Interactions of flavonoids and other phytochemicals with adenosine receptors

Flavone derivatives and other phytochemicals were found to bind to three subtypes of adenosine receptors in the micromolar range. Affinity was determined in radioligand binding assays at rat brain A(1) and A(2A) receptors using [H-3]-N-6-PIA ([H-3]-(R)-N-6-phenylisopropyladenosine) and [H-3]CGS21680 ([H-3]-2-[[4-(2-carboxyethyl)phenyl]ethyl respectively. Affinity was determined at cloned human and rat brain A(3) receptors using [I-125]-AB-MECA [N-6-(4-amino-3-iodobenzyl)adenosine-5'(N-methyluronamide)]. A structure-activity analysis indicated that the hydroxyl groups of naturally occurring flavones are not essential for affinity at adenosine receptors. Galangin, 14, displayed K-i values of 1 mu M at both rat A(1) and A(2A) receptors and 3 mu M at human A(3) receptors. Methylation but not acetylation of the hydroxyl groups of galangin enhanced A(3) affinity. Pentamethylmorin, 20, appeared to bind with 14-17-fold selectivity for human A(3) receptors vs rat A(1) and A(2A) receptors, with a K-i value of 2.65 mu M. Two flavone derivatives (14 and 15) showed 14-fold greater affinity at human vs rat A(3) receptors. Reduction of the 2,3-olefinic bond, as in (+/-)-dihydroquercetin, or glycosidation, as in robinin, greatly diminished affinity. An isoflavone, genistein, also bound only very weakly at 47 receptors. alpha-Naphthoflavone had greater receptor affinity (0.79 mu M at A(1) receptors) than the beta-isomer. Other natural products of plant origin, including oxogalanthine lactam, hematoxylin, and arborinine were found to bind to A(1) adenosine receptors with K-i values of 3-13 mu M. These findings indicate that the flavones, flavonols, flavanones, and other phytochemicals may provide leads for the development of novel adenosine antagonists. The unexpected finding of considerable affinity of flavones at both rat and human A(3) receptors may explain some of the previously observed biological effects of these compounds.