NMDAR pharmacology

Since the NMDA receptor was first identified by the selective activation by N-methyl-D-aspartate (NMDA), it has become clear that many different subtypes of this receptor could be expressed due to the multiple combinations of individual subunits. The challenge for medicinal chemists in recent years has thus been to synthesis compounds that can discriminate between these multiple sub-types. In addition to the glutamate (NMDA) binding site, there are also multiple binding sites on the NMDA receptor for modulatory compounds. Efficient NMDA receptor activation requires not only NMDA but also a co-agonist, glycine. Activation can also be modulated by the binding of polyamines. Each of the binding sites (glutamate, glycine, ployamine) has been used as a potential target for the development of both receptor and sub-type selective compounds. Many of the agonists and competitive antagonists described here have been developed by Prof. Jeff Watkins and Dr David Jane, here at Bristol, most recently under the auspices of the MRC centre for Synaptic Plasticity. Few agonists other that NMDA itself have been widely used in the study of NMDAR function. However, a number of conformationally constrained analogues of NMDA have been developed as competitive antagonists. While it has proved very difficult to generate subtype selective antagonists, progress is now being made in this direction.

NMDAR Competitive Antagonists

These compounds act at the glutamate binding site. (R)-AP5 is perhaps the most commonly used NMDA receptor antagonist, developed more than 20 years ago by Jeff Watkins. This compound shows a tendency to selectivity for GluN2A andGluN2B containing NMDA receptor complexes, with a 5-fold lower Ki value for GluN1/2A over GluN1/2D complexes. (R)-CPP-ene follows a similar pattern. However, the degree of selectivity is greater with the Ki value for GluN1/2D complexes nearly 20-fold higher than that for GluN1/2A receptors, although it cannot discriminate between GluN2A and GluN2B-containing complexes. (-)PPDA (UBP150), on the other hand displays 3-fold selectivity for GluN2C and D over GluN2A and B. The related compound, UBP141, while a lower affinity antagonist, displays a greater degree of selectivity for GluN2D over UBP150.

Table 1
 AP5(R)-CPPeneNVP-AAM007
  structure of AP5 structure of CPPene Structure of NVP-AAM007
  Ki Values (μM) for displacement of [3H]Glutamate IC50 for inhibition of Ca2+ influx
GluN1/2A 0.3 0.11 0.045
GluN1/2B 0.5 0.14 0.66
GluN1/2C 1.6 1.5  
GluN1/2D 1.7 1.8  
      Bartlett et al, 2007
Table 2
 (-)PPDA (UBP150)UBP141UBP145
  structure of PPDA Structure of UBP141
  Ki Values (μM) for inhibition of NMDAR evoked responses expressed in X. Laevis oocytes
GluN1/2A 0.27 22.0 16.5
GluN1/2B 0.16 17.2 10.9
GluN1/2C 0.045 5.24 1.70
GluN1/2D 0.093 2.36 1.53
  Costa et al, 2009, Morley et al, 2005

NMDAR Channel Blockers

These compounds act by binding to the pore of the NMDA receptor channel and are thus non-competitive antagonists. The highest affinity compound available to date is MK-801 with a binding affinity of 18 nM. Both Memantine and Ketamine are lower affinity antagonists. These compounds have proved very effective NMDA receptor antagonists and MK-801 has been used in a radiolabelled form to label NMDA receptor populations in brain slices. However, none of these compounds shows subunit selectivity.

Table 3
MK801Memantine
Structure of MK801 Structure of Memantine
IC50 values for the displacement of [3H]MK801 (μM)
0.018 0.3

NMDAR Polyamine Site Antagonists

Polyamines modulate NMDA receptor function. For instance, spermine stimulates NMDA receptors and increases the affinity for glycine. Conversly, it may aslo decrease the affinity of the receptor for glutamate and generate a voltage dependent inhibition of the channel. The effects of ployamine site antagonists are specific for channels containing the GluN2B subunit. Hence, ployamine site antagonists would be very useful as GluN2B selective ligands. The highest affinity antagonists for this site is currently Ro 25-6981 that has a binding affinity of 6 nM.

Table 4
Ro25-6981Ifenprodil
Structure of Ro25-6981 Structure of Ifenprodil
IC50 Values for displacement of [3H]-Ro 25-6981 (µM)
0.006 0.02