Search

The development of tool compounds for the ionotropic glutamate receptors (iGluRs) remains an important research objective, as these are essential for the study and understanding of the roles of these receptors in health and disease. Herein, we report on the pharmacological characterization of ( S )-2-hydroxyhistidine ( 2a ) and ( S )-2-mercaptohistidine ( 2b ) as mediators of glutamatergic neurotransmission. While 2a displayed negligible binding affinity or activity at all glutamate receptors and transporters investigated, 2b displayed selectivity for homomeric GluK3 with binding affinities in the low micromolar range ( K i = 6.42 ± 0.74 μM). The iGluR subtype selectivity ratio for 2b was calculated at ∼30-fold for GluK1/GluK3, GluA3/GluK3, and GluA4/GluK3 and >100-fold for GluK2/GluK3, GluA1/GluK3, and GluA2/GluK3. Unexpectedly, functional characterization of 2b revealed that the compound is an antagonist ( K b = 7.6 μM) at homomeric GluK3 receptors while exhibiting only weak agonist activity at GluA2 (EC 50 = 3.25 ± 0.55 mM). The functional properties of 2b were explored further in electrophysiological recordings of mouse hippocampal neurons.

In the present study, screening of a library of 49,087 compounds at the excitatory amino acid transporter subtype 3 (EAAT3) led to the identification of 2-(furan-2-yl)-8-methyl- N -( o -tolyl)imidazo[1,2- a ]pyridin-3-amine ( 3a ) which showed a >20-fold preference for inhibition of EAAT3 (IC 50 = 13 μM) over EAAT1,2,4 (EAAT1: IC 50 ∼ 250 μM; EAAT2,4: IC 50 > 250 μM). It was shown that a small lipophilic substituent (methyl or bromine) at the 7- and/or 8-position was essential for activity. Furthermore, the substitution pattern of the o -tolyl group (compound 5b ) and the chemical nature of the substituent in the 2-position (compound 7b ) were shown to be essential for the selectivity toward EAAT3 over EAAT1,2. The most prominent analogues to come out of this study are 3a and 3e that display ∼35-fold selectivity for EAAT3 (IC 50 = 7.2 μM) over EAAT1,2,4 (IC 50 ∼ 250 μM).

Development of pharmacological tools for the ionotropic glutamate receptors (iGluRs) is imperative for the study and understanding of the role and function of these receptors in the central nervous system. We report the synthesis of 18 analogues of (2 S,3 R)-2-carboxy-3-pyrrolidine acetic acid (3a), which explores the effect of introducing a substituent on the ε-carbon (3c-q). A new synthetic method was developed for the efficient synthesis of racemic 3a and applied to give expedited access to 13 racemic analogues of 3a. Pharmacological characterization was carried out at native iGluRs, cloned homomeric kainate receptors (GluK1-3), NMDA receptors (GluN1/GluN2A-D), and excitatory amino acid transporters (EAAT1-3). From the structure-activity relationship studies, several new ligands emerged, exemplified by triazole 3p-d1, GluK3-preferring (GluK1/GluK3 K i ratio of 15), and the structurally closely related tetrazole 3q-s3-4 that displayed 4.4-100-fold preference as an antagonist for the GluN1/GluN2A receptor ( K i = 0.61 μM) over GluN1/GluN2B-D ( K i = 2.7-62 μM).