Multi-kinetics and site-specific release of gabapentin and flurbiprofen from oral fixed-dose combination: in vitro release and in vivo food effect

In this work, a fixed-dose combination of gabapentin and flurbiprofen formulated as multilayer tablets has been designed, developed and studied in vitro and in vivo. The aim was to construct a single dosage form of the two drugs, able to perform a therapeutic program involving three release kinetics and two delivery sites, i.e., immediate release of gabapentin, intra-gastric prolonged release of gabapentin and intestinal (delayed) release of flurbiprofen. An oblong three-layer tablet was manufactured having as top layer a floating hydrophilic polymeric matrix for gastric release of gabapentin, as middle layer a disintegrating formulation for immediate release of a gabapentin loading dose and as bottom layer, an uncoated hydrophilic polymeric matrix, swellable but insoluble in gastric fluids, for delayed and prolonged release of flurbiprofen in intestinal environment. The formulations were studied in vitro and in vivo in healthy volunteers. The in vitro release rate assessment confirmed the programmed delivery design. A significant higher bioavailability of gabapentin administered 30min after meal, compared to fasting conditions or to dose administration 10min before meal, argued in favor of the gastro-retention of gabapentin prolonged release layer. The two drugs were delivered at different anatomical sites, since the food presence prolonged the gastric absorption of gabapentin from the floating layer and delayed the flurbiprofen absorption. The attainment of a successful delayed release of flurbiprofen was realized by a matrix based on a polymers' combination. The combined use of three hydrophilic polymers with different pH sensitivity provided the dosage form layer containing flurbiprofen with gastro-resistant characteristics without the use of film coating.

Impact of oral supplementation of Glutamate and GABA on memory performance and neurochemical profile in hippocampus of rats

Glutamate (GLU) and gamma-amino butyric acid (GABA) are essential amino acids (AA) for brain function serving as excitatory and inhibitory neurotransmitter respectively. Their tablets are available in market for improving gut function and muscle performance. Despite of having a major role during memory formation and processing, effects of these tablets on brain functioning like learning and memory have not been investigated. Therefore, present study is aimed to investigate the effects of orally supplemented GLU and GABA on learning and memory performance and further to monitor related effects of these orally supplemented GLU and GABA on brain levels of these AA. Three groups of rats were supplemented orally with drinking water (control group) or suspension of tablets of GABA and Glutamate, respectively for four weeks. Cognitive performance was determined using behavioral tests (Novel object recognition test, Morris water maze, Passive avoidance test) measuring recognition, spatial reference and aversive memory. Levels of GLU, GABA and acetylcholine (ACh) were estimated in rat hippocampus. Results showed that chronic oral administration of GLU and GABA tablets has a significant impact on brain function and can alter GLU and GABA content in rat hippocampus. Compared to GABA, GLU supplementation specifically enhances memory performance via increasing ACh. Thus, GLU can be suggested as a useful supplement for improving learning and memory performance and neurochemical status of brain and in future could be effective in the treatment of neurological disorders affecting learning and memory performance.

Pharmacokinetics of Compounded Intravenous and Oral Gabapentin in Hispaniolan Amazon Parrots ( Amazona ventralis )

Neuropathic pain is a manifestation of chronic pain that arises with damage to the somatosensory system. Pharmacologic treatment recommendations for alleviation of neuropathic pain are often multimodal, and the few reports communicating treatment of suspected neuropathic pain in avian patients describe the use of gabapentin as part of the therapeutic regimen. To determine the pharmacokinetics of gabapentin in Hispaniolan Amazon parrots ( Amazona ventralis ), compounded gabapentin suspensions were administered at 30 mg/kg IV to 2 birds, 10 mg/kg PO to 3 birds, and 30 mg/kg PO to 3 birds. Blood samples were collected immediately before and at 9 different time points after drug administration. Plasma samples were analyzed for gabapentin concentration, and pharmacokinetic parameters were calculated with both a nonlinear mixed-effect approach and a noncompartmental analysis. The best compartmental, oral model was used to simulate the concentration-time profiles resulting from different dosing scenarios. Mild sedation was observed in both study birds after intravenous injection. Computer simulation of different dosing scenarios with the mean parameter estimates showed that 15 mg/kg every 8 hours would be a starting point for oral dosing in Hispaniolan Amazon parrots based on effective plasma concentrations reported for human patients; however, additional studies need to be performed to establish a therapeutic dose.

Analgesic Efficacy and Transdermal Penetration of Topical Gabapentin Creams: Finding an Optimal Dose and Pre-treatment Time

Many patients with chronic neuropathic pain continue to suffer despite traditional pharmacotherapy. As a result, pharmacists commonly compound gabapentin into creams, gels, and ointments as an alternative treatment option. In this study, various concentrations (1%, 5%, and 10%) of topical gabapentin compounded in Lipoderm were applied at various pre-treatment times (30 minutes, 1 hour, and 4 hours) to investigate what gabapentin concentration and pre-treatment time best attenuates formalin-induced nociceptive behaviors in a rodent model. A 30-minute pre-treatment with 5% gabapentin demonstrated maximum attenuation of nociceptive behaviors in this in vivo preclinical pain model. Nociceptive behaviors unexpectedly increased when gabapentin concentration or pre-treatment time was increased, suggesting both antinociceptive and pronociceptive effects of transdermal gabapentin administration. Gabapentin permeation into the skin and deeper tissues of the hindpaw was measured following the in vivo study. Skin and deep tissue permeation of gabapentin was both dose and time-dependent. Maximum deep-tissue permeation occurred within 30 minutes of topical application. Skin concentrations increased with a longer 1-hour pre-treatment. Minimal skin and deeper tissue levels were found following a 4-hour pre-treatment. These results suggest that topical gabapentin may be antinociceptive in a rodent formalin model at specific doses and pre-treatment intervals.

Skin permeation and antinociception of topical gabapentin formulations

Chronic pain affects greater than 116 million Americans each year. Even with the best pain management approaches, many chronic pain patients still suffer from moderate to severe pain. An alternative therapy to treat chronic pain includes compounded topical formulations of common analgesics. Compounded dosage forms of gabapentin are commonly used for pain management, however, the penetration and efficacy of gabapentin in these compounded topical formulations has not been fully studied. In this study, the transdermal penetration of gabapentin was studied in Franz diffusion cells using porcine skin. Gabapentin was compounded in two commercially available bases; Lipobase, Lipoderm, and a standard poloxamer lecithin organogel. The penetration and retention of gabapentin in the skin was dependent upon the base. The most rapid and greatest penetration and retention of gabapentin in the skin occurred with a poloxamer lecithin organogel base. Lipobase and Lipoderm bases produced slow and smaller penetration and retention of gabapentin as compared to poloxamer lecithin organogel base. Gabapentin 1% and 5% compounded in Lipoderm were tested in the in vivo preclinical formalin pain model. Topical 5% gabapentin produced a similar reduction in nociception in both Phase 1 and Phase 2 of the formalin response as systemic subcutaneous gabapentin (100 mg/kg). Topical 1% gabapentin reduced Phase 2, but not Phase 1 formalin-induced nociceptive behaviors. These findings suggest that topical administration of gabapentin may produce local antinociception.

Pregabalin: a review of its use in adults with generalized anxiety disorder

Pregabalin (Lyrica(®)), a well established anxiolytic agent, has been approved in the EU for the treatment of generalized anxiety disorder (GAD) in adults. It has a distinct mechanism of action relative to other anti-anxiety agents (α2δ binding at presynaptic voltage dependent calcium channels leading to inhibition of excitatory neurotransmission), a rapid onset of effect (typically ≤1 week) and broad spectrum activity against both the psychic and somatic symptoms of GAD. In long-term studies, pregabalin maintained improvements in anxiety symptoms that occurred in response to short-term treatment and delayed the time to relapse of GAD compared with placebo. Common comorbidities of GAD, such as insomnia, gastrointestinal symptoms and subsyndromal depression, have no effect on the anxiolytic efficacy of, and moreover are specifically improved by, pregabalin. Treatment with pregabalin is generally well tolerated; the drug has an adverse event profile that includes dizziness, somnolence and weight gain. The potential for abuse of pregabalin is low; the risk of withdrawal symptoms is generally low when the drug is discontinued gradually (over 1 week). Alongside selective serotonin reuptake inhibitors (SSRIs) and serotonin-noradrenaline reuptake inhibitors (SNRIs), pregabalin is considered a first-line agent for the long-term treatment of GAD by the World Federation of Societies of Biological Psychiatry. It should be stressed, however, that definitive head-to-head studies comparing pregabalin with SSRI/SNRIs, including in patients with GAD and co-morbid major depressive disorder, are currently lacking. Recently, a study of SSRI/SNRI augmentation with pregabalin yielded positive results, while another study of switching from long-term benzodiazepine therapy to pregabalin was inconclusive; further investigations on these topics are warranted.

Ex vivo percutaneous absorption of ketamine, bupivacaine, diclofenac, gabapentin, orphenadrine, and pentoxifylline: comparison of versatile cream vs. reference cream

This ex vivo human percutaneous absorption study evaluated a set of six model drugs (ketamine hydrochloride, bupivacaine hydrochloride, diclofenac sodium, gabapentin, orphenadrine citrate, pentoxifylline) from two popular formulations for topically applied compounding preparations. The compounded preparations used in this study were Versatile cream and a reference cream. Each formulation was applied to human trunk skin mounted on Franz Diffusion Cells, 50 mg/chamber (or 28.2 mg/cm2). Serial dermal receiver solutions were collected for 48 hours. Analysis of the resultant data supports the concept that the Versatile base formulation provides improved characteristics relative to the reference base. This is of key importance where the patient does not show clinical improvement when a conventional topical delivery vehicle is used in the formulation. From the results, it is reasonable to anticipate that, relative to the reference formulation, the Versatile formulation provides enhanced transdermal delivery of some analgesic medications.

Semi-mechanistic modelling of the analgesic effect of gabapentin in the formalin-induced rat model of experimental pain

PURPOSE

The formalin-induced rat model of nociception involves moderate continuous pain. Formalin-induced pain results in a typical repetitive flinching behaviour, which displays a biphasic pattern characterised by peaks of pain. Here we described the time course of pain response and the analgesic effect of gabapentin using a semi-mechanistic modelling approach.

METHODS

Male Sprague-Dawley rats received gabapentin (10-100 mg/kg) or placebo 1 h prior to the formalin injection, as per standard protocol. A reduction in the frequency of the second peak of flinching was used as a behavioural measure of gabapentin-mediated anti-nociception. The flinching response was modelled using a mono-exponential function to characterise the first peak and an indirect response model with a time variant synthesis rate for the second. PKPD modelling was performed using a population approach in NONMEM v.7.1.2.

RESULTS

The time course of the biphasic response was adequately described by the proposed model, which included separate expressions for each phase. Gabapentin was found to reversibly decrease, but not suppress the flinching frequency of the second response peak only. The mean IC50 estimate was 7,510 ng/ml, with relative standard error (RSE%) of 40%.

CONCLUSIONS

A compartmental, semi-mechanistic model provides the basis for further understanding of the formalin-induced flinching response and consequently to better characterisation of the properties of gabapentin, such as the potency in individual animals. Moreover, despite high exposure levels, model predictions show that gabapentin does not completely suppress behavioural response in the formalin-induced pain model.

Combined treatment with gabapentin and drugs affecting the renin-angiotensin system against electroconvulsions in mice

Recent experimental data suggest that certain angiotensin-converting enzyme (ACE) inhibitors and angiotensin AT1 receptor antagonists may possess anticonvulsant activity. The purpose of this study was to examine the effects of two ACE inhibitors, captopril and enalapril, and two AT1 receptor antagonists, losartan and telmisartan, on the protective action of gabapentin in the maximal electroshock seizure threshold test in mice. Additionally, the effects of the combined treatment with gabapentin and antihypertensive drugs on memory retention in the passive avoidance task and motor coordination in the chimney test were assessed. All drugs were injected intraperitoneally. Losartan (50mg/kg) significantly increased the convulsive threshold for gabapentin. The other antihypertensive drugs, captopril (50mg/kg), enalapril (30 mg/kg) and telmisartan (30 mg/kg), did not affect the anticonvulsant activity of gabapentin. The observed interaction between gabapentin and losartan could be pharmacokinetic in nature. Losartan increased plasma and total brain concentrations of gabapentin. In the chimney test, losartan (50mg/kg) administered with gabapentin (50mg/kg) caused motor impairment. In the passive avoidance test, memory retention was not affected by the combined treatment with gabapentin and antihypertensive drugs. It is suggested that the use of captopril, enalapril and telmisartan in epileptic patients receiving gabapentin is presumed neutral upon its anticonvulsant action. The utmost caution is advised when combining losartan and gabapentin in clinical practice due to the appearance of pharmacokinetic interactions between losartan and gabapentin as well as motor impairment evoked by these drugs in mice.

[Antioxidant agents in neuroprotection treatment of glaucoma]

Purpose was to study efficiency and safety of mexidol in combined therapy in patients with primary open-angle glaucoma (POAG). 94 patients (185 eyes) at the age of 18-75 years old with POAG I-III stages were divided into 3 groups: 50 patients received combined therapy of mexidol 100 mg and picamilon 150 mg, 22 patients received combined therapy of mexidol 300 mg and picamilon 150 mg, 22 patients received only picamilon 150 mg. All medicine was administered qd during 14 or 21 days. Examination included standard ophthalmologic methods, perimetry, electroretinography, retinal and optic nerve heard arterial blood flow. Improvement of visual acuity, perimetric, electrofisiological indicies and increased blood flow velocity of central retinal artery were registrated. Combined mexidol therapy allows improving results in treatment of patients with POAG.

The intestinal absorption mechanism of gabapentin makes it appropriate for gastroretentive delivery

Gabapentin is approved for the treatment of postherpetic neuralgia (PHN) and epilepsy. The pharmacokinetic (PK) properties of gabapentin, including absorption, distribution, metabolism, and excretion (ADME), were investigated during the development of Neurontin®, an immediate-release (IR) formulation of gabapentin that is orally administered three-times daily. Recently, a gastroretentive (GR) once-daily formulation of gabapentin (Gralise®) has been developed and marketed for the treatment of PHN. This review focuses on the ADME properties of gabapentin and illustrates how GR delivery enhances its absorption compared with IR formulations and allows once-daily dosing with the evening meal for the treatment of PHN. It includes the following aspects: 1) the mechanism of gastroretention of gabapentin GR tablets, 2) in vitro dissolution profiles of the GR and IR formulations, 3) site of absorption of gabapentin in the human intestine, 4) studies of the mechanism of gabapentin absorption using intestinal tissue preparations, 5) human PK studies to examine the effects of dose and formulations on PK profiles and the bioavailability of gabapentin at therapeutically relevant doses, and 6) efficacy and safety of gastroretentive gabapentin in patients with PHN. The data reviewed support that GR delivery of gabapentin optimizes its absorption via a saturable uptake mechanism. The prolonged residence of the GR tablets in the stomach coupled with the gradual release of gabapentin attenuates saturation of the transporter, thus enhancing absorption and increasing bioavailability, especially at therapeutically relevant doses. The net result is a once-daily formulation of gabapentin that is well tolerated and efficacious for the treatment of PHN.

[Hepatoduodenal circulation and excretion of the new GABA derivative citrocard]

Pharmacokinetic investigation of a new gamma-aminobutyric acid (GABA) derivative cirtocard showed that, upon the intravenous introduction, the drug is determined in high concentrations in organs of elimination--the liver and kidneys. The tissue accessibility amounts to 1.341 for the liver and 4.053 for the kidneys and the separation factor is 1.041 for the liver and 4.486 for the kidneys. The study of drug excretion showed that cirtocard is determined in the urine for 48 h, its nephritic clearance being 0.047 L/h and extra-nephritic clearance, 0.33 L/h. For the unchanged substance, a large significance ofhepatoduodenal circulation is low probable, since no more than 1 - 2% of the introduced dose was isolated with bile over entire experiment. It is established that the removal of the unchanged substance does not exceed 10% of the introduced dose. There is high probability of hepatoduodenal circulation and excretion of the preparation in the form of metabolites.

Application of ED-optimality to screening experiments for analgesic compounds in an experimental model of neuropathic pain

In spite of the evidence regarding high variability in the response to evoked pain, little attention has been paid to its impact on the screening of drugs for inflammatory and neuropathic pain. In this study, we explore the feasibility of introducing optimality concepts to experimental protocols, enabling estimation of parameter and model uncertainty. Pharmacokinetic (PK) and pharmacodynamic data from different experiments in rats were pooled and modelled using nonlinear mixed effects modelling. Pain data on gabapentin and placebo-treated animals were generated in the complete Freund's adjuvant model of neuropathic pain. A logistic regression model was applied to optimise sampling times and dose levels to be used in an experimental protocol. Drug potency (EC(50)) and interindividual variability (IIV) were considered the parameters of interest. Different experimental designs were tested and validated by SSE (stochastic simulation and estimation) taking into account relevant exposure ranges. The pharmacokinetics of gabapentin was described by a two-compartment PK model with first order absorption (CL = 0.159 l h(-1), V(2) = 0.118 l, V(3) = 0.253 l, Ka = 0.26 h(-1), Q = 1.22 l h(-1)). Drug potency (EC(50)) for the anti-allodynic effects was estimated to be 1400 ng ml(-1). Protocol optimisation improved bias and precision of the EC50 by 6 and 11.9. %, respectively, whilst IIV estimates showed improvement of 31.89 and 14.91 %, respectively. Our results show that variability in behavioural models of evoked pain response leads to uncertainty in drug potency estimates, with potential impact on the ranking of compounds during screening. As illustrated for gabapentin, ED-optimality concepts enable analysis of discrete data taking into account experimental constraints.

Pharmacokinetic of gabapentin 600 mg tablet in Thai healthy subjects

BACKGROUND

Gabapentin is an antiepileptic drug. It is structurally similar to yaminobutyric acid (GABA), which crosses the blood-brain barrier. Gabapentin is absorbed into the blood by the L-amino acid transport system. The oral bioavailability of gabapentin displays dose-dependence. Plasma concentrations ofgabapentin are not directly proportional to dose. Therefore, pharmacokinetic of gabapentin is essential for patients who have to receive gabapentin 600 mg.

OBJECTIVE

To investigate the pharmacokinetic of gabapentin 600 mg in Thai healthy subjects.

MATERIAL AND METHOD

The present study was performed on 24 healthy Thai male subjects who received a single oral dose of 600 mg gabapentin tablet. Serial blood samples were collected before and to 48 hours after drug administration. Plasma gabapentin concentrations were determined by automated High Performance Liquid Chromatography (HPLC) with UV detector after deproteinized with acetonitrile followed by derivatization with 1-fluoro-2,4-dinitrobenzene. The relevant pharmacokinetic parameters were determined.

RESULTS

The mean values of pharmacokinetic parameters (mean +/- SD) were 3.17 +/- 0.80 hour (1.5 to 5.0 hour) for T; 4,853.58 +/- 1,369.67 ng/ml for Cm; 0.11 +/- 0.02 hour for Kel, 6.62 +/- 1.87 hour (4.89 to 11.41 hour) for T1/2; 47,712.88 +/- 12,853.61 ng.hour/ml for AUC0-t, 48,713.20 +/- 12,909.78 ng.hour/ml for AUC0-inf, 5.24 +/- 1.32 L/hour for CI, and 49.28 +/- 15.98 L for Vd.

CONCLUSION

The data show the pharmacokinetic parameters of gabapentin 600 mg. These data should be used to support the assignment of therapeutic purposes for patients who have to receive gabapentin 600 mg.

[Pregabalin--a drug with abuse potential?]

A case of pregabalin misuse associated with delusional ideas in a drug addict is reported. Pregabalin has been approved as an adjunct therapy for epilepsy, but also for neuropathic pain and generalized anxiety disorders and is widely used today. It has also been used in clinical trials to study its potential utility as a treatment for tobacco, alcohol and benzodiazepine addiction. Web sites, case reports and an epidemiological study (Swedish National Register of Adverse Drug Reactions) suggest that the drug may be abused, especially by substance-dependent individuals. Pregabalin was analyzed by LC/MS/MS following precipitation of serum proteins. Vigabatrin was used as internal standard. The concentration of 25 pg pregabalin/mL serum determined in the present case is the second highest value published so far after misuse of the substance. Due to paradoxical agitation, anxiety attacks and abnormal thinking, the man was exculpated. Further studies are required to assess the actual abuse potential of pregabalin.

[Pantogam and pantogam active: qualitative and quantitative features of the interaction with neurotransmitter receptors in vitro]

We conducted a comparative study on the effect of active compounds of pantogam and pantogam active (calcium D(R)-homopantothenate and calcium DL(RS)-homopantothenate) and its L(S)-isomer on the receptors of main brain neuromediators in rats using in vitro radioligand binding analysis. All three compounds interact with binding sites of specific GABA-A and, in particular, GABA-B receptor ligands. Racemate and S-enantiomer, but not its R-form, competed to a moderate degree for D2-receptor binding sites. In all cases, degrees of interaction with receptors were ranged as follows: S-isomer>racemate>R-isomer. These qualitative and quantitative differences are assumed to contribute to pharmacological activity of both drugs.

Clinical pharmacokinetics of gabapentin after administration of gabapentin enacarbil extended-release tablets in patients with varying degrees of renal function using data from an open-label, single-dose pharmacokinetic study

BACKGROUND

Gabapentin enacarbil, a transported acyloxyalkylcarbamate prodrug of gabapentin, provides predictable and dose-proportional gabapentin exposure (AUC). Gabapentin is cleared via renal excretion, and its elimination is proportional to creatinine clearance (CrCL); CrCL can, therefore, be used as a predictor of gabapentin renal clearance. Gabapentin produced from hydrolysis of gabapentin enacarbil is also eliminated via the renal clearance pathway. It was, therefore, anticipated that the pharmacokinetics of gabapentin derived from gabapentin enacarbil would also be affected by renal function.

OBJECTIVE

The objective of this study was to describe a population pharmacokinetic analysis of gabapentin enacarbil in patients with varying degrees of renal function, using data from an open-label study of gabapentin enacarbil in patients with renal impairment (XenoPort, Inc. protocol XP066), to determine whether dosage adjustments are necessary in patients with renal impairment.

METHODS

Men and women >18 years of age with a body mass index ≤34 kg/m(2) and who were, in general, healthy with the exception of renal impairment were enrolled All patients received a single 600-mg gabapentin enacarbil extended-release tablet under fed conditions. After dosing, plasma, urine, and dialysate samples were analyzed. Safety profile evaluations included adverse events, vital signs, ECGs, and laboratory values. Pharmacokinetic data were compared with those from Phase I-III studies in subjects with normal renal function to evaluate the relationship between gabapentin oral clearance (CL/F) and CrCL.

RESULTS

Fifteen patients (11 men and 4 women) were enrolled. One patient had moderate renal impairment (CrCL 30-59 mL/min), 7 patients had severe renal impairment (CrCL <30 mL/min), and 7 patients had end-stage renal disease (CrCL <15 mL/min). Ten patients were white, 4 were African American, and 1 was American Indian or Alaskan Native. Their mean (range) age was 55 (28-76) years, weight was 85.6 (62-134) kg, and body mass index was 28.3 (22-34) kg/m(2). Mean maximum plasma gabapentin concentration was 5.77 μg/mL in patients with moderate and severe renal impairment, and 5.59 μg/mL in patients with end-stage renal disease who were undergoing hemodialysis. Based on the population pharmacokinetic analysis, gabapentin CL/F after administration of gabapentin enacarbil was proportionally related to CrCL, with an approximately 1.6-fold decrease in CL/F for every 2-fold decrease in CrCL. The most frequent adverse event was dizziness (4 of 15 patients). Other adverse events that were assessed as possibly or probably related to treatment were defecation urgency, extremity pain, feeling of relaxation, and muscle weakness; each occurred in 1 patient only. All events were mild or moderate and resolved without sequelae.

CONCLUSIONS

The data suggest that dosage adjustment for gabapentin enacarbil is necessary in patients with impaired renal function. Gabapentin enacarbil, 600 mg, seemed to be well tolerated in this small selected population.

[LC-MS/MS method for quantification and pharmacokinetic study of gabapentin in human plasma]

A sensitive, rapid and specific liquid chromatographic-tandem mass spectrometric (LC-MS/MS) method for quantification of gabapentin in human plasma has been developed. After a single plasma protein precipitation with methanol, gabapentin and metformin (internal standard) were chromatographed on a Inertsil ODS-3 column (50 mm x 2.1 mm ID, 3 microm) with mobile phase consisting of methanol-0.2% formic acid aqueous solution (80:20, v/v) at a flow-rate of 0.2 mL x min(-1). Electrospray ionization (ESI) source was applied and operated in the positive ion mode. Multiple reaction monitoring (MRM) mode with the transitions of m/z 172 --> m/z 154 and m/z 130 --> m/z 71 were used to quantify gabapentin and metformin, respectively. The run time was 2.2 min. The linear calibration curve was obtained in the concentration range of 40.8-8.16x10(3) ng x mL(-1). The lower limit of quantification was 40.8 ng x mL(-1). The intra- and inter-day precision (RSD) was less than 12%, and the accuracy (RE) was within +/-6.4% calculated from quality control (QC) samples. The method was used to determine the concentration of gabapentin in human plasma after a single oral administration of 600 mg gabapentin capsule to 20 healthy male Chinese volunteers. The method was proved to be selective, sensitive, rapid and suitable for pharmacokinetic study of gabapentin in human plasma.

Part 2: Design, synthesis and evaluation of hydroxyproline-derived α2δ ligands

Conformational constraint has been used to design a potent series of α(2)δ ligands derived from the readily available starting material (2S,4R)-hydroxy-l-proline. The ligands have improved physicochemistry and potency compared to their linear counterparts (described in our earlier publication) and the lead compound has been progressed to clinical development.

[Bioequivalence of anvifen and phenibut]

An open randomized crossover test on 18 healthy volunteers was used to study the pharmacokinetics of anvifen (acting substance, aminophenylbutyric acid) manufactured in capsules at the Joint-Stock Company "Antiviral" (Russia). The test was performed after single peroral administration in comparison with phenibut tablets of the same manufacturer. The concentration of unchanged aminophenylbutiric acid in the blood plasma was measured by HPLC with UV detection. It is established that anvifen and phenibut are bioequivalent in terms of pharmacokinetics.

Efficacy and safety of pregabalin for treating neuropathic pain associated with diabetic peripheral neuropathy: a 14 week, randomized, double-blind, placebo-controlled trial

AIMS

To evaluate the efficacy, safety and pharmacokinetics of pregabalin in treating neuropathic pain associated with diabetic peripheral neuropathy in Japanese patients.

METHODS

A randomized, double-blind, placebo-controlled, multicentre 14 week clinical trial was conducted. Japanese patients with diabetic peripheral neuropathy (n = 317) were randomized to receive placebo or pregabalin at 300 or 600 mg/day. The primary efficacy measure was a change of mean pain score from baseline to end-point from patients' daily pain diaries.

RESULTS

Significant reductions in pain were observed in patients treated with pregabalin at 300 and 600 mg/day vs. placebo (P < 0.05). Improvements in weekly pain scores were observed as early as week 1 and were sustained throughout the study period (300 and 600 mg/day difference from placebo at study end-point, -0.63 and -0.74, respectively). Pregabalin produced significant improvements in weekly sleep interference scores, the short-form McGill Pain Questionnaire, the Medical Outcomes Study-Sleep Scale, the 36-item Short-Form Health Survey scale, and the Patient and Clinical Global Impression of Change. Patient impressions of numbness, pain and paraesthesia were also significantly improved. Regarding treatment responders, 29.1 and 35.6% of patients treated with 300 and 600 mg/day, respectively, reported ≥ 50% improvement in mean pain scores (vs. 21.5% for placebo). Pregabalin was well tolerated; somnolence (26%), dizziness (24%), peripheral oedema (13%) and weight gain (11%) were the most common adverse events and generally were reported as mild to moderate.

CONCLUSIONS

Pregabalin was effective in reducing pain and improving sleep disturbances due to pain, and was well tolerated in Japanese patients with painful DPN.

[Pharmacokinetics of hopantenic acid upon peroral administration]

A simple, specific and sensitive RP-HPLC method with UV detection for the determination of hopantenic acid in human blood plasma has been developed. The pharmacokinetics of drug pantocalcin upon single peroral administration was investigated on 18 healthy volunteers. The peak of hopantenic acid in blood plasma was achieved at 1.56 h and the elimination half life was 6.68 h. No hopantenic acid in blood plasma is found in 48 h.

Tri-partite complex for axonal transport drug delivery achieves pharmacological effect

BACKGROUND

Targeted delivery of pharmaceutical agents into selected populations of CNS (Central Nervous System) neurons is an extremely compelling goal. Currently, systemic methods are generally used for delivery of pain medications, anti-virals for treatment of dermatomal infections, anti-spasmodics, and neuroprotectants. Systemic side effects or undesirable effects on parts of the CNS that are not involved in the pathology limit efficacy and limit clinical utility for many classes of pharmaceuticals. Axonal transport from the periphery offers a possible selective route, but there has been little progress towards design of agents that can accomplish targeted delivery via this intraneural route. To achieve this goal, we developed a tripartite molecular construction concept involving an axonal transport facilitator molecule, a polymer linker, and a large number of drug molecules conjugated to the linker, then sought to evaluate its neurobiology and pharmacological behavior.

RESULTS

We developed chemical synthesis methodologies for assembling these tripartite complexes using a variety of axonal transport facilitators including nerve growth factor, wheat germ agglutinin, and synthetic facilitators derived from phage display work. Loading of up to 100 drug molecules per complex was achieved. Conjugation methods were used that allowed the drugs to be released in active form inside the cell body after transport. Intramuscular and intradermal injection proved effective for introducing pharmacologically effective doses into selected populations of CNS neurons. Pharmacological efficacy with gabapentin in a paw withdrawal latency model revealed a ten fold increase in half life and a 300 fold decrease in necessary dose relative to systemic administration for gabapentin when the drug was delivered by axonal transport using the tripartite vehicle.

CONCLUSION

Specific targeting of selected subpopulations of CNS neurons for drug delivery by axonal transport holds great promise. The data shown here provide a basic framework for the intraneural pharmacology of this tripartite complex. The pharmacologically efficacious drug delivery demonstrated here verify the fundamental feasibility of using axonal transport for targeted drug delivery.

BL-1020, an oral antipsychotic agent that reduces dopamine activity and enhances GABAA activity, for the treatment of schizophrenia

In schizophrenia, a psychiatric disorder that affects approximately 1% of the global population and is associated with substantial disability, a significant proportion of patients (usually estimated to be at least 30%) fail to respond to treatment, and many patients have difficulty continuing treatment because of side effects. At the time of publication, all available medications for schizophrenia targeted dopamine and other monoamine neurotransmitters. However, a substantial amount of research suggests that patients with schizophrenia have underlying deficits within the GABA neurotransmitter system. BL-1020, being developed by BioLineRx Ltd under license from Tel Aviv University Ltd and Bar-Ilan Research & Development Co Ltd, is a novel compound consisting of the well-established antipsychotic drug perphenazine and GABA. Preclinical studies of BL-1020 indicated that the compound penetrated the brain and was efficacious in rodents, with a significant reduction in side effects compared with the administration of perphenazine. BL-1020 was well tolerated in all clinical trials conducted, and clinically meaningful improvements were demonstrated in phase II trials in patients with schizophrenia. Further data from phase II and subsequent phase III trials will be required to derive conclusions for BL-1020 regarding overall efficacy.

[Pregabalin--profile of efficacy and tolerability in neuropathic pain]

Pregabalin (PGB), like its predecessor gabapentin, is a structural analogue (but not functional) of the gammaaminobutyric acid (GABA). PGB has analgesic, antoconvulsivant, and ansiolytic activities. Neuropathic pain (NP) initial recommended dose is 150 mg per day. Depending on the patient response or bodily resistance to the drug the initial dose can be increased up to 300 mg per day (divided in 2-3 daily doses) during a one-week period. PGB is quickly absorbed in the digestive system with high oral biodisponibility. The drug is eliminated almost exclusively by renal excretion (98%). PGB efficacy was evaluated by several studies by means of visual analog scales (VAS) in several NP conditions such as post-herpetic neuropathy (PHN) or painful diabetic neuropathy (PDN). When compared with placebo, PGB provided significant benefit with 150 mg doses in the treatment of PHN pain. In studies developed in patients with PDN, the significant difference between placebo and treatment group only was achieved in individuals with daily doses of 300 mg/day, and the response was clearly dose-dependent. Tolerability information obtained from a number of studies has shown that the drug has a very good safety and tolerability profile. Side effects were mild to moderate and dose-dependent. Based on controlled studies, the main adverse effects observed with PGB are dizziness (23.1%), drowsiness (14.6%), and peripheral aedema (10.4%). As these side effects are dosedependent, they can be easily managed by a simple dose reduction, with no need to discontinue the therapy. Thus, according to efficacy and tolerability data, PGB is an important therapeutic option in NP treatment.

Pharmacokinetic and pharmacodynamic drug interactions during treatment with vigabatrin

Pharmacokinetic drug interactions take place when one drug interacts with another at the level of metabolism, absorption or excretion. Pharmacodynamic interactions take place at the level of receptor sites, where they may have additive or potentiating effects. Vigabatrin is relatively free of pharmacokinetic interactions, and though it is associated with about a 20% decrease in serum levels of concomitantly administered phenytoin, the reduction is of little clinical significance. The mechanism underlying this effect is unknown. Because vigabatrin increases GABA-mediated inhibition in the brain (an action that is believed to account for its anticonvulsant effects), it might be expected to potentiate the CNS effects of benzodiazepines and alcohol. However, very sensitive eye movement studies have failed to detect any evidence of such an interaction. Overall, vigabatrin appears to be remarkably free of drug interactions. As a result, it is easier to use in clinical practice than older anti-epilepsy agents. Perhaps the most important finding of the interaction studies with vigabatrin is that there is no need for patients receiving the drug to be told to avoid alcohol.

Vigabatrin: effect on brain GABA levels measured by nuclear magnetic resonance spectroscopy

Vigabatrin is undoubtedly one of the most exciting anti-epilepsy drugs in use today. Many open and controlled clinical trials have confirmed that it is particularly effective in controlling partial epileptic seizures with or without secondary generalization. Vigabatrin acts to increase GABA levels in the presynaptic nerve terminal by inhibiting the activity of GABA-transaminase. There is no direct correlation between the blood or brain concentration of vigabatrin and its clinical effect, so monitoring vigabatrin levels is not predictive of patient response. However, it is possible to relate the activity of vigabatrin to levels of GABA in the brain, measured by nuclear magnetic resonance spectroscopy (NMRS). NMRS studies show that following administration of vigabatrin, brain concentrations of GABA rise to about 2-3 times their baseline values. This 'extra' GABA is held within the nerve terminal, and is only released during synaptic transmission. Although there appears to be a clear dose-response relationship up to 3 g/day, it is not well documented if higher doses result in proportionately higher brain GABA levels. This finding seems to support the results of clinical studies suggesting that the optimal dose of vigabatrin may be 3 g/day. There is also some evidence for a correlation between the concentration of GABA in the brain and the clinical outcome. Continuing investigations using NMRS aim to confirm these preliminary findings, and to determine the time course and extent of changes in brain GABA levels after vigabatrin administration.

Gabapentin enacarbil, a gabapentin prodrug for the treatment of the neurological symptoms associated with disorders such as restless legs syndrome

Restless Legs Syndrome (RLS) is a sleep-related movement disorder characterized by an urge to move the legs accompanied by unpleasant symptoms. Dopaminergic agents currently represent the first-line therapy for RLS. However, some patients are unable to continue with this pharmacological approach in the long term because of augmentation or other adverse events. Limited studies with the anticonvulsant/analgesic agent gabapentin have demonstrated that this drug might be useful in the treatment of primary and uremic RLS. Unfortunately, gabapentin has an unfavorable pharmacokinetic profile characterized by a dose-dependent bioavailability. Gabapentin enacarbil, a prodrug of gabapentin, is currently being developed by XenoPort Inc/Astellas Pharma Inc/GlaxoSmithKline plc in order to overcome this limitation. In vitro and in vivo studies have demonstrated that gabapentin enacarbil provides a dose-proportional exposure to gabapentin when orally administered. Gabapentin enacarbil has proved to be beneficial in treating RLS symptoms in several phase II and III clinical trials. In addition, these trials have demonstrated that gabapentin enacarbil is safe and well tolerated, causing transient and mild or moderate adverse events. Gabapentin enacarbil is an interesting compound, which is potentially useful in treating RLS patients who report severe adverse events under dopaminergic agents. The author concludes that additional studies are required in order to better assess the efficacy and safety of gabapentin enacarbil on RLS.

Drug-induced encephalopathy secondary to non renal dosing of common medications in Two dialysis patients

The U.S. end-stage renal disease (ESRD) population continues to increase. Adjustment of several drugs administered to dialysis patients is mandatory because of decreased--and sometimes totally absent--renal clearances. Gabapentin, a newer anticonvulsant increasingly used for several other clinical indications, is excreted in the urine unchanged. Its half-life of 5 - 9 hours is increased to up to 132 hours in anuric patients. Excretion of acyclovir an antiviral agent, occurs predominantly through the kidney (glomerular filtration and tubular secretion). Its normal plasma half-life is 2 - 3 hours; dosage modifications are obligatory in renal insufficiency. In 2008, we encountered 2 ESRD patients on dialysis who exhibited significant neurotoxicity with encephalopathy after gabapentin and acyclovir were given at the usual adult doses. Following prompt drug discontinuation and continued daily hemodialysis or peritoneal dialysis respectively, both patients were discharged home, in normal clinical condition, after 3 days. With the increasing ESRD (and CKD) populations, health care providers other than nephrologists will have increasing contact with these patients. Those providers must consider drug dose modifications according to residual renal function. Algorithms for appropriate renal drug dosing are available from various sources. Pragmatic tactics of this kind will avoid significant morbidity and mortality. In addition, this approach will save millions of now increasingly scarce health care dollars.

Retention rates of new antiepileptic drugs in localization-related epilepsy: a single-center study

OBJECTIVES

We evaluated long-term retention rates of newer antiepileptic drugs (AED) in adults with localization-related epilepsy retrospectively.

METHODS

We estimated retention rates by Kaplan-Meier method in all 222 patients (age > or = 16) with localization-related epilepsy exposed to new AED at the Tampere University Hospital.

RESULTS

There were 141 patients exposed to lamotrigine, 78 to levetiracetam, 97 to topiramate, 68 to gabapentin, and 69 to tiagabine. Three-year retention rate for lamotrigine was 73.5%, levetiracetam 65.4%, topiramate 64.2%, gabapentin 41.7%, and tiagabine 38.2%. The most common cause for withdrawal of these AED was lack of efficacy.

CONCLUSIONS

Our study suggests that there are clinically significant differences among gabapentin, lamotrigine, levetiracetam, tiagabine, and topiramate as treatment for focal epilepsy in everyday practice. Gabapentin and tiagabine seem to be less useful than the other three AED. Furthermore, our study supports the value of retention rate studies in assessing outcome of the drugs in clinical practice.

New antiepileptic drugs in pediatric epilepsy

New antiepileptic drugs (AEDs), introduced since 1993, provide more diverse options in the treatment of epilepsy. Despite the equivalent efficacy and better tolerability of these drugs, more than 25% of patients remain refractory to treatment. Moreover, the issues for pediatric patients are different from those for adults, and have not been addressed in the development and application of the new AEDs. Recently published evidence-based treatment guidelines have helped physicians to choose the most reasonable AED, although they cannot fully endorse new AEDs because of the lack of well-designed, randomized controlled trials. We review the mechanisms of action, pharmacokinetic properties, adverse reactions, efficacy, and tolerability of eight new AEDs (felbamate, gabapentin, lamotrigine, levetiracetam, oxcarbazepine, topiramate, vigabatrin, and zonisamide), focusing on currently available treatment guidelines and expert opinions regarding pediatric epilepsy.

Cyclothiazide binding to the GABAA receptor

In order to explore the molecular interaction between cyclothiazide (CTZ) and gamma-aminobutyric acidA (GABAA) receptors, possibly underlying inhibition of GABAA receptor currents, [3H]-CTZ was synthesized. Binding of [3H]-CTZ to rat brain synaptic membranes could be observed only in the presence of the GABAA receptor antagonist (-)[1S,9R]-bicuculline methiodide (BMI) (EC(50,BMI)=500+/-80microM). GABA decreased [(3)H]-CTZ binding induced by the presence 300microM and 3mM BMI with IC(50,GABA) values of 300+/-50microM and 5.0+/-0.7mM, respectively. Binding of CTZ to [3H]-CTZ labeled sites was characterized by IC(50,CTZ) values of 0.16+/-0.03muM ([BMI]=300microM) and 7.0+/-0.5microM ([BMI]=3mM). Binding of the diastereomeric fraction [3H]-(3R,1'S,4'S,5'R+3S,1'R,4'R,5'S)-CTZ induced by 3mM BMI was quantitatively the more significant in cerebrocortical and hippocampal membranes. It was characterized by IC(50,CTZ)=80+/-15nM and IC(50,GABA)=13+/-3mcapital EM, Cyrillic. In the absence of BMI, CTZ (1mM) significantly decreased GABA-induced enhancement of [3H]-flunitrazepam binding. Our findings suggest that functional inhibition may occur through binding of CTZ to an allosteric site of GABAA receptors. This allosteric site is possibly emerged in the receptor conformation, stabilized by BMI binding.

Effect of L-arginine on synaptosomal mitochondrial function

OBJECTIVE

Specific aim of this study is to elucidate the direct effects of L-arginine on the synaptosomal neurotransmission related to the mitochondrial respiratory function.

METHODS

Using isolated endbrains from wild-type mice (ICR), crude synaptosome was analyzed for their concentration of gamma-aminobutyric acid (GABA) and glutamate (Glu) with/without addition of L-arginine. We analyzed the contents of releasing amino acids evoked by high potassium condition and uptake of them in three separated fractions (cytosol, vesicles, and intact mitochondria). The oxygen consumption was also measured by oxygen electrode.

RESULTS

The entire uptakes of GABA and Glu were inhibited by rotenone (about 30 nmol/mg protein) with dose-dependent manner and showed a plateau at about 70% of total uptake. L-arginine inhibited the uptake of Glu logarithmically, however it showed no change in uptake of GABA. The contents of GABA and Glu in synaptosome were decreased in the presence of L-arginine. L-arginine enhanced the respiration of state II by succinate on synaptosomal respiration, although the respiration of synaptosomal mitochondrial fraction and the respiratory chains enzyme activities were almost unaffected by L-arginine. In the presence of rotenone, L-arginine decreased the uptake of Glu without changing the uptake of GABA, increased the releasing of GABA, and may modulate the excitability of neuronal state on the cytosol, cytomembrane, and/or organelles except for mitochondria.

CONCLUSIONS

L-arginine may modulate excitation by neurotransmitters at nerve endings, in relation to potentiated respiratory metabolism of succinate in synaptosomes. Such effects might contribute to alleviation of stroke-like symptoms in MELAS.

Gabapentin Extended-Release - Depomed: Gabapentin ER, Gabapentin Gastric Retention, Gabapentin GR

Depomed is developing an extended-release (ER) oral formulation of gabapentin, a GABA receptor agonist commonly used for the treatment of epilepsy and seizures, neuropathic pain and hot flushes. Gabapentin ER is based on the company's proprietary AcuForm drug delivery technology, which is part of the Gastric Retention (GR) family of technologies; this offers improved drug absorption and bioavailability compared with the existing immediate-release formulation of gabapentin (Neurontin), making gabapentin ER suitable for twice-daily dosing. The product is in clinical development for the treatment of postherpetic neuralgia and diabetic neuropathies in the US. Additionally, Depomed has commenced a phase II trial of gabapentin ER in postmenopausal patients with hot flushes. Depomed's AcuForm platform is based on polymer technology that provides targeted drug delivery for a variety of compounds. Following ingestion, AcuForm tablets swell and are retained for 6-8 hours in the stomach, enabling controlled and prolonged release of gabapentin to the upper intestinal tract; this extends the time of drug delivery to the small intestine for complete and safe elimination via the lower intestinal track. Gabapentin ER is available for licensing. Depomed acquired exclusive development and commercialisation rights to gabapentin ER in September 2003 via its subsidiary, Depomed Development Ltd (DDL). Depomed is not required to pay upfront license fees, but will make royalty and milestone payments to DDL upon successful commercialisation of gabapentin ER. Gabapentin ER was originally developed by DDL, a joint venture between Depomed and Elan established in January 2000 to design products using the GR family of technologies. However, in efforts to restructure joint venture relationships, Elan withdrew from operational involvement of DDL in September 2003, and Depomed has gained full ownership of DDL. Depomed sublicensed exclusive rights to a US patent (held by the University of Rochester) covering the use of gabapentin in the treatment of hot flushes from PharmaNova in October 2006. Under the agreement, Depomed paid PharmaNova an upfront fee of US dollars 500 000. PharmaNova is also entitled to milestone payments and royalties on sales of gabapentin ER in this indication only. Depomed has reported significant safety and efficacy benefits from gabapentin ER in its phase II trial. This study was initiated in February 2005 following positive results from a phase I trial in which gabapentin ER demonstrated a pharmacokinetic profile suitable for twice-daily dosing. In two pharmacokinetic studies, gabapentin ER achieved improved bioavailability at higher doses. This result supports Depomed's development of a once- or twice-daily product with potentially fewer adverse events. The basic US patents relating to gabapentin expired in 2000. Depomed holds exclusive rights to a US patent (No. 6 310 098) held by the University of Rochester covering the use of gabapentin to treat hot flushes.Additionally, Depomed was issued a US patent (No. 6 723 340) in May 2004 that covers proprietary polymer combinations (as used in AcuForm tablets) to create improved formulations of existing drugs.

Pregabalin for the treatment of generalized anxiety disorder: a novel pharmacologic intervention

Pregabalin is the first anxiolytic pharmacologic alternative for the treatment of generalized anxiety disorder (GAD) to be introduced in more than 10 years. GAD is a significant psychiatric condition with lifetime prevalence rates ranging between 5.7 and 6.4%. It causes significant impairment in quality of life and functional abilities equivalent to those associated with major depression. Randomized, controlled trials confirm that pregabalin is superior to placebo and comparable with lorazepam, alprazolam and venlafaxine for the treatment of patients with moderate-to-severe GAD. The onset of anxiolytic activity for pregabalin is apparent within 1 week following initiation of treatment, which is more rapid than that obtained with paroxetine and venlafaxine. Additionally, pregabalin has demonstrated potential for the prevention of relapse of GAD. Recently, the efficacy, safety and tolerability of pregabalin were also shown in a placebo-controlled study with elderly patients. Safety and tolerability profiles are favorable, with transient dizziness and somnolence of mild-to-moderate severity being the most commonly reported adverse events. Pregabalin has minimal potential for drug-drug interactions and does not provoke a clinically significant withdrawal response. Furthermore, pregabalin has low potential for abuse and dependence, unlike other classes of medications used for the treatment of GAD. Clinicians may consider the use of pregabalin in lieu of benzodiazepines as an alternative therapy for their patients with GAD.

Effects of genetic variation in the novel organic cation transporter, OCTN1, on the renal clearance of gabapentin

Gabapentin is an anticonvulsant that is widely prescribed for epilepsy and other neuropathic disorders. The pharmacokinetics, particularly the absorption and renal elimination, of gabapentin appear to involve membrane transporters. In this study, we tested the hypothesis that organic cation transporter 1 (OCTN1), a multispecific transporter expressed at the apical membrane in intestine and kidney, plays a role in gabapentin pharmacokinetics and that the common variant of OCTN1, OCTN1-L503F, contributes to variation in the pharmacokinetics of the drug. We observed that OCTN1 facilitates the Na+-independent transport of gabapentin, and that the OCTN1-L503F variant is deficient in gabapentin transport activity in stably transfected HEK-293 cells (fourfold enhanced uptake of gabapentin by OCTN1-503L vs twofold enhanced uptake by OCTN1-L503F, compared to mock-transfected cells). In clinical studies, we found that in subjects homozygous for the L503F variant, gabapentin renal clearance (CL(R)) approximates the glomerular filtration rate (mean+/-SE: 110+/-12 ml/min, n=9), whereas in subjects homozygous for the reference allele, gabapentin undergoes net secretion in the kidney (141+/-7.8 ml/min, n=11, P<0.05). Creatinine clearance and OCTN1 genotype accounted for 56% of the variation in CL(R) and were the only significant predictors of CL(R) (P<0.05). Importantly, OCTN1 genotype was the only significant predictor of net secretion of gabapentin (P<0.008). Oral bioavailability of gabapentin was not affected by OCTN1 genotype. We conclude that OCTN1 contributes to active tubular secretion of gabapentin, and that this effect may be diminished or absent in individuals carrying the OCTN1-L503F polymorphism. These results provide clinical evidence of the role of genetic variation in renal drug transporters in active drug secretion in vivo.

Validated HPLC method for the determination of gabapentin in human plasma using pre-column derivatization with 1-fluoro-2,4-dinitrobenzene and its application to a pharmacokinetic study

A rapid, sensitive and accurate high-performance liquid chromatographic method with UV detection was developed and validated for the quantification of gabapentin in human plasma. Gabapentin was quantified using pre-column derivatization with 1-fluoro-2,4-dinitrobenzene following protein precipitation of plasma with acetonitrile. Amlodipine was used as internal standard. The chromatographic separation was carried out on a Nova-Pak C(18) column using a mixture of 50 mM NaH(2)PO(4) (pH=2.5)-acetonitrile (30:70, v/v) as mobile phase with UV detection at 360 nm. The flow rate was set at 1.5 ml/min. The method was linear over the range of 0.05-5 microg/ml of gabapentin in plasma (r(2)>0.999). The within-day and between-day precision values were in the range of 2-5%. The limit of quantification of the method was 0.05 microg/ml. The method was successfully used to study the pharmacokinetics of gabapentin in healthy volunteers.

Safety and efficacy of anticonvulsants in elderly patients with psychiatric disorders: oxcarbazepine, topiramate and gabapentin

Few controlled studies are available to guide the clinician in treating potentially assaultive elderly individuals with psychiatric disorders. Safety concerns limit the use of benzodiazepines and antipsychotic medications in the elderly individual, making anticonvulsants an attractive alternative. This paper reviews three specific anticonvulsants for this purpose: gabapentin, oxcarbazepine and topiramate, describing safety and efficacy in elderly patients with severe agitation from psychosis or dementia. Gabapentin, renally excreted, with a half-life of 6.5-10.5 h, may cause ataxia. Oxcarbazapine, hepatically reduced, may cause hyponatremia, and topiramate may cause significant cognitive impairment. Nonetheless, these are important medications to consider in the treatment of agitation.

The effect of MDR1 G2677T/A polymorphism on pharmacokinetics of gabapentin in healthy Korean subjects

This study evaluated the relationship between the genetic polymorphism in the MDR1 (exon 21) genes and the pharmacokinetics of gabapentin (GBP) in healthy Korean subjects. The healthy Koreans were genotyped with respect to MDR1 (exon 21, 30 subjects) using polymerase chain reaction-based diagnostic testing. The pharmacokinetic profile of GBP was examined. A single oral dose of 300 mg GBP in a capsule was administered to the subjects and the blood samples were taken during a 24 h post-dose interval. The serum GBP concentration was measured using an HPLC-fluorescence detector system. There were no significant (P < 0.05) differences between the genotypes and pharmacokinetic parameters such as AUC(0-1h), AUC(0-1.5h), AUC(0-infinity), Cmax and t1/2. However, there was a trend toward higher values of the absorptive phase characterized by the AUC(0-1h) and AUC(0-1.5h) in 2677T/A subjects. In conclusion, this study suggests that GBP may be a weak substrate for the P-glycoprotein (P-gp) transporter.

l-serine and GABA uptake by synaptosomes during postnatal development of rat

Postnatal development changes in mechanisms of synaptosomal amino acid transport have been studied in rat cerebral cortex. Specific uptake of radiolabeled L-serine was examined and compared with that of radiolabeled GABA using synaptosomes-enriched fractions freshly prepared from cerebral cortex at different postnatal days from the birth to young adulthood. The preparations were incubated with 10 nM of [3H]L-serine and 10 nM of [3H]-GABA in either the presence or absence of NaCl, KCl or choline chloride, at 2 and 30 degrees C, for different periods up to 30 min. The uptake of [3H]l-serine was temperature dependent in synaptosomal fractions prepared from cerebral cortex of rats in postnatal days 5, 7, 13 and 21, but stronger dependence was observed in adult brain, irrespective of the presence of Na+, K+ or choline ions. At all postnatal ages studied, [3H]-GABA uptake showed a high activity in the presence of Na+ ions and at 30 degrees C. The values of Km were 90-489 microM in L-serine uptake. However, in the uptake of GABA the values of Km were 80-150 microM. The highest values of Vmax were obtained at 5 and 21 postnatal days for both transport systems. These results indicate that the uptake of l-serine and GABA are regulated differentially during postnatal development.

In Vivo Effects of Glycyl-Glutamate and Glycyl-Sarcosine on Gabapentin Oral Absorption in Rat

PURPOSE

The objective of this study was to evaluate the in vivo consequences of glycyl-glutamate coadministration on gabapentin oral absorption.

METHODS

Rats were administered gabapentin (10 mg/kg plus radiotracer) by gastric gavage, in the absence and presence of dipeptides, and by intravenous administration. Serial blood samples were obtained over 6 h and the pharmacokinetics of gabapentin were determined by noncompartmental analysis.

RESULTS

Glycyl-glutamate coadministration increased the Cmax of gabapentin by 86% as compared to gabapentin alone. In agreement, the oral absorption of gabapentin, relative to the intravenous dose, was 79% after glycyl-glutamate loading but only 47% when drug was administered alone. However, when glycyl-sarcosine was added to the orally administered admixture of gabapentin plus glycyl-glutamate, values for Cmax and AUC(0-6 h) reverted back to that of control. In contrast, the tmax and terminal half-life of gabapentin did not change after oral dosing for all treatments.

CONCLUSIONS

These findings are unique in demonstrating that under physiologic, in vivo conditions, the luminal presence of glycyl-glutamate could dramatically enhance the Cmax and AUC(0-6 h) of gabapentin. The results are consistent with previous in situ intestinal perfusion studies in rat, and establish a functional interaction between the activities of PEPT1 and amino acid exchangers.

Isobolographic analysis of interaction between drugs with nonparallel dose-response relationship curves: a practical application

The objective of this study was to characterize the anticonvulsant and acute adverse-effect potentials of topiramate (TPM) and gabapentin (GBP)-two second-generation antiepileptic drugs administered alone and in combination in the maximal electroshock (MES)-induced seizures and chimney test in mice. The anticonvulsant and acute adverse effects of the combination of TPM with GBP at the fixed ratio of 1:1 were determined using the type I isobolographic analysis for nonparallel dose-response relationship curves (DRRCs). To ascertain any pharmacokinetic contribution to the observed interaction between TPM and GBP, total brain concentrations of both drugs were determined. The isobolographic analysis of interaction for TPM and GBP, whose DRRCs were not parallel in both MES and chimney tests, was accompanied with a presentation of all required calculations allowing the determination of lower and upper lines of additivity. The isobolographic analysis revealed that TPM combined with GBP at the fixed-ratio combination of 1:1 interacted supraadditively (synergistically) in terms of suppression of MES-induced seizures, and simultaneously, the combination produced additive interaction with respect to motor coordination impairment (adverse effects) in the chimney test. The evaluation of pharmacokinetic characteristics of interaction for the combination of TPM with GBP revealed that neither TPM nor GBP affected their total brain concentrations in experimental animals, and thus, the observed interaction in the MES test was pharmacodynamic in nature. In conclusion, the combination of TPM with GBP, because of supraadditivity in the MES test and additivity in terms of motor coordination impairment in the chimney test as well as lack of pharmacokinetic interactions between drugs, fulfilled the criterion of a favorable combination, worthy of recommendation in further clinical practice.

Uptake and release of [(3)H]GABA in human dental pulp

The purpose of this study was to determine whether (a) an uptake system for gamma-aminobutyric acid (GABA) exists in human dental pulp, (b) GABA can be released from nerves in this tissue, and (c) GABA(B) autoreceptors modulate release of this transmitter. Segments of vital pulp were incubated in [(3)H]GABA (0.1-10 microM) for up to 120 min, washed, and the retained [(3)H] extracted and assayed. Some tissues were treated with GABA uptake inhibitors (nipecotic acid or NO-711) prior to incubation. At concentrations of 0.1 and 1.0 microM the uptake of [(3)H]GABA was saturated after 90 min of incubation. At 10 microM, at least two uptake compartments were apparent, and the amount of [(3)H]GABA retained was five-fold greater than 0.1 microM. The uptake inhibitors reduced [(3)H]GABA accumulation by more than 80%. In the release study, pulp was incubated in [(3)H]GABA (0.5 microM) for 90 min, and superfused with Krebs solution containing NO-711 (5 microM). Electrical stimulation increased the overflow of [(3)H]; a GABA(B) autoreceptor agonist (baclofen) inhibited, whilst an antagonist, Sch 50911, enhanced this release. The effects of baclofen were reversed by Sch 50911. These results imply that GABA can be taken up and bound firmly in compartments within human dental pulp, GABA can be released from isolated pulp segments by electrical stimulation, and this release is modulated by GABA(B) autoreceptors.

Pregabalin and duloxetine for the treatment of neuropathic pain disorders

Clinical studies investigating the use of pregabalin and duloxetine for the management of diabetic peripheral neuropathy and post-herpetic neuralgia are reviewed. The benefits and potential drawbacks associated with these agents are discussed.