Developing New 4-PIOL and 4-PHP Analogues for Photoinactivation of γ-Aminobutyric Acid Type A Receptors.

The critical roles played by GABAA receptors as inhibitory regulators of excitation in the central nervous system has been known for many years. Aberrant GABAA receptor function and trafficking deficits have also been associated with several diseases including anxiety, depression, epilepsy, and insomnia. As a consequence, important drug groups such as the benzodiazepines, barbiturates, and many general anesthetics have become established as modulators of GABAA receptor activity. Nevertheless, there is much we do not understand about the roles and mechanisms of GABAA receptors at neural network and systems levels. It is therefore crucial to develop novel technologies and especially chemical entities that can interrogate GABAA receptor function in the nervous system. Here, we describe the chemistry and characterization of a novel set of 4-PIOL and 4-PHP analogues synthesized with the aim of developing a toolkit of drugs that can photoinactivate GABAA receptors. Most of these new analogues show higher affinities/potencies compared with the respective lead compounds. This is indicative of cavernous areas being present near their binding sites that can be potentially associated with novel receptor interactions. The 4-PHP azide-analogue, 2d, possesses particularly impressive nanomolar affinity/potency and is an effective UV-inducible photoinhibitor of GABAA receptors with considerable potential for photocontrol of GABAA receptor function in situ.

4-Hydroxy-1,2,3-triazole moiety as bioisostere of the carboxylic acid function: a novel scaffold to probe the orthosteric γ-aminobutyric acid receptor binding site.

The correct application of bio(iso)steric replacement, a potent tool for the design of optimized compounds, requires the continuous development of new isosters able to respond to specific target requirements. Among carboxylic acid isosters, as the hydroxylated pentatomic heterocyclic systems, the hydroxy-1,2,3-triazole represents one of the most versatile but less investigated. With the purpose to enlarge its bioisosteric application, we report the results of a study devoted to obtain potential biomimetics of the γ-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the central nervous system (CNS). A series of N1- and N2- functionalized 4-hydroxy-1,2,3-triazole analogues of the previous reported GABAAR ligands, including muscimol, 4-PIOL, and 4-PHP has been synthesized and characterized pharmacologically. Furthermore, this study led to development of straightforward chemical strategies directed to decorate the hydroxytriazole core scaffold, opening for further elaborative studies based on this system. The unsubstituted N1- and N2-piperidin-4-yl-4-hydroxy-1,2,3-triazole analogues (3a, 4a) of 4-PIOL and 4-PHP showed weak affinity (high to medium micromolar range), whereas substituting the 5-position of the triazole core with a 2-naphthylmethyl or 3,3-diphenylpropyl led to binding affinities in the low micromolar range. Based on electrostatic analysis and docking studies using a α1ß2γ2 GABAAR homology model we were able to rationalize the observed divergence in SAR for the series of N1- and N2- piperidin-4-yl-4-hydroxy-1,2,3-triazole analogues, offering more detailed insight into the orthosteric GABAAR binding site.

Exploration of the molecular architecture of the orthosteric binding site in the α4ß2 nicotinic acetylcholine receptor with analogs of 3-(dimethylamino)butyl dimethylcarbamate (DMABC) and 1-(pyridin-3-yl)-1,4-diazepane.

X-ray crystal structures of acetylcholine binding proteins (AChBPs) have revealed two different possible extensions to the classical ligand binding pocket known to accommodate various nicotinic agonists. One of the pockets is limited in size while the other is of considerable dimensions and protrudes along the interfacial cleft between subunits. To probe these putative extensions in functional nicotinic acetylcholine receptors (nAChRs), elongated analogs of 3-(dimethylamino)butyl dimethylcarbamate (DMABC) and 1-(pyridine-3-yl)-1,4-diazepane were prepared and characterized pharmacologically at neuronal heteromeric nAChRs. Although the new analogs, relative to parent compounds, displayed lower binding affinities, functional characterization of selected compounds revealed that they had retained partial α4ß2 nAChR agonist activity. The structure-activity relationship data did not indicate an upper limit to the size of substituents as would have been expected if the ligand was bound in the smaller pocket. The data were better in agreement with a binding mode in which substituents protrude along the interfacial cleft of the receptor. This was further supported by docking into a homology model of the α4-ß2 nAChR interface and by surface plasmon resonance biosensor analysis of binding of the compounds to acetylcholine-binding proteins, where they exhibit preference for Lymnaea stagnalis ACh binding protein (Ls-AChBP) over the Aplysia california ACh binding protein (Ac-AChBP). These results suggest new opportunities for expanding chemical space in the development of partial agonist and may be of interest in relation to development of novel smoking cessation aids.

GABAA receptor partial agonists and antagonists: structure, binding mode, and pharmacology.

A high degree of structural heterogeneity of the GABAA receptors (GABAARs) has been revealed and is reflected in multiple receptor subtypes. The subunit composition of GABAAR subtypes is believed to determine their localization relative to the synapses and adapt their functional properties to the local temporal pattern of GABA impact, enabling phasic or tonic inhibition. Specific GABAAR antagonists are essential tools for physiological and pharmacological elucidation of the different type of GABAAR inhibition. However, distinct selectivity among the receptor subtypes (populations) has been shown for only a few orthosteric ligands. Still, these examples show that it is indeed possible to obtain orthosteric subtype selectivity and they serve as models for further development in the orthosteric GABAAR ligand area. This review presents the very few existing structural classes of orthosteric GABAAR antagonists and describes the development of potent antagonists from partial agonists originally derived from the potent GABAAR agonist muscimol. In this process, several heterocyclic aromatic systems have been used in combination with structural models in order to map the orthosteric binding site and to reveal structural details to be used for obtaining potency and subtype selectivity. The challenges connected to functional characterization of orthosteric GABAAR partial agonists and antagonists, especially with regard to GABAAR stoichiometry and alternative binding sites are discussed. GABAAR antagonists have been essential in defining the tonic current but both remaining issues concerning the GABAARs involved and the therapeutic possibilities of modulating tonic inhibition underline the need for GABAAR antagonists with improved selectivity.

5-(Piperidin-4-yl)-3-hydroxypyrazole: A novel scaffold for probing the orthosteric γ-aminobutyric acid type A receptor binding site.

A series of bioisosteric N1- and N2 -substituted 5-(piperidin-4-yl)-3-hydroxypyrazole analogues of the partial GABAA R agonists 4-PIOL and 4-PHP have been designed, synthesized, and characterized pharmacologically. The unsubstituted 3-hydroxypyrazole analogue of 4-PIOL (2 a; IC50 ∼300 µM) is a weak antagonist at the α1 ß2 γ2 GABAA R, whereas substituting the N1- or N2- position with alkyl or aryl substituents resulted in antagonists with binding affinities in the high nanomolar to low micromolar range at native rat GABAA Rs. Docking studies using a α1 ß2 γ2 GABAA R homology model along with the obtained SAR indicate that the N1 -substituted analogues of 4-PIOL and 4-PHP, 2 a-k, and previously reported 3-substituted 4-PHP analogues share a common binding mode to the orthosteric binding site in the receptor. Interestingly, the core scaffold of the N2 -substituted analogues of 4-PIOL and 4-PHP, 3 b-k, are suggested to flip 180° thereby adapting to the binding pocket and addressing a cavity situated above the core scaffold.

Exploring the orthosteric binding site of the γ-aminobutyric acid type A receptor using 4-(Piperidin-4-yl)-1-hydroxypyrazoles 3- or 5-imidazolyl substituted: design, synthesis, and pharmacological evaluation.

A series of 4-(piperidin-4-yl)-1-hydroxypyrazole (4-PHP) 3- or 5-imidazolyl substituted analogues have been designed, synthesized, and characterized pharmacologically. All analogues showed binding affinities in the low micro- to low nanomolar range at native rat GABAA receptors and were found to be antagonists at the human α1ß2γ2s receptor. The structure-activity relationship of the compound series demonstrates distinct differences in size and architecture of previously discovered cavities in the vicinity of the 4-PHP scaffold in the orthosteric binding site.