**1. Introduction**

The new covid-19 pandemic was reported in Wuhan, China in December, 2019 [1]. This disease is caused by corona virus also known as SARS-CoV-2 and characterized by severe acute respiratory distress syndrome [2]. As per the recent report of WHO on 20 December, 2020, there have been over 75 million cases and 1.6 million deaths reported worldwide since the start of the pandemic [3]. In 2002, SARS-CoV (Severe Acute Respiratory Syndrome Coronavirus) outbreak was reported in China then spread worldwide, whereas, MERS-CoV (Middle East Respiratory Syndrome Coronavirus) emerged in Saudi Arabia in 2012 with 37% mortality rate. Similar to SARS and MERS, newly identified severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) belongs to beta-coronaviridae family and showed close resemblance with them [4]. These three zoonotic viruses have pandemic potential and able to produce severe respiratory infection in humans [5].

The SARS-CoV-2 infection is transmitted through respiratory secretions either in droplet or aerosol form from one person to another [6]. Apart from respiratory secretions, urine, stool and close proximity with patients may be the sources for dissemination of SARS-CoV-2 [7, 8]. Based on phylogenetic studies, the genomic sequence of SARS-CoV-2 virus is 96% similar to bat corona viruses so these bats may be potential reservoir host for human corona virus [9]. However, there are

no clear evidences which have suggested virus transmission directly from bats to human population [10]. Further, some studies suggested that pangolins can be considered as intermediate hosts between bats and human [11, 12]. Severe infectious covid-19 cases have rapidly progressed to dyspnoea, shock and acute respiratory distress [13]. In addition, the other organ dysfunctions have also been reported from patients including severe cardiac injury, acute renal, gastrointestinal, liver injury, neurological defect along with coagulation impairment and death [13].

It is important to understand the virus biology, and replication cycle to identify effective therapies against SARS-CoV-2, because most of therapies are directly targeting the stages in the virus life cycle. Highly pathogenic SARS-CoV-2 are enveloped, single-stranded positive sense RNA betacoronavirus with size ranging from 80–120 nm, and their genomes encode non-structural proteins (nsps), structural proteins, and several accessory proteins [14]. Genome of RNA virus contains ten open reading frames (ORF1–10) and has total 29,903 nucleotides [15]. Among the ten ORFs ORF2–10 generates four structural proteins S (spike), N (nucleocapsid), E (Envelop protein), M (Membrane protein) along with auxillary proteins. However, large replicase polyproteins (PP1a/b) encoded by ORF1ab further gets cleaved by proteolytic enzymes into non structural proteins (nsp1–16) [15].

The SARS-CoV-2 virus entry in host cell is mediated with attachment of the spike (S) glycoprotein with the host angiotensin-converting enzyme 2 (ACE2) receptor thereby infection process starts [16]. Further, virus S protein cleaved by the cathepsin L proteases which get activated in a pH- dependant manner allows the release of viral genome into host cell cytoplasm [17]. In addition, other host cell proteases like TMPRSS2 (Transmembrane Protease Serine Type-2) and TMPRSS11D (Airway trypsin like protease) participate in the cleavage of spike protein into its constituents (S1 and S2) which further lead to entry of virus genome into host cell through non endocytic pathway [18]. S1 subunit of spike protein possesses receptor binding domain (RBD) which binds with host receptors and S2 subunit favors fusion of viral membrane with host cell [19]. Once the virus genome released inside the host cell, then host ribosomes are involved in the process of translation of virus genome containing ORF1ab into replicase polyproteins PP1ab [20]. These PP1ab further cleaved by important viral proteases include 3CLpro (3 chymotrypsin like proteases) and PLpro (papain like proteases) to generate nsp2–16 [20]. These nsp2–16 are participated in virus replication and transcription complex, while virus structural proteins are translated from another ORF2–10 containing viral genome and contribute to outer structure of virus [21]. At last, the newly born virions are delivered outside the infected cell by exocytosis after completion of their structural assembling in the endoplasmic reticulum golgi bodies complex [22].

The WHO (world health organization) has declared covid-19 a public health emergency due to its high spreading potential across the world. Although, vaccine development trial has almost finished and vaccination drive is going to be started. However, till now there are no effective therapies or specific drug candidates against this communicable disease. Thus, it is required to understand detail biology of virus (SARS-CoV-2) to further elucidate novel drug therapeutics.

#### **2. Therapeutic agents to tackle the covid-19 infection**

#### **2.1 ACE-2 modulators**

Like SARS-CoV, it is confirmed that SARS-CoV-2 virus also interacts with ACE-2 human enzyme for entry and replication into the host cell. SARS-CoV-2 spike protein has high binding affinity with ACE-2 enzyme present in respiratory epithelial

*Different Therapeutic Strategies to Tackle the Infection Associated with COVID-19 DOI: http://dx.doi.org/10.5772/intechopen.96899*

cell of host [23]. Hence, the therapeutics which inhibit spike protein-ACE-2 interaction would be considered effective therapy against SARS-CoV-2. ACE-2 enzyme is a zinc metalloproteinase which contains two domains in its structure which include amino terminal domain and carboxy terminal domain [5]. ACE-2 enzymes are exhibits in type-I and type-II alveolar cells of respiratory tract, liver, kidney, testes, heart and intestine [24]. Wu et al. [25] have found that ACE-2 enzymes are highly expressed in alveolar epithelial type-II cells in an around 83% which indicate these cell can be served as reservoir for virus. ACE-2 enzyme is a key regulator protein of RAS (Renin-Angiotensin System) system which contributes to vascular homeostasis [26]. In RAS system, angiotensinogen glycoprotein is cleaved by renin enzyme present in kidney to angiotensin-I, which has converted into angiotensin-II (Ang-II) by ACE-1. Further, Ang-II binds to angiotensin receptor (ATR1) and produces vasoconstriction, cell proliferation, inflammation, thrombosis and vascular constriction [27]. For the counterbalance of AngII- ATR1 axis effect, AngII is cleaved by ACE-2 enzymes into Ang1–7 peptides [28]. These angiotensin peptides further act on MASR (mitochondrial assembly receptor) and exhibits protective effects such as anti-inflammatory, anti-apoptotic and vasodilatation. Rothlin and co-worker [29] have reported the protective effects of ACE inhibitors and angiotensin receptor blockers (ARBs). They have found that patients with Covid-19 infection are taking ACE inhibitors and angiotensin receptor blockers exhibited lower mortality as compared with non-user patients. Previous study revealed that SARS-CoV virus down-regulates the ACE-2 enzymes present in host cell surface and increased ACE enzyme activity which lead to severe lung injury [30]. Increased ACE enzyme activity has been observed in SARS-CoV-2 patients. Hence, it has been proposed that delivery of soluble ACE-2 recombinant protein compete with host ACE-2 enzymes for the SARS-CoV-2 spike protein and ultimately neutralize the virus load and further, protect the patients from lung injury. For this purpose, recombinant human ACE-2 such as APN01 and GK2586881 have been analyzed and found effective in patient suffered from acute severe respiratory syndrome (**Figure 1**) [31].

#### **2.2 TMPRSS2 inhibitors**

Transmembrane serine protease-2 (TMPRSS-2) is present in host epithelium cells of various tissues [32]. It is involved in the pathogenesis of SARS-CoV-2 through cleavage of spike protein and facilitates virus entry into host cell [33]. Matsuyama et al. [34] demonstrated that over-expressed protein TMPRSS-2 containing vero E6 cell lines are susceptible for SARS-CoV virus infection and used as pharmacological tool for SARS-CoV-2 research. Thus any drug candidate which inhibits TMPRSS-2 protease may be effective in SARS-CoV-2 infection. In this regard, *in vitro* study conducted against SARS-CoV-2 to check the efficacy of compound camostat mesylate blocked the spike mediated virus entry in caco-2 cells [35]. In addition, some repurposing studies have been conducted to evaluate the efficacy of other proteases inhibitors such as nafamostat, 4-(2 aminomethyl) benzenesulfonyl fluoride and mucolytic drug bromhexine which can offer new therapeutic option for this pandemic (**Figure 1**) [36, 37].

#### **2.3 JAK–STAT inhibitors**

In covid-19 infection, patients suffer from severe acute respiratory syndrome in which huge amounts of various cytokines are released from immune cells leading to multiorgan failure and death [38]. Moreover, JAK–STAT signaling pathway is involved in SARS-CoV-2 virus entry into host cell which has linked with AAK1 (Adaptor-associated protein kinase-1) related clathrin mediated endocytosis [39].

**Figure 1.**

*Diagram showing the therapeutics approach to treat the infection of covid-19 patients.*

Activation of JAK–STAT pathway through Ang-II/AT1R may generate cytokines storm including IL-1, IL-2, IL-6, IL-7, IL-10 and TNF-α [39]. Thus, it may be suggested that JAK–STAT inhibitors could be potential therapeutics for the covid-19 infection. Further, Junior and co-workers [40] have shown that baricitinib, a JAK–STAT inhibitor, has potential to prevent generation of cytokines through inhibition of JAK–STAT signaling and also block the entry of SARS-CoV-2 via inhibiting AAK1 related clathrin mediated endocytosis (**Figure 1**).
