PAR1 antagonists inhibit thrombin-induced platelet activation whilst leaving the PAR4-mediated response intact

Abstract

Thrombin-induced platelet activation is initiated by PAR1 and PAR4 receptors. Vorapaxar, a PAR1 antagonist, has been assessed in patients with acute coronary syndromes (ACS) and stable atherosclerotic disease in addition to standard-of-care treatment. In clinical trials, vorapaxar has been observed to reduce the frequency of ischaemic events in some subgroups though in others has increased the frequency of bleeding events. Among patients undergoing CABG surgery, which is associated with excess thrombin generation, bleeding was not increased. The aim of these studies was to investigate the effects of selective PAR1 antagonism on thrombin-induced platelet activation in patients receiving vorapaxar or placebo in the TRACER trial and to explore the roles of PAR1 and PAR4 in thrombin-induced platelet activation in healthy volunteers. ACS patients receiving vorapaxar or placebo in the TRACER trial were studied at baseline and 4 hours, 1 and 4 months during drug administration. Thrombin-induced calcium mobilisation in platelet-rich plasma was assessed by flow cytometry. In vitro studies were performed in healthy volunteers using the PAR1 antagonist SCH79797 or PAR4 receptor desensitisation. Vorapaxar treatment significantly inhibited thrombin-induced calcium mobilisation, leaving a residual, delayed response. These findings were consistent with calcium mobilisation mediated via the PAR4 receptor and were reproduced in vitro using SCH79797. PAR4 receptor desensitization, in combination with SCH79797, completely inhibited thrombin-induced calcium mobilisation confirming that the residual calcium mobilisation was mediated via PAR4. In conclusion vorapaxar selectively antagonises the PAR1-mediated component of thrombin-induced platelet activation, leaving the PAR4-mediated response intact, which may explain why vorapaxar is well tolerated in patients undergoing CABG surgery since higher thrombin levels in this setting may override the effects of PAR1 antagonism through PAR4 activation, thus preserving haemostasis. Further assessment may be warranted.

• Acute coronary syndromes
• platelet aggregation inhibitors
• protease-activated receptors
• thrombin
• vorapaxar

Declaration of interest

The ex vivo studies were funded by Merck & Co. Inc. NJ, USA and this funding was administered by CirQuest Labs. In vitro studies were funded by the University of Sheffield.

RF Storey reports receiving research grants from AstraZeneca, Eli Lilly/Daiichi Sankyo, and Merck; research support from Accumetrics; honoraria from AstraZeneca, Eli Lilly/Daiichi Sankyo, Merck, Novartis, Iroko, Sanofi-Aventis, BMS, Accumetrics, and Medscape; consultancy fees from AstraZeneca, Daiichi Sankyo, Merck, Novartis, Accumetrics, Regeneron and Roche. Dr. Moliterno declares past consultancy and research funding from Merck-Schering-Plough. Dr. Kotha declares institutional contracts from Merck, Portola and Johnson and Johnson. LK Jennings reports receiving research grants from AstraZeneca and Merck; consultancy fees from Portola. The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this article.