Free Phenytoin Research Articles

Clinical pharmacokinetics and pharmacodynamics of bromfenac.

Bromfenac is a nonsteroidal anti-inflammatory drug whose peak plasma concentration is reached 0.5 hours after oral administration. Bromfenac binds extensively to plasma albumin. The area under the plasma concentration-time curve is linearly proportional to the dose for oral doses up to 150 mg. The relationship between the total plasma and analgesic effect has been established. Only small amounts of bromfenac are eliminated unchanged, with the remaining drug being biotransformed into glucuronide metabolites which are excreted in urine and bile. Rapid elimination occurs in healthy individuals (half-life 0.5 to 4.0 h). Renal disease, hepatic disease and aging alter the disposition kinetics of bromfenac, and dosage adjustment may be advisable. Bromfenac modestly decreases free phenytoin concentrations. Bromfenac can cause idiosyncratic hepatic toxicity and has been withdrawn by its manufacturer pending further investigation of these case reports.

Standards of laboratory practice: antiepileptic drug monitoring

Discussion and development of standards for appropriate monitoring led to the following key recommendations for ordering, sampling, and analyzing antiepileptic drugs: Monitoring should usually be done on trough specimens after steady-state has been reached and always with an appropriate medical indication; non-steady-state concentrations may be indicated in selected situations. Monitoring of free phenytoin and free valproic acid is indicated in specific situations and should be done in serum. The metabolite of primidone, phenobarbital, should be measured concurrently with parent drug, but the active metabolite of carbamazepine does not need to be monitored unless the patient is exhibiting an unusual toxic response that cannot be otherwise explained. Assays used for antiepileptic drug monitoring should display a long-term CV of <10% and preferably <5%. Subtherapeutic and supratherapeutic drug concentrations should be investigated on a regular basis as part of a quality assurance process.

Elevated free phenytoin and free valproic acid concentrations in sera of patients infected with human immunodeficiency virus.

Seizures are common in patients infected with human immunodeficiency virus (HIV). Phenytoin and valproic acid are common anticonvulsants, and both drugs are strongly bound to serum albumin. Because patients infected with HIV are often on polytherapy, using homeopathic medicines, and may also have hypoalbuminemia, elevated free drug concentrations may occur in these patients. The authors prepared one serum pool from patients infected with HIV but receiving no bactrim and the other pool from HIV patients receiving bactrim. They supplemented both HIV pools and normal pool (diluted with 0.9% saline to mimic albumin concentration of HIV pools) with a known concentration of phenytoin or valproic acid. After incubation at 37 degrees C for 3 hours, they measured free phenytoin and free valproic acid concentrations in the protein free ultrafiltrates using fluorescence polarization immunoassays. The total drug concentrations in original sera were measured by microparticle enzyme immunoassays. None of the patients had any significant liver or renal disease. The aliquots of HIV pools and normal pool were supplemented with the same concentration of phenytoin or valproic acid. The concentration of free phenytoin and free valproic acid were significantly elevated in patients with HIV (mean = 2.52, SD = 0.11 micrograms/ml for phenytoin; mean = 41.5, SD = 1.5 micrograms/ml for valproate) compared to controls (mean = 1.50, SD = 0.0 7 micrograms/ml for phenytoin; mean = 19.9, SD = 0.5 micrograms/ml for valproate). The concentrations of both free phenytoin and valproic acid were further elevated in patients prepared in the HIV pool who were receiving bactrim (mean = 2.81, SD = 0.09 micrograms/ml for phenytoin; mean = 44.0, SD = 1.1 micrograms/ml for valproate), but when normal serum pool was supplemented with 4.4 mg/dl of bactrim (concentration of bactrim in HIV pool) and supplemented with the same concentration of phenytoin or valproic acid, the observed free concentrations were much lower (mean = 1.65, SD = 0.05 micrograms/ml for phenytoin; mean = 26.1, SD = 1.4 micrograms/ml for valproate). This indicates that hypoalbuminemia and bactrim concentrations do not account for the observed free drug concentrations in patients with HIV. The authors also observed elevated free phenytoin and valproic acid in sera from three individual patients with AIDS compared to normals (normal serum diluted with 0.9% saline to mimic the albumin concentration of serum collected from a patient with HIV and then both specimens supplemented with the same concentration of phenytoin or valproic acid.

Kinetics and proposed mechanism of the reaction of an immunoinhibition, particle-enhanced immunoassay

We report kinetic studies on the reaction of a latex agglutination immunoassay used to quantify phenytoin in serum. In this assay, polystyrene particles with a covalently attached analog of phenytoin react with an antiphenytoin monoclonal antibody to form light-scattering aggregates, with the rate of this reaction being decreased by addition of phenytoin from sample. In the absence of free (sample) phenytoin, this reaction did not exhibit a maximum rate of agglutination in the presence of excess antibody, i.e., an equivalence point. Furthermore, agglutination was inhibitable by free phenytoin even when the latter was added after agglutination of particles with antibody had begun. Most significantly, the immunoagglutination proceeded in an identical fashion with monovalent F(ab) fragment. These data are consistent with low-affinity immunospecific particle-antibody complexation, which then induces colloidal aggregation, without requiring immunospecific bridging by antibody molecules. The described mechanism is not generalizable to all latex agglutination immunoassays, although disturbance of colloidal stability may be a component in most assays.

Elevated free fatty acid concentrations in lipemic sera reduce protein binding of valproic acid significantly more than phenytoin.

Higher concentrations of free valproic acid and phenytoin have been reported in patients with uremia and liver disease. Free fatty acids also displace valproic acid and phenytoin. This is a study of the magnitude of displacement of valproic acid and phenytoin from protein binding by free fatty acid in lipemic sera. Higher concentrations of free fatty acids in lipemic sera affected protein binding of valproic acid significantly more than that of phenytoin. Supplementing normal sera with free fatty acids also increased the free concentrations of both valproic acid and phenytoin as expected, but the observed effect was several times higher in magnitude with valproic acid. There was an increased free fraction of valproic acid in patients who received valproic acid and had hypertriglyceridemia. In a patient with uremia, there was also a significant increase in free valproic acid concentration after routine hemodialysis caused by an increase in free fatty acid concentration secondary to hemodialysis. Increased protein binding of valproic acid in sera was observed after treatment with activated charcoal because charcoal can remove free fatty acid. Because higher free fatty acid concentration significantly affects protein binding of valproic acid, careful monitoring of free valproic acid in patients with lipid disorder may be beneficial.

Elevated Free Fatty Acid Concentrations in Lipemic Sera Reduce Protein Binding of Valproic Acid Significantly More Than Phenytoin

AbstractHigher concentrations of free valproic acid and phenytoin have been reported in patients with uremia and liver disease. Free fatty acids also displace valproic acid and phenytoin. This is a study of the magnitude of displacement of valproic acid and phenytoin from protein binding by free fatty acid in lipemic sera. Higher concentrations of free fatty acids in lipemic sera affected protein binding of valproic acid significantly more than that of phenytoin. Supplementing normal sera with free fatty acids also increased the free concentrations of both valproic acid and phenytoin as expected, but the observed effect was several times higher in magnitude with valproic acid. There was an increased free fraction of valproic acid in patients who received valproic acid and had hypertriglyceridemia. In a patient with uremia, there was also a significant increase in free valproic acid concentration after routine hemodialysis caused by an increase in free fatty acid concentration secondary to hemodialysis. Increased protein binding of valproic acid in sera was observed after treatment with activated charcoal because charcoal can remove free fatty acid. Because higher free fatty acid concentration significantly affects protein binding of valproic acid, careful monitoring of free valproic acid in patients with lipid disorder may be beneficial.

Metabolite and matrix interference in phenytoin immunoassays

The major phenytoin metabolite, 5-(p-hydroxyphenyl)-5-phenylhydantoin glucuronide (HPPG), was primarily responsible for the positive bias noted when uremic specimens were assayed with the Abbott TDx Free Phenytoin fluorescence polarization immunoassay. The amount of bias depended on both HPPG and phenytoin concentration, increasing with increases in either concentration. The new Abbott TDx II assays for phenytoin and free phenytoin exhibited no significant cross-reactivity with HPPG and no bias in clinical specimens from uremic patients. Both assays correlated well with Emit-based assays (r >0.98), had CVs of <3.5%, and had minimum detection limits of <0.1 mg/L. Calibration curves were stable for at least 6 weeks. All of the TDx assays cross-reacted with another metabolite, 5-(p-hydroxyphenyl)-5-phenylhydantoin (HPPH), but expected HPPH concentrations are too low to cause a clinically significant bias. The Emit-based phenytoin assay exhibited a significant matrix effect when calibrators were prepared in defibrinated plasma processed to resemble serum.

Pharmacology and pharmacokinetics of fosphenytoin

Fosphenytoin sodium, a phosphate ester prodrug of phenytoin, was developed as a replacement for parenteral phenytoin sodium. Unlike phenytoin, fosphenytoin is freely soluble in aqueous solutions, including standard i.v. solutions, and is rapidly absorbed by the i.m. route. Fosphenytoin is metabolized (conversion half-life of 8 to 15 min) to phenytoin by endogenous phosphatases. Therapeutic free (unbound) and total plasma phenytoin concentrations are consistently attained after i.m. or i.v. administration of fosphenytoin loading doses. Fosphenytoin has fewer local adverse effects (e.g., pain, burning, and itching at the injection site) after i.m. or i.v. administration than parenteral phenytoin. Systemic effects related to the CNS are similar for both preparations, but transient paresthesias are more common with fosphenytoin.

In vitro displacement of phenytoin from protein binding by nonsteroidal antiinflammatory drugs tolmetin, ibuprofen, and naproxen in normal and uremic sera.

Displacement of phenytoin (90% bound to albumin) by other highly albumin-bound drugs like salicylate has been well documented. Other widely used nonsteroidal antiinflammatory drugs like tolmetin, ibuprofen, and naproxen are also strongly bound to albumin and can potentially displace phenytoin. However, phenytoin-ibuprofen interaction has been poorly studied in the past, and interaction of phenytoin with tolmetin or naproxen has not been studied before. For normal serum pool (albumin 3.7 g/dl), we observed significant increases in free phenytoin concentrations only with antiinflammatory drug concentrations at the upper end of therapeutic or above therapeutic concentrations. However, for the uremic pool (albumin 2.9 g/dl), displacement of phenytoin was significant even at the lower end of therapeutic concentrations of those antiinflammatory drugs. Of the three antiinflammatory drugs we studied, ibuprofen caused the highest displacement of phenytoin.

Diminished protein binding capacity of uremic sera for valproate following hemodialysis: role of free fatty acids and uremic compounds.

We previously reported higher free phenytoin concentration in predialysis serum compared to postdialysis serum. In our present study with valproic acid, we observed an opposite effect, where free valproic acid concentration was higher in postdialysis serum. The increased free valproic acid concentrations in postdialysis sera (15.4-77.4% higher in postdialysis sera) were probably related to a 2-to 4-fold increase in free fatty acid concentrations in postdialysis sera. On the other hand, when an aliquot of predialysis serum pool was subjected to in vitro equilibrium dialysis and then both undialyzed and dialyzed serum pools were supplemented with valproic acid and incubated, the concentration of valproic acid in dialyzed serum was significantly lower than the undialyzed serum. This is probably related to removal of some uremic compounds during in vitro dialysis. Treatment of both pre- and postdialysis sera with activated charcoal corrected the binding deficiency of valproic acid due to removal of both uremic compounds and free fatty acids.

Phenytoin as a Risk Factor in Gingival Hyperplasia

Fifty-four out-patients with epilepsy who had been taking phenytoin for more than one year were examined for gingival hyperplasia. Approximately 76% of patients showed either mild or no gingival hyperplasia. Lesion severity was then compared statistically to phenytoin dosage and drug concentrations as well as to other clinical and laboratory parameters. There was a tendency for gingival hypertrophy to be associated with both increasing dosage of phenytoin per unit of body weight and the duration of phenytoin administration. All patients followed had a statistically significant progressive trend to increasing gingival hyperplasia with higher total and free phenytoin concentration.

Pharmacokinetics of phenytoin in children with acute neurotrauma.

To determine the pharmacokinetics of intravenous phenytoin in critically ill infants and children with acute neurologic injury. A prospective, descriptive study. A pediatric intensive care unit. Sixteen children, 0.5 to 16 yrs of age (mean 7.6), with various types of acute neurologic injuries, receiving intravenous phenytoin therapy. Blood samples were collected to measure total and free phenytoin concentrations in plasma. A 24-hr urine collection was made to determine the concentrations of the major metabolite of phenytoin. In 12 children who survived the acute illness, a lower-than-predicted Michaelis-Menten constant (Km) and higher-than-predicted maximum rate of metabolism (Vmax) were observed. Initial free phenytoin fractions ranged between 0.08 and 0.15. In the eight patients who had additional free fractions measured, six patients demonstrated an increase (9.1% to 34% increase) in free fraction, while two patients demonstrated a decrease (1.8% and 19.8% decrease) in free fraction. The ratio of amount of phenytoin to phenytoin plus 5-(p-hydroxyphenyl)-5-phenylhydantoin excreted in the urine in a 24-hr urine collection demonstrated a wide inter-patient variability. There was no correlation in the difference between the predicted and calculated Km and Vmax values and Glasgow Coma Score, circulating albumin concentration, or concomitant medications. Based on the average Km and Vmax values of the children enrolled in our study, it appears that children with neurologic injury between the ages of 0.5 and 9 yrs may require dosages of at least 8 to 10 mg/kg/day, and children aged 10 to 16 yrs may require 6 to 8 mg/kg/day to attain therapeutic phenytoin concentrations.

Phenytoin free fraction determination: comparison of an improved direct serum injection high-performance liquid chromatographic method to ultrafiltration coupled with fluorescence polarization immunoassay

Recent developments in restricted-access media (RAM) liquid chromatography make the simultaneous determination of total and free phenytoin concentrations possible by direct injection of drug-containing serum samples. A comparison of phenytoin free fraction determination by ultrafiltration coupled with fluorescence polarization immunoassay (TDX) to an improved direct injection RAM-HPLC method is presented. Our improved method differs from those previously reported with regard to column type, mobile-phase composition, and column temperature. Replicate samples analyzed by each method yielded similar values for serum phenytoin free fraction.

Diamminebis(5,5-diphenylhydantoinato)zinc(II) Hemihydrate

In the title compound, diamminebis(5,5-diphenyl-2,4-imidazolidinedionato-N 3 )zinc(II) hemihydrate, [Zn(C 15 H 11 N 2 O 2 ) 2 (NH 3 ) 2 ].1/2H 2 O the zinc(II) ion coordinates with two deprotonated hydantoin and two ammine ligands in a 4-N tetrahedral arrangement. The complex has C s symmetry with the Zn cation and the two ammonia N atoms located in the symmetry plane. Geometric changes in the hydantoin ring compared with free phenytoin show the effect of the Zn coordination. The arrangement of phenyl rings in this complex is different from those in free phenytoin and the analogous copper and nickel complexes.

Rebound of Plasma Free Phenytoin Concentration Following Plasmapheresis in a Patient with Thrombotic Thrombocytopenic Purpura

To measure the total and free serum phenytoin concentrations and phenytoin clearance in a patient with thrombotic thrombocytopenic purpura undergoing plasmapheresis. A dose of phenytoin 400 mg i.v. was administered daily 2.5 hours after the completion of plasmapheresis. Intravenous samples were obtained to measure the free and total phenytoin concentrations on 4 consecutive days. These samples were obtained before, at the midpoint, upon completion, and 30, 60, and 120 minutes after completion of plasmapheresis. The samples were centrifuged and stored at 4 degrees C until analysis using the same fluorescence polarization immunoassay kit. The free phenytoin was separated by ultracentrifugation. Despite a decrease in total and free serum phenytoin concentrations measured during the procedure, a marked rebound in the free phenytoin concentration was noted 30 minutes after completion of the procedure. However, by 120 minutes after the procedure, the free phenytoin concentration had decreased to the preprocedure value. The total amount of phenytoin removed by the procedure was less than 5%. Despite the decrease in total and free serum phenytoin concentrations observed during plasmapheresis, patients undergoing plasmapheresis may not need supplemental phenytoin, as a rapid reequilibration of the therapeutically active free serum phenytoin occurs following the completion of the procedure. In addition, less than 5% of the total body phenytoin is removed. Second, if the patient experiences symptoms of phenytoin toxicity immediately after the procedure, a free serum phenytoin concentration may be obtained, as this concentration may reflect the reason for toxicity despite a decreased total phenytoin concentration.

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.

USA — Measurement with a self-contained fiber-optic immunosensor: In ANAL. CHEM. (66/10 (1675–1682) 1994)) J.R. Astles and W.G. Miller of the Medical College of Virginia report on Measurement of free phenytoin in blood with a self-contained fiber-optic immunosensor

USA — Measurement with a self-contained fiber-optic immunosensor: In ANAL. CHEM. (66/10 (1675–1682) 1994)) J.R. Astles and W.G. Miller of the Medical College of Virginia report on Measurement of free phenytoin in blood with a self-contained fiber-optic immunosensor

Phenytoin and Plasmapheresis

The effect of plasmapheresis (PP) on total and free phenytoin clearance is reported. An obese patient with the diagnosis of thrombotic thrombocytopenic purpura (TTP) was treated with PP. Twelve episodes of PP, having exchange volumes of 1.5-2.25 times the plasma volume with a mean +/- SD 7.7 +/- 0.8 L of plasma removed, were studied. A significant (p < 0.05) difference was observed with a mean change in plasma phenytoin concentrations from pre- to end-PP of 7.32 +/- 2.5 mg/L compared to 1.98 +/- 0.7 mg/L observed pre-PP to 1 h post-PP. These values corresponded to 48.4 +/- 11.6 and 15.0 +/- 6.7% decreases in phenytoin concentrations at the two aforementioned time periods. To prevent misinterpretation of plasma phenytoin concentrations, samples should not be obtained for at least 2 h after PP.

Measurement of free phenytoin in blood with a self-contained fiber-optic immunosensor.

We describe the measurement of the anticonvulsant drug phenytoin (5,5-diphenylhydantoin) in human blood and plasma using a self-contained fiber-optic immunosensor which gave a reversible response to changes in concentration. No regenerative treatment of the immunosensor was required between measurements. The analytical signal depended on the degree of energy transfer from B-phycoerythrin labeled with phenytoin to Texas Red labeled anti-phenytoin antibody. Dextran 70K was added to the reagent system to equalize the oncotic pressure across the encapsulation membrane. A gas chromatography reference method was used to measure free drug concentrations. Regression analysis for plasma samples gave the relationship (sensor) = 1.02 (reference) + 0.07 microM; Syx = 1.00 microM; r = 0.953; sensor mean 4.45 microM; reference mean 4.31 microM. Sensor performance in plasma and whole blood was essentially equivalent. We demonstrate the feasibility of measuring free phenytoin directly in blood and suggest that the sensor design is generally applicable for the measurement of other haptens in blood.

A Rational Basis for the Measurement of Free Phenytoin Concentration in Critically I11 Trauma Patients

Phenytoin binding to serum proteins and factors influencing protein binding were investigated in 38 critically ill trauma patients. In 24% of these patients, the free fraction of phenytoin was < or = 10%, whereas in 76%, the free phenytoin fraction was increased > 10%--up to 24%. Nonantiepileptic comedication, sex, or age had no influence on phenytoin binding in any of the 38 patients. Elevated free phenytoin fraction was found in those with hypoalbuminemia and hepatic and renal impairments. In these patients, the free phenytoin fraction should be measured routinely.