Allosteric modulators of A(1) and A(2A) adenosine receptors have been described; however, for the A(3) adenosine receptor, neither an allosteric site nor a compound with allosteric effects has been described. In this study, the allosteric modulation of human A(3) adenosine receptors by a series of 3-(2-pyridinyl)isoquinoline derivatives was investigated by examining their effects on the dissociation of the agonist radioligand, [(125)I]N(6)-(4-amino-3-iodobenzyl)-5'-N-methylcarboxamidoadenosine (I-AB-MECA), from the receptor. Several 3-(2-pyridinyl)isoquinoline derivatives, including VUF5455, VUF8502, VUF8504, and VUF8507, slowed the dissociation of the agonist radioligand [(125)I]I-AB-MECA in a concentration-dependent manner, suggesting an allosteric interaction. These compounds had no effect on the dissociation of the radiolabeled antagonist [(3)H]PSB-11 from the A(3) adenosine receptor, suggesting a selective enhancement of agonist binding. By comparison, compounds of similar structure (VUF8501, VUF8503, VUF8505), the classical adenosine receptor antagonist CGS15943 and the A(1) receptor allosteric enhancer PD81723 did not significantly influence the dissociation rate of [(125)I]I-AB-MECA. The effect of agonist on forskolin-induced cAMP production was significantly enhanced by VUF5455. When the subtype-selectivity of the allosteric enhancement was tested the compounds had no effect on the dissociation of either [(3)H]N(6)-[(R)-phenylisopropyl]adenosine from the A(1) adenosine receptor or [(3)H]CGS21680 from the A(2A) adenosine receptor. Probing of structure-activity relationships suggested that a carbonyl group is essential for allosterism but preferred only for competitive antagonism. The presence of a 7-methyl group decreased the competitive binding affinity without a major loss of the allosteric enhancing activity, suggesting that the structural requirements for allosteric enhancement might be distinct from those for competitive antagonism.