The current study describes the synthesis and biological evaluation of a novel series of 2-amino-3-naphthoylthiophenes, with variable modifications at the 4- and 5-position of the thiophene as well as the naphthoyl ring. Allosteric enhancer activity was measured in several ways: (1) evaluating the effect on forskolin-stimulated cAMP accumulation in the presence of an A(1)-adenosine agonist (CPA) in Chinese hamster ovary (CHO) cells expressing the cloned human A(1)-adenosine receptor (hA(1)AR); (2) ability of these compounds to displace the binding of [H-3]DPCPX, [H-3]ZM 241385, and [H-3]MRE 3008F20 to the ligand binding site of CHO cells expressing the hA(1), hA(2)A, and hA(3) adenosine receptors, respectively; (3) effect on the binding of [H-3]CCPA to membranes from CHO cells expressing hA(1)AR, to rat brain and human cortex membrane preparations containing native adenosine A, receptors; (4) kinetics of the dissociation of [H-3]CCPA from CHO-hA(1) membranes. The pharmacological assays compared the various activities to that of the reference compound PD 81,723 (compound 1). Several compounds appeared to be better than PD 81,723 to enhance the effect of CPA (and thus reduce cAMP content) in the CHO:hA(1) assay. The effect of these compounds at a concentration of 10 muM was slightly greater than that of the same concentration of the PD 81,723 and substantially greater than that of PD 81,723 when responses to 1 muM of each compound were compared. These include compounds 23, 25-29, 31-34, 38, 39, 43, and 58. Cycloalkylthiophenes tended to be more potent then their 4,5-dimethyl analogues, and in the series of cycloalkylthiophenes, tetrahydrobenzo[b]thiophene derivatives appeared to be more potent than the dihydrocyclopentadien[b]thiophene counterparts. Some of the most potent compounds were tested at a concentration of 10 muM for their affinity as competitors to the antagonist binding site of CHO cells expressing hA(1,) hA(2)A, and hA(3) adenosine receptors. None inhibited binding at the hA(2A)-AR, but most of them inhibited binding to the hA(1)AR to varying extents (0-19%) as well as to the hA(3)AR to a substantial degree (0-57%). At a concentration of 10 muM, the compounds 31, 34, 37, 38, and 39 were more active than PD 81,723 to increase the binding of [H-3]CCPA to CHO:bA(1), human brain and rat cortex membranes. Compound 37 was the most active compound increasing the binding to CHO:hAj, human brain, and rat cortex membranes by 149, 43, and 27%, respectively (51, 15, and 22%, respectively, for PD 81,723). A good correlation was found between the increments [H-3]CCPA binding to A(1) receptors expressed in different systems. Unlike the effect on agonist binding, the tested compounds did not increase the binding of the antagonist [H-3]DPCPX on hCHO-A(1) membranes. Ligand dissociation studies revealed that two compounds (22 and 39) were more potent than 1 to slow the dissociation of [H-3]CCPA from CHO:hA(1)AR membranes. No clear-cut structure -activity relationship can be observed based on data from the functional assay, but we have identified several compounds, in particular 37 and 39, which appeared to be more potent than 1 and that may be selected for further development.