CGP 37849 and CGP 39551: novel and potent competitive N-methyl-D-aspartate receptor antagonists with oral activity

1. The pharmacological properties of CGP 37849 (DL-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid; 4-methyl-APPA) and its carboxyethylester, CGP 39551, novel unsaturated analogues of the N-methyl-D-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonopentanoate (AP5), were evaluated in rodent brain in vitro and in vivo. 2. Radioligand binding experiments demonstrated that CGP 37849 potently (Ki 220 nM) and competitively inhibited NMDA-sensitive L-[3H]-glutamate binding to postsynaptic density (PSD) fractions from rat brain. It inhibited the binding of the selective NMDA Receptor Antagonist, [3H]-((+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP), with a Ki of 35 nM, and was 4, 5 and 7 fold more potent than the antagonists [+/-)-cis-4-phosphonomethylpiperidine-2-carboxylic acid) (CGS 19755), CPP and D-AP5, respectively. Inhibitory activity was associated exclusively with the trans configuration of the APPA molecule and with the D-stereoisomer. CGP 39551 showed weaker activity at NMDA receptor recognition sites and both compounds were weak or inactive at 18 other receptor binding sites. 3. CGP 37849 and CGP 39551 were inactive as inhibitors of L-[3H]-glutamate uptake into rat brain synaptosomes and had no effect on the release of endogenous glutamate from rat hippocampal slices evoked by electrical field stimulation. 4. In the hippocampal slice in vitro, CGP 37849 selectively and reversibly antagonized NMDA-evoked increases in CA1 pyramidal cell firing rate. In slices bathed in medium containing low Mg2+ levels, concentrations of CGP 37849 up to 10 microM suppressed burst firing evoked in CAl neurones by stimulation of Schaffer collateral-commissural fibres without affecting the magnitude of the initial population spike; CGP 39551 exerted the same effect but was weaker. In vivo, oral administration to rats of either CGP 37849 or CGP 39551 selectively blocked firing in hippocampal neurones induced by ionophoreticallyapplied NMDA, without affecting the responses to quisqualate or kainate. 5. CGP 37849 and CGP 39551 suppressed maximal electroshock-induced seizures in mice with ED50 s of 21 and 4 mg kg'- p.o., respectively. 6. CGP 37849 and CGP 39551 are potent and competitive NMDA receptor antagonists which show significant central effects following oral administration to Animals. As such, they may find value as tools to elucidate the roles of NMDA receptors in brain function, and potentially as therapeutic agents for the treatment of neurological disorders such as epilepsy and ischaemic brain damage in man.