Synthesis and biological effects of novel 2-amino-3-(4-chlorobenzoyl)-4-substituted thiophenes as allosteric enhancers of the A1 adenosine receptor

• Compounds 3a–d appeared to be more active than PD 81,723 in the functional assay.
• None of compounds 3a–aq inhibited antagonist binding at the hA1AR, hA2AR, or hA3AR.
• Derivatives 3a–c, 3e and 3n were the most active compounds in binding experiments.

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 [3H]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.

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