Dear editor,
Since December 2019, a novel disease COVID-19 caused by severe acute respiratory syndrome
coronavirus 2 (SARS-CoV-2) rapidly spread to over 200 countries and infected over
1.50 million people including 92,798 deaths (data as of April 10, 2020). On March
11, the World Health Organization (WHO) characterized COVID-19 as a pandemic, and
called for accelerating diagnostics, vaccines, and drugs developments to combat this
novel disease. Apart of the new coronavirus, influenza virus infections have been
a consistent threat to the global public health over the years. In the United States
alone, the Centers for Disease Control and Prevention (CDC) estimates that, so far
during the 2019–2020 winter season, there have been at least 39 million illnesses,
400,000 hospitalizations and 24,000 deaths from influenza (https://www.cdc.gov/flu/weekly/index.htm).
Considering the current concomitant circulation of SARS-CoV-2 and influenza virus
infections, the exploration of available and viable anti-influenza drugs to treat
both diseases is of great interest.
Actually, in the early stages of the outbreak of COVID-19, some anti-flu drugs (for
example, oseltamivir) have been applied for the treatment of COVID-19 patients
1,2
. Previously, we reported that favipiravir (T705), an anti-influenza drug approved
in Japan and China, showed a certain efficacy against SARS-CoV-2 in vitro
3
. In addition, arbidol, an anti-influenza drug targeting the viral hemagglutinin (HA)
is being used in a clinical trial against COVID-19 (ChiCTR2000029573) and has been
recently added to the Guidelines for the Diagnosis and Treatment of COVID-19 (sixth
and seventh editions) in China. A recent retrospective study suggested that arbidol
treatment showed tendency to improve the discharging rate and decrease the mortality
rate of COVID-19 patients
4
. However, to our knowledge, there has been no systematical analysis about the efficacy
of anti-influenza drugs against SARS-CoV-2.
In this study, we evaluated six currently available and licensed anti-influenza drugs
against SARS-CoV-2. The drugs include arbidol, baloxavir, laninamivir, oseltamivir,
peramivir, and zanamivir
5,6
. The M2 inhibitors (amantadine and rimantadine) were not considered in this study
since they were not recommended for treating influenza by WHO due to drug resistance.
First, the cytotoxicity of the compounds in African green monkey kidney cells, Vero
E6 (ATCC-1586) was measured by a standard cell counting kit-8 (CCK8) assay. Then,
the cells were infected with SARS-CoV-2 at a multiplicity of infection (MOI) of 0.05
in the presence of either compound or dimethyl sulfoxide (DMSO) control. The dose–response
curves were determined by quantification of viral RNA copy numbers in the supernatant
of infected cell at 48 h post infection (p.i.). As demonstrated in Fig. 1a, arbidol
efficiently inhibited virus infection in vitro. The 50% maximal effective concentration
(EC50) and the 50% cytotoxic concentration (CC50) of arbidol was 4.11 (3.55–4.73)
and 31.79 (29.89–33.81) μM, respectively, and the selectivity index (SI = CC50/EC50)
was 7.73. Baloxavir partially inhibited SARS-CoV-2 infection (~29%) at a high concentration
of 50 μM (Fig. 1a). In contrast, laninamivir, oseltamivir, peramivir, and zanamivir
did not exhibit anti-SARS-CoV-2 activity even at the highest drug concentrations (Fig.
1a). The antiviral effect of the compounds was also evaluated by observing cytopathic
effects (CPE) and immunofluorescence staining of infected cells. As shown in Supplementary
Fig. S1, at 48 h p.i. only in cells treated with arbidol, but not with the other five
drugs, viral NP expression and CPE due to SARS-CoV-2 was substantially reduced. To
be noted, we also tried some human lung cell lines, for example human embryo lung
fibroblasts MRC-5 and lung cancer cell line Calu-3, however, they were not very efficient
for SARS-CoV-2 replication, and therefore were not used for this study.
Fig. 1
Comparative antiviral efficacy of anti-influenza drugs and the mode of actions of
arbidol against SARS-CoV-2 infection in vitro.
a Antiviral activities of the drugs. The antiviral efficacy was evaluated in Vero
E6 cells by qRT-PCR analysis of virus yield at 48 h p.i. Data represent the mean ± standard
deviation (SD) from two independent repeats. b, c Time-of-addition experiment of arbidol.
Three experimental groups (Full-time, Entry, and Post-entry) were set up as described
in the Supplementary Methods. At 16 h p.i., virus yield in the cell supernatant was
quantified by qRT-PCR (b), and the expression of NP in infected cells was analyzed
by western blots (c). The values below the blot represent the relative band intensity
(NP/GAPDH) normalized to that of the DMSO group. d Impact of arbidol on SARS-CoV-2
binding. Vero E6 cells were treated with arbidol (10 μM) or DMSO for 1 h prior to
infection with SARS-CoV-2 at 4 °C for 1 h. The supernatant (unbound virions) and the
cells containing bound virions (bound virions) were collected for quantification of
viral RNA copies by qRT-PCR. e, f Effect of arbidol on intracellular trafficking of
SARS-CoV-2. The co-localization of virions with EEs or LEs was analyzed by immunofluorescence
assays as described in the Supplementary Methods. e The portion of virions that co-localized
with EEs or ELs in each group (n > 150 cells) was quantified by Image J. f Representative
confocal microscopic images of virions (red) and LAMP1+ ELs (green) in each group.
The nuclei (blue) were stained with Hoechst 33258 dye. White arrows: virions co-localized
with ELs; bars: 10 μm. For (b) and (e), statistical analysis was performed using a
one-way analysis of variance (ANOVA) with GraphPad Prism. For (d), statistical analysis
was performed and calculated by unpaired two-tailed t test. *P < 0.05; ***P < 0.001;
ns, not significant.
Apart from influenza virus, arbidol was reported to inhibit a wide array of viruses
by interfering with multiple steps of the virus replication cycle
7
. The stage of SARS-CoV-2 replication targeted by arbidol was explored by conducting
a preliminary time-of-addition experiment using virus at an MOI of 0.05. Arbidol was
incubated with cells during the virus entry process (Entry), the post-entry stages
(Post-entry), or the entire process of infection (Full-time) and progeny virus yield
was quantified by qRT-PCR. The data revealed that arbidol efficiently blocked both
viral entry and post-entry stages. It had a profound impact on virus Entry (~75% inhibition)
with a lesser effect on Post-entry events (~55% inhibition rate) (Fig. 1b). In addition,
western blot analysis (Fig. 1c) and immunofluorescence microscopy (Supplementary Fig.
S2) confirmed that the expression level of viral NP was reduced drastically at Full-time
(13% of the DMSO group, Fig. 1c), and showed more inhibitory effect at the Entry stage
(41%) than at the Post-entry stage (61%).
The details of how arbidol blocks the entry of SARS-CoV-2 into cells were further
investigated. Virus (MOI = 0.05) was allowed to bind to Vero E6 cells at 4 °C for
1 h in the presence of arbidol (10 μM) or DMSO control. Virus particles bound to the
cell (bound virions) and those in the supernatant (unbound virions) were analyzed
by qRT-PCR. The results showed that arbidol treatment led to a significantly decreased
binding efficiency (67%) compared with the control group (P < 0.05) (Fig. 1d). Correspondingly,
the portion of unbound virions increased significantly to 156% of the control group
after arbidol treatment (P < 0.001) (Fig. 1d).
Next, we analyzed viral intracellular trafficking. As we reported recently, within
infected cells, SARS-CoV-2 underwent vesicle transportation, which was first carried
out by early endosomes (EEs) then further transported to endolysosomes (ELs)
8
. Co-localization of virions with EEs or ELs was visualized by immunofluorescence
microscopy and statistically analyzed (n > 150 cells). As shown in Fig. 1e and Supplementary
Fig. S3, in each tracked time points, there was no significant difference in the amounts
of virions co-localized with EEs when comparing the DMSO- and arbidol-treated groups,
although as time of infection went on (30, 60, and 90 min p.i.), the levels of co-localization
considerably decreased in both DMSO- (24.0%, 5.1%, and 3.2%) and arbidol- (21.4%,
4.1%, and 2.8%) treated groups, suggesting that some virions were already transported
from EEs to the next stage of vesicle transportation. By contrast, at 60 min p.i.,
a slightly higher percentage of virions were transported to ELs in the arbidol-treated
group (22.4%) than in the DMSO group (18.3%) (P < 0.05) (Fig. 1e, f). At 90 min p.i.,
significantly fewer virions (~13.5%) were detected in ELs in the DMSO group; whereas
significantly higher proportions of virions (~23.6%) remained within ELs in the arbidol-treated
group, suggesting the drug trapped the virus in the ELs (P < 0.001) (Fig. 1e, f).
Taken together, these results suggested that arbidol impeded not only viral attachment,
but also release of SARS-CoV-2 from intracellular vesicles (ELs).
Among the drugs tested, laninamivir, oseltamivir, peramivir, and zanamivir are neuraminidase
(NA) inhibitors, which are most widely prescribed for prophylaxis and treatment of
influenza. Although no NA analog exists in SARS-CoV-2, NA inhibitors such as oseltamivir
nevertheless are being used clinically in treating COVID-19 patients
1,2
. Our data showed these NA inhibitors were not active against SARS-CoV-2 (Fig. 1a),
which is consistent with the finding that oseltamivir and zanamivir were ineffective
in inhibiting SARS-CoV
9
. Baloxavir marboxil is a new anti-influenza drug, which selectively inhibits the
endonuclease activity of the viral polymerase responsible for snatching capped primers
from host mRNAs to initiate viral mRNA transcription. However, this “cap-snatching”
mechanism of the endonuclease is not shared by coronaviruses that encode their own
enzymes to form 5ʹ-mRNA cap structures
10
. This may explain why baloxavir failed to block SARS-CoV-2 infection (Fig. 1a). During
the review process of this study, Choy et al. also showed that oseltamivir and baloxavir
failed to inhibit SARS-CoV-2 in vitro
11
.
Arbidol, an indole-derivative, has been licensed for decades in Russia and China against
influenza. It is a broad-spectrum drug against a wide range of enveloped and non-enveloped
viruses. Arbidol interacts preferentially with aromatic amino acids, and it affects
multiple stages of the virus life cycle, either by direct targeting viral proteins
or virus-associated host factors
7
. For example, in influenza virus, crystal structures showed that arbidol inserted
into a hydrophobic pocket of the fusion subunit of HA, thus hindering low-pH conformational
change of HA and blocking the fusion process
12
. In hepatitis C virus, arbidol impaired both virus attachment and intracellular vesicle
trafficking
13
. Likewise, we found arbidol plays a role in interfering SAS-CoV-2 binding (Fig. 1d)
and intracellular vesicle trafficking (Fig. 1e, f). Arbidol can also bind to lipid
membranes and may alter membrane configuration of the cytoplasm or the endosome, which
are crucial for viral attachment and fusion
7
. It could be further investigated whether arbidol targets virus or/and cells by using
published method
14
.
In summary, among the six anti-influenza drugs, only arbidol efficiently inhibited
SARS-CoV-2 infection. Functionally, it appears to block virus entry by impeding viral
attachment and release from the ELs. Although the SI of arbidol is relatively low
(SI = 7.73), as a repurposed drug, its pharmacokinetics profile such as maximal concentration
(Cmax) is more important for predicting efficacy. It is generally believed that if
the Cmax achieves EC90, the drug is very likely to be effective; while if the Cmax
achieves EC50, the drug is possibly effective in vivo. In humans, a single oral administration
of 800 mg of arbidol results in Cmax of ~4.1 μM
15
, and this dosage is efficacious and safe against different influenza viruses with
EC50 values ranging from 2.5–20 μM
7,16
. Arbidol also showed anti-inflammatory activity, which may enhance its efficacy in
vivo
16
. Considering the EC50 (4.11 μM) of arbidol against SARS-CoV-2 is comparable to, or
even lower than those of influenza viruses, we, therefore, suggest that arbidol is
potentially effective to treat COVID-19 patients. However, the current dose of arbidol
(200 mg, 3 times/day) recommended by the Chinese Guidelines may not be able to achieve
an ideal therapeutic efficacy to inhibit SARS-CoV-2 infection, and should be elevated.
This needs to be verified by clinical trials.
Supplementary information
Supplementary Figs. S1-S3