Truncated (N)-Methanocarba Nucleosides as Partial Agonists at Mouse and Human A₃ Adenosine Receptors: Affinity Enhancement by <i>N</i>⁶-(2-Phenylethyl) Substitution

Abstract

Dopamine-derived <i>N</i>⁶-substituents, compared to <i>N</i>⁶-(2-phenylethyl), in truncated (N)-methanocarba (bicyclo[3.1.0]hexyl) adenosines favored high A₃ adenosine receptor (AR) affinity/selectivity, e.g., C2-phenylethynyl analogue <b>15</b> (MRS7591, <i>K</i>_i = 10.9/17.8 nM, at human/mouse A₃AR). <b>15</b> was a partial agonist in vitro (hA₃AR, cAMP inhibition, 31% <i>E</i>_max; mA₃AR, [³⁵S]GTP-γ-S binding, 16% <i>E</i>_max) and in vivo and also antagonized hA₃AR in vitro. Distal H-bonding substitutions of the <i>N</i>⁶-(2-phenylethyl) moiety particularly enhanced mA₃AR affinity by polar interactions with the extracellular loops, predicted using docking and molecular dynamics simulation with newly constructed mA₃AR and hA₃AR homology models. These hybrid models were based on an inactive antagonist-bound hA₁AR structure for the upper part of TM2 and an agonist-bound hA_2AAR structure for the remaining TM portions. These species-independent A₃AR-selective nucleosides are low efficacy partial agonists and novel, nuanced modulators of the A₃AR, a drug target of growing interest.