Hydroxy-1,2,5-oxadiazolyl Moiety as Bioisoster of the Carboxy Function. Synthesis, Ionization Constants, and Pharmacological Characterization of γ-Aminobutyric Acid (GABA) Related Compounds Marco L. Lolli, † Suzanne L. Hansen, ‡ Barbara Rolando, † Birgitte Nielsen, § Petrine Wellendorph, § Karsten Madsen, ‡ Orla Miller Larsen, ‡ Uffe Kristiansen, ‡ Roberta Fruttero, † Alberto Gasco,* ,† and Tommy N. Johansen* ,§ Dipartimento di Scienza e Tecnologia del Farmaco, UniVersita ` degli Studi di Torino, Via Pietro Giuria 9, 10125 Torino, Italy, and Departments of Pharmacology and Medicinal Chemistry, The Danish UniVersity of Pharmaceutical Sciences, 2 UniVersitetsparken, DK-2100 Copenhagen, Denmark ReceiVed December 28, 2005 Three 4-substituted 1,2,5-oxadiazol-3-ols containing aminoalkyl substituents (analogues and homologues of γ-aminobutyric acid (GABA)) were synthesized to investigate the hydroxy-1,2,5-oxadiazolyl moiety as a bioisoster for a carboxyl group at GABA receptors. The pK a values of the target compounds were close to those of GABA. At GABA A receptors of cultured cerebral cortical neurons, weak agonist and partial agonist profiles were identified, demonstrating the 4-hydroxy-1,2,5-oxadiazol-3-yl unit to be a nonclassical carboxyl group bioisoster. Introduction Isosteric replacement of functional groups in a lead compound is a widely used approach to study receptor chemistry and to develop new drugs with optimized behavior. 1 When this replacement affords products with broadly similar biological properties, the groups are called bioisosters. 2,3 A number of clear bioisosteric relationships have been established for many functional groups, in particular for the carboxyl group, which successfully has been substituted by heterocycles such as tetrazole, 3-hydroxyisoxazole, 3-hydroxyisothiazole, 3-hydroxy- 1,2,5-thiadiazole, and 3-cyclobutene-1,2-dione. These cyclic systems have been used extensively to design amino acid mimetics active at subtypes of central nervous system (CNS) receptors. 4-7 The 1,2,5-oxadiazole (furazan) system and its 2-oxide (furoxan) are heterocyclic rings whose pharmacochem- istry some of us have been studying for many years. 8 The former is a classical isoster of the 1,2,5-thiadiazole ring. Similar to the hydroxy-substituted 1,2,5-thiadiazoles, the hydroxy-1,2,5-oxa- diazoles are known to display marked acidic properties. 9 Consequently, the 4-hydroxy-1,2,5-oxadiazol-3-yl moiety could reasonably behave as the bioisoster of the carboxy function. In this first paper we report the results of a work devoted to obtain potential biomimetics of the γ-aminobutyric acid (GABA), the major inhibitory neurotransmitter in CNS. In GABA neurotransmission, synaptically released GABA exerts its effects through activation of ionotropic GABA A and metabotropic GABA B receptors. After unbinding from the receptor, GABA is taken up by GABA transporters of which four subtypes have been cloned (GAT1-4). 10 To pharmacologi- cally characterize the GABA A receptors, a number of ligands bioisosterically derived from GABA, such as the selective agonists muscimol (1) and 4,5,6,7-tetrahydroisoxazolo[5,4-c]- pyridine-3-ol (THIP, gaboxadol, 2) and the antagonist gabazine (3), have been developed over the years 11 (Figure 1). THIP, which has a particular partial agonist profile at cloned GABA A receptors, is currently undergoing phase III clinical investigation for treatment of sleep disorders. 12 Whereas THIP shows very potent nonopioid analgesic effects and novel hypnotic effects in human clinical studies, it seems likely that partial GABA A agonists showing lower levels of efficacy such as 5-(4- piperidyl)-3-isoxazolol (4-PIOL, 4) 13 may be of therapeutic interest in certain CNS disorders such as schizophrenia. 14 Information about the mechanism of receptor-ligand interac- tions resulting in partial agonism at the molecular level is still not available, thus making the design of new GABA A agonists with a range of different efficacies relevant. In this paper we report the synthesis, the ionization constants, and the pharmacological characterization at GABA receptors and GABA transporters of new analogues and homologues of GABA, 5-7 (Figure 1), in which the carboxyl group has been replaced by a 4-hydroxy-1,2,5-oxadiazol-3-yl moiety. Results and Discussion Chemistry. The synthetic pathway for preparing the final products 6 and 7 is depicted in Scheme 1. The common starting material was the 3,4-diphenylsulfonyl-1,2,5-oxadiazole 8. By * To whom correspondence should be addressed. For A.G.: phone, 0039 011 6707670; fax, 0039 011 6707286; e-mail, alberto.gasco@unito.it. For T.N.J.: phone, +45 35306412; fax, +45 35306040; e-mail: tnj@dfuni.dk. † Universita ` degli Studi di Torino. ‡ Department of Pharmacology, The Danish University of Pharmaceutical Sciences. § Department of Medicinal Chemistry, The Danish University of Pharmaceutical Sciences. Figure 1. Structures of the GABAA receptor agonists muscimol (1), THIP (2), and 4-PIOL (4), the GABAA antagonist gabazine (3), and the new 1,2,5-oxadiazol-4-ols (5-7). 4442 J. Med. Chem. 2006, 49, 4442-4446 10.1021/jm051288b CCC: $33.50 © 2006 American Chemical Society Published on Web 06/21/2006