Structural determinants of efficacy at A 3 adenosine receptors: modification of the ribose moiety

Abstract

We have found previously that structural features of adenosine derivatives, particularly at the N 6- and 2-positions of adenine, determine the intrinsic efficacy as A 3 adenosine receptor (AR) agonists. Here, we have probed this phenomenon with respect to the ribose moiety using a series of ribose-modified adenosine derivatives, examining binding affinity and activation of the human A 3 AR expressed in CHO cells. Both 2′- and 3′-hydroxyl groups in the ribose moiety contribute to A 3 AR binding and activation, with 2′-OH being more essential. Thus, the 2′-fluoro substitution eliminated both binding and activation, while a 3′-fluoro substitution led to only a partial reduction of potency and efficacy at the A 3 AR. A 5′-uronamide group, known to restore full efficacy in other derivatives, failed to fully overcome the diminished efficacy of 3′-fluoro derivatives. The 4′-thio substitution, which generally enhanced A 3 AR potency and selectivity, resulted in 5′-CH 2OH analogues ( 10 and 12) which were partial agonists of the A 3 AR. Interestingly, the shifting of the N 6-(3-iodobenzyl)adenine moiety from the 1′- to 4′-position had a minor influence on A 3 AR selectivity, but transformed 15 into a potent antagonist ( 16) ( K i =4.3 nM). Compound 16 antagonized human A 3 AR agonist-induced inhibition of cyclic AMP with a K B value of 3.0 nM. A novel apio analogue ( 20) of neplanocin A, was a full A 3 AR agonist. The affinities of selected, novel analogues at rat ARs were examined, revealing species differences. In summary, critical structural determinants for human A 3 AR activation have been identified, which should prove useful for further understanding the mechanism of receptor activation and development of more potent and selective full agonists, partial agonists and antagonists for A 3 ARs.