At the end of 2019, a new type of pneumonia emerged in Wuhan, central China, and rapidly spread through the entire world. Lacking effective drugs for this disease significantly hindered the treatment at the very beginning. Not until Lu Roujian et al. unveiled the results of pathogen isolation and genomic characterization [1], a new human-infected beta-coronavirus was found and named as SARS-CoV-2 by ICTV (International Committee on Taxonomy of Viruses, ICTV) [2]. Currently, coronavirus infectious disease-19 (COVID-19) has been announced as a pandemic by the WHO and became the focus of the world as the number of patients infected with COVID-19 has increased up to over 4,400,000, with a mortality rate over 6% by May 15th, 2020. SARS-CoV-2 shares some similarities with MERS-CoV (with about 50% identity) and SARS-CoV (about 79% identity), and notably it shares a similar receptor-binding domain structure to that of SARS-CoV. Consequently, antiviral drugs that had proved to be effective in the previous SARS or MERS treatment have been tested in COVID-19 treatment, though SARS-CoV and MERS-CoV disappeared suddenly and no further antiviral studies were continued. To deal with the pandemic crisis, scientists have registered hundreds of clinical trials around world, with varied antiviral drugs, study designs, and endpoints et al.

Additionally, preliminary results showed that remdesivir (REM) had vigorous antiviral activity against SARS-CoV-2 in vitro [3]. Besides, due to lack of effective agents, some other antiviral agents, such as darunavir (DRV), was used to test the efficacy against SARS-CoV-2. However, in vitro study showed DRV had no antiviral activity against SARS-CoV-2 at clinically relevant concentrations (EC50 > 100 µM) [4]. Currently, the NIH COVID-19 treatment guidelines updated the antiviral section [5]: REM was recommended as the investigational antiviral agent as it has not yet been approved by the FDA and it was available through an FDA emergency use authorization for the treatment of COVID-19 in hospitalized patients with severe disease. Some latest results of clinical trials have been released, and favipiravir (FPV) and arbidol (ARB; umefenovir) showed potential antiviral efficacy against COVID-19. However, due to the varied clinical research design, it is crucial to conduct a systematic review of the published data, in order to provide rationale evidence for clinicians in choosing anti-viral drugs to combate COVID-19.

Though the COVID-19 pandemic is currently under control in some countries, the WHO has announced COVID-19 as an international public health emergency. Thanks to the dedication of hard-working scientists and clinicians around the world, novel agents, or new activity of currently used drugs i.e. drug repurposing, such as chloroquine (CQ) and its analogue hydroxychloroquine (HCQ) were investigated for their anti-viral activity [3]. However, the specific pharmacokinetic characteristics (long time elimination, CYP3A4 inhibition that may induce various drug-to-drug interactions, etc.) and safety concerns (e.g. cardiac side effects) of CQ and HCQ limited their clinical application [18, 19]. REM was first developed as an antiviral drug against SARS-CoV and MERS-CoV [20, 21], both viruses of which suddenly disappeared in humans, thus markedly diminishing clinical research. LPV/RTV as well as ARB were proven to have activity either in in vitro studies as well as in animal models or for post-exposure prophylaxiss [22, 23], and they were being investigated in hundreds of registered clinical trials worldwide. According to our network meta-analysis, FPV was the best candidate antiviral drug in terms of both efficacy (i.e. CT improvements and nucleic acid positive-to-negative conversion rate) and safety.

FPV was first approved in Japan and a recent research unveiled its activity against COVID-19 [14]. FPV, as a novel RNA-dependent RNA polymerase (RdRp) inhibitor, shows inhibitory effects on a wide range of RNA viruses, such as the influenza virus. Retrospective analysis of patients with Ebola virus disease (EVD) indicated that FPV treatment showed advantages in overall survival [24]. The genomic characteristics of SARS-CoV-2 is that it contains a single-stranded RNA beta-coronavirus genome harboring the RdRp gene, which shares great similarity with SARS-CoV as well as MERS-CoV [1]. Preclinical research demonstrated the efficacy of FPV against SARS-CoV-2 in vitro (EC50 = 61.9 µM determined using monkey African green kidney Vero E6 cells) [3]. Two clinical trials have investigated the comparative effectiveness and safety of FPV versus ritonavir-boosted lopinavir or ARB [14, 15], with randomized or controlled study, suggesting moderate-to-high level evidence. The most common adverse events of FPV were digestive tract reactions (nausea and diarrhea) and increased serum uric acid, side effects of which were irreversible. CFDA (China Food and Drug Administration, CFDA) approved FPV on the 16^th Feb of 2020 as an antiviral agent for COVID-19. Consequently, the results of CT improvements, nucleic acid positive-to-negative rate, the efficacy and safety of FPV were apparently encouraging. These results provide clinicians with an apparently promising candidate for COVID-19 treatment.

REM was first developed as an adenosine analogue against EBOLA virus, which undergoes incorporation into nascent viral RNA chains and results in premature termination of viral replication. Actually, it is a prodrug of an adenosine analogue that has a broad antiviral spectrum, including all human and animal coronaviruses tested in vitro. As the first antiviral drug proved to be effective against SARS-CoV-2 in vitro [3], REM got high expectations especially after the first case in the United States recovered by REM treatment [25]. There are 8 registered clinical trials in the NIH, 3 of which have been completed and the results reported. However, the results of REM seem controversial: a multicenter, randomized, double-blind clinical trial launched in severe COVID-19 patients in China indicates REM is not associated with statistically significant clinical benefits, though it shortened the time to clinical improvements [16] (NCT04257656). The clinical trial of REM in adults with mild and moderate COVID-19 was suspended due to lack of patients. NIH clinical trials named “Adaptive COVID-19 Treatment Trial” (ACTT) launched in the United States showed that REM accelerated patient recovery from advanced COVID-19. Patients who received REM had a 31% faster time to recovery than those who received placebo. Besides, a favorable mortality rate was also observed in REM group (p = 0.059) (NCT04280705). The ACTT trial designed to be completed on April 1^st, 2023, so we have only included Cao Bin's research in this manuscript due to lack of full data of another clinical study. In fact, the inconsistency of the REM results is related to the inconsistent criteria for efficacy evaluation. The clinical study launched by Cao Bin et al designed a six-point scale to evaluate the clinical improvements by at least two-point reduction in patients' admission status within 28 days after randomization. The primary clinical outcome of ACTT was the time to recovery within 29 days, which defined a three-point scale. Due to the circumstances of COVID-19 pandemic, Gilead Inc announced the latest news that the Japanese Ministry of Health, Labor and Welfare (MHLW) has approved REM as a treatment for severe patients COVID-19 patients. Generally, the activity of REM to COVID-19 patients in clinical improvements, mortality, as well as frequencies of adverse effects, needs to be confirmed in large-sample clinical study.

As for LPV/RTV, the drug-to-drug interaction and side effects could not be ignored. LPV/RTV were first designed and developed as a peptidomimetic HIV protease inhibitors and tested against COVID-19 treatment due to its previous studies on SARS-CoV [19, 26] and its availability at the very beginning of the epidemic in Wuhan. The clinical trial completed by Cao Bin et al focused on severe COVID-19 patients, though LPV/RTV treatment could reduce the time to clinical improvement (15 days versus 16 days of standard care), LPV/RTV was not associated with statistically significant clinical benefits [17]. When compared with ARB, the latter was more superior to LPV/RTV in nucleic acid positive-to-negative conversion [11]. Additionally, a clinical trial launched by Guangzhou Eighth People's Hospital demonstrated that LPV/RTV, as well as ARB had little benefit in improving the clinical outcome of mild/moderate COVID-19, and LPV/RTV was associated with adverse reactions [13] , that was a major concern of LPV/RTV in clinical application. Ritonavir-boosted lopinavir could increase the bioavailability of lopinavir through inhibition of the CYP3A enzyme [27]. Lymphopenia was the most frequent and severe adverse effect of LPV/RTV during the treatment of COVID-19 [17]. The digestive tract side effects including diarrhea, vomiting, and nausea, were more common than that observed upon FPV treatment [25]. As the CYP3A enzyme was the principal enzyme of metabolism, LPV/RTV might induce various drug-to-drug interactions [28]. Some commonly applied agents in COVID-19 treatment including CQ, HCQ as well as moxifloxacin had strong interactions with LPV/RTV, leading to increased the serum concentration of those agents. That may also result in increasing side effects of LPV/RTV. Additionally, LPV/RTV monotherapy was inferior to other interventions concerning CT improvements, nucleic acid conversion rates, and adverse reactions et al.

ARB displayed better efficacy than LPV/RTV in nucleic acid conversion rate and fewer safety concerns. ARB is a non-nucleoside analogue designed and developed by Russian scientists. As a hemagglutinin inhibitor, it could block the entry of viruses and induce the secretion of interferon and display broad antiviral activity against influenza, human respiratory syncytial (HRSV), adenovirus, Coxsackie B5, parainfluenza, Ebola (EBOV), and hepatitis B and C viruses [29]. Previous studies showed its efficacy in inhibiting SARS-CoV as well as MERS-CoV [30, 31]. Based on this, professor Li Lanjuan et al., tested the antiviral activity of ARB and showed that it could significantly inhibit SARS-CoV-2 at the concentrations of ~10-30 µM in vitro. Due to the COVID-19 crisis, ARB was then recommended in the Chinese Clinical Guidance for the treatment COVID-19. According to our meta-analysis results, ARB was superior to LPV/RTV in the 7 days’ treatment in the nucleic acid conversion rate. The incidence of adverse effects of ARB was less than LPV/RTV, though no statistical significance was found.

The current literature review study has some limitations: (ⅰ) There is limited antiviral clinical research included due to lack of published data, and only three randomized clinical trials were included in our review, which may increase the risks of bias; (ⅱ) Only one REM clinical trial was included and due to varied assessment scale, there is limited data in 28-days mortality and direct comparison. The therapeutic efficacy and safety of REM need to be validated in future studies; (ⅲ) The majority of the sample size in most of the studies was small which may be due to the drug accessibility and the pandemic crisis.

Additionally, recent study have unveiled the fact that antioxidants and heme oxygenase contribute a lot in pathogenesis and progression of COVID-19 [32-34]. Clinical observations also reported the paradox of COVID-19 hospital admissions that fewer cigarette-smokers are admitted compared with non-smokers in general population, which could be explained by the fact that long-time smoke could induce the expression of HO-1, which plays a vital role in antioxidant and cytoprotective properties and could limit the infection’s damage [32]. Ahmed S. et al. found that alveoloar type Ⅱ cell SOD3 was the top-ranked gene that was most down-regulated in the elderly compared with young healthy donors through single cell RNA sequencing [33]. Taken together, the decreased ability in antioxidant may aggravate COVID-19 progression, especially in the elderly. Therapies with novel anti-oxidant agents as well as promisiong antiviral agents should be taken into consideration in order to prevent or treat COVID-19 disease.

In conclusion, FPV was superior to other antiviral agents in clinical improvement indicators (CT improvements, cough relief, nucleic acid conversion rate, and incidence of adverse reactions). ARB had advantages in nucleic acid conversion rate and the incidence of side effects. The activity of REM needs to be validated in more clinical research. Above all, the encouraging results of FPV may provide clinicians a rationale choice for COVID-19 treatment.

We would like to express our gratitude to all those who dedicated themselves in the COVID-19 pandemics and thanks to all the peer reviewers and editors for their opinions and suggestions.

The authors declare no conflict of interest.