Fraction Of Phenytoin Research Articles

Effect of low doses of heparin on the plasma binding of phenytoin and prazosin in normal man.

The effect of low doses of heparin on the binding of phenytoin and prazosin to plasma proteins was evaluated in four normal subjects. Heparin activates the hydrolysis of triglycerides in plasma. The ensuing increase in non-esterified fatty acids (NEFA) was more marked in vitro than in vivo and increased the free fraction (FF) of phenytoin and prazosin in plasma. The higher FF caused a change in the plasma to whole blood ratio (P/B ratio) of both drugs. The changes in FF and P/B ratio after heparin were small, but could be of significance in pharmacokinetic studies.

Plasma protein binding of phenytoin in 100 epileptic patients.

The plasma protein binding of phenytoin was investigated in 100 epileptic patients, using equilibrium dialysis at 37 degrees C. The unbound fractions of phenytoin in plasma formed a skewed distribution, with a range of 9.7 to 24.7% and a median value of 12.3%. Most (80%) patients appeared to form one group with free phenytoin fractions from 9.7 to 14.5%, while the remainder formed a group with elevated free fractions (greater than 14.5%). Total and unbound plasma concentrations of phenytoin were strongly correlated (r=0.95, P less than 0.0001). There was a weak correlation between increasing age and the unbound phenytoin fraction (r=0.28, P less than 0.01). The results indicate that measurement of the total phenytoin concentration in plasma should usually provide a reliable index of anticonvulsant effect. However, determination of the unbound phenytoin fraction would be beneficial in the management of those patients in whom this fraction may be elevated, due to interacting drugs or biochemical abnormalities.

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.

Gas-chromatographic assay of free phenytoin in ultrafiltrates of plasma: test of a new filtration apparatus and specimen stability.

I describe a gas-chromatographic procedure involving a nitrogen detector for "free" (or "unbound") phenytoin in the concentration range 0.2-4.0 mg/L in ultrafiltrates of plasma. The CV (n = 10) for the assay was 2.9% for a 0.49 mg/L concentration, 1.5% for 2.02 mg/L. The sensitivity was 0.05 mg/L. The ultrafiltration process ("Ultra-Free" filters) used to separate the free phenytoin fraction from the bound fraction was tested. The mean of and CV for free phenytoin concentrations measured in eight replicate serum filtrates were 0.68 mg/L and 15%; for eight replicate plasma filtrates the values were 0.42 mg/L and 2.9%. Thus serum is not suitable for this procedure. Storage of plasma before filtration under a wide variety of times and temperatures has an insignificant effect upon the filtration process. Some evidence was found of occasional protein "leakage" through the filters.

Interaction between phenytoin and valproic acid: Plasma protein binding and metabolic effects

The effect of sodium valproate (400 mg three times daily) on the disposition kinetics of intravenous phenytoin (250 mg) was investigated in seven normal subjects. After valproate, the free (unbound) fraction of phenytoin in serum rose from 9.6 +/- 0.9% (SD) to 15.6 +/- 1.4% on average (p < 0.001). The effect was associated with an increase in systemic clearance and apparent volume of distribution of total drug. There was a strong positive correlation between percent increment in each of these parameters and percent increment in unbound drug in serum. Free phenytoin concentration in serum and phenytoin concentration in saliva increased during valproate administration. As a result, both the clearance and the apparent volume of distribution of free drug were reduced. There was an increase in the renal excretion of unchanged phenytoin during valproate administration, but the effect was too small to have an appreciable influence on the overall clearance of the drug. There were no consistent changes in the excretion of the major metabolite 5, p-hydroxyphenyl, 5-phenyl, hydantoin (pHPPH), in the urine. These results suggest that valproic acid may have two separate and opposing effects on phenytoin disposition: (1) displacing phenytoin from plasma protein binding sites, thereby enhancing the systemic clearance of total drug, and (2) inhibiting phenytoin metabolism, thereby increasing the concentration of free drug in the serum.

Interactions of valproic acid with phenytoin.

The interaction of phenytoin and valproic acid was studied in four adults. We studied serial changes in total phenytoin concentrations, protein binding, urinary hydroxyphenylphenylhydantoin (HPPH) excretion, and half-life. In all four patients valproic acid caused an increase in the free fraction of phenytoin. Total phenytoin plasma concentrations decreased transiently in three patients and remained low throughout the study period in one patient. HPPH excretion increased transiently and then decreased, corresponding to changes in total phenytoin plasma concentrations. Biologic half-life transiently decreased in three patients (not statistically significant) and subsequently increased significantly in all four patients. The data suggest that valproic acid displaced phenytoin from protein-binding sites in all four patients and subsequently inhibited phenytoin metabolism in three patients.

Decreased plasma protein binding of phenytoin in patients on valproic acid.

1 Plasma protein binding of phenytoin and of valproic acid were measured in ten epileptic patients on this drug combination. Ten other epileptics not on valproic acid served as controls. All patients had normal kidney function. 2 The measured free fraction of phenytoin among the patients on valproic acid ranged from 12.5 to 23.2% and after recalculation to a plasma albumin level of 45 g/l from 12.5 to 20.0 (median 15.4%). This differed significantly (P = 0.002, Mann- Whitney U-test) from the control patients where the normalized values ranged from 9.9 to 13.9% with a median value of 11.8%. 3 The measured free fractions of phenytoin and of valproic acid showed a significant correlation which, however, was due to the quantitative relation between the degree of binding of both these drugs and the concentration of plasma albumin. There was no discernable relation in this material between the free concentration of valproic acid and the free fraction of phenytoin. 4 It is concluded that patients on combined treatment with phenytoin and valproic acid have an unpredictably raised free fraction of phenytoin. This drug interaction therefore can complicate the important plasma level monitoring of phenytoin in epileptic patients unless the free concentration of this drug can be analysed or estimated.

Solubility and ionization characteristics of phenytoin.

The solubility of phenytoin was determined in pH 7.4 and 5.4 phosphate buffers at five temperatures; in hydroalcoholic solutions, 0--4% methanol; and in pH 4.8--8.4 buffer solutions. From the temperature data, the enthalpy and entropy of solution of this nonideal system were calculated and were similar at both pH values. The data obtained from the buffer solutions were used to calculate the apparent dissociation constant, pKa', of phenytoin as 8.06. A GLC method with on-column methylation was used to quantitate phenytoin with 5-(p-methyl-phenyl)-5-phenylhydantoin as an internal standard. The assay uses chloroform of extraction of the drug from aqueous solutions. The ratio of peak heights was adjusted for weights of aqueous and organic layers, and results were calculated in micrograms per gram of sample and mole fraction of phenytoin. Although hydroalcoholic solutions enhanced drug solubility, there is a potentially significant disadvantage in using alcohol for clinical studies.

Plasma protein binding of phenytoin after cholecystectomy and neurosurgical operations.

The purpose of the present study was to investigate whether changes in the protein binding of phenytoin occur postoperatively. Fourteen patients were studied after cholecystectomy and six patients after operations for intracranial aneurysm or meningeoma. It was found that the fraction of phenytoin not bound to plasma proteins rose significantly after operation. The total concentration, on the other hand, decreased, so the concentration of free phenytoin did not change much. Possible explanations to the changes observed are discussed, as well as the clinical usefulness of determination of the protein binding of phenytoin.

Frequency distribution of free warfarin and free phenytoin fraction values in serum of healthy human adults.

The protein binding of racemic warfarin was determined in serum from 57 normal adults (27 men and 30 women). The free fraction of warfarin (total concentration, 0.8 mug/ml) ranged from 0.0050 to 0.0186 and was log-normally distributed. The frequency distribution differs from that in rats in which the serum free fraction values of warfarin are trimodally distributed. The protein binding of phenytoin was determined in serum from 39 of the subjects. The free fraction of phenytoin (total concentration, 15 mug/ml) ranged from 0.111 to 0.155 and was unimodally distributed. There was no apparent correlation between the extent of protein binding of warfarin and phenytoin in individual serum samples. The pronounced intersubject variation in serum free fraction of warfarin observed in consistent with our previous finding that serum protein binding is an important determinant of interindividual differences in the total clearance of warfarin in man. On the other hand, the relatively narrow distribution of free phenytoin fraction values suggests that differences in serum protein binding of phenytoin are not an important cause of the prounounced interindividual differences in the total clearance of phenytoin by subjects with normal renal function.

Two-fold interindividual variation in plasma protein binding of phenytoin in patients with epilepsy.

Plasma protein binding of phenytoin (diphenylhydantoin) in 63 epileptic patients was investigated with an ultrafiltration technique at room temperature using 14C-labelled phenytoin. A strong correlation was found between the total and the unbound drug concentration (r = 0.97, p less than 0.001). The unbound phenytoin fraction was 7.1 +/- 1.0% with a range of 4.9 to 10.2%. This variation is considerably less than that reported recently by different authors. Individual phenytoin binding was reproducible when the determination was repeated several weeks later. Salivary phenytoin concentrations in 33 epileptic patients were significantly correlated to the unbound (r = 0.83) and total concentrations (r = 0.82) of phenytoin in plasma. This study confirms that the clinical practice of monitoring total phenytoin plasma concentrations is sufficient, since the unbound phenytoin fraction has only a 2-fold interindividual variation in epileptic patients, provided that they do not suffer from renal or hepatic disease.