Abstract

We have identified a series of 1H-imidazo-[4,5-c]quinolines as selective allosteric enhancers of human A₃ adenosine receptors. Several of these compounds potentiated both the potency and maximal efficacy of agonist-induced responses and selectively decreased the dissociation of the agonistN ⁶-(4-amino-3-[¹²⁵I]iodobenzyl)-5′-N-methylcarboxamidoadenosine from human A₃ adenosine receptors. There was no effect on the dissociation of the antagonist [³H]8-ethyl-4-methyl-2-phenyl-(8R)-4,5,7,8-tetrahydro-1H-imidazo[2.1-i]purin-5-one (PSB-11) from the A₃ receptors, as well as [³H]N ⁶-[(R)-phenylisopropyl]adenosine from rat brain A₁ receptors and [³H]2-[p-(2-carboxyethyl)phenyl-ethylamino]-5′-N-ethylcarboxamidoadenosine from rat striatal A_2A receptors, suggesting the selective enhancement of agonist binding at A₃ receptors. The analogs were tested as antagonists of competitive binding at human A₃ receptors, and K _i values ranging from 120 nM to 101 μM were observed; as for many allosteric modulators of G protein-coupled receptors, an orthosteric effect was also present. The most promising leads from the present set of analogs seem to be the 2-cyclopentyl-1H-imidazo[4,5-c]quinoline derivatives, of which the 4-phenylamino analog DU124183 had the most favorable degree of allosteric modulation versus receptor antagonism. The inhibition of forskolin-stimulated cyclic AMP accumulation in intact cells that express human A₃ receptors was employed as a functional index of A₃ receptor activation. The enhancer DU124183 caused a marked leftward shift of the concentration-response curve of the A₃ receptor agonists in the presence of antagonist and, surprisingly, a potentiation of the maximum agonist efficacy by approximately 30%. Thus, we have identified a novel structural lead for developing allosteric enhancers of A₃ adenosine receptors; such enhancers may be useful for treating brain ischemia and other hypoxic conditions.