Use of imidazo[1,5-a]quinoline scaffold as the pharmacophore in the design of bivalent ligands of central benzodiazepine receptors

The imidazo[1,5-a]quinoline scaffold of central benzodiazepine receptor (CBR) ligands was used as the pharmacophore in the design of bivalent ligands bearing spacers showing variable length and different physicochemical features. The newly designed compounds were synthesized along with the corresponding reference monovalent compounds bearing the corresponding spacers terminated with a tert-butoxycarbonyl group. The novel compounds were tested in binding assays with different CBR preparations such as the cerebral cortex from male CD-1 albino mice or the human recombinant α1β3γ2 and α2β3γ2 γ-aminobutyric acid type A receptors (GABA_ARs) stably expressed in mouse L(tk-) cells. The tested compounds showed IC₅₀ values from the sub-micromolar up to the nanomolar range with very similar inhibition constants values for the two isoforms of GABA_ARs. The similarity in the affinity between the bivalent ligands and the corresponding monovalent ones appeared to rule out any bivalent interactions of these ligands with the two isoforms of GABA_ARs. Similarly, both series were able to inhibit the binding of radiolabeled flumazenil to GABA_ARs in cortical membranes of albino CD-1 mice, but most of the tested compounds showed biphasic inhibition curves, suggesting the existence of two well-distinct populations of binding sites. Finally, some CBR ligands selected from the bivalent ligands (i.e. 6a,c) and from the reference monovalent ligands (i.e. 7a) were then tested in vivo for their potential pharmacological effects, evaluating four classical benzodiazepine actions such as anticonvulsant, anxiolytic, locomotor, and anti-amnesic activities. All the tested compounds showed anticonvulsant and anxiolytic properties with neither muscle relaxant effect nor learning and memory impairments.