SynthesisandInVitroPharmacologyatAMPAandKainate PreferringGlutamateReceptorsof4-Heteroarylmethylidene GlutamateAnalogues Jon Valgeirsson, a Jeppe K. Christensen, c Anders S. Kristensen, b Darryl S. Pickering, b Birgitte Nielsen, a Christina H. Fischer, a Hans Bra¨uner-Osborne, a Elsebet Ø. Nielsen, c Povl Krogsgaard-Larsen a and Ulf Madsen a, * a Department of Medicinal Chemistry, The Danish University of Pharmaceutical Sciences, 2 Universitetsparken, DK-2100 Copenhagen, Denmark b Department of Pharmacology, The Danish University of Pharmaceutical Sciences, 2 Universitetsparken, DK-2100 Copenhagen, Denmark c NeuroSearch A/S, 93 Pederstrupvej, DK-2750 Ballerup, Denmark Received 30 May 2003; accepted 18 July 2003 Abstract—2-Amino-3-[3-hydroxy-5-(2-thiazolyl)-4-isoxazolyl]propionic acid (1) is a potent AMPA receptor agonist with moderate affinity for native kainic acid (KA) receptors, whereas (S)-E-4-(2,2-dimethylpropylidene)glutamic acid (3) show high affinity for the GluR5 subtype of KA receptors and much lower affinity for the GluR2 subtype of AMPA receptors. As an attempt to develop new pharmacological tools for studies of GluR5 receptors, (S)-E-4-(2-thiazolylmethylene)glutamic acid (4a) was designed as a structural hybrid between 1 and 3. 4a was shown to be a potent GluR5 agonist and a high affinity ligand and to indiscriminately bind to the AMPA receptor subtypes GluR1–4 with lower affinities. Compounds 4b–h, in which the 2-thiazolyl substituent of 4a was replaced by other heterocyclic rings, which have previously been incorporated as 5-substituents in AMPA analogues, as exemplified by 1 were also synthesized. Compounds 4b–h were either inactive (4e,f) or weaker than 4a as affinity ligands for GluR1–4 and GluR5 with relative potencies comparable with those of the corresponding AMPA analogues as AMPA receptor agonists. Compounds 4a– h may be useful tools for the progressing pharmacophore mapping of the GluR5 agonist binding site. # 2003 Elsevier Ltd. All rights reserved. Introduction Receptors for the major central excitatory amino acid neurotransmitter, (S)-glutamic acid (Glu) are divided into two heterogeneous classes, the ionotropic Glu receptors (iGluRs) and the metabotropic Glu receptors (mGluRs). 1 3 The iGluRs are subdivided into N- methyl-d-aspartic acid (NMDA), 2-amino-3-(3- hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA), and kainic acid (KA) receptors. The eight mGluRs, so far identified, are subdivided into group I (mGluR1,5), group II (mGluR2,3), and group III (mGluR4,6,7,8). AMPA receptors are homo- or heteromeric assembles of GluR1–4 subunits and KA receptors of GluR5–7, KA1 and KA2 subunits. 1,4,5 (S)-AMPA is the classical AMPA receptor agonist, 6 and a large number of het- erocyclic Glu analogues, most of which are derived from AMPA, are potent and selective agonists at AMPA receptors. 3 Thus, (S)-2-amino-3-[3-hydroxy-5- (2-pyridyl)-4-isoxazolyl]propionic acid [(S)-2-Py- AMPA] 7 and 2-amino-3-[3-hydroxy-5-(2-thiazolyl)-4- isoxazolyl]propionic acid (1) 8 (Fig. 1) are examples of very potent AMPA agonists, whereas (R)-2-Py-AMPA, 3-Py-AMPA (2), and 4-Py-AMPA are essentially inac- tive. 7,9 (S)-2-Amino-3-(5-tert-butyl-3-hydroxy-4-iso- xazolyl)propionic acid [(S)-ATPA], but not (R)- ATPA, 10 12 on the other hand, is a very potent agonist at GluR5 receptors, but a relatively weak agonist at AMPA receptor subtypes. 10 Compound 3, which is an acyclic analogue of (S)-ATPA, shows a very similar 0968-0896/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0968-0896(03)00485-1 Bioorganic & Medicinal Chemistry 11 (2003) 4341–4349 *Corresponding author. Tel.: +45-3530-6243; fax: +45-3530-6040; e-mail: um@dfh.dk