Selective allosteric enhancement of agonist binding and function at human A3 adenosine receptors by a series of imidazoquinoline derivatives

We have identified a series of 1H-imidazo-[4,5-c] quinolines as selective allosteric enhancers of human A(3) adenosine receptors. Several of these compounds potentiated both the potency and maximal efficacy of agonist-induced responses and selectively decreased the dissociation of the agonist N-6-(4-amino-3-[ I-125] iodobenzyl)-5'-N-methylcarboxamidoadenosine from human A3 adenosine receptors. There was no effect on the dissociation of the antagonist [H-3]8-ethyl-4-methyl-2-phenyl( 8R)-4,5,7,8-tetrahydro-1H-imidazo[2.1-i] purin-5-one (PSB-11) from the A(3) receptors, as well as [H-3] N-6-[(R)-phenylisopropyl] adenosine from rat brain A(1) receptors and [H-3] 2-[p-(2-carboxyethyl) phenyl-ethylamino]-5'-N-ethylcarboxamidoadenosine from rat striatal A(2A) receptors, suggesting the selective enhancement of agonist binding at A(3) receptors. The analogs were tested as antagonists of competitive binding at human A(3) receptors, and K-i values ranging from 120 nM to 101 muM 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 A(3) receptors was employed as a functional index of A(3) receptor activation. The enhancer DU124183 caused a marked leftward shift of the concentration-response curve of the A(3) 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 A(3) adenosine receptors; such enhancers may be useful for treating brain ischemia and other hypoxic conditions.