IDENTIFICATION OF POTENT, SELECTIVE P-2Y-PURINOCEPTOR AGONISTS - STRUCTURE-ACTIVITY-RELATIONSHIPS FOR 2-THIOETHER DERIVATIVES OF ADENOSINE 5'-TRIPHOSPHATE

Study of P-2-purinoceptor subtypes has been difficult due to the lack of potent and selective ligands. With the goal of developing high affi nity P-2-purinoceptor-selective agonists, we have synthesized a series of analogues of adenine nucleotides modified on the purine ring as ch ain-extended 2-thioethers or as N-6-methyl-substituted compounds. Chem ical functionality incorporated in the thioether moiety included cyano alkyl, 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 carbacho l-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 r abbit saphenous artery and of the guinea pig vas deferens and urinary bladder. All 11 of the 2-thioethers of ATP stimulated the production o f inositol phosphates with K-0.5 values of 1.5-770 nM, with an (aminop henyl)ethyl derivative being most potent. Two adenosine diphosphate an alogues were equipotent to the corresponding ATP analogues. Adenosine monophosphate analogues were full agonists, although generally 4 order s of magnitude less potent. ATP 2-thioethers displayed pD(2) 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 com pounds between the pK(0.5) values for inositol phosphate production an d the pD(2) 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 observe d in the rabbit saphenous artery, but variable degrees of activity wer e observed in the guinea pig vas deferens and bladder depending on dis tal substituents of the thioether moiety. N-6-Methyl-ATP was inactive at P-2X-receptors, and approximately equipotent to ATP at taenia coli P-2Y-receptors. This suggested that hybrid N-6-methyl and 2-thioether ATP derivatives might be potent and selective for certain P-2Y-recepto rs, as was shown for one such derivative, N-6-methyl-2-(5-hexenylthio) -ATP.