Two α-amino acid-functionalized
quinoxalines, <b>1a</b> (CNG-10301) and <b>1b</b> (CNG-10300),
of a quinoxaline moiety
coupled to an amino acid moiety were designed, synthesized, and characterized
pharmacologically. While <b>1a</b> displayed low affinity at
native AMPA, KA, and NMDA receptors, and at homomeric GluK1,3 receptors,
the affinity for GluK2 was in the midmicromolar range (<i>K</i><sub>i</sub> = 136 μM), <b>1b</b> displayed low to midmicromolar
range binding affinity at all the iGluRs (<i>K</i><sub>i</sub> = 9–126 μM). In functional experiments (outside-out
patches excised from transfected HEK293T cells), 100 μM <b>1a</b> partially blocked GluK1 (33% peak response), while GluK2
was unaffected (96% peak response). Furthermore, <b>1a</b> was
shown not to be an agonist at GluK1 and GluK2 at 100 μM. On
the other hand, 100 μM <b>1b</b> fully antagonized GluK1
(8% peak response) but only partially blocked GluK2 (33% peak response).
An X-ray structure at 2.3 Å resolution of <b>1b</b> in
the GluK1-LBD (ligand-binding domain) disclosed an unexpected binding
mode compared to the predictions made during the design phase; the
quinoxaline moiety remains to act as an amino acid bioisostere, but
the amino acid moiety is oriented into a new area within the GluK1
receptor. The structure of the GluK1-LBD with <b>1b</b> showed
a large variation in domain openings of the three molecules from 25°
to 49°, demonstrating that the GluK1-LBD is capable of undergoing
major domain movements
