N6-Substituted adenosine derivatives: selectivity, efficacy, and species differences at A3 adenosine receptors

Abstract

The activation of the human A(3) adenosine receptor (AR) by a wide range of N(6)-substituted adenosine derivatives was studied in intact CHO cells stably expressing this receptor. Selectivity of binding at rat and human ARs was also determined. Among N(6)-alkyl substitutions, small N(6)-alkyl groups were associated with selectivity for human A(3)ARs vs. rat A(3)ARs, and multiple points of branching were associated with decreased hA(3)AR efficacy. N(6)-Cycloalkyl-substituted adenosines were full (</=5 carbons) or partial (>/=6 carbons) hA(3)AR agonists. N(6)-(endo-Norbornyl)adenosine 13 was the most selective for both rat and human A(1)ARs. Numerous N(6)-arylmethyl analogues, including substituted benzyl, tended to be more potent in binding to A(1) and A(3) vs. A(2A)ARs (with variable degrees of partial to full A(3)AR agonisms). A chloro substituent decreased the efficacy depending on its position on the benzyl ring. The A(3)AR affinity and efficacy of N(6)-arylethyl adenosines depended highly on stereochemistry, steric bulk, and ring constraints. Stereoselectivity of binding was demonstrated for N(6)-(R-1-phenylethyl)adenosine vs. N(6)-(S-1-phenylethyl)adenosine, as well as for the N(6)-(1-phenyl-2-pentyl)adenosine, at the rat, but not human A(3)AR. Interestingly, DPMA, a potent agonist for the A(2A)AR (K(i)=4nM), was demonstrated to be a moderately potent antagonist for the human A(3)AR (K(i)=106nM). N(6)-[(1S,2R)-2-Phenyl-1-cyclopropyl]adenosine 48 was 1100-fold more potent in binding to human (K(i)=0.63nM) than rat A(3)ARs. Dual acting A(1)/A(3) agonists (N(6)-3-chlorobenzyl- 29, N(6)-(S-1-phenylethyl)- 39, and 2-chloro-N(6)-(R-phenylisopropyl)adenosine 53) might be useful for cardioprotection.

2-Substituted adenosine derivatives: affinity and efficacy at four subtypes of human adenosine receptors

“…Similar to previous studies of the N 6 -position [8] and the ribose moiety [9,30,31], structural determinants at the 2-position of adenosine have been found to be critical for A 3 AR recognition and activation. Fig.…”

“…Radioactivity was determined in a Beckman 5500B γ-counter. The binding of [ 3 H]R-PIA to A 1 receptors and the binding of [ 3 H]CGS21680 to A 2A receptors were as previously described [8].…”

“…Binding at the A 1 , A 2A , and A 3 ARs was carried out using standard agonist radioligands [8,23] in membranes of transfected CHO cells [14]. The activation of A 1 and A 3 ARs (G icoupled) by the 2-substituted adenosine derivatives (Table 1) was examined by measuring the inhibition of forskolin-stimulated cyclic AMP accumulation in intact CHO cells stably expressing these receptors.…”

“…The intrinsic efficacy of adenosine derivatives in activation of the A 3 AR is more variable than at other subtypes [7][8][9][10]. Specific groups placed at the N 6 -position and on the ribose moiety have reduced or completely abolished the ability to activate this receptor, while maintaining high binding affinity.…”

Structural determinants of efficacy at A3 adenosine receptors: modification of the ribose moiety

“…In an effort to develop potent agonists and antagonists for A 3 ARs, a wide range of N 6 -and 2′-substituted adenosine derivatives have recently been pharmacologically characterized [8][9][10]. We found that various adenosine derivatives, which were previously demonstrated to be agonists at A 1 and A 2A ARs and assumed to be full/partial agonists at A 3 ARs, are indeed antagonists at this subtype.…”