Main Text
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).
Protease-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, 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 and when activated, 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 action of vorapaxar in inhibiting platelet
aggregation 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 , Armstrong et
al., 2020).
Based on the expression and physiological effects of PAR1 (and
potentially, PAR4) in cell types relevant to COVID-19 pathobiology, the
following questions arise:
- Does PAR1 (and perhaps PAR4) contribute to the pathophysiology of
COVID-19?
- Might antagonism of PAR1 (or perhaps PAR4) reduce pathological effects
of COVID-19, especially ones associated with thrombosis and related
features of the infection?
- 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 dose-dependent effect of thrombin on
endothelial cells: at low concentrations, thrombin (via PAR1) is
protective, whilst at high concentrations, thrombin promotes endothelial
dysfunction and disruption (Bae et al., 2009; Jose et al., 2014; Jose &
Manuel, 2020). Data for effects of PAR1 in alveolar and bronchial
epithelial cells in the lung suggest that PAR1 activation drives a
pathological phenotype, including epithelial-to-mesenchymal transition
(EMT), apoptosis and secretion of inflammatory factors (e.g.,
Asokananthan et al., 2002; Suzuki et al., 2005; Song et al., 2013;
Atanelishvili et al., 2014). PAR1 also promotes lung fibrosis, including
by stimulating pro-fibrotic processes in fibroblasts, increasing their
transformation to myofibroblasts and secretion of extracellular matrix
proteins (e.g., Blanc-Brude et al, 2005; Atanelishvili et al., 2014;
Jose et al., 2014). As discussed previously (Sriram & Insel, 2020),
dysregulation of the angiotensin pathway , in particular, elevated ANG
II signaling is likely a key mediator of COVID-19 pathobiology. The
effects of PAR1 on fibroblasts, endothelial cells and epithelial cells
are similar to those of ANG II via AGTR1 (Figure , adapted from
Sriram & Insel, 2020), raising the possibility that such effects may be
additive or synergistic.
By contrast, sparse direct evidence exists for effects of PAR1 on immune
cells. The lack of immune-associated adverse effects with use of
vorapaxar (Morrow et al., 2012) suggests that an impact on immune cells
is unlikely a major component of effects of thrombin-PAR1 signaling in
COVID-19. As recently noted (Jose & Manuel, 2020), studies with mice
suggest that inhibition of PAR1 may also reduce inflammation and enhance
host immune response, including with viral infection, although the cell
types involved in these responses are unclear.
The pathobiology of COVID-19 thus suggests that PAR1 may be a
therapeutic target in COVID-19. Importantly, one could repurpose
vorapaxar or expand trials with atopaxar. However, concerns regarding
the safety of PAR1 antagonists require pre-clinical validation of a role
of PAR1 in COVID-19 models prior to clinical trials. Increased bleeding
risk, which can include fatal bleeding events, is the major adverse
effect of vorapaxar (Morrow et al., 2012). An advantage of atopaxar is
fewer bleeding events and its shorter half-life compared to that of
vorapaxar, which has such a slow rate of metabolism (Statkevich et al.,
2010; Heuberger & Schuepbach, 2019) that its effects, including
potential adverse effects, are essentially irreversible within the time
frame (~7 days) relevant to treatment of the acute,
imminently life-threatening effects of COVID-19.
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 drugsin-vivo 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. 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.