Vol. 62, Issue 1, 81-89, July 2002

Selective Allosteric Enhancement of Agonist Binding and Function at Human A₃ Adenosine Receptors by a Series of Imidazoquinoline Derivatives

Zhan-Guo Gao, Seong Gon Kim, Kelly A. Soltysiak, Neli Melman, Adriaan P. IJzerman, and Kenneth A. Jacobson

Molecular Recognition Section, Laboratory of Bioorganic Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland (Z.-G.G., S.G.K., K.A.S., N.M., K.A.J.); Division of Medicinal Chemistry, Leiden/Amsterdam Center for Drug Research, Leiden University, Leiden, the Netherlands (A.P.I.)

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 agonist N⁶-(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]N⁶-[(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.