SYNTHESIS AND STRUCTURE-ACTIVITY RELATIONSHIP OF NEW SUBTYPE SELECTIVE KAINATE RECEPTOR LIGANDS

Glutamate receptors are the principal mediators of the excitatory transmission in the CNS. These proteins are divided into two big families: metabotropic and ionotropic receptors (mGluRs and iGluRs). Among iGluRs, kainate receptors (KARs) are of remarkable interest because they have a modulatory role and, for this reason, they could be ideal drug targets. Moreover, since kainate receptors are formed by five different type of subunits (GluK1-5) and each of them seems to be related to different neorological disorder, is much more fascinating to search for subunit-selective compounds. On this ground, I have designed and synthesized a series of compounds using the structure/backbone of the non-selective AMPA/KA agonists L-Glutamate, L-Tricholomic acid and CIP-AS. The design strategy included the homologation of the amino acidic chain and the increase of molecular complexity, which aimed at producing respectively a modification of the profile from agonist to antagonist and selectivity toward KARs subunits. All the new derivatives were pharmacologically characterized through the evaluation of their binding affinity for native ionotropic glutamate receptors and for recombinant homomeric GluK1-3 receptors. The attained results support the proposed strategy, since some very selective compounds have been identified. In particular, compound 1f show a very high GluK1 selectivity (Ki= 0.464 µM), while amino acids 9a and 9d are GluK3 selective ligands (Ki= 0.206 μM and 0.117 μM, respectively). Notably, compound 9a displayed a 10-fold selectivity for GluK3 versus GluK2 and a 35-fold selectivity for GluK3 versus GluK1. Even more interestingly, compound 9d showed a 26-fold selectivity for GluK3 versus GluK2 and a 70-fold selectivity for GluK3 versus GluK1, being the most selective GluK3 ligand yet ever discovered. Therefore, compound 9d represents a valuable probe for characterizing the molecular determinants of subunit-selective binding and could represent an ideal pharmacological tool for investigating the patho-physiological role played by the least characterized GluK3 subtype. In conclusion, amino acid chain homologation coupled to the insertion of bulkier substituents into the L-Glutamate/CIP-AS backbones has allowed the establishment of additional interactions/steric clashes with specific amino acidic residues, which have in turn determined selectivity for the specific KAR subtype GluK1 or GluK3. The molecular determinants of the observed selectivity will be clarified through crystallographic studies on the GluK1 and GluK3 LBD, which are currently in due course, and these results will pave the way to the development of further highly potent and selective GluK receptor antagonists.