| Abstract
| - The human P2Y1 receptor is widely distributed in many tissues and has a classical structureof a G protein-coupled receptor. Activated by adenosine-5‘-diphosphate (ADP), this receptor isessential for platelet aggregation. In the present paper, we describe the synthesis of novelP2Y1 antagonists that could be of interest at least as tools to define the physiological roles ofthe P2Y1 receptor, at best as new antithrombotic agents. Thus, we prepared the 2,N6-dimethyl-2‘-deoxyadenosine-3‘,5‘-bisphosphate derivative, 1e. The biological activity was demonstratedby the ability of compound 1e to inhibit ADP-induced platelet aggregation, shape change, andintracellular calcium rise. This compound was a full antagonist at the P2Y1 receptor with apA2 value of 7.11 ± 0.11 and was found to be 4-fold more potent than the reference N6-methyl-2‘-deoxyadenosine-3‘,5‘-bisphosphate (1a, pA2 = 6.55 ± 0.05), revealing the potency-enhancingeffects of the 2-methyl group. The better activity of 1e as compared to 1a was analyzed usingboth potentiometric and nuclear magnetic resonance titration techniques, which highlightedspecific conformational features of this compound. These results clearly indicate the preferencefor both compounds for an anti conformation at the N−glycosyl linkage. Furthermore, thepercentage of S conformer of 1e is close to that of 1a, which is nearly 70% at pH = 2.8 andincreases dramatically when pH increases. From the macroprotonation constants, it can benoted that compound 1e is significantly more basic than 1a. This is indeed expected for theN1 adenine nitrogen due to the electron-donating character of the methyl moiety. By consideringthe microconstants of the phosphate groups, the higher basicity of P3 and P5 for 1e may bedue to the decrease in the local dielectric constant induced by the substitution of the hydrogenatom by a more lipophilic methyl group. Thus, it may be suggested that the gain in activity of1e when compared to the reference compound 1a would result from its gain in basicity ratherthan steric and conformational modifications. The synthesis of the first selective radioligandacting at the P2Y1 receptor ([33P]-N6-methyl-2‘-deoxyadenosine-3‘,5‘-bisphosphate, 17) is alsoreported and will be used in the future for efficient screening needed for drug optimization.
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