Identification of Potent, Selective P_2Y-Purinoceptor Agonists: Structure-Activity Relationships for 2-Thioether Derivatives of Adenosine 5′-Triphosphate

Study of P2-purinoceptor subtypes has been difficult due to the lack of potent and selective ligands. With the goal of developing high affinity P₂-purinoceptor-selective agonists, we have synthesized a series of analogues of adenine nucleotides modified on the purine ring as chain-extended 2-thioethers or as N⁶-methyl-substituted compounds. Chemical functionality incorporated in the thioether moiety included cyanoalkyl, nitroaromatic, amino, thiol, cycloalkyl, n-alkyl, and olefinic groups. Apparent affinity of the compounds for P_2Y-purinoceptors was established by measurement of P_2Y-purinoceptor-promoted phospholipase C activity in turkey erythrocyte membranes and relaxation of carbachol-contracted smooth muscle in three different preparations (guinea pig taenia coli, rabbit aorta, and rabbit mesenteric artery). Activity at P_2x-purinoceptors was established by measurement of contraction of rabbit saphenous artery and of the guinea pig vas deferens and urinary bladder. All 11 of the 2-thioethers of ATP stimulated the production of inositol phosphates with K_0.5 values of 1.5–770 nM, with an (aminophenyl)ethyl derivative being most potent. Two adenosine diphosphate analogues were equipotent to the corresponding ATP analogues. Adenosine monophosphate analogues were full agonists, although generally 4 orders of magnitude less potent. ATP 2-thioethers displayed pD2 values in the range of 6–8 in smooth muscle assay systems for activity at P_2Y-receptors. There was a significant correlation for the 2-thioether compounds between the pK_0.5 values for inositol phosphate production and the pD₂ values for relaxation mediated via the P_2Y-purinoceptors in the guinea pig taenia coli, but not for the vascular P_2Y-receptors or for the P_2x-receptors. At P_2x-receptors, no activity was observed in the rabbit saphenous artery, but variable degrees of activity were observed in the guinea pig vas deferens and bladder depending on distal substituents of the thioether moiety. N⁶-Methyl-ATP was inactive at P_2x-receptors, and approximately equipotent to ATP at taenia coli P_2Y-receptors. This suggested that hybrid N⁶-methyl and 2-thioether ATP derivatives might be potent and selective for certain P_2Y-receptors, as was shown for one such derivative, N⁶-methyl-2-(5-hexenylthio)-ATP.