Mark G. Vartanian

Potent and stereospecific anticonvulsant activity of 3-isobutyl GABA relates to in vitro binding at a novel site labeled by tritiated gabapentin.

3-Isobutyl GABA is a derivative of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) and is also structurally related to the novel anticonvulsant gabapentin. The S(+) enantiomer of 3-isobutyl GABA blocks maximal electroshock seizures in mice and also potently displaces tritiated gabapentin from a novel high-affinity binding site in rat brain membrane fractions. The R(-) enantiomer is much less active in both assays, suggesting that the gabapentin binding site is involved in the anticonvulsant activity of 3-isobutyl GABA.

Gabapentin anticonvulsant action in rats: disequilibrium with peak drug concentrations in plasma and brain microdialysate

The concentration-time profile of [14C]gabapentin (GBP) in plasma and brain interstitial fluid (ISF) was determined following a single 15 mg/kg intravenous bolus dose to rats. Brain ISF was sampled with a microdialysis probe in striatum. Blood was also collected serially to 4 h postdose. At termination, brain was sectioned into regions and [14C]GBP concentrations were determined. Anticonvulsant effects were determined by maximal electroshock in rats with identical dosing. Plasma [14C]GBP declined linearly after dosing while brain ISF [14C]GBP concentration peaked at approximately 1 h and then declined in parallel with plasma concentration. Throughout, brain ISF [14C]GBP concentration was approximately 3-6% of [14C]GBP concentration in plasma. However, at 4 h postdose, whole brain tissue [14C]GBP concentration was equal to or greater than the concentration of [14C]GBP in plasma. Maximal anticonvulsant effect lagged behind both plasma and brain ISF GBP concentrations. The anticonvulsant effect of GBP is delayed by time-dependent events other than distribution from blood to brain.

3-alkyl GABA and 3-alkylglutamic acid analogues: two new classes of anticonvulsant agents.

Recently we showed that 3-alkyl-4-aminobutanoic acids are in vitro activators of brain L-glutamic acid decarboxylase (GAD) that show anticonvulsant activity. Since activation of GAD leads to increased concentrations of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in vitro, these compounds could represent a new class of anticonvulsant agents. Here it is shown that 3-alkylglutamic acid analogues also activate GAD and that all of the compounds in both series are active anticonvulsant agents against low intensity electroshock in mice. The most active compound, 3-isobutyl GABA, was tested further against maximal electroshock in mice and was shown to be very potent after both intravenous and oral administration without causing ataxia. It is not known if brain GABA levels are elevated in vivo by administration of these compounds or if the mechanism of anticonvulsant activity is related to their ability to activate GAD.

Phenytoin pretreatment prevents hypoxic-ischemic brain damage in neonatal rats

This study was performed to investigate whether the anticonvulsant phenytoin has neuroprotective effect in a model of hypoxia-ischemia with neonatal rats. The left carotid artery of each rat was ligated, followed by 3 h of hypoxic exposure (8% O2) in a temperature-regulated environment (36 degrees C). Two weeks later, brain damage was assessed by measuring loss of brain hemisphere weight. Phenytoin had no effect on body temperature or plasma glucose, but attenuated brain damage in a dose-dependent manner (3, 10, and 30 mg/kg i.p.) when administered before the hypoxic episode. Phenytoin administered during or after hypoxia did not alter hypoxic brain damage significantly. A parallel experiment using histological examination of frozen brain sections demonstrated less brain infarction after phenytoin treatment (30 mg/kg i.p.). In an additional experiment measuring breakdown of an endogenous brain calpain substrate, spectrin, phenytoin treatment reduced this measure of early cellular damage. Our results indicate that pretreatment with phenytoin is neuroprotective at a plasma phenytoin concentration of approximately 12 micrograms/ml. These results are consistent with the hypothesis that blockade of voltage-dependent sodium channels reduces brain damage following ischemia.

Probenecid pretreatment enhances anticonvulsant action of NBQX in mice

NBQX (2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline) was studied following intravenous administration to mice. Maximal electroshock seizures were suppressed by low doses of NBQX (ED50 = 13 mg/kg) but effects had dissipated by 10 min after a dose of 25 mg/kg. Coadministration of the transport inhibitor probenecid (p-(dipropylsulphamoyl)-benzoic acid) enhanced and prolonged the anticonvulsant action of NBQX and also enhanced and prolonged ataxia. NBQX may be rapidly eliminated by a process sensitive to probenecid.

Different stereoselectivity of the enantiomers of HA-966 (3-amino-1-hydroxy-2-pyrrolidinone) for neuroprotective and anticonvulsant actions in vivo

HA-966 is an antagonist at the glycine modulatory site of N-methyl-D-aspartate (NMDA) receptors. Neuroprotective effects of HA-966 were determined in postnatal day (PND) 7 rats that received intrastriatal injections of NMDA (15 nmol) and then were administered either racemic HA-966, or the purified (R) or (S) enantiomer 15 min later. The (R)-enantiomer dose-dependently attenuated NMDA-induced brain injury, whereas the (S)-enantiomer was ineffective. When given intravenously to mice, racemic HA-966 and the (S)-enantiomer prevented tonic extensor seizures from low-intensity electroshock (ED50 = 13.2 and 8.8 mg/kg, respectively). Anticonvulsant effects of the (R)-enantiomer were much less potent (ED50 = 105.9 mg/kg). Ataxia measured by inverted screen fall-off was 17 times more potent with the (S)-enantiomer than with the (R)-enantiomer. Since previous published work indicates that glycine-site NMDA antagonist activity is primarily due to the (R)-enantiomer, our results suggest neuroprotective action for the (R)-enantiomer and anticonvulsant action, not related to a glycine antagonist mechanism for the (S)-enantiomer.

Structure-activity relationship of N-methyl-N-aralkyl-peptidylamines as novel N-type calcium channel blockers

Selective N-type voltage sensitive calcium channel (VSCC) blockers have shown efficacy in several animal models of stroke and pain. In the process of searching for small molecule N-type calcium channel blockers, we have identified a series of N-methyl-N-aralkyl-peptidylamines with potent functional activity at N-type VSCCs. The most active compound discovered in this series is PD 173212 (11, IC50 = 36 nM in the IMR-32 assays). SAR and pharmacological evaluation of this series are described.

N,N-Dialkyl-dipeptidylamines as novel N-type calcium channel blockers

Selective N-type voltage sensitive calcium channel (VSCC) blockers have shown utility in several models of stroke and pain. We are especially interested in small molecule N-type calcium channel blockers for therapeutic use. Herein, we report a series of N,N-dialkyl-dipeptidylamines with potent functional activity at N-type VSCCs and in vivo efficacy. The synthesis, SAR, and pharmacological evaluation of this series are discussed.

N-sulfonyl derivatives of 6,7-dichloro 3,4-dihydro-3-oxo-quinoxalinecarboxylate as glycine-site NMDA and AMPA antagonists

Abstract N-Phenylsulfonyl and N-methylsulfonyl derivatives of 6,7-dichloro-3,4-dihyro-3-oxo-2-quinoxalinecarboxamide have been synthesized. Both compounds have been characterized as antagonists, at the glycine-site of the NMDA receptor and AMPA receptor.

Synthesis and anticonvulsant activity of the (+)- and (−)-enantiomers of 1,2,3,4-tetrahydro-5-(2-phosphonoethyl)-3-isoquinolinecarboxylic acid, a competitive NMDA antagonist

Abstract The (+)- and (−)-enantiomers of 1,2,3,4-tetrahydro-5-(2-phosphonoethyl)-3-isoquinoline-carboxylic acid ( 12 and 13 ) were prepared. The anticonvulsant activity of 12 and 13 and their parent ( 14 ) was compared to the reference agent, CPP, in the maximal electroshock assay in mouse. 12 was similarly potent to CPP, three-fold more potent than the racemate and twenty-fold more potent than 13 in this assay.