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PubMed2004

Determination of gabapentin-lactam in serum of patients under gabapentin therapy.

Gabapentin (1-(aminomethyl)cyclohexane acetic acid, CAS 601 42-96-3, GBP, Neurontin) and its derivative gabapentin-lactam (8-aza-spiro[5,4]decan-9-one, GBP-L) were determined by HPLC in the serum of patients with focal epilepsy treated with GBP. In patients in whom serum was acquired within 3 h after oral intake, GBP-L could be detected at concentrations up to 8.2 micromol/l. As GBP-L has been previously shown to exert neuroprotective effects in a similar concentration range, this finding suggests that clinically relevant effects of GBP-L may occur in patients treated with GBP. The possible neuroprotective efficacy of GBP-L should be the subject of further preclinical and clinical investigations.

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PubMed2001

Gabapentin and gabapentin monohydrate.

Gabapentin [1-(aminomethyl)cyclohexaneacetic acid, C9H17NO2] is a zwitterion in the solid state. Its crystal structure involves extensive hydrogen bonding between the NH3(+) and COO(-) groups of neighboring molecules. The structure of gabapentin monohydrate [1-(aminomethyl)cyclohexaneacetic acid monohydrate, C9H17NO2-H2O] also involves such hydrogen bonding and, in addition, has a hydrogen-bonding network comprising the water molecules and both the NH3(+) and COO(-) groups.

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PubMed2000

Gabapentin-lactam, a close analogue of the anticonvulsant gabapentin, exerts convulsant activity in amygdala kindled rats.

The cyclic GABA analogue gabapentin (GBP), which recently has been marketed for treatment of epilepsy, is particularly effective against complex-partial seizures as occurring in temporal lobe epilepsy. In the present study, we compared the effects of GBP and its lactam analogue (GBP-L) in the amygdala kindling model of temporal lobe epilepsy. In fully kindled rats, GBP (50 mg/kg and 100 mg/kg i.p.) dose-dependently increased the threshold for focal seizures and inhibited the progression from focal to generalized seizures. This effect was not associated with any marked adverse effects. In contrast, GBP-L (10-50 mg/kg) induced myoclonic activity and generalized clonic seizures in kindled rats, demonstrating a striking qualitative difference between the two compounds. By comparison with non-kindled rats it was shown that kindling markedly enhanced the sensitivity of rats to the convulsant activity of GBP-L. The finding that the anticonvulsant efficacy of GBP is lost by lactam formation indicates that GBP and GBP-L differ in their mechanism(s) of action.

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PubMedDate not supplied

Gabapentin (Neurontin) as add-on therapy in patients with partial seizures: a double-blind, placebo-controlled study. The International Gabapentin Study Group.

A multicenter, double-blind, randomized, placebo-controlled study evaluated the efficacy and safety of gabapentin (Neurontin, GBP) as add-on therapy in 272 patients with refractory partial seizures who were receiving one to two standard antiepileptic drugs (AEDs). Efficacy assessments compared the frequency of partial seizures during the 12-week treatment phase (T) and the 12-week baseline period (B). The primary analysis compared data for patients receiving GBP 900 mg/day with placebo; the GBP 1,200-mg/day group provided dose-response data. Efficacy criteria were percentage of change in seizure frequency (PCH), responder rate (percentage of patients with > or = 50% reduction in seizure frequency), and response ratio, where RRatio = (T-B)/(T + B). Median PCH was -21.8% in the 900-mg/day group and -17.8% in the 1,200-mg/day group as compared with -0.3% in the placebo group. Responder rate was 22.9% in the 900-mg/day group and 10.1% in the placebo group (p = 0.020, Fisher's exact test). Adjusted mean RRatio was -0.136 in the 900-mg/day group and -0.025 in the placebo group (p = 0.0046, analysis of variance ANOVA). Results showed slightly greater improvement for the 1,200-mg/day than for the 900-mg/day group (RRatio = -0.157, responder rate 28.0%). Adverse events (AE) occurred in 69% of patients in the 900-mg/day group and in 64% in the 1,200-mg/day group as compared with 52% in patients receiving placebo as add-on therapy. The most frequent AE among patients treated with GBP were somnolence, dizziness, and fatigue. Clinical laboratory evaluations showed no clinically important trends and no evidence of hepatic or hematopoietic effects.(ABSTRACT TRUNCATED AT 250 WORDS)

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PubMed1999

Gabapentin as add-on therapy in children with refractory partial seizures: a 12-week, multicentre, double-blind, placebo-controlled study. Gabapentin Paediatric Study Group.

PURPOSE: To evaluate the efficacy and safety of gabapentin (Neurontin; GBP) as add-on therapy for refractory partial seizures in paediatric patients aged 3-12 years. METHODS: After a 6-week baseline period, 247 patients (54 centres) entered a 12-week double-blind phase and were randomized to receive either GBP (t.i.d., titrated to 23-35 mg/kg/ day) or placebo. Seizure activity and type were recorded daily. Efficacy variables included Response Ratio (RRatio), responder rate, and percentage change in frequency (PCH) for all partial seizures; PCH and RRatio for individual types of partial seizures; and investigator and parent/guardian global assessments of seizure frequency and patient well-being. RESULTS: RRatio for all partial seizures was significantly lower (better) for GBP-treated patients (p = 0.0407). Responder rate favored GBP, but the difference between treatment groups was not statistically significant. Median PCH for all partial seizures for the GBP treatment group (-17.0%) was better than that for the placebo group (-6.5%). Median PCH for specific seizure types showed GBP to be most effective in controlling complex partial seizures (-35%) and secondarily generalized seizures (-28%) when compared with placebo (-12%, +13%, respectively). A greater percentage of GBP-treated patients exhibited improvement according to investigator and parent/guardian global assessments, with a statistically significant difference observed in the parent/guardian global assessment of seizure-frequency reduction (p = 0.046). Three GBP patients and one placebo patient were seizure free during the double-blind treatment period. GBP was well tolerated. CONCLUSIONS: GBP was effective and well tolerated as an add-on therapy for partial seizures in paediatric patients with previously drug-resistant seizures.

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PubMedDate not supplied

[Gabapentin used in 559 patients with partial seizures. A multicenter observation study. Spanish Gabapentin Work Group].

INTRODUCTION: Gabapentin (GBP) is a new antiepileptic drug whose efficacy and tolerability have been evaluated in clinical trials, although there is little data on its use in everyday clinical practice. OBJECTIVES: To evaluate the characteristics of GBP in an observational study when used in patients with uncontrolled partial seizures. PATIENTS AND METHODS: An open multicentric study in which GBP was used in 559 patients of over 12 years of age with uncontrolled partial crises in whom the efficacy, tolerability and quality of life (QOLIE-10) over a period of 6 months were analysed. RESULTS: The response rate (> 50% reduction in seizures) was 71% with 35.3% seizure-free patients. There were no differences related to age, aetiology, previous frequency of seizures or duration of the epilepsy. In 18.8% there were adverse effects (somnolence, dizziness, headache, blurred vision, diplopia and nausea), which were generally well-tolerated, but in 4.5% of the patients led to the drug being suspended. In the 65 patients in whom quality of life was evaluated there was improvement, both overall and in each aspect studied. CONCLUSION: Under normal conditions of everyday clinical practice, GBP is an effective drug which is well tolerated by adults and adolescents with refractory partial seizures.

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PubMed2004

Gabapentin: a pooled analysis of adverse events from three clinical trials in patients with postherpetic neuralgia.

BACKGROUND: Gabapentin has been shown to be well tolerated and effective in the management of the pain associated with postherpetic neuralgia (PHN). It is assumed that adverse events occurring with gabapentin are dose related, their frequency and severity increasing with increasing doses. OBJECTIVE: The aim of this study was to assess the dose dependence of adverse events with gabapentin by determining the relationship between increasing doses of gabapentin and the onset and/or worsening of adverse events in patients with PHN. METHODS: Data were pooled from 3 randomized, double-blind, placebo-controlled, parallel-group studies of gabapentin that focused on or included patients with PHN. Gabapentin was initiated at 300 mg/d and titrated to maintenance doses of 1800 to 3600 mg/d by day 12 to 24. The analysis of adverse events was based on 3 distinct groups: patients who received gabapentin <1800 mg/d, those who received gabapentin >or=1800 mg/d, and those who received placebo. Patients who were given higher doses of gabapentin had already received lower doses. An adverse event was recorded at the dose of its first onset and recorded again if its severity worsened at a higher dose. RESULTS: This study included data from 603 patients with PHN: 358 patients (196 [54.7%] women, 162 [45.3%] men; mean [SD] age, 72.3 [10.3] years) received gabapentin, and 245 (133 [54.3%] women, 112 [45.7%] men; mean [SD] age, 73.3 [10.7] years) received placebo. The 3 most common adverse events were dizziness, somnolence, and peripheral edema. Patients receiving gabapentin >or=1800 mg/d had a higher incidence of peripheral edema (7.5%) than those receiving gabapentin <1800 mg/d (1.4%) or placebo (1.6%) (P<0.002, gabapentin >or=1800 mg/d vs placebo). In contrast, the incidence of dizziness and somnolence was not higher in patients receiving gabapentin >or=1800 mg/d compared with those in the other groups. Compared with placebo recipients, patients receiving gabapentin <1800 mg/d reported a significantly greater frequency of dizziness (20.2% gabapentin <1800 mg/d vs 7.4% placebo; P<0.002) and somnolence (14.9% vs 5.8%, respectively; P=0.005). However, at >or=1800 mg/d, rates of dizziness (9.7%) and somnolence (6.9%) were comparable to those with placebo. Discontinuation rates were comparable between patients receiving gabapentin and those receiving placebo. CONCLUSIONS: In this pooled analysis of adverse-event data from 3 clinical trials in patients with PHN, the incidence of peripheral edema was increased when gabapentin was titrated to >or=1800 mg/d. Dizziness and somnolence, the other most commonly occurring adverse events, were transient and did not occur more frequently or worsen with titration to >or=1800 mg/d. Based on these findings, it does not appear that safety concerns should limit titration of gabapentin to achieve optimal efficacy.

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PubMed1997

Spermine modulation of specific [3H]-gabapentin binding to the detergent-solubilized porcine cerebral cortex alpha 2 delta calcium channel subunit.

1. Recent studies have identified the [3H]-gabapentin-binding protein, purified from porcine cerebral cortical membranes, as the alpha 2 delta subunit of voltage-sensitive calcium channels (Gee et al., 1996). The present study investigates the influence of the polyamine spermine on specific [3H]-gabapentin binding to detergent-solubilized porcine cerebral cortical membranes. 2. Spermine, spermidine, 1,10 diaminodecane, Mg2+ and Zn2+, all divalent cations, displaced [3H]-gabapentin binding to detergent-solubilized membranes in a concentration-dependent manner with a maximal inhibition of 65-75%. Radioligand binding studies showed that spermine did not directly interact with the [3H]-gabapentin-binding site. Spermine inhibited [3H]-gabapentin binding by interacting with a polyamine-sensitive allosteric site on the membrane protein. The steep concentration-dependence of spermine inhibition of [3H]-gabapentin binding may suggest multi-site co-operativity. 3. Prolonged dialysis of cerebral cortical membranes and Tween 20-solubilized membranes resulted in a > 2.0 fold increase in [3H]-gabapentin binding. The increase in binding was due to the removal of a heat stable, low molecular weight (< 12,000Da) endogenous molecule which influences [3H]-gabapentin binding competitively. 4. Dialysis of detergent-solubilized cerebral cortical membranes also resulted in a decrease in the maximum inhibition of [3H]-gabapentin binding by spermine. Since the rates of the increase in [3H]-gabapentin binding and the loss of the ability of spermine to inhibit [3H]-gabapentin binding on dialysis were different it was inferred that a second endogenous ligand was removed during dialysis. 5. During initial steps of purification of the [3H]-gabapentin-binding protein there was a decrease in the maximum inhibition of [3H]-gabapentin binding by spermine. The loss of the second endogenous molecule during initial purification would reasonably explain the reduction in inhibition of binding by spermine. However, spermine stimulation of [3H]-gabapentin binding to material that eluted from the gel-filtration column later in the purification scheme does not appear to be due to removal of a dialysable endogenous factor or to the dissociation of other calcium channel subunit(s). 6. Adding back dialysate, before or after boiling, to detergent solubilized membranes resulted in a dose-dependent restoration of the inhibition of [3H]-gabapentin binding and of the maximal inhibition [3H]-gabapentin binding by spermine. This result is consistent with the re-addition of two endogenous heat stable ligands. 7. The findings that [3H]-gabapentin binding to the pure alpha 2 delta subunit was stimulated by spermine indicates that the alpha 2 delta subunit of voltage-sensitive calcium channels bears a modulatory spermine site. Such a spermine site has not been identified before. Spermine stimulation of [3H]-gabapentin binding to the purified protein was reversed to inhibition after adding back dialysate. Thus the inhibitory spermine effect in membranes is also probably due to one or more modulatory sites on the alpha 2 delta subunit.

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PubMedDate not supplied

Impact of a clinical pharmacy consult service on guideline adherence and management of gabapentin for neuropathic pain.

OBJECTIVE: Our objectives were to (1) determine whether a computerized clinical pharmacy approval and follow-up consult process for ordering new prescriptions for gabapentin for the treatment of neuropathic pain decreased the number of patients without documented treatment benefit while increasing follow-up and documentation of effectiveness, and (2) describe gabapentin use patterns at a Veterans Affairs (VA) Medical Center, including the use of first-line therapies prior to gabapentin therapy for neuropathic pain. METHODS: The clinical pharmacy intervention included review of (1) the indication for gabapentin; (2) the required use and failure or contraindication of 3 first-line therapies: nonsteroidal anti-inflammatory drugs (NSAIDs), tricyclic antidepressants (TCAs), and capsaicin cream; (3) the initial pain assessment; and (4) patient follow-up in 4 to 6 weeks, with repeat pain assessment. A retrospective chart review was performed for all patients who received a new prescription for gabapentin from October 2002 to April 2003 at the Portland VA Medical Center (PVAMC). The outcomes of interest for the provider group versus the clinical pharmacy managed group included follow-up at 6 weeks or less versus follow-up at more than 6 weeks, documentation of treatment benefit, how many of the 3 first-line therapies were tried before gabapentin, and whether the gabapentin therapy was discontinued. RESULTS: There were 237 patients who received a new prescription for gabapentin between October 2002 and April 2003. Of these gabapentin prescriptions, 61% (n=144) were prescribed for neuropathic pain. Of the new gabapentin prescriptions for neuropathic pain, 61% (n=88) were made from approved clinical pharmacy consults, 38% (n=54) were ordered without a clinical pharmacy consult, and 1% (n=2) were not included because the patient received the drug despite denial by the clinical pharmacy consult. The rate of follow-up to assess documentation of benefit of therapy with gabapentin was 87% (n=62) in the clinical pharmacy consult group compared with 51% (n=27) in the provider-managed group (chi2=18.07, P<0.001). Of the patients who were assessed by follow-up, 89% (n=55) of the clinical pharmacy consult group received follow-up within 6 weeks versus 52% (n=14) of the provider-managed group (chi2=12.63, P <0.001). Compared with the patients managed by clinical pharmacists, 43% (n=23) of the gabapentin patients in the provider-managed group had no evidence of prior use of any of the 3 agents required by the gabapentin neuropathic pain guideline, 55% (n=29) had evidence of prior use of 1 or 2 first-line agents, and only 2% (n=1) had evidence of prior use of all 3 required first-line agents, versus 100% (n=71) of the patients managed by clinical pharmacy consult. There was no difference in the rate of continuation of gabapentin therapy in the group of patients who received clinical pharmacy consults (65%) compared with the provider-managed group (68%, chi2=0.11, P=0.718). Of the 148 pharmacy consults for new gabapentin prescriptions that were completed during the 7-month period from October 2002 through April 2003, 60 (40%) were denied, which resulted in the lack of gabapentin use in these 60 patients. CONCLUSIONS: A clinical pharmacy intervention as part of the management of a treatment guideline for appropriate gabapentin use promotes documentation of drug therapy effectiveness in neuropathic pain and prevention of gabapentin use prior to a trial with alternative first-line therapies.

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PubMed2006

Gabapentin-lactam induces dendritic filopodia and motility in cultured hippocampal neurons.

Gabapentin is currently used as a therapeutic agent against epilepsy as well as neuropathic pain. In contrast to gabapentin, its derivative gabapentin-lactam has a pronounced neuroprotective activity. We have studied in cultured hippocampal neurons whether gabapentin-lactam has also neurotrophic effects. Gabapentin-lactam enhanced the formation of dendritic filopodia, which are necessary for synapse formation. It also induced a network of F-actin-containing neurites. In studies with time lapse microscopy, gabapentin-lactam increased the addition but also the elimination of new branches. Affinity precipitation assays showed that gabapentin-lactam increased the GTP binding of the small GTPases Rac and Cdc42, which facilitate branch addition. Gabapentin-lactam also activated RhoA and phosphatidylinositol 3-kinases. In neurons transfected with dominant-negative RhoA or treated with the RhoA-inactivating C3 toxin, gabapentin-lactam increased the number of dendrites and branches. In the presence of Y-27632, which inhibits Rho kinase, newly added branches induced by gabapentin-lactam were no longer eliminated so that gabapentin-lactam increased the number of branches. Y-27632 [(+)-(R)-trans-4-(1-aminoethyl)-N-(4-pyridyl) cyclohexanecarboxamide] also prevented the gabapentin-lactam induced activation of phosphatidylinositol 3-kinases. The phosphatidylinositol 3-kinase inhibitor LY294002 [2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride] reduced the elimination of newly added branches caused by gabapentin-lactam and thus facilitated branch formation. In contrast to gabapentin-lactam, gabapentin had no effect on dendritic filopodia or motility. The effects exerted by gabapentin-lactam on dendritic arborization may be of potential therapeutic interest.

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PubMed2002

Gabapentin-mediated inhibition of voltage-activated Ca2+ channel currents in cultured sensory neurones is dependent on culture conditions and channel subunit expression.

We have used the whole cell patch clamp method and fura-2 fluorescence imaging to study the actions of gabapentin (1-(aminoethyl) cyclohexane acetic acid) on voltage-activated Ca(2+) entry into neonatal cultured dorsal root ganglion (DRG) neurones and differentiated F-11 (embryonic rat DRG x neuroblastoma hybrid) cells. Gabapentin (2.5 microM) in contrast to GABA (10 microM) did not influence resting membrane potential or input resistance. In current clamp mode gabapentin failed to influence the properties of evoked single action potentials but did reduce the duration of action potentials prolonged by Ba(2+). Gabapentin attenuated high voltage-activated Ca(2+) channel currents in a dose- and voltage- dependent manner in DRG neurones and reduced Ca(2+) influx evoked by K(+) depolarisation in differentiated F-11 cells loaded with fura-2. The sensitivity of DRG neurones to gabapentin was not changed by the GABA(B) receptor antagonist saclofen but pertussis toxin pre-treatment reduced the inhibitory effects of gabapentin. Experiments following pre-treatment of DRG neurones with a PKA-activator and a PKA-inhibitor implicated change in phosphorylation state as a mechanism, which influenced gabapentin action. Sp- and Rp-analogues of cAMP significantly increased or decreased gabapentin-mediated inhibition of voltage-activated Ca(2+) channel currents. Culture conditions used to maintain DRG neurones and passage number of differentiated F-11 cells also influenced the sensitivity of Ca(2+) channels to gabapentin. We analysed the Ca(2+) channel subunits expressed in populations of DRG neurones and F-11 cells that responded to gabapentin had low sensitivity to gabapentin or were insensitive to gabapentin, by Quantitative TaqMan PCR. The data obtained from this analysis suggested that the relative abundance of the Ca(2+) channel beta(2) and alpha(2)delta subunit expressed was a key determinant of gabapentin sensitivity of both cultured DRG neurones and differentiated F-11 cells. In conclusion, gabapentin inhibited part of the high voltage-activated Ca(2+) current in neonatal rat cultured DRG neurones via a mechanism that was independent of GABA receptor activation, but was sensitive to pertussis toxin. Gabapentin responses identified in this study implicated Ca(2+) channel beta(2) subunit type as critically important to drug sensitivity and interactions with alpha(1) and alpha(2)delta subunits may be implicated in antihyperalgesic therapeutic action for this compound.

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PubMed2003

The use of gabapentin for the treatment of postherpetic neuralgia.

BACKGROUND: Varicella-zoster virus causes chickenpox and can reemerge later in life to cause herpes zoster or shingles. One of the most common and disabling complications of herpes zoster is postherpetic neuralgia (PHN). OBJECTIVES: This article reviews the current primary literature about the efficacy and tolerability of gabapentin for the treatment of PHN. Gabapentin pharmacokinetics and drug interactions are also reviewed. METHODS: A literature search in the English language was conducted using OVID Web, which contained the following databases: MEDLINE (1966-present), EMBASE (1980-2002), Current Contents/Clinical Medicine (1999-2002), Cochrane Controlled Trials Register (1898-present), Cochrane Database of Systemic Reviews (fourth quarter, 2002), and International Pharmaceutical Abstracts (1970-2002). Search terms used were postherpetic neuralgia; zoster; gabapentin; neuropathic pain; pain; pharmacoeconomic; cost; controlled clinical trial; randomized, controlled trial; postherpetic neuralgia and gabapentin; gabapentin and pain; treatment and postherpetic neuralgia; gabapentin and age; gabapentin and gender; gabapentin and ethnicity; and gabapentin and pharmacokinetics. RESULTS: Gabapentin displays nonlinear absorption kinetics, is minimally protein bound (< 3%), has a high mean (SD) volume of distribution (50.4 [8.0] L), and is excreted via the kidneys as unchanged drug. Two randomized, placebo-controlled, parallel-group, multicenter clinical trials demonstrated the effectiveness of gabapentin at doses of up to 3600 mg/d to significantly reduce pain (P < 0.01 and P < 0.001), improve sleep (P < 0.01), and improve some parameters on the Short Form-McGill Pain Questionnaire (P < 0.05). Dizziness and somnolence were the most common side effects leading to withdrawal from the trials. The recommended dosage in adults is 300 mg at bedtime on day 1,300 mg BID on day 2, and 300 mg TID on day 3, titrating up as needed to 2400 to 3600 mg/d. To reduce adverse events in patients with renal impairment, the dose should be adjusted based on the patient's creatinine clearance. CONCLUSIONS: Gabapentin appears to be effective and well tolerated for the short-term treatment of PHN. However, future controlled studies are needed to determine whether the effectiveness of gabapentin for PHN is maintained for > 2 months, to establish the optimal dose of gabapentin for PHN, and to compare the efficacy of gabapentin with that of other pharmacologic agents used for the treatment of PHN.

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PubMed1996

Comparison of the uptake of [3H]-gabapentin with the uptake of L-[3H]-leucine into rat brain synaptosomes.

1. Gabapentin is a novel anticonvulsant with an unknown mechanism of action. Homogenate binding studies described elsewhere have suggested that [3H]-gabapentin binds to a site in brain similar to the large neutral amino acid (LNAA) uptake site, termed system-L. 2. This study describes an investigation into the uptake of [3H]-gabapentin into a crude synaptosomal preparation from cerebral cortex of rat brain. Characterization studies showed that [3H]-gabapentin is taken up into synaptosomes by a system that is similar to that responsible for the uptake of L-[3H]-leucine. This system is sodium-independent, temperature-sensitive and requires ATP for function. 3. Kinetic studies of [3H]-gabapentin uptake produced a Michaelis constant (KM = 160 microM) similar to that observed for L-[3H]-leucine (KM = 110.3 microM). Vmax values were 837.1 pmol mg-1 protein min-1 and 2.192 nmol mg-1 protein min-1 respectively. 4. Gabapentin and L-leucine mutually inhibit their uptake. Lineweaver-Burke plots of these data demonstrate that inhibition occurs by a competitive mechanism. Further to this the Dixon transformation of the data illustrates that these two substrates share a common uptake site by the similarity between their calculated Ki and KM values (gabapentin inhibition of L-[3H]-leucine uptake: Ki = 160 microM; L-leucine inhibition of [3H]-gabapentin uptake: Ki = 262 microM). 5. Studies into the effect of gabapentin, the system-L-specific ligand 2-(-)-endoamino-bicycloheptane-2-carboxylic acid (BCH), and the system-A-specific ligand alpha-(methyl-amino)-isobutyric acid (MeAIB), on the initial rate of uptake of [3H]-glycine, L-[3H]-glutamate, L-[3H]-glutamine, and L-[3H]-leucine were performed. At 100 microM, gabapentin significantly inhibited initial rate of uptake of [3H]-glycine (29%), L-[3H]-glutamate (22%) and L-[3H]-leucine (40%). 6. Gabapentin is taken up into synaptosomes by a system similar to system-L, responsible for the uptake of large neutral amino acids. Gabapentin will also inhibit the uptake of certain excitatory amino acids in this synaptosomal preparation. The implications of these findings for the mechanism of action for gabapentin are unclear. The data presented here may suggest an intracellular site for mechanism of action for this compound. Similarly changes in levels of amino acid pools may be involved in the mechanism of gabapentin's anticonvulsant action.

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PubMedDate not supplied

Examination of the evidence for off-label use of gabapentin.

OBJECTIVES: (1) Describe the relevance of off-label use of gabapentin to managed care pharmacy; (2) summarize recent FDA warnings and media reports related to off-label gabapentin use; (3) review medical information pertaining to the off-label use of gabapentin; (4) outline alternatives to off-label use of gabapentin in an evidence-based fashion, where literature exists to support such alternatives; and (5) encourage key clinicians and decision makers in managed care pharmacy to develop and support programs that restrict the use of gabapentin to specific evidence-based situations. SUMMARY: Gabapentin is approved by the U.S. Food and Drug Administration (FDA) for adjunctive therapy in treatment of partial seizures and postherpetic neuralgia. Various off-label (unapproved) uses have been reported, and the use of gabapentin for off-label purposes has reportedly exceeded use for FDAapproved indications. Pharmaceutical marketing practices and physician dissatisfaction with currently available pharmacological treatment options may be key factors that contribute to this prescribing trend. Recently, the media has focused on these issues, noting that many cases of reported safety and effectiveness of gabapentin for off-label use may have been fabricated. A thorough review of the medical and pharmacy literature related to off-label use of gabapentin was performed, and a summary of the literature for the following conditions is presented: bipolar disorder, peripheral neuropathy, diabetic neuropathy, complex regional pain syndrome, attention deficit disorder, restless legs syndrome, trigeminal neuralgia, periodic limb movement disorder of sleep, migraine headaches, and alcohol withdrawal syndrome. A common theme in the medical literature for gabapentin is the prevalence of open-label studies and a lack of randomized controlled clinical trials for all but a small number of indications. CONCLUSIONS: In the majority of circumstances where it has reported potential for.off-label. use, gabapentin is not the optimal treatment. The off-label use of gabapentin for indications not approved by the FDA should be reserved for cases where there is solid research support (e.g., diabetic neuropathy and prophylaxis of frequent migraine headaches). Managed care pharmacists should develop programs to restrict the use of gabapentin to these specific evidence-based situations, and key decision makers in managed care practice should feel confident in supporting these use restrictions for gabapentin.

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PubMed1995

Disposition of gabapentin in anuric subjects on hemodialysis.

Gabapentin is an anticonvulsant drug, which in man is cleared solely by renal excretion and is not bound to plasma proteins. Because the clearance of gabapentin is dependent on renal function, the pharmacokinetics of gabapentin were investigated in anuric subjects maintained on hemodialysis. Plasma samples were obtained over an 8-day period after administration of single oral 400-mg doses of gabapentin. Pre- and post-dialyzer plasma samples and dialysate samples from quantitative collection of dialyzer effluent were obtained during hemodialysis sessions performed 2, 4, and 7 days after dosing. A mean (SD) maximum gabapentin plasma concentration of 6.0 (2.4) micrograms/mL was achieved at 4.7 (2.1) hours post-dose. The elimination half-life of gabapentin on non-hemodialysis days averaged 132 hours. Approximately 35% of the gabapentin dose was recovered in dialysate, and mean hemodialysis clearance of gabapentin was 142 (26) mL/min; approximately 93% of the dialyzer creatinine clearance. Gabapentin elimination half-life during hemodialysis was approximately 4 hours. Systemic plasma gabapentin concentrations increased approximately 30% during the first 2 hours after hemodialysis as a result of drug redistribution in the body. It is recommended that patients with end-stage renal disease maintained on hemodialysis receive an initial 300-mg to 400-mg gabapentin loading dose. Plasma gabapentin concentrations can be maintained by giving 200 to 300 mg of gabapentin after every 4 hours of hemodialysis.

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PubMed1995

Effects of anticonvulsant drug gabapentin on the enzymes in metabolic pathways of glutamate and GABA.

Gabapentin is a novel anticonvulsant drug. The anticonvulsant mechanism of gabapentin is not known. Based on the amino acid structure of gabapentin we explored its possible effects on glutamate and gamma-aminobutyric acid (GABA) metabolism in brain as they may relate to its anticonvulsant mechanisms of action. Gabapentin was tested for its effects on seven enzymes in the metabolic pathways of these two neurotransmitters: alanine aminotransferase (AL-T), aspartate aminotransferase (AS-T), GABA aminotransferase (GABA-T), branched-chain amino acid aminotransferase (BCAA-T), glutamine synthetase (Gln-S), glutaminase (GLNase), and glutamate dehydrogenase (GDH). In the presence of 10 mM gabapentin, only GABA-T, BCAA-T, and GDH activities were affected by this drug. Inhibition of GABA-T by gabapentin was weak (33%). The Ki values for inhibition of cytosolic and mitochondrial forms of GABA-T (17-20 mM) were much higher than the Km values for GABA (1.5-1.9 mM). It is, therefore, unlikely that inhibition of GABA-T by gabapentin is clinically relevant. As with leucine, gabapentin stimulated GDH activity. The GDH activity in rat brain synaptosomes was activated 6-fold and 3.4-fold, respectively, at saturating concentrations (10 mM) of leucine and gabapentin. The half-maximal stimulation by gabapentin was observed at approximately 1.5 mM. Gabapentin is not a substrate of BCAA-T, but it exhibited a potent competitive inhibition of both cytosolic and mitochondrial forms of brain BCAA-T. Inhibition of BCAA-T by this drug was reversible. The Ki values (0.8-1.4 mM) for inhibition of transamination by gabapentin were close to the apparent Km values for the branched-chain amino acids (BCAA) L-leucine, L-isoleucine, and L-valine (0.6-1.2 mM), suggesting that gabapentin may significantly reduce synthesis of glutamate from BCAA in brain by acting on BCAA-T.

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PubMed1998

A summary of mechanistic hypotheses of gabapentin pharmacology.

Although the cellular mechanisms of pharmacological actions of gabapentin (Neurontin) remain incompletely described, several hypotheses have been proposed. It is possible that different mechanisms account for anticonvulsant, antinociceptive, anxiolytic and neuroprotective activity in animal models. Gabapentin is an amino acid, with a mechanism that differs from those of other anticonvulsant drugs such as phenytoin, carbamazepine or valproate. Radiotracer studies with [14C]gabapentin suggest that gabapentin is rapidly accessible to brain cell cytosol. Several hypotheses of cellular mechanisms have been proposed to explain the pharmacology of gabapentin: 1. Gabapentin crosses several membrane barriers in the body via a specific amino acid transporter (system L) and competes with leucine, isoleucine, valine and phenylalanine for transport. 2. Gabapentin increases the concentration and probably the rate of synthesis of GABA in brain, which may enhance non-vesicular GABA release during seizures. 3. Gabapentin binds with high affinity to a novel binding site in brain tissues that is associated with an auxiliary subunit of voltage-sensitive Ca2+ channels. Recent electrophysiology results suggest that gabapentin may modulate certain types of Ca2+ current. 4. Gabapentin reduces the release of several monoamine neurotransmitters. 5. Electrophysiology suggests that gabapentin inhibits voltage-activated Na+ channels, but other results contradict these findings. 6. Gabapentin increases serotonin concentrations in human whole blood, which may be relevant to neurobehavioral actions. 7. Gabapentin prevents neuronal death in several models including those designed to mimic amyotrophic lateral sclerosis (ALS). This may occur by inhibition of glutamate synthesis by branched-chain amino acid aminotransferase (BCAA-t).

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PubMed1997

Gabapentin (neurontin) and S-(+)-3-isobutylgaba represent a novel class of selective antihyperalgesic agents.

1. Gabapentin (neurontin) is a novel antiepileptic agent that binds to the alpha 2 delta subunit of voltage-dependent calcium channels. The only other compound known to possess affinity for this recognition site is the (S)-(+)-enantiomer of 3-isobutylgaba. However, the corresponding (R)-(-)-enantiomer is 10 fold weaker. The present study evaluates the activity of gabapentin and the two enantiomers of 3-isobutylgaba in formalin and carrageenan-induced inflammatory pain models. 2. In the rat formalin test, S-(+)-3-isobutylgaba (1-100 mg kg-1) and gabapentin (10-300 mg kg-1) dose-dependently inhibited the late phase of the nociceptive response with respective minimum effective doses (MED) of 10 and 30 mg kg-1, s.c. This antihyperalgesic action of gabapentin was insensitive to naloxone (0.1-10.0 mg kg-1, s.c.). In contrast, the R-(-)-enantiomer of 3-isobutylgaba (1-100 mg kg-1) produced a modest inhibition of the late phase at the highest dose of 100 mg kg-1. However, none of the compounds showed any effect during the early phase of the response. 3. The s.c. administration of either S-(+)-3-isobutylgaba (1-30 mg kg-1) or gabapentin (10-100 mg kg-1), after the development of peak carrageenan-induced thermal hyperalgesia, dose-dependently antagonized the maintenance of this response with MED of 3 and 30 mg kg-1, respectively. Similar administration of the two compounds also blocked maintenance of carrageenan-induced mechanical hyperalgesia with MED of 3 and 10 mg kg-1, respectively. In contrast, R-(-)-3-isobutylgaba failed to show any effect in the two hyperalgesia models. 4. The intrathecal administration of gabapentin dose-dependently (1-100 micrograms/animal) blocked carrageenan-induced mechanical hyperalgesia. In contrast, administration of similar doses of gabapentin into the inflamed paw was ineffective at blocking this response. 5. Unlike morphine, the repeated administration of gabapentin (100 mg kg-1 at start and culminating to 400 mg kg-1) over 6 days did not lead to the induction of tolerance to its antihyperalgesic action in the formalin test. Furthermore, the morphine tolerance did not cross generalize to gabapentin. The s.c. administration of gabapentin (10-300 mg kg-1), R-(-) (3-100 mg kg-1) or S-(+)-3-isobutylgaba (3-100 mg kg-1) failed to inhibit gastrointestinal motility, as measured by the charcoal meal test in the rat. Moreover, the three compounds (1-100 mg kg-1, s.c.) did not generalize to the morphine discriminative stimulus. Gabapentin (30-300 mg kg-1) and S-(+)-isobutylgaba (1-100 mg kg-1) showed sedative/ataxic properties only at the highest dose tested in the rota-rod apparatus. 6. Gabapentin (30-300 mg kg-1, s.c.) failed to show an antinociceptive action in transient pain models. It is concluded that gabapentin represents a novel class of antihyperalgesic agents.

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