Mode of action of P2Y12 antagonists as inhibitors of platelet function

P2Y12 receptor antagonists are antithrombotic agents that inhibit platelet function by blocking the effects of adenosine diphosphate (ADP) at P2Y12 receptors. However, some P2Y12 receptor antagonists may affect platelet function through additional mechanisms. It was the objective of this study to investigate the possibility that P2Y12 antagonists inhibit platelet function through interaction with G-protein-coupled receptors other than P2Y12 receptors. We compared the effects of cangrelor, ticagrelor and the prasugrel active metabolite on platelet aggregation and on phosphorylation of vasodilator-stimulated phosphoprotein (VASP). We compared their effects with those of selective IP, EP4 and A2A agonists, which act at Gs-coupled receptors. All three P2Y12 antagonists were strong inhibitors of ADP-induced platelet aggregation but only partial inhibitors of aggregation induced by thrombin receptor activating peptide (TRAP) or the thromboxane A2 mimetic U46619. Further, after removing ADP and its metabolites using apyrase and adenosine deaminase, the P2Y12 antagonists produced only minor additional inhibition of TRAP or U46619-induced aggregation. Conversely, the Gs-coupled receptor agonists always produced strong inhibition of aggregation irrespective of whether ADP was removed. Other experiments using selective receptor agonists and antagonists provided no evidence of any of the P2Y12 antagonists acting through PAR1, TP, IP, EP4, A2A or EP3 receptors. All three P2Y12 antagonists enhanced VASP-phosphorylation to a small and equal extent but the effects were much smaller than those of the IP, EP4 and A2A agonists. The effects of cangrelor, ticagrelor and prasugrel on platelet function are mediated mainly through P2Y12 receptors and not through another G-protein-coupled receptor.