key: cord-0907065-fi993f4n
authors: Tao, Kaiming; Tzou, Philip L.; Nouhin, Janin; Bonilla, Hector; Jagannathan, Prasanna; Shafer, Robert W.
title: SARS-CoV-2 Antiviral Therapy
date: 2021-07-28
journal: Clin Microbiol Rev
DOI: 10.1128/cmr.00109-21
sha: da9b84fcaeb610019a11cc2f093110d13e70cec4
doc_id: 907065
cord_uid: fi993f4n

The development of effective antiviral therapy for COVID-19 is critical for those awaiting vaccination, as well as for those who do not respond robustly to vaccination. This review summarizes 1 year of progress in the race to develop antiviral therapies for COVID-19, including research spanning preclinical and clinical drug development efforts, with an emphasis on antiviral compounds that are in clinical development or that are high priorities for clinical development. The review is divided into sections on compounds that inhibit SARS-CoV-2 enzymes, including its polymerase and proteases; compounds that inhibit virus entry, including monoclonal antibodies; interferons; and repurposed drugs that inhibit host processes required for SARS-CoV-2 replication. The review concludes with a summary of the lessons to be learned from SARS-CoV-2 drug development efforts and the challenges to continued progress.

W hen the COVID-19 pandemic began, the development of effective antiviral treatments appeared promising. Since the 2002-2003 SARS-CoV pandemic and the multiple MERS-CoV outbreaks that began in 2012, many novel and repurposed compounds have been found to possess anticoronavirus activity in vitro, and it was expected that one or more effective antiviral treatments would be deployed as a useful stopgap measure pending vaccine development (1) (2) (3) (4) (5) (6) . One year later, highly effective vaccines have now been introduced and are beginning to slow the spread of SARS-CoV-2. However, despite the emergency use authorization (EUA) by the U.S. Food and Drug Administration (FDA) of one nucleoside analog and three monoclonal antibody (MAb) preparations, antiviral therapy has had little impact on COVID-19 clinical outcomes for most patients globally.

The development of effective SARS-CoV-2 antiviral therapy, however, remains critical for those awaiting vaccination, as well as for the estimated millions of immunocompromised persons who are unlikely to respond robustly to vaccination. Moreover, the ongoing emergence and spread of immune-escape variants means that even immunocompetent persons are likely to have higher rates of vaccine failure than what was observed in clinical trials conducted earlier in the pandemic (7) (8) (9) (10) . Finally, antiviral therapies that target conserved viral proteins are likely to be effective against future pandemic coronaviruses.

Here, we review 1 year of progress in the race to develop antiviral therapies for COVID-19. The review summarizes research spanning preclinical and clinical drug development efforts with an emphasis on antiviral compounds that are in clinical development or that are high priorities for clinical development. The review is divided into four main largely nonoverlapping sections: (i) compounds that inhibit SARS-CoV-2 enzymes, including its polymerase and Main protease (Mpro); (ii) compounds that inhibit virus entry, including MAbs; (iii) interferons (IFNs); and (iv) repurposed drugs that inhibit host processes required for SARS-CoV-2 replication.

RNA-dependent RNA polymerases (RdRps) catalyze phosphodiester bond formation between nucleoside triphosphates in an RNA-templated manner. RdRps are highly conserved in their structural and functional features, even among diverse RNA viruses belonging to different families (11) . Nucleoside analog polymerase inhibitors are the most common antiviral compounds comprising a plurality of all licensed antivirals. Because of their broad spectrum of activity, nucleoside analog polymerase inhibitors have been the only successful repurposed directly acting antivirals. For example, tenofovir and lamivudine are among the mainstays of therapy for treating human immunodeficiency virus and hepatitis B infections (12) . Most antiviral polymerase inhibitors lack a 39-hydroxyl group and act as nucleoside analog chain terminators (12) . Others contain a 39-hydroxyl group and yet still result in immediate or delayed chain termination. Finally, some nucleoside analogs are incorporated into viral genomes and inhibit replication by introducing mutations during subsequent rounds of virus replication.

Coronavirus RNA polymerization is more complex than that of other viruses. First, coronaviruses contain a 39-to-59 exoribonuclease (ExoN; nsp14), which is required for replication fidelity (13) . ExoN is responsible for the intrinsic resistance of coronavirus species to ribavirin and several other nucleoside analogs (14, 15) . Second, coronavirus genomes are three times larger than most other RNA genomes and thus require increased processivity, which may explain why RdRp (nsp12) requires several accessory proteins, including nsp7 and nsp8. Third, in addition to copying the full virus genome, the coronavirus RdRp transcribes multiple subgenomic mRNAs. Several cryo-electron microscopy (cryo-EM) structures of the nsp12-nsp7-nsp8 replication-transcription complex of SARS-CoV-2 have been published (Fig. 1A ) (16) (17) (18) (19) .

Remdesivir (GS-5734, Veklury) is the monophosphate prodrug of the parent 1'cyano-substituted adenine C-nucleoside analogue GS-441524 (20) . The presence of the phosphate group allows for more efficient metabolism of the prodrug to the active nucleoside triphosphate form by bypassing the rate-limiting initial phosphorylation step. Remdesivir contains a 39-OH group and is therefore a nonobligate chain terminator. It causes delayed chain termination due to a steric clash with S861 after the addition of three trailing nucleosides (Fig. 1B) (21) (22) (23) (24) (25) (26) . The fact that chain termination does not occur until additional nucleoside triphosphates have been added likely explains how remdesivir eludes the coronavirus exonuclease.

Remdesivir has broad spectrum activity against multiple RNA viruses and inhibits SARS-CoV, MERS-CoV, and SARS-CoV-2 with half-maximal effective concentrations by 100 mg daily for 5 to 10 days. After intravenous administration, the achievable maximum remdesivir plasma concentration is predicted to be at least twice as high as most reported EC 50 s. Several research groups have argued that remdesivir penetrates poorly into the lungs and that either GS-441524 itself or other GS-441524 prodrugs may be superior to remdesivir (34) (35) (36) (37) (38) . However, the most detailed pharmacokinetic study has found that at currently approved dosing, remdesivir results in sufficiently high intracellular concentrations of the active triphosphate form GS-443902 in peripheral blood mononuclear cells (39) .

As of February 2021, there have been four randomized controlled trials of remdesivir containing a placebo arm (40) (41) (42) (43) , of which two were blinded (40, 42) . The NIH Adaptive Covid-19 Treatment Trial (ACTT-1) randomized 1,063 persons with severe disease to remdesivir for 10 days versus placebo. Persons receiving remdesivir had a median recovery time of 10 days versus 15 days for the placebo group (rate ratio for recovery, 1.29; 95% confidence interval [CI] =1.12 to 1.49; P , 0.001). Kaplan-Meier estimates of mortality were 6.7% with remdesivir and 11.9% with placebo by day 15 and 11.4% versus 15.2% at day 29 (hazard ratio [HR] = 0.73; 95% CI = 0.52 to 1.03). A subgroup analysis showed that response to therapy was greater in those not requiring supplemental oxygen or receiving oxygen via nasal cannula than in those requiring high flow oxygen or mechanical ventilation. These results led to an EUA, followed by the eventual approval of remdesivir by the FDA (40, 42) .

The open-label WHO Solidarity trial which included 2,750 persons randomized to remdesivir and 4,800 to standard-of-care detected no reduction in mortality, requirement for ventilation, or reduction in hospital stay for those receiving remdesivir (41) . It has been proposed that Solidarity trial may not have observed shortened hospital stays with remdesivir because it was an open-label trial that studied patients who received highly heterogenous routine care and because it required patients receiving remdesivir to remain hospitalized until they completed the full 10-day course of intravenous treatment (44, 45). Another randomized controlled trial compared remdesivir for 10 days versus 5 days versus placebo in persons with moderate disease detected no difference between each of the three arms (43) . Remdesivir was not associated with reductions in either upper or lower respiratory tract virus load levels in the one study that examined this outcome (42), possibly because virus levels typically have already begun to decrease by the time patients require hospitalization.

b-D-N 4 -hydroxycytidine (NHC) is a cytidine analogue that exerts its activity primarily through viral mutagenesis (Fig. 1C ). It incorporates into new RNA strands and results in the introduction of many mutations during subsequent rounds of replication (5, 28) . Molnupiravir (b-D-N 4 -hydroxycytidine-59-isopropyl ester) is an orally available NHC prodrug that has also been known as MK-4482 and EIDD-2801. The fact that molnupiravir is not a chain terminator may explain the mechanism by which it eludes the proofreading function of coronavirus exonucleases. Biochemical and deep sequencing studies have confirmed that viral mutagenesis is the main mechanism by which molnupiravir inhibits coronaviruses (28, 46, 47) . NHC can be metabolized into deoxy-NHC and cause DNA mutations in host cells (48).

Molnupiravir has broad-spectrum antiviral activity against multiple viruses, including SARS-CoV, MERS-CoV, and SARS-CoV-2, with most EC 50 s below 1 mM (28, 49). It is active in primary human airway epithelial cells, and it reduces virus levels, disease, and lung damage in mouse models of SARS-CoV and MERS-CoV (28, 50) and in hamster (46) and ferret models (51) of SARS-CoV-2. Two first-in-human pharmacokinetic studies have been performed, including a phase 1 dose-ranging study of a 5-day course of oral therapy in healthy adults (52) and phase 1b/2a dose-escalating placebo-controlled trials among adult COVID-19 outpatients within 5 days of symptom onset (53).

A phase II trial examined virological endpoints among persons receiving molnupiravir 200 mg twice daily (BID), 400 mg BID, and 800 mg BID compared to placebo in 176 nonhospitalized COVID-19 patients with fever and/or signs of a respiratory illness (NCT04405570). Among 74 patients with positive baseline cultures, 6/25 (24%) placebo patients versus 0/49 pooled molnupiravir patients (P = 0.001) had positive cultures at day 5 (371) . Based on this trial, a dose of 800 mg BID was selected for further study. Two large phase II/III trials in hospitalized (NCT04575584) and nonhospitalized (NCT04575597) patients with COVID-19 began in October 2020. The study in hospitalized patients was discontinued for futility after interim data were reviewed by the data safety monitoring board (https://www.businesswire.com/ news/home/20210415005258/en/). The study in nonhospitalized patients is anticipated to be completed by September/October 2021. Molnupiravir is not being studied in pregnant women or women who might become pregnant because of its mutagenic potential (48, 54).

AT-527 is an oral nucleoside analog prodrug of AT-511 that has been previously studied for the treatment of HCV. It inhibits SARS-CoV-2 with an EC 90 of ;0.5 mM in human airway epithelial cells (55-57). The mechanism by which it retains activity in the face of coronavirus exonuclease activity has not been described. AT-527 is being evaluated in two phase II placebo-controlled trials of patients with mild-to-moderate disease (NCT04396106 and NCT04709835).

Favipiravir (T-705) is a purine analog prodrug that is ribosylated and phosphorylated intracellularly to form the active metabolite ribofuranosyl-59-triphosphate (T-705-RTP). It has broad spectrum activity against multiple viral RNA polymerases and appears to act by causing viral mutagenesis (58) (59) (60) (61) . However, it demonstrates little inhibitory activity in vitro against SARS-CoV-2 with EC 50 s ranging from 60 to .100 mM (54, 60, 62-64) consistent with a low rate of favipiravir-RTP incorporation into the RdRp catalytic site (65) . It is active in a hamster model but only when used at high doses (66) . Several small open-label randomized studies have demonstrated little or no clinical or virological benefit associated with its use (67) (68) (69) (70) .

The FDA-approved anti-hepatitis C virus (HCV) nucleotide analog sofosbuvir is being studied in several COVID-19 clinical trials. Although it has reported to inhibit SARS-CoV-2 in biochemical studies (71) (72) (73) , it has little or no inhibitory activity in cell culture (54, 64, 74). One patient-level meta-analysis of three open-label studies totaling 176 patients reported that clinical recovery within 14 days was significantly greater among patients receiving sofosbuvir plus daclastavir (an HCV NS5A inhibitor) than among those receiving standard of care (75) . If this preliminary finding is validated, it is possible that the response is due to the fact that daclastavir itself has in vitro activity against SARS-CoV-2, although the mechanism for this activity is not known (76) .

Although ribavirin 59-monophosphate can be incorporated during RNA synthesis, it is readily excised by the coronavirus exonuclease (15) . As a result, ribavirin has little, if any, in vitro or in vivo activity against coronaviruses, including SARS-CoV, MERS-CoV, and SARS-CoV-2 (54, [77] [78] [79] .

Coronaviruses contain two protease enzymes: 3 chymotrypsin-like cysteine protease (3CLpro or Mpro; nonstructural protein 5 [nsp5]) and papain-like serine protease (PLpro; nsp3). Mpro cleaves polyprotein 1a/b at 11 sites. It is conserved among several families of RNA viruses, and its cleavage site specificity is similar to the picornavirus family of 3C proteases (80) . Mpro contains 306 amino acids and functions as a homodimer. It shares 96% amino acid identity with SARS-CoV Mpro (81) . PLpro is part of a 1,922-amino-acid multidomain transmembrane protein. It cleaves polyprotein 1a/b at the nsp1/2, nsp2/3, and nsp3/4 boundaries and several host proteins important for innate immunity such as ubiquitin interferon-stimulated gene product 15 (80) .

There are more candidate Mpro inhibitors than PLpro inhibitors because of this enzyme's similarities to proteases of other virus species and because Mpro is smaller, less complicated, and easier to produce in large quantities (80) . There have been more than one hundred published Mpro structures, but only a limited number of published PLpro structures (80, (82) (83) (84) . Although HIV-1 protease inhibitors were used to treat SARS-CoV-2 early in the pandemic, they possess either little or no anticoronavirus activity in biochemical and cell culture studies (62, 63, (85) (86) (87) , animal models (85, 88) , and clinical trials (41, 89, 90) .

Drug screens and structure-based designs targeting SARS-CoV-2 Mpro have identified a variety of compounds that inhibit SARS-CoV and SARS-CoV-2 biochemically and in cell culture with 50% inhibitory concentrations (IC 50 s) and 50% effective concentrations (EC 50 s) ranging from 0.01 to 1 mM. These compounds belong to several families of reversible and covalently binding peptidomimetic inhibitors, many of which have been cocrystallized with SARS-CoV-2 Mpro (81, (91) (92) (93) (94) (95) (96) (97) . These compounds are of interest as chemical scaffolds for potential therapeutic agents; however, some of these compounds may not be sufficiently selective for Mpro, increasing their risk of off-target effects (98, 99) . In addition, most current inhibitors bind Mpro covalently, which also increases the risk of off-target effects (80) .

PF-00835231 is among the most potent investigational Mpro inhibitors (Fig. 2 ). It has an IC 50 in enzymatic assays of 0.0003 mM and an EC 50 in cell culture of 0.2 mM (96, 100). It has undergone safety studies and pharmacokinetic profiling in rats, dogs, and monkeys (101) . PF-07304814 is an intravenously administered phosphate PF-00835231 prodrug that is being studied in a phase I trial of hospitalized patients with mild to moderate COVID-19 disease (NCT04535167). PF-07321332 is an oral prodrug that is being studied in another phase 1 trial (NCT04756531).

Ebselen is an investigational synthetic organoselenium drug with anti-inflammatory and antioxidant properties that has been studied for the treatment of a variety of illnesses. It was found to inhibit Mpro in a high-throughput drug screen and to have IC 50 s in biochemical assays and EC 50 s in cell culture assays of about 1 mM (92). It appears to inhibit Mpro allosterically (102) and to also inhibit PLpro (103, 104) . It is being studied as an oral drug in two small phase II placebo-controlled trials of patients with mild-to-moderate (NCT04484025) and severe (NCT04483973) COVID-19. GC376 has been effectively used for treating cats with the rapidly fatal coronavirus disease feline infectious peritonitis (105) . It forms covalent bonds with the Mpro active site cysteine and inhibits multiple coronaviruses (106) . Its SARS-CoV-2 Mpro IC 50 ranges from 0.03 to 1.5mM, while its cell culture EC 50 ranges from 0.2 to 3.4 mM (107-110). GC376 is considered a promising compound for further development (Fig. 2 ) (111) (112) (113) .

The approved HCV protease inhibitor boceprevir inhibits Mpro biochemically and in cell culture and is also considered a promising compound for further development (97, 107, 110, 114, 115) .

The spike glycoprotein is responsible for attachment to host cells and for fusion of viral and cellular membranes. It is a trimer comprising three identical subunits. Each monomer has an exposed S1 attachment domain and a partially hidden S2 fusion domain. The receptor-binding domain (RBD), which is part of S1, alternates between a closed/down position and an open/up position that enables it to bind to the human angiotensin converting enzyme 2 (ACE2) receptor (116, 117) . S1 binding occurs on the outer surface of ACE2, whereas angiotensin substrates bind in a deep cleft containing the active site (118) (119) (120) (121) . RBD-ACE2 binding results in structural changes that lead to S1 dissolution, S2 exposure, and virus-cell fusion (122) .

The proteolytic activation of coronavirus spike proteins by host cell proteases is required for the virus spike to transition from receptor attachment to cell fusion. The spike protein has two cleavage sites: one at the S1/S2 boundary and one within S2 referred to as S29. The sequences of coronavirus spike cleavage sites, the host enzymes required for their cleavage, and the cellular locations where cleavage occurs influence cell tropism and transmissibility (123, 124) . For SARS-CoV-2, the S1/S2 cleavage site is a polybasic furin site that is usually posttranslationally cleaved during viral biosynthesis (116, (125) (126) (127) (128) (129) . Cleavage at the S29 site is carried out by the host protease cathepsin B/ L within endosomes and by TMPRSS2 at the plasma membrane (130) . Accumulating data suggest that TMPRSS2-mediated S29 cleavage is more important for SARS-CoV-2 cell fusion whereas endosomal cathepsins may have been relatively more important for SARS-CoV (130) (131) (132) (133) (134) .

S2 contains a fusion peptide and two complementary heptad repeat regionsdesignated heptad repeat 1 (HR1) and heptad repeat 2 (HR2)-which are alpha helices with repeated patterns of seven amino acids. HR1 and HR2 are complementary allowing them to bind to one other. After RBD-ACE2 binding, the fusion peptide inserts into the host cell membrane. The three HR1 domains then associate with the three HR2 domains to create a six-helix bundle which creates a hairpin that brings the viral and host cell membranes together (123, 135, 136) .

Neutralizing antibodies can block the entry of virus into host cells and recruit host effector pathways to destroy virus-infected cells (Fig. 3) . Neutralizing MAbs are effective at preventing or treating multiple viral infections, including those caused by respiratory syncytial virus (137, 138) , influenza (139) , Ebola virus (140) , and MERS-CoV (141) . The presence of neutralizing antibodies targeting the SARS-CoV-2 spike RBD correlate with protection in animal models and in previously infected and vaccinated persons, although cellular immune responses and potentially nonneutralizing antibodies are also likely to have contributed to protection in these studies (142) (143) (144) (145) (146) (147) (148) (149) (150) . Paradoxically, the highest levels of neutralizing antibodies are detected in patients experiencing severe COVID-19 infections (151-154), suggesting that they may play a protective role in patients for whom the initial immunologic response to infection fails to prevent severe disease (155) (156) (157) . The most common neutralizing antibodies emerging in patients target the S1 RBD (156, (158) (159) (160) (161) .

Neutralizing MAbs have been isolated most commonly from the memory B cells of persons recovered from SARS-CoV-2, from immunized transgenic mice, and from combinatorial protein display libraries (162) . Regardless of their source, antibody-producing cells are screened for their ability to bind either the S trimer or just the RBD. The most potent SARS-CoV-2 MAbs have IC 50 s between 1 and 15 ng/ml. Since standard MAbs have a molecular weight of 150 kDa, an IC 50 below 150 ng/ml indicates picomolar activity. In addition to preventing viral entry into target cells by directly binding to SARS-CoV-2, MAbs also elicit Fc-effector functions such as antibody-dependent cellular cytotoxicity, complement-dependent cytotoxicity, and antibody-dependent cellular phagocytosis (163) . Although no two MAbs share identical epitopes, those binding the RBD are usually classified according to where on the RBD they bind. Two main classes of MAbs bind the ACE2-binding region of the RBD referred to as the receptor-binding motif (RBM), and two classes bind a separate part of the RBD referred to as the RBD core (164) (165) (166) . The RBD core is more evolutionarily conserved than the ACE2-binding residues in the RBM and MAbs that bind this region can often neutralize SARS-CoV and other SARSrelated coronaviruses (167) (168) (169) (170) (171) . However, non-ACE2-competing MAbs are usually somewhat less inhibitory than those that compete with ACE2 binding. Several laboratories have described potent MAbs that recognize the S1 N-terminal domain rather than the RBD (172, 173) . In macaques, hamsters, and various mouse models, the administration of neutralizing MAbs shortly before or after infection with SARS-CoV-2 has consistently resulted in reduced respiratory tract virus levels and signs of illness.

At least seven MAb preparations are in phase III trials. Four of these are MAb combinations: casirivimab plus imdevimab (174) (175) (176) , bamlanivimab plus etesevimab (177, 178) , cilgavimab (COV2-2130) plus tixagevimab (COV2-2196) (179) , and BRII-196 plus BRII-198. Three are single MAb agents: sotrovimab (VIR-7831) (171), regdanvimab (CT-P59) (180) , and TY027. More than 10 additional MAb preparations are in phase I/II trials (181, 182) . Four MAb preparations have received FDA EUAs for use in nonhospitalized patients at high-risk of severe COVID-19 illness: bamlanivimab monotherapy, bamlanivimab plus etesevimab, casirivimab plus imdevimab, and sotrovirmab. However, because of an increasing number of reports of SARS-CoV-2 variants that are resistant to bamlanivimab alone, the FDA recently revoked the EUA for bamlanivimab monotherapy.

Ten studies of MAb preparations have been published (149, (183) (184) (185) (186) (187) (188) (189) (190) 372) (Table  1) . Bamlanivimab plus etesevimab (186) , casirivimab plus imdevimab (189) , and sotrovirmab (190) have been reported to reduce the risk of hospitalization and mortality in nonhospitalized persons with risk factors for severe COVID-19. Bamlanivimab monotherapy (187) and casirivimab plus imdevimab (188) have been reported to reduce the risk of symptomatic and/or overall infection in persons at high risk of infection. Bamlanivimab monotherapy was studied in nursing home residents and staff while casirivimab plus imdevimab was studied in household contacts of infected persons. A study of bamlanivimab monotherapy in hospitalized patients was terminated prematurely as an interim analysis pointed to the unlikelihood of achieving benefit (184) . In contrast, one of two studies of casirivimab plus imdevimab in seronegative hospitalized patients (RECOVERY trial) reported a reduction in 28-day mortality compared with patients receiving standard of care (Table 1) (372) .

As of June 2021, five SARS-CoV-2 variants have been designated variants of concern (VOCs) by the WHO and/or the U.S. Centers for Disease Control and Prevention (CDC) because they are associated with increased transmissibility, more severe disease, and/or a reduction in antibody neutralization: the U.K. origin B. (191) (192) (193) (https://www.fda.gov/ media/145802/download). The combination of casirivimab plus imdevimab appears to retain full susceptibility against each of the VOCs (192, 194) (https://www.fda.gov/media/ 145611/download) likely because casirivimab binds to the RBD receptor binding motif, while imdevimab binds to the more conserved RBD core. Sotrovimab, which binds to the RBD core, also appears to be fully active against B.1.1.7, B.1.351, P.1, and B.1.427/9 (191, 192, 195, 196) (https://www.fda.gov/media/149534/download). It has not yet been evaluated against B.1.617. Although sotrovimab appears fully active against all VOCs to which has been tested, its maximal percent inhibition of B.1.1.7 was below 90% in two studies (195, 197) .

Single-domain Abs (sdAbs) are heavy-chain only antibodies that occur naturally in camelids and are increasingly being developed as potential therapeutics (198, 199) . (199) . sdAbs are easier to manufacture then standard MAbs because they can be expressed in bacterial or yeast cells. They can also often be delivered by inhalation. Although they are generally less potent than complete MAbs, their activity can be increased when engineered in multimeric forms. Many SARS-CoV-2-neutralizing sdAbs have been isolated (200) (201) (202) (203) (204) (205) (206) (207) (208) (209) , and several have demonstrated efficacy in animal models (208, 210, 211) . However, so far none have been evaluated in a clinical trial.

There has been one large observational trial and several open label and placebocontrolled trials of convalescent plasma. In the subset of patients in the observational trial that received units with known anti-SARS-CoV-2 antibody levels, mortality was inversely proportional to antibody titer: 115 of 515 patients (22.3%) in the high-titer group, 549 of 2,006 patients (27.4%) in the medium-titer group, and 166 of 561 patients (29.6%) in the low-titer group (212) . The benefit of high titer convalescent plasma, however, was observed only among those not receiving mechanical ventilation. Among the placebo-controlled trials, there has generally been no benefit associated with convalescent plasma (213, 214) with the exception of one trial in which high-risk patients older than 75 were treated within 72 h of the onset of COVID-19 symptoms (215) . In this trial, the risk of progression to severe disease was reduced in those receiving convalescent plasma: 13 of 80 (16%) versus 25 of 80 (31%), with a relative risk = 0.52 (95% CI = 0.29 to 0.94; P = 0.03).

Polyclonal antibody preparations are expected to be less expensive than MAb preparations, although they will have a lower mean binding affinity (216) . Several clinical trials are evaluating hyperimmune polyclonal antibody preparations, including IgG fractionated from convalescent patients with high neutralization antibody titers (217) and the more scalable solution of IgG fractionated from immunized animals. The polyclonal antibody preparations from immunized animals include purified equine F(ab) 2 preparations (INM005) (218) , swine glyco-humanized IgG (XAV-19) (219), and bovine transchromosomic IgG (SAB-185). Each of these technologies has been shown in phase 1 and 2 human trials to avoid the risk of serum sickness reactions (218) (219) (220) .

Peptides that mimic SARS-CoV-2 spike HR2 can block virus-cell fusion by preventing the interaction between HR1 and HR2. Several HR2 mimics have been shown to potently inhibit infection with SARS-CoV, MERS-CoV, and SARS-CoV-2 (135, (221) (222) (223) (224) (225) (226) . Interest in coronavirus fusion inhibitors is partly motivated by the high degree of conservation among S2 subunits. Although SARS-CoV and SARS-CoV-2 S2 differ in about 10% of their amino acids overall, their HR1 domains differ by only 7%, and their HR2 domains are essentially identical (227) .

Prior to the SARS-CoV-2 pandemic, an HR2-mimicking peptide, EK1, was identified as a potential broad-spectrum coronavirus inhibitor because it inhibited both SARS-CoV and MERS-CoV at submicromolar levels and was safe and protective against MERS-CoV when administered intranasally to mice (225) . Based on SARS-CoV-2 structural studies, several EK1 amino acid modifications were made and a cholesterol group was added to improve its pharmacokinetic and inhibitory activity (227, 228) . This new lipopeptide, named EK1C4, inhibits SARS-CoV-2 fusion about 150 times more potently than EK1, resulting in an EC 50 of 0.04 mM.

Several additional highly potent HR2-mimicking SARS-Cov-2 fusion inhibitors have been described, including IPB02 and IPB04 (226) 

The ;750-amino-acid soluble recombinant human ACE2 (rhACE2) protects lungs from injury during the acute respiratory distress syndrome (ARDS) (231) (232) (233) (234) . rhAce2 is safe in human subjects and was being developed as an ARDS treatment prior to the SARS-CoV-2 pandemic (235) . It inhibits the binding of SARS-CoV and SARS-CoV-2 to ACE2-expressing cells and also appears to prevent the loss of lung protective effects associated with the internalization of ACE2 following SARS-CoV-2 binding (119, 236). APN1 (Apeiron Biologics) is a clinical-grade soluble rhACE2 preparation that has demonstrated safety in 89 non-SARS-CoV-2 patients and volunteers and is now being evaluated in a 200-person placebo-controlled study of hospitalized SARS-CoV-2 patients (NCT04335136) (234) A press release from Apeiron Biologics announced that preliminary data from this trial showed that persons receiving APN1 experienced reductions in virus load and required mechanical ventilation for fewer days than those receiving placebo (237) .

Several research groups have shown that proteins designed to mimic the ACE2 ectodomain are highly potent SARS-CoV-2 inhibitors in cell culture and in pseudovirus entry experiments. Although ACE2 mimics cannot recruit immune cells, they can potentially bind to SARS-CoV-2 as tightly as MAbs and can be easier to manufacture as they do not require expression in mammalian cells. ACE2 mimics have also been fused to Ig Fc domains and to scaffolds to create bivalent or trivalent inhibitors (238) (239) (240) (241) (242) (243) . CTC-445.2d is a dimeric ACE2 mimic that has been shown to protect SARS-CoV-2infected hamsters from weight loss and death (244) .

An additional strategy to create even smaller ACE2 mimics have involved identifying those ACE2 regions that contribute most strongly to RBD binding (239, 240, (244) (245) (246) . One of these strategies has involved the use of linked designed ankyrin repeat domains (DARPin) as scaffolds (247, 248) . Each domain is about one-tenth of the size of an MAb. The binding portion of each domain is optimized using ribosomal display libraries (247) . One trispecific DARPin, ensovibep (MP0420), contains an ACE2 mimicking domain and two additional domains targeting other parts of the spike RBD. It has been shown to potently inhibit SARS-CoV-2 in vitro and to reduce weight loss and virus loads in hamsters (248) . A phase 1 safety trial of intravenous ensovibep has been completed and a phase 2/3 outpatient trial has begun (NCT04828161).

Glycosaminoglycans are widely distributed on the surface of mammalian cells and they serve as attachment sites for SARS-CoV-2 and other viruses (249) (250) (251) . Several compounds that inhibit the interaction between gylcosaminoglycans and viruses have antiviral activity, including lactoferrin and the anticoagulants heparin and enoxaparin (250, (252) (253) (254) (255) . The intranasal administration of heparin has been proposed as a possible way to prevent the spread of SARS-CoV-2 after initial infection without increasing the risk of bleeding (253) . Heparin and enoxaparin have also been studied in COVID-19 clinical trials to prevent the thromboembolic complications associated with severe disease.

In response to cellular changes suggestive of a viral infection, interferons (IFNs) induce many genes encoding proteins that inhibit viral replication by slowing cellular metabolism, interfering with the membrane formation required for virus replication, and inducing cytokines that promote adaptive immunity (256) . Although there are three IFN families (257), the innate immune sensing of viral nucleic acids leads specifically to the production of type I and type III IFNs (258) . Type I IFNs include 13 related IFN-a subtypes, IFN-b, and several poorly defined single gene products. Although both type I and type III IFNs activate the same dominant JAK-STAT signaling pathway, their cognate receptor expression differs; receptors for type I IFN have near ubiquitous expression throughout the mammalian host, whereas type III IFN receptors are largely thought to be confined to tissues of the respiratory and gastrointestinal tracts (259) .

The importance of IFNs for combatting SARS-CoV-2 is underscored by the fact that many viruses, particularly coronaviruses, encode multiple proteins that antagonize cellular IFN signaling pathways (6, (260) (261) (262) . The finding that about 10% of patients with life-threatening COVID-19 have neutralizing type I IFN autoantibodies (263) supports a role for IFNs in protection against SARS-CoV-2. SARS-CoV-2 infection has been associated with a reduced IFN response in some studies (264, 265) and an increased response in other studies (266) . IFN-a has also been reported to increase ACE2 expression in upper airway cells (267) .

IFN-a, IFN-b, and IFN-l each demonstrate inhibitory activity against SARS-CoV-2 at low concentrations of 100 to 1,000 IU/ml in Vero and Calu3 cell lines and in primary human alveolar cells (268) (269) (270) (271) (272) . IFN-a and IFN-b have demonstrated protective effects against SARS-CoV and MERS-CoV in mice (273, 274) and macaques (79, 275, 276) , while IFN-l has demonstrated protective effects against SARS-CoV-2 in mice (277) . Several studies, however, have reported worse outcomes in mouse models of SARS-CoV and MERS-Cov with delayed administration of type I IFNs (85, 278, 279) .

The IFN clinical trials have differed in the formulations studied, route of administration, and design. There has been one phase II randomized placebo-controlled trial of a nebulized IFN-b preparation called SNG001 in nonventilated hospitalized patients with COVID-19 receiving supplementary oxygen (280) , two phase II randomized placebocontrolled trials of subcutaneous IFN-l in outpatients with mild disease (281, 282) , two randomized open-label trials of IFN-b (including the SOLIDARITY trial) (41, 283) , one open-label trial of IFN-a (284) , and one open-label trial of IFN-b combined with lopinavir/r and ribavirin (285) . The nebulized IFN-b preparation SNG001 was found to be associated with an improved outcome, as 6 of 48 patients in the IFN group compared to 11 of 51 patients in the placebo group developed severe disease or died (280) . The larger of two phase II IFN-l studies showed no virological benefit in outpatients with mild to moderate SARS-CoV-2 infection (281), whereas the smaller study reported lower virus loads at day 7 among individuals with high baseline viral loads (282) . Among the open-label trials, the strongest signal of efficacy was a shorter time to viral clearance and more rapid clinical improvement in patients receiving IFN-b combined with lopinavir/r and ribavirin (285) .

There is currently one ongoing phase III trial for inhaled SNG001 (NCT04732949), one phase III trial of subcutaneous IFN-b given along with remdesivir (ACTT-3, NCT04492475), and one planned study for inhaled Novaferon (IFN-a) in hospitalized patients with moderate to severe disease (NCT04669015). Subcutaneous IFN-l is being studied in a third phase II trial of outpatients (NCT04344600). The NIH treatment guidelines panel recommends against the use of IFNs for the treatment of severely ill patients with COVID-19 but provide no recommendation for the use of IFNs in earlier disease (https://www.covid19treatmentguidelines.nih.gov/).

Repurposed drugs that have been approved or are being studied for other indications often target host processes required for viral replication. Such host-targeting compounds may be able to inhibit multiple viruses because different viruses often depend on similar host factors and pathways. Although such compounds may have a higher risk of toxicity than those specifically targeting a virus protein, such toxicity may be acceptable for the relatively short time required to treat an acute infection. Many host-targeting compounds that inhibit coronaviruses in vitro have been identified. They may act by inhibiting a cellular protein, influencing a signaling pathway, or modifying a cellular organelle (Fig. 4) . However, for many inhibitory compounds, the mechanism of action is not known. This section reviews compounds that appear to act primarily by influencing the host rather than the virus and that have favorable safety profiles.

The proteolytic activation of coronavirus spike proteins by host cell proteases is required for the virus spike to transition from receptor attachment to cell fusion. These proteases include cell surface proteases such as transmembrane serine protease 2 (TMPRSS2) and other related transmembrane serine proteases, furin, and endosomal cathepsins (123, 135) . Accumulating data suggest that furin and TMPRSS2 mediate cleavage at the S1/S2 and S29 sites of SARS-CoV-2, respectively (130, 131) . The endosomal cysteine proteases, cathepsins B and L, appear to be less important for SARS-CoV-2 than for SARS-CoV (130-134, 286, 287) .

Camostat and nafamostat are serine protease inhibitors used in Japan for the treatment of pancreatitis and disseminated intravascular coagulation. They inhibit TMPRSS2 in biochemical assays and coronaviruses in cell culture (Fig. 4A) (288, 289) . In Calu-3 and Caco-2 cells, camostat inhibits SARS-CoV and SARS-CoV-2 with EC 50 s generally below 1 mM, while the EC 50 s for nafamostat are generally ;10-fold lower (130, 287, 

Clinical Microbiology Reviews 290, 291). These compounds, however, are largely inactive in Vero cells because Vero cells do not require TMPRSS2 for virus entry (63, 131, 287) . Camostat has protected mice from fatal SARS-CoV infection (292) . In animal models and humans, camostat is rapidly converted into the active metabolite 4-(4-guanidinobenzoyloxy) phenylacetic acid (GBPA), which inhibits SARS-CoV-2 entry with nearly the same efficacy as camostat (288, 289) . In a phase 1 study in healthy adults, administration of camostat 600 mg every 6 h in a fasted state or 1 h before meals was associated with a plasma GBPA concentration above EC 50 for approximately 12 h (293). In a phase 2 placebo-controlled trial of 205 patients hospitalized for #48 h who were randomized to receive camostat 200 mg orally three times daily or placebo, the median time to a 2-point improvement on a 7point ordinal scale (the primary endpoint) was 5 days in both arms (294) . The proportion of patients dying or requiring mechanical ventilation was 10% in the camostat arm and 18% in the placebo arm (secondary endpoint; odds ratio [OR] = 0.54; 95% CI = 0.25 to 1.18; P = 0.1). Among those with paired samples, the reduction in virus levels between the camostat and placebo arms were not different. Camostat is currently being studied in the large phase 3 NIH-sponsored adaptive platform treatment trial for outpatients with COVID-19 (Adapt Out COVID) trial (NCT04518410) at a dosage of 200 mg every 6 h for 7 days and in a smaller trial at a dosage of 600 mg every 6 h in patients not receiving oxygen (NCT04657497). Nafamostat, which requires intravenous administration, is associated with higher risk of toxicity compared to camostat. It was associated with hyperkalemia in four consecutive critically ill patients with SARS-CoV-2 (295) . Nafamostat is primarily being studied in small open-label trials.

Alpha-1 antitrypsin (a1-AT) is an endogenous protease inhibitor used as a treatment for persons with severe a1-AT insufficiency. It has been reported to inhibit TMPRSS2 biochemically (296) and to inhibit SARS-CoV-2 in vitro in two studies (297, 298) but not in a third study (299) . a1-AT is being studied in at least three placebo-controlled trials of hospitalized non-ICU patients (NCT04495101, NCT0457140, and NCT04385836) (300). Aprotinin is a 58-amino-acid serine protease inhibitor that inhibits SARS-CoV and SARS-CoV-2 with an EC 50 of ;1 mM (299). It has been used in Russia intravenously and inhalationally for the treatment of respiratory viral infections. There is one small ongoing open-label trial examining the response to intravenous and inhalational aprotinin (NCT04527133). Bromehexine is an oral mucolytic that has been identified as a TMPRSS2 inhibitor in a high-throughput screen for the treatment of prostate cancer (301) . However, it did not inhibit TMPRSS2 in a recent biochemical study (289) . Neither bromehexine nor its metabolite ambroxol have been shown to inhibit SARS-CoV-2 in vitro (302) . However, in a randomized open-label trial of 78 patients, oral administration was reported to significantly reduce ICU admissions, mechanical ventilation, and death (303) .

Dihydroorotate dehydrogenase (DHODH) is an enzyme in the pyrimidine synthesis pathway. Its inhibition limits the availability of nucleoside triphosphates required for viral replication (Fig. 4B) (304) . DHODH inhibitors have also been used to treat autoimmune diseases, organ rejection, and cancer. Leflunomide is a DHODH inhibitor licensed for the treatment of rheumatoid and psoriatic arthritis. Its active metabolite, teriflunomide, has weak in vitro SARS-CoV-2-inhibitory activity (305) . However, leflunomide did not increase the rate of viral clearance in a randomized open-label trial of 50 patients with prolonged postsymptomatic viral shedding (306) . PTC299 is an investigational DHODH inhibitor which has an EC 50 in the low nanomolar range and a high selectivity index (307) . It is an oral drug that is being developed for oncological indications. A phase 2 placebo-controlled trial of PTC299 in 380 hospitalized patients with COVID-19 began in July (NCT04439071). IMU-838 is another investigational DHODH inhibitor (308) being studied in a phase 2 placebo-controlled trial of 230 patients (NCT04379271).

Plitidepsin is a marine-derived cyclic peptide that inhibits eukaryotic translation elongation factor eEF1A. It has limited clinical approval for the treatment of multiple myeloma (Fig. 4C) . eEF1A has been shown to interact with SARS-CoV-2 nsp9 and plitidepsin has been shown to inhibit SARS-CoV-2 in vitro by inhibiting eEF1A. It has an EC 90 of 3 nM in pneumocyte-like cells and a selectivity index of 40 (309) . It has activity comparable to remdesivir in a mouse model (309) . A phase 1 trial of three different doses of intravenous plitidepsin in 45 hospitalized COVID-19 patients found the drug to be well tolerated, although anaphylaxis occurred in one patient (310) . A phase 3 trial in hospitalized patients with moderate disease is planned (NCT04784559).

Apilimod is an investigational compound that has been studied in human clinical trials as an anticancer agent. Apilimod inhibits the intracellular trafficking of several viruses, including SARS-CoV, MERS-COV, and SARS-CoV-2, during the early stages of virus replication (132, 311) . It inhibits a kinase enzyme, phosphatidylinositol-3-phosphate 5-kinase (PIKfyve), involved in endosomal membrane formation (Fig. 4D) . Although it does not alter the pH of endosomes or inhibit cathepsin B or L, it blocks entry of multiple viruses in both pseudovirus entry experiments and in cell culture assays (132, 311) . In two large-scale screening assays, apilimod was found to have EC 50 s ranging from 10 to 90 nM (312, 313) . It has been noted, however, that apilimod may interfere with the cellular immune response to SARS-CoV-2 by interfering with antigen processing in macrophages and T cells (314) . Apilimod is being studied in a phase 2 trial for the treatment of mild to moderate COVID-19 (NCT04446377).

Chloroquine analogs are weak bases that, in their nonprotonated form, concentrate within acidic intracellular organelles such as endosomes and interfere with the trafficking of viruses that require a pH-dependent step for entry into the cytoplasm (315, 316) . Chloroquine and hydroxychloroquine have modest in vitro activity against MERS-CoV (317, 318), SARS-CoV (318) (319) (320) , and SARS-CoV-2, with most EC 50 s ranging between 1 and 10 mM in Vero cells, and higher levels in other cell lines. However, multiple large randomized placebo-controlled (41, (321) (322) (323) and open-label (324-326) trials showed no clinical or virological benefit from the use of hydroxychloroquine for COVID-19 treatment. Two additional randomized trials showed that hydroxychloroquine was also ineffective for postexposure prophylaxis (327, 328) . Two factors likely explain the lack of clinical efficacy of chloroquine analogs. First, cytoplasmic entry of SARS-CoV-2, particularly in lung cells, depends primarily on the membrane-based enzyme TMPRSS2 rather than on endosomal fusion and acidification making it likely that the weak in vitro activity observed in Vero cells was not clinically relevant (132, 329) . Second, even at high dosages, chloroquine and hydroxychloroquine may not achieve the drug levels required to inhibit SARS-CoV-2 in vivo.

Niclosamide is an oral anti-helminthic that exerts its antiparasitic effects by inhibiting oxidative phosphorylation and stimulating mitochondrial ATPase activity (330) . In high-throughput drug repurposing screens, niclosamide has been found to have additional biological effects that appear to be associated either with an effect on endosomal acidification, preventing the disruption of autophagy, or inhibiting syncytium formation (331, 332) . Niclosamide was also the most active inhibitor of SARS-CoV-2 syncytium formation in a high-throughput screen of 3,000 approved drugs, suppressing the activity of TMEM16/Anoctamin6, a calcium-activated ion channel, and the scramblases responsible for phosphatidylserine cell surface exposure (332) . Niclosamide inhibits SARS-CoV, MERS-CoV, and SARS CoV-2 in cell culture with EC 50 s consistently below 1mM predominantly in Vero cells (333) (334) (335) . Oral, inhalational, and injectable formulations of niclosamide are being studied in several small phase 1 and 2 trials (336) (NCT04399356, NCT04749173, and NCT04603924). Prophylactic intranasal niclosamide is being studied in one phase 3 trial (NCT04870333).

Clofazimine is an antileprosy drug discovered to have anticoronavirus activity in two high-throughput drug screens (312, 337) . It has been shown to reduce lung virus loads in Syrian hamsters (338) . One study suggests that, in a manner similar to niclosamide, it inhibits the calcium channel pathways required for syncytium formation (332) .

Nitazoxanide is licensed for the treatment of cryptosporidium infections. It has been reported to inhibit several viruses in vitro by interfering with host pathways involved in viral replication, including those involving interferon or mTORC1 (339) . Nitazoxanide has also been reported to interfere with SARS-CoV-2-associated syncytium formation (340) . The EC 50 for SARS-CoV-2 in Vero cells is reported to be about 1 to 5 mM (341, 342) . In two randomized placebo-controlled trials of nearly 800 outpatients with mild-to-moderate SARS-CoV-2 infections, nitazoxanide did not influence time to symptom resolution (343, 344) , but one trial reported that those receiving nitazoxanide had slightly more rapid virus load reductions (343) , and the other reported that those receiving nitazoxanide had a nonstatistically significant reduction in progression to severe disease (344) .

Emetine is an FDA-approved drug for treating amebiasis. It has been shown to inhibit multiple coronaviruses in vitro, including SARS-CoV, MERS-CoV, and SARS-CoV-2, with EC 50 s below 1.0 mM (62, 317, 345, 346) . Its mechanism of action is uncertain, although it did inhibit MERS-CoV in an entry inhibitor assay using a pseudotype virus expressing S protein (345) . After oral administration, its levels in the lungs are much higher than in the plasma and well above its reported EC 50 levels (347) . Emetine is currently not being studied in any SARS-CoV-2 clinical trials.

Ivermectin is an antiparasitic agent that acts by binding to glutamate-gated chloride channels in the membranes of invertebrate nerve and muscle cells. It has been reported to also inhibit human importin alpha/beta-1 nuclear transport proteins, which viruses hijack to enhance infection by suppressing the host antiviral response (348) . Ivermectin weakly inhibits SARS-CoV-2 in Vero cells with a reported EC 50 of about 2 mM (349). Plasma and lung levels of ivermectin after standard oral dosing have been estimated to be 10-to 100-fold lower than required to inhibit virus infection in vivo (350, 351) . A retrospective observational study of 173 hospitalized patients found that treatment with ivermectin 200 mg/kg was associated with reduced mortality compared to 107 contemporaneous patients not receiving ivermectin, particularly in the subgroup with severe pulmonary disease, which was defined as requiring an FiO 2 of $50%, highflow oxygen, or mechanical ventilation (352) . However, two randomized placebo-controlled trials of ivermectin-one of 300 mg/kg per day for 5 days in 400 patients with symptoms for fewer than 7 days and another of 12 mg per day for 2 days in 112 patients with mild-moderate disease-found no significant virological or clinical improvement associated with the use of ivermectin (353, 354) . A meta-analysis that included three additional randomized placebo-controlled trials and five non-placebocontrolled studies also found no clinical benefit associated with ivermectin treatment for COVID-19 (355) .

Ciclesonide is an inhaled corticosteroid discovered in a high-throughput drug screen to inhibit coronavirus replication in the low-micromolar range (63, 356, 357) . Although its mechanism of action is not known, several SARS-CoV-2 passage experiments resulted in nsp3 and nsp4 mutations that were subsequently shown to reduce ciclesonide susceptibility. The mutated regions of nsp3 and nsp4 are thought to be associated with double membrane formation. There is one ongoing randomized phase 3 placebo-controlled trial of inhaled ciclesonide in patients with mild-to-moderate COVID-19 infection (NCT04377711).

The sigma-1 endoplasmic reticulum receptor (S1R) has been identified as a target for antiviral therapy in two large proteomic studies designed to detect SARS-CoV-2 host dependency factors. However, compounds that influence the activity of this receptor have not been evaluated for their effects on SARS-CoV-2 in cell culture (358, 359) . Fluvoxamine, an FDA-approved antidepressant, is an S1R agonist. Although fluvoxamine may inhibit SARS-CoV-2 by interfering with endosomal viral trafficking, most studies suggests that its main benefit is likely to be as an inhibitor of excess cytokine production (360) . Indeed, fluvoxamine has also been shown to modulate the response to bacterial sepsis in a beneficial manner in a mouse model (361) . In a double-blind, randomized, placebo-controlled study of 152 outpatients with confirmed SARS-CoV-2 infection, none of 80 patients receiving fluvoxamine compared to 6 of 72 patients receiving placebo experienced clinical deterioration over 15 days (P = 0.009) (362) . A larger study of fluvoxamine in 1,100 patients is currently recruiting participants (NCT04668950).

Antiviral development for SARS-CoV-2 has been disappointing. The most notable therapeutic success has been the development of MAbs targeting the SARS-CoV-2 spike protein, which have been shown to prevent infection in persons with high-risk exposures and to prevent hospitalization and mortality in COVID-19 outpatients. Remdesivir, a broad-spectrum nucleoside analog has been found to accelerate clinical improvement in hospitalized COVID-19 patients who do not require high-flow oxygen or mechanical ventilation. Convalescent plasma has been shown to reduce disease severity, but only if it contains high titers of neutralizing antibodies and is administered within the first 3 days of symptom development. A small number of additional drugs are in phase 3 clinical trials while many more are in earlier stages of development.

There are several lessons that can be learned from this disappointing progress. The first lesson is that drugs that are ineffective in preclinical studies and lack a biologically plausible mechanism of action will not be effective in clinical studies. Examples of these drugs included chloroquine analogs, ivermectin, favipiravir, antiretroviral protease inhibitors, and a long tail of other compounds, many of which have not been reviewed here. A second lesson is that uncoordinated, poorly designed, and underpowered clinical trials are wasteful, often produce misleading results, and arguably unethical (363) . Many clinical trials of antiviral therapies that were eventually found to be ineffective initially reported positive clinical outcomes, likely indicating researcher or publication bias.

A third lesson is there is currently no model for drug development in the setting of a pandemic. Traditional drug development is an iterative process aimed at maximizing selectivity while improving pharmacokinetic and pharmacodynamic properties (364) . However, in the face of a global public health threat, many researchers anticipated that drugs that were already approved or studied in humans for other indications could be repurposed to treat COVID-19, thus bypassing traditional drug discovery approaches. Although high-throughput screens identified several compounds with in vitro SARS-CoV-2-inhibitory activity (313, 358, 359, (365) (366) (367) , the process of creating a clinical trials pathway to investigate existing drugs for new indications has been slow and lacking in transparency. Drug development is expensive and the companies with the financial ability to launch the necessary clinical trials may have lacked economic incentives to foster the development of drugs that were already approved for other indications. Moreover, treatment for acute infectious diseases has not been a priority for the pharmaceutical industry for the past 2 decades (368) .

A fourth lesson is that antiviral therapy for COVID-19 must be administered early in the course of infection. This was largely expected based on the finding that virus loads peak early in infection and that severe COVID-19 disease manifestations arise at a time when virus levels have usually begun to decrease. Indeed, neutralizing MAbs, remdesivir, and molnupiravir have been most effective when administered early in the course of infection. Antiviral therapy for influenza is also optimal when administered early; however, the requirement for prompt treatment appears to be stricter for SARS-CoV-2 than for influenza (369, 370) . The timely initiation of antiviral therapy for SARS-CoV-2 will require the widespread availability of simple, affordable, and self-administered tests for the early diagnosis of SARS-CoV-2 infection. Moreover, for those persons at highest risk of severe COVID-19 infection, postexposure prophylaxis strategies will need to be considered.

The final lesson is that surrogate markers predictive of clinical endpoints are needed. Without such markers, clinical trials must be large and expensive. Most of the clinical trials demonstrating the efficacy of MAbs and remdesivir required more than 500 patients per arm. With the EUA and likely approval of several neutralizing MAb preparations for SARS-CoV-2-infected persons at high risk of disease progression, it will no longer be unethical to conduct placebo-controlled trials in this population in regions where neutralizing MAbs are available. This development will necessitate larger clinical trials enrolling low-risk patients or trials in which new treatments are studied in combination with a neutralizing MAb.

In conclusion, the main therapeutic advance during the pandemic has been the development of highly potent neutralizing MAbs for the prevention and treatment of SARS-CoV-2. The administration of remdesivir and possibly the early use of convalescent plasma may have a role in a subset of patients. Several additional treatments that appear promising based on small clinical trials, including molnupiravir, inhaled IFN-b, and fluvoxamine. Other treatments have a compelling mechanism of action but are further behind in clinical development, including the oral nucleoside analog AT-527, the oral protease inhibitor PF-07321332, polyclonal antibody preparations, single-domain antibodies, and molecular decoys that bind SARS-CoV-2 spike. Several repurposed drugs that target host processes are also in phase 2 or 3 clinical trials, including camostat, apilimod, the DHODH inhibitor PTC-299, nitazoxanide, and niclosamide. Although these repurposed drugs do not have the same in vitro potency as directly acting antivirals, they may prove useful in combination with a directly acting antiviral. Despite the slow start in anti-SARS-CoV-2 drug development and the possible cresting of the pandemic in upper-income countries, continued antiviral drug development remains critical to developing affordable treatments for patients in low-and middleincome countries and for patients at risk of developing COVID-19 despite vaccination. In addition, some of the treatments in clinical development are also likely to be effective against future pandemic viruses. 

Coronaviruses: drug discovery and therapeutic options

Therapeutic options for the 2019 novel coronavirus (2019-nCoV)

Antiviral drugs specific for coronaviruses in preclinical development

Human monoclonal antibodies as candidate therapeutics against emerging viruses

Nucleoside analogues for the treatment of coronavirus infections

Global virus outbreaks: interferons as 1st responders

Efficacy of ChAdOx1 nCoV-19 (AZD1222) vaccine against SARS-CoV-2 variant of concern 202012/ 01 (B.1.1.7): an exploratory analysis of a randomised controlled trial

Efficacy of NVX-CoV2373 Covid-19 vaccine against the B.1.351 variant

Efficacy of the ChAdOx1 nCoV-19 Covid-19 vaccine against the B.1.351 variant

Effectiveness of the BNT162b2 COVID-19 vaccine against the B.1.1.7 and B.1.351 variants

A comprehensive superposition of viral polymerase structures

Approved antiviral drugs over the past 50 years

Discovery of an RNA virus 39!59 exoribonuclease that is critically involved in coronavirus RNA synthesis

Coronaviruses lacking exoribonuclease activity are susceptible to lethal mutagenesis: evidence for proofreading and potential therapeutics

Structural and molecular basis of mismatch correction and ribavirin excision from coronavirus RNA

Structural basis for RNA replication by the SARS-CoV-2 polymerase

Structure of the RNA-dependent RNA polymerase from COVID-19 virus

Structure of replicating SARS-CoV-2 polymerase

Structural basis for inhibition of the RNA-dependent RNA polymerase from SARS-CoV-2 by remdesivir

Discovery and synthesis of a phosphoramidate prodrug of a pyrrolo[2,1-f][triazin-4-amino] adenine C-nucleoside (GS-5734) for the treatment of Ebola and emerging viruses

The antiviral compound remdesivir potently inhibits RNA-dependent RNA polymerase from Middle East respiratory syndrome coronavirus

Remdesivir is a direct-acting antiviral that inhibits RNAdependent RNA polymerase from severe acute respiratory syndrome coronavirus 2 with high potency

Remdesivir and SARS-CoV-2: structural requirements at both nsp12 RdRp and nsp14 exonuclease activesites

Template-dependent inhibition of coronavirus RNA-dependent RNA polymerase by remdesivir reveals a second mechanism of action

Mechanism of SARS-CoV-2 polymerase stalling by remdesivir

Remdesivir is a delayed translocation inhibitor of SARS-CoV-2 replication

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

An orally bioavailable broad-spectrum antiviral inhibits SARS-CoV-2 in human airway epithelial cell cultures and multiple coronaviruses in mice

Coronavirus susceptibility to the antiviral remdesivir (GS-5734) is mediated by the viral polymerase and the proofreading exoribonuclease

Remdesivir inhibits SARS-CoV-2 in human lung cells and chimeric SARS-CoV expressing the SARS-CoV-2 RNA polymerase in mice

Off-target in vitro profiling demonstrates that remdesivir is a highly selective antiviral agent

Prophylactic and therapeutic remdesivir (GS-5734) treatment in the rhesus macaque model of MERS-CoV infection

Clinical benefit of remdesivir in rhesus macaques infected with SARS-CoV-2

Tissue distributions of antiviral drugs affect their capabilities of reducing viral loads in COVID-19 treatment

Remdesivir for treatment of COVID-19: combination of pulmonary and IV administration may offer additional benefit

Advantages of the parent nucleoside GS-441524 over remdesivir for COVID-19 treatment

Pharmacokinetics of orally administered GS-441524 in dogs

Remdesivir metabolite GS-441524 effectively inhibits SARS-CoV-2 infection in mouse models

Pharmacokinetic, pharmacodynamic, and drug-interaction profile of remdesivir, a SARS-CoV-2 replication inhibitor

Remdesivir for the treatment of COVID-19: final report

Repurposed antiviral drugs for COVID-19: interim WHO solidarity trial results

Remdesivir in adults with severe COVID-19: a randomised, double-blind, placebocontrolled, multicentre trial

CoV-2 Antiviral Therapy Clinical Microbiology Reviews promising oral antiviral for treatment of COVID-19

Favipiravir (T-705), a novel viral RNA polymerase inhibitor

Favipiravir as a potential countermeasure against neglected and emerging RNA viruses

Rapid incorporation of Favipiravir by the fast and permissive viral RNA polymerase complex results in SARS-CoV-2 lethal mutagenesis

Favipiravir: pharmacokinetics and concerns about clinical trials for 2019-nCoV infection

Remdesivir, lopinavir, emetine, and homoharringtonine inhibit SARS-CoV-2 replication in vitro

Identification of antiviral drug candidates against SARS-CoV-2 from FDAapproved drugs

A nanoluciferase SARS-CoV-2 for rapid neutralization testing and screening of anti-infective drugs for COVID-19

Structure of the SARS-CoV-2 RNA-dependent RNA polymerase in the presence of favipiravir-RTP

Favipiravir at high doses has potent antiviral activity in SARS-CoV-2-infected hamsters, whereas hydroxychloroquine lacks activity

Efficacy and safety of favipiravir, an oral RNA-dependent RNA polymerase inhibitor, in mild-to-moderate COVID-19: a randomized, comparative, open-label, multicenter, phase 3 clinical trial

A prospective, randomized, open-label trial of early versus late favipiravir therapy in hospitalized patients with COVID-19

AVIFAVIR for treatment of patients with moderate coronavirus disease 2019 (COVID-19): interim results of a phase II/ III multicenter randomized clinical trial

Clinical outcomes and plasma concentrations of baloxavir marboxil and favipiravir in COVID-19 patients: an exploratory randomized, controlled trial

A library of nucleotide analogues terminate RNA synthesis catalyzed by polymerases of coronaviruses that cause SARS and COVID-19

Sofosbuvir terminated RNA is more resistant to SARS-CoV-2 proofreader than RNA terminated by remdesivir

Nucleotide analogues as inhibitors of SARS-CoV-2 polymerase, a key drug target for COVID-19

In vitro antiviral activity of the anti-HCV drugs daclatasvir and sofosbuvir against SARS-CoV-2, the aetiological agent of COVID-19

Sofosbuvir/ daclatasvir regimens for the treatment of COVID-19: an individual patient data meta-analysis

The in vitro antiviral activity of the anti-hepatitis C virus (HCV) drugs daclatasvir and sofosbuvir against SARS-CoV-2

A new mouse-adapted strain of SARS-CoV as a lethal model for evaluating antiviral agents in vitro and in vivo

SARS: systematic review of treatment effects

Inhibition of novel b coronavirus replication by a combination of interferon-a2b and ribavirin

Targeting SARS-CoV-2 proteases and polymerase for COVID-19 treatment: state of the art and future opportunities

Crystal structure of SARS-CoV-2 main protease provides a basis for design of improved a-ketoamide inhibitors

Potential SARS-CoV-2 main protease inhibitors

Structure of papain-like protease from SARS-CoV-2 and its complexes with non-covalent inhibitors

Mechanism and inhibition of the papain-like protease, PLpro

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

Atazanavir, alone or in combination with ritonavir, inhibits SARS-CoV-2 replication and proinflammatory cytokine production

Lack of antiviral activity of darunavir against SARS-CoV-2

Antiviral efficacies of FDAapproved drugs against SARS-CoV-2 infection in ferrets

A trial of lopinavir-ritonavir in adults hospitalized with severe COVID-19

Saudi Critical Care Trials Group. 2020. Interferon b1b and lopinavir-ritonavir for Middle East respiratory syndrome

Structure-based design of antiviral drug candidates targeting the SARS-CoV-2 main protease

Structure of M pro from SARS-CoV-2 and discovery of its inhibitors

Crystal structure of SARS-CoV-2 main protease in complex with the non-covalent inhibitor ML188

GRL-0920, an indole chloropyridinyl ester, completely blocks SARS-CoV-2 infection

A small molecule compound with an indole moiety inhibits the main protease of SARS-CoV-2 and blocks virus replication

Discovery of ketone-based covalent inhibitors of coronavirus 3CL proteases for the potential therapeutic treatment of COVID-19

Malleability of the SARS-CoV-2 3CL Mpro active-site cavity facilitates binding of clinical antivirals

Challenges for targeting SARS-CoV

Antiviral Therapy Clinical Microbiology Reviews proteases as a therapeutic strategy for COVID-19

ALG-097111, a potent and selective SARS-CoV-2 3-chymotrypsin-like cysteine protease inhibitor exhibits in vivo efficacy in a Syrian hamster model

A comparative analysis of SARS-CoV-2 antivirals characterizes 3CLpro inhibitor PF-00835231 as a potential new treatment for COVID-19

Discovery of a novel inhibitor of coronavirus 3CL protease as a clinical candidate for the potential treatment of COVID-19

Molecular characterization of ebselen binding activity to SARS-CoV-2 main protease

Ebselen derivatives are very potent dual inhibitors of SARS-CoV-2 proteases: PL pro and M pro in in vitro studies

Identification of ebselen and its analogues as potent covalent inhibitors of papain-like protease from SARS-CoV-2

Reversal of the progression of fatal coronavirus infection in cats by a broad-spectrum coronavirus protease inhibitor

Broad-spectrum antivirals against 3C or 3C-like proteases of picornaviruses, noroviruses, and coronaviruses

Boceprevir, GC-376, and calpain inhibitors II, XII inhibit SARS-CoV-2 viral replication by targeting the viral main protease

Feline coronavirus drug inhibits the main protease of SARS-CoV-2 and blocks virus replication

Structure basis for inhibition of SARS-CoV-2 by the feline drug GC376

Both Boceprevir and GC376 efficaciously inhibit SARS-CoV-2 by targeting its main protease

Structure-guided design of potent and permeable inhibitors of MERS coronavirus 3CL protease that utilize a piperidine moiety as a novel design element

Lead compounds for the development of SARS-CoV-2 3CL protease inhibitors

Structure and inhibition of the SARS-CoV-2 main protease reveals strategy for developing dual inhibitors against Mpro and cathepsin L

Identification of 14 known drugs as inhibitors of the main protease of SARS-CoV-2

SARS-CoV-2 Mpro inhibitors with antiviral activity in a transgenic mouse model

Structure, function, and antigenicity of the SARS-CoV-2 spike glycoprotein

Cryo-EM structure of the 2019-nCoV spike in the prefusion conformation

Structure of the SARS-CoV-2 spike receptor-binding domain bound to the ACE2 receptor

Structural and functional basis of SARS-CoV-2 entry by using human ACE2

Structural basis of receptor recognition by SARS-CoV-2

Conformational dynamics of SARS-CoV-2 trimeric spike glycoprotein in complex with receptor ACE2 revealed by cryo-EM

Distinct conformational states of SARS-CoV-2 spike protein

Host cell proteases: critical determinants of coronavirus tropism and pathogenesis

Structural insights into coronavirus entry

The spike glycoprotein of the new coronavirus 2019-nCoV contains a furin-like cleavage site absent in CoV of the same clade

Receptor binding and priming of the spike protein of SARS-CoV-2 for membrane fusion

Furin inhibitors block SARS-CoV-2 spike protein cleavage to suppress virus production and cytopathic effects

Loss of furin cleavage site attenuates SARS-CoV-2 pathogenesis

A multibasic cleavage site in the spike protein of SARS-CoV-2 is essential for infection of human lung cells

SARS-CoV-2 cell entry depends on ACE2 and TMPRSS2 and is blocked by a clinically proven protease inhibitor

TMPRSS2 and furin are both essential for proteolytic activation of SARS-CoV-2 in human airway cells

Characterization of spike glycoprotein of SARS-CoV-2 on virus entry and its immune cross-reactivity with SARS-CoV

Wild-type human coronaviruses prefer cell-surface TMPRSS2 to endosomal cathepsins for cell entry

Inhibitors of cathepsin L prevent severe acute respiratory syndrome coronavirus entry

Coronavirus membrane fusion mechanism offers a potential target for antiviral development

Pan-coronavirus fusion inhibitors as the hope for today and tomorrow

Respiratory syncytial virus entry and how to block it. 4

Expert consensus on palivizumab use for respiratory syncytial virus in developed countries

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COVID-19-neutralizing antibodies predict disease severity and survival

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Convergent antibody responses to SARS-CoV-2 in convalescent individuals

SARS-CoV-2 RNA shedding in recovered COVID-19 cases and the presence of antibodies against SARS-CoV-2 in recovered COVID-19 cases and close contacts

Antibody responses to SARS-CoV-2 in patients with novel coronavirus disease

Dissecting antibody-mediated protection against SARS-CoV-2. 7

COVID-19 survival associates with the immunoglobulin response to the SARS-CoV-2 spike receptor binding domain

Delayed production of neutralizing antibodies correlates with fatal COVID-19

The receptor binding domain of the viral spike protein is an immunodominant and highly specific target of antibodies in SARS-CoV-2 patients

Mapping neutralizing and immunodominant sites on the SARS-CoV-2 spike receptor-binding domain by structure-guided high-resolution serology

Comprehensive mapping of mutations in the SARS-CoV-2 receptor-binding domain that affect recognition by polyclonal human plasma antibodies

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Neutralizing monoclonal antibodies for treatment of COVID-19

Recognition of the SARS-CoV-2 receptor binding domain by neutralizing antibodies

SARS-CoV-2 neutralizing antibody structures inform therapeutic strategies

Structural analysis of neutralizing epitopes of the SARS-CoV-2 spike to guide therapy and vaccine design strategies

A human monoclonal antibody blocking SARS-CoV-2 infection

A highly conserved cryptic epitope in the receptor-binding domains of SARS-CoV-2 and SARS-CoV

Structural basis for the neutralization of SARS-CoV-2 by an antibody from a convalescent patient

Broad neutralization of SARS-related viruses by human monoclonal antibodies

Cross-neutralization of SARS-CoV-2 by a human monoclonal SARS-CoV antibody

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

Potent neutralizing antibodies against multiple epitopes on SARS-CoV-2 spike

Studies in humanized mice and convalescent humans yield a SARS-CoV-2 antibody cocktail

REGN-COV2 antibodies prevent and treat SARS-CoV-2 infection in rhesus macaques and hamsters

Antibody cocktail to SARS-CoV-2 spike protein prevents rapid mutational escape seen with individual antibodies

The neutralizing antibody, LY-CoV555, protects against SARS-CoV-2 infection in non-human primates

A human neutralizing antibody targets the receptor binding site of SARS-CoV-2

Potently neutralizing and protective human antibodies against SARS-CoV-2

A therapeutic neutralizing antibody targeting receptor binding domain of SARS-CoV-2 spike protein

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Versatile and multivalent nanobodies efficiently neutralize SARS-CoV-2

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

Identification of human single-domain antibodies against SARS-CoV-2

Rapid identification of a human antibody with high prophylactic and therapeutic efficacy in three animal models of SARS-CoV-2 infection

Structure-guided multivalent nanobodies block SARS-CoV-2 infection and suppress mutational escape

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

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RBD-specific polyclonal F(ab¨)2 fragments of equine antibodies in patients with moderate to severe COVID-19 disease: a randomized, multicenter, double-blind, placebo-controlled

Pharmacokinetics and safety of XAV-19, a swine glyco-humanized polyclonal anti-SARS-CoV-2 antibody, for COVID-19-related moderate pneumonia: a randomized, double-blind, placebo-controlled, phase IIa study

Safety and tolerability of a novel, polyclonal human anti-MERS coronavirus antibody produced from transchromosomic cattle: a phase 1 randomised, double-blind, single-dose-escalation study

Suppression of SARS-CoV entry by peptides corresponding to heptad regions on spike glycoprotein

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Structure-based discovery of Middle East respiratory syndrome coronavirus fusion inhibitor

De novo design of a-helical lipopeptides targeting viral fusion proteins: a promising strategy for relatively broadspectrum antiviral drug discovery

A pan-coronavirus fusion inhibitor targeting the HR1 domain of human coronavirus spike

Design of potent membrane fusion inhibitors against SARS-CoV-2, an emerging coronavirus with high fusogenic activity

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

A lipopeptide HIV-1/2 fusion inhibitor with highly potent in vitro, ex vivo, and in vivo antiviral activity

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Intranasal fusion inhibitory lipopeptide prevents direct-contact SARS-CoV-2 transmission in ferrets

Angiotensin-converting enzyme 2 in acute respiratory distress syndrome

A crucial role of angiotensin converting enzyme 2 (ACE2) in SARS coronavirus-induced lung injury

A pilot clinical trial of recombinant human angiotensin-converting enzyme 2 in acute respiratory distress syndrome

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

Pharmacokinetics and pharmacodynamics of recombinant human angiotensin-converting enzyme 2 in healthy human subjects

Susceptibility to SARS coronavirus S protein-driven infection correlates with expression of angiotensin converting enzyme 2 and infection can be blocked by soluble receptor

Apeiron's APN01 shows clinical benefits for severely ill COVID-19 patients in phase 2 trial

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

Stapled peptides based on human angiotensin-converting enzyme 2 (ACE2) potently inhibit SARS-CoV-2 infection in vitro

Engineered ACE2 receptor traps potently neutralize SARS-CoV-2

A trimeric human angiotensin-converting enzyme 2 as an anti-SARS-CoV-2 agent

An engineered decoy receptor for SARS-CoV-2 broadly binds protein S sequence variants

Engineering human ACE2 to optimize binding to the spike protein of SARS coronavirus 2

De novo design of potent and resilient hACE2 decoys to neutralize SARS-CoV-2

Identification of critical determinants on ACE2 for SARS-CoV entry and development of a potent entry inhibitor

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

Beyond antibodies: the DARPin drug platform

Highly potent anti-SARS-CoV-2 multi-DARPin therapeutic candidates

Inhibition of SARS pseudovirus cell entry by lactoferrin binding to heparan sulfate proteoglycans

SARS-CoV-2 infection depends on cellular heparan sulfate and ACE2

Lactoferrin as potential preventative and treatment for COVID-19

The in vitro antiviral activity of lactoferrin against common human coronaviruses and SARS-CoV-2 is mediated by targeting the heparan sulfate coreceptor

Effective inhibition of SARS-CoV-2 entry by heparin and enoxaparin derivatives

Heparin inhibits cellular invasion by SARS-CoV-2: structural dependence of the interaction of the spike S1 receptor-binding domain with heparin

Characterization of heparin and severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) spike glycoprotein binding interactions

Type 1 interferons as a potential treatment against COVID-19

Type I interferons in infectious disease

Type I and type III interferons: induction, signaling, evasion, and application to combat COVID-19

Shared and distinct functions of type I and type III interferons

SARS and MERS: recent insights into emerging coronaviruses

SARS coronavirus pathogenesis: host innate immune responses and viral antagonism of interferon

Interplay between SARS-CoV-2 and the type I interferon response

Auto-antibodies against type I IFNs in patients with lifethreatening COVID-19

Imbalanced host response to SARS-CoV-2 drives development of COVID-19

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

Heightened innate immune responses in the respiratory tract of COVID-19 patients

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2020. Type I interferon susceptibility distinguishes SARS-CoV-2 from SARS-CoV

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

Inhibition of SARS-CoV-2 by type I and type III interferons

2020. Type I and type III interferons restrict SARS-CoV-2 infection of human airway epithelial cultures

Antiviral activity of type I, II, and III interferons counterbalances ACE2 inducibility and restricts SARS-CoV-2

Evaluation of immunomodulators, interferons and known in vitro SARS-coV inhibitors for inhibition of SARS-coV replication in BALB/c mice

Single-dose intranasal administration with mDEF201 (adenovirus vectored mouse interferonalpha) confers protection from mortality in a lethal SARS-CoV BALB/c mouse model

Pegylated interferon-alpha protects type 1 pneumocytes against SARS coronavirus infection in macaques

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

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Peginterferon lambda for the treatment of outpatients with COVID-19: a phase 2, placebo-controlled randomised trial

Efficacy and safety of interferon b-1a in treatment of severe COVID-19: a randomized clinical trial

SARS-CoV-2 clearance in COVID-19 patients with novaferon treatment: a randomized, open-label, parallel group trial

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

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The anticoagulant nafamostat potently inhibits SARS-CoV-2 S protein-mediated fusion in a cell fusion assay system and viral infection in vitro in a cell-type-dependent manner

Camostat mesylate inhibits SARS-CoV-2 activation by TMPRSS2-related proteases and its metabolite GBPA exerts antiviral activity

An enzymatic TMPRSS2 assay for assessment of clinical candidates and discovery of inhibitors as potential treatment of COVID-19

Nafamostat mesylate blocks activation of SARS-CoV-2: new treatment option for COVID-19

Differentially conserved amino acid positions may reflect differences in SARS-CoV-2 and SARS-CoV behaviour

Protease inhibitors targeting coronavirus and filovirus entry

A phase I study of high dose camostat mesylate in healthy adults provides a rationale to repurpose the TMPRSS2 inhibitor for the treatment of COVID-19

Efficacy of the TMPRSS2 inhibitor camostat mesilate in patients hospitalized with Covid-19-a double-blind randomized controlled trial

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In plain sight: the role of alpha-1-antitrypsin in COVID-19 pathogenesis and therapeutics

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Aprotinin inhibits SARS-CoV-2 replication

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Novel and potent inhibitors targeting DHODH are broad-spectrum antivirals against RNA viruses including newly-emerged coronavirus SARS-CoV-2

Treatment of COVID-19 patients with prolonged postsymptomatic viral shedding with leflunomide: a single-center, randomized, controlled clinical trial

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IMU-838, a developmental DHODH inhibitor in phase II for autoimmune disease, shows anti-SARS-CoV-2 and broad-spectrum antiviral efficacy in vitro

Plitidepsin has potent preclinical efficacy against SARS-CoV-2 by targeting the host protein eEF1A

Plitidepsin has a positive therapeutic index in adult patients with COVID-19 requiring hospitalization

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Discovery of SARS-CoV-2 antiviral drugs through large-scale compound repurposing

The global phosphorylation landscape of SARS-CoV-2 infection

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Effects of chloroquine on viral infections: an old drug against today's diseases?

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Screening of an FDA-approved compound library identifies four small-molecule inhibitors of Middle East respiratory syndrome coronavirus replication in cell culture

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In vitro inhibition of severe acute respiratory syndrome coronavirus by chloroquine

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A clusterrandomized trial of hydroxychloroquine for prevention of Covid-19

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Niclosamide: beyond an antihelminthic drug

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Inhibition of severe acute respiratory syndrome coronavirus replication by niclosamide

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A randomized, double-blind, placebo-controlled phase 1 trial of inhaled and intranasal niclosamide: a broad spectrum antiviral candidate for treatment of COVID-19

Identification of potent and safe antiviral therapeutic candidates against SARS-CoV-2

Clofazimine broadly inhibits coronaviruses including SARS-CoV-2

Nitazoxanide, a new drug candidate for the treatment of Middle East respiratory syndrome coronavirus

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Early use of nitazoxanide in mild Covid-19 disease: randomised, placebo-controlled trial

Early treatment with nitazoxanide prevents worsening of mild and moderate COVID-19 and subsequent hospitalization

High-throughput screening and identification of potent broad-spectrum inhibitors of coronaviruses

Screening of FDA-approved drugs using a MERS-CoV clinical isolate from South Korea identifies potential therapeutic options for COVID-19

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The broad spectrum antiviral ivermectin targets the host nuclear transport importin a/b1 heterodimer

The FDAapproved drug ivermectin inhibits the replication of SARS-CoV-2 in vitro

Ivermectin and novel coronavirus disease (COVID-19): keeping rigor in times of urgency

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Ivermectin as a potential treatment for mild to moderate COVID-19: a double blind randomized placebo-controlled trial

Effect of ivermectin on time to resolution of symptoms among adults with mild COVID-19: a randomized clinical trial

Ivermectin for the treatment of COVID-19: a systematic review and meta-analysis of randomized controlled trials

The inhaled steroid ciclesonide blocks SARS-CoV-2 RNA replication by targeting the viral replication-transcription complex in cultured cells

Screening of FDA-approved drugs using a MERS-CoV clinical isolate from South Korea identifies potential therapeutic options for COVID-19

A SARS-CoV-2 protein interaction map reveals targets for drug repurposing

Comparative host-coronavirus protein interaction networks reveal panviral disease mechanisms

Fluvoxamine: a review of its mechanism of action and its role in COVID-19

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The continuing unethical conduct of underpowered clinical trials

Accelerated preclinical paths to support rapid development of COVID-19 therapeutics

Lessons from the COVID-19 pandemic for advancing computational drug repurposing strategies

Exploring the SARS-CoV-2 virus-host-drug interactome for drug repurposing

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Kaiming Tao obtained his Bachelor of Engineering in Electrical Engineering and Automation from Jinling Institute of Technology, Nanjing, China. Since obtaining his degree, he has worked for Springer Nature as a software developer and marketing technologist to support digital publication and digital marketing. He has certifications of Deep Learning Specialization and MIT7.00x. His main interests includes applying deep learning methods on bioinformatics and data engineering. In early 2020, he joined the Stanford HIV Drug Resistance Database and worked on developing software pipelines for analyzing antiviral research data. He is a cocreator of the Coronavirus Antiviral and Resistance Database. Robert W. Shafer, M.D., is a professor of medicine, and by courtesy, pathology in the Division of Infectious Diseases at Stanford University. Dr. Shafer received his undergraduate education at Columbia University and his medical degree at New York University. He has studied the mechanisms and consequences of virus evolution with a focus on antiretroviral drug therapy and drug resistance. In 2000, he created the Stanford HIV Drug Resistance Database, a publicly available resource for those performing HIV drug resistance surveillance, interpreting HIV drug resistance tests, and developing new antiretroviral drugs. His work is focused on the clinical management, epidemiology, and laboratory science of HIV drug resistance. In 2020, Dr. Shafer created the SARS-CoV-2 Antiviral and Resistance Database to track the in vitro activity of candidate SARS-CoV-2 antiviral drugs and the impact of SARS-CoV-2 spike mutations on the efficacy of monoclonal antibodies, risk of rereinfection, and vaccine efficacy.