Exploring distal regions of the A3 adenosine receptor binding site: sterically constrained N6-(2-phenylethyl)adenosine derivatives as potent ligands

Abstract

We synthesized phenyl ring-substituted analogues of N 6 -(1S,2R)-(2-phenyl-1-cyclopropyl)adenosine, which is highly potent in binding to the human A 3 AR with a K i value of 0.63 nM. The effects of these structural changes on affinity at human and rat adenosine receptors and on intrinsic efficacy at the hA 3 AR were measured. A 3-nitrophenyl analogue was resolved chromatographically into pure diastereomers, which displayed 10-fold stereoselectivity in A 3 AR binding in favor of the 1S,2R isomer. A molecular model defined a hydrophobic region (Phe168) in the putative A 3 AR binding site around the phenyl moiety. A heteroaromatic group (3-thienyl) could substitute for the phenyl moiety with retention of high affinity of A 3 AR binding. Other related N 6 -substituted adenosine derivatives were included for comparison. Although the N 6 -(2-phenyl-1-cyclopropyl) derivatives were full A 3 AR agonists, several other derivatives had greatly reduced efficacy. N 6 -Cyclopropyladenosine was an A 3 AR antagonist, and adding either one or two phenyl rings at the 2-position of the cyclopropyl moiety restored efficacy. N 6 -(2,2-Diphenylethyl)adenosine was an A 3 AR antagonist, and either adding a bond between the two phenyl rings (N 6 -9-fluorenylmethyl) or shortening the ethyl moiety (N 6 -diphenylmethyl) restored efficacy. A QSAR study of the N 6 region provided a model that was complementary to the putative A 3 AR binding site in a rhodopsin-based homology model. Thus, a new series of highaffinity A 3 AR agonists and related nucleoside antagonists was explored through both empirical and theoretical approaches.

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

“…Several of the nucleosides studied here were found to have K i values at the A 3 AR of approximately 50 nM and compared favorably in selectivity to other analogues of adenosine in which only single sites have been modified [11][12][13]21]. For example, compound 41 was >100-fold selective for the A 3 AR in comparison to both the A 1 and A 2B ARs.…”

“…In the absence of a physically-determined structure for the A 3 AR, the use of molecular modeling [12,13,34] will be necessary to understand the structural basis for the loss of efficacy associated with certain 2-ether groups, such as benzyl, 2,2-diphenylethyl, and S-2-phenylbutyl. There were parallels between the effects on A 3 AR binding and activation of the same substitution at either the 2-position (in the form of an ether) or at the N 6 -position (in the form of a secondary arylamine).…”

“…There were parallels between the effects on A 3 AR binding and activation of the same substitution at either the 2-position (in the form of an ether) or at the N 6 -position (in the form of a secondary arylamine). For example, 2-(2,2-diphenylethyloxy)-adenosine and N 6 -(2,2-diphenylethyl)adenosine were both antagonists of the A 3 AR [12].…”

“…Thus, it has been possible to design nucleoside-based antagonists [11], which in many cases are selective for both the human and rat A 3 ARs. Such A 3 AR antagonists include adenosine derivatives bearing: steric constraints of the ribose moiety or its 5′-amide modification [11], N 6 -groups such as 2,2-diphenylethyl and cyclopropyl [12], and the combination of substituted N 6 -benzyl groups and various small substituents at the adenine 2-position (such as chloro, cyano, and methoxycarbonyl) [11,13]. A radically altered analogue in which the properly functionalized adenine moiety was shifted from the 1′-position to the 4′-carbon proved to be an A 3 AR selective antagonist [9].…”

Receptores A3 da adenosina: uma nova abordagem terapêutica no câncer

“…23 Esta variabilidade é considerada de particular importância para os estudos de relação estrutura-atividade (REA).…”