Allosteric enhancers for the A1 adenosine receptor represent a novel and unique drug design strategy to augment the response to endogenous adenosine in a site- and event-specific manner. We have previously investigated a detailed structure-activity relationship study around a wide series of 2-amino-3-aroyl-4-[(4-arylpiperazin-1-yl)methyl]thiophene derivatives as potent allosteric enhancers of the A1 adenosine receptor. In this manuscript we report our investigation on the influence on allosteric enhancer activity of further substitution at the 4-position of the 2-amino-3-(4-chlorobenzoyl)-thiophene system to explore bulk tolerance by replacement of the arylpiperazine moiety with a series of fused indole nuclei corresponding to 1,2,3,4-tetrahydropyrazino[1,2-a]indole, 1,2,3,4,10,10a-hexahydropyrazino[1,2-a]indole, tetrahydro-γ-carboline, tetrahydroisoquinoline, spiro-1,3-benzodioxolepiperidine, aliphatic tertiary amine, N-alkylaniline, aryl ether and aryl thioether templates. The 1,2,3,4-tetrahydropyrazino[1,2-a]indole derivatives 3a-c and 3e were the most active compounds in binding (saturation and competition) and functional cAMP studies, being able to potentiate agonist [(3)H]CCPA binding to the A1 receptor. This study also shows that it is possible to obtain a good separation between allosteric enhancement and antagonistic activity at the A1 adenosine receptor.

2-Amino-3-benzoyl thiophenes have been widely reported to act as allosteric enhancers at the A1 adenosine receptor. Their activity can be increased considerably by appropriate substitutions at the 4- and 5-positions of the thiophene ring. Substituent size at the thiophene C-4 position seemed to be a factor closely related to activity, with the 4-neopentyl (2,2-dimethylpropyl) substitution showing the greatest enhanced activity. A wide series of 2-amino-3-aroyl-4-neopentylthiophene derivatives with general structure 3, characterized by the presence of different substituents (bromine, aryl and heteroaryl) at the 5-position of the thiophene ring, have been identified as potent AEs at the A1AR. With only one exception, all of the synthesized compounds proved to be superior to the reference compound PD 81,723 in a functional assay. Derivatives 3p, 3u, 3am, 3ap and 3ar were the most active compounds in binding (saturation and competition) and functional cAMP studies, being able to potentiate agonist [(3)H]CCPA binding to the A1 receptor.