Pharmacodynamic profile of antiplatelet agents: marked differences between single versus costimulation with platelet...

In pharmacodynamic studies with antiplatelet agents, platelets are usually activated in vitro with single agonists (e.g., ADP) solely. We questioned whether differences occur between single and combined stimulation of platelets [involving the major thrombin-receptors, protease-activated receptors (PAR)1 and PAR4], and whether the pharmacodynamic response to common antiplatelet drugs vary when a combined stimulus is applied instead of a single agonist.

We investigated the influence of different antiplatelet agents (aspirin [500 mg]) in vivo, the P2Y12-antagonist AR-C 69931MX (4 nM) and the GPII/IIIa-antagonist (abciximab ([5 microg/ml] in vitro) on the degranulation response (CD62) and expression of the activated GPIIb/IIIa-receptor (PAC-1) after stimulation with ADP (2 microM), collagen (4 microg/ml), a PAR1-activating peptide (3 microM TRAP) and a PAR4-activating peptide (200 microM AYPGKF) alone or in a combination of each two agonists by flow cytometry in healthy subjects.

(1) Combined activation of TRAP with AYPGKF resulted in synergistic CD62 and PAC-1 expression. Only AYPGKF but neither TRAP nor ADP acted synergistically with collagen. (2) AR-C 69931MX inhibited platelet degranulation (CD62) in all inducer combinations with ADP or the combination TRAP with AYPGKF. The effect was considerably smaller or absent for the combination of collagen with a second inducer. (3) Aspirin intake reduced platelet degranulation and PAC-1 expression only for AYPGKF costimulation with collagen.

Because a variety of different agonists influence platelet activation and its distinct functions at a time, investigations which regard the concert of these agonists might be closer to the in vivo situation and better reflect the pharmacodynamic profile of an antiplatelet agent than using one single inducing agent.