title: Exploring distal regions of the A3 adenosine receptor binding site: sterically constrained N6-(2-phenylethyl)adenosine derivatives as potent ligands.
abstract: We synthesized phenyl ring-substituted analogues of N(6)-(1S,2R)-(2-phenyl-1-cyclopropyl)adenosine, which is highly potent in binding to the human A(3)AR with a Ki 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 (Phe168) in the putative A(3)AR binding site around the phenyl moiety. A heteroaromatic 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-1-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.
authors.affiliation: Molecular Recognition Section, Laboratory of Bioorganic Chemistry, National Institute of Diabetes & Digestive & Kidney Diseases, National Institutes of Health, Department of Health, and Human Services, Bethesda, MD 20892-0810, USA.
authors.forename: Susanna
authors.initials: S
authors.lastname: Tchilibon
authors.forename: Soo-Kyung
authors.initials: SK
authors.lastname: Kim
authors.forename: Zhan-Guo
authors.initials: ZG
authors.lastname: Gao
authors.forename: Brian A
authors.initials: BA
authors.lastname: Harris
authors.forename: Joshua B
authors.initials: JB
authors.lastname: Blaustein
authors.forename: Ariel S
authors.initials: AS
authors.lastname: Gross
authors.forename: Heng T
authors.initials: HT
authors.lastname: Duong
authors.forename: Neli
authors.initials: N
authors.lastname: Melman
authors.forename: Kenneth A
authors.initials: KA
authors.lastname: Jacobson
mesh_terms: D001665:Binding Sites; D002851:Chromatography, High Pressure Liquid; D008024:Ligands; D008958:Models, Molecular; D021281:Quantitative Structure-Activity Relationship; D043684:Receptor, Adenosine A3 / Q000378:metabolism*; D013057:Spectrum Analysis
chemical_list: D008024:Ligands; D043684:Receptor, Adenosine A3