PAR-1 antagonists prevent platelet aggregation and adhesion without affecting thrombin time

Dr. Nadal-Wollbold, Florence , Dr. Létienne, Robert , Dr. Perez, Michel , Dr. Le Grand, Bruno. Institut de Recherche Pierre Fabre CV2, 17 Avenue Jean Moulin, 81106 Castres, France.

Background: Thrombin is an important agonist for platelet activation and mediates its cellular effects principally through protease-activated receptor type 1 (PAR1). Several studies demonstrated that only guinea-pigs and non-human primates expressed PAR-1 and several lines of evidence highlighted the potential role of PAR1 antagonists as promising targets for development of new antithrombotic drugs.

Experimental approach: platelet aggregation was assessed using a lumino-aggregometer on human and guinea pig washed platelets. Adhesion studies were carried out using static method. Thrombin time experiments were evaluated on a coagulometer.

Key results: we characterized the effects of two PAR1 antagonists, SCH 203099 and ER 121958 on both guinea pig and human platelets adhesion and aggregation as well as the fibrin generation on guinea pig platelets. SCH 203099 and ER 121958 blocked the SFLLR-induced guinea pig platelet aggregation in a concentration-dependent manner with an IC50 value of 1.74 [0.84 ; 3.95] µM and 2.74 [1.75 ; 4.16] nM respectively and human platelet aggregation with an IC50 value of 2.36 [nd ; 3.98] µM and 11.9 [10.7 ; 14.7] nM, respectively. Similarly, SCH 203099 and ER 121958 blocked SFLLR-induced guinea pig platelet adhesion in a concentration dependant manner with an IC50 value of 93 [32 ; 254] nM and 1.73 [1.121 ; 2.53] nM, respectively and human platelet adhesion with and IC50 value of 127 [nd ; 652] nM and 2.91 [2.16 ; 5.42] nM. However, SCH 203099 and ER 121958 failed to antagonise the thrombin-induced fibrin formation.

Conclusion: These data demonstrated that antithrombotic properties of PAR1 antagonists could be attributed to an action on both early and/or late processes of haemostasis (adhesion and aggregation). Moreover, these results showed that guinea-pig is a good model to study PAR1 antagonists effects, since SCH203099 and ER121958 had a similar effect on platelet adhesion in human and guinea-pigs as well as on platelet aggregation.