Synthesis and anti-renal fibrosis activity of conformationally locked truncated 2-hexynyl-N(6)-substituted-(N)-methanocarba-nucleosides as A3 adenosine receptor antagonists and partial agonists.

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 A3AR binding, but only an unsubstituted 6-amino group with a C2-hexynyl group promoted high hA2AAR affinity. A small hydrophobic alkyl (4b and 4c) or N(6)-cycloalkyl group (4d) showed excellent binding affinity at the hA3AR and was better than an unsubstituted free amino group (4a). A3AR affinities of 3-halobenzylamine derivatives 4f-4i did not differ significantly, with Ki values of 7.8-16.0 nM. N(6)-Methyl derivative 4b (Ki = 4.9 nM) was a highly selective, low efficacy partial A3AR agonist. All compounds were screened for renoprotective effects in human TGF-ß1-stimulated mProx tubular cells, a kidney fibrosis model. Most compounds strongly inhibited TGF-ß1-induced collagen I upregulation, and their A3AR binding affinities were proportional to antifibrotic effects; 4b was most potent (IC50 = 0.83 µM), indicating its potential as a good therapeutic candidate for treating renal fibrosis.

Structure-activity relationships of truncated C2- or C8-substituted adenosine derivatives as dual acting A2A and A3 adenosine receptor ligands.

Truncated N(6)-substituted-4'-oxo- and 4'-thioadenosine derivatives with C2 or C8 substitution were studied as dual acting A(2A) and A(3) adenosine receptor (AR) ligands. The lithiation-mediated stannyl transfer and palladium-catalyzed cross-coupling reactions were utilized for functionalization of the C2 position of 6-chloropurine nucleosides. An unsubstituted 6-amino group and a hydrophobic C2 substituent were required for high affinity at the hA(2A)AR, but hydrophobic C8 substitution abolished binding at the hA(2A)AR. However, most of synthesized compounds displayed medium to high binding affinity at the hA(3)AR, regardless of C2 or C8 substitution, and low efficacy in a functional cAMP assay. Several compounds tended to be full hA(2A)AR agonists. C2 substitution probed geometrically through hA(2A)AR docking was important for binding in order of hexynyl > hexenyl > hexanyl. Compound 4g was the most potent ligand acting dually as hA(2A)AR agonist and hA(3)AR antagonist, which might be useful for treatment of asthma or other inflammatory diseases.

Discovery of New Human A(2A) Adenosine Receptor Agonists: Design, Synthesis, and Binding Mode of Truncated 2-Hexynyl-4'-thioadenosine.

The truncated C2- and C8-substituted-4'-thioadenosine derivatives 4a-d were synthesized from D-mannose, using palladium-catalyzed cross coupling reactions as key steps. In this study, an A(3) adenosine receptor (AR) antagonist, truncated 4'-thioadenosine derivative 3 was successfully converted into a potent A(2A)AR agonist 4a (K(i) = 7.19 ± 0.6 nM) by appending a 2-hexynyl group at the C2-position of a derivative of 3 that was N(6)-substituted. However, C8-substitution greatly reduced binding affinity at the human A(2A)AR. All synthesized compounds 4a-d maintained their affinity at the human A(3)AR, but 4a was found to be a competitive A(3)AR antagonist/A(2A)AR agonist in cyclic AMP assays. This study indicates that the truncated C2-substituted-4'-thioadenosine derivatives 4a and 4b can serve as a novel template for the development of new A(2A)AR ligands.