Structure-Guided Design of A3 Adenosine Receptor-Selective Nucleosides: Combination of 2-Arylethynyl and Bicyclo[3.1.0]hexane Substitutions

(N)-Methanocarba adenosine 5′-methyluronamides containing known A3 AR (adenosine receptor)-enhancing modifications, i.e., 2-(arylethynyl)adenine and N6-methyl or N6-(3-substituted-benzyl), were nanomolar full agonists of human (h) A3AR and highly selective (Ki ∼0.6 nM, N6-methyl 2-(halophenylethynyl) analogues 13 and 14). Combined 2-arylethynyl-N6-3-chlorobenzyl substitutions preserved A3AR affinity/selectivity in the (N)-methanocarba series (e.g., 3,4-difluoro full agonist MRS5698 31, Ki 3 nM, human and mouse A3) better than that for ribosides. Polyaromatic 2-ethynyl N6-3-chlorobenzyl analogues, such as potent linearly extended 2-p-biphenylethynyl MRS5679 34 (Ki hA3 3.1 nM; A1, A2A, inactive) and fluorescent 1-pyrene adduct MRS5704 35 (Ki hA3 68.3 nM), were conformationally rigid; receptor docking identified a large, mainly hydrophobic binding region. The vicinity of receptor-bound C2 groups was probed by homology modeling based on recent X-ray structure of an agonist-bound A2AAR, with a predicted helica...