Methanocarba analogues of purine nucleosides as potent and selective adenosine receptor agonists

Adenosine receptor agonists have cardioprotective, cerebroprotective, and antiinflammatory properties. We report that a carbocyclic modification of the ribose moiety incorporating ring constraints is a general approach for the design of A(1) and A(3) receptor agonists having favorable pharmacodynamic properties. While simple carbocyclic substitution of adenosine agonists greatly diminishes potency, methanocarba-adenosine analogues have now defined the role of sugar puckering in stabilizing the active adenosine receptor-bound conformation and thereby have allowed identification of a favored isomer. In such analogues a fused cyclopropane moiety constrains the pseudosugar ring of the nucleoside to either a Northern (N) or Southern (S) conformation, as defined in the pseudorotational cycle. In binding assays at A(1), A(2A), and A(3) receptors, (N)-methanocarba-adenosine was of higher affinity than the (S)-analogue, particularly at the human A(3) receptor (N/S affinity ratio of 150). (N)-Methanocarba analogues of various N-6-substituted adenosine derivatives, including cyclopentyl and 3-iodobenzyl, in which the parent compounds are potent agonists at either A(1) or A(3) receptors, respectively, were synthesized. The N-6-cyclopentyl derivatives were A(1) receptor-selective and maintained high efficacy at recombinant human but not rat brain A(1) receptors, as indicated by stimulation of binding of [S-35] GTP-gamma-S. The (N)-methanocarba-N-6-(3-iodobenzyl)adenosine and its 2-chloro derivative had K-i values of 4.1 and 2.2 nM at A(3) receptors, respectively, and were highly selective partial agonists. Partial agonism combined with high functional potency at A(3) receptors (EC50 < 1 nM) may produce tissue selectivity. In conclusion, as for P2Y(1) receptors, at least three adenosine receptors favor the ribose (N)-conformation.