Diphenylhydantoin: an alternative ligand of a glucocorticoid receptor affecting prostaglandin generation in A/J mice

Evidence for the binding of 5,5-diphenylhydantoin and glucocorticoids to a common receptor is presented for pulmonary and hepatic cytosols and thymocytes of A/J female mice. The 5,5-diphenylhydantoin-protein complex is absorbed by DNA cellulose, and is incorporated into nuclei, 5,5-Diphenylhydantoin, like glucocorticoids, inhibits the production of prostaglandins in thymocytes. Thus a common receptor is probably responsible for the inhibitory and teratogenic effects of these drugs.

Drug interactions with valproic acid

Valproic acid undergoes drug-drug interactions with most of the commonly used anticonvulsants. Since it possesses a wide range of indications, concomitant use with other anticonvulsants, and hence interactions, are not infrequent. Many of these interactions are reciprocal and may have important therapeutic consequences. Valproate acts as a protein binding displacer and/or metabolic inhibitor with respect to a number of other anticonvulsants (phenobarbitone, primidone, phenytoin). Inhibition of metabolism would, in most instances, result in a decrease of the dose requirements of the affected drugs. Valproate is a low clearance drug primarily eliminated by metabolism. Its metabolism is highly inducible by some of the major anticonvulsants (e.g. carbamazepine, phenytoin). Valproate is also highly protein bound in plasma and thus is displaced by salicylates and free fatty acids. However, displacement alone, unlike induced metabolism, should not affect the drug's dose-response relationship.

Phenytoin excretion in human breast milk and plasma levels in nursed infants

Phenytoin excretion into human breast milk was studied in six nursing women with epilepsy. The average ratio between the areas under the plasma (and milk) concentration versus time curves (AUC) was 0.13. There was a good (r = 0.97) correlation between the mean plasma and milk concentrations of phenytoin, and an even better relation (r = 0.99) between the AUC for phenytoin in plasma and the mean milk concentration. The ratio between unconjugated and conjugated 4-OH-phenytoin (the main metabolite) in plasma was 0.08-0.09. The corresponding ratio in milk was considerably higher. The present data do not argue against breast feeding during phenytoin therapy, not even when weighed against the potential risks for toxicity of the parent compound. Only two of six infants had a measurable, yet very low plasma concentration of phenytoin. The calculated body weight--related doses of phenytoin secreted into milk will be less than 5% of the dose to infants and small children.

Evidence for an arene oxide-NIH shift pathway in the metabolic conversion of phenytoin to 5-(4-hydroxyphenyl)-5-phenylhydantoin in the rat and in man

To determine whether the hydroxylation of 5,5-diphenylhydantoin (DPH) to 5-(4-hydroxyphenyl)-5-phenylhydantoin (p-HPPH) occurs by an arene oxide-NIH shift process, racemic 5-(4-deuteriophenyl)-5-phenylhydantoin (p-2H-DPH) was subjected to in vivo metabolic experiments in the rat and in man. After enzymatic hydrolysis of the urine, para-hydroxylated metabolites were separated by HPLC. Deuterium retention in the isolated metabolites determined by gas chromatography-mass spectrometry, was 68-72%. The results are interpreted as the predominance of an arene oxide-NIH shift pathway in those two metabolic systems. Induction of rats with phenobarbital or 3-methylcholanthrene showed no effect on the value of deuterium retention.

Anticonvulsants during pregnancy and lactation. Transplacental, maternal and neonatal pharmacokinetics

Few data are available on placental transfer of anticonvulsants during early pregnancy. Nevertheless, it has been demonstrated that at this early stage of gestation, considerable amounts of phenytoin, primidone/phenobarbitone and carbamazepine as well as some of their metabolites are already present in fetal tissues. Potentially reactive metabolites of anticonvulsants can be formed by the fetal liver and accumulate in some organs. At term, most anticonvulsants are present in neonatal plasma in concentrations similar to those in maternal plasma. Valproic acid, on the other hand, can accumulate in fetal blood, for still unknown reasons. Elimination by the neonate is variable and is dependent on several factors, such as clinical state, pre- or perinatal enzyme induction, absorption of the drugs and their plasma protein binding. Neonatal acquisition of anticonvulsants via breast-feeding does not seem to be harmful for the neonate. In the case of phenobarbitone, however, the drug may accumulate in nursing neonates to levels approaching or even exceeding those of their mothers. Significant drug levels can also build up in neonates and infants nursed by carbamazepine- and ethosuximide-treated mothers. This review contains relevant pharmacokinetic data on anticonvulsant drugs widely used during pregnancy and the neonatal period. The differences between pregnant and non-pregnant adults as well as between neonates and older age groups are emphasized. Some pharmacokinetic data are correlated with clinical manifestations, such as seizure frequency, neonatal depression and withdrawal symptoms.

Single-point prediction methods: a critical review

This article presents a critical review of a proposed method of dose prediction. Previously published experiences with this method are reviewed. New applications with drugs that have short elimination half-lives and drugs that demonstrate nonlinearity of clearance are discussed. Potential sources of error are demonstrated, using computer simulations.

Blood-brain barrier transfer and cerebral uptake of antiepileptic drugs

The permeability across the blood-brain barrier of phenobarbital, phenytoin, clonazepam, and diazepam was determined in a total of 29 patients with the double-indicator dilution method. Cerebral blood flow was measured with the 133Xe intra-arterial injection method. The unidirectional extraction (E) of the four drugs was 0.07, 0.11, 0.42, and 0.42, respectively. Permeability surface area products (PS) calculated for the drugs depended on E as well as on the plasma protein binding of the drugs and the cerebral blood flow and was calculated as 0.1, 0.5, 0.5, and 2.6 ml gm-1 min-1, respectively. A mathematic model of cerebral uptake and concentration is presented. The brain concentration of each drug is then calculated for two different states, one with a sudden rise from zero to an arterial concentration, which remains constant, and the other with the arterial concentration, which is achieved after rapid intravenous injection. The cerebral uptake rate of clonazepam and diazepam was much more rapid than that of phenobarbital and phenytoin. After intravenous clonazepam or diazepam injection, half-maximal gray matter concentration is reached about 15 sec after the drug arrives at the brain.

Ethnic differences in drug metabolism

Interethnic differences in drug-metabolising capacity may be substantial, and they are sufficiently frequent to warrant attention. Such differences may consist of different mean values of quantitative traits in separate populations, or of different frequency distributions as produced by the occurrence of genetic enzyme variants. The collection of population data requires the investigation of substantial numbers of subjects. This may be no problem if drug-metabolising enzymes occur in blood or are sufficiently stable in their tissues to allow investigation in vitro. However, if investigations require the use of probe drugs, new efforts are needed to adapt pharmacokinetic methods to make them suitable for population studies. This development of methods is further called for because genetic variants seem to be more easily detected through the assessment of particular metabolites than through the determination of pharmacokinetic parameters of the parent drug. Many studies with probe drugs comparing different populations have given results that are equivocal in terms of the nature-nurture interplay. However, a set of data with antipyrine has pointed to environmental factors as the principal determinant of differences in metabolising capacity, while data with debrisoquine have indicated monogenically controlled variation of one facet of the cytochrome P-450 system. In several instances, statistically significant differences between population means have been established by testing small numbers of subjects, numbers insufficient to establish distribution patterns that would allow the recognition of genetic polymorphism. The populations studied range from Greenlanders to South African Blacks, but most comparisons pertain to Caucasians and Orientals.

The metabolism of phenytoin by isolated hepatocytes and hepatic microsomes from male rats

Previous in vivo and in vitro studies have indicated that the Km for phenytoin hydroxylation is about 30 microM. Yet, the drug shows dose-dependent kinetics suggesting a Km of about 5 microM. The present studies indicate the discrepancy is not due to active transport of the drug in the hepatocyte or a decrease in the Km due to the low pO2 of the portal vein leading to uncompetitive inhibition. Studies in both hepatocytes and microsomes indicate the presence of a high affinity hydroxylase with a Km of 2 to 5 microM. These data suggest that this enzyme is the one primarily involved in the metabolism of phenytoin.

Phenytoin metabolism in subjects with long and short plasma half-lives

Normal adult men with long and short phenytoin plasma half-lives were given 300-mg oral doses of phenytoin once daily for 15 days. Plasma levels of phenytoin (DPH) and its major metabolite (p-HPPH) were measured during the period of drug administration and for 5 days thereafter. Average steady-state plasma levels of DPH rose to 13.4 micrograms/ml in the long half-life group, compared with 3.6 micrograms/ml in the short half-life group. HPPH levels in the long half-life group were about one half of those observed in the short half-life group. The DPH/HPPH ratios in plasma specimens showed excellent correlation with the plasma half-lives of DPH and average steady-state levels, suggesting that this ratio could provide guidance in the selection of optimum dosage regimens for problem patients.

[Feasibility of the in vitro evaluation of bioavailability. 3: Method of operation of a newly-developed absorption model and results obtained with it]

In connection with the function of a newly constructed absorption model the problem of interpretation of values obtained by models in regard of bioavailability is discussed. Wrong interpretations can be obtained if nonanalogous values are compared. An interpretation is proposed to calculate a value which is analogous to the relative bioavailability (calculated bioavailability). The efficiency of the absorption model and of the interpretation method is demonstrated by three examples of preparations of phenytoin, digoxin and chloramphenicol. We compared these results with the bioavailability of these preparations and we found a good correspondence.

Rapid and slow release phenytoin in epileptic patients at steady state: assessment of relative bioavailability utilizing Michaelis-Menten parameters

The bioavailability of phenytoin from rapid release capsule and oral solution formulations relative to that of a slow release capsule formulation was assessed in five patients who had participated in a three-way crossover study performed at steady state. The subjects then underwent dosage adjustment utilizing the slow release formulation, and estimates of their Michaelis-Menten parameters thus obtained were utilized in calculating the relative bioavailabilities. In addition, expected changes in steady-state plasma phenytoin concentrations were calculated assuming initial levels of 15 mg/liter, with increases and decreases in bioavailability of 10%. The consequences of such alterations in the extent of phenytoin absorption or average content of the dosage form may be clinically significant, particularly where the initial phenytoin level is equal to or greater than the patient's operative Km.

Rapid and slow release phenytoin in epileptic patients at steady state: comparative plasma levels and toxicity

Phenytoin plasma level and toxicity data were compared in a three-way crossover study performed in 18 patients at steady state. Formulations compared were a rapid and a slow release capsule and an oral solution. Plasma concentration-time integrals and maximum plasma phenytoin levels were significantly greater for the rapid release capsule and solution than for the slow release capsule. The incidence of nystagmus and toxicity did not differ for the three treatments, but the occurrence of mental symptoms was more frequent for the oral solution, possibly because of the solvent used in this formulation.

Biopharmaceutical studies on hydantoin derivatives. III. Physico-chemical properties, dissolution behavior, and bioavailability of the molecular compound of 1-benzenesulfonyl-5,5-diphenylhydantoin and antipyrine

Physico-chemical properties, dissolution profiles, and bioavailability of the molecular compound composed of equimolar amount of 1-benzenesulfonyl-5, 5-diphenylhydantoin (BSDH) and antipyrine were studied. The spectral and thermal analyses suggest that the intermolecular force in the molecular compound is attributed to the hydrogen bond between 3-NH of BSDH and 5-C = 0 of antipyrine. In the aqueous media, the molecular compound decomposed to the constituents and there was no interaction between them. The dissolution behavior of BSDH from the molecular compound was characteristic, the high supersaturation being present for a long time, while the dissolution of antipyrine from the molecular compound was slow and continued for a long time. The dissolution profile of the molecular compound was truly reflected on the bioavailability in dogs. With respect to BSDH, the rate and the extent of bioavailability of the molecular compound were excellent. On the other hand, the absorption of antipyrine from the molecular compound was sustained for a long time. BSDH and antipyrine had no influence one another on the intrinsic rate constant of transport through the intestinal membrane and on the pharmacokinetic constants such as the volume of distribution and the elimination rate constant.

The effect of pregnancy and estradiol-17 beta treatment on the biliary transport maximum of dibromosulfophthalein, and the glucuronide conjugates of 5-phenyl-5-p-hydroxyphenyl[14C]hydantoin and [14C]morphine in the isolated perfused rat liver

The biliary transport maximum (Tm) of three organic axions was determined in the isolated perfused livers of untreated female (control), estradiol-17 beta (E2)-treated female (1 mg/kg/day, s.c. for 14 days), and pregnant (19-21 days of gestation) rats. Dibromosulfophthalein (DBSP), 5-phenyl-5-p-hydroxyphenyl[14C]hydantoin (HPPH) and [14C]morphine were infused continuously into the perfusate for a total dose of 41.2, 18, or 40.5 mumol, respectively. The concentration of [14C]HPPH and [14C]morphine declined in the perfusate, whereas the concentrations of [14C]HPPH glucuronide and [14C]morphine glucuronide increased during the 90-min experiment, indicating that the rate of formation of the glucuronide exceeded its rate of excretion in bile. E2 treatment decreased the Tm (nmol/min/g liver) for [14C]HPPH glucuronide and [14C]morphine glucuronide but not for DBSP, whereas pregnancy decreased the Tm for all three organic anions. Pregnancy, and to a lesser extent E2 treatment, increased liver weight. When expressed per whole liver, the Tm was not altered by pregnancy for any of three organic anions. E2 treatment increased the Tm for DBSP, had no effect on the Tm for HPPH glucuronide and decreased the Tm for [14C]morphine glucuronide. These data suggest the presence of multiple carriers for organic anions which are differentially affected by estrogen treatment and pregnancy.

Combined phenytoin and phenobarbital overdose

A 32-year-old female attempted suicide by consuming large quantities of both phenytoin (P) and phenobarbital (Pb). On hospital admission, serum drug concentrations (SDC), measured by immunoassay, were 46 micrograms/ml for P and 86 micrograms/ml for Pb. The patient remained unconscious, requiring mechanical ventilation, for the next two weeks. Serial SDC demonstrated: (1) continued P absorption over the next five days; (2) different elimination patterns for the two drugs (Pb exhibited first-order elimination while P elimination represented Michaelis-Menten kinetic); and (3) the limited effectiveness of forced alkaline diuresis in lowering Pb serum concentration, though resulting in an episode of pulmonary edema. The patient's clinical status subsequently improved and correlated well with SDC decline into the usual therapeutic range.

A comparison of diphenylhydantoin metabolism in different tissues using high performance liquid chromatography

We have quantified diphenylhydantoin metabolism in vitro using high performance liquid chromatography. Five metabolites from rat liver post-mitochondrial supernates were assayed (in order of concentrations): pHPPH, dihydrodiol, pHPPHglucuronide, mHPPH, and an unknown. Catechol was rarely identified and 3-0-methyl catechol was not found. Induction of DPH metabolism by phenobarbital, beta-naphthoflavone, and DPH was studied. While phenobarbital increased synthesis of all the products, beta-naphthoflavone decreased DPH metabolism. DPH itself stimulated only dihydrodiol formation. Studies of dog liver showed that microsomes were more efficient metabolizers of DPH than post-mitochondrial supernatants, with predominance of mHPPH over pHPPH. This finding differed from that with rat liver. Rat thymus lymphocytes and cultured human peripheral blood lymphocytes converted less than 1% of DPH and yielded HPLC profiles indistinguishable from cell-free blanks.

Diurnal oscillations in plasma protein binding of valproic acid

In view of the observed variation of valproic acid (VPA) free fraction (fp) during a dosing interval and the competitive binding effect of free fatty acids (FFA) in vitro, this study was designed to address the existence of diurnal variations in the fp of VPA. Six subjects were hospitalized at 7 a.m. for 25 h, and plasma samples were collected every 2 h. The protocol was repeated in 4 of the 6 subjects one week later. In vitro binding of VPA (100 micrograms/ml) was determined by equilibrium dialysis (14C-VPA), and FFAs were assayed colorimetrically. Phenytoin (PHT) binding was also determined for comparison. VPA fp ranged from 8.10 +/- 1.16 to 9.63 +/- 1.54. Intrasubject variability was also measured by the ratio of maximum to minimum fp values (fp max/fp min) over 24 h: This ratio ranged from 1.30 to 1.68 (mean +/- %SD = 1.51 +/- 7.7%, n = 10). For PHT, fp ranged from 10.88 +/- 0.50 to 12.39 +/- 1.07, and fp max/fp min from 1.09 to 1.31 (1.17 +/- 5.1%, n = 10). The fp max was observed between 2 and 6 a.m. in 7 out of 10 cases for VPA and 5 out of 10 cases for PHT. FFA levels, although in the normal range, varied two- to fourfold within 24 h. A significant correlation was observed between mean FFA levels at each sampling time and the corresponding fp values for VPA (p less than 0.001), but not for PHT.

Interference of oral phenytoin absorption by continuous nasogastric feedings

Inhibition of phenytoin absorption by continuous nasogastric tube feeding was studied in 20 neurosurgery patients and 5 normal subjects. Ten patients receiving phenytoin suspension 300 mg per day coadministered with continuous nasogastric feedings had a mean phenytoin serum concentration of 2.59 micrograms per milliliter. When the feedings were discontinued, the average concentration rose to 10.22 micrograms per milliliter in 7 days. In 10 other patients stabilized on phenytoin suspension 300 mg per day, the average serum concentration decreased from 9.80 microgram per milliliter to 2.72 microgram per milliliter in 7 days when continuous tube feedings were started. Five normal subjects received a single oral dose of phenytoin suspension alone and while drinking a nasogastric tube feeding preparation orally at a rate of 100 ml per hour; phenytoin serum levels decreased an average of 71.6% when the tube feeding was taken concurrently.