The Discovery of Vorapaxar (SCH 530348), a Thrombin Receptor (Protease Activated Receptor-1) Antagonist with Potent Antiplatelet Effects - Accounts in Drug Discovery Case Studies in Medicinal Chemistry - page 31

LDPR SFLLRNP

LDPR

NRLLFS

Signal

Figure 2.2 The tethered ligand mechanism of thrombin receptor activation. Throm-
bin binds to the extracellular domain of the receptor and cleaves it at
Arg41-Ser42. The newly generated serine N-terminus binds intramolecu-
larly to the second loop of the receptor, triggering cellular activation.

2.4.2 Early Preclinical Evidence Suggesting the Antiplatelet
Effect of a Thrombin Receptor Antagonist with Improved
Safety Margin with Regard to Bleeding
A number of early preclinical experiments pointed to the fact that PAR-1
antagonism could indeed engender strong antithrombotic effects without the
attendant bleeding that is common to most anticoagulants and antiplatelet
agents. Early PAR-1 antagonists were peptides and peptidomimetics modeled
after the tethered ligand of PAR-1. Several of these compounds showed potent
PAR-1 anity in radioligand binding assays against PAR-1 isolated from
platelet membrane. In platelet aggregation functional assays, these compounds
exhibited potent inhibition of aggregation caused by either TRAP or a-
thrombin. 51,52 These compounds also blocked thrombin-induced calcium
mobilization and smooth muscle cell proliferation mediated by the activa-
tion of PAR-1. Two salient examples from these early studies are illustrated in
Figure 2.3. 53,54
Due to the absence of PAR-1 in mouse and rat platelets, non-human primate
models have been often used to test the antiplatelet effect of PAR-1 antagon-
ism. In vivo studies carried out by PAR-1 antagonists of peptide and non-
peptide origin have shown inhibition of platelet-rich thrombus formation in
both baboon and guinea pig thrombosis models, suggesting the promise of a
PAR-1 antagonist for inhibiting arterial thrombosis. 55,56 In another important
study, an antibody to the PAR-1 N-terminus was reported to inhibit
mechanical injury-induced thrombosis in a baboon carotid artery model
without affecting bleeding time and coagulation parameters. 57 The peptido-
mimetic PAR-1 antagonist 2 (RWJ-58259) was tested in a cynomolgus monkey