Structure-activity analysis of a novel NR2C/NR2D-preferring NMDA receptor antagonist: 1-(phenanthrene-2-carbonyl)piperazine-2,3-dicarboxylic acid

1 (2S*,3R*)-1-(biphenyl-4-carbonyl)piperazine-2,3-dicarboxylic acid (PBPD) is a moderate affinity, competitive N-methyl-D-aspartate (NMDA) receptor antagonist with an atypical pattern of selectivity among NMDA receptor 2 subunit (NR2) subunits. We now describe the activity of several derivatives of PBPD tested at both rat brain NMDA receptors using L-[H-3]-glutamate binding assays and at recombinant receptors expressed in Xenopus oocytes. 2 Substituting various branched ring structures for the biphenyl group of PBPD reduced NMDA receptor activity. However, substituting linearly arranged ring structures-fluorenone or phenanthrene groups-retained or enhanced activity. 3 Relative to PBPD, the phenanthrene derivative (2S*,3R*)-1-(phenanthrene-2-carbonyl)piperazine-2,3-dicarboxylic acid (PPDA) displayed a 30- to 78-fold increase in affinity for native NMDA receptors. At recombinant receptors, PPDA displayed a 16-fold (NR2B) to 94-fold (NR2C) increase in affinity over PBPD. 4 Replacement of the biphenyl group of PBPD with a 9-oxofluorene ring system resulted in small changes in receptor affinity and subtype selectivity. 5 2'-Bromo substitution on the biphenyl group of PBPD reduced antagonist affinity 3- to 5-fold at NR2A-, NR2B- and NR2D-containing receptors, but had little effect on NR2C-containing receptors. In contrast, 4'-fluoro substitution of the biphenyl ring of PBPD selectively increased NR2A affinity. 6 The aromatic rings of PBPD and PPDA increase antagonist affinity and appear to interact with a region of the NMDA receptor displaying subunit heterogeneity. PPDA is the most potent and selective NR2C/NR2D-preferring antagonist yet reported and thus may be useful in defining NR2C/NR2D function and developing related antagonists with improved NMDA receptor subtype selectivity.