Modulation of the in vivo effects of gabapentin by vigabatrin and SKF89976A

A comparison of the pharmacokinetics and pharmacodynamics of pregabalin and gabapentin

Pregabalin and gabapentin share a similar mechanism of action, inhibiting calcium influx and subsequent release of excitatory neurotransmitters; however, the compounds differ in their pharmacokinetic and pharmacodynamic characteristics. Gabapentin is absorbed slowly after oral administration, with maximum plasma concentrations attained within 3-4 hours. Orally administered gabapentin exhibits saturable absorption--a nonlinear (zero-order) process--making its pharmacokinetics less predictable. Plasma concentrations of gabapentin do not increase proportionally with increasing dose. In contrast, orally administered pregabalin is absorbed more rapidly, with maximum plasma concentrations attained within 1 hour. Absorption is linear (first order), with plasma concentrations increasing proportionately with increasing dose. The absolute bioavailability of gabapentin drops from 60% to 33% as the dosage increases from 900 to 3600 mg/day, while the absolute bioavailability of pregabalin remains at > or = 90% irrespective of the dosage. Both drugs can be given without regard to meals. Neither drug binds to plasma proteins. Neither drug is metabolized by nor inhibits hepatic enzymes that are responsible for the metabolism of other drugs. Both drugs are excreted renally, with elimination half-lives of approximately 6 hours. Pregabalin and gabapentin both show dose-response relationships in the treatment of postherpetic neuralgia and partial seizures. For neuropathic pain, a pregabalin dosage of 450 mg/day appears to reduce pain comparably to the predicted maximum effect of gabapentin. As an antiepileptic, pregabalin may be more effective than gabapentin, on the basis of the magnitude of the reduction in the seizure frequency. In conclusion, pregabalin appears to have some distinct pharmacokinetic advantages over gabapentin that may translate into an improved pharmacodynamic effect.

Anti-glutamatergic effect of riluzole: comparison with valproic acid

Riluzole, an anti-amyotrophic lateral sclerosis drug, known to decrease presynaptic glutamate release, is viewed as a candidate supplementary medication for epilepsy. In the present study, we compared the effects of riluzole and valproate (VPA) in the pilocarpine-induced limbic seizure model and in the gamma-hydroxybutyrate lactone (GBL)-induced absence seizure model. We applied immunohistochemical study for vesicular transporter 1 (VGLUT1) and extracellular recording in the rat dentate gyrus of both pilocarpine- and GBL-induced seizure models to measure effects of riluzole and VPA. Both VPA and riluzole treatments reduced VGLUT1 immunoreactivity. Riluzole treatment completely inhibited pre-ictal spikes and spike-wave discharges in the pilocarpine- and GBL-induced epilepsy models, whereas VPA partially inhibited these phenomena. In both seizure models, the anti-epileptic effects of VPA and riluzole are basically related to anti-glutamatergic (reducing field excitatory postsynaptic potential slope and excitability ratio), not GABAergic (paired-pulse inhibition) effect. Riluzole was more effective at reducing seizure activity in both epilepsy models than VPA. These results suggest that riluzole is a potential antiepileptic drug with activity against limbic seizure and absence seizure.

Pharmacology and mechanism of action of pregabalin: the calcium channel alpha2-delta (alpha2-delta) subunit as a target for antiepileptic drug discovery

Pregabalin (Lyrica) is a new antiepileptic drug that is active in animal seizure models. Pregabalin is approved in US and Europe for adjunctive therapy of partial seizures in adults, and also has been approved for the treatment of pain from diabetic neuropathy or post-herpetic neuralgia in adults. Recently, it has been approved for treatment of anxiety disorders in Europe. Pregabalin is structurally related to the antiepileptic drug gabapentin and the site of action of both drugs is similar, the alpha2-delta (alpha2-delta) protein, an auxiliary subunit of voltage-gated calcium channels. Pregabalin subtly reduces the synaptic release of several neurotransmitters, apparently by binding to alpha2-delta subunits, and possibly accounting for its actions in vivo to reduce neuronal excitability and seizures. Several studies indicate that the pharmacology of pregabalin requires binding to alpha2-delta subunits, including structure-activity analyses of compounds binding to alpha2-delta subunits and pharmacology in mice deficient in binding at the alpha2-delta Type 1 protein. The preclinical findings to date are consistent with a mechanism that may entail reduction of abnormal neuronal excitability through reduced neurotransmitter release. This review addresses the preclinical pharmacology of pregabalin, and also the biology of the high affinity binding site, and presumed site of action.

The effect of topiramate on GABAB receptor, vesicular GABA transporter and paired-pulse inhibition in the gerbil hippocampus

To extend our understanding of the properties of topiramate (TPM), we investigated the effect of TPM on GABAergic transmission in the dentate gyrus of gerbil. TPM treatment (> or = 40 mg/kg) dramatically decreased GABA(B)R2, not GABA(B)R1, immunoreactivity in hilar interneurons. In contrast, TPM treatment increased vesicular GABA transporter immunoreactivity and the paired-pulse inhibition in the dentate gyrus of seizure prone gerbils. Furthermore, TPM effectively prevented the reduction of paired-pulse inhibition induced by baclofen treatment. These findings suggest that TPM may enhance GABA release in the dentate gyrus of gerbils by down-regulation of GABA(B) autoreceptor expression. Therefore, these properties of TPM may be another possible antiepileptic effect, which plays an important role in preventing the spread of seizure activity without proconvulsive effects.

Isobolographic characterisation of interactions among selected newer antiepileptic drugs in the mouse pentylenetetrazole-induced seizure model

The aim of this study was to characterise the types of interactions between gabapentin (GBP), tiagabine (TGB) and three second-generation antiepileptic drugs (AEDs) with different mechanisms of action (felbamate [FBM], loreclezole [LCZ], and oxcarbazepine [OXC]) by isobolographic analysis. Anticonvulsant and acute neurotoxic adverse effect profiles of combinations of GBP and TGB with other AEDs at fixed ratios of 1:3, 1:1 and 3:1 were investigated in pentylenetetrazole (PTZ)-induced seizures and the chimney test (as a measure of motor impairment) in mice so as to identify optimal combinations. Protective indices (PIs) and benefit indices (BIs) were calculated for each combination in order to properly classify the investigated interactions. Isobolographic analysis revealed that only the combination of GBP with OXC at the fixed ratio of 1:1 exerted supra-additive (synergistic) interaction (P<0.05) against PTZ-induced seizures. The other combinations tested between GBP and OXC (1:3 and 3:1), as well as all combinations of GBP with FBM or LCZ (1:3, 1:1 and 3:1) were additive in the PTZ test. Similarly, all combinations of TGB with FBM LCZ, and OXC (at the fixed ratios of 1:3, 1:1 and 3:1) were associated with additive interactions against PTZ-induced seizures in mice. In the chimney test, the isobolographic analysis revealed that the combinations of GBP and OXC (at the fixed ratios of 1:3 and 1:1), GBP and LCZ (at 1:1), as well as TGB and OXC (at 1:3 and 1:1) were sub-additive (antagonistic; P<0.05 and P<0.01). In contrast, only one combination tested (TGB and LCZ at the fixed ratio of 1:1) was supra-additive (synergistic; P<0.05) in the chimney test, whereas the other combinations of GBP and TGB with OXC, FBM, and LCZ displayed barely additivity. Based upon the current preclinical data, GBP and OXC appear to be a particularly favourable combination. Also, the combinations of GBP with FBM, GBP with LCZ, and TGB with OXC are beneficial. In contrast, during the combining of TGB with FBM, or TGB with LCZ, the utmost caution is advised because of their unfavourable profiles in this preclinical study.