1 INTRODUCTION
The little known respiratory infection by a novel coronavirus 2019-nCoV
(or SARS-CoV-2) in December 2019 in Wuhan, China has now spread to
become a global pandemic. On 11 February 2020, the World Health
Organization (WHO) named the disease as COVID-19, while the
International Committee on Taxonomy of Viruses renamed the virus based
on phylogeny, and taxonomy, as severe acute respiratory syndrome
coronavirus-2 (SARS-CoV-2) (1). At the time of preparing this
manuscript, WHO reported over two million confirmed positive cases of
COVID-19 and the number of global death now exceeds 157, 847
(Coronavirus disease 2019 (COVID-19) Situation Report – 91.
https://www.who.int/docs/default-source/coronaviruse/situation-reports/20200420-sitrep-91-covid-19.pdf?sfvrsn=fcf0670b_4).
The death rate of COVID-19 is estimated to be 2% (2), which is less
than SARS and the Middle East Respiratory Syndrome (MERS), but
SARS-CoV-2 is proven to be more contagious (3). The resulting
hospitalization, death and measure of social distancing leading to
complete shutdown of the human socioeconomic activity brought up
unprecedented level of global concern.
SARS-CoVs, MERS-CoVs and SARS-CoV-2 are members of the beta-coronavirus
family (4). Recent reports have shown that SARS-CoV-2 has 79%
nucleotide similarity to SARS-CoV and 51.8% to MERS-CoV (5). In
addition to non-structural proteins, the viral genome encodes structural
proteins such as the membrane (M), envelope (E), nucleocapsid (N) and
spike (S) proteins. Playing key role for coronavirus pathogenicity (6),
these structural proteins are involved in viral entry into host cells
and replication (7). The similarity of SARS-CoV-2 structural proteins
with SARS-CoV was noted, implying similar entry mechanism through
attachment to the host receptor, angiotensin-converting enzyme 2 (ACE2).
For membrane fusion, the interaction between spike protein and ACE2
appears to be a critical step in a cascade of viral-host cell
communications (7). The RNA-dependent RNA polymerase (RdRp) has a role
for replication of SARS-CoV-2 RNA genome. The packing of new virions
have been shown to be dependent on two polyproteins which are processed
by 3C-like and papain-like proteases (6).
Although the symptoms of COVID-19 could vary depending on age and
underlying conditions (8). Most patients exhibit symptoms such as fever,
dry cough, myalgia, tiredness, and diarrhea. People over the age of 60
and with past medical history of hypertension, diabetes, chronic
obstructive pulmonary disease (COPD), cardiovascular, cerebrovascular,
hepatic, renal, and gastrointestinal diseases are at higher risk for
developing SARS-CoV-2 infection with a substantial mortality rate (6).
Patients with COVID-19 are also more susceptible to thromboembolic
diseases because of immobility, inflammation, hypoxia, and disseminated
intravascular coagulation (DIC). In one study, the incidence of
thrombotic complications in patients admitted to the intensive care unit
(ICU) with COVID-19 was reported to reach 31% (9). In this connection,
hyperfibrinolysis related to an increase in D-dimer levels was observed
in 97% of patients with COVID-19 at the time of hospital admission and
it continued to increase in all patients before death. Fibrin
degradation products also significantly increased throughout the course
of the disease. In severe cases of COVID-19 or in dying patients, a
significant drop in platelet levels was observed (6). Numerous reports
also outlined that COVID-19 patients may experience a severe form of the
disease during a few days, which is often manifested as an acute lung
injury/acute respiratory distress syndrome (ALI/ARDS), respiratory
failure, heart failure, or sepsis (8). In animal model of SARS-CoV and
MERS-CoV, the significant levels of inflammatory and immune responses
cause “cytokine storm”, and apoptosis of epithelial and endothelial
cells. This is followed by an increase in vascular permeability and
leakage, abnormal T cell and macrophages responses, and ALI/ARDS that
could lead to death (10).
In patients with COVID-19, the inflammatory cytokine storm is closely
associated with the development and progression of ARDS (11). In these
patients, the high levels of expression of inflammatory cytokines such
as interleukin (IL)-1β, interferon (IFN)-γ-induced protein (IP-10), and
monocyte chemoattractant protein 1 (MCP-1) could result in activated
T-helper-1 (Th1) response. Furthermore, SARS-CoV-2 infection, unlike
that of SARS-CoV, induces the release of T-helper-2 (Th2) cytokines,
such as IL-4 and IL-10, with inflammation suppression ability. Further
comprehensive studies are however essential to determine the Th1 and Th2
cells responses in COVID-19. When compared to hospitalized COVID-19
patients at general wards, those in the ICU appears to show higher level
of granulocyte colony-stimulating factor, IP-10, MCP-1, macrophage
inflammatory protein-1A, and tumor necrosis factor-alpha (TNF-α).
Several studies emphasized this positive correlation between cytokine
storm and COVID-19 severity (12). Furthermore, an elevated level of IL-6
was shown to be a predictor of poor outcome in severe COVID-19 with
pneumonia and ARDS (13). Hence, the inflammatory cytokine storm play key
role both for the development of ARDS and extra-pulmonary organ failure
(11).
ARDS is a severe type of acute lung injury characterized by massive
infiltration of neutrophils, monocytes and lymphocytes. The diffuse
bilateral edema followed by reduced lung compliance, alveolar damage,
and bronchoalveolar lumen hyaline deposition in ARDS result in hypoxic
respiratory failure. Degranulating neutrophils have a key role in the
development of capillary injury, leakage, and hyaline deposition. These
events may progress ARDS to a more fatally diffuse alveolar damage (14).
The key role of neutrophils in the pathogenesis of ALI/ARDS has also
been shown in animal and clinical studies. Histological assay on autopsy
samples of ARDS patients have illustrated a significant accumulation of
polymorphonuclear cells (PMN) including neutrophils in the damaged
alveoli and in the interstitial tissues (15). Till now, there is no
specific treatment for ARDS except for supportive care using low-tidal
volume ventilators that limit trans-pulmonary pressures (16).
Unfortunately, low-tidal volume ventilation might result in hypercapnia
and higher hospital mortality (17).
Nuclear Factor kappa-light-chain-enhancer of activated B cells (NF-κB)
is a basic transcription factor that is essential for the expression of
inflammation-related genes such as inducible nitric oxide synthase
(iNOS) and inflammatory cytokines (18). In patients with ARDS, the
activation of NF-κB leads to increased expression of immunoregulatory
and pro-inflammatory cytokines (19).
Viral infection has also the potential to induce the production of
oxidized products or oxidative stress that aggravates the
inflammation-mediated COVID-19 pathology. For example, oxidized low
density lipoprotein under SARS-induced ALI activate the innate immune
response leading to overproduction of IL-6 in alveolar macrophages via
the Toll-like receptor 4 (TLR4)/NF-κB signaling pathway (20). With
active viral infection, the retinoic acid-inducible gene I (RIG-I) also
senses viral RNA and triggers signaling cascades, adaptor proteins (MAVS
and TRAF), and different transcription factors (NF-κB and IRF3/IRF7) at
host pattern recognition receptors (PRRs). This constitutes the
beginning of transcription of antiviral type I interferon and
pro-inflammatory cytokines (21).
As major components of inflammatory responses to endothelial injury,
neutrophils have proteolytic, and pro-apoptotic properties through the
action of several enzymes (22). Among them is the serine protease,
neutrophil elastase (NE), which has antimicrobial properties due to its
ability to degrade phagocytosed pathogens (23). It also contributes to
inflammation by increasing vascular permeability (24) and induction of
the release of pro-inflammatory cytokines such as IL-6 and IL-8 (25). In
this regard, NE is needed for the function of neutrophil both under
regulated inflammatory response and tissue damage during sepsis. Under
normal physiological conditions, the function of NE is rigorously
regulated by endogenous protease inhibitors (25). Under exaggerated
inflammatory conditions, however, NE is enabled to attack the
endothelial barrier and infiltrate to bronchoalveolar space, since
protease inhibitors are inactivated by neutrophil oxidants (26). Thus,
excessive activity of NE may lead to tissue damage and remodeling in a
number of pulmonary diseases such as community acquired pneumonia,
ventilator-associated pneumonia, exacerbated COPD, cystic fibrosis,
bronchiectasis, and ALI/ARDS (27). Furthermore, in those patients with
ALI/ARDS, plasma levels of NE are significantly higher in comparison to
healthy subjects (28). The significant level of proteolytic activity of
NE was also observed in bronchoalveolar lavage (BAL) of ARDS patients
(29).
Sivelestat, also known in the scientific literature as ONO-5046, is a
selective, reversible and competitive neutrophil elastase inhibitor.
Hence, it has no effect on the function of other proteases in the body
(30). Its protective effects in attenuating ALI/ARDS has been described
in several models of lung injury. In different preclinical and animal
models of lung injury, sivelestat mitigated the elevation in the lung
vasculature permeability, pulmonary artery pressure (PAP), increase in
the lung tissue wet or dry weight ratio, and neutrophil count (31-33).
Furthermore, sivelestat improved pathogen clearance, the decrease in
PaO2, and prevented digestion of surfactant protein D (34, 35).