The recent outbreak of the coronavirus disease 2019 outbreak could force to impose major changes to health system across the globe and possible bring a major challenges to human being for the survival followed struggling to keep the global economy on correct pace, if the spread of the virus is not eff ectively controlled. [5, 6] The causative agent of this devastating disease COVID-19 belongs to Beta coronavirus which shares 89.1% nucleotide similarity with acute respiratory syndrome coronavirus (SARS-CoV). [7] The key amino acid residues are conserved in many viral key drug targets, including those found in both SARS-CoV and COVID-19 pathogens.
Consequently, many common viral targets are structurally similar to SARS-CoV and likely to be inhibited by the same compounds. The emergence of COVID-19 has severely compromised the arsenal of antiviral and antibiotic drugs. New compounds and targets are needed to meet the growing threat from Beta coronavirus. The possibility that COVID-19 and other viruses/bacteria can be perniciously engineered as biowarfare agents creates another demand for new treatment options. Drug discovery is a multistep process with a high attrition rate, substantial cost and slow pace of development of new molecules into the clinical candidate. On the other side, drug repurposing also called old drugs for new uses, is an attractive alternative approach for the new application of FDA approved or investigational drugs that are outside the scope of the original medical condition. [8] There is a plethora of literature evidence indicated the vital application of drug repurposing for various infectious diseases. [9] [10] [11] Given the current scenario of COVID-19 outbreak across the world and the complex nature of the disease repurposing of existing FDA approved drugs is the first line of the treatment.
Given the pandemic nature of the disease, there is a hard-pressing need to uncover the possible treatment/s as early as possible. The researchers and physicians have been putting sincere efforts to understand this new virus, the pathophysiology of the disease to and the possible therapeutic effective agents and vaccines. Tremendous effects have been done to study the newly emerged virus and find potent drugs for clinical usage. A PubMed search with key work COVID-19 resulted in more than 900 publications from November 01, 2019, to April 31, 2020. A glance at the title of these papers indicated that the majority are focused on manifestations and treatment options.
It has been shown in the literature that the FDA approved antimalarial and autoimmune disease drug chloroquine (CQ) and hydroxychloroquine (HCQ), known to inhibit viral infection by raising endosomal pH necessary for the interaction of virus/cell fusion. [12, 13] [14] CQ and HCQ interferes ACE2 glycosylation or glycosyltransferases within human cells or inhibits sialic acid biosynthesis by inhibiting quinone reductase 2. The immunomodulatory property of chloroquine synergistically involved with the anti-viral property. The antiviral drug remdesivir (RDV) can inhibit the growth of COVID-19 and found to be efficacious in the clinic in combination. [13, 15, 16] [17]The mechanism of action of RDV is under investigation, however, the high sequence similarity (>95%) between COVID-19 and SARS-CoV RNA-dependent RNA polymerase (RdRp) indicating the inhibition of RdRp. Similarly, the broad-spectrum virus-host cell fusion inhibitor marketed antiviral drug arbidol (ABD), prevents the entry of the virus entry into the host cell acting through ACE2 has entered into a clinical trial for the treatment of COVID-19. [18, 19] Inspired from the current pandemic situation and our experience in drug repurposing/drug design through computational and medicinal chemistry tools, we have carried docking studies with selected FDA approved drugs against COVID-19
selected key protein targets. Given the complex nature of this rapidly and severely infecting lower respiratory system disease, our hypothesis is to identify FDA approved drug combinations targeting the different key viral proteins . The selection of the key   proteins is based on the virus life cycle starting from attachment onto the human host, replication, viral latency (invasion from immunity), release and complex nature of the disease. The selection of four key targets: PDB ID: 6LZG which is the structure of novel coronavirus spike receptor-binding domain complexed with its receptor ACE2;
PDB ID: 6NUR a recently identified SARS-Coronavirus NSP12 bound to NSP7 and Further to expand the specificity, we performed online docking by using "The COVID- We will further improve our next publications. Table 6 ).
The receptor pocket and top 10 conformers are shown in figure 6 A. A.
B. ( Figure 8B) . The negative free energy calculation (-6.31 kcal/mole) as shown in table   5 is an indication of the interaction of ABD with nonstructural viral proteins. Our docking studies of ABD with NSP16-NSP10, however, our docking methodology did not work very well with this target resulted in the high positive values of binding energy (data are not shown here). We will further improve our next publications.
A. Here, we report the preliminary docking studies of the selected FDA approved drugs which have shown promising anti-COVID-19 activity. Literature evidences are indicating a single drug is probably not effective for the treatment of this disease. The viral polymerases and proteases are known to be the most suitable targets for the treatment of viral born disease. Therefore, we selected the targets to cover a range of targets starting from the entry point to the multiplication and release of the viruses. In 