Novel 2- and 4-Substituted 1H-Imidazo[4,5-c]quinolin-4-amine Derivatives as Allosteric Modulators of the A3 Adenosine Receptor

4-Arylamino and 2-cycloalkyl (including amino substitution) modifications were made in a series of 1H-imidazo[4,5-c]quinolin-4-amine derivatives as allosteric modulators of the human A3 adenosine receptor (AR). In addition to allosteric modulation of the maximum functional efficacy (in [S-35]GTP gamma S G protein binding assay) of the A(3)AR agonist C1-IB-MECA (15), some analogues also weakly inhibited equilibrium radioligand binding at ARs. 4-(3,5-Dichlorophenylamino) (6) or 2-(1-adamantyl) (20) substitution produced allosteric enhancement (twice the maximal agonist efficacy), with minimal inhibition of orthosteric AR binding. 2-(4-Tetrahydropyranyl) substitution abolished allosteric enhancement but preserved inhibition of orthosteric binding. Introduction of nitrogen in the six-membered ring at the 2 position, to improve aqueous solubility and provide a derivatization site, greatly reduced the allosteric enhancement. 2-(4-(Benzoylamino)cyclohexyl) analogues 23 and 24 were weak negative A3AR modulators. Thus, consistent with previous findings, the allosteric and orthosteric inhibitory A3AR effects in imidazoquinolines are structurally separable, suggesting the possible design of additional derivatives with enhanced positive or negative allosteric A3AR activity and improved selectivity in comparison to inhibition of orthosteric binding.