) with epilepsy on PHT monotherapy. Their age ranged from 3 to 64 years (mean (SD), 23.3 (16.3) years). Protein binding of PHT was evaluated by ultrafiltration under current routine laboratory conditions (25 +/- 3 degrees C) or at a temperature of 37 degrees C. The in vivo binding parameters of PHT to serum proteins were determined using a binding equation derived from the Scatchard equation for a one-site binding model. Significant differences were observed in serum concentrations of unbound PHT between paired data (P < 0.05). The mean association constant (K) of PHT to serum proteins is 0.011 microM-1 at 25 +/- 3 degrees C and 0.006 microM-1 at 37 degrees C, while mean total concentration of binding sites (n(Pt)) is 1002 microM for 25 +/- 3 degrees C and 1112 microM for 37 degrees C. Significant differences were observed in the binding kinetics of PHT to serum proteins for the different temperature conditions of ultrafiltration (P < 0.05). Our study confirms that binding affinity for PHT-serum protein interaction is approximately 45% lower at 37 degrees C than at 25 +/- 3 degrees C and consequently, binding potential (K.n(Pt)) is approximately 39% lower at 37 degrees C than at 25 +/- 3 degrees C.     

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

Background Total serum drug levels are routinely determined for the therapeutic drug monitoring of selected, difficult-to-dose drugs. For some of these drugs, however, knowledge of the free fraction is necessary to adapt correct dosing. Phenytoin, with its non-linear pharmacokinetics, >90 % albumin binding and slow elimination rate, is such a drug requiring individualization in patients, especially if rapid intravenous loading and subsequent dose adaptation is needed. In a prior long-term investigation, we showed the excellent performance of pharmacy-assisted Bayesian forecasting support for optimal dosing in hospitalized patients treated with phenytoin. In a subgroup analysis, we evaluated the suitability of the Sheiner-Tozer algorithm to calculate the free phenytoin fraction in hypoalbuminemic patients. Objective To test the usefulness of the Sheiner-Tozer algorithm for the correct estimation of the free phenytoin concentrations in hospitalized patients. Setting A Swiss tertiary care hospital. Method Free phenytoin plasma concentration was calculated from total phenytoin concentration in hypoalbuminemic patients and compared with the measured free phenytoin. The patients were separated into a low (35 ≤ albumin ≥ 25 g/L) and a very low group (albumin <25 g/L) for comparing and statistically analyzing the calculated and the measured free phenytoin concentration. Main outcome measures Calculated and the measured free phenytoin concentration. Results The calculated (1.2 mg/L (SD = 0.7) and the measured (1.1 mg/L (SD = 0.5) free phenytoin concentration correlated. The mean difference in the low and the very low albumin group was: 0.10 mg/L (SD = 1.4) (n = 11) and 0.13 mg/L (SD = 0.24) (n = 12), respectively. Although the variability of the data could be a bias, no statistically significant difference between the groups was found: t test (p = 0.78), the Passing–Bablok regression, the Spearman’s rank correlation coefficient of r = 0.907 and p = 0.00. The Bland–Altman plot including the regression analysis revealed no systematic differences between the calculated and the measured value [M = 0.11 (SD = 0.28)]. Conclusion In absence of a free phenytoin plasma concentration measurement also in hypoalbuminemic patients, the Sheiner-Tozer algorithm represents a useful tool to assist therapeutic monitoring to calculate or control free phenytoin by using total phenytoin and the albumin concentration.     

Changes in serum glucocorticoid levels and thymic atrophy induced by phenytoin administration in mice

The weights of spleen and thymus were recorded and serum glucocorticoid (GC) levels examined in mice injected intraperitoneally with 60 mg phenytoin (PHT) kg body wt. for 3, 8, and 30 days. In mice injected with PHT, the serum GCs were elevated about 4- to 5-fold in 3 days; the higher levels were maintained and accompanied thymic atrophy throughout the experiment. Moreover, dose-dependent changes in serum GC levels and thymus weight were also observed at 3 days. These results indicate that GCs may play an important role in thymic atrophy induced by PHT.     

Clinical evaluation of plasma free phenytoin measurement and factors influencing its protein binding

The relationship between free phenytoin concentrations and clinical responses, and the factors influencing protein binding of phenytoin were investigated. A total of 119 plasma samples from 70 patients treated orally with phenytoin were analysed. The mean free phenytoin concentration was significantly higher in the patients who received phenytoin monotherapy and were classified as having a complete response (1.25 +/- 1.09 microg/ml) than that in the partial response group (0.59 +/- 0.07 microg/ml), whereas the mean total concentrations were not significantly different between the two groups. Samples were divided into three groups based on the free fraction of phenytoin, i.e. low, <5%; medium, 5%-10%; high, > 10%. The mean age (55.3 +/- 10.9 years) was significantly higher in the high group than in the low (42.7 +/- 21.2 years) and medium (42.8 +/- 16.0 years) groups. The mean creatinine clearance (CLcr) (55.3 +/- 10.9 ml/min) and serum albumin concentration (3.30 +/- 1.25 g/dl) were significantly lower in the high group than the low (88.3 +/- 29.0 ml/min and 4.08 +/- 0.50 g/dl, respectively) and medium (95.0 +/- 32.8 ml/min and 3.95 +/- 0.92 g/dl, respectively) groups. These results suggest that the free phenytoin concentration, rather than the total concentration, is more useful for monitoring antiepileptic effects in patients receiving phenytoin monotherapy. It was also found that the free phenytoin fraction was significantly influenced by aging, CLcr and serum albumin levels.