Determination of phenobarbital, p-hydroxyphenobarbital and phenobarbital-N-glucoside in urine by gas chromatography chemical ionization mass spectrometry

Host factors affecting antiepileptic drug delivery-pharmacokinetic variability

Antiepileptic drugs (AEDs) are the mainstay in the treatment of epilepsy, one of the most common serious chronic neurological disorders. AEDs display extensive pharmacological variability between and within patients, and a major determinant of differences in response to treatment is pharmacokinetic variability. Host factors affecting AED delivery may be defined as the pharmacokinetic characteristics that determine the AED delivery to the site of action, the epileptic focus. Individual differences may occur in absorption, distribution, metabolism and excretion. These differences can be determined by genetic factors including gender and ethnicity, but the pharmacokinetics of AEDs can also be affected by age, specific physiological states in life, such as pregnancy, or pathological conditions including hepatic and renal insufficiency. Pharmacokinetic interactions with other drugs are another important source of variability in response to AEDs. Pharmacokinetic characteristics of the presently available AEDs are discussed in this review as well as their clinical implications.

Antiepileptic drugs--best practice guidelines for therapeutic drug monitoring: a position paper by the subcommission on therapeutic drug monitoring, ILAE Commission on Therapeutic Strategies

Although no randomized studies have demonstrated a positive impact of therapeutic drug monitoring (TDM) on clinical outcome in epilepsy, evidence from nonrandomized studies and everyday clinical experience does indicate that measuring serum concentrations of old and new generation antiepileptic drugs (AEDs) can have a valuable role in guiding patient management provided that concentrations are measured with a clear indication and are interpreted critically, taking into account the whole clinical context. Situations in which AED measurements are most likely to be of benefit include (1) when a person has attained the desired clinical outcome, to establish an individual therapeutic concentration which can be used at subsequent times to assess potential causes for a change in drug response; (2) as an aid in the diagnosis of clinical toxicity; (3) to assess compliance, particularly in patients with uncontrolled seizures or breakthrough seizures; (4) to guide dosage adjustment in situations associated with increased pharmacokinetic variability (e.g., children, the elderly, patients with associated diseases, drug formulation changes); (5) when a potentially important pharmacokinetic change is anticipated (e.g., in pregnancy, or when an interacting drug is added or removed); (6) to guide dose adjustments for AEDs with dose-dependent pharmacokinetics, particularly phenytoin.

Metabolism of verapamil: 24 new phase I and phase II metabolites identified in cell cultures of rat hepatocytes by liquid chromatography–tandem mass spectrometry

Verapamil is a widely prescribed calcium antagonist, but suffers from extensive first pass metabolism. Despite its frequent use in drug metabolism a complete understanding of its metabolic pathway is still lacking. We thus investigated verapamil's metabolism in cultures of primary rat hepatocytes and isolated metabolites from cell culture media by solid phase extraction (SPE). In detail, we investigated their structure in multiple liquid chromatography-mass spectrometry (LC-MSn) experiments and found 25 phase I and 14 phase II metabolites. We showed many metabolites to be produced by oxidative dealkylation, and several yet unknown metabolites were identified that stem from hydroxylation and dealkylation reactions. Furthermore, we identified an array of glucuronides and, additionally, a glucoside. Finally, we investigated the enantioselective biotransformation of verapamil and found preferential metabolism of the S-enantiomers. In conclusion, this illustrates again the true complexity of verapamil's disposition.

High-performance liquid chromatographic analysis of phenobarbital and phenobarbital metabolites in human urine

A HPLC assay using UV detection and post-column alkalinization was developed to quantify possible urinary excretion products of phenobarbital in human urine. After filtration the urine was injected directly onto the HPLC column for analysis of phenobarbital, p-hydroxyphenobarbital, phenobarbital N-glucosides and phenobarbital N-glucuronides. The accuracy and precision of the assay were within +/- 15% and the limit of detection (LOD) was 1 microM, suitable for pharmacokinetic studies. Phenobarbital was administered orally to five male subjects and urine was collected for a period of 96-108 h. Phenobarbital, p-hydroxyphenobarbital, and phenobarbital N-glucosides were detected and quantified in the urine of all five subjects. The phenobarbital N-glucuronides were not detected in the urine. This assay provides a rapid method with improved selectivity to analyze urine for phenobarbital and its metabolites.

Analysis of antiepileptic drugs

This review discussed various analytical methods for the determination of antiepileptic drugs and their metabolites in biological tissues. The emphasis was on the reports published since their last review [J. T. Burke and J. P. Thenot, J. Chromatogr., 340 (1985) 199]. Both chromatographic and immunological procedure were cited and compared. Methods for individual and simultaneous quantitation of standard antiepileptic drugs and their metabolites were considered. In addition, a discussion of free drug determination and procedures for new candidate antiepileptic drugs were included.

Stereochemical characterization of the diastereomers of the phenobarbital N-beta-D-glucose conjugate excreted in human urine

The absolute configuration of the N-beta-D-glucoside metabolites of phenobarbital was determined by methylation of the diastereomers to make mephobarbital N-beta-D-glucosides, followed by oxidative removal of glucose to give the optical isomers of mephobarbital. Following a single oral dose of phenobarbital to two male subjects, both phenobarbital N-beta-D-glucosides were excreted in the urine. The absolute configuration (C-5 position) of the major phenobarbital N-beta-D-glucoside excreted in the urine was the S form. A pronounced stereoselective formation and/or urinary excretion occurs for the N-glucoside conjugates of phenobarbital in humans.

LC determination of the diastereomers of 1-(beta-D-glucopyranosyl)phenobarbital in human urine

The "product enantioselectivity" associated with the urinary excretion of the phenobarbital N-glucoside conjugates has not been determined previously. A liquid chromatography method using gradient elution was developed for quantifying both phenobarbital N-glucoside conjugates, phenobarbital, and p-hydroxyphenobarbital. Following a single oral dose of phenobarbital to male Caucasian and Oriental subjects, both phenobarbital N-glucoside conjugates were observed in the urine. In seven subjects, 3.3-10.6% of the phenobarbital dose was detected as a single phenobarbital N-glucoside (S configuration at the C-5 position of the barbiturate ring). The other phenobarbital N-glucoside diastereomer accounted for less than 1.5% of the phenobarbital dose. The urinary excretion of the major phenobarbital N-glucoside diastereomer paralleled the urinary excretion of phenobarbital and was comparable in both Caucasian and Oriental subjects. These results indicate a pronounced selectivity for the formation and/or urinary excretion of the phenobarbital N-glucosides.

Synthesis of N-beta-D-glucopyranosyl derivatives of barbital, phenobarbital, metharbital, and mephobarbital

The condensation of per(trimethyl)silylbarbital and -phenobarbital with 1,2,3,4,6-penta-O-acetyl-beta-D-glucopyranose in the presence of stannic chloride in dichloroethane gave moderate yields of the beta-coupled barbiturate N-D-glucopyranosyl derivatives. Reaction of metharbital and mephobarbital under the same conditions was unsuccessful. The homologous N-methylglucosides were prepared by reaction of the barbital and phenobarbital N-glucosyl derivatives with diazomethane. The diastereomers of the phenobarbital and mephobarbital derivatives were resolved by use of C-18 reverse-phase h.p.l.c. 1H- and 13C-n.m.r. spectroscopy, and thermospray 1.c.-m.s. proved to be the most useful methods for characterizing the barbiturate glucosides.

Stability of phenobarbital N-glucosides: identification of hydrolysis products and kinetics of decomposition

The two diastereomers of 1-(1-beta-D-glucopyranosyl)phenobarbital, (1A) and (1B), decompose to 1-(1-beta-D-glucopyranosyl)-3-(2-ethyl-2-phenylmalonyl)urea (2A or 2B) followed by decarboxylation to 1-(1-beta-D-glucopyranosyl)-3-(2-phenylbutyryl)urea (3A and 3B) under physiological conditions of temperature and pH. The sigmoidal pH-rate profile and the Arrhenius parameters indicate that degradation takes place by hydroxide ion attack on the undissociated and monoanion forms of 1A and 1B. The rates of hydrolysis of the nonionized species of 1A and 1B are more than two orders of magnitude faster than those of common 5,5-disubstituted or 1,5,5-trisubstituted barbiturates. Molecular modeling studies suggest that rate enhancement is due to intramolecular hydrogen bonding in the transition state of the C2' hydroxyl with the tetrahedral hydrated C6 carbonyl as well as hindered rotation around the N1-C1' of phenobarbital and glucose. Based on these studies it is recommended that any data related to the quantitation of 1A and 1B be reevaluated depending on how the samples were collected, stored, and analyzed.