A series of tricyclic quinoxalinediones, 5,6-dihydro-1H-pyrrolo[1,2,3-de]quinoxaline-2,3-diones and 6,7-dihydro-1H,5H-pyrido[1,2,3-de]quinoxaline-2,3-diones, were synthesized and was evaluated for their affinity for the glycine binding site of the NMDA receptor using a [3H]-5,7-dichlorokynurenic acid binding assay. The six-membered ring-fused tricyclic quinoxalinedione 18g (Ki = 9.9 nM) displayed high affinity for the glycine site. The anilide derivative 20g (Ki = 2.6 nM) was 4-fold more potent than 18g and as potent as L-689,560, one of the most potent glycine antagonists so far prepared. Although the carboxylic acid derivative of the corresponding five-membered ring-fused tricyclic quinoxalinedione 18e (Ki = 7.3 nM) had affinity comparable to that of 18g, the anilide derivative 20e largely decreased in the affinity in contrast to 20g. Enantiomers 23g, 24g, 25g, and 26g were prepared and tested. Only the S enantiomer 25g (Ki = 0.96 nM) retained the affinity among the anilide derivatives, whereas both enantiomers 23g (Ki = 2.3 nM) and 24g (Ki = 9.6 nM) were active among the carboxylic acid derivatives. The origin of the high affinity of carboxylic acid derivatives such as 18e and 18g would be a charge-charge interaction between the anionic carboxylate residues of the compounds and the cationic proton-donor site in the receptor.

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A series of tricyclic indole-2-carboxylic acid derivatives were synthesized and evaluated by the radioligand binding assay and the anticonvulsant effects in the mouse NMDA-induced seizure model. Among them, derivatives of 3S-(-)-4 such as 3a, 3f, and 3g which had certain zwitterionic anilides showed high affinity to the NMDA-glycine binding site. The absolute configuration of 3S-(-)-4 was confirmed by X-ray crystallographic analysis. In particular, 3g (SM-31900) was found to be a highly active glycine antagonist for both in vitro and in vivo assays (K(i) = 1.0 +/- 0.1 nM, ED(50) = 2.3 mg/kg, iv) and also showed high selectivity for the glycine site. In addition, 3g was soluble enough in aqueous media (>10 mg/mL at pH 7.4) to use for medications by intravenous injection.

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