Vladislava Nikonenko

Comparative Quantitative Structure-Activity Relationship Studies (QSAR) on Non-Benzodiazepine Compounds Binding to Benzodiazepine Receptor (BzR)

 

D. Hadjipavlou itina, Rajni Garg and Corwin Hansch

 

Department of Pharmaceutical Chemistry, School of Pharmacy, Aristotelian University of Thessaloniki, Thessaloniki, Greece, Chemistry Department, Clarkson University, Potsdam, New York 13699-5810, and Chemistry Department, Pomona College, Claremont, California 91711

 

1.    Introduction

 

Benzodiazepines (BDZs) are the drugs of choice in the pharmacotherapy of anxiety and related emo­tional disorders, sleep disorders, status epilepticus, and other convulsive states; they are used as cen­trally acting muscles relaxants, for premedication, and as inducing agents in anesthesiology. They act via the benzodiazepine receptor site (BzR) on the γ-aminobutyric acid receptor (GABAa) family and have been subjected to extensive quantitative struc­ture-activity relationship (QSAR) studies.^1-4 GABAer-gic inhibition is one of the most rapidly developing topics in neuropharmacology.^5a New therapeutic op­portunities arise due to increasing insights into the molecular architecture and diversity of the compo­nents involved in signal transduction such as GABAa receptors, GABAb receptors, and GABA transporters. GABAa receptors are the major inhibitory neu-rotransmitter receptors in the brain, in the site of action of many clinically important drugs, and are important drug targets representing the sites of action of benzodiazepines, barbiturates, and neuro-steroids. These receptors are ligand-gated chloride channels composed of five subunits that can belong to eight different subunit classes. All subunits pos­sess an extracellulaar amino-terminal domain con­taining a conserved disulfide bridge, followed by four Iransmembrane segments. GABAa receptors belong to the superfamily of pentameric ligand-gated ion channels ("cys-loop receptors").^5b At synapses GABAa receptors are activated by a brief nonequilibrium exposure to high concentrations of GABA. On the basis of the presence of 7 subunit families comprising at least 18 subunits in the central nervous system (α _1-6, β_1-3, γ_1-3, €, θ, ρ_1-3), the GABAa receptors display an extraordinary structural heterogeneity. Most GABAa receptor sybtypes in vivo are believed to be composed of α-, β-, and γ-subunits.The benzo-diazepine site is thought to be located at the interface of the respective α-subunit (α _1-3, α₅) and the γ₂-subunit. The classical benzodiazepine site is mainly found on GABA_A receptors at the interface between the α- and γ₂-subunits and can be rendered diaz-epam-insensitive by a point mutation in the a-sub­unit in which a histidine residue is placed by an arginine residue in recombinant receptors.^5с

When BDZs bind to their receptors, they appear to induce a conformational change leading to an increase in the availability of GABAa receptors for GABAa, leading to higher chloride influx and hyper-

polarization. BDZs interact with two classes of rec­ognition sites, "central" and "peripheral" (mitochon-drial) types. Recently a novel low-affinity benzodiaze­pine site was identified on recombinant GABAa receptors (α₁β₃γ₂).^5b

Receptors containing the α_1-5-subunits in combina­tion with any of the β-subunits and the γ₂-subunit are most prevalent in the brain. These receptors are sensitive to benzodiazepine modulation. The major receptor subtype is assembled from the subunits α₁β₂γ₂ (diazepam-sensitive GABAa receptors). GABA_A receptors that do not respond to clinically used ligands, such as diazepam, flunitrazepam, clon-azepam, and zolpidem, are oflow abundance in the brain and are largely characterized by the α₄- and α₆-subunits (diazepam-insensitive GABAa receptors). L-838.417.SL65.149, and CL 284,846 are some novel subtype-selective benzodiazepine site ligands, whereas |³H]RY 80 can be used as a radioligand to examine the properties of GABAa receptors containing α₅ subunits.^5d

The central receptors located in the neuronal tissues ^6,7a,b are functionally linked to a GABAa recep­tor chloride ionophore complex⁷ and are apparently located on synaptic membranes.⁸ Central BDZ recep­tors mediate classical pharmacological properties of the clinically widely used BDZs⁶ (anxiolytic, anti-convulsants, sedative, and muscle relaxants). The GABA_A-independent⁹'¹⁰ peripheral or "mitochondria!" benzodiazepine receptors (MBR) have been identi­fied in a wide range of peripheral tissues as well as in the central nervous system,¹¹ and their subcel-lular location has been reported to be mainly "mitochondria!",^l2-14 nuclear,^10,11 and in the plasma membrane.^15,16 During the past decade the MBR has been the object of several studies aimed to under­stand its physiological role. The peripheral benzodi­azepine   receptor   (PBR)   is  a  multimeric  protein complex located on the outer milochondrial mem­brane of astroglial cells and is expressed in both central and peripheral tissues. The physiological role of mitochondrial BzR is still not clear. PBR-selecLive ligands known to date belong to structurally unre­lated classes of compounds such as BDZs, isoquino-lines, imidazopyridines, 2-aryl-3-indoleacetamides, benzofuracetamides, and benzothiazepines. They are involved in various cellular functions such as the inhibition of oxidative phosphorylation,⁹ the inhibi­tion of cell proliferation, and steroidogenesis.^16,17 The BzR is unique in the way it responds to three different types of ligands, which act as allosteric modulators of the GABAa receptor complex. GABA-or benzodiazepine-induced conformational changes originate in the extracellular domain and are trans­duced to other, allosteric binding sites and the ion channel. The initial trigger that drives an allosteric motion is thought to entail some rearrangement in the binding site itself. In fact, allosteric modulators can either enhance (agonists) or reduce (inverse agonists) the GABA_A-induced Cl^- ion flux. A third group of ligands, interacting with the allosteric site of GABAa receptor, does not influence GABAA-induced ion flux but antagonizes (antagonists) the actions of the agonists and inverse agonists. The interrelationships of these three types of BzR ligands can be explained on the basis of changes in the conformation of the receptor from its unoccupied resting state.^18,19 An argument for the homogeneity of BzR binding sites might lie in the activities displayed after minor structural modifications of compounds with similar binding interactions. Thus, all compounds that bind to the BzR should have certain common characteristics that allow for recog­nition by the receptor regardless of the type of (in vivo) activity. Many types of compounds have been shown to bind at the BzR, for example, BDZs, arylpyrazolo-quinolines, β-carbolines, imidazopy-ridazines, and cyclo-pyrrolones. BDZs agonists are believed to bind to sites associated with the GABA_A receptor, an ion channel linked receptor. GABAa acts on at least two different receptor types.^20-24 The action of BDZs seems to be restricted to synaptic effects of GABAa, which are mediated by the GABAa receptors. The conformational form of the receptor complex that binds BDZs agonists (e.g., diazepam) has a little affinity for GABAa at its associated site. In equilibrium is a conformational form of the recep­tor complex that binds BDZ inverse agonists (e.g., β-carbolines) that have a low affinity for GABAa and thus is not associated with the opening of the associated ion channel. Antagonist drugs at the BzR will prevent the binding of either agonists or inverse agonists. Currently, only two different BzR subtypes, Bz₁R and Bz₂R, can be distinguished pharmacologi­cally.

Comparative modeling (synonymous with the term "homology modeling") is based on the observation that in protein families, structure is more conserved than sequence. Due to the absence of several bulky side chains, the volume of the benzodiazepine pocket is larger than that of the GABAa pocket. Competitive antagonists inhibit agonist action by binding into a partially overlapping pocket. Because they are larger than agonists, the pocket geometry requires that they extend further into the membrane-near par of the cleft and thus block allosteric changes that possibly involve motions on the complementary side on the principal side.^5b Molecular mechanics approaches combined with comparative modeling may provide additional and complementary information with re­spect to the conformational changes proposed from the electron crystallography study. Molecular model­ing studies²⁵ have determined that all BDZ ligands share the presence of an aromatic or heteroaromatic A ring, believed to undergo π/π stacking with aro­matic amino acid residues within the receptor, as well as a proton-accepting group that exists in the same plane of the aromatic A ring and interacts with a histidine residue on the receptor. A 5-phenyl aro­matic group, C, may contribute steric or hydrophobic interactions with the receptor. For an agonist, sub­stitution of the para-position on ring C is sterically unfavorable. The amide nitrogen, its methyl sub-stituent, and the 4,5-(methyleneimino) group are not required for in vitro binding of ligands. Substitution of the methylene 3-position or the imine nitrogen is sterically unfavorable for agonist activity but does not affect antagonists.

 

 

In continuation of our previous quantitative struc­ture—activity relationship studies⁴ on BDZs, we present a new QSAR study on some non-BDZs binding to GABA_A/Bz receptors.

 

Источник: http://ximia.org/encyklopedia/1248.html