The NMDA antagonists, CPP and CGS 19755, lack affinity for central benzodiazepine receptors.

CPP (3-(2-carboxypiperazin-4-yl-propyl-1-phosphonic acid), a rigid analog of AP7 (2-amino-7-phosphonoheptanoate), previously shown to be a selective antagonist of the NMDA (N-methyl-D-aspartate) receptor (IC50 = 209 nM) has been reported to be exceptionally active (IC50 = 430 pM) at benzodiazepine binding sites. Re-examination of CPP, and the rigid AP5 analog, CGS 19755 (cis-4-phosphonomethyl-2-piperidine carboxylic acid; 0.001-10,000 nM), showed that, as previously reported, neither compound affected the binding of [3H]flunitrazepam. These compounds are thus selective NMDA receptor antagonists.