Truncated (N)-Methanocarba Nucleosides as Partial Agonists at Mouse and Human A₃ Adenosine Receptors: Affinity Enhancement by <i>N</i>⁶‑(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, Ki = 10.9/17.8 nM, at human/mouse A3AR). 15 was a partial agonist in vitro (hA3AR, cAMP inhibition, 31% Emax; mA3AR, [35S]­GTP-γ-S binding, 16% Emax) 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 hA2AAR 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.与N6-(2-苯乙基)取代基相比，多巴胺衍生的N6位取代基在截短的(N)-甲烷碳环（双环[3.1.0]己基）腺苷类化合物中，更倾向于展现出对A3型腺苷受体（A3 adenosine receptor, A3AR）的高亲和力与选择性；例如C2-苯乙炔基类似物15（MRS7591，人源/鼠源A3AR的抑制常数Ki分别为10.9/17.8 nM）。15在体外（人源A3AR：环磷酸腺苷（cAMP）抑制实验中最大效应Emax为31%；鼠源A3AR：[35S]标记的GTPγS结合实验中Emax为16%）与体内均为部分激动剂，同时在体外还可拮抗人源A3AR。对N6-(2-苯乙基)基团进行远端氢键取代修饰，可通过与细胞外环的极性相互作用显著提升鼠源A3AR的亲和力，该结论通过分子对接与分子动力学模拟得到验证，且模拟所用的鼠源与人类A3AR同源建模模型为新构建所得。这些混合模型以结合了非活性拮抗剂的人源A1型腺苷受体（A1 adenosine receptor, A1AR）结构作为跨膜结构域2（Transmembrane domain 2, TM2）的上部模板，其余跨膜区段则以结合了激动剂的人源A2A型腺苷受体（A2A adenosine receptor, A2AAR）结构作为模板。这类物种非依赖型的A3AR选择性核苷类化合物属于低效价部分激动剂，同时也是A3AR这一日益受到关注的药物靶点的新型精细化调节剂。