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

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 e-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 (less than or equal to5 carbons) or partial (greater than or equal to6 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 I and A3 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 A3AR 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 N6-(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 = 4 nM), was demonstrated to be a moderately potent antagonist for the human A(3)AR (K-i = 106 nM). N-6-[(1S,2R)-2-Phenyl-1-cyclopropyl]adenosine 48 was 1100-fold more potent in binding to human (K-i = 0.63 nM) 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. (C) 2003 Elsevier Science Inc. All rights reserved.