GluN2A-Selective NMDA Receptor Antagonists: Mimicking the U-Shaped Bioactive Conformation of TCN-201 by a [2.2]Paracyclophane System

Under physiological conditions, N-Methyl-D-Aspartate (NMDA) receptors play a crucial role for synaptic plasticity, long-term potentiation and long-term depression. However, overactivation of NMDA receptors can result in excitotoxicity, which is associated with various neurological and neurodegenerative diseases. The physiological properties of NMDA receptors are strongly dependent on the GluN2 subunit incorporated into the heterotetrameric NMDA receptor. Therefore, subtype selective NMDA receptor modulators are of high interest. Since prototypical GluN2A-NMDA receptor antagonists TCN-201 and its MPX-analogs adopt a U-shaped conformation within the binding pocket, paracyclophanes were designed containing the phenyl rings in an already parallel orientation. Docking studies of the designed paracyclophanes show a similar binding pose as TCN-201. [2.2]Paracyclophanes with a benzoate or benzamide side chain were prepared in four-step synthesis, respectively, starting with a radical bromination in benzylic 1-position of [2.2]paracyclophane. In two-electrode voltage clamp experiments using Xenopus laevis oocytes transfected with cRNAs for the GluN1-4a and GluN2A subunits, the esters and amides (conc. 10 μM) did not show considerable inhibition of ion flux. It can be concluded that the GluN2A-NMDA receptor does not accept ligands with a paracyclophane scaffold functionalized in benzylic 1-position, although docking studies had revealed promising binding poses for benzoic acid esters and benzamides.

Synthesis of GluN2A-selective NMDA receptor antagonists with an electron-rich aromatic B-ring

Glutamatergic N-Methyl-d-aspartate (NMDA) receptors are heterotetrameric ion channels that can be comprised of different subunits. GluN2A subunit-containing NMDA receptors are associated with diseases like anxiety, depression, and schizophrenia. However, the exact contribution of these NMDA receptor subtypes is still unclear. To understand better the role of the GluN2A-containing receptors, novel ligands were designed. In co-crystallization with the isolated binding site, TCN-201 (1) and analogs adopt a U-shape conformation with parallel orientation of rings A and B. In order to increase the π/π-interactions between these rings, ring B of TCN-201 was replaced bioisosterically by different electron-rich thiazole, oxazole, and isoxazole heterocycles. The inhibitory activity was measured by two-electrode voltage clamp experiments with Xenopus laevis oocytes expressing GluN2A-containing NMDA receptors. It was found that 21c, 31a, 37a, and 37b were able to inhibit the ion channel. The isoxazole derivative 37b was the most potent negative allosteric modulator displaying 40% of the TCN-201 activity at a concentration of 10 μM.

Systematic variation of the benzenesulfonamide part of the GluN2A selective NMDA receptor antagonist TCN-201

GluN2A subunit containing N-methyl-d-aspartate receptors (NMDARs) are highly involved in various physiological processes in the central nervous system, but also in some diseases, such as anxiety, depression and schizophrenia. However, the role of GluN2A subunit containing NMDARs in pathological processes is not exactly elucidated. In order to obtain potent and selective inhibitors of GluN2A subunit containing NMDARs, the selective negative allosteric modulator 2 was systematically modified at the benzenesulfonamide part. The activity of the test compounds was recorded in two electrode voltage clamp experiments using Xenopus laevis oocytes expressing exclusively NMDARs with GluN1a and GluN2A subunits. It was found that halogen atoms in 3-position of the benzenesulfonamide part result in high GluN2A antagonistic activity. With an IC50 value of 204 nM the 3-bromo derivative 5i (N-{4-[(2-benzoylhydrazino)carbonyl]benzyl}-3-bromobenzenesulfonamide) has 2.5-fold higher antagonistic activity than the lead compound 2 and represents our new lead compound.

Diverse modes of NMDA receptor positive allosteric modulation: Mechanisms and consequences

NMDA Receptors (NMDARs) play key roles in synaptic physiology and NMDAR hypofunction has been implicated in various neurological conditions. In recent years an increasing number of positive allosteric modulators (PAMs) of NMDARs have been discovered and characterized. These diverse PAM classes vary not only in their binding sites and GluN2 subunit selectivity profiles, but also in the nature of their impacts on channel function. Major differences exist in the degree of slowing of channel deactivation and shifting of apparent agonist affinity between different classes of PAMs. Here we review the diverse modes of potentiation by the currently known classes of NMDAR PAMs and discuss the potential consequences of different types of potentiation in terms of desirable and undesirable effects on brain function. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.