Exploring distal regions of the A 3 adenosine receptor binding site: sterically constrained N6-(2-phenylethyl)adenosine derivatives as potent ligands

Abstract

We synthesized phenyl ring-substituted analogues of N 6-(1 S,2 R)-(2-phenyl-1-cyclopropyl)adenosine, which is highly potent in binding to the human A 3AR 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 3AR were measured. A 3-nitrophenyl analogue was resolved chromatographically into pure diastereomers, which displayed 10-fold stereoselectivity in A 3AR binding in favor of the 1 S,2 R isomer. A molecular model defined a hydrophobic region (Phe168) in the putative A 3AR binding site around the phenyl moiety. A heteroaromatic group (3-thienyl) could substitute for the phenyl moiety with retention of high affinity of A 3AR binding. Other related N 6-substituted adenosine derivatives were included for comparison. Although the N 6-(2-phenyl-1-cyclopropyl) derivatives were full A 3AR agonists, several other derivatives had greatly reduced efficacy. N 6-Cyclopropyladenosine was an A 3AR 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 3AR 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 3AR binding site in a rhodopsin-based homology model. Thus, a new series of high-affinity A 3AR agonists and related nucleoside antagonists was explored through both empirical and theoretical approaches.