New derivatives of PD 81,723, an allosteric enhancer of agonist binding to the A(1)-adenosine receptor, have been synthesized and evaluated in an intact cell assay. Compounds 3a, 3o and 3p appeared to be more potent than PD 81.723 and at a concentration of 0.1 mu M caused significant reductions of cAMP content of CHO cells expressing the human A(1)-adenosine receptor. Compounds 4e and 4o appeared to be allosteric enhancers at a low concentration and antagonists at a higher concentration, whereas compounds 3c, 3g, 3s and ill appeared to be weak antagonists that are also allosteric enhancers at the: higher concentration of 10 mu M. (C) 2000 Elsevier Science Ltd. All rights reserved.