Synthesis and biological evaluation of 7,8,9,10-tetrahydroimidazo[1,2-c]pyrido[3,4-e]pyrimdin-5(6H)-ones as functionally selective ligands of the benzodiazepine receptor site on the GABA(A) receptor

Pyrazole-containing pharmaceuticals: target, pharmacological activity, and their SAR studies

Pyrazole is a five-membered heterocycle bearing two adjacent nitrogen atoms. Both pharmaceutical agents and natural products with pyrazole as a nucleus have exhibited a broad spectrum of biological activities. In the last few decades, more than 40 pyrazole-containing drugs have been approved by the FDA for the treatment of a broad range of clinical conditions including celecoxib (anti-inflammatory), CDPPB (antipsychotic), difenamizole (analgesic), etc. Owing to the unique physicochemical properties of the pyrazole core, pyrazole-containing drugs may exert better pharmacokinetics and pharmacological effects compared with drugs containing similar heterocyclic rings. The purpose of this paper is to provide an overview of all the existing drugs bearing a pyrazole nucleus that have been approved or in clinical trials, involving their pharmacological activities and SAR studies.

GABAA receptors in GtoPdb v.2021.3

The GABA_A receptor is a ligand-gated ion channel of the Cys-loop family that includes the nicotinic acetylcholine, 5-HT₃ and strychnine-sensitive glycine receptors. GABA_A receptor-mediated inhibition within the CNS occurs by fast synaptic transmission, sustained tonic inhibition and temporally intermediate events that have been termed 'GABA_A, slow' [45]. GABA_A receptors exist as pentamers of 4TM subunits that form an intrinsic anion selective channel. Sequences of six α, three β, three γ, one δ, three ρ, one ε, one π and one θ GABA_A receptor subunits have been reported in mammals [278, 235, 236, 283]. The π-subunit is restricted to reproductive tissue. Alternatively spliced versions of many subunits exist (e.g. α4- and α6- (both not functional) α5-, β2-, β3- and γ2), along with RNA editing of the α3 subunit [71]. The three ρ-subunits, (ρ1-3) function as either homo- or hetero-oligomeric assemblies [359, 50]. Receptors formed from ρ-subunits, because of their distinctive pharmacology that includes insensitivity to bicuculline, benzodiazepines and barbiturates, have sometimes been termed GABA_C receptors [359], but they are classified as GABA _A receptors by NC-IUPHAR on the basis of structural and functional criteria [16, 235, 236]. Many GABA_A receptor subtypes contain α-, β- and γ-subunits with the likely stoichiometry 2α.2β.1γ [168, 235]. It is thought that the majority of GABA_A receptors harbour a single type of α- and β - subunit variant. The α1β2γ2 hetero-oligomer constitutes the largest population of GABA_A receptors in the CNS, followed by the α2β3γ2 and α3β3γ2 isoforms. Receptors that incorporate the α4- α5-or α 6-subunit, or the β1-, γ1-, γ3-, δ-, ε- and θ-subunits, are less numerous, but they may nonetheless serve important functions. For example, extrasynaptically located receptors that contain α6- and δ-subunits in cerebellar granule cells, or an α4- and δ-subunit in dentate gyrus granule cells and thalamic neurones, mediate a tonic current that is important for neuronal excitability in response to ambient concentrations of GABA [209, 272, 83, 19, 288]. GABA binding occurs at the β+/α- subunit interface and the homologous γ+/α- subunits interface creates the benzodiazepine site. A second site for benzodiazepine binding has recently been postulated to occur at the α+/β- interface ([254]; reviewed by [282]). The particular α-and γ-subunit isoforms exhibit marked effects on recognition and/or efficacy at the benzodiazepine site. Thus, receptors incorporating either α4- or α6-subunits are not recognised by 'classical' benzodiazepines, such as flunitrazepam (but see [356]). The trafficking, cell surface expression, internalisation and function of GABA_A receptors and their subunits are discussed in detail in several recent reviews [52, 140, 188, 316] but one point worthy of note is that receptors incorporating the γ2 subunit (except when associated with α5) cluster at the postsynaptic membrane (but may distribute dynamically between synaptic and extrasynaptic locations), whereas as those incorporating the δ subunit appear to be exclusively extrasynaptic. NC-IUPHAR [16, 235, 3, 2] class the GABA_A receptors according to their subunit structure, pharmacology and receptor function. Currently, eleven native GABA_A receptors are classed as conclusively identified (i.e., α1β2γ2, α1βγ2, α3βγ2, α4βγ2, α4β2δ, α4β3δ, α5βγ2, α6βγ2, α6β2δ, α6β3δ and ρ) with further receptor isoforms occurring with high probability, or only tentatively [235, 236]. It is beyond the scope of this Guide to discuss the pharmacology of individual GABA_A receptor isoforms in detail; such information can be gleaned in the reviews [16, 95, 168, 173, 143, 278, 216, 235, 236] and [9, 10]. Agents that discriminate between α-subunit isoforms are noted in the table and additional agents that demonstrate selectivity between receptor isoforms, for example via β-subunit selectivity, are indicated in the text below. The distinctive agonist and antagonist pharmacology of ρ receptors is summarised in the table and additional aspects are reviewed in [359, 50, 145, 223]. Several high-resolution cryo-electron microscopy structures have been described in which the full-length human α1β3γ2L GABA_A receptor in lipid nanodiscs is bound to the channel-blocker picrotoxin, the competitive antagonist bicuculline, the agonist GABA (γ-aminobutyric acid), and the classical benzodiazepines alprazolam and diazepam [198].

Prescribed drugs containing nitrogen heterocycles: an overview

Heteroatoms as well as heterocyclic scaffolds are frequently present as the common cores in a plethora of active pharmaceuticals natural products. Statistically, more than 85% of all biologically active compounds are heterocycles or comprise a heterocycle and most frequently, nitrogen heterocycles as a backbone in their complex structures. These facts disclose and emphasize the vital role of heterocycles in modern drug design and drug discovery. In this review, we try to present a comprehensive overview of top prescribed drugs containing nitrogen heterocycles, describing their pharmacological properties, medical applications and their selected synthetic pathways. It is worth mentioning that the reported examples are actually limited to current top selling drugs, being or containing N-heterocycles and their synthetic information has been extracted from both scientific journals and the wider patent literature.

Identification of anxiolytic/nonsedative agents among indol-3-ylglyoxylamides acting as functionally selective agonists at the gamma-aminobutyric acid-A (GABAA) alpha2 benzodiazepine receptor

Anxioselective agents may be identified among compounds binding selectively to the alpha(2)beta(x)gamma(2) subtype of the gamma-aminobutyric acid-A (GABA(A))/central benzodiazepine receptor (BzR) complex and behaving as agonists or among compounds binding with comparable potency to various BzR subtypes but eliciting agonism only at the alpha(2)beta(x)gamma(2) receptor. Because of subtle steric differences among BzR subtypes, the latter approach has proved much more successful. A biological screening within the class of indol-3-ylglyoxylamides 1-3 allowed us to identify compounds 1c and 2b as potential anxiolytic/nonsedative agents showing alpha(2) selective efficacy in vitro and anxioselective effects in vivo. According to molecular modeling studies, and consistently with SARs accumulated in the past decade, 5-NO(2)- and 5-H-indole derivatives would preferentially bind to BzR by placing the indole ring in the L(Di) and the L(2) receptor binding sites, respectively.

Population Patch-Clamp Electrophysiology Analysis of Recombinant GABAA α1β3γ2 Channels Expressed in HEK-293 Cells

γ-Amino butyric acid (GABA)—activated Cl— channels are critical mediators of inhibitory postsynaptic potentials in the CNS. To date, rational design efforts to identify potent and selective GABAA subtype ligands have been hampered by the absence of suitable high-throughput screening approaches. The authors describe 384-well population patch-clamp (PPC) planar array electrophysiology methods for the study of GABAA receptor pharmacology. In HEK293 cells stably expressing human α1β3γ2 GABAA channels, GABA evoked outward currents at 0 mV of 1.05 ± 0.08 nA, measured 8 s post GABA addition. The IGABA was linear and reversed close to the theoretical ECl (—56 mV). Concentration-response curve analysis yielded a mean pEC50 value of 5.4 and Hill slope of 1.5, and for a series of agonists, the rank order of potency was muscimol > GABA > isoguvacine. A range of known positive modulators, including diazepam and pentobarbital, produced concentration-dependent augmentation of the GABA EC 20 response (1 µM). The competitive antagonists bicuculline and gabazine produced concentration-dependent, parallel, rightward displacement of GABA curves with pA2 and slope values of 5.7 and 1.0 and 6.7 and 1.0, respectively. In contrast, picrotoxin (0.2-150 µM) depressed the maximal GABA response, implying a non-competitive antagonism. Overall, the pharmacology of human α1β3γ2 GABAA determined by PPC was highly similar to that obtained by conventional patch-clamp methods. In small-scale single-shot screens, Z′ values of >0.5 were obtained in agonist, modulator, and antagonist formats with hit rates of 0% to 3%. The authors conclude that despite the inability of the method to resolve the peak agonist responses, PPC can rapidly and usefully quantify pharmacology for the α1β3γ2 GABAA isoform. These data suggest that PPC may be a valuable approach for a focused set and secondary screening of GABAA receptors and other slow ligand-gated ion channels. ( Journal of Biomolecular Screening 2009:769-780)

Antiseizure activity of novel gamma-aminobutyric acid (A) receptor subtype-selective benzodiazepine analogues in mice and rat models

The antiseizure activity of benzodiazepines (BDZs) 1-5 in mice and rats as animal models is described. These BDZs have selective efficacy for alpha2beta3gamma2 and alpha3beta3gamma2 GABA(A)-receptors. Significant anticonvulsant activity with little or no motor impairment and therapeutic indexes (TI) of 2.8-44 (mice, ip) were observed for compounds 2-4 in the subcutaneous metrazole seizure (scMET) test. In rats, orally (po) the TI was >5 to 105. These compounds represent novel leads in the search for anticonvulsants devoid of sedative, ataxic, and amnestic side effects.

Azaflavones compared to flavones as ligands to the benzodiazepine binding site of brain GABA(A) receptors

A series of azaflavone derivatives and analogues were prepared and evaluated for their affinity to the benzodiazepine binding site of the GABA(A) receptor, and compared to their flavone counterparts. Three of the compounds, the azaflavones 9 and 12 as well as the new flavone 13, were also assayed on GABA(A) receptor subtypes (alpha(1)beta(3)gamma(2s), alpha(2)beta(3)gamma(2s), alpha(4)beta(3)gamma(2s) and alpha(5)beta(3)gamma(2s)), displaying nanomolar affinities as well as selectivity for alpha1- versus alpha2- and alpha3-containing receptors by a factor of between 14 and 26.

Synthesis of GABAA receptor agonists and evaluation of their alpha-subunit selectivity and orientation in the GABA binding site

Drugs used to treat various disorders target GABA A receptors. To develop alpha subunit selective compounds, we synthesized 5-(4-piperidyl)-3-isoxazolol (4-PIOL) derivatives. The 3-isoxazolol moiety was substituted by 1,3,5-oxadiazol-2-one, 1,3,5-oxadiazol-2-thione, and substituted 1,2,4-triazol-3-ol heterocycles with modifications to the basic piperidine substituent as well as substituents without basic nitrogen. Compounds were screened by [(3)H]muscimol binding and in patch-clamp experiments with heterologously expressed GABA A alpha ibeta 3gamma 2 receptors (i = 1-6). The effects of 5-aminomethyl-3 H-[1,3,4]oxadiazol-2-one 5d were comparable to GABA for all alpha subunit isoforms. 5-piperidin-4-yl-3 H-[1,3,4]oxadiazol-2-one 5a and 5-piperidin-4-yl-3 H-[1,3,4]oxadiazol-2-thione 6a were weak agonists at alpha 2-, alpha 3-, and alpha 5-containing receptors. When coapplied with GABA, they were antagonistic in alpha 2-, alpha 4-, and alpha 6-containing receptors and potentiated alpha 3-containing receptors. 6a protected GABA binding site cysteine-substitution mutants alpha 1F64C and alpha 1S68C from reacting with methanethiosulfonate-ethylsulfonate. 6a specifically covalently modified the alpha 1R66C thiol, in the GABA binding site, through its oxadiazolethione sulfur. These results demonstrate the feasibility of synthesizing alpha subtype selective GABA mimetic drugs.

2,3,7-Trisubstituted pyrazolo[1,5-d][1,2,4]triazines: Functionally selective GABAA α3-subtype agonists

Novel synthetic routes have been devised for the preparation of previously inaccessible 2,3,7-trisubstituted pyrazolo[1,5-d][1,2,4]triazines 2. These compounds are high affinity ligands for the GABA(A) benzodiazepine binding site and some analogues show functional selectivity for agonism at alpha3-containing receptors over alpha1-containing receptors with the lead compound being 32.

4-quinolone derivatives: high-affinity ligands at the benzodiazepine site of brain GABA A receptors. synthesis, pharmacology, and pharmacophore modeling

The 3-ethoxycarbonyl-4-quinolone compound 1 has previously been identified via a database search as an interesting lead compound for ligand binding at the benzodiazepine site of GABA(A) receptors (Kahnberg et al. J. Mol. Graphics Modelling 2004, 23, 253-261). Pharmacophore-guided optimization of this lead compound yielded a number of high-affinity ligands for the benzodiazepine site including compounds 20 and 23-25 displaying sub-nanomolar affinities. A few of the compounds have been tested on the alpha(1)beta(2)gamma(2S) and alpha(3)beta(2)gamma(2S) GABA(A) receptor subtypes, and two of the compounds (5 and 19) display selectivity for alpha(1)- versus alpha(3)-containing receptors by a factor of 22 and 27, respectively. This selectivity for alpha(1)beta(2)gamma(2S) is in the same range as that for the well-known alpha(1) subunit selective compound zolpidem.

Interaction of novel condensed triazine derivatives with central and peripheral type benzodiazepine receptors: synthesis, in vitro pharmacology and modelling

Structurally related sets of triazino-quinoline, triazino-isoquinoline and pyrido-triazine derivatives were synthesised and their binding interactions with central (CBR)- and peripheral-type (PBR) benzodiazepine binding sites have been characterised. Of 33 compounds tested, a new compound, 2-(4-methylphenyl)-3H- [1,2,4] triazino [2, 3-a] quinolin-3-one (1 g) showed the lowest CBR binding inhibition constant (K(i) = 42 +/- 9 nM) and the highest CBR over PBR selectivity (>1300). All but the 4-methylphenyl (1 g) structural modifications decreased the affinity and selectivity of the parent compound, 2-phenyl-3H- [1,2,4]triazino[2,3-a]quinolin-3-one (1d) (K(i) = 69 +/- 9 nM, selectivity >890). Molecular interactions between selected ligands (standards and triazine derivatives) and alpha(1)gamma(2) subunit-interface residues in a GABA(A) receptor extracellular domain homology model have been calculated. Comparing data with calculations confirmed hydrogen bonding to gamma(2)Thr142 and hydrophobic interaction with alpha(1)His101 as being essential for high-affinity CBR binding.

Synthesis, Pharmacology, and Structure−Activity Relationships of Novel Imidazolones and Pyrrolones as Modulators of GABAA Receptors

New series of imidazolones and pyrrolones were synthesized. The compounds were tested regarding their anxiolytic properties due to modulation of the GABAA receptor response. Several derivatives exhibit considerable pharmacological activity while lacking the typical side effects of benzodiazepine receptor agonists. 1-(4-chlorophenyl)-4-morpholin-1-yl-1,5-dihydro-imidazol-2-one (2) and 1-(4-chlorophenyl)-4-piperidin-1-yl-1,5-dihydro-imidazol-2-one (3) were protective in the pentylenetetrazole test in rats with oral ED50 of 27.4 and 12.8 mg/kg and TD50 (rotarod) of >500 and 265 mg/kg, respectively. The minimum effective dose in the Vogel conflict test was 3 mg/kg for both compounds. Common structure-activity relationship and comparative molecular field analysis models of the various series of derivatives could be established which are in accordance with a GABAA mediated pharmacological action. The findings fit well into an established pharmacophore model. This model is refined by an additional steric restriction feature.

Pyrazolopyridinones as functionally selective GABAA ligands

2,5-Dihydro-3H-pyrazolo[4,3-c]pyridin-3-ones are GABAA receptor benzodiazepine binding site ligands, which can exhibit functional selectivity for the alpha3 subtype over the alpha1 subtype. SAR studies to optimize this functional selectivity are described.

7-(1,1-Dimethylethyl)-6-(2-ethyl-2H-1,2,4- triazol-3-ylmethoxy)-3-(2-fluorophenyl)- 1,2,4-triazolo[4,3-b]pyridazine: A Functionally Selective γ-Aminobutyric AcidA (GABAA) α2/α3-Subtype Selective Agonist That Exhibits Potent Anxiolytic Activity but Is Not Sedating in Animal Models

There is increasing evidence that compounds with selectivity for gamma-aminobutyric acid(A) (GABA(A)) alpha2- and/or alpha3-subtypes may retain the desirable anxiolytic activity of nonselective benzodiazepines but possess an improved side effect profile. Herein we describe a novel series of GABA(A) alpha2/alpha3 subtype-selective agonists leading to the identification of the development candidate 17, a nonsedating anxiolytic in preclinical animal assays.

Selective antagonism of GABAA receptor subtypes: an in vivo approach to exploring the therapeutic and side effects of benzodiazepine-type drugs

Benzodiazepines (BZs) are clinically used as anxiolytic, hypnotic, anticonvulsant, and antispasmodic drugs. Research using transgenic mouse models has suggested that the effects of BZs involve multiple subtypes of the gamma-aminobutyric acid type A (GABAA) receptor, identified by specific a subunits (alpha1, alpha2, alpha3, alpha5). This review discusses the experimental uses of b-carboline-3-carboxylate-t-butyl ester (betaCCT), a drug that binds preferentially to the GABAA alpha1 subtype but exerts no action (ie, is a pharmacologic antagonist at the GABAA alpha1 subtype receptor). betaCCT blocks the anxiolytic-like effects of BZs, although studies in primates suggests this antagonism may reflect multiple receptor populations. betaCCT antagonized the ataxic but not muscle relaxant effects of BZs, a finding that implicates the GABAA alpha1 subtype receptor in ataxia but not muscle relaxation. The potential clinical utility of betaCCT is discussed, both in terms of treatment (ie, hepatic encephalopathy) and as a diagnostic imaging agent. Altogether, these results indicate that subtype-selective antagonists represent a useful approach to studying receptor mechanisms underlying the behavioral effects of BZ-type drugs.

2,5-Dihydropyrazolo[4,3-c]pyridin-3-ones: functionally selective benzodiazepine binding site ligands on the GABAA receptor

2,5-Dihydropyrazolo[4,3-c]pyridin-3-ones are GABAA receptor benzodiazepine binding site ligands with functional selectivity for the alpha3 subtype over the alpha1 subtype. SAR studies to optimise this functional selectivity are described.

3-Phenyl-6-(2-pyridyl)methyloxy-1,2,4-triazolo[3,4-a]phthalazines and Analogues: High-Affinity γ-Aminobutyric Acid-A Benzodiazepine Receptor Ligands with α2, α3, and α5-Subtype Binding Selectivity over α1

Studies with our screening lead 5 and the literature compound 6 led to the identification of 6-benzyloxy-3-(4-methoxy)phenyl-1,2,4-triazolo[3,4-a]phthalazine 8 as a ligand with binding selectivity for the gamma-aminobutyric acid-A (GABA-A) alpha 3- and alpha 5-containing receptor subtypes over the GABA-A alpha 1 subtype (K(i): alpha 2 = 850 nM, alpha 3 = 170 nM, alpha 5 = 72 nM, alpha 1 = 1400 nM). Early optimization studies identified the close analogue 10 (K(i): alpha 2 = 16 nM, alpha 3 = 41 nM, alpha 5 = 38 nM, alpha 1 = 280 nM) as a suitable lead for further study. High-affinity ligands were identified by replacing the 6-benzyloxy group of compound 10 with 2-pyridylmethoxy (compound 29), but binding selectivity was not enhanced (K(i): alpha 2 = 1.7 nM, alpha 3 = 0.71 nM, alpha 5 = 0.33 nM, alpha 1 = 2.7 nM). Furthermore, on evaluation in xenopus oocytes,(22) 29 was discovered to be a weak to moderate inverse agonist at all four receptor subtypes (alpha 1, -7%; alpha 2, -5%; alpha 3, -16%; alpha 5, -5%). Replacement of the 3-phenyl group of 29 with alternatives led to reduced affinity, and smaller 3-substituents led to reduced efficacy. Methyl substitution of the benzo-fused ring of 29 at the 7-, 8-, and 10-positions resulted in increased efficacy although selectivity was abolished. Increased efficacy and retention of selectivity for alpha 3 over alpha 1 was achieved with the 7,8,9,10-tetrahydro-(7,10-ethano)-phthalazine 62. Compound 62 is currently one of the most binding selective GABA-A alpha 3-benzodiazepine-site partial agonists known, and although its selectivity is limited, its good pharmacokinetic profile in the rat (33% oral bioavailability after a 3 mg/kg dose, reaching a peak plasma concentration of 179 ng/mL; half-life of 1 h) made it a useful pharmacological tool to explore the effect of a GABA-A alpha 2/alpha 3 agonist in vivo.

Synthesis of 2-aryl-oxazolo[4,5-c]quinoline-4(5H)-ones and 2-aryl-thiazolo[4,5-c]quinoline-4(5H)-ones

[reaction: see text] Novel and highly efficient syntheses of oxazolo[4,5-c]quinoline-4(5H)-ones (1) and thiazolo[4,5-c]quinoline-4(5H)-ones (2) from ethyl 2-chlorooxazole-4-carboxylate (4) and ethyl 2-bromo-5-chlorothiazole-4-carboxylate (13), respectively, are described.