Synthesis and Anti-Renal Fibrosis Activity of Conformationally Locked Truncated 2-Hexynyl-N(6)-Substituted-(N)-Methanocarba-nucleosides as A(3) Adenosine Receptor Antagonists and Partial Agonists

[Image: see text] Truncated N(6)-substituted-(N)-methanocarba-adenosine derivatives with 2-hexynyl substitution were synthesized to examine parallels with corresponding 4′-thioadenosines. Hydrophobic N(6) and/or C2 substituents were tolerated in A(3)AR binding, but only an unsubstituted 6-amino group with a C2-hexynyl group promoted high hA(2A)AR affinity. A small hydrophobic alkyl (4b and 4c) or N(6)-cycloalkyl group (4d) showed excellent binding affinity at the hA(3)AR and was better than an unsubstituted free amino group (4a). A(3)AR affinities of 3-halobenzylamine derivatives 4f–4i did not differ significantly, with K(i) values of 7.8–16.0 nM. N(6)-Methyl derivative 4b (K(i) = 4.9 nM) was a highly selective, low efficacy partial A(3)AR agonist. All compounds were screened for renoprotective effects in human TGF-β1-stimulated mProx tubular cells, a kidney fibrosis model. Most compounds strongly inhibited TGF-β1-induced collagen I upregulation, and their A(3)AR binding affinities were proportional to antifibrotic effects; 4b was most potent (IC(50) = 0.83 μM), indicating its potential as a good therapeutic candidate for treating renal fibrosis.