Direct Comparison of (N)-Methanocarba and Ribose-Containing 2‑Arylalkynyladenosine Derivatives as A₃ Receptor Agonists

A side-by-side pharmacological comparison of ribose and (N)-methanocarba (bicyclo[3.1.0]­hexane) nucleosides as A₃AR agonists indicated that the bicyclic pseudoribose ring constraint provided higher affinity/selectivity at human and mouse A₃AR. The mean affinity enhancement for 5 pairs of 5′-methylamides was 11-fold at hA₃AR and 42-fold at mA₃AR. Novel C2-(5-fluorothien-2-ylethynyl) substitution enhanced affinity in the methanocarba but not ribose series, with highly hA₃AR-selective <b>16</b> (MRS7334) displaying K_i 280 pM and favorable pharmacokinetics and off-target activity profile. Molecular dynamics comparison of <b>16</b> and its corresponding riboside <b>8</b> suggested a qualitative entropic advantage of <b>16</b> in hA₃AR binding. The 5-F substitution tended to increase hA₃AR affinity (cf. 5-Cl) for methanocarba but not ribose derivatives. A representative methanocarba agonist <b>4</b> was shown to interact potently exclusively with A₃AR, among 240 GPCRs and 466 kinases. Thus, despite added synthetic difficulty, the (N)-methanocarba modification has distinct advantages for A₃AR agonists, which have translational potential for chronic disease treatment.