Contribution of specific binding to the central benzodiazepine site to the brain concentrations of two novel benzodiazepine site ligands.

The in vivo occupancy of brain benzodiazepine binding sites by compounds A and B was measured using a [(3)H]Ro 15-1788 binding assay and related to plasma and brain drug concentrations. The plasma concentration associated with 50% occupancy was higher for compound A than compound B (73 and 3.7 nM, respectively), however, there was little difference in the brain concentrations required (73 and 63 nM).

Comparison of in vivo and ex vivo [3H]flumazenil binding assays to determine occupancy at the benzodiazepine binding site of rat brain GABAA receptors.

In the present study, the occupancy of flumazenil (Ro 15-1788; 1-30mg/kg p.o.) at the benzodiazepine site of rat brain GABA(A) receptors was compared using in vivo and ex vivo binding methodologies with [(3)H]flumazenil as the radioligand.

A pyridazine series of alpha2/alpha3 subtype selective GABA A agonists for the treatment of anxiety.

The development of a series of GABA(A) alpha2/alpha3 subtype selective pyridazine based benzodiazepine site agonists as anxiolytic agents with reduced sedative/ataxic potential is described, including the discovery of 16, a remarkably alpha3-selective compound ideal for in vivo study. These ligands are antagonists at the alpha1 subtype, with good CNS penetration and receptor occupancy, and excellent oral bioavailability.

Discovery of imidazo[1,2-b][1,2,4]triazines as GABA(A) alpha2/3 subtype selective agonists for the treatment of anxiety.

The identification of a series of imidazo[1,2-b][1,2,4]triazines with high affinity and functional selectivity for the GABA(A) alpha3-containing receptor subtype is described, leading to the identification of a clinical candidate, 11. Compound 11 shows good bioavailability and half-life in preclinical species, and it is a nonsedating anxiolytic in both rat and squirrel monkey behavioral models.

Rat pharmacokinetics and pharmacodynamics of a sustained release formulation of the GABAA alpha5-selective compound L-655,708.

The pharmacokinetic and pharmacodynamic (i.e., receptor occupancy) properties of L-655,708, a compound with selectivity for alpha5-over alpha1-, alpha2-, and alpha3-containing GABA(A) receptors, were examined in rats with the aim of developing a formulation that would give sustained (up to 6 h) and selective occupancy of alpha5-containing GABA(A) receptors suitable for behavioral studies.

The in vivo properties of pagoclone in rat are most likely mediated by 5'-hydroxy pagoclone.

The cyclopyrrolone pagoclone binds with roughly equivalent high affinity (0.7-9.1nM) to the benzodiazepine binding site of human recombinant GABA(A) receptors containing either an alpha1, alpha2, alpha3 or alpha5 subunit.

Imidazo[1,2-a]pyrimidines as functionally selective and orally bioavailable GABA(A)alpha2/alpha3 binding site agonists for the treatment of anxiety disorders.

A series of high-affinity GABA(A) agonists with good oral bioavailability in rat and dog and functional selectivity for the GABA(A)alpha2 and -alpha3 subtypes is reported. The 7-trifluoromethylimidazopyrimidine 14g and the 7-propan-2-olimidazopyrimidine 14k are anxiolytic in both conditioned and unconditioned animal models of anxiety with minimal sedation observed at full BZ binding site occupancy.

Imidazo[1,2-b][1,2,4]triazines as alpha2/alpha3 subtype selective GABA A agonists for the treatment of anxiety.

Imidazo[1,2-a]pyrimidines and imidazo[1,2-b][1,2,4]triazines are ligands for the benzodiazepine binding site of GABA(A) receptors that are functionally selective for the alpha2/alpha3 subtypes over the alpha1 subtype. SAR studies to optimise this functional selectivity, pharmacokinetic and behavioural data are described.

Pharmacokinetics and metabolism studies on (3-tert-butyl-7-(5-methylisoxazol-3-yl)-2-(1-methyl-1H-1,2,4-triazol-5-ylmethoxy) pyrazolo[1,5-d][1,2,4]triazine, a functionally selective GABA(A) alpha5 inverse agonist for cognitive dysfunction.

(3-tert-Butyl-7-(5-methylisoxazol-3-yl)-2-(1-methyl-1H-1,2,4-triazol-5-ylmethoxy)pyrazolo[1,5-d][1,2,4]triazine (1) was recently identified as a functionally selective, inverse agonist at the benzodiazepine site of GABA(A) alpha5 receptors and enhances performance in animal models of cognition. The routes of metabolism of this compound in vivo in rat have been well characterised, the identities of the major metabolites are confirmed by synthesis and their biological profiles were evaluated. An unusual oxidation of the pyrazolo[1,5-d][1,2,4]triazine core to the corresponding pyrazolo[1,5-d][1,2,4]triazin-4(5H)-one scaffold by aldehyde oxidase has been observed.

Evidence for a significant role of alpha 3-containing GABAA receptors in mediating the anxiolytic effects of benzodiazepines.

The GABA(A) receptor subtypes responsible for the anxiolytic effects of nonselective benzodiazepines (BZs) such as chlordiazepoxide (CDP) and diazepam remain controversial. Hence, molecular genetic data suggest that alpha2-rather than alpha3-containing GABA(A) receptors are responsible for the anxiolytic effects of diazepam, whereas the anxiogenic effects of an alpha3-selective inverse agonist suggest that an agonist selective for this subtype should be anxiolytic. We have extended this latter pharmacological approach to identify a compound, 4,2'-difluoro-5'-[8-fluoro-7-(1-hydroxy-1-methylethyl)imidazo[1,2-á]pyridin-3-yl]biphenyl-2-carbonitrile (TP003), that is an alpha3 subtype selective agonist that produced a robust anxiolytic-like effect in both rodent and non-human primate behavioral models of anxiety.

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 gamma-aminobutyric acid(A) (GABA(A)) alpha2/alpha3-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 labelling of diazepam-insensitive GABAA receptors in vivo using [3H]Ro 15-4513.

Classical benzodiazepines (BZs), such as diazepam, bind to GABAA receptors containing alpha1, alpha2, alpha3 or alpha5 subunits that are therefore described as diazepam-sensitive (DS) receptors. However, the corresponding binding site of GABAA receptors containing either an alpha4 or alpha6 subunit do not bind the classical BZs and are therefore diazepam-insensitive (DIS) receptors; a difference attributable to a single amino acid (histidine in alpha1, alpha2, alpha3 and alpha5 subunits and arginine in alpha4 and alpha6). Unlike classical BZs, the imidazobenzodiazepines Ro 15-4513 and bretazenil bind to both DS and DIS populations of GABAA receptors.

TPA023 [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], an agonist selective for alpha2- and alpha3-containing GABAA receptors, is a nonsedating anxiolytic in rodents and primates.

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 (TPA023) is a triazolopyridazine that binds with equivalent high (subnanomolar) affinity to the benzodiazepine binding site of recombinant human GABA(A) receptors containing an alpha1, alpha2, alpha3, or alpha5 subunit but has partial agonist efficacy at the alpha2 and alpha3 subtypes and essentially antagonist efficacy at the alpha1 and alpha5 subtypes. In rats, TPA023 gave time- and dose-dependent occupancy after oral dosing, with 50% occupancy corresponding to a dose of 0.42 mg/kg.

In vivo labelling of alpha5 subunit-containing GABA(A) receptors using the selective radioligand [(3)H]L-655,708.

L-655,708 is an imidazobenzodiazepine possessing 30-70-fold selectivity for the benzodiazepine binding site of GABA(A) receptors containing an alpha5 rather than alpha1, alpha2 or alpha3 subunit. In the present study, [(3)H]L-655,708 was used to label mouse brain benzodiazepine binding sites in vivo. When compared to inhibition of in vivo binding of the non-selective ligand [(3)H]Ro 15-1788, the pharmacology of mouse in vivo [(3)H]L-655,708 binding was consistent with selective in vivo labelling of alpha5 subunit-containing GABA(A) receptors.

Discovery of functionally selective 7,8,9,10-tetrahydro-7,10-ethano-1,2,4-triazolo[3,4-a]phthalazines as GABA A receptor agonists at the alpha3 subunit.

We have previously identified the 7,8,9,10-tetrahydro-7,10-ethano-1,2,4-triazolo[3,4-a]phthalazine (1) as a potent partial agonist for the alpha(3) receptor subtype with 5-fold selectivity in binding affinity over alpha(1). This paper describes a detailed investigation of the substituents on this core structure at both the 3- and 6-positions. Despite evaluating a wide range of groups, the maximum selectivity that could be achieved in terms of affinity for the alpha(3) subtype over the alpha(1) subtype was 12-fold (for 57).

Anxiogenic properties of an inverse agonist selective for alpha3 subunit-containing GABA A receptors.

Alpha3IA (6-(4-pyridyl)-5-(4-methoxyphenyl)-3-carbomethoxy-1-methyl-1H-pyridin-2-one) is a pyridone with higher binding and functional affinity and greater inverse agonist efficacy for GABA(A) receptors containing an alpha3 rather than an alpha1, alpha2 or alpha5 subunit. If doses are selected that minimise the occupancy at these latter subtypes, then the in vivo effects of alpha3IA are most probably mediated by the alpha3 subtype. Alpha3IA has good CNS penetration in rats and mice as measured using a [(3)H]Ro 15-1788 in vivo binding assay.

An orally bioavailable, functionally selective inverse agonist at the benzodiazepine site of GABAA alpha5 receptors with cognition enhancing properties.

(3-tert-Butyl-7-(5-methylisoxazol-3-yl)-2-(1-methyl-1H-1,2,4-triazol-5-ylmethoxy)pyrazolo[1,5-d][1,2,4]triazine (13) has been identified as a functionally selective, inverse agonist at the benzodiazepine site of GABA(A) alpha5 receptors. 13 is orally bioavailable, readily penetrates the CNS, and enhances performance in animal models of cognition. It does not exhibit the convulsant, proconvulsant, or anxiogenic activity associated with nonselective GABA(A) inverse agonists.

3,4-Dihydronaphthalen-1(2H)-ones: novel ligands for the benzodiazepine site of alpha5-containing GABAA receptors.

A series of substituted 3,4-dihydronaphthalen-1(2H)-ones with high binding affinity for the benzodiazepine site of GABAA receptors containing the alpha5-subunit has been identified. These compounds have consistently higher binding affinity for the GABAA alpha5 receptor subtype over the other benzodiazepine-sensitive GABAA receptor subtypes (alpha1, alpha2 and alpha3). Compounds with a range of efficacies for the benzodiazepine site of alpha5-containing GABAA receptors were identified, including the alpha5 inverse agonist 3,3-dimethyl-8-methylthio-5-(pyridin-2-yl)-3,4-dihydronaphthalen-1(2H)-one 22 and the alpha5 agonist 8-ethylthio-3-methyl-5-(1-oxidopyridin-2-yl)-3,4-dihydronaphthalen-1(2H)-one 19.

3-phenyl-6-(2-pyridyl)methyloxy-1,2,4-triazolo[3,4-a]phthalazines and analogues: high-affinity gamma-aminobutyric acid-A benzodiazepine receptor ligands with alpha 2, alpha 3, and alpha 5-subtype binding selectivity over alpha 1.

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.

Tricyclic pyridones as functionally selective human GABAA alpha 2/3 receptor-ion channel ligands.

A series of tricyclic pyridones has been evaluated as benzodiazepine site ligands with functional selectivity for the alpha(3) over the alpha(1) containing subtype of the human GABA(A) receptor ion channel. This investigation led to the identification of a high affinity, functionally selective, orally bioavailable benzodiazepine site ligand that demonstrated activity in rodent anxiolysis models and reduced sedation relative to diazepam.