Vorapaxar is an antagonist of the
protease activated receptor-1 (PAR-1), the principal platelet
thrombin receptor. The
Thrombin Receptor Antagonist for Clinical Event Reduction (TRACER) trial evaluated
vorapaxar compared to placebo in non-ST-elevation (NSTE)-
acute coronary syndrome (ACS) patients. It was the study's objective to assess the pharmacodynamic effects of
vorapaxar versus placebo that included
aspirin or a
thienopyridine or, frequently, a combination of both agents in NSTE-ACS patients. In a substudy involving 249 patients, platelet aggregation was assessed by light transmittance aggregometry (LTA) in 85 subjects (41 placebo, 44
vorapaxar) using the agonists
thrombin receptor activating peptide (TRAP, 15 μM),
adenosine diphosphate (
ADP, 20 μM), and the combination of
collagen-related peptide (2.5 μg/ml) +
ADP (5 μM) + TRAP (15 μM) (CAT). VerifyNow® IIb/IIIa and
vasodilator-stimulated phosphoprotein (VASP) phosphorylation assays were performed, and platelet PAR-1 expression, plasma platelet/endothelial and inflammatory
biomarkers were determined before and during treatment. LTA responses to TRAP and CAT and VerifyNow results were markedly inhibited by
vorapaxar. Maximal LTA response to TRAP (median, interquartile range) 2 hours post loading dose: placebo 68% (53-75%) and
vorapaxar 3% (2-6%), p<0.0001.
ADP inhibition was greater in the
vorapaxar group at 4 hours and one month (p<0.01). In contrast to the placebo group,
PAR-1 receptor number in the
vorapaxar group at one month was significantly lower than the baseline (179 vs 225; p=0.004). There were significant changes in selected
biomarker levels between the two treatment groups. In conclusion,
vorapaxar caused a potent inhibition of PAR-1-mediated platelet aggregation. Further studies are needed to explore
vorapaxar effect on P2Y12 inhibition, PAR-1 expression and
biomarkers and its contribution to clinical outcomes.