key: cord-0986972-rszu1c48
authors: Twomey, Julianne D.; Luo, Shen; Dean, Alexis Q.; Bozza, William P.; Nalli, Ancy; Zhang, Baolin
title: COVID-19 update: The race to therapeutic development
date: 2020-10-24
journal: Drug Resist Updat
DOI: 10.1016/j.drup.2020.100733
sha: f838b1f4f9261b1b1d11f29bbca760ddfb3be644
doc_id: 986972
cord_uid: rszu1c48

The COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), represents an unprecedented challenge to global public health. At the time of this review, COVID-19 has been diagnosed in over 40 million cases and associated with 1.1 million deaths worldwide. Current management strategies for COVID-19 are largely supportive, and while there are more than 2000 interventional clinical trials registered with the U.S. National Library of Medicine (clinicaltrials.gov), results that can clarify benefits and risks of candidate therapies are only gradually becoming available. We herein describe recent advances in understanding SARS-COV-2 pathobiology and potential therapeutic targets that are involved in viral entry into host cells, viral spread in the body, and the subsequent COVID-19 progression. We highlight two major lines of therapeutic strategies for COVID-19 treatment: 1) repurposing the existing drugs for use in COVID-19 patients, such as antiviral medications (e.g., remdesivir) and immunomodulators (e.g., dexamethasone) which were previously approved for other disease conditions, and 2) novel biological products that are designed to target specific molecules that are involved in SARS-COV-2 viral entry, including neutralizing antibodies against the spike protein of SARS-COV-2, such as REGN-COV2 (an antibody cocktail) and LY-COV555, as well as recombinant human soluble ACE2 protein to counteract SARS-COV-2 binding to the transmembrane ACE2 receptor in target cells. Finally, we discuss potential drug resistance mechanisms and provide thoughts regarding clinical trial design to address the diversity in COVID-19 clinical manifestation. Of note, preventive vaccines, cell and gene therapies are not within the scope of the current review.

Fusion peptide (FP) and heptad repeat (HR) regions within the S2 subunit mediate membrane fusion and release of viral RNA. Therapeutic potential of these regions gained attention during previous efforts towards treatments of respiratory illness caused by the closely related coronavirus, SARS-CoV. It is important to note that the S2 subunit is less exposed than S1 and may only be accessible following conformational changes (Coutard et al., 2020; Xia et al., 2020) . Nevertheless, examining virus binding and entry as a critical checkpoint within the virus life cycle reveals several targets that are likely beneficial to reducing virus spread within the host system.

After releasing viral RNA into the cytoplasm, the virus hijacks the host translational machinery to translate the RNA into two large polyproteins, which will ultimately produce the replicasetranscriptase complex (RTC) through viral protease cleavage . The importance of viral replication proteins, particularly the viral proteases, main protease (Mpro), papain-like protease (PLpro), as well as RdRp activity of the RTC is evidenced by the functionality conservation across coronaviruses (Li and De Clercq, 2020) . Blockade of these proteins with inhibitors, such as boceprevir (an inhibitor of Mpro) and antivirals that block viral RNA polymerase activity may alleviate SARS-CoV-2 infection (Choy et al., 2020; . The RTC drives production of both negative sense RNA and positive sense RNA with RdRp and helicase functionality Thiel et al., 2003) . Discontinuous transcription of negative sense RNA yields the subgenomic RNA (sgRNA) to be translated into viral accessory and structural proteins, including the S, M, N, and E proteins. Most of these proteins localize to the endoplasmic reticulum (ER) where they are folded and post-translationally modified before being transported to the ER-Golgi intermediate compartment (Fehr and Perlman, 2015) . Mature virions are an assembly of nucleocapsid-coated genomic RNA and structural proteins that are released from the cell via exocytosis. Once released, virion particles may interact with other ACE2 expressing cells in various organs, including the heart, liver, and kidney .

A critical approach to treating COVID-19 patients, particularly those with severe respiratory or cardiovascular complications may be through regulating the virus-mediated immunopathology and host immune response to the viral infection. The initial host immune response involves the activation of the type I and III interferon (IFN) responses, followed by activation of IFN-stimulated J o u r n a l P r e -p r o o f genes (ISGs) as well as later release of pro-inflammatory cytokines and chemokines, which mount an antiviral defense against the viral infection (Cardone et al., 2020) . However, coronaviruses, including the novel SARS-CoV-2, have been documented to employ strategies towards blocking and evading this response, resulting in dampening of antiviral mechanisms and increases in viral replication, titer, and spread (Blanco-Melo et al., 2020; Park and Iwasaki, 2020) . Given the critical role of the IFN system in viral defense, exogenous IFN as a therapeutic has been explored and will be discussed in greater detail in the following sections.

Other mechanisms employed by the immune system to combat the virus can also play a role in disease severity (Cardone et al., 2020) . With continuing viral replication, infected cells can become apoptotic, releasing increased levels of pro-inflammatory cytokines or damage-associated molecular patterns (DAMPs) , which may lead to increased tissue damage (Cardone et al., 2020) .

In some settings, the release of high levels of pro-inflammatory cytokines may be associated with severe symptoms and virus persistence (FIG. 1) . Elevations have included IL-6, IL-1β, and TNFα, with high levels of IL-7, IL-8, IL-9, IL-10, IFN-γ, TNF, MCP1, MIP1A, MIP1B, G-CSF, GM-CSF having also been observed in COVID-19 patient plasma . In some patients, this event is believed to promote cytokine release syndrome (CRS) or "cytokine storm" resulting in the recruitment of hyperactive macrophages and monocytes to sites of infection (FIG.   1) . The continuous activation of immune cells leaves damaging effects ranging from capillary damage to multiorgan failure. In severe patients, initial lung injury can progress into ARDS, a major contributor to COVID-19 mortality . Thus, to mitigate the harmful effects of the host immune response, a considerable portion of therapeutic programs are aimed towards reducing the overproduction of pro-inflammatory cytokines and associated cytokine storm.

The current review highlights the potential therapeutic strategies for the treatment of COVID-19, including small molecule drugs and therapeutic proteins to target the SARS-CoV-2 viral entry, viral amplification or the host immune responses. However, preventive vaccines, cell and gene therapies are not within the scope of this review.

While treatment options that prevent infection or viral replication of SARS-CoV-2 are yet to be approved, the availability of therapeutics with antiviral, anti-inflammatory, or immunomodulatory J o u r n a l P r e -p r o o f activities provide many potential candidates (Table 1) . Using drugs previously approved for other indications takes advantage of existing detailed information on human pharmacology and toxicology to expedite clinical trials and regulatory review. Indeed, the majority of ongoing clinical trials for COVID-19 are aimed to evaluate the safety and efficacy of the existing drugs, previously 

One way these drugs are being used is to target the endolysosomal pathway that SARS-CoV-2 uses to enter target cells. Chloroquine (CQ) and hydroxychloroquine (HCQ), the antimalarial drugs that increase pH in the endosome, lysosome, and Golgi apparatus have also been reported to alter ACE2 terminal glycosylation, impacting SARS-CoV binding to ACE2 (Fox, 1993; Mauthe et al., 2018; Vincent et al., 2005; Yao et al., 2020) . Azithromycin, a broad-spectrum macrolide antibiotic that induces lysosomal alkalization, has been reported to have potential in vitro activity against H1N1 influenza virus by interfering with its internalization (Renna et al., 2011; Tran et al., 2019) . To prevent S priming, camostat, a pancreatitis drug that inhibits TMPRSS2, was found to inhibit SARS-CoV-2 infection in human lung cells . While camostat and azithromycin are currently under investigation, the potency of CQ/HCQ in COVID-19, which initially showed promise, was later challenged due to a lack of efficacy by larger studies (Boulware et al., 2020; Horby et al., 2020b; Kupferschmidt, 2020; World Health Organization, 2020d ). An Emergency Use Authorization (EUA) for CQ and HCQ, which had been authorized by FDA to allow certain uses of those drugs in the context of initial reports of potential benefit and widespread expert interest, was revoked based on FDA's ongoing assessment of available scientific information associated with the emergency use of those products (Hinton, 2020) . Specifically, FDA determined that CQ and HCQ are unlikely to be effective in treating COVID-19 for the authorized uses in the EUA. Additionally, in light of ongoing serious cardiac adverse events and other serious side effects, FDA determined that the known and potential benefits of CQ and HCQ no longer outweigh the known and potential risks for the authorized use.

Many clinical strategies to inhibit viral replication and release involve the use of repurposed antivirals (Artese et al., 2020; Drożdżal et al., 2020; Pawar, 2020) . Drugs that have been under study for potential uses in COVID-19 include remdesivir, a broad-spectrum RdRp inhibitor that was previously studied for Ebola treatment, and favipiravir (obtained conditional marketing approval in Japan for the treatment of influenza), and the approved HIV protease inhibitor combination lopinavir/ritonavir (Beigel et al., 2020; Furuta et al., 2017; Grein et al., 2020; McMahon et al., 2020; Sheahan et al., 2017; .

Preclinical studies have shown that remdesivir inhibited replication of SARS-CoV, MERS-CoV, and SARS-CoV-2 in vitro (Agostini et al., 2018; Brown et al., 2019; Sheahan et al., 2017; Sheahan et al., 2020; , which prompted several clinical studies including the ACTT trial sponsored by the U.S. National Institute of Allergy and Infectious Diseases (NCT04280705) and the SOLIDARITY trial sponsored by WHO (World Health Organization, 2020c). Preliminary report from randomized, double-blind, placebo-controlled phase-3 ACTT trial showed that remdesivir shortened the recovery time in hospitalized patients with COVID-19 (Beigel et al., 2020) . As a result, remdesivir was granted EUA in the US (U.S. Food and Drug Administration, 2020a), Special Approval for Emergency in Japan (Pharmaceuticals and Medical Devices Agency, 2020), and Conditional Marketing Authorization in the EU (European Medicines Agency, 2020), where the safety and efficacy will be continuously evaluated through ongoing trials. However, the interim results from the randomized open-label SOLIDARITY trial reported little or no effect of remdesivir on overall mortality, initiation of ventilation and duration of hospital stay among hospitalized patients (Pan et al., 2020) . Moreover, the lopinavir/ritonavir combination was reportedly ineffective in hospitalized COVID-19 patients also based on results from randomized open-label trials RECOVERY Collaborative Group, 2020; World Health Organization, 2020d) .

Exogenous IFN treatments, which have been previously approved for multiple sclerosis, are being tested for their antiviral and immunomodulatory properties. Clinical trials are focused on testing subcutaneous administration or nebulized IFN β, with direct inhalation into the lungs of patients, to stimulate an immune response. IFN β is an endogenously produced cytokine protein with antiviral effects, which may be deficient in lung cells of COVID-19 patients. Therefore, restoring J o u r n a l P r e -p r o o f type I IFN levels might reduce the viral load in COVID-19 patients (Mantlo et al., 2020) . However, the interim results from the WHO SOLIDARITY trial reported that IFN-β1a was ineffective in hospitalized patients (Pan et al., 2020) . This has only been tested in small trials, with larger trials being necessary to fully understand the treatment results. Pegylated IFN λ, which is known for its antiviral activity independent of type I interferons, is being tested in clinical trials to prevent infection and to slow viral replication while reducing inflammatory damage (Prokunina-Olsson et al., 2020) . IFN λ, may have a narrow therapeutic window due to the heightened risk of bacterial infections, impacting the duration or timing of administration (Broggi et al., 2020).

As described above, COVID-19 patients with severe illness often present increased levels of cytokines, chemokines, T lymphocytes, NK cells, and growth factors in plasma (Huang et al., 2020b) . The use of immunomodulators to reduce serum levels of pro-inflammatory cytokines might be helpful in mitigating immune-related symptoms. Severe COVID-19 cases are also linked to high levels of serum IL-6, which can be a component of cytokine release syndrome (Filocamo et al., 2020; . Several clinical trials are evaluating the potential use of the existing anti-inflammatory therapeutic proteins (Guaraldi et al., 2020) including (i) IL-6R blockers tocilizumab (NCT04381936), indicated for treatment of rheumatoid, giant cell, and juvenile idiopathic arthritis, as well as cytokine release syndrome, and sarilumab (NCT04315298), indicated for rheumatoid arthritis (RA); (ii) IL-1R blocker anakinra (NCT04330638), a recombinant protein drug also treating RA; (iii) TNF-α blocker infliximab (NCT04344249), another biologic for RA; and (iv) IFN-γ blocker emapalumab (NCT04324021) approved for primary hemophagocytic lymphohistiocytosis treatment (Filocamo et al., 2020; Spinelli et al., 2020) . However, recent results from a phase-3 trial of tocilizumab (Roche, 2020) and sarilumab (Sanofi, 2020) were both disappointing. Additional therapies that are commonly used for allergies or heartburn are being tested to potentially reduce cytokine storm in pulmonary tissues (Hogan et al., 2020) . Histamine blockers (the H1 receptor antagonist Cetirizine and H2 receptor antagonist Famotidine) may control the hyper-immune response .

Several small molecule drugs with anti-inflammatory activity are also being tested for COVID-19.

Among these, the corticosteroid dexamethasone has been reported to decrease the risk of death in COVID-19 patients with severe respiratory complications (Horby et al., 2020a) . The FDA recently added dexamethasone to the list of drugs for temporary compounding by outsourcing facilities and pharmacy compounders during the COVID-19 public health emergency to help prevent demand J o u r n a l P r e -p r o o f and supply interruptions (U.S. Food and Drug Administration, 2020d; University of Oxford, 2020a). WHO also strongly recommends corticosteroids (i.e. dexamethasone, hydrocortisone or prednisone) for the treatment of patients with severe and critical COVID-19 (World Health Organization, 2020b) . Other anti-inflammatory small molecule drugs include the anti-microtubule agent colchicine (NCT04322682) to block elongation of microtubules and interfere with cytokine and chemokine secretion, and the Janus kinase (JAK) inhibitors ruxolitinib and baricitinib (NCT04345289) to inhibit the release of pro-inflammatory cytokines (Spinelli et al., 2020) .

Heparin, previously approved as a blood thinner, was originally used in COVID-19 clinical trials to limit lung injury and prevent blood clot formation (Ayerbe et al., 2020; . Due to the affinity of the spike protein to cell surface glycans, Heparin, which is a secretory glycosaminoglycan (GAG), is now being tested as a decoy protein with administration in a spray and nebulized formulation (Dixon et al., 2020; Kim et al., 2020) . By binding to the spike protein within the blood, heparin may prevent the interaction of S protein with ACE2 preventing initial infection or viral spread.

The development of novel biotechnology products mainly focuses on preventing viral entry through targeting the interaction between the S protein and the ACE2 host cell receptor (FIG. 2C, Table 2 ). Several strategies are under clinical investigation, including recombinant proteins and monoclonal antibodies against the key molecules involved in SARS-CoV-2 viral entry.

Recombinant human soluble ACE2 (rhsACE2) protein represents a novel approach to preventing binding of SARS-CoV-2 to ACE2, by introducing a soluble form in excess to compete with endogenous membrane-bound ACE2 for S protein binding (Table 2) . Upon entering the bloodstream, the exogenous rhsACE2 is intended to bind and neutralize circulating SARS-CoV-2 virions, serving as a scavenger or decoy (Monteil et al., 2020; Tai et al., 2020; . rhsACE2 proteins were previously explored to treat acute lung injury, pulmonary hypertension, and ARDS, as a soluble additive protein to replace deficient ACE2 and as a potential soluble decoy protein to SARS-CoV (Hemnes et al., 2018; Imai et al., 2005) . In addition to inhibiting virus infection, treatment with rhsACE2 may reduce interleukins and TNFα production, and supplement endogenous ACE2 enzymatic activity, contributing to the regulation of the renin-J o u r n a l P r e -p r o o f angiotensin system (RAS) to prevent further lung injury (Hemnes et al., 2018; .

rhsACE2 was reported to be safe in healthy subjects and in patients diagnosed with ARDS, which led to multiple rhsACE2 therapies advancing to later phase clinical trials (Table 2) (Haschke et al., 2013; Ju et al., 2020; Khan et al., 2017) . A similar protein to rhsACE2 in development is a recombinant bacterial carboxypeptidase (B38-CAP), an ACE2 receptors-like enzyme that was explored previously for hypertension and cardiac dysfunction (Minato et al., 2020) .

Expanding on soluble ACE2, Fc-ACE2 fusion proteins are being designed with IgG1 Fc portion to prolong the circulating half-life of the rhsACE2 and modulate the immune system response Designing a therapy to target multiple epitopes of the SARS-CoV-2 spike protein may increase the potential of binding and neutralizing the virus. The ACE-MAB bispecific fusion protein has two functional arms, with one arm being a fully human antibody targeting the spike protein, to prevent binding to ACE2, and the other arm being a truncated rhsACE2 protein (Cel-Sci, 2020a).

Through targeting three parts of the spike protein, the anti-COVID-19 DARPin® fusion protein candidates are proposed to potentially block the binding of the spike protein to ACE2, block protease activation of the spike protein, and "cuff" the protein to prevent conformational changes (Balfour, 2020). Many of these Fc fusion proteins aim to not only be used as a treatment but to have prophylactic potential.

Small peptide-based therapies are also being developed that target the N protein and elicit cytolytic T cell response (Amawi et al., 2020; Cel-Sci, 2020a, b) . By targeting N protein antigens, those agents may have a broader antiviral potential as the N protein shares 90% amino acid homology with SARS-CoV and 48% with MERS-CoV (Grifoni et al., 2020) . Masking the sialic acid glycans on the host cell receptors in a patient's airway may reduce the binding of the spike protein and J o u r n a l P r e -p r o o f lower the risk of SARS-CoV-2 infection, potentially offering prophylactic activity (Pneumagen, 2020) . Preliminary data reported a reduction in SARS-CoV-2 infectivity with the multivalent carbohydrate binding molecule (mCBMs) Neumifil. Additional targeted therapies are under development, not specifically to prevent the viral infection, but to treat downstream pathology of COVID, such as acute lung injury, pneumonia, and ARDS. Recombinant human plasma gelsolin (rhu-pGSN) is under investigation to treat COVID-19-induced pneumonia (Bioaegis Therapeutics, 2020) and has been reported to reduce inflammation (DiNubile et al., 2020; Yang et al., 2015) .

Controlling inflammation to decrease potential organ injury may be important to improve recovery from COVID-19 infection.

Neutralizing antibodies employ the same strategy as the decoy ligands, aiming to block the interaction of the virus with the ACE2 host cell receptor, thereby inhibiting viral entry . Antibodies can also alter the conformation of a protein needed for viral entry or destroy infected cells through effector functions. Several antibodies are being developed against the spike protein (Table 3) , with the aim of blocking receptor binding to the host cell membrane (Eli Lilly, 2020; Wong, 2020) . Such antibodies have been designed based on convalescent serum isolated from patients who have recovered from severe SARS-CoV-2 infection, from phage display libraries, transgenic mice engineered to express the trimeric SARS-CoV-2 spike protein, B cell isolation, and 3D printed lymph nodes (3DPrint, 2020; BusinessWire, 2020; Hansen et al., 2020; Ju et al., 2020; Snyder, 2020; Zimmer, 2020) . Anti-SARS-CoV-2 antibodies are being isolated from wide populations of patients who have recovered from infection, with the hope of identifying a strong candidate that will effectively bind and neutralize the virus.

Using antibodies from patients who have recovered from SARS-CoV-2 infection as a molecular template follows a similar strategy as using convalescent serum from recovered patients; to provide a potential boost to the immune system or to provide a prophylactic immunity prior to infection. (Table 3 ). These cocktails have demonstrated an activity to reduce viral load in vitro as well as in animal models with lung inflammation and injury (Hansen et al., 2020; Regeneron, 2020; Snyder, 2020; Zimmer, 2020) .

Antibody therapy development is advancing towards cocktails against all coronavirus strains, projecting past development for SARS-CoV-2 (Taylor, 2020) .

Several antibody products are also being developed to control cytokine storm and treat COVID-19-induced pneumonia. These therapies include targeting of IL-6R (Levilimab) (NCT04397562), TLR4 (EB05) (NCT04401475), CXCL10 (EB06) and angiopoietin-2 (Ang2) (LY3127804) (NCT04342897). Targeting TLR4, which is upstream of IL-6, IL-8 and type I and type III interferons, or IL-6R may control the proinflammatory response in patients and suppress cytokine storm (Sallenave and Guillot, 2020) . EB05 and Levilimab both aim to inhibit cytokine release through blocking the inflammatory signaling cascade. Anti-CXCL10 aims to reduce the high expressions of the chemokine CXCL10, which has been associated with long-term lung injury (Oliviero et al., 2020) . Controlling excessive inflammation and cytokine release may reduce progression to ARDS in infected patients. Meplazumab, an anti-CD147 mAb, was developed to prevent the binding of the SARS-CoV-2 spike protein to CD147 host cells and to regulate cytokine secretion in patients with COVID-19 acquired pneumonia (Bian et al., 2020). CD147 was believed to be an additional host cell receptor to ACE2, which the virus utilized for internalization, though recent studies have demonstrated a lack of binding of CD147 to the spike protein (Shilts and Wright, 2020) .

Modulation of the immune system to selectively remove cells infected by SARS-CoV-2 may have potential in reducing the spread of the virus. A Tri-specific NK cell engager (TriKE) antibody is being developed, with the aim to engage NK cells and eliminate SARS-CoV-2 infected cells, before further virions are released into the blood stream (GT Biopharma, 2020). A similar approach involves a bi-specific antibody targeting the spike protein and the NKp46 NK cell receptor (Cytovia Therapeutics, 2020).

While most of the antibody therapeutics being developed are IgG, development of IgM and IgA therapies may potentially increase binding power as these antibodies have a greater number of J o u r n a l P r e -p r o o f binding domains than IgG (IgM Bio, 2020). IgM and IgA antibodies are based on naturally occurring antibodies following severe SARS-CoV-2 infection produced from single B cells. These antibodies can undergo transport across mucosal membranes, which may have potential for treating infected lung tissue.

Single-domain antibodies (sdAbs), or nanobodies, may potentially be used to neutralize the spike protein, through targeting a cryptic epitope in the trimeric spike protein interface (Table 3 ) (Wrapp et al., 2020; Wu et al., 2020c) . These nanobodies, designed from screening nanobody libraries and shark variable domain of immunoglobulin new antigen receptor (VNAR) phage display libraries, are composed of the complementary determining regions with molecular weights of 12-15 kDa (Jaimes et al., 2020; Kupferschmidt, 2020; Wu et al., 2020c) . Nanobodies are being developed against S protein and N protein to prevent viral protein-ACE2 receptor interactions. Nanobodies have demonstrated successful blocking of ACE2 binding and were shown to recognize RBD in different structural conformations (Huo et al., 2020) . These therapies have been proposed for use as inhalation agents and may potentially neutralize the virus in a dose-dependent manner.

The concerted effort of the global scientific community to address the COVID-19 pandemic has been remarkable. This includes the rapid launch of a large volume of clinical trials to investigate potential therapeutics for COVID-19 treatment, as well as preventive vaccines (not included in this review). To mount an immediate response to the health crisis, many existing drugs are being tested for use in COVID-19. These include antiviral drugs and immunomodulators that were previously approved for the treatment of other disease conditions. These drugs have a regulatory record and established safety profiles which help expedite the development process. The novel targeted therapies, including neutralizing antibodies and Fc-fusion proteins, are mainly targeting the SARS-CoV-2 viral entry step. Some of these products may prove to be helpful in preventing the spread of the virus in the body, thereby accelerating recovery after infection or providing a means of prophylaxis.

Still, the SARS-CoV-2 virus remains novel and many aspects of transmission, infection, and

treatment are yet to be unraveled. The current strategies under development mainly target ACE2 and S protein interaction for viral entry. Other potential therapeutic targets may include the sugar J o u r n a l P r e -p r o o f moieties as additional receptors on the host cells (Qing et al., 2020) , mediation of viral entry processes by accessory proteins, secondary infection to organs outside the respiratory system, and the dynamic immune response to the infection Liu et al., 2014) .

Further research into the pathophysiology of COVID-19 is needed to understand the mechanisms that result in severe manifestation of the disease. Since the beginning of the SARS-CoV-2 pandemic, the virus has been undergoing various mutations potentially impacting the effectiveness of the repurposed drugs or novel therapeutics, with constant surveillance identifying mutations in the RdRp and spike proteins (Pachetti et al., 2020; van Dorp et al., 2020) . Therapeutic development for COVID-19 will need to monitor the changing viral genome to stay ahead of potential drug resistance and understand how the virus evades the human immune system (Pachetti et al., 2020; van Dorp et al., 2020) .

Clinical trial design constitutes a critical component in the development of therapeutics against COVID-19. The FDA has established provisions for timely interaction and responses to sponsor inquiries and submissions to facilitate early discussion of efficient approaches to product development (U.S. Food and Drug Administration, 2020c). The course of COVID-19 disease is diverse, ranging from asymptomatic to fatal respiratory failure. It is therefore important to evaluate therapeutics in adequate and well-controlled clinical trials with defined populations to generate a safety and efficacy database that will best inform the clinical use of the drug or biologic.

This article reflects the views of the authors and should not be construed to represent FDA's views or policies.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

This work was funded by the U.S. Food and Drug Administration. The funder had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.

As there is a huge volume of therapeutic approaches for COVID-19, we may not cover all available therapeutic development programs. In addition, research results are dynamic and change as new evidence emerges. Second, the clinical trial information was derived from the U.S. National Library of Medicine ClinicalTrials.gov, which was last accessed on 17 October 2020. Third, only articles/publications/translations from English were analyzed so some relevant international data might be missing. 1) SARS-CoV-2 attachment to host receptor ACE2, and release of genomic RNA, through cleavage with TMPRSS2 or endo-/lysosomal proteases characterizes initial infection. Several agents can be employed to inhibit these interactions including neutralizing antibodies, recombinant human soluble ACE2 (rhsACE2), protease inhibitors, and endosomal pH modulators (Fehr and Perlman, 2015) . 2) Replication and translation of genomic RNA into structural proteins to assemble mature virions for release results in viral amplification and increases in infection. Antivirals inhibiting viral proteases and RdRp are being evaluated against this checkpoint (Fehr and Perlman, 2015) . 3) Due to viral replication, host immune responses are triggered that can cause hyperactivation of immune cells and constant production of pro-inflammatory cytokines, resulting in more severe disease presentation (Cardone et al., 2020; Huang et al., 2020b) . Novel and repurposed immunomodulators are under investigation to mitigate harmful immune responses. Angiotensin-converting enzyme 2 (ACE2); Transmembrane protease serine 2 (TMPRSS2); RNA-dependent RNA polymerase (RdRp); Main protease (Mpro); Papain-like protease (PLpro); IL (interleukin); Tumor necrosis factor α (TNFα) 

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Azithromycin blocks autophagy and may predispose cystic fibrosis patients to mycobacterial infection

Roche provides an update on the phase III COVACTA trial of Actemra/RoActemra in hospitalised patients with severe COVID-19 associated pneumonia

Innate Immune Signaling and Proteolytic Pathways in the Resolution or Exacerbation of SARS-CoV-2 in Covid-19: Key Therapeutic Targets?

Sanofi and Regeneron provide update on Kevzara® (sarilumab) Phase 3 U.S. trial in COVID-19 patients

A small molecule oxocarbazate inhibitor of human cathepsin L blocks SARS and Ebola pseudotype virus infection into HEK 293T cells

Broad-spectrum antiviral GS-5734 inhibits both epidemic and zoonotic coronaviruses

Comparative therapeutic efficacy of remdesivir and combination lopinavir, ritonavir, and interferon beta against MERS-CoV

No evidence for basigin/CD147 as a direct SARS-CoV-2 spike binding receptor

Vanderbilt

HiJAKing SARS-CoV-2? The potential role of JAK inhibitors in the management of COVID-19

Effect of Remdesivir vs Standard Care on Clinical Status at 11 Days in Patients With Moderate COVID-19: A Randomized Clinical Trial

SystImmune, 2020. COVID-19 Pandemic: Development of Recombinant Human ACE2 Fusion Protein Drug (SI-F019) for the Treatment of Patients with Novel Coronavirus Pneumonia

Characterization of the receptor-binding domain (RBD) of 2019 novel coronavirus: implication for development of RBD protein as a viral attachment inhibitor and vaccine

Anticoagulant treatment is associated with decreased mortality in severe coronavirus disease 2019 patients with coagulopathy

Celltrion plans July COVID-19 trial, advances 'super antibody

Mechanisms and enzymes involved in SARS coronavirus genome expression

Azithromycin, a 15-membered macrolide antibiotic, inhibits influenza A(H1N1)pdm09 virus infection by interfering with virus internalization process

Coronavirus (COVID-19) Update: FDA Issues Emergency Use Authorization for Potential COVID-19 Treatment

FDA Coronavirus Treatment Acceleration Program (CTAP)

Food and Drug Administration, 2020c. FDA Guidance for Industry -COVID-19: Developing Drugs and Biological Products for Treatment or Prevention

Food and Drug Administration, 2020d. Temporary Policy for Compounding of Certain Drugs for Hospitalized Patients by Outsourcing Facilities During the COVID-19 Public Health Emergency

Dexamethasone reduces death in hospitalised patients with severe respiratory complications of COVID-19

This national clinical trial aims to identify treatments that may be beneficial for people hospitalised with suspected or confirmed COVID-19

Emergence of genomic diversity and recurrent mutations in SARS-CoV-2, Infection

Chloroquine is a potent inhibitor of SARS coronavirus infection and spread

A human monoclonal antibody blocking SARS-CoV-2 infection

Remdesivir and chloroquine effectively inhibit the recently emerged novel coronavirus (2019-nCoV) in vitro

Phase I start marks first for anti-SARS-CoV-2 mAbs, first for AbCellera

World Health Organization, 2020a. Coronavirus -World Health Organization

Coronavirus disease (COVID-19): Dexamethasone

Solidarity" clinical trial for COVID-19 treatments

World Health Organization, 2020d. WHO discontinues hydroxychloroquine and lopinavir/ritonavir treatment arms for COVID-19

Structural Basis for Potent Neutralization of Betacoronaviruses by Single-Domain Camelid Antibodies

Risk Factors Associated With Acute Respiratory Distress Syndrome and Death in Patients With Coronavirus Disease

Analysis of therapeutic targets for SARS-CoV-2 and discovery of potential drugs by computational methods

Identification of Human Single-Domain Antibodies against SARS-CoV-2

Fusion mechanism of 2019-nCoV and fusion inhibitors targeting HR1 domain in spike protein

Plasma gelsolin improves lung host defense against pneumonia by enhancing macrophage NOS3 function

In Vitro Antiviral Activity and Projection of Optimized Dosing Design of Hydroxychloroquine for the Treatment of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2)

Rational Use of Tocilizumab in the Treatment of Novel Coronavirus Pneumonia

Perspectives on therapeutic neutralizing antibodies against the Novel Coronavirus SARS-CoV-2

First Antibody Trial Launched in COVID-19 Patients

FDA reissued emergency use authorization with revisions to allow for remdesivir to be used only to treat adults and children with suspected or laboratory confirmed COVID-19 administered in an in-patient hospital setting

Footnote: Selected clinical trials may have expanded or been withdrawn since publication as this is a rapidly expanding field. mAb, monoclonal antibody

ribonucleic acid; TMPRSS2, transmembrane serine protease 2

HIV, human immunodeficiency virus; IL, interleukin; TNF, tumor necrosis factor

The 

Endosome, lysosome, and Golgi apparatus Interferes with ACE2 glycosylation, virus-ACE2 interaction, and virus endocytosis WHO SOLIDARITY trial (international, >12000), NCT04381936/RECOVERY (UK, 4716) , NCT04334148 (US, 2000) , NCT04303507 (UK/Thailand, 40000) (Hinton, 2020; Horby et al., 2020b; US National Library of Medicine, 2020; World Health Organization, 2020c, d) CamostatSmall molecule Footnote: Listed therapeutics and clinical trials may have expanded or been withdrawn since publication as this is a rapidly expanding field. Therapeutics have been identified from Clinicaltrials.gov and publicly available literature searches. ACE2, Angiotensin converting enzyme 2; mAbs, monoclonal antibodies; IgM, Immunoglobulin M; IgA, Immunoglobulin A; sdAbs, single domain antibodies; IL-6R, interleukin-6 receptor; TLR4, Toll-like receptor 4 protein; CXCL10, C-X-C motif chemokine 10; C5a, complement component 5a; TriKE, Tri-specific NK cell Engager; n/a indicates that a clinical trial has not been registered with ClinicalTrials.gov