RPR-104632

The effects of five glycine site antagonists were comparatively examined on maximal and plateau currents evoked by 200 microM N-methyl-D-aspartate (NMDA) in the presence of 1 microM glycine in cultured cerebrocortical cells of the rat using whole-cell patch-clamp technique. 5,7-Dichlorokynurenic acid, ACEA-1021 (5-nitro-6,7-dichloro-quinoxalinedione), L-695,902 (methyl 7-chloro-4-hydroxy 2(1H)-quinolone-3-carboxylate), LY-294,619 (5,7-dichloro-3-(4-hydroxphenyl)-4-hydroxyquinolin-2(1H)-one ) and RPR-104,632 (2-(3-bromo-benzyl)-6,8-dichloro-3,4-dihydro-2H-1,2,4-benzothiadiazine 1,1-dioxide-3-carboxylic acid) caused concentration-dependent inhibition of NMDA-activated currents. However, antagonists showed different relative efficacies to block peak currents and plateau currents, characterised by the following IC50 ratios: L-695,902: 0.98; RPR-104,632: 1.06; ACEA-1021: 1.69; LY-294,619: 1.71; and 5,7-dichlorokynurenic acid: 3.42. Our findings indicate a heterogeneity of glycine site antagonists in affecting NMDA receptor desensitisation, and suggest potential differences in their pharmacologies.

The NMDA antagonist and neuroprotective effects of RPR 104632 (2H-1,2,4-benzothiadiazine-1-dioxide-3-carboxylic acid), a new benzothiadiazine derivative, with affinity for the glycine site of the NMDA receptor-channel complex are described. RPR 104632 antagonized the binding of [3H]5,7-dichlorokynurenic acid to the rat cerebral cortex, with a Ki of 4.9 nM. This effect was stereospecific, since the (-)-isomer was 500-fold more potent than the (+)-isomer. The potent affinity of RPR 104632 for the glycine site was confirmed by the observation that RPR 104632 inhibited [3H]N-[1-(2-thienyl)cyclohexyl]-3,4-piperidine ([3H]TCP) binding in the presence of N-methyl-D-aspartate (NMDA) (IC50 = 55 nM), whereas it had no effect on the competitive NMDA site or on the dissociative anaesthetic site. RPR 104632 inhibited the NMDA-evoked increase in guanosine 3',5'-cyclic monophosphate (cGMP) levels of neonatal rat cerebellar slices (IC50 = 890 nM) in a non-competitive manner and markedly reduced NMDA-induced neurotoxicity in rat hippocampal slices and in cortical primary cell cultures. These results suggest that RPR 104632 is a high-affinity specific antagonist of the glycine site coupled to the NMDA receptor channel with potent neuroprotective properties in vitro.