1-, 3- and 8-substituted-9-deazaxanthines as potent and selective antagonists at the human A2B adenosine receptor Original Research Article

A large series of piperazin-, piperidin- and tetrahydroisoquinolinamides of 4-(1,3-dialkyl-9-deazaxanthin-8-yl)phenoxyacetic acid were prepared through conventional or multiple parallel syntheses and evaluated for their binding affinity at the recombinant human adenosine receptors, chiefly at the hA2B and hA2A receptor subtypes. Several ligands endowed with high binding affinity at hA2B receptors, excellent selectivity over hA2A and hA3 and a significant, but lower, selectivity over hA1 were identified. Among them, piperazinamide derivatives 23 and 52, and piperidinamide derivative 69 proved highly potent at hA2B (Ki = 11, 2 and 5.5 nM, respectively) and selective towards hA2A (hA2A/hA2B SI = 912, 159 and 630, respectively), hA3 (hA3/hA2B SI = > 100, 3090 and >180, respectively) and hA1 (hA1/hA2B SI = > 100, 44 and 120, respectively), SI being the selectivity index. A number of selected ligands tested in functional assays in vitro showed very interesting antagonist activities and efficacies at both A2A and A2B receptor subtypes, with pA2 values close to the corresponding pKis. Structure–affinity and structure–selectivity relationships suggested that the binding potency at the hA2B receptor may be increased by lipophilic substituents at the N4-position of piperazinamides and that an ortho-methoxy substituent at the 8-phenyl ring and alkyl groups at N1 larger than the ones at N3, in the 9-deazaxanthine ring, may strongly enhance the hA2A/hA2B SI.