Eliprodil: a novel neuroprotective agent acting at a modulatory site of the NMDA receptor

Abstract

The NMDA receptor, composed of heteromeric assemblies of NR1 and NR2 (A-D) subunits, is coactivated by glutamate and glycine and positively modulated by spermine and spermidine via at least two modulatory polyamine sites with distinct actions. Polyamines increase glycine site affinity but also increase NMDA responses in the presence of saturating concentrations of glycine. The piperidine-ethanol derivatives ifenprodil and eliprodil have been shown to block the NMDA receptor via an interaction with a modulatory polyamine site. They partially displace [3H]CPP, [3H]glycine or [3H]MK801 at submicromolar concentrations but antagonize the stimulatory effects of polyamines on the binding of these ligands. In different brain areas, their NMDA antagonistic effects have been shown to be total, biphasic or partial and can be reversed either by the polyamines or by glycine, suggesting action at the polyamine site that regulates glycine site affinity. [3H]ifenprodil or eliprodil label a site on the NMDA receptor which is totally displaced by spermine and spermidine in a competitive manner. Recent studies have shown that ifenprodil and eliprodil selectively antagonize a specific NMDA receptor subtype. In transfected Xenopus oocytes ifenprodil antagonises NR1A/NR2B receptors with high affinity (IC50=340nM) in a glycine reversible and voltage-independent manner and blocks the NR1A/NR2A subtype (IC50=146μM) in a voltage-dependent and glycine-insensitive manner (Williams, Mol.Pharmacol. 44, 851–859, 1993). The selectivity of ifenprodil for a subtype of NMDA receptor is supported by the distribution of polyamine-sensitive [3H]ifenprodil binding sites which closely matches that of NR2B receptor mRNA. In striatal slices, ifenprodil and eliprodil block the stimulatory effects of NMDA on acetylcholine release, but are without effect on NMDA evoked spermidine release. This profile is shared by magnesium, polyamine spider toxins and the relatively less psychostimulant NMDA channel blockers dextrorphan and dextromethorphan while dizocilpine, phencyclidine, CGP37849 and L-689,560 block both responses with equal potency. Finally, in immature rat cerebellar Purkinje cells, dizocilpine, 2APV and 7-Chlorokynurenate totally block NMDA-induced calcium entry, which is however only partially blocked by ifenprodil or eliprodil.Ifenprodil or Eliprodil protect against the neurotoxic effects of NMDA receptor activation in vitro; their protective effects in cortical culture can be reversed by spermine or spermidine. Eliprodil has been shown to be neuroprotective in mouse (ip,po), rat (po) and cat (iv) models of focal ischaemia and also provides extensive neuroprotection in a rat model of cerebral trauma. The window of therapeutic opportunity in the mouse focal ischaemia model is from 2-4hrs post-occlusion and considerably longer in cerebral trauma (∼18hrs).Antagonists at each other site of the NMDA receptor (channel, glutamate and glycine) have also been shown to exert neuroprotective effects in animal models of stroke or cerebral trauma. Unfortunately, NMDA receptor antagonism can be accompanied by a battery of undesirable side effects including, in varying degree, phencyclidine-like psychostimulation, amnesia, neurotoxicity, hypertension and tachycardia. These effects are particularly intense with certain NMDA channel blockers (although less so in the case of dextrorphan-like compounds), and can also be observed with competitive NMDA antagonists. Glycine antagonists appear to have a more favourable profile at neuroprotective doses, while eliprodil is completely devoid of such side-effects. The reasons for this favourable profile are likely to reside in its mechanism of action - antagonism of the effects of modulatory polyamines whose influence is increased in the ischaemic or traumatised brain. The fact that ifenprodil and eliprodil act at a specific NMDA receptor subtype, and that non-stimulant channel blockers also show a degree of native NMDA receptor subtype selectivity suggests that NMDA receptor subtype targetting could provide neuroprotective NMDA antagonists with a favourable therapeutic index.