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 A1 and A2A receptors using [3H]-N6-PIA ([3H]-(R)-N6-phenylisopropyladenosine) and [3H]CGS21680 ([3H]-2-[[4-(2-carboxyethyl)phenyl]ethylamino]-5'- (N-ethylcarbamoyl)adenosine), respectively. Affinity was determined at cloned human and rat brain A3 receptors using [125I]-AB-MECA [N6-(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 Ki values of 1 microM at both rat A1 and A2A receptors and 3 microM at human A3 receptors. Methylation but not acetylation of the hydroxyl groups of galangin enhanced A3 affinity. Pentamethylmorin, 20, appeared to bind with 14-17-fold selectivity for human A3 receptors vs rat A1 and A2A receptors, with a Ki value of 2.65 microM. Two flavone derivatives (14 and 15) showed 14-fold greater affinity at human vs rat A3 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 A3 receptors. alpha-Naphthoflavone had greater receptor affinity (0.79 microM at A1 receptors) than the beta-isomer. Other natural products of plant origin, including oxogalanthine lactam, hematoxylin, and arborinine were found to bind to A1 adenosine receptors with Ki values of 3-13 microM. 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 A3 receptors may explain some of the previously observed biological effects of these compounds.