key: cord-0786374-lx06jm0f authors: Sriram, Krishna; Insel, Paul A. title: Proteinase‐activated receptor 1 (PAR1): A target for repurposing in the treatment of COVID‐19? date: 2020-07-08 journal: Br J Pharmacol DOI: 10.1111/bph.15194 sha: d341a5439826720d354c413b5d977702bb1c6102 doc_id: 786374 cord_uid: lx06jm0f 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 signaling. Recent data indicate that key findings in COVID‐19 patients include thrombosis and endothelitis. Activation of PAR1 (Proteinase‐activated receptor 1), 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. Here, we discuss evidence implying a possible beneficial role for vorapaxar in the treatment of COVID‐19 patients and in addition, other as‐yet non‐approved antagonists of PAR1 and PAR4. Recent studies have provided evidence that the pathobiology of COVID-19 includes thrombosis and endothelitis (Ackermann et al., 2020; Joly et al., 2020; Magro et al., 2020 ; hematology.org). Such evidence includes data indicating infection by SARS-CoV-2 of ACE2-expressing endothelial cells in infected tissue (Ackermann et al., 2020) , widespread endothelial inflammation and disruption (Ackermann et al., 2020; Joly et al., 2020) , and alterations in the coagulation cascade that can result in thrombosis in COVID-19 patients. This thrombosis is associated with increased circulating levels of fibrin and D-dimer (indicative of increased coagulation cascade activity), in parallel with increased levels of inflammatory markers (Joly et al., 2020; Magro et al., 2020; hematology.org) . Reports that indicate an association of COVID-19 with risk of stroke, even in younger patients (Oxley et al., 2020) , are consistent with the idea that the pathophysiology of COVID-19 involves damage to the endothelium and associated thrombosis. Thrombosis can be a feature of pulmonary infections, acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) (Frantzeskaki et al., 2017) . Hence, the administration of anti-coagulants in ALI and ARDS (besides in COVID-19) is under study (Camprubí-Rimblas et al., 2018). Proteinase-activated receptor 1 (PAR1), a G protein-coupled receptor (GPCR), is a key mediator of the aggregation and activation of platelets and as such, an initiator of the coagulation cascade. Thrombin, a serine protease and the major physiologic agonist of PAR1, cleaves its amino terminal, thereby exposing a tethered ligand that self-activates the receptor; other proteases can also activate PAR1 (Heuberger & Schuepbach, 2019) . PAR1 is widely expressed, including in cell types relevant to COVID-19 pathobiology, including pneumocytes, endothelial cells, fibroblasts and platelets. PAR4, another PAR receptor, is also expressed on human platelets and other cell types; agonist stimulation of PAR4 also activates platelets (Heuberger & Schuepbach, 2019) . Vorapaxar is a selective PAR1 antagonist that is approved for the treatment of patients with myocardial infarction and/or peripheral arterial disease. The inhibition of platelets by vorapaxar is thought to be the main therapeutic action of this drug, with limited data on effects of vorapaxar on other cell types (Heuberger & Schuepbach, 2019) . Atopaxar, another PAR1 antagonist, has a shorter half-life than vorapaxar and has been tested in phase II trials. No approved drugs currently target PAR4 but several PAR4 inhibitors have been identified (guidetopharmacology.org; Alexander et al. especially ones associated with thrombosis and related features of the infection? c) Do patients being treated with vorapaxar and exposed to SARS-CoV-2 have an altered susceptibility to developing COVID-19, its clinical features and course? Thrombin is produced in-vivo from pro-thrombin, as part of the coagulation cascade. A key component of the extrinsic mechanism of this cascade is the production of tissue factor (TF) (Figure) . Hyperinflammation associated with severe COVID-19 disease can promote TF production by endothelial cells, macrophages and fibroblasts (Joly et al., 2020) . Hence, this disease setting is likely associated with elevated TF and thrombin production and PAR1 activation, as in patients with ALI/ARDS, as noted above. Besides its action in platelets, PAR1 regulates endothelial function. The emerging paradigm is a dosedependent effect of thrombin on endothelial cells: at low concentrations, thrombin (via PAR1) demonstrated a protective role for thrombin in maintaining segregation between intestinal epithelium and gut microbiota (an effect that may occur in other epithelia, including in the lung), implying that inhibition of thrombin may have deleterious effects related to the microbiome. Such potential effects require further study in pre-clinical models. Given these hazards, caution is necessary in evaluating the potential of PAR1 inhibition as a means to treat COVID-19 patients. Preclinical studies need to evaluate effects of PAR1 antagonists (and similarly, for tool compounds for inhibition of PAR4) on alveolar epithelial cells, endothelial cells and fibroblasts along with assessment of these drugs in animal models of COVID-19. It is as-yet unclear if clinical features of COVID-19, associated with thrombosis are replicated in animal models. The development of animal models for COVID-19 is an ongoing effort, including work with ferrets, rhesus monkeys and other organisms (Sriram & Insel, 2020) . We anticipate greater clarity on the suitability of specific animal models for recapitulating clinical features of COVID-19, as data on these animal models accumulate. Pre-clinical studies with animal models should help to further define the mechanisms for potential beneficial effects of PAR1 inhibition in-vivo, for example, actions on platelets or other cell types. Such data are available in mouse models of ARDS/ALI (e.g., reviewed by Frantzeskaki et al., 2017) and can be extended to COVID-19, in relevant animal models as these become better established. In-vivo studies should optimally be complemented by ex-vivo studies with isolated cells from treated animals, in order to confirm target engagement and functional effects in specific cellular populations. Nevertheless, the growing recognition of endothelitis and thrombosis in COVID-19 patients provides a strong incentive to determine the potential utility of PAR1 (and perhaps PAR4) inhibitors to improve the outcome of such patients. Besides investigating such approaches, it would be of interest to assess methods to directly deliver PAR receptor antagonists to the lungs, via inhalation-based methods, as a possible way to mitigate systemic adverse effects. COVID-19 pulmonary pathobiology is driven by dysregulation of angiotensin signaling (adapted from Sriram & Insel, 2020), which results in feedback between various cell types, leading to increased inflammation and cell death. These conditions are associated with increased Factor X activation, resulting in formation of thrombin, which has actions on platelets, endothelial cells, fibroblasts and alveolar epithelial cells inducing similar effects to those of ANG II in several cell types and promoting thrombosis, which exacerbates pulmonary injury along with that of other organs. ACE2: Angiotensin converting enzyme 2; ANG II: Angiotensin II; EMT: Epithelial-to-Mesenchymal transition. 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