Cyclobutane Quisqualic Acid Analogues as Selective mGluR5a Metabotropic Glutamic Acid Receptor Ligands Louis Littman, † Christopher Tokar, ‡ Shankar Venkatraman, ‡ Robert J. Roon, § James F. Koerner, § Michael B. Robinson, † and Rodney L. Johnson* ,‡ Departments of Medicinal Chemistry and Biochemistry, University of Minnesota, Minneapolis, Minnesota 55455, and The Children’s Seashore House, Children’s Hospital of Philadelphia, and Departments of Pediatrics and Pharmacology, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Received December 8, 1998 The conformationally constrained cyclobutane analogues of quisqualic acid ( Z)- and (E)-1-amino- 3-[2′-(3′,5′-dioxo-1′,2′,4′-oxadiazolidinyl)]cyclobutane-1-carboxylic acid, compounds 2 and 3, respectively, were synthesized. Both 2 and 3 stimulated phosphoinositide (PI) hydrolysis in the hippocampus with EC 50 values of 18 ( 6 and 53 ( 19 µM, respectively. Neither analogue stimulated PI hydrolysis in the cerebellum. The effects of 2 and 3 were also examined in BHK cells which expressed either mGluR1a or mGluR5a receptors. Compounds 2 and 3 stimulated PI hydrolysis in cells expressing mGluR5a but not in those cells expressing mGluR1a. The EC 50 value for 2 was 11 ( 4 µM, while that for 3 was 49 ( 25 µM. Both 2 and 3 did not show any significant effect on cells expressing the mGluR2 and mGluR4a receptors. In addition, neither compound blocked [ 3 H]glutamic acid uptake into synaptosomal membranes, and neither compound was able to produce the QUIS effect as does quisqualic acid. This pharmacological profile indicates that 2 and 3 are selective ligands for the mGluR5a metabotropic glutamic acid receptor. Introduction The acidic amino acids L-glutamic acid and L-aspartic acid are the major excitatory neurotransmitters in the mammalian central nervous system (CNS). These ex- citatory amino acids (EAAs) activate both receptors coupled to ion channels (ionotropic) and receptors coupled to second-messenger systems (metabotropic). 1 The ionotropic glutamic acid receptors have been di- vided into the following subtypes based on agonist specificity: N-methyl-D-aspartate (NMDA), kainic acid, and R-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. These receptors have been implicated in mediating fast synaptic events, are in- volved in long-term potentiation, and can cause neu- ronal degeneration upon excessive activation. 1-3 To date, eight distinct metabotropic glutamic acid receptors (mGluRs) have been cloned (mGluR1-mGluR8) with multiple splice variants existing for several of these receptor subtypes. 1,4,5 The mGluRs have been catego- rized into three groups based on agonist specificity and second-messenger coupling. Group 1 receptors (mGluR1a-d, mGluR5a,b) are coupled to phospho- inositide (PI) hydrolysis and are stimulated by the EAA analogues quisqualic acid (1) and (1S,3R)-1-amino-1,3- cyclopentanedicarboxylic acid [(1S,3R)-ACPD]. 6,7 Group 2 receptors (mGluR2, mGluR3) are coupled to inhibition of cyclic AMP (cAMP) formation and are stimulated by (+)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (LY354740) 8 and (2S,2′R,3′R)-2-(2′3′-dicarboxycyclopro- pyl)glycine (DCG-IV). 9 Group 3 receptors (mGluR4a,b, mGluR6, mGluR7, mGluR8) are also coupled to inhibi- tion of cAMP formation but are stimulated by L-2-amino- 4-phosphonobutanoic acid (L-AP4). 10 The mGluRs have been implicated in mediating development of the ner- vous system, 11,12 long-term potentiation, 13,14 modulation of neuronal activity, 15,16 and regulation of glutamic acid release. 17,18 Great success, in the past, has been achieved in the synthesis of selective metabotropic receptor ligands by synthesizing conformationally constricted analogues of glutamic acid and L-AP4. Since quisqualic acid is one of the most potent agonists for group 1 mGluRs, the synthesis of conformationally constrained analogues of 1 has been undertaken in hopes of developing high- affinity ligands which can be used to define the func- tions of these receptors in the mammalian CNS. In the present study, we describe the synthesis and selective mGluR5a receptor agonist activity of the cyclobutyl analogues of quisqualic acid, compounds 2 and 3. Chemistry The synthesis of 2 and 3 is outlined in Scheme 1. The synthetic strategy for the two cyclobutyl quisqualic acid analogues was centered around the construction of the oxadiazolidinedione ring on a suitable cyclobutane pre- cursor and employed methodology similar to that used in the synthesis of L-quisqualic acid. 19,20 In our ap- proach, condensation of cyclobutanone 4 21 with O-ben- zylhydroxylamine in the presence of sodium cyanoboro- hydride gave the N-benzyloxyamino derivative as a ‡ Department of Medicinal Chemistry, University of Minnesota. § Department of Biochemistry, University of Minnesota. † Children’s Hospital of Philadelphia and University of Pennsylva- nia. 1639 J. Med. Chem. 1999, 42, 1639-1647 10.1021/jm9806897 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/21/1999