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

Abstract

(<i>N</i>)-Methanocarba adenosine 5′-methyluronamides containing known A₃ AR (adenosine receptor)-enhancing modifications, i.e., 2-(arylethynyl)­adenine and <i>N</i>⁶-methyl or <i>N</i>⁶-(3-substituted-benzyl), were nanomolar full agonists of human (h) A₃AR and highly selective (<i>K</i>_i ∼0.6 nM, <i>N</i>⁶-methyl 2-(halophenylethynyl) analogues <b>13</b> and <b>14</b>). Combined 2-arylethynyl-<i>N</i>⁶-3-chlorobenzyl substitutions preserved A₃AR affinity/selectivity in the (<i>N</i>)-methanocarba series (e.g., 3,4-difluoro full agonist MRS5698 <b>31</b>, <i>K</i>_i 3 nM, human and mouse A₃) better than that for ribosides. Polyaromatic 2-ethynyl <i>N</i>⁶-3-chlorobenzyl analogues, such as potent linearly extended 2-<i>p</i>-biphenylethynyl MRS5679 <b>34</b> (<i>K</i>_i hA₃ 3.1 nM; A₁, A_2A, inactive) and fluorescent 1-pyrene adduct MRS5704 <b>35</b> (<i>K</i>_i 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 A_2AAR, with a predicted helical rearrangement requiring an agonist-specific outward displacement of TM2 resembling opsin. Thus, the X-ray structure of related A_2AAR is useful in guiding the design of new A₃AR agonists