Direct Comparison of (N)-Methanocarba and Ribose-Containing 2-Arylalkynyladenosine Derivatives as A 3 Receptor Agonists.

A side-by-side pharmacological comparison of ribose and (N)-methanocarba (bicyclo[3.1.0]hexane) nucleosides as A3 AR agonists indicated that the bicyclic pseudoribose ring constraint provided higher affinity/selectivity at human and mouse A3 AR. The mean affinity enhancement for 5 pairs of 5'-methylamides was 11-fold at hA3 AR and 42-fold at mA3 AR. Novel C2-(5-fluorothien-2-ylethynyl) substitution enhanced affinity in the methanocarba but not ribose series, with highly hA3 AR-selective 16 (MRS7334) displaying Ki 280 pM and favorable pharmacokinetics and off-target activity profile. Molecular dynamics comparison of 16 and its corresponding riboside 8 suggested a qualitative entropic advantage of 16 in hA3 AR binding. The 5-F substitution tended to increase hA3 AR affinity (cf. 5-Cl) for methanocarba but not ribose derivatives. A representative methanocarba agonist 4 was shown to interact potently exclusively with A3 AR, among 240 GPCRs and 466 kinases. Thus, despite added synthetic difficulty, the (N)-methanocarba modification has distinct advantages for A3 AR agonists, which have translational potential for chronic disease treatment.