The P2Y12 Antagonists, 2-Methylthioadenosine 5′-Monophosphate Triethylammonium Salt and Cangrelor (ARC69931MX), Can Inhibit Human Platelet Aggregation through a Gi-independent Increase in cAMP Levels

ADP plays an integral role in the process of hemostasis by signaling through two platelet G-protein-coupled receptors, P2Y(1) and P2Y(12). The recent use of antagonists against these two receptors has contributed a substantial body of data characterizing the ADP signaling pathways in human platelets. Specifically, the results have indicated that although P2Y(1) receptors are involved in the initiation of platelet aggregation, P2Y(12) receptor activation appears to account for the bulk of the ADP-mediated effects. Based on this consideration, emphasis has been placed on the development of a new class of P2Y(12) antagonists (separate from clopidogrel and ticlopidine) as an approach to the treatment of thromboembolic disorders. The present work examined the molecular mechanisms by which two of these widely used adenosine-based P2Y(12) antagonists (2-methylthioadenosine 5'-monophosphate triethylammonium salt (2MeSAMP) and ARC69931MX), inhibit human platelet activation. It was found that both of these compounds raise platelet cAMP to levels that substantially inhibit platelet aggregation. Furthermore, the results demonstrated that this elevation of cAMP did not require G(i) signaling or functional P2Y(12) receptors but was mediated through activation of a separate G protein-coupled pathway, presumably involving G(s). However, additional experiments revealed that neither 2MeSAMP nor ARC69931MX (cangrelor) increased cAMP through activation of A2a, IP, DP, or EP2 receptors, which are known to couple to G(s). Collectively, these findings indicate that 2MeSAMP and ARC69931MX interact with an unidentified platelet G protein-coupled receptor that stimulates cAMP-mediated inhibition of platelet function. This inhibition is in addition to that derived from antagonism of P2Y(12) receptors.