Blocking protease-activated receptor 4 alleviates liver injury induced by brain death.

Brain death (BD) induces a systemic inflammatory response that influences donor liver quality. Protease-activated receptor 4 (PAR4) is a thrombin receptor that mediates platelet activation and is involved in inflammatory and apoptotic processes. Therefore, we investigated the role of PAR4 blockade in liver injury induced by BD and its associated mechanisms. In this study, we constructed a BD rat model and treated rats with TcY-NH2, a selective PAR4 antagonist, to block PAR4 signaling at the onset of BD induction. Our results revealed that PAR4 protein expression increased in the livers of rats with BD. PAR4 blockade alleviated liver injury induced by BD, as indicated by lower serum ALT/AST levels and an improvement in histomorphology. Blood platelet activation and hepatic platelet accumulation in BD rats were reduced by PAR4 blockade. Additionally, PAR4 blockade attenuated the inflammatory response and apoptosis in the livers of BD rats. Moreover, the activation of NF-κB and MAPK pathways induced by BD was inhibited by PAR4 blockade. Thus, our results suggest that PAR4 contributes to liver injury induced by BD by regulating inflammation and apoptosis through the NF-κB and MAPK pathways. Thus, PAR4 blockade may provide a feasible approach to improve the quality of organs from BD donors.

Synthesis and evaluation of novel and potent protease activated receptor 4 (PAR4) antagonists based on a quinazolin-4(3H)-one scaffold.

Protease activated receptor 4 (PAR4) is an important target in antiplatelet therapy 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 aggregation, and may provide a safer alternative to other antiplatelet agents. To date, only two PAR4 antagonists, BMS-986120 and BMS-986141 have entered clinical trials for thrombosis. Thus, the development of a potent and selective PAR4 antagonist with a novel chemotype is highly desirable. In this study, we explored the activity of quinazolin-4(3H)-one-based PAR4 antagonists, beginning with their IDT analogues. By repeated structural optimisation, we developed a series of highly selective PAR4 antagonists with nanomolar potency on human platelets. Of these, 13 and 30g, with an 8-benzo[d]thiazol-2-yl-substituted quinazolin-4(3H)-one structure, showed optimal activity (h. PAR4-AP PRP IC50 = 19.6 nM and 6.59 nM, respectively) on human platelets. Furthermore, 13 and 30g showed excellent selectivity for PAR4 versus PAR1 and other receptors (IC50s > 10 µM) on human platelets. And 13 and 30g were lack of cross-reactivity for PAR1 or PAR2 (PAR1 AP FLIPR IC50 > 3162 nM, PAR2 AP FLIPR IC50 > 1000 nM) in the calcium mobilization assays. Metabolic stability assays and cytotoxicity tests of 13 and 30g indicated that these compounds could sever as promising drug candidates for the development of novel PAR4 antagonists. In summary, the quinazolin-4(3H)-one-based analogues are the first reported chemotypes with excellent activity and selectivity against PAR4, and, in the current study, we expanded the structural diversity of PAR4 antagonists. The two compounds, 13 and 30g, found in our study could be promising starting points with great potential for further research in antiplatelet therapy.

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.

Antithrombotic Effects of Combined PAR (Protease-Activated Receptor)-4 Antagonism and Factor Xa Inhibition.

OBJECTIVE: PAR (protease-activated receptor)-4 antagonism has antiplatelet effects under conditions of high shear stress. We aimed to establish whether PAR4 antagonism had additive antithrombotic activity in the presence of factor Xa inhibition in an ex vivo model of acute arterial injury. Approach and Results: Fifteen healthy volunteers (29±6 years, 7 women) completed a phase zero double-blind randomized controlled crossover trial. Ex vivo platelet activation, platelet aggregation, and thrombus formation were measured following blood perfusion of low shear and high shear stress chambers. Upstream of the chambers, extracorporeal blood was admixed with (1) vehicle, (2) low-dose apixaban (20 ng/mL), (3) high-dose apixaban (80 ng/mL), (4) BMS-986141 (400 ng/mL), (5) BMS-968141 and low-dose apixaban, or (6) BMS-968141 and high-dose apixaban in 6 sequential studies performed in random order. Compared with vehicle, BMS-986141 demonstrated selective inhibition of PAR4-AP (agonist peptide)-stimulated platelet aggregation, platelet-monocyte aggregates, and P-selectin expression (P≤0.01 for all). Total thrombus area was reduced under both low shear and high shear stress conditions for all drug infusions (P<0.0001 for all versus vehicle). BMS-968141 reduced total (≤44.4%) and platelet-rich (≤39.3%) thrombus area, whereas apixaban reduced total (≤42.9%) and fibrin-rich (≤31.6%) thrombus area. Combination of BMS-986141 with apixaban caused a further modest reduction in total thrombus area (9.6%-12.4%), especially under conditions of high shear stress (P≤0.027). CONCLUSIONS: In the presence of factor Xa inhibition, PAR4 antagonism with BMS-986141 further reduces thrombus formation, especially under conditions of high shear stress. This suggests the potential for additive efficacy of combination PAR4 antagonism and factor Xa inhibition in the prevention of atherothrombotic events.

Proteinase-activated receptor 1: A target for repurposing in the treatment of COVID-19?

In the search to rapidly identify effective therapies that will mitigate the morbidity and mortality of COVID-19, attention has been directed towards the repurposing of existing drugs. Candidates for repurposing include drugs that target COVID-19 pathobiology, including agents that alter angiotensin signalling. Recent data indicate that key findings in COVID-19 patients include thrombosis and endotheliitis. Activation of proteinase-activated receptor 1 (PAR1), in particular by the serine protease thrombin, is a critical element in platelet aggregation and coagulation. PAR1 activation also impacts on the actions of other cell types involved in COVID-19 pathobiology, including endothelial cells, fibroblasts and pulmonary alveolar epithelial cells. Vorapaxar is an approved inhibitor of PAR1, used for treatment of patients with myocardial infarction or peripheral arterial disease. We discuss evidence for a possible beneficial role for vorapaxar in the treatment of COVID-19 patients and other as-yet non-approved antagonists of PAR1 and proteinase-activated receptor 4 (PAR4). LINKED ARTICLES: This article is part of a themed issue on The Pharmacology of COVID-19. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.21/issuetoc.

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.

Discovery of Potent Protease-Activated Receptor 4 Antagonists with in Vivo Antithrombotic Efficacy.

In an effort to identify novel antithrombotics, we have investigated protease-activated receptor 4 (PAR4) antagonism by developing and evaluating a tool compound, UDM-001651, in a monkey thrombosis model. Beginning with a high-throughput screening hit, we identified an imidazothiadiazole-based PAR4 antagonist chemotype. Detailed structure-activity relationship studies enabled optimization to a potent, selective, and orally bioavailable PAR4 antagonist, UDM-001651. UDM-001651 was evaluated in a monkey thrombosis model and shown to have robust antithrombotic efficacy and no prolongation of kidney bleeding time. This combination of excellent efficacy and safety margin strongly validates PAR4 antagonism as a promising antithrombotic mechanism.

Selective Inhibition of PAR4 (Protease-Activated Receptor 4)-Mediated Platelet Activation by a Synthetic Nonanticoagulant Heparin Analog.

Objective- PAR4 (protease-activated receptor 4), one of the thrombin receptors in human platelets, has emerged as a promising target for the treatment of arterial thrombotic disease. Previous studies implied that thrombin exosite II, known as a binding site for heparin, may be involved in thrombin-induced PAR4 activation. In the present study, a heparin octasaccharide analog containing the thrombin exosite II-binding domain of heparin was chemically synthesized and investigated for anti-PAR4 effect. Approach and Results- PAR4-mediated platelet aggregation was examined using either thrombin in the presence of a PAR1 antagonist or γ-thrombin, which selectively activates PAR4. SCH-28 specifically inhibits PAR4-mediated platelet aggregation, as well as the signaling events downstream of PAR4 in response to thrombin. Moreover, SCH-28 prevents thrombin-induced ß-arrestin recruitment to PAR4 but not PAR1 in Chinese Hamster Ovary-K1 cells using a commercial enzymatic complementation assay. Compared with heparin, SCH-28 is more potent in inhibiting PAR4-mediated platelet aggregation but has no significant anticoagulant activity. In an in vitro thrombosis model, SCH-28 reduces thrombus formation under whole blood arterial flow conditions. Conclusions- SCH-28, a synthetic small-molecular and nonanticoagulant heparin analog, inhibits thrombin-induced PAR4 activation by interfering with thrombin exosite II, a mechanism of action distinct from other PAR4 inhibitors that target the receptor. The characteristics of SCH-28 provide a new strategy for targeting PAR4 with the potential for the treatment of arterial thrombosis.

Thrombin and the Protease-Activated Receptor-1 in Organophosphate-Induced Status Epilepticus.

Organophosphates (OP) are a major threat to the health of soldiers and civilians due to their use as chemical weapons in war and in terror attacks. Among the acute manifestations of OP poisoning, status epilepticus (SE) is bearing the highest potential for long-term damages. Current therapies do not prevent brain damage and seizure-related brain injuries in OP-exposed humans. Thrombin is a serine protease known to have a fundamental function in the clotting cascade. It is highly expressed in the brain where we have previously found that it regulates synaptic transmission and plasticity. In addition, we have found that an excess of thrombin in the brain leads to hyperexcitability and therefore seizures through a glutamate-dependent mechanism. In the current study, we carried out in vitro, ex vivo, and in vivo experiments in order to determine the role of thrombin and its receptor PAR-1 in paraoxon-induced SE. Elevated thrombin activity was found in the brain slices from mice that were treated (in vitro and in vivo) with paraoxon. Increased levels of PAR-1 and pERK proteins and decreased prothrombin mRNA were found in the brains of paraoxon-treated mice. Furthermore, ex vivo and in vivo electrophysiological experiments showed that exposure to paraoxon causes elevated electrical activity in CA1 and CA3 regions of the hippocampus. Moreover, a specific PAR-1 antagonist (SCH79797) reduced this activity. Altogether, these results reveal the importance of thrombin and PAR-1 in paraoxon poisoning. In addition, the results indicate that thrombin and PAR-1 may be a possible target for the treatment of paraoxon-induced status epilepticus.

Synthesis and biological evaluation of BMS-986120 and its deuterated derivatives as PAR4 antagonists.

BMS-986120 is a PAR4 antagonist that is being investigated as an antiplatelet agent in phase I clinical trial. An improved synthesis of BMS-986120 has been developed. Based on the novel synthetic approach to BMS-986120, a series of deuterated derivatives of BMS-986120 have been synthesized and biologically evaluated to search for more potent antiplatelet agents. The in vitro antiplatelet assay by turbidimetry demonstrated that PC-2 and PC-6 had IC50 values of 6.30 nM and 6.97 nM, respectively, versus BMS-986120 with an IC50 of 7.80 nM. The result of in vitro metabolic stability study showed that all of the deuterated compounds had similar half-life (T1/2) and intrinsic clearance (Clint) in comparison with BMS-986120. Further probing the metabolic profile of BMS-986120 is worth being conducted.

A function-blocking PAR4 antibody is markedly antithrombotic in the face of a hyperreactive PAR4 variant.

Thrombin activates human platelets via 2 protease-activated receptors (PARs), PAR1 and PAR4, both of which are antithrombotic drug targets: a PAR1 inhibitor is approved for clinical use, and a PAR4 inhibitor is in trial. However, a common sequence variant in human PAR4 (rs773902, encoding Thr120 in place of Ala120) renders the receptor more sensitive to agonists and less sensitive to antagonists. Here, we develop the first human monoclonal function-blocking antibody to human PAR4 and show it provides equivalent efficacy against the Ala120 and Thr120 PAR4 variants. This candidate was generated from a panel of anti-PAR4 antibodies, was found to bind PAR4 with affinity (KD ≈ 0.4 nM) and selectivity (no detectable binding to any of PAR1, PAR2, or PAR3), and is capable of near-complete inhibition of thrombin cleavage of either the Ala120 or Thr120 PAR4 variant. Platelets from individuals expressing the Thr120 PAR4 variant exhibit increased thrombin-induced aggregation and phosphatidylserine exposure vs those with the Ala120 PAR4 variant, yet the PAR4 antibody inhibited these responses equivalently (50% inhibitory concentration, 4.3 vs 3.2 µg/mL against Ala120 and Thr120, respectively). Further, the antibody significantly impairs platelet procoagulant activity in an ex vivo thrombosis assay, with equivalent inhibition of fibrin formation and overall thrombus size in blood from individuals expressing the Ala120 or Thr120 PAR4 variant. These findings reveal antibody-mediated inhibition of PAR4 cleavage and activation provides robust antithrombotic activity independent of the rs773902 PAR4 sequence variant and provides rationale for such an approach for antithrombotic therapy targeting this receptor.

Clinical features and outcomes of patients with type 2 myocardial infarction: Insights from the Thrombin Receptor Antagonist for Clinical Event Reduction in Acute Coronary Syndrome (TRACER) trial.

BACKGROUND: Type 2 myocardial infarction (MI) is characterized by an imbalance between myocardial blood supply and demand, leading to myocardial ischemia without coronary plaque rupture, but its diagnosis is challenging. METHODS: In the TRACER trial, patients with non-ST-segment elevation acute coronary syndromes were included. We aimed to describe provoking factors, cardiac biomarker profiles, treatment patterns, and clinical outcomes of patients with type 2 MIs. MI events during trial follow-up were adjudicated by an independent clinical events classification committee (CEC) and were classified according to the Third Universal Definition of MI. Using available source documents retrieved as part of the CEC process, we performed a retrospective chart abstraction to collect details on the type 2 MIs. Cox regression models were used to explore the association between MI type (type 1 or type 2) and cardiovascular death. RESULTS: Overall, 10.3% (n=1327) of TRACER participants had a total of 1579 adjudicated MIs during a median follow-up of 502 days (25th and 75th percentiles [IQR] 349-667). Of all MIs, 5.2% (n=82) were CEC-adjudicated type 2 MIs, occurring in 76 patients. The incidence of type 2 MI was higher in the first month following randomization, after which the distribution became more scattered. The most frequent potential provoking factors for type 2 MIs were tachyarrhythmias (38.2%), anemia/bleeding (21.1%), hypotension/shock (14.5%), and hypertensive emergencies (11.8%). Overall, 36.3% had a troponin increase >10× the upper limit of normal. Coronary angiography was performed in 22.4% (n=17) of patients during hospitalizations due to type 2 MIs. The hazard of cardiovascular death was numerically higher following type 2 MI (vs. no MI, adj. HR 11.82, 95% CI 5.71-24.46; P<.0001) than that of type 1 MI (vs. no MI, adj. HR 8.90, 95% CI 6.93-11.43; P<.0001). CONCLUSIONS: Type 2 MIs were more prevalent in the first month after ACS, were characterized by the presence of triggers and infrequent use of an invasive strategy, and were associated with a high risk of death. Further efforts are needed to better define the role and implications of type 2 MI in both clinical practice and research.

Protease-Activated Receptor 4 (PAR4): A Promising Target for Antiplatelet Therapy.

Cardiovascular diseases (CVDs) are currently among the leading causes of death worldwide. Platelet aggregation is a key cellular component of arterial thrombi and major cause of CVDs. Protease-activated receptors (PARs), including PAR1, PAR2, PAR3 and PAR4, fall within a subfamily of seven-transmembrane G-protein-coupled receptors (GPCR). Human platelets express PAR1 and PAR4, which contribute to the signaling transduction processes. In association with CVDs, PAR4 not only contributes to platelet activation but also is a modulator of cellular responses that serve as hallmarks of inflammation. Although several antiplatelet drugs are available on the market, they have many side effects that limit their use. Emerging evidence shows that PAR4 targeting is a safer strategy for preventing thrombosis and consequently may improve the overall cardiac safety profile. Our present review summarizes the PAR4 structural characteristics, activation mechanism, role in the pathophysiology of diseases and understanding the association of PAR4 targeting for improved cardiac protection. Conclusively, this review highlights the importance of PAR4 antagonists and its potential utility in different CVDs.

PAR4 (Protease-Activated Receptor 4) Antagonism With BMS-986120 Inhibits Human Ex Vivo Thrombus Formation.

OBJECTIVE: BMS-986120 is a novel first-in-class oral PAR4 (protease-activated receptor 4) antagonist with potent and selective antiplatelet effects. We sought to determine for the first time, the effect of BMS-986120 on human ex vivo thrombus formation. APPROACH AND RESULTS: Forty healthy volunteers completed a phase 1 parallel-group PROBE trial (Prospective Randomized Open-Label Blinded End Point). Ex vivo platelet activation, platelet aggregation, and thrombus formation were measured at 0, 2, and 24 hours after (1) oral BMS-986120 (60 mg) or (2) oral aspirin (600 mg) followed at 18 hours with oral aspirin (600 mg) and oral clopidogrel (600 mg). BMS-986120 demonstrated highly selective and reversible inhibition of PAR4 agonist peptide (100 µM)-stimulated P-selectin expression, platelet-monocyte aggregates, and platelet aggregation (P<0.001 for all). Compared with pretreatment, total thrombus area (µm2/mm) at high shear was reduced by 29.2% (95% confidence interval, 18.3%-38.7%; P<0.001) at 2 hours and by 21.4% (9.3%-32.0%; P=0.002) at 24 hours. Reductions in thrombus formation were driven by a decrease in platelet-rich thrombus deposition: 34.8% (19.3%-47.3%; P<0.001) at 2 hours and 23.3% (5.1%-38.0%; P=0.016) at 24 hours. In contrast to aspirin alone, or in combination with clopidogrel, BMS-986120 had no effect on thrombus formation at low shear (P=nonsignificant). BMS-986120 administration was not associated with an increase in coagulation times or serious adverse events. CONCLUSIONS: BMS-986120 is a highly selective and reversible oral PAR4 antagonist that substantially reduces platelet-rich thrombus formation under conditions of high shear stress. Our results suggest PAR4 antagonism has major potential as a therapeutic antiplatelet strategy. CLINICAL TRIAL REGISTRATION: URL: http://www.clinicaltrials.gov. Unique identifier: NCT02439190.

Anti-platelet effects of vitamin supplements in age-related macular degeneration: an in vitro study.

OBJECTIVE: The purpose of this experimental study was to investigate the role of vitamin supplements (Ocuvite, Vitalux Omega, and Nutrof Total) as possible inhibitors of the onset of age-related macular degeneration (AMD). MATERIALS AND METHODS: The anti-aggregating effect of each vitamin was determined against four accumulative factors namely, platelet activating factor (PAF), adenosine diphosphate (ADP), thrombin receptor-activating peptide (TRAP), and arachidonic acid (AA) in the platelet rich plasma (PRP) of healthy volunteers. RESULTS: Ocuvite, Vitalux Omega, and Nutrof Total were more potent inhibitors against PAF and ADP compared to TRAP and AA. Among the three vitamins, Nutrof Total displayed more potent inhibitions against TRAP and AA, while against PAF and ADP all the three vitamins revealed similar IC50 values. CONCLUSIONS: The vitamins Ocuvite, Vitalux Omega, and Nutrof Total have anti-aggregating effects and therefore can be used against AMD in healthy volunteers.

Protease-Activated Receptor PAR-4: An Inducible Switch between Thrombosis and Vascular Inflammation?

Thrombin triggers activation of platelets through protease-activated receptor 1 (PAR-1) and PAR-4. Both receptors are widely expressed and exert multiple platelet-independent functions. PAR signalling contributes to healing responses after injury, by promoting cytokine activity and cellular growth and mobility. Uncontrolled PAR activation, however, can prevent timely resolution of inflammation, enhance thrombogenic endothelial function and drive adverse remodelling. The specific role of PAR-4 in thromboinflammatory vascular disease has been largely underestimated, given the relatively limited expression of PAR-4 in non-platelet cells under healthy conditions. However, unlike PAR-1, PAR-4 expression adapts dynamically to numerous stimuli associated with thromboinflammation, including thrombin, angiotensin II, sphingosine-1-phosphate (S1P), high glucose and redox stress, suggesting expression is switched on 'at need'. Prostacyclin negatively regulates PAR-4 expression at the post-transcriptional level, which may serve to fine-tune thrombin responses and limit these to the injury site. PAR-4 elicits inflammatory, mitogenic and proliferative actions not only in response to thrombin but also to numerous other inflammatory proteases, and can cross-talk with other receptor systems such as S1P and adenosine receptors. Accordingly, PAR-4 has emerged as a candidate player in vessel disease and cardiac post-infarction remodelling. Currently, PAR-4 is a particularly promising target for safer anti-thrombotic therapies. Recent studies with the PAR-4 antagonist BMS-986120 lend support to the concept that selective antagonism of PAR-4 may offer both an effective and safe anti-thrombotic therapy in the acute thrombotic setting as well as an anti-inflammatory strategy to prevent long-term progressive atherosclerotic disease in high-risk cardiovascular patients.

Blockade of protease-activated receptor-4 (PAR4) provides robust antithrombotic activity with low bleeding.

Antiplatelet agents are proven efficacious treatments for cardiovascular and cerebrovascular diseases. However, the existing drugs are compromised by unwanted and sometimes life-threatening bleeding that limits drug usage or dosage. There is a substantial unmet medical need for an antiplatelet drug with strong efficacy and low bleeding risk. Thrombin is a potent platelet agonist that directly induces platelet activation via the G protein (heterotrimeric guanine nucleotide-binding protein)-coupled protease-activated receptors PAR1 and PAR4. A PAR1 antagonist is approved for clinical use, but its use is limited by a substantial bleeding risk. Conversely, the potential of PAR4 as an antiplatelet target has not been well characterized. Using anti-PAR4 antibodies, we demonstrated a low bleeding risk and an effective antithrombotic profile with PAR4 inhibition in guinea pigs. Subsequently, high-throughput screening and an extensive medicinal chemistry effort resulted in the discovery of BMS-986120, an orally active, selective, and reversible PAR4 antagonist. In a cynomolgus monkey arterial thrombosis model, BMS-986120 demonstrated potent and highly efficacious antithrombotic activity. BMS-986120 also exhibited a low bleeding liability and a markedly wider therapeutic window compared to the standard antiplatelet agent clopidogrel tested in the same nonhuman primate model. These preclinical findings define the biological role of PAR4 in mediating platelet aggregation. In addition, they indicate that targeting PAR4 is an attractive antiplatelet strategy with the potential to treat patients at a high risk of atherothrombosis with superior safety compared with the current standard of care.

Contributions of Protease-Activated Receptors PAR1 and PAR4 to Thrombin-Induced GPIIbIIIa Activation in Human Platelets.

Human platelets display a unique dual receptor system for responding to its primary endogenous activator, α-thrombin. Because of the lack of efficacious antagonists, the field has relied on synthetic peptides and pepducins to describe protease-activated receptor PAR1 and PAR4 signaling. The precise contributions of each receptor have not been established in the context of thrombin. We took advantage of newly discovered PAR antagonists to contrast the contribution of PAR1 and PAR4 to thrombin-mediated activation of the platelet fibrin receptor (GPIIbIIIa). PAR1 is required for platelet activation at low but not high concentrations of thrombin, and maximal platelet activation at high concentrations of thrombin requires PAR4. As the concentration of thrombin is increased, PAR1 signaling is quickly overcome by PAR4 signaling, leaving a narrow window of low thrombin concentrations that exclusively engage PAR1. PAR4 antagonism reduces the maximum thrombin response by over 50%. Thus, although the PAR1 response still active at higher concentrations of thrombin, this response is superseded by PAR4. Truncation of a known PAR4 antagonist and identification of the minimum pharmacophore converted the mechanism of inhibition from noncompetitive to competitive, such that the antagonist could be outcompeted by increasing doses of the ligand. Fragments retained efficacy against both soluble and tethered ligands with lower cLogP values and an increased free fraction in plasma. These reversible, competitive compounds represent a route toward potentially safer PAR4 antagonists for clinical utility and the development of tools such as radioligands and positron emission tomography tracers that are not currently available to the field for this target.

Identification of the minimum PAR4 inhibitor pharmacophore and optimization of a series of 2-methoxy-6-arylimidazo[2,1-b][1,3,4]thiadiazoles.

This letter describes the further deconstruction of the known PAR4 inhibitor chemotypes (MWs 490-525 and with high plasma protein binding) to identify a minimum PAR4 pharmacophore devoid of metabolic liabilities and improved properties. This exercise identified a greatly simplified 2-methoxy-6-arylimidazo[2,1-b][1,3,4]thiadiazole scaffold that afforded nanomolar inhibition of both activating peptide and γ-thrombin mediated PAR4 stimulation, while reducing both molecular weight and the number of hydrogen bond donors/acceptors by ∼50%. This minimum PAR4 pharmacophore, with competitive inhibition, versus non-competitive of the larger chemotypes, allows an ideal starting point to incorporate desired functional groups to engender optimal DMPK properties towards a preclinical candidate.

Development of a Series of (1-Benzyl-3-(6-methoxypyrimidin-3-yl)-5-(trifluoromethoxy)-1H-indol-2-yl)methanols as Selective Protease Activated Receptor 4 (PAR4) Antagonists with in Vivo Utility and Activity Against γ-Thrombin.

Here, we describe the development of a series of highly selective PAR4 antagonists with nanomolar potency and selectivity versus PAR1, derived from the indole-based 3. Of these, 9j (PAR4 IC50 = 445 nM, PAR1 response IC50 > 30 µM) and 10h (PAR4 IC50 = 179 nM, PAR1 response IC50 > 30 µM) maintained an overall favorable in vitro DMPK profile, encouraging rat/mouse in vivo pharmacokinetics (PK) and activity against γ-thrombin.