New drugs for the treatment of epilepsy

Seizure freedom and plasma levels of newer generation antiseizure medications

OBJECTIVES

Contrary to older antiseizure medications (ASM), correlation between plasma levels and seizure freedom is not well defined for newer generation ASM. We assessed correlations between efficacy and newer generation ASM plasma levels in patients with epilepsy.

MATERIALS AND METHODS

Plasma medication levels were measured over two years in consecutive patients taking lamotrigine, levetiracetam, oxcarbazepine, topiramate, zonisamide, lacosamide, perampanel or pregabalin. Seizure freedom was defined as three times the longest inter-seizure pre-treatment interval, or at least one year. Each medication level was stratified according to its position in relation to its proposed reference range (below or in lower half vs upper half or above).

RESULTS

168 patients on stable therapy were included. ASM plasma levels of seizure-free patients were lower than those with ongoing seizures; 45/48 (93.7%) were in the lower half or below the reference ranges, compared to 86/106 (81.1%; p = .004). Lamotrigine plasma levels were significantly lower in seizure-free patients (median 2.4 mg/L range 0.4-6.5 mg/L, none above 6.5 mg/L) compared with those with ongoing seizures (5 mg/L, 0.5-14.2 mg/L; p < .0001). Levetiracetam showed similar results (7.2 mg/L, 1.6-15.1 mg/L; none above 15.1 mg/L in seizure-free patients vs 16.4 mg/L, 0.6-47.7 mg/L; p = .005). Demographics, epilepsy type and polytherapy did not influence the results.

CONCLUSIONS

Efficacy of newer generation ASMs seems to be reached at the lower part or at times even below the reference ranges in drug responsive patients; this could inform regarding titrations of these treatments.

Correlation between measured and calculated free phenytoin serum concentration in neurointensive care patients with hypoalbuminemia

PURPOSE

In critically ill patients, monitoring free phenytoin concentration is a valuable method for phenytoin-dosage adjustment. However, due to technical difficulties and the high cost of these methods, the Sheiner-Tozer equation is routinely used for estimating free phenytoin concentration in clinical practice. There have been conflicting results concerning accuracy and precision of the Sheiner-Tozer equation for prediction of free phenytoin concentration in various patient populations. Therefore, this study was conducted to evaluate the accuracy and correlation of measured and calculated free phenytoin concentrations in neurointensive care patients with hypoalbuminemia.

METHODS

A total of 65 adult neurointensive care patients with hypoalbuminemia who were receiving phenytoin for prevention or treatment of seizures were recruited in this study. In addition to measuring free phenytoin concentration by HPLC, free phenytoin concentration was calculated using both conventional and revised Sheiner-Tozer equations. Eventually, the correlation and level of agreement between measured and calculated free phenytoin concentrations were evaluated.

RESULTS

The mean albumin concentration of studied patients was 2.63±0.57 g/dL. There was a significant but weak-moderate correlation between measured and calculated free phenytoin concentration using conventional and revised Sheiner-Tozer equations (r=0.45 and r=0.43, respectively). Conventional and revised Sheiner-Tozer equations were not able to predict free phenytoin concentrations accurately in 33.85% and 35.4% of patients, respectively. Although the sex of patients did not have a significant impact on the level of agreement, younger patients had a higher level of agreement.

CONCLUSION

Although there was a moderate correlation between calculated and measured free phenytoin concentration, the Sheiner-Tozer equation was not able to predict free phenytoin concentration accurately in all patients, especially in older patients. Therefore, monitoring free phenytoin serum concentration besides clinical outcomes should be considered for phenytoin-dose adjustment in critically ill patients.

One-Pot Solvent-Free Three-Component Synthesis of Conjugated Enaminones Containing Three Alkyl Carboxylate Groups

An effective, one-pot, multicomponent, and solvent-free reaction for synthesis of conjugated enaminones containing three alkyl carboxylate groups is described. The reaction of primary amine, alkyl acetoacetate, and dialkyl acetylenedicarboxylate obtained the title compound in good yields in a short time.

QSAR study and molecular design of open-chain enaminones as anticonvulsant agents

Present work employs the QSAR formalism to predict the ED₅₀ anticonvulsant activity of ringed-enaminones, in order to apply these relationships for the prediction of unknown open-chain compounds containing the same types of functional groups in their molecular structure. Two different modeling approaches are applied with the purpose of comparing the consistency of our results: (a) the search of molecular descriptors via multivariable linear regressions; and (b) the calculation of flexible descriptors with the CORAL (CORrelation And Logic) program. Among the results found, we propose some potent candidate open-chain enaminones having ED₅₀ values lower than 10 mg·kg⁻¹ for corresponding pharmacological studies. These compounds are classified as Class 1 and Class 2 according to the Anticonvulsant Selection Project.

Titrimetric and spectrophotometric assay of oxcarbazepine in pharmaceuticals using N-bromosuccinimide and bromopyrogallol red

Titrimetric and spectrophotometric methods are described for the determination of oxcarbazepine (OXC) in bulk drug and in tablets. The methods use N-bromosuccinimide (NBS) and bromopyrogallol red (BPR) as reagents. In titrimetry (method A), an acidified solution of OXC is titrated directly with NBS using methyl orange as indicator. Spectrophotometry (method B) involves the addition of known excess of NBS to an acidified solution of OXC followed by the determination of the unreacted NBS by reacting with BPR and measuring the absorbance of the unreacted dye at 460 nm. Titrimetry allows the determination of 6–18 mg of OXC and follows a reaction stoichiometry of 1 : 1 (OXC : NBS), whereas spectrophotometry is applicable over the concentration range of 0.8–8.0 μg mL⁻¹. Method B with a calculated molar absorptivity of 2.52 × 10⁴ L mol⁻¹ cm⁻¹ is the most sensitive spectrophotometric method ever developed for OXC. The optical characteristics such as limits of detection (LOD), quantification (LOQ), and Sandell's sensitivity values are also reported for the spectrophotometric method. The accuracy and precision of the methods were studied on intraday and interday basis. The methods described could usefully be applied to routine quality control of tablets containing OXC. No interference was observed from common pharmaceutical adjuvants. Statistical comparison of the results with a reference method shows an excellent agreement and indicates no significant difference in accuracy and precision. The reliability of the methods was further ascertained by recovery studies in standard addition procedure.

Development and validation of a high performance liquid chromatographic method for the determination of oxcarbazepine and its main metabolites in human plasma and cerebrospinal fluid and its application to pharmacokinetic study

An isocratic reversed-phase HPLC-UV procedure for the determination of oxcarbazepine and its main metabolites 10-hydroxy-10,11-dihydrocarbamazepine and 10,11-dihydroxy-trans-10,11-dihydrocarbamazepine in human plasma and cerebrospinal fluid has been developed and validated. After addition of bromazepam as internal standard, the analytes were isolated from plasma and cerebrospinal fluid by liquid-liquid extraction. Separation was achieved on a X-TERRA C18 column using a mobile phase composed of 20 mM KH(2)PO(4), acetonitrile, and n-octylamine (76:24:0.05, v/v/v) at 40 degrees C and detected at 237 nm. The described assay was validated in terms of linearity, accuracy, precision, recovery and lower limit of quantification according to the FDA validation guidelines. Calibration curves were linear with a coefficient of variation (r) greater than 0.998. Accuracy ranged from 92.3% to 106.0% and precision was between 2.3% and 8.2%. The method has been applied to plasma and cerebrospinal fluid samples obtained from patients treated with oxcarbazepine, both in monotherapy and adjunctive therapy.

AEDs Availability and Professional Practices in Delivery Outlets in a City Center in Southern Vietnam

In developing countries, from 80 to 90% of the people with active epilepsy do not have access to treatment. A multitude of factors such as nonavailability of antiepileptic drugs (AEDs) contributes to the treatment gap in epilepsy. Our study carried out in 2003 in southern Vietnam showed that 57% of pharmacies had AEDs. A majority of these pharmacies were located in specific areas like market area or hospital area. The pharmacist in charge was present in only 24% of the pharmacies. The different kinds of AEDs available were carbamazepine (94%), phenytoin (61%), valproate (56%) and diazepam (16%). The maximum stock of a drug was two box. The availability of AEDs in southern Vietnam can be regarded as sufficient but does not allow an adequate treatment for a long time. An effort must be made to sensitize professional health workers to decrease the treatment gap in epilepsy.

Development of capillary zone electrophoresis-electrospray ionization-mass spectrometry for the determination of lamotrigine in human plasma

A method of coupling capillary zone electrophoresis (CZE) with electrospray ionization-mass spectrometry (ESI-MS) detection has been developed for monitoring an antiepileptic drug, lamotrigine (LTG) in human plasma. The CZE-MS was developed in three stages: (i) CZE separation and ESI-MS detection of LTG and tyramine (TRM, internal standard) were simultaneously optimized by studying the influence of CZE background electrolyte (BGE) pH, BGE ionic strength, and nebulizer pressure of the MS sprayer; (ii) sheath liquid parameters, such as pH, ionic strength, organic modifier content, and flow rate of the sheath liquid, were systematically varied under optimum CZE-MS conditions developed in the first stage; (iii) MS sprayer chamber parameters (drying gas temperature and drying gas flow rate) were varied for the best MS detection of LTG. The developed assay was finally applied for the determination of LTG in plasma samples. The linear range of LTG in plasma sample assay was between 0.1-5.0 microg/mL with a limit of detection as low as 0.05 microg/mL and run time less than 6 min. Finally, the concentration-time profile of LTG in human plasma sample was found to correlate well when CZE-ESI-MS was compared to a more established method of high-performance liquid chromatography with ultraviolet detection.

Long-Term Follow-Up Using a Higher Target Range for Lamotrigine Monitoring

The aims of the study were (1) to review the clinical application of the higher target plasma lamotrigine (LTG) concentration of 3-14 mg/L previously proposed by our therapeutic drug monitoring (TDM) laboratory following our initial study 7 years earlier, and (2) to survey clinical application of LTG assays by experienced neurologists (n = 11) who frequently use LTG. There was a 2.9-fold increase in LTG assay requests received by our laboratory from 1996 to 2003. By comparison, data for the number of LTG prescriptions filled throughout Australia were limited to the 4 years from 1997 to 2000, where a 1.7-fold increase was seen. LTG assay requests increased 1.5-fold in this same 4-year period (r2 = 0.97), indicating that the growth in assay requests paralleled the growth in prescriptions. The distribution of LTG concentrations measured in 2003 was compared with those for 1996 and 1997. This indicated there was a significantly increased (P < 0.01) clinical usage of the higher LTG target range. This result was reinforced by questionnaire responses. Respondents (100% of those surveyed), (1) considered the target LTG concentration (3-14 mg/L) to be one of the primary parameters applied in individualizing LTG dosage regimens, (2) were using target concentrations above 7 mg/L in 75% of patients, and (3) reported dose-limiting toxicities in some (but not all) patients typically at concentrations above, or well above, 13 mg/L. In conclusion, the growth in LTG assay requests received by our laboratory paralleled prescribing of this drug. The clinical use of the higher LTG target concentration range was increased during the 7 years since its introduction, indicating clinical acceptance and therapeutic benefit as well as the absence of long-term adverse effects associated with higher plasma LTG concentrations.

Anticonvulsant profile of flunarizine and relation to Na(+) channel blocking effects

The present study will summarize our findings concerning the anticonvulsant properties of the Ca2+ channel blocker flunarizine in a variety of experimental models of epilepsy. Flunarizine exhibits anticonvulsant effects against tonic seizures induced by electroshock or various chemoconvulsants in mice, however, did not protect against pentylenetetrazol-induced clonic seizures. In the MES test, the efficacy of clinically established antiepileptics was increased by co-medication. In the rotarod test, a minimal "neurotoxic" dose (TD50) of 18.0 mg/kg intraperitoneally was determined. In models of complex partial seizures like the hippocampal stimulation and the amygdala kindling in rats, flunarizine showed only a moderate activity. Thus, it can be suggested that the anticonvulsant potency of flunarizine in various screening tests is lower than that of standard antiepileptics such as carbamazepine and phenytoin. Concerning the possible mode of action, whole-cell patch-clamp experiments with cultured neonatal rat cardiomyocytes showed that flunarizine depressed the fast inward Na+ current in a concentration- and frequency-dependent manner well comparable with the action of phenytoin. It is concluded that the use-dependent inhibition of voltage-dependent Na+ channels may essentially contribute to the anticonvulsant activity of flunarizine in models for generalized tonic-clonic seizures. The clinical efficacy as add-on therapy is critically discussed in view of the present data.

LC method for the determination of oxcarbazepine in pharmaceutical preparations

A simple and accurate HPLC method for determination of oxcarbazepine (OXC) in a new tablet formulation is described. Chromatographic separation was achieved on a Diamonsil C18 column using a mobile phase consisting of acetonitrile, potassium phosphate monobasic buffer (pH 6.8) and water (36:8:56, v/v) at a flow rate of 1.0 ml/min. Absorbance was monitored at 255 nm where OXC has maximum absorption. The linear range of detection for OXC was from 9.96 to 99.6 microg/ml. The proposed method was validated for selectivity, precision, accuracy and limits of detection and quantitation, etc.

LC determination of oxcarbazepine and its active metabolite in human serum

Twenty-five percent of epileptic patients present refractory seizures to current frontline antiepileptic drugs, needing new treatments and leading to the introduction of several new AEDs, among which is oxcarbazepine (Trileptal). This 10-ketoanalogue of carbamazepine seems to be a weaker inducer of cytochrome P450 3A4. However, pharmacokinetic interactions with clinical significance have already been reported, before the marketing of Trileptal in France. The aim of this study was to develop and validate a HPLC method allowing simultaneous dosage of oxcarbazepine, 10-hydroxycarbamazepine, epoxycarbamazepine, carbamazepine, phenobarbital and phenytoïn. After plasma defecation by acetonitrile, dosage was obtained by analysis of the supernatants on a C(18) reversed-phase column coupled with UV detection (240 nm). The statistical validation was performed according to the recommendations of a European technical commission. This method seems to provide a quite good selectivity from the psychotropic therapeutics, which is commonly coprescribed with AEDs. Linearity was established for the whole concentration range, whatever the compound. Quantization limits of oxcarbazepine, 10-hydroxycarbamazepine, epoxycarbamazepine, carbamazepine, phenobarbital and phenytoïn are 0.58, 3.5, 2.35, 0.66, 1.02 and 3.13 microg/ml, respectively, and absolute recoveries are 105.15, 84.76, 94.45, 96.52, 98.62 and 95.08%, respectively.

Therapeutic drug monitoring of lamotrigine

OBJECTIVE

To evaluate the usefulness of routine monitoring of serum lamotrigine concentration.

DATA SOURCE

Literature was accessed through MEDLINE (1990-January 2001). Key search terms included lamotrigine, pharmacokinetics, and epilepsy.

DATA SYNTHESIS

A decision-making algorithm was used to evaluate the clinical evidence to support or refute the routine use of serum lamotrigine concentrations to adjust doses. The value of serum lamotrigine concentration monitoring remains controversial, primarily because clear relationships between concentration and pharmacologic response (either efficacy or toxicity) have not been demonstrated.

CONCLUSIONS

Serum concentration monitoring of lamotrigine is not recommended as a tool for routine dose adjustment.

Simultaneous determination of four antiepileptic drugs in serum by high-performance liquid chromatography

A high-performance liquid chromatographic method has been developed for the simultaneous determination of oxcarbazepine, 10-hydroxycarbamazepine, epoxycarbamazepine, carbamazepine, phenobarbital and phenytoin. After protein precipitation by acetonitrile, the supernatant was analysed on a C18 reversed-phase HPLC column. Antiepileptic drugs and oxazepam (internal standard) were detected by ultraviolet absorbance at 240 nm. Linearity was established for the whole concentration range for each compound. Quantitation limits of oxcarbazepine, 10-hydroxycarbamazepine, epoxycarbamazepine, carbamazepine, phenobarbital and phenytoin were 0.58, 3.5, 2.35, 0.66, 1.02 and 3.13 microg/mL, respectively, and mean recoveries added to serum were 105.15, 84.76, 94, 45, 96.52, 98.62 and 95.08%, respectively. This method has been used for the simultaneous determination of steady-state serum concentration of antiepileptic drugs in patients treated by one or more anticonvulsive treatment.

New antiepileptic drugs: review on drug interactions

During the Past decade, nine new antiepileptic drugs (AEDs) namely, Felbamate, Gabapentin, Levetiracetam, Lamotrigine, Oxcarbazepine, Tiagabine, Topiramate, Vigabatrin and Zonisamide have been marketed worldwide. The introduction of these drugs increased appreciably the number of therapeutic combinations used in the treatment of epilepsy and with it, the risk of drug interactions. In general, these newer antiepileptic drugs exhibit a lower potential for drug interactions than the classic AEDs, like phenytoin, carbamazepine and valproic acid, mostly because of their pharmacokinetic characteristics. For example, vigabatrin, levetiracetam and gabapentin, exhibit few or no interactions with other AEDs. Felbamate, tiagabine, topiramate and zonisamide are sensitive to induction by known anticonvulsants with inducing effects but are less vulnerable to inhibition by common drug inhibitors. Felbamate, topiramate and oxcarbazepine are mild inducers and may affect the disposition of oral contraceptives with a risk of failure of contraception. These drugs also inhibit CYP2C19 and may affect the disposition of phenytoin. Lamotrigine is eliminated mostly by glucuronidation and is susceptible to inhibition by valproic acid and induction by classic AEDs such as phenytoin, carbamazepine, phenobarbital and primidone.

Zonisamide in pediatric epilepsy: review of the Japanese experience

Zonisamide is a novel anticonvulsant that is structurally and mechanistically unique, compared with other antiepilepsy drugs. Available in Japan and South Korea since 1989, it was approved in the United States in the year 2000 as adjunctive therapy for partial seizures in adults. There has been extensive clinical trial and clinical practice experience with zonisamide therapy in Japanese children. Open-label data from pediatric clinical trials conducted in Japan suggest that zonisamide is well tolerated and effective against partial- and generalized-onset seizures in children. Despite this wealth of open-label data, no formal pharmacokinetic studies and only one well-controlled trial of zonisamide's efficacy and safety in Japanese children have been completed to date. No controlled clinical trials of zonisamide in children have been completed in the United States or Europe. Additional controlled trials in children with partial- or generalized-onset seizures, infantile spasms, and Lennox-Gastaut syndrome are warranted to further delineate zonisamide's broad spectrum of efficacy and tolerability in children.

The chemistry, toxicology, and identification in rat and human urine of 4-hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one: a reactive metabolite in felbamate bioactivation

4-Hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one has been proposed to be a reactive metabolite of the anti-epileptic drug felbamate [Thompson et al. (1996) Chem. Res. Toxicol. 9, 1225-1229]. 4-Hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one exists in equilibrium with 3-oxo-2-phenylpropyl aminooate, which is known to eliminate to generate 2-phenylpropenal. Thus, this species is postulated to be a latent form of the ultimate reactive metabolite, 2-phenylpropenal. The chemistry of 4-hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one is proposed to parallel that of 4-hydroxycyclophosphamide, the bioactivated form of cyclophosphamide that undergoes ring-opening to aldophosphamide and subsequent elimination to afford 2-propenal (acrolein). The work presented here reports the chemical synthesis of 4-hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one and demonstrates that under buffered conditions it exists in equilibrium with 3-oxo-2-phenylpropyl aminooate. The rate-limiting step in the decomposition of 4-hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one is the irreversible beta-elimination from 3-oxo-2-phenylpropyl aminooate to 2-phenylpropenal. We have found the half-life of 4-hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one to be 4.6 +/- 0.4 h under in vitro conditions that mimic the physiological setting. As a consequence of the relatively long half-life of 4-hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one, we have sought evidence for the significance of this pathway in experimental and clinical conditions. We report here the observation of this metabolite in the urine of rats being treated with 3-hydroxy-2-phenylpropyl aminooate, the esterase-mediated metabolite of felbamate, and in the urine of patients undergoing felbamate therapy. In addition, we have shown that 4-hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one is toxic to cultured cells in a time-dependent manner, most likely as a result of its decomposition to 2-phenylpropenal. Taken together, the data support the hypothesis that 4-hydroxy-5-phenyl-1,3-oxazaperhydroin-2-one represents a "time-release" form of 2-phenylpropenal capable of traveling to distal sites from its locus of bioactivation and thereby mediates felbamate associated toxicities.

Treatment of epilepsy in the multiply handicapped

The medical management of epilepsy in the multi-handicapped patient requires careful evaluation, classification, and pharmacologic treatment. It is estimated that 20-40% of patients with mental retardation and cerebral palsy have epilepsy. This review reports the clinical trial data and personal experience related to the use of newer AEDs in the chronic management of epilepsy syndromes in children and adults, as well as information available on the treatment of seizures in individuals with mental retardation and associated handicaps. Furthermore, clusters of seizures, prolonged seizures and status epilepticus are more commonly seen in the multiply handicapped and mentally retarded population and require special attention. The new antiepileptic drugs felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, tiagabine, topiramate, vigabatrin and zonisamide show specific advantage in some multiply handicapped patients, be it for seizure control or medication tolerance. Furthermore, new modalities of treatment for prolonged seizures allow better efficacy both outside of hospital and within hospital facilities. The treatment of epilepsy in multi-handicapped and retarded adults and children has significantly advanced in the past few years, and much of this improvement can be attributed to improved knowledge and monitoring of new antiepileptic drugs. Conventional anticonvulsants remain first line therapy for most clinicians, but newer AEDs must broaden the therapeutic option and do allow improved therapy for some multiply handicapped patients.

Plasma level monitoring of oxcarbazepine in epileptic patients

BACKGROUND

Despite the wide use of oxcarbazepine (OXC) there is little data concerning the usefulness of plasma level monitoring with this drug in Mexican patients with epilepsy. The purpose of the present study was to determine whether OXC levels correlate with dose, age, weight, or drugs used concomitantly.

METHODS

Plasma levels of the antiepileptic drug OXC were evaluated in 214 patients with epilepsy. In each patient, plasma MHD (10-hydroxycarbazepine, the main metabolite of OXC) concentration was determined. Additionally, plasma protein binding was determined in 30 patients and affinity to red blood cells (RBCs) was evaluated in 50 patients.

RESULTS

Our results showed that the mean plasma level of MHD was 15.34 microg/mL, mean protein binding ranged between 30-40%, and the mean RBC concentration was 18.38 microg/mL. A relationship between dose/weight and plasma concentration was found (r = 0.5149, p <0.001). In addition, a linear relationship between plasma and RBC concentration was established (r = 0.8806, p <0.0001).

CONCLUSIONS

These results suggest that for OXC, routine RBC concentrations are not necessary to make drug adjustments.

Quantification in patient urine samples of felbamate and three metabolites: acid carbamate and two mercapturic acids

PURPOSE

Previously we proposed and provided evidence for the metabolic pathway of felbamate (FBM), which leads to the reactive metabolite, 3-carbamoyl-2-phenylpropion-aldehyde. This aldehyde carbamate was suggested to be the reactive intermediate in the oxidation of 2-phenyl-1,3-propanediol monocarbamate to the major human metabolite 3-carbamoyl-2-phenylpropionic acid. In addition, the aldehyde carbamate was found to undergo spontaneous elimination to 2-phenylpropenal, commonly known as atropaldehyde. Moreover, atropaldehyde was proposed to play a role in the development of toxicity during FBM therapy. Evidence for atropaldehyde formation in vivo was reported with the identification of modified N-acetyl-cysteine conjugates of atropaldehyde in both human and rat urine after FBM administration. Identification of the atropaldehyde-derived mercapturic acids in urine after FBM administration is consistent with the hypothesis that atropaldehyde is formed in vivo and that it reacts with thiol nucleophiles. Based on the hypothesis that the potential for toxicity will correlate to the amount of atropaldehyde formed, we sought to develop an analytic method that would quantify the amount of relevant metabolites excreted in patient urine.

METHODS

We summarize the results of an LC/MS method used to quantify FBM, 3-carbamoyl-2-phenylpropionic acid and two atropaldehyde-derived mercapturic acids in the patient population.

RESULTS

Analysis was performed on 31 patients undergoing FBM therapy. The absolute quantities of FBM and three metabolites were measured.

CONCLUSIONS

This method demonstrated sufficient precision for the identification of patients exhibiting "abnormal" levels of atropaldehyde conjugates and may hold potential for patient monitoring.

GABA (gamma-amino-butyric acid) neurotransmission: identification and fine mapping of the human GABAB receptor gene

GABA (gamma-amino-butyric acid) receptors are a family of proteins involved in the GABAergic neurotransmission of the mammalian central nervous system (CNS). They have physiological importance and clinical relevance in several diseases. We report the identification, cloning, and fine mapping of the human cDNA for GABAB receptor. A 4.2-Kb cDNA containing an open reading frame for a predicted protein of 960 aa was isolated from a fetal brain cDNA library. It had a strong identity (91.5%) with the rat GABAB receptor (rGB1A) nucleotide sequence, that corresponded to 98.6% identity at the amino acid level. Expression of the GABAB at the transcription level was detected by Northern analysis in all brain areas examined. The GABAB receptor has been mapped to human chromosome 6p21.3 within the HLA class I region close to the HLA-F gene. Susceptibility loci for multiple sclerosis, epilepsy, and schizophrenia have been suggested to map in this region.

The diversity of GABAA receptors. Pharmacological and electrophysiological properties of GABAA channel subtypes

The amino acid gamma-aminobutyric-acid (GABA) prevails in the CNS as an inhibitory neurotransmitter that mediates most of its effects through fast GABA-gated Cl(-)-channels (GABAAR). Molecular biology uncovered the complex subunit architecture of this receptor channel, in which a pentameric assembly derived from five of at least 17 mammalian subunits, grouped in the six classes alpha, beta, gamma, delta, sigma and epsilon, permits a vast number of putative receptor isoforms. The subunit composition of a particular receptor determines the specific effects of allosterical modulators of the GABAARs like benzodiazepines (BZs), barbiturates, steroids, some convulsants, polyvalent cations, and ethanol. To understand the physiology and diversity of GABAARs, the native isoforms have to be identified by their localization in the brain and by their pharmacology. In heterologous expression systems, channels require the presence of alpha, beta, and gamma subunits in order to mimic the full repertoire of native receptor responses to drugs, with the BZ pharmacology being determined by the particular alpha and gamma subunit variants. Little is known about the functional properties of the beta, delta, and epsilon subunit classes and only a few receptor subtype-specific substances like loreclezole and furosemide are known that enable the identification of defined receptor subtypes. We will summarize the pharmacology of putative receptor isoforms and emphasize the characteristics of functional channels. Knowledge of the complex pharmacology of GABAARs might eventually enable site-directed drug design to further our understanding of GABA-related disorders and of the complex interaction of excitatory and inhibitory mechanisms in neuronal processing.

Effect of chronic ethanol treatment in vivo on excitability in mouse cortical neurones in vitro

1. The effects of cessation of chronic ethanol ingestion on seizure activity in vivo and on the characteristics of the evoked synaptic potentials in cortical neurones in vitro have been investigated in mice. Withdrawal from chronic ethanol treatment increased handling seizure ratings in mice between 4 and 16 h post-withdrawal. This ethanol-induced increase in seizure rating was unaffected by carbamazepine (30 mg kg(-1)) but significantly reduced at a higher concentration (130 mg kg(-1)). 2. Intracellular recordings were made from cortical layer II neurones in vitro from control mice and from mice following chronic ethanol ingestion. Evoked synaptic potentials were generated in these neurones through intralaminar stimulation. 3. Neurones from control mice displayed an evoked potential consisting of a fast excitatory postsynaptic potential (e.p.s.p.) mediated by AMPA-type glutamate receptors and an inhibitory postsynaptic potential (i.p.s.p.) mediated via GABA(A) receptors. Application of pentylenetetrazole (PTZ) or bicuculline onto these neurones inhibited the i.p.s.p., caused a large increase in both the amplitude and duration of the e.p.s.p. and initiated spontaneous excitatory activity. The resulting large evoked e.p.s.p. was mediated via both NMDA- and AMPA-type glutamate receptors. 4. Most neurones (77%) from ethanol treated mice displayed an evoked potential which comprised a large e.p.s.p. and no i.p.s.p. The e.p.s.p. consisted of several distinct components and in addition these neurones displayed spontaneous paroxysmal depolarizing shifts. This multi-component e.p.s.p. was mediated through both NMDA- and AMPA-type glutamate receptors. A population (23%) of neurones from ethanol treated mice exhibited evoked potentials which possessed both inhibitory and excitatory components and these neurones were effectively identical to those obtained from control mice. 5. Carbamazepine reduced the duration of the e.p.s.p. in neurones from ethanol treated mice and in PTZ-treated control neurones. 6. Prolonged ethanol ingestion is known to create a neurochemical imbalance in cortical neurones resulting in abnormal neurotransmission. The present study highlights the functional consequences that arise as a result of these neurochemical changes leading to over-excitation of neurones and pronounced epileptiform activity.

Optimized high-performance liquid chromatographic method for determination of lamotrigine in serum with concomitant determination of phenytoin, carbamazepine, and carbamazepine epoxide

Lamotrigine (LG), phenytoin (PY), carbamazepine (CM), and carbamazepine epoxide (CE) are measured with an optimized procedure that uses thin sorbent extraction disks and a highly selective, sterically protected bonded silica high-performance liquid chromatography (HPLC) column. Routinely, serum (200 microliters at pH 6.8 with cyheptamide as internal standard) is applied to an Empore octyl (C8) solid-phase extraction disk to isolate the drugs. a water wash removes interferences, and the retained drugs are eluted with a small volume of solvent. The eluate is directly injected onto a Zorbax Stable Bond cyanopropyl HPLC column with quantification at 214 nm. Evaporation-concentration steps are unnecessary. Overall, for all drugs, between-run precision coefficients of variation (n = 16 each) ranged from 2.1% to 4.9% at concentrations from 0.75 to 20.5 mg/l; extraction recoveries fell within a range of 96% to 110% at concentrations of 2, 10, and 30 mg/l tested for each drug; the lowest limit of detection was 0.15 to 0.35 mg/l. The analytical response was linear for each drug > 80 mg/l (LG) and > 50 mg/l (PY, CM, and CE). Optimization graphs are presented to illustrate the rationale for selection of test parameters for a robust method. In addition, a comparison study between two commercial laboratories demonstrates accuracy problems associated with LG testing.

Identification of modified atropaldehyde mercapturic acids in rat and human urine after felbamate administration

3-Carbamoyl-2-phenylpropionaldehyde has recently been proposed [Thompson et al. (1996) Chem. Res. Toxicol. 9, 1225-1229] as a potential reactive metabolite of the anti-epileptic drug felbamate. This aldehyde was found to undergo rapid elimination to generate 2-phenylpropenal and reversible cyclization to generate 4-hydroxy-5-phenyltetrahydro-1,3-oxazin-2-one at physiological pH. 2-Phenylpropenal, an alpha,beta-unsaturated aldehyde commonly termed atropaldehyde, is a potent electrophile and undergoes rapid conjugation with glutathione. We sought to demonstrate the formation of atropaldehyde in vivo through the identification of mercapturic acids in rat and human urine after felbamate administration. In this paper, we describe the identification of both the reduced (N-acetyl-S-(2-phenylpropan-3-ol)-L-cysteine) and oxidized (N-acetyl-S-(2-phenyl-3-propanoic acid)-L-cysteine) mercapturic acids of atropaldehyde in rat and human urine. The reduced species was the more abundant in human (approximately 2:1) and rat (approximately 6:1) urine. These findings establish the possibility that atropaldehyde is formed from felbamate in vivo, undergoes glutathione conjugation, and is ultimately excreted in urine in the form of mercapturic acids. Thus, the proposed pathway of felbamate biotransformation, if confirmed in patients, could contribute to our understanding of the toxicities observed during felbamate treatment.

New antiepileptic drugs in children: present and future

The pharmacologic management of epilepsy has progressed greatly during the last decade. New medications are available for the management of refractory patients, and more are being developed. For some patients, these productions offer added efficacy, better tolerability, and some pharmacodynamic advantages. Since preapproval studies include few pediatric trials, the final role of these medications in the treatment of childhood epilepsy will be dictated by additional studies and postmarketing experience.

Synthesis and in vitro reactivity of 3-carbamoyl-2-phenylpropionaldehyde and 2-phenylpropenal: putative reactive metabolites of felbamate

We propose that 3-carbamoyl-2-phenylpropionaldehyde is an intermediate in the metabolism of felbamate, an anti-epileptic drug with a unique profile of the therapeutic activity, and undergoes a cascade of chemical reactions responsible for the toxic properties of the parent drug. To test this hypothesis, we have synthesized 3-carbamoyl-2-phenylpropionaldehyde and evaluated its in vitro reactivity. This molecule was found to be highly unstable at physiological pH (t1/2 < or = 30 s) and to undergo facile elimination to 2-phenylpropenal, an alpha, beta-unsaturated aldehyde commonly termed atropaldehyde. However, the predominant reaction pathway for 3-carbamoyl-2-phenylpropionaldehyde was reversible cyclization to generate 4-hydroxy-5-phenyltetrahydro-1,3-oxazin-2-one, a urethane that has a considerably longer half-life at physiological pH (t1/2 > or = 5 h) and may serve as a stable reservoir of the reactive aldehyde both in vitro and in vivo. Atropaldehyde is a potent electrophile and was found to exhibit cytotoxicity to cultured fibroblasts (50% growth inhibition (GI50) = 4.1 +/- 1.1 microM) comparable to the known unsaturated aldehyde toxins, 4-hydroxy-2-nonenal and acrolein. 3-Carbamoyl-2-phenylpropionaldehyde also exhibited significant cytotoxicity (GI50 = 53 +/- 8 microM), whereas 2-phenyl-1,3-propanediol monocarbamate (GI50 > 500 microM) and 3-carbamoyl-2-phenylpropionic acid (GI50 > 500 microM) were nontoxic. We have additionally demonstrated the formation of a glutathione-atropaldehyde conjugate from the in vitro incubation of 3-carbamoyl-2-phenylpropionaldehyde with glutathione. Thus, the potent cytotoxicity and potential allergenicity of atropaldehyde implicate this unsaturated aldehyde as a possible causative agent in the toxicities observed with felbamate treatment.