Exposure and toxicity of green tea polyphenols in fasted and non-fasted dogs

Standardized green tea extract was evaluated for exposure and toxicity in Beagle dogs following oral dosing by capsules. The main component (-)-epigallocatechin gallate (EGCG) accounted for 56-72% of the material. A 9-month chronic study (0, 200, 500, and 1000 mg/kg/day) was done in fasted dogs to take advantage of the reported improved catechin bioavailability with fasting. Extensive morbidity, mortality, and pathology of many major organs led to its early termination at 6.5 months and prevented identification of the toxicity mechanisms. A follow-up 13-week study examined the exposure to and toxicity of the extract. In general, toxicities were less severe than in the chronic study during the same interval. Dosing in a fed state resulted in considerably lower and less variable exposure than found under fasted conditions. Toxicity was less frequent and of lesser severity with lower exposure but limited sample size and large variability prevented reaching that definitive conclusion. Differences in mortality and morbidity between the preliminary terminated chronic and follow-up subchronic studies with the same dose of the same drug lot and similar exposure were not fully resolved as there may be other as yet unclear confounding factors.

Comparing pharmacokinetic and pharmacodynamic profiles in female rats orally exposed to lovastatin by gavage versus diet

The objective of this study was to assess how the dosing method (i.e., gavage versus diet) affects the absorption and disposition of lovastatin, as well as its effect on two biological markers of exposure, such as serum levels of cholesterol and triglycerides. In preclinical safety studies the test agent is normally administered by gavage, but in chemoprevention efficacy studies the test agent is usually administered with the diet. Therefore, extrapolation of safety and efficacy data from laboratory animals to humans should consider the influence of the method of administration on the absorption, disposition and effect of the drug. Lovastatin, a blood cholesterol-lowering drug with a short elimination half-life in humans, was used to assess the influence of two different dosing methods on the drug pharmacokinetics and pharmacodynamics. Plasma and liver concentrations of lovastatin and its active metabolite lovastatin-Na were measured in female rats at sequential times after administration. Serum concentrations of triglycerides and cholesterol were measured at similar times and used as biomarkers of effect. Significant differences in pharmacokinetics and pharmacodynamics were observed after administration of lovastatin by the two oral dosing paradigms. In general, oral gavage resulted in higher peak and lower trough concentrations of lovastatin and lovastatin-Na in plasma and liver, lower area under the concentration-time curve of lovastatin-Na in plasma and liver, and less of an effect on the serum concentrations of triglycerides and cholesterol than the corresponding diet dosing. Although no inverse linear relationship was observed between pharmacokinetic and pharmacodynamic markers, in the case of serum cholesterol a visual trend could be observed which might have proven significant had data from a larger number of dose levels been available. As in our previous study with sulindac, this study illustrates potential limitations in trying to extrapolate from data obtained using different dosing schemes to potential safety and efficacy in humans.

Computer-aided drug discovery and development (CADDD): in silico-chemico-biological approach

It is generally recognized that drug discovery and development are very time and resources consuming processes. There is an ever growing effort to apply computational power to the combined chemical and biological space in order to streamline drug discovery, design, development and optimization. In biomedical arena, computer-aided or in silico design is being utilized to expedite and facilitate hit identification, hit-to-lead selection, optimize the absorption, distribution, metabolism, excretion and toxicity profile and avoid safety issues. Commonly used computational approaches include ligand-based drug design (pharmacophore, a 3D spatial arrangement of chemical features essential for biological activity), structure-based drug design (drug-target docking), and quantitative structure-activity and quantitative structure-property relationships. Regulatory agencies as well as pharmaceutical industry are actively involved in development of computational tools that will improve effectiveness and efficiency of drug discovery and development process, decrease use of animals, and increase predictability. It is expected that the power of CADDD will grow as the technology continues to evolve.

Effects of oral dosing paradigms (gavage versus diet) on pharmacokinetics and pharmacodynamics

In cancer chemopreventive studies, test agents are typically administered via diet, while the preclinical safety studies normally employ oral gavage dosing. Correspondence in pharmacokinetic and pharmacodynamic profiles between the two dosing approaches cannot be assumed a priori. Sulindac, a non-steroidal anti-inflammatory agent with potential chemopreventive activity, was used to assess effects of the two oral dosing paradigms on its pharmacokinetics and pharmacodynamics. Time-dependent concentrations of sulindac and its sulfone metabolite were determined in plasma and potential target organ, mammary gland. Prostaglandin E(2) was used as a pharmacodynamic biomarker and measured in mammary gland. An inverse linear relationship was detected between pharmacodynamic and pharmacokinetic markers, area under the curve for prostaglandin E(2) levels and sulindac sulfone concentrations, respectively, in the mammary tissue. Marked differences in pharmacokinetics and pharmacodynamics were observed after administration of sulindac by the two oral dosing paradigms. In general, oral gavage resulted in higher peak and lower trough concentrations of sulindac in plasma and mammary tissue, higher area under concentration-time curve in plasma and mammary tissue, and greater effect on prostaglandin E(2) levels than the corresponding diet dosing. This study illustrates potential pitfalls and limitations in trying to generalize based on data obtained with different oral dosing schemes and their extrapolation to potential efficacy and health risks in humans.

Chiral high-performance liquid chromatographic analysis of enantiomers of losigamone, a new candidate antiepileptic drug

An assay based on a single-step liquid-liquid extraction from human plasma followed by high-performance liquid chromatography on a chiral column was developed for the measurement of enantiomers of a racemic new candidate antiepileptic drug. Excellent intra- and inter-assay accuracy and precision and recovery were demonstrated in the desired concentration range of 0.031 to 5.00 microg/ml. The method is free from interferences by other anticonvulsant drugs and their metabolites. The method is being used in a clinical trial of losigamone.

Potentially reactive cyclic carbamate metabolite of the antiepileptic drug felbamate produced by human liver tissue in vitro

Felbamate (FBM) is a novel antiepileptic drug that was approved in 1993 for treatment of several forms of epilepsy. After its introduction, toxic reactions (aplastic anemia and hepatotoxicity) associated with its use were reported. It is unknown whether FBM or one of its metabolites is responsible for these idiosyncratic adverse reactions. Although the metabolism of FBM has not been fully characterized, three primary metabolites of FBM have been identified, i.e. 2-hydroxy, p-hydroxy, and monocarbamate metabolites. In addition, the monocarbamate metabolite leads to a carboxylic acid, which is the major metabolite of FBM in humans. Formation of the hydroxylated products of FBM involves cytochrome P450 enzymes, but the enzymes involved in the formation and further metabolism of the monocarbamate have not yet been elucidated. Recently, mercapturate metabolites of FBM have been identified in human urine, and a metabolic scheme involving reactive aldehyde metabolite formation from the monocarbamate metabolite has been proposed. The present study confirmed the formation of the proposed metabolites using human liver tissue in vitro. The aldehyde intermediates were trapped as oxime derivatives, and the cyclic equilibrium product (proposed as a storage and transport form for the aldehydes) was monitored directly by HPLC or GC/MS. Formation of putative toxic aldehyde intermediates and the major carboxylic acid metabolite of FBM was differentially effected with the cofactors NADP+ and NAD+. It is possible that the cofactors may influence the relative metabolism via activation and inactivation pathways.

In vitro glucuronidation of D-23129, a new anticonvulsant, by human liver microsomes and liver slices

1. The metabolic profile of D-23129, a new anticonvulsant agent, was studied in vitro using human liver microsomes and fresh liver slices. 2. Oxidative metabolism appeared to be minimal with D-23129. The percent mean total radioactivity not associated with the parent compound recovered from oxidative metabolism studies from three individual liver donors was 0.7% +/- 0.6 SD and was not significantly different from [14C]-D-23129 incubated with heat inactivated microsomes, mean = 0.5% +/- 0.4 SD. 3. Phase II conjugation dominated the metabolism of D-23129 producing two distinct N-glucuronides as the primary metabolites. These metabolites were identified by electrospray ionization LC/MS. 4. The apparent Km for one of the glucuronide metabolites was determined in human liver microsome preparations from two individual liver donors to be 131 and 264 microM respectively, Vmax determined for the same microsomal preparations yielded 48.9 and 59.9 pmol/min/mg protein.

Stereoselective metabolism of a new anticonvulsant drug candidate, losigamone, by human liver microsomes

Losigamone (LSG) is a new candidate anticonvulsant drug under going preclinical and clinical development. Metabolism of racemic (+/-)-LSG and its two enantiomers, AO-242 [(+)-LSG] and AO-294 [(-)-LSG], was studied using human liver microsomes and recombinant cytochrome P450 isozymes. HPLC with both UV and electrochemical detection was used for analysis of the incubation media. Five metabolites (M1, M2, M3, M4, and M5) were generated from racemic (+/-)-LSG by both human liver microsomes and recombinant enzymes. Stereoselective metabolism was observed when each enantiomer was incubated separately with human liver microsomes. M1 was the major metabolite produced from (+)-LSG, whereas M3, M4, and M5 were primarily produced from (-)-LSG. The production of M1 from (+)-LSG was markedly inhibited by (-)-LSG, indicating a metabolic enantiomer/enantiomer interaction. (+/-)-LSG enantiomers were selectively metabolized by recombinant cytochrome P450 2A6, and the metabolism of (+)-LSG and (-)-LSG by human liver microsomes was preferentially inhibited by coumarin, a cytochrome P450 2A6-selective compound.

The effects of D-23129, a new experimental anticonvulsant drug, on neurotransmitter amino acids in the rat hippocampus in vitro

D-23129 [N-(2-amino-4-(4-fluorobenzylamino)phenyl)carbamic acid ethyl ester] and D-20443 (dihydrochloride of D-23129) are promising anticonvulsant compounds with a broad spectrum activity in animal models of epilepsy. Their effects on de novo synthesis of excitatory (glutamate and aspartate) and inhibitory (GABA) amino acids were studied in rat hippocampal slices. Like phenytoin, carbamazepine, lamotrigine, losigamone, U54494A, and flupirtine, D-23129 and D-20443 were effective in preventing the effects of a chemoconvulsant, 4-aminopyridine, on de novo synthesis of the three amino acids. However, unlike the other compounds, D-23129 and D-20443 also preferentially increased the concentrations of newly synthesized GABA. Their effect on the neosynthesis of GABA was unique, dose dependent, and not tetrodotoxin sensitive. A total of 15 compounds (including standard, new and candidate anticonvulsants) either had no effect on new GABA or decreased it. Therefore, D-23129 and D-20443 exhibited two different effects on de novo synthesis of neurotransmitter amino acids, both of which could potentially be anticonvulsant in nature.

The effects of anticonvulsant compounds on 4-aminopyridine-induced de novo synthesis of neurotransmitter amino acids in rat hippocampus in vitro

4-Aminopyridine, a voltage-dependent potassium channel blocker, causes tonic-clonic and electrographic seizures in vivo and evokes epileptiform activity and release of glutamate, aspartate and GABA in vitro. This study examined the effects of 4-aminopyridine (4AP) on de novo synthesis of neuroactive amino acids and a subsequent response to various anticonvulsant compounds (phenytoin, carbamazepine, phenobarbital, valproate, ethosuximide, diazepam, lamotrigine, felbamate, losigamone, U54494A, CPP, MK801 and CNQX) using a hippocampal slice preparation. 4-Aminopyridine had a minimal effect on total tissue concentrations of glutamate, aspartate, and GABA, but caused a significant increase in their de novo synthesis. Phenytoin, carbamazepine, lamotrigine, losigamone and U54494A were the only compounds which were effective in blocking the 4AP-induced increase in all newly synthesized amino acids. It appears that these compounds inhibit 4AP effects in this paradigm by blocking depolarization, probably at use-dependent voltage-sensitive sodium channels. Therefore, this paradigm may be useful in selectively identifying anticonvulsants which act by blocking depolarization.

The effects of anticonvulsant agents on 4-aminopyridine induced epileptiform activity in rat hippocampus in vitro

Six anticonvulsant drugs, phenytoin (PHT), carbamazepine (CBZ), valproate (VPA), U-54494A, losigamone (LOS), and D-20443, were studied using rat hippocampal slices and standard electrophysiological techniques. The K+ channel blocker, 4-aminopyridine (4-AP), was used as neuronal stimulant. The extracellular parameters evaluated in areas CA3 and CA1 were: (1) interictal-type bursting, (2) evoked population spike (PS) amplitude, (3) latency to PS onset, and (4) duration of the excitatory postsynaptic potential (EPSP). VPA was ineffective in altering any of the parameters. PHT and CBZ partially reversed the increase in EPSP duration produced by 4-AP in area CA3, while the spontaneous bursting was not affected. The experimental drugs, U-54494A, LOS, and D-20443 (dihydrochloride salt of D-23129 from Asta Medica), tended to reverse to varying degrees the 4-AP effects, especially the increase in the EPSP duration. U-54494A tended to depress responses even under control conditions. LOS partially reversed the 4-AP excitation, but abolished bursting in only one of five slices. D-20443 abolished bursting in all slices. It also partially reversed the 4-AP induced increase in the EPSP duration without depressing the normal evoked potential. The results show that 4-AP induced changes in vitro can help differentiate drugs with similar in vivo spectrums of anticonvulsant activity. While the drug induced changes may not truly define the mechanisms of action of these promising new agents, these experimental anticonvulsants can be differentiated from standard agents using the experimental paradigm in this study.

A system for testing the development and reversal of anticonvulsant tolerance to benzodiazepines in mice

Tolerance to the anticonvulsant effects of benzodiazepines limits their use in epilepsy treatment. Animal models producing tolerance have been developed, but they require repetitive injections over several days or use silastic capsules which must be made for each drug and do not provide a constant infusion rate. Alzet 2001 osmotic pumps deliver at a constant rate (1 microliter/h) and dosage can be easily adjusted. Various solvents, PEG 400, propylene glycol, 2% Tween, 50% DMSO, saline, Molecusol, and 0.5% methyl cellulose, were tried and found unsuitable because benzodiazepines were not maintained in solution or proconvulsant activity was seen. Tetraglycol was chosen as it did not demonstrate these shortcomings. Anticonvulsant activity was evaluated by PTZ i.v. tail infusion using forelimb clonus as the endpoint. This study describes a simple method for testing the development of tolerance and its reversal with flumazenil or ZK 93426. At 72 h of pump infusion with diazepam or flunitrazepam, tolerance to anticonvulsant activity was evident. Acute treatment with flumazenil or ZK 93426 reversed this tolerance. When flumazenil or ZK 93426 was given to diazepam tolerant mice, this reversal was complete. In flunitrazepam tolerant mice reversal with flumazenil was partial, but significant. When flumazenil was chronically coinfused with diazepam or flunitrazepam, anticonvulsant activity was antagonized. Similarly, when ZK 93426 was coinfused with diazepam, anticonvulsant activity was antagonized. The method described is suitable for screening putative anticonvulsant drugs for development of tolerance and the reversal of tolerance by other compounds.

Time-related loss of glutamine from hippocampal slices and concomitant changes in neurotransmitter amino acids

A dramatic, time-dependent loss of L-glutamine was observed in mouse and rat hippocampal slices equilibrated in normal artificial CSF under static (no-flow) and superfused (constant-flow) conditions. Concomitant with the decline in L-glutamine, there was a significant, but less pronounced, decrease in levels of the neurotransmitter amino acids, gamma-aminobutyric acid, L-aspartate, and L-glutamate. The disappearance of L-glutamine was a result of diffusion from the tissue to the artificial CSF rather than chemical or biochemical transformation. The loss of amino acids from the hippocampal slices was prevented to different degrees by the addition of 0.5 mM exogenous L-glutamine to the artificial CSF. The levels of newly synthesized amino acids were also determined, because they may be more indicative of the neuronal activity than the total tissue levels of amino acids. The effects of perturbations in glutamine (length of the equilibration time and addition of exogenous glutamine) on newly synthesized glutamate were more pronounced under 4-aminopyridine-stimulated than control (unstimulated) conditions. Therefore, a loss of L-glutamine from the hippocampal slices may have neurophysiological effects and warrants further investigation.

GABA-mediated synaptic interaction between the visual and vestibular pathways of Hermissenda

The synaptic convergence of the eyes and the vestibular hair cells in the nudibranch mollusc Hermissenda has been shown previously to mediate the learning of simple visual-vestibular associations. The neurotransmitter mediating this interaction between the visual and vestibular organs was characterized. HPLC chromatography, confirmed by mass spectroscopic analysis, demonstrated endogenous GABA in the statocysts, in a concentration approximately 150 times greater than in the whole CNS. Additional confirmation was provided by immunocytochemical localization of GABA in hair cell axons and branches that converge with photoreceptor terminal branches. Depolarization of the hair cells in the caudal region of the statocyst in response to positive current injection or vibratory stimulation caused a hyperpolarization and a cessation of the type B photoreceptor impulse activity. The inhibition of the B cell was unaffected by addition to the artificial sea water bath of the adrenergic antagonist yohimbine (250 microM), the cholinergic antagonist atropine (250 microM), and the serotonergic antagonist imipramine (50 microM). In contrast, the GABAA antagonist bicuculline (250 microM) significantly reduced the inhibitory interaction. Moreover, the GABA reuptake inhibitor guvisine (250 microM) increased the hyperpolarization. Pressure microapplication of GABA (12.5 or 25 microM) onto the terminal branches of the B cell resulted in a concentration-dependent hyperpolarization and cessation of spikes in the B cell. Depolarization of the caudal hair cell, or direct GABA application, decreased input resistance across the B cell soma membrane. Moreover, removal of chloride from the extracellular solution reduced inhibition of the B cell induced by GABA application or hair cell stimulation. Furthermore, application of the GABAB agonist baclofen hyperpolarized the type B cell and reduced or eliminated spontaneous impulse activity at the resting membrane potential. The reversal potentials for inhibition induced in all three procedures ranged from -70 to -80 mV and were consistent with mixed Cl- and K+ conductances. These results implicate GABA as the endogenous neurotransmitter mediating visual-vestibular interactions in this animal, and suggest a possible role of GABA in visual-vestibular associative learning.

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.

Antagonism between intracerebroventricularly administered N-methyl-D-aspartate and bicuculline methiodide in induction of clonic seizures in mice

N-Methyl-D-aspartate and bicuculline were administered alone or as a combination by intracerebroventricular injection to mice, and their convulsant activity was monitored. Both of these compounds elicited clonic seizures, though by different mechanisms. However, their simultaneous administration resulted in less than additive induction of clonic activity.

Use of stable isotopes and gas chromatography-mass spectrometry in the study of different pools of neurotransmitter amino acids in brain slices

A method was developed for simultaneous determination of endogenous and newly synthesized neurotransmitter amino acids (4-aminobutyric acid, glutamate and aspartate) and glutamine in brain in vitro. Brain slices were incubated in artificial cerebrospinal fluid in the presence of 13C-labeled precursors (glucose, pyruvate or acetate). After the incubation, the slices were homogenized in cold 80% ethanol and the supernatants were evaporated to dryness. The resultant residues were derivatized with N-methyl-N-(tert.-butyldimethylsilyl)trifluoroacetamide and analyzed by capillary gas chromatography-mass spectrometry in the electron-impact mode. N(O)-tert.-Butyldimethylsilyl derivatives of the naturally occurring amino acids, their 13C-enriched counterparts and deuterated internal standards were detected as their [M-57]+ fragments using selected-ion monitoring. The method was shown applicable to studying compartmentation of neurotransmitter amino acids.

Effects of pharmacological manipulations on basal and newly synthesized levels of GABA, glutamate, aspartate and glutamine in mouse brain cortex

Concentrations of basal and newly synthesized inhibitory (gamma-aminobutyric acid, GABA) and excitatory (glutamate and aspartate) neurotransmitter amino acids and glutamine were determined in mouse brain cortex. Isotopic enrichment following an intravenous infusion of a stable-labeled precursor, [13C6]D-glucose, was used to estimate the newly synthesized amino acid content. Effects of various pharmacological agents (valproate, aminooxyacetic acid, 3-mercaptopropionic acid, N-methyl-D-aspartate, and 2-amino-7-phosphonohepatanoic acid) were evaluated. The effects of 3-mercaptopropionic acid (an inhibitor of glutamate decarboxylase, a GABA-synthesizing enzyme) were restricted to the GABAergic system. On the other hand, N-methyl-D-aspartate (an agonist of a glutamate receptor subtype) was selective for the glutamate-glutamine system, and its effects were prevented by its selective antagonist, 2-amino-7-phosphonoheptanoic acid. In some cases, divergent effects were observed on basal and new amino acids. This suggested that basal and new amino acids may represent different compartments. The anticonvulsant drug valproate caused an increase in basal but a decrease in newly synthesized GABA. Aminooxyacetic acid caused a dramatic increase in basal GABA without affecting the newly synthesized GABA. This approach may be useful in studying compartmentation and fluxes of neurotransmitters.

Pharmacokinetic profile of flunarizine after single and multiple dosing in epileptic patients receiving comedication

This preliminary clinical study describes the pharmacokinetic characteristics of flunarizine (FLN) following single and multiple dosing in epileptic patients receiving comedication. Three groups [phenytoin (PHT) only, carbamazepine (CBZ) only, and PHT plus CBZ] of four patients each were studied. Large interindividual differences, but no statistically significant differences in pharmacokinetic parameters, were observed between the three groups. Following a single dose, mean values (and ranges) for apparent clearance, volume distribution, and elimination half-life (t1/2) were 0.504 L/h/kg (0.086-1.119), 12,431 L (1,959-20,920), and 308 h (61-506), respectively. FLN had no effect on PHT or CBZ steady-state levels but PHT or CBZ appeared to induce the metabolic disposition of FLN. The effect of dose on FLN kinetics could not be evaluated in this preliminary study.

Determination of 4-aminobutyric acid, aspartate, glutamate and glutamine and their 13C stable-isotopic enrichment in brain tissue by gas chromatography-mass spectrometry

A selected-ion monitoring method was developed for measuring 4-aminobutyric acid, aspartate, glutamate, and glutamine in brain tissue. Natural isotopes of these amino acids and their stable-isotopic enrichment following intravenous infusion of a precursor, [13C]glucose, were quantitated. Frozen mouse brain tissue was homogenized in cold 80% ethanol, and the supernatant, equivalent to 1 mg of wet weight brain tissue, was extracted using solid-phase bonded silica ion-exchange columns. Aspartate and glutamate (dicarboxylic acids) were isolated from strong anion-exchange columns, whereas 4-aminobutyric acid and glutamine (neutral amino acids) were isolated from strong-cation exchange columns. n-Butyl ester pentafluoropropionyl amide derivatives of these amino acids were analyzed by gas chromatography-mass spectrometry using a methane positive chemical ionization mode after gas chromatographic separation on a wide-bore, fused-silica capillary column. The method is applicable to determination of brain concentrations of these amino acids as well as their fluxes following administration of a stable-isotopic tracer.

Single and multiple dose kinetics of a new antiepileptic drug, Org 6370, and its desmethyl metabolite, Org 6363

The pharmacokinetic parameters of the new antiepileptic drug, Org 6370, and its desmethyl metabolite, Org 6363, were studied in healthy male volunteers. Plasma concentrations of the compounds were determined by a new method using liquid-solid extraction and capillary gas-chromatographic separation with a nitrogen-selective detector. The kinetic parameters obtained after a single oral dose of Org 6370 were not good predictors of multiple-dose parameters. With long-term treatment, there was unanticipated accumulation of the parent drug and especially the metabolite. The clinical implication of these findings is that caution must be exercised in clinical trials of Org 6370.

Determination in plasma of a new antiepileptic drug, dl-(5 alpha,9 alpha,11S*)-5,6,9,10-tetrahydro-N,N-dimethyl-5,9-methanobenzocycloo cten-11-amine hydrochloride, and its N-desmethyl metabolite by liquid-solid extraction and capillary gas chromatography

We have developed a sensitive and accurate method for the determination in plasma of the antiepileptic drug dl-(5 alpha,9 alpha,11S*)-5,6,9,10- tetrahydro-N,N-dimethyl-5,9-methanobenzocycloocten-11-amine hydrochloride and its N-desmethyl metabolite. The extraction procedure utilizes base-treated disposable C2 solid-phase columns, with the analyte eluted with organic solvent. Nitrogen-selective gas chromatography is used for detection. Linear regression analysis showed that the method is linear between 4 and 1500 ng/ml for the parent drug and between 8 and 3000 ng/ml for the N-desmethyl metabolite. Intra- and inter-day variability, as shown by the coefficient of variation, is less than 8% for both compounds. The method is applicable to routine plasma determination of both these compounds in clinical pharmacokinetic studies.

Inhibition of microsomal phenytoin metabolism by nafimidone and related imidazoles. Potency and structural considerations

Nafimidone and other 1-imidazoles were shown to be potent inhibitors of phenytoin p-hydroxylation in rat hepatic microsomes, being very effective even at submicromolar concentrations. The inhibitory potency of these 1-imidazoles was similar to that of SKF 525-A and considerably greater than that observed for other imidazoles (4,5-diphenylimidazole, cimetidine, metronidazole), metyrapone, or other anticonvulsants. The effects of structural modification on the inhibitory activity were examined. Except at the 2-position on the imidazole, alkyl substitution increased the inhibitory potency, probably because of increased lipophilicity. Substitution at the 2-position caused marked diminution in inhibitory activity, possibly due to steric hindrance. The hydroxy analogs of nafimidone exhibited greater inhibitory activity than the corresponding keto compounds. Furthermore, pretreatment of the rats with nafimidone resulted in greater Vmax values for both low affinity and high affinity metabolic sites.

Nafimidone, an imidazole anticonvulsant, and its metabolite as potent inhibitors of microsomal metabolism of phenytoin and carbamazepine

The effects of nafimidone and its metabolite (reduced nafimidone) on p-hydroxylation of phenytoin were examined by using hepatic microsomes from rats pretreated with phenytoin. Both nafimidone and reduced nafimidone inhibited p-hydroxylation of phenytoin in a concentration-dependent manner and at both high and low affinity metabolic sites. Both compounds were effective inhibitors at submicromolar concentrations. The inhibitory pattern at both metabolic sites was consistent with "mixed type" inhibition. The inhibition constants, Ki, calculated for reduced nafimidone, were about 0.2 microM at both sites. Submicromolar concentrations of nafimidone and its metabolite also inhibited carbamazepine epoxidation. Thus, nafimidone and its metabolite were shown to be potent inhibitors of major biotransformation pathways of two important antiepileptic drugs.

Methadone in brain and its effects on locomotor activity of young and aged mice

Plasma and brain levels of methadone produced by subcutaneous injections of the chloride salt (7.5, 15.0 or 22.5 mg/kg) and resultant changes in locomotor activity were determined in young (6-8 months) and aged (30-32 months) C57BL/6J mice. Methadone elevated locomotor activity of young and aged mice above control levels to about the same extent. The degree of activity elevation was inversely related to dose for both age groups, however, the reduction in stimulation with increasing dose was greater for young than aged mice; and only young mice exhibited a significant biphasic response to the high drug dose. Although the behavioral results suggest that aged mice were less responsive than young mice to methadone, brain concentrations of the drug were higher in aged than young mice by approximately one hour after injection. The age difference noted in brain concentration was not observed for plasma levels of methadone; hence cannot be accounted for by reduced drug metabolism in aged mice. The results of this study indicate that compared to young adults, aged mice are less responsive behaviorally to methadone despite higher brain concentrations of the drug.

Analysis of the antimalarial, mefloquine, in blood and plasma using high-performance liquid chromatography

An analytical method is described for the quantitation of mefloquine, a new antimalarial agent, in plasma and blood. A structurally similar quinolinemethanol compound, WR 184,806, is used as the internal standard. The method employs a three-step extraction procedure followed by reversed-phase high-performance liquid chromatography, and octanesulfonate is used as an ion-pairing reagent. Detection is achieved at 222 nm. The entire procedure is relatively simple and requires only 1 ml of sample. Good accuracy and precision are obtained over the wide concentration range tested.

Phenobarbital pharmacokinetics in rat as a function of age

Biodisposition of phenobarbital was examined in male Fischer-344 rats of different ages (3-4, 11-12, 23-24, and 32-34 months). Phenobarbital was administered intraperitoneally as a bolus (20 mg/kg) or continuously for 5 days from an implanted osmotic minipump (4.73 mg/day). Phenobarbital concentrations were determined by gas chromatography with a nitrogen-selective detector after extraction from plasma, plasma ultrafiltrate, and brain (cortex and cerebellum). Higher plasma and brain concentrations were found in the aged animals after a bolus injection or during continuous administration, and were related to decreases in the apparent plasma clearance of the drug. Apparent clearance was greater during continuous administration than following a bolus in 3- to 4-month- and in 32- to 34-month-old rats, which suggested autoinduction of phenobarbital metabolism. In addition, the plasma/brain distribution ratio of phenobarbital was elevated in the older animals.

Age effects on haloperidol pharmacokinetics in male, Fischer-344 rats

Biodisposition of haloperidol was examined in male, Fischer-344 rats in four age groups, 3 to 4, 11 to 12, 23 to 24 and 32 to 34 months. Haloperidol was administered i.p. as a bolus (0.50 mg/kg) or continuously during 4 days from an implanted osmotic minipump (0.15 mg/day). Plasma and regional brain concentrations of haloperidol were determined by gas chromatography with a nitrogen-phosphorus detector. After a bolus administration, plasma and brain concentrations were significantly higher at later time points, the plasma elimination half-life was significantly longer and the estimated plasma clearance was lower in 32- to 34- than in 3- to 4-month-old animals. The plasma and regional brain concentrations 6 hr after a bolus increased as a function of age, up to 23 to 24 months. At a steady state after continuous infusion, plasma and brain concentrations also rose with age up to 23 to 24 months. After either bolus or continuous infusion, the brain/plasma concentration ratios were lower at 11- to 12- and 23- to 24-months-of-age than at either of the other age groups. The results indicate that a reduced apparent plasma clearance of haloperidol is primarily responsible for higher plasma and brain concentrations of haloperidol in the older animals and that plasma concentrations are not always predictive of brain concentrations.