Protease-activated Receptor-1 Antagonist Research Articles

Abstract P273: Treatment Patterns Among Vorapaxar New Users in a US Outpatient Clinical Setting

Introduction: Vorapaxar is a once daily oral protease-activated receptor-1 (PAR-1) antagonist for the reduction of thrombotic events in patients with a history of myocardial infarction or peripheral arterial disease. Efficacy and safety was evaluated in a large randomized clinical trial (TRA 2°P TIMI 50), with a median follow-up of 30 months. This is the first descriptive study of vorapaxar treatment patterns in routine clinical practice. Objectives: To evaluate medication adherence and persistence among vorapaxar new users. Methods: Vorapaxar new users ≥18 years of age were identified with an index prescription date between June 2014 and October 2015 with ≥1 years of follow-up through October 2016. The qualifying prescription was identified from Symphony Health’s US outpatient pharmacy claims linked to private practitioner claims and hospital data. Baseline characteristics were assessed using a 1-year look-back prior to the index prescription date. Adherence was defined as a medication possession ratio (MPR) ≥80%, and persistence was defined allowing for up to a 7-day gap in days supply. Sensitivity analysis with 15-day gap to define persistence was performed. Results: The 529 vorapaxar new users included in the analysis were predominantly male (67%), 11% African American and 70% Caucasian, and had a mean (SD) age of 65.2 (10.5) years. At the end of 3, 6, and 12 months, 61%, 45%, and 32%, respectively, of new users were adherent on vorapaxar. A majority (68%) of vorapaxar users were persistent at 3 months, 48% at 6 months, and 29% at 12 months. An additional 10% restarted therapy by the end of the first year after a period of supply interruption. Results of the sensitivity analysis for persistence using 15-day allowable gap were within 1-2% of primary analysis. The median (IQR) time to discontinuation of therapy during the first year was 2.6 (1.2-6.1) months. During the first year, a median (IQR) of 5 (3-9) prescriptions per patient were filled- majority 30 day fills. Conclusions: Medication adherence and persistence of use decreased over time. One-third of patients were adherent and/or persistent at one year. Further research is warranted to understand treatment pattern differences by indication of use and factors affecting adherence and discontinuation.

SCH79797 improves outcomes in experimental bacterial pneumonia by boosting neutrophil killing and direct antibiotic activity.

The role of protease-activated receptor 1 (PAR1) in the pathogenesis of pneumonia and sepsis is ambiguous given the existing literature. As PAR1 is classically activated by the coagulation-based protease thrombin and leads to vascular leakage, our hypothesis was that PAR1 blockade with SCH79797 would be therapeutically beneficial in an experimental model of murine Gram-negative pneumonia. In this study, we administered SCH79797 via the intrapulmonary route 6 h after the establishment of Escherichia coli pneumonia and observed a significant improvement in survival, lung injury, bacterial clearance and inflammation. We focused on neutrophils as a potential target of the PAR1 antagonist, since they are the predominant inflammatory cell type in the infected lung. Neutrophils appear to express PAR1 at low levels and the PAR1 antagonist SCH79797 enhanced neutrophil killing. Part of this effect may be explained by alterations in the generation of reactive oxygen species (ROS). SCH79797 also led to robust neutrophil extracellular trap (NET) generation and cathelicidin-related antimicrobial peptide (CRAMP) release by neutrophils. Surprisingly, SCH79797 also had a potent, direct antibiotic effect with disruption of the E. coli cell membrane. However, the newer-generation PAR1 antagonist, vorapaxar (SCH530348), had no appreciable effect on neutrophil activity or direct bacterial killing, which suggests the effects seen with SCH79797 may be PAR1 independent. In summary, we observed that intrapulmonary treatment with SCH79797 has significant therapeutic effects in a model of E. coli pneumonia that appear to be due, in part, to both neutrophil-stimulating and direct antibacterial effects of SCH79797.

Antimicrobial Peptide Cathelicidin-BF Inhibits Platelet Aggregation by Blocking Protease-Activated Receptor 4

Cathelicidin-BF (BF-30), a peptide isolated from the snake venom of Bungarus fasciatus, exhibits multiple biological functions, including antimicrobial, anticancer and anti-inflammatory. However, the effect of BF-30 on platelet and thrombus formation was reported rarely. In this study, we investigated the antiplatelet and antithrombotic effects of BF-30 and its underlying mechanism. Our results showed that BF-30 potently inhibited thrombin-induced platelet aggregation, and further specifically blocked protease-activated receptor 4 (PAR4). It also reduced P-selectin expression, AktSer473 phosphorylation and platelet spreading on fibrinogen. Furthermore, BF-30 exhibited potent inhibitory activity on thrombus formation in vivo: it decreased death of mice with acute pulmonary thrombosis and attenuated thrombosis weight in arterio-venous shunt model. Additionally, a tail cutting bleeding time assay revealed that BF-30 did not prolong bleeding time in mice at efficient dosage. Taken together, BF-30 is a PAR4 antagonist, and inhibits thrombus formation without obvious bleeding risk in vivo. We believe that this study may provide a source for the development of PAR4 antagonist for the treatment of thrombotic disorders.

Designing Natural Product Hybrids Bearing Triple Antiplatelet Profile and Evaluating Their Human Plasma Stability.

Cardiovascular diseases (CVDs) are becoming major contributors to the burden of disease due to genetic and environmental factors. Despite current standard oral care, cardiovascular risk remains relatively high. A triple antiplatelet therapy with a cyclooxygenase-1 (COX-1) inhibitor, a P2Y12 receptor antagonist, and a protease-activated receptor-1 (PAR-1) antagonist has been established in the secondary prevention of atherothrombosis in patients with acute myocardial infraction and in those with peripheral artery disease. However, due to the combinatorial use of three different drugs, patients receiving this triple therapy are exposed to enhanced risk of bleeding. Conforming to polypharmacology principles, the discovery of a single compound that can simultaneously block the three platelet activation pathways (PAR-1, P2Y12, and COX-1) is of importance. Natural products have served as an inexhaustiblesource of bioactive compounds presenting a diverse pharmaceutical profile, including anti-inflammatory, antioxidant, anticancer, and antithrombotic activity. Indeed, principal component analysis indicated that natural products have the potential to inhibit the three aforementioned pathways, though existed reports refer to single inhibition mechanism on specific receptor(s) implicated in platelet activation. We thus set out to explore possibilities thattake advantage of this potential of natural products and shape the basis to produce novel compounds that could simultaneously target PAR-1, P2Y12, and COX-1 platelet activation pathways. Polyunsaturated fatty acids (PUFAs) have multiple effects leading to improvements in blood pressure and cardiac function and arterial compliance. A promising approach to achieve the desirable goal is the bioconjugation of natural products with PUFAs. Herein, we describe the principles that should be followed to develop molecular hybrids bearing triple antiplatelet activity profile.

A Potent Antagonist of Protease-Activated Receptor 2 That Inhibits Multiple Signaling Functions in Human Cancer Cells.

Protease-activated receptor 2 (PAR2) is a cell surface protein linked to G-protein dependent and independent intracellular signaling pathways that produce a wide range of physiological responses, including those related to metabolism, inflammation, pain, and cancer. Certain proteases, peptides, and nonpeptides are known to potently activate PAR2. However, no effective potent PAR2 antagonists have been reported yet despite their anticipated therapeutic potential. This study investigates antagonism of key PAR2-dependent signaling properties and functions by the imidazopyridazine compound I-191 (4-(8-(tert-butyl)-6-(4-fluorophenyl)imidazo[1,2-b]pyridazine-2-carbonyl)-3,3-dimethylpiperazin-2-one) in cancer cells. At nanomolar concentrations, I-191 inhibited PAR2 binding of and activation by structurally distinct PAR2 agonists (trypsin, peptide, nonpeptide) in a concentration-dependent manner in cells of the human colon adenocarcinoma grade II cell line (HT29). I-191 potently attenuated multiple PAR2-mediated intracellular signaling pathways leading to Ca2+ release, extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation, Ras homologue gene family, member A (RhoA) activation, and inhibition of forskolin-induced cAMP accumulation. The mechanism of action of I-191 was investigated using binding and calcium mobilization studies in HT29 cells where I-191 was shown to be noncompetitive and a negative allosteric modulator of the agonist 2f-LIGRL-NH2 The compound alone did not activate these PAR2-mediated pathways, even at high micromolar concentrations, indicating no bias in these signaling properties. I-191 also potently inhibited PAR2-mediated downstream functional responses, including expression and secretion of inflammatory cytokines and cell apoptosis and migration, in human colon adenocarcinoma grade II cell line (HT29) and human breast adenocarcinoma cells (MDA-MB-231). These findings indicate that I-191 is a potent PAR2 antagonist that inhibits multiple PAR2-induced signaling pathways and functional responses. I-191 may be a valuable tool for characterizing PAR2 functions in cancer and in other cellular, physiological, and disease settings.

Protease-activated receptor 1 inhibition protects mice against thrombin-dependent respiratory syncytial virus and human metapneumovirus infections.

Protease-activated receptor 1 (PAR1) has been demonstrated to be involved in the pathogenesis of viral diseases. However, its role remains controversial. The goal of our study was to investigate the contribution of PAR1 to respiratory syncytial virus (RSV) and human metapneumovirus (hMPV) infections. Pharmacological approaches were used to investigate the role of PAR1 during RSV and hMPV infection, in vitro using epithelial A549 cells and in vivo using a mouse model of virus infection. In vitro, the PAR1 antagonist RWJ-56110 reduced the replication of RSV and hMPV in A549 cells. In agreement with these results, RWJ-56110-treated mice were protected against RSV and hMPV infections, as indicated by less weight loss and mortality. This protective effect in mice correlated with decreased lung viral replication and inflammation. In contrast, hMPV-infected mice treated with the PAR1 agonist TFLLR-NH2 showed increased mortality, as compared to infected mice, which were left untreated. Thrombin generation was shown to occur downstream of PAR1 activation in infected mice via tissue factor exposure as part of the inflammatory response, and thrombin inhibition by argatroban reduced the pathogenicity of the infection with no additive effect to that induced by PAR1 inhibition. These data show that PAR1 plays a detrimental role during RSV and hMPV infections in mice via, at least, a thrombin-dependent mechanism. Thus, the use of PAR1 antagonists and thrombin inhibitors may have potential as a novel approach for the treatment of RSV and hMPV infections.

A protease-activated receptor-1 antagonist protects against podocyte injury in a mouse model of nephropathy

The kidney expresses protease-activated receptor-1 (PAR-1). PAR-1 is known as a thrombin receptor, but its role in kidney injury is not well understood. In this study, we examined the contribution of PAR-1 to kidney glomerular injury and the effects of its inhibition on development of nephropathy. Mice were divided into 3 groups: control, doxorubicin + vehicle (15 mg/kg doxorubicin and saline) and doxorubicin + Q94 (doxorubicin at 15 mg/kg and the PAR-1 antagonist Q94 at 5 mg/kg/d) groups. Where indicated, doxorubicin was administered intravenously and PAR-1 antagonist or saline vehicle by subcutaneous osmotic mini-pump. PAR-1 expression was increased in glomeruli of mice treated with doxorubicin. Q94 treatment significantly suppressed the increased albuminuria in these nephropathic mice. Pathological analysis showed that Q94 treatment significantly attenuated periodic acid–Schiff and desmin staining, indicators of podocyte injury, and also decreased glomerular levels of podocin and nephrin. Furthermore, thrombin increased intracellular calcium levels in podocytes. This increase was suppressed by Q94 and Rox4560, a transient receptor potential cation channel (TRPC)3/6 antagonist. In addition, both Q94 and Rox4560 suppressed the doxorubicin-induced increase in activities of caspase-9 and caspase-3 in podocytes. These data suggested that PAR-1 contributes to development of podocyte and glomerular injury and that PAR-1 antagonists have therapeutic potential.

Investigating detailed interactions between novel PAR1 antagonist F16357 and the receptor using docking and molecular dynamic simulations

Currently, Vorapaxar is the only recently FDA-approved antiplatelet drug targeting Protease-activated receptor 1 (PAR1). However, a novel antagonist, F16357, has been shown to prevent painful bladder syndrome, also known as interstitial cystitis (IC). Unfortunately, there is no high resolution structure of the F16357-receptor complex, hindering its optimization as a therapeutic agent. In this study, we used docking and molecular dynamic (MD) simulations to investigate the detailed interactions between F16357 and PAR1 at a molecular level. The recently solved crystal structure of human PAR1 complexed with Vorapaxar was used in our docking of F16357 into the binding pocket of the receptor. To enhance binding pose selection, F16357 was docked first without constraints and then with a positional constraint to invert its orientation to become similar to that of Vorapaxar. The three systems, with crystal Vorapaxar, F16357 and an inverted F16357, were subjected to 3.0μs MD simulations. The MM-GBSA binding energy analysis showed that F16357 binds more strongly in a pose obtained from an unrestrained docking than in the inverted pose from a restrained docking; and Vorapaxar binds more strongly than F17357. This ordering is consistent with the experimental pIC50 values. Our structural data showed subtle changes in the binding pose between Vorapaxar and F16357. Transmembrane helices 1, 2, 5, and 7 were most significantly affected; most notably a large kink at F2795.47 in TM helix 5 of the Vorapaxar complex was completely absent in the F16357 complex. The results of this study facilitate the future development of other therapeutic PAR1 antagonists.

Paradigm of Biased PAR1 (Protease-Activated Receptor-1) Activation and Inhibition in Endothelial Cells Dissected by Phosphoproteomics.

Thrombin is the key serine protease of the coagulation cascade and mediates cellular responses by activation of PARs (protease-activated receptors). The predominant thrombin receptor is PAR1, and in endothelial cells (ECs), thrombin dynamically regulates a plethora of phosphorylation events. However, it has remained unclear whether thrombin signaling is exclusively mediated through PAR1. Furthermore, mechanistic insight into activation and inhibition of PAR1-mediated EC signaling is lacking. In addition, signaling networks of biased PAR1 activation after differential cleavage of the PAR1 N terminus have remained an unresolved issue. Here, we used a quantitative phosphoproteomics approach to show that classical and peptide activation of PAR1 induce highly similar signaling, that low thrombin concentrations initiate only limited phosphoregulation, and that the PAR1 inhibitors vorapaxar and parmodulin-2 demonstrate distinct antagonistic properties. Subsequent analysis of the thrombin-regulated phosphosites in the presence of PAR1 inhibitors revealed that biased activation of PAR1 is not solely linked to a specific G-protein downstream of PAR1. In addition, we showed that only the canonical thrombin PAR1 tethered ligand induces extensive early phosphoregulation in ECs. Our study provides detailed insight in the signaling mechanisms downstream of PAR1. Our data demonstrate that thrombin-induced EC phosphoregulation is mediated exclusively through PAR1, that thrombin and thrombin-tethered ligand peptide induce similar phosphoregulation, and that only canonical PAR1 cleavage by thrombin generates a tethered ligand that potently induces early signaling. Furthermore, platelet PAR1 inhibitors directly affect EC signaling, indicating that it will be a challenge to design a PAR1 antagonist that will target only those pathways responsible for tissue pathology.

Discovery of Potent Orally Active Protease-Activated Receptor 1 (PAR1) Antagonists Based on Andrographolide.

Protease-activated receptor-1 (PAR1), a G-protein-coupled receptor, plays a critical role in thrombin-mediated platelet aggregation. It is regarded as a promising antithrombosis target that is unlikely to result in bleeding. Here, we describe the synthesis of a series of novel PAR1 antagonists by borrowing the chiral fragment of andrographolide, an easily accessible natural molecule from Andrographis paniculata, to produce natural product/synthesis hybrids. An in vitro PAR1 inhibition assay and an in vivo pharmacokinetic profile led to the identification of compound 39 as the best PAR1 inhibitor. The further in vitro and ex vivo antiplatelet aggregation assays of compound 39 indicated that compound 39 was a potent antiplatelet agent. In addition, this compound is metabolically stable and displays a favorable pharmacokinetic profile with an elimination half-life of 3.1 h, which could be treated as a promising candidate for further clinical development.

Synthesis of Arylpiperazine Derivatives as Protease Activated Receptor 1 Antagonists and Their Evaluation as Antiproliferative Agents.

Protease activated receptor-1 (PAR1) is a G-coupled receptor activated by α-thrombin and other proteases. Several reports have demonstrated the PAR1 involvement in tumorigenesis and tumor progression. In order to investigate on potential use of PAR1 antagonists as antiproliferative agents. We have identified a series of arylpiperazine derivatives acting as PAR1 antagonists; the selected molecules have been evaluated for their antiproliferative properties. All the compounds inhibited the growth of a panel of cell lines expressing PAR1; two of them, compounds 13 and 15, were able to inhibit, in a dose dependent manner, the growth of the selected cell lines with the lowest IC50 values, and were further characterized to define the mechanism responsible for the observed antiproliferative effect. This study directed us to the identification of two interesting leads that may help to further validate PAR1 as an important therapeutic target for cancer treatment.

Vorapaxar in the long-term secondary prevention of atherothrombotic events: a profile of its use in the USA

Vorapaxar (Zontivity®), is a first-in-class protease-activated receptor-1 antagonist, is approved for use with standard care antiplatelet therapy for the secondary prevention of cardiovascular thrombotic events in patients with a history of myocardial infarction or with peripheral artery disease. In the TRA 2°P-TIMI 50 trial, oral vorapaxar 2.08 mg once daily + standard antiplatelet therapy significantly reduced the risk of major cardiovascular events in stable patients with established atherosclerosis; however, this treatment benefit was accompanied by an increased risk of bleeding. The net clinical benefit of treatment appears to be greatest in patients with increased atherothrombotic risk, such as those with diabetes.

The role of spinal thrombin through protease-activated receptor 1 in hyperalgesia after neural injury.

OBJECTIVE Painful neuropathic injuries induce blood-spinal cord barrier (BSCB) breakdown, allowing pro-inflammatory serum molecules to cross the BSCB, which contributes to nociception. The goal of these studies was to determine whether the blood-borne serine protease thrombin also crosses a permeable BSCB, contributing to nociception through its activation of protease-activated receptor-1 (PAR1). METHODS A 15-minute C-7 nerve root compression, which induces BSCB breakdown and painful behaviors by Day 1, was administered in the rat (n = 10); sham operation (n = 11) and a 3-minute compression (n = 10) that does not induce sensitivity were administered as controls. At Day 1 after root compression, spinal cord tissue was co-immunolabeled for fibrin/fibrinogen, the enzymatic product of thrombin, and IgG, a serum protein, to determine whether thrombin acts in areas of BSCB breakdown. To determine whether spinal thrombin and PAR1 contribute to hyperalgesia after compression, the thrombin inhibitor hirudin and the PAR1 antagonist SCH79797, were separately administered intrathecally before compression injuries (n = 5-7 per group). Rat thrombin was also administered intrathecally with and without SCH79797 (n = 6 per group) to determine whether spinal thrombin induces hypersensitivity in naïve rats through PAR1. RESULTS Spinal fibrin(ogen) was elevated at Day 1 after root compression in regions localized to BSCB breakdown and decreased in those regions by Day 7. Blocking either spinal thrombin or PAR1 completely prevented compression-induced hyperalgesia for 7 days. Intrathecal thrombin induced transient pain that was prevented by blocking spinal PAR1 before its injection. CONCLUSIONS The findings of this study suggest a potent role for spinal thrombin and its activation of PAR1 in pain onset following neuropathic injury.

Vorapaxar: The Current Role and Future Directions of a Novel Protease-Activated Receptor Antagonist for Risk Reduction in Atherosclerotic Disease.

IntroductionDespite the current standard of care, patients with cardiovascular disease remain at a high risk for recurrent events. Inhibition of thrombin-mediated platelet activation through protease-activated receptor-1 antagonism may provide reductions in atherosclerotic disease beyond those achievable with the current standard of care.ObjectiveOur primary objective is to evaluate the clinical literature regarding the role of vorapaxar (Zontivity™) in the reduction of cardiovascular events in patients with a history of myocardial infarction and peripheral artery disease. In particular, we focus on the potential future directions for protease-activating receptor antagonists in the treatment of a broad range of atherosclerotic diseases.Data SourcesA literature search of PubMed and EBSCO was conducted to identify randomized clinical trials from August 2005 to June 2016 using the search terms: ‘vorapaxar’, ‘SCH 530348’, ‘protease-activated receptor-1 antagonist’, and ‘Zontivity™’. Bibliographies were searched and additional resources were obtained.ResultsVorapaxar is a first-in-class, protease-activated receptor-1 antagonist. The Thrombin Receptor Antagonist for Clinical Event Reduction (TRACER) trial did not demonstrate a significant reduction in a broad primary composite endpoint. However, the Thrombin-Receptor Antagonist in Secondary Prevention of Atherothrombotic Ischemic Events (TRA 2°P-TIMI 50) trial examined a more traditional composite endpoint and found a significant benefit with vorapaxar. Vorapaxar significantly increased bleeding compared with standard care. Ongoing trials will help define the role of vorapaxar in patients with peripheral arterial disease, patients with diabetes mellitus, and other important subgroups. The use of multivariate modeling may enable the identification of subgroups with maximal benefit and minimal harm from vorapaxar.ConclusionVorapaxar provides clinicians with a novel mechanism of action to further reduce the burden of ischemic heart disease. Identification of patients with a high ischemic risk and low bleeding risk would enable clinicians to maximize the utility of this unique agent.Electronic supplementary materialThe online version of this article (doi:10.1007/s40268-016-0158-4) contains supplementary material, which is available to authorized users.

A protease-activated receptor 1 antagonist protects against global cerebral ischemia/reperfusion injury after asphyxial cardiac arrest in rabbits.

Cerebral ischemia/reperfusion injury is partially mediated by thrombin, which causes brain damage through protease-activated receptor 1 (PAR1). However, the role and mechanisms underlying the effects of PAR1 activation require further elucidation. Therefore, the present study investigated the effects of the PAR1 antagonist SCH79797 in a rabbit model of global cerebral ischemia induced by cardiac arrest. SCH79797 was intravenously administered 10 minutes after the model was established. Forty-eight hours later, compared with those administered saline, rabbits receiving SCH79797 showed markedly decreased neuronal damage as assessed by serum neuron specific enolase levels and less neurological dysfunction as determined using cerebral performance category scores. Additionally, in the hippocampus, cell apoptosis, polymorphonuclear cell infiltration, and c-Jun levels were decreased, whereas extracellular signal-regulated kinase phosphorylation levels were increased. All of these changes were inhibited by the intravenous administration of the phosphoinositide 3-kinase/Akt pathway inhibitor LY29004 (3 mg/kg) 10 minutes before the SCH79797 intervention. These findings suggest that SCH79797 mitigates brain injury via anti-inflammatory and anti-apoptotic effects, possibly by modulating the extracellular signal-regulated kinase, c-Jun N-terminal kinase/c-Jun and phosphoinositide 3-kinase/Akt pathways.

PAR1 Antagonists Development and Clinical Utility

Thrombin is a potent activator of platelets and other cells. The mechanism by which thrombin, a protease, regulates cellular behaviors like a hormone was revealed by expression cloning of a G protein-coupled receptor now known as protease-activated receptor-1 (PAR1). Thrombin activates PAR1 by cleaving its N-terminal ectodomain to reveal a new N-terminus that then serves as a tethered peptide ligand, binding to the heptahelical domain to trigger G protein activation. A synthetic hexapeptide mimicking this new N-terminus activates PAR1 without receptor cleavage. Point mutations that prevent receptor cleavage render PAR1 unactivatable by thrombin without altering activation by exogenous tethered ligand peptide. Replacement of the thrombin cleavage site with sites for other proteases allows PAR1 to signal in response to those other proteases. Removal of sequence N-terminal to the cleavage site and creation of the new protonated amino group at N-terminal of the tethered ligand sequence is necessary for its function, explaining how the tethered ligand is silent in the intact receptor and activated by receptor cleavage. Thus, PARs are, in essence, peptide receptors that carry their own ligands, which can be unmasked by receptor cleavage. Mammalian cells utilize four PARs to respond to coagulation proteases and other proteases with trypsin-like specificity. Three, PAR1, PAR3 and PAR4 can mediate responses to thrombin, with PAR3 and PAR4 mediating platelet activation by thrombin in mice and PAR1 and PAR4 platelet activation in humans. Inhibition of thrombin signaling via PARs in platelets decreases thrombus formation in animal models of platelet-dependent thrombosis. These and other studies led to development of the PAR1 antagonist vorapaxar (Zontivity), a first-in-class antiplatelet agent approved for secondary prevention of atherothrombotic events in selected patients with prior myocardial infarction and peripheral arterial disease. What was and was not learned from animal and human studies and remaining questions related to clinical utility of PAR inhibition will be discussed. The structure-function studies outlined above support the tethered ligand model, but a structure that reveals how the tethered ligand binds and how binding leads to transmembrane signaling and G protein activation is needed. Vorapaxar provided a tool for stabilizing PAR1 protein during solubilization, purification and crystallization. In collaboration with Brian Kobilka and colleagues, we solved a crystal structure of off-state PAR1 in complex with its antagonist vorapaxar, which explains pharmacological properties of vorapaxar and efforts to solve a complementary on-state structure are ongoing. DisclosuresCoughlin:Merck: Research Funding; Novartis: Consultancy; Portola Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees.

Inverse PAR1 Activity of Hematopoietic Stem Cells and BM Stromal Cells Mediates G-CSF-Induced Mobilization By Regulation of Nitric Oxide Generation

Hematopoietic stem and progenitor cell (HSPC) egress from the bone marrow (BM) to the circulation is tightly regulated and is accelerated during stress conditions, a process utilized for BM harvest. Recently, we demonstrated that mouse long term repopulating hematopoietic stem cell (LT-HSC) BM retention and their rapid release to the blood circulation are governed by a switch in nitric oxide (NO) generation via distinct coagulation-related protease activated receptor 1 (PAR1) cascades (Gur-Cohen S. et al., NM, 2016).Herein we report that surface PAR1 expression can be exploited and serve as a positive predictive marker for the efficiency of human CD34+ HSPC mobilization among healthy donors in clinical G-CSF-induced mobilization for matched allogeneic transplantations. We found that PAR1 expression on circulating leukocytes before G-CSF administration was positively correlated with higher yields of mobilized leukocytes after stimulation (P<0.001). In addition, the percentage of PAR1-expressing CD34+ HSPCs in the blood and their absolute numbers before initiation of mobilization positively correlated with peripheral blood leukocyte counts (P<0.001) and yield of mobilized CD34+ HSPCs (P=0.002). Importantly, tracing the consequences of the follow-up BM transplantation, we found accelerated neutrophil recovery in patients transplanted with mobilized cells expressing higher surface PAR1 levels at baseline (P=0.074), shortening the time for neutrophil recovery (neutrophil count>0.5 x 109/L) from an average of 14.6 days to an average of 11 days post-transplant. In addition, a trend of accelerated platelet production was documented to be related with higher PAR1 expression by circulating leukocytes prior to G-CSF stimulation. Consequently, poor mobilizer donors were characterized by extremely low surface PAR1 expression on circulating CD34+ cells prior to G-CSF stimulations. Herein we present a case report of a thrombophilic donor carrying the MTHFR mutation, expressing exceedingly low PAR1 levels at baseline, with the outcome of inadequate numbers of mobilized CD34+ HSPC in the blood following G-CSF treatments.To further gain insight into the role played by PAR1 signaling in the regulation of G-CSF-induced HSPC mobilization, we used mice as a functional preclinical small animal model. We found that antagonizing PAR1 signaling attenuated both steady state release and G-CSF-induced HSPC mobilization. Furthermore, co-administration of G-CSF with PAR1 antagonist attenuated secretion of BM stromal CXCL12 and abrogated upregulation of surface CXCR4 and PAR1 expression by BM HSPCs, all leading to significantly reduced HSPC migration, differentiation and mobilization. In support, PAR1-/- mice failed to efficiently mobilize HSPCs in response to G-CSF compared to wild type counterparts. Enforced HSPC recruitment by G-CSF treatments dramatically accelerated PAR1-dependent NO production by eNOS, known to promote TACE-mediated EPCR shedding and rapid LT-HSC mobilization. Concomitantly, circulating steady state and G-CSF-mobilized stem cells lack surface EPCR expression. Intriguingly, while EPCR expression by primitive BM stem cells was transiently reduced after G-CSF treatments, antagonizing PAR1 signaling along with G-CSF stimuli blocked NO generation and synchronically expanded BM EPCR+ LT-HSC and their supportive stromal progenitor cells (MSPCs), as confirmed by increased repopulation in transplanted mice. Finally, we report an inverse PAR1 expression and regulation by BM HSPC and stromal MSPCs in mediating G-CSF-induced mobilization. G-CSF induced elevation of PAR1 expression on BM HSPCs, providing the driving force for their enhanced NO mediated migration, proliferation, differentiation and recruitment to the circulation. Contrary, the levels of PAR1 expression were reduced on MSPCs in response to G-CSF treatment, and following NO generation by eNOS activity and CXCL12 secretion, resulted in reduced mesenchymal differentiation leading to accumulated numbers of immature mesenchymal (CFU-F) and osteoblast (CFU-OB) progenitor cells.Taken together, our study identifies and highlights inverse PAR1 signaling and NO generation as essential regulator of G-CSF induced HSPC mobilization and MSPC development opening new avenues to advance therapeutics for enhancing clinical G-CSF induced stem cell mobilization and transplantation protocols. DisclosuresRuf:Iconic Therapeutics: Consultancy.

Humanizing the Protease-Activated Receptor (PAR) Expression Profile in Mouse Platelets by Knocking PAR1 into the Par3 Locus Reveals PAR1 Expression Is Not Tolerated in Mouse Platelets

Anti-platelet drugs are the mainstay of pharmacotherapy for heart attack and stroke prevention, yet improvements are continually sought. Thrombin is the most potent activator of platelets and targeting platelet thrombin receptors (protease-activated receptors; PARs) is an emerging anti-thrombotic approach. Humans express two PARs on their platelets–PAR1 and PAR4. The first PAR1 antagonist was recently approved for clinical use and PAR4 antagonists are in early clinical development. However, pre-clinical studies examining platelet PAR function are challenging because the platelets of non-primates do not accurately reflect the PAR expression profile of human platelets. Mice, for example, express Par3 and Par4. To address this limitation, we aimed to develop a genetically modified mouse that would express the same repertoire of platelet PARs as humans. Here, human PAR1 preceded by a lox-stop-lox was knocked into the mouse Par3 locus, and then expressed in a platelet-specific manner (hPAR1-KI mice). Despite correct targeting and the predicted loss of Par3 expression and function in platelets from hPAR1-KI mice, no PAR1 expression or function was detected. Specifically, PAR1 was not detected on the platelet surface nor internally by flow cytometry nor in whole cell lysates by Western blot, while a PAR1-activating peptide failed to induce platelet activation assessed by either aggregation or surface P-selectin expression. Platelets from hPAR1-KI mice did display significantly diminished responsiveness to thrombin stimulation in both assays, consistent with a Par3-/- phenotype. In contrast to the observations in hPAR1-KI mouse platelets, the PAR1 construct used here was successfully expressed in HEK293T cells. Together, these data suggest ectopic PAR1 expression is not tolerated in mouse platelets and indicate a different approach is required to develop a small animal model for the purpose of any future preclinical testing of PAR antagonists as anti-platelet drugs.

Peripheral Revascularization in Patients WithPeripheral Artery Disease WithVorapaxar: Insights From the TRA 2°P-TIMI 50 Trial.

The aim of this study was to determine whether the reduction in peripheral revascularization with vorapaxar in patients with peripheral artery disease (PAD) is directionally consistent across indications, including acute limb ischemia, progressively disabling symptoms, or both. The protease-activated receptor-1 antagonist vorapaxar reduces peripheral revascularization in patients with PAD. The TRA 2°P-TIMI 50 (Thrombin Receptor Antagonist in Secondary Prevention of Atherothrombotic Ischemic Events-Thrombolysis in Myocardial Infarction 50) trial randomized 26,449 patients with histories of myocardial infarction, stroke, or symptomatic PAD to vorapaxar or placebo on a background of standard therapy. A total of 5,845 patients had a known history of PAD at randomization. Peripheral revascularization procedures reported by the site were a pre-specified outcome. We explored whether the benefit of vorapaxar was consistent across indication and type of procedure. Of the 5,845 patients with known PAD, a total of 934 (16%) underwent at least 1 peripheral revascularization over 2.5 years (median). More than one-half (55%) were for worsening claudication, followed by critical limb ischemia (24%), acute limb ischemia (16%), and asymptomatic severe stenosis (4%). Vorapaxar significantly reduced peripheral revascularization (19.3% for placebo, 15.4% for vorapaxar; hazard ratio: 0.82; 95% confidence interval: 0.72 to 0.93; p=0.003), with a consistent pattern of efficacy across indication. Vorapaxar reduces peripheral revascularization in patients with PAD. This benefit of vorapaxar is directionally consistent across type of procedure and indication. (Trial to Assess the Effects of Vorapaxar [SCH 530348; MK-5348] in Preventing Heart Attack and Stroke in Patients With Atherosclerosis [TRA 2°P - TIMI 50] [P04737]; NCT00526474).

Vorapaxar in patients with coronary artery bypass grafting: Findings from the TRA 2°P-TIMI 50 trial.

Vorapaxar is a first-in-class protease-activated receptor-1 antagonist indicated for the reduction of cardiovascular death, myocardial infarction, and stroke in stable patients with prior atherothrombosis, who have not had a prior stroke or transient ischemic attack. The aims of this study were to investigate: 1) the role of vorapaxar in patients with severe coronary artery disease treated previously with coronary artery bypass grafting (CABG); and 2) safety in patients undergoing CABG while receiving vorapaxar. TRA 2°P-TIMI 50 was a randomized, double-blinded, placebo-controlled trial of vorapaxar in 26,449 stable patients with prior atherothrombosis followed for a median of 30 months. We 1) investigated the efficacy of vorapaxar among patients with a history of CABG prior to randomization ( n=2942); and 2) assessed the safety among 367 patients who underwent a new CABG during the trial. Patients with a prior CABG were at higher risk for cardiovascular death, myocardial infarction, or stroke at three years compared with patients without a prior CABG (13.7% vs. 7.8%, p<0.001). Among patients with a prior CABG, vorapaxar significantly reduced the risk of cardiovascular death, myocardial infarction, or stroke (11.9% vs. 15.6%, hazard ratio 0.71, 95% confidence interval 0.58-0.88, p=0.001; number-needed-to-treat = 27). In patients undergoing CABG while receiving vorapaxar, the rate of Thrombolysis in Myocardial Infarction CABG major bleeding was 6.3% vs. 4.1% with placebo (hazard ratio 1.53, 95% confidence interval 0.58-4.01, p=0.39). In patients with a prior CABG, vorapaxar significantly reduced the risk of recurrent major cardiovascular events. In patients undergoing CABG while receiving vorapaxar, bleeding risk appeared similar to that seen in the overall trial population.