Truncated (N)-Methanocarba Nucleosides as Partial Agonists at Mouse and Human A3 Adenosine Receptors: Affinity Enhancement by N6-(2-Phenylethyl) Substitution

Dopamine-derived N⁶-substituents, compared to N⁶-(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 15 (MRS7591, K_i = 10.9/17.8 nM, at human/mouse A₃AR). 15 was a partial agonist in vitro (hA₃AR, cAMP inhibition, 31% E_max; mA₃AR, [³⁵S]GTP-γ-S binding, 16% E_max) and in vivo and also antagonized hA₃AR in vitro. Distal H-bonding substitutions of the N⁶-(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.