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

Article abstract of DOI:10.1016/j.bmc.2004.02.037

We synthesized phenyl ring-substituted analogues of N⁶-(1S,2R)- (2-phenyl-1-cyclopropyl)adenosine, which is highly potent in binding to the human A₃AR with a K_i value of 0.63nM. The effects of these structural changes on affinity at human and rat adenosine receptors and on intrinsic efficacy at the hA₃AR were measured. A 3-nitrophenyl analogue was resolved chromatographically into pure diastereomers, which displayed 10-fold stereoselectivity in A₃AR binding in favor of the 1S,2R isomer. A molecular model defined a hydrophobic region (Phe168) in the putative A₃AR binding site around the phenyl moiety. A heteroaromatic group (3-thienyl) could substitute for the phenyl moiety with retention of high affinity of A₃AR binding. Other related N⁶-substituted adenosine derivatives were included for comparison. Although the N ⁶-(2-phenyl-1-cyclopropyl) derivatives were full A₃AR agonists, several other derivatives had greatly reduced efficacy. N ⁶-Cyclopropyladenosine was an A₃AR antagonist, and adding either one or two phenyl rings at the 2-position of the cyclopropyl moiety restored efficacy. N⁶-(2,2-Diphenylethyl)adenosine was an A ₃AR antagonist, and either adding a bond between the two phenyl rings (N⁶-9-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 A₃AR binding site in a rhodopsin-based homology model. Thus, a new series of high-affinity A₃AR agonists and related nucleoside antagonists was explored through both empirical and theoretical approaches.