Virus-, host-, immune-based targets for COVID-19 therapy.

A clinical case with pneumonia-like symptoms caused by SARS-CoV-2 was first recorded in the Wuhan city of Hubei province, China [1]. Within a few weeks of its emergence, more than 90, 395 Chinese individuals, including the health care professionals and other common civilians were affected by COVID-19 until Sep 1st, 2020. Anti-viral agents against different targets had exhibited profound therapeutic effect on SARS-CoV-2 through which the clinicians were able to control the COVID-19 outbreak. Indeed, it is alarming to realize that more than 200 countries globally had been affected by the COVID-19 virulence of which about 25, 525, 569 COVID-19 cases were recorded till Sep 1st, 2020. Countries like the United States of America, Russia, Italy, India and Germany got immensely affected by the COVID-19 outbreak with the increased mortality.

Clinicians worldwide have been voraciously seeking for a potential anti-COVID-19 drug of all modules such as vaccines; target-specific monoclonal antibodies; viral oligonucleotide-based peptide drugs; interferons and other small bio-actives [2]. Unfortunately, there aren’t any proficient anti-COVID-19 drugs discovered yet through the clinical trials conducted on COVID-19 patients across global countries. Including SARS-CoV-2, till-date, there are seven potential viral strains of Coronaviridae family have invaded the Sapiens that include: HCoV-229E and HCoV-NL63 (of α-corona virus genera); HCoV-OC43 and HCoV-HKU1 (of β-corona virus genera); Severe Acute Respiratory Syndrome Virus (SARS-CoV); and Middle East Respiratory Syndrome Virus (MERS-CoV). The genome-based analysis revealed SARS-CoV-2 sharing a close phylogenetic relationship with 88% genome identity with the SARS-CoV viral strains of bat origin: Bat-SL-CoVZC45 and Bat-SL-CoVZXC21. Alternatively, SARS-CoV-2 genome shares 79% gene homology with SARS-CoV (of Sarbecovirus subgenus) and 50% gene homology with MERS-CoV (of Merbecovirus subgenus) [3].

The genome structure of SARS-CoV-2 comprised of minimum 10 open reading frames (ORFs) with two-third (approx. 67%) of the genome encodes two types of replicase polyprotein defined as pp1a and pp1b. Whereas the remaining one-third (approx. 33%) of the SARS-CoV-2 encodes for four major structural proteins that are essential for its replication such as Spike (S); Envelope (E); Membrane (M); and Nucleocapsid (N) proteins. The spike protein of SARS-CoV-2 constitutes two subunits S1 and S2 that paves the viral attachment to the host during its initial stages of colonization in the upper bronchial system. Alike SARS-CoV, SARS-CoV-2 binds to the angiotensin converting enzyme-2 receptor (ACE2) and/or dipeptidyl peptidase 4 receptor (DPP4) during its early stages of invasion of the host in the upper respiratory system. Along with these crucial viral proteins, the other non-structural proteins of SARS-CoV-2 such as papain-like protease (PLpro), 3-chymotrypsin-like protease (3CLpro), RNA dependent RNA polymerase (RdRp), and ATP-dependent Helicase (Hel) have been targeted to evade the initial colonization of SARS-CoV-2 on the host. It is both the innate and the adaptive immune response gets triggered upon the SARS-CoV-2 invasion. However, the innate and the adaptive immune response might get comprised in the immune-compromised individuals compared with the other healthy counterparts.

The replication pattern of SARS-CoV-2 mimics the other viruses of Coronaviridae family whereby it replicates in this sequential order: 1) Invasion 2) Replication; 3) Genome-assembly 4) Exocytose of virus expulsion in the cytoplasm [4] (Supplemental Fig. 1). Coronaviruses invade the host by both endosomal and non-endosomal (at the cell surface) pathways. Henceforth, the viral protein kinases and its associated signalling cascades have been targeted to attenuate the coronaviruses replication, especially SARS-CoV-2.

Clinicians being pertinently seeking for the drugs with a higher proficiency to break the human to human transmission chain of COVID-19; here we are the first to classify the therapeutic targets of COVID-19 into three major types (Supplemental Fig. 1): 1) Virus-based targets 2) Host-based targets 3) Immune-based targets. All promising therapeutic targets and its significance have been discussed briefly under each of these classified therapeutic targets to benefit experts from different fields.

Virus-based targets

S protein being the crucial protein facilitating the SARS-CoV-2 entry into the host has been preferred as a potential therapeutic target of interests as it could be spliced into two individual peptides by the furin-like proteases [5]. Arbidol and Griffithsin are the noticeable peptides targeting different regions of S protein. Also, the former peptide hinders the collision of the endosomal membrane of COVID-19 with the host preventing the viral entry. Commonly, drugs like mycophenolic acid that primarily targets the viral replication have been classified multifaceted because of its ability to exhibit its various modules of anti-viral activity. The other novel drug-like K22 that inhibits the viral-dependent RNA synthesis exhibited strong anti-replicative activity against the coronaviruses in an in-vitro set-up. The first chosen anti-viral target against SARS-CoV-2 was PLpro whose inhibitor lopinavir has effectively controlled the SARS-CoV-2 replication. Lopinavir and ritonavir being the active inhibitors of 3CLpro exhibited an effective anti-COVID-19 activity on the study patients. Clinical drugs like favipiravir, galidesivir and BCX-4430 that inhibits RdRp, have been tested on the COVID-19 patients. Concomitantly, ribavirin, a potent inhibitor of RdRp, produced a strong inhibitory action against the COVID-19 replication. Following remdesivir has also been proven to be effective in attenuating the COVID-19 virulence on the affected patients [6]. Moreover, siRNA molecules by inhibiting RdRp controlled the SARS-CoV-2 multiplication in an in-vitro condition that could benefit the COVID-19 patients. Ivermectin and HE602 drugs that target Hel which is crucial for the unwinding of oligonucleotides duplexes of SARS-CoV-2 during its replication has been recommended for the treatment of COVID-19 virulence. Baloxavir that inhibits cap-dependent endonuclease, and LJ001 a membrane-binding photosensitizer that intervenes the SARS-CoV-2 collision with the host cell membrane have been suggested for the COVID-19 management.

Host-based targets

The SARS-CoV entry has been facilitated by two host-dependent pathways such as endosomal and non-endosomal. During the endosomal entry, the host-mediated cathepsin synthesis, a pH-sensitive cysteine protease cleaves the S-glycoprotein of SARS-CoV-2. While, with the non-endosomal entry, the host-dependent transmembrane serine protease 2 (TMPRSS2) paves the SARS-CoV invasion by cleaving its S-glycoprotein [7]. The idea of preventing the viral entry by blocking TMSPSS2 using the recently developed therapeutic bio-actives like ABBV-075, bicalutamide, apalutamide, and bromhexine hydrochloride could break the human to human transmission chain of COVID-19 in both the weak and healthy individuals. Seeking bio-actives targeting cysteine proteases are under progress. While the other potential clinical agents like dec-RVKR-CMK, agmatine and andrographolide could attenuate the COVID-19 virulence by inhibiting furin (a host-dependent serine endo-protease) secretions. SARS-CoV-2 showing a higher binding affinity for angiotensin-converting enzyme-2 (ACE2) receptor could be treated by chloroquine phosphate, a potent inhibitor of ACE2 receptor glycosylation; that gathered wide attention among the clinicians to tackle the COVID-19 invasion. Additionally, there are other efficient clinical bio-actives like MLN-4760, N-(2-aminoethyl)-1-aziridine-ethanamine (NAAE) and glycyrrhizin that disrupts the interaction of SARS-CoV-2 with ACE2 has been highly preferred by the clinicians to control the COVID-19 virulence. However, the mechanism of MLN-4760 is clearly understood; that induces the conformational changes of ACE2. Alternatively, the mechanism of the other two bio-actives shown above remains unveiled. Chlorpromazine and ouabain as an effective inhibitor of clathrin-mediated SARS-CoV-2 binding to the host surface receptor could inhibit the proliferation of SARS-CoV-2.

Immune-based targets

Stimulation of innate immune response is crucial for controlling the SARS-CoV-2 replication and its virulence on the infected hosts [8]. The activation of apoptosis by Fas to Fas receptor binding is also essential for enhancing innate immunity in the host. Mesenchymal stem cells that are exclusively engaged with the tissue repair process controls the immune response through enhanced cytokines and growth factor secretions. Thymosin together with the other clinical drugs could enhance the host immune response against the viral invasion. Parallelly, the interferons also mediate the synthesis of many anti-viral peptides, including, protein kinase R (PKR) in the host. Interferons are efficient intermediate signalling proteins secreted by the human body; ameliorates the body resistance to the pathogens by inhibiting the supportive host-protein synthesis essential for the pathogen colonization. Also, interferon by stimulating the RNAse L secretions degrades the viral RNA, which is crucial for the host-dependent viral protein secretions and its associated replication. Being a potential regulator of interferons, the CYP-calcineurin-NFAT could exhibit a profound anti-COVID-19 response on the affected patients. Both cyclosporin A and cyclophilins could abolish the viral replication by triggering the strong immune response and by inhibiting the calcineurin secretions, respectively. Convalescent plasma therapy using the recruited patient's blood plasma with adequate antibody titer has been used to treat the severely ill COVID-19 patients. Site-specific monoclonal antibodies such as CR3022, tocilizumab, fingolimod, adalimumab, eculizumab, sarilumab, ixekizumab, and meplazumab have been prioritized for the clinical trials to subsidize the SARS-CoV-2. Along with these drugs that target the host immune response, there are other vaccines namely mRNA-1273, Ad5-nCoV and aAPC have been preferred for the clinical trials [9]. To date, close to 200 vaccines for the disease are under study, and several candidates have moved to phase III human studies.

Exploring the potential clinical targets for attenuating COVID-19 is essential for its long-term therapeutic management. Multifaceted drugs could help us to overcome the pathogenesis of COVID-19 and its unprecedented transmission among the global population in the nearest future.

Funding

This work was supported by the (No. 2020M670083ZX).

Declaration of competing interest

The authors declare no conflict of interests.