Current Topics in Medicinal Chemistry 2005, 5, 31-42 31

1568-0266/04 $??.00+.00 © 2005 Bentham Science Publishers Ltd.

AMPA Receptor Antagonists as Potential Anticonvulsant Drugs

Giovambattista De Sarro1, Rosaria Gitto2, Emilio Russo1, Guido Ferreri Ibbadu1, Maria LetiziaBarreca2, Laura De Luca2 and Alba Chimirri2,*

1Chair of Pharmacology, Department of Experimental and Clinical Medicine, Faculty of Medicine and Surgery,University of Catanzaro, Italy, 2 Department of Medicinal Chemistry, Faculty of Pharmacy, University of Messina, Italy.

Abstract: Over the last years α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid glutamate receptors (AMPARs)have been intensively studied owing to their crucial role in physiological and pathological processes. Efforts targetingAMPAR have been focused on identification of ligands as potential therapeutic agents useful in the prevention andtreatment of a variety of neurological and non-neurological diseases. In particular, extensive work was addressed to thediscovery of selective antagonists some of which proved to be potent anticonvulsant agents.

Key Words: Glutamate, AMPA receptor antagonists, AMPAR, anticonvulsants.

1. INTRODUCTION

Approximately 1% of the world’s population (~50million people) is affected by epilepsy, a seriousneurological disorder that typically manifests as spontaneousconvulsions and/or a loss of consciousness. These symptomsare caused by the appearance of abnormal electrical seizuredischarges, characterized by episodic high frequency firingof impulses by a group of neurones within the brain as aresult of an imbalance between excitatory and inhibitorysynaptic processes [1].

Often, therapeutic regimens for epileptic patients willinvolve a change of first-line and/or add-on antiepilepticdrugs. Most antiepileptic drugs are associated with adverseeffects, such as sedation, ataxia and weight loss (e.g.topiramate) or weight gain (e.g. valproate, tiagabine, andvigabatrin). Rare adverse effects can be life threatening suchas rashes leading to Stevens-Johnson syndrome (e.g.lamotrigine) or aplastic anaemia (e.g. felbamate) [2]. Asabout 30 % of people affected by epilepsy have uncontrolledseizures, the development of safer and more effective newantiepileptic drugs (AEDs) is necessary. Despite theexcitement that has accompanied the launch of newalternative drugs in the last 20 years, they have made littleimprovement on the number of patients who suffer fromchronic and refractory epilepsy [1,3,4]. The role of noveldrugs in the treatment of newly diagnosed epilepsy is notcompletely clear at this point, because their chronic side-effects and their efficacy in refractory epilepsy have not yetbeen established.

More than 100 neurotransmitters or neuromodulatorshave been shown to play a role in neuronal processes. Somespecific neurotransmitters that relate to epilepsy are γ-aminobutyric acid (GABA), norepinephrine, endogenousopioid peptides, and the excitatory amino acids, such asglutamate (1) and aspartate, although the most widely

*Address correspondence to this author at Dipartimento Farmaco-Chimico,Facoltà di Farmacia, Università di Messina, Viale Annunziata I-98168Messina, Italy; Tel: +39 0906766412; Fax: +39 090355613; E-mail:chimirri@pharma.unime.it

studied have been GABA and glutamate acting at more thanhalf the neuronal synapses in the brain [5].

Present clinically efficacious antiepileptics act byinducing prolonged inactivation of the Na+ channel, byblocking Ca2+ channel currents or by enhancing inhibitoryGABAergic neurotransmission. Some of the “newer” anticon-vulsant agents act via a number of different mechanisms,which may include antagonism of glutamatergicneurotransmission [6].

Glutamatergic neurotransmission involves ionotropic andmetabotropic receptors (iGluRs and mGluR) that are activatedunder differing circumstances.

The iGluRs are ligand gated ion channels which arefurther subdivided into three classes based on their affinityfor specific agonists: the N-methyl-D-aspartic acid (NMDA,2) , the kainic acid (KA, 3) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA, 4) receptorsubtypes allow for sodium, potassium and calcium flux uponglutamate binding.

Fig. (1). iGluR agonists.

The iGluRs comprise of different subunits which presentdifferent distribution in the brain: NR1, NR2A-D, NR3A-B

(NMDA), GluR1-4 (AMPA) and GluR5-7, KA1-2 (KA)(Figure 2) [7].

NHCH 3

CO2HHO2C

NH 2

HO2C CO2 H

ON

H2N

CH3

OHHO2 C

NH

CH3

CH2CO2H

CO2H

1 (S )-Glutamic acid

3 Kaini c acid

2 NMDA

4 AMPA

32 Current Topics in Medicinal Chemistry, 2005, Vol. 5, No. 1 Chimirri et al.

Initially, it was thought that synaptically releasedglutamate acted only on iGluRs opening cation-permeablechannels. However, from 1985 onwards, evidenceaccumulated that glutamate, like acetylcholine, dopamine,serotonin, and noradrenaline, could also act via G-protein-coupled mGluRs to induce phosphoinositide hydrolysis [8,9]or to decrease adenylate cyclase activity [10]. Since thecloning and sequencing of mGlu1 in 1991 [11] seven othermGluRs have been characterised and sequenced. These eightreceptors, termed mGluR1-8, fall into three groups accordingto their sequence homology, transduction mechanisms andagonist pharmacology [12]. In general, mGluRs modulateglutamatergic excitations by pre-synaptic, post-synaptic andglial mechanisms (Figure 2).

2. AMPA RECEPTORS

AMPA glutamate receptors (AMPARs) have structuralfeatures that allow for multiple sites to which ligands can actto modulate receptor functions [13]. AMPARs are tetramersbuilt from closely related subunits, called both GluR1-4 andGluRA-D [14], assembled from homo- or heteromericcomplexes surrounding a central cation-conducting poremediating fast excitatory postsynaptic potentials by the fluxof Na+ and Ca2+ ions [15].

Release of glutamate from the presynaptic neuron and itsbinding to AMPA receptors of the postsynaptic neuron leadsto cations influx into the cells, but also causes the receptor todesensitize thus preventing excitotoxic processes.

Each of GluR1-4 subunit can exist as two forms, flip andflop, due to variable gene splicing and consists of a longextracellular amino terminus, jointed to three transmembranespanning domains (TM1, TM3 and TM4), a membraneimbedded re-entry loop (M2) that connects TM1 and TM3,and a short intracellular C-terminus as showed in Figure 3for GluR2. Two discontinuous extracellular domains (S1S2)contain the glutamate binding site, responsible for bindingboth the neurotransmitter and the competitive agonists/antagonists [16].

The application of X-ray diffraction has allowed thestructure of the GluR2 bound with a series of competitiveagonists/antagonists to be determined, providing somedetails of ligand recognition and of the activation/deactivation mechanism [17]. On the contrary, no X-raystructure of any complex between non-competitiveantagonists and their binding site has been reported.Therefore an homology model study has been recentlycarried out in the attempt to decipher the mechanism of

action and the localization of the binding pocket for AMPARallosteric modulators [18]. It has been hypothesized thatAMPA and NMDA receptors have similar structuralorganization and that, similarly to the allosteric binding siteof NMDA antagonists [19], the distal N-terminus regionmight have the binding site for AMPAR non-competitiveligands. The results of homology modelling and moleculardocking experiments identified the LIVBP-like region in theN-terminal domain as a plausible binding site and indicatedthe bind mode and the preferred disposition of AMPARallosteric ligands. Moreover, it has been postulated thepresence of at least other two binding-sites: a polyaminerecognition site within the ion channel for a particular groupof antagonists and an allosteric site, different to that ofpositive modulators, at which non-competitive antagonistscan bind [20].

The present review is addressed to AMPAR antagonistswith particular attention to molecules effective for treatmentand prevention of epileptic seizures.

3. COMPETITIVE AMPA RECEPTOR ANTAGONISTS

Quinoxaline derivatives are an interesting class ofspecific and potent competitive non-NMDA glutamatereceptor antagonists [13,20,21]. Some quinoxaline-2,3-

Fig. (2). Glutamate receptors.

Fig. (3). GluR2 subunit of AMPA receptor.

AMPA Receptor Antagonists as Potential Anticonvulsant Drugs Current Topics in Medicinal Chemistry, 2005, Vol. 5, No. 1 33

diones such as DNQX (5), NBQX (6), YM-90K (7), YM-872 (8) have been found to be neuroprotective in variousmodels of ischemia and to have anticonvulsant properties indifferent models of epilepsy.

Fig. (4). Quinoxalinedione derivatives.

Using these molecules as templates the synthesis ofdifferent quinoxaline derivatives was thus pursued, togetherwith extensive structure activity relationships (SAR) andpharmacophore modelling studies on this class ofcompounds. Recently, the X-ray structure of the competitiveantagonist ATPO in complex with the GluR2 ligand-bindingcore has been solved and compared with the only previouscomplex with DNQX. It has been thus observed that non-covalent interactions between the two molecules and thereceptor subunit stabilize an open form of the ligand-bindingcore, contrarily to agonists which induce substantial domainclosure [17,22]. Molecular modelling studies were alsoperformed with the aim to highlight the key residuesinvolved in ligand recognition and to estimate the differencesof binding mode between agonists and antagonists.

Fig. (5). Quinoxalinedione derivatives.

Furthermore it has also been demonstrated that thepresence of suitable substituents on the quinoxaline skeletoninfluenced the selectivity against iGluRs as well aspharmacokinetic properties. In particular, some 5-aminoalkylsubstituted quinoxaline-2,3-diones were reported as AMPAreceptor antagonists, where their affinity is depending uponthe orientation of substituent. For instance, compound 9showed efficacy in maximal electroshock seizures but didnot show selectivity between AMPA and kainate receptors[23].

In order to increase the water solubility of thesederivatives, an acidic functionality has been introducedthus affording the identification of {[(7-nitro-2,3-dioxo-1, 2, 3, 4 - tetrahydro - quinoxalin-5-ylmethyl)-amino]-methyl}-phosphonic acid (10, AMP397A). This compound displayedhigh affinity ([3H]CNQX binding, IC50=11 nM) andselectivity for AMPAR. Moreover, AMP397A showed goodin vivo potency in different animal models and oral activityas anticonvulsant agent (Table 1) [24].

Some novel fused 2,3-quinoxalines were obtained aspotential therapeutic agents for the treatment of epilepsy,among which compound 11 proved to be antagonist ofAMPA and kainate receptors, with IC50 binding affinityvalues of 0.24 µM and 1.62 µΜ, respectively [25] and activein the maximal electroshock seizure (MES) assay in mice.

Table 1. Anticonvulsant Properties of Compound 10(AMP397A).

ED50(mg/kg)

MES 9.0 po

PTZ 14.0 ip

Audiogenic seizures 5.4 po

The syntheses of a series of 6,7-disubstituted-2-(1H)-oxoquinolines bearing different acidic functions in the3-position and their salts have also been reported. Inparticular, 6,7-dichloro-2-(1H)-oxoquinoline-3-phosphonicacid (S17625, 12) was a potent, water soluble and selectiveAMPA antagonist but nephrotoxic and without anticonvul-sant effects [26]. More recently, exploiting SAR it wasdemonstrated that the replacement of chlorine in position 6by a sulfonylamine moiety led to very potent AMPAantagonists endowed with good in vivo activity and lackingnephrotoxicity potential. Compounds 13 and 14 manifestedsignificant anticonvulsant efficacy in audiogenic seizures inDBA/2 mice when compared with NBQX (Table 2) [27].

Fig. (6). 2-Oxoquinoline derivatives.

NH

HN

O

O

O2 N

CH3

N

CH3

H3CO2S

NH

HN

O

O

O2 N

N

HH2O3P

NH

HN

O

O

O2 N

N

O

9 10, AMP397A

11

NH

OCl

PO3H2Cl

NH

O

S

Cl

COOHNH

O O

R

12, S17625 13 R = H14 R = NHCOPh

NH

N

O

ON

N

O2N

R

NH

HN

O

O

O2N

H2NO2S

NH

HN

O

O

O2N

O2N

7 R =H, YM90K8 R = CH2CO2 H, YM872

6, NBQX5, DNQX

34 Current Topics in Medicinal Chemistry, 2005, Vol. 5, No. 1 Chimirri et al.

Table 2. Anticonvulsant Efficacy Against AudiogenicSeizures.

cpd ED50 (mg/kg)

13 3.32

14 3.00

NBQX 12.6

A new series of hydrosoluble AMPAR antagonistscontaining 4, 5 - dihydro - 4 - oxo - 10H - imidazo[1,2-a]indeno[1,2-e]pyrazine system have been synthesized [28-34]. SARstudies demonstrated that both the position and the nature ofthe substituents on the tetracyclic skeleton influence theactivity. Particularly advantageous is the presence of thecarboxy substituent or its bioisosters such as tetrazole orphosphonic acid groups at 2 and 9 positions. Compounds 15(RPR119990), 16 (RPR117824) and 17 exhibited potentanticonvulsant effects following ip and iv administration andare considered members of a new generation of AMPAantagonists with high solubility and better duration of actionthan quinoxalinedione series (Table 3).

Fig. (7). Imidazo[1,2-a]indeno[1,2-e]pyrazines.

Table 3. Anticonvulsant Efficacy Against MES Test.

cpd ED50 (mg/kg, ip)

15 3.5

16 1.2

17 1.0

YM-90K 12.0

NBQX 36.0

4. NON-COMPETITIVE ANTAGONISTS

The non-competitive AMPAR antagonists, interactingwith an allosteric AMPA binding site, have the advantage ofremaining effective independently of the level of glutamateor the polarization state of the synaptic membrane during aneurological diseases [20,35]. Moreover, they do notinfluence the normal glutamatergic activity also afterprolonged use. Thus, in recent years some important classesof these ligands have been developed.

The first non-competitive AMPA antagonist was1 - (4 - aminophenyl) - 4 - methyl - 7,8 - methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466, 18) [36], discovered in 1989and used as template to develop novel more potent and lesstoxic AMPAR modulators (Figure 8). In detail, 3-N-substituted 3,4-dihydro-2,3-benzodiazepine analogues havebeen developed to prevent the excitotoxic action of highextracellular glutamate levels [37,38]. The importance ofstereoselectivity in AMPA receptor recognition is alsoconfirmed by the higher activity of R-enantiomers such as (-)GYKI 53733 (19, also named LY300164 or talampanel) and(-)GYKI 53784 (20) [39,40].

Talampanel emerged as highly active molecule and iscurrently under phase II clinical trials in the US in patientswith severe epilepsy not responsive to other drugs [41].Animal studies have shown it to have a broad spectrum ofanticonvulsant activity. Its mean plasma half life is about 7hours, the protein binding ranges from 67 to 88%, andmoreover the plasma concentrations are affected byacetylator status [42]. It is an irreversible inhibitor ofCYP3A and so may increase concentrations ofconcomitantly administered carbamazepine. A double-blind,placebo-controlled add-on trial in 49 patients with refractorypartial epilepsy showed a mean seizure reduction of 21%compared with placebo. Dizziness and ataxia were the mostcommon adverse effects [42].

Other derivatives were synthesized by introducingdifferent functionalities on the 2,3-benzodiazepine system(21-23) obtaining more active, less toxic and longer lastinganticonvulsant agents [43-53].

These results showed that the introduction of a lactamfunction and the modification of the methylenedioxy moietywere well-tolerated by AMPA receptor. Furthermore, whilecompounds with either a methoxy or an halogen group at 8-position retained considerable AMPA antagonist potency,the introduction of the same substituents at 7-positionnegatively influenced the anticonvulsant activity [35]. Thesolid phase synthesis techniques have also been successfullyapplied to the preparation of 1-aryl-7,8-dimethoxy-3,5-dihydro-2,3-benzodiazepin-4-ones [54].

The synthesis of several 2,3-benzodiazepines containingan additional heterocyclic ring fused to the “c” edge of the 7-membered diazepine system allowed the discovery of newnon-competitive AMPA receptor antagonists such asimidazo-2,3-benzodiazepine derivative 24 (GYKI 47621)and the triazolo-2,3-benzodiazepines which demonstratedinteresting pharmacological properties [44,45,55-57].

SAR studies emphasized that appropriate chemicalfeatures able to participate in hydrogen bond interactions are

NH

N N

O

R

HO2C

NH

N N

O

PO3H2

NHN

N N

15, R = PO3H2, RPR-11999016, R = CO2H, RPR-117824

17

AMPA Receptor Antagonists as Potential Anticonvulsant Drugs Current Topics in Medicinal Chemistry, 2005, Vol. 5, No. 1 35

key structural requirements for the anticonvulsant activity ofthis class of molecules.

In particular, 6-(4’-bromophenyl)-8,9-dimethoxy-11H-[1,2,4]triazolo[4,5-c][2,3]benzodiazepine-3(2H)-one (25) wasalmost 10-fold more active than GYKI 52466 (18), and 3.5-fold more than CFM-2 (22) and talampanel (19) in audiogenicseizure test, acting via negative allosteric modulation of theAMPA receptor as confirmed by electrophysiological tests[57].

Taking the 2,3-benzodiazepine derivatives as a mould,different arylphthalazines have been developed as negativeAMPAR modulators [58-62]. It was reported that somemolecules, such as SYM 2206 (26) and SYM 2189 (27),demonstrated efficient protection of both mice and rats inMES test (35 and 52 mg/kg ip respectively). Moreover, 4-aryl-1,2-dihydrophthalazin-1(2H)-one derivatives (28 and

29) structurally related to 2,3-benzodiazepin-4-one derivat-ives have been also developed, and the most interestingcompound (29) of this series showed potent anticonvulsantefficacy [60]. By analogy with findings for 2,3-benzo-diazepine, a five-membered heterocyclic nucleus was alsointroduced on the phthalazine skeleton; some monomethoxy-substituted analogues proved to be AMPA receptorantagonists [35], while methylenedioxyphthalazines did notshow any effect of AMPA antagonism.

Other non-competitive AMPAR antagonists containingquinazolin-4-one skeleton were developed by Pfizerresearch group [63-68]. Compound CP-465,022 (+)-(aS)-(2-chlorophenyl) - 2 - [(E) - 2 - [6 - (diethylaminomethyl)pyridin-2-yl]-vinyl]-6-fluoroquinazolin-4(3H)-one 30 was tested indifferent pharmacological assays and showed anticonvulsantefficacy when tested against pentylentetrazole and AMPA

Fig. (8). 2,3-Benzodiazepine derivatives.

Fig. (9). Phthalazine derivatives.

N

NR 2

NH2

O

R1O

R2ON

NCONH-n-P rR 1

R 2

Me

NH2

28, R1 = R 2 = Me, R3 = H29, R1-R2 = -CH2-, R3 = CONH-nBu

26, R1-R2 = -OCH 2O-; SYM 220627, R1 = H, R2 = OMe; SYM 2189

N

N

Me

O

O

H2N

N

NH

O

H2N

R 1

R 2

N

N

R 2

Me

O

R 3

H2 N

R 1

NCl

N

N

H2N

Me

N

N

NHN

Br

O

MeO

MeO

18, GYKI 52466 (-) 19, R 1-R2 = -OCH2O-; R3 = Me GYKI 53733 talampanel (-) 20, , R 1 -R 2 = -OCH 2O-; R3 = NHMe GYKI53784 (-) 21, R1 = H, R2 = C l, R3 = H, R3 = Me

22, CFM-2 R1 = R2 = MeO23, R1 - R 2 = -OCH 2O-

24, GYKI 47621 25

36 Current Topics in Medicinal Chemistry, 2005, Vol. 5, No. 1 Chimirri et al.

induced seizures (4.0 mg/kg sc) [65,67]. Compound CP-526,427 (31) was also radiolabeled in order to elucidate theinhibitory binding site of allosteric modulators [68].

The different potency of this class of compounds wasexplained on the basis of a pharmacophore model whichconsisted of three features: (1) the quinazolin-4-one ring,with a small C-6 substituent, (2) the orthogonal N-3 phenylring containing a single ortho substituent, and (3) the arylring attached to C-2 through a two-atom spacer. This lastfeature was identified as the linking unit that could greatlyinfluence the AMPAR antagonism potency [63].

Recently important new classes of non-competitiveAMPA receptor antagonists containing different hetero-cyclic skeleton have been identified. The 2-[N-(4-chloro-phenyl)-N-methylamino]-4H-pyrido[3,2-e]-1,3-thiazin-4-one(32, YM928) exerts significant anticonvulsant effects invarious seizures models (e.g. MES, ED50 = 4.0-7.4 mg/kgp.o. both in mice and rats), it is orally active and doesn’tinduce tolerance after subchronic administration [69,70].Moreover, compound 32 demonstrated to prevent audiogenicseizures in DBA/2 mice after oral administration at 3mg/kg.

Another compound, irampanel (33, BIIR 561CL,dimethyl-[2-[2-(3-phenyl-[1,2,4]oxadiazol-5-yl)-phenoxyl]-ethyl]amine hydrochloride) showed anticonvulsant effect; itsmechanism is due to the combination of antagonistic actionat AMPA receptors and Na+ channel blocking properties[71].

Fig. (10). Different non-competitive AMPA receptor antagonists

Irampanel suppressed tonic seizures in a maximalelectroshock model in mice with an ED50 value of 2.8 mg/kgafter subcutaneous administration and protected mice againstAMPA-induced toxicity with an ED50 value of 4.5 mg/kgfollowing subcutaneous administration. Despite itsinteresting anticonvulsant effects, by October 1999 thedevelopment was only ongoing for stroke [72]; theneuroprotection provided by this molecule is comparable tothe effects of NBQX [73].

The rational design of new N-acetyl-1-aryl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline derivatives aspotent anticonvulsant agents [74-76] was suggested bystudies of pharmacophore analysis. [74]

The three-dimensional pharmacophore model includedtwo hydrophobic, one hydrogen-bond acceptor and onearomatic features which were considered to be important inobtaining potent AMPAR non-competitive antagonistactivity [77]. The most interesting molecule of this series ofcompounds was 34 (Figure 10) which proved to be in vivoand in vitro more potent than other known AMPAantagonists such as GYKI 52466, CFM-2 and talampanel[74].

Table 4. Anticonvulsant Efficacy Against MES Test.

cpd ED50(mg/kg, po)

GYKI 52466 37.4

GYKI 53733 8.6

GYKI 47621 24.0

21 9.3

Finally, comprehensive quantitative structure-activityrelationship (QSAR) studies on an extensive set of negativeallosteric modulators have also been reported, in the hope ofgaining further insight into the structural requirements forthe optimal anticonvulsant effect [78].

A highly predictive QSAR model was thus obtained,revealing high correlation between some electrotopologicaldescriptors and anticonvulsant activity.

5. PHARMACOLOGICAL ACTIVITY OF AMPARECEPTOR ANTAGONISTS AGAINST EPILEPSY

It has been shown that AMPA receptor antagonists maybe effective for symptomatic treatment of epileptic seizuresand in preventing permanent brain damage resulting fromprolonged seizure activity. Both competitive and non-competitive antagonists proved to block seizure activityinduced by MES and chemical convulsants in many animalmodels of epilepsy [38,48,50,52,53,79-95].

Nevertheless, a potential problem associated with the useof AMPAR antagonists as anticonvulsants is that thetherapeutic dose for protection against MES-inducedseizures is very close to the toxic dose, giving rise to motorimpairments [83,84,93]. Furthermore some compounds (e.g.NBQX) are ineffective in blocking seizure activity caused bysome convulsants [92]. Anyway some reports found asynergism between NBQX and other anticonvulsants in theMES test [96] or against kindling-induced seizures[83,84,97,98], and this may help to reduce the level of theseadverse effects. It has to be noted that the therapeutic marginis wider when these compounds are tested in other models ofepilepsy [3,44,46-50,52,53,56,57,60,61,74-76,80,99,100];however, the relevance of these models for the developmentof clinically effective anticonvulsants has to be betterclarified.

N S

O

N

Me

Me

N

NO

O

N Me

MeN

Cl

MeO

MeO Ac

N

N

O

F

Cl

N R 1

R 2

32, YM928

33, Irampanel 34

30, R1 = CH2NEt2 , R2 = H; CP-465,02231, R1 = H, R2 = CN; CP-526,427

AMPA Receptor Antagonists as Potential Anticonvulsant Drugs Current Topics in Medicinal Chemistry, 2005, Vol. 5, No. 1 37

The effects of AMPA receptor antagonists againstvarious model of kindling have been reported in literatureand it has been observed that the pentylenetetrazole inducedkindling is more sensible to antiseizure effects of AMPAantagonists than kindling induced by electrical stimulation oflimbic seizures [3,50,81,83,84,86].

Furthermore, different effects of AMPA receptorantagonists against some models of absence epilepsy havebeen reported: CNQX and NBQX seem to be effective [101]whereas more selective non-competitive AMPAR antagonists(LY 300164 and GYKI 52466) do not significantly changethe frequency and the total number of absence epilepticdischarges [102,103]. Furthermore, LY 300164 was able toexert additive effects on the antiabsence activity of CGP36742, a GABAB receptor antagonist, in WAG/Rij geneticmodel of absence epilepsy [102].

The short duration of action of currently available AMPAreceptor antagonists is a further problem for prophylacticuse.

In addition, sclerosis in the hippocampus and otherlimbic areas is a common pathological finding in brains frompatients with temporal lobe epilepsy (TLE) or partialcomplex seizures [104-108]. While controversy rages overwhether this is a cause or a result of seizure activity[104,109], it is clear from animal models that prolongedseizure activity causes neuronal loss [105,108].

AMPAR antagonists might provide a symptomatictreatment against epileptic seizure activity and, in addition,may be effective in preventing permanent brain damageresulting from prolonged seizure activity.

However, the neuronal damage produced by prolongedperiods of seizures, such as that caused by kainate andAMPA, is poorly prevented by competitive AMPARantagonist NBQX [110-112] whereas NMDA receptorantagonists provide substantial protection [113]. Neverthe-less, NBQX (20 to 40 mg/kg) has been reported to protectagainst the brain damage induced by electrically-inducedseizures [114].

6. ROLE OF AMPA RECEPTOR ANTAGONISTS INEPILEPSY

Excessive glutamatergic neurotransmission is understoodto be one of the primary metabolic and pathological mecha-nisms behind the aetiology of numerous types of epilepsy[115]. A number of early studies showed that glutamate andkainate were capable of inducing epilepsy in animals thatcorrelated with human symptoms [105]. Since then, anumber of functional changes in excitatory amino acidneurotransmission have been reported in seizure-susceptibleanimals including increased excitatory amino acid-inducedCa2+ influx, altered excitatory amino acid binding, enhancedglutamate and aspartate release, and modulation of glutamatetransporter expression and function [6].

Because AMPAR ligands are relatively novelanticonvulsant agents compared for example tobenzodiazepines or Na+ channel inhibitors, the potential ofAMPA receptor antagonists to attenuate epileptic seizureshas not yet been fully investigated. At present, talampanel isthe only AMPA receptor antagonist in phase II clinical trialuse for the amelioration of epileptic seizures.

Interest in iGluR antagonists as potential antiepilepticdrugs increased with the discovery of competitive and non-competitive NMDA receptor antagonists, such as D-CPPene,(E)-4-(3-phosphonopropyl)piperazine-2-carboxylic acid andMK-801, (5S,10R)-(+)-5-methyl-10,11-dihydro5H-dibenzo[a,d]cyclohepten-5,10-imine maleate, and the noncompe-titive AMPA receptor antagonist GYKI 52466. All NMDAreceptor antagonists showed therapeutic potential in animalmodels of epilepsy [80,85] but they failed early clinicaltrials. Ionotropic glutamate AMPA receptor antagonistscontinue, however, to be investigated as possible therapeuticagents and to further understand the role of glutamate in theaetiology of epilepsy.

It is known that AMPA receptors are expressed in thekey epileptogenic regions of the brain including the cerebralcortex, the thalamus, the amygdala, the hippocampus, andeven the basal ganglia which receives inputs from theseregions [116,117].

Table 5. Anticonvulsant Effects in Different Seizure Models.

ED50 (µmol/kg, ip)cpd

Audiogenic seizures AMPA-induced seizures MES PTZ

GYKI 52466 35.8 40.5 35.7 68.3

GYKI 53733 13.4 29.1 28.8 56.3

22 15.0 25.0 15.9 22.6

23 21.8 37.9 32.1 71.8

25 3.65 17.7 5.93 13.8

29 3.25 47.4 33.1 41.9

34 4.20 7.90 5.17 9.2

38 Current Topics in Medicinal Chemistry, 2005, Vol. 5, No. 1 Chimirri et al.

7. COMMENTS

The focus of this review was to describe competitive andnon-competitive AMPAR antagonists able to prevent and/orblock the epileptic seizures in different animal models.

The possible therapeutic efficacy in epilepsy of AMPAreceptor antagonists in animal models of generalized seizures(including clonic, clonic-tonic) and temporal lobe epilepsy(such as those induced by kainate or kindling) suggest apossible role against partial and generalized seizures. Noclear effect has been observed against absence seizures.Further studies are requested in order to better characterizethe efficacy and safety of AMPAR antagonists, as possibleanticonvulsants in animal models of genetic epilepsy, otherthan GEPRs, WAG/Rij and DBA/2 mice.

However, more experimental studies are necessary inorder to compare the several AMPAR antagonists inpreclinical animal models. Many compounds weresynthesized but only a few were adequately screened againstmodels different from those of generalized seizures andpartial epilepsy; this approach could better indicate apossible clinical use. Furthermore, in the DBA/2 mousemodel of primary generalized seizures some AMPA receptorligands show equal or greater potency than clinicalantiepileptic drugs when administered intraperitoneally.Another important consideration when assessing thetherapeutic potential of AMPA receptor ligands is toestablish the therapeutic index of such ligands, i.e. theanticonvulsant potency of the ligand compared to its ataxicor sedative potency. Current clinical antiepileptic drugs suchas diazepam, carbamazepine, lamotrigine, and sodiumvalproate exhibit therapeutic indexes of approximately 41,15, 23, and 7 in DBA/2 mice, respectively. Similarly, someAMPAR antagonists demonstrate a 2-6-fold therapeuticindex in the same assay.

Other considerations towards evaluating the therapeuticpotential of AMPAR antagonists as antiepileptic drugsinclude drug interactions, absorption with food, metabolism,and protein binding which are yet to be investigated.Preclinical studies positively support the therapeuticpotential of AMPAR antagonists in epilepsy and more dataare required concerning the efficacy of subtype specificagents in different epilepsy models.

Pharmacological data are conspicuously lacking inanimal models of epileptogenesis. Prolonged epilepticseizures produce a similar histopathological pattern to that ofischaemic damage. Some studies have investigated thecorrelation of the antiepileptic effect of AMPAR antagonistswith neuroprotection. It is possible that neuroprotectiveeffects may be associated with administration of antiepilepticdoses of AMPAR antagonists and therefore add to thetherapeutic potential of this class of drugs.

Another important consideration regarding thetherapeutic relevancy of AMPA receptor antagonists arosefrom the studies [76,118] where action on multiple targetsinvolved in glutamatergic neurotransmission was found to bemore efficacious than action on only one target. Testing withthe recent1y developed AMPA receptor antagonists mayprovide even more opportunity for obtaining the maximum

therapeutic value. It could be interesting to observe whichcombination of glutamate ligands produces the besttherapeutic value according to the animal model of seizures.Whereas selective inactivation of AMPA receptors canprovide information of the therapeutic contribution of eachreceptor subtype alone and help to map epileptic circuitry inthe brain, ultimately, antiepileptic drugs directed at theglutamatergic system are likely to be most beneficial whenthey involve a combination of agents including AMPAreceptor modulators.

ABBREVATIONS

AEDs = antiepileptic drugs

AMP397A = {[(7-nitro-2,3-dioxo-1,2,3,4-tetrahydro-quinoxalin-5-ylmethyl)-amino]-methyl}-phosphonic acid

AMPA = α -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid

AMPAR = α -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor

ATPO = (S)-2-Amino-3-[5-tert-butyl-3-(phosphonomethoxy)-4-isoxazolyl]propionic acid

BIIR 561CL,Irampanel = dimethyl-[2-[2-(3-phenyl-

[1,2,4]oxadiazol-5-yl)-phenoxyl]-ethyl]amine hydrochloride

CGP36742 = (3-Aminopropyl)butylphosphinic acid

CNQX = 6-cyano-7-nitroquinoxaline-2,3-dione

CP-465,022 = (+)-(aS)-(2-chlorophenyl)-2-[(E)-2-[6-(diethylaminomethyl)pyridin-2-yl]-vinyl]-6-fluoroquinazolin-4(3H)-one

CP-526,427 = 3-Pyridinecarbonitrile, 2-[2-[3-(2-chlorophenyl-4-t)-6-fluoro-3,4-dihydro-4-oxo-2-quinazolinyl]ethenyl]- (9CI)

D-CPPene = (E)-4-(3-phosphonopropyl)piperazine-2-carboxylic acid

DNQX = 6,7-dinitroquinoxaline-2,3-dione

iGluR = ionotropic glutamate receptor

GABA = γ-aminobutyric acid

GYKI 47621 = 6-(4-aminophenyl)-8-chloro-2-methyl-11H-imidazo[1,2-c][2,3]benzodiazepine

GYKI 52466 = 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine

GYKI 53733,talampanel,LY300164 = [(R)-7-acetyl-5-(4-aminophenyl)-8,9-

dihydro-8-methyl-7H-1,3-dioxolo[4,5-h][2,3] benzodiazepine

GYKI 53784 = LY303070 [(–)1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-4,5-dihydro-3-methylcarbamoyl-2,3-benzodiazepine]

AMPA Receptor Antagonists as Potential Anticonvulsant Drugs Current Topics in Medicinal Chemistry, 2005, Vol. 5, No. 1 39

KA = kainic acid

LIVBP = leucine/ isoleucine/valine bindingprotein

MES = maximal electroshock

mGluR = metabotropic glutamate receptor

MK-801 = (5S,10R)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-iminemaleate

NBQX = 1,2,3,4-Tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide

NMDA = N-methyl-D-aspartic acid

PTZ = Pentylentetrazol

QSAR = quantitative structure-activityrelationship

RPR117824 = 9-carboxymethyl-4-oxo-5H,10H-imidazo[1,2-a]indeno[1,2-e]pyrazin-2-carboxylic acid

RPR119990 = 9-carboxymethyl-4-oxo-5H,10H-imidazo[1,2-a]indeno[1,2-e]pyrazin-2-phosphonic acid

S17625 = 6,7-dichloro-2-(1H)-oxoquinoline-3-phosphonic acid

SAR = structure activity relationship

SYM 2189 = 4-(4-aminophenyl)-1-methyl-6-methoxy-N-propyl-1,2-dihydrophthalazine-2-carboxamide,

SYM 2206 = 4-(4-aminophenyl)-1-methyl-6,7-methylenedioxy-N-propyl-1,2-dihydrophthalazine-2-carboxamide,

TLE = temporal lobe epilepsy

YM-90K = 6-(1H-imidazol-1-yl)-7-nitro-2,3-(1H,4H)-quinoxalinedionehydrochloride

YM-872 = 1(2H)-Quinoxaline acetic acid, 3,4-dihydro-7-(1H-imidazol-1-yl)-6-nitro-2,3-dioxo

YM-928 = 2-[N-(4-chlorophenyl)-N-methylamino]-4H-pyrido[3,2-e]-1,3-thiazin-4-one

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