Overview of the status of the development of antiviral drugs for COVID-19

Remdesivir was originally developed by Gilead Sciences in 2009 for the treatment of hepatitis C and was then repurposed for Ebola virus disease. Remdesivir has been recently recognized as a promising antiviral drug against a broad spectrum of RNA viruses in cultured cells, mice, and non‐human primate models (Scavone et al., 2020). The antiviral mechanism of remdesivir includes delayed cessation of the nascent viral RNA chain (Al-Tawfiq et al., 2020). Cao et al. (2020a) reported that SARS-CoV-2 entered target cells by allowing the S protein to bind to the angiotensin-converting enzyme-2 receptor on the cell surface and remdesivir, the nucleotide analogue that acts as a RNA-dependant RNA polymerase (RdRp) inhibitor, blocked RNA replication (Cao et al., 2020) (Table 2).

First, Gilead Science reported that in this cohort of hospitalized patients with severe COVID-19 that were treated with compassionate-use remdesivir, clinical improvement was observed in 36 out of 53 patients (68%). Measurement of efficacy will require ongoing randomized, placebo-controlled trials of remdesivir therapy (Grein et al., 2020). Wang et al. (2020a) reported that two compounds, remdesivir and chloroquine, potently blocked viral infection at low-micromolar concentrations and showed high selectivity index (Wang et al., 2020). Choy et al. (2020) reported that remdesivir inhibited SARS-CoV-2 replication in Vero E6 cells at a half-maximal effective concentration (EC₅₀₎ of 23.15 μM. Lopinavir, but not ritonavir, inhibited SARS-CoV-2 replication at an EC₅₀ of 26.63 μM. Homoharringtonine and emetine inhibited SARS-CoV-2 replication at EC₅₀ values of 2.55 μM and 0.46 μM, respectively (Choy et al., 2020). A combination of remdesivir and emetine showed synergistic effects in vitro.

Williamson et al. (2020) reported that animals treated with remdesivir showed reduced pulmonary infiltrates on radiographs and no signs of respiratory disease (Williamson et al., 2020). Virus titres in bronchoalveolar lavages were significantly reduced as early as 12 h after administration of the first treatment. On performing necropsy on day 7 after inoculation, lung viral loads of remdesivir-treated animals were significantly lower and a clear reduction in lung tissue damage was observed. Thus, these findings supported early remdesivir treatment initiation in COVID-19 patients to prevent progression to severe pneumonia (Williamson et al., 2020).

Wang et al. (2020c) reported that remdesivir treatment was not associated with a difference in the time to clinical improvement (Wang et al., 2020). Although not statistically significant, a numerically faster time to clinical improvement was observed in remdesivir-treated patients than in placebo-treated patients who showed symptoms for at most 10 days. Adverse events were reported in 102 out of 155 (66%) remdesivir recipients, compared to 50 out of 78 (64%) placebo recipients. Adverse events occurred in 18 (12%) patients for whom remdesivir was stopped early, compared to 4 (5%) patients for whom placebo was stopped early. Wang et al. (2020) concluded that in this study of adult hospitalized COVID-19 patients, remdesivir was not associated with statistically significant clinical benefits. However, the numerical reduction in the time to clinical improvement in those treated with this drug earlier requires confirmation using a larger number of samples.

According to the results of our survey, even though the effect of remdesivir was not as high as we expected, it is considered the most valuable antiviral drug till date. For this reason, the FDA approved remdesivir to be used as a COVID-19 treatment, and the Korean government is urgently approving its use and pushing for its purchase. Although some researchers recently argued that remdesivir was ineffective for Asians, we assume that it will most likely be used as a treatment for COVID-19, especially if no new drug candidates are identified in the near future. Nevertheless, earlier clinical trials at the Centers for disease control and prevention (CDC) reported side effects of remdesivir, such as nausea, vomiting, stomach paralysis, liver damage, and temporary gastrointestinal symptoms including rectal bleeding. Therefore, further verification of the exact effects of remdesivir is required.

Chloroquine is used to prevent and treat malaria, and is being studied for COVID-19 treatment (Cortegiani et al., 2020). Chloroquine inhibits the pre-entry step in the viral cycle by interfering with the binding of the viral particles to their cellular surface receptors (Devaux et al., 2020). Chloroquine inhibits activation of cells by MAP kinase and post-translational modification of M proteins, thereby altering viral assembly and budding (Kumar et al., 2020). For this reason, it is being studied as a candidate for COVID-19 treatment, and currently, 23 clinical trials of chloroquine are underway in China (Cortegiani et al., 2020).

In early in vitro studies, chloroquine was found to block COVID-19 infection at low-micromolar concentrations, an EC₅₀ value of 1.13 μM, and a half-cytotoxic concentration >100 μM (Gao et al., 2020). Keyaerts et al. (2004) reported that chloroquine inhibited SARS-CoV replication in Vero E6 cells (Keyaerts et al., 2004). Since immunopathological factors may play a significant role in SARS-CoV infection, it will be of interest to study whether chloroquine is also effective in modulating the inflammatory responses to SARS-CoV infection (Keyaerts et al., 2004). Wang et al. (2020a) also found that chloroquine functioned at both the entry and post-entry stages of the 2019-nCoV infection in Vero E6 cells (Wang et al., 2020).

Gao et al. (2020) suggested that approximately 100 chloroquine-treated COVID-19 patients, compared to the controls, experienced a more rapid decline in fever and an improvement in the lung computed tomography (CT) images and required a shorter time to recover, showing no obvious serious adverse effects (Gao et al., 2020). The Chinese medical advisory board has suggested the inclusion of chloroquine in the SARS-CoV-2 treatment guidelines (Devaux et al., 2020). Chen et al. (2020c) reported that the body temperature recovery and cough remission times were significantly shortened in the hydroxychloroquine treatment group. Besides, more patients with improved pneumonia were observed in the hydroxychloroquine treatment group (80.6%, 25 out of 31) than in the control group (54.8%, 17 out of 31) (Chen et al., 2020). However, two patients in the hydroxychloroquine treatment group developed mild adverse reactions. Therefore, Chen et al. (2020c) suggested that hydroxychloroquine treatment could significantly shorten the time to clinical recovery and promote the absorption of pneumonia (Chen et al., 2020).

In France, the combination of hydroxychloroquine and azithromycin was used for the treatment of 1061 COVID-19 patients (46.4% male, mean age 43.6 years, age range 14–95 years) (Million et al., 2020). Good clinical outcomes and virological cure were observed in 973 patients within 10 days (91.7%). Prolonged viral carriage was observed in 47 patients (4.4%), and was associated with a higher viral load at diagnosis; however, viral culture was negative on day 10. A poor clinical outcome was observed for 46 patients (4.3%), and 8 patients (0.75%, age range 74–95 years) died. Therefore, Million et al. (2020) suggested that the administration of the hydroxychloroquine and azithromycin combination before the occurrence of COVID-19 complications was safe and was associated with a very low patient fatality rate (Million et al., 2020).

In contrast, Mehra et al. (2020) reported that compared with the control group, the hydroxychloroquine, hydroxychloroquine plus a macrolide, chloroquine, and chloroquine plus a macrolide treatment groups independently showed an association with an increased risk of de-novo ventricular arrhythmia during hospitalization (Mehra et al., 2020). Mehra et al. (2020) confirmed no benefits of hydroxychloroquine or chloroquine, when used alone or with a macrolide, in the in-hospital COVID-19 outcomes (Mehra et al., 2020). Each of these drug regimens, when used for COVID-19 treatment, was associated with decreased in-hospital survival and an increased frequency of ventricular arrhythmias (Mehra et al., 2020). Although a number of in vivo clinical trials are underway, there is limited evidence of the in vitro activity of chloroquine/hydroxychloroquine against SARS-CoV-2 (Gbinigie and Frie, 2020). The empirical data available from two of these trials revealed conflicting results (Gbinigie and Frie, 2020). Both trials were characterized by a small number of participants (n=30 and n=36, respectively), and suffered methodological limitations(Gbinigie and Frie, 2020). No medium- or long-term follow-up data was available (Gbinigie and Frie, 2020). Additionally, there are several reports on the side effects of chloroquine, such as severe diarrhoea, hearing loss, and increased burden on the heart. Hence, further verification of the exact effects of chloroquine/hydroxychloroquine is required.

Niclosamide is an FDA-approved anthelminthic drug that has been widely used in humans to treat tapeworm infections for several decades, and is currently listed on the World Health Organization’s list of essential medicines (Xu et al., 2020). After screening a small-marketed drug library, Wu et al. (2020) suggested that niclosamide inhibited SARS-CoV replication and completely abolished viral antigen synthesis at a concentration of 1.56 μM. Niclosamide suppressed the cytopathic effect (CPE) of SARS-CoV at a concentration as low as 1 μM and inhibited SARS-CoV replication at an EC₅₀ value < 0.1 μM in Vero E6 cells (Wu et al., 2004). Gassen et al. (2019) reported that niclosamide inhibited the S-Phase kinase-associated protein 2, enhanced autophagy, and reduced MERS-CoV replication (Gassen et al., 2019). This could be the potential antiviral mechanism of niclosamide against SARS-CoV-2. Gassen et al. (2020) also found that niclosamide inhibited SARS34 CoV-2 propagation by 85%, 88%, and > 99%, respectively in vitro (Gassen et al., 2020). Jeon et al. (2020) screened a panel of 48 FDA-approved SARS-CoV-2 drugs, which were pre-selected by a SARS-CoV assay, and identified 24 potential antiviral drug candidates against SARS-CoV-2 infection. Niclosamide exhibited highly potent antiviral activity against SARS-CoV-2 at a half-maximal inhibitory concentration [IC₅₀] of 0.2871 μM (Jeon et al., 2020). Since there are insufficient studies on niclosamide, further clinical trials must be conducted to verify its efficacy.

Ivermectin is an FDA-approved broad-spectrum anti-parasitic agent, and has been shown to have antiviral activity against a broad range of viruses in vitro (Caly et al., 2020). Caly et al. (2020) reported that viral RNA was reduced (99.98%) after 49 h of ivermectin (5 μM) treatment in SARS-CoV-2 infected cells (Caly et al., 2020). Taken together, these results demonstrated that a single dose of ivermectin showed antiviral action against the SARS-CoV-2 clinical isolate in vitro within 24–48 h by controlling viral replication. Caly et al. (2020) hypothesized that ivermectin possibly inhibited importin α/β1 heterodimer (IMPα/β1)-mediated nuclear import of viral proteins (Caly et al., 2020). Paz et al. (2020) suggested that ivermectin was capable of docking with the 3CL protease and the HR2 domain, possibly promoting structural changes in these proteins by inducing unfolding/folding (Paz et al., 2020). Specifically, ivermectin brings the protease to a significantly more deployed conformational state and the HR2 domain to a more compact state, compared to their native states (Paz et al., 2020). These results suggested a possible inhibitory effect of ivermectin on SARS-CoV-2 survival due to the synergistic role of this drug in spontaneously binding with two important proteins involved in viral proliferation (Paz et al., 2020). In contrast, Momekov and Momekova (2020) reported that the available pharmacokinetic data on ivermectin indicated that the SARS-CoV-2 inhibitory concentrations were not practically attainable at the doses routinely used for parasitic disease management (Momekov and Momekova, 2020). Therefore, Momekov and Momekova (2020) suggested that because ivermectin failed to demonstrate antiviral effects beyond the in vitro level, its application in COVID-19 patients must be decisively discouraged until the paucity of reliable data from controlled studies has been resolved.

Lopinavir/ritonavir is a medication used in combination with other medications to treat human immunodeficiency virus (HIV)-1 infection in adults and children over 14 days of age (Dong et al., 2020). There are some reports on the use of lopinavir/ritonavir for COVID-19 treatment (Kim et al., 2020). Lim et al. (2020) reported that within a day of lopinavir/ritonavir administration (2 tablets [lopinavir 200 mg/ritonavir 50 mg]), the β-coronavirus load started to decrease and no detectable or lesser coronavirus titres were observed (lopinavir/ritonavir was started on day 8 of hospitalization [day 10 of illness]) (Lim et al., 2020). Wang et al. (2020b) reported that after lopinavir/ritonavir treatment of four COVID-19 patients, three patients showed significant improvement of pneumonia-associated symptoms, two of whom were then confirmed to be COVID-19 negative and discharged. The remaining patient was negative for the virus according to the first test (Wang et al., 2020). Yao et al. (2020) suggested that this study showed the positive effects of lopinavir/ritonavir therapy (Yao et al., 2020). Ye et al. (2020b) reported that compared with the treatment using pneumonia-associated adjuvant drugs alone, the combination treatment using lopinavir/ritonavir and adjuvant drugs showed a more evident therapeutic effect in lowering the body temperature and restoring normal physiological mechanisms, with no evident toxic and side effects (Ye et al., 2020). Therefore, Ye et al. (2020b) suggested that the use of lopinavir/ritonavir, combined with pneumonia-associated adjuvant drugs, for the clinical treatment of COVID-19 patients should be promoted. Yu et al. (2020) also suggested that the combination of lopinavir/ritonavir, interferon (IFN), and arbidol could be a good choice for antiviral therapy, especially in adults (Yu et al., 2020). Deng et al. (2020) analysed 16 patients who received oral arbidol and lopinavir/ritonavir in the combination group and 17 patients who received only oral lopinavir/ritonavir in the monotherapy group; after 14 days, SARS-CoV-2 could not be detected in 15 out of 16 (94%) and 9 out of 17 (52.9%) patients, respectively (Deng et al., 2020). The chest CT scans improved for 11 out of 16 (69%) patients in the combination group, compared with 5 out of 17 (29%) patients in the monotherapy group, after 7 days. Based on these results, Deng et al. (2020) suggested that arbidol combined with lopinavir/ritonavir could delay the progression of lung lesions and lower the possibility of respiratory and gastrointestinal transmission by decreasing the COVID-19 viral load and the high faecal COVID-19 viral concentration.

In contrast, Cao et al. (2020b) recently reported the results of a clinical trial that included 199 patients with laboratory-confirmed SARS-CoV-2 infection (Cao et al., 2020). They found that lopinavir/ritonavir treatment, compared to standard care, was not associated with a difference in the time to clinical improvement and mortality at 28 days, and viral the RNA percentage was similar in both the lopinavir/ritonavir treatment and standard care groups. However, in a modified intention-to-treat analysis, lopinavir/ritonavir treatment, compared to standard care, led to a median time to clinical improvement that was shorter by 1 day. Gastrointestinal adverse events were more common in the lopinavir/ritonavir treatment group, but serious adverse events were more common in the standard care group. Therefore, Cao et al. (2020b) concluded that no benefit beyond standard care was observed in patients on lopinavir/ritonavir treatment. As mentioned above, Dalerba et al. (2020) reported different experimental results of the antiviral effects of lopinavir/ritonavir treatment (Dalerba et al., 2020). They reported that after reviewing the findings of Cao et al. (2020b), many clinicians were abandoning the use of lopinavir/ritonavir for COVID-19 treatment. This action was considered premature (Dalerba et al., 2020). According to Dalerba et al. (2020), it was crucial to realize that the trial was statistically underpowered to show a significantly better time to clinical improvement in lopinavir/ritonavir-treated severe COVID-19 patients, compared to patients administered standard care. Dalerba et al. (2020) also suggested that because the SARS-CoV-2 pandemic diffusion is causing a shortage in alternative drugs, lopinavir/ritonavir must be retained as a treatment option against COVID-19. We strongly agree with this suggestion by Dalerba et al. (2020). Even though several previous studies have shown that lopinavir/ritonavir does not show therapeutic effects against COVID-19, future studies on lopinavir/ritonavir must be continued. Further clinical trials for various patient groups (different gender, age, health status, race, or vaccination status) are required in the near future. It is also necessary to study the efficacy of lopinavir/ritonavir further through different combinations of various drugs, which could be used if another virus emerges in the future.

Favipiravir is a pyrazine carboxamide derivative and a broad-spectrum antiviral drug approved in Japan for influenza treatment. Cai et al. (2020) examined the effects of favipiravir and lopinavir/ritonavir for COVID-19 treatment (Cai et al., 2020). COVID-19 patients who received oral favipiravir plus IFN-α by aerosol inhalation were included in the favipiravir treatment group in this study, whereas patients who received lopinavir/ritonavir plus IFN-α by aerosol inhalation were included in the control group. The favipiravir treatment group, compared to the control group, showed significant improvement in chest images. Multivariable Cox regression showed that favipiravir was independently associated with faster viral clearance. Additionally, fewer adverse events were observed in the favipiravir-treated patients than in the controls. Therefore, Cai et al. (2020) suggested that favipiravir showed better therapeutic responses to COVID-19 in terms of disease progression and viral clearance.

Shinoda et al. (2020) reported that on days 3 and 4 after favipiravir administration, the polymerase chain reaction (PCR) results were negative, and the preliminary results of the clinical studies showed that favipiravir showed promising potency in treating Chinese SARS-CoV-2 patients. Subjects treated with favipiravir tested negative for the virus in the PCR analysis at the early stage; it showed potential in preventing acute respiratory distress syndrome. Based on these results, Shinoda et al. (2020) concluded that favipiravir administration in a patient with rapidly progressing hypoxemia caused the symptoms to be alleviated rapidly and PCR to yield negative results early (Shinoda et al.). Thus, favipiravir was effective for COVID-19 treatment, but it is necessary to conduct trials on more cases to ascertain its efficacy (Shinoda et al.). Chen et al. (2020a) conducted clinical trials of favipiravir (116 patients) and umifenovir (120 patients) for COVID-19 treatment in Wuhan, China. They found that favipiravir (1,600 mg twice on day 1 and then 600 mg twice daily for another 7–10 days) showed superior efficacy in terms of increased rate of clinical recovery by day 7 and reduced incidence of fever and cough. The clinical recovery rate at day 7 was 55.8% in the umifenovir group and 71.4% in the favipiravir group. Despite these findings, some media spokespersons and experts raised concerns about the government rushing to approve favipiravir and undermining strict medical procedures in place to authorize its use. Even though favipiravir was approved by the National Medical Products Administration of China as the first anti-COVID-19 drug in China (Yanai, 2020) in March 2020, the Japanese government decided to postpone the approval of favipiravir (Avigan^®) for COVID-19 treatment until at least June on the grounds of insufficient clinical tests and the presence of side effects (including birth defects). Based on this survey, we believe that since the side effects of favipiravir are being reported, it is still early to decide whether favipiravir can be used for COVID-19 treatment.

β-D-N⁴-hydroxycytidine (NHC; EIDD-1931), an available oral prodrug, is a ribonucleoside analogue with broad-spectrum antiviral activity against multiple RNA viruses, including the current SARS-CoV-2, SARS-CoV-1, and MERS-CoV (Neerukonda and Katneni, 2020). Sheahan et al. (2020) reported that NHC showed a broad-spectrum antiviral activity against SARS-CoV-2, MERS-CoV, SARS-CoV, and a related zoonotic group 2b or 2c bat-coronavirus. In SARS-CoV- or MERS-CoV-infected mice, both prophylactic and therapeutic administration of EIDD-2801, a bioavailable oral NHC prodrug (β-D-N⁴-hydroxycytidine-5′-isopropyl ester), improved pulmonary function and reduced viral titre and body weight loss (Sheahan et al., 2020). NHC/EIDD-2801 potency against multiple coronaviruses and oral bioavailability highlight its potential utility as an effective antiviral agent against SARS-CoV-2 and other future zoonotic coronaviruses (Sheahan et al., 2020). The FDA has approved an Investigational New Drug application for EIDD-2801, allowing human clinical testing to be initiated in the USa.

Convalescent plasma has been used for the treatment of SARS, pandemic 2009 influenza A (H1N1), avian influenza A (H5N1), several haemorrhagic fevers such as Ebola fever, and other viral infections (Roback and Guarner, 2020). Deploying passive antibody therapies against the rapidly increasing number of COVID-19 cases provides an open window for the clinical testing of antiviral drugs (Roback and Guarner, 2020). Convalescent plasma or immunoglobulins (Igs) have been used as a last resort to improve the survival rate of SARS patients, whose condition continued to deteriorate despite treatment with pulsed methylprednisolone (Chen et al., 2020). One possible explanation for the efficacy of convalescent plasma therapy is that the antibodies from convalescent plasma possibly suppress viraemia (Chen et al., 2020). The administration of convalescent plasma is not a common treatment, but it is an important treatment in the absence of specific treatment agents for new infectious diseases (Ahn et al., 2020). Ahn et al. (2020) reported that convalescent plasma was obtained from a male donor (in his 20s) who had recovered from COVID-19 for 18 days. In this study, two patients presented with severe pneumonia and acute respiratory distress syndrome and showed a favourable outcome after convalescent plasma and systemic corticosteroid administration.

In a study by Shen et al. (2020), patients received a transfusion of convalescent plasma with a SARS-CoV-2-specific antibody (IgG) that had been obtained from five patients that had recovered from COVID-19 (Sheahan et al., 2020). Following plasma transfusion, the body temperature normalized within 3 days, the sequential organ failure assessment score decreased, and Pao₂/Fio₂ ratio increased within 12 days in four out of five patients. Viral loads also decreased and became negative within 12 days after the transfusion. Based on these results, Shen et al. (2020) concluded that administration of convalescent plasma that contained neutralizing antibodies was followed by an improvement in the patient clinical status. Ye et al. (2020a) reported positive findings of convalescent plasma test, with no adverse effects being observed during the treatment (Ye et al., 2020). Convalescent plasma transfusion led to the resolution of the ground glass opacities and consolidations in the patient chest images. In two patients whose throat swabs tested positive for SARS-CoV-2, convalescent plasma therapy elicited virus elimination. Serologic analysis indicated an immediate increase in the anti-SARS-CoV-2 antibody titres in the two patients. Thus, based on these results, Ye et al. (2020a) suggested that convalescent plasma therapy was effective and specific for COVID-19.

Meanwhile, Zeng et al. (2020) performed a clinical test using convalescent plasma and reported that six patients with COVID-19 and respiratory failure received convalescent plasma 21.5 days (median) after viral shedding was first detected (Zeng et al., 2020). All patients tested negative for SARS-CoV-2 RNA within 3 days of plasma infusion, and five patients eventually died. Thus, based on these results, Zeng et al. (2020) concluded that convalescent plasma treatment could eliminate SARS-CoV-2 shedding, but could not reduce the mortality rate of critically ill patients with end-stage COVID-19. Hence, treatment should be initiated at an earlier stage. According to our survey, there remains a lack of research on the treatment of COVID-19 using convalescent plasma to date. Clinical trials of convalescent plasma require convalescent plasma donation from a sufficient number of patients who have recovered from COVID-19. To date, more than 10,000 people in Korea have recovered, and approximately 150 recovered patients have donated their convalescent plasma. Therefore, it appears that a global collaboration is required to obtain convalescent plasma until the development of effective antiviral drugs.