Synthesis of novel imidazobenzodiazepines selective for the alpha 5 beta 2 gamma 2 (Bz5) GABA(A)/benzodiazepine receptor subtype

The synthesis and in vitro activity of a series of novel imidazobenzodiazepines at five GABA(A)/BzR subtypes (alpha 1 beta 2 gamma 2, alpha 2 beta 2 gamma 2 , alpha 3 beta 2 gamma 2, alpha 5 beta 2 gamma 2, and alpha 6 beta 2 gamma 2) expressed from human cell lines are described. The results indicate that prototypical imidazobenzodiazepines such as Ro15-4513 and Ro15-1310 exhibited moderate selectivity for alpha 5 beta 2 gamma 2 receptor subtypes. When an acetylenyl or its equivalent moiety was substituted for the azido group of Ro15-4513, the ligands (e.g. 3c, 4a, and 4c) thus synthesized exhibited high selectivity at the alpha 5 beta 2 gamma 2 GABA(A)/BzR subtype. These ligands (e.g. 3c, 4c) negatively modulated GABA induced function at the alpha 5 beta 2 gamma 2 GABA(A)/BzR subtype, expressed in Xenopus oocytes in a manner similar to that of DMCM, an inverse agonist. In addition, it was found that increasing the size of substituents at position-8 of imidazobenzodiazepines (e.g. 3a) dramatically diminished the affinity of these ligands at most subtypes, but still provided a ligand with good affinity at alpha 5 beta 2 gamma 2 sites. However, ligands such as 3b with very large substituents at C(8) do not bind to any of the BzR subtypes described herein. These results demonstrated that chemical modification of imidazobenzodiazepines at position-8 had a profound effect on selectivity of ligands at the GABA(A)/BzR subtype and thus provide a basis for the search for even more selective ligands at GABA(A)/BzR subtypes.