SARS-CoV-2 and COVID-19: What are our options? Where should we focus our attention on to find new drugs and strategies?

Dear Editor,

Since the outbreak of COVID-19 many efforts are being made to engineer new drugs and to find ways of fighting this pandemic. We write this letter to focus everybody's attention on the possible and viable strategies to find new drugs and therapeutic compounds. In doing so we describe the virus cycle of infection, showing for each phase a possible compound of therapeutic use.

• (1) Adhesion and Viral Entry: This first phase consist of membrane interaction between the host cell and the virus, allowed by the ACE2 receptor. Cleavage and activation of SARRS-CoV spike protein (S) by a host cell protease is essential for viral entry, this host cell protease is called TMPRSS2. Potential in vitro proven inhibitor of TMPRSS2 is camostat mesylate [1].
• (2) Endocytosis: This second phase allows the virus inside the host cell. One known regulator of endocytosis is the AP2-associated protein kinase 1 (AAK1). Using Artificial Intelligence it was possible to find a good inhibitor of AAK1 like Baricitinib, a known Janus kinase inhibitor which can reduce both viral entry and inflammation in patients [2]. Another option in altering the endocytosis process is hydroxychloroquine, a derivate of chloroquine, which alters pH (by increasing it) of endosome and lysosome essential for membrane fusion between host cell and the virus. An in vitro experiment showed that in chloroquine treated cells endosomes vesicles were abnormally enlarged. This indicates an altered maturation process of endosomes, blocking endocytosis, resulting in failure of further transport of virions to the replication site [3].
• (3) Replication: Targeting the RNA-dependent RNA polymerase (RDRp) showed low specificity and low potency, nevertheless the most promising drug belonging to this class is Remdesivir. Therapeutic and prophylactic use of Remdesivir has recently been tested in nonhuman primate model of MERS-CoV infection. Administration of the drug 24 hours prior to inoculation prevented the virus from inducing clinical disease and prevented replication in respiratory tissue, thus preventing lung lesions to form. Therapeutic usage of remdesivir (12 hours after inoculation) showed similar results [4]. Human clinical trials are ongoing.
• (4) Protein Protease: Targeting the Main protease protein is another suitable option, preventing the virus from building its proteins. One example in doing so is the combination of Lopinavir/Ritonavir: which has shown to be of some use for SARS-CoV and MERS-CoV infected patients in the past.
• (5) Cytokine Storm: This has been shown to be the result of the secretion of many cytokines, during the infection, especially in patients with extensive lung injuries. SARS-CoV, MERS-CoV and SARS-CoV-2 show a relatively higher mortality rates then other coronaviruses, this might suggest that inflammatory responses might play a role in the pathogenesis. If that's the case, targeting the coronavirus alone with antiviral therapy might not be enough to reverse highly pathogenic infections. Among all SARS-CoV structural proteins (N, S, E and M) only the nucleocapsid protein (N) significantly induced the activation of interleukin-6 (IL-6) promotor in human airway epithelial cell cultures [5]. IL-6 gene expression is activated by the N protein which binds to the NF-kB regulatory element on IL-6 promoter and facilitates its translocation from cytosol to nucleus. The N protein is essential for IL-6 secretion to happen, since deletion of the C-terminus of the N protein resulted in the loss of function in the activation of IL-6 [5]. This shows how high levels of IL-6 are directly induced by SARS-CoV-2, besides being also associated with extensive lung injuries, making IL-6 a good therapeutic target, with drugs like Tocilizumab a known IL-6 inhibitor.
• 6) Free circulation: Targeting viral components such as the spike proteins of the virus with specific antibodies is another therapeutic option, this happens during free circulation of the virus. Coronaviruses neutralizing antibodies usually target the spike proteins(S) on the viral surface of the virus which allow it to enter host cells. The S protein has two subunits: one that mediates cell binding S1, and one that mediates cell fusion, the S2 domain. The most neutralizing antibodies are shown to target the receptor interaction site in S1 subunit. The latest developed antibody showing neutralizing properties is the Human 47D11 antibody, which was shown to target the S1B receptor binding domain of SARS-S and SARS2-S. Despite its capacity to inhibit infected cells with SARS-S and SARS2-S, the binding activity did not compete with S1B to the ACE2 receptor. This seems to indicate that 47D11 neutralizes SARS-CoV and SARS-CoV-2 through a yet unknown mechanism different from binding interference [6]. This study shows how, despite the high capability of the virus to mutate, targeting conserved core structure of the S1B receptor is a good strategy. This also provides evidence for the design of a SARS-CoV-2 vaccine composed of viral structural proteins, like the S proteins.

In conclusion we showed all phases of the virus cycle that can possibly be targeted, showing the most promising targets. We aimed to shed some light on the future drugs and therapies that need to be engineered in the future to come.

Declaration of competing interest

The author declares no conflict of interest.