Exploring distal regions of the A3 adenosine receptor binding site:: sterically constrained N6-(2-phenylethyl)adenosine derivatives as potent ligands

We synthesized phenyl ring-substituted analogues of N-6-(1S,2R)-(2-phenyl-l-cyclopropyl)adenosine, which is highly potent in binding to the human A(3)AR with a K-i value of 0.63 nM. The effects of these structural changes on affinity at human and rat adenosine receptors and on intrinsic efficacy at the hA(3)AR were measured. A 3-nitrophenyl analogue was resolved chromatographically into pure diastereomers, which displayed 10-fold stereoselectivity in A(3)AR binding in favor of the 1S,2R isomer. A molecular model defined a hydrophobic region (Phel68) in the putative A(3)AR binding site around the phenyl moiety. A hetero-aromatic group (3-thienyl) could substitute for the phenyl moiety with retention of high affinity of A(3)AR binding. Other related N-6-substituted adenosine derivatives were included for comparison. Although the N-6-(2-phenyl-l-cyclopropyl) derivatives were full A(3)AR agonists, several other derivatives had greatly reduced efficacy. N-6-Cyclopropyladenosine was an A(3)AR antagonist, and adding either one or two phenyl rings at the 2-position of the cyclopropyl moiety restored efficacy. N-6-(2,2-Diphenylethyl)adenosine was an A(3)AR antagonist, and either adding a bond between the two phenyl rings (N-6-9-fluorenylmethyl) or shortening the ethyl moiety (N-6-diphenylmethyl) restored efficacy. A QSAR study of the N-6 region provided a model that was complementary to the putative A(3)AR binding site in a rhodopsin-based homology model. Thus, a new series of high-affinity A(3)AR agonists and related nucleoside antagonists was explored through both empirical and theoretical approaches. (C) 2004 Elsevier Ltd. All rights reserved.