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).