Two series of 5-substituted 2-amino-4-(3-trifluoromethylphenyl)thiophenes were prepared and evaluated as allosteric enhancers at the A(1) adenosine receptor (A(1)AR). In the 3-benzoyl series, a 5-phenyl group was found to confer the greatest potency (9a: ED(50)=2.1 microM, AE score=18%). However, the analogue with no 5-substituent (6b: ED(50)=15.8 microM, AE score=77%) proved to be the most efficacious. In the 3-ethoxycarbonyl series, the 5-(4-chlorophenyl) analogue was clearly the most potent and efficacious (9l: ED(50)=6.6 microM, AE score=57%). The antagonist activity of all compounds was measured using a [(3)H]CPX competitive binding assay.