In our search for novel selective I₂ / a₂ ligands we have prepared 13 benzimidazole derivatives shown in figure 1. 2-(4,5-Dihydroimidazol-2-yl)benzimidazoles 1, 2, 4 have been identified as selective imidazoline I₂ / a₂-adrenoceptor ligands. 4-Methyl derivative 2 displays I₂ affinity at nanomolar concentration (K_i = 4.4 nM) and a high I₂/a₂ selectivity ratio = 4226. Analysis of the structure - activity relationships indicates that for this class of imidazoline ligands the presence of a weakly acidic NH group of benzimidazole ring and the basic NH group of imidazoline ring connected by a two atom C-C bridge is an important motif responsible for binding to I₂ receptors. Moreover, an evidence has been obtained that pKa value of imidazoline ligands can influence their a₂-adrenoceptor activity.

Figure 1.

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