Drugs targeting protease-activated receptor-4 improve the anti-thrombotic therapeutic window

Discovery of Potent and Selective Quinoxaline-Based Protease-Activated Receptor 4 (PAR4) Antagonists for the Prevention of Arterial Thrombosis

PAR4 is a promising antithrombotic target with potential for separation of efficacy from bleeding risk relative to current antiplatelet therapies. In an effort to discover a novel PAR4 antagonist chemotype, a quinoxaline-based HTS hit 3 with low μM potency was identified. Optimization of the HTS hit through the use of positional SAR scanning and the design of conformationally constrained cores led to the discovery of a quinoxaline-benzothiazole series as potent and selective PAR4 antagonists. The lead compound 48, possessing a 2 nM IC50 against PAR4 activation by γ-thrombin in platelet-rich plasma (PRP) and greater than 2500-fold selectivity versus PAR1, demonstrated robust antithrombotic efficacy and minimal bleeding in the cynomolgus monkey models.

Discovery of 7, 4'-dimethoxy-3-hydroxyflavone as a protease-activated receptor 4 antagonist with antithrombotic activity and less bleeding tendency in mice

There is growing evidence of the importance of protease-activated receptor 4 (PAR4), one of thrombin receptors, as a therapeutic target in thrombotic cardiovascular diseases. In the present study, we utilized ligand-based virtual screening, bioassay, and structure-activity relationship study to discover PAR4 antagonists with new chemical scaffolds from natural origin, and examined their application as antiplatelet agents. By using these approaches, we have identified a flavonoid, 7, 4'-dimethoxy-3-hydroxyflavone, that exhibits anti-PAR4 activity. 7, 4'-Dimethoxy-3-hydroxyflavone inhibited PAR4-mediated human platelet aggregation, GPIIb/IIIa activation, and P-selectin secretion. Also, it inhibited PAR4 downstream signaling pathways, including Ca2+/protein kinase C, Akt, and MAP kinases ERK and p38, in human platelets, and suppressed PAR4-mediated β-arrestin recruitment in CHO-K1 cells exogenously expressed human PAR4. In a microfluidic system, 7, 4'-dimethoxy-3-hydroxyflavone reduced thrombus formation on collagen-coated chambers at an arterial shear rate in recalcified whole blood. Furthermore, mice treated with 7, 4'-dimethoxy-3-hydroxyflavone were significantly protected from FeCl3-induced carotid arterial occlusions, without significantly affecting tail bleeding time. In conclusion, 7, 4'-dimethoxy-3-hydroxyflavone represents a new class of nature-based PAR4 antagonist, it shows effective in vivo antithrombotic properties with less bleeding tendency, and could be a potential candidate for developing new antiplatelet agents.

Protease activated receptor 4 (PAR4) antagonists: Research progress on small molecules in the field of antiplatelet agents

Protease activated receptor 4 (PAR4) is a key target in antiplatelet medication to reduce the risk of heart attack and thrombotic complications in stroke. PAR4 antagonists can prevent harmful and stable thrombus growth while retaining initial thrombus formation by acting on the late diffusion stage of platelet activation, which may provide a safer alternative than other antiplatelet agents. Currently, research on PAR4 antagonists is of increasing interest in the field of antiplatelet agents. This article provides an overview of the discovery and development of small-molecule antagonists of PAR4 as novel antiplatelet agents, including structure-activity relationship (SAR) analysis, progress of structure and bioassay optimization, and the latest structural and/or clinical information of representative small-molecule antagonists of PAR4.

Analysis of the F2LR3 (PAR4) Single Nucleotide Polymorphism (rs773902) in an Indigenous Australian Population

The F2RL3 gene encoding protease activated receptor 4 (PAR4) contains a single nucleotide variant, rs773902, that is functional. The resulting PAR4 variants, Thr120, and Ala120, are known to differently affect platelet reactivity to thrombin. Significant population differences in the frequency of the allele indicate it may be an important determinant in the ethnic differences that exist in thrombosis and hemostasis, and for patient outcomes to PAR antagonist anti-platelet therapies. Here we determined the frequency of rs773902 in an Indigenous Australian group comprising 467 individuals from the Tiwi Islands. These people experience high rates of renal disease that may be related to platelet and PAR4 function and are potential recipients of PAR-antagonist treatments. The rs773902 minor allele frequency (Thr120) in the Tiwi Islanders was 0.32, which is similar to European and Asian groups and substantially lower than Melanesians and some African groups. Logistic regression and allele distortion testing revealed no significant associations between the variant and several markers of renal function, as well as blood glucose and blood pressure. These findings suggest that rs773902 is not an important determinant for renal disease in this Indigenous Australian group. However, the relationships between rs773902 genotype and platelet and drug responsiveness in the Tiwi, and the allele frequency in other Indigenous Australian groups should be evaluated.

Using PAR4 Inhibition as an Anti-Thrombotic Approach: Why, How, and When?

Protease-activated receptors (PARs) are a family of four GPCRs with a variety of cellular functions, yet the only advanced clinical endeavours to target these receptors for therapeutic gain to date relates to the impairment of platelet function for anti-thrombotic therapy. The only approved PAR antagonist is the PAR1 inhibitor, vorapaxar—the sole anti-platelet drug against a new target approved in the past 20 years. However, there are two PARs on human platelets, PAR1 and PAR4, and more recent efforts have focused on the development of the first PAR4 antagonists, with first-in-class agents recently beginning clinical trial. Here, we review the rationale for this approach, outline the various modes of PAR4 inhibition, and speculate on the specific therapeutic potential of targeting PAR4 for the prevention of thrombotic conditions.