Recent developments on triazole nucleus in anticonvulsant compounds: a review

Abstract Epilepsy is one of the common diseases seriously threatening life and health of human. More than 50 million people are suffering from this condition and anticonvulsant agents are the main treatment. However, side effects and intolerance, and a lack of efficacy limit the application of the current anticonvulsant agents. The search for new anticonvulsant agents with higher efficacy and lower toxicity continues to be the focus and task in medicinal chemistry. Numbers of triazole derivatives as clinical drugs or candidates have been frequently employed for the treatment of various types of diseases, which have proved the importance of this heterocyclic nucleus in drug design and discovery. Recently many endeavours were made to involve the triazole into the anticonvulsants design, which have brought lots of active compounds. This work is an attempt to systematically review the research of triazole derivatives in the design and development of anticonvulsant agents during the past two decades.


Introduction
Epilepsy, a group of neurological disorders characterised by epileptic seizures, afflict over 50 million people around the world 1,2 . The cause of most cases of epilepsy is unknown 3 . Some cases occur as the result of brain injury, stroke, brain tumours, infections of the brain, and birth defects, through a process known as epileptogenesis [3][4][5] . Known genetic mutations are directly linked to a small proportion of cases 6,7 . The acceptable pathogenesis of epileptic seizures is the imbalance of excitatory and inhibitory neurotransmitters in central nervous system, which lead an abnormal nerve cell activity and neuronal discharge resulting in seizures 4 . Because the exact mechanism of epilepsy is unknown, there is still no medical cure for epilepsy (epileptogenesis) 8,9 . The anticonvulsants (also commonly known as antiepileptic drugs or antiseizure drugs), "symptomatic" agents that suppress the symptoms of epilepsy (i.e. seizures), was the main strategy for epilepsy treatment.
Unfortunately, about 30% of people continue to have seizures despite anticonvulsants treatment 15 , and the side effects of anticonvulsant agents follow up. Until now, the existing drugs are far from ideal, being consistently effective in fewer than 70% of patients and tending to produce a variety of side-effects in more than 50% of patients 8 . Toxicity, intolerance, and a lack of efficacy represent the limitations of the available anticonvulsants, which stimulated the continual attempts for discovery of new anticonvulsants.
Since 1975, the Epilepsy Branch of the National Institute of Neurological Disorders and Stroke, National Institutes of Health, through its Antiepileptic Drug Development (ADD) Program, has collaborated with the pharmaceutical industry and academic individual in developing new therapeutic agents for the treatment of seizure disorders 16 . The program has made important contributions to the development of several FDA-approved drugs for epilepsy, including felbamate (Felbatol), topirimate (Topamax), lacosamide (Vimpat), and retigabine (Potiga) 17 . In the ADD program, maximal electroshock seizure (MES) and chemical induced seizures (scPTZ) along with toxicity screen (rotorod in mice, positional sense and gait in rats) are employed to screen the new anticonvulsants, which gradually became the most widely used animal models world-wide for the discovery of new anticonvulsant drugs at the initial stage [18][19][20] . Except the in vivo models, some mechanistically related in vitro methods are also used to evaluate the anticonvulsant potential via obtaining the compound's affinity with excitatory (glutamate), inhibitory (GABA) receptor and other related enzymes 21,22 .
A triazole refers to the heterocyclic compounds with molecular formula C 2 H 3 N 3 , having a five-membered ring of two carbon atoms and three nitrogen atoms. There are two sets of isomers that differ in the relative positions of the three nitrogen atoms. Each of these has two tautomers that differ by which nitrogen has a hydrogen bonded to it (Figure 1). The aromaticity and electron rich property of triazole endows it to readily bind with various kinds of enzymes and receptors via weak interactions such as hydrogen bonds, coordination bonds, ion-dipole, cation-p, hydrophobic effect, van der Waals force and so on, and thus makes it widely employed in various field 23,24 .
The triazole ring is also an important isostere of oxazole, thiazole, imidazole, pyrazole, and so on. Based on the excellent properties of triazole, a huge number of triazole-based derivatives have been prepared and studied for their biological activities, especially for the research and development of new drugs [25][26][27][28][29][30] .
Since the approval and sales of triazolam and alprazolam (which have triazole moiety) as anticonvulsants in the 1980s, more and more medicinal chemists paid their attention to design and synthesis the triazole-derivatives for their anticonvulsant activity. Recently, several scholars have reviewed the medicinal attributes of triazoles 23,28,[31][32][33][34] , part of which focused on the anticonvulsant agents 23,34 . However, these reviews focus on the work of part laboratories and short time periods, which limited the overall and systematic understanding of the medicinal attributes of triazoles in anticonvulsant agents for readers. In view of this, we intended to review the triazole derivatives with anticonvulsant activity systematically and comprehensively from the reported literatures in recent 20 years. These researches can be classified into the following categories: (1) 1-substituted-1,2,4-triazoles, (2) 4-disubstituted-4H-1,2,4-triazoles, (3) polysubstituted-4H-1,2,4-triazoles, (4) substituted-triazolones and substituted-triazolthiones, (5) fused-triazoles, (6) fused-triazolones, and (7) 1,2,3-triazoles, according to the difference of structures.
1. 1-Substituted-1,2,4-triazoles 1-Substituted-1,2,4-triazoles, usually prepared by the reaction of halide and 1,2,4-triazoles (Scheme 1), were reported frequently as anticonvulsive compounds. In 2006, Shafiee's team reported several triazolylchromans as novel anticonvulsant agents. The anticonvulsant activities of these compounds were evaluated by determining seizure latency and protective effect against pentylenetetrazole (PTZ)-induced seizures in mice. Among of them, 3-(1H-1,2,4-triazol-1-yl)chroman-4-one (1, Figure 2) exhibited significant activity in delaying seizures as well as effective protection against PTZ-induced hind limb tonic extension (HLTE), and deaths at a dose of 5 mg/kg 35 . To further confirm the treatment for epilepsy of triazolylchromans, the lithium-epilocarpine induced seizure and PTZ-induced kindling models were used to study the anticonvulsive and antiepileptogenic properties of compound 1. The results suggested that compound 1 showed highly effective at the dose of 5 mg/kg against acute PTZ-induced convulsions, while exhibited limited effects in PTZ-induced kindling model which had considerable effect on seizure index only at the highest dose of 30 mg/kg. The effectiveness of compound 1 against lithium-epilocarpine induced status epilepticus, suggesting the potential application of it in the treatment of status epilepticus 36 .
In 1988, Dalkara et al. synthesised 1-(2-naphthyl)-2-(1,2,4-triazol-1-yl)ethanone (2, Figure 2), which has a triazole instead of an imidazole ring in the nafimidone (an anticonvulsant possessing activity profile similar to phenytoin or carbamazepine). It possessed noticeable anticonvulsant activity at dose of 30 mg/kg in the PTZ model 37 . As a continue of the above work, they prepared some new oxime ether derivatives (3, Figure 2) of 1-(2-naphthyl)-2-(1,2,4triazol-1-yl)ethanones and tested their anti-MES and anti-PTZ activities. The pharmacological experiment suggested that the size and structure of the O-substituents in oxime ethers were found to be important for the activity. Compounds with small alkyl groups such as methyl, ethyl, propyl, allyl were active in 100 mg/kg dose levels, whereas larger groups such as benzyl and substituted benzyl molecules resulted in a lack of anticonvulsant activity and a decrease in toxicity. Compound 3a (with R ¼ CH 3 ) was the most active one, which exhibited significant protection activity against MES and PTZ induced seizure in 0.5 and 4 h interval, at the dose of 100 mg/kg 38 .
Based on the potent anticonvulsant activity of the oxime ester derivatives of nafimidone containing triazole, Sari et al. prepared 14 novel 1-(2-naphthyl)-2-(1H-1,2,4-triazol-1-yl)ethanone oxime ester derivatives (4, Figure 2) through modifications on the oxime ester function. Their anticonvulsant and neurotoxic effects were evaluated in mice using MES, PTZ and rotarod tests. In order to get insights into the effects of these compounds on VGSCs and Atype GABA receptors (GABAARs), a docking studies using homology model of Na þ channel inner pore and GABA A R were carried out in this study. It was found that the compounds bound VGSCs in similar ways as phenytoin, carbamazepine, and lamotrigine, and showed strong affinity to benzodiazepine (BZD) binding site. Pharmacology test showed that some of these compounds displayed anti-MES and anti-PTZ activity at 30 or 100 mg/kg 39 . Srivastava et al. described the synthesis and anticonvulsant activity of several 1-disubstituted-4H-1,2,4-triazoles bearing thiadiazole moiety (5 and 6, Figure 3). All compounds showed anticonvulsant activities against PTZ induced seizures with 20-80% protection 40 .
A series of 3-and 5-aryl-1,2,4-oxadiazoles containing triazole (7 and 8, Figure 3) were prepared and tested for anticonvulsant activity in a variety of models. These compounds showed a remarkable in vivo activity against electrically and chemically induced seizures. Compound 8a (R ¼ p À F) gave a median effective dose (ED 50 ) of 15.2 mg/kg in the rat PTZ test in p.o. Additionally, a sufficient therapeutic index (PI) was achieved by 8b (R ¼ H) with a PI value bigger than 13 and 24 for PTZ and MES models, respectively. Mechanism studies on selected compounds revealed that the modulating of the GABA receptor was via a binding site but not the benzodiazepine site, and sodium channel blocking are both involved in the mechanisms 41 .
Several 9-subsituted-6-(1H-1,2,4-triazol-1-yl) (14, Figure 4) and (3H-1,2,4-triazol-3-ylthio)-9H-purines (15, Figure 4) were synthesised as new anticonvulsant agents. The results of MES tests indicated that the series of 14 showed good activity, while the series of 15 did not show protection at the dose of 100 mg/kg. Compounds 14a was the most potent compound, with an ED 50 of 23.4 mg/kg and a PI value of more than 25.6 after intraperitoneal administration in mice. Furthermore, it showed significant oral activity in MES test in mice, with an ED 50 of 39.4 mg/kg and a PI above 31.6 46 .
Quan's team reported the design and synthesis of numbers of triazolebenzo[d]oxazoles as anticonvulsant agents. Among of them, 2-phenyl-6-(4H-1,2,4-triazol-4-yl)benzo[d]oxazole (16, Figure  5) was the most active and also had the lowest toxicity. In the anti-MES potency test, it showed median effective dose (ED 50 ) of 29.5 mg/kg, a median toxicity dose (TD 50 ) of 285 mg/kg, and a protective index (PI) of 9.7, which is greater than the reference drug,  (13)(14)(15). carbamazepine, whose PI value was 6.4. This attempt suggested that the mono substituted triazoles also meet the request of the anticonvulsant activity 50 .
Based on the expectation that combining the anticonvulsant activity of benzothiazole and triazole, a series of triazole derivatives (17, Figure 5) containing benzothiazole were synthesised by Quan's team. The compounds tested in the MES screens were all effective. Among of which, the most promising compound was identified as 17a ( Figure 5), with an ED 50 value of 39.4 mg/kg and a PI value of 17.3. It is worth mentioning that these compounds had long duration of anticonvulsant activity with the 17b ( Figure  5) keeping protection activity in MES test more than 6 h. Moreover, compound 17a significantly inhibited the clonic seizures, tonic seizures and lethality with the rates of 80, 100, and 100%, respectively; and the result of PTZ test indicated that compound 17a could be used as a lead compound for the development of drugs treating absence seizures 51 .
Deng et al. embarked on a program to combine the antidepressant and anticonvulsant activities of 3,4-dihydro-2(1H)-quinolinone and triazole through the preparation of some triazolecontaining quinolinones (18, Figure 5). The pharmacology screen results showed that several compounds showed antidepressant and anticonvulsant activities. Compounds 18a and 18b ( Figure 5) showed 100% protection against MES-induced seizures at the dose of 100 mg/kg. No neurotoxicity was found at the dose of 100 mg/kg 52 .
Recently, a series of benzo[d]oxazoles containing triazole were designed and synthesised as new anticonvulsant agents. The pharmacology results showed that most compounds exhibited anticonvulsant activity in MES and Sc-PTZ models. Among them, compound 19 ( Figure 5) was the most potent with ED 50 value of 11.4 and 31.7 mg/kg in MES and Sc-PTZ models, respectively. The TD 50 value of 19 was 611.0 mg/kg, which resulted in the protective index (PI) value of 53.6 and 19.3. Further, the pre-treatment of thiosemicarbazide (an inhibitor of c-aminobutyric acid synthesis enzyme) significantly decreased the activity of 19 in MES, which suggested that the GABAergic system may contribute at least in part to the anticonvulsive action 53 . Meanwhile, another series of triazole-containing benzo[d]oxazoles were prepared via altering the position of triazole. In this study, compound 20 was obtained with an ED 50 of 12.7 mg/kg and 29.5 mg/kg in MES and Sc-PTZ models, respectively. The rotarod test showed the TD 50 of 491.0 mg/kg for 20 54 .
Several 2-arylisoindoline-1,3-dione derivatives were prepared and evaluated for their anticonvulsant activities. The in vivo screening data indicated that the 4-triazoly derivative (23, Figure  5) was the most promising one, which could increase the tonic seizure threshold significantly in the PTZ model. Docking studies using the model of sodium channel has revealed that compound 23 interacted mainly with residues II-S6 of NaV1.2 by making hydrogen bonds and had additional hydrophobic interactions with domain I and II in the channel's inner pore 56 .
Recently, Quan et al. designed and synthesised several novel triazoleindole derivatives with tryptamine as the starting material. Among the compounds studied, compound 25 ( Figure 5) was found to be the most potent compound with an ED 50 value of 10.2 mg/kg in MES test in intraperitoneal administrated mice, possessed better anticonvulsant activity and higher safety than carbamazepine 58 . relationship of triazoles and indeed provided a large amount of compounds with prominent anticonvulsant effects.

Polysubstituted-4H-1,2,4-triazoles
Alprazolam and triazolam were two of the promising anticonvulsant agents that containing triazole (26a and 26b, Figure 6). Around the triazole-benzodiazepine structure, many analogues were prepared to find new anticonvulsant agents. Gall et al. Amongst all pharmacodynamic models suggested for binding to the benzodiazepine receptor at least two features are common: an aromatic ring and a coplanar proton accepting group in suitable distance. Also, the presence of a second out-of-plane, aromatic ring could potentiate binding to the receptor. Abbas Shafiee and his team launched a project to design simple non-rigid structures with benzodiazepine activity based on the proposed SAR. A series of 4H-3-(2-phenoxy)phenyl-1,2,4-triazole derivatives (29, Figure 6), with a simple non-rigid structure, were designed, which had all the suggested requirements for binding to the benzodiazepine receptors. Their benzodiazepine effects and anticonvulsant activity were evaluated. Among the tested compounds, compound 29a with dichloro-substituent showed the best anticonvulsant activity with an ED 50 of 12.0 mg/kg in the PTZ models. To clarify whether the designed compounds could mimic the structure of a benzodiazepine agonist, conformational analysis on designed molecules as well as a known benzodiazepine agonist estazolam was performed followed by superimposition of energy minima conformers. The results showed that the main proposed pharmacofores were well matched. In addition, the activity of the compounds is significantly reduced by flumazenil, a benzodiazepine antagonist, which further confirms that this effect is mediated through benzodiazepine receptors.
From the pharmacological results, the follow SARs were obtained: Analogous with chloro substituent on position 2 of phenoxy group and position 4 of phenyl ring are more potent than the corresponding unsubstituted compounds. These two positions are well matched to positions 2 0 and 7 of the benzodiazepine ring; it has been established that electron withdrawing substituent on these positions enhance the activity.
The study indicates that some synthesised 1,2,4-triazoles with a simple non-rigid structure can also show benzodiazepine activity comparable with diazepam, which lead us to the new class of benzodiazepine receptor ligands 61 .
Followed the above study, several new 2-substituted-5-[2-(2fluorophenoxy)phenyl]-1,3,4-oxadiazoles and 1,2,4-triazoles were prepared to explore the necessity or not of the 2-NH 2 for the anticonvulsant activity. The results suggested that compounds with substituents other than amino group did not have significant activity in PTZ and MES models. Only compounds with OEt (30, Figure 6) showed mild effects in PTZ test with an ED 50 of 84.84 mg/kg 62 .
Bioisosterism is a good method to obtain derivatives with similar or better activity. With the purpose to evaluate the effects of different substitutents on pharmacological activity, a series of 1,2,4-triazole derivatives were prepared by bioisosteric replacement of oxygen with sulphur in the compounds 30. Unfortunately, none of them, except (31, Figure 6) showed anticonvulsant effect in dose of 100 mg/kg 63 .
Several new N-4-substituted triazolylthiazoles (33, Figure 7) were prepared and screened for anticonvulsant activity against PTZ-induced seizures. Most compounds exhibited protection against PTZ-induced seizures in the range doses of 25-200 mg/kg. Pre-treatment of animals with flumazenil (10 mg/kg, i.p.) as a benzodiazepine receptors antagonist did not have any significant effect on their anticonvulsant activity, which suggested that the anticonvulsant activity of N 4-substituted triazolyl thiazoles is not mediated by activation of benzodiazepine receptors 65 .
In order to discuss the effect of different substituted groups in the third position of 4-(4-alkoxyphenyl)-4H-1,2,4-triazoles to anticonvulsant activity, a series of 3-substituted-4-(4-hexyloxyphenyl)-4H-1,2,4-triazoles (36, Figure 7) was designed and prepared. Their anticonvulsant activities and neurotoxicity were evaluated by MES test and rotarod assay, respectively. Compound 36a with n-propyl was the most active compound amongst all 19 derivatives, with an ED 50 value of 5.7 mg/kg, which was greater than that of the reference drug phenytoin. In addition, it was also safer than phenytoin with a PI value of 11.5 68 .
Sahoo et al. reported a series of 3,5-substituted-2-amino-1,2,4triazole derivatives as anticonvulsant agents. All the compounds were screened for their anticonvulsant activity in the MES model and were compared with phenytoin sodium and carbamazepine.
Some of the tested compounds showed comparable anti-MES activity to phenytoin and carbamazepine after 0.5 h. And compound 38 was more potent than carbamazepine after 4 h 70 .
In another study, some 3-(arylalkylthio)-4-alkyl/aryl-5-(4-aminophenyl)-4H-1,2,4-triazole derivatives (41 and 42, Figure 7) were synthesised and screened for their anticonvulsant activity. Some valuable compounds have emerged with activity in the dose of 100 mg/kg in both MES and scMET tests at 0.5 and 4 h. The obtained results demonstrated that either small alkyl substitution at triazole ring or primary aromatic amino group were essential for bioactivity. The replacement of small alkyl by aryl substitution at the same position completely blocked activity in both MES and scPTZ models. The complete loss of activity was observed when the NH 2 function was replaced with an arylideneamino (-N¼CH-Ar) moiety 73 .

Substituted-triazolones and substituted-triazolthiones
Classical methods for the synthesis of triazolones and triazolthiones are by heating the substituted isothiosiyanates or semicarbazide in the presence of alkaline medium (Scheme 3). Another method for preparing triazolones is utilising a wide range of substituted primary amines (arylamine or aliphatic amine), methyl carbazate, and triethyl orthoformate in the presence of alkaline medium (Scheme 3). Many triazolones and triazolthiones were prepared by the two methods and reported with the potent anticonvulsant activity.
A series of 5-pyrazolinones containing triazolone or triazolthione (44, Figure 8) were prepared by K€ uc¸€ ukg€ uzel and his coworkers. These compounds were tested at 100 mg/kg in pentylenetetrazole induced seizures in mice, and protections ranging from 10% to 40% were obtained 75 .
A series of alkoxyaryltriazolones were synthesised by Quan's team, and tested for their anticonvulsant activity and neurotoxicity in mice by MES test and rotarod test, respectively. All target compounds exhibited anticonvulsant activity in varying degrees in the MES test. 4-(3-Benzyloxy-phenyl)-2,4-dihydro- [1,2,4]triazol-3-one (48, Figure 9) was the most promising compound with an ED 50 value of 30.5 mg/kg and a TD 50 value of 568.1 mg/kg, which gave a protective index value (PI) of 18.63. Quan et al. suggested that these alkoxytriazolones acted via inhibiting of voltage-gated ion channels and modulating of GABAergic activity 78 .
In another work, several new alkoxyaryltriazolones were prepared as anticonvulsant agents by altering the site of alkoxy group of 48. Based on the results, 4-(2-octyloxy-phenyl)-2,4-dihydro-3H-1,2,4-triazol-3-one (49, Figure 9) was the most promising compound with the ED 50 of 23.7 mg/kg, the TD 50 of 611.0 mg/kg, and the protective index (PI) of 25.8. In addition, the potency of compound 49 against seizures induced by pentylenetetrazole, 3-mercaptopropionic acid, and bicuculline were also established in this work 79 .
Meanwhile, another series of 4-(4-alkoxyphenyl)-1H-1,2,4-triazol-5(4H)-ones were prepared and evaluated the anticonvulsant activity in MES test in mice. Among of which, compound 51 ( Figure 9) was found to possess better anti-MES activity and higher safety than marketed drug carbamazepine with an ED 50 value of 25.5 mg/kg and protective index (PI) value above 48.8 81 .
A series of 4-butyl-5-(4-alkoxyphenyl)-2H-1,2,4-triazol-3(4H)-ones were also synthesised and screened for their anticonvulsant effects by MES test. Among the synthesised compounds, compound 52 ( Figure 9) was the most potent with ED 50 value of 27.4 mg/kg and a protective index value of 12.0. Besides the anti-MES efficacy, the potency of 52 against seizures induced by pentylenetetrazole, 3mercaptopropionic acid, and bicuculline was also established, which suggested that enhancing of GABAergic activity might be involved in its mechanisms of action 82 .
Khanum reported the synthesis of 3-(2-aroylaryloxy)methyl-5mercapto-4H-1,2,4-triazole analogues (59, Figure 10) by intramolecular cyclisation of (2-aroylaryloxy)acetates with thiosemicarbazide. The anticonvulsant activities of these compounds were screened by MES model in rats. The results showed that compounds with chloro in the R 2 and R 4 substituents were more active than others and showed comparative activity to that of phenytoin 86 .
A series of 5-substituted Isatin-imino derivatives (60, Figure 10) containing triazolthione was prepared for their anticonvulsant properties by Pandeya et al. Some of them showed the anti-MES and anti-PTZ activities 87 .
Bhat et al. reported the synthesis and anticonvulsant properties of coumarin incorporated 1,2,4-triazole-5-thione derivatives. Compound 63 ( Figure 10) showed significant anticonvulsant activity. In MES test, compound 63 showed protection from seizures at the dose of 30 mg/kg after 0.5 h and it kept the activity after 4.0 h at higher dose of 100 mg/kg. In the scPTZ screen, compound 63 was active at 100 mg/kg after 0.5 h, and was also found to be active after 4.0 h at higher dose 300 mg/kg 90 .
Kumudha et al. synthesised numbers of acetohydrazide Schiff's bases containing 1,2,4-triazole-5-thione (64, Figure 11), which was further cyclised with thioglycolic acid to get some thiazolidinones (65, Figure 11). Parts of these compounds were evaluated for anticonvulsant, CNS depressant activities and neurotoxicity. All the tested compounds showed potent anticonvulsant activity, with good protection against PTZ and MES induced seizures at 20 mg/ kg. No toxicity was found at the same dose in the rotarod test 91 .
Some new phenytoin-triazole derivatives (66, Figure 11) were prepared using hybrid approach as anticonvulsant agents. These compounds exhibited weaker anticonvulsant activity that phenytoin in the PTZ model, but comparative protection in the MES test 92 . A series of substituted 4,5-diphenyl 4H-1,2,4-triazole-3-thiols (67, Figure 11 Chromatographic tests allow one to state that the lack of permeability through the blood-brain barrier was the reason for the lack of activity of some compounds in this series 94 . Several bis-1,2,4-triazoles derivatives (69, Figure 11) were prepared by Kumudha et al. to evaluate their anticonvulsant. Both MES method and PTZ model confirmed the moderate to good anticonvulsant activity of these compounds. No neurotoxicity was found at the dose of 100 mg/kg 95 .
Several 1,2,4-triazol-3-thiols (70, Figure 11) containing 5-amino-1,3,4-thiadiazole were prepared by Kumudha's group to evaluate their anticonvulsant, CNS depressant activities and neurotoxicity. The target compounds showed excellent anticonvulsant activity in both MES and PTZ tests, which significantly reduced the duration of  tonic hind limb extensor in the MES model and significantly prolonged the onset of seizure against convulsions induced by PTZ 96 .
Several 1,2,4-triazole-3-thione derivatives (71 and 72, Figure 12) were prepared by Plech's group to evaluate their anticonvulsant activity. Rapid onset of action and long lasting effect of these compounds were found in the MES model. Addition, a SAR was obtained that the anticonvulsant activity request at least one of the phenyl rings attached to 1,2,4-triazole nucleus, and the phenyl had a substituent at the para position. The radio-ligand binding assay (with benzodiazepine binding sites and GABAa receptors complex) suggested that a direct involvement of GABAA receptors or bind with BDZ-binding sites was excluded for their molecular mechanism 97 .
To obtain more SARs and higher activity of anticonvulsant agents, Plech's group prepared another 1,2,4-triazole-3-thione derivatives (73, Figure 12) via the cyclisation of 1-acyl-4-alkylthiosemicarbazides. Taking into the account of the activity, toxicity and the time-course changes of ED 50 , 73a ( Figure 12) was proposed as the best tolerated and the most promising potential drug candidate (ED 50 ¼ 72.1 mg/kg and TD 50 > 1000 mg/kg at 15 min interval). Like the previous work, the interactions (direct or allosteric) with GABAA receptor complex and/or the affinity to benzodiazepine (BDZ) binding sites was excluded for its molecular mechanism 98 .
Several indole derivatives (74, 75, and 76, Figure 12) containing triazolethione were synthesised and screened for their anticonvulsant activity by MES test in rats. All the compounds showed the protection against MES induced seizures in varying degrees. Among them, compound 76a ( Figure 12) substituted with 2-chloro showed interesting anticonvulsant activity with a considerable percentage of protection (90%) against MES induced seizure at the dose of 15 mg/kg, which exhibited better anticonvulsant activity than standard drug. The newly synthesised compounds were also tested for approximate lethal dose (ALD 50 ) and were found to exhibit a higher value of ALD 50 i.e. more than 1000 mg/kg, i.p. except compound 76a which exhibited ALD 50 of more than 2000 (maximum dose tested), indicating the safer nature of these compounds 99 .
Prakash et al. synthesised some isatin containing congeners of 1,2,4-triazole Schiff and Mannich bases with a view to explore their potency as better anticonvulsant agents. All the synthesised compounds were screened for its anti-epileptic activity by MES and scPTZ methods using phenytoin and ethosuximide as standards. The results showed that some of the synthesised compounds were exhibited significant activity. The most active was 77 ( Figure  12

Fused-triazoles
There are various methods for synthesis of fused-triazoles. The most general method to introduce triazole ring for the fused-triazoles is cyclisation of halogenated heterocyclic or thio-substituted heterocyclic compounds with hydrazines (Scheme 4) 102,103 . Many fused-triazoles exhibited higher anticonvulsant activity because of the raise of the electronic cloud density in the triazole ring.  clonic seizures in mice were evaluated. Compound 79 ( Figure 13) was the most active one with ED 50 of 10 mg/kg in this model, which displayed more potent than carbamazepine (28 mg/kg) 102 .
As part of the program finding new anticonvulsant agents in heterocyclic fused triazoles, several [1,2,4]triazolo[4,3-f]pyrimidine derivatives were designed and synthesised by Guan and coworkers. The anticonvulsant activity screens showed that the compound 80 (Figure 13) was the most active agent with an ED 50 value of 34.7 mg/kg, a TD 50 of 262.9 mg/kg, and PI value of 7.6 103 .
A series of [1,2,4]triazolo[4,3-a]pyrimidine derivatives were synthesised in good yields by the microwave-assisted one-pot protocol in very short reaction time. A prediction of anticonvulsant activities applying computer program PASS indicated that compound 81 ( Figure 13) was a promising anticonvulsant agent 104 .
Recently, several novel BZD agonists were designed and synthesised based on the pharmacophore/receptor model of BZD binding site of GABAA receptor. Their affinity in vitro for the central BZD receptor was determined. Most of the novel compounds had better affinity for the BZD site of action on GABAA receptor complex than diazepam. Compound 82 ( Figure 13) with the best affinity in radio-ligand receptor binding assay (K i ¼ 0.42 nM and IC 50 ¼ 0.68 nM) was selected as candidate for in vivo evaluation. This compound showed significant hypnotic activity and anticonvulsant effect [ED 50 ¼ 15.01 (MES model), 56.57 (PTZ model)] with no impairment on learning and memory performance in mouse. The pharmacological effects of the compound 82 were antagonised by flumazenil, a BZD antagonist, which confirms the involvement of BZD receptors in the biological effects of the novel ligand 105 .
Another triazolopyrimidines were synthesised by Jiang et al. through incorporating triazole moiety into the pyrimidine ring, which were expected to have the synergistic effect in dealing with the epilepsy. Their anticonvulsant activities against MES induced seizures were measured in mice. Carbamazepine and valproate were considered as positive control drugs with anticonvulsant effects [ED 50 ¼ 11.8 and 272 mg/kg]. Amongst these compounds, compound 83 ( Figure 13) showed potent anticonvulsant activity with ED 50 84.9 mg/kg, which was weaker than carbamazepine, but better than valproate 106 Figure 13) were carried out by Guan and coworkers. Based on the MES test and rotarod test, compounds 86a ( Figure 13) and 86b ( Figure 13) showed promising anticonvulsant activity with an ED 50 of 13.2 and 15.8 mg/kg and had a protective index value of 4.8 and 6.9, respectively 109 .

Triazolothiazoles, triazolothiadiazines, triazolothiadiazoles, and triazolothiazepines
Narayana and Vijaya Raj reported a smooth synthesis of a series of thiazolotriazolones and the anticonvulsant activity screening of part compounds. Compounds 87 and 88 ( Figure 14) exhibit promising anticonvulsant activity with a significant reduction of the duration of seizure in PTZ test at the dose of 4 mg/kg 110 .
Various triazolothiadiazines were designed and synthesised by Quan's team. All the compounds were evaluated for their anticonvulsant activity against MES-induced seizures. Among of which, compound 89 ( Figure 14) was the most promising compound with an ED 50 value of 40.9 mg/kg and a PI value of 6.5. The SARs indicated that any substituent of R' would decrease the anticonvulsant activity 111 .
In the next work, numbers of triazolotriazoles 90 ( Figure 14) were synthesised and tested for their anticonvulsant activities using the MES and PTZ screens. In the MES test, compound 90a ( Figure 14) was found to be the most active compound, which gave an ED 50 of 49.1 mg/kg. In the PTZ model the most active compound of tested compounds was 90b (Figure 14), which showed an ED 50 of 63.4 mg/kg. In this work, compounds holding big lipophilicity (CLoP > 3.9) did not show anticonvulsant activity, which was explained by the interrupting of the absorption and distribution 112 .
A series of 3,6-disubstituted-1,2,4-triazolo-1,3,4-thiadiazole derivatives (91, Figure 14 (Figure 14) with 4-chlorophenyl substituent emerged as the most promising candidate based on its favourable ED 50 value of 23.7 mg/kg and PI value of 10.8. In addition, the potency of compound 92a against seizures induced by PTZ, 3mercaptopropionic acid, and bicuculline suggested that compound 92a displayed broad-spectrum activity, and it might exert its anticonvulsant activity through affecting the GABAergic system 114 .
Comparing the anticonvulsant activity of compounds 89, 91, 92, and 93, it can be found that the conjugation between the heterocycle and triazole is very important for their anticonvulsant activity. The SAR information was described as below (Scheme 5).
Kamble and Sudha reported an efficient synthesis of pharmacologically active derivatives of 1,3,4-oxadiazoles (94, Figure 14), in which some triazole-containing derivatives were included. Several triazole-containing derivatives were found to possess very good activity against MES induced convulsions in rats when compared to that of standard phenyntoin 115 .

Triazoloquinolines
In 2005, the team of Quan launched a study for new anticonvulsant agents by incorporating triazole with various heterocyclic. Based on the weak activity of 6-benzyloxy-3,4-dihydro-1Hquinoline-2-one, a series of 1-substituted-7-benzyloxy-4,5-dihydro-[1,2,4]triazolo[4,3-a]quinolines (95, Figure 15) were prepared by incorporating triazole with 6-benzyloxy-3,4-dihydro-1H-quinolinone for more potent anticonvulsant activity. The results indicated that the introduction of triazole increased the activity, but any substitutions at the first position decreased the anticonvulsant activity markedly. Compound 95a with H in the 1-position was the most active one with an ED 50 of 17.3 and 24 mg/kg, respectively in the MES and PTZ model tests 116 .
As a continuation of the above work, 7-alkoxyl-4,5-dihydro-[1,2,4]triazolo[4,3-a]quinoline derivatives (96, Figure 15) were then prepared. Most of them displayed potent anticonvulsant activity against the MES and scPTZ-induced seizures. The SAR indicated that the lengthening of the alkyl chain at position 7 influenced the anticonvulsant activity obviously, in which the n-hexyl-substituted compound was found to be the most active. Among all the compounds in this series, 96a (Figure 15) was found to be the most promising one with ED 50 of 11.8, and 6.7 mg/kg in the MES and PTZ model tests, respectively, although the neurotoxicity was also followed with a TD 50 of 54.5 mg/kg in the rotarod test 117 .
In order to obtain a novel anticonvulsant agent having more potency and lower neurotoxicity, a series of 7-substituted-benzylamino-4, 5-dihydro- [1,2,4]triazolo [4, 3-a]quinoline derivatives (98, Figure 15) was synthesised and evaluated for their anticonvulsant activity. Compound 98a (Figure 15) was the most effective one in PTZ test with an ED 50 of 5. 0 mg/kg and the PI of 20.7, which was also safer than the reference drugs. In MES test, compound 98b ( Figure 15) was the most promising one with an ED 50 of 15.3 mg/ kg and the PI of 7.2 119 .
A number of 1-aminocarbonyl-triazolo[4,3-a]quinoline derivatives was prepared by Quan's team via linking the key group of anticonvulsant agents, i.e. carboxamide group to the triazolo[4,3a]quinoline. The anticonvulsant effect and neurotoxicity of the compounds was evaluated with MES test and rotarod tests in mice. The SAR indicated that alkyl substitutes was good for activity than the benzyl substitutes. Compound 99 (Figure 15) was the most active one with an ED 50 of 30.1 mg/kg, and had the lowest toxicity with an TD 50 of 286 mg/kg, which gave the greater PI value of 9.5 than the reference drug carbamazepine (PI ¼ 6.0) 120 .
In order to obtain compounds with better anticonvulsant activity, structural modification was made on the compounds 100 i.e. the introduction of a double bond into the fourth and fifth positions to give the compound 102 ( Figure 16). A further modification was made via introducing a methoxy group in the seventh position, and then an alkyl (or aryl) in the first position of the 1,2,4-triazolo[4,3-a]-quinolines to give the compounds 103 ( Figure  16). Compound 102 (ED 50 ¼ 28.4 mg/kg) showed higher activity than compound 100 b (R ¼ H, ED 50 ¼ 54.8 mg/kg) in MES test. While in the series of 103, compound 103a ( Figure 16), with ED 50 of 9.2 mg/kg, was the most promising one with the anticonvulsant activity comparable to that of the reference drug phenytoin (ED 50 ¼ 9.9 mg/kg) in MES test. In addition, compound 103a showed lower neurotoxicity than phenytoin with a TD 50 of 151 mg/kg, and gave a PI value of 16.6 in the MES test, which was higher than the PI value of phenytoin. Compound 103a also antagonised PTZ and isoniazid-induced seizures with an ED 50 of 21.1 mg/kg and 83.3 mg/kg, respectively, which suggested that compound 103a might exert anticonvulsant activity by impacting GABA-ergic neurotransmission and the glycine system 122 .
A series of new derivatives of compound 102 was prepared to obtain more SARs of [1,2,4] higher than the PI of the reference drug phenytoin (PI ¼ 6.9) 123 124 .
In the previous work, the introduction of the double bond was considered as beneficial based on the improvement of the anticonvulsant activity. So, in the next work, a series of 5-alkoxy- [1,2,4]triazolo[4,3-a]quinoline derivatives were prepared by Quan's team. The hypothesis is that the electron density at triazole ring will be increased because of the lone pair electrons of oxygen atom of 5-alkoxy via the conjugation effect, so as the affinity of compounds to the receptor will be increased. The pharmacological results revealed that most of the compounds showed remarkable anticonvulsant activity, but the serious toxicity also followed. Taking consideration of the safety, compound 105 ( Figure 16) was considered as the most promising one with ED 50 of 22.8, TD 50 of 273.9, and PI value of 12.0. Additional, compound 105 produced significant protection activity against seizures induced by PTZ, 3-MP, thiosemicarbazide and isoniazid, but not strychnine suggested that the compound 105 might have effects on GABAergic neurotransmission and activate GAD or inhibit GABA-T in the brain 125 .

Triazolo-quinazoline, phthalazine, and quinoxaline derivatives
Two series of 5-substituted [1,2,4]triazolo[4,3-a]quinazolines (106 and 107, Figure 17) were synthesised to screen their anticonvulsant activity in MES test. The pharmacology results showed that compound 106a (Figure 17) with pentyl group was the most potent with ED 50 value of 19.7 m g/kg and PI value of 6.2. It is interesting that alkyl amino substituted derivatives (107) exhibited low activity and high toxicity. It suggested the avoiding of this group in the later design of anticonvulsants 126 .
Kawade et al. synthesised another series of triazolopyrimidine-5(4H)-ones (109, Figure 17) to screen their anticonvulsant activity. The results indicated that all compounds showed activity with ED 50 range from 152 to 667 mg/kg in MES test. Furthermore, it has been found that the ED 50 and TD 50 values of test compounds increase significantly at t ¼ 4 h, when compared to t ¼ 30 min, in contrast to the standard Phenytoin 128 .
In the work of Zheng et al. for preparing same quinazolintriazoles as anticonvulsant agents, 5-(2-bromophenyl)-[1,2,4]triazolo[4,3-c]quinazolin-3-amine (110, Figure 17) was obtained, which showed an ED 50 value of 27.4 mg/kg and a TD 50 value of 157.8 in the MES and rotarod test, respectively. This work found that the introducing of NH 2 to the triazole was beneficial to the anticomvulsant activity for these quinazolinetriazoles 129 .
In another work, a series of novel 6-alkyoxyl [1,2,4]triazolo[1,5a]quinazoline derivatives were synthesised and evaluated for anticonvulsant activity using MES tests. The pharmacological results showed that some of the compounds displayed positive anticonvulsant activity. Among them, compound 111 ( Figure 17) was the most active compound with an ED 50 value of 78.9 mg/kg and a PI value of 9.0 130 .
As the continue of the above works, another triazolopyrimidine derivatives (112, Figure 17) were synthesised by Quan's team and evaluated for their anticonvulsant activity and neurotoxicity. In the MES test, among the compounds studied, 112a ( Figure 17) and 112b ( Figure 17) showed potent anti-MES activity with an ED 50 of 27.4 and 26.9 mg/kg, and wide margins of safety with PI value of above 25.5 and 26.9, respectively. The two compounds were also found to have potent activity against seizures induced by PTZ and bicuculline at 50 mg/kg in mice 131 .
Several [1,2,4]triazolo [4,3-g]purines (113, Figure 17) were also prepared by Quan's group to more effective AEDs. Compounds 113a and 113b were considered as the most promising compounds with an ED 50 of 51.2 and 51.9 mg/kg in MES test, respectively 46 . Meanwhile, a series of triazolopyrimidines were prepared and evaluated for their anticonvulsant activities. The SARs analysis revealed that the most effective structural motif involves a substituted phenol, especially when substituted with a single chlorine, fluorine or trifluoromethyl group (at the meta-position), or two chlorine atoms. The most active compound in this series was 114 ( Figure 17) with ED 50 values of 11.5 mg/kg (MES) and 58.9 mg/kg (scPTZ) 132 .
A new series of 6-alkoxy- [1,2,4]triazolo[3,4-a]phthalazines (115, Figure 18) were synthesised as an isostere of compounds 106, which held potent anticonvulsant activity as shown above. In this series, the most promising compounds 115a ( Figure 18) and 115b ( Figure 18) showed a median effective dose of 7.1 and 11.0 mg/kg, and had protective index value of 5.2 and 8.0, respectively. The two compounds were further found to have potent activity against seizures induced by pentylenetetrazole, isoniazid, thiosemicarbazide, 3-mercaptopropionic acid but not seizures induced by strychnine, indicating that the two compounds might function by enhancing gamma-aminobutyric acid neurotransmission 133 .
Bian et al. described the synthesis and anticonvulsant activity evaluation of 6-substituted- [1,2,4]triazolo[3,4-a]phthalazines (116, Figure 18). Most of the synthesised compounds exhibited potent anticonvulsant activities in the maximal electroshock test (MES). The most promising compound 116a (Figure 18) showed significant anticonvulsant activity in MES test with ED 50 value of 9.3 mg/ kg. It displayed a wide margin of safety with protective index much higher than the standard drug Carbamazepine (91.7 versus 6.4) 134 .
A series of 6-substituted-pyrido[3,2-d]pyridazine derivatives containing triazole was synthesised as the isosteres of 115. Their anticonvulsant activities were evaluated by the MES and their neurotoxicity were measured by the rotarod test. The results demonstrated that compound 117 (Figure 18 (Figure 18) was the most potent and selective AMPA receptor antagonists. It displayed its ability to prevent sound-induced seizures in DBA/2 mice with 83% protection against tonic seizures at 30 mg/kg 136 .
A series of 1,2,4-triazolo(4,3-a)quinoxalin-4-5H-ones 119 ( Figure  18) were prepared as AMPA receptor antagonists. Their anticonvulsant activity was confirmed by the protection activity against PTZ induced seizures. Compound 119a (Figure 18) was the most potent one with a ED 50 of 12.5 mg/kg in PTZ induced seizures model. There was a strong correlation between the results of the anticonvulsant activity and molecular modelling, which suggested that the AMPA receptor antagonism was involved in their anticonvulsant activity 137 .
1,4-Disubstituted [1,2,4]triazolo[4,3-a]quinoxalines (120, Figure  18) were prepared by Wagle et al. as potent anticonvulsants. Some of the compounds exhibited 100% protection for the animals against PTZ induced convulsions at the dose of 10 mg/kg. The SARs study revealed that compounds bearing CF 3 , H or CH 3 group in position-1 and 4-fluorophenyl moiety or 4-methoxyphenyl substituents at C-4 of these compounds have shown good anticonvulsant activity in comparison with standard drug diazepam 138 .
Based on the promising anticonvulsant activity of [1,2,4]  (121 and 122, Figure 18) showed the best anticonvulsant activities with an ED 50 value of 30 mg/kg in the PTZ model 139 .

Triazolobenzothiazines and triazolobenzothiazoles derivatives
A series of triazolobenzothiazines was designed and synthesised by Quan's team as an isostere of compounds 95. Among these compounds, 123 (Figure 19) was the most active compound with an ED 50 of 17.0 mg/kg and a protective index of 14.3 in the MES test 140 . Next, a series of 7-alkoxy-triazolo-[3,4-b]benzo[d]thiazoles was designed and synthesised as the ring contraction analogues of compounds 123 through removal of a CH 2 in the compounds 123, were anticipated to possess a better anticonvulsant activity. Among these compounds, 124 ( Figure 19) was found to be the most potent compound with an ED 50 of 8.0 mg/kg and a PI value of 15.0, possessing better anticonvulsant activity and higher safety than marketed drugs carbamazepine and phenytoin. The mechanism study of compound 124 showed that it displayed broad spectrum activity in several models, and it is likely to have several mechanisms of action (including inhibiting voltage-gated ion channels and GABAergic activity) 141 .

Tricyclic fused-triazoles containing 7-membered ring
For the past three decades, five-atom heterocyclic fused benzodiazepine ring systems occupy a prominent place among drugs for treatment of CNS disorders 142 . The introduction of alprazolam and triazolam in epilepsy therapy has enhanced the interest in preparation triazole-benzodiazepine derivatives. Numerous analogues of alprazolam and triazolam along with their pharmacological profiles have been described. Based on the SAR for the triazole-benzodiazepine class of compounds that an electronegative substituent at C-7 is an essential requirement for anticonvulsant, Phillips et al. designed and synthesised a pyrimido [4,5-f] [1,2,4]triazolo [4,3-a] [1,4]diazepine derivative (125, Figure 20) and evaluated its anticonvulsant activity to find more potent anticonvulsant. Compound 125 showed moderately activity against MES-induced seizures with ED 50 of 79.5 mg/ kg, and high activity against PTZ-induced seizures with ED 50 of 6.2 mg/kg. It was non-toxic under the dose of 150 mg/kg (TD 50 >150 mg/kg i.p. in mice). The affinity for the benzodiazepine receptor was observed, but it was weaker than the reference drug clonazepam (IC 50 : 72 mM versus 0.00372 mM) 143 .
A series of 10-alkoxy-5,6-dihydro-triazolo [4,3-d]benzo[f] [1,4]oxazepines (132, Figure 22) were designed and synthesised by Deng et al., as the ring enlargement analogues of compounds 95. Among of which, compound 132a was found to possess better anticonvulsant activity and higher safety than marketed drugs carbamazepine and phenytoin with an ED 50 value of 6.9 mg/kg a PI value of 9.5. To explain the possible mechanism of anticonvulsant activity, compound 132a was tested in pentylenetetrazole, isoniazid, thiosemicarbazide, 3-mercaptopropionic acid, and Bicuculline induced seizures tests. The results suggested that compound 132a exerts anticonvulsant activity through GABA-mediated mechanism 148 .
Based on the potent anticonvulsant activity of compounds 123, a series of ring-expanding products (133, Figure 22) and its carbonyl derivatives (134, Figure 22) were designed and synthesised. Their anticonvulsant activities and neurotoxicity were evaluated using the MES-induced seizure model and the rotarod assay in mice, respectively. Based on the results, compound 133a containing benzyl group was the most active one with an ED 50 of 20.7 mg/kg in MES test 149 Figure 22) with ED 50 of 17.5 mg/kg in the MES test was more potent than phenobarbital (ED 50 ¼ 21.8 mg/kg) but slightly less than that of carbamazepine (ED 50 ¼ 8.8 mg/kg) 150 .
As   Figure 22) was the most promising compound, with an ED 50 of 19.0 mg/kg, and PI value of 14.8 in the MES test, which is much higher than the PI value of the prototype antiepileptic drug carbamazepine (PI ¼ 8.1), phenytoin (PI ¼ 6.9), phenobarbital (PI ¼ 3.2), and sodium valproate (PI ¼ 1.6) 151 . 2-Amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) receptor is one of Glutamate receptors (GluRs), which is an important target for the treatment of epilepsy. Several AMPA receptor antagonists have been reported in the literature and show promise in terms of their therapeutic potential for the prevention and treatment of a broad range of acute and chronic neurological diseases [152][153][154][155] .
Taking compound 137c ( Figure 23) and 139a ( Figure 23) as an example, the ED 50 of 137c and 139a against audiogenic seizures was 78.0 and 32.1 mg/kg, respectively. In addition, the property of compounds 139 that antagonising the AMPAinduced seizures, and the reverse of anticonvulsant activity by pre-treatment with aniracetam, suggested the involvement of AMPA receptors in their mechanism of action 160 .
As an attempt to further examine the role of cyclo-functionalisation and to gain more information from SAR studies, other compounds in the series of 139 were prepared by Gitto and his team in 2003. The promising one 139b ( Figure 23) was found with an ED 50 of 3.65, 5.93, and 13.8 mg/kg against audiogenic seizures, MES induced seizure and PTZ induced seizure, respectively 161 .
As a continuation of the project, a series of 3-ethoxycarbonyl-11H- [1,2,4]triazolo [4,5-c] [2,3]benzodiazepines were then synthesised, the potent anticonvulsant activities were also established against AMPA-induced seizures and audiogenic seizures in mice. Compound 140 (Figure 23) was the most active among the series with ED 50 of 35.1 and 26.2 mg/kg against audiogenic seizures in clonic phase and tonic phase in mice, respectively 162 .  (Figure 24), which completely protected the mice test against PTZ induced clonic convulsions at 6 mg/kg 164 .

Tetracyclic fused-triazoles
As a continuation of the above work, several pyrano[4 0 ,3 0 :4,5]thieno[3,2-e]triazolo [3,4-b]pyrimidine derivatives were synthesised and tested for their anticonvulsant activity. None of the compounds studied protected the experimental animals from the convulsion-inducing action of nicotine, arecoline, and MES. However, all drugs prevented PTZ induced convulsions. In the PTZ induced convulsions models, compound 143 ( Figure 24) with the ED 50 of 49 mg/kg, TD 50 of 850 mg/kg and PI value of 17.3 was the most promising anticonvulsant agent 165 .
In the fused-triazoles, triazoloquinolines was the most representative classes covering a large number of derivatives with potent anticonvulsant activity. Based on the available reports, the below SAR scheme could be drawn (Scheme 6).
In the next part of the above work, 2-substituted-7-heptyloxy-4,5-dihydro- [1,2,4]triazolo[4,3-a]quinoline-1(2H)-ones (146, Figure  25) were prepared to investigate the contribution of different acyl and alkyl groups at position 2 of the 146 to the anticonvulsant activity. The results revealed that neither alkylation nor acylation of compound 144 at the 2 position markedly decreased the anticonvulsant activities except small acyl group. Compound 146a (Figure 25) was the most active one with an ED 50 values of 7.2 mg/kg, and PI value of 12.2, which exhibited more potent anticonvulsant activity than compound 144a and the reference drugs phenytoin and carbamazepine. Compound 146 b (Figure 25) was the safest therapeutic compound with ED 50 Figure 25) were synthesised as the ring enlargement product of 144. Their anticonvulsant activities were evaluated by the MES test, PTZ test, and their neurotoxicity was evaluated by the rotarod neurotoxicity test. The results of these tests demonstrated that 147a ( Figure 25) and 147b ( Figure 25) were the most promising compounds, with median effective dose (ED 50 ) of 17.6 and 17.9 mg/kg, and protective index (PI) of greater than 63.4 and 62.4 in the MES test, respectively. These PI values were higher than the PI value of the prototype antiepileptic drug carbamazepine. Sc-PTZ tests showed that 147a was the most potent with ED 50 value of 38.0 mg/kg and PI value of greater than 29.4, which is much safer than marketed drug carbamazepine 169 .
Another series of triazolones containing phthalazine were prepared and evaluated for their anticonvulsant activity against MESinduced seizure. The result illustrated that 149 (Figure 25) possessed the most potential anticonvulsant activity with the 100% protection at 100 mg/kg in MES test 171 .
Next, a number of benzo[b] [1,2,4]triazolo [4,3-d][1,4]-thiazin-1one derivatives were synthesised as the bioisosterism of 144, and tested for their anticonvulsant activity by MES test in mice. Based on the results of MES test and rotarod test, 150 ( Figure 25) was considered as the most promising one, which showed an ED 50 value of 9.2 mg/kg and a PI value of 15.4. The safety of compound 150 was superior to the standard drug carbamazepine 172 .
Following the above work, a series of triazolo [3,4-b]benzothiazol-3(2H)-ones was synthesised as the ring-reduce produces of 150. In this series, the compound 151a ( Figure 25) and 151b ( Figure 25) showed high levels of anticonvulsant activity in MES test with ED 50 of 11.4 and 13.6 mg/kg, respectively. It is worth mentioning that compound 151b showed no neurotoxicity at 700 mg/kg, which led to a high protective index (PI > 51). In addition, the potency of compound 151b against seizures induced by PTZ, 3-mercaptopropionic acid, and bicuculline suggested that it might exert its anticonvulsant activity through affecting the GABAergic system 173 .
A series of novel triazolones containing benzimidazole were synthesised and screened for their anticonvulsant activities. Among the synthesised compounds, 152 ( Figure 25) was the most active compound with an ED 50 of 31.69 mg/kg, a TD 50 above 350 mg/kg, and PI value bigger than 11.04 by intraperitoneal administration in the mice. Meanwhile, compound 152 exhibited Introducing of alkyl, amino, aryl, and acyl group on 1st position all decrease the anticonvulsant. The larger the substituent, the lower the activity. Introducing of carbonyl and amide group on 1st position keep the activity. This ring replacing by other heterocycle (including five-membered, six-membered and seven-membered ring) remained the activity. Introduing of a conjugated double bond between the fourth and fifth positions raised the activity. This ring replacing by other heterocycle (including pyridine, thiofuran, imidazole) remained the activity. Introducing of alkoxy group on this ring with appropriate size remarkably increase the activity .

1,2,3-Triazoles
Rufinamide (154, Figure 26), a 1,2,3-triazole derivative, was developed in 2004 and approved in 2008. It is frequently used to treat various seizure disorders, especially the seizures associated with Lennox-Gastaut syndrome in children 4 years and older and adults 176 . Based on the SAR of rufinamide, several 1-(2,6-difluorobenzyl)-1H-1,2,3-triazole were prepared by Shukla et al., and evaluated for CNS depressant and anticonvulsant activities by photoactometer, rotarod, and PTZ-induced convulsion method, respectively, in mice. The most promising one 155 exhibited the potent anticonvulsant activity in PTZ model with an ED 50 of 5.5 mg/kg while the diazepam showed an ED 50 of 1.0 mg/kg 177 .
It is well known that sodium channel is one of the important targets for antiepileptic drugs. Many marketed anticonvulsant agents are involved in the action mechanism of sodium channel inhibition. In 2012, a novel series of 1,4-disubstituted-[1,2,3]triazole derivatives (156, Figure 26) were prepared via copper-catalysed click chemistry as inhibition of NaV1.6 sodium channel currents. In electrophysiological evaluation, many of the molecules can block the rNav1.6 currents at 10 lM by over 20%, displaying IC 50 values ranging from 19.6 to 125.4 lM. These founding will be exploited in the preparation of new compounds and could result in potentially useful AEDs 178 .  Figure 26) as well as their intermediates. The anticonvulsant study was carried out by MES and scPTZ screening methods, while their toxicity study was performed by rotarod method. Some of these compounds exhibited complete protection against seizure at 20 mg/kg, which were comparable with standard drug diazepam. The c logP values of target compounds are in the range of 3.5-5.3, which confirm their lipophilic nature 179 .
Recently, a novel series of chalcone derivatives containing 1,2,3-triazole were synthesised via Claisen-Schmidt condensation reaction. All compounds 158 ( Figure 26) and 159 ( Figure 26) revealed anticonvulsant activity, and compounds 159 were found to be more active than compounds 158. This activity may be attributed to the presence of the azepine nucleus in compounds 159 which has sedative like effect and increases the activity of these compounds as anticonvulsant, also the presence of the two cyano groups in compounds 159 makes the compounds more lipophilic than compounds 158. The compound 159a (Figure 26) was the most active one in this study with an ED 50 of 25 and 28 mg/kg in the MES test and PTZ test, respectively 180 .

Conclusion
In this paper, an attempt was made to systematically review the researches of triazole derivatives in the design and development of anticonvulsant agents in recent 20 years. From the review, we can see that big numbers of triazole derivatives were synthesised and identified with promising anticonvulsant activity. Their anticonvulsant activities were confirmed mainly but not limited to the in vivo screening methods. Their structures were focused on triazole moiety but abundant with various kinds of structures. From the presented review, it could be concluded that the triazole is an important fragment and has an affinity for some targets related to epilepsy-treatment. Many of triazole derivatives showed potent anticonvulsant activities with high selectivity and low toxicity, which can be used as lead compound to design and develop new anticonvulsant drugs. It seems that the role of triazole nucleus for designing anticonvulsants remains unclear, because it has no specifically target. But what does that matter? Because of the dimness of epilepsy pathogenesis and the complex mechanisms of the existing AEDs, the effectiveness and safety in vivo remain the most important factors in the discovery of new AEDs. Triazole nucleus plays different roles in these compounds for their anticonvulsants mediated by different molecular targets. Therefore, the introduction of triazole nucleus is relevant for designing newer compounds (new chemical entities) regardless of mechanism of action.