Pharmacophore/Receptor Models for GABA_A/BzR Subtypes (α1β3γ2, α5β3γ2, and α6β3γ2) via a Comprehensive Ligand-Mapping Approach

Abstract

Pharmacophore/receptor models for three recombinant GABA(A)/BzR subtypes (alpha1beta3gamma2, alpha5beta3gamma2, and alpha6beta3gamma2) have been established via an SAR ligand-mapping approach. This study was based on the affinities of 151 BzR ligands at five distinct (alpha1-3,5,6beta3gamma2) recombinant GABA(A)/BzR receptor subtypes from at least nine different structural families. Examination of the included volumes of the alpha1-, alpha5-, and alpha6-containing subtypes indicated that region L(2) for the alpha5-containing subtype appeared to be larger in size than the analogous region of the other receptor subtypes. Region L(Di), in contrast, appeared to be larger in the alpha1 subtype than in the other two subtypes. Moreover, region L(3) in the alpha6 subtype is either very small or nonexistent in this diazepam-insensitive subtype (see Figure 16 for details) as compared to the other subtypes. Use of the pharmacophore/receptor models for these subtypes has resulted in the design of novel BzR ligands (see 27) selective for the alpha5beta3gamma2 receptor subtype. alpha5-Selective ligand 27 when injected directly into the hippocampus did enhance memory in one paradigm (Bailey et al., unpublished observations); however, systemic administration of either 9 or 27 into animals did not provide an observable enhancement. This result is in complete agreement with the observation of Liu (1996). It has been shown (Liu, 1996; Wisden et al., 1992) that in the central nervous system of the rat (as well as monkeys and pigeons) there are several native subtypes of the GABA(A) receptor which exhibit different functions, regional distributions, and neuronal locations. Although 27 binds more potently at alpha5beta3gamma2 receptor subtypes and is clearly an inverse agonist (Liu et al., 1996; Liu, 1996), it is possible that this ligand acts as an agonist at one or more subtypes. Liu (1996) clearly showed that a number of imidazobenzodiazepines were negative modulators at one subtype and agonists at another. Therefore, selectivity for a particular subtype at this point is not sufficient to rule out some physiological effect at other GABA(A)/BzR subtypes. The inability of 27 to potentiate memory when given systemically is again in support of this hypothesis, especially since alpha1beta2gamma2 subtypes are distributed throughout the brain (Wisden et al., 1992). A drug delivered systemically is far more likely to interact with all subtypes than one delivered to a specific brain region. This observation (systemic vs intrahippocampal) provides further support for the design of more subtype-specific ligands at the BzR to accurately define their pharmacology, one key to the design of new drugs with fewer side effects.

Discriminative Stimulus Effects of Benzodiazepine (BZ)1 Receptor-Selective Ligands in Rhesus Monkeys

“…In addition, substitution for flumazenil occurs with other drugs that antagonize diazepam at BZ receptors [e.g., Ro 14-4513] or that functionally antagonize diazepam at other sites on the GABA A receptor complex (e.g., pentylenetetrazol; Gerak and France, 1999). The similar although not identical binding profile of ␤-CCt and flumazenil at BZ receptors suggests that ␤-CCt antagonizes chronic diazepam at BZ receptors to precipitate withdrawal (Huang et al, 2000). ␤-CCt is approximately 100-fold less potent than flumazenil in diazepam-treated monkeys, a difference com- parable with that observed in untreated monkeys (see below; Paronis et al, 2001).…”

“…BZs such as diazepam and midazolam are nonselective (Huang et al, 2000), whereas non-BZ compounds such as zaleplon and zolpidem are approximately 10-fold selective for BZ 1 receptors compared with other GABA A receptors in binding assays in vitro (Dä mgen and Lü ddens, 1999;Huang et al, 2000). Selectivity at BZ 1 receptors might be responsible for zaleplon and zolpidem having behavioral effects that differ from those of nonselective BZs.…”

“…However, in primate species, zaleplon and zolpidem share discriminative stimulus and subject-rated effects with other positive GABA A modulators (Griffiths et al, 1992;Rush and Griffiths, 1996;Rowlett and Woolverton, 1997;Rush et al, 1997Rush et al, , 1999Rowlett et al, 1999Rowlett et al, , 2000Ator, 2000). Among antagonists (e.g., neutral GABA A modulators) at BZ receptors, ␤-carboline-3-carboxylate-t-butyl ester (␤-CCt) is approximately 20-fold selective for BZ 1 receptors (Huang et al, 2000). Receptor selectivity could be responsible for the ability of ␤-CCt to antagonize some (e.g., anxiolytic, sedative, anticonvulsant, and discriminative stimulus effects) but not other (e.g., muscle relaxation) effects of nonselective BZ agonists (Shannon et al, 1984(Shannon et al, , 1988Griebel et al, 1999;Rowlett et al, 2001).…”

Dopamine and Benzodiazepine-Dependent Mechanisms Regulate the EtOH-Enhanced Locomotor Stimulation in the GABAA α1 Subunit Null Mutant Mice

“…Thus, bCCt exhibits the greatest binding selectivity of the currently available a1 receptor ligands McKernan et al, 2000;Cox et al, 1998). In contrast, flumazenil is a nonselective BDZbinding antagonist at the diazepam-sensitive sites (Huang et al, 2000). In relation to physiological efficacy (ie, potentiation of GABAergic activity), Xenopus oocyte studies have reported that both bCCt and flumazenil demonstrate a neutral or low efficacy agonist response profile across the a1, a2, a3, a4, and a5 receptors .…”

Selective targeting of the α5-subunit of GABAA receptors relaxes airway smooth muscle and inhibits cellular calcium handling

“…A novel approach to achieve this goal was developed by Cook and coworkers in the 1980s (1,25) that employed a pharmacophore/receptor model based on the binding affinity of rigid ligands to BDZ/GABA A receptor sites (8). From this series of receptor models for ␣ 1-6 ␤3␥2 subtypes a robust pharmacophore for ␣5-subtype selective ligands emerged resulting in the synthesis of a novel ␣5␤3␥2 partial agonist modulator: (R)-ethyl 8-ethynyl-6-(2-fluorophenyl)-4-methyl-4H-benzo[f]imidazo [1,5-a] [1,4]diazepine-3-carboxylate (or SH-053-2=-F-R-CH 3 ) (10).…”