2-Amino-3-benzoylthiophene Allosteric Enhancers of A1 Adenosine Agonist Binding:  New 3, 4-, and 5-Modifications

2-Amino-3-aroylthiophenes are agonist allosteric enhancers (AE) at the A1 adenosine receptor (A1AR). Here we report the syntheses of three kinds of novel 2-aminothiophenes and assays of their AE activity at the human A1AR (hA1AR), namely, (1) 2-amino-4,5-diphenylthiophene-3-carboxylates, 3a−h, (2) 2-amino-3-benzoyl-4,5-diphenylthiophenes, 7a−p, and (3) 2-amino-5-bromo-3-benzoyl-4-phenylthiophenes, 10a−h. An in vitro assay employing the A1AR agonist [125I]ABA and membranes from CHO−K1 cells stably expressing the hA1AR measured an index of AE activity, the ability of a candidate AE to stabilize the agonist-A1AR-G protein ternary complex, scored as the percentage of ternary complex remaining after 10 min of dissociation initiated by CPX and GTPγS. The AE activity score of 2-amino-4,5-dimethyl-3-(3-trifluoromethylbenzoyl)thiophene (PD 81,723), which was 19%, served as a standard for comparison. Two 3-carboxythiophene 3-trifluoromethylbenzyl esters, 3d (49%) and 3f (63%), had substantial AE activity. The 3-(1-naphthoyl) substituent of 7e (52%) also supported AE activity. Compounds in series 3 tended to be more potent, 10a and 10c having scores of 91 and 80%, respectively. The activity of 2-amino-5-bromo-3-ethoxycarbonyl-4-(3-nitrophenyl)thiophene, 10h (26%), is an exception to the rule that a 3-ethoxycarbonyl substituent cannot support AE activity.