Free Phenytoin Concentrations Research Articles

Monitoring Phenytoin Therapy Using Citric Acid-Stimulated Saliva in Infants and Children

Two factors have limited the use of saliva in monitoring phenytoin therapy: availability of adequate volume of clear saliva and lack of a sensitive phenytoin assay. The applicability of citric acid-stimulated saliva and of a sensitive analytical assay (fluorescence polarization immunoassay, "TDx" Abbott) was evaluated in this study. Phenytoin was measured in paired plasma-saliva specimens from epileptic children during the long-term or the initial phase of phenytoin therapy. Analysis was carried out in plasma and in the clear supernatant of saliva (following centrifugation). Pooled-estimate SD of the analytical assay variability was 0.175 micrograms/ml for plasma total phenytoin, 0.063 for plasma free phenytoin, and 0.009 for saliva phenytoin. Recovery measurements of phenytoin spiked into saliva samples gave a coefficient of variation of less than 5%. Correlations between saliva and total plasma phenytoin levels and between saliva and free plasma phenytoin levels were strong and highly significant (r = 0.99, p less than 0.01). The percentage of temporal fluctuation (as determined by saliva phenytoin profiles) during 10-24 h ranged between 25.5-177 (mean, 58.3; SD, 47.3). Ratios of plasma total phenytoin/saliva phenytoin and of plasma free phenytoin/saliva phenytoin levels were 9.54 +/- 1.05 and 0.71 +/- 0.09, respectively. Dialysis experiments showed no binding of phenytoin to saliva supernatant. The greater saliva phenytoin concentrations as compared to plasma free phenytoin concentrations could be due to active transport of phenytoin from plasma to saliva. Measurement of phenytoin in citric acid-stimulated saliva by fluorescent polarization immunoassay is a reliable, noninvasive, and convenient method for monitoring phenytoin therapy in children.

Severe Phenytoin Intoxication as a Result of Altered Protein Binding in Aids

Alterations in plasma protein binding may alter patient response to pharmaceutical agents because only free drug is considered to be pharmacologically active. Such alterations appear to be more significant with highly bound agents such as phenytoin. Traditionally, most drug assays monitor total drug concentrations and do not quantitate free drug. When binding alterations are present, total drug concentrations may mislead clinicians in evaluating patient response. We describe a case in which profound hypoalbuminemia (0.2 g/dL), associated with focal segmental glomerulosclerosis, produced toxic free phenytoin concentrations (4.9 micrograms/mL) in an HIV-positive 25-year-old black woman. At such a high serum concentration of free phenytoin, the patient exhibited seizure-like effects. Renal abnormalities and hypoalbuminemia associated with acquired immunodeficiency syndrome (AIDS) may place patients at risk for elevated free fractions of phenytoin and subsequent toxicity.

Protein Binding of Antiepileptic Drugs During Pregnancy, Labor, and Puerperium

Twenty-four epileptic women were followed-up during late pregnancy, labor, and early puerperium in order to detect possible alterations in serum protein binding of antiepileptic drugs (AEDs). The total and free concentrations of carbamazepine (CBZ), phenytoin (PHT), and valproate (VPA) in maternal serum were measured. In addition, the concentrations of albumin, alpha 1-acid glycoprotein (AGP), and free fatty acids (FFA) were also measured. Total AED concentrations during labor were influenced by changes in drug dosages; total PHT increased during the first puerperal weeks. During labor the free fraction of CBZ remained stable, whereas PHT and particularly VPA free fractions increased. This phenomenon was parallel to the increase in FFA concentration; FFA concentrations decreased again during the first days postpartum. Albumin and AGP concentrations were low during pregnancy and labor, and increased after delivery. The total umbilical CBZ and PHT concentrations were not significantly different from maternal concentrations. The total VPA concentration in umbilical serum was significantly higher than that in maternal serum. The free fraction of CBZ was higher and that of PHT and VPA lower in umbilical than in maternal serum at delivery. Umbilical cord serum had a higher albumin but a lower AGP and FFA concentration than maternal serum. The changes in PHT and particularly VPA free fraction associated with changes in FFA concentration should be considered when assessing the total concentration of these drugs in maternal and umbilical serum.

Effect of Acute and Chronic Terfenadine on Free and Total Serum Phenytoin Concentrations in Epileptic Patients

Terfenadine is a unique H1-receptor antagonist devoid of adverse central nervous system (CNS) effects. Terfenadine is highly protein bound and has been shown to stimulate hepatic microsomal enzymes, making an interaction with phenytoin (PHT) possible. After assuring constant PHT levels for 7 days, 12 epileptic patients were studied with PHT alone, after a single daily dose of terfenadine (60 mg twice daily), and again after 2 weeks of chronic terfenadine therapy. Samples for PHT were drawn over 24 h, and urine was collected for determination of the PHT metabolite 5-(p-hydroxyphenyl)-5-phenylhydantoin (HPPH). The pharmacokinetic parameters calculated for total and free PHT and HPPH were area under the time concentration curve (AUC), the maximum serum concentration (Cmax), the minimum serum concentration (Cmin), and the percentage of fluctuation between maximum and minimum concentrations. A factor analysis was used to clarify interrelationships. Five dependent variables were found to represent the data. These were assessed by multi-variate analysis of variance (MANOVA); p less than 0.05 was considered significant. No significant differences were observed for any of the parameters for the acute or chronic effects of terfenadine. We conclude that terfenadine does not interfere with PHT.

Derivation and Evaluation of an Equation for Prediction of Free Phenytoin Concentration in Patients Co-Medicated with Valproic Acid

The relationship between free phenytoin fraction (F-PHT-F) and valproic acid (VPA) was studied under two conditions: (a) in serum samples from 43 institutionalized patients (212 serial data points) at plateau steady state, and (b) in plasma samples from 50 outpatients coming for regular visits to our clinic throughout the day. Results for both groups led to identical linear regression equations relating F-PHT-F to VPA: F-PHT-F = 0.095 + 0.001 (VPA). Nevertheless, as expected, the resulting equation gave an unreliable prediction of F-PHT-F due to a variable physiological matrix. Substituting the parameter F-PHT-F by its equivalent (F-PHT/PHT) gave the equation F-PHT = [0.095 + 0.001 (VPA)] PHT. The predictive power of this equation was evaluated by the comparison of obtained and predicted F-PHT concentrations. For the combined patient group (n = 93), an excellent agreement (r = 0.972 and F = 999.9; p less than 0.001) was obtained. Although the empirically derived constants of the equation are not unique and may vary depending on the conditions of different methodologies, the fundamental relationship has been established and can be used to reliably predict F-PHT concentration from plasma VPA and PHT concentrations.

Accuracy of two equations in determining normalized phenytoin concentrations.

The accuracy of two equations in normalizing total phenytoin concentrations in the presence of renal failure or hypoalbuminemia was evaluated in 11 renal failure and 23 hypoalbuminemic patients. Blood samples were obtained from hospitalized patients receiving phenytoin and were assayed for free and total phenytoin concentrations. Estimated normalized phenytoin concentrations based on free drug concentration were compared statistically with normalized concentrations calculated from the two equations via Student's t-test. The equation for normalizing phenytoin concentrations in hypoalbuminemic patients significantly underpredicted normalized phenytoin concentrations 15.7 +/- 8.5 versus 19.9 +/- 12.1 mg/L (p less than 0.001). In patients with renal failure, the mean phenytoin concentration from the respective equations and that based on free concentration were 14.1 +/- 6.2 and 14.0 +/- 7.9 mg/L, respectively. However, in 5 of 11 renal failure patients the equation resulted in over- or underprediction by at least 25 percent. Neither equation should be used clinically to normalize phenytoin concentrations in these patient populations.

Fiber optic immunochemical sensor for continuous, reversible measurement of phenytoin.

This fiber optic sensor, based on a homogeneous fluorescence energy-transfer immunoassay, operates in a continuous, reversible manner to quantify the anticonvulsant drug phenytoin. B-Phycoerythrin-phenytoin and Texas Red-labeled antibody to phenytoin were sealed inside a short length of cellulose dialysis tubing, which was cemented to the distal end of an optical fiber. When the sensor was alternately placed into solutions with various concentrations of free phenytoin, the drug crossed the dialysis membrane and displaced a fraction of the B-phycoerythrin-phenytoin from the antibody. The resulting change in fluorescence signal was measured with a fiber optic fluorometer. A typical competitive-binding calibration curve was seen between 5 and 500 mumol of phenytoin per liter. Equilibrium response time ranged from 5 to 30 min for different sensors. Replicate equilibrium measurements with one sensor, alternated eight times between two solutions of phenytoin, gave a CV of 2.1% (n = 16) at 0 mumol/L and 2.4% (n = 13) at 100 mumol/L. A reversible immunochemical sensor can be made that has a response time suitable for continuous concentration measurements.

Protein binding of phenytoin and lidocaine in pediatric patients with type I diabetes mellitus.

We evaluated serum protein binding of phenytoin and lidocaine, the extent of albumin and hemoglobin glycation, and alpha-1-acid glycoprotein concentrations in 47 children and adolescents (9-17 years) with type I diabetes mellitus (DM). Serum was incubated with phenytoin (n = 47) and lidocaine (n = 32) to yield total concentrations of 19.4 +/- 1.34 and 4.6 +/- 0.5 mg/l, respectively. A serum ultrafiltrate was prepared from an aliquot of each sample by membrane centrifugation. Total and free concentrations of phenytoin (free fraction 8.3 +/- 1.0%) and lidocaine (free fraction 25.8 +/- 11.6%) were determined using a fluorescence polarization immunoassay technique. A linear relationship (r = 0.63, p = 0.0001, y = 40.1 + [-0.2x]) was also found between the alpha-1-acid glycoprotein concentration (68.5 +/- 20.9 mg%, range 47.2-134.7 mg%) and lidocaine-free fraction (n = 32). No relationship existed between the extent of glycated albumin and the lidocaine-free fraction, in contrast to the linear correlation (r = 0.49, p = 0.0005, y = 3.7 + 0.4x) found between the extent of glycated albumin and the free fraction of phenytoin in serum (n = 47). A similar correlation (r = 0.49, p = 0.014, y = 3.7 + 0.3x) was found between glycated albumin (%) and the phenytoin-free fraction (3.9 +/- 0.9%) when examined in a separate (n = 17) set of patient samples spiked to contain a total serum phenytoin concentration of 32.6 +/- 1.4 (range 29.4-35.7) mg/l. Our data demonstrate a predictable increase in the free fraction of phenytoin as the extent of albumin glycation increases in pediatric patients with type I DM who are in poor glycemic control. This relationship was not found for lidocaine, a representative basic compound. These findings suggest that glycation of albumin alters the binding of phenytoin in children and adolescents with type I DM.

Comparison of the Amicon Centrifree Micropartition System with the Sartorius SM 13249E Centrisart I Device to Determine Protein Free Phenytoin Concentrations

The Amicon Centrifree micropartition system was compared with the Sartorius SM 13249E Centrisart I ultrafiltration device to determine non-protein-bound phenytoin levels. Phenytoin was measured by an enzyme multiplied immunoassay technique (EMIT), adapted to a Cobas Bio centrifugal analyzer, and by a radiometric assay. From pooled human plasma with total phenytoin concentrations of 19.1, 44.0, and 95.1 microM, the following percentages of free phenytoin levels in the ultrafiltrate were obtained with Amicon: 10.6 +/- 1.3, 10.5 +/- 0.4, and 8.8 +/- 0.2. The corresponding figures with Sartorius were 5.4 +/- 0.8, 5.4 +/- 0.5, and 4.9 +/- 0.4, respectively (n = 5). Similar results were obtained with 3H-labeled phenytoin, ruling out the possibility that the discrepancies were due to interference with the EMIT assay. Phenytoin binding to the Amicon membrane was less than or equal to 5% of the free phenytoin concentration, whereas the Sartorius membranes absorbed 80%. With the Amicon device, the free phenytoin concentration remained constant as serial ultrafiltrate volumes from the same sample were collected, whereas with Sartorius, free phenytoin increased from 2.5 +/- 1.8% of the total concentration in the first fraction to 13.4 +/- 1.6% in the last collection (n = 6). The results indicate that the Amicon Centrifree filter is an excellent tool to obtain protein-free phenytoin ultrafiltrates, whereas the Sartorius Centrisart I device is not suited for this purpose because of excessive adsorption of phenytoin.

Phenytoin pharmacokinetics in therapeutic plasmapheresis.

Total and free serum phenytoin concentrations and net phenytoin removal were examined in two stable patients (henceforth referred to as patient D.K. and patient C.L.) receiving therapeutic plasmapheresis for myasthenia gravis at Mercy Hospital. Similar serum determinations were obtained on the days prior to and after plasmapheresis in order to determine if a single patient plasma volume exchange replaced isovolumetrically with a 4.0 g% albumin solution would significantly alter serum concentrations of phenytoin.

Time-dependent interaction between phenytoin and valproic acid.

The diurnal variation in total and free plasma phenytoin (PHT) concentration at steady state was examined in eight epileptic patients receiving combination therapy with tid valproic acid (VPA) as sodium salt. Eight patients treated with PHT, but not with VPA, were studied for comparison purposes. In the absence of VPA coadministration, total and free PHT concentrations did not change significantly during the day and showed only minor intrapatient fluctuations (14 and 13%, respectively). In patients receiving VPA, the mean total PHT did not change significantly, whereas the free PHT increased during the day (p less than 0.05). The fluctuations in total and free PHT in these patients were 16 and 17% on average. In the presence of VPA, the free PHT fraction was higher than in controls (13.9 +/- 2.3% versus 8.3 +/- 1.9%; p less than 0.01) and fluctuated to a greater extent (29 versus 14% in controls; p less than 0.01), mainly as a result of combined opposite swings in both total and free concentration. The diurnal changes in free PHT concentration and fraction correlated positively with the changes in plasma VPA. An inverse relationship between total PHT concentration and plasma VPA was found in some patients. These data demonstrate that the displacement interaction between PHT and VPA is subject to diurnal variation, probably as a result of the fluctuation in plasma VPA. The implications of these findings are discussed.

Monitoring free plasma concentrations of phenytoin.

British Journal of Clinical PharmacologyVolume 18, Issue 6 p. 971-973 Free Access Monitoring free plasma concentrations of phenytoin. GM Peterson, GM PetersonSearch for more papers by this authorS McLean, S McLeanSearch for more papers by this author GM Peterson, GM PetersonSearch for more papers by this authorS McLean, S McLeanSearch for more papers by this author First published: December 1984 https://doi.org/10.1111/j.1365-2125.1984.tb02576.xAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. References Barth, N., Alvan, G., Borga, O. & Sjoqvist, F. (1976). Two-fold interindividual variation in plasma protein binding of phenytoin in patients with epilepsy. Clin. Pharmacokin., 1, 444– 452. DeMonaco, H. J. & Lawless, L. M. (1983). Variability of phenytoin protein binding in epileptic patients. Arch. Neurol., 40, 481– 483. Perucca, E. (1980). Plasma protein binding of phenytoin in health and disease: relevance to therapeutic drug monitoring. Ther. Drug Monit., 2, 331– 344. Peterson, G. M., McLean, S., Aldous, S., Von Witt, R. J. & Millingen, K. S. (1982). Plasma protein binding of phenytoin in 100 epileptic patients. Br. J. clin. Pharmac., 14, 298– 300. Rimmer, E. M., Buss, D. C., Routledge, P. A. & Richens, A. (1984). Should we routinely measure free plasma phenytoin concentration Br. J. clin. Pharmac., 17, 99– 102. Walther, H. & Meyer, F. P. (1979). Interindividual differences of protein binding in man. Int. J. clin. Pharmac. Biopharm., 17, 392– 395. Yacobi, A., Lampman, T. & Levy, G. (1977). Frequency distribution of free warfarin and free phenytoin fraction values in serum of healthy human adults. Clin. Pharmac. Ther., 21, 283– 286. Volume18, Issue6December 1984Pages 971-973 ReferencesRelatedInformation

Effect of valproate on free plasma phenytoin concentrations.

The plasma protein binding of phenytoin was studied in nine epileptic patients before and during addition of sodium valproate to the drug therapy. The free phenytoin fraction in plasma was significantly greater during sodium valproate treatment. The mean free fraction rose from 0.135 +/- 0.019 (s.d.) to 0.182 +/- 0.030. Total plasma phenytoin concentration fell significantly from a range of 4.3-26.2 micrograms/ml to 3.4-19.8 micrograms/ml during sodium valproate treatment. Neither the free plasma concentration nor the saliva concentration of phenytoin was significantly altered by sodium valproate. No significant correlation was found between plasma valproic acid concentrations and the change in phenytoin binding. We conclude that valproic acid displaces phenytoin from plasma protein binding sites but does not inhibit its metabolism.

Effects of penicillins on binding of phenytoin to plasma proteins in vitro and in vivo.

Effects of penicillins on the binding of phenytoin to plasma proteins were examined in vitro and in vivo. The results from in vitro studies showed that the penicillins including oxacillin and dicloxacillin were effective in displacing phenytoin from its binding sites. In vivo, the total phenytoin concentration in serum decreased during penicillin administration, while the free phenytoin concentration increased. As a result, penicillins caused a significant increase in the apparent volume of distribution and in the total body clearance of phenytoin. These results can be explained on the basis of the displacement of phenytoin from its plasma protein binding site by penicillins.

Should we routinely measure free plasma phenytoin concentration?

The plasma protein binding of phenytoin was investigated in 56 epileptic patients attending the outpatient clinic. The free phenytoin fraction was measured by equilibrium dialysis at 37 degrees C and the total concentration by a homogenous enzyme immunoassay technique. The free fraction ranged from 0.123 to 0.177 (median 0.144, mean +/- s.d. = 0.145 +/- 0.12). Distribution was consistent with normality. Four of the patients were also taking sodium valproate. The median free fraction of phenytoin in these patients was 0.174, 21% higher than that of the total group (P less than 0.05). The total concentration of phenytoin varied from 0.3 to 29.4 micrograms/ml (median 12 micrograms/ml, mean +/- s.d. = 13.31 +/- 6.13 micrograms/ml) and the free fraction was not related to the total drug concentration. There was a highly significant relationship between free phenytoin concentration and total phenytoin concentration (r = 0.986, P less than 0.001). There appears to be very little variability in protein binding of phenytoin in epileptic patients and thus total plasma phenytoin concentration closely reflects the free (unbound) drug concentration. Routine estimation of free plasma phenytoin concentration is therefore unnecessary and should be reserved for those patients where alteration in binding is likely, e.g. renal or hepatic disease or where adverse effects occur at unexpectedly low total phenytoin concentrations.

Single Feedback Bayesian Forecasts with Free and Total Phenytoin Concentrations

Single Feedback Bayesian Forecasts with Free and Total Phenytoin Concentrations

Variability of phenytoin protein binding in epileptic patients.

The variability of protein binding of phenytoin in epileptic patients was evaluated using a rapid ultrafiltration system. Thirty-one pairs of total and free (unbound) phenytoin plasma concentrations were reviewed. Filtered plasma for free phenytoin determination was obtained using an ultrafiltration membrane system. All samples were analyzed by an enzyme-multiplied immunoassay technique. A good correlation was obtained for total phenytoin concentration v free phenytoin concentration. Comparison of obtained and predicted free phenytoin values differed significantly. Of nine patients demonstrating toxic effects, six had total phenytoin concentrations within the accepted therapeutic range with free concentrations above the therapeutic range. The results of this study indicate a wide variation of phenytoin protein binding in chronic epileptic patients, implying that total phenytoin concentration must be interpreted cautiously, and use of free phenytoin concentration appears to be a more appropriate guide to therapy. The availability of a rapid method of ultrafiltration makes free phenytoin plasma levels readily available to the clinician.

Urine and plasma free phenytoin concentration correlation.

In an attempt to find a simple, noninvasive method for estimating the plasma free concentration (CPf) of phenytoin (PHT), the relationship between urine PHT concentration (Cu) and CPf was studied in 40 epileptic patients who were 6 to 50 yr old. Cu was determined by gas-liquid chromatography and CPf by equilibrium dialysis at 37 degrees. Cu was generally greater than CPf, but correlation between the two was strong (r2 = 0.876) and in the same range as previously published results for saliva and CPf. Correlation of Cu with CPf was not influenced by patient age, urine pH, or urine flow rate. In 24 patients timed urine collections were obtained and calculated renal clearances of PHT were shown to increase in proportion to urine flow rates. Although it is not known if renal impairment and albuminuria will alter the relationship between Cu and CPf it appears that Cu may be useful in estimating the concentration of pharmacologically active PHT in plasma.

Serum protein binding and free concentration of phenytoin and phenobarbitone in pregnancy.

1 The effect of pregnancy on the binding of phenytoin and phenobarbitone to serum proteins was studied in normal women and in drug treated epileptic women. 2 The binding of both drugs was reduced during pregnancy. The reduction correlated positively with the gestational age and negatively with the serum albumin concentration. 3 In spite of the increase in unbound fraction, both the total and free serum concentrations of phenytoin were decreased in late pregnancy.

Effect of short- and long-term alcohol use on phenytoin kinetics in chronic alcoholics.

Phenytoin kinetics during long-term alcohol use and withdrawal were studied in 11 male alcoholics with a history of withdrawal seizures and no evidence of chronic liver disease. Ethanol, 20% v/v, was given for 6 days after admission to maintain the blood alcohol level between 500 and 800 mg/l and phenytoin suspension, 150 mg, was given orally or intravenously (on three occasions) every 12 hr for 20 days. The mean (+/- SD) total phenytoin clearance in 9 of 11 subjects was 0.023 +/- 0.006 l/kg/hr during the alcohol ingestion period. Clearance rose to 0.033 +/- 0.013 l/kg/hr (P less than 0.05) during alcohol withdrawal. Total steady-state concentration after 3 wk ranged from 3.4 to 29.9 mg/l, while the weight-corrected dose range was only 3.7 to 5.5 mg/kg/day. Inter- and intra-subject variation in bioavailability was small (0.93 to 1.03). Phenytoin free fractions ranged from 9.09% to 17.75% and changes in total and free phenytoin concentration correlated (r2 = 0.92, P less than 0.001). The data are compatible with the hypothesis that increased phenytoin clearance during alcohol withdrawal is due to the increased metabolic rate of the drug secondary to enzyme induction by ethanol, which becomes unmasked on cessation of drinking. In most alcoholics standard-dose phenytoin (300 mg/l) will induce lower than usual plasma concentrations.