key: cord-1025749-hsu1w9ce
authors: Tian, Xiaolong; Li, Cheng; Wu, Yanling; Ying, Tianlei
title: Insights into biological therapeutic strategies for COVID-19
date: 2021-02-04
journal: nan
DOI: 10.1016/j.fmre.2021.02.001
sha: aa6fe3d76e9915dd434e16b767bda410340ff607
doc_id: 1025749
cord_uid: hsu1w9ce

The worldwide pandemic of novel coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that emerged in late December 2019 requires the urgent development of therapeutic options. So far, numerous studies have investigated and uncovered the underlying epidemiology and clinical characteristics of COVID-19 infections in order to develop effective drugs. Compared with antiviral small-molecule inhibitors, biotherapeutics have unique advantages such as few side effects by virtue of their high specificity, and thus can be rapidly developed for promising treatments of COVID-19. Here, we summarize potential biotherapeutics and their mechanisms of action, including convalescent plasma, therapeutic antibodies, peptides, engineered ACE2, interferons, cytokine inhibitors, and RNAi-based therapeutics, and discuss in depth the advancements and precautions for each type of biotherapeutics in the treatment of COVID-19.

In late December 2019, a previously undetermined acute respiratory disease called coronavirus disease 2019 (COVID-19) emerged, which is caused by severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2) [1] . By March 11, 2020, COVID-19 was declared a pandemic by the World Health Organization, and at the time of this publication, resulted in over 44,000,000 infections and more than 1,100,000 deaths worldwide (http://covid19.who.int) [2] . SARS-CoV-2 is an enveloped positive-sense RNA virus and belongs to the lineage B betacoronavirus that also includes the highly pathogenic coronaviruses MERS-CoV and SARS-CoV. Based on the phylogenetic analysis, SARS-CoV-2 shares high sequence identity with that of SARS-like coronaviruses (89.1% nucleotide similarity) and SARS-CoV (79% nucleotide similarity) [3, 4] . Like other coronaviruses, the structure of SARS-CoV-2 is composed of 16 nonstructural proteins (Nsps) (Nsp1-16), 5--8 accessory proteins, and 4 structural proteins including the spike (S), membrane (M), envelope (E) and nucleocapsid (N) proteins (Figure 1a , b) [5] . The S protein is a heavily glycosylated type I membrane protein and uniformly arranged as trimers anchored in the viral membrane [6] . The trimeric S protein consists of two fragments: the receptor-binding fragment S1 and the fusion fragment S2 (Figure 1a ) [7] , and is crucial for viral fusion, entry, and transmission. The critical step for SARS-CoV-2 entering host cells and establishing infection is receptor binding and membrane fusion. The receptor-binding domain (RBD) of S1 interacts directly with angiotensin converting enzyme 2 (ACE2) on the surface of host cells [8] . The S protein is naturally in a closed conformation while the helices in the S2 component are capped by the neighboring RBD. Following cleavage by furin between the S1 and S2 domains, S trimers are able to accommodate the RBD in an open, ACE2-binding conformation [9] . Binding of the ACE2 receptor to open the RBD leads to a more open trimer conformation. Then, the S2 fusion region is exposed and inserted into the host cell membrane [10, 11] , priming the internalization of SARS-CoV-2 into host cells by receptor mediated endocytosis [12] . Once inside the host cell endosomal compartment, there is an increase in H + influx into the endosome which activates cathepsin L to facilitate viral membrane fusion and release of RNA out of the endosome. Alternatively, following recognition by ACE2, proteolytic cleavage of the S protein by type II transmembrane serine protease (TMPRSS2) on the surface of host cell can induce direct fusion of the viral and plasma membranes leading to release of the viral RNA into the cytoplasm [13] . Next, polyproteins, such as pp1a and pp1ab, are translated, and cleaved by the Papain-like protease (Pl pro) and 3C-like protease (3CL pro) to form functional Nsps as a helicase or the RNA replicase-transcriptase complex (RdRp) [14] . Using these viral replicative enzymes, the viral RNA acts as a messenger RNA (mRNA) and then generates new RNA and the mRNAs for SARS-CoV-2 genome replication. RNA polymerization also relies on the RdRp. Subsequently, structure proteins of SARS-CoV-2 are translated by RdRp [15] , and fuse with the virus precursor which is then transported from the endoplasmic reticulum through the Golgi apparatus to the cell surface via small vesicles. Finally, SARS-CoV-2 is released from the infected cell through exocytosis and can infect other host cells (Figure 2 ) [13] . Noteworthily, increasing mutations occurring within the S protein-encoded genome have been detected. Among them, D614G shows fitness advantage and increases infectivity of the SARS-CoV-2, but not with disease severity [16] . Double mutations containing not only D614G but also P1263L, S943T, S939F, D936F, I472V, K458R, V341I, or L5F, as well as the single mutation A520S are also significantly more infectious, whereas most variants with amino acid change at RBD are less infectious, and V341I and investigational glycosylation mutant (N331Q+N343Q) have no infectivity [17] .

SARS-CoV-2 infection in humans can be asymptomatic or result in a range of mild to fatal disease [18] . Generally, COVID-19 patients with pneumonia present with fever, fatigue, dyspnea and a cough [19] . During infection, a local immune response is triggered by damaged lung cells. Clinical reports revealed that CD4 + T cells are rapidly activated to form pathogenic T helper (Th) 1 cells and produce granulocyte-macrophage colony-stimulating factor (GM-CSF). In most COVID-19 infections, increased plasma concentrations of inflammatory cytokines, such as interleukins (IL-2, IL-7, IL-10), chemokine CCL2 and TNF-α, are found, especially in critically ill patients [20] [21] [22] . Moreover, these cytokines contribute to CD14 + and CD16 + monocyte recruitment and IL-6 secretion, which further aggravate the inflammatory response ( Figure 3 ) [23] . In severe COVID-19 infections, the overreactive immune response results in cytokine storm and development of severe acute respiratory distress syndrome (ARDS), which can cause respiratory failure, multi-organ failure, and death [2] .

As an airborne virus, SARS-CoV-2 is transmitted through respiratory droplets and aerosols [24] . Moreover, SARS-CoV-2 infection can be asymptomatic, particularly in the younger population [25] , and highly contagious before symptom onset [26] . These altogether contribute to the spread of SARS-CoV-2 worldwide and make it more challenging to curb SARS-CoV-2 transmission compared to other respiratory viruses [27, 28] . Current clinical management of COVID-19 largely relies on infection prevention and supportive care. Epidemiological research showed that universal masking, extending social distancing and timely identification of infected individuals are the most effective preventive strategies in reducing the spread of COVID-19 [29] . Generally, 80% of patients with COVID-19 recover within 1 to 3 weeks without specific treatment. However, ∼20% of patients rapidly deteriorate within ∼7 to 10 days after symptom onset, and a small proportion (~5%) develop severe illness such as ARDS, ultimately resulting in death even under proper supportive care, which constitute a significant health and economic burden [30] . Thus, there is a critical need to develop vaccines and drugs to protect against and treat COVID-19.

Antifibrotic therapy is deemed to be effective in attenuating the progress of fibrosis and the progressive decline of lung function in COVID-19 patients when used early in SARS-CoV-2 infection [31] . A phase III trial showed that pirfenidone, an antifibrotic drug, reduced disease progression in patients with idiopathic pulmonary fibrosis, yet with no significant differences in dyspnea scores or rates of death when compared with the placebo group (NCT01366209). As an agonist of angiotensin II receptor type 2 that had been proved for inhibiting experimental acute lung injury and IL-6 [32] , C21 has been approved for a phase II study in COVID-19 (EudraCT 2017-004923-63). Corticosteroid compound, a type of anti-inflammatory drug, has also been suggested as a potential drug against the COVID-19, in light of that ciclesonide suppressed SARS-CoV-2 replication in cultured cells [33] and COVID-19 patients receiving dexamethasone showed significantly lower mortality and hospitalization duration than controls [34] . The antiviral drugs such as remdesivir, chloroquine, hydroxychloroquine, lopinavir/ritonavir, ribavirin, favipinavir and umifenovir, have been considered as attractive options for COVID-19 therapy [35] .

Among them, remdesivir, the first treatment for COVID-19 approved by FDA, was able to shorten the time to recovery from mild to fatal COVID-19 infections (NCT04280705), improve clinical status with five-day injection in moderate COVID-19 patients (NCT04292730), but couldn't change recovery rates or mortality rates in severe COVID-19 patients (NCT04292899). Similarly, chloroquine was highly effective in the control of SARS-CoV-2 in vitro [36] , yet administration of hydroxychloroquine showed higher adverse events and a non-significantly higher probability of negative conversion than in non-recipients [37, 38] . The combination use of antiviral drugs with traditional Chinese medicines has been found effective against the COVID-19 infection [39] .

Unlike most conventional drugs, biotherapeutics present higher potency and fewer side effects because of their high specificity. Biotherapeutics targeting COVID-19 have been rapidly developed and show promising clinical outcomes. In this review, we will review COVID-19 biotherapeutics involved in blocking viral fusion and entry, degrading the viral genome, preventing viral replication, enhancing antiviral innate immunity, and ameliorating the inflammatory cytokine storm. [40, 41] . Convalescent plasma treatment for SARS and severe influenza also appears safe and reduces mortality, especially if administered early in the illness [42] . For COVID-19, five patients at the Shenzhen Third People's Hospital in Shenzhen, China, were first treated with convalescent plasma, and all five patients were discharged or stable from respiratory failure following the transfusion. The second study in ten patients with severe disease reported that three had been discharged and the others were ready for discharge following transfusion. In contrast, among ten matched historical controls with similar baseline characteristics, three died, one improved and six stabilized. Sean et al. [43] performed a propensity score-matched case-control study to assess the effectiveness of convalescent plasma therapy in 39 patients, and results showed that supplemental oxygen requirements were reduced with increased survival in plasma recipients compared to the matched controls. The mass convalescent plasma clinical trials have since been initiated; however, many hospitals deployed convalescent plasma without clinical support data, mostly due to the urgency of the pandemic [44] .

It is worth noting that the plasma component can differ significantly among convalescent patients. The viral neutralization titer of collected plasma against SARS-CoV-2 should be measured before treatment. Otherwise, transfusions of plasma without neutralizing antibodies will have no impact on treatment [45] , and may even result in adverse immune responses caused by non-specific antibodies. Therefore, more large-scale clinical trials still need to be conducted for the evaluation of curative effects. Moreover, due to limited sources, convalescent plasma should only be used in emergency cases.

The antiviral activity of antibodies is mediated by the inhibition of viral entry into host cells (neutralization) and by the effector functions of antibodies as they recruit other components of the immune response ( Figure 4 ) [46, 47] . Human monoclonal antibodies are the most common therapeutic strategy for human viral infections due to their high specificity, strong neutralizing activity and potentially low immunogenicity. Trimeric S protein exposed on the viral surface, which mediates receptor binding and viral entry into host cells, is a dominant target for SARS-CoV-2 neutralizing antibodies [11] . Currently, multiple SARS-CoV-2 specific monoclonal antibodies against the S protein have been developed, and three monoclonal antibodies have been evaluated in Phase III trials (Table 1) . However, no therapeutic antibodies have been approved to treat COVID-19 to date.

The S protein shares high sequence similarity between SARS-CoV-2 and SARS-CoV, suggesting the possibility of conserved immunogenic surfaces on the RBD domain by cross-neutralizing antibodies. One antibody (named S309), potently neutralizing SARS-CoV-2 and SARS-CoV pseudoviruses as well as authentic SARS-CoV-2 [48] , was engineered to have a longer half-life and is currently in a phase III clinical trial, known as VIR-7831 and VIR-7832.

Besides, some of broadly neutralizing antibodies were effective in vitro. A previously identified SARS-CoV-specific human monoclonal antibody from a convalescent SARS patient [49] , CR3022, was confirmed to bind potently to the SARS-CoV-2 RBD [50] . The neutralizing activity of CR3022 against SARS-CoV-2 has not been conclusively defined, although one study reported neutralization through destruction of the prefusion S protein conformation [51] . In another study, H2L2 mice were immunized with the S protein of human coronavirus OC43 (HCoV-OC43), SARS-CoV, and MERS-CoV, which resulted in the identification of one monoclonal antibody, 47D11, with cross-neutralizing activity against SARS-CoV-2 and SARS-CoV [52] . Several monoclonal antibodies have also been identified that target the S glycoprotein of SARS-CoV-2 from memory B cells of an individual who was infected with SARS-CoV in 2003 [48] . Similarly, using single B-cell sorting, multiple human monoclonal antibodies against the viral spike protein of SARS-CoV-2 were isolated from the memory B cells of a survivor infected with SARS-CoV [53] . Eight RBD-targeted antibodies showed potent and broad neutralization against SARS-CoV-2, SARS-CoV, and representative SARS-like virus WIV1 by blocking receptor attachment and inducing S1 shedding, as demonstrated cryo-EM structure [53] .

SARS-CoV-2 infected patients produce SARS-CoV-2 specific antibodies, which can be rapidly isolated to develop therapeutic neutralizing antibodies. Several of them have been developed and tested in clinical trials. Human monoclonal antibody CB6 was collected from recovered COVID-19 patients using SARS-CoV-2 RBD as the antigen. It can strongly neutralized SARS-CoV-2 with an IC 50 of 0.036 μg/mL. Moreover, CB6 greatly decreased the viral load in the respiratory tract of SARS-CoV-2-infected rhesus monkeys [54] and has entered a phase II clinical trial in China and USA with the name as JS016. A cocktail of two potent neutralizing antibodies, REGN10987 and REGN10933, targeting non-overlapping epitopes on the SARS-CoV-2 spike protein, were derived from genetically humanized mice immunized by SARS-CoV-2 protein and convalescent humans, respectively [55] .

These two antibodies can greatly reduce viral load in lower and upper airways and decrease virus induced pathological sequelae when administered prophylactically or therapeutically in rhesus macaques. Similarly, administration in hamsters decreases lung titers and evidence of pneumonia in the lungs [56] . This antibody cocktail is one of the most potent therapeutic antibodies against SARS-CoV-2 with low pM activity and is being evaluated in a phase III clinical trial. Baum et al. tested natural variants and possible emergence of escape mutants following antibody treatment, indicating that a combination of antibodies binding to distinct and non-overlapping regions of the viral target is a powerful way to minimize mutational escape [57] . To tackle the escaping SARS-CoV-2 variants that may emerge, various epitope-targeted antibodies should be developed and combined as therapeutics. Many natural mutations have already been identified in the spike protein of SARS-CoV-2 (https://bigd.big.ac.cn/ncov/variation/statistics?lang=en). Among them, a variant with the D614G mutation has rapidly become the dominant pandemic form probably due to its fitness advantage [58] . Indeed, the emergence of antibody or convalescent plasma-resistant SARS-CoV-2 variants has been confirmed [59, 60] . Therefore, antibody cocktails are a promising strategy to decrease the potential for the emergence of virus escape mutants.

There are also a number of antibodies that have shown protective efficacy in animal models. A group isolated 8,558 antigen-binding IgG1 + clonotypes from 60 convalescent patients, and 14 potent neutralizing antibodies were identified, with the most potent one, BD-368-2, targeting an ACE2 binding site and exhibiting an IC 50 of 1.2 and 15 ng/mL against pseudotyped and authentic SARS-CoV-2, respectively.

BD-368-2 also displayed strong therapeutic and prophylactic efficacy in SARS-CoV-2-infected hACE2-transgenic mice [61] . A combination of BD-368-2 and BD-629 represents a potent cocktail of two distinct epitope-binding antibodies and can rescue mutation-induced neutralization escapes of BD-368-2 [62] . Another study also reported the identification of over 1800 antibodies from a cohort of SARS-CoV-2 recovered participants using a novel high-throughput antibody discovery platform.

One human mAb CC12.1 has an in vitro IC 50 neutralization of 0.019 μg/mL and provides protection against SARS-CoV-2 infection in Syrian hamsters [63] . with SARS-CoV. RBD-specific mAbs H4 and B38 were isolated from a convalescent patient and found to inhibit viral infection by blocking the RBD from binding to ACE2. Since H4 and B38 bind to different epitopes, the two antibodies can bind simultaneously and exhibit additive viral inhibition effects [65] In some coronaviruses, the N-terminal domain (NTD) may recognize specific sugar moieties upon initial attachment and might play an important role in the pre-fusion to post-fusion transition of the S protein. The NTD of the MERS-CoV S protein can serve as a critical epitope for neutralizing antibodies [66] . So far, several SARS-CoV-2 neutralizing antibodies against NTD also have been isolated from convalescent patients [53, 67] . For example, a fully human neutralizing mAb, 4A8, recognizes a vulnerable epitope of the NTD on the S protein of SARS-CoV-2 and acts through a mechanism that is independent of receptor binding inhibition but may restrain the conformational changes of the S protein. These NTD-targeting antibodies may be useful for combining with RBD-targeting antibodies in therapeutic cocktails.

Phage-display technology has also been applied in the biopanning of SARS-CoV-2 antibodies. H014, screened from a scFv phage-display library that was prepared from the mice immunized with recombinant SARS-CoV protein, prevents attachment of SARS-CoV-2 to ACE2, and protects against SARS-CoV-2 in the ACE2 humanized mouse model [68] .

An attractive alternative for mAbs is single-domain antibodies from camelid immunoglobulins, termed VHH, or nanobody (Nb), which are the smallest naturally occurring antibody with a molecular weight of 12-15 kilodaltons (kDa). The small size and favorable biophysical characteristics make nanobodies particularly suitable for the treatment of the respiratory diseases COVID-19 by inhaled delivery. Fully human Nbs W25 [69] , Sb23 [70] , Nb11-59 [71] , and MR3 [72] were found to potently bind to the SARS-CoV-2 RBD and neutralize SARS-CoV-2 infection by blocking the RBD-ACE2 interaction. Fully human Nbs n3088 and n3130, which were found to be synergistic with n3113, neutralize SARS-CoV-2 by targeting a cryptic epitope located in the spike trimeric interface, demonstrating the advantage of the small-size of Nbs [73] . By directly interfering with ACE2 binding, the fully human bivalent ab8 [74] , trivalent Nb6 [75] , Nb20 and Nb21 [76] exhibited higher avidity to the S protein, and correlated with a stronger neutralization of SARS-CoV-2 than the respective monomeric Nbs. VHH-72 [77] , Ty1 [78] , H11-D4 and H11-H4 [79] , isolated from dromedary llamas or alpaca, recognize epitopes on the RBD and have been demonstrated to neutralize pseudotyped and/or authentic SARS-CoV-2. In light of the discovery that Nb can be humanized to reduce the risk of immunogenicity [80] , multimerize or cooperate with other mAbs by a variety of means to enhance half-life and avidity [81] , and expressed in high quantities in bacteria or yeast, Nb is an excellent candidate for COVID-19 treatment from a biopharmaceutical manufacturing perspective.

With such a great number of SARS-CoV-2 antibodies developed, some characteristics of SARS-CoV-2 neutralizing antibodies have also been discovered.

Some studies found that neutralizing mAbs targeting SARS-CoV-2 S protein are minimally mutated [82] and show little somatic mutation over time [83] . For instance, it was found that the immunoglobulin heavy-chain variable region 3-53 gene was the most frequently used among 294 RBD-targeted antibodies, which have few somatic mutations [84] . Besides, through analysis of humoral responses in SARS-CoV-2 infected patients, most antibodies in plasma target non-neutralizing epitopes that are outside the RBD [83] . However, the most potent neutralizing antibodies are SARS-CoV-2 RBD-specific that account for 90% of plasma neutralizing activity [85] , indicating SARS-CoV-2 RBD is immunodominant. Collectively, the epitope of the ACE2-binding site on RBD, named receptor binding motif (RBM), dominates SARS-CoV-2 polyclonal neutralizing antibody responses [85] and induces strongly neutralizing antibodies [86] . These findings should facilitate the design of antigens that elicit specific neutralizing antibody responses.

As week [89] . Since exogenous ACE2 has already been developed for treatment of SARS-CoV, it can be rapidly deployed for the treatment of COVID-19.

The structural-based computational design of small peptides provides an efficient way to quickly identify potential therapeutics for emerging diseases. The autologous nature of ACE2-based drugs eliminates the risk of immunogenicity, and their small-molecular weight makes them optimal for inhalation administration for direct delivery to the lung. Nevertheless, it is worth noting that the effect on the ACE2/angiotensin axis with regards to blood pressure and kidney

function has yet to be determined.

The trimeric hairpin structure formed by HR1 and HR2 regions in the S2 subunit of SARS-CoV-2 plays a key role during the viral membrane fusion process, which makes it an attractive target for drug design. 

The [107] . Therefore, Type III IFNs may help to achieve a sustained antiviral state that limits viral spread in the lower airway as well as the lung.

Taken together, IFNs are promising as repurposed drugs for COVID-19 treatment. Early administration prior to viral peak or as a prophylactic treatment may offer maximal protection without appreciable pathology. Combination administration with other antiviral drugs could result in more potent therapeutic effects.

responses with the release of a large amount of pro-inflammatory cytokines in an event known as "cytokine storm", which leads to ARDS aggravation and widespread tissue damage resulting in multi-organ failure and death. Therapeutic strategies targeting cytokines, accompanied by other anti-inflammation methods, during the management of COVID-19 patients could improve survival rates and reduce mortality (Table 2) .

IL-6 is one of the major cytokines that amplifies the immune response and mediates lung damage and respiratory failure during cytokine storm. In COVID-19

patients, the IL-6 level was observed to be almost two-fold higher in severe patients compared with mildly symptomatic patients [108] . Inhibition of IL-6 was speculated to be effective in treating severe COVID-19 patients. As a humanized monoclonal antibody against IL-6 receptor (IL-6R), Tocilizumab (Actemra, Roche) has been approved for the treatment of severe COVID-19 patients with elevated levels of IL-6.

In a retrospective study, 20 severe or critical patients received five days of Tocilizumab treatment, and it was found that the clinical symptoms are effectively improved, as demonstrated by significantly lower C-reactive protein levels and percentage of peripheral lymphocytes [109] . However, the study did not include a 

IL-1β is another pro-inflammatory interleukin produced following immune recognition of SARS-CoV-2. A cohort study enrolling 96 patients found that 10-day subcutaneous administration of Anakinra, an IL-1 receptor antagonist, reduced the mortality and need for invasive mechanical ventilation significantly in patients with COVID-19-related bilateral pneumonia, typical lung infiltrates, or signs of respiratory failure [110] . Another retrospective cohort study also reported that high-dose intravenous Anakinra benefited COVID-19 patients by decreasing mortality, serum C-reactive protein, and improving clinical status [111] . Anakinra has shown a survival benefit without increased adverse events in sepsis patients with hyperinflammation in a phase III randomized controlled trial [112] , which may be helpful in controlling hyperinflammation status in COVID-19 patients.

GM-CSF can induce macrophages and neutrophils to secret pro-inflammatory cytokines such as IL-1, IL-6 and IL-23, as well as stimulate multiple downstream signal pathways that have effects on activation and differentiation of myeloid cells after binding to its receptor [113] . Given the critical role of GM-CSF in inflammation, an antibody against GM-CSF receptor-α (GM-CSFRα) to block downstream signaling was considered an option for treating hyperinflammation in COVID-19 patients.

Mavrilimumab is an antibody targeting GM-CSFRα that has undergone phase I and phase II efficacy and safety clinical trials in patients with rheumatoid arthritis [114] .

In a COVID-19 cohort study, the Mavrilimumab treated group showed earlier improvement from pneumonia and systemic hyperinflammation, as well as lower mortality than the controlled patients receiving only standard care [115] . Compared with other potential anti-cytokine agents like IL-6 inhibitors, inhibition of GM-CSFRα exhibits effects on the upstream inflammatory cascades, which could yield robust results, though placebo-controlled randomized trials are needed to confirm these initial findings.

Severe COVID-19 patients have been reported to possess significant complement activation in their lung and sera, and the complement cascade has also been speculated as a promoter of cytokine storm, lung inflammation, and thrombotic microangiopathy (TMA) in COVID-19 [116] . Several complement-targeted therapeutic candidates have been developed and undergone various stages of clinical trials. A complement C3 inhibitor, AMY-101, was reported to successfully restore the normal lung function in a COVID-19 patient with severe ARDS after 14 days of treatment [117] . Anti-complement C5 therapy with Eculizumab also downregulated inflammatory markers and promoted recovery of four ICU COVID-19 patients [118] .

Several complement therapeutics targeting C3 or C5, such as APL-9, AMY-101, Zilucoplan, Eculizumab, and Ravulizumab are in phase II or III clinical trials [119] to evaluate their immune modulatory efficacy for COVID-19.

CD6 is a co-stimulatory molecule required for optimal T-cell stimulation by antigen-presenting cells, which is crucial in T-cell proliferation to form Th1 and Th17 for treatment of moderate to severe chronic plaque psoriasis [120] . Considering its unique mechanism of action in ameliorating CRS, which is the leading cause of death in COVID-19, itolizumab has been repurposed for COVID-19. In a trial conducted in Cuba, 94.7% of the patients were discharged after two weeks. Similarly, a prospective, randomized, placebo-controlled phase II trial was conducted in 30 severe COVID-19 patients in India and showed therapeutic benefits, as evidenced by significant improvement in blood oxygen levels, reduced levels of proinflammatory cytokines, and reduced mortality rate (CTRI Number: CTRI/2020/05/024959). Due to these positive results, the Indian Central Drug Standard Control Organisation (CDSCO) approved Itolizumab for "restricted emergency use" for the treatment of CRS in moderate to severe ARDS patients with COVID-19.

Taken together, hyperinflammatory status with incapacitated defense against viral invasion was found in many COVID-19 patients and can be eased by anti-inflammation therapeutics. However, anti-inflammation drugs should be used only for a limited period of time, and patients should be carefully monitored to avoid severe infections [121] . The existing large-scale, randomized clinical trials may not support the beneficial outcome of some agents, but anti-inflammation therapy is probably needed for patients with life-threatening COVID-19 disease.

A promising approach for a more specific anti-viral therapy could be based on endogenous RNA interference (RNAi) mechanisms whose physiological goal is to regulate protein synthesis events. RNAi has been adopted for anti-viral therapy using 

Strong transmission capacity coupled with no effective prevention and therapeutics has resulted in the continuing threat of COVID-19. Currently, several vaccine candidates have entered phase III trials and might be available within months [123] . Despite this, viral evolution might hamper the protective effects of vaccines, and some elderly or immunocompromised individuals may not develop strong immune responses after vaccination [124] . Therefore, the situation demands an urgent need to explore all potential therapeutic strategies that can be made available to prevent the disease progression and improve patient outcomes.

Currently, a total of 212 drug sets and 1796 unique drugs, including biotherapeutics related to COVID-19 research, have been collected in the COVID-19

Gene and Drug Set Library (https://amp.pharm.mssm.edu/covid19/) [125] . The viral RNA is then translated to produce the polyproteins pp1a and pp1ab (5) , which are cleaved (6) to yield the 16 NSPs that form the RNA replicase-transcriptase complex (7) for the genome replication (8) . The viral mRNA encoding structural proteins is then transcribed (9) for the final virus assemble (10) . In the end, the new virion is released (11) . 

Blockade of IL-1β receptor and its downstream pro-inflammatory pathways Anakinra A case-control study of 19 COVID-19 patients with moderate to severe ARDS, and hyperinflammation indicated efficacy [110] GM-CSF inhibitors Blockade of GM-CSF receptor-α on an upstream of inflammatory cascades Mavrilimumab A case-control study of 13 COVID-19 patients with pneumonia, hypoxia, and systemic hyperinflammation indicated efficacy [115] 

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A noncompeting pair of human neutralizing antibodies block COVID-19 virus binding to its receptor ACE2

Structural definition of a neutralization epitope on the N-terminal domain of MERS-CoV spike glycoprotein

A neutralizing human antibody binds to the N-terminal domain of the Spike protein of SARS-CoV-2

Structural basis for neutralization of SARS-CoV-2 and SARS-CoV by a potent therapeutic antibody

Potent neutralization of clinical isolates of SARS-CoV-2 D614 and G614 variants by a monomeric, sub-nanomolar affinity Nanobody

Selection, biophysical and structural analysis of synthetic nanobodies that effectively neutralize SARS-CoV-2

A potent neutralizing nanobody against SARS-CoV-2 with inhaled delivery potential

Potent synthetic nanobodies against SARS-CoV-2 and molecular basis for neutralization

3D mapping and accelerated super-resolution imaging of the human genome using in situ sequencing

High potency of a bivalent human VH domain in SARS-CoV-2 animal models

An ultra-high affinity synthetic nanobody blocks SARS-CoV-2 infection by locking Spike into an inactive conformation

Multivalent Nanobody Cocktails Efficiently Neutralize SARS-CoV-2. bioRxiv 2020

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

An alpaca nanobody neutralizes SARS-CoV-2 by blocking receptor interaction

Neutralizing nanobodies bind SARS-CoV-2 spike RBD and block interaction with ACE2

General strategy to humanize a camelid single-domain antibody and identification of a universal humanized nanobody scaffold

Nanobody engineering: toward next generation immunotherapies and immunoimaging of cancer

Longitudinal isolation of potent near-germline SARS-CoV-2-neutralizing antibodies from COVID-19 patients

Analysis of a SARS-CoV-2-Infected Individual Reveals Development of Potent Neutralizing Antibodies with Limited Somatic Mutation

Structural basis of a shared antibody response to SARS-CoV-2

Mapping Neutralizing and Immunodominant Sites on the SARS-CoV-2 Spike Receptor-Binding Domain by Structure-Guided High-Resolution Serology

Structural and Functional Basis of SARS-CoV-2 Entry by Using Human ACE2

Inhibition of SARS-CoV-2 infections in engineered human tissues using clinical-grade soluble human ACE2

Neutralization of SARS-CoV-2 spike pseudotyped virus by recombinant ACE2-Ig

Novel ACE2-Fc chimeric fusion provides long-lasting hypertension control and organ protection in mouse models of systemic renin angiotensin system activation

Computational Design of ACE2-Based Peptide Inhibitors of SARS-CoV-2

De novo design of picomolar SARS-CoV-2 miniprotein inhibitors

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

Inhibition of SARS-CoV-2 (previously 2019-nCoV) infection by a highly potent pan-coronavirus fusion inhibitor targeting its spike protein that harbors a high capacity to mediate membrane fusion

Impaired type I interferon activity and inflammatory responses in severe COVID-19 patients

Activation and evasion of type I interferon responses by SARS-CoV-2

Antiviral activities of type I interferons to SARS-CoV-2 infection

Retrospective multicenter cohort study shows early interferon therapy is associated with favorable clinical responses in COVID-19 patients

Diagnosis and treatment of 2019 novel coronavirus infection in children: a pressing issue

Type I and Type III Interferons Restrict SARS-CoV-2 Infection of Human Airway Epithelial Cultures

A Randomized Clinical Trial of the

Efficacy and Safety of Interferon β-1a in Treatment of Severe COVID-19

UK Biotech Corporate Finance Trends Post COVID-19

Triple combination of interferon beta-1b, lopinavirritonavir, and ribavirin in the treatment of patients admitted to hospital with COVID-19: an open-label, randomised, phase 2 trial

IFNλ is a potent anti-influenza therapeutic without the inflammatory side effects of IFNα treatment

The Superiority of IFN-λ as a Therapeutic Candidate to

Control Acute Influenza Viral Lung Infection

A mouse-adapted model of SARS-CoV-2 to test COVID-19 countermeasures

Type III interferons disrupt the lung epithelial barrier upon viral recognition

Dysregulation of Immune Response in Patients With Coronavirus

Effective treatment of severe COVID-19 patients with tocilizumab

Anakinra for severe forms of COVID-19: a cohort study

Interleukin-1 blockade with high-dose anakinra in patients with COVID-19, acute respiratory distress syndrome, and hyperinflammation: a retrospective cohort study

Interleukin-1 Receptor Blockade Is Associated With Reduced Mortality in Sepsis Patients With Features of Macrophage Activation Syndrome: Reanalysis of a Prior Phase III Trial*

GM-CSF-Dependent Inflammatory Pathways

Mavrilimumab, a human monoclonal antibody targeting GM-CSF receptor-α, in subjects with rheumatoid arthritis: a randomised, double-blind, placebo-controlled, phase I, first-in-human study

GM-CSF blockade with mavrilimumab in severe COVID-19 pneumonia and systemic hyperinflammation: a single-centre, prospective cohort study

Will Complement Inhibition Be the New Target in Treating COVID-19-Related Systemic Thrombosis?

The first case of COVID-19 treated with the complement C3 inhibitor AMY-101

Eculizumab treatment in patients with COVID-19: preliminary results from real life ASL Napoli 2 Nord experience

A phase 3 open-label, randomized, controlled study to evaluate the efficacy and safety of intravenously administered ravulizumab compared with best supportive care in patients with COVID-19 severe pneumonia, acute lung injury, or acute respiratory distress syndrome: a structured summary of a study protocol for a randomised controlled trial

Itolizumab -a humanized anti-CD6 monoclonal antibody with a better side effects profile for the treatment of psoriasis

The many faces of the anti-COVID immune response

Nucleic acid-based therapy for coronavirus disease

SARS-CoV-2 vaccines in development

Antibodies may curb pandemic before vaccines

The COVID-19 Gene and Drug Set Library

The Curated CORona Drug InTERactions Database for SARS-CoV-2

Structurally Resolved SARS-CoV-2 Antibody Shows High Efficacy in Severely Infected Hamsters and Provides a Potent Cocktail Pairing Strategy

Analysis of a SARS-CoV-2-Infected Individual Reveals Development of Potent Neutralizing Antibodies with Limited Somatic Mutation

A therapeutic non-self-reactive SARS-CoV-2

antibody protects from lung pathology in a COVID-19 hamster model

The first case of COVID-19 treated with the complement C3 inhibitor AMY-101

Currently he is a Ph.D. candidate under the supervision of Prof. Tianlei Ying at the School of Basic Medical Science, Fudan University

Yanling Wu is an associate professor at the School of Basic Medical Science, Fudan University. She obtained her Ph.D. degree from the School of Basic Medical Science

Tianlei Ying is a full professor at the School of Basic Medical Science, Fudan University. He received his B.S. and Ph.D. degree from Fudan University. His research focuses on design, engineering and clinical application of novel antibody constructs and synthetic biotherapeutics

The authors declare that they have no conflict of interest.A case reported successful treatment of a COVID-19 patient with ARDS [130] Zilucoplan Under phase II clinical trial (EudraCT 2020-001736- 95) Eculizumab Successful treatment of four COVID-19 patients with ARDS or severe pneumonia [118] Ravulizumab Under phase III clinical trial (NCT04369469)

Blockade of CD6 to attenuate T cell infiltration and cytokine expression Itolizumab Approved by CDSCO for 'restricted emergency use' for the treatment of CRS in moderate to severe ARDS patients with COVID-19.