key: cord-0870991-j1yy16sh
authors: Españo, Erica; Kim, Dajung; Kim, Jiyeon; Park, Song-Kyu; Kim, Jeong-Ki
title: COVID-19 Antiviral and Treatment Candidates: Current Status
date: 2021-02-15
journal: Immune Netw
DOI: 10.4110/in.2021.21.e7
sha: c9e1376419b140bc60a249cf11d7624b9e01c2b9
doc_id: 870991
cord_uid: j1yy16sh

The coronavirus disease 2019 (COVID-19) pandemic caused by the severe acute respiratory syndrome coronavirus 2 has severely impacted global health and economy. There is currently no effective approved treatment for COVID-19; although vaccines have been granted emergency use authorization in several countries, they are currently only administered to high-risk individuals, thereby leaving a gap in virus control measures. The scientific and clinical communities and drug manufacturers have collaborated to speed up the discovery of potential therapies for COVID-19 by taking advantage of currently approved drugs as well as investigatory agents in clinical trials. In this review, we stratified some of these candidates based on their potential targets in the progression of COVID-19 and discuss some of the results of ongoing clinical evaluations.

The coronavirus disease 2019 (COVID- 19) was first reported in December 2019 in Wuhan, Hubei, China as cases of respiratory illness leading to pneumonia of unknown etiology. Viral isolation and genetic characterization revealed the causative agent to be closely related (79% nucleotide identity) to the severe acute respiratory syndrome coronavirus (SARS-CoV) of the genus Betacoronavirus of the Coronaviridae family (1). This family includes several veterinary and human viruses, including 4 human coronaviruses that cause the common cold (HCoV-NL63, HCoV-229E, HCoV-OC43, and HCoV-HKU1) and the Middle East respiratory syndrome coronavirus (MERS-CoV). Owing to its genetic relationship to SARS-CoV, the COVID-19 agent was named SARS-CoV-2 by the International Committee on Taxonomy of Viruses. Further phylogenetic analyses showed that SARS-CoV-2 shares 96.2% of its genome with a SARS-like CoV (RaTG13) isolated from the intermediate horseshoe bat Rhinolophus affinis in 2013, suggesting that SARS-CoV-2 is zoonotic in nature and emerged from a spillover event from bats (2) . SARS-CoV-2 has spread at a much larger scale than either SARS-CoV or MERS-CoV, eventually leading the World Health Organization (WHO) to declare a COVID-19 pandemic on March 11, 2020. At the time of writing, the number of cases has breached 90 million, with more than 1.9 million deaths (https://coronavirus.jhu.edu/map.html) (3) . Apart ERGIC, endoplasmic reticulum-to-Golgi intermediate compartment.

RDV (GS-5374) is considered one of the leading candidates in the search for drugs against SARS-CoV-2 ( Table 1) . RDV is a prodrug with a triphosphate form (RDV-TP) that closely resembles ATP and has been reported to be slightly more preferentially incorporated than ATP into the nascent RNA strand by the viral RNA-dependent RNA polymerase (RdRp) (14) . Unlike typical RNA chain terminators, RDV causes delayed termination at i+3 and i+5 positions (where i is the RDV-TP insertion position), likely due to steric strain at the RdRp active site (15) .

RDV was initially reported to have the potential to inhibit filoviruses. It progressed to phase 2/3 clinical trials against the Ebola virus (EBOV) but was found to be inferior to other treatments (16) . Several in vitro studies have also reported the activity of RDV against other RNA viruses such as paramyxoviruses (e.g., the Nipah virus), pneumoviruses, and coronaviruses (e.g., SARS-CoV and MERS-CoV), indicating the potential of RDV as a broadspectrum antiviral (17) . The promise of RDV as an agent against coronaviruses was further extended to SARS-CoV-2 in an in vitro study (18) . Based on its good safety profile in the EBOV trials, RDV was used against COVID-19 under the compassionate use protocol and advanced to phase 2/3 clinical trials. An early trial in China has shown that RDV treatment resulted in faster improvement in severe COVID-19 patients, but the effects were not significantly different from those of placebo (19) . Compassionate use of RDV in hospitalized patients also increased recovery rate in the cohort (20) . The interim results of the National Institute of Allergy and Infectious Diseases (NIAID) double-blind, randomized, placebo-controlled phase 3 trial (NCT04280705) showed that a 10-day course of RDV reduced the time to recovery of hospitalized COVID-19 patients (21) . The final results of this trial show that RDV treatment improved clinical outcomes, including shorter time to improvement in an ordinal scale of patient categories, shorter time to recovery, shorter time for oxygen supplementation, and lower mortality compared to placebo (21) . This trial also suggests that, while RDV is more beneficial when given early into the illness, RDV provides benefits even when administered later in the course of COVID-19. In contrast, results of the WHO Solidarity trials show that RDV does not benefit hospitalized COVID-19 patients in terms of mortality, progression to ventilation, and length of hospital stay (22) . Whether RDV treatment is beneficial to mild to moderate COVID-19 patients (outpatients), to pediatric patients, and when used with other agents are still being evaluated (e.g., NCT04501952, NCT04431453, and NCT04409262).

Lopinavir-ritonavir (LPVr) is a combination of HIV protease inhibitors used for AIDS treatment. Most of the protease inhibitory effects are attributed to lopinavir, while ritonavir is used to elevate systemic levels of lopinavir (23) . An in silico study suggests that LPVr bind the SARS-CoV 3C-like protease (3CL pro ) (24) . In line with this, a study has shown that both drugs can inhibit SARS-CoV infection in vitro, and treatment with LPVr has clinical benefits against SARS in a small cohort (25) . Similarly, post-exposure prophylaxis with LPVr and ribavirin has been associated with protectivity against MERS in a retrospective study (26) .

Given the high sequence conservation of the CoV protease, LPVr was expected to have COVID-19 treatment benefits (27). Based on the previous guidelines for MERS and SARS management, LPVr was given to patients in South Korea and in China early into the SARS-CoV-2 outbreak and appeared to have treatment benefits (28, 29) . However, the results of The Efficacy of Lopinavir Plus Ritonavir and Arbidol Against Novel Coronavirus Infection (30) and Randomised Evaluation of COVID-19 Therapy (RECOVERY) (31) suggest that LPVr provides little to no benefit to hospitalized COVID-19 patients ( Table 1) . The WHO's Solidarity trial revealed similar findings, leading to the WHO's decision to halt the LPVr arms of the Solidarity trial (22, 32) . However, whether LPVr is effective against the early stages of SARS-CoV-2 infection or as a prophylactic agent is still currently being explored in some trials (e.g., NCT04372628 and NCT04328285).

Ribavirin is a guanosine analog clinically used against the hepatitis C virus (HCV). The reported mechanisms for the antiviral activity of ribavirin include: competitive inhibition of inosine monophosphate dehydrogenase, a rate-limiting enzyme for GTP synthesis (33) ; and mutagenesis of the viral genome via the incorporation of ribavirin triphosphate instead of GTP, which results in lower virus viability (34) . Ribavirin has also shown immunomodulatory activity in cases of HCV infection (35).

Ribavirin has displayed antiviral effects against SARS-CoV in vitro and has shown synergistic effects with type I IFNs (36,37). Ribavirin alone or in combination with IFN-β1b improved the clinical scores and promoted viral clearance in MERS-CoV-infected rhesus macaques (38). The combination of ribavirin with LPVr and IFNs, rather than treatment with ribavirin alone, has been seen to improve clinical outcomes in some MERS cases (39,40). In line with this, a retrospective study has shown that treatment with ribavirin alone did not improve outcomes in COVID-19 patients (41). Thus, most COVID-19 trials involving ribavirin test its combination with other agents, particularly IFNs, for optimal effects. A prospective randomized study has shown that early treatment with the combination of ribavirin with IFN-β1b and LPVr alleviates symptoms and shortens the duration of viral shedding in patients with mild to moderate COVID-19 (42). In contrast, a randomized open-labeled prospective study suggests that ribavirin+IFN-α, LPVr+IFN-α, and ribavirin+LPVr+IFN-α do not have significantly different antiviral effects, and that the combination of ribavirin+LPVr may have adverse effects in patients with mild to moderate COVID-19 (43). The results of ongoing trials (e.g., NCT04551768, NCT04494399, and NCT04563208) are needed to determine whether ribavirin alone is beneficial to or enhances the benefits of other (e.g., IFNs) agents in COVID-19 treatment.

SOF and DCV are direct-acting antivirals used in combination to treat HCV infection. DCV inhibits the HCV nonstructural (NS) 5A protein and is hypothesized to affect HCV replication, assembly, and secretion (44) . Meanwhile, SOF is a nucleotide analog that inhibits the HCV polymerase, NS5B (45) . SOF has also demonstrated activity against Zika virus (ZIKV), dengue virus (DENV), yellow fever virus, and chikungunya virus (46) .

The conserved nature of the RdRp in positive-sense RNA viruses prompted the interest in SOF for COVID-19 treatment. In silico studies have shown that the SARS-CoV-2 RdRp can bind SOF, suggesting that SOF may be used to inhibit SARS-CoV-2 replication (47, 48) . Indeed, a study has demonstrated that the SARS-CoV-2 RNA strand terminated by the incorporation of SOF was more resistant to the SARS-CoV-2 exonuclease proofreading activity than the RNA strand terminated by the incorporation of RDV (49) . DCV was likewise shown to bind the SARS-CoV-2 3CL pro (50) . An unpublished study also suggests that treatment with DCV or SOF inhibits SARS-CoV-2 production in vitro (51) .

A series of small clinical trials to evaluate the benefits of the SOF/DCV combination against COVID-19 have been performed in Iran. One of these studies, a randomized, controlled trial, suggests that SOF/DCV shortens the time to recovery and hospital stay of severe COVID-19 patients relative to standard of care (LPVr or hydroxychloroquine [HCQ]), but without significant effects on mortality (52) . SOF/DCV was superior to ribavirin in terms of safety, symptom improvement, mortality, and hospital stay in severe COVID-19 cases in another study (53) . A double-blind, randomized parallel, active-controlled study on outpatients (IRCT20200403046926N1) shows that SOF/DCV with HCQ had a tendency to reduce the rate of hospital admission and a tendency towards faster resolution of appetite loss (54) . Furthermore, SOF/DCV significantly improved dyspnea and fatigue within the 30-day follow-up period, suggesting that SOF/DCV may help patients who suffer from the long-term effects of COVID-19, which include both symptoms. However, these studies are small and are not placebo-controlled; larger randomized trials with placebo controls will have to be conducted to form definite conclusions regarding the treatment benefits of SOF/DCV against COVID-19. More trials to determine the effects of SOF with DCV and with other agents (e.g., NCT04497649, NCT04530422, NCT04460443, and NCT04468087) have been registered.

Favipiravir (T-705) is a nucleoside analog that has been approved for the treatment of novel influenza virus strains in Japan. It is believed to inhibit viral RdRp either by incorporation into the nascent RNA strand as a pseudo-purine or by direct binding to the RdRp (55,56). Although the mechanisms underlying the antiviral activity of favipiravir have not yet been fully elucidated, studies across several other viruses, including, chikungunya virus (57), Rift Valley fever virus (58), HCV (59), and the West Nile virus (60) suggest that it can induce lethal mutagenesis after incorporation into the RNA chain and that it has a broad range of targets.

An in vitro study reports that favipiravir exerts inhibitory effects on SARS-CoV-2 as well (61) . Additionally, a study using a hamster SARS-COV-2 infection model has shown that high doses of favipiravir reduced infectious viral titers in the lungs and reduced transmission to favipiravir-treated hamsters (62) . In an open-labeled comparative controlled study in China, favipiravir treatment led to faster SARS-CoV-2 clearance and to improvement in chest CT scans compared to LPVr treatment (63) . Another trial has reported that while favipiravir did not significantly reduce mortality and improve the overall outcome, it alleviated some of the symptoms, especially cough and pyrexia, suggesting that favipiravir may be beneficial to patients with mild COVID-19 (64) . A prospective, randomized, open-label study on early (day 1 of study participation) or late (day 6 of participation) treatment with favipiravir (jRCTs041190120) on patients with asymptomatic SARS-CoV-2 infection or with mild disease showed that early treatment tended to accelerate viral clearance and defervescence, although the differences between the treatment groups were not significant (65) . Remarkably, reduction in body temperature was observed as early as the day after initiation of treatment in both groups. Larger placebo-controlled clinical trials are needed to further evaluate the treatment benefits of favipiravir. Other clinical trials (e.g., NCT04359615, NCT04464408, NCT04402203, NCT04346628, JapicCTI-205238) have been registered to evaluate the effectivity of favipiravir against the various degrees of COVID-19 severity.

Umifenovir is a non-nucleoside antiviral licensed in China and Russia for the prophylaxis and treatment of influenza. It binds hemagglutinin (HA) on the envelop of the influenza virus to prevent pH-induced conformational changes to HA, thereby inhibiting viral fusion with the host (66) . It has also been demonstrated to inhibit the early stages of infection by disrupting endocytosis of several viruses including the respiratory syncytial virus, hepatitis B virus, adenoviruses, and EBOV, and to inhibit the replication stage of human herpesvirus 8 (67) (68) (69) . The inhibitory effects of umifenovir on SARS-CoV-2 has already been demonstrated in vitro (70) . However, a retrospective study has shown that umifenovir did not improve patient outcomes based on time to reach a double-negative result and on time to symptom recovery (71) . In contrast, a retrospective study in China has reported that the combination of umifenovir and IFN-α2b significantly improved clinical symptoms and CT scans of patients with COVID-19 compared to treatment with IFN-α2b alone, although viral clearance and time to recovery did not differ between combinatorial and single therapy (72) . Additionally, the results of a randomized, controlled trial comparing the effects of HCQ+umifenovir with those of HCQ+LPVr show that umifenovir treatment led to significantly shorter hospital stay 7/24 https://doi.org/10.4110/in.2021.21.e7

Updates on COVID-19 Treatment https://immunenetwork.org and lower ICU admission rates among hospitalized COVID-19 patients (73) . More trials will have to be performed to evaluate the effects of umifenovir on COVID-19 treatment.

CQ is a quinine used for malaria prophylaxis. HCQ is also a quinine used for rheumatoid arthritis and lupus, but it can also be used as prophylaxis against CQ-sensitive malaria. While both are effective against malaria, HCQ is better tolerated than CQ (74) . Both have displayed antiviral activities against a broad range of viruses including DENV, ZIKV, filoviruses, SARS-CoV, etc. in vitro and in animal models, suggesting that it may also exert inhibitory effects on SARS-CoV-2 (75). In most of these viruses, CQ and HCQ are believed to increase endosomal pH, thereby interfering with viral endocytosis, which requires acidic conditions (76). A study has suggested that HCQ with azithromycin accelerates SARS-CoV-2 clearance (77) . However, further studies show that CQ and HCQ do not benefit hospitalized COVID-19 patients, leading the WHO to halt the CQ and HCQ arms of the Solidarity trial (32) . The final results of the RECOVERY group's HCQ trial also show that HCQ does not lower the 28-day mortality rate among hospitalized COVID-19 patients (78) . Given that CQ and HCQ target early stages of infection, several trials are still ongoing to determine whether they can be used as prophylactic agents or for early stages of SARS-CoV-2 infection ( Table 1) .

Statins are cholesterol-lowering agents mainly used to prevent primary or secondary cardiovascular disease. However, they have demonstrated inhibitory effects on several viruses, including ZIKV, DENV, influenza, HIV, and EBOV in vitro (79) . Much of the statins' antiviral capacity is attributed to their ability to inhibit the synthesis of cholesterol, which is important for the formation of lipid rafts that are needed in different stages of viral infection (80) . Angiotensin-converting enzyme (ACE) 2, the primary receptor used by SARS-CoV-2 ( Fig. 1) , is embedded in lipid rafts, suggesting that destabilizing lipid rafts through the inhibition of cholesterol production may inhibit SARS-CoV-2 infection (81) . Moreover, statins have been suggested to have cholesterol-independent anti-inflammatory effects (82) . Furthermore, although evidence have so far been mixed, an observational suggests that continuous or prior use of statins have benefits to patients with sepsis-related ARDS (83) .

Statins seem to have multiple potential targets in the progression of COVID-19: the infection stage, the hyper-inflammatory stage, and ARDS; therefore, statins may be effective against COVID-19 (Table 1) . Indeed, a large retrospective study has associated the in-hospital use of statins with lower morbidity and mortality among COVID-19 patients (84) . Another retrospective study has associated the use of statins with asymptomatic SARS-CoV-2 infection and has shown that the combination of statins with ACE inhibitors and angiotensin II receptor blockers significantly reduced the risks of symptoms and serious disease (85) . Another study suggests that prior statin use reduced the risk of progression to severe COVID-19 and could be linked to faster recovery of severe COVID-19 patients (86) . Supporting these, an unpublished study reports that selective statins, particularly fluvastatin, inhibits SARS-CoV-2 entry (87) . Several controlled trials to evaluate different statins against COVID-19 have already been registered.

Similar to SARS-CoV, SARS-CoV-2 fusion with the host cell requires priming of the spike (S) protein by a cellular protease, transmembrane protease serine 2 (Fig. 1) ; camostat mesylate, a clinically approved serine protease inhibitor, was demonstrated to partially block priming of the S protein (88) . Similarly, nafamostat mesylate, another serine protease inhibitor has been shown to inhibit SARS-CoV-2 S priming and subsequently block virus-host fusion in vitro (89) . Furthermore, nafamostat mesylate was reported to have more potent effects than camostat mesylate. Together, these reports suggest that protease inhibitors are potential candidates for COVID-19 treatment (Table 1) . Additionally, nafamostat has anticoagulatory properties, suggesting that it may be beneficial to the management of the thrombotic stage of COVID-19 (90, 91) . Clinical trials evaluating the effectivity of nafamostat mesylate and camostat mesylate in COVID-19 treatment are ongoing.

IFNs are cytokines involved in the host antiviral defense system, especially in the early stages of infection. They are classified into 3 families: type I (IFN-I: 13-14 IFN-α subtypes, IFN-β, IFN-ε, IFN-κ, IFN-ω, IFN-δ, IFN-ζ, and IFN-τ), type II (IFN-γ), and type III (IFN-λ subtypes) and generally differ by the receptors and signaling pathways they activate.

An early study has suggested that SARS-CoV-2 does not stimulate an IFN response in patients, which may lead to disease progression (92) . Supporting this, another study has shown that mild to moderate COVID-19 patients had higher systemic type I IFN levels than severe patients (93) . Furthermore, mutations in type I IFN-related genes and auto-Abs against type I IFNs have been correlated with critical COVID-19 cases (94, 95 ). An in vitro study also shows that although SARS-CoV-2 infection did not induce IFN production in primary human airway epithelial (pHAE) cells, pretreatment with type I and III IFNs reduced SARS-CoV-2 replication in the pHAE cells (96) . In contrast, IFN-stimulated genes have been reported to be upregulated in COVID-19 patients (97) . Although these studies provide contradictory evidence for the role of IFN in COVID-19 progression, a few clinical studies have already been completed and suggest that type I IFNs alone or in combination with other agents are beneficial to the treatment of mild to moderate COVID-19 ( Table 2 ) (42,98). In particular, early administration of IFN-α2b and IFN-β1 reduced the COVID-19-associated mortality but did not improve recovery time (99, 100) . Additionally, a prospective observational study in Cuba suggests that the use of IFN-α2b in addition to standard treatment (LPVr and CQ) increased the likelihood of recovery and survival among hospitalized COVID-19 patients (101) . Remarkably, adding IFN-β1b to standard LPVr therapy reduced the recovery time and mortality of severe COVID-19 patients (102) . However, the results of the WHO Solidarity trial suggest that IFN-β1 treatment is not beneficial to hospitalized COVID-19 patients (22) . In contrast to this, however, nebulized IFN-β1a increased the likelihood of improvement based on an ordinal scale and led to faster recovery of hospitalized COVID-19 patients in a phase 2 randomized, double-blind, placebo-controlled study (103) . Given the contradictory results of IFN for COVID-19, several clinical trials are ongoing to further explore the benefits of IFN treatment ( 

Passive immunization has a long history of use in the treatment of infectious diseases. Plasma from convalescent patients carry neutralizing Abs (NAbs) and a host of other factors, such as clotting factors and cytokines, that may contribute to therapy. Thus, passive immunization can be used for post-exposure prophylaxis and in disease management. The use of plasma from convalescent COVID-19 patients is one of the treatments that has been considered early into the COVID-19 pandemic ( Table 2) . Case reports and case series have suggested the benefits of convalescent plasma (CP) to severe and critical COVID-19 patients (104, 105) . A randomized controlled trial in China shows that the rate of negative results after 72 hours was higher in the CP group than in the control group among patients with severe disease, and the rate of negative results was higher at certain timepoints among critically ill patients treated with CP (106) . However, other outcomes, such as time to clinical improvement, time to discharge, and 28-day mortality, were not significantly affected by CP treatment. The largest completed CP study to date has reported no significant differences in the outcomes of COVID-19 patients with severe pneumonia treated with CP and with placebo (107) .

Potential risks that may arise from CP therapy include: Ab-dependent enhancement of infection or of disease, exacerbation of the hyper-coagulable state in COVID-19 by clotting factors in the plasma, and transfusion-related lung injury (104) . Moreover, NAb titers from different individuals vary, thereby requiring titration of NAbs per donor and pooling of plasma from a few donors. Thus, to minimize the adverse effects of plasma therapy, to have consistent Ab titers, and to maximize neutralization, the use of mAbs and mAb cocktails have been proposed. Several studies have already identified NAbs that may be used for COVID-19 treatment (108) (109) (110) (111) . The interim analysis of Eli-Lilly's BLAZE-1 trial (NCT04427501) suggests that the administration of LY-CoV555 (bamlanivimab), an IgG1 that targets the SARS-CoV-2 S protein, at early stages of mild-to-moderate COVID-19 results in faster reduction of viral load and in the reduction of hospitalization rates (112) . The combination of LY-CoV555 with LY-CoV016 (etesivimab), which also binds the S protein, is also being evaluated in the same trial. Regeneron's REGN-COV2, a cocktail of 2 anti-SARS-CoV-2 S protein Abs (REGN10933+REGN10987), has also been reported to accelerate viral clearance and symptom recovery in COVID-19 outpatients, especially among those who were seronegative for SARS-CoV-2 Abs or those who had high viral loads at baseline (113) . The results of the phase 1 placebo-controlled trials for Celltrion's CT-P59 (regdanvimab), a mAb that binds the receptor-binding domain of the SARS-CoV-2 S protein, show that CT-P59 accelerated the recovery of patients with mild COVID-19 (114) . A phase 2/3 trial for CT-P59 in COVID-19 outpatients has already been initiated (NCT04602000).

Because high serum IL-6 levels have been consistently observed in severe COVID-19, IL-6 has been proposed as a marker for progression to severe disease and is being widely considered as a target for treatment ( Table 2 ) (115) . Tocilizumab is a humanized mAb that binds the IL-6 receptor and is primarily used for the management of rheumatoid arthritis. Retrospective studies have associated subcutaneous and intravenous tocilizumab administration with improved clinical outcomes and reduced mortality in severe and critical COVID-19 patients (116) (117) (118) (119) . COVIDOSE, a phase 2, single-armed trial, has reported the alleviation of inflammation and faster defervescence following treatment with low-dose tocilizumab in non-critical hospitalized COVID-19 patients (120) . Similarly, the results of EMPACTA, a global phase 3, placebo-controlled trial for tocilizumab, suggest that tocilizumab reduced the likelihood of progression to mechanical ventilation and death in COVID-19 pneumonia 11/24 https://doi.org/10.4110/in.2021.21.e7

Updates on COVID-19 Treatment https://immunenetwork.org patients (121) . However, another randomized, double-blind placebo-controlled study shows that tocilizumab did not reduce the likelihood of death and progression to mechanical ventilation (122) . Similarly, an observational study suggests that tocilizumab does not help in the management of cytokine storm in severe COVID-19 patients (123) . These contradicting results emphasize the need for larger placebo-controlled studies to evaluate the treatment benefits of tocilizumab in COVID-19. On the other hand, the trials for sarilumab, which also targets the IL-6 receptor, have been halted after failing to improve clinical outcomes in severe COVID-19 patients (124) .

Abs that directly bind IL-6 are also being considered for COVID-19 therapy. The unpublished results of SISCO, a phase 2 observational, control cohort study, suggest that siltuximab, a mAb that binds IL-6, reduced the death of COVID-19 ARDS patients who required mechanical support (125); a phase 3 trial (NCT04616586) has therefore been initiated. A phase 2/3 trial to evaluate the effects of olokizumab, another IL-6 inhibitor, has also been recently completed with pending results (NCT04380519). Trials comparing the effects of different IL-6 antagonists have also been registered (NCT04330638, NCT04486521).

Inhibitors of other cytokines implicated in the COVID-19 cytokine storm are also being evaluated (126) . A preliminary study on mavrilimumab, a human mAb that binds the GM-CSFRα, has also been reported to improve clinical outcomes (127) . Furthermore, early results of the phase 2 portion of Kiniksa's phase 2/3 placebo-controlled trial on mavrilimumab (NCT04399980) suggest reduced mortality and shorter duration of mechanical ventilation among patients with severe COVID-19 pneumonia and hyperinflammation (128) . Anakinra, an IL-1 antagonist, has been reported to improve respiratory function, dampen inflammation, and reduce progression to mechanical ventilation in severe COVID-19 patients (129) (130) (131) . Cytokine inhibitor cocktails have also been proposed to maximize the benefits of modulating the immune response (126) . However, more controlled studies will have to be performed to determine the benefits and risks associated with the use of cytokine inhibitor cocktails.

JAK1, JAK2, JAK3, and tyrosine kinase 2 are members of the JAK family of non-receptor tyrosine kinases. They mediate cytokine signaling through the JAK/STAT pathway, making JAK inhibition a plausible option for regulating cytokine-stimulated inflammatory responses in COVID-19 ( Table 2) . Ruxolitinib, which binds the kinase domain of JAK1 and JAK2, was the first approved JAK inhibitor and is currently used for the management of myelofibrosis, hemophagocytic lymphohistiocytosis, and graft-versus-host disease. It has been shown to reduce C-reactive protein, TNF-α, and IL-6 plasma levels in myelofibrosis cases, suggesting that it can be used to reduce inflammation in COVID-19 patients. In a pilot case series, a low starting dose of ruxolitinib was shown to improve clinical scores of patients without major signs of toxicity (132) . Several other studies have suggested that ruxolitinib is beneficial to hospitalized COVID-19 patients, with indications that it can dampen the hyperinflammatory state in patients (133) (134) (135) . However, Novartis has reported that ruxolitinib did not improve clinical outcomes in hospitalized COVID-19 patients in their phase 3 trial (136) . Notably, a study suggests that the combination of ruxolitinib and steroids is beneficial to patients with COVID-19 pneumonia (137) . Whether this combination is an effective COVID-19 treatment will have to be verified in larger studies.

Baricitinib, another JAK1/2 inhibitor, has been reported to significantly reduce the fatality rate and ICU admission rate; accelerate viral clearance; and increase discharge rates in 12/24 https://doi.org/10.4110/in.2021.21.e7

Updates on COVID-19 Treatment https://immunenetwork.org COVID-19 patients with moderate pneumonia, compared to standard-of-care (138) . The NIAID-sponsored ACTT-2 phase 3 trial for baricitinib in combination with RDV suggests that the combination shortens the median recovery time and reduces the 28-day mortality of hospitalized COVID-19 patients; in particular, the combination reduced the time to recovery of patients under non-invasive oxygen support from 18 days (RDV only) to 10 days (139) . Several trials to evaluate the COVID-19 treatment benefits of baricitinib (e.g., NCT04421027, NCT04373044, NCT04640168) have already been registered.

One potential drawback to the use of JAK1/2 inhibitors against COVID-19 is their ability to target several types of cytokines, some of which (i.e., IFNs) are needed for viral clearance. Thus, other JAK inhibitors that can selectively target certain cytokines are also being considered. Tofacitinib, a potent inhibitor of JAK3, which is not involved in the IFNγ pathway, is already being evaluated in phase 2 trials for COVID-19 (e.g., NCT04415151, NCT04469114). Likewise, fedratinib a JAK2 inhibitor, is expected to not disrupt the type I IFN pathways and has already been seen to reduce cytokine production by Th17 cells in vitro, suggesting that it can be used for COVID-19 treatment (140).

Corticosteroids were initially not recommended for COVID-19 treatment based on existing data on MERS-CoV and SARS-CoV wherein corticosteroids delayed viral clearance; under the assumption that disease severity is associated with high viremia, the use of corticosteroids in COVID-19 was hypothesized to lead to severe disease and viral sepsis (141) . However, given the current evidence that inflammation plays a primary role in COVID-19 progression, anti-inflammatory drugs were deemed viable candidates for the treatment of severe COVID-19 ( Table 2) .

A controlled, open-label trial (RECOVERY) has shown that a 10-day course of dexamethasone reduced the 28-day mortality among COVID-19 patients who were receiving respiratory support (142) . Moreover, dexamethasone reduced the risk of progression to invasive ventilation in patients receiving oxygen support. Notably, improvements were not observed among patients who did not receive respiratory support. These findings suggest that dexamethasone is beneficial to COVID-19 patients in later stages of infection, where pulmonary and systemic inflammatory damage is present. Although another trial (COVID-19 Dexamethasone) shows that while dexamethasone failed to significantly reduce in mortality of COVID-19 patients with moderate or severe ARDS, dexamethasone significantly increased the number of days alive and the number of days the patients were free of mechanical ventilation (143) .

Due to the announcement of the RECOVERY findings, enrollment for corticosteroid trials were halted, but a few trials have reported their results. A randomized clinical study shows that hydrocortisone did not significantly improve outcomes in terms of mortality and persistent mechanical ventilation in critical COVID-19 cases, but the study may have been underpowered due to premature termination of enrolment (144) . The REMAP-CAP randomized clinical trial, on the other hand, suggested probable superiority of a fixed dose and shock-dependent dosing of hydrocortisone in patients with severe COVID-19, although definitive conclusions could not be made (145) . In light of the RECOVERY findings, the WHO Rapid Evidence Appraisal for COVID-19 Therapies (REACT) working group pooled data from 7 randomized controlled studies and performed meta-analysis to evaluate the effects of corticosteroids (dexamethasone, hydrocortisone, and methylprednisolone) in critically ill COVID-19 patients. The REACT meta-analysis associated the use of systemic corticosteroids with lower 28-day mortality among COVID-19 patients, leading to the WHO's strong recommendation to use corticosteroids in severe and critical COVID-19 patients and to a conditional recommendation to use corticosteroids in non-severe cases (146, 147) . The benefits and risks of corticosteroids in non-severe COVID-19 patients; the long-term effects of corticosteroid use in COVID-19 survivors; and the optimal timing of corticosteroid administration will still have to be evaluated in future studies.

The COVID-19 pandemic has spurred global cooperation for the speedy evaluation of drug and vaccine candidates. In a short span of time, several therapeutic candidates have already been tested, discontinued, reconsidered, or recommended based on the results of collaborative efforts between research institutes, clinical practitioners, and manufacturing companies. However, the current efforts still leave much to be desired. For example, the conflicting results of the WHO Solidarity and RECOVERY trials on RDV emphasize the need for standardized study designs and clinical outcomes to obtain coherent and conclusive evidence in large clinical studies.

Additionally, stratification of patients based on the different phases of COVID-19 is important in determining the optimal timepoint for any intervention. As we have presented, COVID-19 treatment candidates can be grouped based on modes of action (Fig. 1) and target stages in COVID-19 progression (Tables 1 and 2) . The early stages of COVID-19 can be targeted using agents that promote viral clearance, which include antivirals (e.g. nucleoside analogs), approved drugs with non-antiviral indications but with antiviral potential (e.g. statins, CQ/HCQ, camostat mesylate, and nafamostat mesylate), IFNs, and NAbs. The middle stages of COVID-19, which is characterized by the decline in viral replication and the start of the hyperinflammatory response, can be targeted using agents that can dampen the inflammatory response, such as cytokine inhibitors, JAK inhibitors, and corticosteroids. Finally, the late stages of COVID-19, which is characterized by hyperinflammation, thrombosis, and other critical manifestations, can be managed using anti-inflammatory and anticoagulatory (e.g. nafamostat mesylate, camostat mesylate) agents.

Understandably, because of the initial surge in hospital burden early into the COVID-19 pandemic, most of the therapeutic approaches, including antivirals and potential antivirals, were tested on hospitalized patients. However, this may explain why most of the candidates failed to show COVID-19 treatment benefits; antivirals and IFNs are expected to be beneficial early into the course of the disease, where viral replication is at its peak, and would be less helpful in alleviating inflammation and other complications that arise in later stages of COVID-19. This may also be the reason why corticosteroids, which target a stage of COVID-19 that coincides with patient hospitalization, are, thus far, the only treatment with strong conclusive evidence. Recently, trials have been directed to COVID-19 outpatients and to mild-to-moderate COVID-19 patients. Some of the agents (e.g. CQ/HCQ, LPVr) are also being tested as prophylaxis for individuals with high risks of exposure to SARS-CoV-2 (e.g. healthcare workers). Various combinations of the candidates are also being looked into for potential synergistic effects. Hopefully, the results of all these studies will reveal early 14/24 https://doi.org/10.4110/in.2021.21.e7

Updates on COVID-19 Treatment https://immunenetwork.org treatments that will accelerate viral clearance and prevent COVID-19 progression to reduce the hospital burden, morbidity, and mortality associated with SARS-CoV-2 infection.

Although SARS-CoV-2 vaccines have been granted emergency-use authorization in various countries, several months are needed for the global vaccine roll out and to reach the desired level of community immunity. Furthermore, long-term data are needed to ascertain whether vaccination confers long-term protection or will have to be frequently administered (e.g., every few years). Thus, prophylactic and therapeutic agents are expected to fill in the gap in virus control measures left by the ongoing vaccination efforts. Clinical evaluations of candidates for COVID-19 treatment therefore remain invaluable to the management of the SARS-CoV-2 pandemic. Hopefully, the learnings from this pandemic, especially the organization of rapid but thorough clinical evaluations for pharmaceutical interventions and the effective strategies in discovering and developing antiviral treatments, will arm us for future large viral outbreaks.

Genomic characterisation and epidemiology of 2019 novel coronavirus: implications for virus origins and receptor binding

A pneumonia outbreak associated with a new coronavirus of probable bat origin

The Johns Hopkins Coronavirus Resource Center (CRC). COVID-19 dashboard by the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University (JHU)

The incubation period of coronavirus disease 2019 (COVID-19) from publicly reported confirmed cases: estimation and application

Clinical characteristics of coronavirus disease 2019 in China

Multisystem inflammatory syndrome in children: a systematic review

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

Remdesivir is a directacting antiviral that inhibits RNA-dependent RNA polymerase from severe acute respiratory syndrome coronavirus 2 with high potency

A randomized, controlled trial of Ebola virus disease therapeutics

Remdesivir against COVID-19 and other viral diseases

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

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

Compassionate use of remdesivir for patients with severe COVID-19

Remdesivir for the treatment of COVID-19 -final report

Repurposed antiviral drugs for COVID-19 -interim WHO Solidarity trial results

Kaletra (lopinavir/ritonavir)

In silico prediction of potential inhibitors for the main protease of SARS-CoV-2 using molecular docking and dynamics simulation based drug-repurposing

Role of lopinavir/ritonavir in the treatment of SARS: initial virological and clinical findings

Post-exposure prophylaxis for Middle East respiratory syndrome in healthcare workers

Clinical characteristics and therapeutic procedure for four cases with 2019 novel coronavirus pneumonia receiving combined Chinese and Western medicine treatment

Case of the index patient who caused tertiary transmission of coronavirus disease 2019 in Korea: the application of lopinavir/ritonavir for the treatment of COVID-19 pneumonia monitored by quantitative RT-PCR

Efficacy and safety of lopinavir/ritonavir or arbidol in adult patients with mild/moderate COVID-19: an exploratory randomized controlled trial

Lopinavir-ritonavir in patients admitted to hospital with COVID-19 (RECOVERY): a randomised, controlled, open-label, platform trial

Solidarity" clinical trial for COVID-19 treatments

The predominant mechanism by which ribavirin exerts its antiviral activity in vitro against flaviviruses and paramyxoviruses is mediated by inhibition of IMP dehydrogenase

RNA virus error catastrophe: direct molecular test by using ribavirin

Modeling shows that the NS5A inhibitor daclatasvir has two modes of action and yields a shorter estimate of the hepatitis C virus half-life

Sofosbuvir, a NS5B polymerase inhibitor in the treatment of hepatitis C: a review of its clinical potential

Beyond members of the Flaviviridae family, sofosbuvir also inhibits chikungunya virus replication

Anti-HCV, nucleotide inhibitors, repurposing against COVID-19

Sofosbuvir as a potential alternative to treat the SARS-CoV-2 epidemic

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

Structure-based virtual screening of phytochemicals and repurposing of FDA approved antiviral drugs unravels lead molecules as potential inhibitors of coronavirus 3C-like protease enzyme

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

Sofosbuvir and daclatasvir compared with standard of care in the treatment of patients admitted to hospital with moderate or severe coronavirus infection (COVID-19): a randomized controlled trial

The impact of sofosbuvir/daclatasvir or ribavirin in patients with severe COVID-19

Sofosbuvir and daclatasvir for the treatment of COVID-19 outpatients: a double-blind, randomized controlled trial

Extinction of West Nile virus by favipiravir through lethal mutagenesis

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

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

Experimental treatment with favipiravir for COVID-19: an open-label control study

Favipiravir versus arbidol for COVID-19: a randomized clinical trial

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

Structural basis of influenza virus fusion inhibition by the antiviral drug Arbidol

The synthetic antiviral drug arbidol inhibits globally prevalent pathogenic viruses

Antiviral activity of arbidol against influenza A virus, respiratory syncytial virus, rhinovirus, coxsackie virus and adenovirus in vitro and in vivo

Identification of compounds targeting hepatitis B virus core protein dimerization through a split luciferase complementation assay

The anti-influenza virus drug, arbidol is an efficient inhibitor of SARS-CoV-2 in vitro

Umifenovir treatment is not associated with improved outcomes in patients with coronavirus disease 2019: a retrospective study

Arbidol/IFN-α2b therapy for patients with corona virus disease 2019: a retrospective multicenter cohort study

Effect of Arbidol (umifenovir) on COVID-19: a randomized controlled trial

New insights on the antiviral effects of chloroquine against coronavirus: what to expect for COVID-19?

Hydroxychloroquine and azithromycin as a treatment of COVID-19: results of an openlabel non-randomized clinical trial

Effect of hydroxychloroquine in hospitalized patients with COVID-19

Antiviral effects of statins

Function of membrane rafts in viral lifecycles and host cellular response

Coronavirus interplay with lipid rafts and autophagy unveils promising therapeutic targets

The anti-inflammatory effects of statins on coronary artery disease: an updated review of the literature

Impact of statin therapy on mortality in patients with sepsis-associated acute respiratory distress syndrome (ARDS) depends on ARDS severity: a prospective observational cohort study

In-hospital use of statins is associated with a reduced risk of mortality among individuals with COVID-19

The effects of ARBs, ACEis, and statins on clinical outcomes of COVID-19 infection among nursing home residents

Relation of statin use prior to admission to severity and recovery among COVID-19 inpatients

Effect of statins on SARS-CoV-2 infection

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

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

The use of nafamostat mesilate as an anticoagulant during continuous renal replacement therapy for children with a high risk of bleeding

Nafamostat mesilate as an anticoagulant during continuous renal replacement therapy in patients with high bleeding risk: a randomized clinical trial

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

Inborn errors of type I IFN immunity in patients with life-threatening COVID-19

Autoantibodies against type I IFNs in patients with life-threatening COVID-19

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

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

A clinical pilot study on the safety and efficacy of aerosol inhalation treatment of IFN-κ plus TFF2 in patients with moderate COVID-19

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

A randomized clinical trial of the efficacy and safety of interferon β-1a in treatment of severe COVID-19

Therapeutic effectiveness of interferon-α2b against COVID-19: the cuban experience

Interferon β-1b in treatment of severe COVID-19: a randomized clinical trial

Safety and efficacy of inhaled nebulised interferon beta-1a (SNG001) for treatment of SARS-CoV-2 infection: a randomised, double-blind, placebo-controlled, phase 2 trial

Convalescent plasma therapy for COVID-19: state of the art

Life-saving effect of convalescent plasma treatment in COVID-19 disease: clinical trial from eastern Anatolia

Effect of convalescent plasma therapy on time to clinical improvement in patients with severe and life-threatening COVID-19: a randomized clinical trial

A randomized trial of convalescent plasma in COVID-19 severe pneumonia

A human monoclonal antibody blocking SARS-CoV-2 infection

Human-IgG-neutralizing monoclonal antibodies block the SARS-CoV-2 infection

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

Potent neutralizing antibodies from COVID-19 patients define multiple targets of vulnerability

SARS-CoV-2 neutralizing antibody LY-CoV555 in outpatients with COVID-19

REGN-COV2, a neutralizing antibody cocktail, in outpatients with COVID-19

Celltrion presents efficacy and safety data for potential COVID-19 treatment candidate CT-P59 in patients with mild symptoms

An inflammatory cytokine signature predicts COVID-19 severity and survival

Tocilizumab in patients with severe COVID-19: a retrospective cohort study

Tocilizumab among patients with COVID-19 in the intensive care unit: a multicentre observational study

Effective treatment of severe COVID-19 patients with tocilizumab

Tocilizumab therapy reduced intensive care unit admissions and/or mortality in COVID-19 patients

COVIDOSE: a phase II clinical trial of low-dose tocilizumab in the treatment of noncritical COVID-19 pneumonia

Tocilizumab in patients hospitalized with COVID-19 pneumonia

Efficacy of tocilizumab in patients hospitalized with COVID-19

Impact of tocilizumab administration on mortality in severe COVID-19

Sanofi provides update on Kevzara (sarilumab) phase 3 trial in severe and critically ill COVID-19 patients outside the U.S

IL-6 signalling pathway inactivation with siltuximab in patients with COVID-19 respiratory failure: an observational cohort study

Focus shifts to antibody cocktails for COVID-19 cytokine storm

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

Kiniksa announces data from U.S. investigator-initiated study of mavrilimumab in severe COVID-19 pneumonia and hyperinflammation

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

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

Anakinra in hospitalized patients with severe COVID-19 pneumonia requiring oxygen therapy: results of a prospective, open-label, interventional study

The Janus kinase 1/2 inhibitor ruxolitinib in COVID-19 with severe systemic hyperinflammation

Ruxolitinib rapidly reduces acute respiratory distress syndrome in COVID-19 disease. Analysis of data collection from RESPIRE protocol

Ruxolitinib in treatment of severe coronavirus disease 2019 (COVID-19): a multicenter, single-blind, randomized controlled trial

Ruxolitinib for tocilizumab-refractory severe COVID-19 infection

Novartis provides update on RUXCOVID study of ruxolitinib for hospitalized patients with COVID-19

Low-dose ruxolitinib plus steroid in severe SARS-CoV-2 pneumonia

Beneficial impact of Baricitinib in COVID-19 moderate pneumonia; multicentre study

Baricitinib plus remdesivir for hospitalized adults with COVID-19

TH17 responses in cytokine storm of COVID-19: an emerging target of JAK2 inhibitor Fedratinib

Clinical evidence does not support corticosteroid treatment for 2019-nCoV lung injury

Dexamethasone in hospitalized patients with COVID-19 -preliminary report

Effect of dexamethasone on days alive and ventilator-free in patients with moderate or severe acute respiratory distress syndrome and COVID-19: the CoDEX randomized clinical trial

Effect of hydrocortisone on 21-day mortality or respiratory support among critically ill patients with COVID-19: a randomized clinical trial

Effect of hydrocortisone on mortality and organ support in patients with severe COVID-19: the REMAP-CAP COVID-19 corticosteroid domain randomized clinical trial

Association between administration of systemic corticosteroids and mortality among critically ill patients with COVID-19: a meta-analysis

Corticosteroids for COVID-19: living guidance

This work was supported by Korea University Research Grants (K1826621, K2005541 and K2023291). We thank Younsik Kim for the illustration.