Current Topics in Medicinal Chemistry 2002, 2, 817-832 817 GABAA Receptor Ligands and their Therapeutic Potentials Bente Frølund,a,* Bjarke Ebert,b Uffe Kristiansen,c Tommy Liljeforsa and Povl Krogsgaard- Larsena aDepartments of Medicinal Chemistry and cPharmacology The Royal Danish School of Pharmacy, DK 2100 Copenhagen, Denmark and bDepartment of Molecular Pharmacology, H. Lundbeck A/S, 9 Ottiliavej, DK-2500 Valby, Denmark. Abstract: The GABAA receptor system is implicated in a number of neurological diseases, making GABAA receptor ligands interesting as potential therapeutic agents. Only a few different classes of structures are currently known as ligands for the GABA recognition site on the GABAA receptor complex, reflecting the very strict structural requirements for GABAA receptor recognition and activation. Within the series of compounds showing agonist activity at the GABAA receptor site that have been developed, most of the ligands are structurally derived from the GABAA agonists muscimol, THIP or isoguvacine. Using recombinant GABAA receptors, functional selectivity has been shown for a number of compounds such as the GABAA agonists imidazole-4-acetic acid and THIP, showing highly subunit-dependent potency and maximal response. In the light of the interest in partial GABAA receptor agonists as potential therapeutics, structure-activity studies of a number of analogues of 4-PIOL, a low-efficacy partial GABAA agonist, have been performed. In this connection, a series of GABAA ligands has been developed showing pharmacological profiles from moderately potent low-efficacy partial GABAA agonist activity to potent and selective antagonist effect. Only little information about direct acting GABAA receptor agonists in clinical studies is available. Results from clinical studies on the effect of the GABAA agonist THIP on human sleep pattern shows that the functional consequences of a direct acting agonist are different from those seen after administration of GABAA receptor modulators. Keywords. GABAA receptor, partial agonist, functional selectivity, THIP, 4-PIOL, IAA, muscimol. INTRODUCTION bind to a group of GABA receptors which was insensitive to both BMC and baclofen [6, 8]. These receptors were named 4-Aminobutyric acid (GABA), the major inhibitory GABAC receptors or non-GABAA, non-GABAB (NANB) neurotransmitter in the central nervous system (CNS) receptors for GABA [9, 10]. operates through two different classes of receptors consisting of the ionotropic GABAA and GABAC receptors and the The pharmacological characterization of the GABAA metabotropic GABAB receptors. This receptor classification receptors was accelerated by the development of specific has been under development over a period of 40 years, GABAA agonists such as isoguvacine, 4,5,6,7-tetrahydro- starting in the early 1950s and is primarily based on the isoxazolo[5,4-c]pyridin-3-ol (THIP) and piperidin-4- selective pharmacology observed for the ligands depicted in sulphonic acid (P4S) [11, 12]. Following the observation Fig. (1). The identification of the alkaloid bicuculline [1] that the binding site for the benzodiazepines (BZDs) was and its quaternized analogue bicuculline methochloride associated with the GABAA receptors the exploration of the (BMC) [2] as competitive GABA antagonists in CNS GABAA receptors was dramatically intensified [13-17]. After tissues initiated the pharmacological characterization of cloning of a large number of GABAA subunits, this area of GABA receptors. The observation that the antispastic effect the pharmacology is developing rapidly. of baclofen, which was designed as a lipophilic GABA analogue, could not be antagonized by BMC led to the A number of compounds with selective action at the recognition of the GABAB receptors as a distinct class [3-5]. GABAB receptors, have been described [18, 19]. The number Subsequently, in connection with the design of of selective GABAC receptor ligands is still limited, making conformationally restricted analogues of GABA, cis- this receptor group the least characterized of the GABA aminobut-2-enoic acid (CACA) was developed [6]. CACA receptors [10]. and the structurally related analogue, cis-2-aminomethyl- cyclopropanecarboxylic acid (CAMP) [7], were shown to THE GABAA RECEPTOR COMPLEX *Address correspondence to this author at the Department of Medicinal The GABAA receptor is a member of the superfamily of Chemistry, The Royal Danish School of Pharmacy, 2 Universitetsparken, ligand-gated ion channels. The structure and function of this DK-2100 Copenhagen, Denmark; Tel: (+45) 35306495; Fax: (+45) 35306040; E-mail: [email protected] group of receptors display a high degree of complexity, of 1568-0266/02 $35.00+.00 © 2002 Bentham Science Publishers Ltd. 818 Current Topics in Medicinal Chemistry, 2002, Vol. 2, No. 8 Frølund et al. OH H2N OH OH O HO O OH OH P CH HN HN 3 O N O O O H2 N H N Cl 2 H2N Isoguvacine THIP (R)-Baclofen CGP-35024 CACA CAMP GABA GABAA GABAB C OCH3 O OH HO O O CH3 O N P OH P P OH O CH H N H 3 2 O H CH3 O HN O H2N OH N O N O Cl O O NH BMC SR 95531 (R)-Phaclofen CGP-35348 TPMPA Fig. (1). Schematic illustration of the different classes of GABA receptors and the structures of some key agonists (upper part) and antagonists (lower part) used for pharmacological characterization of these receptors. which only a few aspects of particular relevance to the development of GABAA receptor ligands will be mentioned. The postsynaptic GABAA receptor is a receptor complex containing a considerable number of modulatory binding sites, as reflected by the structural diversity of compounds acting on GABAA receptors, including important drugs such as benzodiazepines, barbiturates, and neurosteroids (Fig. 2) [10]. The heterogeneity of GABAA receptors in the brain is pronounced due to the large number of different GABAA receptor subunits. At least 17 different subunits (a 1–6, b1–4, g1–4, d, e, and p) have been identified [20, 21]. GABAA receptors are built up as pentameric assemblies of different families of receptor subunits, making the existence of a very large number of heteromeric GABAA receptors possible [22, 23]. The assembly, which in most receptors includes two a subunits, two b subunits and one g or d subunit, determines the pharmacology of the functional receptor [9, 21]. Site- directed mutagenesis studies have shown that the binding site for benzodiazepines is located at the interface between a and g subunits in the GABAA receptor complex, whereas Fig. (2). (A) Schematic model of the pentameric structure of the the binding site(s) for GABA, GABAA agonists and GABAA receptor complex and (B) a schematic illustration of the competitive antagonists are located at the interface between GABAA receptor complex indicating the chloride ion channel a and b subunits [24]. and additional binding sites. The number of physiologically relevant GABAA that approximately twenty different subunit combination receptors, their subunit combinations, and their regional make up the major subtypes of GABAA receptors, with the distributions are far from being fully mapped out and studies a 1b2/3g2 receptor combination as the most abundant [27, in this area are in rapid progress [20, 25, 26]. It is estimated 28]. To further elucidate the physiological importance of the GABAA Receptor Ligands and their Therapeutic Potentials Current Topics in Medicinal Chemistry, 2002, Vol. 2, No. 8 819 various GABAA receptor subunits and their preferred although BMC has recently been shown to lack the combinations, GABAA ligands with subunit specific effects selectivity of bicuculline as a GABAA antagonist [33]. are needed as important tools. New structural classes of GABAA receptor antagonists This review will focus exclusively on ligands for the have been developed (Fig. 3). A series of arylaminopyrida- GABA recognition site on the GABAA receptor complex. zine analogues of GABA, notably SR 95531, show potent and selective competitive GABAA antagonist effects [34, 35]. These compounds bind tightly to GABAA receptor GABAA RECEPTOR LIGANDS sites, and tritiated SR 95531 is now used as a standard receptor ligand. Although SR 95531 and related compounds Only very few naturally occurring compounds have been containing a GABA structure element show convulsant effect identified as GABAA receptor agonists. Among these the after systemic administration [36], these zwitterionic most important one is the potent GABAA agonist compounds do not easily penetrate the BBB. Compound 3, muscimol, a constituent of the mushroom Amanita in which the GABA structure element has been replaced by a muscaria (Fig. 3) [29]. The histamine metabolite imidazole- thiomuscimol unit (see Fig. 4), is the most potent GABAA 4-acetic acid (IAA) represents another structural class of antagonist in the arylaminopyridazine series [36]. The ligands and has been shown to be a relatively potent bicyclic 5-isoxazolol compound, iso-THAZ, derived from GABAA agonist and a GABAC antagonist [30]. IAA readily THIP, is a moderately potent GABAA antagonist [37]. penetrates the blood-brain barrier (BBB) and may play a role as a central and/or peripheral endogenous GABAA receptor ligand. Like the imidazole group of IAA, the structurally DEVELOPMENT OF GABAA RECEPTOR related isothiouronium element of (RS)-2-amino-2-thiazoline- AGONISTS 4-acetic acid (1) is effectively recognized by the GABAA receptors [31]. (1S,3S)-3-Aminocyclopentane-1-carboxylic In order to determine the structural requirements for acid (2), a GABA analogue containing a conformationally activation of the GABAA receptor a number of structural restricted carbon backbone, is also a specific GABAA modifications of the molecule of GABA have been made, agonist [32]. some of which are shown in Fig. (4). Conformational restriction of various parts of the molecule and isosteric The classical GABAA antagonist bicuculline and BMC replacements of the carboxyl group have led to a broad have played a key role in studies on GABAA receptors, spectrum of specific GABAA agonists. Some of these A OH OH OH OH O O O H2 N N O N NH S N NH2 H2N Muscimol IAA 1 2 B O OCH3 O O N O CH3 H H OH O OH N N N O N N O S O O NH NH Bicuculline SR 95531 3 OH HN O N Iso-THAZ Fig.