Separation of phenytoin and its metabolites by high-performance liquid chromatography

Phenytoin - An anti-seizure drug: Overview of its chemistry, pharmacology and toxicology

Phenytoin is a long-standing, anti-seizure drug widely used in clinical practice. It has also been evaluated in the context of many other illnesses in addition to its original epilepsy indication. The narrow therapeutic index of phenytoin and its ubiquitous daily use pose a high risk of poisoning. This review article focuses on the chemistry, pharmacokinetics, and toxicology of phenytoin, with a special focus on its mutagenicity, carcinogenicity, and teratogenicity. The side effects on human health associated with phenytoin use are thoroughly described. In particular, DRESS syndrome and cerebellar atrophy are addressed. This review will help in further understanding the benefits phenytoin use in the treatment of epilepsy.

Probabilistic approach to the establishment of maximal content limits of impurities in drug formulations: the case of parenteral diphenylhydantoic acid

Diphenylhydantoic acid (DPHA) is a degradation product in parenteral formulations of the anticonvulsant phenytoin and the prodrug fosphenytoin. DPHA has also been reported to be a minor metabolite of phenytoin. Levels found in the urine of various species, including humans, after oral or intravenous (iv) phenytoin ranged from undetected to a few percent of administered dose. In the present analysis, the toxicologic profile of DPHA was integrated with exposure data in order to characterize its safety under recommended clinical regimens of fosphenytoin administration. In preclinical safety studies, DPHA was without effect in the Ames assay and at concentrations up to 3000 microg/plate in the presence or absence of metabolic activation, and in the in vitro micronucleus test with acute and 2-week repeated dose studies in Wistar rats at iv doses up to 15 mg/kg. In 4-week studies conducted in rats and dogs receiving fosphenytoin containing DPHA levels up to 1.1%, and in an in vitro structural chromosome aberration test with DPHA levels up to 2.0%, all findings were consistent with known effects of phenytoin (such as CNS signs and increased liver weight), and none were attributed to DPHA. Reports in the literature indicate that in murine in vivo and in vitro models, DPHA has much lower potential for reproductive toxicity than phenytoin. A no-observed-effect level (NOEL) of 15 mg/kg established from the 2-week study in rats was used with probabilistic techniques to estimate tolerable daily doses (TDDs) of DPHA. In this approach, interspecies correction was performed by allometrically scaling the NOEL based on a distributional power of body weight while intraindividual variability was accounted for by selecting the lower percentiles of the population-based distribution of TDDs. The results indicate that a DPHA content limit of 3.0% in an administered dose of fosphenytoin is unlikely to cause adverse effects in patients.

Vigabatrin-induced decrease in serum phenytoin concentration does not involve a change in phenytoin bioavailability

The possibility that vigabatrin (VGB) decreases serum phenytoin (PHT) concentration by lowering the oral bioavailability of PHT was investigated in 21 patients with epilepsy. Each patient was switched from oral to intravenous PHT for 5 days before and after combined treatment with VGB. After VGB (2-3.5 g day(-1) for at least 5 weeks), serum PHT concentrations decreased slightly from 87 +/- 25 to 76 +/- 31 micromol l(-1) (means +/- s.d., P < 0.05), but in a subgroup of seven patients the decrease was more prominent (from 72 +/- 22 to 49 +/- 17 micromol l(-1), P < 0.005). At baseline (before VGB), serum PHT remained unaffected (85 +/- 30 micromol l(-1)) after switching PHT dosage to the intravenous route, indicating that the oral availability of the drug was virtually complete. During VGB treatment, serum PHT was also unchanged (74 +/- 34 micromol l(-1)) after switching from oral to intravenous therapy, and this was also true for the subgroup of patients showing a prominent interaction (48 +/- 18 micromol l(-1)). The urinary recoveries of PHT and its metabolites pHPPH and mHPPH remained constant throughout the study. It is concluded that the oral availability of PHT is unaffected by VGB and that the VBG-induced decrease in serum PHT is mediated by alternative mechanisms.

Two-dimensional high-performance liquid chromatographic method to assay p-hydroxyphenylphenylhydantoin enantiomers in biological fluids and stereoselectivity of enzyme induction in phenytoin metabolism

A two-dimensional high-performance liquid chromatographic method was developed to assay the enantiomers of a major phenytoin metabolite, p-hydroxyphenylphenylhydantoin (p-HPPH). Racemic p-HPPH was first separated from phenytoin and other interfering peaks by a reversed-phase column and monitored by an ultraviolet detector. At the retention time of p-HPPH, the racemic p-HPPH peak was automatically transferred to a chiral ligand-exchange column to separate R-p-HPPH and S-p-HPPH by a time-programmed column-switching valve. The ratio of enantiomers was measured by a second ultraviolet detector. The method can be used to assay R- and S-p-HPPH enantiomers with reasonable sensitivity and reproducibility. By using this method, the stereoselectivity of enzyme induction and inhibition of phenytoin metabolism was investigated. Male rats were treated with phenobarbital, 3-methylcholanthrene, acetone, Aroclor 1254, pregnenolone-16 alpha-carbonitrile, dexamethasone and isosafrole. Microsomes were prepared from the rat liver and phenytoin hydroxylation was measured. Pretreatment with phenobarbital, pregnenolone-16 alpha-carbonitrile or acetone induced phenytoin metabolism non-stereoselectively. Pretreatment with dexamethasone decreased R-p-HPPH formation without affecting the formation of S-p-HPPH. Liver microsomes from female rats showed a higher S-p-HPPH formation, whereas R-p-HPPH formation remained the same. Various inhibitors were added to inhibit phenytoin metabolism by control microsomes. Sulphaphenazole, ketoconazole, 4,4-di(p-methoxyphenyl)hydantoin, cimetidine and diazepam inhibited the formation of R- and S-p-HPPH. Quinidine, tolbutamide and mephenytoin showed no significant inhibitory activity. None of these inhibitors showed stereoselectivity.

Simultaneous determination of p-hydroxylated and dihydrodiol metabolites of phenytoin in urine by high-performance liquid chromatography

Accurate urinary measurements of the two major metabolites of phenytoin, 5-(p-hydroxyphenyl)-5-phenylhydantoin (p-HPPH) and 5-(3,4-dihydroxy-cyclohexa-1,5-dienyl)-5-phenylhydantoin (dihydrodiol, DHD), are necessary for pharmacokinetic and drug-interaction studies of this commonly used antiepileptic drug. We describe a simple, rapid, acid hydrolysis, with liquid-liquid extraction and simultaneous isocratic reversed-phase high-performance liquid chromatography of p-HPPH and 5-(m-hydroxyphenyl)-5-phenylhydantoin (m-HPPH) (hydrolytic end product of DHD). p-HPPH and m-HPPH were quantitated against their separate respective internal standards of alphenal and tolylbarb. The mobile phase consisted of water-dioxane-tetrahydrofuran (80:15:5, v/v/v) at 2 ml/min and at 50 degrees C, with detection at 225 nm. Baseline separation was achieved by use of a 16 cm x 3.9 mm Nova-Pak C18 column and total analysis time of 12 min. p-HPPH and m-HPPH concentrations ranged from 10 to 200 and from 2 to 30 micrograms/ml, respectively, with between-day coefficients of variations of 3.3-4.5% and 2.2-5.1% for controls. All standard curves were linear with r values greater than 0.993. The DHD concentration was determined by multiplying m-HPPH concentrations by 2.3.

Identification of a teratogenic drug-protein complex in sera of phenytoin-treated monkeys

Whole rat embryos were cultured for 48 h on sera drawn from monkeys before and 10 h after phenytoin gavage (275 mg/kg body weight). Sera from treated monkeys caused exencephaly, anophthalmia, microcephaly, and incomplete ventral curvature when used as culture media, whereas sera drawn from the same monkeys before treatment supported normal embryonic development. To identify the cause of serum teratogenicity, isolated constituents of teratogenic sera were added to nonteratogenic sera for testing by embryo culture. Serum extracts containing free phenytoin and its free metabolites were not teratogenic. Teratogenicity was found associated with serum proteins. Using polyacrylamide gel electrophoresis (PAGE) and an antibody that recognized phenytoin and its metabolites, we were able to demonstrate that phenytoin was bound to a protein of 80,000 daltons. Addition of this same antibody to teratogenic sera from dosed monkeys improved the development of cultured embryos and provided additional support for this complex as the proximal teratogen. Use of the antibody to follow the uptake and distribution of phenytoin in cultured embryos suggested that only the phenytoin-protein complex (and not phenytoin itself) was able to pass through the yolk sac and reach the tissues of the embryo proper. These results suggested that a drug-protein complex may serve to transport drugs from their site of activation in the maternal liver to the developing embryo.

Urinary dihydrodiol metabolites of phenytoin: high-performance liquid chromatographic assay of diastereomeric composition

Diastereomeric dihydrodiol metabolites of phenytoin, (5S)-5-[(3R,4R)-3,4-dihydroxy-1,5-cyclohexadien-1-yl]-5-phenylh yda ntoin, (S)-DHD, and (5R)-5-[(3R,4R)-3,4-dihydroxy-1,5-cyclohexadien-1-yl]-5-phenylh yda ntoin, (R)-DHD, have been resolved from each other and from urinary constituents with reversed-phase HPLC columns and acetonitrile-water gradients. Recoveries of DHD isomers from urine averaged 99.1% and it was demonstrated that known mixtures of DHD diastereomers added to blank urine were not altered by the assay procedures. The relative diastereomeric content of DHD was determined from integration of the chromatographic peaks. Assay of urine samples from patients on chronic phenytoin therapy and from volunteers indicated that both DHD isomers were present in all samples, and stereoselectivity favored the production of (S)-DHD.

Routine determination of hydroxyphenytoin in urine by high-performance liquid chromatography using an automatic column-switching technique

A high-performance liquid chromatographic method for the determination of the main phenytoin metabolite, hydroxyphenytoin, in the urine of epileptic patients is described. The use of an automated column-switching technique greatly simplifies the pretreatment steps. Thereby, both time and chemicals are saved. The possibility of error arising during the several pretreatment steps is considerably reduced. Following acid hydrolysis of the hydroxyphenytoin glucuronic acid conjugate the sample is diluted with water and after centrifugation is injected onto the pre-column. After washing for a short time with water, the substances which were absorbed on the head of the pre-column were backflushed with water--acetonitrile as eluent onto the analytical column. Separation is achieved by gradient elution using an ODS reversed-phase column with a particle size of 5 microns.

High-performance liquid chromatographic determination of phenytoin and hydroxyphenytoin in human urine

Simple and rapid quantitative analysis utilizing high-performance liquid chromatography was performed to determine the concentration of the antiepileptic agent phenytoin, and its main metabolite hydroxyphenytoin, in human urine. For the purposes of simple and rapid determination, the commercially available Extrelut columns were used with diethyl ether and chloroform as extraction solvents. High-performance liquid chromatography was performed on a LiChrosorb RP-8 column, with a mobile phase of methanol-0.02% ammonium acetate (1:1). The internal standard was 5-(4-methylphenyl)-5-phenylhydantoin. The method was applied to patients' urine, to examine the influence of concomitant drugs. Also, the results obtained using the commercially available enzymatic immunoassay method were compared with those from the present method, and it was concluded that a simple and rapid microanalysis is possible with a high extraction ratio.

Decreased phenytoin level during antineoplastic therapy: a case report

We report a case of interaction between anticonvulsant and antineoplastic drugs in one male patient with seizures from brain tumour. The patient was treated with phenytoin (PHT), phenobarbital (PB), and an antineoplastic protocol based on a combination regimen with carmustine (BCNU), vinblastin (VLB), methotrexate (MTX), and radiotherapy. Plasma concentrations of PHT fell from 9.4 to 5.6 micrograms/ml 24 h after VLB administration, and remained low for at least 10 days. During this period, partial seizures occurred. Plasma concentrations of PB were unchanged during the period of observation. It is suggested that impaired absorption of PHT, caused by VLB or MTX or both, is responsible for this interaction.